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Contents

Authors Preface

1. Introduction Gary D. Hammer, MD, PhD, & Stephen J. McPhee, MD

2. Genetic Disease Shane C. Quinonez, MD, & Gregory Barsh, MD, PhD

3. Disorders of the Immune System Jeffrey L. Kishiyama, MD, Jennifer J. Chang, MD, & Suzanne M. Donovan, MD, MPH

4. Infectious Diseases Christina T. Fiske, MD, MPH, & Karen C. Bloch, MD, MPH

5. Neoplasia Mark M. Moasser, MD, & Weiyun Z. Ai, PhD, MD

6. Blood Disorders J. Ben Davoren, MD, PhD, & Gerald Hsu, MD, PhD

7. Nervous System Disorders Vikram G. Shakkottai, MD, PhD, & Catherine Lomen-Hoerth MD, PhD

8. Diseases of the Skin Laura B. Pincus, MD, & Timothy H. McCalmont, MD

9. Pulmonary Disease

Thomas H. Sisson, MD, Dru Claar, MD, Mark S. Chesnutt, MD, & Thomas J. Prendergast, MD

10. Cardiovascular Disorders: Heart Disease Fred M. Kusumoto, MD, FACC

11. Cardiovascular Disorders: Vascular Disease Igor Mitrovic, MD

12. Disorders of the Adrenal Medulla Lauren Fishbein, MD, PhD, & Tobias Else, MD

13. Gastrointestinal Disease Jason C. Mills, MD, PhD, AGAF, & Matthew A. Ciorba, MD

14. Liver Disease Nizar A. Mukhtar, MD, & Mandana Khalili, MD, MAS

15. Disorders of the Exocrine Pancreas Timothy L. Frankel, MD, & Christopher J. Sonnenday, MD, MHS

16. Renal Disease Rachel L. Perlman, MD, & Michael Heung, MD, MS

17. Disorders of the Parathyroids & Calcium and Phosphorus Metabolism Dolores M. Shoback, MD

18. Disorders of the Endocrine Pancreas Janet L. Funk, MD

19. Disorders of the Hypothalamus & Pituitary Gland Tobias Else, MD, & Gary D. Hammer, MD, PhD

20. Thyroid Disease Nazanene H. Esfandiari, MD, & Stephen J. McPhee, MD

21. Disorders of the Adrenal Cortex Tobias Else, MD, & Gary D. Hammer, MD, PhD

22. Disorders of the Female Reproductive Tract Erika B. Johnston-MacAnanny, MD, Bansari G. Patel, MD, & Robert N. Taylor, MD, PhD

23. Disorders of the Male Reproductive Tract Mikkel Fode, MD, PhD, Jens Sønksen, MD, PhD, & Dana A. Ohl, MD

24. Inf lammatory Rheumatic Diseases Allan C. Gelber, MD, MPH, PhD, Stuart M. Levine, MD, FACP, & Erika Darrah, PhD

25. Case Study Answers Yeong S. Kwok, MD

Index

Authors

Weiyun Z. Ai, PhD, MD Assistant Clinical Professor of Medicine Division of Hematology/Oncology Helen Diller Family Comprehensive Cancer Center University of California, San Francisco School of Medicine San Francisco, California [email protected] Neoplasia

Gregory Barsh, MD, PhD Professor of Genetics and Pediatrics, Emeritus Stanford University School of Medicine Stanford, California [email protected] Genetic Disease

Karen C. Bloch, MD, MPH Associate Professor of Medicine and Health Policy Division of Infectious Diseases Department of Medicine Vanderbilt University School of Medicine Nashville, Tennessee [email protected] Infectious Diseases

Jennifer J. Chang, MD, AAHIVS Fellow in HIV Medicine David Geffen School of Medicine at the University of California, Los Angeles Los Angeles, California

[email protected] Disorders of the Immune System

Mark S. Chesnutt, MD Professor of Medicine Pulmonary & Critical Care Medicine Department of Medicine and Dotter Department of Interventional Radiology Oregon Health & Science University Director, Critical Care VA Portland Health Care System Portland, Oregon [email protected] Pulmonary Disease

Matthew A. Ciorba, MD Associate Professor of Medicine Director, Inflammatory Bowel Disease Program Division of Biology & Biomedical Sciences Division of Gastroenterology Washington University School of Medicine St. Louis, Missouri [email protected] Gastrointestinal Disease

Dru Claar, MD House Officer, Internal Medicine (Pulmonary) Pulmonary & Critical Care Medicine University of Michigan Hospitals Ann Arbor, Michigan [email protected] Pulmonary Disease

Erika Darrah, PhD Assistant Professor of Medicine Division of Rheumatology Department of Medicine Johns Hopkins University School of Medicine Baltimore, Maryland

[email protected] Inflammatory Rheumatic Diseases

J. Ben Davoren, MD, PhD Clinical Professor of Medicine Associate Chief of Staff, Clinical Informatics San Francisco Veterans Affairs Medical Center Division of Hematology/Oncology Department of Medicine University of California, San Francisco School of Medicine San Francisco, California [email protected] Blood Disorders

Suzanne M. Donovan, MD, MPH Clinical Professor of Medicine Division of Infectious Diseases Department of Medicine David Geffen School of Medicine at the University of California, Los Angeles [email protected] Disorders of the Immune System

Tobias Else, MD Assistant Professor of Internal Medicine Division of Metabolism, Endocrinology & Diabetes Department of Internal Medicine University of Michigan Medical School Ann Arbor, Michigan [email protected] Disorders of the Adrenal Medulla, Disorders of the Hypothalamus & Pituitary Gland, Disorders of the Adrenal Cortex

Nazanene H. Esfandiari, MD Associate Professor of Internal Medicine Division of Metabolism, Endocrinology & Diabetes Department of Internal Medicine University of Michigan Medical School Ann Arbor, Michigan

[email protected] Thyroid Disease

Lauren Fishbein, MD, PhD Assistant Professor of Medicine Division of Endocrinology, Metabolism and Diabetes University of Colorado School of Medicine Aurora, Colorado [email protected] Disorders of the Adrenal Medulla

Christina T. Fiske, MD, MPH Assistant Professor of Medicine Division of Infectious Diseases Department of Medicine Vanderbilt University School of Medicine Nashville, Tennessee [email protected] Infectious Diseases

Mikkel Fode, MD, PhD Department of Urology Herlev & Gentofte Hospital University of Copenhagen Herlev, Denmark [email protected] Disorders of the Male Reproductive Tract

Timothy L. Frankel, MD Assistant Professor of Surgery Department of Surgery University of Michigan Medical School Ann Arbor, Michigan [email protected] Disorders of the Exocrine Pancreas

Janet L. Funk, MD Professor of Medicine Division of Endocrinology

University of Arizona School of Medicine Tucson, Arizona [email protected] Disorders of the Endocrine Pancreas

Allan C. Gelber, MD, MPH, PhD Professor of Medicine Division of Rheumatology Department of Medicine Johns Hopkins University School of Medicine Baltimore, Maryland [email protected] Inflammatory Rheumatic Diseases

Gary D. Hammer, MD, PhD Millie Schembechler Professor of Adrenal Cancer Director, Endocrine Oncology Program Rogel Cancer Center University of Michigan Ann Arbor, Michigan [email protected] Introduction, Disorders of the Hypothalamus & Pituitary Gland, Disorders of the Adrenal Cortex

Michael Heung, MD, MS Associate Professor of Internal Medicine Division of Nephrology Department of Internal Medicine University of Michigan Medical School Ann Arbor, Michigan [email protected] Renal Disease

Gerald Hsu, MD, PhD Assistant Clinical Professor of Medicine Division of Hematology/Oncology Department of Medicine San Francisco Veterans Affairs Medical Center University of California, San Francisco School of Medicine

San Francisco, California [email protected] Blood Disorders

Erika B. Johnston-MacAnanny, MD Attending Physician Shady Grove Fertility Center Women’s Hospital 1602 Richmond, VA [email protected] Disorders of the Female Reproductive Tract

Mandana Khalili, MD, MAS Professor of Medicine Director of Clinical Hepatology Zuckerberg San Francisco General Hospital Department of Medicine University of California San Francisco School of Medicine San Francisco, California [email protected] Liver Disease

Jeffrey L. Kishiyama, MD Associate Clinical Professor Division of Allergy and Immunology Department of Medicine University of California, San Francisco School of Medicine San Francisco, California Disorders of the Immune System

Fred M. Kusumoto, MD, FACC Professor of Medicine Division of Cardiovascular Diseases Department of Medicine Mayo Clinic Jacksonville Jacksonville, Florida [email protected] Cardiovascular Disorders: Heart Disease

Yeong S. Kwok, MD Clinical Assistant Professor Division of General Internal Medicine Department of Internal Medicine University of Michigan Medical School Ann Arbor, Michigan [email protected] Case Studies, Case Study Answers

Stuart M. Levine, MD, FACP President & Chief Medical Officer MedStar Harbor Hospital Senior Vice President MedStar Health Baltimore, Maryland [email protected] Inflammatory Rheumatic Diseases

Catherine Lomen-Hoerth, MD, PhD Professor of Neurology ALS Research Center at UCSF Department of Neurology University of California, San Francisco School of Medicine San Francisco, California [email protected] Nervous System Disorders

Timothy H. McCalmont, MD Professor of Pathology, Dermatology, and Oral Medicine Departments of Pathology and Dermatology University of California, San Francisco San Francisco, California [email protected] Diseases of the Skin

Stephen J. McPhee, MD Professor of Medicine, Emeritus Division of General Internal Medicine Department of Medicine

University of California, San Francisco School of Medicine San Francisco, California [email protected] Introduction, Thyroid Disease

Jason C. Mills, MD, PhD, AGAF Professor of Medicine Division of Gastroenterology Departments of Medicine, Pathology & Immunology, Developmental Biology

Washington University School of Medicine St. Louis, Missouri [email protected] Gastrointestinal Disease

Igor Mitrovic, MD Jack and DeLoris Lange Endowed Chair in Systems Physiology I Professor of Physiology Department of Physiology University of California, San Francisco School of Medicine San Francisco, California [email protected] Cardiovascular Disorders: Vascular Disease

Mark M. Moasser, MD Professor of Medicine Division of Hematology/Oncology Department of Medicine Helen Diller Family Comprehensive Cancer Center University of California, San Francisco School of Medicine San Francisco, California [email protected] Neoplasia

Nizar A. Mukhtar, MD Transplant Hepatologist Director, Liver Tumor Board Liver Center and Organ Transplant Program Swedish Medical Center

Seattle, Washington [email protected] Liver Disease

Dana A. Ohl, MD Professor of Urology Chief, Division of Andrology and Urologic Health Department of Urology University of Michigan Medical School Ann Arbor, Michigan [email protected] Disorders of the Male Reproductive Tract

Bansari G. Patel, MD Assistant Professor of Obstetrics and Gynecology–Reproductive Medicine Department of Obstetrics and Gynecology–Reproductive Medicine Wake Forest Baptist Medical Center Wake Forest University School of Medicine Winston-Salem, North Carolina [email protected] Disorders of the Female Reproductive Tract

Rachel L. Perlman, MD Associate Professor of Internal Medicine Division of Nephrology Department of Internal Medicine University of Michigan Medical School Ann Arbor, Michigan [email protected] Renal Disease

Laura B. Pincus, MD Associate Professor of Dermatology Departments of Dermatopathology and Oral Pathology University of California, San Francisco School of Medicine San Francisco, California [email protected] Diseases of the Skin

Thomas J. Prendergast, MD Professor of Medicine Section Chief, Pulmonary & Critical Care Medicine Portland Veterans Affairs Medical Center Oregon Health & Science University Portland, Oregon [email protected] Pulmonary Disease

Shane C. Quinonez, MD Clinical Assistant Professor of Pediatrics and Medical Genetics Co-Director, Biochemical Laboratory, Michigan Medical Genetics Laboratory

University of Michigan Medical School Ann Arbor, Michigan [email protected] Genetic Disease

Vikram G. Shakkottai, MD, PhD Associate Professor of Neurology and Molecular & Integrative Physiology University of Michigan Medical School Ann Arbor, Michigan [email protected] Nervous System Disorders

Dolores M. Shoback, MD Professor of Medicine Endocrine Research Unit San Francisco Veterans Affairs Medical Center Department of Medicine University of California, San Francisco School of Medicine San Francisco, California [email protected] Disorders of the Parathyroids & Calcium and Phosphorus Metabolism

Thomas H. Sisson, MD Professor of Pulmonary & Critical Care Medicine Department of Internal Medicine University of Michigan Medical School

Ann Arbor, Michigan [email protected] Pulmonary Disease

Jens Sønksen, MD, PhD Professor of Urology Department of Urology Herlev & Gentofte Hospital University of Copenhagen Herlev, Denmark [email protected] Disorders of the Male Reproductive Tract

Christopher J. Sonnenday, MD, MHS Associate Professor of Surgery Department of Surgery University of Michigan Medical School Ann Arbor, Michigan [email protected] Disorders of the Exocrine Pancreas

Robert N. Taylor, MD, PhD Professor, Reproductive Endocrinology & Infertility Department of Obstetrics & Gynecology University of Utah School of Medicine Salt Lake City, Utah [email protected] Disorders of the Female Reproductive Tract

Sunny Wang, MD Assistant Clinical Professor of Medicine Division of Hematology/Oncology University of California San Francisco VA Medical Center San Francisco, California [email protected] Blood Disorders

Preface

Goal and Audience The goal of Pathophysiology of Disease: An Introduction to Clinical Medicine, as outlined in the introductory chapter (Chapter 1), is to introduce students to clinical medicine by reviewing the pathophysiologic basis of the symptoms and signs of various common diseases.

The book has proved useful as a text for both Pathophysiology and Introduction to Clinical Medicine courses in medical schools, and it has been popular in similar courses in nursing schools, physician assistants’ training programs, and other allied health programs. It is valuable to students early in their medical school years by highlighting the clinical relevance of their basic science courses, and in preparation for their USMLE Step 1 examinations. The book is also helpful to students engaged in their internal medicine and surgery clerkships, and to house officers as an up-to-date summary of relevant physiology and a source of key references. Practitioners (both general internists and specialists who provide generalist care) will find it beneficial as a refresher text, designed to update their knowledge of the mechanisms underlying 132 commonly encountered diseases and disorders. Nurses, nurse-practitioners, physician assistants, and other allied health practitioners have found that its concise format and broad scope facilitate their understanding of these basic disease entities. Pathophysiology of Disease has been widely adopted in the United States,

Canada, and the United Kingdom, and it has been translated into Spanish, Italian, Chinese, Japanese, Greek, and Turkish. Both the text and its Case Study Questions and Answers are also available online at accessmedicine.mhmedical.com, the online version of McGraw-Hill’s many medical textbooks (search under “Books, Library, Basic Science” for “Pathophysiology,” listed alphabetically).

New Features for This Edition

In preparation for this eighth edition, the editors and authors reviewed the entire book. There have been many text revisions aimed at updating information, improving clarity, and eliminating minor errors. With emphasis on recent pertinent reviews, references have been entirely updated, as have figures and tables. “Checkpoints,” collections of review questions, continue to appear throughout the chapters and have been revised.

Examples of Substantive New Content Found in This Eighth Edition

• Update on components and physiology of normal immunity • Most recent surveillance case definition for HIV infection • Explication of the concepts of innate immunity and pathogen-associated

molecular patterns • Totally revised chapter on neoplasia, including 19 new figures and 4 new

tables • New figure illustrating iron transport and regulation in the duodenal

enterocyte • New chapter section on urticaria (perivascular dermatitis) • New chapter section on various forms of spinocerebellar ataxia • Clarification in text and figures of regional alterations in the overall

distribution of ventilation and perfusion referred to as mismatch, including concepts of anatomic versus alveolar (wasted ventilation) dead space and right-to-left shunt

• Update on genetic factors implicated in asthma risk, as well as allergic versus nonallergic asthma

• Newly rewritten section on idiopathic pulmonary fibrosis as a prototypic restrictive (interstitial) lung disease

• Extensive revision of sections on pulmonary edema, adult respiratory distress syndrome (ARDS), and pulmonary venous thromboembolism

• Expanded material on paragangliomas • New figures on mechanisms leading to nonalcoholic fatty liver disease and to

hepatic steatosis • New table summarizing adverse prognostic signs in acute pancreatitis derived

from the Acute Pancreatitis Classification Working Group’s 2012 classification, a revision of the Atlanta international consensus classification and definitions of acute pancreatitis

• New table summarizing genetic syndromes associated with pancreatic cancer

• New table summarizing the prevalence of various causes of end-stage renal disease for U.S. Medicare recipients in the 2016 U.S. Renal Data System

• Revised flowchart summarizing the pathogenesis of bone diseases in chronic kidney disease

• Updated information on familial hypocalcuric hypercalcemia, malignant hypercalcemia, autosomal dominant hypocalcemia, and the autoimmune polyendocrine failure syndromes

• The American Diabetes Association’s new diagnosis and etiologic classification of diabetes mellitus

• New figure illustrating the amino acid sequence and covalent structure of human proinsulin

• New schematic diagram of glucose-stimulated insulin release from the pancreatic β cell

• New figure showing how pancreatic islet cell secretions of glucagon (from α cells) and insulin (from β cells), which are reciprocally regulated by glucose, play key roles in maintaining glucose homeostasis

• Diagram demonstrating stages in the development of type 1 diabetes mellitus with the appearance of β-cell autoantibodies, followed by dysglycemia, insulinopenia, and then frank hyperglycemia

• Review of mechanisms of the newest classes of pharmacologic agents approved for type 2 diabetes mellitus

• New figure illustrating modifiable cardiovascular risk factors in individuals with diabetes

• New figure illustrating the control of energy homeostasis by arcuate nucleus neurons, both stimulatory and inhibitory neurons with regard to food intake

• Updated information on fine-needle aspiration biopsy of thyroid nodules • Update on thyroid disorders in pregnancy • New schematic drawing of the sequence of changes that occur in the alveolar

secretory units and duct system of the female breast before, during, and after pregnancy and lactation

• Update of indicators of mild to moderate versus severe preeclampsia- eclampsia

• Incorporation of new International Federation of Gynecology and Obstetrics classification system for pathogenesis of abnormal uterine bleeding

• Update to pathogenesis and pathophysiology of inflammatory myopathies and rheumatoid arthritis

• New chapter section on spondyloarthropathies, including ankylosing spondylitis, reactive arthritis, inflammatory bowel disease–associated arthritis, and psoriatic arthritis

• Updated references throughout the text, including articles mainly from 2015, 2016, and 2017

• Revised and updated figure citations throughout the text

Case Study Questions and Answers Each chapter ends with a collection of Case Studies. These clinical problems give readers an opportunity to test their understanding of the pathophysiology of each clinical entity discussed and to apply their knowledge to exemplar clinical situations. In this eighth edition, Yeong S. Kwok, MD, of the University of Michigan, has added an additional 12 Case Studies with questions, bringing the total number to 132, or one for each of the clinical entities discussed in the book’s 24 chapters. New Case Study topics include:

• Down syndrome as a result of a balanced robertsonian translocation • Chronic granulomatous disease • Malignant hypercalcemia • Cerebellar ataxia • Urticaria • Ulcerative colitis • Type 1 diabetes mellitus • Subclinical hypothyroidism • Subclinical hyperthyroidism • Amenorrhea caused by polycystic ovary syndrome • Abnormal vaginal bleeding as a result of endometrial cancer • Spondyloarthropathy as a result of ankylosing spondylitis

As before, detailed analyses of the Cases appear in Chapter 25: Case Study Answers. There, Dr. Kwok has updated the Answers to the existing 120 Case Study Questions to reflect the changes made by chapter authors in their revisions, and he has added Answers to the Questions for the 12 new Case Studies.

Changes in Authors With this eighth edition, the authorship of many chapters has evolved and transitioned—this should not be surprising given the book’s multiple editions since it was first published in 1992 and given the transition in location of the book’s lead editor from the University of California, San Francisco (Stephen J. McPhee, MD), to the University of Michigan (Gary Hammer, MD, PhD). The

editors wish to welcome aboard the following new contributors who are joining the book’s authors and to thank the following past contributors who are now departing the book:

• Shane C. Quinonez, MD, of the University of Michigan, has taken over the current revision of Chapter 2, Genetic Disease, from Greg Barsh, MD, PhD, of Stanford University; Dr. Barsh, who has been this chapter’s author since the book’s first edition, will continue as a coauthor for this eighth edition only

• Jennifer J. Chang, MD, and Suzanne M. Donovan, MD, MPH, joined Jeffrey L. Kishiyama, MD, in revising Chapter 3, Disorders of the Immune System. Sadly, Dr. Kishiyama died during the final production process for this edition; please see the In Memoriam below

• Christina T. Fiske, MD, MPH, of Vanderbilt University, has joined Karen C. Bloch, MD, MPH, in revising Chapter 4, Infectious Diseases

• Weiyun Z. Ai, PhD, MD, of UCSF, has assisted chapter author Mark M. Moasser, MD, by revising the hematologic disorders section of Chapter 5, Neoplasia

• J. Ben Davoren, MD, PhD, of UCSF, has replaced Sunny Wang, MD, with Gerald Hsu, MD, PhD, as coauthor of Chapter 6, Blood Disorders; we would like to thank Dr. Wang for her revisions for the sixth and seventh editions

• Vikram G. Shakkottai, MD, PhD, of the University of Michigan, has taken over the current revision of Chapter 7, Nervous System Disorders, from Catherine Lomen-Hoerth, MD, PhD, of UCSF; Dr. Lomen-Hoerth will continue only once more as coauthor for this eighth edition

• Laura B. Pincus, MD, of UCSF, has replaced Melissa M. Meier, MD, as the coauthor, with Timothy H. McCalmont, MD, of Chapter 8, Diseases of the Skin; we would like to thank Dr. Meier for her revisions for the seventh edition

• Thomas H. Sisson, MD, and Dru Claar, MD, of the University of Michigan, have taken over the current revision of Chapter 9, Pulmonary Disease, from Mark S. Chesnutt, MD, and Thomas J. Prendergast, MD, of Oregon Health and Sciences University; Drs. Chesnutt and Prendergast will continue as coauthors for this eighth edition only. We would like to acknowledge Dr. Prendergast for his role in coauthoring the original chapter in the book’s first edition and for his revisions for the next seven editions, and Dr. Chesnutt for his assistance with the seventh and eighth editions

• Lauren Fishbein, MD, PhD, of the University of Colorado, is now coauthor of Chapter 12, Disorders of the Adrenal Medulla, with Tobias Else, MD, of the University of Michigan; after the sixth and seventh editions, Gary Hammer,

MD, PhD, has now “retired” from coauthorship of this chapter • Matthew A. Ciorba, MD, of the Washington University at St. Louis, has now

joined his colleague Jason C. Mills, MD, PhD, as coauthor of Chapter 13, Gastrointestinal Disease; we are thankful for the past contributions of Thaddeus S. Stappenbeck, MD, PhD, to its revision for the sixth and seventh editions

• Mandana Khalili, MD, MAS, of UCSF, is now working with Nizar A. Mukhtar, MD, of Seattle’s Swedish Medical Center in producing the current revision of Chapter 14, Liver Disease; we are grateful to Blaire Burman, MD, of UCSF for assistance with the seventh edition revision

• Timothy L. Frankel, MD, has now joined his University of Michigan colleague Christopher J. Sonnenday, MD, MHS, in producing the current revision of Chapter 15, Disorders of the Exocrine Pancreas

• Joachim H. Ix, MD, has now “retired” from coauthorship of Chapter 16, Renal Disease, which is now coauthored by Rachel Leah Pearlman, MD, and Michael Heung, MD, MS; we thank Dr. Ix for his revisions in the fifth, sixth, and seventh editions

• Deborah E. Sellmeyer, MD, has now “retired” from coauthorship with Dolores M. Shoback, MD, of Chapter 17, Disorders of the Parathyroids & Calcium and Phosphorus Metabolism; we thank Dr. Sellmeyer for her contributions to the fifth, sixth, and seventh editions

• Nazanene H. Esfandiari, MD, of the University of Michigan, has replaced Douglas C. Bauer, MD, of UCSF, as the coauthor, with Stephen J. McPhee, MD, of Chapter 20, Thyroid Disease; we thank Dr. Bauer for his revisions for the second through seventh editions and note that Dr. McPhee, who has been this chapter’s author or coauthor for the prior seven editions, will continue as coauthor for this eighth edition only

• Bansari G. Patel, MD, at Wake Forest University, now joins Erika B. Johnston-MacAnanny, MD, now in Richmond, VA, and Robert N. Taylor, MD, PhD, now at the University of Utah, as a coauthor of Chapter 22: Disorders of the Female Reproductive Tract

• Mikkel Fode, MD, PhD, and Jens Sønksen, MD, PhD, of Herlev & Gentofte Hospital of the University of Copenhagen in Herlev, Denmark, and Dana A. Ohl, MD, of the University of Michigan, have assumed authorship of Chapter 23, Disorders of the Male Reproductive Tract; after seven editions, Stephen J. McPhee, MD, has now “retired” from authorship or coauthorship of this chapter

• Allan C. Gelber, MD, MPH, PhD, and Stuart M. Levine, MD, have been joined by a new coauthor, Erika Darrah, PhD, of Johns Hopkins University,

for Chapter 24, Inflammatory Rheumatic Diseases; the chapter authors and textbook editors gratefully acknowledge the intellectual contributions of Antony Rosen, MB, ChB, BSc (Hons), to the content of this chapter in the book’s third through seventh editions

With these transitions, the content of more than two-thirds of this eighth edition has greatly benefited from the new contributors’ viewpoints and inputs (for instance, by including 36 new illustrations in the book’s attractive four-color design and layout).

In Memoriam: Jeffrey L. Kishiyama, MD We are saddened to report that, following submission of the revision of Chapter 3, Disorders of the Immune System, for the eighth edition of Pathophysiology of Disease, its lead author, Jeffrey L. Kishiyama, MD, died.

A graduate of Stanford University with a degree in biology and economics, Dr. Kishiyama received his MD from Creighton University School of Medicine. He did his internal medicine residency at Northwestern University, after which he completed a fellowship in allergy and immunology at the University of California, San Francisco (UCSF). Thereafter, Dr. Kishiyama spent many years on the faculties of both UCSF and Stanford University, where he served in a variety of positions, including director of the UCSF Clinical Immunology Laboratory and director of the UCSF Stanford Allergy and Immunology training program. In addition to his academic positions, Dr. Kishiyama maintained an active clinical practice, treating patients at Allergy Asthma Associates of Northern California.

Jeff first joined the authors of Pathophysiology of Disease as a coauthor with Richard Shames, MD, for its third and fourth editions, published in 2000 and 2003. Thereafter, Jeff continued as a solo author for the fifth edition in 2006, the sixth edition in 2010, and the seventh edition in 2014. For the eighth edition, he recruited coauthors Jennifer J. Chang, MD, Fellow in HIV Medicine, and Suzanne M. Donovan, MD, MPH, Clinical Professor of Medicine, Division of Infectious Diseases, of the UCLA School of Medicine.

Jeff was a superb contributor to Pathophysiology, helping to make understandable the increasingly complex field of immunology. Through the years, our readers, particularly our student readers, have been most grateful for his gift in this regard. As one small example, Jeff more than tripled the size of the introductory table of acronyms and abbreviations used throughout both Chapter 3 and the entire book.

We will greatly miss having Jeff as an outstanding (and timely!) contributor to our wonderful book.

With the publication of this eighth edition, the editors want to extend special thanks, not only to the contributors old and new, but also to the students and colleagues who have offered helpful comments and criticisms for each of the previous editions. The authors and editors continue to welcome comments and recommendations for future editions, in writing or via email. The editors’ and authors’ institutional and email addresses are given in the Authors section.

Gary D. Hammer, MD, PhD Ann Arbor, Michigan

Stephen J. McPhee, MD San Francisco, California

CHAPTER

1

Introduction Gary D. Hammer, MD, PhD, & Stephen J. McPhee, MD

A man cannot become a competent surgeon without the full knowledge of human anatomy and physiology, and the physician without physiology and chemistry flounders along in an aimless fashion, never able to gain any accurate conception of disease, practicing a sort of popgun pharmacy, hitting now the malady and again the patient, he himself not knowing which.

Sir William Osler (1849–1919)

Osler expresses particularly well the relationship between the basic sciences and clinical medicine in the aphorism cited above. Indeed, ever since the Middle Ages, wise physicians and others concerned with the sick and their care have realized that most human disease may be understood in a real sense as disordered physiology (pathophysiology). Something (eg, a mutation [pathogenic variant] in a gene or invasion by a bacterial organism) triggers an illness, and the body reacts with molecular, cellular, and systemic responses that are the symptoms and signs of the disease. Therefore, with proper knowledge of the body’s normal structure and function, and the ways in which these can become disordered, comes the ability to understand disease and to design rational and effective treatment. In addition, of course, the relationship between pathophysiology and disease is a two-way street. Diseases may be viewed as “experiments of nature” that may uncover previously unknown or unappreciated physiologic mechanisms, and the investigation of these physiologic mechanisms in normal individuals advances our fundamental biomedical knowledge. Therefore, it is important that students understand normal structure and function, and how they can become disordered, and apply this knowledge to disease.

The aim of this book is to provide students with an introduction to clinical medicine through the study of diseases as manifestations of pathophysiology.

The authors (all experts in their respective fields) have provided a brief review of the relevant normal structure and function of each system in the body, followed by a description of the underlying pathophysiologic mechanisms that underlie several common diseases related to that system. With this approach comes an explication of the symptoms and signs of each disease state and an essential framework for the student’s later mastery of treatment strategies. Several subject areas that are not restricted to a single body system are also covered (eg, neoplasia and infectious disease), but the same approach is used in these instances as well. For the most part, diagnosis and treatment are not covered here but are left for later, more detailed study and textbooks such as the annually updated Current Medical Diagnosis & Treatment. No attempt is made here to be comprehensive or complete; the pathophysiology section of each chapter discusses one to five relevant clinical entities, based either on their frequency (eg, coronary artery disease and hypertension) or on their importance to understanding how physiologic systems may become disordered (eg, fragile X mental retardation or pheochromocytoma). The aim is to introduce students to diseases as manifestations of disordered function and to start them thinking about the related symptoms and signs in terms of their pathophysiologic basis.

This is the eighth edition of this basic science textbook, first published in 1992. It has grown from 20 to 25 chapters, with the number of clinical entities discussed increasing and the number of case study problems increasing gradually from 38 when debuted in the third edition, to 89 in the fourth and fifth editions, to 111 in the sixth, to 120 in the seventh, and now to 132 in this eighth edition. In addition, the authorship of chapters has gradually transitioned, with 18 new authors or co-authors in this eighth edition alone (compared to the seventh edition). Finally, with the rapid expansion in our understanding of the genetic and genomic origin of many pathophysiologic entities, the amount of content devoted to this particular disease mechanism has greatly increased. In this new edition, for example, a revised Table 15–9 gives a much longer listing of the genetic syndromes associated with pancreatic cancer. And a newly rewritten Chapter 5 provides a detailed explanation of the genetic and genomic origins of neoplastic disorders—including several types of epithelial neoplasms (carcinomas); mesenchymal, neuroendocrine, and germ cell neoplasms (neuroendocrine tumors, testicular germ cell cancers, and sarcomas); and hematologic neoplasms (lymphomas and acute and chronic leukemias).

CHAPTER

2

Genetic Disease Shane C. Quinonez, MD, & Gregory Barsh, MD, PhD

Mechanisms of cellular and tissue dysfunction in genetic diseases are as varied as the organs they affect. To some extent, these mechanisms are similar to those that occur in nonheritable disorders. For example, a fracture resulting from decreased bone density in osteoporosis heals in much the same way as one caused by a defective collagen gene in osteogenesis imperfecta, and the response to coronary atherosclerosis in most individuals does not depend on whether they have inherited a defective low-density lipoprotein (LDL) receptor. Thus, the pathophysiologic principles that distinguish genetic disease focus not so much on the affected organ system as on the mechanisms of genetic and genomic changes, inheritance, and molecular pathways from genotype to phenotype.

This chapter begins with a discussion of the terminology used to describe inherited conditions, the prevalence of genetic disease, and some major principles and considerations in medical genetics. Important terms and key words used throughout the chapter are defined in Table 2–1.

TABLE 2–1 Glossary of terms and keywords.

Next, a group of disorders caused by pathogenic variants (formerly termed “mutations”) in collagen genes is discussed (ie, osteogenesis imperfecta). Although osteogenesis imperfecta is often considered a single entity, different pathogenic variants and different genes subject to alteration lead to a wide spectrum of clinical phenotypes. The different types of osteogenesis imperfecta exhibit typical patterns of autosomal dominant or autosomal recessive inheritance and are, therefore, examples of so-called mendelian conditions. To show how environmental factors can influence the relationship between genotype and phenotype, another mendelian condition, phenylketonuria, is discussed. This serves as a paradigm for newborn screening programs and for treatment of genetic disease. Several genetic conditions have been found to depend not only on the gene being inherited but also on the phenotype or the sex of the parent. As an example of a condition that exhibits non-autosomal inheritance, fragile X–associated mental retardation syndrome is discussed. This syndrome not only is the most common inherited cause of mental retardation but also illustrates how different types of pathogenic variants can explain the perplexing phenomenon of genetic anticipation, in which the severity of a mendelian syndrome appears to progress with every generation of inheritance. Another group of disorders that depend on the phenotype and sex of the parent consists of those that affect the mitochondrial genome. As examples, Leber hereditary optic neuropathy (LHON) and myoclonic epilepsy with ragged red fibers (MERRF) are considered. These illustrate the principles of mitochondrial inheritance and its pathophysiology. Aneuploidy is discussed as one of the most common causes of human genetic disorders that does not affect DNA structure but instead alters the normal chromosome content per cell. The example that is considered, Down syndrome, has had a major impact on reproductive medicine and reproductive decision making and serves to illustrate general principles that apply to many aneuploid conditions. Finally, this chapter considers how genome sequences and sequencing are improving our understanding of pathophysiology for many diseases. With the completion of the annotation of the human genome and technological advances that allow

individual genomes to be sequenced rapidly and inexpensively, prospects are at hand to identify genetic components of any human phenotype and to provide medical care that is truly personalized.

UNIQUE PATHOPHYSIOLOGIC ASPECTS OF GENETIC DISEASES

Although the phenotypes of genetic diseases are diverse, their causes are not. The primary cause of any genetic disease is a change in the sequence or cellular content of DNA that ultimately deranges gene expression. Most genetic diseases are caused by an alteration in DNA sequence that alters the synthesis of a single gene product. However, some genetic diseases are caused by (1) structural rearrangements that result in deletion or duplication of a group of closely linked genes or (2) abnormalities during mitosis or meiosis that result in an abnormal number of chromosomes per cell. In most genetic diseases, every cell in an affected individual carries the mutated gene or genes as a consequence of its inheritance via a mutant egg or sperm cell (gamete). However, mutation of the gametic cell may have arisen during its development, in which case somatic cells of the parent do not carry the variant, and the affected individual is said to have a “de novo variant.” In addition, some variants may arise during early embryogenesis, in which case tissues of the affected individual contain a mixture, or mosaic, of abnormal and normal cells. Depending on the time of embryogenesis and cell type in which a new variant arises, an individual may carry the alteration in some but not all of their germ cells (germline mosaicism), some but not all of their somatic cells (somatic mosaicism), or both.

It is helpful to begin with a brief review of terms that are commonly used in discussing genetic disease with patients and their families. Although genes were recognized and studied long before the structure of DNA was known, it has become common to regard a gene as a short stretch of DNA, usually but not always <100,000 base pairs (bp) in length, that encodes a product (usually protein) responsible for a function within the cell. DNA length is typically measured in base pairs, kilobase pairs (kb), or megabase pairs (Mb); chromosomes vary in length from about 46 Mb to 245 Mb. The locus is where a particular gene lies on its chromosome. A gene’s DNA sequence nearly always has slight differences when compared across many unrelated individuals within a population. These variant versions of a gene are referred to as different alleles of that gene. A genetic variant arises via a biochemical event such as a nucleotide

change, deletion, or insertion that has produced a new allele. The historic terms “polymorphism” and “mutation” are rarely used in current scientific and clinic discourse. A “polymorphism” was defined as a benign variant when it exhibited a population frequency greater than 1% in DNA sequence and did not contribute to disease manifestation. A “mutation” was defined as a pathogenic change in DNA sequence that did contribute to disease manifestation. The term “variant” is now the recommended term to define a change in DNA sequence from the population norm and is used with the following modifiers: “pathogenic,” “likely pathogenic,” “uncertain significance,” “likely benign,” and “benign.” Many changes in the DNA sequence of a gene, such as those within introns or at the third “wobble” position of codons for particular amino acids, do not affect the structure or expression of the gene product; therefore, although all variants result in a biochemical or molecular biologic phenotype (ie, a change in DNA), only some of them result in a clinically abnormal phenotype.

At the molecular level, variant sequences are usually detected through laboratory evaluation (DNA sequencing) and are referred to as a single nucleotide variant (SNV) if a single base pair change has occurred. At the clinical level, variant alleles are recognized by their effect on a phenotype such as human leukocyte antigen (HLA) type or hair color. Two copies or alleles of each autosomal gene (ie, genes on chromosomes 1–22) are present per cell. Individuals carrying identical copies are homozygous, whereas individuals whose two copies differ from each other are heterozygous. These terms —“homozygous” and “heterozygous”—can apply to the DNA sequence, the protein product, or the clinical phenotype. In other words, an individual may be heterozygous for a single nucleotide polymorphism (SNP) that does not alter the protein product, heterozygous for a deletion that causes a genetic disease, or heterozygous for a DNA sequence alteration that causes a change in protein structure but does not cause disease.

This discussion helps to illustrate the use of the word phenotype, which refers simply to any characteristic that can be measured, with the type of measurement depending on the characteristic. Hair color and height are phenotypes readily apparent to a casual observer that are not obviously associated with disease; diabetes and coronary artery disease are disease phenotypes that typically require clinical investigation to be recognized; and restriction fragment length polymorphisms (RFLPs), simple sequence length polymorphisms (SSLPs), and SNPs are molecular biologic phenotypes that can be detected only with a laboratory test.

PENETRANCE & EXPRESSIVITY

One of the most important principles of human genetics is that two individuals with the same mutated gene may have different phenotypes. For example, in the genetic condition of type I osteogenesis imperfecta, pedigrees may occur in which there are both a phenotypically affected grandparent and an affected grandchild even though the obligate carrier parent of the grandchild with the identical genetic variant is asymptomatic (Figure 2–1). Given a set of defined criteria, phenotypic expression of the condition in individuals known to carry the pathogenic variant gene is described as penetrance. In other words, if 7 of 10 individuals older than 40 with the type I osteogenesis imperfecta mutation have an abnormal bone density scan, the condition is said to be 70% penetrant by that criterion. Penetrance may vary both with age and according to the set of criteria being used; for example, type I osteogenesis imperfecta may be 90% penetrant at age 40 when the conclusion is based on a bone density scan in conjunction with laboratory tests for abnormal collagen synthesis. Reduced penetrance, or age- dependent penetrance, is a common feature of dominantly inherited conditions (see below) that often have a relatively high fitness (the extent to which individuals carrying an altered allele produce offspring relative to individuals who carry a normal allele); Huntington disease and polycystic kidney disease are examples of disorders with low fitness. Genetic conditions that result in a high degree of embryonic lethality have low fitness.

FIGURE 2–1 Penetrance and expressivity in type I osteogenesis imperfecta. In this schematic

pedigree of the autosomal dominant condition of type I osteogenesis imperfecta, nearly all affected individuals exhibit different phenotypic features that vary in severity (variable expressivity). As is shown, type I osteogenesis imperfecta is fully penetrant, because every individual who transmits the pathogenic variant is phenotypically affected to some degree. However, if mild short stature in the individual indicated with the arrow had been considered to be a normal variant, then the condition would have been nonpenetrant in this individual. Thus, in this example, judgments about penetrance or nonpenetrance depend on the criteria for normal and abnormal stature.

The same altered gene giving rise to a spectrum of different phenotypes is referred to as variable expressivity. For example, blue scleras and short stature may be the only manifestations of type I osteogenesis imperfecta in a particular individual, whereas a sibling who carries the identical mutation may be confined to a wheelchair as a result of multiple fractures and deformities. The pathogenic variant is penetrant in both individuals, but its expression is variable. Both reduced penetrance and variable expressivity may occur in individuals who carry the same altered allele; therefore, phenotypic differences between these individuals must be due to the effects of other “modifier” genes, to environmental interactions, or to chance.

MECHANISMS OF MUTATION & INHERITANCE PATTERNS

Genetic variants can be characterized both by their molecular nature—nucleotide deletion, insertion, substitution—and by their effects on gene activity (ie, no effect [neutral or silent], complete loss of function [amorphic variant], partial loss of function [hypomorphic variant], gain of function [hypermorphic variant], or acquisition of a new property [neomorphic variant]). Geneticists who study experimental organisms frequently use specific deletions to ensure that an altered allele causes a loss of function, but human geneticists rely on biochemical or cell culture studies to assess both losses and gains of function. Amorphic and hypomorphic variants are probably the most frequent types of pathogenic variant in human genetic disease because there are many ways to interfere with a protein’s function.

For autosomal genes, the fundamental difference between dominant and recessive inheritance is that, with dominant inheritance, the disease state or phenotypic trait being measured is apparent when one copy of the altered allele and one copy of the normal allele are present. With recessive inheritance, two copies of the altered allele must be present for the disease state or trait to be apparent. However, for genes that lie on the X chromosome, the situation is

slightly different because females have two X chromosomes and males have only one. X-linked dominant inheritance occurs when one copy of an abnormal gene causes the disease phenotype (in males and females); X-linked recessive inheritance occurs when two copies of an abnormal gene cause the disease phenotype (in females). Because most pathogenic variants are amorphic or hypomorphic, however, one copy of an X-linked abnormal allele in males is not “balanced” with a normal allele, as it would be in females; therefore, in X-linked recessive inheritance, one copy of an abnormal allele is sufficient to produce a disease phenotype in males, a situation referred to as hemizygosity.

RECESSIVE INHERITANCE & LOSS-OF- FUNCTION PATHOGENIC VARIANTS

Most recessive pathogenic variants are due to loss of function of the gene product, which can occur from a variety of causes, including failure of the gene to be transcribed or translated and failure of the translated gene product to function correctly. There are two general principles to keep in mind when considering loss-of-function variants. First, because expression from the normal allele usually does not change (ie, there is no dosage compensation), gene expression in a heterozygous carrier of a loss-of-function allele is reduced to 50% of normal. Second, for most biochemical pathways, a 50% reduction in enzyme concentration is not sufficient to produce a disease state. Thus, most diseases resulting from enzyme deficiencies such as phenylketonuria (Table 2–2) are inherited in a recessive fashion.

TABLE 2–2 Phenotype, inheritance, and prevalence of selected genetic disorders.

DOMINANT INHERITANCE & LOSS-OF- FUNCTION PATHOGENIC VARIANTS

If 50% of a particular product is not enough for the cell or tissue to function normally, then a loss-of-function variant in this gene produces a dominantly inherited phenotype. Such pathogenic variants often occur in structural proteins; an example is type I osteogenesis imperfecta, which is considered in detail later. Most dominantly inherited phenotypes are actually semidominant, which means that an individual who carries two copies of the abnormal allele is affected more severely than someone who carries one abnormal and one normal copy. However, for most dominantly inherited conditions, individuals homozygous for two pathogenic variants are rarely observed. For example, inheritance of achondroplasia, the most common genetic cause of very short stature, is usually

described as autosomal dominant. However, rare matings between two affected individuals have a 25% probability of producing offspring with two copies of the abnormal gene. This results in homozygous achondroplasia, a condition that is very severe and usually fatal in the perinatal period; thus, achondroplasia exhibits semidominant inheritance. Huntington disease, a dominantly inherited neurologic disease, is the only known human condition in which the homozygous abnormal phenotype is identical to the heterozygous abnormal phenotype (sometimes referred to as a “true dominant”).

DOMINANT NEGATIVE GENE ACTION

A special kind of pathophysiologic mechanism, referred to as dominant negative, occurs frequently in human genetic diseases that involve proteins that form oligomeric or polymeric complexes. In these disorders, the abnormal allele gives rise to a structurally abnormal protein that interferes with the function of the normal allele. Note that any molecular lesion (ie, deletion, nonsense, missense, or splicing) can produce a loss-of-function allele. However, only molecular lesions that yield a protein product (ie, splicing, missense, or nonsense variants) can result in a dominant negative allele. Type II osteogenesis imperfecta, described later, is an example of a dominant negative pathogenic variant.

Although the terms “dominant” and “recessive” are occasionally used to describe specific pathogenic variants, a DNA sequence alteration itself cannot, strictly speaking, be dominant or recessive. The terms are instead appropriate to the effect of a pathogenic variant on a particular trait. Therefore, in characterizing a particular pathogenic variant as “recessive,” one is referring to the effect of the variant on the trait being studied.

MUTATION RATE & THE PREVALENCE OF GENETIC DISEASE

At the level of DNA sequence, nucleotide variants (substitutions, small insertions, or small deletions) in humans occur at a rate of approximately 2 × 10– 8 per nucleotide per human generation, or 150 new variants per diploid genome. However, only about 5% of the human genome is protein coding, so most new variants have no effect. Still, with approximately 23,000 genes in the human

genome and an estimated deleterious “per locus” mutation rate of 10–5 per generation, the chance of a new deleterious variant occurring in any one individual is about 20%. Furthermore, assuming 10 billion new births in the last millennium, every gene in the human genome has probably been mutated (in a deleterious manner) about 100,000 different times. However, from a clinical perspective, only about 6000 single-gene disorders have been recognized to cause a human disease. In considering possible explanations for this disparity, it seems likely that deleterious variants of many single genes are lethal very early in development and thus not clinically apparent, whereas deleterious variants in other genes do not cause an easily recognizable phenotype. The overall frequency of disease attributable to defects in single genes (ie, mendelian disorders) is approximately 1% of the general population.

Table 2–2 lists the major symptoms and signs (phenotypes), genetic mechanisms, and prevalence of the diseases considered in this chapter as well as of several others. The most common conditions, such as neurofibromatosis, cystic fibrosis, and fragile X–associated mental retardation syndrome, will be encountered at some time by most health care professionals regardless of their field of interest. Other conditions, such as Huntington disease and adenosine deaminase deficiency, although of academic and pathophysiologic interest, are not likely to be seen by most practitioners.

Many common conditions such as atherosclerosis and breast cancer, while in a minority of cases are caused by a single-gene disorder, usually do not show strictly mendelian inheritance patterns but have some genetic component evident from familial aggregation or twin studies. These conditions are usually described as multifactorial, which means that the effects of one or more mutated genes and environmental differences all contribute to the likelihood that a given individual will manifest the phenotype.

ISSUES IN CLINICAL GENETICS

Most patients with genetic disease present during early childhood with symptoms that ultimately give rise to a diagnosis such as fragile X–associated mental retardation or Down syndrome. The major clinical issues at presentation are arriving at the correct diagnosis and counseling the patient and family regarding the natural history and prognosis of the condition. It is important to assess the likelihood that the same condition will occur again in the family and determine whether it can be diagnosed prenatally. These issues are the subject

matter of genetic counseling by medical geneticists and genetic counselors. Understanding the pathophysiology of genetic diseases that interfere with

specific metabolic pathways—so-called inborn errors of metabolism—has led to effective treatments for selected conditions such as phenylketonuria, maple syrup urine disease, and homocystinuria. Many of these diseases are rare, but efforts are underway to develop treatments for common single-gene disorders such as Duchenne muscular dystrophy, cystic fibrosis, and hemophilia. Some forms of therapy are directed at replacing the mutant protein, whereas others are directed at ameliorating its effects.

CHECKPOINT

1. Define gene, locus, allele, variant, heterozygosity, hemizygosity, and phenotype.

2. How is it possible for two individuals with the same pathogenic variant to experience differences in the severity of an abnormal phenotype?

3. Explain the pathophysiologic difference between variants that act via loss of function and those that act via dominant negative gene action.

PATHOPHYSIOLOGY OF SELECTED GENETIC DISEASES

OSTEOGENESIS IMPERFECTA

Osteogenesis imperfecta is a condition inherited in mendelian fashion that illustrates many principles of human genetics. It is a heterogeneous and pleiotropic group of disorders characterized by a tendency toward bone fragility. Advances in the last two decades demonstrate two genetically different groups: the “classical” group, in which more than 90% of cases are caused by pathogenic variants of the COL1A1 or COL1A2 genes, which encode the subunits of type I collagen, proα1(I) and proα2(I), respectively, and a newer group, caused by loss- of-function pathogenic variants in other proteins required for proper folding, processing, and collagen secretion. More than 100 different abnormal alleles have been described for osteogenesis imperfecta; the relationships between

different DNA sequence alterations and the type of disease (genotype–phenotype correlations) illustrate several pathophysiologic principles in human genetics.

Clinical Manifestations The clinical and genetic characteristics of osteogenesis imperfecta are summarized in Table 2–3, in which the timing and severity of fractures, radiologic findings, and presence of additional clinical features help to distinguish four different subtypes. This classification was presented more than 30 years ago. Over the past decade, it has become clear that there are more than a dozen different genes in which pathogenic variants can cause osteogenesis imperfecta, and that the utility of alternative or more extended nosologic approaches depends on whether the condition is being considered from the perspective of patients, caregivers, or molecular geneticists.

TABLE 2–3 Subtypes of dominant osteogenesis imperfecta.

All forms of osteogenesis imperfecta are characterized by increased susceptibility to fractures (“brittle bones”), but there is considerable phenotypic heterogeneity, even within individual subtypes. Individuals with type I or type IV osteogenesis imperfecta present in early childhood with one or a few fractures of long bones in response to minimal or no trauma; x-ray films may reveal mild osteopenia, little or no bony deformity, and often evidence of earlier subclinical fractures. However, most individuals with type I or type IV osteogenesis imperfecta do not have fractures in utero. Type I and type IV

osteogenesis imperfecta are distinguished by the severity (less in type I than in type IV) and by scleral hue, which indicates the thickness of this tissue and the deposition of type I collagen. Individuals with type I osteogenesis imperfecta typically have blue scleras, whereas the scleras of those with type IV are normal or slightly gray. In type I, the typical number of fractures during childhood is 10–20; fracture incidence decreases after puberty, and the main features in adult life are mild short stature, a tendency toward conductive hearing loss, and occasionally dentinogenesis imperfecta. Individuals with type IV osteogenesis imperfecta generally experience more fractures than those with type I and have significant short stature caused by a combination of long bone and spinal deformities, but they often are able to walk independently. Approximately 60% of the cases of type I and type IV osteogenesis imperfecta represent de novo previously undescribed pathogenic variants; in the remainder, the history and examination of other family members reveal findings consistent with autosomal dominant inheritance. In most cases, identified pathogenic variants are unique to an individual or family.

Type II osteogenesis imperfecta presents at or before birth (diagnosed by prenatal imaging) with multiple fractures, bony deformities, increased fragility of nonbony connective tissue, and blue scleras, and usually results in death in infancy. Two typical radiologic findings are the presence of isolated “islands” of mineralization in the skull (wormian bones) and a beaded appearance to the ribs. Nearly all cases of type II osteogenesis imperfecta represent a new dominant pathogenic variant, and there is no family history. Death usually results from respiratory difficulties.

Type III osteogenesis imperfecta presents at birth or in infancy with progressive bony deformities, multiple fractures, and blue scleras. It is intermediate in severity between types II and IV; most affected individuals will require multiple corrective surgeries and lose the ability to ambulate by early adulthood. Unlike other forms of osteogenesis imperfecta, which are nearly always due to variants that act dominantly, type III may be inherited, very rarely, in a recessive manner.

Although different subtypes of osteogenesis imperfecta can often be distinguished biochemically, the classification presented in Table 2–3 is primarily clinical rather than molecular, and the disease phenotypes for each subtype show a spectrum of severities that overlap one another. For example, a few individuals diagnosed with type II osteogenesis imperfecta based on the presence of severe bony deformities in utero will survive for many years and thus overlap the type III subtype. Similarly, some individuals with type IV

osteogenesis imperfecta have fractures in utero and develop deformities that lead to loss of ambulation. Distinguishing this presentation from type III osteogenesis imperfecta may be possible only if other affected family members exhibit a milder course.

Additional subtypes of osteogenesis imperfecta have been suggested for individuals that do not match types I–IV, and there are additional disorders associated with congenital fractures that are usually not considered to be “classic” osteogenesis imperfecta. In particular, work over the past several years has identified 11 additional genes in which pathogenic variants can cause autosomal recessive osteogenesis imperfecta and has provided additional insight into the genetic pathophysiology. In general, recessively inherited osteogenesis imperfecta is caused by loss-of-function variants in genes whose protein product is required for proper protein folding, intracellular processing, and trafficking of type I collagen.

Pathophysiology Osteogenesis imperfecta is a disease of type I collagen, which constitutes the major extracellular protein in the body. It is the major collagen in the dermis, the connective tissue capsules of most organs, and the vascular and gastrointestinal (GI) adventitia and is the only collagen in bone. A mature type I collagen fibril is a rigid structure that contains multiple type I collagen molecules packed in a staggered array and stabilized by intermolecular covalent cross-links. Each mature type I collagen molecule contains two α1 chains and one α2 chain, encoded by the COL1A1 and COL1A2 genes, respectively (Figure 2–2). The α1 and α2 chains are synthesized as larger precursors with amino and carboxyl terminal “propeptide” extensions, assemble with each other inside the cell, and are ultimately secreted as a heterotrimeric type I procollagen molecule. During intracellular assembly, the three chains wind around each other in a triple helix that is stabilized by interchain interactions between hydroxylated proline and adjacent carbonyl residues. There is a dynamic relationship between the post- translational action of prolyl hydroxylase and assembly of the triple helix, which begins at the carboxyl terminal end of the molecule. Increased levels of hydroxylation result in a more stable helix, but helix formation prevents further prolyl hydroxylation. The nature of the triple helix causes the side chain of every third amino acid to point inward, and steric constraints allow only a proton in this position. Thus, the amino acid sequence of virtually all collagen chains in the triple-helical portion is (Gly-X-Y)n, where Y is proline about one-third of the time.

FIGURE 2–2 Molecular assembly of type I procollagen. Type I procollagen is assembled in the endoplasmic reticulum from three proα chains that associate with each other beginning at their carboxyl terminals. An important requirement for proper assembly of the triple helix is the presence of a glycine residue at every third position in each of the proα chains. After secretion, the amino and carboxyl terminal propeptides are proteolytically cleaved, leaving a rigid triple helical collagen molecule with very short non– triple-helical domains at both ends. (Modified and reproduced, with permission, from Alberts BA. Molecular Biology of the Cell, 4th ed. Garland Science, 2002. [Fig. 19-47].)

The fundamental defect in most individuals with type I osteogenesis imperfecta is reduced synthesis of type I collagen resulting from loss-of-function variants in COL1A1. In most cases, the abnormal COL1A1 allele gives rise to greatly reduced (partial loss-of-function) or no (complete loss-of-function) mRNA. Because the normal COL1A1 allele continues to produce mRNA at a normal rate (ie, there is no dosage compensation), heterozygosity for a complete loss-of-function variant results in a 50% reduction in the total rate of proα1(I) mRNA synthesis, whereas heterozygosity for a partial loss-of-function variant results in a less severe reduction. A reduced concentration of proα1(I) chains

limits the production of type I procollagen, leading to (1) a reduced amount of structurally normal type I collagen and (2) an excess of unassembled proα2(I) chains, which are degraded inside the cell (Figure 2–3).

FIGURE 2–3 Molecular pathogenesis of type I and type II osteogenesis imperfecta (OI). The COL1A1 gene normally produces twice as many proα chains as the COL1A2 gene. Therefore, in normal cells, the ratio of proα1 to proα2 chains is 2:1, which corresponds to the ratio of α1 and α2 chains in intact collagen molecules. In type I osteogenesis imperfecta, a variant (X) in one of the COL1A1 alleles (*) results in failure to produce proα1 chains, leading to a 50% reduction in the total number of proα1 chains, a 50% reduction in the production of intact type I collagen molecules, and an excess of unassembled proα2 chains, which are degraded inside the cell. In type II osteogenesis imperfecta, a pathogenic variant in one of the COL1A1 alleles results in a structural alteration that blocks triple-helix formation and secretion of partially assembled collagen molecules containing the abnormal chain. (Adapted from Nussbaum RL et al. Thompson & Thompson Genetics in Medicine, 7th ed. Saunders Elsevier, 2007.)

There are several potential molecular defects responsible for COL1A1 pathogenic variants in type I osteogenesis imperfecta, including alterations in a regulatory region leading to reduced transcription, splicing abnormalities leading to reduced steady-state levels of RNA, and deletion of the entire COL1A1 gene. However, in many cases, the underlying defect is a single base pair change that

creates a premature stop codon (also known as a “nonsense mutation”) in an internal exon. In a process referred to as nonsense-mediated decay, partially synthesized mRNA precursors that carry the nonsense codon are recognized and degraded by the cell. With collagen and many other genes, production of a truncated protein (as might be predicted from a nonsense mutation) would be more damaging to the cell than production of no protein at all. Thus, nonsense- mediated decay, which has been observed to occur for mutations in many different multiexon genes, serves as a protective phenomenon and is an important component of the genetic pathophysiology.

An example of these principles is apparent from considering type II osteogenesis imperfecta, which is caused by structurally abnormal forms of type I collagen and is more severe than type I osteogenesis imperfecta. Pathogenic variants in type II osteogenesis imperfecta can be caused by defects in either COL1A1 or COL1A2 and usually are missense alterations of a glycine residue that allow the abnormal peptide chain to bind to normal chains in the initial steps of trimer assembly (see Figure 2–3). However, triple-helix formation is ineffective, often because amino acids with large side chains are substituted for glycine. Ineffective triple-helix formation leads to increased post-translational modification by prolyl hydroxylase, a reduced rate of secretion, and activation of the unfolded protein stress response. These appear to be critical events in the cellular pathogenesis of type II osteogenesis imperfecta, because glycine substitutions toward the carboxyl terminal end of the molecule are generally more severe than those at the amino terminal end.

These considerations help to explain why type II osteogenesis imperfecta is more severe than type I and exemplify the principle of dominant negative gene action. The effects of an amino acid substitution in a proα1(I) peptide chain are amplified at the levels of both triple-helix assembly and fibril formation. Because every type I procollagen molecule has two proα1(I) chains, only 25% of type I procollagen molecules will contain two normal proα1(I) chains, even though only one of the two COL1A1 alleles is mutated. Furthermore, activation of the unfolded protein stress response appears to be a key event in the pathophysiology of the disease, as discussed further below. Finally, because each molecule in a fibril interacts with several others, incorporation of an abnormal molecule can have disproportionately large effects on fibril structure and integrity.

Collagen pathogenic variants that cause type III and type IV osteogenesis imperfecta are diverse and include glycine substitutions in the amino terminal portion of the collagen triple helix, a few internal deletions of COL1A1 and

COL1A2 that do not significantly disturb triple helix formation, and some unusual alterations in the non–triple-helical extensions at the amino and carboxyl terminals of proα chains.

Recessively inherited osteogenesis imperfect can be caused by loss of function for a key prolyl hydroxylase encoded by the PLOD2 gene, one of three genes—CRTAP, LEPRE1, and PPIB—that encode members of a protein complex that resides within the rough endoplasmic reticulum and facilitates the folding and processing of type I collagen, as well as several additional genes whose protein products are required for intracellular trafficking and secretion of type I collagen. A common pathway for all types of osteogenesis imperfecta involves a combination of reduced production of type I collagen in the extracellular matrix and/or dysfunctional intracellular collagen processing and maturation.

Genetic Principles As already described, most cases of type I osteogenesis imperfecta are caused by partial or complete loss-of-function variants in COL1A1. Moreover, in approximately 60% of affected individuals, the disease is caused by a de novo previously unidentified pathogenic variant. In addition, there are many ways in which DNA sequence alterations can reduce gene expression. Consequently, there is a wide range of abnormal alleles (ie, allelic heterogeneity), which represents a challenge for the development of molecular diagnostic tests. Using current sequencing strategies, the pathogenic variant is identifiable in almost all patients affected with types I–IV osteogenesis imperfecta. Identifying the pathogenic variant or variants in a patient and their family allows for precise recurrence risk estimates and for prenatal testing, if desired.

For types III and IV osteogenesis imperfecta, pathogenic variants can occur in COL1A1 or COL1A2 (ie, locus heterogeneity), and in this situation, linkage analysis is more difficult because one cannot be sure which locus is abnormal.

For both type I and type IV osteogenesis imperfecta, the most important question in the clinical setting often relates to the natural history of the illness. For example, reproductive decision making in families at risk for osteogenesis imperfecta is influenced greatly by the relative likelihood of producing a child who will never walk and will require multiple orthopedic operations versus a child whose major problems will be a few long bone fractures and an increased risk of mixed sensorineural and conductive hearing loss in childhood and adulthood. As evident from the prior discussion, different abnormal genes and different abnormal alleles, as well as other genes that modify the osteogenesis

imperfecta phenotype, can contribute to this phenotypic heterogeneity. In type II osteogenesis imperfecta, a single copy of the abnormal allele causes

the abnormal phenotype and, therefore, has a dominant mechanism of action. Although the type II phenotype itself is never inherited, there are rare situations in which a phenotypically normal individual harbors a COL1A1 pathogenic variant allele among their germ cells. These individuals with so-called gonadal mosaicism can produce multiple offspring with type II osteogenesis imperfecta (Figure 2–4), a pattern of segregation that can be confused with recessive inheritance. In fact, many other pathogenic variants, including Duchenne muscular dystrophy, which is X linked, and type 1 neurofibromatosis, which is autosomal dominant, also occasionally show unusual inheritance patterns explained by gonadal mosaicism.

FIGURE 2–4 Gonadal mosaicism for type II osteogenesis imperfecta. In this idealized pedigree, the phenotypically normal father (indicated with the arrow) has had two children by different mates, each of whom is affected with autosomal dominant type II osteogenesis imperfecta (OI). Analysis of the father showed that some of his spermatozoa carried a COL1A1 pathogenic variant, indicating that the explanation for this unusual pedigree is germline mosaicism. (Adapted from Cohn DH et al. Recurrence of lethal osteogenesis imperfecta due to parental mosaicism for a dominant mutation in a human type I collagen gene [COL1A1]. Am J Hum Genet. 1990;46:591.)

CHECKPOINT

4. When and how does type II osteogenesis imperfecta present? To what do these individuals succumb?

5. What are two typical radiologic findings in type II osteo-genesis imperfecta?

6. Explain how nonsense-mediated decay can help pro-tect individuals affected by a genetic disease.

PHENYLKETONURIA

Phenylketonuria represents one of the most dramatic examples of how the relationship between genotype and phenotype can depend on environmental variables. Phenylketonuria was first recognized as an inherited cause of intellectual disability in 1934, and systematic attempts to treat the condition were initiated in the 1950s. The term “phenylketonuria” denotes elevated levels of urinary phenylpyruvate and phenylacetate, which occur when circulating phenylalanine levels, normally between 0.06 and 0.1 mmol/L, rise above 1.2 mmol/L. Thus, the primary defect in phenylketonuria is hyperphenylalaninemia, which itself has a number of distinct genetic causes.

The pathophysiology of phenylketonuria illustrates several important principles in human genetics. Hyperphenylalaninemia itself is caused by substrate accumulation, which occurs when a normal intermediary metabolite fails to be eliminated properly and its concentrations become elevated to toxic levels. The most common cause of hyperphenylalaninemia is deficiency of the enzyme phenylalanine hydroxylase, which catalyzes the conversion of phenylalanine to tyrosine. Individuals with pathogenic variants in phenylalanine hydroxylase usually do not suffer from the absence of tyrosine because this amino acid can be supplied to the body by mechanisms independent of phenylalanine hydroxylase. In other forms of phenylketonuria, however, additional disease manifestations occur as a result of end-product deficiency, which occurs when the downstream product of a particular enzyme is required for a key physiologic process.

A discussion of phenylketonuria also helps to illustrate the rationale for, and application of, population-based screening programs for genetic disease. More than 10 million newborn infants per year are tested for phenylketonuria, and the focus today in treatment has shifted in several respects. First, “successful” treatment of phenylketonuria by dietary restriction of phenylalanine is, in general, accompanied by subtle neuropsychologic defects that have been recognized only in the last decade. Thus, current investigations focus on alternative treatment strategies such as somatic gene therapy, as well as on the social and psychologic factors that affect compliance with dietary management. Second, a generation of females treated for phenylketonuria are now bearing children, and the phenomenon of maternal phenylketonuria has been recognized in which in utero exposure to maternal hyperphenylalaninemia results in congenital abnormalities regardless of fetal genotype. The number of pregnancies at risk has risen in proportion to the successful treatment of

phenylketonuria and represents a challenge to public health officials, physicians, dieticians, and geneticists in the future.

Clinical Manifestations The incidence of hyperphenylalaninemia varies among populations. In African Americans, it is about 1:50,000; in Yemenite Jews, about 1:5000; and in most Northern European populations, about 1:10,000. Post-natal growth restriction, moderate-to-severe intellectual disability, recurrent seizures, hypopigmentation, and eczematous skin rashes constitute the major phenotypic features of untreated phenylketonuria. However, with the advent of widespread newborn screening programs for hyperphenylalaninemia, the major phenotypic manifestations of phenylketonuria today occur when treatment is partial or when it is terminated prematurely during late childhood or adolescence. In these cases, there is a variety of neurocognitive deficits and psychiatric problems that can develop, including deficits in executive functioning and anxiety, depression, and phobias.

Newborn screening for phenylketonuria is performed on a small amount of dried blood obtained at 24–72 hours of age. From the initial screen, there is about a 1% incidence of positive or indeterminate screening results, and in such cases, a more quantitative measurement of plasma phenylalanine is then performed, ideally before 2 weeks of age. In neonates who undergo this confirmatory testing, the diagnosis of phenylketonuria is ultimately validated in about 1%, providing an estimated phenylketonuria prevalence of 1:10,000, although there is great geographic and ethnic variation (see prior discussion).

Infants in whom a diagnosis of phenylketonuria is confirmed are usually placed on a dietary regimen in which a semisynthetic formula low in phenylalanine can be combined with regular breast feeding. This regimen is adjusted empirically to maintain a plasma phenylalanine concentration at or below 1 mmol/L, which is still several times greater than normal but similar to levels observed in so-called benign hyperphenylalaninemia (see later discussion), a biochemical diagnosis which is not associated with phenylketonuria and has no clinical consequences. Phenylalanine is an essential amino acid, and even individuals with phenylketonuria must consume small amounts to avoid protein starvation and a catabolic state. Most children require between 25–50 mg/kg/d of phenylalanine, and these requirements are met by combining natural foods with commercial products designed for phenylketonuria treatment. When dietary treatment programs were first implemented, it was hoped that the risk of neurologic damage from the hyperphenylalaninemia of phenylketonuria would have a limited window and that treatment could be

stopped after childhood. However, it now appears that even mild hyperphenylalaninemia in adults (>1.2 mmol/L) is associated with neuropsychologic and cognitive deficits; therefore, dietary treatment of phenylketonuria should probably be continued indefinitely.

As an increasing number of treated females with phenylketonuria reach childbearing age, a new problem—fetal hyperphenylalaninemia via intrauterine exposure—has become apparent. Newborn infants in such cases exhibit microcephaly and growth restriction of prenatal onset, congenital heart disease, and severe developmental delay regardless of fetal genotype. Rigorous control of maternal phenylalanine concentrations from before conception until birth reduces the incidence of fetal abnormalities in maternal phenylketonuria, but the level of plasma phenylalanine that is “safe” for a developing fetus is 0.12–0.36 mmol/L—significantly lower than what is considered acceptable for phenylketonuria-affected children or adults on phenylalanine-restricted diets.

Pathophysiology The normal metabolic fate of free phenylalanine is incorporation into protein or hydroxylation by phenylalanine hydroxylase to form tyrosine (Figure 2–5). Because tyrosine, but not phenylalanine, can be metabolized to produce fumarate and acetoacetate, hydroxylation of phenylalanine can be viewed both as a means of making tyrosine a nonessential amino acid and as a mechanism for providing energy via gluconeogenesis during states of protein starvation. In individuals with pathogenic variants in phenylalanine hydroxylase, tyrosine becomes an essential amino acid. However, the clinical manifestations of the disease are caused not by absence of tyrosine (most people get enough tyrosine in the diet in any case) but by accumulation of phenylalanine. Transamination of phenylalanine to form phenylpyruvate normally does not occur unless circulating concentrations exceed 1.2 mmol/L, but the pathogenesis of central nervous system (CNS) abnormalities in phenylketonuria, while still being elucidated, is currently felt to be related more to phenylalanine itself than to its metabolites. In addition to a direct effect of elevated phenylalanine levels on energy production, protein synthesis, and neurotransmitter homeostasis in the developing brain, phenylalanine can also inhibit the transport of neutral amino acids across the blood–brain barrier, leading to a selective amino acid deficiency in the cerebrospinal fluid. Thus, the neurologic manifestations of phenylketonuria are felt to be due to a general effect of substrate accumulation on cerebral metabolism. The pathophysiology of the eczema seen in untreated or partially treated phenylketonuria is not well understood, but eczema is a

common feature of other inborn errors of metabolism in which plasma concentrations of branched-chain amino acids are elevated. Hypopigmentation in phenylketonuria is probably caused by an inhibitory effect of excess phenylalanine on the production of dopaquinone in melanocytes, which is the rate-limiting step in melanin synthesis.

FIGURE 2–5 Metabolic fates of phenylalanine. Because catabolism of phenylalanine must proceed via tyrosine, the absence of phenylalanine hydroxylase leads to accumulation of phenylalanine. Tyrosine is also a biosynthetic precursor for melanin and certain neurotransmitters, and the absence of phenylalanine hydroxylase causes tyrosine to become an essential amino acid.

Approximately 90% of infants with persistent hyperphenylalaninemia detected by newborn screening have typical phenylketonuria caused by a defect in phenylalanine hydroxylase (see later discussion). Of the remainder, most have benign hyperphenylalaninemia, in which circulating levels of phenylalanine are between 0.1 mmol/L and 1 mmol/L. However, approximately 1% of infants with persistent hyperphenylalaninemia have defects in the metabolism of tetrahydrobiopterin (BH4), which is a stoichiometric cofactor for the hydroxylation reaction (Figure 2–6). Importantly, BH4 is required not only for phenylalanine hydroxylase but also for tyrosine hydroxylase and tryptophan hydroxylase. The products of these latter two enzymes are catecholaminergic and serotonergic neurotransmitters; thus, individuals with defects in BH4 metabolism suffer not only from phenylketonuria (substrate accumulation) but also from absence of important neurotransmitters (end-product deficiency). Affected individuals develop a severe neurologic disorder in early childhood manifested by hypotonia, inactivity, and developmental regression and are treated not only with dietary restriction of phenylalanine but also with dietary supplementation with BH4, dopa, and 5-hydroxytryptophan.

FIGURE 2–6 Normal and abnormal phenylalanine metabolism. Tetrahydrobiopterin (BH4) is a cofactor for phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase. Consequently, defects in the biosynthesis of BH4 or its metabolism result in a failure of all three hydroxylation reactions. The absence of phenylalanine hydroxylation has phenotypic effects because of substrate accumulation, but the absence of tyrosine or tryptophan hydroxylation has phenotypic effects as a result of end-product deficiency. (6-PTS, 6-pyruvoyltetrahydrobiopterin synthetase; qBH2, quinonoid dihydrobiopterin.)

Genetic Principles Phenylketonuria is one of several mendelian conditions that have a relatively high incidence, others being cystic fibrosis, Duchenne muscular dystrophy, neurofibromatosis type I, and sickle cell anemia (see Table 2–2). These conditions share no single feature: Some are recessive, some dominant, some autosomal, some X-linked; some are lethal in early childhood, but others have very little effect on reproduction (and transmission to subsequent generations). In fact, the incidence of a mendelian condition is determined by a balance of factors, including the rate at which new variants occur and the likelihood that an individual carrying a pathogenic variant will transmit it to their offspring. As mentioned earlier, the latter characteristic—the probability, compared with the general population, of transmitting one’s genes to the next generation—is called fitness. Reduced fitness exhibited by many genetic conditions such as Duchenne muscular dystrophy or type 1 neurofibromatosis is balanced by an appreciable new mutation rate, so that the incidence of the condition remains constant in

successive generations. For recessive conditions like phenylketonuria or sickle cell anemia (or X-

linked recessive conditions such as Duchenne muscular dystrophy), another factor that can influence disease incidence is whether heterozygous carriers experience a selective advantage or disadvantage for survival to reproductive age compared with homozygous normal individuals. For example, the relatively high incidence of sickle cell anemia in individuals of West African ancestry is due in part to heterozygote advantage, conferring resistance to malaria. Because the detrimental effects of homozygosity for the hemoglobin B sickle allele (HBBS) are balanced by the beneficial effects of heterozygosity, the overall frequency of the HBBS allele has increased over time in populations in which malaria is endemic.

A final factor that may contribute to the high incidence of a mendelian disease is genetic drift, which refers to the fluctuation of gene frequencies due to random sampling over many generations. The extent of fluctuation is greatest in very small populations. A related phenomenon is the founder effect, which occurs when a population founded by a small number of ancestors has, by chance, a high frequency of a deleterious gene. A founder effect and genetic drift can operate together to produce large changes in the incidence of mendelian diseases, especially in small populations founded by a small number of ancestors. In the case of phenylketonuria, the fitness of affected individuals has until recently been very low, and de novo variants are exceedingly rare; however, population genetic studies provide evidence for both a founder effect and heterozygote advantage.

Phenylketonuria is also representative of a class of mendelian conditions for which efforts are under way to develop gene therapy, such as hemophilia and ornithine transcarbamylase deficiency. A thorough understanding of the pathophysiology of these conditions is an important prerequisite to developing treatments. Each of these conditions is caused by loss of function for an enzyme expressed specifically in the liver; therefore, attempts to deliver a normal gene to affected individuals have focused on strategies to express the gene in hepatocytes. However, as is the case for benign hyperphenylalaninemia, individuals with very low levels of enzymatic activity are clinically normal, and successful gene therapy might, therefore, be accomplished by expressing the target gene in only a small proportion of hepatic cells.

CHECKPOINT

7. What are the primary defects in phenylketonuria? 8. Why is dietary modification a less than satisfactory treatment of this

condition? 9. Explain how strategies of dietary treatment for inborn errors of

metabolism depend on whether the patho-physiology is caused by substrate accumulation or end-product deficiency.

10. Explain the phenomenon of maternal phenylketonuria.

FRAGILE X–ASSOCIATED MENTAL RETARDATION SYNDROME

Fragile X–associated mental retardation syndrome produces a combination of phenotypic features that affect the CNS, the testes, and the cranial skeleton. These features were recognized as a distinct clinical entity more than 50 years ago. A laboratory test for the syndrome was developed during the 1970s, when it was recognized that most affected individuals exhibit a cytogenetic abnormality of the X chromosome: failure of the region between bands Xq27 and Xq28 to condense at metaphase. Instead, this appears in the microscope as a thin constriction that is subject to breakage during preparation, which accounts for the designation “fragile X.” Advances in the last decade have helped to explain both the presence of the fragile site and the unique pattern of inheritance exhibited by the syndrome. In some respects, fragile X–associated mental retardation syndrome is similar to other genetic conditions caused by X-linked pathogenic variants: Affected males are impaired more severely than affected females, and the condition is never transmitted from father to son. However, the syndrome appears to break the rules of mendelian transmission in that at least 20% of carrier males manifest no signs of it when family pedigrees are analyzed. Daughters of these nonpenetrant but “transmitting males” are themselves nonpenetrant but produce affected offspring, male and female, with frequencies close to mendelian expectations (Figure 2–7). About half of carrier females (those with one normal and one abnormal X chromosome) exhibit a significant degree of intellectual disability. These unusual features of the syndrome were explained when the subchromosomal region spanning the fragile site was isolated and shown to contain a segment in which the triplet sequence CGG was repeated many times: (CGG)n. The number of triplet repeats is very polymorphic

but normally less than 60. A repeat size between 60 and 200 does not cause a clinical phenotype or a cytogenetic fragile site but is unstable and subject to additional amplification, leading to typical features of the syndrome (Figures 2– 8 and 2–9).

FIGURE 2–7 Likelihood of fragile X–associated mental retardation syndrome in an artificial pedigree. The percentages shown indicate the likelihood of clinical manifestation according to position in the pedigree. Because individuals carrying the abnormal X chromosome have a 50% chance of passing it to their offspring, penetrance is twice that of the values depicted. Penetrance increases with each successive generation owing to the progressive expansion of a triplet repeat element (see text). Expansion is dependent on maternal inheritance of the abnormal allele; thus, daughters of normal transmitting males (indicated with a T in II-4) are nonpenetrant. Obligate carrier females are indicated with a central dot. (Reproduced, with permission, from Nussbaum RL, Ledbetter DH. Fragile X syndrome: a unique mutation in man. Annu Rev Genet. 1986;20:109. Permission conveyed through Copyright Clearance Center.)

FIGURE 2–8 Molecular genetics of fragile X–associated mental retardation syndrome. The cytogenetic fragile site at Xq27.3 is located close to a small region of DNA that contains a CpG island (see text) and the FMR1 gene. Within the 5′ untranslated region of the FMR1 gene lies an unstable segment of repetitive DNA 5′–(CGG)n–3′. The table shows the methylation status of the CpG island, the size of the triplet repeat, and whether the FMR1 mRNA is expressed depending on the genotype of the X chromosome. Note that the inactive X chromosome in normal females has a methylated CpG island and does not express the FMR1 mRNA. The methylation and expression status of FMR1 in premutation and full-mutation alleles applies to males and to the active X chromosome of females; premutation and full-mutation alleles on the inactive X chromosome of females exhibit methylation of the CpG island and fail to express the FMR1 mRNA.

FIGURE 2–9 Transmission and amplification of the fragile X–associated mental retardation

syndrome triplet repeat. The heavy arrows show expansion of the triplet repeat, which is thought to occur postzygotically after the premutation or full mutation is transmitted through the female germline. The dashed arrows represent potential phenotypic consequences. Daughters with the full mutation may not express the fragile X–associated mental retardation phenotype, depending on the proportion of cells in which the mutant allele happens to lie on the inactive X chromosome. (Adapted from Tarleton JC et al. Molecular genetic advances in fragile X syndrome. J Pediatr. 1993;122:169.)

Clinical Manifestations Fragile X–associated mental retardation syndrome is usually recognized in affected boys because of developmental delay apparent by 1–2 years of age, small joint hyperextensibility, mild hypotonia, and a family history of intellectual disability in maternally related males. Affected females generally have either milder intellectual disability (when compared to affected males) or only subtle impairments of visuospatial ability, and the condition may not be evident or diagnosed until it is suspected after identification of an affected male relative. In late childhood or early adolescence, affected males begin to exhibit large testes and characteristic facial features, including mild coarsening, large ears, a prominent forehead and mandible, a long face, and relative macrocephaly (considered in relation to height). The syndrome is extremely common and affects about 1:1500–1:1000 males. Virtually all affected males are born to females who are either affected or carry the premutation, and there are no well- recognized cases of new premutations in males or females.

The inheritance of fragile X–associated mental retardation syndrome exhibits several unusual features and is often described in terms of empiric risk figures (see Figure 2–7). In particular, the likelihood that an individual carrying an abnormal chromosome will manifest clinical features depends on the number of generations through which the abnormal chromosome has been transmitted and the sex of the transmitting parent. For example, nonpenetrant transmitting males tend to occur in the same sibship with each other and with nonpenetrant carrier females. This is reflected in low risk figures for brothers and sisters of transmitting males: 9% and 5%, respectively, compared with 40% and 16% for their maternal grandsons and granddaughters. This latter observation, in which the penetrance or expressivity (or both) of a genetic disease seems to increase in successive generations, is sometimes referred to more generally as genetic anticipation.

Genetic anticipation in fragile X–associated mental retardation syndrome is caused by progressive expansion of the triplet repeat. A similar phenomenon occurs in several neurodegenerative disorders such as Huntington disease and spinocerebellar ataxia (ie, grandchildren are affected more severely than

grandparents). The neurodegenerative disorders are caused by production of abnormal proteins; fragile X–associated mental retardation is caused by failure to produce a normal protein. Although the biochemical mechanisms are different, the underlying molecular causes of genetic anticipation are identical and involve progressive expansion of an unstable triplet repeat.

In addition to triplet repeat expansion, genetic anticipation can be caused by bias of ascertainment, which occurs when a mild or variably expressed condition first diagnosed in grandchildren from a three-generation pedigree is then easily recognized in siblings of the grandchildren who are available for examination and testing. In contrast to genetic anticipation caused by expansion of a triplet repeat, anticipation caused by bias of ascertainment affects the apparent, rather than the actual, penetrance.

Pathophysiology Amplification of the (CGG)n repeat at the fraXq27.3 site affects both methylation and expression of the FMR1 gene. This gene and the unstable DNA responsible for the expansion were isolated on the basis of their proximity to the cytogenetic fragile site in Xq27.3. FMR1 encodes an RNA-binding protein that regulates translation of mRNA molecules carrying a characteristic sequence in which four guanine residues can form intramolecular bonds, a so-called G quartet structure.

The (CGG)n repeat is located in the 5′ untranslated region of the FMR1 gene (see Figure 2–8). This segment is highly variable in length; the number of repeats, n, is equal to about 30 ± 25 in individuals who are neither affected with nor carriers for fragile X–associated mental retardation syndrome. In transmitting males and in unaffected carrier females, the number of repeats is usually between 70 and 100. Remarkably, alleles with fewer than 50 repeats are very stable and almost always transmitted without a change in repeat number. However, alleles with 55 or more repeats are unstable and often exhibit expansion after maternal transmission; these individuals are said to carry a premutation. Although premutation carriers do not develop a typical fragile X– associated mental retardation syndrome, studies indicate that female premutation carriers exhibit a 20% and 15% incidence of premature ovarian failure and fragile X–associated tremor/ataxia syndrome (FXTAS), respectively. Male premutation carriers are at an increased risk for FXTAS compared to female carriers, with an overall incidence of 45% in individuals older than 50 years. In both cases, the mechanism is likely to be explained by somatic expansion of the

premutation (see later discussion). The degree of expansion is related to the number of repeats; premutation alleles with a repeat number less than 60 rarely are amplified to a full mutation, but premutation alleles with a repeat number greater than 90 are usually amplified to a full mutation. The number of repeats in the full mutation—observed both in affected males and in affected females—is always greater than 200 but is generally heterogeneous, suggesting that once this threshold is reached, additional amplification occurs frequently in somatic cells.

Expansion from a premutation to a full mutation has two important effects: FMR1 gene transcription is shut off, and DNA surrounding the transcriptional start site of the FMR1 gene becomes methylated. The clinical phenotype is caused by failure to produce FMR1; in addition, methylation of surrounding DNA has important implications for molecular diagnosis. Methylation occurs in a so-called CpG island, a several-hundred-base-pair segment just upstream of the FMR1 transcriptional start site that contains a high frequency of 5′-CG-3′ dinucleotides compared with the rest of the genome. Methylation of the CpG island and expansion of the triplet repeat can be easily detected with molecular biologic techniques, which are the basis of the common diagnostic tests for individuals at risk.

Genetic Principles In addition to the tendency of (CGG)n premutation alleles to undergo further amplifications in length, the molecular genetics of fragile X–associated mental retardation syndrome exhibit several unusual features. As described previously, each phenotypically affected individual carries a full mutation defined by a repeat number greater than 200, but the exact repeat number exhibits considerable heterogeneity in different cells and tissues.

Diagnostic testing for the number of CGG repeats is usually performed on lymphocytes taken from a small amount of peripheral blood. In individuals who carry a repeat number less than 50, each cell has the same number of repeats. However, in phenotypically affected males or females (ie, those with a repeat number greater than 200), many of the cells may have a different number of repeats. This situation, often referred to as somatic mosaicism, indicates that at least some of the amplification is postzygotic, meaning that it occurs in cells of the developing embryo after fertilization. In addition to the DNA methylation associated with an abnormal FMR1 gene, methylation of many genes is a normal process during development and differentiation that helps to regulate gene expression. Cells in which a particular gene should not be expressed frequently shut off that gene’s expression by alterations to chromatin structure that include

DNA modification by methylation and histone modification by methylation and/or acetylation. For example, globin should be expressed only in reticulocytes; albumin should be expressed only in hepatocytes; and insulin should be expressed only by pancreatic β cells. During gametogenesis and immediately after fertilization, specific patterns of chromatin modification characteristic of differentiated cells are erased, only to be reestablished in fetal development. In this way, DNA methylation and other chromatin modifications provide a reversible change in gene structure that can be inherited during mitosis of differentiated cells yet erased during meiosis and early development. This type of alteration—a heritable phenotypic change that is not determined by DNA sequence—is broadly referred to as epigenetic.

Analysis of fragile X–associated mental retardation syndrome pedigrees reveals that one of the most important factors influencing whether a premutation allele is subject to postzygotic expansion is the sex of the parent who transmits the premutation allele (see Figures 2–7 and 2–9). As discussed, a premutation allele transmitted by a female expands to a full mutation with a likelihood proportionate to the length of the premutation. Premutation alleles with a repeat number between 52 and 60 rarely expand to a full mutation, and those with a repeat number greater than 90 nearly always expand. In contrast, a premutation allele transmitted by a male rarely if ever expands to a full mutation regardless of the length of the repeat number.

The notion that alleles of the same DNA sequence can behave very differently depending on the sex of the parent who transmitted them is closely related to the concept of gametic imprinting, which describes the situation that occurs when expression of a particular gene depends on the sex of the parent who transmitted it. Gametic imprinting affects a handful of genes involved in fetal or placental growth, including insulin-like growth factor 2 (IGF2) and the type 2 IGF receptor (IGF2R); for example, the IGF2 gene is expressed only on the paternally derived chromosome, whereas in some individuals, the IGF2R gene is expressed only on the maternally derived chromosome. The mechanisms responsible for gametic imprinting depend on biochemical modifications to the chromosome that occur during gametogenesis; these modifications do not affect the actual DNA sequence but are stably transmitted for a certain number of cell divisions (ie, they are epigenetic and contribute to the pathogenesis of certain types of cancer).

CHECKPOINT

11. Explain why fragile X–associated mental retardation syndrome exhibits an unusual pattern of inheritance.

12. What is genetic anticipation? What are two explanations for it? 13. What is an epigenetic change?

LEBER HEREDITARY OPTIC NEUROPATHY, MYOCLONIC EPILEPSY WITH RAGGED RED FIBERS & OTHER MITOCHONDRIAL DISEASES

In nearly every cell in the body, the indispensable job of turning nutrients into energy takes place in mitochondria, ubiquitous subcellular organelles with their own genomes and unique rules of gene expression. Over the past decade, defects in mitochondrial function have become increasingly recognized as important human causes of diseases, from rare conditions whose study has led to a deeper understanding of pathophysiologic mechanisms to common conditions such as diabetes and deafness. On one level, the consequences of defective mitochondrial function are predictable and nonspecific: Inability to generate sufficient adenosine triphosphate (ATP) leads to accumulation of lactic acid, weakness, and, eventually, cell death. However, every mitochondrion contains multiple mitochondrial genomes; every cell contains multiple mitochondria; the requirements for energy production vary from one tissue to another; and, most importantly, pathogenic variants in mitochondrial DNA affect only a fraction of mitochondrial genomes within a given individual. Because of these characteristics, defects in mitochondrial function present clinically with symptoms and signs that are both specific and protean. In addition, mitochondrial DNA is transmitted by eggs but not by sperm, leading to a unique and characteristic pattern of inheritance.

Clinical Manifestations First described by a German physician in 1871, Leber hereditary optic neuropathy (LHON) presents as painless bilateral loss of vision that occurs in young adults, more commonly in males. Loss of vision can be sudden and complete or subacute and progressive, proceeding from central scotomas to blindness over 1–2 years and usually affecting both eyes within 1–2 months. The condition is occasionally associated with neurologic findings, including ataxia,

dysarthria, or symptoms of demyelinating disease, and may be associated also with cardiac conduction abnormalities. Ophthalmologic examination shows peripapillary telangiectasia, microangiopathy, and vascular tortuosity; in patients with neurologic findings (and some without), CNS imaging studies may reveal abnormalities of the basal ganglia and corpus striatum.

By contrast to LHON, myoclonic epilepsy with ragged red fibers (MERRF) was recognized as a distinct clinical entity relatively recently. The usual presenting symptoms are episodic epilepsy, periodic muscle jerking (myoclonus), and progressive skeletal weakness, but the onset and severity of the symptoms are variable. The term “ragged red fibers” refers to the histologic appearance of muscle from affected individuals, in which abnormal mitochondria accumulate and aggregate in individual muscle fibers. Other symptoms may include sensorineural hearing loss, ataxia, cardiomyopathy, and dementia.

Pathophysiology The central energy-producing machinery of the mitochondria, complexes I–V of the electron transport chain, contains approximately 90 proteins. The majority are encoded by the nuclear genome and, like proteins required for replication, transcription, and translation of the mitochondrial genome, are imported into the mitochondria after translation. The mitochondrial genome itself (mtDNA) is 16,569 bp in length and encodes 13 proteins that are transcribed and translated in mitochondria; mtDNA also encodes mitochondrial ribosomal RNA and 22 mitochondrial tRNA species. Complexes I, III, IV, and V of the electron transport chain contain subunits encoded by both mtDNA and the nuclear genome, whereas the proteins that form complex II are encoded entirely in the nuclear genome.

LHON and MERRF are both caused by pathogenic variants in mtDNA; LHON is caused by pathogenic variants in a component of the electron transport chain, whereas MERRF is caused by pathogenic variants of mitochondrial tRNA, usually tRNALys. Thus, from a biochemical perspective, LHON is caused by a specific inability to generate ATP, whereas MERRF is caused by a general defect in mitochondrial protein synthesis. However, the pathophysiologic mechanisms that lead from defective mitochondrial function to specific organ abnormalities are not completely understood. In general, organ systems affected by mitochondrial diseases are those in which ATP production plays a critical role, such as skeletal muscle and the CNS. In addition, defects in electron transport can cause excessive production of toxic free radicals, leading to

oxidative damage and cell death, and may contribute to age-related dementia. Finally, several proteins that normally reside within mitochondria play key roles in the control of apoptosis; thus, primary abnormalities in mitochondrial integrity can contribute to disease both by decreasing energy production and by increasing programmed cell death.

Genetic Principles For mitochondrial proteins encoded by the nuclear genome and imported into mitochondria after translation, defects that cause disease are inherited in a typical mendelian fashion. mtDNA, however, is transmitted by the egg and not the sperm, in part because the egg contains more than 1000 times more mtDNA molecules than the sperm. Therefore, for diseases like LHON and MERRF caused by defects in mtDNA, the conditions show a characteristic pattern of maternal inheritance (Figure 2–10) in which all offspring of an affected female are at risk but affected males never transmit the condition.

FIGURE 2–10 Maternal inheritance. An idealized pedigree illustrating maternal inheritance, which occurs in disease caused by pathogenic variants in mitochondrial DNA. Mothers transmit the variant mtDNA to all of their offspring, but fathers do not. Variable expressivity and reduced penetrance are a consequence of different levels of heteroplasmy.

A second unique feature of diseases caused by mutations in mtDNA is the mosaic nature of the pathogenic variants within individual cells. Typically, a single cell contains 10–100 separate mtDNA molecules; in the case of an mtDNA pathogenic variant, usually only a fraction of the molecules carry the

variant, a situation referred to as heteroplasmy. The levels of heteroplasmy may vary considerably among different individuals and among different tissues; furthermore, a female primordial germ cell with a mixture of normal and abnormal mtDNA molecules can transmit different proportions to daughter eggs (Figure 2–11). An mtDNA pathogenic variant present in all molecules is said to be homoplasmic. For both LHON and MERRF, levels of abnormal mtDNA may vary from about 50% to about 90%; in general, the severity of the condition correlates with the extent of heteroplasmy.

FIGURE 2–11 Heteroplasmy and variable expressivity. The fraction of variant mtDNA molecules within a cell is determined by a combination of random chance and selection at the cellular level during embryonic development. Adult tissues are mosaic for cells with different fractions of variant mtDNA molecules, which helps to explain why mitochondrial dysfunction can produce different phenotypes and different levels of severity.

A final principle that is apparent from the pathophysiology of mitochondrial diseases is genetic interaction between the nuclear and mitochondrial genomes. One of the best examples is the sex difference in LHON, which affects four to five times as many males as females. This observation suggests that there may be a gene on the X chromosome that modifies the severity of a mitochondrial tRNALys pathogenic variant and underscores the observation that, even though mtDNA itself encodes for a set of key mitochondrial components, most mitochondrial proteins are encoded by the nuclear genome.

DOWN SYNDROME

The clinical features of Down syndrome were described over a century ago. Although the underlying cause—an extra copy of chromosome 21—has been known for more than four decades, the nearly complete DNA sequence of chromosome 21—some 33,546,361 base pairs—has only recently been determined, and the relationship of genotype to phenotype is just beginning to be understood. Down syndrome is broadly representative of aneuploid conditions, those caused by a deviation from the normal chromosome complement (euploidy). Chromosome 21, which contains a little less than 2% of the total genome, is one of the acrocentric autosomes (the others are 13, 14, 15, and 22), one in which nearly all the DNA lies on one side of the centromere. In general, aneuploidy may involve part or all of an autosome or sex chromosome. Most individuals with Down syndrome have 47 chromosomes (ie, one extra chromosome 21, or trisomy 21) and are born to parents with normal karyotypes. This type of aneuploidy is usually caused by nondisjunction during meiotic segregation: the failure of two homologous chromosomes to separate (disjoin) from each other at anaphase. In contrast, aneuploid conditions that affect part of an autosome or sex chromosome must at some point involve DNA breakage and reunion. DNA rearrangements are an infrequent but important cause of Down syndrome and are usually evident as a karyotype with 46 chromosomes in which one chromosome 21 is fused via its centromere to another acrocentric chromosome. This abnormal chromosome is described as a robertsonian translocation and can sometimes be inherited from a carrier parent (Figure 2– 12). Thus, Down syndrome may be caused by a variety of different karyotypic abnormalities, which have in common a 50% increase in gene dosage for nearly all of the genes on chromosome 21.

FIGURE 2–12 Mechanisms leading to Down syndrome: a pedigree in which the mother is phenotypically normal yet is a balanced carrier for a 14;21 robertsonian translocation. She transmits both the translocation chromosome and a normal chromosome 21 to her son, who also inherits a normal chromosome 21 from his father. Three copies of chromosome 21 in the son cause Down syndrome. (Adapted from Nussbaum RL et al. Thompson & Thompson Genetics in Medicine, 7th ed. Saunders Elsevier, 2007.)

Clinical Manifestations Down syndrome is the most common identifiable cause of intellectual disability, occurring in approximately once in every 700 live births and accounting for 15– 20% of the intellectually disabled population. The likelihood of conceiving a child with Down syndrome is related exponentially to increasing maternal age. Historically, because screening programs were offered to pregnant women older than 35 years (Figure 2–13), most children with Down syndrome have been born to women younger than 35 years. Updated recommendations, though, recommend that screening be offered to all pregnant women, regardless of age. When not identified prenatally, Down syndrome is usually suspected shortly after birth from the presence of characteristic facial and dysmorphic features such as brachycephaly, epicanthal folds, small ears, transverse palmar creases,

and hypotonia (Table 2–4). Approximately 50% of affected children have congenital heart defects that come to medical attention in the immediate perinatal period because of cardiorespiratory problems. Strong suspicion of the condition on clinical grounds is usually confirmed by molecular testing within 2–3 days.

FIGURE 2–13 Relationship of Down syndrome to maternal age. The frequency of Down syndrome rises exponentially with increasing maternal age. The frequency at amniocentesis (blue symbols) is slightly higher than in live-born infants (black symbols) because miscarriages are more likely in fetuses with Down syndrome. (Data from Scriver CR et al, eds. The Metabolic and Molecular Bases of Inherited Disease, 8th ed. McGraw-Hill, 2001.)

TABLE 2–4 Phenotypic features of trisomy 21.

A great many minor and major anomalies occur with increased frequency in Down syndrome, yet two affected individuals rarely have the same set of anomalies, and many single anomalies can be seen in unaffected individuals. For example, the incidence of a transverse palmar crease in Down syndrome is about 50%, ten times that of the general population, yet most individuals in whom transverse palmar creases are the only unusual feature do not have Down syndrome or any other genetic disease.

The natural history of Down syndrome in childhood is characterized mainly by developmental delay, growth restriction, and immunodeficiency. Developmental delay is usually apparent by 3–6 months of age as failure to attain age-appropriate developmental milestones and affects all aspects of motor

and cognitive function. The mean IQ is between 30 and 70 and declines with increasing age. However, there is a considerable range in the degree of intellectual disability in adults with Down syndrome, and many affected individuals can live semi-independently. In general, cognitive skills are more limited than affective performance, and only a minority of affected individuals are severely impaired. Restriction of linear growth is moderate, and most adults with Down syndrome have statures 2–3 standard deviations below that of the general population. In contrast, weight growth in Down syndrome exhibits a mild proportionate increase compared with that of the general population, and most adults with Down syndrome are overweight. Although increased susceptibility to infections is a common clinical feature at all ages, the nature of the underlying abnormality is not well understood, and laboratory abnormalities can be detected in both humoral and cellular immunity.

One of the most prevalent and dramatic clinical features of Down syndrome —premature onset of Alzheimer disease—is not evident until adulthood. Although frank dementia is not clinically detectable in all adults with Down syndrome, the incidence of typical neuropathologic changes—senile plaques and neurofibrillary tangles—is nearly 100% by age 35. The major causes of morbidity in Down syndrome are congenital heart disease, infections, and leukemia. Life expectancy depends to a large extent on the presence of congenital heart disease; survival to ages 10 and 30 years is approximately 60% and 50%, respectively, for individuals with congenital heart disease and approximately 85% and 80%, respectively, for individuals without congenital heart disease.

Pathophysiology The advent of molecular markers for different portions of chromosome 21 provided considerable information about when and how the extra chromosomal material arises in Down syndrome, and the Human Genome Project has provided a list of the approximately 230 genes found on chromosome 21. In contrast, much less is known about why increased gene dosage for chromosome 21 should produce the clinical features of Down syndrome.

For trisomy 21 (47,XX+21 or 47,XY+21), cytogenetic or molecular markers that distinguish between the maternal and paternal copies of chromosome 21 can be used to determine whether the egg or the sperm contributed the extra copy of chromosome 21. There are no obvious clinical differences between these two types of trisomy 21 individuals, which suggests that gametic imprinting does not play a significant role in the pathogenesis of Down syndrome. If both copies of

chromosome 21 carried by each parent can be distinguished, it is usually possible to determine whether the nondisjunction event leading to an abnormal gamete occurred during anaphase of meiosis I or meiosis II (Figure 2–14). Studies such as these show that approximately 75% of cases of trisomy 21 are caused by an extra maternal chromosome, that approximately 75% of the nondisjunction events (both maternal and paternal) occur in meiosis I, and that both maternal and paternal nondisjunction events increase with advanced maternal age.

FIGURE 2–14 Nondisjunction has different consequences depending on whether it occurs at meiosis I or meiosis II. The abnormal gamete has two copies of a particular chromosome. When nondisjunction occurs at meiosis I, each of the copies originates from a different chromosome; however, when nondisjunction occurs at meiosis II, each of the copies originates from the same chromosome. Both cytogenetic and molecular polymorphisms can be used to determine the stage and the parent in which nondisjunction occurred. (Adapted, with permission, from Thompson MW et al. Genetics in Medicine, 5th ed. Saunders, 1991. Copyright © Elsevier.)

Several theories have been proposed to explain why the incidence of Down syndrome increases with advanced maternal age (see Figure 2–13). Most germ cell development in females is completed before birth; oocytes arrest at prophase of meiosis I (the dictyotene stage) during the second trimester of gestation. One proposal suggests that biochemical abnormalities that affect the ability of paired chromosomes to disjoin normally accumulate in these cells over time and that,

without a renewable source of fresh eggs, the proportion of eggs undergoing nondisjunction increases with maternal age. However, this hypothesis does not explain why the relationship between the incidence of trisomy 21 and advanced maternal age holds for paternal as well as maternal nondisjunction events.

Another hypothesis proposes that structural, hormonal, and immunologic changes that occur in the uterus with advanced age produce an environment less able to reject a developmentally abnormal embryo. Thus, an older uterus would be more likely to support a trisomy 21 conceptus to term regardless of which parent contributed the extra chromosome. This hypothesis can explain why paternal nondisjunction errors increase with advanced maternal age. However, it does not explain why the incidence of Down syndrome resulting from chromosomal rearrangements (see later discussion) does not increase with maternal age.

These and other hypotheses are not mutually exclusive, and it is possible that a combination of factors is responsible for the relationship between the incidence of trisomy 21 and advanced maternal age. A number of environmental and genetic factors have been considered as possible causes of Down syndrome, including exposure to caffeine, alcohol, tobacco, or radiation, and the likelihood of carrying one or more genes that would predispose to nondisjunction. Although it is difficult to exclude all of these possibilities from consideration as minor factors, there is no evidence that any of these factors plays a role in Down syndrome.

The recurrence risk for trisomy 21 is not altered significantly by previously having affected children. However, approximately 5% of Down syndrome karyotypes have 46 rather than 47 chromosomes as a result of robertsonian translocations that usually involve chromosomes 14 or 22. As described, this type of abnormality is not associated with increased maternal age; however, in about 30% of such individuals, cytogenetic evaluation of the parents reveals a so-called balanced rearrangement such as 45,XX,+t(14q;21q). Because the robertsonian translocation chromosome can pair with both of its component single acrocentric chromosomes at meiosis, the likelihood of segregation leading to unbalanced gametes is significant (Figure 2–15), and the recurrence risk to the parent with the abnormal karyotype is much higher than for trisomy 21 (Table 2– 5). Approximately 1% of Down syndrome karyotypes show mosaicism in which some cells are normal and some abnormal. Somatic mosaicism for trisomy 21 or other aneuploid conditions may initially arise either prezygotically or postzygotically, corresponding to nondisjunction in meiosis or mitosis, respectively. In the former case (in which a zygote is conceived from an

aneuploid gamete), the extra chromosome is presumably lost mitotically in a clone of cells during early embryogenesis. The range of phenotypes seen in mosaic trisomy 21 is great, ranging from mild intellectual disability with subtle dysmorphic features to “typical” Down syndrome, and does not correlate with the proportion of abnormal cells detected in lymphocytes or fibroblasts. Nonetheless, on average, intellectual disability in mosaic trisomy 21 is generally milder than in nonmosaic trisomy 21.

FIGURE 2–15 Types of gametes produced at meiosis by a carrier of a robertsonian translocation. In a balanced carrier for a robertsonian translocation, different types of segregation at meiosis lead to several different types of gametes, including ones that are completely normal (A), ones that would give rise to other balanced translocation carriers (B), and ones that would give rise to aneuploid progeny (C).

TABLE 2–5 Risk for Down syndrome depending on parental sex and karyotype.

Genetic Principles A fundamental question in understanding the relationship between an extra chromosome 21 and the clinical features of Down syndrome is whether the phenotype is caused by abnormal gene expression or an abnormal chromosomal constitution. An important principle derived from studies directed at this question is that of gene dosage, which states that the amount of a gene product produced per cell is proportionate to the number of copies of that gene present. In other words, the amount of protein produced by all or nearly all genes that lie on chromosome 21 is 150% of normal in trisomy 21 cells and 50% of normal in monosomy 21 cells. Thus, unlike the X chromosome, there is no mechanism for dosage compensation that operates on autosomal genes.

Experimental evidence generally supports the view that the Down syndrome phenotype is caused by increased expression of specific genes and not by a nonspecific detrimental effect of cellular aneuploidy. Rarely, karyotypic analysis of an individual with Down syndrome reveals a chromosomal rearrangement (usually an unbalanced reciprocal translocation) in which only a very small portion of chromosome 21 is present in three copies per cell (Figure 2–16). These observations suggest that there may be a critical region of chromosome 21, which, when present in triplicate, is both sufficient and necessary to produce Down syndrome.

FIGURE 2–16 Down syndrome (DS) critical region. Rarely, individuals with Down syndrome will have chromosomal rearrangements that cause trisomy for just a portion of chromosome 21. The APP, SOD1, ETS2, CRYA1, and CBS genes encode proteins (amyloid precursor, superoxide dismutase, the Ets2 transcription factor, crystallin, and cystathionine beta-synthase, respectively) that may play a role in the pathogenesis of Down syndrome. Analysis of two sets of individuals (indicated by the two vertical lines) suggests that the genes responsible for Down syndrome lie in the region of overlap. (Reproduced, with permission, from Thompson MW et al. Genetics in Medicine, 5th ed. Saunders, 1991.)

The concept that altered gene dosage of a group of closely linked genes can produce a distinct clinical phenotype is also supported by the observation that an increasing number of congenital anomaly syndromes have been found to be caused by so-called copy number or structural variants, often mediated by homologous segments of DNA that lie at both ends of deletion and/or insertion breakpoints. Such structural variants, which can be easily detected with molecular genetic techniques, result in an increase and/or decrease in gene copy number for one or more genes. Contiguous gene syndromes, described in Table 2–6, are generally rare, but they have played important roles in expanding our understanding of the pathophysiology of aneuploid conditions.

TABLE 2–6 Phenotype and karyotype of some contiguous gene syndromes.

Carriers for robertsonian translocations that involve chromosome 21 can produce several different types of unbalanced gametes (see Figure 2–15). However, the empiric risk for such a carrier bearing an infant with Down syndrome is higher than for other aneuploid conditions, in part because embryos with other types of aneuploidies are likely to result in miscarriages early in development. Thus, the consequences of trisomy for embryonic and fetal development are proportionate to the number of genes expressed to 150% of their normal levels. Because monosomy for chromosome 21 (and other autosomes) is virtually never seen in live-born infants, a similar line of reasoning suggests that a 50% reduction in gene expression is more severe than a 50% increase. Finally, female robertsonian translocation carriers exhibit much higher empiric recurrence risks than male carriers, which suggests that (1) selective responses against aneuploidy can operate on gametic as well as somatic cells and (2) spermatogenesis is more sensitive to aneuploidy than oogenesis.

CHECKPOINT

14. What are the common features of the variety of different karyotypic abnormalities resulting in Down syndrome?

15. What are the major categories of anomalies in Down syndrome, and what is their natural history?

16. Explain why trisomy 21 is associated with such a wide range of phenotypes from mild intellectual disability to that of “typical” Down syndrome.

IMPACT OF THE HUMAN GENOME PROJECT AND GENOME SEQUENCING ON PATHOPHYSIOLOGY

The major goal of the Human Genome Project has been to determine the identity and gain an understanding of all the genes of human beings and to apply this information to the diagnosis and treatment of human disease. An international collaboration, in which U.S. efforts were coordinated by the National Human Genome Research Institute, achieved a primary milestone in 2003 when the approximately 3 billion–nucleotide human genome DNA sequence was determined.

Understanding the function of all human genes has been facilitated by determining genome sequences for other living organisms. Some are closely related to humans on an evolutionary time scale, such as the chimpanzee, whose genome is approximately 98% the same as humans and whose last common ancestor with humans lived approximately 6 million years ago. Others are more distantly related, such as the laboratory mouse, the fruit fly, or baker’s yeast, but nonetheless serve as valuable model organisms for experimental biologists. Even the laboratory mouse, whose last common ancestor with humans lived approximately 100 million years ago, shares more than 95% of its genes with the human genome. These considerations underscore the important genetic principle that the processes of evolution have left valuable molecular footprints that can be used to learn more about human biology.

One important advance of the Human Genome Project in the last decade has been a catalog of common human genetic variation, usually referred to as the HapMap (for “Haplotype Map”), in which millions of SNPs have been genotyped among individuals of diverse genetic ancestry, including populations from Asia, Africa, the Americas, and Europe. Because common genetic differences are a major determinant of susceptibility to conditions such as diabetes mellitus, hypertension, obesity, and schizophrenia, a principal goal of

the HapMap is to develop a molecular understanding of those determinants. Importantly, the HapMap catalog of common human genetic variation makes it possible to predict DNA sequence variation for specific segments of the genome, even when that sequence has not been measured directly. The underlying reason is that, in most cases, closely linked SNPs are not independently distributed among humans but are nonrandomly associated in clusters known as haplotype blocks. For example, if two closely linked SNPs are each found at a frequency of 30%, chromosomes that carry both SNPs may exist at a frequency considerably different from 9%, which would be the prediction if the two SNPs were completely independent. This phenomenon, referred to as allelic association or linkage disequilibrium, is a result of human evolutionary and population history; the extent to which new SNPs (that arise by mutation) become separated from closely adjacent SNPs (by recombination) depends on the distance between adjacent SNPs and the effects of population history on the chances for recombination.

The idea that measuring human genetic variation on a genome-wide scale could provide insight into common diseases such as hypertension, schizophrenia, and cancer underscores the perspective that there is a spectrum of genetic disease from rare conditions inherited in a mendelian fashion (which have been the major subject of this chapter) to so-called complex genetic or multifactorial conditions, for which the incidence of the disease is influenced by a combination of genes, environment, and chance. Identifying genetic components of multifactorial conditions is an important goal of the field of genetic epidemiology, in which epidemiology-based study designs are applied to populations whose familial structure is uncertain or unknown, and SNP measurements in candidate genes are treated as hypothetical risk factors. For example, the epsilon 4 allele of the apolipoprotein E gene (APOE4) is found in approximately 15% of the population and increases the risk of both atherosclerosis and late-onset Alzheimer disease. However, APOE4 is just one of many genes that influence susceptibility to these important conditions, and a major goal of the HapMap is to identify and characterize those genes, both to develop new treatments and to provide as much information as possible to physicians and their patients regarding disease susceptibility as a function of genetics.

Indeed, there is much excitement today about the potential of personalized genetic medicine, in part due to recent advances in several different areas. First, technological advances now make it possible to efficiently measure variation at millions of SNPs in individual patient samples as a routine laboratory test. These

kinds of tests have been applied to thousands of individuals in so-called case– control studies to identify particular SNPs that occur more or less frequently in cases versus controls. Second, advances in the design and analysis of this type of approach, known as a genome-wide association study (GWAS), have been very successful in identifying new genetic determinants for obesity, diabetes, inflammatory bowel disease, coronary artery disease, and other common conditions.

A second important advance in the Human Genome Project has been the drive to develop new technological approaches for efficient and inexpensive DNA sequencing. So-called next-generation sequencing instruments use an innovative combination of molecular biologic, computational, and optical principles and have revolutionized our approach to biomedical research and medical care. The scale of technological advance is staggering: Sequencing the first human genome cost several billion dollars and required the effort of several thousand scientists over a decade; today, a single laboratory technician can sequence a genome on a benchtop instrument for a thousand dollars. This advanced sequencing technology has resulted in the generation of vast amounts of sequence data, which requires equally advanced bioinformatic technologies for analysis and interpretation.

The availability and low cost of genome sequencing are having an enormous impact on our approach to the diagnosis and pathophysiologic understanding of genetic disease. For example, the ability to compare entire genome sequences (or partial sequences of the protein-coding regions, or exomes) of individuals affected with rare syndromes is rapidly leading to the identification of pathogenic variants that cause thousands of different conditions, including recessively inherited forms of osteogenesis imperfecta, many unexplained syndromes that involve intellectual disability, and neuropsychiatric conditions such as autism. In addition, the ability to compare genome sequences of different tissues or biopsy samples from the same individual allows unprecedented insight into the pathophysiology of many cancers, identifying, for example, a catalog of DNA sequence alterations that have occurred and, in some cases, have helped to drive the progression of blood cancers, brain tumors, breast cancer, prostate cancer, and melanoma.

The future of genetic medicine will be greatly informed by these advances; many scientists envision that powerful but inexpensive laboratory tests that measure genetic variation across the entire genome will soon be used routinely to predict individual susceptibility to common and rare diseases and take appropriate steps to intervene and/or modify the course of those conditions. For

example, individuals at high risk for certain types of cancer may benefit from aggressive screening programs.

Genetic differences may also help identify subgroups of patients whose course is likely to be more or less severe and who may respond to a particular treatment. The latter approach is part of the larger field of pharmacogenomics, in which sequence variation in the hundreds of genes that influence drug absorption, metabolism, and excretion is a major determinant of the balance between pharmacologic efficacy and toxicity. One might imagine, for example, that tests for specific nucleotide differences in a set of genes unique to a particular situation might be used to help predict the pathophysiologic response to alcoholic liver damage, type of regimen used to treat leukemia, and course of infectious diseases like tuberculosis or HIV infection.

CASE STUDIES Yeong Kwok, MD

(See Chapter 25, p. 741–44 for answers)

CASE 1

A 4-year-old boy is brought in with pain and swelling of the right thigh after a fall in the home. An x-ray film reveals an acute fracture of the right femur. Questioning of the mother reveals that the boy has had two other known fractures—left humerus and left tibia—both with minimal trauma. The family history is notable for a bone problem during childhood in the boy’s father that got better as he grew into adulthood. A diagnosis of osteogenesis imperfecta is entertained.

Questions

A. What are the four types of osteogenesis imperfecta? How are they genetically transmitted?

B. Which two types are most likely in this patient? How might they be distinguished clinically?

C. Further workup results in a diagnosis of type I osteogenesis imperfecta.

What clinical features may the boy expect in adult life? D. What is the pathogenesis of this patient’s disease?

CASE 2

A newborn girl tests positive for phenylketonuria (PKU) on a newborn screening examination. The results of a confirmatory serum test done at 2 weeks of age are also positive, establishing the diagnosis of PKU.

Questions

A. What are the metabolic defects in persons with PKU? B. How do these defects lead to clinical disease in persons not treated with

dietary restrictions appropriate for PKU? C. What is the genetic pattern of inheritance? What are some possible

explanations for why the gene for the condition has persisted in the gene pool despite the obvious disadvantages for affected individuals?

CASE 3

A young woman is referred for genetic counseling. She has a 3-year-old boy with developmental delay and small joint hyperextensibility. The pediatrician has diagnosed fragile X–associated mental retardation. She is currently pregnant with her second child at 14 weeks of gestation. The family history is unremarkable.

Questions

A. What is the genetic mutation responsible for fragile X–associated mental retardation? How does it cause the clinical syndrome of developmental delay, joint hyperextensibility, large testes, and facial abnormalities?

B. Which parent is the probable carrier of the genetic mutation? Explain

why this parent and the grandparents are phenotypically unaffected. C. What is the likelihood that the unborn child will be affected?

CASE 4

A 16-year-old boy presents with worsening vision for the past 2 months. He first noticed that he was having trouble with central vision in his right eye, seeing a dark spot in the center of his visual field. The dark spot had gotten larger over time, and he had also developed a central loss of vision in his left eye. Two of his maternal uncles had loss of vision, but his mother and another maternal uncle and two maternal aunts had no visual difficulties. No one on his father’s side was affected. Physical examination reveals microangiopathy and vascular tortuosity of the retina. Genetic testing confirms the diagnosis of Leber hereditary optic neuropathy.

Questions

A. What is the central defect in Leber hereditary optic neuropathy (LHON)?

B. How is this disorder inherited, and what is the principle of heteroplasmy?

C. What explains the fact that males are much more likely to be affected than females?

CASE 5

A 40-year-old woman, recently married and pregnant for the first time, comes to the clinic with a question about the chances of having “a Down syndrome baby.”

Questions

A. What is the rate of occurrence of Down syndrome in the general

population? What are some of the common clinical features? B. What major genetic abnormalities are associated with Down syndrome?

How might these abnormalities lead to the clinical features of the syndrome?

C. How might this woman’s age contribute to her risk of having a child with Down syndrome?

CASE 6

A 25-year-old woman recently gave birth to a baby diagnosed with Down syndrome. Neither she nor the baby’s father have a personal or family history of Down syndrome. Karyotyping of the parents shows that the mother has a balanced robertsonian translocation: 45,XX,+t(14q;21q).

Questions

A. What are common phenotypic features of babies with Down syndrome? B. What is the chance that this couple would have a subsequent child with

Down syndrome? C. Why does the extra genetic material of chromosome 21 lead to Down

syndrome?

REFERENCES

Osteogenesis Imperfecta Forlino A et al. Osteogenesis imperfecta. Lancet. 2016 Apr 16;387(10028):1657–71. [PMID: 26542481] Marom R et al. Pharmacological and biological therapeutic strategies for osteogenesis imperfecta. Am J Med Genet C Semin Med Genet. 2016 Dec;172(4):367–83. [PMID: 27813341]

Van Dijk FS et al. Osteogenesis imperfecta: clinical diagnosis, nomenclature and severity assessment. Am J Med Genet A. 2014 Jun;164A (6):1470–81. [PMID: 24715559]

Phenylketonuria

Blau N. Genetics of phenylketonuria: then and now. Hum Mutat. 2016 Jun;37(6):508–15. [PMID: 26919687]

Hood A et al. Variability in phenylalanine control predicts IQ and executive abilities in children with phenylketonuria. Mol Genet Metab. 2014 Apr;111(4):445–51. [PMID: 24568837]

Thomas J et al; PRISM investigators. Pegvaliase for the treatment of phenylketonuria: results of a long-term phase 3 clinical trial program (PRISM). Mol Genet Metab. 2018 May;124(1):27–38. [PMID: 29653686]

Singh RH et al. Recommendations for the nutrition management of phenylalanine hydroxylase deficiency. Genet Med. 2014 Feb;16(2):121–31. [PMID: 24385075]

Vockley J et al. Phenylalanine hydroxylase deficiency: diagnosis and management guideline. Genet Med. 2014 Feb;16(2):188–200. [PMID: 24385074]

Fragile X–Associated Mental Retardation Syndrome Grigsby J. The fragile X mental retardation 1 gene (FMR1): historical perspective, phenotypes, mechanism, pathology, and epidemiology. Clin Neuropsychol. 2016 Aug;30(6):815–33. [PMID: 27356167]

Kidd SA et al. Fragile X syndrome: a review of associated medical problems. Pediatrics. 2014 Nov;134(5):995–1005. [PMID: 25287458]

Tassone F. Advanced technologies for the molecular diagnosis of fragile X syndrome. Expert Rev Mol Diagn. 2015;15(11):1465–73. [PMID: 26489042]

LHON, MERFF & Other Mitochondrial Diseases Gorman GS et al. Mitochondrial diseases. Nat Rev Dis Primers. 2016 Oct 20;2:16080. [PMID: 27775730] Meyerson C et al. Leber hereditary optic neuropathy: current perspectives. Clin Ophthalmol. 2015 Jun 26;9:1165–76. [PMID: 26170609]

Parikh S et al. Diagnosis and management of mitochondrial disease: a consensus statement from the Mitochondrial Medicine Society. Genet Med. 2015 Sep;17(9):689–701. [PMID: 25503498]

Down Syndrome Grieco J et al. Down syndrome: cognitive and behavioral functioning across the lifespan. Am J Med Genet C Semin Med Genet. 2015 Jun;169(2):135–49. [PMID: 25989505]

Taylor-Phillips S et al. Accuracy of non-invasive prenatal testing using cell-free DNA for detection of Down, Edwards and Patau syndromes: a systematic review and meta-analysis. BMJ Open. 2016 Jan 18;6(1):e010002. [PMID: 26781507]

Vermeesch JR et al. Prenatal and pre-implantation genetic diagnosis. Nat Rev Genet. 2016;17(10):643–56. [PMID: 27629932]

The Human Genome Project & Human Genetic Variation Ashley EA. Towards precision medicine. Nat Rev Genet. 2016 Aug 16;17(9):507–22. [PMID: 27528417] den Dunnen JT et al. HGVS recommendations for the description of sequence variants: 2016 update. Hum

Mutat. 2016 Jun;37(6):564–9. [PMID: 26931183] Goodwin S et al. Coming of age: ten years of next-generation sequencing technologies. Nat Rev Genet. 2016 May 17;17(6):333–51. [PMID: 27184599]

Richards S et al. Standards and guidelines for the interpretation of sequence variants: a joint consensus recommendation of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology. Genet Med. 2015 May;17(5):405–24. [PMID: 25741868]

Vissers LE et al. Genetic studies in intellectual disability and related disorders. Nat Rev Genet. 2016 Jan;17(1):9–18. [PMID: 26503795]

CHAPTER

3

Disorders of the Immune System Jeffrey L. Kishiyama, MD, Jennifer J. Chang, MD, & Suzanne M. Donovan, MD, MPH

The function of the immune system is to protect the host from invasion of foreign organisms by distinguishing “self” from “non-self.” A well-functioning immune system not only protects the host from external factors such as microorganisms or toxins but also prevents and repels attacks by endogenous factors such as tumors and participates in tissue repair. A normal immune response relies on the careful coordination of a complex network of biological factors, specialized cells, tissues, and organs necessary for the recognition of pathogens and subsequent elimination of foreign antigens. Dysfunction or deficiency of components of the immune system leads to a variety of clinical diseases of varying expression and severity, ranging from atopic disease to autoimmune disease, primary immunodeficiency, and cancer. This chapter introduces the intricate physiology of the immune system and abnormalities that lead to diseases of hypersensitivity and immunodeficiency.

The immune system consists of both antigen-specific and nonspecific components that have distinct yet overlapping functions. The antibody-mediated and cell-mediated immune systems are adaptive, providing specificity and memory of previously encountered antigens. The nonspecific or innate defenses include epithelial barriers, mucociliary clearance, phagocytes, dendritic cells, innate lymphoid cells, mast cells, and complement proteins. Despite being phylogenetically primitive and lacking specificity, these components are essential because they are largely responsible for natural immunity to a vast array of environmental threats and microorganisms. Knowledge of the

components and physiology of normal immunity is essential for understanding the pathophysiology of diseases of the immune system.

NORMAL STRUCTURE & FUNCTION OF THE IMMUNE SYSTEM

ANATOMY

Cells of the Immune System The major cellular components of the immune system consist of monocytes and macrophages, lymphocytes, and the family of granulocytic cells, including neutrophils, eosinophils, basophils, and mast cells. Derived from hematopoietic stem cells, these fully differentiated effector cells have membrane receptors for various chemoattractants, immunoglobulins, soluble mediators, and cell-surface proteins, which facilitate cellular homing, the activation or destruction of target cells. Furthermore, it is increasingly recognized that many immune cell-types have subsets, with distinct cytokine profiles and surface markers.

Mononuclear phagocytes play a central role in the immune response. Tissue macrophages are derived from blood monocytes and participate in antigen processing and secretion of mediators vital to initiating specific immune responses. Abundant near mucosal surfaces and recruited to sites of inflammation, these cells phagocytose and internalize microorganisms and debris, then travel to secondary lymphoid organs where they process and present that antigen in a form recognizable to T lymphocytes. In addition, macrophages function as effector cells for certain types of tumor immunity and participate in tissue repair through promotion of angiogenesis and fibrosis.

Macrophages are activated by binding of extracellular molecules to surface- bound receptors. Receptors for complement component C3b (bound fragments of complement, activated by microbes and antigen-bound immunoglobulins; ie, immune complexes) and the Fc portion of both immunoglobulins G and E (IgG and IgE) molecules facilitate activation and phagocytosis through both antigen- specific and nonspecific immune pathways. Constitutively expressing toll-like receptors (TLRs), macrophages also recognize pathogen-associated molecular patterns and bind associated pathogen components that augment macrophage activation against intracellular pathogens, enhancing microbial ingestion and

killing through synthesis of proteolytic enzymes, arachidonic acid metabolites, and reactive oxygen metabolites. Activated macrophages can synthesize and release cytokines, such as tumor necrosis factor (TNF), IL-1, and IL-6, which shape immune responses but also trigger inflammation in both health and disease.

Dendritic cells share duties as sentinel and antigen-presenting cells (APCs)

of the innate immune system. Many dendritic cells (eg, Langerhans cells, oligodendrocytes, Kupffer cells) encounter microbes at epithelial surfaces, share a common hematopoietic precursor, and function to process and transport antigen from skin, respiratory, and gastrointestinal (GI) surfaces to regional lymphoid tissues. Like macrophages, dendritic cells are highly efficient at presenting antigen to T lymphocytes, initiating adaptive immune responses and complementing their innate immune functions.

Lymphocytes express specialized receptors, responsible for the initial recognition and binding to specific antigens. They are functionally and phenotypically divided into B and T lymphocytes. Structurally, B and T lymphocytes cannot be distinguished visually from each other under the microscope. They can be enumerated by flow cytometric phenotyping or by immunohistochemical methods. Approximately 70–80% of circulating blood lymphocytes are T cells (CD3), and 10–15% are B cells (CD19); the remainder are referred to as natural killer (NK) cells (CD56, CD161; also known as NK cells or null cells). The thymus-derived cells (T lymphocytes or T cells) are involved in cellular immune responses. B lymphocytes or B cells are involved in humoral or antibody responses.

During embryonic development, T cell precursors migrate to the thymus, where they develop some of the functional and cell surface characteristics of mature T cells. Through positive and negative selection, clones of autoreactive T cells are eliminated, and mature T cells migrate to the peripheral lymphoid tissues. There, they enter the pool of long-lived lymphocytes that recirculate from the blood to the lymph. Immune tolerance may occur centrally in the thymus or peripherally through mechanisms of induced anergy (functional hyporesponsiveness), suppression by regulatory T cells (Treg), or deletion through apoptosis (programmed cell death).

Numerous subpopulations of T cells are now appreciated, heterogeneous with respect to their cell surface markers and functional characteristics. The T-cell receptor (TCR) complex, expressed on helper-inducer T cells (CD4+), recognizes antigenic peptides, processed and presented on the surface of APCs. Modulated by a host of co-stimulatory factors, CD4+ T cells enhance phagocytic cell function, amplify B-cell production of immunoglobulin, recruit leukocytes, regulate inflammation and promote T cell (CD8+)–mediated cytotoxicity. Activated CD4+ T cells regulate immune responses through cell-to-cell contact and by elaboration of soluble factors or cytokines.

Subsets of CD4+ T cells are differentiated based on the cytokine milieu, antigen concentration, degree of TCR binding affinity, and nature of the APC

and can be identified on the basis of their pattern of cytokine production. Often, CD4+ T cell subsets display autocrine characteristics, promoting their own and reciprocally inhibiting other subset development. One subset, TH1 cells, develop in the presence of IL-12, secreted from activated macrophages, especially in the presence of infection with intracellular organisms. TH1 cells elaborate interferon-γ (IFN-γ) and TNF, stimulating macrophage ingestion and microbe killing. Pathogenetically, TH1 cells contribute to type IV delayed hypersensitivity reactions, tuberculin granulomatous reactions, and autoimmune disorders such as rheumatoid arthritis and multiple sclerosis. TH2 cell subsets develop in the presence of IL-4 and secrete IL-4, IL-5, and IL-13, which facilitate humoral responses and defense against helminths. TH2 subsets also play a key role in atopic diseases. Because IL-4 and IL-13 promote IgE production, and IL-5 is a proliferation and differentiation factor for eosinophils, TH2 cells have been implicated in eosinophil-mediated reactions and response to allergens. Via elaboration of IL-17, TH17 cell subsets appear to boost early phagocytic cell responses by recruiting neutrophils to sites of infection and acute inflammation. In the primary immunodeficiency known as the hyper-IgE syndrome, defective development of TH17 cells presents clinically with increased susceptibility to skin and pulmonary bacterial and fungal infections.

Cytotoxic or “killer” T cells (CTLs) are CD8+ effector T cells, generated after interaction with certain foreign antigens, presented by dendritic cells, in the presence of CD4+ T cell–derived cytokines. CTLs are responsible for defense against intracellular pathogens (eg, viruses), tumor immunity, and organ graft rejection. Most killer T cells exhibit the CD8+ phenotype, although in certain circumstances, CD4+ T cells can also be cytotoxic. CTLs may kill their target through cell-to-cell adhesion, osmotic lysis, or by inducing apoptosis. CTLs participate in delayed hypersensitivity reactions and, in some cases, destroying infected cells. The resultant tissue damage may contribute to immunopathology.

A number of additional T-helper subsets have been discovered that contribute to immune regulation. Regulatory T cells (Treg) modulate and inhibit immune responses, thereby regulating homeostasis and tolerance versus inflammation, allergy, and autoimmunity. Treg cells suppress activated T-effector cells by secreting transforming growth factor-β (TGF-β) and IL-10, both inhibitory cytokines. The best characterized Treg cell expresses high-affinity receptors for IL-2 (CD25) and FOXP3, a transcription factor that may suppress autoimmune disease. On the surface of FOXP3+ Treg cells, CTLA-4 (cytotoxic T lymphocyte

antigen 4) binds to B7, a co-receptor on APCs, leading to suppression of APC- mediated T cell responses. Mutations of FOXP3 have been associated with the unchecked inflammatory autoimmune disease, known as immunodysregulation, polyendocrinopathy, enteropathy X-linked syndrome (IPEX). Induction of antigen-specific Treg may be a major mechanism of action of allergen immunotherapy for the treatment of allergic rhinitis, asthma, and food, and venom hypersensitivity. The major role of B cells is to differentiate into antibody-secreting plasma cells. B-lymphocyte maturation proceeds in antigen- independent and antigen-dependent stages. Antigen-independent development occurs in the marrow where pre-B cells mature into immunoglobulin-bearing naive B cells (cells that have not been exposed to antigen previously). Surface immunoglobulin functions as B cell antigen-binding receptors. During the maturation of lymphocyte-committed hematopoietic stem cells, the genes encoding the antigen-combining sites on immunoglobulin molecules are rearranged, generating the extreme immunologic diversity seen in pre-B cells. Immature B cells initially express IgM on their surface and later in development co-express IgD. While most naive B cells that do not encounter antigen survive only a matter of months, a portion of those activated through T cell–dependent processes become long-lived memory B cells.

Memory B cells and plasma cells, found predominantly in primary follicles and germinal centers of the lymph nodes and spleen, express other immunoglobulin isotypes, such as IgA and IgG, and possess the capacity for brisk secondary immune responses upon re-exposure to antigen. In addition to their role synthesizing both membrane-bound and soluble immunoglobulin, B cells may also release cytokines and function as antigen-presenting cells (APCs).

Null cells probably include a number of different cell types, including a group called NK cells. These cells appear distinct from other lymphocytes in that they are slightly larger, with a kidney-shaped nucleolus, have a granular appearance (large granular lymphocytes), and express distinct cell surface markers (CD56, CD161), but lack antigen-specific T-cell receptors (CD3, or TCRs). Recruited to sites of inflammation, NK cells possess membrane receptors for the IgG molecules (FcγR), facilitating antibody-dependent cell-mediated cytotoxicity (ADCC). Binding of an antibody-coated cell or foreign substance triggers release of perforin, a pore-forming protein that causes cytolysis. Other NK cell functions include antibody-independent cellular killing, inducing apoptosis in Fas-expressing cells, and immunomodulating responses to viruses, malignancy, and transplanted tissue through a potent release of IFN-γ, TNF, and other key cytokines. Innate lymphoid cells (ILC) morphologically and functionally

resemble T cells but lack TCR and therefore also lack antigen-specificity. Producing cytokines, they contribute to innate immunity, with roles in defense against viruses and helminths, but may also be immunopathogenic in allergic inflammation and asthma.

Polymorphonuclear leukocytes (neutrophils) are granulocytes that phagocytose and destroy foreign antigens and microbial organisms. In acute inflammatory reactions, they are attracted to the site of antigen by chemotactic factors, including plasma-activated complement 5 (C5a), leukotriene B4 (LTB4), granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-8, and platelet-activating factor (PAF). The presence of receptors for complement C3b and invariant/constant regions of IgG molecules (Fcγ) on the surface of neutrophils also facilitates the clearance of opsonized microbes through the reticuloendothelial system. Smaller antigens are phagocytosed and destroyed by lysosomal enzymes. Locally released lysosomal enzymes destroy particles too large to be phagocytosed. Neutrophils contain or generate a number of antimicrobial factors, including oxidative metabolites, superoxide, and hydrogen peroxide, as well as myeloperoxidase, which catalyzes the production of hypochlorite, and proteolytic enzymes, including collagenase, elastase, and cathepsin B.

Eosinophils are often found in inflammatory sites or at sites of immune reactivity and play a crucial role in the host’s defense against parasites. Despite many shared functional similarities to neutrophils, eosinophils are considerably less efficient than neutrophils at phagocytosis. Eosinophils play both a proactive and a modulatory role in inflammation. They are attracted to the site of the antigen–antibody reactions by PAF, C5a, chemokines, histamine, and LTB4. When stimulated, they release numerous inflammatory factors, including major basic protein (MBP), eosinophil-derived neurotoxin, eosinophil cationic protein (ECP), eosinophil peroxidase, lysosomal hydrolases, and LTC4. MBP destroys parasites, impairs ciliary beating, and causes exfoliation of respiratory epithelial cells; it may also trigger histamine release from mast cells and basophils. Eosinophil-derived products may play a role in the development of airway hyperreactivity. Interleukin-5 (IL-5) is a primary growth factor for eosinophils, and current pharmacologic IL-5 antagonists appear to be clinically useful in the treatment of eosinophil-derived asthma and airways diseases.

Mast cells, found chiefly in connective and subcutaneous tissue, are basophilic cells that interface with the environment (eg, skin, conjunctiva, respiratory and gastrointestinal tracts). They have prominent granules that are the source of many mediators of immediate hypersensitivity and have 30,000–

200,000 cell surface membrane receptors for the Fc fragment of IgE. When an allergen molecule cross-links two adjacent mast cell surface–associated IgE antibodies, calcium-dependent cellular activation leads to the release of both preformed and newly generated mediators. Vasoactive mediators (eg, histamine), lipid mediators (eg, prostaglandins/leukotrienes), cytokines, and proteases contribute to the acute inflammation and tissue damage seen with mast cell activation. Beyond IgE, mast cells also have surface receptors for “anaphylatoxins” (activated complement fragments, C3a, C4a, and C5a), cytokines, and neuropeptides, such as substance P. Activation by these non–IgE- mediated mechanisms may contribute to host immunity and provide ties between the immune and neuroendocrine systems. Mast cell–deficient mice display a particular vulnerability to sepsis and rapid death after peritonitis, possibly owing to insufficient TNF production during bacterial infection. Mast cells also appear in areas of wound healing and in fibrotic lung disease. Experimentally, mast cell–derived mediators have been shown to promote angiogenesis and fibrogenesis, suggesting their presence in these sites is pathologically relevant.

Basophils are circulating granulocytes with properties similar to tissue mast cells. Possessing high-affinity receptors for IgE (FcεRI), they mediate both immediate- and late-phase allergic responses. These cells release many of the potent mediators of allergic inflammatory diseases, including histamine, leukotrienes (LTs), prostaglandins (PGs), and PAF, all of which have significant effects on the vasculature and on the inflammatory response.

Organs of the Immune System Several tissues and organs play roles in host defenses and are functionally classified as the immune system. In mammals, the primary lymphoid organs are the thymus and the bone marrow. All cells of the immune system are originally derived from bone marrow. Pluripotent stem cells differentiate into lymphocyte, granulocyte, monocyte, erythrocyte, and megakaryocyte populations. Deficiency or dysfunction of the pluripotent stem cell or the various cell lines developing from it can result in immune deficiency disorders of varying expression and severity.

The thymus, derived from the third and fourth embryonic pharyngeal pouches, functions to produce T lymphocytes and is the site of initial T- lymphocyte differentiation. Its reticular structure allows a significant number of lymphocytes to migrate through it to become fully immunocompetent thymus- derived cells. Developing T cells in the thymic cortex are first positively selected for their ability to recognize self-peptides (ie, major histocompatibility

complex, MHC). In subsequent negative selection, T cells that avidly recognize self-peptides are destroyed, thus removing deleterious self-reactive clones. In some murine models, autoimmune diseases such as systemic lupus erythematosus may develop in mice with defective apoptotic pathways in T cells recognizing self-antigen. The thymus also regulates immune function by secreting multiple bioactive substances that promote T-lymphocyte differentiation and are essential for T-lymphocyte–mediated immunity.

In mammals, the lymph nodes, spleen, and gut-associated lymphoid tissue are secondary lymphoid organs connected by blood and lymphatic vessels. Lymph nodes are strategically dispersed throughout the vasculature and are the principal organs of the immune system that deliver and localize antigen, promoting adaptive immunity through cell–cell interaction and lymphocyte activation. Lymph nodes have a framework of reticular cells and fibers that are arranged into a cortex and medulla. B lymphocytes, the precursors of antibody- producing cells, or plasma cells, are found in the cortex (the follicles and germinal centers), as well as in the medulla. T lymphocytes are found chiefly in the medullary and paracortical areas of the lymph node (Figure 3–1). The spleen filters and processes antigens from the blood and is functionally and structurally divided into B-lymphocyte and T-lymphocyte areas, similar to those of the lymph nodes. The spleen also removes damaged blood cells and immune complexes, including antibody-coated microbes. Loss of the spleen, through traumatic causes or therapeutic splenectomy, increases host susceptibility to encapsulated bacteria, which are normally cleared through opsonization and phagocytosis in splenic tissue.

FIGURE 3–1 Anatomy of a normal lymph node. (Redrawn, with permission, from Chandrasoma P et al, eds. Concise Pathology, 3rd ed. Originally published by Appleton & Lange. Copyright © 1998 by The McGraw-Hill Companies, Inc.)

Gut-associated lymphoid tissue, adjacent to the mucosal epithelium, includes the tonsils, the Peyer patches of the small intestine, and the appendix, all of which facilitate immune responses to the multitude of ingested antigens and commensal microbes. Like the lymph nodes and spleen, these tissues exhibit separation into B-lymphocyte–dependent and T-lymphocyte–dependent areas. Mucosal immune responses tend to generate antigen-specific IgA, and, with some orally administered antigens, T-cell anergy or tolerance may occur rather than immune stimulation. Increasingly, the commensal microbiome of the gastrointestinal tract has been recognized to shape both local and systemic innate and adaptive immune responses, influencing both health and disease (such as allergy or autoimmunity).

Inflammatory Mediators Mediators are released or generated during immune responses to coordinate and regulate immune cell activities to generate physiological or cytotoxic responses. Targeting many diverse cell types, they can have antiviral, pro-inflammatory, or anti-inflammatory activities, can act locally or systemically, and can be redundant in their actions (Table 3–1). Mediators exist either in a preformed state in the granules of mast cells and basophils or are newly synthesized at the time of cellular activation. Increased awareness of the immunologic and physiologic effects of mediators has led to a better understanding of immunopathology and provides potential targets for future pharmacotherapies.

TABLE 3–1 Cytokines and their functions.

Preformed mediators include histamine, eosinophil and neutrophil chemoattractants, proteoglycans (heparin, chondroitin sulfate), and various proteolytic enzymes. Histamine is a bioactive amine, packaged in dense intracellular granules, that when released binds to membrane-bound H1, H2, and H3 receptors, resulting in significant physiologic effects. Binding to H1 receptors causes smooth muscle contraction, vasodilatation, increased vascular permeability, and stimulation of nasal mucous glands. Stimulation of H2 receptors causes enhanced gastric acid secretion, mucus secretion, and leukocyte chemotaxis. Histamine is important in the pathogenesis of allergic rhinitis, allergic asthma, and anaphylaxis.

Newly generated mediators include kinins, PAF, and arachidonic acid metabolites. In many immune cells, arachidonic acid, liberated from membrane phospholipid bilayers, is metabolized either by the lipoxygenase pathway to form leukotrienes (LTs) or by the cyclooxygenase pathway to form prostaglandins (PGs) and thromboxanes A2 and B2 (TXA2 and TXB2). LTB4 is a potent chemoattractant for neutrophils. LTC4, LTD4, and LTE4 constitute a

slow-reacting substance of anaphylaxis, which has a bronchial smooth muscle spasmogenic potency 100–1000 times that of histamine, and which also causes vascular dilation and vascular permeability.

Almost all nucleated cells generate PGs. The most important are PGD2, PGE2, PGF2, and PGI2 (prostacyclin). Human mast cells produce large amounts of PGD2, which causes vasodilatation, vascular permeability, and airway constriction. Activated polymorphonuclear neutrophils and macrophages generate PGF2α, a bronchoconstrictor, and PGE2, a bronchodilator. PGI2 causes platelet disaggregation. TXA2 causes platelet aggregation, bronchial constriction, and vasoconstriction.

Macrophages, neutrophils, eosinophils, and mast cells generate PAF, which causes platelet aggregation, vasodilatation, increased vascular permeability, and bronchial smooth muscle contraction. PAF is the most potent eosinophil chemoattractant described and plays a role in anaphylaxis. The kinins are vasoactive peptides formed in plasma when kallikrein, released by basophils and mast cells, digests plasma kininogen. Kinins, including bradykinin, contribute to human angioedema and anaphylaxis by causing slow, sustained contraction of bronchial and vascular smooth muscle, vascular permeability, secretion of mucus, and stimulation of pain fibers. Bradykinin antagonists may be used therapeutically to treat life-threatening swelling in patients with hereditary angioedema.

Cytokines Many immune functions are regulated or mediated by cytokines, which are soluble factors secreted by activated immune cells. Cytokines can be functionally organized into groups according to their major activities: (1) those that promote inflammation and mediate natural immunity, such as IL-1, IL-6, IL- 8, TNF, and IFN-α; (2) those that support allergic inflammation, such as IL-4, IL-5, and IL-13; (3) those with immunoregulatory activity, such as IL-10, IL-12, TGF-β, and IFN-γ; and (4) those that act as hematopoietic growth factors, such as IL-3, IL-7, and GM-CSF (see Table 3–1). A group of chemotactic factors called chemokines regulate homing and migration of immune cells to sites of inflammation. Human immunodeficiency virus (HIV) may exploit certain chemokine receptors to infect host cells, and natural mutations in these same chemokine co-receptors may confer a susceptibility or resistance to infection. Targeting specific immune cells or pathogenetic mechanisms, therapeutic applications of cytokines are now available for a variety of autoimmune,

inflammatory, atopic and hematologic disorders.

Complement Cascades The complement system is a family of serum proteins, which, when activated through a series of proteolytic enzymes, enhances microbe opsonization, leukocyte recruitment, and target cell lysis. Immune complex formation (ie, the union of antigen with IgG or IgM antibody) initiates classic complement pathway activation. Complement-fixing sites on the Fc portion the immunoglobulin molecules are exposed, allowing binding of the first component of the complement sequence, C1q. Other components of the complement sequence are subsequently bound and activated, leading to significant amplification of the immune response. Important byproducts of the classic pathway include activated cleavage products, the anaphylatoxins C3a, C5a, and less-potent C4a. C5a is a potent leukocyte chemotactic factor that also causes mediator release from mast cells and basophils. C4b and C3b mediate immune complex binding to phagocytic cells, facilitating opsonization.

Activation of the complement sequence by the alternative pathway is initiated by a number of agents, including lipopolysaccharides (LPS; eg, bacterial endotoxin), trypsin-like molecules, aggregated IgA and IgG, and cobra venom. Activating the alternative pathway does not require the presence of antigen–antibody immune complexes or use the early components of the complement sequence, C1, C4, and C2. Ultimately, activating either the classic or alternative pathway leads to formation of the membrane attack complex, resulting in cell lysis, and/or tissue inflammation. Soluble inhibitors regulate the complement pathway to prevent unchecked activation and prolonged inflammation. Deficiency of one factor, C1-esterase inhibitor, leads to recurrent, potentially life-threatening attacks of facial, laryngeal, and gastrointestinal (GI) swelling in hereditary angioedema.

CHECKPOINT

1. What are the specific and nonspecific components of the cellular and noncellular limbs of the immune system?

2. What is the role of macrophages in the immune system, and what are some of the products they secrete?

3. What are the categories of lymphocytes, and how are they distinguished?

4. What is the role of lymphocytes in the immune system, and what are some of the products they secrete?

5. What is the role of eosinophils in the immune system, and what are some of the products they secrete?

6. What is the role of basophils in the immune system, and what are some of the products they secrete?

7. What is the role of epithelial cells in the immune system, and what are some of the products they secrete?

8. What are the primary and secondary lymphoid organs, and what roles do they play in the proper functioning of the immune system?

PHYSIOLOGY

1. Innate & Adaptive Immunity Living organisms exhibit two levels of response against external invasion: an innate system of natural immunity and an adaptive system that is acquired. Innate immunity is present from birth, does not require previous antigenic exposure, and is nonspecific in its activity. The skin and epithelial surfaces serve as the first line of defense of the innate immune system, whereas enzymes, the alternative complement system pathway, acute-phase proteins, phagocytic, NK cells, and cytokines provide additional layers of protection.

Microbial cell walls or nucleic acids contain nonmammalian patterns or motifs that can bind to cell-associated pattern recognition receptors (ie, Toll-like receptors [TLRs]) on innate immune cells including macrophages and dendritic and epithelial cells. These structures are highly conserved, and different TLRs binds to specific pathogen-related products, such as LPS, viral RNA, microbial DNA, and fungal wall mannon proteins. Binding triggers transcription of proinflammatory factors and cytokine synthesis (TNF, IFNγ, IL-1, IL-6, and IL- 12, among others) prior to adaptive responses. Some of these released cytokines are endogenous pyrogens, others antiviral, and still others shape adaptive immune responses by modulating the differentiation of naive T cells into specific T-helper subtypes or effector cells.

Through a series of proteolytic activations, the serum and membrane components of the complement cascade amplify and regulate microbial killing and inflammation. Despite the lack of specificity, innate immunity is largely responsible for protection against a vast array of environmental microorganisms

and foreign substances. Higher organisms have evolved an adaptive immune system, which is

triggered by encounters with foreign agents that have evaded or penetrated the innate immune defenses. The adaptive immune system is characterized both by specificity for individual foreign agents and by immunologic memory, which makes possible an intensified response to subsequent encounters with the same or closely related agents. Primary adaptive immune responses require clonal expansion, leading to a delayed response to new exposures. Secondary immune responses are more rapid, larger, and more efficient. Stimulation of the adaptive immune system triggers a complex sequence of events that initiate the activation of lymphocytes, the production of antigen-specific antibodies (humoral immunity) and effector cells (cellular or cell-mediated immunity), and ultimately, the elimination of the inciting substance. Although adaptive immunity is antigen specific, the repertoire of responses is tremendously diverse, with an estimated 109 antigenic specificities.

2. Antigens (Immunogens) Foreign substances that induce an immune response are called antigens or immunogens. Immunogenicity implies that the substance has the ability to react with antigen-binding sites on antibody molecules or TCRs. Complex foreign agents possess distinct and multiple antigenic determinants or “epitopes,” dependent on the peptide sequence and conformational folding of immunogenic proteins. Most immunogens are proteins, although pure carbohydrates may be immunogenic as well. It is estimated that the human immune system can respond to 107–109 different antigens, an amazingly diverse repertoire.

3. Immune Response The primary role of the immune system is to discriminate self from non-self and to eliminate foreign substances. The physiology of the normal immune response to antigen is summarized in Figure 3–2. A complex network of specialized cells, organs, and biologic factors is necessary for the recognition and subsequent elimination of foreign antigens. Both T and B cells need to migrate throughout the body to increase the likelihood that they will encounter an antigen to which they have specificity. Soluble antigens are transported to regional lymph tissues through afferent lymphatic vessels, while other antigens are carried by phagocytic dendritic cells. Regional, peripheral lymphoid organs and the spleen are sites for concentrated immune responses to antigen by recirculating

lymphocytes and APCs. Antigens encountered via inhaled or ingested routes activate cells in the respiratory mucosa– or gut-associated lymphoid tissues. The major pathways of antigen elimination include the direct killing of target cells by cytotoxic T lymphocytes (CTLs; cellular response) and the elimination of antigen through antibody-mediated events arising from T- and B-lymphocyte interactions (humoral response). The series of events that initiate the immune response includes antigen processing and presentation, lymphocyte recognition and activation, cellular or humoral immune responses, and antigenic destruction or elimination.

FIGURE 3–2 The normal immune response. Cytotoxic T-cell response is shown on the left side of the figure, and the helper T-cell response is shown on the right side. As depicted on the left, most CD8 T cells recognize processed antigen presented by MHC class I molecules and destroy infected cells, thereby preventing viral replication. Activated T cells secrete interferon-γ, which, along with interferon-α and

interferon-β secreted by infected cells, produces cellular resistance to viral infection. On the right and at the bottom, CD4 helper (TH1 and TH2) cells recognize processed antigen presented by MHC class II molecules. TH1 cells secrete interferon-γ and interleukin-2, which activate macrophages and cytotoxic T cells to kill intracellular organisms; TH2 cells secrete interleukin-4, 5, and 6, which help B cells secrete protective antibodies. B cells recognize antigen directly or in the form of immune complexes on follicular dendritic cells in germinal centers.

Antigen Processing & Presentation Most foreign immunogens are not recognized by the immune system in their native form and require capture and processing by professional APCs, which constitutively express class II MHC molecules and accessory co-stimulatory molecules on their surfaces. Such specialized cells include macrophages, dendritic cells in lymphoid tissue, Langerhans cells in the skin, Kupffer cells in the liver, microglial cells in the nervous system, and B lymphocytes.

Following an encounter with immunogens, the APCs internalize the foreign substance by phagocytosis or pinocytosis, modify the parent structure, and display antigenic fragments of the native protein on its surfaces in association with MHC class II molecules (see later discussion). T-cell–independent antigens such as polysaccharides can activate B cells without assistance from T cells by binding to B-cell receptors (BCRs, or surface-bound antibody), leading to rapid IgM responses, without generating memory cells or long-lived plasma cells. Most antigens, however, require internalization and processing by B cells or other APCs with subsequent recognition by CD4 T cells.

T-Lymphocyte Recognition & Activation The recognition of processed antigen by specialized T lymphocytes known as helper T (CD4) lymphocytes and the subsequent activation of these cells constitute the critical events in the immune response. The helper T lymphocytes orchestrate the many cells and biologic signals (cytokines) necessary to carry out the immune response. Helper T lymphocytes recognize processed antigen displayed by APCs only in association with polymorphic cell surface proteins called the major histocompatibility complex (MHC). With haplotypic mendelian inheritance, MHC genes are highly polymorphic. Known also as human leukocyte antigens (HLAs), these proteins participate in the co- presentation of processed peptide antigens to T cells, thereby facilitating the essential distinction of “self” from “non-self.” During cell-to-cell contact between T helper cells and APCs, the combined antigen–MHC complex functions as the binding epitope, interacting with the TCR and its associated

accessory molecules, CD4 or CD8. Although HLA typing is used primarily for determining transplant compatibility, HLAs also determine immune responsiveness to foreign agents and confer susceptibility to certain autoimmune disorders. All somatic cells express MHC class I, whereas only the specialized APCs can express MHC class II. Exogenous foreign antigens are expressed in association with MHC class II structures, expressed only by specialized APCs.

Besides binding to modified antigen, T cell activation depends on the co- stimulation of accessory molecules. Accessory molecules on T cells bind to ligands found on APCs, epithelial cells, vascular endothelium, and extracellular matrix, controlling the subsequent T-cell function or homing (Table 3–2). In the absence of such signals, the T cell may be “tolerized” or may undergo apoptosis instead of being activated. Biologic products that block some of these co- stimulatory pathways are currently being investigated as potential therapeutic agents to prevent organ rejection in transplantation and in the management of some autoimmune diseases.

TABLE 3–2 T-cell and APC surface molecules and their interactions.

Before an activated T cell can differentiate, proliferate, produce cytokines, or

participate in cell killing, the activation signal must be transduced into the cytoplasm or nucleus of the cell. The presence of immunoreceptor tyrosine activation motifs associated with each TCR complex facilitates and amplifies intracellular signaling. The binding of zeta-associated protein 70 (ZAP-70), a Syk-family protein tyrosine kinase (PTK), to CD3ε and ζ-subunits after they are phosphorylated is critical for downstream signaling. Another important enzyme in the activation of T cells is CD45, a protein tyrosine phosphatase. The critical nature of these enzymes in lymphocyte development is underscored by the discovery of ZAP-70 and CD45 deficiency syndromes, disorders that result in various forms of severe combined immunodeficiency disease (SCID; see Primary Immunodeficiency Diseases).

T-cell activation does not occur in isolation but also depends on the cytokine milieu. In true autocrine fashion, the APCs involved in antigen presentation release IL-1, which induces the release of both IL-2 and IFN-γ from CD4 cells. IL-2 feeds back to stimulate the expression of additional IL-2 receptors on the surface of the CD4 cells and stimulates the production of various cell growth and differentiation factors (cytokines) by the activated CD4 cells. Induction of IL-2 expression is particularly critical for T cells. Cyclosporine and tacrolimus, two immunosuppressive agents used to prevent organ transplant rejection, function by downregulating IL-2 production by T cells.

CD8 Effector Cells (Cellular Immune Response) CTLs eliminate target cells (virally infected cells, tumor, or foreign tissues), thus constituting the cellular immune response. CTLs differ from helper T lymphocytes in their expression of the surface antigen CD8 and by the recognition of antigen complexed to cell surface proteins of MHC class I. All somatic cells can express MHC class I molecules. Pathogenic microorganisms, whose proteins gain access to the cell cytoplasm (eg, malarial parasites) or by de novo gene expression in the infected cell cytoplasm (eg, viruses) stimulate CD8 class I MHC-restricted T-cell responses. Killing of target cells by CTLs requires direct cell-to-cell contact. Two major mechanisms for killing target cells have been described: (1) CTL secretion of a pore-forming protein (perforin) that inserts in the plasma membrane of target cells along with serine proteases called granzymes, leading to osmotic lysis; and (2) expression of the Fas ligand on the surface of CTLs that bind to Fas on the target cell membrane inducing apoptosis. In addition to killing infected cells directly, CD8 T cells can elaborate a number of cytokines, including TNF and lymphotoxin. Memory CTLs may be long-lived to provide “recall” responses and immunity against latent or persistent viral

infections.

Activation of B Lymphocytes (Humoral Immune Response) The primary function of mature B lymphocytes is to synthesize antibodies. Like T-cell activation, B-lymphocyte activation is triggered after antigen binds to BCRs (ie, surface-bound immunoglobulin) and is regulated through concomitant co-receptor binding. In secondary lymphoid tissues, release of cytokines IL-2, IL-4, IL-5, and IL-6 by activated helper T lymphocytes promotes the proliferation and terminal differentiation of B cells into high-rate antibody- producing cells called plasma cells, which secrete antigen-specific immunoglobulin. If complement fragments bind B-cell surface complement receptors at the same time antigen engages BCRs, cellular responses are heightened. T cells also modulate humoral immunity through their activation- dependent membrane expression of CD40 ligand protein. During direct T- and B-cell contact, CD40 ligand binds to the CD40 receptor on the surface of B cells, inducing apoptosis or activation of immunoglobulin synthesis, depending on the situation. The importance of CD40 ligand–CD40 binding in normal humoral immunity is highlighted by the congenital immunodeficiency X-linked hyper-IgM syndrome. A defect in the synthesis of CD40 ligand on activated T cells results in impaired “isotype switching” and hyper-IgM, with subsequent deficient IgG and IgA production and impaired humoral immunity.

Although their primary function is immunoglobulin synthesis, B lymphocytes may also bind and internalize foreign antigen directly, process that antigen, and present it to CD4 T lymphocytes. A pool of activated B lymphocytes may differentiate into memory cells, which respond more rapidly and efficiently to subsequent encounters with identical or closely related antigenic structures.

Antibody Structure & Function Antibodies (immunoglobulins) are proteins that possess “specificity,” enabling them to combine with one particular antigenic structure. Antigen-binding sites for immunoglobulin will recognize three-dimensional structures, whereas TCRs bind short peptide segments without tertiary structure. Humoral (antibody- mediated) immune responses result in the production of a diverse repertoire (estimated 109–1011) of antibody specificities, providing the ability to recognize and bind with a broad range of antigens. This diversity is a function of somatic recombination of gene segments within B lymphocytes early in ontogenetic development. Rearrangement occurs at the genetic loci for the immunoglobulin

and TCR antigen binding sites through a process called variable-diversity- joining (VDJ) gene recombination. Somatic recombination, in both T cells and B cells, is dependent on recombination-activating genes (RAG1 and RAG2), deficiency of which leads to a lack of T and B lymphocytes, an autosomal recessive form of SCID. Somatic mutations occurring after antigenic stimulation lead to a phenomenon called “affinity maturation,” whereby the average affinity of antibody binding increases throughout the immune response.

All immunoglobulin molecules share a four-chain polypeptide structure consisting of two heavy and two light chains (Figure 3–3). Each chain includes an amino-terminal portion, containing the variable (V) region, and a carboxyl terminal portion, containing four or five constant (C) regions. V regions are highly variable structures that form the antigen-binding sites, whereas the C domains support effector functions of the molecules. The five classes (isotypes) of immunoglobulins are IgG, IgA, IgM, IgD, and IgE and are defined on the basis of differences in the C region of the heavy chains. The isotype expressed by a particular B lymphocyte depends on the state of cellular differentiation and “isotype switching,” a process characterized by splicing of heavy chain mRNA prior to translation and leading to synthesis of different immunoglobulin classes. Different isotypes contribute to different effector functions on the basis of the ability of the molecule to bind to specific receptors and their efficiency in fixing serum complement. IgG is the predominant immunoglobulin in serum with the longest half-life. Four subclasses—IgG1, IgG2, IgG3, and IgG4—differ in their relative quantities and targets (protein vs. carbohydrate antigens). IgA is the predominant immunoglobulin on mucous membrane surfaces. It exists predominantly as a monomer in serum and as a dimer or trimer when secreted on mucous membrane surfaces. IgA antibodies protect the host from foreign antigens on mucous membrane surfaces, but they do not fix complement by the classic pathway. IgM is a pentamer found almost exclusively in the intravascular compartment. IgM is expressed early in immune responses, providing rapid adaptive immunity, and detection of antigen-specific IgM can be used diagnostically during certain infections. IgD is a monomeric immunoglobulin. Its biological function is unknown. IgE is the heaviest immunoglobulin monomer, with a normal concentration in serum varying from 20 to 100 IU, but the concentration may be 5 times normal or even higher in an atopic individual. The Fc portion of IgE binds to receptors on the surfaces of mast cells and basophils. IgE antibodies play an important role in immediate hypersensitivity reactions.

FIGURE 3–3 Structure of a human IgG antibody molecule. Depicted are the four-chain structure with two heavy chains (H) and two light chains (L). Each light chain consists of a variable and constant region. (V, variable region; C, constant region.) The sites of pepsin and papain cleavage are shown. (Reproduced, with permission, from Parslow TG, et al, eds. Medical Immunology, 10th ed. McGraw-Hill, 2001.)

Humoral Mechanisms of Antigen Elimination Antibodies induce the elimination of foreign antigen through a number of different mechanisms. Binding of antibody to bacterial toxins or foreign venoms may cause neutralization or promote elimination of these antigen–antibody immune complexes through the reticuloendothelial system. Antibody-coated bacteria are more easily phagocytosed by macrophages in a process known as

opsonization. Some classes of antibodies may complex with antigen and activate the complement cascade (“complement fixation”), culminating in lysis of the target cell. Finally, the major class of antibody, IgG, can bind to NK cells that subsequently complex with target cells and release cytotoxins (see prior discussion of antibody-dependent cellular cytotoxicity). IgG passes transplacentally, providing passive immunization of neonates.

After the successful elimination of antigen, the immune system uses several mechanisms to return to basal homeostasis. IgG can switch off its own response to antigen through the binding of immune complexes that transmit inhibitory signals into the nuclei of B cells.

Mechanisms of Inflammation Elimination of foreign antigen by cellular or humoral processes is integrally linked to the inflammatory response, in which cytokines and antibodies trigger the recruitment of additional cells and the release of endogenous vasoactive and proinflammatory enzymatic substances (inflammatory mediators).

Inflammation may have both positive and deleterious effects. Tight control of inflammatory mechanisms promotes efficient elimination of foreign substances, killing microbes, infected cells, and tumors. Uncontrolled lymphocyte activation and unregulated antibody production, however, can lead to tissue damage and organ dysfunction. Pathogenic immune dysfunction is responsible for hypersensitivity reactions, immunodeficiency, and many of the clinical effects of autoimmunity. Imbalances in the inflammatory system may result from genetic defects, infection, neoplasms, and exposure to environmental triggers, although precise mechanisms that promote abnormal regulation and persistence of inflammatory processes are complex and poorly understood.

Hypersensitivity Immune Responses Gell and Coombs classified the mechanisms of immune responses to antigen into four distinct types of reaction to allow for a clearer understanding of the immunopathogenesis of disease.

A. Type I

Clinical allergy represents an IgE-mediated hypersensitivity response arising from deleterious inflammation in response to the presence of normally harmless antigens, such as pollen, animal dander, or foods. Anaphylactic or immediate

hypersensitivity reactions occur after binding of antigen to IgE antibodies attached to the surface of the mast cell or basophil and result in the release of preformed and newly generated inflammatory mediators that produce the clinical manifestations. Examples of type I–mediated reactions include anaphylactic shock, allergic rhinitis, allergic asthma, and allergic food and drug reactions.

B. Type II

Cytotoxic reactions involve the binding of either IgG or IgM antibody to antigens covalently bound to cell membrane structures. Antigen–antibody binding activates the complement cascade and results in destruction of the cell to which the antigen is bound. Examples of tissue injury by this mechanism include immune hemolytic anemia and Rh hemolytic disease in the newborn. An example of the type II–mediated disease process without cell death is autoimmune hyperthyroidism, a disorder in which anti-thyroid antibodies bind to and stimulate thyroid tissue.

C. Type III

Antigen binding to antibodies with complement fixation forms immune complexes. Complement-bound immune complexes facilitate opsonization by phagocytes and ADCC. Complexes are usually cleared from the circulation in the reticuloendothelial system. However, deposition of these complexes in tissues or in vascular endothelium can produce immune complex–mediated tissue injury through complement activation, anaphylatoxin generation, polymorphonuclear leukocyte chemotaxis, mediator release, and tissue injury. Cutaneous Arthus reaction, systemic serum sickness, some aspects of clinical autoimmunity, and certain features of infective endocarditis are clinical examples of type III–mediated diseases. In the earliest reported cases of serum sickness, passive immunization with horse serum triggered the formation of human–anti-horse serum immune complexes, leading to fever, rash, renal dysfunction, and inflammatory arthritis.

D. Type IV

Cell-mediated immunity is responsible for host defenses against intracellular pathogenic organisms, although abnormal regulation of this system may result in delayed-type hypersensitivity. Type IV hypersensitivity reactions are mediated not by antibody but by antigen-specific T lymphocytes. Classic examples are tuberculin skin test reactions and contact dermatitis.

IgE Synthesis in Allergic Reactivity Atopy or Ig-E-mediated immediate hypersensitivity results from the inappropriate and sustained production of IgE in response to allergen. TH2 cytokines IL-4 and IL-13 are critical to isotype switching through induction of germline transcription of IgE heavy chain genes. IL-13 has about 30% structural homology with IL-4 and shares much of the activities of IL-4 on mononuclear cells and B lymphocytes. There is a strong genetic predisposition toward the development of atopic disease. Evidence has been found for the linkage of 5q31.1 and the IL-4, IL-5, IL-9, and IL-13 receptor genes, suggesting that a nearby gene in this chromosome locale regulates overall IgE production.

In contrast, TH1-generated IFN-γ inhibits IL-4–dependent IgE synthesis in humans. Thus, an imbalance favoring IL-4 over IFN-γ may induce IgE formation. In one study, reduced cord blood IFN-γ at birth was found to be associated with clinical atopy at age 12 months.

In allergic inflammatory processes, TH2 lymphocytes, innate lymphoid cells (ILCs), and follicular T cells represent a source of IL-4, as well as secondary signals necessary to drive the production of IgE by B lymphocytes. Another TH2 cytokine, IL-5, promotes maturation, activation, chemotaxis, and prolongation of survival in eosinophils. In situ hybridization analyses of T-cell mRNA in airway mucosal biopsies from allergic rhinitis and asthma patients show a distinct TH2 pattern. The demonstration of allergen-specific T-cell lines that proliferate and secrete large amounts of IL-4 on exposure to relevant antigen in vitro further supports the existence of specific TH2-like clones. The original source of the IL- 4 responsible for TH2 differentiation is unclear, although some observations suggest that a TH2 bias exists during fetal development in both atopic and nonatopic individuals. The “hygiene hypothesis” posits that environmental exposures, possibly to bacterial products, such as endotoxin, bacterial DNA, or the gut commensal microbiome, encourage a shift toward TH1 and a subsequent reduced risk of clinical atopic disease. Mononuclear phagocytes are the major source of IL-12, suggesting a mechanism whereby antigens more likely to be processed by macrophages, including bacterial antigens and intracellular pathogens, produce TH1 responses. Epidemiologic studies of children suggest those exposed to daycare at early ages and those with numerous siblings are at reduced risk for atopy and asthma.

Since the discovery of IgE more than 5 decades ago, scientists have considered various therapeutic strategies to selectively inhibit IgE antibody

production and action. Recent data suggest that conventional and modified immunotherapy (allergy shots) eliminate (“anergize”) rather than stimulate TH2 responses to environmental allergen, potentially through generation of immunosuppressive Treg. Current research has focused on understanding the mechanisms controlling IgE production, including the molecular events of B-cell switching to IgE synthesis, IL-4 and IL-13 signaling, T- and B-cell surface receptor interactions, and mechanisms driving TH2 differentiation. Soluble cytokine receptors and genetically engineered monoclonal antibodies are currently under development for the purpose of cytokine neutralization in allergic diseases. The U.S. Food and Drug Administration (FDA) has approved several immunomodulatory agents, including omalizumab (anti-IgE), benralizumab, mepolizumab and reslizumab (all anti-IL-5 agents), and dupilumab (anti-IL-4 and anti-IL-13), for the treatment of severe refractory asthma and of atopic dermatitis.

CHECKPOINT

9. What are the components of and distinctions between the innate and adaptive forms of immunity?

10. Indicate the primary role of the immune system and the major classes of events by which this is accomplished.

11. What is the phenomenon of MHC restriction? 12. What signals are necessary to activate helper T lymphocytes? 13. What two signals are necessary to activate cytotoxic T lymphocytes? 14. What are the common structural features of antibodies? 15. Name four different mechanisms by which antibodies can induce the

elimination of foreign antigens. 16. What are the four types of immune reactions in the Gell and Coombs

classification scheme, and what are some examples of disorders in which each is involved?

17. What is the critical factor in switching Ig synthesis to the IgE isotype? What are some secondary factors that con-tribute to, or inhibit, IgE synthesis?

PATHOPHYSIOLOGY OF SELECTED IMMUNE

DISORDERS

ALLERGIC RHINITIS

Clinical Presentation Allergic airway diseases such as allergic rhinitis and asthma are characterized by local tissue damage and organ dysfunction in the upper and lower respiratory tract arising from an abnormal hypersensitivity immune response to normally harmless and ubiquitous environmental allergens. Allergens that cause airway disease are predominantly seasonal tree, grass, and weed pollens or perennial inhalants (eg, house dust mite or cockroach antigen, mold, animal dander, and some occupational protein antigens). Allergic disease is a common cause of pediatric and adult acute and chronic airway problems. Both allergic rhinitis and asthma account for significant morbidity, and atopic disorders have increased in prevalence over the past few decades. In a Danish survey, the prevalence of skin test–positive allergic rhinitis in persons 15–41 years of age increased from 12.9% in 1990 to 22.5% in 1998. Allergic rhinitis is discussed here as a model for the pathophysiology of IgE-mediated allergic airway disease.

Etiology Allergic rhinitis implies the existence of type I (IgE-mediated) immediate hypersensitivity to environmental allergens that impact the upper respiratory mucosa directly. Particles larger than 5 μm are filtered almost completely by the nasal mucosa. Because most pollen grains are at least this large, few intact particles would be expected to penetrate the lower airway when the nose is functioning normally. The allergic or atopic state is characterized by an inherited tendency to generate IgE antibodies to specific environmental allergens and the physiologic responses that ensue from inflammatory mediators released after the interaction of allergen with mast cell–bound IgE. The clinical presentation of allergic rhinitis includes nasal, ocular, and palatal pruritus, paroxysmal sneezing, rhinorrhea, and nasal congestion. A personal or family history of other allergic diseases such as asthma or atopic dermatitis supports a diagnosis of allergy. Confirmation of allergic rhinitis requires the demonstration of specific IgE antibodies to common allergens by in vitro immunoassay or in vivo (skin) testing in patients with a history of symptoms with relevant exposures.

Pathology & Pathogenesis Inflammatory changes in the airways are recognized as critical features of both allergic rhinitis and chronic asthma. Cross-linking of surface-bound IgE by antigen activates tissue mast cells and basophils, inducing the immediate release of preformed mediators and the synthesis of newly generated mediators. Mast cells and basophils also have the ability to synthesize and release proinflammatory cytokines, growth and regulatory factors that interact in complex networks. The interaction of mediators with various target organs and cells of the airway can induce a biphasic allergic response: an early phase mediated chiefly by release of histamine and other stored mediators (tryptase, chymase, heparin, chondroitin sulfate, and TNF), whereas late-phase events are induced after generation of arachidonic acid metabolites (LTs and PGs) and PAF and de novo cytokine synthesis.

The early-phase response occurs within minutes after exposure to an antigen. After intranasal challenge or ambient exposure to a relevant allergen, the allergic patient begins sneezing and develops an increase in nasal secretions. After approximately 5 minutes, the patient develops mucosal swelling leading to reduced airflow. These changes are secondary to the effects of vasoactive and smooth muscle constrictive mediators, including histamine, N-α-p-tosyl-L- arginine methylester-esterase (TAME), LTs, PGD2, and kinins and kininogens from mast cells and basophils. Histologically, the early response is characterized by vascular permeability, vasodilatation, tissue edema, and a mild cellular infiltrate of mostly granulocytes.

The late-phase allergic response may follow the early-phase response (dual response) or may occur as an isolated event. Late-phase reactions begin 2–4 hours after initial exposure to antigen, reach maximal activity at 6–12 hours, and usually resolve within 12–24 hours. If the exposure is frequent or ongoing, however, the inflammatory response becomes chronic. The late-phase response is characterized by erythema, induration, heat, burning, and itching and microscopically by a significant cellular influx of mainly eosinophils and mononuclear cells. Changes consistent with airway remodeling and tissue hyperreactivity may also occur.

Mediators of the early-phase response—except for PGD2—reappear during the late-phase response in the absence of antigen re-exposure. Absence of PGD2, an exclusive product of mast cell release, in the presence of continued histamine release suggests that basophils and not mast cells are an important source of mediators in the late-phase response. There is an early accumulation of

neutrophils and eosinophils, with later accumulation of activated T cells, synthesizing TH2 cytokines. Inflammatory cells infiltrating tissues in the late- phase response may further elaborate cytokines and histamine-releasing factors that may perpetuate the response, leading to sustained hyperresponsiveness, mucus hypersecretion, IgE production, eosinophilia, and disruption of the target tissue (eg, bronchi, skin, or nasal mucosa).

There is strong circumstantial evidence that eosinophils are key proinflammatory cells in allergic airway disease. Eosinophils are frequently found in secretions from the nasal mucosa of patients with allergic rhinitis and in the sputum of asthmatics. Products of activated eosinophils such as MBP and eosinophilic cationic protein, which are destructive to airway epithelial tissue and predispose to persistent airway reactivity, have also been localized to the airways of patients with allergic disease.

The recruitment of eosinophils and other inflammatory cells to the airway is largely a product of activated chemokines and adhesion molecules. There are two subfamilies of chemokines, which differ in the cells they primarily attract and in the chromosome location of their genes. The C-C chemokines, including RANTES, MCP-1, MCP-3, and eotaxin, are located on chromosome segment 7q11-q21 and selectively recruit eosinophils. Leukocytes attach to vascular endothelial cells through receptor–ligand interaction of cell surface adhesion molecules of the integrin, selectin, and immunoglobulin supergene family. The interaction of these adhesion molecules and their counter-receptors mediates a sequence of events that includes margination of leukocytes along the walls of the microvasculature, adhesion of leukocytes to the epithelium, transmigration of leukocytes through vessel walls, and migration along a chemotactic gradient to reach tissue compartments. Both chemokine production and adhesion molecule expression are upregulated by soluble inflammatory mediators. For instance, endothelial cell adhesion molecule receptors, ICAM-1, VCAM-1, and E-selectin, are upregulated by IL-1, TNF, and LPS.

Clinical Manifestations The clinical manifestations of allergic airway disease (Table 3–3) arise from the interaction of mast cell and basophil mediators with target organs of the upper and lower airway. The symptoms of allergic rhinitis appear immediately after exposure to a relevant allergen (early-phase response), although many patients experience chronic and recurrent symptoms on the basis of the late-phase inflammatory response. Complications of severe or untreated allergic rhinitis include sinusitis, auditory tube dysfunction, hyposmia, sleep disturbances,

asthma exacerbations, and chronic mouth breathing.

TABLE 3–3 Clinical manifestations of allergic rhinitis.

A. Sneezing, Pruritus, and Mucus Hypersecretion

Patients with allergic rhinitis develop chronic or episodic paroxysmal sneezing; nasal, ocular, or palatal pruritus; and watery rhinorrhea triggered by exposure to a specific allergen. Patients may demonstrate signs of chronic pruritus of the upper airway, including a horizontal nasal crease from frequent nose rubbing (“allergic salute”) and palatal “clicking” from rubbing the itching palate with the tongue. Many tissue mast cells are located near terminal sensory nerve endings. Pruritus and sneezing are caused by histamine-mediated stimulation of these c- fibers. Mucus hypersecretion results primarily from excitation of parasympathetic–cholinergic pathways. Early-phase symptoms are best treated with avoidance of relevant allergens and oral or topical antihistamines, which competitively antagonize H1 receptor sites in target tissues. Anti-inflammatory

treatment can reduce cellular inflammation during the late phase, providing more effective symptom relief than antihistamines alone. Allergen immunotherapy (hyposensitization) has shown effectiveness in reducing symptoms and airway inflammation by inhibiting both early- and late-phase allergic responses. Diverse mechanisms of immunotherapy have been observed, including reducing seasonal increases in IL-4 and allergen-specific IgE, inducing allergen-specific IgG1 and IgG4 (blocking antibodies), modulating T-cell cytokine synthesis by enhancing TH1 and inhibiting TH2 responses, and upregulating Treg and downregulating eosinophilic and basophilic inflammatory responses to allergen. One trial found that immunotherapy administered to patients with grass-pollen allergy for 3–4 years induced prolonged clinical remission accompanied by a persistent alteration in immunologic reactivity that included sustained reductions in the late skin response and associated T-cell infiltration and IL-4 mRNA expression.

B. Nasal Stuffiness

Symptoms of nasal obstruction may become chronic as a result of persistent late- phase allergic mechanisms. Nasal mucous membranes may appear pale blue and boggy. Children frequently show signs of obligate mouth breathing, including long facies, narrow maxillae, flattened malar eminences, marked overbite, and high-arched palates (so-called adenoid facies). These symptoms are not mediated by histamine and are, therefore, poorly responsive to antihistamine therapy. Oral sympathomimetics that induce vasoconstriction by stimulating α-adrenergic receptors are often used in conjunction with antihistamines to treat nasal congestion. Topical decongestants may be used to relieve acute congestion but have limited value in patients with chronic allergic rhinitis because frequent use results in rebound vasodilation (rhinitis medicamentosa).

C. Airway Hyperresponsiveness

The phenomenon of heightened nasal sensitivity to reduced levels of allergen after initial exposures to the allergen is known as priming. Clinically, priming may be observed in patients who develop increased symptoms late in the pollen season compared with early in the season. Late-phase inflammation induces a state of nasal airway hyperresponsiveness to both irritants and allergens in patients with chronic allergic rhinitis and asthma. Airway hyperreactivity can cause heightened sensitivity to both environmental irritants such as tobacco smoke and noxious odors, as well as to allergens such as pollens. There are no standardized clinical tools to accurately assess late-phase hyperresponsiveness in

allergic rhinitis as there are for asthma (methacholine or histamine bronchoprovocation challenge). Genetic markers for bronchial airway hyperresponsiveness, however, have been identified. It also appears that late- phase cellular infiltration and eosinophil byproducts may inflict airway epithelial damage, which in turn can predispose to upper and lower airways hyperreactivity.

Accumulating evidence supports a relationship between allergic rhinitis and asthma. Many patients with rhinitis alone demonstrate nonspecific bronchial hyperresponsiveness, and prospective studies suggest that nasal allergy may be a predisposing risk factor for developing asthma. Treatment of patients with allergic rhinitis may result in improvement of asthma symptoms, airway caliber, and bronchial hyperresponsiveness to methacholine and exercise. Finally, mechanistic studies of airway physiology have demonstrated that nasal disease may influence pulmonary function via both direct and indirect mechanisms. Such mechanisms may include the existence of a nasal–bronchial reflex (with nasal stimulation causing bronchial constriction), postnasal drip of inflammatory cells and mediators from the nose into the lower airways, absorption of inflammatory cells and mediators into the systemic circulation and ultimately to the lung, and nasal blockage and subsequent mouth breathing, which may facilitate the entry of asthmagenic triggers to the lower airway.

D. In Vivo or In Vitro Measurement of Allergen-Specific IgE

This is the primary tool for the confirmation of suspected allergic disease. In vivo skin testing with allergens suspected of causing hypersensitivity constitutes an indirect bioassay for the presence of allergen-specific IgE on tissue mast cells or basophils. Percutaneous or intradermal administration of dilute concentrations of specific antigens elicits an immediate wheal-and-flare response in a sensitized individual. This response marks a “local anaphylaxis” resulting from the controlled release of mediators from activated mast cells. Positive skin test results to airborne allergens, combined with a history and examination suggestive of allergy, strongly implicate the allergen as a cause of the patient’s symptoms. Negative skin test results with an unconvincing allergy history argue strongly against an allergic origin. Major advantages to skin testing include simplicity, rapidity of performance, and low cost.

In vitro tests provide quantitative assays of allergen-specific IgE in the serum. In these assays, patient serum is reacted initially with antigen bound to a solid- phase material and then labeled with a radioactive or enzyme-linked anti-IgE antibody. These immunoallergosorbent tests show a 70–80% correlation with

skin testing to pollens, dust mites, and danders and are useful in patients receiving chronic antihistamine therapy who are unable to undergo skin testing and in patients with extensive dermatitis.

E. Complications of Allergic Rhinitis

Serous otitis media and sinusitis are major comorbidities in patients with allergic rhinitis. Both conditions occur secondarily to the obstructed nasal passages and sinus ostia in patients with chronic allergic or nonallergic rhinitis. Complications of chronic rhinitis should be considered in patients with protracted rhinitis unresponsive to therapy, refractory asthma, or persistent cough. Serous otitis results from auditory tube obstruction by mucosal edema and hypersecretion. Children with serous otitis media can present with conductive hearing loss, delayed speech, and recurrent otitis media associated with chronic nasal obstruction.

Sinusitis may be acute, subacute, or chronic depending on the duration of symptoms. Obstruction of osteomeatal drainage in patients with chronic rhinitis predisposes to bacterial infection in the sinus cavities. Patients manifest symptoms of persistent nasal discharge, cough, sinus discomfort, and nasal obstruction. Examination may reveal chronic otitis media, infraorbital edema, inflamed nasal mucosa, and purulent nasal discharge. Radiographic diagnosis by x-ray film or computed tomographic (CT) scan reveals sinus opacification, membrane thickening, or the presence of an air-fluid level. Effective treatment of infectious complications of chronic rhinitis requires antibiotics, systemic antihistamine and decongestants, and perhaps intranasal or systemic corticosteroids.

CHECKPOINT

18. What are the major clinical manifestations of allergic rhinitis? 19. What are the major etiologic factors in allergic rhinitis? 20. What are the pathogenetic mechanisms in allergic rhinitis?

PRIMARY IMMUNODEFICIENCY DISEASES

There are many potential sites where developmental aberrations in the immune system can lead to abnormalities in immunocompetence manifesting as an increased susceptibility to infection (Figure 3–4; Tables 3–4 and 3–5). When these defects are genetic in origin, they are referred to as primary immunodeficiency disorders. This is in contrast to compromised immunity secondary to pharmacologic therapy, HIV, malnutrition, or systemic illnesses such as systemic lupus erythematosus or diabetes mellitus.

FIGURE 3–4 Simplified schema of defects in cell surface receptor–dependent activation leading to different primary immunodeficiency disorders. Table 3–4 lists the syndromes and immunologic deficits seen with a variety of these humoral, cellular, neutrophil, or combined immunodeficiency disorders.

TABLE 3–4 Primary immunodeficiency disorders.

TABLE 3–5 Relationship of various pathogens to infection in primary immunodeficiency disorders.

Clinical investigations of various congenital defects have helped characterize many aspects of normal immune physiology. Defects in host immunity place the susceptible individual at high risk for a variety of infectious, malignant, and autoimmune diseases and disorders. The nature of the specific functional defect will significantly influence the susceptibility to infections caused by specific pathogens and their associated clinical features. Table 3–5 lists some of the typical organisms causing infection in patients with various immunodeficiency disorders. Any immunopathogenic mechanism that impairs T-lymphocyte function, or cell-mediated immunity, predisposes the host to the development of serious chronic and potentially life-threatening opportunistic infections with viruses, mycobacteria, fungi, and protozoa involving any or all organ systems. Similarly, immunopathogenic dysfunction of B lymphocytes resulting in antibody deficiency will predispose the host to pyogenic sinopulmonary and mucosal infections. Defects in innate immunity adversely influence phagocytic function, leukocyte adhesion and migration, NK cell function, and/or TLR signaling, leading to ineffective microbial killing. As the molecular bases of many primary immunodeficiency disorders are being discovered, it has become apparent that different molecular defects can result in common clinical phenotypes.

As T lymphocytes play a central role in inducing and coordinating immune responses, dysfunction can be associated with an increased incidence of autoimmune phenomena. These include diseases clinically similar to rheumatoid arthritis, systemic lupus erythematosus, and immune hematologic cytopenias. Patients with impaired immune responses are also at greater risk for certain malignancies than the general population. The occurrence of cancer may be related to an underlying impairment of tumor surveillance, dysregulation of cellular proliferation and differentiation, chromosomal translocations during defective antigen receptor gene rearrangement, or the presence of infectious agents predisposing to or causing cellular transformation. Non-Hodgkin lymphoma or B-cell lymphoproliferative disease, skin carcinomas, and gastric carcinomas are the most frequently occurring tumors in patients with immunodeficiency.

Traditionally, the primary immunodeficiencies are classified according to which component of the immune response is principally compromised: the humoral response, cell-mediated immunity, innate immunity, complement, or phagocytic cell function (see Table 3–4). Distinct developmental stages characterize the maturation and differentiation of the cellular components of the immune system. The underlying pathophysiologic abnormalities leading to

primary immunodeficiency are diverse and include the following: (1) early developmental defects in cellular maturation; (2) specific enzyme defects; (3) abnormalities in cellular proliferation and functional differentiation; (4) abnormalities in cellular regulation; and (5) abnormal responses to cytokines.

COMBINED IMMUNODEFICIENCY

Severe Combined Immunodeficiency Disease Clinical Presentation Many primary immunodeficiency disorders present early in the neonatal period. In patients with severe combined immunodeficiency disease (SCID), there is an absence of normal thymic tissue, and the lymph nodes, spleen, and other peripheral lymphoid tissues are devoid of lymphocytes. In these patients, the complete or near-complete failure of development of both the cellular and the humoral component of the immune system results in severe infections. The spectrum of infections is broad because these patients may also suffer from overwhelming infection by opportunistic pathogens, disseminated viruses, and intracellular organisms. Failure to thrive may be the initial presenting symptom, but mucocutaneous candidiasis, chronic diarrhea, and pneumonitis are common. Vaccination with live viral vaccines or bacillus Calmette–Guérin (BCG) may lead to disseminated disease. Without immune reconstitution by bone marrow transplantation, SCID is inevitably fatal within 1–2 years.

Pathology and Pathogenesis

SCID is a heterogeneous group of disorders characterized by a failure in the cellular maturation of lymphoid stem cells, resulting in reduced numbers and function of both B and T lymphocytes and hypogammaglobulinemia. The molecular basis for many types of SCID have been discovered (see Table 3–4). The genetic and cellular defects can occur at many different levels, starting with surface membrane receptors but also including deficiencies in signal transduction or metabolic biochemical pathways. Although the different molecular defects may cause clinically indistinguishable phenotypes, identification of specific mutations allows for improved genetic counseling, prenatal diagnosis, and carrier detection. Moreover, specific gene transfer offers hope as a future therapy.

1. Defective Cytokine Signaling— X-linked SCID (XSCID) is the most prevalent form of SCID, resulting from a genetic mutation in the common γ chain of the trimeric (αβγ) IL-2 receptor. This defective chain is shared by the receptors for IL-4, IL-7, IL-9, and IL-15, leading to dysfunction of all of these cytokine receptors. Defective signaling through the IL-7 receptor appears to block normal maturation of T lymphocytes. Circulating B-cell numbers may be preserved, but defective IL-2 responses inhibit proliferation of T, B, and NK cells, explaining the combined immune defects seen in XSCID patients. A defect in the α chain of the IL-7 receptor can also lead to an autosomal recessive form of SCID through mechanisms similar to XSCID but with intact NK cells.

2. Defective T-Cell Receptor and TCR signaling—The genetic defects for several other forms of autosomal recessive SCID have also been identified. A deficiency of ZAP-70, a protein tyrosine kinase important in signal transduction through the TCR, leads to a total absence of CD8 T lymphocytes. ZAP-70 plays an essential role in thymic selection during T-cell development. Consequently, these patients possess functionally defective CD4 T lymphocytes and no circulating CD8 T lymphocytes. B-lymphocyte and NK cell functions remain intact, but immunoglobulin production is impaired, in part owing to a lack of T- helper activity. Mutations of CD3δ, CD3γ, and CD3ε subunits may lead to partially arrested development of TCR expression and severe T-cell deficiency. Deficiencies of both p56lck and Janus kinase 3 (Jak3) can also lead to SCID through defective signal transduction. P56lck is a TCR-associated tyrosine kinase essential for T-cell differentiation, activation, and proliferation. Jak3 is a cytokine receptor–associated signaling molecule.

3. Defective Receptor Gene Recombination—Patients have been identified with defects in enzymes participating in VDJ recombination. Recombination- activating genes (RAG1 and RAG2) initiate recombination of antigen-binding receptor genes in both immunoglobulins and TCRs. Defects in RAG-1 and RAG-2 lead to a failure in VDJ receptor gene rearrangement and, subsequently, a severe quantitative and functional deficiency of T and B lymphocytes. NK cells are not antigen specific and for that reason are unaffected. Artemis and DNA ligase-4 proteins are involved in double-stranded DNA breakage and repair during VDJ recombination of T-cell receptors and BCRs. Artemis mutations may also lead to increased sensitivity to ionizing radiation. Because NK cells are invariant, their numbers are typically preserved, even as T- and B-cell numbers are severely deficient.

4. Defective Nucleotide Salvage Pathway—Approximately 20% of SCID cases are caused by a deficiency of adenosine deaminase (ADA), which is an enzyme in the purine salvage pathway, responsible for the metabolism of adenosine. Absence of the ADA enzyme results in an accumulation of toxic adenosine metabolites within the cells. These metabolites inhibit normal lymphocyte proliferation and lead to extreme cytopenia of both B and T lymphocytes. The combined immunologic deficiency and clinical presentation of this disorder, known as SCID-ADA, is identical to that of the other forms of SCID. Skeletal abnormalities and neurologic abnormalities may be associated with this disease. In similar fashion, purine nucleoside phosphorylase deficiency leads to an accumulation of toxic deoxyguanosine metabolites. T-cell development is impaired, possibly through induced apoptosis of double-positive thymocytes in the corticomedullary junction of the thymus. B-cell dysfunction is more variable.

CELL-MEDIATED IMMUNODEFICIENCY

DiGeorge Syndrome (22q11.2 Deletion Syndrome) Clinical Presentation and Pathogenesis The clinical manifestations of DiGeorge syndrome reflect the defective embryonic development of organs derived from the third and fourth pharyngeal arches, including the thymus, parathyroids, and cardiac outflow tract. Occasionally, the first and sixth pharyngeal pouches may also be involved. Cytogenetic abnormalities, most commonly chromosome 22q11 deletions, are associated with DiGeorge syndrome, especially in patients manifesting cardiac defects. DiGeorge syndrome is classified as complete or partial depending on the presence or absence of immunologic abnormalities. In this syndrome, the spectrum of immunologic deficiency is wide, ranging from immune competency to conditions in which there are life-threatening infections with organisms typically of low virulence. Patients affected by the complete syndrome have a profound T lymphocytopenia resulting from congenital thymic aplasia with impaired T-lymphocyte maturation, severely depressed cell-mediated immunity, and decreased suppressor T-lymphocyte activity. B lymphocytes and immunoglobulin production are unaffected in most patients, although in rare instances patients may present with mild hypogammaglobulinemia and absent or poor antibody responses to neoantigens. In this subset of patients, inadequate helper-T function as a result of dysfunctional T- and B-cell interaction and

inadequate cytokine production leads to impaired humoral immunity. DiGeorge syndrome is truly a developmental anomaly and can be associated

with structural abnormalities in the cardiovascular system such as truncus arteriosus or right-sided aortic arch. Parathyroid abnormalities may lead to hypocalcemia, presenting with neonatal tetany or seizures. In addition, it is common for patients to exhibit facial abnormalities such as micrognathia, hypertelorism, low-set ears with notched pinnae, and a short philtrum.

HUMORAL IMMUNODEFICIENCY

X-Linked Agammaglobulinemia Clinical Presentation Formerly called Bruton agammaglobulinemia, X-linked agammaglobulinemia (XLA) is thought to be pathophysiologically and clinically more homogeneous than SCID. It is principally a disease of childhood, presenting clinically within the first year of life, after the disappearance of passively transferred maternal IgG, with multiple and recurrent sinopulmonary infections caused primarily by pyogenic bacteria and, to a much lesser extent, viruses. Because encapsulated bacteria require antibody binding for efficient opsonization, these humoral immune–deficient patients suffer from sinusitis, pneumonia, pharyngitis, bronchitis, and otitis media secondary to infection with Streptococcus pneumoniae, other streptococci, and Haemophilus influenzae. Although infections from fungal and opportunistic pathogens are rare, patients display a unique susceptibility to a rare but deadly enteroviral meningoencephalitis.

Pathology and Pathogenesis

Patients with XLA have pan-hypogammaglobulinemia, with decreased levels of IgG, IgM, and IgA. They exhibit poor to absent responses to antigen challenge, even though virtually all demonstrate normal functional T-lymphocyte responses to in vitro and in vivo tests (eg, delayed hypersensitivity skin reactions). The basic defect in this disorder appears to be arrested cellular maturation at the pre- B-lymphocyte stage. Indeed, normal numbers of pre-B lymphocytes can be found in the bone marrow, although in the circulation, B lymphocytes are virtually absent. Lymphoid tissues lack fully differentiated B lymphocytes (antibody-secreting plasma cells), and lymph nodes lack developed germinal centers. The gene defective in XLA has been isolated. The defective gene

product, Bruton tyrosine kinase (BTK), is a B-cell–specific signaling protein belonging to the cytoplasmic tyrosine kinase family of intracellular proteins. Gene deletions and point mutations in the catalytic domain of the BTK gene block normal BTK function, necessary for B-cell maturation.

Common Variable Immunodeficiency Disease Clinical Presentation Common variable immunodeficiency disease is often referred to as acquired or adult-onset hypogammaglobulinemia. It is the most common serious primary immune deficiency disorder in adults. In North America, for example, it affects an estimated 1:100,000 to 1:10,000 individuals. The clinical spectrum is broad, and patients usually present within the first 2 decades of life. Affected individuals commonly develop recurrent sinopulmonary infections, including sinusitis, otitis, bronchitis, and pneumonia. Common pathogens are encapsulated bacteria such as S pneumoniae, H influenzae, and Moraxella catarrhalis. Bronchiectasis can be the result of recurrent serious respiratory infections, leading to infection with more virulent pathogens, including Staphylococcus aureus and Pseudomonas aeruginosa, which in turn can change the long-term prognosis. A number of important noninfectious disorders are commonly associated with common variable immunodeficiency, including GI malabsorption, autoimmune disorders, and neoplasms. The most frequently occurring malignancies are lymphoreticular, but gastric carcinoma and skin cancer also occur. Autoimmune disorders occur in 20–30% of patients and may precede the recurrent infections. Autoimmune cytopenias occur most frequently, but rheumatic diseases can also be seen. Serologic testing for infectious or autoimmune disease is unreliable in hypogammaglobulinemia. As in XLA, infusions of intravenous or subcutaneous immunoglobulin can reconstitute humoral immunity, decrease infections, and improve quality of life.

Pathology and Pathogenesis

Common variable immunodeficiency is heterogeneous disorder in which the primary immunologic abnormality is a marked reduction in antibody production. The vast majority of patients demonstrate an in vitro defect in terminal differentiation of B lymphocytes. Peripheral blood lymphocyte phenotyping demonstrates normal or reduced numbers of circulating B lymphocytes, but antibody-secreting plasma cells are conspicuously sparse in lymphoid tissues. In sharp contrast to XLA, no single gene defect can be held accountable for the

multitude of defects known to cause common variable immunodeficiency. In many patients, the defect is intrinsic to the B-lymphocyte population. Approximately 15% of patients with common variable immunodeficiency disease demonstrate defective B-cell surface expression of transmembrane activator and calcium-modulator and cyclophilin ligand interactor (TACI), a member of the TNF receptor family. Lacking a functional TACI, the affected B cells will not respond to B-cell–activating factors, resulting in deficient immunoglobulin production. Another defect that may lead to common variable immunodeficiency disease involves deficient expression of B-cell surface marker CD19. When complexed with CD21 and CD81, CD19 facilitates cellular activation through BCRs. B-cell development is not affected, but humoral function is deficient. A variety of T-cell abnormalities may also lead to immune defects with subsequent impairment of B-cell differentiation. A mutation of inducible T-cell co-stimulator gene (ICOS), expressed by activated T cells and responsible for B-cell activation/antibody production, may be the molecular defect in some cases of common variable immunodeficiency disease. More than 50% of patients also have some degree of T-lymphocyte dysfunction as determined by absent or diminished cutaneous responses to recall antigens. Immune dysregulation may contribute to the morbidity and the myriad autoimmune manifestations associated with common variable immunodeficiency.

Hyper-IgM Immunodeficiency (CD40 and CD40 Ligand Deficiency) Clinical Presentation In patients with hyper-IgM immunodeficiency, serum levels of IgG and IgA are very low or absent, but serum IgM (and sometimes IgD) levels are normal or elevated. Inheritance of this disorder may be autosomal, although it is most often X-linked. Clinically, this syndrome is manifested by recurrent pyogenic respiratory infections, predisposition to Pneumocystis jiroveci pneumonia and an array of autoimmune phenomena such as Coombs-positive hemolytic anemia and immune thrombocytopenia.

Pathology and Pathogenesis

The principal abnormality is the defective expression of CD40-ligand (CD40L), a T-lymphocyte activation surface marker (also known as CD154 or gp39). In the course of normal immune responses, CD40L interacts with CD40 on B-cell

surfaces during cellular activation, initiating proliferation and immunoglobulin isotype switching. In hyper-IgM syndrome, defective CD40 co-receptor stimulation during T- and B-cell interactions leads to impairment of B-cell isotype switching and subsequent production of IgM, but very low or no production of IgG, IgA, and IgE. CD40L–CD40 interaction also promotes dendritic cell maturation and IL-12 and IFN-γ secretion, so CD40L deficiency can be associated with impaired cell-mediated immunity and increased risk of opportunistic infection.

Selective IgA Deficiency This is the most common primary immunodeficiency in adults, with a prevalence of 1:700 to 1:500 individuals. Most affected individuals have few or no clinical manifestations, but there is an increased incidence of upper respiratory tract infections, allergy, asthma, and autoimmune disorders. Whereas serum levels of the other immunoglobulin isotypes are typically normal, serum IgA levels in these individuals are markedly depressed, often less than 5 mg/dL.

As in common variable immunodeficiency, the primary functional defect is an inability of B cells to terminally differentiate to IgA-secreting plasma cells. An associated deficiency of IgG subclasses (mainly IgG2 and IgG4) and low- molecular-weight monomeric IgM is not uncommon and can be clinically significant. Because of the role of secretory IgA in mucosal immunity, patients with this immunodeficiency frequently develop significant infections involving the mucous membranes of the gut, conjunctiva, and respiratory tract. There is no specific therapy, but prompt antibiotic treatment is necessary in patients with recurrent infections. A subset of patients may recognize IgA as a foreign antigen. These patients are at risk for transfusion reactions to unwashed red blood cells or other blood products containing trace amounts of IgA.

DISORDERS OF PHAGOCYTIC CELLS & INNATE IMMUNITY

Defective phagocytic cell function presents with infections at sites of interface between the body and the outside world. Recurrent skin infections, abscesses, gingivitis, lymphadenitis, and poor wound healing are seen in patients with macrophage or neutrophil disorders. More difficult to assay, clinical immunodeficiency can occur through defects in phagocytic cell migration,

adhesion, opsonization, or killing.

Chronic Granulomatous Disease Clinical Presentation Chronic granulomatous disease is typically X-linked and is characterized by impaired granulocyte function. This disorder of phagocytic cell function presents with recurrent skin infections, abscesses, and granulomas at sites of chronic inflammation. Abscesses can involve skin or viscera and may be accompanied by lymphadenitis. Catalase-positive organisms predominate; S aureus is thus the most common pathogen, although infections with Nocardia species, gram- negative Serratia marcescens, and Burkholderia cepacia can also occur. Aspergillus species and Candida represent common fungal pathogens in chronic granulomatous disease. Sterile noncaseating granulomas resulting from chronic inflammatory stimuli can lead to GI or genitourinary tract obstruction. Chronic granulomatous disease typically presents in childhood, although cases presenting in adulthood are occasionally reported.

Pathology and Pathogenesis

Defects in the gene coding for nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibit oxidative metabolism and severely compromise neutrophil-killing activity. NADPH oxidase is assembled from two membrane and two cytosolic components after phagocytic cell activation, leading to catalytic conversion of molecular oxygen into superoxide. Oxidative burst and intracellular killing rely on production of superoxide, which is later converted to hydrogen peroxide and sodium hypochlorite (bleach). In patients with chronic granulomatous disease, other neutrophil functions such as chemotaxis, phagocytosis, and degranulation remain intact, but microbial killing is deficient. Catalase-negative bacteria are effectively killed because microbes produce small amounts of peroxide, concentrated in phagosomes, leading to microbial death. Catalase-positive organisms scavenge these relatively small amounts of peroxide and are not killed without neutrophil oxidative metabolism. X-linked inheritance is most frequently seen, but autosomal recessive forms and spontaneous mutations can also lead to clinical disease.

Leukocyte Adhesion Deficiency, Type 1 Integrins and selectins are specialized molecules that play a role in leukocyte

homing to sites of inflammation. These adhesion molecules facilitate cell–cell and cell–extracellular matrix interactions, allowing circulating leukocytes to stick and roll along endothelial cell surfaces prior to diapedesis into extravascular tissues. An autosomal recessive train, leukocyte adhesion deficiency type 1, and defective expression of β2-integrin (CD11/CD18) adhesion molecules result in impaired leukocyte trafficking, leading to recurrent infections, lack of pus formation, and poor wound healing. Leukocytosis occurs because cells cannot exit the circulation, and recurrent infections of skin, airways, bowels, perirectal area, and gingival and periodontal areas are common.

Hyper-IgE Syndrome Clinical Presentation Hyper-IgE syndrome (HIES) is often referred to as “Job syndrome” because affected individuals suffer from recurrent boils as the tormented biblical figure did. The initial description of this immunodeficiency disorder was in two fair- skinned girls with recurrent staphylococcal “cold” skin abscesses associated with furunculosis, cellulitis, recurrent otitis, sinusitis, pneumatoceles, and a coarse facial appearance. The predominant organism isolated from sites of infection is S aureus, although other organisms such as H influenzae, pneumococci, gram- negative organisms, Aspergillus sp., and C albicans are also often identified. Characteristically, patients have a chronic pruritic eczematoid dermatitis, defective shedding of primary teeth, growth retardation, coarse facies, scoliosis, osteopenia, vascular abnormalities, and hyperkeratotic fingernails. Extremely high IgE levels (>3000 IU/mL) have also been observed in patients’ serum.

Pathology and Pathogenesis

The high IgE levels are thought to be a consequence of dysregulated immunologic responsiveness to cytokines, yet it is unclear whether the hyper- IgE contributes to the observed susceptibility to infection or is simply an immunologic epiphenomenon. Autosomal dominant forms have been associated with mutations in STAT3, a transcriptional factor involved in the activation of cytokine and growth factor receptors. Responses to numerous cytokines do appear impaired, along with decreased TH17 function. Autosomal recessive forms have been associated with mutations in the dedicator of cytokinesis-8 (DOCK8) gene, which adds an increased susceptibility to cutaneous viruses and lymphopenia to the clinical presentation. A spectrum of immune abnormalities may be observed in HIES. Poor antibody responses to neoantigens, deficiency of

IgA antibody against S aureus, and low levels of antibodies to carbohydrate antigens suggest humoral immunodeficiency. T-lymphocyte functional abnormalities are suggested by decreased absolute numbers of suppressor T lymphocytes, poor in vitro proliferative responses, and defects in cytokine production. Several reports have also documented highly variable abnormalities in neutrophil chemotaxis.

Toll-Like Receptor 3 Deficiency Patients with toll-like receptor 3 (TLR3) deficiency have shown specific susceptibility to herpes simplex 1 (HSV1) encephalitis. Typically, binding of pathogen-associated molecular patterns to TLR will activate transcription factors, such as nuclear factor kappa beta (NF-κβ), IFN regulatory factors, and activator protein 1, leading to immune responsiveness. Defects in this pathway impair viral immunity. In TLR3 deficiency, defective IFN-α, IFN-β, and IFN-λ synthesis leads to uninhibited HSV1 replication in neurons and oligodendritic cells. A similar phenotype is seen in autosomal recessive UNC-93b deficiency. UNC-93b is required for TLR3 function, as it translocates TLR3 to its endosomal site of action.

CHECKPOINT

21. What are the major clinical manifestations of each of the five categories of primary immune deficiency?

22. What are the major pathogenetic mechanisms in each category of primary immune deficiency?

AIDS

Human immunodeficiency virus (HIV) infection is one of the greatest epidemics in human history. HIV is the agent responsible for acquired immunodeficiency syndrome (AIDS), the most common immunodeficiency disorder worldwide. AIDS, as the consequence of a chronic retroviral infection with HIV, produces severe, life-threatening CD4 helper T-lymphocyte dysfunction, opportunistic infections, and malignancies. AIDS is defined by serologic evidence of HIV infection in the presence of a variety of indicator diseases associated with

clinical immunodeficiency. Tables 3–6 and 3-7 list criteria for defining and diagnosing AIDS. HIV is transmitted by exposure to infected body fluids or to sexual or even perinatal contact. Vertical transmission from mother to infant may occur in utero, during childbirth, and through breastfeeding. Transmissibility of the HIV virus is related to subtype virulence, viral load, and immunologic host factors.

TABLE 3–6 Surveillance case definition for human immunodeficiency virus (HIV) infection among adults and adolescents (age ≥13 years), United States, 2008

TABLE 3–7 AIDS-defining conditions.

Acute HIV infection may present as a self-limited, febrile viral syndrome characterized by fatigue, pharyngitis, myalgias, maculopapular rash, lymphadenopathy, and significant viremia, without detectable anti-HIV antibodies. Acute HIV infection may be detected with modern fourth- or fifth- generation immunoassays that utilize combination HIV p24 antigen and HIV antibody assays. These fourth- or fifth-generation immunoassays may detect acute HIV infection an average of 14 days following infection. Less commonly, primary HIV infection may also be associated with orogenital or esophageal ulcers, meningoencephalitis, or opportunistic infection. After an initial viremic phase, patients seroconvert and a period of clinical latency is usually seen. Lymph tissues become centers for massive viral replication during the latent, asymptomatic, stage of HIV infection. HIV viremia may be detectable by PCR assay; however, there may or may not be clinical symptoms of HIV infection until CD4 reserves become profoundly diminished. Over time, there is a progressive decline in CD4 T lymphocytes, a reversal of the normal CD4:CD8 T-lymphocyte ratio, and numerous other immunologic derangements. The clinical manifestations are directly related to HIV tissue tropism and defective immune function. Marked immune deficiency is signaled by development of neurologic complications, opportunistic infections, or malignancies. The time course for progression is highly variable, but the median time before appearance of clinical disease is about 10 years in untreated individuals. Approximately 10% of those infected manifest rapid progression to AIDS (within 5 years after infection). A minority of individuals are termed “elite controllers” (HIV quantitative serum viral load <50/mL) or “viremic controllers” (detectable but low-level viral loads). In these individuals, genetic factors, host cytotoxic immune responses, viral load, and virulence appear to have an impact on susceptibility to infection and the rate of disease progression. Although not curative, modern antiretroviral therapies can suppress viral replication and restore immune function, leading to clinical recovery and markedly extended life expectancy.

Pathology and Pathogenesis

HIV is a single-stranded, positive-sense RNA lentivirus, whose RNA encodes for nine genes (Table 3–8). Chemokines (chemoattractant cytokines that regulate leukocyte trafficking to sites of inflammation) play a significant role in the pathogenesis of HIV disease. During the initial stages of infection and viral proliferation, virion entry and cellular infection require binding to two co- receptors on target T lymphocytes and monocyte/macrophages. All HIV strains

express the envelope protein gp120, which binds to CD4 surface receptor molecules, but different viral strains display tissue “tropism” or specificity on the basis of the co-receptor they recognize. Changes in the viral phenotype during the course of an individual’s HIV infection may lead to changes in tropism and cytopathology at different stages of their disease. Viral strains isolated in the early stages of infection and associated with mucosal and intravenous transmission (eg, R5-trophic viruses) bind macrophages expressing chemokine receptor CCR5. In the later stages of disease, CXCR-4-tropic strains of HIV are more commonly seen. X4-trophic viruses bind to chemokine receptor CXCR4, more broadly expressed on T cells, and are associated with syncytium formation. Since chemokine receptors play a role in viral cell entry, specific inherited polymorphisms of chemokine receptors influence susceptibility to infection and disease progression.

TABLE 3–8 HIV genes and gene products.

Following virion entry into the CD4 T lymphocyte, the reverse transcription step of HIV replication is error prone owing to a lack of proofreading mechanisms during and after viral reverse transcription. Mutations occur frequently, and even within an individual patient, HIV heterogeneity develops rapidly. Patients may be infected with more than one quasispecies, and through mechanisms of recombination, genes from separate strains may intermingle, contributing to genetic diversity. The development of antigenically and phenotypically distinct strains contributes to progression of disease, clinical drug resistance, and lack of efficacy of early vaccines.

Cellular activation is critical for viral infectivity and reactivation of integrated proviral DNA. After viral entry and capsid disassembly, HIV reverse transcriptase converts uncoated viral RNA into double-stranded viral DNA. Using several host proteins, including HIV integrase enzyme, the double- stranded viral DNA complex is transported into the host cell nucleus and integrates into the host chromosome. Integrated HIV DNA is referred to as “provirus” and may remain latent or become transcriptionally active, depending on the activation state of the host cell. Cellular activation triggers NF-κβ, a cytoplasmic transcription factor that migrates to the nucleus, initiating viral gene expression. HIV protein Nef enhances viral replication and reduces host antiviral immune responses. New infectious virions are assembled. Viral proteins and RNA are packaged at the infected cell’s exterior membrane through a process called budding.

Although only 2% of mononuclear cells are found peripherally, lymph nodes from HIV-infected individuals can contain large amounts of virus sequestered among infected follicular dendritic cells in the germinal centers. For patients infected through vaginal or rectal mucosa, gut-associated lymphoid tissue is a major site of viral replication and persistence. Gut-associated lymphoid tissue harbors the majority of the host’s T cells, and when HIV-infected epidermal Langerhans cells migrate to draining lymph nodes, large numbers of lymphocytes encounter surface-bound virus. The persistence of virus in these secondary lymphoid structures triggers cellular activation and massive, irrevocable depletion of CD4 T-lymphocyte reservoirs, as well as disease latency. The marked decline in CD4 T-lymphocyte counts is due to several mechanisms: (1) direct HIV-mediated infection and destruction of CD4 T lymphocytes during viral replication; (2) depletion by fusion and formation of multinucleated giant cells (syncytium formation); (3) toxicity of viral proteins to CD4 T lymphocytes and hematopoietic precursors; (4) loss of T-lymphocyte co- stimulatory factors such as CD28; and (5) induction of apoptosis of uninfected T

cells. CD8 CTL activity is initially brisk and effective at controlling viremia but later induces the generation of viral escape mutations. Ultimately, viral proliferation outstrips host responses, and HIV-induced immunosuppression leads to disease progression. Neutralizing antibodies are generated very late, but mutations in HIV-envelope proteins overcome protective humoral responses. Over time, the infection is characterized by systemic, generalized cytokine dysregulation and immune activation. Hyperactivity of the immune system increases naive T-cell infection. Ultimately, these events prove deleterious to maintenance of lymphatic organs, bone marrow integrity, and effective immune responses.

In addition to the cell-mediated immune defects, B lymphocyte function is altered such that many HIV-infected individuals have marked hypergammaglobulinemia but impaired specific antibody responses. Both anamnestic responses and those to neoantigens can be impaired.

The development of plasma HIV RNA quantification as an assay to measure viral burden has led to a better understanding of HIV dynamics and has provided a tool for assessing response to therapy. Despite viral suppression in peripheral blood, replication and activation of infected T-lymphocytes may continue within lymph nodes and the central nervous system (CNS), in a phenomenon referred to as “CNS escape.” The risk of progression to AIDS appears correlated with an individual’s viral load after seroconversion. Data from several large clinical cohorts have shown a direct correlation between the CD4 T-lymphocyte count and the risk of AIDS-defining opportunistic infections and malignancy. The viral load and the degree of CD4 T-lymphocyte depletion serve as important clinical indicators of immune status in HIV-infected individuals. The CD4 T-lymphocyte count is perhaps better for disease staging, and viral load better as a proxy measure for disease progression or monitoring response to therapy. Prophylaxis for opportunistic infections such as pneumocystis pneumonia (PCP) is recommended when the CD4 T-lymphocyte counts reach <200 cells/μL. Patients with HIV infection with a CD4 T lymphocyte count of <50 cells/μL are at significantly increased risk for cytomegalovirus (CMV) retinitis and M avium complex (MAC) infection. Unfortunately, some complications of HIV infection, including tuberculosis infection, non-Hodgkin lymphoma, and cardiovascular, hepatic, and neurocognitive diseases, may occur even with robust CD4 counts.

Monocytes, macrophages, and dendritic cells also express HIV receptors (CD4) and can be infected with HIV. This facilitates virus transfer to lymphoid tissues and immunoprivileged sites, such as the CNS. HIV-infected monocytes will also release large quantities of the acute-phase reactant cytokines, including

IL-1, IL-6, and TNF, contributing to constitutional symptomatology. TNF, in particular, has been implicated in the severe wasting syndrome seen in patients with advanced disease. Concomitant infections may serve as cofactors for HIV infection, facilitate mucosal entry, and increase HIV expression through enhanced cytokine production, co-receptor surface expression, or increased cellular activation mechanisms. Epidemiologic studies of HSV-2–infected patients demonstrate a 2- to 7-fold increased risk of acquiring HIV compared with similar cohorts.

Clinical Manifestations

The clinical manifestations of AIDS are the direct consequence of the progressive and severe immunologic deficiency induced by HIV. Patients are susceptible to a wide range of atypical or opportunistic infections with bacterial, viral, protozoal, and fungal pathogens. Common nonspecific symptoms include fever, night sweats, and weight loss. Weight loss and cachexia can be due to nausea, vomiting, anorexia, or diarrhea. They often portend a poor prognosis.

The risk of opportunistic infections increases as the CD4 T-lymphocyte count declines. Fungal pathogens may affect immunocompetent hosts but frequently cause opportunistic infections in immunosuppressed HIV-infected patients. Infections with Cryptococcus neoformans meningoencephalitis, disseminated Histoplasma capsulatum, and disseminated Coccidioides immitis are typically seen in late stages of disease, when CD4 counts are below 200 cells/μL. Symptoms of C. neoformans meningoencephalitis include fever, malaise, headache, photophobia, and nausea. Presentation with altered mental status or elevated intracranial pressure is associated with a higher risk of death (or lifelong neurologic sequelae following survival). Occasionally, a cerebral cryptococcoma presents as a CNS mass lesion.

Found in regional soil contaminated with bird and bat droppings, H capsulatum infection is characterized by prominent constitutional symptoms, frequent pulmonary symptoms, and subacute meningoencephalitis symptoms. Disseminated disease may represent reactivation of latent disease when cellular immunity fails.

Previously thought to be a protozoan but now classified as a fungus, Pneumocystis jirovecii is one of the most common opportunistic infections in AIDS patients. Patients present clinically with fever, cough, shortness of breath, and hypoxemia, ranging in severity from mild to life-threatening. PCP may represent new acquisition of infection or reactivation of prior infection. A diagnosis of PCP can be made by substantiating the clinical and radiographic

findings with direct fluorescent antibodies against PCP in expectorated sputum samples. Wright–Giemsa or silver methenamine staining of induced sputum samples may also be required for diagnosis. Further diagnostic testing such as bronchoalveolar lavage or fiberoptic transbronchial biopsy may be required to establish the diagnosis in the context of equivocal initial testing but strong suspicion of disease. Complications of PCP include pneumothoraces, progressive parenchymal disease with severe respiratory insufficiency, and, most commonly, adverse reactions to the medications used for treatment and prophylaxis.

As a consequence of chronic immune dysfunction, HIV-infected individuals are also at high risk for other pulmonary infections, including bacterial pneumonias with S pneumoniae, H influenzae, and P aeruginosa; mycobacterial infections with Mycobacterium tuberculosis or Mycobacterium avium-intracellulare complex (MAC); and fungal infections with C neoformans, H capsulatum, Aspergillus sp., or C immitis. Clinical suspicion followed by early diagnosis of these infections should lead to aggressive treatment.

The risk of M tuberculosis reactivation is estimated to be 5–10% per year in HIV-infected patients compared with a lifetime risk of 10% in those without HIV. Furthermore, because of anergic skin responses, diagnosis of latent tuberculosis infection in the interest of prevention of active disease may be delayed. The development of active tuberculosis is significantly accelerated in HIV infection as a result of compromised cellular immunity. Respiratory symptoms of cough, dyspnea, or pleuritic chest pain may be associated with the insidious onset of fever, malaise, weight loss, and anorexia. Extrapulmonary manifestations occur in up to 70% of HIV-infected patients with tuberculosis, with miliary tuberculosis and tuberculous meningitis representing more serious complications. The emergence of multidrug resistance may compound the problem. M avium-intracellulare is a less virulent pathogen than M tuberculosis, and disseminated infections usually occur only with severe clinical immunodeficiency. M avium-intracellulare survives intracellularly within macrophages owing to defective cytokine (IFN-γ, IL-2, IL-12, TNF) synthesis, leading to impaired killing of phagocytosed organisms. Symptoms of MAC are nonspecific and typically consist of fever, weight loss, anemia, and GI distress with diarrhea.

The presence on physical examination of oral candidiasis (thrush) and hairy leukoplakia is highly correlated with HIV infection and portends rapid progression to AIDS. Oral candidiasis develops when reduced local and

systemic immune function, sometimes combined with metabolic imbalances, contributes to opportunistic outgrowth of Candida, which is commonly a commensal organism. HIV-infected individuals with oral candidiasis are at much greater risk for esophageal candidiasis, which may present as substernal pain and dysphagia. This infection and its characteristic clinical presentation are so common that most practitioners treat these symptoms with empiric oral antifungal therapy. Should the patient not respond rapidly, other explanations for the esophageal symptoms should be explored, including herpes simplex and CMV esophagitis. Epstein–Barr virus (EBV) is the cause of hairy leukoplakia, another oral complication of HIV, manifested by white thickening of mucosal folds, most prominent on the buccal mucosa, the soft palate, and the floor of the mouth.

Diarrhea has been a hallmark feature of AIDS and leads to significant wasting, morbidity, and mortality. Persistent diarrhea, especially when accompanied by high fevers and abdominal pain, may signal infectious enterocolitis. The degree of CD4 lymphopenia is significantly correlated with the risk of opportunistic GI tract infections. The list of potential pathogens in such cases is long and includes bacteria, MAC, protozoans (Cryptosporidium, Microsporidia, Isospora belli, Entamoeba histolytica, Giardia lamblia), and even HIV itself. Because of their reduced gastric acid concentrations, patients also have an increased susceptibility to nonopportunistic infectious gastroenteritis with Campylobacter, Salmonella, and Shigella. Co-infection with viral hepatitis (HBV, HCV, CMV) can lead to accelerated cirrhosis and end-stage liver disease, but fortunately, instituting antiretroviral therapy can lead to a reduction in clinical disease.

Skin lesions commonly associated with HIV infection are typically classified as infectious (viral, bacterial, fungal), neoplastic, or nonspecific. Herpes simplex virus (HSV) and herpes zoster virus (HZV) may cause chronic persistent or progressive lesions in patients with compromised cellular immunity. HSV commonly causes oral and perianal lesions but can be an AIDS-defining illness when involving the lung or esophagus. The risk of disseminated HSV or HZV infection and the presence of molluscum contagiosum appear to be correlated with the extent of immunosuppression. Seborrheic dermatitis caused by Pityrosporum ovale and fungal skin infections (C albicans, dermatophyte species) are also common in HIV-infected patients. Staphylococcus aureus infections, including methicillin-resistant S aureus, can cause folliculitis, furunculosis, and bullous impetigo in HIV-infected patients and may require aggressive treatment to prevent dissemination and sepsis. Bacillary

angiomatosis is a potentially fatal dermatologic disorder of tumor-like proliferating vascular endothelial cell lesions, the result of infection by Bartonella quintana or Bartonella henselae. The lesions may resemble those of Kaposi sarcoma but respond to treatment with tetracyclines.

CNS manifestations in HIV-infected patients include CNS infections and malignancies. Toxoplasmosis frequently presents with space-occupying lesions, causing headache, altered mental status, seizures, or focal neurologic deficits. Cryptococcal meningitis commonly manifests as headache and fever. Up to 90% of patients with cryptococcal meningitis exhibit a positive serum test for C neoformans antigen.

Patients with HIV-associated neurocognitive disorder typically have difficulty with cognitive tasks, poor short-term memory, slowed motor function, personality or affective changes, and waxing-and-waning dementia. In its severe form, AIDS dementia may be characterized by severe psychomotor retardation, akinesis, and language impairment, associated with widespread cortical atrophy and ventricular enlargement on imaging by CT or MRI. Up to 50% of patients with AIDS suffer from this disorder, perhaps caused by glial or macrophage infection by HIV resulting in destructive inflammatory changes within the CNS. R5 viruses are trophic for cells of monocytic lineage in the CNS. The differential diagnosis of CNS changes is broad and includes metabolic disturbances and toxic encephalopathy resulting from drugs. Other causes of altered mental status include neurosyphilis, CMV or herpes simplex encephalitis, mycobacterial or cryptococcal meningitis, lymphoma, and progressive multifocal leukoencephalopathy, a progressive demyelinating disease caused by a JC papovavirus.

Peripheral nervous system manifestations of HIV infection include sensory, motor, and inflammatory polyneuropathies. Almost 33% of patients with advanced HIV disease develop peripheral numbness, tingling, and pain in their extremities. These symptoms are likely a result of loss of nerve axons from direct neuronal infection by HIV. Alcoholism, thyroid disease, syphilis, vitamin B12 deficiency, drug toxicity (eg, stavudine or didanosine), CMV-associated ascending polyradiculopathy, and transverse myelitis also cause peripheral neuropathies. Less commonly, HIV-infected patients can develop an inflammatory demyelinating polyneuropathy similar to Guillain–Barré syndrome; however, unlike the sensory neuropathies, this inflammatory demyelinating polyneuropathy typically presents before the onset of clinically apparent immunodeficiency. The origin of this condition is not known, although an autoimmune reaction is suspected. CMV retinitis is the most common cause

of rapidly progressive visual loss in HIV-infected patients. The diagnosis can be difficult to make because Toxoplasma gondii infection, microinfarction, and retinal necrosis can all cause visual loss.

HIV-related malignancies commonly seen in AIDS include Kaposi sarcoma, non-Hodgkin lymphoma, primary CNS lymphoma, invasive cervical carcinoma, and anal squamous cell carcinoma. Impairment of immune surveillance and defense and increased exposure to oncogenic viruses appear to contribute to the development of these neoplasms.

Kaposi sarcoma is the most common HIV-associated cancer. In San Francisco, 15–20% of HIV-infected men who have sex with men develop this tumor during the progression of their disease. For reasons that are unclear, Kaposi sarcoma is uncommon in women and children. Unlike classic Kaposi sarcoma, which affects elderly men in Africa and the Mediterranean region, the disease in HIV-infected patients may present with either localized cutaneous lesions or lymphatic or disseminated visceral involvement. It is often a progressive disease, and pulmonary involvement can be fatal. Histologically, the lesions of Kaposi sarcoma consist of a mixed cell population that includes vascular endothelial cells and spindle cells within a collagen network. Kaposi sarcoma is caused by human herpesvirus 8 (HHV-8), which in immunocompromised patients appears to promote angiogenesis through growth factor and proinflammatory gene product production. HIV itself appears to induce cytokines and growth factors that stimulate tumor cell proliferation rather than causing malignant cellular transformation. Clinically, cutaneous Kaposi sarcoma typically presents as a purplish nodular skin lesion or painless oral lesion. Sites of visceral involvement include the lung, lymph nodes, liver, and GI tract. In the GI tract, Kaposi sarcoma can result in either acute hemorrhage or chronic blood loss. In the lung, it often presents as bilateral coarse nodular infiltrates, frequently associated with pleural effusions.

Non-Hodgkin lymphoma is particularly aggressive in HIV-infected patients and is usually indicative of significant immune compromise. The majority of these tumors are high-grade B-cell lymphomas with a predilection for dissemination. Chronic B-cell stimulation, immune dysfunction, and loss of immunoregulation of EBV-infected cells are all risk factors for transformation of clonally selected cells and development of non-Hodgkin lymphoma. Large-cell and Burkitt-type lymphoma are often associated with EBV but only account for about half of cases. Many cases are diagnosed at advanced stages of disease, and the CNS is frequently involved either as a primary site or as an extranodal site of widespread disease.

Anal dysplasia and anal squamous cell carcinoma are more commonly found in HIV-infected men who have sex with men. These conditions are associated with concomitant anal or rectal infection with oncogenic types of human papillomavirus (HPV). In HIV-infected women, the incidence of HPV- related cervical dysplasia is as high as 40%, and dysplasia, and without proper screening for early detection and treatment, can progress to invasive cervical carcinoma.

In HIV-infected patients, adherence to multidrug treatment regimens remains a challenge, but clearly such antiretroviral therapy (ART) improves immune function. For reasons that are unclear, HIV-infected patients have an unusually high rate of adverse reactions to a wide variety of antibiotics and frequently develop severe, debilitating cutaneous reactions. Drug hypersensitivity and toxicity can be severe, potentially life-threatening, and life-limiting with certain agents.

Immune reconstitution syndrome is a reaction that occurs days to weeks after initiation of ART. Clinical relapse or worsening of mycobacterial, pneumocystis, hepatitis, or neurological infections occurs as a result of a resurgence of immune activity, causing paradoxical worsening of inflammation, possibly as residual antigens or subclinical pathogens are attacked.

Other complications of HIV infection include arthritides, myopathy, GI syndromes, dysfunction of the adrenal and thyroid glands, hematologic cytopenias, and nephropathy. As patients are living longer due to potent ART, cardiovascular complications have become more prominent. ART has been associated with dyslipidemia, insulin resistance, and other metabolic abnormalities. HIV infection itself may be atherogenic, both through effects on lipids and through proinflammatory mechanisms.

Since the disease was first described in 1981, medical knowledge of the underlying pathogenesis of HIV infection and AIDS has increased at a rate unprecedented in medical history. This knowledge has led to the rapid development of therapies directed at controlling HIV infection as well as the multitude of complicating opportunistic infections and cancers.

CHECKPOINT

23. What are the major clinical manifestations of AIDS? 24. What are the major steps in developing AIDS after infec-tion with HIV?

CASE STUDIES Yeong Kwok, MD

(See Chapter 25, p. 744–46 for Answers)

CASE 7

A 40-year-old woman comes to the clinic with a history of worsening nasal congestion and recurrent sinus infections. She had been healthy until about 1 year ago when she first noticed persistent rhinorrhea, sneezing, and stuffiness. She noted that when she went on a 2-week vacation to Mexico, her rhinorrhea disappeared, only to return when she came home again. She has lived in the same house for the past 5 years along with her husband and one child. They have had a dog for 4 years and a cat for 1 year. On physical examination, she has boggy, swollen nasal turbinates and a cobblestone appearance of her posterior pharynx.

Questions

A. What are the pathophysiologic mechanisms in allergic rhinitis? B. What are the symptoms and signs of allergic rhinitis? C. What are possible complications of allergic rhinitis?

CASE 8

A 2-month-old child is admitted to the ICU with fever, hypotension, tachycardia, and lethargy. The medical history is notable for a similar hospitalization at 2 weeks of age. Physical examination is notable for a temperature of 39°C, oral thrush, and rales in the right lung fields. Chest x- ray film reveals multilobar pneumonia. Given the history of recurrent severe infection, the pediatrician suspects an immunodeficiency disorder.

Questions

A. What is the most likely immunodeficiency in this child? Why? B. What are the underlying genetic and cellular defects associated with

this disease? C. What is the overall prognosis for patients with this disorder?

CASE 9

An 18-month-old boy is brought to the emergency department by his parents with a high fever, shortness of breath, and cough. The boy was well until he was 6 months old. Since then, he has had four bouts of otitis media, and because of their severity and recurrence, he was placed on prophylactic antibiotics for several months. He was recently taken off the antibiotics to see how he would do. The day before presentation, he developed a cough that has quickly progressed into an illness with high fevers and lethargy. Both of his parents are healthy, and he has a healthy older sister. His father’s family history is unremarkable, but his maternal uncle died of pneumonia in infancy. Examination is remarkable for a normally developed toddler who is lethargic and tachypneic. His temperature is 39°C, and he has decreased breath sounds at both lung bases. Chest x-ray film shows consolidation of the right and left lower lobes, as well as bilateral pleural effusions. He is admitted to the hospital, and the boy’s blood cultures grow out Streptococcus pneumoniae the next day. Immunologic testing shows very low levels of IgG, IgM, and IgA antibodies in the serum, and flow cytometry shows the absence of circulating B lymphocytes.

Questions

A. What is the likely diagnosis in this patient? Why? B. What is the primary pathophysiologic defect in the condition, and how

does it lead to this clinical presentation? C. Why are the affected children generally fine until they reach 4–6

months of age?

CASE 10

An 18-year-old man presents with complaints of fever, facial pain, and nasal congestion consistent with a diagnosis of acute sinusitis. His medical history is notable for multiple sinus infections, two episodes of pneumonia, and chronic diarrhea, all suggestive of a primary immunodeficiency syndrome. Workup establishes a diagnosis of common variable immunodeficiency syndrome.

Questions

A. What are the common infectious manifestations of common variable immunodeficiency syndrome?

B. What are the underlying immunologic abnormalities responsible for these infectious manifestations?

C. What other diseases is this patient at increased risk for? D. What treatment is indicated?

CASE 11

A 2-year-old boy is hospitalized with recurrent Staphylococcus aureus abscesses. This is his fourth hospitalization; two previous hospitalizations were also with S aureus abscesses, and one was with Serratia marcescens pyelonephritis. His parents, sister, and brother have never been hospitalized for infections. The recurrent infections lead to a workup, and the boy is diagnosed with chronic granulomatous disease.

Questions

A. What is the likely mechanism of inheritance of chronic granulomatous disease?

B. What is the fundamental immune defect in chronic granulomatous disease?

C. Why are certain infections more common in individuals with this

condition?

CASE 12

A 31-year-old male injection drug user presents to the emergency department with a chief complaint of shortness of breath. He describes a 1- month history of intermittent fevers and night sweats associated with a nonproductive cough. He has become progressively more short of breath, initially dyspneic only with exertion but now dyspneic at rest. He appears to be in moderate respiratory distress. His vital signs are abnormal, with a temperature of 39°C, heart rate of 112 bpm, respiratory rate of 20/min, and oxygen saturation of 88% on room air. Physical examination is otherwise unremarkable but notable for the absence of abnormal lung sounds. Chest x-ray film reveals a diffuse interstitial infiltrate characteristic of pneumocystis pneumonia, an opportunistic infection.

Questions

A. What is the underlying disease most likely responsible for this man’s susceptibility to pneumocystis pneumonia?

B. What is the pathogenesis of the immunosuppression caused by this underlying disease?

C. What is the natural history of this disease? What are some of the common clinical manifestations seen during its progression?

REFERENCES

General Abbas AK et al. Basic Immunology: Functions and Disorders of the Immune System, 5th ed. Elsevier, 2015. Akdis M et al. Interleukins, from 1 to 38, interferons, transforming growth factor-β and TNF-α: receptors, functions, and roles in diseases. J Allergy Clin Immunol. 2016 138:984–1010.

DeFranco AL et al. Immunity: The Immune Response in Infectious and Inflammatory Disease. New Science Press Ltd, 2007.

Noval Rivas M et al. Regulatory T cells in allergic diseases. J Allergy Clin Immunol. 2016 Sep;138(3):639–

52. [PMID: 27596705] O’Shea JJ et al. JAKs and STATs in immunity, immunodeficiency, and cancer. N Engl J Med. 2013 Jan 10;368(2):161–70. [PMID: 23301733]

Schwartz RS. Shattuck lecture: diversity of the immune repertoire and immunoregulation. N Engl J Med. 2003 Mar 13;348(11):1017–26. [PMID: 12637612]

van de Veen W et al. Role of regulatory B cells in immune tolerance to allergens and beyond. J Allergy Clin Immunol. 2016 Sep;138(3):654–65. [PMID: 27596706]

Allergic Rhinitis Adkinson FN et al, eds. Middleton’s Allergy Principles and Practice, 8th ed. Mosby, 2013. Akdis CA et al. Mechanisms and treatment of allergic disease in the big picture of regulatory T cells. J Allergy Clin Immunol. 2009 Apr;123(4):735–46. [PMID: 19348912]

Bernstein DI et al. Allergic rhinitis: mechanisms and treatment. Immunol Allergy Clin North Am. 2016 May;36(2):261–78. [PMID: 27083101]

Durham SR et al. Long-term clinical efficacy of grass-pollen immunotherapy. N Engl J Med. 1999 Aug 12;341(7):468–75. [PMID: 10441602]

Jutel M et al. International consensus on allergy immunotherapy. J Allergy Clin Immunol. 2015 Sep;136(3):556–68. [PMID: 26162571]

Kariyawasam HH et al. Allergic rhinitis, chronic rhinosinusitis and asthma: unravelling a complex relationship. Curr Opin Otolaryngol Head Neck Surg. 2013 Feb;21(1):79–86. [PMID: 23241653]

Wheatley LM et al. Clinical practice. Allergic rhinitis. N Engl J Med. 2015 Jan 29;372(5):456–63. [PMID: 25629743]

Primary Immunodeficiency Diseases Bonilla FA et al. International Consensus Document (ICON): common variable immunodeficiency disorders. J Allergy Clin Immunol Pract. 2016 Jan-Feb;4(1):38–59. [PMID: 26563668]

Bousfiha A et al. The 2015 IUIS phenotypic classification for primary immunodeficiencies. J Clin Immunol. 2015 Nov;35(8): 727–38. [PMID: 26445875]

Notarangelo LD. Primary immunodeficiencies. J Allergy Clin Immunol. 2010 Feb;125(2 Suppl 2):S182–94. [PMID: 20042228]

Ochs HD et al, eds. Primary Immunodeficiency Diseases: A Molecular and Genetic Approach, 3rd ed. Oxford University Press, 2014.

Parvaneh N et al. Primary immunodeficiencies: a rapidly evolving story. J Allergy Clin Immunol. 2013 Feb;131(2):314–23. [PMID: 23374262]

Starr SP. Immunology update: primary immunodeficiency diseases. FP Essent. 2016 Nov;450:35–53. [PMID: 27869441]

Sullivan KE et al. Stiehm’s Immune Deficiencies. Academic Press, 2014.

AIDS Boassa A et al. Chronic innate immune activation as a cause of HIV-1 immunopathogenesis. Clin Immunol. 2008 Mar;126(3):235–42. [PMID: 17916442]

Cary DC et al. Molecular mechanisms of HIV latency. J Clin Invest. 2016 Feb;126(2):448–54. [PMID:

26731470] Centers for Disease Control and Prevention. HIV/AIDS. https://www.cdc.gov/hiv/. Accessed April 2, 2018. Centers for Disease Control and Prevention. Revised surveillance case definition for HIV infection—United States, 2014. MMWR Recomm Rep. 2014 Apr 11;63(RR-03):1–10. [PMID: 24717910]

Fauci AS. The acquired immune deficiency syndrome: the ever-broadening clinical spectrum. JAMA. 2016 Jul 12;316(2):230. [PMID: 27404204]

Jones RB et al. HIV-specific CD8+ T cells and HIV eradication. J Clin Invest. 2016 Feb;126(2):455–63. [PMID: 26731469]

Saag MS. The Sanford Guide to HIV/AIDS Therapy 2017: Pocket Edition, 25th ed. Antimicrobial Therapy, 2017.

CHAPTER

4

Infectious Diseases Christina T. Fiske, MD, MPH, & Karen C. Bloch, MD, MPH

Infectious diseases remain one of the leading causes of death in both developed and developing countries. Infections cause significant morbidity and mortality, especially in individuals who are most vulnerable to illness: the very young, the elderly, the immunocompromised, and the disenfranchised.

The pathogenesis of infectious diseases reflects the relationship among the human host, the infectious agent, and the external environment. Figure 4–1 portrays a host–agent–environment paradigm for the study of infectious diseases. The infectious agent can be either exogenous (ie, not normally found on or in the body) or endogenous (ie, one that may be routinely cultured from a specific anatomic site but that does not normally cause disease in the host). Infection results when an exogenous agent is introduced into a host from the environment or when an endogenous agent overcomes innate host immunity to cause disease. Host susceptibility plays an important role in both settings.

FIGURE 4–1 The fundamental relationships involved in the host–agent–environment interaction model. In the host, pathogenetic mechanisms extend from the level of populations (eg, person-to-person transmission) to the level of cellular and molecular processes (eg, genetic susceptibility).

The environment includes vectors (insects and other carriers that transmit infectious agents) and zoonotic hosts or reservoirs (animals that harbor infectious agents and often act to amplify the infectious agent). For example, the white-footed mouse (Peromyscus leucopus) serves as an animal reservoir for Borrelia burgdorferi, the bacterium that causes Lyme disease. The Ixodes tick serves as an insect vector. Infection in the mouse is asymptomatic, and the bacteria can multiply to high levels in this animal. When the tick larva feeds on an infected mouse, it becomes secondarily infected with B burgdorferi, and this infection persists when the tick molts into a nymph. Subsequently, when an infected nymph feeds on a human, the bacterium is transmitted through infected saliva into the host bloodstream, causing disease.

The study of infectious diseases requires an understanding of pathogenesis at the level of the population, the individual, the cell, and the gene. For example, at the population level, the spread of tuberculosis in the community is related to the social interactions of an infectious human host. Outbreaks of tuberculosis have occurred in group settings such as homeless shelters, prisons, and nursing homes when an index case comes in close contact with susceptible persons. At the individual level, tuberculosis results from inhalation of respiratory droplets containing airborne tubercle bacilli. At the cellular level, these bacilli activate T cells, which play a critical role in containing the infection. Individuals with an impaired T-cell response (eg, those infected with human immunodeficiency virus

[HIV]) are at particularly high risk for developing active tuberculosis at the time of the initial infection or for reactivation of latent tuberculosis as their immunity wanes. Finally, at the genetic level, individuals with specific polymorphisms in genes encoding inflammatory cytokines and macrophage proteins may be at significantly higher risk for tuberculosis.

Specific microorganisms have a tendency to cause certain types of infections: Streptococcus pneumoniae commonly causes pneumonia, meningitis, and bacteremia but rarely causes endocarditis (infection of the heart valves); Escherichia coli is a common cause of gastrointestinal (GI) and urinary tract infections; Plasmodium species infect red blood cells and liver cells to cause malaria; Entamoeba histolytica causes amebic dysentery, liver abscesses, and so on. Table 4–1 presents a clinical approach to taking a patient history that considers features of the host and the environment in identifying the most likely microorganisms associated with specific clinical syndromes.

TABLE 4–1 Obtaining a history in the diagnosis of infectious diseases.

HOST DEFENSES AGAINST INFECTION

Humans live in an environment brimming with microorganisms that have potential to cause disease. The human body has the ability to control infection through a number of different mechanisms. Physical barriers impede the entry of bacteria from the external environment and from normally colonized sites in the body into sterile anatomic areas. When these physical defenses are breached, the immune system is activated (Figure 4–2). Innate immunity (also called natural or native immunity), provided by preformed proteins (eg, complement) and immune cells (eg, phagocytes) that are activated by nonspecific foreign proteins, allows an immediate and rapid response to foreign material. Adaptive immunity (also called specific immunity) includes both early and late adaptive responses activated by specific antigenic proteins (eg, production of antibodies active against the specific strains of S pneumoniae contained in the pneumococcal vaccine in a previously vaccinated individual). Induction of these specific immune receptor cells may take several days in the immunologically naive host. Protective immunity, which occurs after initial exposure (via infection or vaccination) through generation of memory lymphocytes and pathogen-specific antibody, allows a much more rapid response to reinfection. These components of the immune response are discussed in detail later.

FIGURE 4–2 Phases of the host response to infection. During the earliest stage of initial infection, nonspecific mediators (complement, phagocytes) predominate. Adaptive immunity (production of antibody, stimulation of lymphocytes) requires clonal expansion after recognition of specific antigens. Once immunity toward a specific agent is induced, the immune response remains primed so that the response to reinfection is much more rapid.

NORMAL MICROBIAL FLORA

Until recently, our view of human microbiology was shaped by what microorganisms were isolated from persons with acute infection. However, over time it has become apparent that the diversity of microorganisms observed from microscopy or genetic sequencing was far greater than the microorganisms that were isolated from traditional culture techniques. The human body harbors numerous species of bacteria, viruses, fungi, and protozoa, referred to as the human microbiota or microbiome. The great majority of these are commensals, defined as organisms that live symbiotically on or within the human host but rarely cause disease (Figure 4–3). Anatomic sites where bacteria are normally found include the skin (staphylococci and diphtheroids), oropharynx (streptococci, anaerobes), large intestine (enterococci, enteric bacilli), and vagina (lactobacilli).

FIGURE 4–3 Commensal bacteria secrete toll-like receptor (TLR) ligands, which bind to TLR on the surface of normal intestinal tissue. This interaction stimulates basal signaling, which protects against cellular injury. Disruption of TLR signaling or antibiotic-associated eradication of commensal bacteria results in compromised ability of the intestinal epithelium to withstand injury and repair cell damage. (Redrawn, with permission, from Madara J. Building an intestine—architectural contributions of commensal bacteria. N Engl J Med. 2004;351:1686. Copyright © 2004 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.)

Microbes do not exist in the human body in isolation but in complex communities and habitats. Imbalances in the composition of these habitats are associated with human infection and disease. One example of an infection that can result from a perturbed microbiota community is that caused by Clostridium difficile. C difficile is an anaerobic gram-negative rod that causes intestinal disease in the setting of antibiotic administration. Antibiotics alter the structure and function of an individual’s gut microbiota, reducing the body’s innate resistance to colonization with C difficile organisms (and other enteric pathogens) and providing them a niche to multiply and elaborate their enterotoxins. Similarly, broad-spectrum antibiotics will destroy normal vaginal flora, such as lactobacilli, and allow overgrowth of Candida (yeast) species, resulting in Candida vaginitis.

Determining when an isolate is a component of the normal flora rather than an invasive pathogen may be difficult. For example, culture of staphylococci from a blood sample may represent skin contamination at the time of phlebotomy or may indicate a potentially life-threatening bloodstream infection. Helpful clues include burden of organism (eg, number of positive blood cultures), symptoms and signs of infection (eg, cough, fever), and the presence of inflammatory cells (eg, polymorphonuclear cells in the sputum and an increased proportion of immature neutrophils in the blood). Isolation of an obligate pathogen such as Mycobacterium tuberculosis from any anatomic site is diagnostic of infection. Fortunately, few microorganisms are absolute pathogens. For example, Neisseria meningitidis, a major bacterial cause of meningitis, can be cultured from the oropharynx of as many as 10% of asymptomatic individuals, in which case it represents transient normal flora. Even if asymptomatic, the host can serve as a carrier, transferring bacteria to susceptible individuals. Infections resulting from commensals that rarely cause disease (eg, Candida albicans) or organisms ubiquitous in the environment that are generally not considered human pathogens (eg, Aspergillus) are termed opportunistic infections. These infections occur almost exclusively in immunocompromised hosts, such as HIV-infected patients or transplant recipients. The agents are opportunists in that they take advantage of impaired host immunity to cause infection but rarely cause disease in a healthy host.

The presence of bacteria on a body surface without causing disease is called colonization, whereas invasion of the tissues by a microorganism that causes disease is termed infection. The site from which an organism is cultured is important in differentiating colonization from infection. Growth of any microorganism from a normally sterile site such as blood, cerebrospinal fluid,

synovial (joint) fluid, or deep tissues of the body is diagnostic of infection. For example, Bacteroides, the predominant genus of bacteria in the colon, may cause intra-abdominal abscesses and sepsis when the integrity of the colonic mucosa is breached. Staphylococcus epidermidis, a common skin commensal, can cause bacteremia after intravascular catheter placement. Knowledge of the common endogenous flora may be useful in determining the cause of an infection and may aid in the choice of empiric antibiotic therapy.

The normal flora prevent colonization through numerous mechanisms. Organisms comprising the normal flora often have a selective advantage over colonizers in that they are already established in an anatomic niche. Colonization of sites that are normally sterile or have very few microbes is generally easier because there is no competition for nutrients from endogenous flora. However, host defenses at these sites are often vigorous. For instance, the stomach is normally sterile because few microbes can survive at the normal gastric pH of 1.5 to 3.5. However, if antacids are used to decrease gastric acidity, colonization of the stomach and adjacent trachea with gram-negative bacteria occurs rapidly. Other host defense mechanisms that serve to inhibit colonization by pathogenic bacteria include (1) mechanical clearance; (2) phagocytic killing; and (3) deprivation of necessary nutrients. Successful colonizers have adapted the ability to evade or overcome these defenses. For example, gonococci, the bacteria that cause gonorrhea, avoid excretion in the urine by adhering to the mucosal epithelium of the urogenital tract with pili. Pneumococci resist phagocytosis by encapsulation within a slime layer that impairs uptake by neutrophils. Some staphylococci elaborate enzymes known as hemolysins, which destroy host red blood cells, thus giving them access to a needed source of iron.

When replacement of the normal flora occurs in the hospital environment, the organisms are said to be nosocomially acquired. The distinction between hospital-acquired and community-acquired infections has blurred in recent years, because of an increase in medical care in the home or skilled nursing facility among patients who previously would have required long-term hospitalization. For this reason, the broader term “health care–associated infections” is used to encompass both hospitalized patients and patients with frequent medical interactions (eg, residence in nursing home, outpatient hemodialysis, home intravenous antibiotics). Health care–associated infections are significant because the organisms involved are often resistant to multiple antibiotics. In the health care setting, colonization may progress to symptomatic infection. For example, individuals hospitalized for extended periods often become colonized with gram-negative bacteria such as Pseudomonas aeruginosa. These

individuals are then at increased risk for life-threatening infections such as Pseudomonas pneumonia.

INNATE IMMUNITY

The innate immune system represents a rapid, multi-pronged mechanism to protect the host from infection. The components of the innate immune system are present at birth and not learned or adapted as a result of exposure to microorganisms. Innate immunity includes physical barriers to prevent microbial entry, activation of inflammatory cells and proteins that target the microorganisms, and activation of the adaptive immune system. The first line of defense are nonspecific barriers against infectious diseases that do not require prior contact with the microorganism. These defenses consist of simple physical (eg, skin) and chemical (eg, acidic gastric secretions) barriers that prevent easy entry of microorganisms into the body. Some infectious agents use a vector (such as an insect) to bypass structural barriers and gain direct access to the blood or subcutaneous tissues of the body. Once an agent has entered the body, the major innate defenses are the acute inflammatory response and the complement system. These defenses can neutralize the organism, recruit phagocytic cells, and induce a more specific response through humoral and cell-mediated immunity. The innate defenses of the body are important from an evolutionary perspective in enabling humans to encounter and adapt to a variety of new and changing environments.

Physical & Chemical Barriers to Infection The squamous epithelium of the skin is the first line of defense against microorganisms encountered in the outside world. As keratinized epithelial surface cells desquamate, the skin maintains its protective barrier by generating new epithelial cells beneath the surface. The skin is also bathed with oils and moisture from the sebaceous and sweat glands. These secretions contain fatty acids that inhibit bacterial growth. Poor vascular supply to the skin may result in skin breakdown and increased susceptibility to infection. For example, chronically debilitated or bedridden patients may suffer from presacral pressure ulcers as a result of constant compression of this dependent body area; these ulcers allow direct entry of skin and enteric bacteria into previously sterile sites and predispose patients to severe infections.

The mucous membranes also provide a physical barrier to microbial invasion.

The mucous membranes of the mouth, pharynx, esophagus, and lower urinary tract are composed of several layers of epithelial cells, whereas those of the lower respiratory tract, the GI tract, and the upper urinary tract are delicate single layers of epithelial cells. These membranes are covered by a protective layer of mucus, which traps foreign particles and prevents them from accessing the lining of epithelial cells. Because mucus is hydrophilic, many substances produced by the body easily diffuse to the surface, including enzymes with antimicrobial activity such as lysozyme and peroxidase.

Inflammatory Response How host cells recognize invading organisms is an area of active research. Pattern recognition receptors (PRRs) in host cells include Toll-like receptors, nucleotide-binding oligomerization domain-like receptors, and other intracellular proteins. PRRs are activated when exogenous organisms interact with host cells through their sensing of pathogen-associated molecular patterns (PAMPs). Detection of PAMPs stimulates activation of a pro-inflammatory cascade that plays a key role in pathogen recognition and inactivation. Importantly, PRRs do not require prior contact with the organism to be activated. This response is usually tightly regulated through feedback mechanisms; however, when this goes awry, the immune cascade may lead to damage to host tissues, termed “autoimmune dysfunction.”

Clinically, signs of inflammation (heat, erythema, pain, and swelling) are the characteristic features of localized infection, secondary tissue injury, and the body’s response to this injury. Blood supply to the affected area increases in response to vasodilation, and the capillaries become more permeable, allowing antibodies, complement, and white blood cells to cross the endothelium and reach the site of injury. An important consequence of inflammation is that the pH of the inflamed tissues is lowered, creating an inhospitable environment for the microbe. The increased blood flow to the area allows continued recruitment of inflammatory cells as well as the necessary components for tissue repair and recovery.

When a microorganism enters host tissue, it activates circulating proteins including complement (see below) and induces the release of pro-inflammatory cytokines. These mediators result in the increased vascular permeability and vasodilation characteristic of inflammation. For example, the anaphylatoxins C3a, C4a, and C5a, produced by the activation of complement, stimulate the release of histamine from mast cells. Histamine dilates the blood vessels and further increases their permeability. Bradykinin is also released, increasing

vascular permeability. Pro-inflammatory cytokines include interleukin-1 (IL-1), IL-6, tumor necrosis

factor, and interferon-γ. These factors, singly or in combination, promote fever, produce local inflammatory signs, and trigger catabolic responses. During severe infection, a change in hepatic synthesis of proteins occurs, resulting in an increase in some proteins and a decrease in others. Most notable is the increase in acute-phase reactants, which include rheumatoid factor, C-reactive protein (CRP), ferritin, and various proteinase inhibitors. The erythrocyte sedimentation rate (ESR), a nonspecific marker of inflammation, also increases, although in a slower fashion. A catabolic state is further augmented by simultaneous increases in levels of circulating cortisol, glucagon, catecholamines, and other hormones.

Mild to moderate inflammatory responses serve important host defense functions. For example, elevated body temperature may inhibit viral replication. Inflammatory hyperemia and systemic neutrophilia optimize phagocyte delivery to sites of infection. Decreased availability of iron inhibits the growth of microbes such as Yersinia that require this element as a nutrient. However, when the inflammatory responses become extreme, extensive tissue damage can result, as in the case of sepsis.

COMPLEMENT SYSTEM

The complement system is composed of a series of plasma protein and cell membrane receptors that are important mediators of host defenses and inflammation (Figure 4–4). Most of the biologically significant effects of the complement system are mediated by the third component (C3) and the terminal components (C5–9). These crucial proteins are activated through one of two mechanisms, termed the classic and alternative pathways. The classic pathway is activated by antigen–antibody complexes or antibody-coated particles, and the alternative pathway is activated by mechanisms independent of antibodies, usually by interaction with bacterial surface components. Both pathways form C3 convertase, which cleaves the C3 component of complement, a key protein common to both pathways. The two pathways then proceed in identical fashion to bind late-acting components to form a membrane attack complex (C5–9), which results in target cell lysis.

FIGURE 4–4 Complement reaction sequence and infections associated with deficiency states. (Redrawn, with permission, from Detrick B. Immunology. In: Carroll KC et al, eds. Jawetz, Melnick, and Adelberg’s Medical Microbiology, 27th ed. McGraw-Hill, 2015.)

Once activated, complement functions to enhance the antimicrobial defenses in several ways. Complement facilitates phagocytosis by serving as an opsonin, binding to invading microorganisms and making them susceptible to engulfment

and destruction by neutrophils and macrophages. The complement-derived membrane attack complex inserts itself into the membrane of a target organism, leading to increased permeability and subsequent lysis of the cell. Complement also acts indirectly by producing substances that are chemotactic for white blood cells.

Inherited disorders of complement are associated with an increased risk of bacterial infection. The specific infections seen in complement-deficient patients relate to the biologic functions of the missing component (see Figure 4–4). Patients with a deficiency of C3 or a component in either of the two pathways necessary for the activation of C3 typically have increased susceptibility to infections with encapsulated bacteria such as S pneumoniae and Haemophilus influenzae. In contrast, patients with deficiencies of C5–9 have normal resistance to encapsulated bacteria because C3b-mediated opsonization is intact. These patients, however, are unusually susceptible to infections with N meningitidis and N gonorrhoeae because they are unable to form a membrane attack complex and, therefore, cannot lyse the Neisseria cell membrane. Recurrent infection with encapsulated bacteria such as Neisseria is an indication for testing for complement deficiency, and if present, immunizations (with vaccines against N meningitis, Haemophilus influenzae, and Streptococcus pneumoniae) to protect against future infection with such encapsulated bacteria.

Phagocytosis After the natural barriers of the skin or mucous membranes have been penetrated, the phagocytic cells—neutrophils, monocytes, and macrophages— constitute an important component of the innate immune system. The process of internalizing organisms by these cells (phagocytosis) involves attachment of the organism to the cell surface. This triggers extension of a pseudopod to enclose the bacterium in an endocytic vesicle, or phagosome.

The circulating polymorphonuclear neutrophil (PMN) is an important component of the host immune response that, in the absence of infection, circulates in a quiescent state. When chemotactic factors, arachidonic acid metabolites, or complement cleavage fragments interact with specific PMN membrane receptors, the neutrophil rapidly becomes activated and moves toward the chemoattractants. After phagocytosis, the mechanisms by which the phagolysosome destroys the microorganism can be divided into oxygen- independent and oxygen-dependent processes. Functional defects in circulating neutrophils or decreases in absolute number of neutrophils are important risk factors for infection.

Neutropenia, defined as an absolute neutrophil count of less than 1000 cells/ μL, is a predisposing factor for life-threatening bacterial and fungal infections. The risk of infection is inversely proportionate to the number of neutrophils, rising significantly with neutrophil counts less than 500 cells/μL, termed “severe neutropenia.” The longer the duration of profound neutropenia, the greater is the risk of infection. At the first sign of infection (eg, fever), these patients should be given broad-spectrum antibacterial agents to cover gram-negative bacterial pathogens. In addition to impaired immunity, neutropenic hosts often have additional risk factors for infection, such as the need for long-term indwelling central venous catheters (predisposing to infection with skin bacteria) and the frequent use of parenteral nutrition (predisposing to fungal infection).

Several inherited disorders of neutrophil function have been described. Chédiak–Higashi syndrome is a rare autosomal recessive hereditary disorder in which the neutrophils have a profound defect in the formation of intracellular granules. Opsonized bacteria such as Staphylococcus aureus are ingested normally, but viable bacteria persist intracellularly, presumably because of the inability of the neutrophil’s intracellular granules to fuse with phagosomes to form phagolysosomes. Patients with Chédiak–Higashi syndrome typically present in childhood with recurrent bacterial infections involving the skin and soft tissues and the upper and lower respiratory tracts.

Myeloperoxidase deficiency is the most common neutrophil disorder, with a prevalence of one case per 2000 individuals. In this autosomal recessive disorder, phagocytosis, chemotaxis, and degranulation are normal, but metabolic pathways are impaired, leading to delayed microbicidal activity. Most patients with this defect are asymptomatic. In contrast, chronic granulomatous disease is a genetically heterogeneous group of inherited disorders characterized by the failure of phagocytic cells to produce superoxides. The defect involves neutrophils, monocytes, eosinophils, and some macrophages. Oxygen-dependent intracellular killing is impaired, and these patients are susceptible to recurrent, often life-threatening infections. Patients with chronic granulomatous disease also tend to form granulomas in tissues, particularly in the lungs, liver, and spleen. Chronic granulomatous disease is usually diagnosed in childhood after recurrent severe infections with S aureus and Aspergillus species, and affected patients benefit from prophylactic antimicrobials.

INDUCED DEFENSES OF THE BODY

Although innate host defenses against infectious agents are generally nonspecific and do not require prior exposure to the invading agent, induced defenses are highly specific and are qualitatively and quantitatively altered by prior antigenic exposure. Details of the pathophysiology of the host immune system are covered in Chapter 3. Recurrent infections or infections with unusual organisms may be clues to an underlying defect in the induced immune response (Table 4–2).

TABLE 4–2 Infections associated with common defects in humoral and cellular immune response.

ESTABLISHMENT OF INFECTION

An infectious disease occurs when a pathogenic organism causes inflammation or organ dysfunction. This may be caused directly by the infection itself, as when the etiologic agent replicates in the host, or indirectly as a result of the host’s inflammatory response. Many infections are subclinical, not producing any obvious manifestations of disease. To cause overt infection, all microorganisms must go through the following stages: The microorganism must (1) encounter the host; (2) gain entry into the host; (3) multiply and spread from the site of entry; and (4) cause host tissue injury, either directly (eg, cytotoxins) or indirectly (host inflammatory response). The severity of infection ranges from asymptomatic to life-threatening, and the course may be characterized as acute, subacute, or chronic. Whether infection is subclinical or overt, the outcome is either (1) resolution (eg, eradication of the infecting pathogen); (2) chronic active infection (eg, HIV, hepatitis); (3) prolonged asymptomatic excretion of the agent (eg, carrier state with Salmonella typhi); (4) latency of the agent within host tissues (eg, latent tuberculosis or varicella zoster virus); or (5) host death from infection.

Except for congenital infections (acquired in utero), humans first encounter microorganisms at birth. During parturition, the newborn comes into contact with microorganisms present in the mother’s vaginal canal and on her skin. Most of the bacteria the newborn encounters do not cause harm, and for those that might cause infection, the newborn usually has passive immunity through maternal antibodies. For example, neonates are protected against infection with H influenzae by maternal antibodies for the first 6 months of life until passive immunity wanes and the risk of infection with this bacterium increases. On the other hand, newborns whose mothers are vaginally colonized with group B streptococci are at increased risk in the perinatal period for serious infections such as sepsis or meningitis with this organism. For this reason, it is recommended that (1) vaginal cultures be done to screen for group B streptococci for all pregnant women; and (2) intrapartum antibiotic prophylaxis be administered to those with positive detection of group B streptococci.

Direct entry of microorganisms into the host (ie, bypassing the usual chemical and physical barriers) may occur when (1) an insect vector directly inoculates the infectious agent into the host (eg, a mosquito transmitting malaria); (2) bacteria gain direct access to host tissues through loss of integrity of the skin or mucous membranes (eg, trauma, surgical wounds); or (3) microbes gain access via instruments or catheters that allow communication between usually sterile sites and the outside world (eg, indwelling venous catheters).

Ingression occurs when an infectious agent enters the host via an orifice contiguous with the external environment. This primarily involves inhalation of infectious aerosolized droplets (eg, M tuberculosis) or ingestion of contaminated foods (eg, Salmonella, hepatitis A virus). Other infectious agents directly infect mucous membranes or cross the epithelial surface to cause infection. This commonly occurs in sexually transmitted diseases. For example, HIV can cross vaginal mucous membranes via penetration of virus-laden macrophages from semen. After the initial encounter with the host, the infectious agent must successfully multiply at the site of entry. The process whereby the newly introduced microorganism successfully competes with normal flora and is able to multiply is termed colonization (eg, pneumococci colonizing the upper respiratory tract). When the microorganism multiplies at a normally sterile site, it is termed infection (eg, pneumococci multiplying in the alveoli, causing pneumonia). Factors that facilitate the multiplication and spread of infection include inoculum size (the quantity of infectious organisms introduced), host anatomic factors (eg, impaired ciliary function in children with cystic fibrosis), availability of nutrients for the microbe, physicochemical factors (eg, gastric

pH), microbial virulence factors, and anatomic sanctuaries (eg, abscesses). An abscess is a special case in which the host has contained the infection but is unable to eradicate it, and these localized infections generally require surgical drainage. Once introduced, infections can spread along the epidermis (eg, impetigo), along the dermis (eg, erysipelas), along subcutaneous tissues (eg, cellulitis), along fascial planes (eg, necrotizing fasciitis), into muscle tissue (eg, myositis), along veins (eg, suppurative thrombophlebitis), into the blood (eg, bacteremia, fungemia, viremia), along lymphatics (eg, lymphangitis), and into organs (eg, pneumonia, brain abscesses, hepatitis).

Infections cause direct injury to the host through a variety of mechanisms. If organisms are present in sufficient numbers and are of sufficient size, mechanical obstruction can occur (eg, children with roundworm gastrointestinal infections may present with bowel obstruction). More commonly, pathogens cause an intense secondary inflammatory response, which may result in life-threatening complications (eg, children with H influenzae epiglottitis may present with mechanical airway obstruction secondary to intense soft tissue swelling of the epiglottis). Some bacteria produce neurotoxins that affect host cell metabolism rather than directly causing cell damage (eg, tetanus toxin antagonizes inhibitory neurons, causing unopposed motor neuron stimulation, manifested clinically as sustained muscle rigidity). Host cell death can occur by a variety of mechanisms. Shigella produces a cytotoxin that causes death of large intestine enterocytes, resulting in the clinical syndrome of dysentery. Poliovirus-induced cell lysis of the anterior horn cells of the spinal cord causes flaccid paralysis. Gram-negative bacterial endotoxin can initiate a cascade of cytokine release, resulting in sepsis syndrome and septic shock.

Many infections that begin as mild and easily treatable conditions readily progress without prompt treatment. Small, seemingly insignificant skin abrasions superinfected with toxic shock syndrome toxin (TSST-1)–producing S aureus can result in fulminant infection and death. Even indolent infections, such as infective endocarditis caused by viridans group streptococci can be fatal unless recognized and appropriately treated.

There are three potential outcomes of infection: recovery, chronic infection, and death. Most infections resolve, either spontaneously (eg, rhinovirus, the leading cause of the common cold) or with medical therapy (eg, streptococcal pharyngitis after treatment with penicillin). Chronic infections may be either saprophytic, in which case the organism does not adversely affect the health of the host, or parasitic, causing tissue damage to the host. An example of a

saprophytic infection is Salmonella typhi, which may be harbored asymptomatically in the gallbladder of about 2% of individuals after acute infection. Chronic infection with the hepatitis B virus may be either saprophytic, in which case the human host is infectious for the virus but has no clinical evidence of liver damage, or parasitic, with progressive liver damage and cirrhosis. A final form of chronic infection is tissue latency. Varicella-zoster virus, the agent causing chicken pox, survives in the dorsal root ganglia, with reactivation causing a dermatomal eruption with vesicles or shallow ulcerations, commonly known as shingles or zoster. When the ability of the immune system to control either the acute or chronic infection is exceeded, the infection may result in host death.

All infectious agents, regardless of specific mechanisms, must successfully reproduce and evade host defense mechanisms. This knowledge helps the physician to prevent infections (eg, vaccinate against influenza virus); to treat and cure infection (eg, administer antibiotic for E coli urinary tract infection); and to prevent further transmission, recurrence, or reactivation (eg, advise use of barrier protection to reduce the sexual spread of genital herpes simplex infection).

CHECKPOINT

1. By what three general mechanisms do hosts resist colo-nization by pathogenic bacteria?

2. What are three ways in which the normal flora contrib-utes to the balance between health and disease?

3. Which specific host defenses against infection do not require prior contact with the infecting organism?

4. What are the categories of outcomes from an infection?

PATHOPHYSIOLOGY OF SELECTED INFECTIOUS DISEASE SYNDROMES

INFECTIVE ENDOCARDITIS

Clinical Presentation Infective endocarditis refers to a bacterial or, rarely, a fungal infection of the cardiac valves. Infection of extracardiac endothelium is termed “endarteritis” and can cause disease that is clinically similar to endocarditis. The most common predisposing factor for infective endocarditis is the presence of structurally abnormal cardiac valves. Consequently, patients with a history of rheumatic or congenital heart disease, a prosthetic heart valve, or a history of prior endocarditis are at increased risk for infective endocarditis. Infection involves the left side of the heart (mitral and aortic valves) almost exclusively, except in patients who are injection drug users or, less commonly, in patients with valve injury from a pulmonary artery (Swan–Ganz) catheter, in whom infection of the right side of the heart (tricuspid or pulmonary valve) may occur.

Etiology The most common infectious agents causing native valve infective endocarditis are gram-positive bacteria, including viridans group streptococci, S aureus, and enterococci. The specific bacterial species causing endocarditis can often be anticipated on the basis of epidemiologic factors. Injection drug users commonly introduce skin bacteria such as S aureus into the blood when nonsterile needles are used or the skin is not adequately cleaned before needle insertion. Patients with recent dental work are at risk for transient bacteremia with normal oral flora, particularly viridians group streptococci, with secondary seeding of endovascular tissue. Genitourinary tract infections with enterococci may lead to bacteremia and subsequent seeding of heart valves. Patients with prosthetic heart valves are also at increased risk for infective endocarditis resulting from skin flora such as S epidermidis or S aureus. Before the availability of antibiotics, infective endocarditis was a uniformly fatal disease. Even with antibiotics, the case fatality rate for left-sided S aureus endocarditis exceeds 25%, and definitive cure often requires both prolonged intravenous antibiotic administration and urgent surgery to replace infected cardiac valves.

Pathogenesis Endothelial damage is a common predisposing factor for the development of endocarditis. This may result from turbulent blood flow, as occurs with valvular regurgitation, or from micro-abrasion, common with injection drug use. In this latter setting, it is thought that impurities present in the injected material cause repeated injury to the tricuspid valve endothelium, or less frequently, pulmonary

valve endothelium, before being cleared from the bloodstream by the pulmonary circulation. Denuded endothelium results in exposure of extracellular matrix proteins expressing fibronectin, leading to a sticky matrix composed of fibrin, platelets, and adhesion proteins. Infective endocarditis occurs when microorganisms are deposited onto these sterile thrombi during the course of bacteremia (Figure 4–5). Not all bacteria adhere equally well to these sites. For example, E coli, a frequent cause of urosepsis, is rarely implicated as a cause of endocarditis. Conversely, virulent organisms such as S aureus can invade intact endothelium, causing endocarditis in the absence of preexisting valvular abnormalities

FIGURE 4–5 Pathogenesis of bacterial valve colonization. Viridans group streptococci adhere to fibrin-platelet clots that form at the site of damaged cardiac endothelium (A). The fibrin-adherent streptococci activate monocytes to produce tissue factor activity (TFA) and cytokines (B). These mediators activate the coagulation pathway, resulting in further recruitment of platelets and growth of the vegetation (C). (Redrawn, with permission, from Moreillon P et al. Pathogenesis of streptococcal and staphylococcal endocarditis. Infect Dis Clin North Am. 2002;16:297. Copyright © Elsevier.)

Once infected, these vegetations enlarge through further deposition of platelets and fibrin, providing the bacteria a sanctuary from host defense mechanisms such as polymorphonuclear leukocytes and complement. Consequently, once infection takes hold, the infected vegetation continues to grow in a largely unimpeded fashion. Infected vegetations also act as mechanical barriers to antibiotic penetration, necessitating the prolonged administration of intravenous (rather than oral) antibiotics that kill (bacteriocidal agents) rather than inhibit (bacteriostatic agents) the responsible organism. Surgical removal of the infected valve is sometimes required for cure, particularly if there is mechanical valve dysfunction with resultant decompensated heart failure, abscess formation around the valve ring, or prosthetic valve infection.

A hallmark of infective endocarditis is sustained high-grade bacteremia, which stimulates both the humoral and cellular immune systems. A variety of immunoglobulins are expressed, resulting in immune complex formation, increased serum levels of rheumatoid factor, and nonspecific hypergammaglobulinemia. Immune complex deposition along the renal glomerular basement membrane may result in the development of acute glomerulonephritis and renal failure.

Clinical Manifestations Infective endocarditis is a multisystem disease with protean manifestations. Table 4–3 summarizes the important features of the history, physical examination, laboratory results, and complications of infective endocarditis. Cutaneous findings suggestive of endocarditis include Osler nodes, painful papules on the pads of the fingers and toes thought to be secondary to deposition of immune complexes; and Janeway lesions, painless hemorrhagic lesions on the palms and soles caused by septic microemboli (Figure 4–6). Symptoms and signs of endocarditis may be acute, subacute, or chronic. The clinical manifestations reflect primarily (1) hemodynamic changes from valvular damage; (2) end-organ symptoms and signs from septic emboli (right-sided emboli to the lungs, left-sided emboli to the brain, spleen, kidney, and extremities); (3) end-organ symptoms and signs from immune complex deposition; and (4) persistent bacteremia with metastatic seeding of infection

(abscesses or septic joints). Death is usually caused by hemodynamic collapse or by septic emboli to the central nervous system, resulting in brain abscesses or mycotic aneurysms and intracerebral hemorrhage. Adverse outcomes are more common among older patients, those with S aureus infections, and those with prosthetic valve infections. Outcomes are also worse in patients with complications from endocarditis (heart failure, valve ring abscess, or emboli) and in patients who have experienced a delay in receiving valvular surgery (those with large vegetations and significant valvular destruction).

TABLE 4–3 Features of infective endocarditis.

FIGURE 4–6 Osler node causing pain within pulp of the big toe in a woman hospitalized with acute bacterial endocarditis. (Osler nodes are painful: remember “O” for “ouch” and “Osler.”) Note the multiple painless flat Janeway lesions over the sole of the foot. (Used, with permission, from David A. Kasper, DO, MBA. Originally published in: Chumley H. Bacterial endocarditis. In: Usatine RP et al, eds. The Color Atlas of Family Medicine, 2nd ed. McGraw-Hill, 2013.)

CHECKPOINT

5. Which patients are at highest risk for infective endocarditis? 6. What are the leading bacterial agents of infective endocarditis? 7. What features characterize infective endocarditis in intravenous drug

users? In patients with prosthetic heart valves?

8. What hemodynamic features predispose to infective endocarditis? 9. What is the outcome of untreated bacterial endocarditis? 10. What are the risk factors for a fatal outcome? What are the most

common causes of death in untreated infec-tive endocarditis?

MENINGITIS

Clinical Presentation Symptoms commonly associated with both bacterial and viral meningitis include acute onset of fever, headache, neck stiffness (meningismus), photophobia, and confusion. Bacterial meningitis causes significant morbidity (neurologic sequelae, particularly sensorineural hearing loss) and mortality and thus requires immediate antibiotic therapy. With rare exceptions, only supportive care with analgesics is necessary for viral meningitis.

Because the clinical presentations of bacterial and viral meningitis may be indistinguishable, laboratory studies of the cerebrospinal fluid are critical in differentiating these entities. Cerebrospinal fluid leukocyte pleocytosis (white blood cells in the cerebrospinal fluid) is the hallmark of meningitis. Bacterial meningitis is generally characterized by neutrophilic pleocytosis (predominance of polymorphonuclear neutrophils in the cerebrospinal fluid). Common causes of lymphocytic pleocytosis include viral infections (eg, enterovirus, West Nile virus), fungal infections (eg, cryptococcus in HIV-infected persons), and spirochetal infections (eg, neurosyphilis or Lyme neuroborreliosis). Noninfectious causes such as cancer, connective tissue diseases, and hypersensitivity reactions to drugs can also cause lymphocytic pleocytosis. The cerebrospinal fluid in bacterial meningitis is generally characterized by marked elevations in protein concentration, an extremely low glucose level, and, in the absence of previous antibiotic treatment, a positive Gram stain for bacteria. However, there is often significant overlap between the cerebrospinal fluid findings in bacterial and nonbacterial meningitis, and differentiating these entities at presentation is a significant clinical challenge.

Etiology Bacterial agents causing meningitis vary according to host age (Table 4–4). Infections in neonates are typically caused by translocation of vaginal flora

during birth. Post-neonatal meningitis is most frequently caused by encapsulated bacteria, specifically S pneumoniae and N meningitidis, although the introduction of a conjugate vaccine against the former has led to a decline in the incidence of S pneumoniae meningitis (in both pediatric and adult populations). Listeria monocytogenes is a food-borne pathogen that has a particular predilection for causing meningitis in elderly, immunocompromised, or pregnant patients. The microbiology of bacterial meningitis varies for postneurosurgery patients (S aureus, gram-negative bacilli, P aeruginosa), patients with ventricular shunts (S epidermidis, S aureus, gram-negative bacilli), and neutropenic patients (gram-negative bacilli, including P aeruginosa), with important implications for empiric therapy. Subacute or chronic meningitis may be caused by M tuberculosis, fungi (eg, Coccidioides immitis, Cryptococcus neoformans), and spirochetes such as Treponema pallidum (the bacterium causing syphilis) or Borrelia burgdorferi (the bacterium causing Lyme disease). The diagnosis of meningitis caused by these organisms may be delayed because many of these pathogens are difficult to culture and require special serologic or molecular diagnostic techniques.

TABLE 4–4 Proportion of cases of bacterial meningitis in the United States by host age, 2003–2007.

Pathogenesis The pathogenesis of bacterial meningitis involves a sequence of events in which virulent microorganisms overcome the host defense mechanisms (Table 4–5).

TABLE 4–5 Pathogenetic sequence of bacterial neurotropism.

Most cases of bacterial meningitis begin with bacterial colonization of the nasopharynx (Figure 4–7, panel A). An exception is Listeria, which enters the bloodstream through ingestion of contaminated food. Pathogenic bacteria such as S pneumoniae and N meningitidis secrete an IgA protease that inactivates host antibody and facilitates mucosal attachment. Many of the causal pathogens also possess surface characteristics that enhance mucosal colonization. N meningitidis binds to nonciliated epithelial cells by finger-like projections known as pili.

FIGURE 4–7 Pathogenic steps leading to pneumococcal meningitis. The pneumococcus adheres to and colonizes the nasopharynx. IgA1 protease protects the pneumococcus from host antibody (A). Once in the bloodstream, the bacterial capsule helps the pneumococcus to evade opsonization (B). The pneumococcus accesses the cerebrospinal fluid through receptors on the endothelial surface of the blood– brain barrier (C). (Redrawn, with permission, from Koedel U et al. Pathogenesis and pathophysiology of pneumococcal meningitis. Lancet Infect Dis. 2002;2:731.)

Once the mucosal barrier is breached, bacteria gain access to the bloodstream,

where they must overcome host defense mechanisms to survive and invade the central nervous system (CNS) (Figure 4–7, panel B). The bacterial capsule, a feature common to N meningitidis, H influenzae, and S pneumoniae, is the most important virulence factor in this regard. Host defenses counteract the protective effects of the pneumococcal polysaccharide capsule by activating the alternative complement pathway, resulting in C3b activation, opsonization, phagocytosis, and intravascular clearance of the organism. This defense mechanism is impaired in patients who have undergone splenectomy, and such patients are predisposed to the development of overwhelming bacteremia and meningitis with encapsulated bacteria. Activation of the complement system membrane attack complex is an essential host defense mechanism against invasive disease by N meningitidis, and patients with deficiencies of the late complement components (C5–9) are at increased risk for meningococcal meningitis.

The blood–brain barrier consists of endothelial cells joined by intercellular tight junctions that prevent access of intravascular organisms into the CNS. The exact mechanism of how pathogens transverse the blood–brain barrier is incompletely understood (Figure 4–7, panel C). Possible routes include (1) transport by infected leukocytes (the “Trojan Horse” hypothesis); and (2) opening of the intercellular junctions following cytokine release. Invasion of the spinal fluid by a meningeal pathogen results in increased permeability of the blood–brain barrier, where local host defense mechanisms are inadequate to control the infection. Normally, complement components are minimal or absent in the cerebrospinal fluid. Meningeal inflammation leads to increased, but still low, concentrations of complement, inadequate for opsonization, phagocytosis, and removal of encapsulated meningeal pathogens. Immunoglobulin concentrations are also low in the cerebrospinal fluid, with an average blood-to- cerebrospinal-fluid IgG ratio of 800:1.

The ability of meningeal pathogens to induce a marked subarachnoid space inflammatory response contributes to many of the pathophysiologic consequences of bacterial meningitis. Although the bacterial capsule is largely responsible for intravascular and cerebrospinal fluid survival, the subcapsular surface components (ie, the cell wall and lipopolysaccharide) of bacteria are more important determinants of meningeal inflammation. The major mediators of the inflammatory process are thought to be IL-1, IL-6, matrix metalloproteinases, and tumor necrosis factor (TNF). Within 1–3 hours after intracisternal inoculation of purified lipopolysaccharide in an animal model, there is a brisk release of TNF and IL-1 into the cerebrospinal fluid, preceding the development of inflammation. Indeed, direct inoculation of TNF and IL-1

into the cerebrospinal fluid produces an inflammatory cascade identical to that seen with experimental bacterial infection. In contrast, experimental injection of purified pneumococcal capsular polysaccharide proteins directly into the cerebrospinal fluid does not result in significant inflammation in animals.

Cytokine and proteolytic enzyme release leads to loss of membrane integrity, with resultant cellular swelling. The development of cerebral edema contributes to an increase in intracranial pressure, resulting in potentially life-threatening cerebral herniation (Figure 4–8). Vasogenic cerebral edema is principally caused by the increase in blood–brain barrier permeability. Cytotoxic cerebral edema results from swelling of the cellular elements of the brain because of toxic factors released by bacteria or neutrophils. Interstitial cerebral edema reflects obstruction in the flow of cerebrospinal fluid, as in hydrocephalus. Neuronal cell death, or apoptosis, is caused both by the immune inflammatory response and by direct toxicity of bacterial components, and clinically may be associated with cognitive impairment as a long-term sequela of meningitis. Cerebrovascular complications including infarction or hemorrhage are common and may be due to localized intravascular coagulation.

FIGURE 4–8 Pathophysiological alterations leading to neuronal injury during bacterial meningitis. (BBB, blood–brain barrier; CBV, cerebral blood volume.) (Redrawn, with permission, from Koedel U et al. Pathogenesis and pathophysiology of pneumococcal meningitis. Lancet Infect Dis. 2002;2:731.)

Understanding the pathophysiology of bacterial meningitis has therapeutic

implications. Although bactericidal antibiotic therapy is critical for adequate treatment, rapid bacterial killing releases inflammatory bacterial fragments, potentially exacerbating inflammation and abnormalities of the cerebral microvasculature. In animal models, antibiotic therapy has been shown to cause rapid bacteriolysis and release of bacterial endotoxin, resulting in increased cerebrospinal fluid inflammation and cerebral edema.

The importance of the immune response in triggering cerebral edema has led researchers to study the role of adjuvant anti-inflammatory medications for bacterial meningitis. Studies evaluating the role of corticosteroids have shown variable results. A meta-analysis has shown a slight decrease in the risk of sensorineural hearing loss and in mortality among adults with pneumococcal meningitis when glucocorticoids are given contemporaneously with the first dose of antibiotics. Most authorities recommend the use of adjuvant corticosteroids for patients in high-income settings with suspected bacterial meningitis. (The benefits of corticosteroids for patients from low-income areas is less clear.)

Clinical Manifestations Among patients who develop community-acquired bacterial meningitis, an antecedent upper respiratory tract infection is common. Patients with a history of head injury or neurosurgery, especially those with a persistent cerebrospinal fluid leak, are at particularly high risk for meningitis. Manifestations of meningitis in infants may be difficult to recognize and interpret; therefore, the physician must be alert to the possibility of meningitis in the evaluation of any febrile neonate.

Most patients with meningitis have a rapid onset of fever, headache, and neck stiffness or pain (meningismus). Nuchal rigidity is frequently noted on physical examination. Other clues seen in a variable proportion of cases include nausea or vomiting, photophobia, Kernig sign (resistance to passive extension of the flexed leg with the patient lying supine), and Brudzinski sign (involuntary flexion of the hip and knee when the examiner passively flexes the patient’s neck). More than half of patients with meningococcemia develop a characteristic petechial or purpuric rash, predominantly on the extremities, although onset of skin lesions may lag behind other symptoms.

Although a change in mental status (lethargy, confusion) is common in bacterial meningitis, up to one-third of patients present with normal mentation. Between 10% to 30% of patients have cranial nerve dysfunction, focal neurologic signs, or seizures. Coma, papilledema, and the Cushing triad (bradycardia, respiratory depression, and hypertension) are ominous signs of

impending herniation (brain displacement through the foramen magnum with brainstem compression), heralding imminent death.

Despite advances in treatment, the case fatality rate of meningitis remains high, and neurologic impairment is common among survivors. Morbidity and mortality may be decreased by rapid initiation of appropriate antibiotics. Any patient suspected of having meningitis requires prompt medical assessment and emergent lumbar puncture for Gram stain and culture of the cerebrospinal fluid, followed immediately by the administration of antibiotics (and consideration of corticosteroids if the cerebrospinal fluid results support bacterial meningitis).

CHECKPOINT

11. What is the typical presentation of bacterial meningitis? 12. What are the major etiologic agents of meningitis, and how do they vary

with age or other host characteristics? 13. What is the sequence of events in development of men-ingitis, and what

features of particular organisms pre-dispose to meningitis? 14. What are the diverse causes of cerebral edema in patients with

meningitis? 15. Why is rapid bacteriolysis theoretically dangerous in meningitis? 16. What are the associated clinical manifestations of untreated bacterial

meningitis?

PNEUMONIA

Clinical Presentation The respiratory tract is the most common site of infection by pathogenic microorganisms. Pneumonia accounts for >1 million hospitalizations each year in the United States, and >50,000 deaths. Pneumonia, together with influenza, is the leading cause of death from an infectious disease in the United States.

Diagnosis and management of pneumonia require knowledge of host risk factors, potential infectious agents, and environmental exposures. Pneumonia is an infection of the lung tissue caused by a number of different bacteria, viruses, and fungi, resulting in inflammation of the lung parenchyma and accumulation

of an inflammatory exudate in the airways. Infection typically begins in the alveoli, with secondary spread to the interstitium, resulting in consolidation and impaired gas exchange. Infection can also extend to the pleural space, causing pleuritis (inflammation of the pleura, characterized by pain on inspiration). The exudative inflammatory response of the pleura to pneumonia is termed a parapneumonic effusion; when bacterial infection is present in the pleura, this is termed empyema.

Etiology Despite technologic advances in diagnosis, no causative agent is identified in more than 50% of cases of community-acquired pneumonia. Even in cases in which a microbiologic diagnosis is made, there is usually a delay of several days before the pathogen can be identified and antibiotic susceptibility determined. Symptoms are nonspecific and do not reliably differentiate the various causes of pneumonia. Therefore, knowledge of the most common etiologic organisms is crucial in determining rational empiric antibiotic regimens. Bacterial causes of community-acquired pneumonia vary by comorbid disease and severity of pulmonary infection (Table 4–6).

TABLE 4–6 Common etiologic agents of community-acquired pneumonia as determined by severity of illness.

Pneumonia can be divided into two categories depending on where the infection was acquired: community-acquired or hospital-acquired (nosocomial).

Nosocomial pneumonia is defined as pneumonia that occurs 48 hours or more after admission to the hospital and that did not appear to be incubating at the time of admission. The distinction of where the infection was acquired is important because nosocomial pneumonia is more likely to be caused by multidrug-resistant bacteria such as methicillin-resistant S aureus (MRSA) or Pseudomonas. Knowledge of where and when the patient became ill with pneumonia can be a clue as to which microorganisms should be empirically treated while awaiting definitive culture results.

Traditionally, S pneumoniae has been the most common organism isolated in community-acquired pneumonia in both immunocompetent and immunocompromised individuals. However, viral pathogens such as influenza and respiratory syncytial virus (RSV) have increasingly been recognized as significant pathogens contributing to community-acquired pneumonia. Several additional organisms require special consideration in specific hosts or because of public health importance (Table 4–7). Understanding and identifying patient risk factors (eg, smoking, HIV infection) and host defense mechanisms (cough reflex, cell-mediated immunity) focus attention on the most likely etiologic agents, guide empiric therapy, and suggest possible interventions to decrease further risk. For example, patients who have suffered strokes and have impaired ability to protect their airways are at risk for aspirating oropharyngeal secretions. Precautions such as avoiding thin liquids in these patients may decrease the risk of future lung infections. Likewise, an HIV-infected patient with a low CD4 lymphocyte count is at risk for pneumocystis pneumonia and should be given prophylactic antibiotics.

TABLE 4–7 Common risk factors and causes of pneumonia in specific adult hosts.

Pathogenesis Pneumonia is disproportionally a disease of the elderly and impaired host; it occurs infrequently in immunocompetent individuals. This can be attributed to the effectiveness of host defenses, including anatomic barriers and cleansing mechanisms in the nasopharynx and upper airways and local humoral and cellular factors in the alveoli.

The lung is constantly exposed to a mixture of particulate matter and microbes that are present in inspired air. Pulmonary pathogens reach the lungs by one of three routes: (1) direct inhalation of infectious respiratory droplets into the lower airways; (2) aspiration of oropharyngeal contents; or (3) hematogenous spread. Acute pneumonia occurs when there is a defect in the host defenses, infection with a particularly virulent microorganism, or exposure to a large inoculum. The pulmonary antimicrobial defense mechanisms consist of innate and adaptive immunity (Figure 4–9). Incoming air with suspended particulate matter is subjected to turbulence in the nasal passages and then to abrupt changes in direction as the airstream is diverted through the pharynx and along the branches of the tracheobronchial tree. Particles larger than 10 mm are trapped in the nose or pharynx; those with diameters of 2–9 mm are deposited on the mucociliary blanket; only smaller particles reach the alveoli. M tuberculosis and Legionella pneumophila are examples of bacteria that are deposited directly in the lower airways through inhalation of small airborne particles.

FIGURE 4–9 Pulmonary defense mechanisms. Abrupt changes in direction of airflow in the nasal passages can trap potential pathogens. The epiglottis and cough reflex prevent introduction of particulate

matter in the lower airway. The ciliated respiratory epithelium propels the overlying mucous layer (right) upward toward the mouth. In the alveoli, cell-mediated immunity, humoral factors, and the inflammatory response defend against lower respiratory tract infections. (C, complement.) (Redrawn, with permission, from Storch GA. Respiratory system. In: Schaechter M et al, eds. Mechanisms of Microbial Disease, 4th ed. Lippincott Williams & Wilkins, 2007.)

The respiratory epithelium that lines the airways and alveoli is an important component of the host defense against respiratory pathogens. Epithelial cells are covered with beating cilia blanketed by a layer of mucus. Each cell has about 200 cilia that beat up to 500 times/min, moving the mucus layer upward toward the larynx. The mucus itself contains antimicrobial compounds such as lysozyme and lactoferrin.

Most bacteria are 0.5–2 μm in size and can reach the terminal bronchioles, alveolar ducts, and alveoli. Upon their arrival, they are opsonized by surfactant, complement, and antibodies. Macrophages recognize the pathogen via pattern recognition receptors such as Toll-like receptors, which leads to generation of inflammatory cytokines such as TNF-α and IL-1. This signaling cascade recruits neutrophils and, ultimately, antigen-specific T and B lymphocytes into the area. Lymphocytes produce antibody and have cytotoxic ability (eg, ability to kill virally infected cells) to further defend against ongoing infection. Another important role of the immune response is the activation of anti-inflammatory mediators, which ensures that the inflammatory response is held in check and that noninvolved areas of the lung are not injured. Impairment at any level of host defenses increases the risk of developing pneumonia. Alteration in the level of consciousness (eg, stroke, alcohol abuse) can compromise epiglottic closure leading to aspiration of bacteria that have colonized the oropharynx. Chronic cigarette smokers have decreased mucociliary clearance secondary to cilia damage and therefore compensate using the cough reflex to clear aspirated material, excess secretions, and foreign bodies. Children with cystic fibrosis have defective ciliary activity and are prone to develop recurrent sinopulmonary infections, particularly with S aureus and P aeruginosa. Patients with neutropenia, whether acquired or congenital, are also susceptible to lung infections with gram-negative bacteria and fungi. HIV-infected patients have depleted CD4 T lymphocyte counts and are predisposed to a variety of bacterial (including mycobacterial) and fungal infections. Respiratory viruses may destroy epithelium, disrupt normal ciliary activity, and cause abnormal neutrophil and macrophage function.

Clinical Manifestations Most patients with pneumonia have fever, cough (usually productive),

tachypnea, tachycardia, and an infiltrate on chest x-ray. Extrapulmonary manifestations that may provide clues to the etiologic agents include pharyngitis (Chlamydia pneumoniae), erythema nodosum rash (fungal and mycobacterial infections), and diarrhea (Legionella).

The following considerations aid in guiding empiric therapy for a patient who presents with symptoms consistent with pneumonia: (1) Is this pneumonia community acquired or hospital acquired? (2) Is this patient immunocompromised (eg, HIV infected, a transplant recipient)? (3) Is this patient an injection drug user? (4) Has this patient had a recent alteration in consciousness (suggestive of aspiration)? (5) Are the symptoms acute (days) or chronic (weeks to months)? (6) Has this patient lived in or traveled through geographic areas associated with specific endemic infections (eg, histoplasmosis, coccidioidomycosis)? (7) Has this patient had recent zoonotic exposures associated with pulmonary infections (eg, psittacosis, Q fever)? (8) Could this patient have a contagious infection of public health importance (eg, tuberculosis)? (9) Could this patient’s pulmonary infection be associated with a common source exposure (eg, Legionella or influenza outbreak)? (10) Does the illness necessitate hospitalization or intensive care admission (eg, pneumonia due to Legionella, S pneumonia, or S aureus)?

CHECKPOINT

17. What are the important pathogens for patients with community-acquired pneumonia based on severity of illness and site of care?

18. What host features influence the likelihood of particular causes of pneumonia?

19. What are the four mechanisms by which pathogens reach the lungs? 20. What are the defenses of the respiratory epithelium against infection?

INFECTIOUS DIARRHEA

Clinical Presentation Each year throughout the world, more than 5 million people—most of them children younger than 1 year—die of acute infectious diarrhea (see also Chapter

13). Although death is a rare outcome of infectious diarrhea in the United States, morbidity is substantial. It is estimated that there are more than 200 million episodes each year, resulting in 1.8 million hospitalizations at a cost of $6 billion per year. The morbidity and mortality attributable to diarrhea are largely due to loss of intravascular volume and electrolytes, with resultant cardiovascular failure. For example, adults with cholera can excrete more than 1 L of fluid per hour. Contrast this with the normal volume of fluid lost daily in the stools (150 mL), and it is clear why massive fluid losses associated with infectious diarrhea can lead to dehydration, cardiovascular collapse, and death.

GI tract infections can present with primarily upper tract symptoms (nausea, vomiting, crampy epigastric pain), small intestine symptoms (profuse watery diarrhea), or large intestine symptoms (tenesmus, fecal urgency, bloody diarrhea). Diarrhea with visible blood or mucus is termed invasive diarrhea (or dysentery). Sources of infection include person-to-person transmission (fecal– oral spread of Shigella), water-borne transmission (Cryptosporidium), food- borne transmission (Salmonella or S aureus food poisoning), and overgrowth after antibiotic administration (C difficile infection). Diarrhea can also be defined in terms of duration: acute (14 days or fewer), persistent diarrhea (14–30 days), or chronic (more than 30 days).

Etiology A wide range of viruses, bacteria, fungi, and protozoa can infect the GI tract. Most cases of acute infectious gastroenteritis are due to viral pathogens. However, the majority of cases of diarrhea are self-limited, and diagnostic evaluation is not performed. Patients presenting to medical attention are biased toward the subset with more severe symptoms (eg, high fevers, hypotension, bloody diarrhea), immunocompromise (eg, HIV, neutropenia), or prolonged duration. An exception is large outbreaks of food-borne illness, in which epidemiologic investigations may detect patients with milder variants of disease.

Pathogenesis A comprehensive approach to GI tract infections starts with the classic host– agent–environment interaction model. A number of host factors influence GI tract infections. Patients at extremes of age and with comorbid conditions (eg, HIV infection) are at higher risk for symptomatic infection. Medications that alter the GI microenvironment or destroy normal bacterial flora (eg, antacids, antibiotics) also predispose patients to infection. Microbial agents responsible

for GI illness can be categorized according to type of organism (bacterial, viral, protozoal), propensity to attach to different anatomic sites (stomach, small bowel, colon), and pathogenesis (enterotoxigenic, cytotoxigenic, enteroinvasive). Environmental factors can be divided into three broad categories based on mode of transmission: (1) water borne; (2) food borne; and (3) person to person. Table 4–8 summarizes these relationships and provides a framework for assessing the pathogenesis of GI tract infections.

TABLE 4–8 Approach to gastrointestinal tract infections.

The spectrum of diarrheal infections is typified by the diverse clinical manifestations and mechanisms through which E coli can cause diarrhea. Colonization of the human GI tract by E coli is universal, typically occurring within hours after birth. However, when the host organism is exposed to pathogenic strains of E coli not normally present in the bowel flora, localized GI disease or even systemic illness may occur. There are five major classes of diarrheogenic E coli (and several newer proposed subgroups): enterotoxigenic (ETEC), enteropathogenic (EPEC), enterohemorrhagic (EHEC), enteroaggregative (EAEC), and enteroinvasive (EIEC) (Table 4–9). Features common to all pathogenic E coli are evasion of host defenses, colonization of intestinal mucosa, and multiplication with host cell injury. This organism, like all GI pathogens, must survive transit through the acidic gastric environment and be able to persist in the GI tract despite the mechanical force of peristalsis and competition for scarce nutrients from existing bacterial flora. Adherence can be

nonspecific (at any part of the intestinal tract) or, more commonly, specific, with attachment occurring at well-defined anatomic areas.

TABLE 4–9 Escherichia coli in diarrheal disease.

The term “gastroenteritis” classically denotes infection of the stomach and proximal small bowel. Symptoms and signs include a combination of diarrhea, vomiting, and abdominal pain. The most common cause is viruses, such as rotavirus and norovirus. Decreasing rates of rotavirus-associated gastroenteritis have been observed among adults due to the rotavirus vaccine being used in children. Other causative organisms include Bacillus cereus, S aureus, and Campylobacter. These bacterial organisms, as well as norovirus, are major causes of food-borne-related gastroenteritis. B cereus and S aureus both produce preformed toxins that, even in the absence of viable bacteria, are capable of causing food-borne disease, often within a few hours of the offending meal. Food poisoning by B cereus causes vomiting and diarrhea, whereas that caused by S aureus is usually associated with vomiting and abdominal cramps. With both, symptoms typically resolve within about 24 hours.

Infectious diarrhea involving the small and large bowel is clinically differentiated into secretory, inflammatory, and hemorrhagic types, with different pathophysiologic mechanisms accounting for these diverse presentations. Secretory (watery) diarrhea is caused by a number of bacteria (eg, Vibrio cholerae, ETEC, EAEC), viruses (eg, rotavirus, norovirus), and protozoa (eg, Giardia, Cryptosporidium). These organisms attach superficially to enterocytes in the lumen of the small bowel. Stool examination is notable for the absence of fecal leukocytes, although in rare cases there is occult blood in the stools. Some of these pathogens elaborate enterotoxins, proteins that increase intestinal cyclic adenosine monophosphate (cAMP) production, leading to net fluid secretion. The classic example is cholera. The bacterium V cholerae attaches to the

epithelium of the small intestine and produces cholera toxin, which causes prolonged activation of epithelial adenylyl cyclase and increased intracellular cyclic AMP. The high cAMP levels cause a dramatic efflux of water and secretion of sodium, chloride, and potassium and bicarbonate ions by intestinal villi, leading to profuse watery diarrhea (Figure 4–10). Clinically, the patient presents with copious diarrhea (“rice-water stools”), progressing to dehydration and vascular collapse, sometimes within 12 hours, unless given vigorous volume resuscitation. ETEC, a common cause of acute diarrheal illness in young children and the most common cause of diarrhea in travelers returning to the United States from developing countries, produces two enterotoxins. The heat- labile toxin (LT) activates adenylyl cyclase, whereas the heat-stable toxin (ST) activates guanylyl cyclase activity. Activation of both enzymes leads to secretion of massive amounts of fluid and electrolytes into the intestinal lumen, similar to the action of the cholera toxin.

FIGURE 4–10 Pathogenesis of Vibrio cholerae and enterotoxigenic E coli (ETEC) in diarrheal disease. V cholerae and ETEC share similar pathogenetic mechanisms in causing diarrheal illness. The bacteria gain entry to the small intestinal lumen through ingestion of contaminated food (left). They elaborate an enterotoxin that is composed of one A subunit and five B subunits. The B subunits bind to the intestinal cell membrane and facilitate entry of part of the A subunit (right). Subsequently, this results in a prolonged activation of adenylyl cyclase and the formation of cyclic adenosine monophosphate (cAMP), which stimulates water and electrolyte secretion by intestinal endothelial cells. (Redrawn, with permission, from Vaughan M. Cholera and cell regulation. Hosp Pract. 1982;17(6):145–52.)

Inflammatory diarrhea is a result of bacterial invasion of the mucosal lumen, with resultant cell death. Patients with this syndrome are usually febrile,

with complaints of crampy lower abdominal pain as well as diarrhea, which may contain visible mucus. The term dysentery is used when significant numbers of fecal leukocytes and gross blood are present. Pathogens associated with inflammatory diarrhea include EIEC, Shigella, Salmonella, Campylobacter, and Entamoeba histolytica. Shigella, the prototypical cause of bacillary dysentery, is transmitted via direct person–person contact or from contaminated food or water. Shigella invades the colonic enterocyte through formation of an endoplasmic vacuole, which is lysed intracellularly. Bacteria then proliferate in the cytoplasm and invade adjacent epithelial cells causing cell death. Production of a cytotoxin, the Shiga toxin, leads to local cell destruction and death. EIEC resembles Shigella both clinically and with respect to the mechanism of invasion of the enterocyte wall through a similar toxin, termed Shigella-like enterotoxin.

Hemorrhagic diarrhea, a variant of inflammatory diarrhea, is primarily caused by EHEC. Infection with E coli O157:H7 has been associated with a number of deaths from the hemolytic-uremic syndrome, with several well- publicized outbreaks related to contaminated foods. EHEC causes a broad spectrum of clinical disease, with manifestations including (1) asymptomatic infection; (2) watery (nonbloody) diarrhea; (3) hemorrhagic colitis (bloody, inflammatory diarrhea); and (4) hemolytic-uremic syndrome (an acute illness, primarily of children, characterized by anemia and renal failure). EHEC does not invade enterocytes; however, it does produce two Shiga-like toxins (Stx1 and Stx2) that closely resemble the Shiga toxin in structure and function. After binding of EHEC to the cell surface receptor, the A subunit of the Shiga toxin catalyzes the destructive cleavage of ribosomal RNA and halts protein synthesis, leading to cell death. Shiga toxin can enter the systemic circulation and is phagocytosed by neutrophils, which are in turn endocytosed by target endothelial cells. This leads to vascular damage and in some patients, a prothrombotic state that precedes the hemolytic-uremic syndrome.

Clinical Manifestations Clinical manifestations of GI tract infections vary depending on the site of involvement (see Table 4–9). For instance, in staphylococcal food poisoning, symptoms develop several hours after ingesting food contaminated with toxin- producing S aureus. The symptoms of staphylococcal food poisoning are profuse vomiting, nausea, and abdominal cramps. Diarrhea is variably present with agents causing gastroenteritis. Profuse watery (noninflammatory, nonbloody) diarrhea is associated with bacteria that have infected the small intestine and elaborated an enterotoxin (eg, Clostridium perfringens, V cholerae). In contrast,

colitis-like symptoms (lower abdominal pain, tenesmus, fecal urgency) and inflammatory or bloody diarrhea occur with bacteria that more commonly infect the large intestine. The incubation period is generally longer (>3 days) for bacteria that localize to the large intestine, and colonic mucosal invasion can occur, causing fever, bacteremia, and systemic symptoms.

CHECKPOINT

21. How many individuals in the world die yearly of infec-tious diarrhea? 22. What are different modes of spread of infectious diar-rhea? Give an

example of each. 23. What are the different mechanisms by which infectious organisms cause

diarrhea?

SEPSIS & SEPTIC SHOCK

Clinical Presentation Sepsis is a clinical syndrome characterized by a dysregulated inflammatory response to infection. It is a leading cause of morbidity and mortality in the United States, with an estimated incidence rate of 300 cases per 100,000 population and a case fatality rate of 20–50%. The medical costs of sepsis in the United States exceed $17 billion annually. Rates of sepsis continue to rise secondary to medical advances such as the widespread use of indwelling intravascular catheters, increased implantation of prosthetic material (eg, cardiac valves and artificial joints), and administration of immunosuppressive drugs and chemotherapeutic agents. These interventions serve to increase the risk of infection and subsequent sepsis.

Sepsis is a continuum of conditions, from infection and bacteremia to sepsis and septic shock. The study of sepsis has been facilitated by the establishment of a standardized case definition. Patients with early sepsis have infection and bacteremia and are at risk for progressing to sepsis and septic shock. Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated immune response to infection. Critical care experts have developed a severity score, called the Sequential (Sepsis-related) Organ Failure Assessment (SOFA) score,

to predict which patients are at highest risk of dying of sepsis. The score is calculated 24 hours after admission to the ICU and every 48 hours thereafter. Septic shock includes patients who, despite fluid resuscitation, require vasopressor support and exhibit signs of organ dysfunction (SOFA score ≥2). Patients with septic shock have a predicted mortality of 40%. Another commonly used term is systemic inflammatory response syndrome (SIRS). SIRS is a clinical syndrome that is a form of dysregulated inflammation and may be seen with infection as well as with noninfectious states such as pancreatitis, pulmonary embolism, and myocardial infarction. Leukopenia and hypothermia, included in the SIRS case definition, are predictors of a poor prognosis when associated with sepsis.

Etiology Although evidence of infection is a diagnostic criterion for sepsis, only 28% of patients with sepsis have bacteremia, and slightly more than 10% will have primary bacteremia, defined as positive blood cultures without an obvious source of bacterial seeding. Common sites of infection among patients with sepsis syndrome (in decreasing order of frequency) include the respiratory tract, the genitourinary tract, abdominal sources (eg, gall bladder, colon), device- related infections, and wound or soft tissue infections.

The bacteriology of sepsis has evolved in the last decade. Gram-negative bacteria (Enterobacteriaceae and Pseudomonas), previously the most common cause of sepsis, have been supplanted by gram-positive organisms, which now cause more than 50% of cases. Staphylococci are the most common bacteria cultured from the bloodstream, presumably because of an increase in the prevalence of chronic indwelling venous access devices and implanted prosthetic material. The incidence of fungal sepsis has risen dramatically owing to an increase in immunosuppressed and neutropenic patients. Sepsis associated with P aeruginosa, Candida, or mixed (polymicrobial) organisms is an independent predictor of mortality.

Pathogenesis The normal host response to infection is recognition via pathogen recognition receptors (eg, Toll-like receptors) and immune cell migration, primarily by neutrophils, to the site of infection, where they release inflammatory mediators. The process is highly regulated, with anti-inflammatory cytokines such as IL-10 suppressing release of inflammatory cytokines (eg, TNF-α). Sepsis occurs when

the inflammatory response overwhelms the local environment and becomes systemic. The cause of the systemic reaction is likely multifactorial and may include the effect of the microorganism, the release of large quantities of pro- inflammatory mediators, and even the genetic susceptibility of the individual. The different stages of sepsis (early sepsis to septic shock) represent a continuum, with patients often progressing from one stage to the next within days or even hours after admission.

Sepsis generally starts with a localized infection. Bacteria may then invade the bloodstream directly (leading to bacteremia and positive blood cultures) or may proliferate locally and release toxins into the bloodstream. These toxins can arise from a structural component of the bacteria (eg, endotoxin) or may be exotoxins, which are proteins synthesized and released by the bacteria. Endotoxin is defined as the lipopolysaccharide (LPS) moiety contained in the outer membrane of gram-negative bacteria. Endotoxin is composed of an outer polysaccharide chain (the O side chain), which varies between species and is not toxic, and a highly conserved lipid portion (lipid A), which is embedded in the outer bacterial membrane. Injection of either purified endotoxin or lipid A is highly toxic in animal models, causing a syndrome analogous to septic shock in the absence of viable bacteria.

Sepsis was initially considered largely to be a result of overstimulation of the host inflammatory response and uncontrolled release of inflammatory mediators. The failure of a number of pharmacologic interventions aimed at blocking endotoxin or the resultant inflammatory cascade suggests that other factors, such as host immunosuppression, play a critical role. Specific stimuli such as organism, inoculum, and site of infection stimulate CD4 T cells to secrete cytokines with either inflammatory (type 1 helper T-cell) or anti-inflammatory (type 2 helper T-cell) properties (Figure 4–11). Among patients who die of sepsis, there is significant loss of cells essential for the adaptive immune response (B lymphocytes, CD4 T cells, dendritic cells). Genetically programmed cell death, termed apoptosis, is thought to play a key role in the decrease in these cell lines and downregulates the surviving immune cells. The clinical consequences of sepsis include hemodynamic changes (tachycardia, tachypnea), inappropriate vasodilation, and poor tissue perfusion, with resultant organ dysfunction (see Figure 4–11).

FIGURE 4–11 Pathogenic sequence of the events in septic shock. Activation of macrophages by endotoxin and other proteins leads to release of inflammatory mediators and immune modulation resulting in host tissue damage and, in some cases, death. (Redrawn and modified, with permission, from Horn DL et al. What are the microbial components implicated in the pathogenesis of sepsis? Clin Infect Dis. 2000;31:852.)

A. Hemodynamic Alterations

All forms of shock result in inadequate tissue perfusion and subsequent cell dysfunction and death (see Chapter 11). In noninfectious forms (such as cardiogenic shock and hypovolemic shock), systemic vascular resistance is elevated as a compensatory mechanism to maintain blood pressure. In the hypoperfused tissues, there is enhanced extraction of oxygen from circulating red blood cells, leading to decreased pulmonary artery oxygenation. In contrast, vasodilation leads to hypotension and hypoperfusion in septic shock. This is an unintended consequence of vasoactive mediators such as nitric oxide, which is released from endothelial cells in response to bacterial endotoxin. Hypovolemia results from inappropriate arterial and venous dilation and leakage of plasma into the extravascular space as a result of increased endothelial permeability. Even with fluid resuscitation, systemic vascular resistance remains low despite a compensatory increase in cardiac output. Inefficient oxygen extraction and tissue hypoperfusion result in an increased pulmonary artery oxygen content.

A hyperdynamic circulatory state, described as distributive shock to emphasize the maldistribution of blood flow to various tissues, is the common hemodynamic finding in sepsis. Distributive shock is characterized by a loss of normal mechanisms of vascular autoregulation, producing imbalances in blood flow with regional shunting and relative hypoperfusion of some organs. Animal studies have documented predictable changes in organ blood flow, with a marked reduction in blood flow to the stomach, duodenum, small bowel, and pancreas; a moderate reduction in blood flow to the myocardium and the skeletal muscles; and relative preservation of perfusion to the kidneys and CNS.

Myocardial depression is a common finding in early septic shock. Initially, patients have low cardiac filling pressures and low cardiac output secondary to volume depletion resulting from vasodilation. After fluid replacement, cardiac output is normal or increased, but ventricular function is abnormal. From 24 to 48 hours after the onset of sepsis, left and right ventricular ejection fractions are reduced, and end-diastolic and end-systolic volumes are increased. This myocardial depression has been attributed to direct toxic effects of nitric oxide, TNF-α, and IL-1. Reduced ejection fraction and consequent myocardial depression are reversible in patients who survive the initial period of septic shock.

B. Vascular and Multiorgan Dysfunction

Most patients who die of septic shock have either refractory hypotension or

multiple-organ failure. Refractory hypotension can occur from two mechanisms. First, some patients cannot sustain high cardiac output in response to the septic state and develop progressive high-output cardiac failure. Second, circulatory failure may be associated with severe vasodilation and hypotension refractory to intravenous fluid resuscitation and vasopressor therapy.

The development of multiple-organ failure represents the terminal phase of a hypermetabolic process that begins during the initial stages of shock. Organ failure results from microvascular injury induced by local and systemic inflammatory responses to infection. Maldistribution of blood flow is accentuated by impaired erythrocyte deformability, with microvascular obstruction. Sepsis is associated with a decrease in the number of functional capillaries, which causes an inability to extract oxygen maximally. Aggregation of neutrophils and platelets may also reduce blood flow. Demargination of neutrophils from vascular endothelium results in a further release of inflammatory mediators and subsequent migration of neutrophils into tissues. Components of the complement system are activated, attracting more neutrophils and releasing locally active substances such as prostaglandins and leukotrienes. The net result of all of these changes is microvascular collapse and, ultimately, organ failure.

The outcome of sepsis depends on the number of organs that fail: Mortality among patients with multiorgan failure (three or more organ systems) averages 70%. Respiratory failure develops in 18% of patients with sepsis. At the most severe end of the spectrum is acute respiratory distress syndrome (ARDS), characterized by endothelial injury in the pulmonary microvasculature. The injury disturbs capillary blood flow and increases endothelial permeability leading to noncardiogenic pulmonary edema, decreased lung compliance, and refractory hypoxia. Renal failure, seen in 15% of cases, is usually a multifactorial process, with additive injury from intra-renal shunting, renal hypoperfusion, and administration of nephrotoxic agents (antibiotics and radiologic imaging dye). Other organs affected by sepsis include the CNS (altered mentation, coma) and the blood (disseminated intravascular coagulation).

Clinical Manifestations The clinical manifestations of sepsis include those related to the systemic response to infections (tachycardia, tachypnea, alterations in temperature and leukocyte count) and those related to specific organ system dysfunction (cardiovascular, respiratory, renal, hepatic, and hematologic abnormalities).

Sepsis sometimes begins with very subtle clues that can be easily confused with more common and less serious illnesses. Awareness of these early signs of sepsis can lead to early recognition and intervention. Clinical guidelines emphasize the use of a systematic approach to the recognition and early treatment of sepsis. Initial responses should include obtaining cultures of blood and other body fluids, empiric administration of broad-spectrum antibiotics, determination of serum lactate as a marker of hypoperfusion, and use of intravenous fluid and vasopressor therapy for patients with sustained hypotension.

CHECKPOINT

24. What is the mortality rate of sepsis and septic shock in the United States?

25. What factors contribute to hospital-related sepsis? 26. Which organisms are most commonly associated with sepsis? 27. What is the role of the host immune system in the pathogenesis of

sepsis? 28. What activates the immune response? 29. What are some distinctive hemodynamic features of septic shock versus

noninfectious shock syndromes?

CASE STUDIES Yeong Kwok, MD

(See Chapter 25, p. 746–49 for Answers)

CASE 13

A 55-year-old man who recently emigrated from China presents to the emergency department with fever. He states that he has had recurring fevers over the past 3 weeks, associated with chills, night sweats, and malaise. Today he developed new painful lesions on the pads of his fingers,

prompting him to come to the emergency department. His medical history is remarkable for “being very sick as a child after a sore throat.” He has recently had several teeth extracted for severe dental caries. He is taking no medications. On physical examination, he has a fever of 38.5°C, blood pressure of 120/80 mm Hg, heart rate of 108 bpm, and respiratory rate of 16/min, with an oxygen saturation of 97% on room air. Skin examination is remarkable for painful nodules on the pads of several fingers and toes. He has multiple splinter hemorrhages in the nail beds and painless hemorrhagic macules on the palms of the hands. Ophthalmoscopic examination is remarkable for retinal hemorrhages. Chest examination is clear to auscultation and percussion. Cardiac examination is notable for a grade 3/6 holosystolic murmur heard loudest at the left lower sternal border, with radiation to the axilla. Abdominal and back examinations are unremarkable.

Questions

A. What is the likely diagnosis? What are some common predisposing factors to this disease? Which is most likely in this patient?

B. Which infectious agents are most likely to be involved? C. What is the name given to the various lesions found on this man’s

hands and feet? What is the pathogenetic mechanism responsible for their formation?

D. What are some other common clinical manifestations of this disease? What are the most common causes of death in this disease? What factors are predictive of a fatal outcome?

E. Why does treatment of endocarditis require prolonged courses of antibiotics to succeed?

CASE 14

A 25-year-old man presents to the emergency department with fever and in a confused, irrational state. He is accompanied by his wife, who provides the history. She states that he had been well until approximately 1 week ago, when he developed symptoms of upper respiratory tract infection that were slow to improve. On the morning of admission, he complained of progressive severe headache and nausea. He vomited once. He became

progressively lethargic as the day progressed, and she brought him to the hospital. He has no other medical problems and takes no medications.

On examination, he is febrile to 39°C, with a blood pressure of 95/60 mm Hg, heart rate of 100 bpm, and respiratory rate of 18/min. He is lethargic and confused, lying with his hand over his eyes. Funduscopic examination shows no papilledema. The neck is stiff, with a positive Brudzinski sign. Heart, lung, and abdominal examinations are unremarkable. Neurologic examination is limited by the patient’s inability to cooperate but appears to be nonfocal. The Kernig sign (resistance to passive extension of the flexed leg with the patient lying supine) is negative.

Questions

A. What infectious diagnosis is suggested? What are the most likely etiologic agents in this patient? What would they be if he were a newborn? If he were a child?

B. What is the pathophysiologic sequence of events in the development of this disease? What features of the pathogens involved facilitate their ability to produce this disease?

C. What are the possible causes of cerebral edema in this patient? D. What tests should be performed to confirm the diagnosis? What kinds

of treatments should be started or considered? Why?

CASE 15

A 68-year-old man presents to the hospital emergency department with acute fever and persistent cough. He has had cough productive of green sputum for 3 days, associated with shortness of breath, left-sided pleuritic chest pain, fever, chills, and night sweats. His medical history is notable for chronic obstructive pulmonary disease (COPD), requiring intermittent oral glucocorticoid use. His medications include albuterol, ipratropium bromide, and corticosteroid inhalers. The patient lives at home and is active. On examination, he is febrile to 38°C, with a blood pressure of 110/50 mm Hg, heart rate of 98 bpm, and respiratory rate of 20/min. Oxygen saturation is 92% on room air. He is a thin man in moderate respiratory distress,

speaking in sentences of three or four words. Lung examination is notable for rales in the left lung base and left axilla and diffuse expiratory wheezes. The remainder of the examination is unremarkable. Chest x-ray film reveals left lower lobe and lingular infiltrates. A diagnosis of pneumonia is made, and the patient is admitted to the hospital for administration of intravenous antibiotics.

Questions

A. On the basis of this patient’s underlying condition and severity of illness, what are the likely pathogens involved in this case? How would your differential change if he required ICU admission?

B. What are the mechanisms by which pathogens reach the lungs? C. What are the normal host defenses against pneumonia? D. What are some common host risk factors for pneumonia? What are the

pathogenetic mechanisms by which they increase the risk of pneumonia? Which of these risk factors are present in this patient?

CASE 16

A 21-year-old woman presents with a complaint of diarrhea. She returned from Mexico the day before her visit. The day before that, she had an acute onset of profuse watery diarrhea. She denies blood or mucus in the stools. She has had no associated fever, chills, nausea, or vomiting. She has no other medical problems and is taking no medications. Examination is remarkable for diffuse, mild abdominal tenderness to palpation without guarding or rebound tenderness. Stool is guaiac negative. Infectious diarrhea is suspected.

Questions

A. What are the different modes of spread of infectious diarrhea? Give an example of each.

B. What is the likely anatomic site of infection in this case? Why? C. What is the most likely pathogen in this case? What is the pathogenetic

mechanism by which it causes diarrhea?

CASE 17

A 65-year-old woman is admitted to the hospital with community-acquired pneumonia. She is treated with intravenous antibiotics and is given oxygen by nasal cannula. A Foley catheter is placed in her bladder. On the third hospital day she is switched to oral antibiotics in anticipation of discharge. On the evening of hospital day 3, she develops fever and tachycardia. Blood and urine cultures are ordered. The following morning, she is lethargic and difficult to arouse. Her temperature is 35°C, blood pressure is 85/40 mm Hg, heart rate is 110 bpm, and respiratory rate is 20/min. Oxygen saturation is 94% on room air. Head and neck examinations are unremarkable. Lung examination is unchanged from admission, with rales in the left base. Cardiac examination is notable for a rapid but regular rhythm, without murmurs, gallops, or rubs. Abdominal examination is normal. Extremities are warm. Neurologic examination is nonfocal. The patient is transferred to the ICU for management of presumed sepsis and given intravenous fluids and antibiotics. Blood and urine cultures are positive for gram-negative rods.

Questions

A. What factors contribute to hospital-related sepsis? B. By what mechanism do gram-negative rods result in sepsis? What role

does the immune response play in the pathogenesis of sepsis? C. Describe the hemodynamic changes that result in septic shock. D. By what mechanisms does sepsis result in multiorgan failure? E. What factors predict a poor outcome in patients with sepsis?

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CHAPTER

5

Neoplasia Mark M. Moasser, MD, & Weiyun Z. Ai, PhD, MD

Cell growth and maturation are normal events in organ development during embryogenesis, growth, and tissue repair, and in remodeling after injury. Disordered regulation of these processes can result in loss of control over cell growth, differentiation, and spatial confinement. Human neoplasia collectively represents a spectrum of diseases characterized by abnormal cell growth, loss of tissue homeostasis, and distorted architecture. Such new growth is called a neoplasm or tumor and can sometimes be a proliferative process confined to one specific tissue site with little systemic manifestations and no threat to the overall state of health. The term “benign” is often used to describe such low- impact tumors, which include many common growths such as dermal nevi, warts, and uterine fibroids. The term “cancer” or “malignant tumor” is used to describe a more advanced form of neoplasia that involves tissue invasion and destruction and defines an inherently progressive biologic process that can culminate in systemic disease and host death. The process of tumorigenesis involves a series of stochastic events in a proliferative context that can generate unlimited diversity in the molecular and phenotypic attributes of tumor cells, both among affected individuals and within a single affected individual. Classification schemes have been devised to provide a framework to reduce this complexity and capture many of the shared attributes of cancers, which are largely based on tissue type and organ of origin. Each type of cancer can exhibit a diversity of biologic behaviors among different patients, and molecular or histologic attributes are used to further sub-classify cancers and identify patterns of behavior.

The recognition of overt malignancy by symptoms or physical examination

findings defines the clinical phase of disease. The clinical phase is preceded by a preclinical phase, which is usually unknown to the patient but may sometimes be identified by screening interventions. Preclinical signs and potential precursors of colon cancer and breast cancer may consist of polyps in the colon and proliferative abnormalities of the breast, respectively. Such precursor lesions usually harbor molecular genetic abnormalities and exhibit features of abnormal cell proliferation without demonstrating invasiveness and may precede the development of an invasive cancer by months to years; or they may not progress to an invasive cancer within the individual’s lifetime. More commonly, the preclinical phase goes undetected until an invasive cancer is present, occasionally with regional or distant metastases. Our understanding of the pathophysiology of various neoplasma is based on clinical and pathologic observations of large series of patients, along with a more recently gained understanding of the cellular and molecular underpinnings of these disorders.

THE CELLULAR & MOLECULAR BASIS OF NEOPLASIA

Neoplasia is a result of stepwise alterations in cellular function. These phenotypic alterations produce morphologic changes that are readily evident by microscopy and may predate tumor development by many years. Morphologic abnormalities may include enlargement of the cell, called cell hypertrophy, reflecting too much protein and membrane synthesis. It may include crowding owing to too much cell division, called cell hyperplasia. It may include cell dysplasia, which reflects a reversion to a more immature cell without a committed identity. It may include metaplasia, which reflects abnormal cell reprogramming to appear and function like a cell of a different type. Such changes in cell behavior and function are a result of molecular abnormalities involving cell signaling that can arise owing to defects in the execution of instructions encoded in the cellular genome. Of these various types of abnormalities, the proliferative abnormalities are the primary forces underlying the progressive nature of many forms of neoplasia that may ultimately lead to uncontrolled growth, invasion, and metastasis, which are the hallmarks of cancer.

Underlying the pathophysiology of neoplasia are changes in the cellular genome that underlie all cellular and biochemical aberrations responsible for the malignant phenotype. While the process of DNA replication is designed to

ensure that all cells of the human body maintain an exact copy of the human genome, this process is ultimately imperfect. With the passing of time, and with stochastic occurrence of errors in DNA replication, accelerated by exposure to environmental carcinogens, cells in various tissues of the body acquire mutations in their genomes. The number of such mutations in various tissues increases with age. Some mutations have no phenotypic consequences for the cells, others may induce changes that lead to cell death, others may induce cellular dysplasia or metaplasia, and yet others may induce changes that lead to increased proliferation. The proliferative phenotype passes the mutated genome to more cells, beginning a self-perpetuating process. Cells that have acquired a growth advantage owing to DNA mutation may repopulate the tissue over time, and additional DNA mutations may select for yet another cell population within this population with even higher growth rates. Repeated cycles of clonal and subclonal selection, called clonal evolution, may eventually lead to a cellular clone with a growth pattern that defies the normal tissue architecture and appears as an outgrowth or new growth in microscopic sections, typically called a neoplasm or tumor (Figure 5–1). Therefore, this process of new growth, called neoplasia, is actually preceded by a years-long process of molecular evolution at the cellular DNA level, which is not apparent by microscopy. A microscopically normal-appearing tissue that bears a high proportion of cells with genetic mutations is sometimes referred to as having a genetic field defect and is considered to have a high risk for the future development of cancer. With increasing age, biopsies of many normal tissues in healthy humans confirm the presence of increasing mutations in cellular DNA. Because of this, it is generally accurate to consider adult cancer as a disease of aging. The pathophysiology of childhood cancer is not as well understood and may be different from that of adult cancer. Childhood cancer may be the endpoint of a series of molecular genetic changes that began very early with errors during the highly proliferative process of development that begins in utero.

FIGURE 5–1 Schematic depiction of the clonal evolution of cancer. Mutations occur stochastically in the tissue. While many are inconsequential, some can contribute to a proliferative phenotype that can expand to form a subclone. Additional mutations within this subclone can confer higher proliferative potential, and sequential cycles of such growth-promoting mutations can eventually repopulate the tissue, creating a genetic field defect in a normal-appearing tissue. Ultimately, mutations that promote continued growth beyond tissue boundaries can lead to the formation of a tumor with morphological and clinical consequences.

In addition to genetic changes involving the nucleic acid sequence of the cellular genome, epigenetic changes are also commonly seen in neoplasia and contribute to the malignant phenotype. Epigenetic phenomena involve chemical modifications of the DNA/protein complex without changes in the genetic sequence. An increasing number of genetic, epigenetic, and proteomic changes are being cataloged from the study of cancer cells, both in vivo, from primary tumors of patients, and in vitro, from established cancer cell lines grown in tissue culture. Some of these changes are specific to a certain tumor type, whereas others are seen more generally across different tumor types. In certain types of cancer, a particular genetic alteration is etiologically linked with, and is pathognomonic of, that cancer type. It may play a significant role as a molecular marker of that disease and as a target for drug development. However, most types of cancers do not have unifying molecular characteristics. Although many of the most common types of cancer are categorized by their primary organ site, such as breast or prostate, an organ-based classification system belies the heterogeneous nature of cancers; thus, in actuality, what is currently called “breast cancer” is in fact a constellation of many diseases with diverse molecular and phenotypic characteristics. Technological advances in high-throughput analysis of the entire cellular genome and of the total cellular gene transcription profiles have allowed the characterization of tumors by in-depth analyses of their

genetic (DNA) and transcriptomic (RNA) features (the transcriptome being the sum of all of the RNA transcripts in the cancer). Such molecular signatures provide important predictive and prognostic clinical information, superior to that provided by simple histologic characterization. The technologies underlying the molecular analysis of tumors are rapidly evolving, both in the depth and scale of the analysis and in their cost and speed. Tens of thousands of different types of cancer have undergone in-depth genomic and transcriptomic analyses, and the data output is expanding exponentially, creating considerable challenges in mass data deposition, organization, sharing, and mining. Reconciling the mass data emerging from the analyses of human cancers is one of the principal challenges in what is being called the Era of Big Data. Numerous repositories have been developed with portals constructed to enable academic and commercial entities to mine these massive data sets. Some of the common databases are listed in Table 5–1. Although molecular profiling of cancers has already led to new classification systems for several types of cancer, these may soon become redundant in favor of a complete individualized molecular tumor characterization for every single patient’s tumor in real time. Many academic institutions and commercial entities are beginning to develop the capabilities to offer such individualized tumor characterization in real time.

TABLE 5–1 Examples of repositories of mass cancer mutation data

Although the progressive phenotypic characteristics of neoplasia result predominantly through sequential molecular alterations and abnormal function of the proliferating tumor cells, it is now clear that at some level abnormal

function of the host stromal cells is fundamentally involved in continued tumor progression. These cells include connective tissue cells, hematopoietic and bone marrow compartment cells, immune system cells, and adipose tissue cells. Also involved are various secreted juxtacrine and paracrine growth factors, matrix proteins, and other soluble factors in the tumor microenvironment. Stromal cell abnormalities can be nonproliferative, such as secretion of requisite growth factors, or proliferative, such as expansion of the blood vessel network to support the growth of enlarging tumors or expansion of the extracellular matrix compartment.

CHECKPOINT

1. What is the preclinical phase of cancer? 2. What causes mutations in the genome? 3. What is a genetic field defect? 4. What is the source of the mass data from human cancers?

GENETIC AND EPIGENETIC CHANGES IN NEOPLASIA

Maintaining genomic integrity is a fundamental cellular task. A complex cellular apparatus serves to recognize DNA damage or errors in DNA replication and to correct them, to activate checkpoints to halt further cell replication while corrective measures take place, or to signal suicidal cell death (apoptosis) if repairs cannot be made. These mechanisms, while extremely competent, are ultimately imperfect, and uncorrected errors lead to the acquisition and accumulation of mutations in the genomic DNA in many cells throughout the human body. The accumulation of such mutations is subject to many influences, including inheritable genetic factors, environmental factors, and stochastic factors. Inherited defects in the very genes involved in the machinery that guards the genome can greatly increase the spontaneous rate at which genomic mutations or structural alterations occur, accelerating the accumulation of mutations. Exposure to the environmental factors of ionizing radiation and chemical carcinogens may initiate or accelerate the accumulation of genomic mutations. Gene mutations inherited from parents that are present in all the cells

in the body are called germline mutations; gene mutations acquired during life in specific cells are called somatic mutations. The frequency of somatic mutations in tumor cells, referred to as the mutational burden, varies greatly among different types of cancer. The relative mutational burden of many cancer types is shown in Figure 5–2.

FIGURE 5–2 The average mutation burden for many different types of cancers is shown for comparison.

Melanomas, lung and oropharyngeal cancers, and cancers of the gastrointestinal (GI) tract have the highest mutational burden of all cancers. This is due to the direct exposure of these tissues to the outside environment and exposure to mutagenic insults such as ultraviolet (UV) light (skin), inhaled carcinogens (lung and oropharynx), and ingested carcinogens and products of endogenous bacterial flora (GI tract). Cancers of mesenchymal tissues and organs without direct environmental exposure have lower mutational burdens. Specific types of DNA mutation (specific nucleotide changes) are linked with specific environmental insults, and such environmental footprints are evident in the altered tumor genome. For example, melanomas of the skin have a high number of specific types of mutation typically induced by UV light. Cancers of the oropharynx and lung have a high number of mutations typically induced by tobacco carcinogens. Some types of mutation are due to overactivity of the APOBEC family of enzymes (ABOBEC being an acronym for “apolipoprotein B mRNA editing enzyme catalytic polypeptide-like”) and likely represent collateral damage from these enzymes of the innate immune system, whose mutagenic functions are intended to infect viruses. Other types of mutation are a

result of spontaneous errors that occur in all normal cells and that accumulate with advancing age. Mutation in DNA can occur in various ways. It can be a change in a single base or in two bases, it can be a deletion of one or a few bases, or it can be insertion of extra bases. Alterations of the genome can also occur in larger ways, with DNA segments as large as kilobases or megabases entirely deleted from the genome, or alternatively, recopied many times (so- called amplification).

Defects in DNA Sequence DNA can incur many types of damage, including chemical modifications or breakages, either to a single strand or to both strands. Cells are endowed with a repertoire of DNA repair mechanisms, each designed for a specific type of damage, to repair and restore the DNA’s nucleotide sequence. Malfunction in one or more of these DNA repair mechanisms is one of the fundamental hallmarks of cancer and is often an early event in tumorigenesis. Faulty DNA repair leads to much more rapid accumulation of genetic defects, providing a genetic diversity that propels a selection process leading to a proliferation of tumor cells with better survival, higher proliferative rates, and more invasive and metastatic phenotypes. The major DNA repair mechanisms implicated in tumorigenesis are summarized in Table 5–2 and discussed below.

TABLE 5–2 DNA repair mechanisms and cancer pathogenesis.

The nucleotides in genome DNA can be chemically altered by exogenous or endogenous carcinogens, radiation, UV light, and other mutagens. Single-strand

defects are typically repaired by the nucleotide-excision repair (NER) and base- excision repair (BER) mechanisms. BER typically repairs bulky DNA lesions caused by oxidation, alkylation, or deamination, whereas NER typically repairs non-bulky lesions such as thymidine dimers caused by UV light. Loss of genes involved in these pathways increases the mutation rate of the genome and, if defective in the germline, may increase the risk of cancers. In particular, germline defects in many of the genes involved in NER lead to the clinical syndrome of xeroderma pigmentosum. Affected individuals are intolerant to sunlight and have a 1000-fold-higher risk of developing UV-induced skin cancers.

Homologous recombination is the preferred and most accurate mode of repairing double-strand breaks in DNA because it uses the identical second copy of the DNA template (available in the G2 phase) to repair the DNA break with complete fidelity. Malfunction in this important DNA repair mechanism (called homologous recombination deficiency [HRD]) can develop as a result of the loss of one of its critical genes (see Table 5–2). Such malfunction forces cells to use more error-prone mechanisms to repair double-strand DNA breaks. Tumors with HRD, such as BRCA1- and BRCA2-deficient tumors, typically have large numbers of structural abnormalities in the tumor genome with many copy number variations. Identifying tumors with HRD has become clinically important because there are specific therapies that are effective in treating HRD tumors. Many commercial assays of HRD have been developed for clinical use in patient samples. The genes involved in homologous recombination repair are sometimes defective in the germline, accounting for familial cancer susceptibility syndromes; others can be somatically inactivated in developing tumors.

Non-homologous end-joining (NHEJ) mechanisms are important in repairing double-strand breaks when an identical chromosome copy is unavailable, such as in the G1 phase of the cell cycle. In addition to its role in correcting insult- induced DNA breaks, NHEJ plays a normal physiologic role in mediating V(D)J recombination, which promotes massive diversity in T-cells and B-cells through genomic rearrangements at the T-cell and B-cell receptor genes. As such, patients with germline defects in genes critical for NHEJ have dysfunctional T cells and B cells, leading to the clinical syndrome of severe combined immunodeficiency (SCID). Since NHEJ joins ends without a template for the correct DNA sequence, it is prone to error and can sometimes join DNA ends that do not belong to each other. This can result in chromosomal rearrangements including chromosomal translocations, in which a segment of one chromosome

is fused to a segment of another chromosome. This imperfect function of NHEJ likely accounts for a large proportion of the chromosomal translocations found in cancer cells. Other mechanisms for repairing double-strand breaks are also error prone and may cause chromosomal translocations.

The process of DNA replication commonly incurs errors in incorporating nucleotides into the newly synthesized DNA strand, leading to mismatches in the double-strand DNA. A family of proteins constitutes the mechanism of DNA mismatch repair that routinely repairs these errors (see Table 5–2). Defects in mismatch repair genes lead to insertion or deletion errors during correction, and tumors with mismatch repair deficiency have tens of thousands of such mutations. This hypermutable phenotype has a genomic footprint that can be detected by polymerase chain reaction (PCR)–based methods, called microsatellite instability (MSI) assays, which detect and measure variations in small genome fragments called microsatellites. These are of uniform length in any individual patient, but they are highly variable in tumors with mismatch repair deficiency, and such tumors are thus called MSI high (whereas most other tumors are MSI low). Commercial assays have been developed to detect MSI in clinical tumor samples because the MSI status of a tumor has prognostic significance and also predicts response or resistance to certain types of therapy. Once again, the genes involved in mismatch repair are sometimes defective in the germline, accounting for familial cancer susceptibility syndromes, or genes can be somatically inactivated in developing tumors.

Microhomology-mediated end-joining (MMEJ) is a mechanism that relies on homology to small overhangs in broken DNA ends to mediate end-joining. This mode of double-strand break repair is inherently error prone and may lead to deletion mutations and chromosomal rearrangements. MMEJ is mostly a back-up mechanism for DNA repair. But in many cancers, overexpression of the genes involved in MMEJ leads to its inappropriate function as a primary mechanism, overriding high-fidelity repair mechanisms and ultimately leading to more mutations. Thus, in contrast to high-fidelity DNA repair mechanisms that are typically dysfunctional in cancers, the error-prone MMEJ mechanism is often overactive in cancers.

CHECKPOINT

5. What is the source of a germline mutation in a patient? 6. What type of cancer has the highest mutation rate?

7. Name several DNA repair mechanisms that can be dysfunctional in cancers.

8. What is the preferred and most accurate mode of repair-ing double- strand breaks in DNA, and what is the name given to its malfunction?

9. What is microsatellite instability?

Defects in Chromatin Structure and Dynamics In addition to the repertoire of DNA repair mechanisms available to correct errors, the integrity of the genome is also preserved and protected by a mega- structural framework consisting of a densely packaged complex of the genome DNA and histone proteins referred to as chromatin. Chromatin is highly structured into repeating units called nucleosomes, and the packing of these units provides protection to the DNA and a mode of regulation that determines access to it. This process is under tight regulation by epigenetic mechanisms that can modify and alter the biochemical properties of the histone proteins. Such epigenetic mechanisms determine whether certain regions of the genome are loosened to provide access for gene transcription or for DNA replication or repair, or tightened for transcriptional silencing, genome protection, or chromosome condensation during mitotic separation (Figure 5–3). Loss of the epigenetic control of chromatin is commonly seen in cancer cells, leading to abnormal expression of many genes, increased susceptibility to DNA damage, and errors in mitotic separation. Alterations in chromatin structure are often visible under light microscopy, and the degree of observed abnormalities in chromatin allows pathologists to assign a nuclear grade when performing a histologic characterization of cancer cells. Underlying the epigenetic deregulation of the genome are defects in the molecular mechanisms that function to regulate chromatin structure. Epigenetic mechanisms involved in chromatin remodeling include the so-called BAF complexes (SWI/SNF proteins) and the polycomb repressive complexes; many of these are known to be mutated, abnormally expressed, or otherwise dysfunctional in cancer cells, underlying the observed epigenetic abnormalities. Another epigenetic mechanism that functions to regulate the expression of genes is the direct methylation of DNA, frequently at promoter regions. DNA methylation suppresses gene transcription and functions to silence certain areas of the genome, while other areas remain transcriptionally active. Abnormal methylation or demethylation of the genome is commonly found in cancer cells. This abnormality constitutes yet another epigenetic mechanism by which some

genes are silenced and other genes are activated to promote the tumorigenic phenotype.

FIGURE 5–3 There are multiple dimensions in the structure and regulation of the genome: the specific nucleotide sequence code; the condensing of genomic regions to keep genes silent and protected; the opening of genomic regions to enable transcriptional activity, replication, or repair; and the complete condensation of the chromosome to prepare for mitosis. The structure and regulation of each dimension of genome integrity and control are frequently altered in cancers. The various types of alteration are reported in the callouts.

Defects in Genome Content Maintaining the integrity of the genome also requires preserving its entire content, despite the fact that the human genome is split into 46 fragments (46 chromosomes). As such, a complex cellular machinery is in place to preserve the integrity of each chromosome, to orchestrate the proper duplication of each of

the chromosomes in every S phase, and the proper allocation and distribution of a full set of chromosomes to each daughter cell during every mitosis. In neoplasia, these mechanisms can fail, leading to abnormalities in the structure or number of chromosomes, which is referred to as chromosome instability. Errors in orchestrating replication or in properly separating and allocating chromosomes during mitosis results in an uneven distribution to daughter cells, producing cell progeny that have increased numbers of some chromosomes and reduced numbers of other chromosomes. Thus, large segments of the genome may be gained or lost in daughter cells. While losses of entire chromosomes can be lethal to cells, gains are often tolerated and selected for during repeated proliferative cycles. As a consequence, tumor cells frequently have many more than the 46 chromosomes of normal cells, an attribute called aneuploidy. In addition to abnormal numbers of whole, intact chromosomes, errors in repairing DNA breaks often lead to chromosome rearrangements, including fusion events between different chromosomes (called chromosomal translocation), inversion of segments of a chromosome, deletions, truncations (shortened chromosomes), or duplications. The various structural alterations of chromosomes are depicted in Figure 5–4.

FIGURE 5–4 Various types of structural chromosomal rearrangements that occur in tumors.

In the scenario of chromosomal translocation, a part of one chromosome is fused with a part of another chromosome, and the resulting abnormal hybrid

chromosome is passed on to all tumor cell progeny thereafter. A chromosomal translocation can be a random event seen in one cancer only, or it can be a specific and recurring event seen in multiple cancers. In fact, a very specific chromosomal translocation can be the defining attribute of some types of cancer. For example, a translocation between chromosomes 9 and 22 creates a shortened chromosome called the Philadelphia chromosome, a defining attribute of chronic myelogenous leukemia.

Defects in Protecting Chromosome Ends The fact that the diploid human genome is fragmented into 46 chromosomes means that there are 92 ends in the human genomic DNA. There are cellular mechanisms in place to hide and protect these loose ends from the DNA repair machinery that would otherwise consider them damaged DNA and inappropriately attempt to fuse them. This protection is accomplished by highly repeated sequences at the ends of chromosomes called telomeres and an associated complex of proteins called shelterin. Since normal DNA replication is unable to proceed to the very end of the telomeres, telomeres shorten with every replication. With continuous cycles of replication, telomeres eventually shorten to nothing; a loss of protective telomeres leads to a telomere crisis consisting of inappropriate DNA damage response, chromosome fusion, and ultimately cell senescence or cell death. Indeed, most cells have limited replicative potential.

The enzyme telomerase can lengthen telomeres and confer unrestricted replicative potential to cells; however, the telomerase gene is silenced in almost all tissues after development is complete. Because of their inherent property of continuous replication, neoplastic cells eventually encounter telomere crisis. While many of these cells may become senescent or die, some survive the crisis through the re-expression of telomerase. The expression of telomerase then allows unrestricted replicative potential, which accounts for the continuous growth and eventual metastasis of malignant tumors. However, with passage through telomere crisis, significant damage to genome integrity occurs, with many chromosome fusions, breaks, and rearrangements; this accounts for many of the chromosomal rearrangements and much of the aneuploidy seen in cancer cells.

CHECKPOINT

10. What regulates chromatin structure? 11. What are some of the chromosomal abnormalities that occur in cancers? 12. What is telomere crisis?

SPECIFIC GENE MUTATIONS IN NEOPLASIA

A principal deleterious effect of DNA mutations is their effects on the functions of the genes encoded by the genome. The cataloging of mutated genes has been a fundamental task of molecular oncology because it identifies genes whose functions are relevant to tumor cells. Genes that confer an advantage to tumor cells through a loss-of-function alteration are named tumor suppressor genes. Genes that confer an advantage through a gain-of-function alteration are named proto-oncogenes, and their altered counterparts are named oncogenes. Tumor suppressor genes are much more common because many types of gene mutation can cause loss of function or loss of expression of the protein product, whereas an increase in the activity of a proto-oncogene protein product requires very specific changes in the amino acid sequence. Mutations that can disrupt tumor suppressor genes include mutations that create a premature stop codon (nonsense mutation), mutations that change the open reading frame of the transcript (frame-shift mutations), and mutations that delete critical parts of the gene (deletion mutations). Alternatively, the gene can be silenced by promoter methylation or can be entirely or partially lost by deletion. Proto-oncogenes can be activated through mutation, gene amplification and overexpression, chromosomal translocation, and possibly other mechanisms. Tables 5–3 and 5– 4 provide examples of oncogenes and tumor suppressor genes, respectively. In general, during the gain-of-function alteration of a proto-oncogene, mutation of only one allele is sufficient to produce the gain of function. In contrast, during the loss-of-function alteration of a tumor suppressor gene, both alleles must be inactivated. In certain cases, loss of one allele can result in a reduction of gene expression. For some genes, this gene-dosage reduction is sufficient to promote tumorigenic growth. In addition to being generated through the mutation of cellular proto-oncogenes, oncogenes can also be acquired through the introduction of foreign genomic material, typically transmitted by viruses. The topic of oncogenic viruses is discussed in more detail later in this chapter.

TABLE 5–3 Representative oncogenes activated in human cancers.

TABLE 5–4 Representative tumor suppressor genes inactivated in human tumors or the human germline.

The diploid human genome naturally contains defective alleles of many

genes, and although defective alleles are for the most part biologically silent, in the case of tumor suppressor genes, a defective allele can confer significant cancer risk to an individual and all family members harboring such an allele. The loss of function of a gene in adult tissues is statistically much more probable when only one functional allele exists in all cells from the beginning of life, and inherited susceptibility to cancer is almost always a result of germline passage of a defective tumor suppressor gene allele. Many of the identified tumor suppressor genes that are frequently inactivated in sporadic human tumors have also been linked to specific hereditary cancer syndromes. In families with these syndromes, a defective allele of the responsible tumor suppressor gene is passed in the germline, and members who harbor this heterozygous genotype inherit a high risk for tumors in which the second allele has also been lost. An inherited mutation in one allele of the TP53 gene can cause the rare Li–Fraumeni syndrome, characterized by the early development of bone, breast, brain, and soft tissue tumors (sarcomas), along with other organ-specific tumors (such as adrenal cancer). Inherited mutations in single alleles of the BRCA1 or BRCA2 gene confer a high risk for breast or ovarian cancers. Table 5–4 lists some hereditary cancer syndromes linked with tumor suppressor genes. The list includes predominantly high-penetrance genes, which confer a very high risk of disease when inherited. Families carrying these alleles are noticeable for their high incidences of cancers associated with these genes. However, many human cancers, possibly a majority, are etiologically linked with germline alleles of moderate- or low-penetrance genes, or possibly a combination of two or more low-penetrance genes. Cancers arising from such genes do not cluster as tightly within families and therefore are not as frequently noted to be clinical familial syndromes. Rather, they are being increasingly recognized by modern-day large- scale germline sequencing efforts.

In contrast to single alleles of defective tumor suppressor genes, single alleles of mutationally activated oncogenes are not biologically silent and, if present in the germline, can have profound clinical manifestations, including even embryonic demise. Because of this fact, inherited syndromes involving the germline transmission of activated oncogenes are rare. One example, however, is the familial syndrome of multiple endocrine neoplasia type II, in which heterozygotes carrying an activated RET oncogene on chromosome 10 are at increased risk of developing pheochromocytoma, medullary carcinoma of the thyroid, and parathyroid tumors (see Table 5–3).

CHECKPOINT

13. What is the difference between an oncogene and a tumor suppressor gene?

14. What are the genetic mechanisms by which onco-genes are activated and tumor suppressor genes are inactivated?

15. What is the molecular basis for most inherited suscepti-bilities to certain cancers?

PROTO-ONCOGENES & TUMOR SUPPRESSOR GENES IN NORMAL PHYSIOLOGY & IN NEOPLASIA

Proteins encoded by proto-oncogenes and tumor suppressor genes perform diverse cellular functions. Not surprisingly, they include proteins that recognize and repair DNA damage, proteins that regulate the cell cycle, proteins that mediate growth factor signal transduction pathways and regulate programmed cell death, proteins involved in cell adhesion to matrix or in cell-to-cell communication, and proteins that regulate the metabolic needs and biomass production of cells. The deregulation of these pathways through mutational events results in increased genomic instability, overactive growth factor signaling, unrestricted proliferation, inactivation of programmed cell death (apoptosis), decreased dependency on cell adhesion, increased energy supplies and protein synthesis, and extracellular proteolysis. Many of these functions may be altered simultaneously through deregulation of transcription factors that regulate many genes.

Examples of tumor suppressor proteins include both the retinoblastoma protein and the p16 cell cycle inhibitor, which function in restricting proliferation at the G1 checkpoint of the cell cycle. Loss of the genes encoding these proteins can result in unchecked progression through the G1/S checkpoint. The TP53 tumor suppressor gene encodes the p53 protein, which is a critical guardian of genomic integrity and serves to recognize DNA damage and consequently to inhibit cell cycle progression and to induce apoptosis. Loss of the p53 protein can result in continued cell replication despite DNA damage and failure to activate apoptosis. The fundamental importance of p53 function and of

genomic stability in the oncogenic process is underscored by the fact that TP53 mutations are the most common mutations in human cancers, being found in more than half of all human tumors. The PTEN tumor suppressor gene encodes the PTEN phosphatase protein, which is involved in the regulation of the Akt and mTor signaling pathway. This protein underlies many cellular functions and reconciles the proliferative, metabolic, and synthetic activities of the cell with inputs including nutrient and energy supply and growth factor signals; thus, it can enable cells to survive periods of stress or starvation. Loss of PTEN can enable sustained proliferative and synthetic activities despite shortages in nutrients and energy supply. Cadherins are proteins involved in cell–cell adhesion. Loss of cadherins can result in reduced cell adhesion, cell detachment, and metastasis. Table 5–4 presents a partial list of examples of tumor suppressor genes. When all human tumor suppressor genes have been fully identified, the list will be much larger.

Proto-oncogenes include proteins involved in various steps of the extracellular growth factor signaling pathway from the membrane receptors to the membrane intermediates to the proteins mediating the cytoplasmic signaling cascades. The receptor tyrosine kinases (RTKs) are some of the most important signaling proteins in cells because they regulate cell behavior in response to extracellular ligands. The RTKs exert their effects through two important downstream signaling pathways: the Ras–Raf–MAPK pathway and the PI3K– Akt pathway. Many, if not most, cancers activate this pathway by one mechanism or another, and indeed many of the genes encoding these signaling proteins are oncogenes or tumor suppressor genes (Figure 5–5).

FIGURE 5–5 Membrane-bound receptor tyrosine kinases (RTKs) activate two important cytoplasmic signaling cascades to affect numerous biological phenotypes, including proliferation, differentiation, survival, protein synthesis, metabolism, and others. Many of the genes involved in this pathway are oncogenes. These are highlighted in the red callouts; when mutationally activated in cancers, they promote excessive signaling. Some of the genes that negatively regulate the pathway are tumor suppressor genes. These are highlighted in the blue callouts; when they are lost in cancers, they allow for unrestricted pathway signaling.

The epidermal growth factor receptor (EGFR) is an RTK that binds a number of extracellular ligands and, in cooperation with its homolog, HER2, signals proliferative and apoptotic pathways. Overactivity of EGFR or HER2 can lead to unregulated growth. The EGFR gene is mutated or amplified in many cancers, including nearly half of all glioblastomas, as well as subsets of lung, breast, oropharyngeal, and other epithelial cancers. The HER2 gene is amplified in 20% of breast cancers and subsets of gastric, esophageal, bladder, and colon cancers; its amplification confers an aggressive phenotype. Ras proteins are a family of membrane-bound signaling switches that function immediately downstream of membrane receptors at a key branch point of cytoplasmic signaling. Mutational activation of Ras proteins causes overactive cytoplasmic signaling and deregulation of proliferative and apoptotic pathways. Ras appears to be critically important in tumorigenesis because nearly one-third of all human tumors harbor

mutationally activated forms of the KRAS, NRAS, or HRAS genes. Raf proteins are a family of serine-threonine kinases that function downstream of Ras proteins. Mutational activation of the BRAF gene can lead to overactive signaling and deregulation of proliferative and apoptotic pathways; its activation is commonly found in many tumors. Table 5–3 presents a partial list of oncogenes identified in human malignancies, along with the tumor types in which they are commonly observed and the cellular function encoded by their proto-oncogene counterparts.

Since the inactivation of a single tumor suppressor gene or the activation of a single oncogene is insufficient for the development of most types of human tumors, the process of tumorigenesis entails the sequential acquisition of a number of “hits” over time leading to sequential cellular phenotypic changes— from atypia to dysplasia to hyperplasia to in situ cancer to invasive cancer, and ultimately to metastatic cancer. The largest body of evidence to support this theory has been generated from the molecular study of colon cancer and identifiable pre-neoplastic lesions, including colonic polyps and adenomas. In this model, the progressive development of neoplasia from premalignant to malignant to invasive lesions is associated with an increasing number of genetic abnormalities, including both oncogene activation and tumor suppressor gene inactivation. This theory of tumorigenesis is further supported by the identification of inherited abnormalities of several tumor suppressor genes, all associated with a strong familial tendency to develop colon cancer at a young age.

A small subset of human cancers appears to be more simplistic in evolution. A translocation of the long arm of chromosome 9 to the long arm of chromosome 22 leads to a fusion of the BCR gene with the c-Abl gene and results in expression of the BCR-Abl oncoprotein seen in chronic myelogenous leukemia (CML). The expression of this oncogene in hematopoietic cells of animal models reproduces the disease. This oncogenic event is seen in virtually 100% of cases of this disease, and a treatment that inhibits the kinase activity of this oncoprotein produces remissions in nearly 100% of affected patients. Thus, in contrast to the multistep process involved in most types of carcinogenesis, the steps necessary for the development of CML may be much simpler.

The identification of tumor suppressor genes and oncogenes as the fundamental enablers of tumorigenesis has led to the hypothesis that cancer can be successfully treated by treatments that counteract the biochemical sequelae of these molecular abnormalities. This hypothesis has fueled attempts to develop therapeutic agents that can inhibit the function of activated oncoproteins or

restore the function of inactivated tumor suppressor proteins. Current pharmaceutical technologies have enabled the development of drugs targeting gain-of-function mutations, and many oncogenes can now be inhibited by such drugs. However, it is not yet possible to restore the function of a lost tumor suppressor gene. But since normal cells may use alternative pathways to compensate for the loss of a tumor suppressor gene, activating the functions of such alternative pathways is a roundabout way to kill cancer cells. Such collateral target genes utilized for therapeutic purposes are called synthetic lethal genes.

NON-CODING RNAs IN NEOPLASIA

In addition to transcribing mRNAs encoding protein products, the genome also transcribes many forms of RNA that do not encode protein products, but rather function to regulate the expression of many other genes. These include microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), both of which are implicated in the pathogenesis of cancer. A single miRNA can target and destabilize many protein-coding genes and can have broad phenotypic effects on diverse cellular processes, such as differentiation, cell cycle regulation, and apoptosis. Overexpression of certain miRNAs is seen in some cancers, and the loss of expression of some miRNAs is seen in others, each with significant tumor-promoting effects. Because of this, miRNAs are typically included in the family of oncogenes (and called oncomiRs) and tumor suppressor genes (although they are not truly genes). The mechanism by which lncRNAs influence cell phenotype is not yet well understood, but they are thought to affect the epigenetic or post-transcriptional regulation of gene expression. lncRNAs also contribute to cancer pathogenesis through overexpression or loss of expression, resulting in biologic phenotypes that favor tumor growth, metabolism, dedifferentiation, and survival. The functions of miRNAs and lncRNAs are also affected by germline or somatic mutations, similar to protein- coding genes, and such mutations may also harbor cancer susceptibility or more direct tumorigenic phenotypes. The study of non-coding RNAs is only in its infancy. Many other small RNA species have been identified and may be important in cancer pathogenesis.

HORMONES AND GROWTH FACTORS IN

NEOPLASIA

Although structurally altered genes, classified as oncogenes or tumor suppressor genes, are key mediators of neoplasia, the role of unaltered genes is not to be dismissed, as such genes are likely equally important in carcinogenesis. The oncogenic process may be driven through abnormalities in signaling of all kinds: abnormal time, duration, or intensity; abnormal tissue expression; or abnormal subcellular compartment localization. The regulation of growth in complex organisms requires specialized proteins for the normal growth, maturation, development, and function of cells and specialized tissues. The complexity of the human organism requires that these proteins be expressed at precisely coordinated points in space and time. An essential component of this regulation is the system of hormones, growth factors, and growth inhibitors. On binding to specific receptor proteins on the cell surface or in the cytoplasm, these factors lead to a complex set of signals that can result in a variety of cellular effects, including mitogenesis, growth inhibition, changes in cell cycle regulation, apoptosis, differentiation, and induction of a secondary set of genes. The actual end effects depend not only on the particular type of interacting factor and receptor, but also on the cell type and milieu in which factor–receptor coupling occurs. This system allows for cell-to-cell interactions, whereby a factor secreted by one cell or tissue can enter the bloodstream and influence another set of distant cells (endocrine action) or act on adjacent cells (paracrine action). An autocrine action is also possible when a cell produces a factor that binds to a receptor on or in the same cell. An altered concentration of these growth factors or receptor overexpression or mutation can change the signaling behavior, contributing to a malignant phenotype. Only a subset of growth factor receptors are proto-oncogenes. However, many additional growth factors and growth factor receptors appear to be important in tumor growth and progression, although they are not classified as proto-oncogenes because they serve tumorigenic causes without incurring mutations or without overexpression.

An important class of growth factor signaling molecules are the RTKs. A number of RTK families exist, and in experimental models, most are capable of transforming cells if activated or overexpressed. Although all of these abnormalities are not necessarily seen in naturally occurring human tumors, the experimental data highlight the potential inherent in these proteins and the important role they may be playing in tumor cells despite lacking the “oncogene” label. Members of the HER family of RTKs are commonly mutated or amplified in human tumors and exemplify the important role of RTKs in

human neoplasia. And despite having a normal sequence and expression level, they likely play an important role in other tumors. For example, HER1 (also called EGFR) is not mutated or overexpressed in colon cancers but is sometimes activated by autocrine signaling in the cancer cells, and EGFR-targeted therapies are used to treat this type of cancer. Other families of RTKs, such as the platelet- derived growth factor (PDGF) receptors, fibroblast growth factor (FGF) receptors, vascular endothelial growth factor (VEGF) receptors, and insulin-like growth factor (IGF) receptors, function similarly to the HER family of RTKs. In general, these receptors are not reported to be mutated or amplified in human tumors. However, there is increased expression in many tumors or aberrant expression in tumors from tissue types that ordinarily do not to express that receptor. Alternatively, there may be excessive production of receptor ligands owing to a variety of mechanisms (eg, loss of epigenetic silencing of the gene coding for the ligand or excessive gene transcription of the same gene). In experimental systems, each of these RTK systems has oncogenic potential, building a circumstantial case that they may be important players in human tumors.

Some growth factor signaling pathways function to inhibit cell growth and provide negative regulation in response to extracellular stimuli. Desensitization of cells to such growth inhibitors is common in tumors. An example of this is the transforming growth factor-β (TGF-β). TGF-β has diverse biological effects. It potently inhibits cell proliferation but also stimulates the production and deposition of extracellular matrix (ECM) and adhesion factors. These functions are important in tissue remodeling during embryogenesis and wound repair. In some tumor types, the anti-proliferative response to TGF-β is lost early on because of mutations in its downstream signaling components. However, continued secretion, and often over-secretion, of TGF-β by the tumor and stromal tissues leads to an increase in the production of ECM and adhesion factors and promotes the invasive and metastatic property of tumors.

Another important class of receptors is the large superfamily of nuclear hormone receptors. These include the cellular receptors for a variety of hormones, among them estrogen and progesterone, androgens, glucocorticoids, thyroid hormone, and retinoids. The actions of estrogen are fundamentally important in the development of breast cancer. In women, oophorectomy early in life offers substantial protection against its development. (In animal models, too, mammary carcinogenesis is significantly retarded in the absence of estrogen.) More than half of all breast cancers are dependent on estrogen for proliferation. Although these data clearly implicate the estrogen signaling pathway in breast

carcinogenesis, specific abnormalities of the estrogen receptor (ER) are not seen in breast cancers; therefore, the ER does not qualify as a tumor suppressor protein or oncoprotein. Although the loss of certain tumor suppressor genes or the activation of certain oncogenes leads to the development of breast cancer, it is possible that continued ER function is essential throughout this process and that, without ER function, it cannot proceed. Alternatively, it is possible that abnormal ER signaling, perhaps as a result of altered cofactors, cross-talk, or phosphorylation status, drives breast carcinogenesis. Although the mechanism by which estrogen and its receptor drive breast cancers has not yet been determined, the fundamental role of estrogen in this disease is well established. Furthermore, treatments that work through inhibiting the production of the active ligand or that inhibit the function of the ER are the most effective therapies for breast cancer yet developed and are highly active in the prevention and treatment of breast cancer. The androgen receptor (AR), similarly, plays a critical role in the development of prostate cancer, although only occasional activating mutations and amplification of the AR have been reported in this cancer.

On the other hand, retinoids (ligands for retinoic acid receptors) are well known to participate in the differentiation of a variety of tissues during development and to cause the differentiation of certain tumors in tissue culture models. These observations have been exploited as a treatment approach for acute promyelocytic leukemia (APL). APL is characterized by a t(15;17) chromosomal translocation resulting in the fusion of the PML gene with the retinoic acid receptor-α (RAR-α) gene. The resulting fusion protein blocks the differentiation of hematopoietic progenitor cells and eventually leads to the development of APL. While this fusion protein is not by itself transforming in experimental models and cannot be categorized as a classic oncogene or tumor suppressor gene, it is etiologically involved in the pathogenesis of APL. Because the fusion protein contains the ligand-binding domain of RAR-α, it remains sensitive to ligand, and treatment with the ligand all-trans retinoic acid results in differentiation of tumor cells and complete remission in most patients with this form of leukemia.

CONTROL OF THE CELL CYCLE IN NEOPLASIA

A hallmark of neoplasia is excessive proliferation inappropriate for the tissue context, which thus leads to architectural distortion and tissue destruction. Several molecular mechanisms contribute to the unrestricted proliferative

capacity of tumor cells. First, excessive growth factor or hormonal signaling stimulates proliferation. Second, telomerase gene activation lifts the natural replicative ceiling pre-existing in most cells. Third, the orderly progression of the phases of the cell cycle is strictly regulated by a large family of cell cycle machinery proteins that can halt cell replication if precise conditions are not met. At the heart of cell cycle regulation are complexes of cyclins and cyclin- dependent kinases (CDKs) that drive the forward direction of cell cycle progression. These are under both positive and negative regulation by a plethora of other proteins and a highly complex, intertwined signaling network that functions to ensure that cell cycle progression proceeds in a desired and healthy manner (Figure 5–6). At each phase of the cell cycle, certain checkpoints are in place to ensure that the tasks of that phase have been properly met. Typically, most non-neoplastic cells proliferate only when instructed to do so by growth factor signals in G1 phase, and only when nutrients and environmental conditions are appropriate. In S-phase, the cell must ensure that DNA replication is completed without damage or errors. In G2 phase, the cell must double its mass by protein and lipid synthesis in preparation for division. In M-phase, the cell must orchestrate the proper spindle attachment and alignment of all the chromosomes in preparation for mitotic separation. This cell cycle machinery is under tight regulation by proteins that function to sense DNA damage, and that can exert a hold on cell cycle progression while DNA is being repaired. In cancers, deregulation of the cell cycle machinery is almost universal, and many cell cycle regulators are oncogenes or tumor suppressor genes that exhibit a gain-of-function or loss-of-function, respectively. Indeed, cell cycle regulators are some of the most frequently mutated, amplified, or otherwise altered genes in tumors of all kinds. In fact, the p53 gene is the single most common tumor suppressor gene; it is mutated in approximately half of all human tumors. The consequences of dysfunctional cell cycle regulation are many. Cells will proliferate with damaged DNA, leading to the accumulation of more and more errors in their genome. They will proliferate despite lack of sufficient nutrients or physical space. They will divide without proper duplication and separation of all chromosomes, such that daughter cells will have abnormal genome content. The RTK signaling pathway, apoptotic pathway, metabolic pathways, and other efferent and afferent biologic signaling pathways that communicate with the cell cycle machinery are also each deregulated in human cancers.

FIGURE 5–6 Progression through each of the four phases of the cell cycle is regulated by distinct sets of cyclin–CDK complexes. These are in turn negatively and positively regulated by a number of cell cycle inhibitors and activators, as shown in the figure. The proteins whose functions promote cell cycle progression, many of which are oncogenes, are shown in green. The proteins whose functions inhibit cell cycle progression, many of which are tumor suppressor genes, are shown in red.

CONTROL OF THE APOPTOTIC PROGRAM IN NEOPLASIA

Despite the extensive mechanisms that exist to protect and repair genomic DNA, and to regulate cell proliferation and maintain metabolic homeostasis, cells invariably experience scenarios in which these imperfect mechanisms fail, leaving no path forward for the proper execution of replicative or metabolic programs. When such scenarios develop, a cell death program is triggered, called apoptosis. The regulation and execution of apoptosis involves a large number of proteins and mitochondria. These are in communication with many input signals that function to report DNA damage, metabolic stress, or other malfunctioning

programs, or with extracellular ligands that instruct a cell to undergo apoptosis. Some integral proteins of the apoptotic machinery are the protease family of caspases and the Bcl-2 family of BH domain proteins. The signaling network that regulates the execution of apoptosis is complex and only partially understood at this time. A number of proteins function to promote apoptosis, and a number of proteins function to inhibit it (Figure 5–7). Whether apoptosis occurs depends greatly on the balance of these stimulatory and inhibitory proteins.

FIGURE 5–7 The decision to execute apoptosis (programmed cell death) is governed by a complex signaling network of proteins. Those that function to promote apoptosis are shown in red, and those that

function to oppose apoptosis are shown in green. Apoptosis can be activated by the intrinsic pathway in response to stressful or catastrophic cellular events or by the extrinsic pathway in response to a signaling instruction from cell death receptors. Cancer cells frequently suppress the apoptotic response by upregulating the anti-apoptotic proteins or downregulating the pro-apoptotic proteins.

Cancer cells are frequently faced with unrepaired DNA, metabolic stress, hypoxia, and other scenarios that would ordinarily trigger apoptosis. The apoptotic program is in fact a tumor-suppressing mechanism. However, one of the hallmarks of cancer cells is their acquisition of methods to disrupt the apoptotic mechanism so as to escape the lethal fate it bestows; thus, cancer cells survive through repeated cycles of DNA damage, metabolic stress, and other stressors. These methods include the upregulation of the inhibitors of apoptotic proteins and the downregulation of their promoters (see Figure 5–7). In particular, upregulation of Bcl-2 is frequently seen in many cancers, and it accounts for the survival potential of these cancers, including their resistance to chemotherapeutic drugs. Many lymphomas have excessive levels of Bcl-2. In follicular lymphomas, for example, a chromosomal translocation places the Bcl- 2 gene next to the immunoglobulin gene, leading to excessively high transcriptional levels of Bcl-2. Another common abnormality in cancer (seen in approximately 50% of all cancers) is loss of the tumor suppressor gene p53 and thus loss of the p53 protein. Since the p53 protein functions to sense DNA damage and trigger activation of the apoptotic cascade, its loss is a key step in allowing cancer cells to survive despite the presence of unrepaired DNA damage.

CHECKPOINT

16. What are some of the oncogenes in the RTK pathway? 17. Is the estrogen receptor an oncogene? 18. Can miRNA be oncogenes? 19. How is cell cycle regulation altered in cancers? 20. How is the apoptotic program affected in cancers? 21. How is apoptosis deregulated in follicular lymphomas?

STROMAL INTERACTIONS & INFLUENCES IN NEOPLASIA

Normal tissues exhibit an orderly architectural structure that supports proper organ function. The establishment and maintenance of this high-level order is governed by two-way communication between the principal cellular components of the specific tissue and numerous stromal elements in their surrounding microenvironment. These stromal interactions ensure the proper initial construction of the tissue architecture during embryogenesis and thus enable tissue remodeling and repair after any damage or wounding. Stromal elements include mesenchymal cells and extracellular matrix proteins and fibrils that provide structural tissue support, stiffness or elasticity, and a skeletal framework for anchorage and adhesion; proteinaceous membrane barriers that separate tissue compartments; and cells of the hematopoietic system that are “on call” to recognize tissue injury or infection and respond by secreting growth factors to initiate necessary immunologic and inflammatory responses. In normal tissues, these stromal interactions function to establish or restore tissue homeostasis and to reach a resting state of equilibrium, but in neoplasms, these two-way interactions continue and proceed in a way that never reaches homeostasis or equilibrium. Thus, tumors are in effect “wounds that do not heal.” Tumor– stromal relationships involve a complex series of molecular and cellular interactions that are not entirely understood, but which involve tumor cell mechanisms to (1) capitalize on stromal influences beneficial to tumor growth and (2) evade stromal influences detrimental to tumor growth.

The extracellular matrix (ECM) is composed of physically robust fibrillary proteins that provide a skeletal framework for cell anchorage. A number of protein families serve to constitute the ECM, to attach cells to the ECM, to attach cells to each other, and to dissolve and re-establish the ECM when necessary. Abnormalities of these proteins frequently occur in later stages of tumorigenesis, accounting for the loss of architecture, and can mediate the invasive and metastatic phenotype of tumor cells. Integrins are a large family of membrane proteins that anchor cells to the ECM and activate intracellular signaling pathways in response to ECM attachment. Cells have the ability to express any of a large repertoire of integrin combinations, but the specificity of integrin expression is not well understood. However, tumor cells can reshuffle their integrin expression profiles in favor of an invasive or metastatic phenotype. Cadherins are a family of membrane proteins that function in epithelial cell-to- cell adhesion. Loss of E-cadherin expression is seen in some human epithelial tumors, leading to a more invasive phenotype. The expression and activity of many secreted and membrane-anchored proteases are increased in tumor cells. These proteases include the matrix metalloprotease family and the serine protease family of proteins. Increased protease activity leads to ECM

degradation, triggering of the plasminogen activation cascade, and activation of transmembrane receptors through cleavage and shedding of their extracellular domains. Through abnormalities in ECM deposition, cell adhesion protein expression, and membrane and secreted protease activity, cancer cells develop an invasive and ultimately a metastatic phenotype.

CANCER AND THE IMMUNE SYSTEM

The process of tumorigenesis is inherently and intricately intertwined with many facets of the immune system. The interactions and relationships are complex and only partially understood at this time. The complexity begins with the causality relationship and extends to the minutiae of cellular and molecular components involved in the interface between a tumor and the host immune system. The complexity in the causal relationship is that the immune system can play a role in causing or preventing cancer, and it can promote the progression or suppress the growth of an existing cancer. The cancer itself can also stimulate or suppress elements of the immune system. As such, the relationship between cancer and the immune system is impossible to define in simple or causal terms; this relationship is best considered a delicate balance of dynamic tumor-promoting and tumor-suppressing forces governed by an amalgam of variables that can change over time.

Tissue injury stimulates the recruitment and activity of numerous elements of the immune system’s process of inflammation (Figure 5–8). The inflammatory response includes innate immune system cells such as resident macrophages, which function to engulf debris or microbes, secrete cytokines to attract other cells of the innate and adaptive immune system, and generate signals to begin the tissue repair process, including both proliferative and angiogenic growth factors. Neutrophils are recruited to secrete a variety of products to kill bacteria and remodel the matrix. This constitutes the normal process of inflammation, which typically reaches its endpoint with the repair and return of the injured tissue to homeostasis. However, homeostasis is not always achieved, and inflammation can sometimes continue for prolonged periods. It is well recognized that chronic inflammation can promote the development of cancer. Examples include inflammatory bowel diseases such as ulcerative colitis and Crohn’s disease, which can lead to the development of colon cancer. The mechanism by which chronic inflammation promotes tumorigenesis is not entirely understood. The inflammatory microenvironment contains many growth

factors that induce proliferative and migratory activities in the wounded epithelium, as well as cytokines such as IL-6, which can induce proliferative and migratory responses. Neutrophils secrete many factors that generate reactive oxygen and nitrogen species, which can not only activate tumorigenic signaling pathways in epithelial cells, but can also induce DNA damage and promote mutagenesis. Neutrophils also secrete products that can affect the epigenetic regulation of epithelial cells, inducing abnormalities in gene expression and genome protection. Myeloid-derived suppressor cells are recruited to sites of inflammation and function to suppress the activation of an adaptive immune response, which can enable the tumorigenic process to evade detection. These inflammatory conditions, by promoting proliferative and genotoxic and immuno- evasive events, can lead to cancer development. But not all inflammatory conditions are cancer promoting; therefore, there is some level of specificity, not yet understood, in chronic inflammation that is associated with tumorigenesis.

FIGURE 5–8 The engagement of immune cells with injured epithelium or tumor cells is shown here, summarized and simplified according to a hypothetical timeline. Inflammation of normal tissues, if prolonged, can promote tumorigenesis. Tumors are frequently engaged with elements of the innate and adaptive immune system throughout their existence. But through the balancing of many factors, some tumors escape from the eradicative effects of immunosurveillance and develop into clinical disease states, albeit in continuous confrontation with many elements of the immune system that exert both anti-tumor and pro-tumor effects. The inflammatory response can also contribute to tumorigenic growth once a tumor has developed.

Inflammation can precede and promote tumorigenesis, but the reverse also

holds: tumors can induce the inflammatory response. Many tumors arise in tissues without a preceding period of inflammation, and in these tissues, the early events in tumor cells can initiate an inflammatory response. The activation of many oncogenes in epithelial cells leads to the activation of signaling pathways that would normally be activated during wounding, which results in an inflammatory response. This response includes the induction of prostaglandin synthesis through upregulation of COX2, the secretion of chemokines (such as CCL2 or CCL20) that recruit monocytes and dendritic cells or that promote angiogenesis (IL-8), and many matrix-degrading proteases. Once tumor growth reaches a point at which it causes tissue injury, an inflammatory reaction may be initiated. Cellular products released into the microenvironment are recognized by pattern recognition receptors (PRRs) of the innate immune system, which can recognize damage-associated molecular patterns (DAMPs). DAMPs, which are stress signals, include ATP and purine metabolites, DNA or RNA, certain chromatin-associated proteins, and certain heat shock proteins, released into the extracellular microenvironment. PRRs include Toll-like receptors (on macrophages and dendritic cells), which can recognize these stress signals and activate adaptive immune responses.

As a tumor develops, the engagement of the adaptive immune system becomes more relevant to the natural course of tumor growth and progression. The genomic mutations that occur in tumor cells lead to new protein epitopes. These epitopes are presented in complex with MHC class I and II molecules in the priming step of adaptive immunity; they are recognized as foreign threats and can elicit a cytotoxic T-cell response. However, not all mutations are equally immunogenic; the degree of immunogenicity likely depends on the extent of sequence divergence of the mutated region, as well as the affinity of the mutated region to bind MHC molecules for presentation. The mutational burden of the cancer cell also determines the potency of the adaptive response, and cancers with many mutated genes are typically more immunogenic. However, the adaptive immune system is regulated by a number of checkpoints designed to avert over-aggressive or autoimmune responses. Among these are the expression of the PD-L1 receptor, which can suppress the actions of cytotoxic T cells (Figure 5–9). Tumor cells commonly demonstrate increased expression of PD- L1, which enables them to evade being targeted by T cells. Tumor cells also downregulate their expression of MHC class I molecules, which also averts recognition and targeting by cytotoxic T cells. Downregulation of MHC molecules can enable tumor cells to evade the adaptive immune system, but the absence of these “self” identifiers comes at the cost of increased susceptibility to natural killer cells of the innate immune system; tumor cells thus walk a fine line

in the regulation of their MHC proteins. It is believed that many tumor cells are cleared by these components of the innate and adaptive immune system early in their evolution through a process called immunosurveillance. However, some tumors eventually develop mechanisms to survive and are able to balance the pro-tumorigenic and anti-tumorigenic elements of the immune system in their favor, enabling them to escape this policing step. This process of immuno- evasion eventually leads to the development of clinical cancers (see Figure 5–8). Reverting the balance between pro-tumorigenic and anti-tumorigenic factors has become the mainstay of modern immunotherapy approaches.

FIGURE 5–9 The immune system can recognize foreign peptides such as mutated residues from cancer cells. Presentation of the peptide results in priming of T cells, as shown on the left. Activated cytotoxic T cells can then identify tumor cells expressing the mutated residue and kill them. This system has checkpoints built in to restrain the immune system from over-aggressive targeting. Both stimulatory and inhibitory interactions are involved in this regulation. Upregulation of PD-L1 by many tumor cells is a mechanism that enables them to suppress the cytotoxic actions of activated T cells and escape the immune

system.

Well-formed tumors are sometimes significantly infiltrated by immune cells, sometimes at the margins and other times deep within the tumor parenchyma. The tumor-infiltrating lymphocytes (TILs) are important in such cases and may include different subsets of regulatory and cytotoxic T cells and B cells. Some tumors demonstrate significant inflammation, which involves many cytokines, including interferons and IL-2, which can enhance the expansion and anti-tumor activity of T cells. Tumors are also often infiltrated by macrophages called tumor-associated macrophages (TAMs) (see Figure 5–8). TAMs are known to promote tumor growth by secreting immunosuppressive factors that inhibit effector T cells, by secreting growth factors that promote tumor growth or stimulate angiogenesis, and by secreting factors and proteases that facilitate matrix remodeling (see Figure 5–9). On the other hand, some tumors experience less infiltration by immune cells and appear not to engage the activities of the immune system as much. Much diversity in the interactions of tumors with the immune system remains to be defined.

CHECKPOINT

22. What are some of the mechanisms by which cancer cell adhesion is altered?

23. What role do tumor-associated macrophages play in tumor biology? 24. How can tumor cells evade being killed by primed cyto-toxic T cells?

ALTERATIONS IN METABOLISM & OXYGENATION IN NEOPLASIA

In addition to abnormalities in cell proliferation and survival, signal transduction, adhesion, and migration, tumor cells exhibit changes in metabolic pathways to meet their increased metabolic requirements. Oxygen pressure is reduced in tumor tissues, and tumor hypoxia signals change in gene expression, mediated through the hypoxia-inducible factor-1 alpha (HIF1α) transcription factor for adaptation to the hypoxic environment. Tumor cells secrete angiogenic growth factors, which signal the proliferation of vascular structures

into tumor tissue for nutrition and oxygenation. The identification of tumor factors that signal pathologic neovascularization has been of particular interest because such factors could be targets for therapeutic drug development for treatments that inhibit tumor angiogenesis. The best-studied pro-angiogenic factor is the vascular endothelial growth factor (VEGF), a mitogen to endothelial cells often secreted by tumor cells that activates the VEGF receptors in endothelial cells, leading to de novo vascularization. Although most cells do not ordinarily express VEGF, malignant transformation often results in the induction of VEGF expression by tumor cells, either directly through the effects of oncogenes or the loss of tumor suppressor genes or indirectly as a result of hypoxia and the induction of hypoxia-induced gene transcription. Other growth factors also have pro-angiogenic effects, including epidermal growth factor, fibroblast growth factor, PDGF, transforming growth factor-α, and others.

Tumor cells needs to generate energy and synthesize biomass at rates much higher than normal cells to sustain their high proliferation rates. As a result, tumor cells frequently have high rates of protein synthesis to supply all the structural and signaling proteins, high rates of fatty acid synthesis to supply structural membrane lipids and signaling lipids, and high rates of nucleotide synthesis to supply enough DNA and RNA. And they need to do this despite reduced nutrient availability owing to lessened and disrupted vascular supplies. In particular, glucose metabolism is uniquely altered in tumor cells. In normal cells, glucose generates some ATP in the initial metabolic steps of glycolysis to generate pyruvate. The resulting pyruvate is then transported to mitochondria, where it undergoes oxidative phosphorylation along the electron transport chain to generate many more ATP molecules. The metabolism of glucose by oxidative phosphorylation requires oxygen but is a highly efficient mode of generating energy from glucose. When oxygen availability is low, cells increase the rate of glycolysis, with ultimate conversion to lactate, to generate ATP. Tumor cells preferentially metabolize glucose through high rates of glycolysis and lactate production, even when oxygen is abundant. This counterintuitive finding is named the Warburg effect (Figure 5–10). The increased energy requirements of tumor cells and their less efficient means of glucose metabolism account for the substantially increased uptake of glucose by tumor cells compared with normal cells. This wide differential in glucose uptake forms the basis for radiologic imaging modalities that can detect tumors by imaging the accumulation of injected radiolabeled glucose analogs at tumor sites throughout the body. One example of this is positron emission tomography (PET) scanning using [18F]- fluorodeoxyglucose as the radiotracer. This imaging technique is widely used in

clinical oncology to locate tumors much more specifically than is possible with conventional computed tomography (CT) scanning.

FIGURE 5–10 Normal cells metabolize glucose using glycolysis followed by oxidative phosphorylation in the mitochondria when oxygen is abundant. This highly efficient mode of metabolism generates plenty of ATP molecules from glucose. If the oxygen supply is limited, cells instead convert the pyruvate to lactate and increase the rate of glycolysis to generate ATP, although this is a far less efficient mode of generating energy. Tumor cells are observed to use this same, less efficient pathway of glycolysis converting pyruvate to lactate even when oxygen is abundant and thus have high glycolytic rates to meet their energy demands. This observation is called the Warburg effect.

The amino acid serine is uniquely important for tumor cells. In addition to its

role in protein synthesis, serine is metabolized by cells to provide one-carbon units that can support the synthesis of nucleotides required for DNA and RNA. The amino acid glutamine is also uniquely important for tumor cells. In addition to its role in protein synthesis, glutamine plays an important role in energy generation, since it is abundant in serum and tissues, it can be readily transported into cells, and its metabolites can generate energy for tumor cells via the Krebs cycle. Although serine and glutamine are not essential amino acids, tumor cells rely heavily on their uptake to meet their enormous needs for synthesis and energy. In addition to avid uptake of these amino acids from serum, another way tumor cells deal with threatened nutrient deprivation is through autophagy, which involves the breakdown of cellular organelles in autophagosomes to generate amino acids and other metabolites.

Tumor cells reprogram their metabolic circuitry primarily to meet their synthesis and energy needs. However, this reprogramming has widespread secondary effects far beyond the production of biomass and energy. Many of the metabolites used to generate biomass or energy are also substrates for the post- translational modification of proteins, including acetylations, methylations, and glycosylations. Such post-translational modifications are the backbone of chromatin modification and form the basis for the epigenetic regulation of the genome. As such, altered tumor metabolism is invariably linked with the epigenetic deregulation of the tumor genome, with widespread consequences for gene expression and genome integrity. Some of the metabolites that are substantially increased in tumor cells and promote tumorigenic properties have been termed oncometabolites (Table 5–5). Pharmacologic approaches to reduce oncometabolite levels are providing a new direction for cancer therapeutics.

TABLE 5–5 Oncometabolites commonly found at high levels in cancers.

Another consequence of altered metabolic programming in cancer cells is a deregulation of reactive oxygen species (ROS) homeostasis. Normal cells use protective mechanisms to minimize ROS levels produced by the electron transport chain, since excessive ROS levels can damage proteins, DNA, and lipids. Increased ROS levels, which create a situation referred to as oxidative stress, are commonly found in tumor cells and appear to play a pro-tumorigenic role. Oxidative stress can function to promote tumorigenesis by increasing DNA damage and genomic instability and by inhibiting protein phosphatases, thus promoting tumorigenic signaling. However, high enough ROS levels can damage proteins and lead to apoptosis. As such, tumor cells also have increased levels of antioxidant proteins to protect against lethal levels of oxidative stress. Neoplastic cells must walk a fine line in the regulation of oxidative stress.

CHECKPOINT

25. What makes tumor cells react to hypoxia? 26. How do tumor cells stimulate the growth of blood vessels? 27. What is the Warburg effect? 28. What is autophagy? 29. What do oncometabolites do that is detrimental?

THE ROLE OF THE MICROBIOME IN NEOPLASIA

The human body encompasses both sterile tissue compartments and a number of nonsterile anatomic compartments. The latter harbor gigantic numbers of bacterial organisms. Indeed, their number vastly exceeds the number of human cells in the body. These nonsterile sites include the skin, oral cavity and nostrils, GI tract, lung, vagina, uterus, and urogenital tract, with the largest population of bacteria residing in the GI tract. The so-called microbiome often engages in a symbiotic relationship with the human host; for example, in facilitating digestion by detoxifying dietary components or by breaking down otherwise indigestible carbohydrates. However, it is now evident that the microbiome also engages in activities relevant to the development of human diseases, including cancer. These activities increase the incidence of certain cancers and reduce the incidence of others.

The microbiome is highly diverse in its content but is also incredibly diverse among individuals. This diversity is associated with dietary, hygienic, behavioral, and environmental factors. This diversity may also account for the large differences observed in cancer incidences among populations from different nations, cultures, continents, and time periods. In addition, the frequent use of antibiotics may alter the composition of the microbiome, which may affect the incidence of various cancers.

The precise mechanisms by which resident bacteria may promote tumorigenesis are not completely understood, but several mechanisms have been proposed. Bacteria can secrete a number of genotoxic products, inducing DNA damage in adjacent epithelial cells. Bacteria can generate metabolites that affect epigenetic programming in nearby epithelial cells. They can secrete factors that directly interact with proteins on the surface of or inside epithelial cells. They can influence cell signaling in ways that promote the development of neoplastic change. And they can secrete factors that interfere with immune system surveillance in an unfavorable manner, perhaps inhibiting immune effector cells that function to detect and eliminate tumor cells.

The most definitive example linking the microbiome to cancer has been the bacterial species Helicobacter pylori. H pylori colonizes the gastric mucosa of a large subset of the human population, and its presence is associated with the development of gastritis and an increased risk of developing gastric epithelial cancer and one type of gastric lymphoma. This is largely attributed to a number

of virulence factors secreted by H pylori that can damage the gastric epithelium as well as to an increased production of free radicals that can damage cell DNA. The microbiome can contribute to cancer development not only by containing a specific cancer-promoting species such as H pylori, but also by altering the relative abundance of various species. For example, an increase in abundance of Fusobacterium species has been linked to the development of colorectal cancers, although the mechanisms are not yet well understood.

VIRAL CAUSES OF NEOPLASIA

Viruses carry genetic material and express their genes when they infect host cells. The expression of these genes in infected cells can lead to proliferative phenotypes that span the spectrum from benign growths to malignant cancers. Different classes of viruses have been associated with human tumorigenesis, including viruses carrying DNA or RNA (Table 5–6). Some of these viruses are highly prevalent and even endemic in certain areas of the world. For example, the hepatitis B and C viruses and the human papillomavirus (HPV) are among the most prevalent; together, they account for a significant burden of global cancer mortality. It is hoped that viral causes of cancer mortality can be reduced by effective vaccination strategies.

TABLE 5–6 Oncogenic human viruses.

Viruses can promote tumorigenesis through different mechanisms. Some viruses carry genes encoding proteins that target and inactivate human tumor suppressor genes. The best studied example is HPV, which is transmitted by sexual contact. HPV accounts for many cancers of the oropharynx and genital regions, particularly cancers of the uterine cervix and anus. Although the HPV genome encodes many genes, among them are two genes, E6 and E7, which are oncoproteins. These oncoproteins interfere with many cellular pathways. For example, the E6 protein can bind and inactivate the p53 tumor suppressor gene

in human cells, and the E7 protein can bind and inactivate the Rb tumor suppressor gene. This oncoprotein interference represents a mechanism other than genomic mutation for the inactivation of tumor suppressors in human cells. The E6 and E7 proteins likely interfere with other cellular pathways as well. There are more than 100 strains of HPV, and only a few are considered “high risk” in terms of promoting cancer. The differences between the “low-risk” and “high-risk” types of HPV appear to be related to differences in the E6 and E7 genes carried by the different types of HPV and perhaps also in their level of expression. Many low-risk types of HPV cause neoplastic growths such as benign warts in the skin, feet, or genital areas but do not cause malignancy. The high-risk types of HPV cause pre-cancerous changes in the cervical epithelium and invasive cancers of the cervix, anus, and oropharynx.

Although some viruses, such as HPV, can promote tumorigenesis directly through oncogenes encoded in their genome, other viruses can promote tumorigenesis indirectly. The hepatitis C virus (HCV) does not encode a protein that inactivates host tumor suppressors. However, its cytotoxicity to hepatocytes leads to chronic inflammation, oxidative stress, and eventual fibrosis and cirrhosis; in this case, it is thought that tumorigenesis is promoted indirectly through persistent chronic inflammation.

CANCER METASTASIS

The growth of a malignant tumor at its primary site can induce local tissue destruction, obstruction, and various other associated symptoms. But, since the primary tumor can often be surgically removed, these problems usually do not account for the cancer’s lethality. Instead, the most lethal attribute of cancer is its propensity for metastasis; that is, the spread of the cancer to other areas of the body where it can generate new tumors. Metastasis does not follow a random distribution pattern; rather, each type of cancer favors metastasis to specific organs. Prostate cancers almost always metastasize to bone. Melanomas have a predilection for the liver and brain. Even for a given cancer, different subtypes typically metastasize to different organs. For example, luminal-type breast cancers (those with cells resembling the inner [luminal] epithelial cells lining the mammary ducts) typically metastasize to bone, whereas basal-like breast cancers (those with cells resembling the basal cells that line the ducts) metastasize to the liver and lung.

The mechanisms that underlie the metastasis of tumor cells to distant organs

are not entirely clear. Tumor cells can acquire increased motility, which enables them to separate from the primary tumor mass. Such rogue cells can then travel through lymphatic channels, through the bloodstream, or directly through connective tissues. Some epithelial tumor cells may undergo a process that fundamentally changes their cytoskeletal and signaling protein expression patterns to temporarily resemble more flexible mesenchymal cells. This reversible process, called epithelial-to-mesenchymal transition, may enable them to traverse the extracellular matrix better by squeezing through tight openings such as in between endothelial cells, effectively enabling them to enter the vasculature and to exit at distant sites, and subsequently to return to their original epithelial state. The reason some tumors appear to prefer certain distant sites of metastasis is not well understood. But it is thought that certain factors at specific sites of metastasis provide a fruitful microenvironment for certain tumor cells. This has been called the “seed-and-soil hypothesis” for the pathogenesis of tumor metastasis. However, the specific elements that constitute the “seed” and the “soil” are not yet well defined. Tumor cells may express certain chemokine receptors, driving them to sites where their chemokine ligands are abundant. Some tumor cells may home into certain distant sites seeking specific growth factors for support. The expression of specific integrin receptors may favor their binding to matrix proteins at specific organ sites. The bone marrow is a common site for the metastasis of many tumors, possibly because of the rich growth factor milieu and easy access from the bloodstream.

While the mechanisms that enable the traveling and homing of tumor cells to specific distant sites are fundamentally important in mediating metastasis, these mechanisms make up only half the metastatic process. Once a tumor cell has arrived at a distant site, its proliferative capacity alone is insufficient to generate a new tumor at the site of metastasis. In fact, most tumor cells are incapable of generating a new tumor at a distant site. The ability to do so is found in only a very small subset of tumor cells, typically called cancer stem cells or tumor- initiating cells. Defining and identifying tumor-initiating cells and determining the mechanisms that enable them to generate an entirely new tumor is a matter of ongoing study.

The process of metastasis follows a continuous pattern of branching and cross-seeding (Figure 5–11). Multiple metastases may originate from a single ancestral clone of cells within a primary tumor or from different ancestral clones within the primary tumor. An established metastasis may then seed many new metastases, essentially serving as a branch point in metastatic dissemination. This process may then proceed with multiple metastases seeding additional

generations of metastases. In addition, cells from one metastasis may travel and land at a site of another pre-existing metastasis, which is referred to as a cross- seeding of metastases. These patterns of tumor dissemination, suspected for decades, have only recently been confirmed by phylogenetic analysis of tumor genomic samples obtained from many different anatomic sites in autopsy studies of deceased patients.

FIGURE 5–11 Selected tumor cell clones from a primary tumor can seed new sites of metastasis. Selected tumor cell clones from these metastases can, in turn, seed additional sites of metastasis, and the formation of new metastases can proceed in a branched fashion, with independent lineages harboring different mutational ancestries. In addition to seeding new metastases, tumor cells disseminating from one metastatic site can cross-seed another already established metastatic site.

CELLULAR HETEROGENEITY IN NEOPLASIA

The molecular changes of neoplastic cells and their phenotypic behavior is a constantly evolving process. Every cell division can result in additional genomic abnormalities with a variety of phenotypic consequences. Certain genotypes result in proliferative, survival, or other biological attributes that favor clonal expansion. Such neoplastic clones can eventually overtake the tumor cell population and change its clinical behavior. This remodeling process occurs repetitively with recurrent cell divisions, recreating a process akin to evolution, albeit in a much shorter time frame. Attributes acquired early in the evolution of a cancer include enhanced proliferation and survival. Changes acquired at the midpoint of evolution include the ability to overcome spatial limitations by invading surrounding tissues, the ability to survive under conditions of low

oxygen and few nutrients, and the ability to evade host immune defenses. Changes acquired later in the progression of neoplasia are the ability to travel to distant organs and the ability to resist anti-cancer treatments.

The evolving nature of a cancer with repeated cell proliferation cycles along constantly expanding cell lineages creates heterogeneity in the whole tumor cell population. Tumor cell heterogeneity is a common characteristic of many types of cancer and introduces significant complexity in the molecular or histologic characterization of cancers. Tumors may have some areas with poor differentiation and other areas that are more differentiated. Certain regions may harbor mutational activation of one oncogene, whereas other regions may demonstrate activation of a different oncogene. Some areas of the tumor may be hypermetabolic or hyperproliferative, whereas other areas may have reduced metabolic or proliferative activities. The inherent heterogeneity of a cancer is a considerable barrier to its effective treatment since each type of treatment may affect only one subpopulation of tumor cells. At one extreme end of tumor heterogeneity is the existence of tumor cells with such low metabolic and proliferative activity that they are considered to be in a state of dormancy. Such tumor cell dormancy can last for a period of many years, after which the cells may recover, leading to late relapses of tumors previously thought to have been eradicated.

Although many (or most) of the cells that arise from cancer cell division themselves proceed to multiply, the changes with repeated cycles of cell division often lead to the loss of some of the more fundamental properties of the ancestral cancer cells. For example, many tumor cells are unable to give rise to a new tumor if isolated and re-implanted. In fact, only a small proportion of cancer cells appear to be capable of starting new colonies of cancer cells if isolated and implanted in, or metastasized to, a new site in the body. Such cells, named cancer stem cells, typically do not proliferate as quickly as most other tumor cells but are capable of self-renewal and of generating daughter cells that can proliferate more quickly and produce new tumors.

CHECKPOINT

30. What are cancer stem cells? 31. What is an example of a bacterial cause of human cancer? 32. How do viruses cause tumors? 33. Can metastases metastasize?

NEOPLASIA CLASSIFICATION

The terms “neoplasia” and “neoplasm” describe a large number of human diseases with extremely diverse characteristics. At the first level of classification, neoplasms can be benign or malignant. Benign neoplasms are relatively straightforward to name and classify, since they are easily identified as an overgrowth of the readily identifiable normal tissue within which they arise. The suffix “-oma” is typically applied to the primary tissue name to describe the benign overgrowth of the tissue. Examples include “lipoma” to describe a benign neoplasm of adipose tissue, “osteoma” to describe a benign neoplasm of bone, and “adenoma” to describe a benign neoplasm of a wide variety of glandular epithelial tissues such as colon, breast, and lung. The classification of malignant neoplasms is much more complex, since their morphology often does not resemble their tissue of origin, and they may also appear at anatomic sites of metastasis not reflecting their organ of origin. Therefore, the classification of malignant neoplasms (cancers) into categories and subcategories is of great value in diagnosing, studying, understanding, and developing treatments for them. Malignant transformation involves the development of abnormal cellular behavior. In some cancers, the abnormalities are subtler and the cancer cells retain many of their specialized tissue functions. These cancers are referred to as “well differentiated.” In other cancers, the abnormalities are extreme, such that the cancer cells bear little resemblance to the normal cells in their primary tissue. Such cancers referred to as “poorly differentiated” if it is difficult to identify their tissue of origin by microscopic examination. In such cases, molecular markers may be required to identify a cancer’s origin.

The broadest classification of tumors relies on the most fundamental characterization of cell types based on their primitive embryologic origins. During early embryonic development, three cell lineages are established: ectoderm, endoderm, and mesoderm. All subsequent cells, including those of adult tumors, can be traced to one of these three cellular origins. As such, tumors are broadly classified into the categories of carcinoma if they originate from ectodermal or endodermal tissues, or sarcomas if they originate from mesodermal tissues. Even if completely unrecognizable by morphologic analysis, fundamental differences in the expression of certain proteins, especially intermediate filaments such as keratins and vimentin, will identify the lineage of origin. Fundamental differences in the biology of endodermal or ectodermal

tissues from that of mesenchymal tissues underlie the vast differences in the cancers that arise from these different cell types. Endodermal and ectodermal origin tissues form much of the “front-line” tissues of the body (eg, skin, GI tract, respiratory tract) that are exposed to the outside environment; are in contact with the microbiome; and have cell structures, polarity, and adhesive functions critical for their “border control” functions. (A notable exception is the nervous system, which also arises early from the ectoderm). On the other hand, mesenchymal origin tissues are sterile tissues, are generally not exposed to the environment, and are more focused toward bodily functions not directly related to the environment.

Carcinomas are the most common type of cancer and include all the common epithelial tissue cancers such as lung, colon, breast, and prostate cancers. Sarcomas arise from mesenchymal cell types, predominantly in the connective tissues. Further classification of carcinomas and sarcomas is based on the organ of origin. In the growing infant and child, mesenchymal tissues are very active in growth and remodeling, and mesenchymal tumors are common (eg, in the muscle, cartilage, bone, and blood). In adults, the mesenchymal tissues are not very active, and epithelial tumors are by far the most common (eg, in the lung, breast, prostate, and colon). Malignancies of blood cells, including leukemias and lymphomas, are technically a subtype of sarcomas because they are of mesenchymal origin. However, because of the highly specialized nature of hematologic cell types, they are generally grouped together under the entity of “hematologic neoplasms.” Developments in the gene expression profiling of tumors have enabled tumor classification based on characteristic molecular portraits, and further work in this area may result in an entirely new classification of human tumors based on their gene expression profiles.

EPITHELIAL NEOPLASIA

Epithelial cells are in constant turnover, arising from a basal layer that continually generates new cells. The mature and functional layer of cells performs specialized tissue or organ functions and, with senescence, is eventually sloughed off. Proliferating epithelial cells normally observe anatomic boundaries such as the basement membrane that underlies the basal layer of cells in the epithelium. The potential to divide, migrate, and differentiate is tightly controlled. The stimulus to divide may be autonomous or exogenous as a response to factors from adjacent or distant cells. Inhibitory signals and factors

may also be present and serve to function as negative regulators to check uncontrolled growth. The neoplastic phenotype of epithelial cells can be seen as a spectrum from hyperplastic to pre-invasive to frankly invasive and metastatic neoplasia, as illustrated in Figure 5–12. Because of their embryonic origins, malignancies of epithelial origin are termed carcinomas. Hyperplasia can be a normal physiologic response in some situations, such as that which occurs in the lining of the uterus in response to estrogens before the ovulatory phase of the menstrual cycle. It can also be a pathologic finding associated with a predisposition to progress to invasive carcinoma. In such instances of hyperplasia, there are usually accompanying disorders of maturation that may be recognizable by microscopic examination. These changes are termed dysplasia, atypical hyperplasia, or metaplasia, depending on the type of epithelium in which they are observed. More aggressive proliferation without the ability to invade through the basement membrane is termed pre-invasive carcinoma or carcinoma in situ. Technically, these cells do not have the capacity to invade the basement membrane and metastasize, although they may over time progress to invasive carcinoma. The term “invasive carcinoma” implies that tissue boundaries, especially the basement membrane, have been breached. Metastatic carcinoma occurs via the lymphatic system to regional lymph nodes and via the bloodstream to distant organs and other tissues. This pattern of metastasis, however, is not unique to epithelial malignancies. In general, epithelial neoplasms have a (variable) propensity to spread to regional nodes as well as to distant sites.

FIGURE 5–12 The phenotypic transition of epithelial cells from normal to hyperplasia to pre- invasive carcinoma in situ to invasive carcinoma to metastatic carcinoma.

From a pathophysiologic standpoint, certain structural and functional characteristics are acquired by malignant cells in the course of their evolution, as outlined in Table 5–7. What follows in the next section is a more in-depth discussion of three epithelial cancers to illustrate some of the pathophysiological principles underlying their development. Colon cancer is an example of an epithelial neoplasm for which stepwise molecular evolution has been well documented, as colonoscopy has enabled precursor lesions to be readily identified, biopsied, and studied. Breast cancer is a type of neoplasm with distinct subtypes driven by distinct biologic drivers. And lung cancer is an epithelial neoplasm directly associated with environmental mutagens and driven by mutationally activated oncogenes.

TABLE 5–7 Phenotypic changes in the progression of neoplasia.

CHECKPOINT

34. What is the difference between carcinoma and sarcoma? 35. What is carcinoma in situ?

1. Colon Carcinoma The model of stepwise genetic alterations in cancer is best illustrated by observation of colonic lesions at different stages of progression to malignancy. Certain genetic alterations are found commonly in early-stage adenomas, whereas others tend to occur more frequently at later stages of tumorigenesis or after the development of invasive carcinoma. The timeline of genotypic and phenotypic changes during the development of colon cancer are shown in Figure 5–13. These changes are in keeping with the concept that serial phenotypic changes, driven by serial genetic alterations, must occur in a cell for it to exhibit full malignant (invasive and metastatic) properties.

FIGURE 5–13 The sequential phases of cellular abnormalities and polyp formation in the colon are shown on a timeline that also depicts the corresponding genetic events associated with each cellular landmark.

The earliest molecular defect in the pathogenesis of colon cancer is activation of the Wnt signaling pathway, which occurs through the development of somatic mutations in the APC tumor suppressor gene in the normal colonic mucosa. This leads to abnormal cell proliferation and the initial steps in neoplasia that lead to formation of polyps in the colon. At an early point in polyp formation, growth factor receptor signaling pathways are activated. This occurs through mutational activation of the KRAS, NRAS, or BRAF signal transduction oncogenes, which further remove restraints on cell proliferation and enable polyps to grow bigger. The next step in tumorigenesis are mutations in genes involved in the TGF-β signaling pathway. This further deregulates growth patterns and properties of enlarging polyps. Deletion or loss of expression of the DCC gene is common in the progression to invasive colon cancers. The DCC protein is a transmembrane protein of the immunoglobulin superfamily and may be a receptor for certain extracellular molecules that guide cell growth and/or apoptosis. Mutational inactivation of TP53 disrupts cell cycle checkpoints, staves off apoptosis, and occurs at a late stage in the progression to an invasive cancer phenotype. The genes specifically responsible for subsequent progression to a metastatic phenotype are not yet well understood.

In parallel to these sequential abnormalities in the regulation of cell proliferation, colon cancers also acquire defects in mechanisms that protect genomic stability. These generally occur along one of two mechanisms. In some colon cancers, the abnormalities are a result of mutations in mismatch repair genes. Mismatch repair genes are a family of genes involved in “proofreading” DNA and correcting incorporation errors during replication; they include MSH2, MLH1, PMS1, and PMS2. Tumors with mutations in mismatch repair genes perform DNA replication with many errors, and thus these tumors develop thousands of mutations, resulting in what is referred to as a hypermutator phenotype. This phenotype can be detected by a PCR-based microsatellite instability (MSI) assay in which the hypermutator tumor phenotype will test MSI high. In colon cancers that are not MSI high, genomic instability occurs as a result of mutations in chromosomal instability (CIN) genes. Because of their impaired ability to preserve chromosome structure and content, these tumors have losses or gains of large segments of chromosomes or entire chromosomes and are highly aneuploid.

The stepwise acquisition of genetic abnormalities described above is associated with alterations in the phenotypic behavior of the colonic mucosa. The earliest change in the progression to colon cancer is an increase in cell number (hyperplasia) on the epithelial (luminal) surface. This produces an

adenoma, which is characterized by gland-forming cells exhibiting increases in size and cell number but no invasion of surrounding structures (Figure 5–14). Presumably, these changes are a result of enhanced proliferation and loss of cell cycle control but occur before acquisition of the capacity to invade the ECM. Additional dysplastic changes, such as loss of mucin production and altered cell polarity, may be present to a variable degree. Some adenomas may continue to enlarge and ultimately progress to invasive carcinoma. An early feature associated with disrupted architecture even before invasion occurs is the development of fragile new vessels or the destruction of existing vessels, which can cause microscopic bleeding. This can be screened for clinically with the fecal occult blood tests used for early diagnosis of pre-invasive and invasive colon cancer. All phenotypic changes cannot be explained by a known genetic abnormality, nor do all identified genetic alterations have a known phenotypic result. However, the stepwise nature of genotypic and phenotypic abnormalities is well established and strongly supports the mechanistic link between these parallel processes.

FIGURE 5–14 Edge of an adenomatous polyp, showing adenomatous change (left), compared with normal mucosal glands (right). Adenomatous change is characterized by increased size and stratification of nuclei and loss of cytoplasmic mucin. Note the arrangement of nuclei of the adenoma perpendicular to the basement membrane (polarity). (Reproduced, with permission, from Chandrasoma P et al. Concise Pathology, 3rd ed. Originally published by Appleton & Lange. Copyright © 1998 by The McGraw-Hill Companies, Inc.)

Further functional changes in the cell and surrounding tissue are also

manifested in the pre-invasive and invasive stages. Once the basement membrane is penetrated by invasive malignant cells, access can be gained to the regional lymphatics and spread to regional pericolic lymph nodes can occur. Entry of cells into the bloodstream can lead to distant spread in a pattern that reflects venous drainage. Therefore, hematogenous spread from primary colon tumors to the liver is common, whereas rectal tumors usually disseminate to the liver, lung, and bone. In addition to anatomic considerations, there may exist specific tropism of malignant cells mediated by surface proteins that cause the cells to preferentially home in on certain organs or sites.

Colonic epithelium is specialized to secrete mucus proteins and to absorb water and electrolytes (Chapter 13). The maintenance of a tight luminal barrier, intracellular charge differences, and the ability to exclude toxins are additional specialized functions. Some of these functions are maintained in the progression to neoplasia and may contribute to a specific phenotype of the malignant cell. One example is the expression of a transporter membrane protein, MDR-1, present on several types of epithelium, including the colon. MDR-1 is known to cause efflux of several compounds out of the cells, presumably as a protective mechanism to exclude toxins. In advanced colon cancer, this protein may contribute to the relative resistance of this and other tumor types to a variety of chemotherapeutic agents transported by MDR-1. In some cases, the activation of a latent gene-encoding carcinoembryonic antigen (CEA) can result in measurable levels of the CEA protein in the serum of patients with localized or metastatic colon cancer (as well as other adenocarcinomas).

The timeline and frequency for the transformation of normal colon epithelial cells to colon cancer is determined by the stochastic nature of genetic mutations and the multitude of such mutations required for the full malignant phenotype to become manifest. This process can be accelerated by both endogenous and exogenous events. Endogenous events account for the inherited susceptibility to colon cancer, whereas exogenous events account for the dietary and environmental contributors to colon cancer.

Endogenously, if one of the required mutations is pre-existing in the germline DNA acquired at conception, then that individual begins life with colon epithelial cells already well along the pathway of tumorigenesis. As an example, the APC gene, which is one of the earliest genes mutated during colon tumorigenesis, is genetically mutated in the germline of some families. Family members who inherit this mutated APC gene develop the clinical syndrome of familial adenomatous polyposis (FAP), characterized by the development of extremely numerous colon polyps and eventual development of colon cancer at a

very young age. Mutations in other genes, such as MSH2 or MLH1, which are involved in DNA mismatch repair, are also seen in the germline of some families. Family members who inherit these mutated genes develop the clinical syndrome of hereditary non-polyposis colorectal cancer (HNPCC). This syndrome is also characterized by a high frequency of developing colon cancer, although with fewer polyps and an older age at onset than seen in FAP.

Exogenously, the development of mutations in the colon epithelial cells can also be accelerated by chemical compounds within the lumen of the gut or within the bloodstream. These include chemical substances derived from bacterial colonic flora, from ingested foods, or from endogenous metabolites such as fecapentaenes, 3-ketosteroids, and benzo[α]pyrenes, which are mutagenic. Levels of these substances can be reduced by low-fat, high-fiber diets; several epidemiologic studies confirm that such diets reduce the risk of colon cancer.

Dietary and environmental factors may also interact with inherited genetic changes to promote the development of colon cancers. In particular, there may be inherited genes, which, by themselves, cause only a slight increase in colon cancer incidence and thus do not create familial clusters of colon cancer cases. But these genes may promote the development of colon cancer in specific individuals exposed to certain dietary or environmental risk factors.

CHECKPOINT

36. What are some of the genes mutated in the evolution of colon cancers? 37. How does mismatch repair deficiency occur in colon cancers? 38. What causes familial adenomatous polyposis?

2. Breast Carcinoma The female breast is a specialized gland that undergoes repeated cycles of growth factor– and hormone-induced changes that define the different stages of breast development (ie, fetal, pubertal, menstrual, pregnancy-associated, lactational growth, and postlactational involution). Deregulation of this complex biology leads to a diverse group of neoplastic breast diseases inherently connected with growth factor or hormonal signaling. These range from benign fibrocystic changes of the breast to malignant tumors of the breast. Three broadly defined molecular pathways appear to be relevant to the

pathophysiology of breast cancers: hormone signaling, growth factor signaling, and DNA repair pathways.

Hormone Signaling

The prolonged use of exogenous estrogen and progesterone therapies, typically used to postpone menopause, is a breast cancer risk factor that clearly implicates the hormonal signaling pathway. In contrast, reduced exposure to estrogen and progesterone protects against the development of breast cancer. This has been demonstrated in ovariectomized animal models of breast carcinogenesis and confirmed by clinical studies demonstrating that women who have undergone oophorectomy at a young age have a significant reduction in their lifetime risk of developing breast cancer. The clinical success of anti-estrogen therapies provides proof of principle of the essential role of estrogen signaling in the pathogenesis of breast cancer. Agents that inhibit the production of estrogen or the ability of estrogen to activate the nuclear estrogen receptor (ER) are highly effective in reducing the development of breast cancer, or in stopping the progression of pre- invasive, invasive, or metastatic breast cancers. However, although the central roles of estrogen and the ER in the pathogenesis of breast cancer are now well established, the evidence to date does not etiologically implicate genetic abnormalities of estrogen, the ER, or downstream ER target genes in the development of breast cancer, and thus neither estrogen nor the ER is considered an oncogene. It appears that ER signaling is a physiological pathway existing in breast epithelial cells whose continued signaling activity is favorable to, or perhaps even necessary for, the transformational steps triggered by as yet unknown drivers of the neoplastic process. However, unlike oncogenes, estrogen signaling is clearly insufficient to initiate tumorigenesis, as the breasts of all healthy women are constantly under active estrogenic signaling without the development of tumors.

The importance of the estrogen signaling pathway does not extend to all breast cancers, however, since half of all breast cancers seem to demonstrate loss of activity of this signaling pathway or no expression of the ER. Some investigators believe that ER-negative breast cancer is a different disease with an alternative pathophysiology. Most likely, there are common early molecular steps in the development of ER-positive and ER-negative breast cancers; however, at an early or intermediate step, these pathways diverge, leading to the development of breast cancers with distinctly different phenotypes.

Growth Factor Signaling

A number of growth factor signaling pathways are implicated in the pathophysiology of breast cancers. The growth factor IGF-1 is an important regulator of mammary gland biology, and elevated serum IGF-1 levels are associated with an increased risk of breast cancers. However, abnormalities of the IGF1 receptor are not seen in breast cancers. Much more directly implicated are the growth factor receptors of the human epidermal growth factor receptor (HER) family. Amplification of the ERBB2 (aka HER2) gene and overexpression of the HER2 protein are common in pre-invasive and invasive breast cancers. Overexpression of the EGFR (aka HER1) gene is also seen with less frequency. The HER family receptors activate a number of downstream signaling pathways, including proliferative, apoptotic, and metabolic pathways. An important downstream pathway is the PI3K/Akt signaling pathway. Components of this pathway are also mutationally activated in breast cancers. In fact, the proto- oncogene PIK3CA, encoding the catalytic subunit of PI3K, is the most commonly mutated gene in breast cancers. The proto-oncogene AKT1 and the tumor suppressor gene PTEN are also mutationally or epigenetically altered in breast cancers, activating this important signaling pathway. The growth factor receptor signaling pathway intermediates are mostly oncogenes and, because of their abnormal activities, are not typically seen in their mutated form in the germline of individuals. However, mutated alleles of the tumor suppressor gene PTEN can be seen in the germline of some familial lineages, causing Cowden syndrome, with its high risk of breast cancer and other tumors.

DNA Repair Pathways

The loss of genomic stability is also a common event in the pathogenesis of many breast cancers. Many of the genes involved in maintaining DNA integrity are tumor suppressor genes and thus are involved both in the pathogenesis of breast cancers at the familial level through germline transmission of mutated alleles, and also in the pathogenesis of sporadic breast cancers through the somatic mutation at the tissue level. The BRCA1 and BRCA2 genes account for a majority of familial breast cancers. Mutated alleles of these genes are seen in 0.2–2% of various ethnic populations and confer a lifetime risk of developing breast cancer of up to 70%. Mutations in these genes are also associated with a high incidence of ovarian cancer in women and increased incidences of prostate cancer, melanomas, and breast cancer in men. Both of these genes function as tumor suppressor genes such that breast tumors contain both the inherited abnormality in one allele and a somatic loss of the remaining allele. Sporadic (nonfamilial) cases of breast cancer may also contain BRCA1 mutations or may

have abnormalities in other proteins that interact with BRCA1 to perform DNA repair functions involving double-strand breaks in DNA. The TP53 gene, also involved in the detection and response to DNA damage, is also frequently mutated in BRCA1- or BRCA2-defective cancers. The existence of a mutated TP53 gene in the germline also carries with it a very high risk and early onset of breast cancer, defining a familial clustering of cancers known as Li–Fraumeni syndrome. Other tumor suppressor genes involved in the detection of or response to DNA damage are also involved in the pathogenesis of breast cancers. The PALB2 gene, which works in partnership with BRCA2 in the repair of DNA, is also mutated in the germline of other familial lineages, conferring a high risk of cancers of the breast and pancreas. The ATM gene, also involved in the response to DNA damage, is mutated in some other breast cancers, and when inactivated in the germline, leads to the clinical syndrome of ataxia-telangiectasia, characterized by extreme sensitivity to radiation and predisposition to cancer. There are many other genes involved in DNA repair, which, when mutated, can contribute to the pathogenesis of breast cancers. However, many of these have only modest or low penetrance (ie, confer only a modest or slight increase in breast cancer risk), and thus their identification and association with cancer risk is more difficult to establish, and it will take many years to identify all such genes. Some that have been identified thus far include RAD51C, CHEK2, and BRIP1.

The Cellular Evolution of Breast Cancer

Cancer of the breast is almost always a result of the malignant transformation of secretory epithelial cells. The normal secretory breast epithelium consists of two layers: the basal myo-epithelial layer and the luminal epithelial layer. These layers can be identified by their characteristic patterns of expression of keratin filaments. The identity of the cell of origin of breast cancers has been debated for decades and has not yet been completely defined. Our current understanding suggests that all breast cancers arise from a cell subset within the luminal epithelial layer. However, in the course of tumorigenesis, some breast cancers can de-differentiate and ultimately develop characteristic features of the basal layer cells.

Abnormalities in the ductal epithelium are evident even before the onset of hyperproliferative states that exhibit excessive numbers and layers of luminal cells. Genetic abnormalities likely underlie the earliest form of ductal pathology, called flat atypia, wherein the normal single-layered epithelium architecture is preserved, although the cell population is replaced by atypical cells. Further

molecular evolution leads to hyperplasia, carcinoma in situ, and invasive cancer (Figure 5–15).

FIGURE 5–15 Early genetic events lead to proliferative ductal epithelial replacement, but not overgrowth. Additional genetic events lead to progressively increased and abnormal overgrowth (hyperplasia, then carcinoma in situ) and eventually to invasive ductal carcinoma. Changes in chromatin are evident in the nuclei early on and are one of the features of atypia.

Breast Cancer Subtyping by Morphology or Receptor Expression

Breast cancers are extremely heterogeneous in their morphology and biology. Such a diverse disease requires classification systems to better understand and manage it. Indeed, many classification systems have been proposed. Figure 5–16 depicts the three most common: the histologic subtypes classification, the clinical subtypes classification, and based on gene expression profiles, the biological (“intrinsic subtypes”) characterization. These different classifications reflect different biologies, prognoses, and responses to specific therapies.

FIGURE 5–16 Breast cancers are frequently classified using different classification systems. Three are shown here—histologic subtypes (top left), intrinsic subtypes (top right), and clinical subtypes (bottom) —with their proportional incidences shown via pie chart. The clinical subtypes, widely used in clinical management, do not divide breast cancers into exclusive categories, such that a cancer can be both ER/PR positive and HER2 positive (ie, HER2 amplified), as shown by the overlapping area in the bottom pie chart.

The oldest system is the histologic subtypes classification of breast cancer. By their distinct morphologies, the majority of breast cancers can be classified as ductal carcinomas, a minority as lobular carcinomas, and a few percent as other rare subtypes. Both in situ and invasive cancers fall into these two subtypes. Ductal carcinomas exhibit varying degrees of tubule formation, but lobular carcinomas have lost the ability to form tubules. This is due to a failure in cell– cell attachment as a consequence of the mutational loss of the cell adhesion protein E-cadherin. While ductal carcinomas make dense solid tumors, lobular carcinomas often do not. Instead, lobular carcinomas grow as disconnected and diffuse infiltrates of cells; consequently, they are typically much harder to detect manually and radiographically.

Another system to classify breast cancers is according to their expression of ER or PR and amplification of HER2. This clinical subtypes system is favored by clinicians because it is directly linked with treatment options. ER- or PR- positive breast cancers are amenable to treatment with anti-estrogen therapies. HER2-amplified breast cancers are clinically labelled HER2 positive and are

amenable to treatment with therapies targeting HER2. Breast cancers that lack all three features are called “triple-negative” breast cancers; they are important to identify because there are as yet no targeted therapy options for them. The clinical subtyping by receptor expression does not define exclusive subtypes, since hormone receptor expression and HER2 amplification can be present in the same cancer; thus, the ER-positive, PR-positive, and HER2-amplified subtypes overlap.

Breast Cancer Subtyping by Transcriptomic Analysis

The development of techniques to simultaneously determine the expression of 10,000 or more genes has revolutionized the way cancers can be classified. This kind of genome-wide transcriptome analysis reveals that breast cancers can be classified by molecular signatures into at least four intrinsic subtypes: basal- like, luminal A, luminal B, and HER2-enriched. These molecular subtypes have strong prognostic significance, with the luminal A subtype having the best prognosis and the basal-like subtype the worst prognosis. The subtypes are also linked with specific mechanistic characteristics. The basal-like subtype lacks a unifying molecular attribute, but its hallmark is a significant amount of genomic instability. The HER2-enriched subtype is linked with the amplification and overexpression of the ERBB2 (HER2) oncogene and the consequent downstream signaling events related to it. The two luminal subtypes are characterized by the expression of ER-linked genes, and ER function plays an important role in these cancers. The four intrinsic subtypes loosely simulate the clinical subtypes of breast cancer since the gene expression profiles are largely driven by ER or HER2.

In addition to its prognostic significance, this analysis of breast cancers by molecular signatures has predictive value regarding the sensitivity of each to various anti-cancer treatments. Indeed, different predictive gene signatures have been developed as commercial assays for clinical breast cancer samples and can provide validated prognostic and predictive scores, enabling more personalized treatment planning for individual patients.

CHECKPOINT

39. What are some of the molecular pathways involved in breast cancer pathogenesis?

40. What are some of the germline genes that cause familial breast cancers? 41. How many types of breast cancer are there? 42. What is the role of HER2 in breast cancer?

3. Lung Carcinoma Lung cancer is the greatest cause of cancer mortality worldwide. This is because the lung epithelium is highly exposed to environmental carcinogens, in particular those in tobacco smoke. Exposure to these carcinogens induces gene mutations in the epithelium along the entire respiratory tract. In fact, in smokers, even normal-appearing respiratory epithelium harbors gene mutations long before the onset of lung cancer. As such, lung cancer is an example of a cancer that is preceded by and arises from a genetic field defect (see Figure 5–1). And since gene mutations persist in the respiratory epithelium for years after cessation of smoking, smokers remain at risk for lung cancer for up to two decades after smoking cessation.

The respiratory tract is lined by different epithelial cell types, with bronchiolar epithelium lining the conducting and protective passages of the upper airways and lower bronchioles, and alveolar epithelium lining the distal alveoli (Figure 5–17). At least four distinct types of cancer develop in the lung, arising from distinct cells of origin in the bronchiolar and alveolar respiratory epithelium. Squamous-cell lung cancers are thought to arise from basal progenitor cells lining the bronchiolar epithelium. These undergo metaplastic change to develop squamous differentiation. Small-cell lung cancers are thought to arise from pulmonary neuroendocrine cells found scattered in the bronchioles. Adenocarcinomas are thought to arise from alveolar type 2 cells, which are thought to have progenitor capability. Large-cell lung cancer is a collective term applied to the remaining rarer subtypes of lung cancer. Consistent with the location of their cells of origin, squamous-cell and small-cell lung cancers are frequently found in more central areas of the lungs, whereas adenocarcinomas frequently arise in the distal regions of the lungs. Since small-cell lung cancers arise from a distinctly different (neuroendocrine) cell of origin and their biologic behavior is also very different, the other three types are usually collectively referred to as non-small-cell lung cancers to clearly differentiate them from small-cell lung cancer.

FIGURE 5–17 The repertoire of cells lining the respiratory system changes from the proximal airways to the distal alveoli. The proximal airways are involved in the passage of air, protection, and drainage of secretions, whereas the distal alveoli are involved in gas exchange. According to our current understanding, the different types of lung cancer arise from different types of cells within the respiratory system.

In contrast to colon cancer, in which gene mutations occur in a specific temporal sequence and govern the development of progressive stages of cellular pathology, the mutations incurred in the process of lung tumorigenesis do not follow a sequential order; instead, the pre-neoplastic respiratory epithelium is perhaps best described as experiencing an accumulation of mutations. For example, lung adenocarcinomas are frequently preceded by pre-neoplastic lesions called atypical adenomatous hyperplasia; their cells frequently harbor the same oncogene mutations found in the later adenocarcinomas.

Numerous oncogenes that are disease drivers for subsets of adenocarcinomas and squamous-cell lung cancers have been identified. These subsets are exclusive of one another, each defining a true molecular subtype of lung cancer (Figure 5–18). Each of these oncogenes is activated by different genomic events, such as gene fusions occurring as a result of chromosome translocation, internal deletions, point mutations, or gene amplifications. The development of the KRAS-mutated molecular subtype is strongly associated with smoking, whereas the development of EGFR- and other kinase-mutated molecular subtypes is more commonly seen in nonsmokers. Tobacco carcinogens induce point mutations in

DNA, and the activation of KRAS occurs through such point mutation. Other oncogenes typically require deletions or fusion events in the DNA to become activated, but these are not typical of tobacco-induced damage. The tumor- driving nature of many of these oncogenes has now been confirmed by the dramatic response to therapy with selective inhibitors of them. Work is ongoing to discover yet other molecular subtypes of lung cancers. Many of these oncogene-defined subtypes of lung cancer are quite rare in incidence, but their identification is critical, since it can lead to rational therapies with substantial therapeutic benefits. Thus, the current diagnostic and therapeutic management of lung cancer is a prime example of personalized medicine.

FIGURE 5–18 The four types of lung cancer are shown in the central pie chart according to their relative incidences. Because small-cell lung cancers derive from a distinctly different cellular origin than the other three types, the other three are collectively called non-small-cell lung cancer. Of these types, adenocarcinomas and squamous-cell lung cancers are driven by any of a number of different oncogenes, as depicted in their respective pie charts. The proportion of these cancers with unknown oncogenes may diminish as more discoveries are made. The relative incidences shown are generalizations, since the actual proportional incidences vary substantially depending on demographics.

CHECKPOINT

43. What is the principal cause of lung cancer? 44. What are some of the oncogenes that drive the biology of lung cancer?

MESENCHYMAL, NEUROENDOCRINE & GERM

CELL NEOPLASIA

Mesenchymal, neuroendocrine, and germ cell neoplasms account for a large proportion of the tumors of childhood and young adulthood, ostensibly because these cells are actively dividing and more subject to mutational events. Table 5–8 provides a representative list of mesenchymal, neuroendocrine, and germ cell tumors, as well as the embryologic cell groups from which they arise. Owing to the extensive migration and convolution of embryonic cell layers during early development, these tumor types may not evolve in specific anatomic sites. Neuroendocrine tumors (NETs) are derived from cells that migrate throughout the body and have developed specific enzymatic capabilities and accumulated cytoplasmic proteins that serve a secretory function. As such, they are frequently identified by certain enzymatic markers, in particular nonspecific esterase. Although they were all originally thought to arise from the neural crest, not all NETs can be traced to the neural crest. Indeed, tumors of this classification may not have a common embryonic ancestry. However, this tumor classification has been maintained because of its unique specialized secretory functions. NETs can secrete biologically active peptides and produce specific clinical syndromes because of their secretory activities. Germ cell tumors can arise within the testes or in extragonadal sites through which germ cells migrate during development. Mesenchymal cells, by virtue of their function, are distributed throughout the body, and mesenchymal tumors can thus arise at any anatomic site.

TABLE 5–8 Mesenchymal, neuroendocrine, and germ cell neoplasia.

1. Neuroendocrine Tumors NETs arise from neural crest tissue and, more specifically, from enterochromaffin cells, whose final resting place after embryonic migration is along the submucosal layer of the intestines and pulmonary bronchi. Reflecting this embryonic origin, neuroendocrine cells can at times express the necessary enzymes to produce bioactive amines, as well as a variety of small peptide hormones. A low-grade NET is classified as a carcinoid tumor (regardless of any hormonal secretion). Cytoplasmic granules typical of neuroendocrine cells are also commonly seen. These features may also be shared by other tumors of neural crest origin. In contrast to epithelial neoplasms, morphologic changes

observed with the light microscope do not distinguish between malignant and benign cells. The anatomic distribution of primary NETs is consistent with embryonic development patterns, as shown in Table 5–9. NETs and other mesenchymal neoplasms demonstrate similar patterns of tissue invasion followed by local and distant spread to regional lymph nodes and distant organs. The characteristics of increased mitotic count (an indicator of rapid proliferation), nuclear pleomorphism, lymphatic and vascular invasion, and an undifferentiated growth pattern are associated with a higher rate of metastases and a less favorable clinical prognosis.

TABLE 5–9 Neuroendocrine tumor location by site of embryonic origin.

A frequent site of NET metastasis is the liver. In this setting, especially with midgut NETs, there can be a constellation of symptoms as a consequence of the secretion of vasoactive substances (including serotonin) into the bloodstream, producing the carcinoid syndrome (Table 5–10). These vasoactive substances reflect the neuroendocrine origin of NETs and the latent machinery that can be activated inappropriately in the malignant state. The serotonin and other peptides secreted can cause intermittent flushing from vasodilation, secretory diarrhea, wheezing, and excessive salivation or lacrimation. Long-term tissue damage can occur by exposure to these substances and their metabolites. Fibrosis of the

pulmonary and tricuspid heart valves, mesenteric fibrosis, and hyperkeratosis of the skin have all been reported in patients with carcinoid syndrome. A urinary marker commonly used to aid in the diagnosis or to monitor patients being treated is a metabolite of serotonin: 5-hydroxyindoleacetic acid (5-HIAA).

TABLE 5–10 Peptides and amines secreted by neuroendocrine tumor cells.

2. Testicular Cancer and Other Germ Cell Neoplasms

Testicular cancer arises chiefly from germ cells within the testes. Germ cells are the population of cells that give rise to spermatozoa through meiotic division and can, therefore, theoretically retain the ability to differentiate into any cell type. Some testicular neoplasms arise from remnant tissue outside the testes owing to the midline migration of germ cells that occurs during early embryogenesis. This is followed by the formation of the urogenital ridge and eventually by the aggregation of germ cells in the ovary or testis. As predicted by this pattern of migration, extragonadal germ cell neoplasms are found in the midline axis of the lower cranium, mediastinum, or retroperitoneum. The pluripotent ability of the germ cell (ie, the ability of one cell to give rise to an entire organism) is most evident in benign germ cell tumors such as mature teratomas. These tumors often contain differentiated elements from all three germ cell layers, including teeth and hair in lesions termed dermoid cysts. Malignant teratomas can also exist as a spectrum bridging other germ cell layer– derived neoplasms such as sarcomas and epithelium-derived carcinomas. Malignant testicular cancers may coexist with benign mature teratomas, and the benign component sometimes becomes apparent only after the malignancy has been eradicated with chemotherapy.

Proteins expressed during embryonic or trophoblastic development, such as alpha-fetoprotein and human chorionic gonadotropin, can be secreted and measured in the serum. Testicular carcinoma follows a lymphatic and hematogenous pattern of spread to regional retroperitoneal nodes and distant organs such as the lung, liver, bone, and brain. The exquisite sensitivity of even advanced testicular cancers to radiation and chemotherapy may be a result of the foreign nature of malignant germ cells when present in a mature organism. This foreign nature may create more specific activity of cytotoxic insults and stimulate a more vigorous immune rejection of the tumor.

CHECKPOINT

45. What products of neuroendocrine tumors reflect their embryonic origin? 46. From what cellular elements of the testes does testicular cancer generally

arise? 47. What are some characteristic markers that may be mon-itored in

testicular tumor progression?

3. Sarcomas The sarcomas consist of a family of neoplasms arising from tissues of mesenchymal origin. They can arise within structures of mesenchymal tissue, such as bone, muscle, cartilage, and connective tissues, or they can arise in areas without a defined tissue from cells that came to rest at such spots during embryonic migration. Compared with epithelial cancers, sarcomas are quite rare, accounting for less than 1% of adult cancers. The reasons for this are not entirely clear, but it may be because tissues of mesenchymal origin have far less proliferative activity than epithelial tissues in adults, have less need to replenish their tissues with new cells, and are thus less prone to DNA replication errors that can introduce mutations or structural alterations in genes. Sarcomas are much more common during childhood, when these tissues do experience proliferative activity to support the growing body.

Although sarcomas are rare in incidence, they are extremely heterogeneous in histologic appearance and biologic behavior and thus represent a challenging case for accurate classification with more than 50 subtypes recognized. Most tumors arising in tissues of mesenchymal origin are benign. Examples of these are lipomas, tumors of adipose tissue very common in adults; hemangiomas, abnormal growths of blood vessels in the skin or various organs; schwannomas, benign growths of schwann cells of peripheral nerves; and myxomas, benign growths within muscle. The distinction of benign versus malignant is more challenging in tumors deriving from mesenchymal tissues than it is for those deriving from epithelial tissues. This is because epithelial tissues have distinct cellular architecture, including ducts, tubules, and alveoli lined by single or double layers of cells exhibiting polarity, adhesion, and enclosure. Such highly ordered architecture is perceptibly disrupted in neoplasia, and malignancy is readily evident at the microscopic level in the form of architectural distortion and invasion of the surrounding connective tissues. Mesenchymal tissues have less complex micro-architecture and no polarity, and the concept of invasion and destructiveness by a tumor is more difficult to assess from examination of microscopic sections. The microscopic characteristic most often used to distinguish benign from malignant tumors of mesenchymal tissue is proliferative activity. This characteristic correlates with the propensity for metastasis and helps guide the management of these tumors. While epithelial tumors can often be seen to metastasize via lymphatic channels to local lymph nodes, sarcomas do not generally migrate via lymphatics. Instead, they metastasize via the systemic blood vessels, most often to the lung.

Sarcomas have a wide range of biologic behavior. Some, such as desmoid

tumors, can grow to a large size and cause local problems owing to their size and pressure but have little capacity for distant metastasis; thus, they are generally considered to be benign. Others, such as Ewing sarcoma, are highly aggressive and metastasize early. The subtyping of sarcomas involves identifying the type of mesenchymal tissue it would most resemble if it were differentiated (see Table 5–8). Sarcoma of bone (osteosarcoma) is generally considered a separate category from all other sarcomas. The remainder, arising in non-bone sites, are collectively called soft-tissue sarcomas. Whereas some sarcomas readily show differentiation toward a particular tissue such as skeletal muscle (rhabdomyosarcoma), smooth muscle (leiomyosarcoma), or fat cells (liposarcoma), other sarcomas show no resemblance to normal tissues and are difficult to classify.

Genomic alterations are common in sarcomas and appear to follow several patterns. Some sarcomas have complex karyotypes with no recurring patterns, implicating genomic instability in their underlying pathophysiology. Other sarcomas have defined and recurrent genomic alterations that implicate specific genes in their pathophysiology. Some of these sarcomas have characteristic chromosomal translocations, whereas others have characteristic activating mutations within specific oncogenes. Chromosomal translocations create gene fusions and consequent novel fusion oncoproteins or increased expression of specific genes. Many of the gene fusion events seen in sarcomas involve a member of the TET family of genes (eg, EWSR1, FUS, TAFII68) fused to a transcription factor. A prototype is Ewing sarcoma in which a chromosomal translocation fuses the EWSR1 gene with a gene from the ETS family of transcription factors. Most of these fusions occur with the FL1 gene and the remainder with other members of the ETS family. Fusion of the EWSR1 gene with non-ETS family genes also occurs and causes other types of sarcoma. Specific gene fusion events have been documented for specific types of sarcomas; occasionally, when subtyping of the sarcoma is difficult by histologic analysis, molecular genomic analysis can identify characteristic gene fusion events that specify the sarcoma subtype. Other sarcomas have characteristic gene mutation events. The best-known examples are the gastrointestinal stromal tumors (GISTs). These sarcomas of the GI tract have characteristic mutations in one of three oncogenes—KIT, PDGFRA, or BRAF—and are typically treated by inhibitors of these oncogenes. Desmoid fibromatosis is often associated with activation of the Wnt signaling pathway, with mutations in the APC or β-catenin genes. Mutational inactivation of the TP53 tumor suppressor gene also occurs in sarcomas, causing increased genomic instability but is so widespread that it does not define a specific subtype of sarcoma. The NF1 tumor suppressor gene is

sometimes mutated in sarcomas, causing excessive G-protein signaling. Mutations of either of these tumor suppressor genes, when present in the germline, cause inheritable syndromes of cancer risk that include sarcomas.

As with carcinomas, sarcomas can be induced by tumor viruses. The best documented example of this is Kaposi sarcoma, a malignant tumor arising in connective tissues of the skin or internal organs, caused by the human herpesvirus type 8 (HHV-8). Kaposi sarcoma predominantly occurs in immunosuppressed patients with HIV infection.

CHECKPOINT

48. Are sarcomas more common in children or adults? 49. Are sarcomas more or less likely to directly invade tis-sues compared

with epithelial malignancies? 50. To what sites do sarcomas commonly metastasize? 51. What is the genetic lesion found in Ewing sarcoma? 52. What are the oncogenes that have characteristic muta-tions in

gastrointestinal stromal tumors?

CANCER STAGING AND GRADING

Once cancer is diagnosed, a myriad of decision algorithms come into consideration, including decisions regarding surgical resection, chemotherapy, radiation therapy, molecular targeted therapy, immunotherapy, and other forms of therapy. The presentation of cancer is highly variable, potentially different in every patient, and the success and outcomes of therapy can vary significantly among patients with the same type of cancer. Furthermore, since treatments for cancer are currently imperfect and many patients ultimately die from it, the treatment of cancer is in continuous evolution, with many improved therapies replacing existing treatments. The enormous diversity of patients and of treatment options has over time made for very complex decision making regarding the optimal treatment of an individual patient. To reduce this complexity, most cancers are generally categorized according to stage (and sometimes according to grade). Most clinical research trials are conducted in patients with particular stages of specific cancers so that clinicians can readily

extrapolate the resulting data, conclusions, and recommendations to their own patients with similar stages of those cancers. Other cancers are now also categorized according to specific molecular and/or genetic markers. This additional categorization will undoubtedly increase in the future.

Cancer Staging For solid tumors, the stage of a cancer describes the anatomic extent of the cancer at and beyond its site of origin. This information has significant prognostic value for most types of solid-tumor cancers. To provide a uniform method of describing the extent of a cancer, a well-defined staging system, referred to as tumor–node–metastasis (TNM) staging, is used. The disease in any given patient is assigned three numbers: T (for tumor stage), N (for node stage), and M (for metastasis stage). The T number describes the size and extent of the tumor. The N number describes the extent of lymph node metastases. The M stage is usually simply 0, if there are no distant metastases, or 1, if distant metastases are present. The numbering system for TNM staging is different for each type of cancer and is defined, revised, and published every few years by the American Joint Committee on Cancer (AJCC). Cancer presentation varies widely among patients and may involve up to 30 different combinations of T, N, and M numbers. Thus, for each cancer type, the various TNM staging numbers are grouped together into four prognostic staging groups denoted by Roman numerals I–IV and alphabetical substages. Categorization into stages is done to reflect the expected prognosis for that stage of disease. Stage I cancers typically have an excellent prognosis, often requiring no treatment other than surgical excision, whereas stage IV cancers have a dismal prognosis with very few long- term survivors despite the use of all available therapies. The staging system can be applied at the time of presentation based on physical examination and radiographic findings; this is called clinical staging. A more accurate staging can be applied after surgical resection of the primary tumor and sampling of the regional lymph nodes. This involves a much more reliable examination of the tissues by pathologists and is called pathologic staging. The N stage is sometimes different for clinical staging and pathologic staging to allow clinicians to stage the disease based on their lymph node palpation findings and pathologists to stage the disease based on their microscopic examination of them. As an example, Figure 5–19 illustrates the system used for breast cancer staging. The current eighth edition of the TNM staging system is being released by the AJCC in 2018.

FIGURE 5–19 The pathologic tumor–node–metastasis (TNM) staging system for breast cancer is shown here in its simplest form, according to the AJCC seventh edition from 2010. (A new 2018 edition is pending.) The detailed staging manual defines many more subdivisions of the T and N stages (eg, T1a, T1b), as well as a clinical staging system. Similar staging systems are defined for all types of solid-tumor cancer.

Cancers of the blood and bone marrow cannot be staged by the TNM system given their tissue of origin. Consequently, each type of hematologic malignancy has its own staging system.

Cancer Grading Cancer grading describes the appearance and behavior of the tumor cells. This information has significant prognostic value of its own, independent of the cancer’s stage. To minimize variability among different pathologists, well- defined criteria have been developed for the tissue analysis of each type of cancer. Unlike the TNM staging of cancers, there is no national or global authority governing all grading systems. Rather, the grading system for each type of cancer has been proposed by specific pathologists and by general consensus has been adopted over time by others in the field. Often the grade assessment is broken down into different categories and independently scored. The degree of differentiation or de-differentiation is the most common category and reflects how much the biologic behavior of the cancer cell has departed from its normal cellular ancestor. Poorly differentiated cancers may have no resemblance to their tissue of origin at all. Another category is the quality of nuclear chromatin, which, if significantly altered, can reflect highly aggressive

biologic behavior. Another category is a quantitative assessment of the proliferative rate, typically in the form of number of observed mitoses within a predefined area. For the management of some cancers, the assessment of grade has only modest value, whereas for other cancers, the assessment of grade is critical for management decisions. For example, in the management of early- stage prostate cancer, the grading system used, the Gleason grading system, is absolutely critical (Figure 5–20). This system is named after the pathologist who developed it in the 1960s, and it is now universally used by pathologists and clinicians today to estimate mortality risk in early-stage prostate cancer and to assist in making treatment decisions.

FIGURE 5–20 The Gleason grading system for prostate cancers. The system is largely based on the uniformity, compactness, shape, and infiltrating features of the malignant glandular structures. Specific morphologic features are used to define scores from 1 to 5. Tumors are often heterogeneous in appearance; in such cases, the two most predominant patterns are scored and combined into a single final score from 2 to 10.

Personalized Definitions of Cancer While describing the cancer affecting a patient by its type, stage, and grade establishes a context that provides considerable prognostic information, there

remains considerable heterogeneity in disease biology. There can be vast differences in the behavior and treatment responsiveness of the disease among different patients. Additional information in the form of receptor markers (for breast cancers), MSI status (for colon cancers), karyotypes (for many blood cancers), and other biomarkers is used in various diseases to predict disease behavior with even more precision. However, it is understood that disease heterogeneity is potentially infinite in scope; in fact, every single patient with cancer has a different, unique disease. An ideal of clinical oncology is to characterize the attributes of each patient’s tumor to such a degree that it defines the unique identity of that individual’s disease. This has become the principal goal of the current era, often labelled “personalized medicine,” “precision medicine,” or “genomic medicine.” A number of technologies are available and many others are in development to characterize a tumor in more depth, including full genomic sequencing, full RNA transcriptomic analysis, and multiplex proteomic analysis. While such technologies to deeply annotate a single tumor are currently available, the predictive and prognostic information they provide to enable the physician to make reliable treatment decisions is still very primitive. Currently, however, a number of commercial tumor genomic and tumor transcriptomic assays are in use, and they are becoming increasingly adopted by clinical oncologists. It is distinctly possible that the molecular definition of a cancer will eventually replace both stage and grade as the principal basis for planning its treatment.

CHECKPOINT

53. What are the three categories involved in staging solid-tumor cancers? 54. What is the system used for grading prostate cancer?

HEMATOLOGIC NEOPLASMS

Hematologic neoplasms are malignancies of cells derived from hematopoietic precursors. The true hematopoietic stem cell has the capacity for self-renewal and the ability to give rise to precursors (colony-forming units) that proliferate and terminally differentiate toward one of any lineage (Figure 5–21). Distinct hematologic neoplasms can arise from each of the cell types along the

differentiation process. Many of these arise in the bone marrow, circulate in the bloodstream, and can infiltrate certain organs and tissues. Others may form tumors in lymphoid tissue, particularly lymphomas, which arise from lymphoblasts. The lineage of a hematopoietic cell and the degree of differentiation along that lineage are associated with the cell surface expression of characteristic proteins, many of which are receptors, others adhesion molecules and proteases, and still others of unknown function. These clusters of differentiation (CD) antigens have become essential diagnostic tools in the management of hematologic neoplasms, and some types of malignancy are defined by characteristic CD expression patterns.

FIGURE 5–21 Classification of leukemias according to cell type and lineage. (Redrawn, with permission, from Chandrasoma P et al. Concise Pathology, 3rd ed. Originally published by Appleton & Lange. Copyright © 1998 by The McGraw-Hill Companies, Inc.)

The cellular ultrastructure and machinery of the malignant cell can somewhat resemble those of its cell of origin. A markedly enhanced proliferative rate and arrest of differentiation are the hallmarks of these neoplasms. Examination of the interphase nucleus of cells, an analysis called cytogenetics, can sometimes reveal chromosomal abnormalities such as deletions (monosomy), duplications

(trisomy), or balanced translocations. Certain types of hematologic neoplasms tend to have stereotypic chromosomal abnormalities. Given their clonal nature, these abnormalities will be evident on all malignant cells. In some cases of chromosomal translocation, a new fusion gene is formed and can result in the production of a fusion protein possessing abnormal function compared with the original gene products (Table 5–11). This function usually involves loss of cell cycle control, abnormal signal transduction, or reprogrammed gene expression as a result of an aberrant transcription factor. In contrast to solid tumors, many hematologic malignancies are specifically linked to certain chromosomal translocations; therefore, karyotype studies are essential in the diagnosis of hematologic malignancies. Other genetic changes described in hematologic malignancies include mutations or deletions of the p53, retinoblastoma (Rb), and Wilms tumor (WT1) suppressor genes and the activation of mutations in the N- ras oncogene. Additional genetic changes can be detected in the clonal evolution of leukemias as disease progresses to a more aggressive form in the patient’s course. This finding lends further support to the theory that neoplasia is the result of stepwise genetic alterations that correspond to the sequential acquisition of additional phenotypic changes that favor abnormal growth, invasion, and resistance to normal host defenses.

TABLE 5–11 Chromosomal translocations of hematologic neoplasms.

1. Lymphoid Neoplasms Lymphoid neoplasms consist of B-cell, T-cell, or natural killer (NK) cell types.

The malignant cells arise from mature and immature B or T cells at different stages of normal maturation. In general, leukemias are derived from immature lymphocytes, lymphomas from mature lymphocytes, and plasma cell (formerly “multiple”) myeloma and related neoplasms from plasma cells (terminally differentiated B lymphocytes).

Acute lymphocytic leukemia (ALL) is a rapidly progressive neoplasm derived from immature lymphocytes named lymphoblasts, which overtake the bone marrow and sometimes infiltrate other organs. Genetic events are common in ALL, and these are linked with biological outcome and used for prognostication. The previous morphologic classification of ALL, used for many years, is now being revised in favor of classification according to B-cell or T-cell lineage and encompassing the spectrum of cytogenetic abnormalities. A distinct genetic alteration, the Philadelphia chromosome [t(9;22)], which is almost universal in CML, can also be seen in some cases of ALL, but its biological role may be different from that in CML because the targeted therapies that block it in ALL are not as effective as they are in CML.

Malignant lymphomas are a diverse group of cancers that result from neoplastic proliferation of mature B or T lymphocytes. These tumors may arise anywhere in the body, most commonly within lymph nodes and occasionally in bone marrow and other organs in which lymphoid elements reside. One subtype of lymphoma composed of malignant cells in a mixture of benign inflammatory cell types with a unique biology is called Hodgkin lymphoma. Hodgkin lymphoma is distinguished by the presence of giant Reed–Sternberg cells of B- cell lineage, which are considered the malignant cells in this neoplasm. The Reed–Sternberg cell constitutes only 1–10% of the total number of cells in pathologic specimens from patients with this disease. It is associated with the infiltration of non-neoplastic inflammatory cells.

All other types of lymphoma are referred to as non-Hodgkin lymphomas. Several factors are associated with their development. These include congenital or acquired immunodeficiency states such as AIDS or iatrogenic immunosuppression (eg, in organ transplantation). Virus infections are associated with some types. For example, most cases of Burkitt lymphoma that occur in Africa (endemic form) are associated with Epstein–Barr virus (EBV) infection (although Burkitt lymphomas that occur in temperate zones are associated with EBV in only 30% of cases). Human T-cell leukemia/lymphoma virus I (HTLV-I) plays a causative role in the genesis of adult T-cell leukemia/lymphoma, and the malignant cells contain the integrated virus. Human herpesvirus-8 (HHV-8) is associated with body cavity–based lymphoma,

a rare B-cell lymphoma that occurs predominantly in patients with HIV infection. Chronic immune stimulation may be a causal mechanism in the development of lymphomas as well. For example, chronic gastritis secondary to Helicobacter pylori infection may give rise to gastric mucosa–associated lymphoid tissue (MALT) lymphomas. Indeed, resolution of gastric MALT lymphoma may occur in the majority of patients with localized disease who are treated with antibiotics effective against H pylori.

The classification of lymphomas has evolved over several decades, as their distinguishing molecular characteristics have been better characterized. The latest classification was devised in 2016 by an international group of lymphoma specialists for the World Health Organization. This scheme characterizes non- Hodgkin lymphomas according to their B-cell or T-cell origin using a combination of criteria: clinical and morphologic features, immunoreactivity with monoclonal antibodies that recognize surface antigens, and specific molecular characteristics including expression of certain genes and characteristic genetic alterations including gene rearrangements and gene mutations. Additionally, precursor-undifferentiated B-cell and T-cell lymphoblastic lymphomas are in a separate class from the more mature B-cell and T-cell lymphomas. Most non-Hodgkin lymphomas originate in B cells and express a B- cell marker, CD20, on their surface. Their monoclonal origin can be inferred by characterization of the specific class of light chain they express. Kappa and lambda B-cell lymphomas are further classified as malignant expansions of cells from the germinal center, mantle zone, or marginal zone of normal lymph nodes. The mature B-cell non-Hodgkin lymphoma classification encompasses more than 20 classes (and smaller subtypes within some of these classes).

Somatic gene rearrangements occur normally during B-cell and T-cell differentiation to form a B-cell or T-cell receptor that serves as a unique identifier for each tumor cell. The B-cell receptor is composed of the immunoglobulin heavy and light chains, both of which are composed of constant and variable regions. The genes for the variable and constant regions of the B- cell receptor are discontinuous in the B-cell germline DNA but are combined by somatic rearrangement to produce a functional antibody molecule. The T-cell receptor gene is analogous to the immunoglobulin molecule in that discontinuous segments of this gene also undergo somatic rearrangement early in T-cell development. Polymerase chain reaction (PCR) and flow cytometry are two common techniques to recognize B-cell or T-cell receptors on each lymphocyte. These techniques allow separation of monoclonal malignant lymphocytes from normal (polyclonal) lymphocytes. DNA hybridization by

Southern blot analysis is another technique that permits recognition of a band of electrophoretic mobility that serves as a “fingerprint” for a monoclonal population of lymphoma cells.

Many non-Hodgkin lymphomas exhibit karyotypic abnormalities. The most prevalent translocations include t(8;14), t(14;18), and t(11;14) (see Table 5–11). Each translocation involves an oncogene with the immunoglobulin heavy-chain gene locus at chromosome 14q32. Identification and cloning of the breakpoints have identified 8q24 as c-myc, 18q21 as bcl-2, and 11q13 as bcl-1. The proximity of these oncogenes to the immunoglobulin gene results in deregulation and increased expression of the oncogene product.

Representative subtypes of non-Hodgkin lymphoma include the indolent lymphomas, such as follicular and marginal zone lymphomas, and the aggressive lymphomas, such as mantle cell, diffuse large-cell, and Burkitt lymphomas.

Follicular lymphomas are low-grade tumors that may be insidious in their presentation and clinical course. The translocation t(14;18)(q32;q21) is found in more than 90% of follicular lymphomas. Mutation of the bcl-2 oncogene that codes for the bcl-2 protein, which blocks apoptosis when overexpressed, results in its overexpression by these cells. In patients whose lymphomas harbor this translocation, the absence of bcl-2 translocation as assessed by the highly sensitive polymerase chain reaction test may be a marker for complete remission status. Spontaneous regression of lymph node size is common in patients with follicular lymphomas. However, follicular lymphomas are not curable with standard chemotherapy, and although patients with it tend to have an indolent clinical course, transformation to a more aggressive grade of lymphoma occurs in 40–50% of them by 10 years.

An important subtype of marginal zone lymphomas are the MALT lymphomas, which may originate in the stomach, lungs, skin, parotid gland, thyroid, breasts, or other extranodal sites, where they characteristically align themselves with epithelial cells. As noted, there is a close association between gastric MALT lymphomas and H pylori infection.

Another indolent B-cell neoplasm is chronic lymphocytic leukemia (CLL) (and its lymphoma equivalent, small lymphocyte lymphoma [SLL]). These are neoplasms of mature B cells. CLL is the most common leukemia in adults. Because CLL results in increased numbers of lymphocytes in the peripheral blood that may not exhibit morphologic abnormalities, assays of clonality are essential in its diagnosis. The disease involves expansion of a neoplastic clone, and clonality can be easily assayed by the exclusively expressed antibody light chains normally present in B cells. CLL and SLL are actually the same disease;

they differ only in where the neoplastic cells accumulate—predominantly in the blood and bone marrow in CLL, and in the lymph nodes and bone marrow in SLL.

In contrast to indolent lymphomas, diffuse large B-cell lymphoma, mantle- cell lymphoma, and Burkitt lymphoma have aggressive clinical courses but are curable diseases. Mantle-cell lymphoma presents histologically as a monotonous population of small- to medium-sized atypical lymphoid cells with irregular nuclear outlines and arrayed in a nodular or diffuse pattern. The diagnosis of mantle-cell lymphoma is based on morphologic criteria with confirmation by monoclonal antibody staining against cyclin D1 (bcl-1) and SOX11. The t(11;14) translocation found in 90% of cases of mantle-cell lymphoma results in juxtaposition of the PRAD1 gene on chromosome 11 with the immunoglobulin heavy chain gene on chromosome 14. This results in overexpression of the PRAD1 gene product, cyclin D1. Cyclin D1 binds to and activates cyclin- dependent kinases, which are thought to facilitate cell cycle progression through the G1 phase. This disease occurs more commonly among older males and presents with adenopathy and hepatosplenomegaly. Mantle-cell lymphomas are significantly more resistant to treatment with combination chemotherapy than follicular lymphomas. They also are incurable.

Diffuse large-cell lymphoma is the most prevalent subtype of non-Hodgkin lymphoma in adults. One-third of patients present with lymphoma involvement of extranodal sites, particularly the head and neck, stomach, skin, bone, testis, and nervous system. Diffuse large B-cell lymphomas commonly harbor mutations or rearrangements of the BCL6 gene.

Virtually all cases of Burkitt lymphoma are associated with alterations of chromosome 8q24, resulting in overexpression of c-myc, an oncogene that encodes a transcriptional regulator of cell proliferation, differentiation, and apoptosis. Adults with Burkitt lymphoma who present with high tumor burdens and elevated serum lactate dehydrogenase levels have a poor prognosis. The high tumor burden may be associated with a hypermetabolic state that can be triggered by treatment as the tumor undergoes sudden lysis. This tumor lysis syndrome can cause life-threatening hyperkalemia, hyperphosphatemia, hyperuricemia, and hypocalcemia.

Like mature B-cell lymphomas, T-cell lymphomas are a group of heterogeneous diseases with many subtypes. Anaplastic large-cell lymphoma is characterized by the proliferation of highly atypical cells that express the CD30 antigen. These tumors have a T-cell phenotype, and more than 50% of cases are associated with the chromosomal translocation t(2;5)(p23;q35), resulting in the

nucleophosmin-anaplastic lymphoma kinase (NPM-ALK) fusion protein. Activation of the ALK receptor tyrosine kinase results in an unregulated mitogenic signal.

Another type of T-cell lymphoma is the adult T-cell leukemia/lymphoma, an aggressive disease associated with HTLV-I infection characterized by generalized adenopathy, polyclonal hypergammaglobulinemia, hypercalcemia, and lytic bone lesions.

Plasma Cell (“Multiple”) Myeloma

Plasma cell myeloma (formerly “multiple myeloma”) is a special type of lymphoid neoplasm that results from malignant proliferation of terminally differentiated B cells (plasma cells) that secrete a monoclonal immunoglobulin called the M protein. Almost all cases of myelomas are preceded by a pre- malignant condition called monoclonal gammopathy of undetermined significance (MGUS). MGUS is, by definition, an asymptomatic, benign condition. But some patients with MGUS do produce a small amount of the monoclonal immunoglobulin M protein. MGUS is quite prevalent, found in 3% of the general population older than 50 years of age. MGUS and myeloma also share certain genetic abnormalities, supporting the notion that myeloma evolves from MGUS. It appears that MGUS progresses to myeloma at a constant rate of about 1% per year. Thus, the pathogenesis of plasma cell myeloma appears to be a two-step process: establishment of the premalignant condition, MGUS, followed by progression to the malignant disease, myeloma.

MGUS appears to result from a variety of cytogenetic abnormalities. For example, approximately half of MGUS cases are found to have chromosome translocation involving the immunoglobulin heavy chain IgH on chromosome 14. In the other half of cases, an increased number of chromosomes (hyperdiploidy) is found. In 40% of myeloma cases, translocation of chromosome 14 brings IgH into juxtaposition with one or more of five oncogenes, leading to overexpression of these oncogenes. Hyperdiploidy is another mechanism to overexpress certain genes important in generating clonal plasma cells in MGUS. Only a small percentage of MGUS will ever progress to myeloma. The constant annual rate of progression from MGUS to myeloma seems to suggest that this progression is instigated by a “random second hit” rather than an accumulation of “hits.” The events that can serve as the “second hit” include additional genetic changes, dysregulation of proliferation, and/or apoptosis and alteration of tumor microenvironment, among others.

Myeloma cells present a unique challenge among cancers. Since they are

neoplasms of immunoglobulin-secreting plasma cells, they place a heavy burden on cell homeostatic mechanisms. The secretion of immunoglobulins proceeds along the secretory pathway and requires their synthesis at the ribosomes, proper folding and maturation by molecular chaperones within the endoplasmic reticulum, and ultimate transport to the membrane for export. When the volume of protein secretion exceeds the capacity of the endoplasmic reticulum to properly fold and maintain them, unfolded or misfolded proteins accumulate. Such dysfunctional proteins are sticky; when they aggregate, they disrupt endoplasmic reticulum function, a condition referred to as endoplasmic reticulum stress (Figure 5–22). Endoplasmic reticulum stress is detected by specific receptors in the endoplasmic reticulum membrane that function to activate a response mechanism called the unfolded protein response. The unfolded protein response exerts many effects to reduce ER stress, including slowing protein translation, slowing cell proliferation, increasing the expression of chaperone proteins and improving protein quality control, inducing the degradation of unfolded proteins, and inducing autophagy, the process by which cellular debris and organelles are digested within lysosomes. If these adaptational responses are unable to sufficiently relieve endoplasmic reticulum stress, the unfolded protein response can alternatively activate apoptosis. The pathogenesis of myeloma frequently involves the activation of the unfolded protein response to enable the neoplastic plasma cells to cope with the high secretory demand in addition to the many metabolic and energetic challenges associated with neoplastic growth. Although some of the effects of the unfolded protein response are beneficial to them, other physiologic effects, including anti- proliferative, anti-synthetic, and pro-apoptotic effects, are detrimental to them. As such, myeloma cells “walk a fine line” in activating the unfolded protein response. We are only now beginning to understand how the unfolded protein response is altered in myeloma cells to enable them to procure its benefits but escape its detriments. To accomplish this, a number of the genes involved in the unfolded protein response are mutated in myeloma plasma cells. Manipulating the unfolded protein response has been one of the most effective strategies in the pharmacologic treatment of myeloma. For example, inhibiting the proteasome in myeloma cells eliminates one of their adaptational mechanisms, increasing endoplasmic reticulum stress beyond their capacity for adaptation and leading to apoptosis.

FIGURE 5–22 The secretory pathway ensures proper translation, folding, and protein secretion through the endoplasmic reticulum (ER) and Golgi apparatus. When protein secretion is taking place at a high output, such as in myeloma cells, the secretory machinery may not be able to keep up with the volume. This can result in unfolded or misfolded proteins accumulating in the ER, which can aggregate and disrupt ER function. This is called ER stress and is sensed by the three receptors, IRE1, PERK, and ATF6, which proceed to activate the unfolded protein response. The unfolded protein response can promote many adaptive responses (shown with pink labels) to enable cells to adapt to the high secretory demand. Alternatively, if the ER stress is beyond the capacity for adaptation, the unfolded protein response can trigger apoptosis. The unfolded protein response is very active in myeloma plasma cells, which are constantly making and secreting immunoglobulins. (ERAD, endoplasmic reticulum–associated degradation.)

2. Myeloid Neoplasms Myeloid neoplasms are derived from hematopoietic precursors. Depending on the precise step in hematopoiesis that is disrupted by the molecular genetic

abnormalities, the cellular expansion can involve cells with features resembling any phase of myelocytic maturation. These cells can be cytologically normal or dysplastic. Myeloid neoplasms are broadly divided into three groups: myeloproliferative neoplasms, myelodysplastic syndromes, and acute myelogenous leukemia (AML). Myeloproliferative neoplasms are marked by proliferation of one or more of the myeloid lineages, such as granulocytes, erythroid cells, and megakaryocytes. Myelodysplastic syndromes are characterized by both the proliferation and apoptosis of hematopoietic cells; affected patients usually present with a hypercellular bone marrow but cytopenias in the peripheral bloodstream. AML is a rapidly progressive neoplasm derived from hematopoietic precursors, or myeloid stem cells, that give rise to granulocytes, monocytes, erythrocytes, and platelets. There are several lines of evidence that genetic events occurring early in stem cell maturation can lead to leukemia. First, there is a lag time of 5–10 years in the development of leukemia after exposure to known causative agents such as chemotherapy, radiation, and certain solvents. Second, many cases of secondary leukemia evolve out of a prolonged “pre-leukemic phase,” manifested as a myelodysplastic syndrome of hypoproduction with abnormal maturation without actual malignant behavior. Finally, examination of precursor cells at a stage earlier than the malignant expanded clone in any given type of leukemia can reveal genetic abnormalities such as monosomy or trisomy of different chromosomes. Additional genetic changes can be found in the malignant clone compared with the morphologically normal stem cell that developmentally precedes it.

Acute myelogenous leukemias have been traditionally classified by morphology and cytochemical staining, as shown in Table 5–12. Auer rods are crystalline cytoplasmic inclusion bodies characteristic of, though not uniformly seen in, all myeloid (but not lymphoid) leukemias. In contrast to mature myeloid cells, leukemic cells have large immature nuclei with open chromatin and prominent nucleoli. The appearance of the individual cell types of AML mirrors the cell type from which they derive. M1 leukemias originate from early myeloid precursors with no apparent maturation toward any terminal myeloid cell type. This is apparent in the lack of granules or other features that mark more mature myeloid cells. M3 leukemias are a neoplasm of promyelocytes, precursors of granulocytes, and M3 cells exhibit abundant azurophilic granules typical of normal promyelocytes. M4 leukemias arise from myeloid precursors that can differentiate into granulocytes or monocytes, whereas M5 leukemias derive from precursors already committed to the monocyte lineage. Therefore, M4 and M5 cells both contain the characteristic folded nucleus and gray cytoplasm of

monocytes, whereas M4 cells also contain granules of a granulocytic cytochemical staining pattern. M6 and M7 leukemias cannot be readily identified on morphologic grounds, but immunostaining for erythrocytic proteins is positive in M6 cells, and staining for platelet glycoproteins is apparent in M7 cells.

TABLE 5–12 Morphologic classification of acute myelogenous leukemias.

Before the introduction of molecular genetic techniques, chromosomal deletions, duplications, and balanced translocations had been noted in the leukemic cells of some patients. Cloning of the regions where balanced translocations occur has, in some cases, revealed a preserved translocation site that reproducibly fuses one gene with another, resulting in the production of a new fusion protein. Acute promyelocytes leukemia is marked by the t(15;17) translocation that juxtaposes the PML gene with the RAR-α gene. RAR-α encodes a retinoic acid steroid hormone receptor, and PML encodes a transcription factor. The fusion protein possesses novel biologic activity that results in enhanced proliferation and a block of differentiation. Interestingly, administration of all- trans retinoic acid (ATRA) can restore differentiation of leukemic cells and induce a temporary remission of acute promeylocytic leukemia L, supporting the importance of the RAR-α–PML fusion protein. Monosomy of chromosome 7 can be seen in leukemias arising out of the myelodysplasia syndrome or in de novo leukemias, and in both cases, this finding is associated with a worse clinical prognosis. Monosomy of chromosome 7 and other serial cytogenetic changes can also be seen with relapse of treated leukemia, a situation characterized by a

more aggressive course and resistance to therapy. With increasing knowledge of the various genetic alterations that dominate the behavior, prognosis, and treatment responses of different forms of AML, the classification of this disease is gradually shifting towards a genetic classification scheme (Table 5–13). This reclassification will continue to evolve as new genetic–clinical links are recognized and established.

TABLE 5–13 Molecular classification of acute myelogenous leukemias.

As hematopoietic neoplasms, acute leukemias involve both abnormal bone marrow and (frequently) abnormal circulating leukemic (blast) cells.

Occasionally, extramedullary leukemic infiltrates known as chloromas can be seen in other organs and on mucosal surfaces. A marked increase in the number of circulating blasts can sometimes cause vascular obstruction accompanied by hemorrhage and infarction in the cerebral and pulmonary vascular beds. This leukostasis results in complications such as stroke, retinal vein occlusion, and pulmonary infarction. In most cases of AML and other leukemias, peripheral blood counts of mature granulocytes, erythrocytes, and platelets are decreased. This is probably due to crowding of the bone marrow by blast cells and the elaboration of inhibitory substances by leukemic cells, or due to alteration of the bone marrow stromal microenvironment and cytokine milieu necessary for normal hematopoiesis. Susceptibility to infections as a result of depressed granulocyte number and function and abnormal bleeding as a result of low platelet counts are common presentations in patients with leukemia.

Chronic myelogenous leukemia (CML) is the most well-known myeloproliferative neoplasm. It is an indolent leukemia manifested by an increased number of immature granulocytes in the marrow and peripheral circulation. One of the hallmarks of CML is the Philadelphia chromosome, a cytogenetic feature that is a result of the balanced translocation of chromosomes 9 and 22, resulting in a fusion gene, bcr-abl, that encodes a kinase that phosphorylates several key proteins involved in cell growth and apoptosis. CML eventually transforms into acute leukemia (blast crisis), accompanied by further cytogenetic changes. Targeted therapies that inhibit the enzymatic function of the bcr-abl kinase by competing with the ATP-binding site induce remissions in most patients in the chronic phase of CML. Furthermore, resistance to these bcr- abl inhibitors can involve amplification of the bcr-abl breakpoint, as well as development (or clonal expansion) of mutations in the ATP-binding pocket of bcr-abl, which prohibit the binding of inhibitors.

THE SYSTEMIC EFFECTS OF NEOPLASIA

Many effects of malignancies are mediated not by the tumor cells themselves but by direct and indirect effects, as outlined in Tables 5–14 and 5–15, respectively. Direct effects (see Table 5–14) include compression or invasion of vital structures, such as blood and lymphatic vessels, nerves, spinal cord or brain, bone, airways, GI tract, and urinary tract. These may cause a typical pain pattern, as well as dysfunction of the involved organ and obstruction of a conduit. On occasion, an inflammatory or desmoplastic host response rather than the tumor

itself can result in the same effect.

TABLE 5–14 Direct systemic effects of neoplasms.

TABLE 5–15 Paraneoplastic syndromes (indirect systemic effects of neoplasms).

Indirect effects (see Table 5–15) are heterogeneous and poorly understood. Likewise, the onset and clinical course of these effects are unpredictable. When

affecting distant targets uninvolved by tumor, they are collectively termed paraneoplastic syndromes. Some of these effects are stereotypic syndromes resulting from the elaboration of peptide hormones or cytokines with specific biologic activity, as shown in Table 5–15. The peptides secreted by a given neoplasm may reflect the tissue of origin or may be the result of activation of latent genes not normally expressed. Common examples of paraneoplastic phenomena include the syndrome of inappropriate antidiuretic hormone (SIADH), occurring most often in small-cell lung cancer. The result of ectopic ADH production is retention of free water and hyponatremia, which can result in altered sensorium, coma, and death. Another peptide secreted in small-cell lung cancer is ACTH, which can cause Cushing syndrome from adrenocorticosteroid excess, with skin fragility, central redistribution of body fat, proximal myopathy, and other features. Hypercalcemia can occur in many types of malignancies. It has several causes, including secretion of a parathyroid hormone–like peptide as a result of activation of the parathyroid hormone–related protein gene (PTHrP), as well as elaboration of locally acting cytokines that increase bone uptake in areas of tumor infiltration of bone. Some paraneoplastic syndromes, particularly those involving the nervous system, are related to antibodies generated in response to cancer cells that cross-react with normal cells in the nervous system.

In some malignancies, such as NETs, several active peptides may act in concert to produce a constellation of symptoms and tissue effects. Cytokines, such as the interleukins and tumor necrosis factor, may be responsible for tumor- related fevers and weight loss. Some paraneoplastic syndromes are associated with the development of autoantibodies. These can derive from an immune response to tumor-associated antigens or from an inappropriate production of antibody, as can occur in lymphoid neoplasms. Finally, the nucleic acid, cytoplasmic, and membrane products of cell breakdown can result in electrolyte and other metabolic abnormalities, as well as coagulopathic disorders, resulting in clotting or bleeding.

CHECKPOINT

55. What are the hallmarks of hematologic malignancies? 56. What are some characteristics of low-grade lymphomas? 57. What are some characteristics of high-grade lymphomas?

CASE STUDIES Yeong Kwok, MD

(See Chapter 25, p. 749–51 for Answers)

CASE 18

A 54-year-old man presents with several weeks of facial flushing and diarrhea. His symptoms began intermittently but are becoming more constant. A 24-hour urine collection reveals an elevated level of 5- hydroxyindoleacetic acid (5-HIAA), a metabolite of serotonin. An abdominal CT scan shows a 2 cm mesenteric mass in the ileum and likely metastatic tumors in the liver.

Questions

A. This patient has the malignant carcinoid syndrome. From what type of tissue do carcinoid tumors arise, and how does this account for the body site where they first appear?

B. What accounts for the frequent association of systemic symptoms, the so-called carcinoid syndrome, with carcinoid tumors?

C. Why is the 24-hour urine collection for 5-HIAA useful in the diagnosis of carcinoid syndrome?

CASE 19

A 54-year-old man presents to the clinic for a routine check-up. He is well, with no physical complaints. The history is remarkable only for a father with colon cancer at age 55 years. Physical examination is normal. Cancer screening is discussed, and the patient is sent home with fecal occult blood testing supplies and scheduled for a colonoscopy. The fecal occult blood test results are positive. The colonoscopy reveals a villous adenoma and a 2 cm carcinoma.

Questions

A. How are the two lesions—adenoma and carcinoma—thought to be related?

B. What are the two principal lines of evidence in favor of such a model? C. Describe the genetic alterations in the stepwise progression of colon

cancer and the phenotypic changes associated with these alterations. D. What is the explanation for the presence of occult blood in stools of

patients with early colorectal cancer?

CASE 20

A 40-year-old woman presents for the evaluation of a left-sided breast lump. She does have a strongly positive family history, with her mother and one older sister both having had breast cancer. Physical examination is notable for a 2 cm lump in the left breast. A biopsy shows invasive ductal carcinoma. The tumor is positive for estrogen receptor expression and HER2 gene amplification.

Questions

A. What genetic factors may have been involved in this patient’s risk for developing breast cancer?

B. What are the two major subtypes of breast cancer? C. Describe the distinction between invasive breast cancer and carcinoma

in situ. D. How is our knowledge of the tumor receptors used in the treatment of

breast cancer?

CASE 21

A 25-year-old man presents with a complaint of testicular enlargement. Examination reveals a hard nodule on the left testicle, 2 cm in diameter.

Orchiectomy is diagnostic of testicular cancer.

Questions

A. From what cellular elements of the testes does testicular cancer generally arise? What is the normal development of these cells?

B. In addition to the testes, where else might testicular cancer arise? What is the explanation for this distribution?

C. What serum markers might be monitored to evaluate disease progression and response to therapy?

CASE 22

A 16-year-old previously healthy teenager presents with a 2-month history of pain and swelling of his knee. He thought it began after a soccer game, but it has not gotten better. Physical examination shows marked swelling of the knee and the distal thigh. Radiographs show a 3 cm partially calcified mass in the distal femur, just above the knee joint. A biopsy reveals an osteosarcoma.

Questions

A. From which tissues do sarcomas arise? B. Why are many sarcomas more common in children, adolescents, and

young adults than in older adults? C. What accounts for the calcifications that can be seen in osteosarcomas?

CASE 23

A 28-year-old woman presents to her primary care physician with complaints of fatigue, intermittent fevers, and 5 pounds of weight loss over a 6-week period. Her medical history is remarkable for a renal transplantation at age 15 years performed for end-stage renal disease as a

result of post-streptococcal glomerulonephritis. Physical examination reveals two enlarged, matted, nontender lymph nodes in the left anterior cervical chain; a firm, nontender 1.5 cm lymph node in the right groin; and an enlarged liver. Biopsy of the lymph nodes in the cervical region reveals follicular, cleaved-cell lymphoma.

Questions

A. One theory states that chronic immune stimulation or modulation may be an early step in lymphomagenesis. What observations support this view?

B. How would one classify this patient’s lymphoma? What are some characteristics of this grade of lymphoma?

C. From which cell line do follicular lymphomas originate? What are some of the common genetic mutations seen with this type of lymphoma? How might one of these mutations contribute to the formation of lymphoma?

D. What is the pathophysiologic mechanism causing this patient’s fever and weight loss?

CASE 24

A 22-year-old woman presents with a 2-week history of fatigue, bleeding from her gums, and very heavy menstrual bleeding. Physical examination reveals a pale woman with an enlarged spleen and petechiae on her legs. A complete blood cell count shows a markedly elevated white cell count (WBC 178,000) with severe anemia (hemoglobin 7.8) and thrombocytopenia (platelet count 25,000). Blast cells (abnormally immature leukemic cells) comprise 30% of the total white cell count. A bone marrow biopsy is positive for AML of the M1 type.

Questions

A. How are leukemias classified in general? More specifically, how are AMLs classified?

B. What accounts for the patient’s symptoms and physical findings? What

other major symptoms or signs may be present? C. What types of genetic abnormalities are responsible for the

development of leukemias? How can this knowledge be used to treat some leukemias?

CASE 25

A 64-year-old man is brought to the emergency room by his family for evaluation of mental status changes. They noticed that he was becoming somewhat confused the day before presentation, describing him as “just not acting like his usual self.” That morning, he was barely arousable and even more confused. He had previously been feeling a bit tired but was otherwise without complaint except for a nagging cough. He had had no falls or head trauma. He had a greater than 50-pack-year smoking history and currently smokes one pack per day. A chest x-ray shows a 3 cm mass in his right upper lung field. Blood chemistries show a serum calcium level of 14 mg/dL. A computed tomography scan of the head is normal.

Questions

A. What is the likely cause of the patient’s mental status changes? B. What can explain the high serum calcium level? C. What is another possible electrolyte abnormality found in individuals

with malignancy that can also lead to a decreased level of consciousness?

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CHAPTER

6

Blood Disorders J. Ben Davoren, MD, PhD, & Gerald Hsu, MD, PhD

NORMAL STRUCTURE & FUNCTION

Blood is an extremely complex fluid, composed of both formed elements (red cells, white cells, platelets) and plasma. Red blood cells (erythrocytes) are the most common formed elements, carrying oxygen to the cells of the body via their main component, hemoglobin. White blood cells are generally present at about 1/700th the number of erythrocytes and function as mediators of immune responses to infection or other stimuli of inflammation. Platelets are the formed elements that participate in coagulation. Plasma is largely water, electrolytes, and plasma proteins. The plasma proteins most important in blood clotting are the coagulation factors. Because blood circulates throughout the body, alterations in normal blood physiology—either formed elements or plasma proteins—may have widespread adverse consequences.

FORMED ELEMENTS OF BLOOD

Anatomy A. Bone Marrow and Hematopoiesis Although the mature formed elements of blood are quite different from each other in both structure and function, all of these cells develop from a common hematopoietic stem cell population, which resides in the bone marrow. The

developmental process is called hematopoiesis and represents an enormous metabolic task for the body. More than 100 billion cells are produced every day. This makes the bone marrow one of the most active organs in the body. In adults, most of the active marrow resides in the vertebrae, sternum, and ribs. In children, the marrow is more active in the long bones.

The process of differentiation from stem cell to mature erythrocyte, granulocyte, lymphocyte, monocyte, or platelet is shown in Figure 6–1. It is not clear exactly what early events lead dividing stem cells down a particular path of development, but many different peptides, called cytokines, are clearly involved (Table 6–1; see also Chapter 3). Perhaps because mature white blood cells have a much shorter half-life in the circulation, white blood cell precursors usually outnumber red blood cell precursors by a ratio of 3:1 in the bone marrow.

FIGURE 6–1 Hematopoiesis: development of the formed elements of blood from bone marrow stem cells. Cells below the horizontal line are found in normal peripheral blood. The principal cytokines that stimulate each cell lineage to differentiate are shown. (CSF, colony-stimulating factor; EPO, erythropoietin; G, granulocyte; IL, interleukin; M, macrophage; SCF, stem cell factor; TPO, thrombopoietin) See Table 6–1

for details. (Redrawn, with permission, from Ganong WF. Review of Medical Physiology, 22nd ed. McGraw-Hill, 2005.)

TABLE 6–1 Cytokines that regulate hematopoiesis.

The major hormone that stimulates the production of erythrocytes (erythropoiesis) is erythropoietin. This peptide is produced by the kidneys and regulates red blood cell production by a feedback system: When blood hemoglobin levels fall (anemia), oxygen delivery to the kidneys falls, and the kidneys produce more erythropoietin, causing the marrow to produce more red cells. When hemoglobin levels rise, the kidneys produce less erythropoietin and the marrow fewer red cells.

For white blood cells, the situation is more complex. The most common cells are the granulocytes, so named because their cytoplasms are filled with granules. Of these, the neutrophils are the most prevalent and the most important cells in producing inflammation. Granulocyte production (myelopoiesis) can be affected by many cytokines at different stages of development. Figure 6–1 shows that interleukin-3 (IL-3), granulocyte colony-stimulating factor (G-CSF), and granulocyte–macrophage colony-stimulating factor (GM-CSF) are the most important. All three proteins have been purified, sequenced, and cloned. The latter two proteins are used therapeutically. Unlike G-CSF, GM-CSF also stimulates the maturation of a different white blood cell line, the monocyte- macrophage line. These cells are part of the immune system as well (ie, they ingest foreign bacteria) and can reside in skin and other tissues, not just blood. Their function, along with that of the B- and T-lymphocyte populations, is discussed more fully in Chapter 3.

Platelets are not cells but fragments of larger multinucleated cells in the marrow called megakaryocytes. Platelets are crucial to normal blood clotting. Platelet production is also stimulated by multiple cytokines but depends mainly on the action of IL-3, IL-6, IL-11, and thrombopoietin (TPO). This peptide is produced by the liver, kidney, skeletal muscle, and marrow stroma. One model of thrombopoiesis proposes that the production of TPO occurs at a constant rate. However, the amount of this hormone free to interact with platelet precursors rises and falls, probably as a result of uptake by TPO receptors (c- Mpl) on existing platelets in the blood. Therefore, a low platelet count (with a lower mass of c-Mpl) stimulates thrombopoiesis as a result of increased circulating levels of TPO. A second model proposes that low platelet levels can induce increased production of TPO in marrow stromal cells via various cytokines, including platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF). These two models are not necessarily mutually exclusive. Inflammation can also lead to thrombocytosis via IL-6-mediated increases in TPO production by the liver.

For all the marrow’s complexity and metabolic activity, there is tremendous regulation of the marrow through the interaction of various cytokines. Normally, only the most mature elements in each cell lineage are released into the general circulation, demonstrating this exquisite control over development. Complex negative-feedback mechanisms must be at work to maintain circulating quantities of each formed element at the consistent levels at which they are found.

Examination of the appropriateness of blood cell development is best undertaken with the microscope, using the thin blood smear (Figure 6–2). Modern technical equipment, which can optically sort cells by size and various optical reflective parameters, gives important information, especially about whether cell numbers are out of the normal ranges (Table 6–2). However, microscopic examination of the blood smear, usually using Wright stain, gives additional information once an abnormality is detected and should always be done when a blood disorder is suspected on clinical grounds.

FIGURE 6–2 Normal thin blood smear, seen at low power (40×) with Wright stain. Erythrocytes predominate and can be seen to be thin disks with central pallor (see text). Platelets are the numerous small, dark bodies. Larger cells with lobulated nuclei are mature neutrophils. Lymphocytes and monocytes are not present on this smear.

TABLE 6–2 Normal values obtained on automated blood count—formed elements of blood.

Physiology A. Erythrocytes Mature red blood cells are biconcave, disk-shaped cells filled with hemoglobin, which function as the oxygen-carrying component of the blood. In contrast to most other cells, they do not have nuclei at maturity; their nuclei are extruded during the final phase of erythrocyte development. The presence of erythrocytes with nuclei in the peripheral blood smear suggests an underlying disease state. Normal red cells are about 8 μm in diameter, a size that is larger than the smallest capillaries. However, their biconcave shape gives them enough flexibility to slip through small capillaries and deliver oxygen to the tissues. Once extruded from the bone marrow, individual erythrocytes function for about 120 days before they are removed from the circulation by the spleen.

In a typical blood smear (stained with Wright stain), erythrocytes dominate

the microscopic field, and their biconcave disk shape resembles that of a doughnut. There is a thicker outer rim that appears red owing to the hemoglobin present and an area of central pallor where the disk is thinnest. Young erythrocytes (reticulocytes) appear bluer (basophilic) because they still contain some ribosomes and mitochondria for a few days after the nuclei are extruded.

Hemoglobin is the most important substance in the erythrocyte. This protein is actually a tetramer, made of two α-protein subunits and two β-protein subunits (in normal adult hemoglobin, called hemoglobin A). Each α- or β-subunit contains the actual oxygen-binding portion of the complex, heme. Heme is a compound whose centrally important atom is iron; it is this atom that binds oxygen in the lungs and subsequently releases it in the tissues of the body. A low level of hemoglobin in the blood, from a variety of causes (see later discussion), is anemia, the most common general blood disorder.

B. Granulocytes: Neutrophils, Eosinophils, and Basophils

The granulocytes are the most common white blood cells; of these, neutrophils are most abundant, followed by eosinophils and basophils (see Table 6–2). Developmentally, all three types are similar: As they mature, their nuclei become more convoluted and multilobed, and each develops a cytoplasm filled with granules. These granules contain a variety of enzymes, prostaglandins, and mediators of inflammation, with specific factors dependent on the cell type. Early progenitor cells for each type of granulocyte (“blasts”) are indistinguishable on microscopic examination of the bone marrow, but under the influence of different cytokines, they become morphologically distinct cell types.

Basophils contain very dark blue or purple granules when stained with either Giemsa or Wright stain. Basophil granules are large and usually obscure the nucleus because of their density. Normally, basophils function in hypersensitivity reactions (as described in Chapter 3). However, their numbers can be increased in diseases not associated with hypersensitivity, such as chronic myelogenous leukemia.

Eosinophils contain large, strikingly “eosinophilic” granules (staining red with Wright or Giemsa stain). Eosinophil nuclei are usually bilobed. Normally, eosinophils function as part of the inflammatory response to parasites too large to be engulfed by individual immune cells. They are also involved in some allergic reactions.

Neutrophils contain granules that are “neutrophilic” (ie, neither eosinophilic nor basophilic). Although they predominate in the blood, their major function is actually in the tissues; they must leave the blood by inserting themselves

between the endothelial cells of the vasculature to reach sites of injury or infection. Their granules contain highly active enzymes such as myeloperoxidase, which, along with the free radical oxygen ions produced by membrane enzymes such as nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, kill bacteria that neutrophils ingest via endocytosis or phagocytosis. They are the “first line of defense” against bacterial pathogens, and low numbers of them (leukopenia) lead directly to a high incidence of significant bacterial infections (see later discussion). Of all the cells produced by the bone marrow, the neutrophils comprise the greatest fraction. Their life span in blood, only 8 hours, is much shorter than that of any other cell type. Evidence of their importance and their short survival is commonly manifested, because microscopic examination of a blood smear from a patient with an active infection may show not only increased numbers of mature, multilobed neutrophils (neutrophilia), but also increased numbers of less mature cells. These less mature cells, released from a large storage pool in the bone marrow, are called bands and have a characteristic horseshoe-shaped nucleus that is not yet fully lobulated. The phenomenon of finding these cells in the peripheral blood is called a left shift of the granulocyte lineage.

C. Other White Blood Cells: Monocytes and Lymphocytes

Both monocytes and lymphocytes arise from the common stem cell. It is the widespread pluripotential ability of stem cells to differentiate into these cells, in addition to the granulocytes, erythrocytes, and platelets, that makes bone marrow transplantation a therapeutic option for immune system disorders and malignancies. Monocytes have a very long life span, probably several months, but spend only about 3 days in the circulation. They mostly reside in tissues and act there as immune cells that engulf (phagocytose) bacteria and subsequently can “present” components of these bacteria to lymphocytes in a way that further amplifies and refines the immune response (see Chapter 3). On blood smear evaluation, monocytes are the largest cells seen, with irregular but not multilobed nuclei and pale blue cytoplasm, often with prominent vacuoles.

Lymphocyte precursors leave the marrow early and require extramedullary (outside of the marrow) maturation to become normally functioning immune cells in either the blood or the lymphatic system (Figure 6–3). Their crucial roles in recognizing “self” versus “nonself” and in modulating virtually all aspects of the immune response are described in Chapter 3. On microscopic examination of the blood smear, lymphocytes are small cells, slightly larger than an erythrocyte, with dark nuclei essentially filling the entire cell; only a thin rim of light blue

cytoplasm is normally seen. Granules are sparse or absent.

FIGURE 6–3 Development of the immune system from the common bone marrow stem cell. (Redrawn, with permission, from Ganong WF. Review of Medical Physiology, 22nd ed. McGraw-Hill, 2005.)

D. Platelets

Platelets are the smallest formed elements in the blood. They are fragments of larger, multinucleated cells, which are the largest discrete constituents of the bone marrow (megakaryocytes), but platelets have no nuclei of their own. Most platelets remain in the circulation, but a substantial minority is trapped in the spleen; this phenomenon becomes important in a variety of immune-mediated decreases in platelet count (thrombocytopenia; see later discussion). In the setting of a normal platelet count, they have a circulatory half-life of about 10 days. In cases of thrombocytopenia, their half-life decreases, as they are consumed in the routine maintenance of vascular integrity.

Platelets are integral components of the coagulation system. Their membranes provide an important source of phospholipids (PLs), which are required for the function of the coagulation system proteins (Figure 6–4), and contain important receptors that allow attachment to endothelial cells (platelet adhesion) so that a platelet plug can be formed in response to blood vessel injury. This prevents further blood loss after trauma and limits the coagulation response to the site of injury rather than letting coagulation proceed inappropriately.

FIGURE 6–4 Coagulation and thrombolytic systems, showing balanced activity between them. (Ca2+, calcium; PAI, plasminogen activator inhibitor; PL, phospholipids; TF, tissue factor [thromboplastin]; TFPI, tissue factor pathway inhibitor; t-PA, tissue plasminogen activator; vWf, von Willebrand factor. All clotting factors are shown as Roman numerals and the addition of an “a” indicates “activated.”

The cytoplasm is also important for platelet function, particularly the intracellular dense granules and alpha granules. The phenomenon of platelet activation is also called “degranulation” and can be initiated by exposure of platelets to the activated blood coagulation factor thrombin, adenosine 5′- diphosphate (ADP), or collagen. This last reaction is probably the most important, occurring when collagen, normally in the basement membrane below the endothelial cells, is exposed to the blood after injury. Platelet activation can also be induced by exposure to platelet-activating factor (PAF) (a neutrophil- derived phospholipid cytokine), thromboxane A2, serotonin, and epinephrine.

During platelet activation, the dense and alpha granules release further activators of platelet activity, such as ADP, and platelet factor 4, which can also

bind to endothelial cells. Platelet factor 4 is clinically relevant because it binds to the most commonly used therapeutic anticoagulant, heparin (see later discussion). After activation, platelets change shape from discoid to spherical with filopodial extensions and finally to a flat shape that allows for adequate coverage of the site of vessel injury. The last step in platelet activity is platelet aggregation, in which platelets stick to each other, firming up the platelet plug. On examination of the blood smear, platelets are small, irregularly shaped blue or purple granular bodies. In conditions in which platelet numbers are rising as a result of increased marrow activity, the more immature platelets can be identified by their larger size.

CHECKPOINT

1. What is the ratio of red blood cells to white blood cells in the bloodstream?

2. What is the number of cells produced daily by the bone marrow? 3. What are the different formed elements of blood, and how can they and

their subtypes be distinguished?

COAGULATION FACTORS & THE COAGULATION SYSTEM

Anatomy The coagulation system, diagrammed in Figure 6–4, is a highly complex, regulated interaction of cells and plasma proteins. The coagulation system provides for immediate activation when control of bleeding (hemostasis) is required and confines its activity to the site of blood loss. Otherwise, coagulation might occur throughout the entire circulatory system, which would be incompatible with life.

The major components of hemostasis are platelets (discussed previously), endothelial cells (lining the blood vessels), other tissue factor (TF)–bearing cells, and the coagulation factors, which are plasma proteins. The end result of the activated coagulation system is the formation of a complex of cross-linked fibrin molecules and platelets that terminate hemorrhage after injury. To

maintain a well-regulated balance between prothrombotic and antithrombotic factors, the sophisticated coagulation system provides several points of control (see Figure 6–4).

The coagulation factors do not generally circulate in active forms. Most of them are enzymes (serine proteases) and remain dormant until needed. This is accomplished by having other enzymes (the other proteases in the coagulation system) available that can cleave the inactive factors into active ones. All factors have roman numerals, and the inactive forms are written without annotation (eg, factor II, also known as prothrombin). The activated forms of the factors are signified by the letter “a” (eg, factor IIa, also known as thrombin).

Most of the coagulation factors are made by the liver, but factor XIII derives from platelets, and factor VIII is made by endothelial cells. Factors II, VII, IX, and X are particularly important factors (Table 6–3) because they are all dependent on the liver enzyme γ-carboxylase. Gammacarboxylase is dependent on vitamin K, and the oral anticoagulant warfarin acts by interfering with vitamin K activity. Two of the anticoagulant proteins, protein S and protein C (see later discussion), are also vitamin K dependent.

TABLE 6–3 Coagulation factors of plasma.

Physiology Hemostasis is divided into three major processes: primary hemostasis, secondary

hemostasis, and fibrinolysis. Primary hemostasis involves vasoconstriction and platelet adhesion and

activation at sites of endothelial injury. Collagen and thrombin activate platelets, leading to an increase in intracellular calcium, secretion of platelet granules, and activation of various signaling pathways.

Secondary hemostasis is the process whereby fibrin is formed. The classical coagulation cascade, involving the intrinsic, extrinsic, and common pathways, better describes coagulation in vitro, as tested by the coagulation assays: activated partial thromboplastin time (aPTT) and prothrombin time (PT). The cell-based model of coagulation has replaced the coagulation cascade as a more accurate depiction of the in vivo coagulation process (see Figure 6–4). Secondary hemostasis is further divided into three overlapping phases: initiation, amplification, and propagation.

Initiation occurs at the surface of injured cells. It starts with the release of TF by the injured cells. TF, also called thromboplastin, is a lipid-rich protein material exposed to plasma upon injury to the vascular wall. It directly activates factor VII, forming the complex TF-VIIa, which activates both factor IX and factor X. Together on the surface of the injured cell, factors Xa (an enzyme) and Va (a cofactor, activated from factor V by factor Xa) catalyze the conversion of prothrombin (II) to thrombin (IIa). Thrombin, a serine protease, cleaves the ubiquitous plasma protein fibrinogen into fibrin monomers, small insoluble proteins that can polymerize with each other to form the complex fibrin; however, the amount of thrombin formed at the site of the injured cell is insufficient by itself to produce enough fibrin to stabilize the platelet plug.

Amplification, unlike the initiation phase, occurs at the surface of platelets. During this phase, thrombin produced in the initiation phase activates platelets and coagulation factors V, VIII, and XI found on the platelet surface. Factor VIII is normally complexed to von Willebrand factor (vWF), the protein that allows platelets to adhere to endothelial cells. Thrombin activates factor VIII by releasing it from vWF. It also activates both V and XI, which allows them to bind to the platelet surface. Factor XIa then catalyzes the activation of IX to IXa, providing supplemental factor IXa at the platelet surface.

Propagation involves activated platelets recruiting other circulating platelets to the site of vessel injury and the formation of two major complexes: tenase and prothrombinase, which are crucial to fibrin production. Factors VIIIa and IXa form the tenase complex on the surface of platelets in the presence of PLs and calcium (VIIIa-IXa-Ca2+-PL). Together, they activate factor X on the platelet surface. Factor Xa then forms the prothrombinase complex with factor Va on the

platelet surface, again in the presence of PLs and calcium (Xa-Va-Ca2+-PL). This complex catalyzes the cleavage of prothrombin (II) to thrombin (IIa) and can convert multiple molecules per complex. As the activated platelets recruit more circulating platelets to the site of injury, a critical mass of platelets leads to a surge of thrombin generation. This, in turn, leads to enough fibrin formation to stabilize the platelet plug. This fibrin polymer is further solidified by chemical cross-links catalyzed by factor XIIIa, which itself is activated by thrombin. Factor XIIIa also incorporates α2-antiplasmin into the clot to protect it from fibrinolytic proteases.

Fibrinolysis involves the process of breaking down fibrin into its degradation products. Plasmin is the main catalytic enzyme in this process. It is a serum protease that cleaves fibrin, resulting in breakup of the clot and creating fibrin degradation products that inhibit thrombin. Thrombin, working in a negative feedback fashion, actually helps catalyze the formation of plasmin from the inactive precursor protein, plasminogen. Plasminogen can also be cleaved by tissue plasminogen activator (t-PA) to form plasmin; t-PA and related proteins are used clinically to break up clots that form in coronary arteries in patients with a new myocardial infarction, as well as in the cerebral arteries of patients with a new stroke. Inhibitors of fibrinolysis include plasminogen activator inhibitor and α2-antiplasmin.

In addition to the fibrinolytic pathway, checks on the coagulation system (ie, the anticoagulant system) also involve various feedback loops and inhibitors. Factor Xa binds to another plasma (and lipid-bound) protein called tissue factor pathway inhibitor (TFPI). TFPI not only inhibits further activity of factor Xa itself but also prevents Xa from binding to the platelet surface, and the combination of factor Xa and TFPI greatly inhibits the TF-VIIa complex. Furthermore, downstream prothrombinase activity can be sustained only if the initial injury continues to generate enough factor IXa and VIIIa (in the form of the tenase complex) to activate more factor X on platelet surfaces.

Other anticoagulants include a group of inhibitors of the coagulation factors. They are composed of antithrombin (AT), protein S, and protein C (see later discussion). AT is a protease inhibitor and physically blocks the action of the serine proteases in the coagulation system. Its activity is enhanced up to 2000- fold by heparin. Protein C, activated by thrombin, cleaves factor Va into an inactive form so that the prothrombinase complex cannot cleave prothrombin (II) into thrombin. Protein C requires protein S as a cofactor. This complex also inactivates factor VIIIa.

LABORATORY TESTING OF THE COAGULATION PROCESS

Assays are available to determine both the absolute level and the activity of each of the coagulation factors, but in practice there are two common in vitro tests of coagulation function, both reported in terms of “seconds required to form a clot”: the PT and the aPTT. The tests are designed in such a way that the results will be prolonged out of the normal range in different pathologic states, but significant alterations in the coagulation pathway inevitably lead to changes in both tests because of the multiple interactions of the involved factors.

PT assesses the “extrinsic” TF-dependent and common pathways of the classical coagulation cascade and is used clinically to monitor the effects of warfarin. Because all vitamin K–dependent factor levels are lowered by warfarin, eventually the aPTT will also become abnormal with high enough doses; but factor VII has the shortest half-life of those factors, so its levels fall first. Because of its critical role in clotting, thrombin is the principal factor whose activity must be reduced to achieve and maintain therapeutic anticoagulation. PT is reported clinically with its companion test, the international normalized ratio (INR), which removes the impact of different prothrombin batch purity on the PT result.

The aPTT assesses the “intrinsic” non-TF-dependent and common pathways and is prolonged most easily when there are reduced levels of factor VIII or factor IX activity, regardless of whether these factors are present at low concentrations or are present at normal concentrations but are being actively inhibited by other molecules. The aPTT is also very sensitive to the presence of heparin bound to AT and is used to monitor the anticoagulant effects of unfractionated heparin. Low-molecular-weight heparins (a specific purified subset of unfractionated heparin) in combination with AT preferentially inhibit factor Xa. In the doses of low-molecular-weight heparins usually given for prevention or treatment of thrombosis, the aPTT will not be prolonged (at least not into the usual “therapeutic range” for unfractionated heparin) despite good evidence of anticoagulation efficacy if factor Xa activity is measured directly. Newer direct oral anticoagulants (thrombin or factor Xa inhibitors) affect aPTT or PT/INR less reproducibly, and drug-specific anti-Xa assays or the thrombin time may be needed in cases in which there are concerns about under- or over-anticoagulation with these agents.

CHECKPOINT

4. Name the vitamin K–dependent clotting factors and the organ in which they are synthesized.

5. What are the two major complexes found on the surface of activated platelets that are important to the surge in thrombin production? Describe the coagulation factors that form these complexes and what the complexes specifically activate.

6. What is the main catalytic enzyme in fibrinolysis? Name two inhibitors of fibrinolysis.

OVERVIEW OF BLOOD DISORDERS

FORMED ELEMENT DISORDERS

Disorders of red cells, white cells, and platelets are separated for discussion because one or the other is found to be the most abnormal during laboratory testing. However, because of the clonal nature of hematopoiesis, many disorders affect all the formed elements of the blood. This is perhaps best demonstrated in the “blast crisis” phase of chronic myelogenous leukemia, in which the majority of both myeloid and lymphoid cells in the blood may be shown to express an identical gene rearrangement, called bcr-abl or the Philadelphia chromosome, that has arisen in a single abnormal progenitor cell.

1. Red Cell Disorders There are many red cell abnormalities, but the principal ones are a variety of anemias. Anemia is defined as an abnormally low hemoglobin concentration in the blood. There are several methods of classification, but the prevailing systems are based on red cell size and shape.

In normal persons, erythrocytes are of uniform size and shape, and the automated blood count shows a mean corpuscular volume (MCV) near 90 fL, which is the estimated volume of a single cell. Automated systems usually report abnormalities of red cells as changes in hemoglobin concentration, red cell number, and MCV. Small cells (with low MCVs) are termed microcytic, and

cells larger than normal are termed macrocytic. The relative nonuniformity of cell shapes (poikilocytosis) or sizes (anisocytosis) can further aid in subclassifying erythrocyte disorders.

The morphologic classification of anemias is set forth in Table 6–4 and Figure 6–5. In general, the microcytic anemias are a result of abnormalities in hemoglobin production, either in number of hemoglobin molecules per cell or in type of hemoglobin molecules (hemoglobinopathies). Iron deficiency anemia resulting from chronic blood loss and the thalassemias are examples of microcytic anemia.

TABLE 6–4 Morphologic classification and common causes of anemia.

FIGURE 6–5 Thin blood smear appearance of erythrocytes in the different morphologic types of anemias. (1, poikilocytosis [variation in shape]; 2, anisocytosis [variation in size].) (Redrawn, with permission, from Chandrasoma P et al. Concise Pathology, 3rd ed. Originally published by Appleton & Lange. Copyright © 1998 by The McGraw-Hill Companies, Inc.)

The macrocytic anemias reflect either abnormal nuclear maturation or a higher fraction of young, large red cells (reticulocytes). When the nuclei of maturing red cells appear too young and large for the amount of hemoglobin in the cytoplasm, the macrocytic anemia is termed megaloblastic. These anemias are most often a result of either vitamin deficiencies (vitamin B12 or folic acid) or drugs that interfere with DNA synthesis. Abnormal nuclear maturation can also be a result of clonal proliferation in the bone marrow, producing

preleukemic states termed the myelodysplastic syndromes. The normocytic anemias can result from multiple causes: decreased numbers

of red cell precursors in the marrow (primary failure called aplastic anemia, replacement of marrow elements with cancer, certain viral infections, or autoimmune inhibition called pure red cell aplasia), low levels of erythropoietin (resulting from chronic kidney disease), or chronic inflammatory diseases that affect the availability of iron in the marrow. Other normocytic anemias can be secondary to decreased life span of the cells that are produced. Examples of this phenomenon are acute blood loss; autoimmune hemolytic anemias, in which antibodies or complement bind to red cells and cause their destruction; sickle cell anemia, in which the abnormal hemoglobin polymerizes and obliterates the usual resilience of the red cell; and hereditary spherocytosis or hereditary elliptocytosis, in which defects in the erythrocyte membrane affect their ability to squeeze through the capillary microcirculation.

Anemias are very common. In contrast, an elevated hemoglobin concentration, termed erythrocytosis, is uncommon. Elevations in hemoglobin concentration can occur as a secondary phenomenon because of increased erythropoietin levels, such as that found in smokers or people who live at high altitudes (whose low blood oxygen levels stimulate erythropoietin production). Some tumors, especially renal tumors, can also make erythropoietin. Primary polycythemia is an abnormality of the bone marrow itself. This myeloproliferative syndrome leads to an increased red cell mass and consequent low erythropoietin levels by the negative-feedback mechanism discussed previously.

2. White Blood Cell Disorders Abnormalities in white cell numbers occur commonly (Table 6–5), whereas abnormalities of function are rare. Neoplastic transformation in the form of leukemia (granulocytes and monocytes) or lymphoma (lymphocytes) is fairly common. The leukemias are discussed in Chapter 5.

TABLE 6–5 Causes of abnormal neutrophil counts.

Changes in neutrophil count are the most common white cell abnormality detected on the automated blood count. Increased numbers of neutrophils (leukocytosis) suggest acute or chronic infection or inflammation but can be a sign of many conditions. These include stress, because adrenal corticosteroids cause demargination of neutrophils from blood vessel walls.

Decreased numbers of neutrophils (neutropenia) can be seen in overwhelming infection and benign diseases such as cyclic neutropenia (see later discussion) but can also be seen when the bone marrow is infiltrated with tumor or involved by the myelodysplastic syndromes. Many drugs can also directly suppress marrow production, and because neutrophils have the shortest half-life in the blood of any cell produced by the marrow, their numbers may fall quickly.

Lymphocyte numbers can vary substantially (Table 6–6). Lymphocyte counts are classically elevated in viral infections, such as infectious mononucleosis. However, persistent elevations suggest malignancies, particularly chronic lymphocytic leukemia, which may not cause any symptoms and be incidentally discovered on a routine blood count.

TABLE 6–6 Causes of abnormal lymphocyte counts.

Decreased lymphocyte counts (lymphopenia) are a common complication of corticosteroid therapy but are most worrisome for immunodeficiency states; HIV directly infects lymphocytes, and the likelihood of opportunistic infections increases as lymphocyte counts fall, resulting in progression from HIV infection to full-blown AIDS.

3. Platelet Disorders Abnormalities in platelet number are fairly common, particularly low counts

(thrombocytopenia). Causes are listed in Table 6–7. Decreased production of platelets occurs when the marrow is affected by a variety of diseases or when TPO production by the liver is impaired, as in cirrhosis. Increased destruction of platelets is much more prevalent. There are three general mechanisms. Because a significant number of platelets normally reside in the spleen, any increase in spleen size or activity (hypersplenism) leads to lower platelet counts. Platelet consumption as a result of ongoing clotting will also lower counts. Most common, however, is immune-mediated consumption caused either by drugs or autoantibodies. The latter are usually directed against the platelet membrane antigen gpIIb/IIIa.

TABLE 6–7 Causes of platelet abnormalities.

Functional platelet disorders are common, especially the acquired disorders resulting from uremia (renal failure) or aspirin, which inhibits the platelet enzyme cyclooxygenase and decreases platelet aggregability. Inherited abnormalities are unusual with the exception of von Willebrand disease, which results from either quantitative or qualitative defect of vWF, the carrier protein for factor VIII. vWF also acts as a bridge between platelets and the endothelium and thus is crucial for the formation of the platelet plug in the coagulation cascade.

Elevations in the platelet count above normal (thrombocytosis) are relatively common and are especially apt to occur in recovery from iron deficiency anemia upon iron repletion. In the myeloproliferative disorders, such as polycythemia, platelet counts are often high. In essential thrombocythemia, platelet counts may be higher than 1,000,000/μL.

COAGULATION FACTOR DISORDERS

The most important coagulation factor disorders are quantitative rather than qualitative and usually hereditary rather than acquired (Table 6–8). Exceptions to this rule are acquired factor inhibitors, which are antibodies that bind to one of the coagulation factors, most often factor VIII. These may or may not cause clinical bleeding problems, but they can be extremely difficult to treat. The quantitative disorders that most commonly cause bleeding are hemophilia A (deficiency of factor VIII) and hemophilia B (deficiency of factor IX). Both are X chromosome–linked recessive traits, and affected males have very low levels of factor VIII or IX. It is not clear why all affected males do not have a complete absence of factor VIII or IX activity. Hemophilia A is more common, with a prevalence of 1:10,000 males worldwide. Both disorders lead to spontaneous and excessive post-traumatic bleeding, particularly into joints and muscles. Females with the trait have 50% of the normal amount of either factor and tend not to have any bleeding problems; in general, one needs only half of the normal quantities of most coagulation factors to clot normally. The aPTT test is usually designed to become abnormal when factor VIII or IX activities fall below 50% of normal.

TABLE 6–8 Coagulation factor deficiencies.

Vitamin K deficiency also leads to quantitative declines in the levels of factors II, VII, IX, and X and proteins C and S; prolongation of the prothrombin time may result.

Quantitative inherited abnormalities of the anticoagulation systems also occur. Protein S deficiency, protein C deficiency, and AT deficiency all occur and lead to abnormal clotting problems, as discussed in the next section.

Finally, the condition of consumptive coagulopathy or disseminated intravascular coagulation (DIC) needs to be included. This condition is generally a result of overwhelming infection, specific leukemias or lymphomas, or massive hemorrhage. In DIC, the coagulation factors become depleted. Often the fibrinolytic system is simultaneously activated, and uncontrolled bleeding may occur throughout the entire circulatory system. PT and aPTT are usually both abnormal.

CHECKPOINT

7. Define anemia, and suggest three causes each for mac-rocytic and microcytic anemia.

8. What are some categories of explanations for a white blood cell count that is substantially increased or decreased compared with the normal range?

9. What are the three general mechanisms of thrombocytopenia? 10. What is the nature of the defects in hemophilia A and B?

PATHOPHYSIOLOGY OF SELECTED BLOOD DISORDERS

RED CELL DISORDERS

1. Iron Deficiency Anemia Etiology Iron deficiency anemia is the most common form of anemia. Although in many developing countries dietary deficiency of iron can occur, in developed nations the main cause is loss of iron, almost always through blood loss from the GI or genitourinary tracts.

Because of recurrent menstrual blood loss, premenopausal women represent the population with the highest incidence of iron deficiency. The incidence in this group is even higher because of iron losses during pregnancy, because the developing fetus efficiently extracts maternal iron for use in its own hematopoiesis. In men or in postmenopausal women with iron deficiency, GI bleeding is usually the cause. Blood loss in this case may result from relatively benign disorders, such as peptic ulcer, arteriovenous malformations, or angiodysplasia (small vascular abnormalities along the intestinal walls). More serious causes are inflammatory bowel disease and malignancy. Endoscopic investigation to exclude malignancy is mandatory in patients without a known cause of iron deficiency.

There are other less common causes of iron deficiency, but most are related to blood loss: Bleeding disorders and hemoptysis are the chief possibilities. When no source of bleeding is uncovered, insufficient dietary intake (eg, vegetarian diets) and GI malabsorption should be considered as a possible cause of iron deficiency anemia. Such malabsorption occurs in patients with celiac disease, Helicobacter pylori infection, partial gastrectomy, or gastric bypass surgery. Other mechanisms of iron deficiency anemia include intravascular hemolysis (paroxysmal nocturnal hemoglobinuria or cardiac valvular disease) and iron depletion in response to erythropoietin treatment.

Pathogenesis

Body iron stores are generally sufficient to last several years, but there is a constant loss of iron in completely healthy persons, such that iron balance

depends on adequate intake and absorption. Dietary iron is primarily absorbed in the duodenum (Figure 6–6). Absorption is increased in the setting of anemia, hypoxia, and systemic iron deficiency. Iron is also recycled from senescent erythrocytes via macrophage phagocytosis and lysis. The export of iron to plasma from these cellular sites is regulated by hepcidin, a 25–amino acid peptide produced by the liver. Hepcidin binds to ferroportin, a transmembrane protein, inducing its internalization and lysosomal degradation. When iron stores are low, hepcidin production is reduced and ferroportin molecules are expressed on the basolateral membrane of enterocytes, where they transfer iron from the cytoplasm of enterocytes to plasma transferrin. Conversely, when iron stores are adequate or elevated, hepcidin production is increased, resulting in the internalization of ferroportin and reduced export of iron into plasma. In inflammatory states, hepcidin production is increased, leading to the internalization of ferroportin on macrophages and the trapping of recycled iron within macrophage stores.

FIGURE 6-6 Iron transport and regulation in the duodenal enterocyte. Under normal conditions (left), iron (Fe) transits from the intestinal lumen into the enterocyte through divalent metal transporter 1 (DMT1)

and is either stored intracellularly bound to ferritin or exported through ferroportin where it is bound to plasma transferrin. Hepcidin (middle and right) binds to ferroportin resulting in its internalization and degradation, thereby decreasing iron export from intracellular stores. (Reproduced, with permission, from Aster JC et al. Pathophysiology of Blood Disorders, 2nd ed. McGraw-Hill, 2017, Figure 5–2.)

Iron is stored in most body cells as ferritin, a combination of iron and the protein apoferritin. It is also stored as hemosiderin, which is ferritin partly stripped of the apoferritin protein shell. Iron is transported in blood bound to its carrier protein, transferrin. Because of the complex interactions between these molecules, a simple measurement of serum iron rarely reflects body iron stores (see later discussion).

Iron is found predominantly in hemoglobin and is also present in myoglobin, the oxygen-storing protein of skeletal muscle. The main role for iron is as the ion in the center of the body’s oxygen-carrying molecule, heme. Held stably in the ferrous form by the other atoms in heme, iron reversibly binds oxygen. Each protein subunit of hemoglobin contains one heme molecule; because hemoglobin exists as a tetramer, four iron molecules are needed in each hemoglobin unit. In iron deficiency, the final step in heme synthesis is interrupted (Figure 6–7). In this step, ferrous iron is inserted into protoporphyrin IX by the enzyme ferrochelatase; when heme synthesis is interrupted, there is inadequate heme production. Globin biosynthesis is inhibited by heme deficiency through a heme-regulated translational inhibitor (HRI). Elevated HRI activity (a result of heme deficiency) inhibits a key transcription initiation factor for heme synthesis, eIF2. Thus, less heme and fewer globin chains are available in each red cell precursor. This directly causes anemia, a decrease in the hemoglobin concentration of the blood.

FIGURE 6–7 Heme synthesis, emphasizing the role of iron and the insertion of heme into individual globin chains to make hemoglobin, and the role of the heme-regulated translational inhibitor (HRI) of globin synthesis. Normal concentrations of heme keep the activity of HRI low, preserving normal globin synthesis.

As noted, heme is also the oxygen acceptor in myoglobin; therefore, iron deficiency will also lead to decreased myoglobin production. Other proteins also depend on iron; most of these are enzymes. Many use iron in the heme molecule, but some use elemental iron. Although the exact implications of iron deficiency on their activity is not known, these enzymes are crucial to metabolism, energy production, DNA synthesis, and even brain function.

Pathology

As iron stores are depleted, the peripheral blood smear pattern evolves. In early iron deficiency, the hemoglobin level of the blood falls, but individual erythrocytes appear normal. In response to a falling oxygen level, erythropoietin levels rise and stimulate the marrow, but the hemoglobin level cannot rise in response because of the iron deficiency. Other hormones are presumably also stimulated, however, and the resulting “revved-up” marrow usually causes an

elevated blood platelet count. An elevated white cell count is less common. Reticulocytes are notably absent.

Eventually, the hemoglobin concentration of individual cells falls, leading to the classic picture of microcytic, hypochromic erythrocytes (see Figure 6–5). This is most commonly found as an abnormally low MCV of red cells on the automated hemogram. There is also substantial anisocytosis and poikilocytosis, seen on the peripheral smear, and target cells may be seen. The target shape occurs because there is a relative excess of red cell membrane compared with the amount of hemoglobin within the cell, so that the membrane bunches up in the center.

Laboratory results are often confusing. A low serum ferritin level is diagnostic of iron deficiency, but even in obvious cases, levels can be normal; ferritin levels rise in acute or chronic inflammation or significant illnesses, which can themselves be the cause of iron (blood) loss. Serum iron levels fall in many illnesses, and levels of its serum carrier, transferrin, fluctuate as well, so neither is a consistent indicator of iron deficiency, nor is their ratio, the transferrin saturation. Nevertheless, serum ferritin is the most sensitive and specific test for iron deficiency. If ferritin levels are not diagnostic, measuring serum-soluble transferrin receptor (sTfR) can help. TfRs are membrane glycoproteins that facilitate iron transport from plasma transferrin into body cells. Erythroid precursors increase their expression of membrane TfR in the setting of iron deficiency but not anemia of chronic disease. Some membrane TfR is released into the serum as sTfR. The amount of sTfR in the serum reflects the amount of membrane TfR. A high ratio of sTfR to ferritin predicts iron deficiency when ferritin is not diagnostically low. Though helpful, this test has had limited adoption in clinical practice.

Other than observing a hematologic response to empiric iron supplementation, bone marrow biopsy can confirm a diagnosis of iron deficiency. Iron is normally found in the macrophages of the marrow, where it supplies erythrocyte precursors; intracellular hemosiderin is easily visualized with Prussian blue stain. These macrophages do not stain at all if there is iron deficiency.

Clinical Manifestations

All anemias lead to classic symptoms of decreased oxygen-carrying capacity (ie, fatigue, weakness, and shortness of breath, particularly dyspnea on exertion), and iron deficiency is no exception. Decreased oxygen-carrying capacity leads to decreased oxygen delivery to metabolically active tissues, which nonetheless

must have oxygen; this leads directly to fatigue. The compensatory mechanisms of the body lead to additional symptoms and signs of anemia. Some patients appear pale not only because there is less hemoglobin per unit of blood (oxygenated hemoglobin is red and gives color to the skin), but also because superficial skin blood vessels constrict, diverting blood to more vital structures. Patients may also respond to the anemia with tachycardia. This increased cardiac output is appropriate because one way to increase oxygen delivery to the tissues is to increase the number of times each hemoglobin molecule is oxygenated in the lungs every hour. This tachycardia may cause benign cardiac murmurs due to the increased blood flow.

Abnormalities of the GI tract occur because iron is also needed for proliferating cells. Glossitis, in which the normal tongue papillae are absent, can occur, as can gastric atrophy with achlorhydria (absence of stomach acid). The achlorhydria may compound the iron deficiency because iron is best absorbed in an acidic environment, but this complication is quite unusual.

In children, there may be significant developmental problems, both physical and mental. Iron-deficient children, mostly in developing regions, perform poorly on tests of cognition compared with iron-replete children. Iron therapy can reverse these findings if started early enough in childhood. The exact mechanism of cognitive loss in iron deficiency is not known. Another unexplained but often observed phenomenon in severe iron deficiency is pica, a craving for nonnutritive substances such as clay or dirt.

Many patients have no specific symptoms or findings at all, and their iron deficiency is discovered because of anemia noted on a blood count obtained for another purpose. It is of interest that mild anemias (hemoglobins of 11–12 g/dL) may be tolerated very well because they develop slowly. In addition to the physiologic compensatory mechanisms discussed previously (increased cardiac output, diversion of blood flow from less metabolically active areas), there is a biochemical adaptation as well. The ability to transfer oxygen from hemoglobin to cells depends partly on a small molecule in erythrocytes called 2,3- biphosphoglycerate (2,3-BPG). In high concentrations, the ability to unload oxygen in the tissues is increased. Chronic anemia leads to elevated 2,3-BPG concentrations in erythrocytes.

Other patients who do not present with symptoms directly related to the anemia present instead with symptoms or signs related directly to blood loss. Because the most common site of unexpected (nonmenstrual) blood loss is the GI tract, patients often have visible changes in the stool. There may be gross blood (hematochezia), which is more common with bleeding sites near the

rectum, or black, tarry, metabolized blood (melena) from more proximal sites. Significant blood loss from the urinary tract is very uncommon.

CHECKPOINT

11. What is the most common form of anemia and its most likely cause in a premenopausal woman? In a man?

12. In what situations might the serum ferritin level be nor-mal or elevated in a patient with iron deficiency?

13. What are some disorders associated with iron deficiency anemia? 14. What are the physiologic adaptations to slowly devel-oping iron

deficiency anemia?

2. Pernicious Anemia Etiology Pernicious anemia is a megaloblastic anemia in which there is abnormal erythrocyte nuclear maturation. Unlike in many other types of anemia, such as that resulting from iron deficiency, hemoglobin synthesis is normal. Pernicious anemia is the end result of a cascade of events that are autoimmune in origin. The ultimate effect is a loss of adequate stores of vitamin B12 (cobalamin), which is a cofactor involved in DNA synthesis. Rapidly proliferating cells are those most often affected, predominantly bone marrow cells and those of the GI epithelium. The nervous system is also affected, demonstrating that this is a systemic disease. Anemia is merely the most common manifestation.

Besides pernicious anemia, cobalamin deficiency can also be a result of bacterial overgrowth in the intestine (because bacteria compete with the host for cobalamin), intestinal malabsorption of vitamin B12 involving the terminal ileum (such as in Crohn’s disease), surgical removal of the antrum of the stomach (gastrectomy), and, rarely, dietary deficiency, which occurs only in strict vegetarians (vegans). In the diet, cobalamin is found mostly in animal products.

Pernicious anemia is most common in older patients of Scandinavian descent and is more commonly found in those of European and African than Asian descent. In the United States, black females are one of the most common groups. Pernicious anemia accounts for only a small percentage of patients with anemia, however.

Pathogenesis

The initial events in the pathogenetic cascade begin in the stomach (Figure 6–8). The gastric parietal cells are initially affected by an autoimmune phenomenon that leads to two discrete effects: loss of gastric acid (achlorhydria) and loss of intrinsic factor. Pernicious anemia interferes with both the initial availability and the absorption of vitamin B12: Stomach acid is required for the release of cobalamin from foodstuffs, and intrinsic factor is a glycoprotein that binds cobalamin and is required for the effective absorption of cobalamin in the terminal ileum. Both stomach acid and intrinsic factor are made exclusively by parietal cells.

FIGURE 6–8 Pathogenesis and effects of pernicious anemia (autoimmune atrophic gastritis). (Redrawn, with permission, from Chandrasoma P et al. Concise Pathology, 3rd ed. Originally published by Appleton & Lange. Copyright © 1998 by The McGraw-Hill Companies, Inc.)

Evidence for the autoimmune destruction of parietal cells is strong: Patients

with pernicious anemia have atrophy of the gastric mucosa, and pathologic specimens show infiltrating lymphocytes, which are predominantly antibody- producing B cells. In addition, 90% or more of patients have antibodies in their serum directed against parietal cell membrane proteins. The major protein antigen appears to be H+-K+ ATPase, the proton pump, which is responsible for the production of stomach acid. Cytotoxic T cells whose receptors recognize H+- K+ ATPase may also contribute to the gastric atrophy. More than half of patients also have antibodies to intrinsic factor itself or the intrinsic factor–cobalamin complex. Furthermore, patients with pernicious anemia have a higher incidence of other autoimmune diseases, such as Graves disease. Lastly, corticosteroid therapy, used as first-line therapy for many autoimmune disorders, may reverse the pathologic findings in pernicious anemia. Despite this evidence, the exact mechanism of the inciting event remains unknown.

Complete vitamin B12 deficiency develops slowly, even after total achlorhydria and loss of intrinsic factor occur. Liver stores of vitamin B12 are adequate for several years. However, the lack of this vitamin eventually leads to alterations in DNA synthesis and, in the nervous system, altered myelin synthesis.

In DNA synthesis, cobalamin, along with folic acid, is crucial as a cofactor in the synthesis of deoxythymidine from deoxyuridine (Figure 6–9). Cobalamin accepts a methyl group from methyltetrahydrofolate, which leads to the formation of two important intracellular compounds. The first is methylcobalamin, which is required for the production of the amino acid methionine from homocysteine. The second is reduced tetrahydrofolate, which is required as the single-carbon donor in purine synthesis. Thus, cobalamin deficiency depletes stores of reduced tetrahydrofolate and impairs DNA synthesis because of lowered purine production. In cobalamin deficiency, other reduced folates may substitute for tetrahydrofolate (and may explain why pharmacologic doses of folic acid can partially reverse the megaloblastic blood cell changes, but not the neurologic changes, seen in pernicious anemia). However, methyltetrahydrofolate, normally the methyl donor to cobalamin, accumulates. This folate cannot be retained intracellularly because it cannot be polyglutamated; the addition of multiple glutamate residues leads to a charged compound that does not freely diffuse out of the cell. Therefore, there is relative folate deficiency in pernicious anemia as well. In addition, methionine may serve as a principal donor of methyl groups to these other “substituting” reduced folates; because methionine cannot be produced in cobalamin deficiency, this compounds the problems in purine synthesis.

FIGURE 6–9 Role of cobalamin (vitamin B12) and folic acid in nucleic acid and myelin metabolism. Lack of either cobalamin or folic acid retards DNA synthesis (A), and lack of cobalamin leads to loss of folic acid, which cannot be held intracellularly unless polyglutamated. Lack of cobalamin also leads to abnormal myelin synthesis, probably via a deficiency in methionine production (B). (Redrawn, with permission, from Chandrasoma P et al. Concise Pathology, 3rd ed. Originally published by Appleton & Lange. Copyright © 1998 by The McGraw-Hill Companies, Inc.)

The exact mechanism of the neurologic consequences of pernicious anemia, with demyelination (loss of the myelin sheaths around nerves), is unknown. Defects in the methionine synthase pathway have been suggested but not proven experimentally. Instead, observations in cobalamin-deficient gastrectomized rats implicate an imbalance of cytokines and growth factors as a potential mediator of nerve damage. The synthesis of the cytokine tumor necrosis factor (TNF) is regulated by S-adenosyl-methionine, a product of methionine. Deficiency of methionine may indirectly lead to neuropathy via unregulated production of TNF, a myelinolytic cytokine, among other mechanisms.

The production of succinyl-coenzyme A (CoA) also depends on the presence of cobalamin. It is unclear whether a decrease in the production of succinyl- CoA, which may affect fatty acid synthesis, is also involved in the demyelinating disease.

Pathology

The gastric disorders associated with pernicious anemia are dominated by the

picture of chronic atrophic gastritis (see Figure 6–8). The normally tall columnar epithelium is replaced by a very thin mucosa, and there is obvious infiltration of plasma cells and lymphocytes. Pernicious anemia also increases the risk for gastric adenocarcinoma. Thus, pathologic examination may also reveal cancer.

The peripheral blood smear picture (see Figure 6–5) varies, depending on the length of time the patient has been cobalamin deficient. In early stages, the patient may have mild macrocytic anemia, and large ovoid erythrocytes (macro- ovalocytes) are commonly seen. In full-blown megaloblastic anemia, however, there are abnormalities in all cell lines. The classic picture reveals significant anisocytosis and poikilocytosis of the red cell line, and there are hypersegmented neutrophils, revealing the nuclear dysgenesis from abnormal DNA synthesis (Figure 6–10). In severe cases of pernicious anemia, the red and white cell series are easily mistaken for acute leukemia because the cells look so atypical.

FIGURE 6–10 Megaloblastic hematopoiesis: morphologic changes visible on microscopic examination of bone marrow or peripheral blood. (Redrawn, with permission, from Chandrasoma P et al. Concise Pathology, 3rd ed. Originally published by Appleton & Lange. Copyright © 1998 by The McGraw-Hill Companies, Inc.)

Bone marrow aspiration and biopsy are not necessary in the diagnosis and results from them may be misleading because the marrow pathology can be confused with acute leukemia, hypercellularity, increased erythroblasts, and even cytogenetic changes. Typical findings in B12 deficiency include megaloblastic changes—nuclei that are too large and immature in cells with mature, hemoglobin-filled cytoplasm—that are seen at each stage of erythrocyte development. These cells are not seen in the peripheral blood because the

abnormal erythrocytes generally are destroyed in the marrow (intramedullary hemolysis) by unexplained processes. This compounds the anemia. Megaloblastic changes can be seen in the marrow even in the absence of obvious changes on the peripheral blood smear.

Spinal cord abnormalities consist of demyelination of the posterolateral spinal columns, called subacute combined degeneration. Peripheral nerves may also show demyelination. Demyelination eventually results in neuronal cell death, which is also obvious on pathologic examination. Because neurons do not divide, new neurons cannot replace the dead ones.

Laboratory findings include elevated lactate dehydrogenase (LDH) and, sometimes, indirect bilirubin consistent with the hemolysis occurring in the bone marrow. LDH is directly released from lysed red cells, and free hemoglobin is metabolized to bilirubin. Serum vitamin B12 levels are usually low, revealing the deficient state. Yet there remain high rates of both false positive and false negative test results because only 20% of total measured serum B12 is bound to the cellular delivery protein, transcobalamin; the rest is bound to haptocorrin, which is not available for cells to use. Antibodies to intrinsic factor are usually detectable. Serum elevations of methylmalonic acid (MMA) and/or homocysteine (see Figure 6–9) are highly predictive of B12 deficiency. The Schilling test, which assesses the oral absorption of vitamin B12 with and without added intrinsic factor, is no longer used, because of lack of availability of radioactively labeled vitamin B12. Typically, the approach is to first measure serum B12 and, if results are equivocal, to obtain serum levels of MMA and/or homocysteine.

Clinical Manifestations

The clinical presentation consists of one or more symptoms related to the underlying deficiency. Anemia is the most commonly encountered abnormality and is often very severe; hemoglobin levels of 4 g/dL (less than a third of normal) can be seen. This degree of anemia is rare with other causes, such as iron deficiency. Typical symptoms are fatigue, dyspnea, or dizziness because a decreased red cell mass equals a decreased oxygen-carrying capacity of the blood. High-output heart failure is relatively common, with tachycardia and signs of left ventricular failure (see Chapter 10). Because oxygen demands are constant (or rise with exercise) and oxygen-carrying capacity is falling, the only way to maintain tissue oxygenation in anemia is to increase cardiac output (ie, the number of times per minute each red cell is fully oxygenated by the lungs).

Eventually, however, the left ventricle fails. However, symptoms may be mild because the anemia develops slowly as a

result of the extensive liver storage of vitamin B12. Patients with anemia usually adapt over time to slow changes in oxygen-carrying capacity. The same changes in 2,3-BPG that encourage oxygen delivery to the tissues from the hemoglobin in red cells in other anemias occur in vitamin B12 deficiency.

GI symptoms are less prevalent and include malabsorption, muscle wasting (unusual), diarrhea (more common), and glossitis (most common). In glossitis, the normal tongue papillae are absent regardless of whether the tongue is painful, red, and “beefy” or pale and smooth.

Neurologic symptoms are least likely to improve with cobalamin replacement therapy. As with other neuropathies involving loss of myelin from large peripheral sensory nerves, numbness and tingling (paresthesias) occur frequently and are the most common symptoms. Demyelination and neuronal cell death in the posterolateral “long tracts” of the spinal cord interfere with delivery of positional information to the brainstem, cerebellum, and sensory cortex. Patients, therefore, complain of loss of balance and coordination. Examination reveals impaired proprioception (position sense) and vibration sense. True dementia may also occur when demyelination involves the brain. Importantly, but somewhat unexpectedly, neurologic symptoms may occur in the absence of any changes in the peripheral blood smear suggestive of pernicious anemia.

Less commonly, vitamin B12 deficiency can manifest with thrombosis and possibly at unusual sites such as cerebral venous sinuses. The prothrombotic state may be secondary to hyperhomocysteinemia seen in severe vitamin B12 deficiency.

CHECKPOINT

15. Name two crucial cofactors in DNA synthesis whose deficiency results in pernicious anemia. In what specific biochemical pathways do they participate?

16. What neurologic defects are observed in prolonged pernicious anemia? 17. What symptoms of pernicious anemia are usually rela-tively mild? 18. Are changes in the peripheral blood smear necessary for neurologic

effects of vitamin B12 deficiency?

WHITE CELL DISORDERS

1. Malignant Disorders The most important leukocyte abnormalities are the malignant disorders leukemia and lymphoma. They are discussed in Chapter 5.

2. Cyclic Neutropenia Absolute neutropenia, characterized by neutrophil counts less than 1500–2000/ μL (>2 SD below the mean in normals), is a commonly encountered problem in medicine and can result from a large number of disease entities (see Table 6–5). Cyclic neutropenia, however, is rare. It is of interest because it provides insight into normal neutrophil production and function. It is characterized by a lifetime history of neutrophil counts that decrease to zero or near zero for 3–5 days at a time every 3 weeks and then rebound. Interestingly, the peripheral blood neutrophil counts and monocyte counts oscillate in opposite phases on this 3- week cycle.

Etiology

Classic, childhood-onset cyclic neutropenia results from heterozygous germline mutations in the gene ELANE (ELAstase, neutrophil expressed), formerly known as ELA2, which encodes for a single enzyme, neutrophil elastase (NE). NE is found in the primary azurophilic granules of neutrophils and monocytes. There are approximately 100 cases of childhood cyclic neutropenia in the literature, most of which are consistent with an autosomal dominant inheritance. However, sporadic adult cases also occur, and these are associated with neutrophil elastase mutations. There does not seem to be a racial predilection or gender bias in incidence.

Pathogenesis

The neutrophil count in blood is stable in normal individuals, reflecting the fact that there is a large storage pool of granulocytes in the marrow. The marrow reserve exceeds the circulating pool of neutrophils by 5- to 10-fold. This large pool is necessary because it takes nearly 2 weeks for the full development of a neutrophil from an early stem cell within the bone marrow, yet the average life

span of a mature neutrophil in blood is less than 12 hours. In cyclic neutropenia, the storage pool is not adequate. Daily measurements

of neutrophil counts in the blood reveal striking variations in their number. Studies of neutrophil kinetics in affected patients reveal that the defect is in abnormal production, rather than abnormal disposition, of neutrophils. Neutrophil production occurs in discrete waves even in normal individuals. As neutrophils differentiate from an early progenitor cell, they produce neutrophil elastase, which is thought to inhibit the differentiation of myeloblasts in a negative feedback loop. This results in an oscillatory wave with peaks and troughs of neutrophil production. As neutrophil numbers increase in the marrow, a peak is obtained in which enough neutrophil elastase causes a drop in neutrophil differentiation. Then, as the number of neutrophils drops again to a nadir, the production of neutrophil elastase also declines, allowing the number of neutrophils to climb once again. In cyclic neutropenia, it is hypothesized that the mutant neutrophil elastase may have an excessive inhibitory effect, causing prolonged trough periods and inadequate storage pools to maintain a normal peripheral neutrophil count. However, once they are extruded from the marrow, the neutrophils appear to have a normal life span (Figure 6–11).

FIGURE 6–11 Feedback loop hypothesis to explain hematopoietic cycling. Neutrophil elastase (NE) is postulated to inhibit further differentiation by a myeloblast. Gray sine wave denotes neutrophil count oscillations. In this model, NE is produced by the terminally differentiating cohort of neutrophils and ultimately feeds back to inhibit further production of neutrophils, which results in loss of the inhibitory cycle—at least for a while, until production of the neutrophils resumes, followed again by the inhibitory action of NE in a cyclic manner. (Redrawn from Horwitz MS et al. Neutrophil elastase in cyclic and severe congenital neutropenia. Blood. 2007 Mar 1;109(5):1817–24. Copyright American Society of Hematology.)

The myeloid progenitor for neutrophils can also produce monocytes. Therefore, during neutrophil nadirs, the myeloid progenitor cell can preferentially differentiate to the monocyte lineage, giving rise to the opposing oscillatory waves of neutrophils and monocytes seen in these patients (Figure 6– 12).

FIGURE 6–12 Regular cyclic variation of monocytes, reticulocytes, and neutrophils in a patient with cyclic neutropenia. Note that monocytes and reticulocytes tend to rise when neutrophils fall. (Redrawn, with permission, from Dale D et al. Cyclic neutropenia: a clinical review. Blood Rev. 1988;2:178. Copyright © Elsevier.)

The waves are remarkably constant in their periodicity. Almost every patient has a cycle between 19 and 22 days, and each patient’s cycle length is constant during his or her lifetime. Neutrophils and monocytes are not the only marrow elements that cycle. Platelet and reticulocyte counts also cycle with the same cycle length, but in contrast to the blood neutrophil count, clinically significant decreases are not observed. This is presumably because the blood life spans of these elements are so much longer than the life span of neutrophils. Because multiple cell lines are seen to cycle, it is believed that neutrophil elastase mutations accelerate the process of apoptosis (programmed cell death) in early progenitor cells, as well, unless they are “rescued” by G-CSF.

Clinically, administration of pharmacologic doses of G-CSF (filgrastim) to affected individuals has three interesting effects that partially overcome the condition. First, although cycling continues, mean neutrophil counts increase at each point in the cycle, such that patients are rarely neutropenic. Second, cycling periodicity decreases immediately from 21 days to 14 days. Third, other cell line fluctuations change in parallel; their cycle periodicity also decreases to 14 days, suggesting that an early progenitor cell is indeed at the center of this illness. However, the fact that cycling does not disappear demonstrates that there are other abnormalities yet to be discovered. It also suggests that there may be an inherent cycling of all stem cells in normal individuals that is modulated by multiple cytokines in the marrow.

Pathology

The pathologic features of cyclic neutropenia are seen mostly in the laboratory. The peripheral blood smear appears normal except for the paucity of neutrophils —mature or immature—during the nadirs of each cycle. Individual neutrophils appear normal. The bone marrow, however, shows striking differences depending on the day of the cycle on which it is examined. During the nadir of each cycle, there are increased numbers of early myeloid precursors such as promyelocytes and myelocytes, and mature neutrophils are rare. This picture is similar to that seen in acute leukemia, but 10 days later, as circulating neutrophil counts are rising, an entirely normal-appearing marrow is typical.

Clinical Manifestations

In general, neutropenia from any cause places patients at risk for severe bacterial infections, generally from enteric organisms, because of the alteration in host defenses in the GI tract. This is especially true when the neutropenia results from administration of chemotherapeutic agents, because chemotherapy also affects the lining of the GI tract. Neutrophils, with their ability to engulf bacteria and deliver toxic enzymes and oxidizing free radicals to sites of infection, normally serve as the first line of host defense against the bacteria that inhabit the gut. Such patients are also at risk for fungal infections if the neutropenia lasts more than several days; this is because it takes longer for fungi to reproduce and invade the bloodstream. Untreated infections of either type can be rapidly fatal, particularly if the neutrophil count is less than about 250/μL.

In cyclic neutropenia, then, recurrent infections are to be expected, and deaths from infections with intestinal organisms have been reported. Each cycle is

characterized by malaise and fever coincident with the time neutrophil counts are falling. Cervical lymphadenopathy is almost always present, as are oral ulcers. These symptoms usually last for about 5 days and then subside until the next cycle.

When infections occur, the site is usually predictable. Skin infections, specifically small, superficial pyogenic abscesses (furunculosis) or bacterial invasion of the dermis or epidermis (cellulitis), are the most common and respond to antibiotic therapy with few sequelae. The next most common infection site is usually the gums, and chronic gingivitis is evident in about half of patients. It is also the most noticeably improved problem when patients receive therapy with filgrastim. Other infections are unusual, but any neutropenic patient is at risk for infection from organisms that reside in the GI system. In the few patients who have required abdominal surgery during their neutropenia, ulcers similar to those seen in the mouth have been noted; this destruction of the normal mucosal barrier presumably eases entry of intestinal bacteria into the bloodstream. Because the period of greatest susceptibility to infection is only a few days in each cycle, most patients grow and develop normally.

CHECKPOINT

19. How long does it take for a neutrophil to develop from a stem cell in the bone marrow? Once fully mature, what is its life span?

20. At what level of neutropenia does the incidence of infection dramatically increase?

21. What are the most common sites and types of infections observed in neutropenic patients?

22. What is the probable underlying abnormality in cyclic neutropenia?

PLATELET DISORDERS

1. Drug-Associated Immune Thrombocytopenia Etiology Thrombocytopenia, defined as the occurrence of platelet levels below the normal

laboratory range, is a commonly encountered abnormality. Although there are many causes (see Table 6–7), the possibility of a drug-induced immune thrombocytopenia should always be considered.

Many drugs have been associated with this phenomenon, and the most common ones are listed in Table 6–9. In practice, the association between a given drug and thrombocytopenia is usually made clinically rather than with specific tests. Thrombocytopenia usually occurs at least 5–7 days after exposure to the drug, if given for the first time. The suspect drug is stopped, and platelet counts rebound within a few days. Rechallenge with the drug, which is rarely done, almost always reproduces the thrombocytopenia.

TABLE 6–9 Common drugs that may cause thrombocytopenia.

Heparin is the most important cause of thrombocytopenia because of its frequent use in hospitalized patients; its use also carries the potential to cause a life-threatening thrombotic syndrome. The pathophysiology of the thrombocytopenia caused by heparin is also the most completely described.

Pathogenesis

Although the phenomenon of drug-induced thrombocytopenia has been known for decades to be immune in nature, the specific mechanisms have long been controversial. The association of antibodies with platelets leads to their destruction via the spleen. The spleen acts as the major “blood filter” and recognizes platelets bound to antibodies as abnormal and thus removes them. Spleen removal also occurs in autoimmune (idiopathic) thrombocytopenia, which is relatively common and difficult to distinguish clinically from drug- induced thrombocytopenia.

There are various mechanisms underlying drug-induced immune thrombocytopenia. Quinine- or NSAID-induced thrombocytopenia involves the tight binding of antibody to normal platelets only in the presence of the sensitizing drug. The antibody usually targets epitopes on the glycoprotein IIb/IIIa or Ib/IX complexes, the major platelet receptors for fibrinogen and vWF, respectively. Penicillin and cephalosporin antibiotics are believed to lead to platelet destruction via hapten-dependent antibodies. The drug acts as a hapten, a small molecule that elicits an immunologic response only when bound to a large carrier molecule or protein. Some drugs (gold salts, procainamide, and possibly sulfonamides) can induce autoantibodies capable of binding to and destroying platelets even in the absence of the sensitizing drug. Finally, antithrombotic agents that block the binding of fibrinogen to gpIIb/IIIa receptors (abciximab, tirofiban, or eptifibatide) can cause an acute immune-mediated thrombocytopenia, in which patients develop severe thrombocytopenia within hours of exposure. The mechanism involves either naturally occurring antibodies that recognize the murine component of abciximab or structural changes to the gpIIb/IIIa receptor caused by the binding of tirofiban or eptifibatide.

For heparin, there is clear evidence of binding to a platelet protein, platelet factor 4 (PF4). PF4 resides in the alpha granules of platelets and is released when they are activated. It binds back onto the platelet surface through a specific PF4 receptor molecule, further increasing platelet activation. It also binds with high affinity to heparin and to heparin-like glycosaminoglycan molecules present on the vascular endothelium. This non-immune-based adhesion to PF4 can lead

to mild thrombocytopenia via promotion of platelet binding to fibrinogen and subsequent aggregation, known as heparin-induced thrombocytopenia (HIT) type I. This can happen in 30% of patients exposed to heparins without clinical sequelae. However, the combination of heparin with PF4 can also act as an antigenic stimulus that provokes the production of immunoglobulin G (IgG) directed against the combination. This immunologic response is known as heparin-induced thrombocytopenia (HIT) type II. About 10–20% of these patients with heparin-PF4 antibodies will develop the serious clinical syndrome of heparin-induced thrombocytopenia (and thrombosis) (HIT[T]), which paradoxically involves both thrombocytopenia 5–10 days after drug exposure and a prothrombotic state via increased platelet activation. There is a 10-fold increased risk for HIT in patients receiving unfractionated heparin (UFH) compared with those receiving low-molecular-weight heparins. Cardiac or orthopedic surgery patients are at higher risk for clinical HIT (1–5%) than medical or obstetric patients (0.1–1%) when receiving UFH. Women are at twice the risk for HIT as men.

Thrombocytopenia occurs in HIT type II after a series of steps. First, PF4 is released from platelets either by heparin itself or by other stimuli. Heparin then binds to PF4, forming an antigenic complex that results in the production of IgG antibodies that can bind directly to this compound. The new complex of IgG– heparin–PF4 binds to platelets through the platelet Fc receptor, via its IgG end. Platelets bound with this antibody complex are then destroyed by the spleen.

Despite the resulting thrombocytopenia, HIT type II leads to a prothrombotic state via the additional binding of the heparin–PF4 portion to the PF4 receptor on platelets, promoting platelet cross-linking, activation, and aggregation (Figure 6–13).

FIGURE 6–13 Pathogenesis of heparin-induced thrombocytopenia (HIT). IgG is the autoantibody against the heparin–PF4 complex. Platelets can bind to each other and become activated via the IgG–Fc receptor interaction, the PF4–PF4 receptor interaction, or both. Aggregation and thrombus formation may thus occur. Furthermore, IgG may bind to the endothelial cell–bound heparin–PF4 construct and cause vascular damage, which may also provoke thrombus formation.

Because each end of this IgG–heparin–PF4 molecule can bind to a platelet, it is possible that platelets can become cross-linked by a single molecule. Many platelets could actually interact in this fashion, leading to further platelet aggregation and activation. Clinically, this decreases the numbers of circulating platelets, but it may also lead to the creation of a thrombus at the site of activation. Thus, despite the fact that heparin is the most commonly used anticoagulant, in this case it may actually provoke coagulation. Furthermore, the activation of platelets via this mechanism leads to increased amounts of circulating PF4, which can bind to more heparin and thus continue the cycle. The excess PF4 can also bind to the endothelial surface via the heparin-like glycosaminoglycans described earlier. It is thus possible that the antibodies to

the heparin–PF4 construct could bind to the endothelial cells as well, which may lead to endothelial cell injury, further increasing the risk of local thrombosis by releasing TF and ultimately thrombin. Lastly, there is some evidence that macrophages may release TF in response to these antibodies, further stimulating coagulation.

Pathology

The peripheral blood smear is not strikingly abnormal unless platelet counts are less than about 75,000/μL, and then it is usually abnormal only because relatively few platelets are seen. Platelet morphology is usually normal, although large platelets can be seen. These large platelets are less mature and are a bone marrow compensation for a low peripheral platelet count, with platelet production from megakaryocytes being increased. Although drugs—heparin in particular—may cause platelet aggregation in vivo and in vitro, this is usually not apparent on review of the blood smear.

The bone marrow usually appears normal, although the megakaryocyte number may be relatively increased, presumably reflecting an attempt to increase the number of platelets (megakaryocyte fragments) in the circulation. In a few cases of immune-mediated thrombocytopenia, however, there may be decreased numbers of megakaryocytes. There are many hypotheses as to why this may occur, but it most likely means that the antigenic combination of drug– platelet protein is also occurring on megakaryocytes, so that they and the platelets in the peripheral circulation are being immunologically destroyed. This destruction would not involve the spleen, of course, but would require antibody- dependent cell killing.

In patients who develop heparin-induced thrombocytopenia and thrombosis, thrombi are seen that are relatively rich in platelets when compared with “typical” thrombi seen in other situations. They are described as “white clots.” The thrombi may be either arterial or venous.

Clinical Manifestations

Despite the fact that the platelet count in immune-mediated thrombocytopenia can be extremely low (<10,000/μL, compared with a normal value of over 150,000/μL), severe bleeding is unusual. More often, there is easy bruising with minimal trauma. With platelet counts of less than about 5000/μL, pinpoint hemorrhages (petechiae) may spontaneously occur in the skin or mucous membranes. These are self-limited because the plasma coagulation factors are

still intact, and only a small number of aggregated platelets are needed to provide adequate phospholipids for clotting.

The relationship between the likelihood of bleeding and the platelet count is not linear. The bleeding time, a test used clinically to evaluate platelet function, does not even begin to be abnormally prolonged until the platelet count is less than 90,000/μL. Spontaneous bleeding is unlikely until platelet counts are less than 20,000/μL but is still uncommon until counts are less than about 5000/μL, assuming that patients do not have other abnormalities of hemostasis. For example, aspirin inhibits platelet aggregation and increases the likelihood of bleeding. When bleeding from thrombocytopenia does occur, it is most often mucosal or superficial in the skin. This is most commonly seen as a nose bleed (epistaxis), but bleeding of the gums, GI tract, or bladder mucosa may also be seen.

As mentioned, however, when immune thrombocytopenia occurs as a result of heparin, paradoxical clotting may occur instead of bleeding. This may create a very confusing picture, because the heparin may have been given therapeutically for another thrombosis; it may be difficult to determine whether the new thrombosis is an extension of the initial clot or a new one resulting from the heparin exposure. However, the occurrence of the simultaneous thrombocytopenia provides a clue.

When heparin-induced thrombocytopenia and thrombosis do occur, the clinical manifestation of the new thrombosis will depend on the site of the thrombus. Most studies of this disorder suggest that when thrombosis occurs, it is at the site of previous vascular injury or abnormality. Thus, in patients with atherosclerotic vascular disease, arterial thromboses are much more common than venous clots. Patients experience a rapid onset of severe pain, usually in an extremity, with a cool, pale limb. Pulses are absent. This can be life-threatening (5–10% mortality rate) or at least extremity threatening because oxygen flow to the affected area is cut off, and emergency clot removal or vascular bypass surgery may be necessary. Venous clots also occur in a manner similar to typical venous clots (see later discussion). In addition to stopping heparin, patients with type II HIT need anticoagulation to prevent and treat thrombosis formation. Direct thrombin inhibitors (argatroban, lepirudin, or bivalirudin) provide a direct means of blocking the effects of thrombin, a primary mediator of the coagulation system.

CHECKPOINT

23. What is the most common category of cause of thrombocytopenia? 24. Name the platelet protein antibodies implicated in the pathogenesis of

heparin-induced thrombocytopenia. 25. By what mechanism can heparin-induced thrombocy-topenia actually

increase clot formation? 26. Why is major bleeding unusual in drug-induced thrombocytopenia?

COAGULATION DISORDERS

1. Inherited Hypercoagulable States Etiology The formation of blood clots in otherwise normal vessels is distinctly abnormal because the coagulation system in mammalian species is both positively and negatively balanced by so many factors. Nonetheless, there are a number of diseases that result in abnormal clotting (thrombosis). Abnormal clotting states may be either primary, in that the abnormalities result from genetic predispositions involving the coagulation factors themselves, or secondary (ie, acquired) because of changes in coagulation factors, blood vessels, or blood flow.

As first noted by the pathologist Virchow more than 150 years ago, there are three possible contributors to the formation of an abnormal clot (thrombus): decreased blood flow, vessel injury or inflammation, and changes in the intrinsic properties of the blood. Persistent physiologic changes in any of these three factors (the Virchow triad) are referred to as the “hypercoagulable states.”

The primary, or inherited, hypercoagulable states are all autosomal dominant genetic defects. This means that carriers (heterozygotes) are affected. Except for hyperprothrombinemia, all lead only to moderate (50%) decreases in the levels of the relevant factors. Despite the relatively modest fall, affected individuals are predisposed to abnormal thrombosis. These disorders are relatively rare in the general population, but they do account for a significant percentage of young patients who come to medical attention with thromboses. The specific states to be discussed are activated protein C (APC) resistance (the most commonly encountered abnormality), protein C deficiency, protein S deficiency, AT deficiency, and the prothrombin 20210 AG abnormality. Hyperhomocystinemia, an inborn error of metabolism, is also an inherited hypercoagulable state, but

because it does not involve the coagulation cascade, it is not further discussed here.

Pathogenesis

In the coagulation cascade, activated factor V (Va) plays a pivotal role (Figure 6–14). It is required for the formation of the prothrombinase complex with factor Xa, which leads to the thrombin burst and fibrin generation during hemostasis. Factor Va thus makes an excellent negative control point, so that once clot formation has begun, it does not go on unchecked.

FIGURE 6–14 The central role of factor V in the control of the coagulation cascade. The action of each of the negative control factors—protein S, protein C, and antithrombin—is shown in color.

Protein C is the major inhibitor of factor Va. Although it is an anticoagulation factor, its production is contingent on vitamin K–dependent γ-carboxylation, just like the coagulation factors II, VII, IX, and X. Protein C, when activated by the presence of clotting that generates thrombin, cleaves factor Va into an inactive form, and activation of factor X is thus slowed. By itself, however, protein C only weakly influences factor Va; its negative effect on factor Va is enhanced by a protein cofactor, protein S.

Factor V does not provide the only negative control point, however. Protein C also inhibits activated factor VIIIa, which is critical to forming the tenase–factor IXa complex, which is needed to activate factor X on activated platelets leading

to prothrombinase generation (see Figure 6–4). Factors II, IX, X, and XI (the serine proteases) are inhibited by a different molecule, AT. The action of AT itself is also regulated and highly dependent on the binding of an accelerator, heparin, or similar molecules present in abundance along the endothelial cells that line the vasculature. Evidence suggests that AT may also inhibit the TF–VIIa complex.

The fact that deficiencies of protein S, protein C, and AT activity cause clinically significant thrombosis demonstrates an important concept: It is the lack of adequate anticoagulant activity, rather than the overproduction of procoagulant activity, that characterizes most of the hypercoagulable states.

A. Activated Protein C Resistance—APC resistance is the most common inherited hypercoagulable state, with as many as 3–7% of the general population heterozygous for the abnormality. Up to 25% of patients who have venous thrombosis without an inciting event are found to have APC resistance in a large patient series. Most cases result from a single DNA base-pair mutation in the factor V gene, where guanine (G) is replaced by adenine (A). This single base change leads to substitution of the amino acid glutamine for arginine at position 506, and the altered factor V is referred to as “factor V Leiden,” named for the town in the Netherlands where it was discovered. This amino acid change alters the three-dimensional conformation of the cleavage site within factor Va, where APC normally binds to inactivate it. Thus, factor Va molecules can continue to enhance factor Xa’s conversion of prothrombin to thrombin (factor IIa), via the prothrombinase complex, and coagulation is not inhibited. This mutation also leads to loss of a cleavage product normally formed when factor V is inactivated by APC, a cofactor important in APC’s inactivation of factor VIIIa. Therefore, loss of this cofactor leads to decreased anticoagulant activity and contributes to the hypercoagulable state.

B. Protein C Deficiency—Protein C deficiency is common; up to 1 of every 200 individuals in the population is a heterozygote. Yet, thrombosis is uncommon among these individuals. The families that are thrombosis prone are thought to carry additional genetic factors, in addition to protein C deficiency, that increase their risk for thrombosis.

As noted earlier, protein C inactivates factors Va and VIIIa but requires protein S for its own action. Protein C also depends on the presence of platelet PL and calcium. In protein C deficiency, there is less inhibition of the prothrombinase complex, leading to relatively unrestricted clot formation. Normally, some of the thrombin generated binds to an endothelial cell protein,

thrombomodulin, and this complex activates protein C in the first place. This “negative feedback loop” is thus lost in protein C deficiency.

Protein C deficiency is not all one disease, however, unlike the factor V Leiden abnormality. Type I deficiency refers to individuals with decreased levels of protein C. Type II deficiency denotes cases with normal protein C levels but low protein C activity.

C. Protein S Deficiency—Protein S deficiency is also an uncommon heterogeneous disorder. Type I protein S deficiency refers to cases with low free and total protein S levels. Type II deficiency, which is the least encountered, refers to an abnormally functioning protein S. Type III deficiency refers only to low levels of free protein S. In the coagulation cascade, when factors Va and Xa are complexed together, the inactivation site on factor Va is “hidden” from protein C. Protein S, not a protease itself, exposes this site so that protein C can cleave Va. Because protein S is so crucial, deficiency of protein S also leads to the unregulated procoagulant action of factor Xa.

D. Antithrombin Deficiency—AT deficiency is less common than any of the previously discussed disorders, with approximately 1 in 2000 cases in the general population. AT binds to and inhibits not just thrombin (thus its name), but also the activated forms of factors IX, X, XI, and XII, and perhaps the factor VII–TF complex as well. Unlike protein C’s proteolytic cleavage of factor Va, AT binds to each factor, directly blocking their activity; it is not an enzyme. This action is accelerated—up to 2000 times—in a reversible manner by the anticoagulant molecule heparin, which binds to AT via its pentasaccharide sequence. The anticoagulant fondaparinux is a synthetic version of this five- saccharide sequence, and thus it can also bind to AT. In AT deficiency, then, multiple coagulation steps are unbalanced, and the coagulation cascade may proceed unrestrained. More than 100 different AT mutations have been reported. Type I molecular defects involve a parallel decrease in antigen and activity, whereas type II defects involve a dysfunctional molecule with decreased activity but normal or near-normal antigen levels.

E. Hyperprothrombinemia—A mutation in the untranslated region of the prothrombin gene (a single base-pair mutation, called 20210 AG) is associated with elevated plasma prothrombin (II) levels and an increased risk of thrombosis. Presumably, this leads to excess thrombin generation when the prothrombinase complex is activated. This is probably the second most common hereditary hypercoagulable state after factor V Leiden. It is the first hereditary

thrombophilia associated with overproduction of procoagulant factors.

Pathology

The pathologic features of thrombi in hypercoagulable states are indistinguishable from those of genetically normal individuals on a gross anatomic or microscopic basis, except that there is a greater likelihood in hypercoagulable states of having a clot in unusual sites (see Clinical Manifestations section).

Most of the pathologic features of the hereditary hypercoagulable states consist of laboratory abnormalities. In the evaluation of patients suspected of having a hereditary hypercoagulable state, there are two basic types of laboratory abnormalities. The first type is quantitative: Specific immunologic assays can define the relative amount of protein C, protein S, AT, or fibrinogen present in a given patient’s serum, but they do not evaluate the function of any of these molecules. The second type is qualitative: The assays for protein C or protein S activity (rather than amount) measure the ability (or inability) of the patient’s protein C or S to prolong a clotting time in vitro. APC resistance can be evaluated with a different clotting assay, but generally the presence of the specific mutation in factor V Leiden is assessed by the polymerase chain reaction because the full sequence of the molecule is known. The polymerase chain reaction is also used for detecting the 20210 AG prothrombin abnormality. Prothrombin levels can also be measured and are consistently in the highest quartile of prothrombin levels found.

Clinical Manifestations

Most thromboembolic events encountered in clinical practice are secondary, not primary. Patients have blood clots usually in the deep veins of the legs for two reasons: (1) because of sluggish blood flow (in high-capacity, low-flow veins) compared with other sites, particularly when inactive (eg, bedridden after surgery or as a result of illness); and (2) because the extremities are more likely to sustain injury than the trunk. Trauma causes blood vessel compression or injury; thus, two elements of the Virchow triad are more readily observed in the legs than elsewhere.

These venous clots in the legs (commonly referred to as deep venous thromboses [DVTs]) usually present with pain, swelling, and redness below the level of the thrombus, with normal arterial pulses and distal extremity perfusion. Because blood return to the central circulation is blocked in these high-capacity

vessels, superficial collateral veins just under the skin may be prominent and engorged. The swelling is mechanical, because normal arterial blood flow continues to the extremity while venous return is compromised, leading to engorgement. Pain occurs primarily as a result of the swelling alone but can also occur from lactic acid buildup in the muscles of the legs. This happens when the pressure in the legs increases to the point that it compromises arterial blood flow and adequate oxygen delivery to those muscles.

Pulmonary emboli, the major source of morbidity and mortality after DVT of the lower extremity, typically present with acute-onset shortness of breath, hypoxemia, and a history suggesting an initial DVT that has now broken off and migrated through the right side of the heart to the pulmonary arterial system. The clot blocks blood flow from the heart to a portion of lung, leading to hypoxemia, which can be exacerbated by underlying lung disease.

The clinical presentations of all hypercoagulable states are similar, but there are some interesting differences. DVTs tend to occur (whether there is a hypercoagulable state or not) in patients with a history of trauma, pregnancy, oral contraceptive use, or immobility, but rarely in adolescents or young adults. Inherited hypercoagulable states are suspected in patients who present with a thromboembolic event, usually because they are young or experience recurrent clots. Events that occur without any specific risks are particularly suspect. Because of the dominant pattern of inheritance, suspicion is aroused when other family members have had clotting problems, underscoring the importance of taking a family history.

Despite the distinct coagulation abnormalities, most thromboses still occur in usual sites (ie, the deep veins of the legs with or without pulmonary embolism). Other unusual sites (eg, the sagittal sinus of the skull or the mesenteric veins in the abdomen) are more likely to be found in patients with underlying coagulation disorders than in those without. Arterial thromboses, however, are extremely rare.

Interestingly, only a minority of patients with an inherited hypercoagulable state develop symptomatic thromboses; this is particularly true for heterozygotes. Each disorder is slightly different, presumably because of the redundancy of the factors in the coagulation cascade. And undoubtedly the penetrance of each state varies in individual patients because of factors we do not yet understand. Heterozygotes who develop thrombosis usually present in the setting of a “typical” risk factor: sustaining an injury, having an extremity immobilized, having surgery, or being pregnant.

Homozygous protein C or protein S deficiencies present the highest

likelihood of causing illness. Both conditions usually result in thrombosis, which is fatal in early life (neonatal purpura fulminans), although some patients may not present until their teens. Heterozygotes for protein C deficiency are unlikely to develop a thrombosis over their lifetimes, although they are about 4–6 times more likely to do so than members of the general population. Heterozygotes for protein S deficiency have a 1- to 10-fold increased relative risk of thrombosis.

AT deficiency is another significant defect in terms of the likelihood of developing thrombosis. These patients have a lifetime 5- to 10-fold increased relative risk for thrombosis.

The situation is complex in the case of APC resistance. Heterozygotes for APC resistance probably represent more than one-third of all patients with familial thromboses. Although there is a 3- to 5-fold increased relative risk of thrombosis for heterozygotes of this mutation, heterozygosity rarely leads to thrombosis unless there is an additional risk factor for hypercoagulability. In heterozygotes, proteins C and S can still cleave factor VIIIa, and the factor V abnormality is a relative rather than absolute insensitivity to APC. There is still negative control of the clotting cascade at the factor X step by TFPI as well.

Even homozygous factor V Leiden does not inevitably cause thrombosis. Families in which homozygous females have had repeated pregnancies without difficulty have been carefully described. This is somewhat surprising because pregnancy, a hypercoagulable state itself, leads to decreases in protein S concentration, which would be expected to amplify the resistance to protein C. Nevertheless, there is at least a 20- to 50-fold increased risk of thrombosis versus the general population for homozygotes for factor V Leiden.

Persons with the prothrombin 20210 AG mutation are nearly all heterozygotes, with about a 2- to 3-fold higher risk of thrombosis than the general population.

CHECKPOINT

27. What constitutes the Virchow triad of factors predispos-ing to the formation of intravascular clots?

28. Deficiencies in which proteins can result in clinically sig-nificant thromboses?

29. What is the basis for activated protein C resistance?

CASE STUDIES Yeong Kwok, MD

(See Chapter 25, p. 751–55 for Answers)

CASE 26

A 65-year-old previously well man presents to the clinic with complaints of fatigue of 3 months’ duration. Questioning reveals diffuse weakness and feeling winded when walking uphill or climbing more than one flight of stairs. All symptoms have slowly worsened over time. There are no other complaints, and the review of systems is otherwise negative. The patient has no significant medical history, social history, or family history. On physical examination, he appears somewhat pale, with normal vital signs. The physical examination is unremarkable except for his rectal examination, which reveals brown, guaiac-positive stool (suggesting the presence of blood in the stool). A blood test reveals anemia.

Questions

A. What is the most likely form of anemia in this man? What is the probable underlying cause?

B. What is the mechanism by which this disorder results in anemia? C. What might one expect to see in the peripheral blood smear? D. What other tests might be ordered to confirm the diagnosis? E. What is the pathophysiologic mechanism of this patient’s fatigue,

weakness, and shortness of breath? Why is he pale?

CASE 27

A 58-year-old black woman presents to the emergency department with complaints of progressive fatigue and weakness for the past 6 months. She is short of breath after walking several blocks. On review of systems, she

mentions mild diarrhea. She has noted intermittent numbness and tingling of her lower extremities and a loss of balance while walking. She denies other neurologic or cardiac symptoms and has no history of black or bloody stools or other blood loss. On physical examination, she is tachycardic to 110 bpm; other vital signs are within normal limits. The head-and-neck examination is notable for pale conjunctivas and a beefy red tongue with loss of papillae. Cardiac examination shows a rapid, regular rhythm with a grade 2/6 systolic murmur at the left sternal border. Lung, abdominal, and rectal examination findings are normal. Neurologic examination reveals decreased sensation to light touch and vibration in the lower extremities. The hematology consultant on call is asked to see this patient because of a low hematocrit level.

Questions

A. What vitamin deficiency is the probable cause of this woman’s anemia? How does this result in anemia?

B. What might one expect the peripheral blood smear to look like? What other blood tests may be ordered, and what results would be anticipated? What test might differentiate the various causes of this vitamin deficiency?

C. The workup reveals pernicious anemia. What is the pathogenesis of this disease? What is the evidence to support an autoimmune origin?

D. What is the pathophysiologic mechanism of this woman’s symptoms of tachycardia, paresthesias, and impaired proprioception?

CASE 28

A 6-year-old boy presents to the pediatric emergency department. His mother states that he has had 3 days of general malaise and fevers to 38.5°C. He has no other localizing symptoms. The medical history is remarkable for multiple febrile illnesses. His mother says, “It seems like he gets sick every month.” The physical examination is notable for cervical lymphadenopathy and oral ulcers. Blood tests reveal a neutrophil count of 200/μL. The patient is admitted to the hospital. Blood, urine, and cerebrospinal fluid cultures are negative, and over 48 hours, the patient’s

neutrophil counts return to normal. He is then discharged.

Questions

A. What is the likely pathogenesis of cyclic neutropenia? What evidence supports this theory?

B. What aspects of this case presentation support the diagnosis of cyclic neutropenia? What is the expected clinical course?

C. Assuming that the diagnosis of cyclic neutropenia is correct, what would one expect the peripheral blood smear to look like? What would you expect the bone marrow examination results to show at admission? What would you expect them to be in 2 weeks?

CASE 29

A 36-year-old man was admitted to hospital after sustaining multiple fractures to the lower extremities after jumping from a three-story building in a suicide attempt. His fractures required surgical repair. He has no significant medical history. Current medications include morphine for pain and subcutaneous heparin for prophylaxis against deep venous thrombosis. Consultation with a hematologist is requested because of a dropping platelet count. On physical examination, the patient has multiple bruises, and his lower extremities are casted bilaterally. Examination is otherwise normal. Laboratory tests from the last several days reveal a platelet count that has dropped from 170,000/μL on admission to 30,000/μL 5 days later.

Questions

A. What is the most likely cause of this man’s thrombocytopenia? B. By what mechanisms does heparin sometimes cause

thrombocytopenia? C. What are the possible clinical consequences of this patient’s

thrombocytopenia?

CASE 30

A 23-year-old woman presents to the emergency department with a chief complaint of acute onset of shortness of breath. It is associated with right- sided chest pain, which increases with inspiration. The patient denies fever, chills, cough, and other respiratory symptoms. She has had no lower extremity swelling. She has not been ill, bedridden, or immobile for prolonged periods. Her medical history is notable for an episode about 2 years ago of deep venous thrombosis in the right lower extremity while taking oral contraceptives. She has been otherwise healthy and is currently taking no medications. The family history is notable for a father who died of a pulmonary embolism. On physical examination, she appears anxious and in mild respiratory distress. She is tachycardic to 110 bpm, with a respiratory rate of 20/min. She has no fever, and blood pressure is stable. The remainder of the physical examination is normal. Chest x-ray film is normal. Ventilation-perfusion scan reveals a high probability of pulmonary embolus. Given her history of deep vein thrombosis, a hypercoagulable state is suspected.

Questions

A. What constitutes the Virchow triad of predisposing factors for venous thrombosis? Which components of the triad may be present in this patient?

B. What are some causes of inherited hypercoagulable states specifically associated with the coagulation cascade? How do they result in hypercoagulability?

C. How might this woman be evaluated for the presence of an inherited hypercoagulable state?

REFERENCES

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Kaushansky K et al. Williams Hematology, 9th ed. McGraw-Hill, 2015. Marder VK et al. Hemostasis ad Thrombosis: Basic Principles and Clinical Practice, 6th ed. Lippincott Williams & Wilkins, 2012.

Iron-Deficiency Anemia Camaschella C. Iron-deficiency anemia. N Engl J Med. 2015 May 7;372(19):1832–43. [PMID: 25946282] Camaschella C. New insights into iron deficiency and iron deficiency anemia. Blood Rev. 2017 Jul;31(4):225–33. [PMID: 28216263]

DeLoughery TG. Iron deficiency anemia. Med Clin North Am. 2017 Mar;101(2):319–32. [PMID: 28189173]

Dev S et al. Overview of iron metabolism in health and disease. Hemodial Int. 2017 Jun;21(Suppl 1):S6– S20. [PMID: 28296010]

Gozzelino R et al. Iron homeostasis in health and disease. Int J Mol Sci. 2016 Jan 20;17(1).pii: E130. [PMID: 26805813]

Kim A et al. New insights into iron regulation and erythropoiesis. Curr Opin Hematol. 2015 May;22(3):199–205. [PMID: 25710710]

Liu J et al. Hepcidin: a promising therapeutic target for iron disorders: a systematic review. Medicine (Baltimore). 2016 Apr;95(14):e3150. [PMID: 27057839]

Peng YY et al. Ferritin and iron studies in anaemia and chronic disease. Ann Clin Biochem. 2017 Jan;54(1):43–8. [PMID: 27701066]

Sangkhae V et al. Regulation of the iron homeostatic hormone hepcidin. Adv Nutr. 2017 Jan 17;8(1):126– 36. [PMID: 28096133]

Silva B et al. An overview of molecular basis of iron metabolism regulation and the associated pathologies. Biochim Biophys Acta. 2015 Jul;1852(7):1347–59. [PMID: 25843914]

Pernicious Anemia Green R. Vitamin B12 deficiency from the perspective of a practicing hematologist. Blood. 2017 May 11;129(19):2603–11. [PMID: 28360040]

Green R et al. Megaloblastic anemias: nutritional and other causes. Med Clin North Am. 2017 Mar;101(2):297–317. [PMID: 28189172]

Minalyan A et al. Autoimmune atrophic gastritis: current perspectives. Clin Exp Gastroenterol. 2017 Feb 7;10:19–27. [PMID: 28223833]

Osborne D et al. Autoimmune mechanisms in pernicious anaemia & thyroid disease. Autoimmun Rev. 2015 Sep;14(9):763–8. [PMID: 25936607].

Shipton MJ et al. Vitamin B12 deficiency—a 21st century perspective Clin Med (Lond). 2015 Apr;15(2):145–50. [PMID: 25824066]

Stabler SP. Clinical Practice. Vitamin B12 deficiency. N Engl J Med. 2013 Jan 10;368(2):149–60. [PMID: 23301732]

Wong CW. Vitamin B12 deficiency in the elderly: is it worth screening? Hong Kong Med J. 2015 Apr;21(2):155–64. [PMID: 25756278]

Cyclic Neutropenia

Makaryan V et al. The diversity of mutations and clinical outcomes for ELANE-associated neutropenia. Curr Opin Hematol. 2015 Jan;22(1):3–11. [PMID: 25427142]

Drug-Induced Thrombocytopenia Al-Nouri ZL et al. Drug-induced thrombotic microangiopathy: a systematic review of published reports. Blood. 2015 Jan 22;125(4):616–8. [PMID: 25414441]

Arman M et al. Human platelet IgG Fc receptor FcγRIIA in immunity and thrombosis. J Thromb Haemost. 2015 Jun;13(6):893–908. [PMID: 25900780]

Greinacher A. Clinical Practice. Heparin-induced thrombocytopenia. N Engl J Med. 2015 Jul 16;373(3):252–61. [PMID: 26176382]

Junqueira DR et al. Unfractionated heparin versus low molecular weight heparins for avoiding heparin- induced thrombocytopenia in postoperative patients. Cochrane Database Syst Rev. 2017 Apr 21;4:CD007557. [PMID: 28431186]

Krzych ŁJ et al. Heparin-induced thrombocytopenia. Anaesthesiol Intensive Ther. 2015;47(1):63–76. [PMID: 25751293]

Nagler M et al. Diagnostic value of immunoassays for heparin-induced thrombocytopenia: a systematic review and meta-analysis. Blood. 2016 Feb 4;127(5):546–57. [PMID: 26518436]

Qiao J et al. The platelet Fc receptor, FcγRIIa. Immunol Rev. 2015 Nov;268(1):241–52. [PMID: 26497525] Staibano P et al. The unique immunological features of heparin-induced thrombocytopenia. Br J Haematol. 2017 Apr;177(2):198–207. [PMID: 28369702]

Warkentin TE. Heparin-induced thrombocytopenia. Curr Opin Crit Care. 2015 Dec;21(6):576–85. [PMID: 26539932]

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Hypercoagulable States Bauer KA et al. Issues in the diagnosis and management of hereditary antithrombin deficiency. Ann Pharmacother. 2016 Sep;50(9):758–67. [PMID: 27281301]

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CHAPTER

7

Nervous System Disorders Vikram G. Shakkottai, MD, PhD, & Catherine Lomen-Hoerth MD, PhD

The major functions of the nervous system are to detect, analyze, and transmit information. Information is gathered by sensory systems, integrated by the brain, and used to generate signals to motor and autonomic pathways for control of movement and of visceral and endocrine functions. These actions are controlled by neurons, which are interconnected to form signaling networks that compose motor and sensory systems. In addition to neurons, the nervous system contains neuroglial cells that serve a variety of immunologic and support functions and modulate the activity of neurons. Understanding the pathophysiology of nervous system disease requires knowledge of neural and glial cell biology and the anatomy of neural networks. The first part of this chapter reviews several basic aspects of histology, cellular physiology, and anatomy of the nervous system.

Understanding the causes of neurologic diseases requires knowledge of molecular and biochemical mechanisms. Discoveries in the fields of molecular biology and genetics have made available important information about the mechanisms of several disease states. Several neurologic disorders for which some of the molecular mechanisms of pathogenesis are known are discussed later in this chapter, including cerebellar ataxia, motor neuron disease, Parkinson disease, myasthenia gravis, epilepsy, Alzheimer disease, and stroke. Exciting advances in our understanding of these diseases, including how they overlap, are leading to new therapeutic targets and the hope of better treating these devastating diseases.

NORMAL STRUCTURE & FUNCTION OF THE NERVOUS SYSTEM

HISTOLOGY & CELL BIOLOGY

Neurons The major function of neurons is to receive, integrate, and transmit information to other cells. Neurons consist of three parts: dendrites, which are elongated processes that receive information from the environment or from other neurons; the cell body, which contains the nucleus; and the axon, which may be up to 1 m long and conducts impulses to muscles, glands, or other neurons (Figure 7–1). Most neurons are multipolar, containing one axon and several dendrites. Bipolar neurons have one dendrite and one axon and are found in the cochlear and vestibular ganglia, retina, and olfactory mucosa. Spinal sensory ganglia contain pseudounipolar neurons that have a single process that emanates from the cell body and divides into two branches, one extending to the spinal cord and the other extending to the periphery. Axons and dendrites usually branch extensively at their ends. Dendritic branching can be very complex, with the result that a single neuron may receive thousands of inputs. Axon branching allows several target cells to simultaneously receive a message from one neuron. Each branch of the axon terminates on the next cell at a synapse, which is a structure specialized for information transfer from the axon to muscle, glands, or another neuron. Synapses between neurons most often occur between axons and dendrites but may occur between an axon and a cell body, between two axons, or between two dendrites.

FIGURE 7–1 Schematic drawing of a Nissl-stained motor neuron. The myelin sheath is produced by oligodendrocytes in the central nervous system and by Schwann cells in the peripheral nervous system. Note the three motor end plates, which transmit the nerve impulse to striated skeletal muscle fibers. (Redrawn, with permission, from Junqueira LC et al, eds. Basic Histology, 11th ed. McGraw-Hill, 2005.)

Signals are propagated electrically along axons. Like other cells, neurons maintain cell size and osmolarity primarily through the action of Na+-K+ ATPase, which actively pumps Na+ out of cells in exchange for K+. This results

in the formation of concentration gradients for Na+ and K+ across the cell membrane. The membrane is practically impermeable to Na+, but the presence of K+ leak channels permits the flow of K+ out of cells. This produces a difference in electrical charge across the membrane that counters transport of K+ from the cell. The flow of ions continues until the opposing electrical force reaches a value that balances the diffusional force and the membrane reaches the equilibrium potential for K+ (EK). EK is calculated by the Nernst equation:

where R = gas constant (2 kcal mol–1°K–1) T = absolute temperature (°K) F = Faraday constant (2.3 × 104 kcal V–1 mol–1) [K+]o = concentration of K

+ outside the cell [K+]i = concentration of K

+ inside the cell

In most neurons, the resting membrane potential (Em) is –50 to –100 mV and lies close to EK since the leak of K

+ is the major determinant of the charge difference across the membrane.

The membrane potential may be altered by increasing the permeability of the membrane to another ion, which drives the resting membrane potential toward the equilibrium potential for that ion. Neurons are highly specialized to use rapid changes in membrane potential to generate electrical signals. This is accomplished by ligand-gated and voltage-gated ion channels that allow the passage of Na+, K+, Ca2+, or Cl– ions in response to electrical or chemical stimuli. These channels are composed of protein complexes embedded in the lipid membrane to form aqueous pores to the inside of the cell. In general, channels are selective for a particular species of ion. An array of charged amino acids within voltage-dependent channels detects changes in voltage and induces a conformational change in the channel to alter ion permeability. Binding sites for neurotransmitters, such as glutamate, γ-aminobutyric acid (GABA), glycine, and acetylcholine, exist on ligand-gated channels and, when occupied, induce a conformational change to open the channel.

Electrical signals are propagated in neurons because a voltage change across

the membrane in one part of a neuron is propagated to other parts. Passive spread of a voltage disturbance weakens with increasing distance from the source unless energy-dependent processes amplify the signal. Passive spread of electrical signals works well over short distances and is a major mechanism of signal propagation in dendrites. However, long-distance communication down axons to nerve terminals requires amplification. This is accomplished through the generation of self-propagating waves of excitation known as action potentials.

An action potential arises primarily from voltage-dependent changes in membrane permeability to Na+ and K+ (Figure 7–2). If a depolarizing stimulus raises the membrane potential to about –45 mV, voltage-gated Na+ channels open, allowing an influx of Na+ and further depolarization toward ENa (±50 mV). Nearby areas of membrane are depolarized to the threshold for Na+ channel activation, propagating a wave of depolarization from the initial site. The resting potential is restored quickly by a combination of events. First, Na+ channels close rapidly and remain in an inactive state until the membrane potential returns to negative levels for several milliseconds. Voltage-dependent K+ channels open as the membrane potential becomes more depolarized, speeding the efflux of K+ from cells and driving the membrane potential back to EK. K

+ channels that participate in the action potential also inactivate, but more slowly than Na+ channels, thereby allowing cell repolarization. Plasma membrane ion exchangers and ion pumps ensure that ionic gradients across the membrane continue to be maintained, by counteracting the ion fluxes that occur in the course of the generation of action potentials.

FIGURE 7–2 Conduction of the nerve impulse through an unmyelinated nerve fiber. In the resting axon, there is a difference of 70 mV between the interior of the axon and the outer surface of its membrane (resting potential). During the impulse passage, more Na+ (thick arrow) passes into the axon interior than the amount of K+ (thin arrow) that migrates in the opposite direction. In consequence, the membrane polarity changes (the membrane becomes relatively positive on its inner surface), and the resting potential is replaced by an action potential (+35 mV here). (Redrawn, with permission, from Junqueira LC et al, eds. Basic Histology, 10th ed. McGraw-Hill, 2003.)

Neurons transmit signals chemically to other cells at synapses (Figure 7–3). Presynaptic and postsynaptic cells are electrically isolated from each other and separated by a narrow synaptic cleft. Signaling across the cleft occurs through the release of neurotransmitters from the terminal of the presynaptic neuron.

Most neurotransmitters are stored in membrane-bound synaptic vesicles and are released into the synaptic cleft by Ca2+-dependent exocytosis. Depolarization of the nerve terminal opens voltage-gated Ca2+ channels, stimulating Ca2+ influx and neurotransmitter release. Neurotransmitters diffuse across the cleft and bind to receptors on ligand-gated ion channels concentrated at the postsynaptic membrane. This produces local permeability changes, altering the membrane potential of the postsynaptic cell. If the response is depolarizing, an action potential may be generated if there are enough voltage-gated Na+ channels nearby and the membrane potential has been raised to the threshold for their activation. Receptor-gated ion channels are highly selective for a particular neurotransmitter and for the type of ions they pass, which determines whether they generate excitatory or inhibitory responses. In general, excitatory neurotransmitters, such as glutamate, open cation channels that allow influx of Na+ or Ca2+ and generate a depolarizing excitatory postsynaptic potential. Inhibitory neurotransmitters, such as GABA and glycine, open Cl– channels and generate an inhibitory postsynaptic potential, keeping the postsynaptic membrane near ECl (= –70 mV). Termination of the signal is achieved by removal of the neurotransmitter from the synaptic cleft. Acetylcholine is hydrolyzed by acetylcholinesterase at the postsynaptic membrane. Other neurotransmitters such as glutamate are removed by specific membrane transporters on nerve terminals or glial cells.

FIGURE 7–3 Schematic drawing of a synaptic terminal. Vesicles pass through the presynaptic membrane and release a transmitter substance into the synaptic cleft. (Redrawn, with permission, from Waxman SG. Clinical Neuroanatomy, 28th ed. McGraw-Hill, 2017.)

Not all neurotransmitter receptors are ion channels. Many receptors are coupled to cellular enzymes that regulate levels of intracellular second messengers to modulate the function of ion channels and many other cell proteins. A major mechanism by which messengers regulate ion channels is promoting phosphorylation of channel subunits. For example, binding of the neurotransmitter norepinephrine to β-adrenergic receptors activates the enzyme adenylyl cyclase and stimulates the production of cyclic adenosine monophosphate (cAMP). The cAMP, in turn, activates a cAMP-dependent protein kinase that can phosphorylate voltage-gated calcium channels. In many cases, this increases the duration of time the channel remains open once it is activated, resulting in increased Ca2+ influx through the channel. Other neurotransmitter receptors, such as α1-adrenergic, muscarinic cholinergic, or metabotropic glutamate receptors, are coupled to the enzyme phospholipase C, which catalyzes the hydrolysis of the membrane lipid phosphatidylinositol-4,5- bisphosphate. Binding of neurotransmitter to the receptor activates phospholipase C to produce two second messengers: 1,2-diacylglycerol and inositol-1,4,5-trisphosphate. Diacylglycerol activates several enzymes of the protein kinase C family, some of which phosphorylate ion channels and either

enhance or suppress their function. Inositol-1,4,5-trisphosphate binds an intracellular receptor that is itself a calcium ionophore, allowing release of calcium from intracellular stores into the cytosol. This calcium signal activates several calcium-dependent enzymes, including phosphatases and kinases that can alter the phosphorylation state and function of several ion channels and other cell proteins.

Astrocytes Astrocytes serve a variety of metabolic, immunologic, structural, and nutritional support functions required for the normal function of neurons. They possess numerous processes that radiate from the cell body, surrounding blood vessels and synapses, and fill the space between neurons in the brain and spinal cord (Figure 7–4). Astrocytes express voltage- and ligand-gated ion channels and regulate K+ and Ca2+ concentrations within the interstitial space. Many synapses are invested with astrocytic processes, and this allows astrocytes to modulate neurotransmission by regulating extracellular concentrations of these cations. Astrocytes provide structural and trophic support for neurons through the production of extracellular matrix molecules such as laminin and through the release of growth factors such as nerve growth factor, fibroblast growth factors, and brain-derived neurotrophic factor. End-feet of astrocytic processes at blood vessels provide sites for the release of cytokines and chemoattractants during central nervous system (CNS) injury. Astrocytes respond to brain injury by increasing in size—and in some cases in number—through a process called reactive astrocytosis. This phenotypic change is characterized by an increase in cells expressing glial-fibrillary acidic protein and by synthesis and release of cytokines that regulate inflammatory responses and entry of hematogenous cells into the CNS. Astrocytes also play an important role in terminating neuronal responses to glutamate, the most abundant excitatory neurotransmitter in the brain. In cell cultures, neurons die in the presence of high levels of glutamate unless astrocytes are present. Glutamate transporters present on astrocyte cell membranes remove glutamate from the synapse. Astrocytes also contain glutamine synthase, which converts glutamate to glutamine, detoxifying the CNS of both glutamate and ammonia.

FIGURE 7–4 Drawings of neuroglial cells as seen in slides stained by metallic impregnation. Observe that only astrocytes exhibit vascular end-feet, which cover the walls of blood capillaries. (Redrawn, with permission, from Mescher AL. Junqueira’s Basic Histology, 14th ed. McGraw-Hill, 2016.)

Oligodendrocytes & Schwann Cells

Plasma membranes of oligodendrocytes in the CNS and Schwann cells in the peripheral nervous system envelop axons. For many axons, the membranes of these glial cells are wrapped layer on layer around the axon, forming a myelin sheath (Figure 7–5). Gaps form between myelin sheaths from neighboring glia and produce nodes of Ranvier where a small portion of the axon is exposed to the interstitial space and where voltage-dependent Na+ channels are clustered in the axonal membrane. Between the nodes, myelin insulates the axon from the extracellular space, allowing efficient spread of depolarization from one node to another. This allows action potentials to propagate rapidly by jumping from node to node in a process called saltatory conduction.

FIGURE 7–5 Myelination of axons. Top left: Unmyelinated axon. Top right: Myelinated axon. Note that the cell membrane of the Schwann cell has wrapped itself around the axon. Bottom: Myelination of several axons in the CNS by an oligodendrogliocyte. (Redrawn, with permission, from Ganong WF. Review of Medical Physiology, 22nd ed. McGraw-Hill, 2005.)

Microglia Although peripheral blood lymphocytes and monocytes enter from the circulation and patrol the CNS, microglia, which reside in the CNS, function as the main immune effector cells. They appear to be derived from bone marrow precursors of macrophage-monocyte lineage and invade the CNS during the perinatal period. Microglia cells are activated by brain injury, infection, or

neuronal degeneration. Activation is characterized by proliferation; migration into damaged tissue; increased or de novo expression of surface receptors, including CD45 (leukocyte common antigen), MHC class I and class II, and immunoglobulin Fc receptors; and secretion of several cytokines, reactive oxygen intermediates, and proteinases. This response functions to remove dead tissue and destroy invading organisms but may contribute to CNS damage, particularly in certain inflammatory and degenerative CNS diseases.

CHECKPOINT

1. What are the primary functions of neurons, astrocytes, and microglia? 2. What role does myelin play in axonal conduction? 3. What is responsible for the resting membrane potential and for the

generation of action potentials? 4. What are some of the major neurotransmitters in the nervous system, and

what effects do they produce when they bind to their receptors?

FUNCTIONAL NEUROANATOMY

To understand neuroanatomy, it is useful to study structures as parts of functional systems.

MOTOR SYSTEM

Large alpha motor neurons of the spinal cord ventral horns and brainstem motor nuclei (facial nucleus, trigeminal motor nucleus, nucleus ambiguus, hypoglossal nucleus) extend axons into spinal and cranial nerves to innervate skeletal muscles. Damage to these lower motor neurons results in loss of all voluntary and reflex movement because they compose the output of the motor system. Neurons in the precentral gyrus and neighboring cortical regions (upper motor neurons) send axons to synapse with lower motor neurons. Axons from these upper motor neurons compose the corticospinal and corticobulbar tracts. The motor cortex and spinal cord are connected with other deep cerebral and brainstem motor nuclei, including the caudate nucleus, putamen, globus pallidus,

red nuclei, subthalamic nuclei, substantia nigra, reticular nuclei, and neurons of the cerebellum. Neurons in these structures are distinct from cortical motor (pyramidal) neurons and are referred to as extrapyramidal neurons. Many parts of the cerebral cortex are connected by fiber tracts to the primary motor cortex. These connections are important for complex patterns of movement and for coordinating motor responses to sensory stimuli.

1. Lower Motor Neurons & Skeletal Muscles Anatomy Each alpha motor neuron axon contacts up to about 200 muscle fibers, and together they constitute the motor unit (Figure 7–6). Axons of the motor neurons intermingle to form spinal ventral roots, plexuses, and peripheral nerves. Muscles are innervated from specific segments of the spinal cord, and each muscle is supplied by at least two roots. Motor fibers are rearranged in the plexuses so that most muscles are supplied by one peripheral nerve. Thus, the distribution of muscle weakness differs in spinal root and peripheral nerve lesions.

FIGURE 7–6 Anatomic components of the motor unit. (Redrawn, with permission, from Aminoff MJ et al, eds. Clinical Neurology, 9th ed. McGraw-Hill, 2015.)

Physiology

The lower motor neurons are the final common pathway for all voluntary movement. Therefore, damage to lower motor neurons or their axons causes flaccid weakness of innervated muscles. In addition, muscle tone or resistance to passive movement is reduced, and deep tendon reflexes are impaired or lost. Tendon reflexes and muscle tone depend on the activity of alpha motor neurons (Figure 7–7), specialized sensory receptors known as muscle spindles, and smaller gamma motor neurons whose axons innervate the spindles. Some gamma motor neurons are active at rest, making the spindle fibers taut and sensitive to stretch. Tapping on the tendon stretches the spindles, which causes them to send impulses that activate alpha motor neurons. These in turn fire, producing the brief muscle contraction observed during the myotactic stretch reflex. Alpha motor neurons of antagonist muscles are simultaneously inhibited through the phenomenon of reciprocal inhibition. Both alpha and gamma motor neurons are influenced by descending fiber systems, and their state of activity determines the level of tone and activity of the stretch reflex.

FIGURE 7–7 Diagram illustrating the pathways responsible for the stretch reflex and the inverse

stretch reflex. Stretch stimulates the muscle spindle, and impulses pass up the Ia fiber to excite the motor neuron. It also stimulates the Golgi tendon organ, and impulses passing up the Ib fiber activate the interneuron to release the inhibitory mediator glycine. With strong stretch, the resulting hyperpolarization of the motor neuron is so great that it stops discharging. (Redrawn, with permission, from Barrett KE et al, eds. Ganong’s Review of Medical Physiology, 25th ed. McGraw-Hill, 2016.)

Each point of contact between nerve terminal and skeletal muscle forms a specialized synapse known as a neuromuscular junction composed of the presynaptic motor nerve terminal and a postsynaptic motor end plate (Figure 7– 8). Presynaptic terminals store synaptic vesicles that contain the neurotransmitter acetylcholine. The amount of neurotransmitter within a vesicle constitutes a quantum of neurotransmitter. Action potentials depolarize the motor nerve terminal, opening voltage-gated calcium channels and stimulating calcium- dependent release of neurotransmitter from the terminal. Released acetylcholine traverses the synaptic cleft to the postsynaptic membrane (end plate), where it binds to nicotinic cholinergic receptors. These receptors are ligand-gated cation channels, and, on binding to acetylcholine, they allow entry of extracellular sodium into the motor end plate. This depolarizes the motor end plate, which in turn depolarizes the muscle fiber through activation of voltage-gated sodium channels. After activation, cholinergic receptors are rapidly inactivated, reducing sodium entry. They remain inactive until acetylcholine dissociates from the receptor. This is facilitated by the enzyme acetylcholinesterase, which hydrolyzes acetylcholine and is present in the postsynaptic zone.

FIGURE 7–8 Sites of involvement in disorders of neuromuscular transmission. Left: Normal transmission involves depolarization-induced influx of calcium (Ca2+) through voltage-gated channels. This stimulates release of acetylcholine (ACh) from synaptic vesicles at the active zone and into the synaptic cleft. ACh binds to ACh receptors and depolarizes the postsynaptic muscle membrane. Right: Disorders of neuromuscular transmission result from blockage of Ca2+ channels (Lambert–Eaton syndrome or aminoglycoside antibiotics), impairment of Ca2+-mediated ACh release (botulinum toxin), or antibody- induced internalization and degradation of ACh receptors (myasthenia gravis). (Redrawn, with permission, from Aminoff MJ et al, eds. Clinical Neurology, 9th ed. McGraw-Hill, 2015.)

Neuromuscular transmission may be disturbed in several ways (see Figure 7– 8). In Lambert–Eaton myasthenic syndrome, antibodies to calcium channels inhibit calcium entry into the nerve terminal and reduce neurotransmitter release. In these cases, repetitive nerve stimulation facilitates calcium accumulation in the nerve terminal and increases acetylcholine release. Clinically, limb muscles are weak, but if contraction is maintained, power increases. Electrophysiologically, the amplitude of the muscle response increases to repetitive nerve stimulation. Aminoglycoside antibiotics also impair calcium

channel function and cause a similar syndrome. Proteolytic toxins produced by Clostridium botulinum cleave specific presynaptic proteins, preventing neurotransmitter release at both neuromuscular and parasympathetic cholinergic synapses. As a result, patients with botulism develop weakness, blurred vision, diplopia, ptosis, and large unreactive pupils. In myasthenia gravis, autoantibodies to the nicotinic acetylcholine receptor (AChR) block neurotransmission by inhibiting receptor function and activating complement- mediated lysis of the postsynaptic membrane. Myasthenia gravis is discussed in greater detail later in this chapter.

Motor nerves exert trophic influences on the muscles they innervate. Denervated muscles undergo marked atrophy, losing more than half of their original bulk in 2–3 months. Nerve fibers are also required to organize the muscle end plate and to cluster cholinergic receptors to that region. Receptors in denervated fibers fail to cluster and become spread across the muscle membrane. Muscle fibers within a denervated motor unit may then discharge spontaneously, giving rise to a visible twitch (fasciculation) within a portion of a muscle. Individual fibers may also contract spontaneously, giving rise to fibrillations, which are not visible to the examiner but can be detected by electromyography. Fibrillations usually appear 7–21 days after damage to lower motor neurons or their axons.

CHECKPOINT

5. From where do lower motor neurons emanate, and to where do they send axons?

6. Describe four mechanisms that can disturb the function of the neuromuscular junction.

2. Upper Motor Neurons Anatomy The motor cortex is the region from which movements can be elicited by electrical stimuli (Figure 7–9). This includes the primary motor area (Brodmann area 4), premotor cortex (area 6), supplementary motor cortex (medial portions of 6), and primary sensory cortex (areas 3, 1, and 2). In the motor cortex, groups of neurons are organized in vertical columns, and discrete groups control

contraction of individual muscles. Planned movements and those guided by sensory, visual, or auditory stimuli are preceded by discharges from prefrontal, somatosensory, visual, or auditory cortices, which are then followed by motor cortex pyramidal cell discharges that occur several milliseconds before the onset of movement.

FIGURE 7–9 Lateral aspect of the cerebrum. The cortical areas are shown according to Brodmann, with functional localizations. (Redrawn, with permission, from Waxman SG. Clinical Neuroanatomy, 28th ed. McGraw-Hill, 2017.)

Cortical motor neurons contribute axons that converge in the corona radiata and descend in the posterior limb of the internal capsule, cerebral peduncles, ventral pons, and medulla. These fibers constitute the corticospinal and corticobulbar tracts and together are known as upper motor neuron fibers (Figure 7–10). As they descend through the diencephalon and brainstem, fibers separate to innervate extrapyramidal and cranial nerve motor nuclei. The lower brainstem motor neurons receive input from crossed and uncrossed corticobulbar fibers, although neurons that innervate lower facial muscles receive primarily crossed fibers.

FIGURE 7–10 Schematic illustration of upper motor neuron pathways. (Redrawn, with permission, from

Ropper AH et al, eds. Adams and Victor’s Principles of Neurology, 10th ed. McGraw-Hill, 2014.)

In the ventral medulla, the remaining corticospinal fibers course in a tract that is pyramidal in shape in cross-section—thus, the name pyramidal tract. At the lower end of the medulla, most fibers decussate, although the proportion of crossed and uncrossed fibers varies somewhat among individuals. The bulk of these fibers descend as crossed fibers in the lateral corticospinal tract of the spinal cord.

Different groups of neurons in the cortex control muscle groups of the contralateral face, arm, and leg. Neurons near the ventral end of the central sulcus control facial muscles, whereas neurons on the medial surface of the hemisphere control leg muscles (see Figure 7–10). Because the movements of the face, tongue, and hand are complex in humans, a large share of the motor cortex is devoted to their control. A somatotopic organization is also apparent in the lateral corticospinal tract of the cervical cord, where fibers to motor neurons that control leg muscles lie laterally and fibers to cervical motor neurons lie medially.

Physiology

Upper motor neurons are the final common pathway between cortical and subcortical structures, such as the basal ganglia, in the planning, initiation, sequencing, and modulation of all voluntary movement. Much has been learned about the normal function of upper motor neurons through the study of animals and humans with focal brain lesions. Upper motor neuron pathways can be interrupted in the cortex, subcortical white matter, internal capsule, brainstem, or spinal cord. Unilateral upper motor neuron lesions spare muscles innervated by lower motor neurons that receive bilateral cortical input, such as muscles of the eyes, jaw, upper face, pharynx, larynx, neck, thorax, and abdomen. Unlike paralysis resulting from lower motor neuron lesions, paralysis from upper motor neuron lesions is rarely complete for a prolonged period of time. Acute lesions, particularly of the spinal cord, often cause flaccid paralysis and absence of spinal reflexes at all segments below the lesion. With spinal cord lesions, this state is known as spinal shock. After a few days to weeks, a state known as spasticity appears, which is characterized by increased tone and hyperactive stretch reflexes. A similar but less striking sequence of events can occur with acute cerebral lesions.

Upper motor neuron lesions cause a characteristic pattern of limb weakness and change in tone. Antigravity muscles of the limbs become more active relative to other muscles. The arms tend to assume a flexed, pronated posture,

and the legs become extended. In contrast, muscles that move the limbs out of this posture (extensors of the arms and flexors of the legs) are preferentially weakened. Tone is increased in antigravity muscles (flexors of the arms and extensors of the legs), and if these muscles are stretched rapidly, they respond with an abrupt catch, followed by a rapid increase and then decline in resistance as passive movement continues. This sequence constitutes the “clasp knife” phenomenon. Clonus—a series of involuntary muscle contractions in response to passive stretch—may be present, especially with spinal cord lesions.

Pure pyramidal tract lesions in animals cause temporary weakness without spasticity. In humans, lesions of the cerebral peduncles also cause mild paralysis without spasticity. It appears that control of tone is mediated by other tracts, particularly corticorubrospinal and corticoreticulospinal pathways. This may explain why the degrees of weakness and spasticity often do not correspond in patients with upper motor neuron lesions.

The distribution of paralysis resulting from upper motor neuron lesions varies with the location of the lesion. Lesions above the pons impair movements of the contralateral lower face, arm, and leg. Lesions below the pons spare the face. Lesions of the internal capsule often impair movements of the contralateral face, arm, and leg equally, because motor fibers are packed closely together in this region. In contrast, lesions of the cortex or subcortical white matter tend to differentially affect the limbs and face because the motor fibers are spread over a larger area of brain. Bilateral cerebral lesions cause weakness and spasticity of cranial, trunk, and limb muscles, which leads to dysarthria, dysphonia, dysphagia, bifacial paresis, and sometimes reflexive crying and laughing (pseudobulbar palsy).

CHECKPOINT

7. Define the motor cortex, and describe its organization. 8. Fibers from which nuclei and in which tracts constitute upper motor

neurons? What is their path? 9. Describe the somatotopic organization of motor neu-rons in the cortex. 10. What are the characteristics of weakness and tone in upper motor neuron

lesions? 11. How is the distribution of paralysis and spasticity affected by the

location of an upper motor neuron lesion?

3. Cerebellum Anatomy The cerebellar cortex can be divided into three anatomic regions (Figures 7–11A and 7–11B). The anterior lobe lies rostral to the primary fissure and includes the remainder of the vermis. It receives proprioceptive input from muscles and tendons via the dorsal and ventral spinocerebellar tracts and influences posture, muscle tone, and gait. The posterior lobe, which composes the remainder of the cerebellar hemispheres, receives major input from the cerebral cortex via the pontine nuclei and middle cerebellar peduncles and is important for the coordination and planning of voluntary skilled movements initiated from the cerebral cortex. The flocculonodular lobe (see Figure 7–11B), composed of the flocculus and the nodulus of the vermis, has connections to vestibular nuclei and is important for the control of posture and eye movement.

FIGURE 7–11 Anatomic divisions of the cerebellum in midsagittal view: (A) unfolded (arrows) and (B) viewed from behind. (Redrawn, with permission, from Aminoff MJ et al, eds. Clinical Neurology, 9th ed. McGraw-Hill, 2015.)

The majority of efferents from the cerebellar cortex project to deep cerebellar nuclei, which in turn project to the cerebrum and brainstem through two main pathways (Figure 7–12). Direct projections from the flocculus and nodule to the vestibular nuclei calibrate vestibulocular responses. The fastigial nucleus receives input from the vermis and sends fibers to bilateral vestibular nuclei and reticular nuclei of the pons and medulla via the inferior cerebellar peduncles. Other regions of the cerebellar cortex send fibers to the dentate, emboliform, and globose nuclei, whose efferents form the superior cerebellar peduncles, enter the upper pons, decussate completely in the lower midbrain, and travel to

the contralateral red nucleus. At the red nucleus, some fibers terminate, whereas others ascend to the ventrolateral nucleus of the thalamus, where thalamic neurons send ascending efferent fibers to the motor cortex of the same side. A smaller group of fibers descend after decussation in the midbrain and terminate in reticular nuclei of the lower brainstem. Thus, the cerebellum controls movement through connections with cerebral motor cortex and brainstem nuclei.

FIGURE 7–12 Cerebellar connections in the superior, middle, and inferior cerebellar peduncles. The peduncles are indicated by dark brown shading, and the areas to and from which they project by blue shading. (Redrawn, with permission, from Aminoff MJ et al, eds. Clinical Neurology, 9th ed. McGraw-Hill, 2015.)

Physiology

The cerebellum is responsible for the coordination of muscle groups, control of stance and gait, and regulation of muscle tone. Rather than causing paralysis, damage to the cerebellum interferes with the performance of motor tasks. The major manifestation of cerebellar disease is ataxia, in which simple movements are delayed in onset and their rates of acceleration and deceleration are decreased, resulting in intention tremor and dysmetria (“overshooting”). Lesions of the cerebellar hemispheres affect the limbs, producing limb ataxia,

whereas midline lesions affect axial muscles, causing truncal and gait ataxia and disorders of eye movement. Cerebellar lesions are often associated with hypotonia as a result of decreased activity of alpha and gamma motor neurons. If a lesion of the cerebellum or cerebellar peduncles is unilateral, the signs of limb ataxia appear on the same side as the lesion. However, if the lesion lies beyond the decussation of efferent cerebellar fibers in the midbrain, the clinical signs are on the side opposite the lesion.

CHECKPOINT

12. What is the overall role of the cerebellum? 13. What are the anatomic regions of the cerebellum, what do they control,

and through which other regions of the brain do they make connections? 14. What are the consequences of damage to the cerebel-lum, and what

symptoms and signs are seen in patients with cerebellar lesions? 15. Below which point do unilateral cerebellar lesions mani-fest on the

opposite side?

4. Basal Ganglia Anatomy Several subcortical, thalamic, and brainstem nuclei are critical for regulating voluntary movement and maintaining posture. These include the basal ganglia (ie, the caudate nucleus and putamen [corpus striatum]) (Figure 7–13), globus pallidus, substantia nigra, and subthalamic nuclei. They also include the red nuclei and the mesencephalic reticular nuclei. The major pathways that involve the basal ganglia form three neuronal circuits. The first is the cortical–basal– ganglionic–thalamic–cortical loop. Inputs mainly from premotor, primary motor, and primary sensory cortices (see Figure 7–9, areas 1, 2, 3, 4, and 6) project to the corpus striatum, which sends fibers to the medial and lateral portions of the globus pallidus. Fibers from the globus pallidus form the ansa and fasciculus lenticularis, which sweep through the internal capsule and project onto ventral and intralaminar thalamic nuclei. Axons from these nuclei project to the premotor and primary motor cortices (areas 4 and 6), completing the loop. In the second loop, the substantia nigra sends dopaminergic fibers to the corpus striatum, which has reciprocal connections with the substantia nigra. The

substantia nigra also projects to the ventromedial thalamus. The third loop is composed of reciprocal connections between the globus pallidus and the subthalamic nucleus. The subthalamic nucleus also sends efferents to the substantia nigra and corpus striatum.

FIGURE 7–13 Diagrammatic representation of the principal connections of the basal ganglia. Solid lines indicate excitatory pathways; dashed lines indicate inhibitory pathways. The transmitters are indicated in the pathways, where they are known. (DA, dopamine; Glu, glutamate.) Acetylcholine is the transmitter produced by interneurons in the striatum (ie, the putamen and the caudate nucleus, which have similar connections). (ES, external segment; IS, internal segment; SNPC, substantia nigra, pars compacta; SNPR, substantia nigra, pars reticulate.) The subthalamic nucleus also projects to the pars compacta of the substantia nigra; this pathway has been omitted for clarity. (Redrawn, with permission, from Barrett KE et al, eds. Ganong’s Review of Medical Physiology, 25th ed. McGraw-Hill, 2016.)

Physiology

Basal ganglia circuits regulate the initiation, amplitude, and speed of movements. Diseases of the basal ganglia cause abnormalities of movement and are collectively known as movement disorders. They are characterized by motor deficits (bradykinesia, akinesia, loss of postural reflexes) or abnormal activation of the motor system, resulting in rigidity, tremor, and involuntary movements (chorea, athetosis, ballismus, and dystonia).

Several neurotransmitters are found within the basal ganglia, but their role in disease states is only partly understood. Acetylcholine is present in high concentrations within the corpus striatum, where it is synthesized and released by large Golgi type 2 neurons (Figure 7–14). Acetylcholine acts as an excitatory transmitter at medium-sized spiny striatal neurons that synthesize and release the inhibitory neurotransmitter GABA and project to the globus pallidus. Dopamine

is synthesized by neurons of the substantia nigra, whose axons form the nigrostriatal pathway that terminates in the corpus striatum. Dopamine released by these fibers inhibits striatal GABAergic neurons. In Parkinson disease, degeneration of nigral neurons leads to loss of dopaminergic inhibition and a relative excess of cholinergic activity. This increases GABAergic output from the striatum and contributes to the paucity of movement that is a cardinal manifestation of the disease. Anticholinergics and dopamine agonists tend to restore the normal balance of striatal cholinergic and dopaminergic inputs and are effective in treatment. The pathogenesis of Parkinson disease is discussed later in this chapter.

FIGURE 7–14 Simplified neurochemical anatomy of the basal ganglia. Dopamine (DA) neurons exert a net inhibitory effect, and acetylcholine (ACh) neurons exert a net excitatory effect on the GABAergic output from the striatum. In Parkinson disease, DA neurons degenerate. The net effect is an increase in GABAergic output from the striatum. (Redrawn, with permission, from Greenberg DA et al, eds. Clinical Neurology, 5th ed. McGraw-Hill, 2002.)

Huntington disease is inherited as an autosomal dominant disorder. When disease onset occurs later in life, patients develop involuntary, rapid, jerky movements (chorea) and slow writhing movements of the proximal limbs and trunk (athetosis). When disease onset occurs earlier in life, patients develop signs of parkinsonism with tremor (cogwheeling) and stiffness. The spiny GABAergic neurons of the striatum preferentially degenerate, resulting in a net decrease in GABAergic output from the striatum. This contributes to the

development of chorea and athetosis. Dopamine antagonists, which block inhibition of remaining striatal neurons by dopaminergic striatal fibers, reduce the involuntary movements. Neurons in deep layers of the cerebral cortex also degenerate early in the disease, and later this extends to other brain regions, including the hippocampus and hypothalamus. Thus, the disease is characterized by cognitive deficits and psychiatric disturbances in addition to the movement disorder.

The gene for Huntington disease is located on chromosome 4p and encodes for a 3144-amino acid protein, huntingtin, which is widely expressed and interacts with several proteins involved in intracellular trafficking and endocytosis, gene transcription, and intracellular signaling. The protein contains a trinucleotide (CAG) repeat of 11–34 copies that encodes a polyglutamine domain and is expanded in patients with the disease. Deletion of the gene in mice causes embryonic death, whereas heterozygous animals are healthy. Transgenic mice with an expanded repeat develop a neurodegenerative disorder, suggesting that the disease results from the toxic effect of a gain-of-function mutation.

The mechanisms by which mutant huntingtin causes disease are not certain. The mutant protein is degraded, and the resulting fragments that contain the glutamine repeats form aggregates, which are deposited in nuclear and cytoplasmic inclusions. These fragments may bind abnormally to other proteins and interfere with normal protein processing or disrupt mitochondrial function. Nuclear fragments may interfere with nuclear functions such as gene expression. For example, in the cerebral cortex, mutant huntingtin reduces the production of brain-derived neurotrophic factor by suppressing its transcription. In addition, normal huntingtin is protective for cortical and striatal neurons and blocks the processing of procaspase 9, thereby reducing apoptosis (programmed cell death). Therefore, both loss of neurotrophic support and enhanced caspase activity could promote striatal cell loss in Huntington disease.

CHECKPOINT

16. Which are the component nuclei of the basal ganglia, and what is their functional role?

17. What are the clinical consequences of lesions in the basal ganglia? 18. What are some of the neurotransmitters within the basal ganglia, and

what is their role in disorders of basal ganglia function?

SOMATOSENSORY SYSTEM

Somatosensory pathways confer information about touch, pressure, temperature, pain, vibration, and the position and movement of body parts. This information is relayed to thalamic nuclei and integrated in the sensory cortex of the parietal lobes to provide conscious awareness of sensation. Information is also relayed to cortical motor neurons to adjust fine movements and maintain posture. Some ascending sensory fibers, particularly pain fibers, enter the midbrain and project to the amygdala and limbic cortex, where they contribute to emotional responses to pain. In the spinal cord, painful stimuli activate local pathways that induce the firing of lower motor neurons and cause a reflex withdrawal. Thus, somatosensory pathways provide tactile information, guide movement, and serve protective functions.

Anatomy A variety of specialized end organs and free nerve endings transduce sensory stimuli into neural signals and initiate the firing of sensory nerve fibers. Fibers that mediate cutaneous sensation from the trunk and limbs travel in sensory or mixed sensorimotor nerves to the spinal cord (Figure 7–15). Cutaneous sensory nerves contain small myelinated Aδ fibers that transmit information about pain and temperature, larger myelinated fibers that mediate touch and pressure sensation, and more numerous unmyelinated pain and autonomic C fibers. Myelinated proprioceptive fibers and afferent and efferent muscle spindle fibers are carried in the larger sensorimotor nerves. The cell bodies of the sensory neurons are in the dorsal root ganglia, and their central projections enter the spinal cord via the dorsal spinal roots. Innervation of the skin, muscles, and surrounding connective tissue is segmental, and each root innervates a region of skin known as a dermatome (Figure 7–16). Cell bodies of the sensory neurons that innervate the face reside in the trigeminal ganglion and send their central projections in the trigeminal nerve to the brainstem. The trigeminal innervation of the face is subdivided into three regions, each innervated by one of the three divisions of the trigeminal nerve.

FIGURE 7–15 Schematic illustration of a spinal cord segment with its dorsal root, ganglion cells, and sensory organs. Sensory organs shown (from top to bottom) are the pacinian corpuscle, muscle spindle, tendon organ, encapsulated ending, and free nerve endings. The somatotopic arrangement of fibers in the dorsal columns, spinothalamic tract, and corticospinal tract is also shown. (Redrawn, with permission, from Waxman SG. Clinical Neuroanatomy, 28th ed. McGraw-Hill, 2017.)

FIGURE 7–16 Segmental distribution of the body viewed in the approximate quadruped position, including sensory distribution of the trigeminal (V) cranial nerve. (Redrawn, with permission, from Waxman SG. Clinical Neuroanatomy, 28th ed. McGraw-Hill, 2017.)

The dorsal roots enter the dorsal horn of the spinal cord (see Figure 7–15). Large myelinated fibers divide into ascending and descending branches and either synapse with dorsal gray neurons within a few cord segments or travel in the dorsal columns, terminating in the gracile or cuneate nuclei of the lower medulla on the same side. Secondary neurons of the dorsal horn also send axons up the dorsal columns. Fibers in the dorsal columns are displaced medially as new fibers are added, so that in the cervical cord, leg fibers are located medially and arm fibers laterally (see Figure 7–15). The gracile and cuneate nuclei send fibers that cross the midline in the medulla and ascend to the thalamus as the medial lemniscus (Figure 7–17). The dorsal column–lemniscal system carries information about pressure, limb position, vibration, direction of movement, recognition of texture and shape, and two-point discrimination.

FIGURE 7–17 Sensory pathways conveying touch, pressure, vibration, joint position, pain, and temperature sensation. (Redrawn, with permission, from Aminoff MJ et al, eds. Clinical Neurology, 9th ed. McGraw-Hill, 2015.)

Thinly myelinated and unmyelinated fibers enter the lateral portion of the dorsal horn and synapse with dorsal spinal neurons within one or two segments. The majority of secondary fibers from these cells cross in the anterior spinal commissure and ascend in the anterolateral spinal cord as the lateral spinothalamic tracts. Crossing fibers are added to the inner side of the tract, so

that in the cervical cord, the leg fibers are located superficially and the arm fibers deeper. These fibers carry information about pain, temperature, and touch sensation.

Sensation from the face is carried by trigeminal sensory fibers that enter the pons and descend to the medulla and upper cervical cord (Figure 7–18). Fibers carrying information about pain and temperature sensation terminate in the nucleus of the spinal tract of cranial nerve V, which is continuous with the dorsal horn of the cervical cord. Touch, pressure, and postural information is conveyed by fibers that terminate in the main sensory and mesencephalic nuclei of the trigeminal nerve. Axons arising from trigeminal nuclei cross the midline and ascend as the trigeminal lemniscus just medial to the spinothalamic tract. Fibers from the spinothalamic tract, medial lemniscus, and trigeminal lemniscus merge in the midbrain and terminate, along with sensory fibers ascending from the spinal cord, in the posterior thalamic nuclei, mainly in the nucleus ventralis posterolateralis. These thalamic nuclei project to the primary somatosensory cortex (see Figure 7–9, areas 3, 1, and 2) and to a second somatosensory area on the upper bank of the Sylvian fissure (lateral cerebral sulcus). The primary somatosensory region is organized somatotopically like the primary motor cortex.

FIGURE 7–18 Schematic drawing of the trigeminal system. (Redrawn, with permission, from Waxman SG. Clinical Neuroanatomy, 28th ed. McGraw-Hill, 2017.)

Physiology A. Pain Free nerve endings of unmyelinated C fibers and small-diameter myelinated Aδ fibers in the skin convey sensory information in response to chemical, thermal, and mechanical stimuli. Intense stimulation of these nerve endings evokes the sensation of pain. In contrast to skin, most deep tissues are relatively insensitive to chemical or noxious stimuli. However, inflammatory conditions can sensitize sensory afferents from deep tissues to evoke pain on mechanical stimulation. This sensitization appears to be mediated by bradykinin, prostaglandins, and leukotrienes released during the inflammatory response. Information from primary afferent fibers is relayed via sensory ganglia to the dorsal horn of the spinal cord and then to the contralateral spinothalamic tract, which connects to thalamic neurons that project to the somatosensory cortex.

Damage to these pathways produces a deficit in pain and temperature discrimination and may also produce abnormal painful sensations (dysesthesias), usually in the area of sensory loss. Such pain is termed neuropathic pain and often has a strange burning, tingling, or electric shock– like quality. It may arise from several mechanisms. Damaged peripheral nerve fibers become highly mechanosensitive and may fire spontaneously without known stimulation. They also develop sensitivity to norepinephrine released from sympathetic postganglionic neurons. Electrical impulses may spread abnormally from one fiber to another (ephaptic conduction), enhancing the spontaneous firing of multiple fibers. Neuropeptides released by injured nerves may recruit an inflammatory reaction that stimulates pain. In the dorsal horn, denervated spinal neurons may become spontaneously active. In the brain and spinal cord, synaptic reorganization occurs in response to injury and may lower the threshold for pain. In addition, inhibition of pathways that modulate transmission of sensory information in the spinal cord and brainstem may promote neuropathic pain.

Pain-modulating circuits exert a major influence on the perceived intensity of pain. One such pathway (Figure 7–19) is composed of cells in the periaqueductal gray matter of the midbrain that receive afferents from the frontal cortex and hypothalamus and project to rostroventral medullary neurons. These in turn project in the dorsolateral white matter of the spinal cord and terminate on dorsal

horn neurons. Additional descending pathways arise from other brainstem nuclei (locus ceruleus, dorsal raphe nucleus, and nucleus reticularis gigantocellularis). Major neurotransmitters used by these systems include endorphins, serotonin, and norepinephrine, providing the rationale for the use of opioids, serotonin agonists, and serotonin and norepinephrine reuptake inhibitors in the treatment of pain.

FIGURE 7–19 Schematic illustration of the pathways involved in pain control. (Used with permission of A. Basbaum.)

B. Proprioception and Vibratory Sense

Receptors in the muscles, tendons, and joints provide information about deep pressure and the position and movement of body parts. This allows one to determine an object’s size, weight, shape, and texture. Information is relayed to the spinal cord via large Aα and Aβ myelinated fibers and to the thalamus by the dorsal column–lemniscal system. Detecting vibration requires sensing touch and rapid changes in deep pressure. This depends on multiple cutaneous and deep sensory fibers and is impaired by lesions of multiple peripheral nerves, the dorsal columns, medial lemniscus, or thalamus, but rarely by lesions of single nerves. Vibratory sense is often impaired together with proprioception.

C. Discriminative Sensation

The primary sensory cortex provides awareness of somatosensory information and the ability to make sensory discriminations. Touch, pain, temperature, and vibration sense are considered the primary modalities of sensation and are relatively preserved in patients with damage to the sensory cortex or its projections from the thalamus. In contrast, complex tasks that require the integration of multiple somatosensory stimuli and of somatosensory stimuli with auditory or visual information are impaired. These include the ability to distinguish two points from one when touched on the skin (two-point discrimination), localize tactile stimuli, perceive the position of body parts in space, recognize letters or numbers drawn on the skin (graphesthesia), and identify objects by their shape, size, and texture (stereognosis).

D. Anatomy of Sensory Loss

The patterns of sensory loss often indicate the level of nervous system involvement. Symmetric distal sensory loss in the limbs, affecting the legs more than the arms, usually signifies a generalized disorder of multiple peripheral nerves (polyneuropathy). Sensory symptoms and deficits may be restricted to the distribution of a single peripheral nerve (mononeuropathy) or two or more peripheral nerves (mononeuropathy multiplex). Symptoms limited to a dermatome indicate a spinal root lesion (radiculopathy).

In the spinal cord, segregation of fiber tracts and the somatotopic arrangement of fibers give rise to distinct patterns of sensory loss. Loss of pain and temperature sensation on one side of the body and of proprioception on the opposite side occurs with lesions that involve one half of the cord on the side of the proprioceptive deficit (Brown–Séquard syndrome; Figure 7–20).

Compression of the upper spinal cord causes loss of pain, temperature, and touch sensation first in the legs, because the leg spinothalamic fibers are most superficial. More severe cord compression compromises fibers from the trunk. In patients with spinal cord compression, the lesion is often above the highest dermatome involved in the deficit. Thus, radiographic studies should be tailored to visualize the cord at and above the level of the sensory deficit detected on examination. Intrinsic cord lesions that involve the central portions of the cord often impair pain and temperature sensation at the level of the lesion because the fibers crossing the anterior commissure and entering the spinothalamic tracts are most centrally situated. Thus, enlargement of the central cervical canal in syringomyelia typically causes loss of pain and temperature sensation across the shoulders and upper arms (Figure 7–21).

FIGURE 7–20 Brown–Séquard syndrome with lesion at the left tenth thoracic level (motor deficits not shown). (Redrawn, with permission, from Waxman SG. Clinical Neuroanatomy, 28th ed. McGraw-Hill, 2017.)

FIGURE 7–21 Syringomyelia (the presence of a cavity in the spinal cord resulting from the breakdown of gliomatous new formations, presenting clinically with pain and paresthesias followed by muscular atrophy of the hands) involving the cervicothoracic portion of the cord. (Redrawn, with permission, from Waxman SG. Clinical Neuroanatomy, 28th ed. McGraw-Hill, 2017.)

Brainstem lesions involving the spinothalamic tract cause loss of pain and temperature sensation on the opposite side of the body. In the medulla, such lesions can involve the neighboring spinal trigeminal nucleus, resulting in a “crossed” sensory deficit involving the ipsilateral face and contralateral limbs. Above the medulla, the spinothalamic and trigeminothalamic tracts lie close

together, and lesions there cause contralateral sensory loss of the face and limbs. In the midbrain and thalamus, medial lemniscal fibers run together with pain and temperature fibers, and lesions are more likely to impair all primary sensation contralateral to the lesion. Because sensory fibers converge at the thalamus, lesions there tend to cause fairly equal loss of pain, temperature, and proprioceptive sensation on the contralateral half of the face and body. Lesions of the sensory cortex in the parietal lobe impair discriminative sensation on the opposite side of the body, whereas detection of the primary modalities of sensation may remain relatively intact.

CHECKPOINT

19. Which fibers carry pain, and how are they segregated from fibers that carry proprioception information in the spinal cord?

20. What are the differences in characteristics of sensory loss at different levels of the nervous system?

21. What is the function of the sensory cortex in the parietal lobe, and what are the clinical features of damage to this region?

VISION & EYE MOVEMENT CONTROL

The visual system provides our most important source of sensory information about the environment. The visual system and pathways for the control of eye movements are among the best characterized pathways in the nervous system. Familiarity with these neuroanatomic features is often extremely valuable in localization of neurologic disease.

Anatomy The cornea and lens of the eye refract and focus images on the photosensitive posterior portion of the retina. The posterior retina contains two classes of specialized photoreceptor cells, rods and cones, which transduce photons into electrical signals. At the retina, the image is reversed in the horizontal and vertical planes so that the inferior visual field falls on the superior portions of the retina and the lateral field is detected by the nasal half of the retina.

Fibers from the nasal half of the retina traverse the medial portion of the optic nerve and cross to the other side at the optic chiasm (Figure 7–22). Each optic tract contains fibers from the same half of the visual field of both eyes. The optic tracts terminate in the lateral geniculate nuclei of the thalamus. Lateral geniculate neurons send fibers to the primary visual cortex in the occipital lobe (calcarine cortex, area 17 of Figure 7–9). These fibers form the optic radiations, which extend through the white matter of the temporal lobes and the inferior portion of the parietal lobes.

FIGURE 7–22 Common visual field defects and their anatomic bases. (1) Central scotoma caused by inflammation of the left optic disk (optic neuritis) or optic nerve (retrobulbar neuritis). (2) Total blindness of the right eye from a complete lesion of the right optic nerve. (3) Bitemporal hemianopia caused by pressure

exerted on the optic chiasm by a pituitary tumor. (4) Right nasal hemianopia caused by a perichiasmal lesion (eg, calcified internal carotid artery). (5) Right homonymous hemianopia from a lesion of the left optic tract. (6) Right homonymous superior quadrantanopia caused by partial involvement of the optic radiation by a lesion in the left temporal lobe (Meyer loop). (7) Right homonymous inferior quadrantanopia caused by partial involvement of the optic radiation by a lesion in the left parietal lobe. (8) Right homonymous hemianopia from a complete lesion of the left optic radiation. (A similar defect may also result from lesion 9.) (9) Right homonymous hemianopia (with macular sparing) resulting from posterior cerebral artery occlusion. (Redrawn, with permission, from Simon RP et al, eds. Clinical Neurology, 10th ed. McGraw-Hill, 2018.)

Eye movements are produced by the extraocular muscles, which function in pairs to move the eyes along three axes (Figure 7–23). These muscles are innervated by the oculomotor (III), trochlear (IV), and abducens (VI) nerves. The oculomotor nerve innervates the ipsilateral medial, superior, and inferior rectus muscles and the inferior oblique muscles. It also supplies the ipsilateral levator palpebrae, which elevates the eyelid. The oculomotor nerve also carries parasympathetic fibers that mediate pupillary constriction (see later discussion). Trochlear nerve fibers decussate before leaving the brainstem, and each trochlear nerve supplies the contralateral superior oblique muscle. The abducens nerve innervates the lateral rectus muscle of the same side.

FIGURE 7–23 Extraocular muscles subserving the six cardinal positions of gaze. The eye is adducted by the medial rectus and abducted by the lateral rectus. The adducted eye is elevated by the inferior oblique and depressed by the superior oblique; the abducted eye is elevated by the superior rectus and depressed by the inferior rectus. (Redrawn, with permission, from Simon RP et al, eds. Clinical Neurology, 10th ed. McGraw-Hill, 2018.)

Cortical and brainstem gaze centers innervate the extraocular motor nuclei and provide for supranuclear control of gaze. A vertical gaze center is located

in the midbrain tegmentum, and lateral gaze centers are present in the pontine paramedian reticular formation. Each lateral gaze center sends fibers to the neighboring ipsilateral abducens nucleus and, via the medial longitudinal fasciculus, to the contralateral oculomotor nucleus. Therefore, activation of the right lateral gaze center stimulates conjugate deviation of the eyes to the right. Rapid saccadic eye movements are initiated by the frontal eye fields in the premotor cortex, which stimulate conjugate movement of the eyes to the opposite side. Slower eye movements involved in pursuit of moving objects are controlled by parieto-occipital gaze centers, which stimulate conjugate gaze to the side of the gaze center. These cortical areas control eye movements through their connections with the brainstem gaze centers.

The size of the pupils is determined by the balance between parasympathetic and sympathetic discharge to the pupillary muscles. The parasympathetic oculomotor nuclei of Edinger–Westphal send fibers in the oculomotor nerves that synapse in the ciliary ganglia within the orbits and innervate the pupillary constrictor muscles.

The motor portion of pupillary dilation is controlled by a three-neuron system (Figure 7–24). It is composed of axons from neurons in the posterolateral hypothalamus that descend through the lateral brainstem tegmentum and the intermediolateral column of the cervical spinal cord to the level of T1. There they terminate on preganglionic sympathetic neurons within the lateral gray matter of the thoracic cord. These neurons send axons that synapse with postganglionic neurons in the superior cervical ganglion. Postganglionic neurons send fibers that travel with the internal carotid artery and the first division of the trigeminal nerve to innervate the iris. The fibers also innervate the tarsal muscles of the eyelids. Damage to these pathways causes Horner syndrome, which consists of miosis, ptosis, and sometimes impaired sweating ipsilateral to the lesion.

FIGURE 7–24 Oculosympathetic pathways. This three-neuron pathway projects from the hypothalamus to the intermediolateral column of the spinal cord, then to the superior cervical (sympathetic) ganglion, and finally to the pupil, the smooth muscle of the eyelids, and the sweat glands of the forehead and face. Interruption of these pathways results in Horner syndrome. (Redrawn, with permission, from Simon RP et al, eds. Clinical Neurology, 10th ed. McGraw-Hill, 2018.)

Physiology A. Vision The rods are sensitive to low levels of light and are most numerous in the peripheral regions of the retina. In retinitis pigmentosa, there is degeneration of the retina that begins in the periphery. Poor twilight vision is thus an early symptom of this disorder. Cones are responsible for perception of stimuli in bright light and for discrimination of color. They are concentrated in the macular region, which is crucial for visual acuity. In disorders of the retina or optic nerve that impair acuity, diminished color discrimination is often an early sign.

Visual processing begins in the retina, where information gathered from rods

and cones is modified by interactions among bipolar, amacrine, and horizontal cells. Amacrine and bipolar cells send their output to ganglion cells, whose axons compose the optic nerve. Photoreceptors convey information about the absolute level of illumination. Retinal processing renders ganglion cells sensitive to simultaneous differences in contrast for detection of edges of objects.

Ganglion cell axons terminate in a highly ordered fashion in well-defined layers of the lateral geniculate nuclei. Because of the separation of fibers in the optic chiasm, the receptive fields of cells in the lateral geniculate lie in the contralateral visual field. Geniculate neurons are arranged in six layers, and ganglion cell axons from each eye terminate in separate layers. Cells in different layers are in register, so that the receptive fields of cells in the same part of each layer are in corresponding regions of the two retinas. A greater proportion of cells are devoted to the macular region of both retinas. This reflects use of the central retina for high acuity and color vision. Some visual processing occurs in the geniculate, particularly for contrast and edge perception and movement detection.

In the primary visual cortex, visual fields from the eyes are also represented in a topographic projection. Cortical neurons are functionally organized in columns perpendicular to the cortical surface. Geniculate fibers terminate within layer IV of the visual cortex, and cells within a column above and below layer IV show the same eye preference and similar receptive fields. Narrow alternating columns of cells supplied by one eye or the other lie next to each other (ocular dominance columns). A tremendous amount of visual processing occurs in the primary visual cortex, including the synthesis of complex receptive fields and the determination of axis orientation, position, and color. The retina is not simply represented as a map on the cortex; rather, each area of the retina is represented in multiple columns and analyzed with respect to the position, color, and orientation of objects. As in the geniculate, a major portion of the primary visual cortex is devoted to analyzing information derived from the macular regions of both retinas. Cortical areas 18 and 19 (and many other areas) provide higher levels of visual processing.

The anatomic organization of the visual system is useful for localizing neurologic disease (see Figure 7–22). Lesions of the retina or optic nerves (prechiasmal lesions) impair vision from the ipsilateral eye. Lesions that compress the central portion of the chiasm, such as pituitary tumors, disrupt crossing fibers from the nasal halves of both retinas, causing bitemporal hemianopia. Lesions involving structures behind the chiasm (retrochiasmal lesions) cause visual loss in the contralateral field of both eyes. Lesions that

completely destroy the optic tract, lateral geniculate nucleus, or optic radiations on one side produce a contralateral homonymous hemianopia. Selective destruction of temporal lobe optic radiations causes superior quadrantanopia, and lesions of the parietal optic radiations cause inferior quadrantanopia. The posterior portions of the optic radiations and the calcarine cortex are supplied mainly by the posterior cerebral artery, although the macular region of the visual cortex receives some collateral supply from the middle cerebral artery. Therefore, a lesion of the primary visual cortex generally causes contralateral homonymous hemianopia, but if it is due to posterior cerebral artery occlusion, it may spare macular vision. In rare cases of bilateral cortical blindness, patients may be unaware of their lack of vision.

B. Eye Movement

Conjugate eye movements are regulated by proprioceptive information from neck structures and information about head movement and position from the vestibular system. This information is used to maintain fixation on a stationary point when moving the head. In a comatose patient, the integrity of these oculovestibular and oculocephalic pathways can be assessed by the “doll’s eye” or vestibulo-ocular reflex maneuver. This is elicited by briskly turning the head, which normally results in conjugate movement of the eyes in the opposite direction in a comatose patient. Irrigation of the ear with 10–20 mL of cold water reduces the activity of the labyrinth on that side and elicits jerk nystagmus, with the fast component away from the irrigated ear in a conscious individual. In coma, the fast saccadic component is lost, and the vestibular influence on eye movements dominates. Cold-water irrigation then results in deviation of the eyes toward the irrigated ear. These caloric responses are lost with midbrain or pontine lesions, with damage to the labyrinths, or with drugs that inhibit vestibular function.

C. Pupillary Function

The size of the pupils is controlled by the amount of ambient light sensed by the retina (Figure 7–25). Fibers from each retina terminate within midbrain pretectal nuclei that send fibers to both Edinger–Westphal nuclei. The fibers mediate pupillary constriction in bright light. In dim light, this reflex is inhibited and the influence of sympathetic fibers predominates, causing pupillary dilatation. The pupillary constrictor fibers release acetylcholine, which activates muscarinic AChRs and thus stimulates contraction of the pupillary sphincter muscle of the

iris. Sympathetic pupillary fibers release norepinephrine, which activates α1- adrenergic receptors, causing contraction of the radial muscle of the iris. Drugs that inhibit muscarinic receptors, such as atropine, or that stimulate α1- adrenergic receptors, such as epinephrine, dilate the pupils, whereas drugs that stimulate muscarinic receptors or block α1-adrenergic receptors cause pupillary constriction.

FIGURE 7–25 Anatomic basis of the pupillary light reflex. The afferent visual pathways from the retina to the pretectal nuclei of the midbrain are represented by dashed lines, the efferent pupilloconstrictor pathways from the midbrain to the retinas by solid lines. Note that illumination of one eye results in bilateral pupillary constriction. (Redrawn, with permission, from Simon RP et al, eds. Clinical Neurology, 10th ed. McGraw- Hill, 2018.)

CHECKPOINT

22. What is the pathway of fibers from the retina to the vis-ual cortex? 23. What is the innervation of the extraocular muscles?

24. Describe how lesions in various parts of the visual path-ways produce characteristic visual field defects.

HEARING & BALANCE

Anatomy Structures of the middle ear serve to amplify and transmit sounds to the cochlea, where specialized sensory cells (hair cells) are organized to detect ranges in amplitude and frequency of sound. The semicircular canals contain specialized hair cells that detect movement of endolymphatic fluid contained within the canals. Similar hair cells in the saccule and utricle detect movement of the otolithic membrane, which is composed of calcium carbonate crystals embedded in a matrix. The semicircular canal hair cells detect angular acceleration, whereas the hair cells of the utricle and saccule detect linear acceleration. Axons from auditory and vestibular neurons compose the eighth cranial nerve, which traverses the petrous bone, is joined by the facial nerve, and enters the posterior fossa through the auditory canal. Auditory fibers terminate in the cochlear nuclei of the pons, and vestibular fibers terminate in the vestibular nuclear complex.

Cochlear neurons send fibers bilaterally to a network of auditory nuclei in the midbrain, and impulses are finally relayed through the medial geniculate thalamic nuclei to the auditory cortex in the superior temporal gyri. Vestibular nuclei have connections with the cerebellum, red nuclei, brainstem gaze centers, and brainstem reticular formation. The vestibular nuclei exert considerable control over posture through descending vestibulospinal, rubrospinal, and reticulospinal pathways.

Physiology A. Hearing There are three types of hearing loss: (1) conductive deafness, resulting from diseases of the external or middle ear that impair conduction and amplification of sound from the air to the cochlea; (2) sensorineural deafness, resulting from diseases of the cochlea or eighth cranial nerve; and (3) central deafness, resulting from diseases affecting the cochlear nuclei or auditory pathways in the CNS. Because of the redundancy of central pathways, almost all cases of hearing loss are due to conductive or sensorineural deafness. Besides hearing loss,

auditory diseases may cause tinnitus, the subjective sensation of noise in the ear. Tinnitus resulting from disorders of the cochlea or eighth cranial nerve sounds like a constant nonmusical tone and may be described as ringing, whistling, hissing, humming, or roaring. Transient episodes of tinnitus occur in most individuals and are not associated with disease. When persistent, tinnitus is often associated with hearing loss.

Conductive and sensorineural deafness may be distinguished by examining hearing with a vibrating 512 Hz tuning fork. In the Rinne test, the tuning fork is held on the mastoid process behind the ear and then placed at the auditory meatus. If the sound is louder at the meatus, the test is positive. Normally the test is positive because sound transmitted through air is amplified by middle-ear structures. In sensorineural deafness, although sound perception is reduced, the Rinne test is still positive because middle-ear structures are intact. In conductive deafness, sounds are heard less well through air, and the test is negative. In the Weber test, the tuning fork is applied to the forehead at the midline. In conductive deafness, the sound is heard best in the abnormal ear, whereas with sensorineural deafness, the sound is heard best in the normal ear. Audiometry can distinguish types of hearing loss. In general, sensorineural deafness causes greater loss of high-pitched sounds, whereas conductive deafness causes more loss of low-pitched sounds.

B. Vestibular Function

In contrast to hearing, vestibular function is commonly disturbed by small brainstem lesions. The vestibular nuclei occupy a large portion of the lateral brainstem, extending from the medulla to the midbrain. Although there are extensive bilateral connections between vestibular nuclei and other motor pathways, these connections are not redundant but highly lateralized and act in concert to control posture, balance, and conjugate eye movement.

Patients with diseases of the vestibular system complain of disequilibrium and dizziness. Cerebellar disease also causes disequilibrium, but this is often described as a problem with coordination rather than a feeling of dizziness in the head. Interpretation of the complaint of dizziness can often be difficult. Many patients use the term loosely to describe sensations of light-headedness, weakness, or malaise. Directed questioning is often required to establish whether there is truly an abnormal sensation of movement (vertigo).

Vertigo may be due to disease of the labyrinth or vestibular nerve (peripheral vertigo) or to dysfunction of brainstem and CNS pathways (central vertigo). In general, peripheral vertigo is more severe and associated with nausea and

vomiting, especially if the onset is acute. Diseases of the semicircular canal neurons or their fibers frequently cause rotational vertigo, whereas diseases involving the utricle or saccule cause sensations of tilting or listing, as on a boat. Traumatic and ischemic lesions may cause associated hearing loss. Dysfunction of one labyrinth often causes horizontal and rotatory jerk nystagmus. The slow phase of the nystagmus is caused by the unopposed action of the normal labyrinth, which drives the eyes to the side of the lesion. The fast-jerk phase is due to a rapid saccade, which maintains fixation.

Vertigo resulting from lesions of the CNS is usually less severe than peripheral vertigo and is often associated with other findings of brainstem dysfunction. In addition, nystagmus associated with central lesions may be present in vertical or multiple directions of gaze. Common causes of central vertigo include brainstem ischemia, brainstem tumors, and multiple sclerosis.

CONSCIOUSNESS, AROUSAL & COGNITION

Anatomy Consciousness is awareness of self and the environment. It has two aspects: arousal, which is the state of wakefulness, and cognition, which is the sum of mental activities. This distinction is useful because neurologic disorders can affect arousal and cognition differently. Arousal is generated by activity of the ascending reticular activating system (Figure 7–26), which is composed of neurons within the central mesencephalic brainstem, the lateral hypothalamus, and the medial, intralaminar, and reticular nuclei of the thalamus. Widespread projections from these nuclei synapse on distal dendritic fields of large pyramidal neurons in the cerebral cortex and generate an arousal response. Cognition is the chief function of the cerebral cortex, particularly of the prefrontal cortex and cortical association areas of the occipital, temporal, and parietal lobes. Some specialized mental functions are localized to specific cortical regions. Several subcortical nuclei in the basal ganglia and thalamus are intimately linked with cortical association areas, and damage to these nuclei or their interconnections with the cortex may give rise to cognitive deficits similar to those observed with cortical lesions.

FIGURE 7–26 Brainstem reticular activating system and its ascending projections to the thalamus and cerebral hemispheres. (Redrawn, with permission, from Aminoff MJ et al, eds. Clinical Neurology, 9th ed. McGraw-Hill, 2015.)

Physiology A. Arousal The reticular activating system is excited by a wide variety of stimuli, especially somatosensory stimuli. It is most compact in the midbrain and can be damaged by central midbrain lesions, resulting in failure of arousal, or coma. Higher nuclei and projections are less localized, and lesions rostrad to the midbrain, therefore, must be bilateral to cause coma.

Less severe dysfunction causes confusional states in which consciousness is clouded and the patient is sleepy, inattentive, and disoriented. Alertness is reduced, and the patient appears drowsy or falls asleep easily without frequent stimulation. More awake patients perceive stimuli slowly but are distractible, assigning important and irrelevant stimuli equal value. Perceptions may be distorted, leading to hallucinations, and the patient may be unable to organize and interpret a complex set of stimuli. The inability to perceive properly

interferes with learning, memory, and problem solving. Thoughts become disorganized and tangential, and the confused patient may maintain false beliefs even in the face of evidence of their falsity (delusions). In some cases, the confusional state presents as delirium, which is characterized by heightened alertness, disordered perception, agitation, delusions, hallucinations, convulsions, and autonomic hyperactivity (sweating, tachycardia, hypertension).

Coma may result from structural or metabolic causes. Some structural lesions of the cerebral hemispheres, such as hemorrhages, large areas of ischemic infarction, abscesses, or tumors, can expand over minutes or a few hours and cause brain tissue to herniate into the posterior fossa (Figure 7–27). If lateral within the temporal lobe, the expanding mass may drive the uncus of the temporal lobe into the ambient cistern surrounding the midbrain, compressing the ipsilateral third cranial nerve (uncal herniation). This causes pupillary dilation and impaired function of eye muscles innervated by that nerve. Continued pressure distorts the midbrain, and the patient lapses into coma with posturing of the limbs. With continued herniation, pontine function is impaired, causing loss of oculovestibular responses. Eventually, medullary function is lost and breathing ceases. Hemispheric lesions closer to the midline compress the thalamic reticular formation structures and can cause coma before eye findings develop (central herniation). With continued pressure, midbrain function is affected, causing the pupils to dilate and the limbs to posture. With progressive herniation, pontine vestibular and then medullary respiratory functions are lost.

FIGURE 7–27 Anatomic basis of herniation syndromes. An expanding supratentorial mass lesion may cause brain tissue to be displaced into an adjacent intracranial compartment, resulting in (1) cingulate herniation under the falx cerebri; (2) downward transtentorial (central) herniation; (3) uncal herniation over the edge of the tentorium; or (4) cerebellar tonsillar herniation into the foramen magnum. Coma and ultimately death result when (2), (3), or (4) produces brainstem compression. (Redrawn, with permission, from Greenberg DA et al, eds. Clinical Neurology, 5th ed. McGraw-Hill, 2002.)

Several nonstructural disorders that diffusely disturb brain function can produce a confusional state or, if severe, coma (Table 7–1). Most of these disorders are acute, and many, particularly those caused by drugs and metabolic toxins, are reversible. Clues to the cause of these “metabolic” encephalopathies are provided by general physical examination, drug screens, and certain blood studies. When these disorders cause coma, pupillary light responses are usually preserved despite impaired oculovestibular or respiratory function. This finding is of great help in distinguishing metabolic from structural causes of coma.

TABLE 7–1 Nonstructural causes of confusional states and coma.

Neurons in the dorsal midbrain and especially nuclei within the pontine reticular formation are important for sleep. Thus, lesions involving the pons may preserve consciousness but disturb sleep. In contrast, diffuse lesions of the neocortex, such as those resulting from global cerebral ischemia, may preserve the reticular activating system and brainstem sleep centers, resulting in a patient with preserved sleep–wake cycles who cannot interact in any meaningful way with the environment (coma vigil or apallic state).

B. Cognition

Several disorders disturb cognition rather than level of consciousness. Specific cortical regions generally mediate different cognitive functions, although there is

considerable overlap and interconnection between cortical and subcortical structures in all mental tasks. When several of these abilities are impaired, the patient is said to suffer from dementia. Dementia is discussed in more detail later in this chapter.

The prefrontal cortex (see Figure 7–9) generally refers to areas 9, 10, 11, 12, 45, 46, and 47 of Brodmann on the superior and lateral surfaces of the frontal lobes and the anterior cingulate, parolfactory, and orbitofrontal cortex inferiorly and mesially. These regions are essential for orderly planning and sequencing of complex behaviors, attending to several stimuli or ideas simultaneously, concentrating and flexibly altering the focus of concentration, grasping the context and meaning of information, and controlling impulses, emotions, and thought sequences. Damage to the frontal lobes or connections to the caudate and dorsal medial nuclei of the thalamus causes frontal lobe syndrome. Patients may suffer dramatic alterations in personality and behavior, whereas most sensorimotor functions remain intact. Some patients become vulgar in speech, slovenly, grandiose, and irascible, whereas others lose interest, spontaneity, curiosity, and initiative. The affect may become apathetic and blunted (abulia). Some patients lose the capacity for creativity and abstract reasoning and the ability to solve problems while becoming excessively concrete in their thinking. Often, they are distractible and unable to focus attention when presented with multiple stimuli. The most dramatic manifestations are seen after bilateral frontal lobe damage; unilateral damage can lead to subtle alterations in behavior that may be difficult to detect. Involvement of premotor areas may lead to incontinence, the inability to perform learned motor tasks (apraxia), variable increases in muscle tone (paratonia), and the appearance of primitive grasp and oral reflexes (sucking, snouting, and rooting).

In about 90% of people, language is a function of the left hemisphere. Whereas 99% of right-handed people are left hemisphere dominant, about 40% of left-handed people are right hemisphere dominant for language. In most left- handed people, hemispheric dominance for language is incomplete, and damage to the dominant hemisphere tends to disturb language less severely than in right- handed individuals. The cortical regions most critical for language include Broca area (area 44), Wernicke area (area 22), the primary auditory cortex (areas 41 and 42), and neighboring frontal and temporoparietal association areas (see Figure 7–9). Injury to these areas or their connections to other cortical regions results in aphasia. Lesions in the frontal speech areas cause nonfluent, dysarthric, halting speech, whereas lesions of the temporal speech area cause fluent speech that contains many errors or may be totally devoid of

understandable words. Patients with damage to temporal speech areas also lack comprehension of spoken words. Isolation of the temporal speech area from the occipital lobes causes an inability to read (alexia). Portions of the parietal lobe adjacent to the temporal lobe are important for retrieval of previously learned words, and damage here may result in anomia. The inferior parietal region is important for the translation of linguistic messages generated in the temporal language areas into visual symbols. Damage to this region may result in an inability to write (agraphia).

Memory requires that information be registered by the primary somatosensory, auditory, or visual cortex. Posterior cortical areas involved in language comprehension are needed for the immediate processing and recall of spoken or written events. The hippocampi and their connections to the dorsal medial nuclei of the thalamus and the mammillary nuclei of the hypothalamus constitute a limbic system network crucial for learning and processing events for long-term storage. When these areas are damaged, the patient is unable to learn new material or retrieve memories from the recent past. The most severe symptoms occur with bilateral lesions; unilateral disease causes subtler learning deficits. Memories that remain with a person for years are considered remote memories and are stored in corresponding association cortex areas (eg, visual cortex for scenes). Remote memories remain intact in patients with damage to limbic structures required for learning. However, they may be lost by damage to cortical association areas. Understanding the mechanisms by which recent memories are transferred from the limbic memory network to the association cortex for long-term storage is a major goal of current research.

The parietal association cortex is the region principally involved in the visuomotor integration of constructional tasks. The visual cortex is required for observation, whereas the auditory cortex and the temporal language cortex are necessary for drawing objects on command. The inferior parietal cortex (areas 39 and 40) integrates visual and auditory information, and the output from this region is translated into motor patterns by the motor cortex. Thus, lesions to the parietal lobes commonly cause constructional impairment. Damage to either hemisphere may result in constructional errors. Drawings may show rotation of objects, disorientation of objects on the background, fragmentation of design, inability to draw angles properly, or omission of parts of a figure presented for copying. It is often difficult to determine which side is damaged, although if language is preserved, a nondominant parietal deficit is more likely.

Calculation ability, abstract reasoning, problem solving, and several other aspects of intelligence are difficult to localize because they require the

integration of several cortical regions. They are frequently disturbed by diseases that cause widespread cortical dysfunction, such as those that cause dementia.

CHECKPOINT

25. What is the network of neurons that maintains normal arousal and consciousness?

26. What are the symptoms and signs of cerebral herniation caused by focal brain lesions?

27. Which cognitive functions are controlled by the frontal lobes and by the parietal association cortex?

28. Which regions of the cortex are important for language and memory?

PATHOPHYSIOLOGY OF SELECTED NEUROLOGIC DISORDERS

Nervous system disease may be caused by a wide variety of degenerative, metabolic, structural, neoplastic, or inflammatory conditions that affect neurons, glia, or both. The resultant dysfunction is expressed by either neuronal hyperactivity, as seen during seizures, or decreased neuronal activity, as observed after a stroke. The specific functional abnormalities observed depend on the network of neurons affected. For example, because amyotrophic lateral sclerosis is a disorder of upper and lower motor neurons, neurologic deficits are limited to the motor system. In Parkinson disease, dopaminergic neurons of the substantia nigra degenerate, causing symptoms of extrapyramidal motor system dysfunction. In patients with ischemic stroke, the particular constellation of deficits is determined by the vascular territory affected. Therefore, an understanding of the pathophysiology of neurologic disease requires an analysis of events occurring at both the cellular level and the level of neural networks.

CEREBELLAR ATAXIA

Clinical Presentation

Cerebellar ataxia is a heterogenous group of disorders that affect the cerebellum and its connections. It is estimated that cerebellar ataxias affect up to 150,000 people in the United States. Clinical features of cerebellar ataxia include the following: (1) ataxic gait, which results in a widened base, staggering, falls, and, if severe, wheelchair confinement; (2) truncal ataxia, which can result in the inability to sit unsupported by the arms; (3) dysmetria, an impaired ability to perform accurate movements during ballistic movements owing to a faulty estimation of distance; (4) limb ataxia, which results in difficulty with coordinated tasks; (5) vertigo with nausea and vomiting, resulting from damage to the vestibulocerebellum; (6) static and kinetic tremor, also called intention tremor; (7) cerebellar dysarthria, also termed scanning speech, which results in slurred speech and, in severe cases, rendering speech unintelligible; and (8) nystagmus and ocular dysmetria, abnormalities in eye movements.

The causes of ataxia are varied. Ataxia may result from vascular insults to the cerebellum, toxic insults (including alcohol), infections, autoimmune disorders, vitamin deficiency (eg, thiamine, vitamin E), and degenerative disorders (inherited or sporadic). The prevalence of inherited ataxias, as a group, may be as high as 25–30 per 100,000 persons. The most common sporadic form of degenerative cerebellar ataxia is multiple system atrophy of the cerebellar type (MSA-C). The heritable ataxias may be inherited in a dominant fashion (the so-called spinocerebellar ataxias [SCAs]) or in a recessive manner. Although mutations in more than 450 genes are associated with cerebellar ataxia, the role of most of these genetic variants in ataxia is unknown. Moreover, although individual genetic causes of ataxia are rare, understanding the basis for cerebellar dysfunction and degeneration in ataxia can give us insight into the disease pathogenesis of other neurodegenerative diseases.

Polyglutamine Ataxia The largest group of dominantly inherited ataxias result from glutamine- encoding CAG repeats in various disease genes. These include SCA types 1, 2, 3, 6, 7, and 17. The pathogenesis of the dominant polyglutamine ataxias is thought to be a gain-of-function mutation causing an expanded number of glutamine repeats in the respective disease proteins. Although the putative roles of these different proteins are diverse, the clinical features of the polyglutamine SCAs are remarkably similar, and it is thus difficult to clinically distinguish one type of SCA from another. The mechanism by which polyglutamine expansion leads to ataxia remains unknown. Several mechanisms have been proposed, including the following:

• Altered protein function owing to polyglutamine expansion • Formation of toxic oligomeric complexes • Transcriptional dysregulation • Aberrant neuronal signaling, including excitotoxicity • Mitochondrial dysfunction • Impaired axonal transport • Impairment in cellular protein homeostasis • RNA toxicity

Recent work in mouse models of SCA1 and SCA2 has suggested that similar transcriptional modules are altered in both disorders, which may account for the similarity in neuronal vulnerability and, hence, phenotypes. It remains to seen whether the convergence of pathological and clinical phenotypes in other cerebellar ataxias is accounted for by a similar downstream effect on transcripts that are particularly important for cerebellar function.

Autoimmune and Paraneoplastic Ataxia Autoimmune cerebellar syndromes represent a small subset of ataxias. However, they are important to recognize for two reasons. First, some of the autoimmune ataxias can help us to understand disease pathogenesis; second, these disorders are treatable, particularly when recognized early. A subset of autoimmune ataxias is associated with an underlying, often occult, malignancy. The ataxia in these cases is thought to be a result of immune cross-reactivity between tumor and cerebellar antigens. The new onset of the neurological symptoms may suggest the presence of a previously unidentified tumor. As a remote effect of an underlying malignancy, the ataxia can thus be considered a paraneoplastic syndrome.

Other immune ataxias that have a parallel genetic etiology are outlined below.

Gluten Ataxia

Cerebellar ataxia is a neurological manifestation of celiac disease, a disorder associated with gluten sensitivity. Clinically, gastrointestinal symptoms of diarrhea, bloating, and malabsorption occur following gluten ingestion, along with characteristic pathologic changes in villus and crypt morphology and inflammation in the epithelium on duodenal biopsies. Antibodies directed against tissue transglutaminase 6 is a serologic marker of the disease.

Interestingly, a rare genetic form of cerebellar ataxia results from mutations in the gene encoding transglutaminase 6 (TGM6). Importantly, a gluten-free diet reduces antibody levels over time, causing resolution of gastrointestinal symptoms and improvement of neurological symptoms.

Cerebellar Ataxias Resulting from Ion-Channel Antibodies

Patients with elevated levels of antibodies directed against calcium channels (P/Q- and N-type) and voltage-gated potassium channels may present with cerebellar ataxia. P/Q-type calcium channels are highly expressed in cerebellar Purkinje neurons, and, in mouse models, reduced channel activity is associated with ataxia. In humans, mutations in CACNA1A, the gene encoding the P/Q calcium channel, results in episodic spinocerebellar ataxia type 2 (SCA2) and spinocerebellar ataxia type 6 (SCA6).

CHECKPOINT

29. What are the clinical features of cerebellar ataxia? 30. Which class of disease genes or proteins is responsible for the most

common dominantly inherited ataxias? 31. What mechanisms may play a role in degeneration in the polyglutamine

spinocerebellar ataxias (SCAs)?

MOTOR NEURON DISEASE

Clinical Presentation Motor neuron diseases predominantly affect the anterior horn cells of the spinal cord and are characterized by wasting and skeletal muscle weakness. Spontaneous discharges of degenerating motor nerve fibers occur, giving rise to muscle twitches known as fasciculations (see prior discussion). Electromyography characteristically shows features of denervation, including increased numbers of spontaneous discharges (fibrillations) in resting muscle and a reduction in the number of motor units detected during voluntary contraction. Sprouting of remaining healthy motor fibers may occur, leading to the appearance of large, polyphasic motor unit potentials (reinnervation).

The spinal muscular atrophies (SMAs) are a heterogeneous group of genetic diseases characterized by the selective degeneration of lower motor neurons. The most common form is autosomal recessive with childhood onset and has a frequency of between 1:6000 and 1:10,000. Childhood SMA has been divided into three types depending on age of onset and clinical progression. SMA I is infantile spinal muscular atrophy (Werdnig–Hoffmann disease), a disorder that usually manifests within the first 3 months of life. Infants with this condition have difficulty sucking, swallowing, and breathing. Atrophy and fasciculations are found in the tongue and limb muscles. SMA I is rapidly progressive, leading to death from respiratory complications usually by the age of 3 years. SMA II begins in the latter half of the first year of life. It progresses more slowly than the infantile form, and patients may survive into adulthood. SMA III (Kugelberg–Welander disease) is a juvenile form that develops after age 2. Patients develop weakness of the proximal limb muscles with relative sparing of bulbar muscles. The pattern of weakness can falsely suggest a myopathy such as limb-girdle dystrophy rather than a motor neuron disease. The course is gradually progressive, leading to disability in adulthood. All three forms of SMA result from deletions or mutations in the survival motor neuron 1 (SMN1) gene on chromosome 5q13. The SMN gene product is expressed in all tissues and appears to be involved in RNA metabolism. Loss of SMN function promotes apoptosis of lower motor neurons. It is not yet known why motor neurons are selectively affected. Some clinical trials have looked at adjusting the levels of the SMN protein in an attempt to modulate disease progression using drugs such as hydroxyurea and valproic acid, but unfortunately these studies failed to show any improvement in the disease. Recent focus has turned to antisense oligonucleotides and stem cell therapies to attempt to slow disease progression.

In adults, motor neuron disease usually begins between the ages of 20 and 80 years, with an average age at onset of 56 years. It is commonly sporadic but is familial in up to 10% of cases. Several varieties have been described, depending on the relative involvement of upper or lower motor neurons and bulbar or spinal anterior horn cells. For example, X-linked spinobulbar atrophy is an X-linked recessive disorder that typically manifests clinically in the fourth or fifth decade and is associated with an expanded CAG repeat in the androgen receptor gene. As with other genetic disorders associated with triplet repeat expansions, the neurodegeneration is associated with neuronal inclusions. Testosterone promotes the development of inclusions, and women homozygous for the mutation develop only mild symptoms. Moreover, female mice carrying the mutation show motor impairment after testosterone administration, whereas castration

reduces impairment in male mice. These findings led to testing the use of gonadotropin-releasing hormone antagonists, which reduce testosterone release from the testes, as treatments for the disease. Unfortunately, the treatments did not improve function and resulted in significantly reduced quality of life secondary to the low testosterone. Current work is focusing on RNAi targeting of the polyQ-AR transcript to reduce the expression and toxicity of the expanded repeat.

The most common form of motor neuron disease in adults is amyotrophic lateral sclerosis (ALS), in which mixed upper and lower motor neuron deficits are found in limb and bulbar muscles. In 80% of patients, the initial symptoms result from weakness of limb muscles. Complaints are often bilateral but asymmetric. Involvement of bulbar muscles causes difficulty with swallowing, chewing, speaking, breathing, and coughing. Neurologic examination reveals a mixture of upper and lower motor neuron signs. There is usually no involvement of extraocular muscles or sphincters. The disease is progressive and generally fatal within 3–5 years, with death usually resulting from pulmonary infection and respiratory failure.

Pathology & Pathogenesis In ALS, there is selective degeneration of motor neurons in the primary motor cortex and the anterolateral horns of the spinal cord. Many affected neurons show cytoskeletal disease with accumulations of intermediate filaments in the cell body and axons. There is only a subtle glial cell response and little evidence of inflammation. The cause is unknown, but biochemical and genetic studies have provided several clues.

A. Glutamate Signaling and RNA Processing

Glutamate (Figure 7–28) is the most abundant excitatory neurotransmitter in the CNS. Glutamate activates a large family of receptors that either open cation channels (ionotropic receptors) or activate phospholipase C (metabotropic receptors), which catalyzes the formation of the second messenger inositol-1,4,5- trisphosphate (IP3). Influx of Na

+ and Ca2+ through glutamate-gated cation channels depolarizes cells, whereas IP3 stimulates the release of Ca

2+ from intracellular storage sites. The net effect of these events is to generate an excitatory postsynaptic potential and raise the concentration of free intracellular Ca2+ in the cytosol of the postsynaptic neuron. This Ca2+ signal activates calcium-sensitive enzymes and is quickly terminated by the removal of

glutamate from the synapse and by mechanisms for calcium sequestration and extrusion in the postsynaptic cell. A breakdown of normal mechanisms for terminating the excitatory signal leads to sustained elevations of intracellular Ca2+, which cause cell death.

FIGURE 7–28 Glutamatergic neurotransmission. Depolarization stimulates release of glutamate from presynaptic terminals into the synaptic cleft, where it binds to ionotropic or metabotropic glutamate receptors, stimulating Ca2+ influx and activation of phospholipase C (PLC). PLC catalyzes hydrolysis of phosphatidylinositol-4,5-bisphosphate (PIP2) to produce inositol-1,4,5-trisphosphate (IP3), which causes

release of Ca2+ from storage sites in smooth endoplasmic reticulum (SER). Synaptic actions of glutamate are terminated mainly by uptake through Na+-dependent glutamate transporters (GT) on glia. In astrocytes, glutamate is converted into glutamine by glutamine synthetase.

Glutamate is removed from synapses by transport proteins on surrounding astrocytes and nerve terminals. In astrocytes, it is metabolized to glutamine and can be shuttled back to neurons for reconversion into glutamate. In 60% of patients with sporadic ALS, there is a large decrease in glutamate transport activity in the motor cortex and spinal cord but not in other regions of the CNS.

This has been associated with a loss of the astrocytic glutamate transporter protein excitatory amino acid transporter 2 (EAAT2), perhaps resulting from a defect in splicing of its messenger RNA. In cultured spinal cord slices, the pharmacologic inhibition of glutamate transport induces motor neuron degeneration. Thus, selective loss of a glutamate transporter may cause excitotoxicity in ALS by increasing extracellular levels of glutamate.

A second alteration in glutamate signaling has been found recently in spinal motor neurons from five patients with ALS. RNA editing is a process whereby gene-specified codons are altered by RNA-dependent deaminases. In GluR2 receptor subunits, this process is virtually 100% efficient, resulting in conversion of glutamine to arginine in the second transmembrane domain of this subunit, which markedly reduces the calcium permeability of a major subclass of glutamate receptors. Editing efficiency was reduced in more than 50% of neurons from the patients with ALS. Because transgenic mice that express GluR2 made artificially more permeable to calcium develop a motor neuron disease late in life, it is possible that defective editing of GluR2 contributes to ALS pathogenesis. These findings suggest that sporadic ALS may be caused by a defect in RNA metabolism.

B. Free Radicals

About 10% of ALS cases are familial, and 20% of these familial cases result from missense mutations in the cytosolic copper-zinc superoxide dismutase (SOD1) gene on the long arm of chromosome 21. SOD1 catalyzes the formation of hydrogen peroxide from superoxide anions. Hydrogen peroxide is then detoxified by catalase or glutathione peroxidase to form water. Not all mutations reduce SOD1 activity, and the disorder is typically inherited as an autosomal dominant trait, suggesting that familial ALS results from a gain, rather than loss, of function. This is supported by the finding that transgenic mice expressing mutant SOD1 develop motor neuron disease analogous to human familial ALS, whereas mice lacking SOD1 do not develop motor neuron disease. One hypothesis suggests that the mutant enzyme has an altered substrate specificity catalyzing the reduction of hydrogen peroxide to yield hydroxyl radicals and using peroxynitrite to produce nitration of tyrosine residues in proteins. This is consistent with the findings of elevated levels of carbonyl proteins in the brain and elevated levels of free nitrotyrosine in the spinal cord of ALS patients. EAAT2 may also be inactivated by mutant SOD1, thereby promoting excitotoxicity. Some mutations also promote the formation of SOD aggregates, which may be neurotoxic.

C. Cytoskeletal Proteins

Motor neurons tend to be very large, with extremely long axons, and cytoskeletal proteins that maintain axonal structure may be critical targets for motor neuron injury. A role for neurofilament dysfunction in ALS is supported by the finding that neurofilamentous inclusions in cell bodies and proximal axons are an early feature of ALS pathology. In addition, mutations in the heavy chain neurofilament subunit (NF-H) have been detected in some patients with sporadic ALS, suggesting that NF-H variants may be a risk factor for ALS. Peripherin is another intermediate filament protein found with neurofilaments in neuronal inclusions in ALS and in mice with SOD1 mutations. Peripherin expression is increased in response to cell injury, and overexpression of peripherin causes a late-onset motor neuron disease in mice. Inclusions containing peripherin and neurofilaments may interfere with axonal transport, resulting in failure to maintain axonal structure and transport of macromolecules such as neurotrophic factors required for motor neuron survival.

D. TDP-43

An exciting discovery of the protein transactive response DNA-binding protein 43 (TDP-43) may offer new clues to the etiology of this disorder. This newly discovered protein is the major component of the ubiquitinated, tau-negative inclusions that are the pathological hallmark of sporadic and familial ALS and frontotemporal dementia (FTD). It is also found in some cases of Alzheimer disease and Parkinson disease. Mutations in this gene, located on chromosome 1, co-segregate with disease in familial forms of ALS and FTD and are not found in SOD1 familial ALS. FTD and ALS overlap in approximately 15–25% of cases, and these disorders are starting to be referred to as “TDP-43 proteinopathies.” Several other genes and gene regions have been identified to cause both FTD and ALS, including TARDBP on chromosome 1p36.2, MAPT on chromosome 7q21, and DCTN1 on chromosome 2p13.

E. C9ORF72

The major genetic cause of ALS and/or FTD was recently discovered. Two independent research teams have identified hexanucleotide repeats in an intron of C9ORF72 on chromosome 9 in 34% of familial ALS cases, 6% of sporadic ALS cases, 26% of familial FTD cases, and 5% of sporadic FTD cases. The protein is of unknown function. These mutations likely induce a gain-of-function mutation similar to other noncoding repeat expansion disorders. This discovery

of another disorder caused by nucleotide repeats may provide an additional rationale for a new drug development paradigm focused on decreasing expression of these toxic repeats.

CHECKPOINT

32. What are the clinical features of motor neuron disease? 33. Which gene is responsible for some cases of familial ALS, and what is a

postulated molecular mechanism by which the mutation causes disease? 34. What two other mechanisms may play a role in motor neuron

degeneration?

PARKINSON DISEASE

Clinical Presentation Parkinsonism is a clinical syndrome of rigidity, bradykinesia, tremor, and postural instability. Most cases are due to Parkinson disease, an idiopathic disorder with a prevalence of about 1–2 per 1000. In the first half of the last century, parkinsonism was a common sequela of von Economo encephalitis. Parkinsonism can also result from exposure to certain toxins such as manganese, carbon disulfide, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and carbon monoxide. Several drugs, particularly butyrophenones, phenothiazines, metoclopramide, reserpine, and tetrabenazine, can cause reversible parkinsonism. Parkinsonism may also result from repeated head trauma or may be a feature of several basal ganglia diseases, including Wilson disease, some cases of early-onset Huntington disease, multiple system atrophy (MSA), and progressive supranuclear palsy. In these disorders, other symptoms and signs are present along with parkinsonism.

Pathology & Pathogenesis In Parkinson disease, there is selective degeneration of monoamine-containing cell populations in the brainstem and basal ganglia, particularly of pigmented dopaminergic neurons of the substantia nigra. In addition, scattered neurons in the basal ganglia, brainstem, spinal cord, and sympathetic ganglia contain

eosinophilic, cytoplasmic inclusion bodies (Lewy bodies). These contain filamentous aggregates of α-synuclein, along with parkin, synphilin, neurofilaments, and synaptic vesicle proteins.

Important clues about the pathogenesis of Parkinson disease have been discovered through study of the potent neurotoxin MPTP. MPTP is a byproduct of synthesis of a synthetic opioid derivative of meperidine. Illicit use of opioid preparations heavily contaminated with MPTP led to several cases of parkinsonism in the early 1980s. MPTP selectively injures dopaminergic neurons in the brain and produces a clinical syndrome very similar to Parkinson disease.

MPTP enters the brain (Figure 7–29) and is converted by monoamine oxidase B present in glia and serotonergic nerve terminals to N-methyl-4- phenyldihydropyridine (MPDP+), which diffuses across glial membranes and then undergoes nonenzymatic oxidation and reduction to the active metabolite N-methyl-4-phenylpyridinium (MPP+). Plasma membrane transporters that normally act to terminate the action of monoamines by removing them from synapses take up MPP+. Internalized MPP+ inhibits oxidative phosphorylation by interacting with complex I of the mitochondrial electron transport chain. This inhibits ATP production and reduces metabolism of molecular oxygen, allowing for increased formation of peroxide, hydroxyl radicals, and superoxide radicals that react with lipids, proteins, and nucleic acids that cause cell injury. In support of a role for mitochondrial dysfunction and oxidative damage in the pathogenesis of Parkinson disease is evidence that the insecticide rotenone, which inhibits mitochondrial complex I, produces parkinsonism in animals with degeneration of nigrostriatal dopaminergic neurons and cytoplasmic inclusions that resemble Lewy bodies. Exposure to paraquat, a common herbicide structurally similar to MPP+ that also inhibits complex I, can lead to selective degeneration of dopaminergic neurons and aggregation of α-synuclein. Furthermore, impaired complex I activity has been observed in cell lines derived from Parkinson disease patients, and one genetic variant of NADH dehydrogenase 3 in complex I is associated with a reduced risk of the disease among Caucasians. Thus, alterations in mitochondrial complex I activity appear to play an important role in the pathogenesis of Parkinson disease.

FIGURE 7–29 Proposed mechanism of MPTP-induced parkinsonism. MPTP enters brain astrocytes and is converted to MPDP+ through the action of monoamine oxidase type B (MAO-B). MPDP+ is then metabolized extracellularly to MPP+, which is taken up through dopamine uptake sites on dopamine nerve terminals and concentrated in mitochondria. The resulting disturbance of mitochondrial function can lead to neuronal death. (Redrawn, with permission, from Greenberg DA et al, eds. Clinical Neurology, 5th ed. McGraw-Hill, 2002.)

The reasons dopaminergic neurons appear selectively vulnerable to complex I inhibition are unclear. Although controversial, some evidence suggests that dopamine can promote neurotoxicity. Addition of exogenous dopamine is toxic to neurons in culture. Dopamine undergoes auto-oxidation to generate superoxide radicals or is metabolized by monoamine oxidase to generate hydrogen peroxide. Superoxide dismutase catalyzes the conversion of superoxide to H2O2, which is converted by glutathione peroxidase and catalase to water. However, H2O2 can also react with ferrous iron to form highly reactive hydroxyl radicals. Thus, dopamine within dopaminergic neurons may provide a source of reactive oxygen species, which, when coupled with reduced complex I function, may promote cell death.

Approximately 5% of Parkinson disease cases are familial. Genetic studies that have identified causative mutations in five genes provide important information about molecular pathways involved in the disease. These genes include the genes for α-synuclein (PARK1), parkin (PARK2), DJ-1 (PARK7), ubiquitin-C-hydrolase-L1 (PARK5), PTEN (phosphatase and tensin homolog deleted on chromosome 10)–induced kinase 1 (PINK1), and leucine-rich repeat kinase 2 (LRRK2).

Mutations in the gene for α-synuclein on chromosome 4q21-23 cause autosomal dominant Parkinson disease. Alpha-synuclein is found in nerve terminals in close proximity to synaptic vesicles. Its normal function is not known. Overexpression of nonmutant human α-synuclein in transgenic mice results in the formation of Lewy bodies, reduced dopaminergic terminals in the striatum, and impaired motor performance resulting from the formation of

abnormal complexes at the synapse with SNARE proteins. Genomic triplication of α-synuclein leading to overexpression has been documented in a human family with autosomal dominant Parkinson disease. This suggests that it is the production of neuronal inclusions containing α-synuclein, rather than a change in α-synuclein function, that contributes to the degeneration of dopaminergic neurons. Interestingly, mice lacking α-synuclein are resistant to the toxic effects of the complex I inhibitor MPTP, suggesting that mitochondrial dysfunction generates an environment that favors α-synuclein aggregation and neurodegeneration.

Misfolded, damaged, or unassembled proteins are generally degraded by a process involving covalent attachment of ubiquitin. Ubiquitin is a 76-residue protein that marks proteins for processing by a proteolytic complex (proteasome). A missense mutation in one component of the ubiquitin– proteasome system, ubiquitin carboxyl terminal hydrolase L1, has been found in one family with autosomal dominant Parkinson disease. Mutations in parkin on chromosome 6q25 have been identified in cases of autosomal recessive juvenile parkinsonism. Parkin is an E3 ubiquitin ligase that catalyzes the addition of ubiquitin to specific proteins to target them for degradation. Known parkin mutations cause loss of function, which presumably leads to a disturbance in protein degradation. However, most patients with parkin mutations lack Lewy bodies, suggesting that other mechanisms, such as increased oxidative stress, cause neurodegeneration in these patients. In support of this mechanism is the finding that Drosophila mutants that lack parkin show mitochondrial pathology.

The most common known genetic form of Parkinson disease was recently discovered. Mutations in the glucocerebrosidase (GCase) enzyme account for 3% of sporadic Parkinson disease cases and 25% of juvenile-onset Parkinson disease cases. This enzyme is involved in lysosomal processing. Enzyme activity is reduced by 58% in the substantia nigra of heterozygous patients and by 33% in patients with sporadic Parkinson disease. Inhibiting this enzyme leads to the accumulation of α-synuclein, which leads to further inhibition of this enzyme.

CHECKPOINT

35. What are the clinical features of parkinsonism? 36. What are some of the causes of this syndrome? 37. What are two major mechanisms proposed to explain the

pathophysiology of Parkinson disease?

MYASTHENIA GRAVIS

Clinical Presentation Myasthenia gravis is an autoimmune disorder of neuromuscular transmission. The major clinical features are fluctuating fatigue and weakness that improve after a period of rest and after administration of acetylcholinesterase inhibitors. Muscles with small motor units, such as ocular muscles, are most often affected. Oropharyngeal muscles, flexors and extensors of the neck, proximal limb muscles, and the erector spinae muscles are involved less often. In severe cases, all muscles are weak, including the diaphragm and intercostal muscles, and death may result from respiratory failure.

About 5% of patients have coexistent hyperthyroidism. Rheumatoid arthritis, systemic lupus erythematosus, and polymyositis are also more common in patients with myasthenia gravis than in the general population, and up to 30% of patients have a maternal relative with an autoimmune disorder. These associations suggest that patients with myasthenia gravis share a genetic predisposition to autoimmune disease.

Pathology & Pathogenesis The major structural abnormality in myasthenia gravis is a simplification of the postsynaptic region of the neuromuscular synapse. The muscle end plate shows sparse, shallow, and abnormally wide or absent synaptic clefts. In contrast, the number and size of the presynaptic vesicles are normal. Scattered collections of lymphocytes, some within the vicinity of motor end plates, may be present. IgG and the C3 component of complement are present at the postsynaptic membrane.

Electrophysiologic studies indicate that the postsynaptic membrane has a decreased response to applied acetylcholine. Studies with iodine-125–labeled α- bungarotoxin, which binds with high affinity to muscle nicotinic AChRs, show a 70–90% decrease in the number of receptors per end plate in affected muscles. Circulating antibodies to the receptor are present in 90% of patients, and the disorder may be passively transferred to animals by administering IgG from affected patients. Moreover, immunization with AChR protein from muscle can produce myasthenia in experimental animals. The antibodies block acetylcholine binding and receptor activation (Figure 7–30). In addition, the antibodies cross- link receptor molecules, increasing receptor internalization and degradation.

Bound antibody also activates complement-mediated destruction of the postsynaptic region, resulting in simplification of the end plate. Many patients who lack antibodies to AChR have autoantibodies instead against the muscle- specific receptor tyrosine kinase (MuSK), which is an important mediator of AChR clustering at the end plate. These antibodies inhibit clustering of receptors in muscle cell culture.

FIGURE 7–30 Pathogenesis of myasthenia gravis. Acetylcholine released at the nerve ending by the nerve impulse normally binds with acetylcholine receptors. This evokes the action potential in the muscle. In myasthenia gravis, antiacetylcholine receptor antibody binds to the acetylcholine receptor and inhibits the action of acetylcholine. Bound antibody evokes immune-mediated destruction of the end plate. (Redrawn, with permission, from Chandrasoma P et al, eds. Concise Pathology, 3rd ed. Originally published by Appleton & Lange. Copyright © 1998 by The McGraw-Hill Companies, Inc.)

During repetitive stimulation of a motor nerve, the number of quanta released from the nerve terminal declines with successive stimuli. Normally, this causes no clinical impairment because a sufficient number of AChR channels are opened by the reduced level of neurotransmitter. However, in myasthenia gravis, in which there is a deficiency in the number of functional receptors, neuromuscular transmission fails at lower levels of quantal release. Electrophysiologically, this is measured as a decremental decline in the compound muscle action potential during repetitive stimulation of a motor

nerve. Clinically, this is manifested by muscle fatigue with sustained or repeated activity.

Treatment has reduced the mortality rate from approximately 30% to 5% in generalized myasthenia gravis. The two basic strategies for treatment that stem from knowledge of the pathogenesis are to increase the amount of acetylcholine at the neuromuscular junction and to inhibit immune-mediated destruction of AChRs.

By preventing acetylcholine metabolism, cholinesterase inhibitors can compensate for the normal decline in released neurotransmitter during repeated stimulation. Therapy with cholinesterase inhibitors can also cause a paradoxical increase in weakness known as a cholinergic crisis, which results from an excess of acetylcholine. At the molecular level, acetylcholine binding first opens nicotinic cation channels, but with continued exposure to the agonist, the channels desensitize and shut down again. The desensitized channels recover their sensitivity to acetylcholine only after the neurotransmitter is removed. Removal of acetylcholine is impaired when cholinesterase activity is inhibited. This can result in a depolarization block of neurotransmission similar to the effect of the depolarizing paralytic agent succinylcholine or organophosphate insecticides and nerve gases that markedly inhibit acetylcholinesterase. Therefore, the dose of cholinesterase inhibitors must be carefully regulated to reduce myasthenia but avoid a cholinergic crisis.

Plasmapheresis, corticosteroids, and immunosuppressant drugs are effective in reducing levels of autoantibody to AChRs and suppressing disease. The thymus is thought to play an important role in the pathogenesis of the disease by supplying helper T cells sensitized against thymic proteins that cross-react with AChRs. In most patients with myasthenia gravis, the thymus is hyperplastic, and 10–15% have thymomas. Thymectomy is indicated if a thymoma is suspected. In patients with generalized myasthenia without thymoma, thymectomy induces remission in 35% and improves symptoms in another 45% of patients.

For patients with AChR antibody–negative myasthenia gravis who test positive for the MuSK antibody, the clinical features and treatment are different. Patients tend to be younger women with bulbar weakness, and muscle atrophy is often seen, particularly in the tongue, making it difficult to distinguish from motor neuron disease. Results of repetitive stimulation studies and single-fiber electromyography (EMG) studies in the limbs are often normal, necessitating facial studies to make a diagnosis. Cholinesterase inhibitors often make these patients worse, but plasma exchange is very effective, as is less conventional immunosuppressive therapy. Thymectomy is not clearly beneficial in this

population. Lastly, there are myasthenia gravis patients with no antibodies for either

AChR antibodies or MuSK, referred to as double sero-negative patients. Recently, a new antibody has been found in 50% of these patients. Antibodies to lipoprotein-related protein 4 (LRP4), which is the agrin-binding receptor of the MuSK complex, disrupt agrin-induced AChR clustering, causing the disease symptoms. The clinical presentation of these patients is similar to that of those with AChR–myasthenia gravis without thymoma.

CHECKPOINT

38. What is the clinical presentation of myasthenia gravis? 39. What causes this disorder? 40. What is the pathophysiology of symptoms in myasthe-nia gravis?

EPILEPSY

Clinical Presentation Seizures are paroxysmal disturbances in cerebral function caused by an abnormal synchronous discharge of cortical neurons. The epilepsies are a group of disorders characterized by recurrent seizures. Approximately 0.6% of people in the United States suffer from recurrent seizures, and idiopathic epilepsy accounts for more than 75% of all seizure disorders. In some forms of idiopathic epilepsy, a genetic basis is apparent. Other forms are secondary to brain injury from stroke, trauma, a mass lesion, or infection. About two-thirds of new cases arise in children, and most of these cases are idiopathic or caused by trauma. In contrast, seizures or epilepsy with onset in adult life is more often due to underlying brain lesions or metabolic causes.

Seizures are classified by behavioral and electrophysiologic data (Table 7–2). Generalized tonic-clonic seizures are attacks characterized by a sudden loss of consciousness followed rapidly by tonic muscle contraction, causing limb extension and arching of the back. The tonic phase lasts 10–30 seconds and is followed by a clonic phase of limb jerking. The jerking builds in frequency to a peak after 15–30 seconds and then slows gradually over another 15–30 seconds.

Thereafter, the patient remains unconscious for several minutes. As consciousness is regained, there is a period of postictal confusion lasting several minutes. In patients with recurrent seizures or an underlying structural or metabolic abnormality, confusion may persist for a few hours. Focal abnormalities may be present on neurologic examination during the postictal period. Such findings suggest a focal brain lesion requiring further laboratory and radiologic study.

TABLE 7–2 Simplified classification of seizures.

Typical absence seizures begin in childhood and usually remit by adulthood. Seizures are characterized by brief lapses in consciousness lasting several seconds without loss of posture. These spells may be associated with eyelid blinking, slight head movement, or brief jerks of limb muscles. Immediately after the seizure, the patient is fully alert. The spells may occur several times throughout the day and impair school performance. The electroencephalogram (EEG) shows characteristic runs of spikes and waves at a rate of three per second, particularly after hyperventilation (Figure 7–31). The disorder is transmitted as an autosomal dominant trait with incomplete penetrance.

FIGURE 7–31 EEG of a patient with typical absence (petit mal) seizures, showing a burst of generalized 3 Hz spike-and-wave activity (center of record) that is bilaterally symmetric and bisynchronous. Odd-numbered leads indicate electrode placements over the left side of the head; even numbers, those over the right side. (Redrawn, with permission, from Aminoff MJ et al, eds. Clinical Neurology, 9th ed. McGraw-Hill, 2015.)

Some forms of epilepsy cause seizures with only a tonic or clonic phase. In others, the seizure is manifested by sudden loss of muscle tone (atonic seizures). In myoclonic epilepsy, sudden, brief contractions of muscles occur. Myoclonic seizures are found in certain neurodegenerative diseases or after diffuse brain injury, as occurs during global cerebral ischemia.

Focal seizures are caused by focal brain disease. Therefore, in general, patients with simple or focal dyscognitive seizures should be investigated for underlying brain lesions. Simple focal seizures begin with motor, sensory, visual, psychic, or autonomic phenomena depending on the location of the seizure focus. Consciousness is preserved unless the seizure discharge spreads to other areas, producing a tonic-clonic seizure (secondary generalization). Focal dyscognitive seizures are characterized by the sudden onset of impaired consciousness with stereotyped, coordinated, involuntary movements (automatisms). Immediately before impairment of consciousness, there may be an aura consisting of unusual abdominal sensations, olfactory or sensory hallucinations, unexplained fear, or illusions of familiarity (déjà vu). Seizures usually last for 2–5 minutes and are followed by postictal confusion. Secondary generalization may occur. The seizure focus usually lies in the temporal or frontal lobe.

Pathogenesis Normal neuronal activity occurs in a nonsynchronized manner, with groups of neurons inhibited and excited sequentially during the transfer of information

between different brain areas. Seizures occur when neurons are activated synchronously. The kind of seizure depends on the location of the abnormal activity and the pattern of spread to different parts of the brain.

Interictal spike discharges are often observed on EEG recordings from patients with epilepsy. These are due to the synchronous depolarization of a group of neurons in an abnormally excitable area of brain. Experimentally, this is known as the paroxysmal depolarizing shift and is followed by a hyperpolarizing afterpotential that is the cellular correlate of the slow wave that follows spike discharges on the EEG. The shift is produced by depolarizing currents generated at excitatory synapses and by the subsequent influx of sodium or calcium through voltage-gated channels.

Normally, discharging excitatory neurons activate nearby inhibitory interneurons that suppress the activity of the discharging cell and its neighbors. Most inhibitory synapses use the neurotransmitter GABA. Voltage-gated and calcium-dependent potassium currents are also activated in the discharging neuron to suppress excitability. In addition, adenosine generated from adenosine triphosphate (ATP) released during excitation further suppresses neuronal excitation by binding to adenosine receptors present on nearby neurons. Disruption of these inhibitory mechanisms by alterations in ion channels, or by injury to inhibitory neurons and synapses, may allow for the development of a seizure focus. In addition, groups of neurons may become synchronized if local excitatory circuits are enhanced by the reorganization of neural networks after brain injury.

Spread of a local discharge occurs by a combination of mechanisms. During the paroxysmal depolarizing shift, extracellular potassium accumulates, depolarizing nearby neurons. Increased frequency of discharges enhances calcium influx into nerve terminals, increasing neurotransmitter release at excitatory synapses by a process known as post-tetanic potentiation. This involves increased calcium influx through voltage-gated channels and through the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor–gated ion channels. NMDA receptor–gated channels preferentially pass calcium ions but are relatively quiescent during normal synaptic transmission because they are blocked by magnesium ions. Magnesium block is relieved by depolarization. In contrast, the effect of inhibitory synaptic neurotransmission appears to decrease with high-frequency stimulation. This may be partly due to rapid desensitization of GABA receptors at high concentrations of released GABA. The net effect of these changes is to recruit neighboring neurons into a synchronous discharge and cause a seizure.

In secondary epilepsy, loss of inhibitory circuits and sprouting of fibers from excitatory neurons appear to be important for the generation of a seizure focus. In several of the idiopathic epilepsies, genetic studies have identified mutations in ion channels. In severe myoclonic epilepsy of infancy, an early-onset form of epilepsy, mutations are most commonly found in SCN1A, an Na+ channel subunit. Benign familial neonatal convulsions have been linked to mutations in two homologous voltage-gated K+ channels: KCNQ2 encoded by a gene on chromosome 20q13.3 and KCNQ3 encoded by a gene on chromosome 8q24. Generalized epilepsy associated with febrile seizures has been linked to mutations in SCN1A and SCN1B, both of which are voltage-gated Na+ channel subunits, and GABRG2, a GABAA receptor subunit gene. Another rare condition, autosomal dominant nocturnal frontal lobe epilepsy, is associated with mutations on chromosome 20q13.2 in the gene for the α4 subunit of neuronal nicotinic cholinergic receptors. Lastly, a genome-wide association study in idiopathic generalized epilepsy revealed the first common genetic risk variants. These variants were found in genes with both known and largely unknown pathways, such as CHRM3, VRK2, ZEB2. Mutations of PNPO result in pyridoxamine 5'-phosphate oxidase (PNPO) deficiency, causing a form of neonatal epileptic encephalopathy. And, as noted above, mutations of SCN1A are present in severe myoclonic epilepsy of infancy.

Animal models have provided clues to the pathogenesis of absence seizures. Absence seizures arise from synchronous thalamic discharges mediated by activation of low-threshold calcium currents (T or “transient” currents) in thalamic neurons. The anticonvulsant ethosuximide blocks T channels and suppresses absence seizures in humans. T channels are more likely to be activated after hyperpolarization of the cell membrane. Activation of GABAB receptors hyperpolarizes thalamic neurons and facilitates T-channel activation. Lethargic (lh/lh) mice demonstrate frequent absence spells accompanied by 5 to 6 Hz spike-wave discharges on the EEG and respond to drugs used in human absence epilepsy. A single mutation in a gene on chromosome 2 results in this autosomal recessive disorder. There is an increase in the number of GABAB receptors in the cerebral cortex in mice with this disorder, and the GABAB agonist baclofen worsens the seizures, whereas antagonists alleviate them. This suggests that abnormal regulation of GABAB receptor function or expression may be important in the pathogenesis of absence seizures. This is supported by the finding that γ-hydroxybutyrate, which causes behavioral and electroencephalographic alterations similar to those seen during absence attacks,

activates GABAB receptors and that GABAB agonists increase and GABAB antagonists reduce spike-wave discharges in rats genetically susceptible to absence seizures (GAERS rats).

The main targets for currently available anticonvulsants are (1) voltage-gated sodium and calcium channels that are involved in the generation of action potentials and neurotransmitter release; and (2) ligand-gated channels that modulate synaptic excitation and inhibition. Many agents act by more than one mechanism. Several anticonvulsants and some of their presumed mechanisms of action are listed in Table 7–3.

TABLE 7–3 Known mechanisms of action of some anticonvulsant drugs.

CHECKPOINT

41. What is the clinical presentation of the major types of seizures? 42. What are some disorders that lead to secondary epi-lepsy, and what

changes in brain structure lead to sec-ondary epilepsy? 43. What kinds of mutations have been associated with idi-opathic

epilepsies?

DEMENTIA & ALZHEIMER DISEASE

1. Clinical Features of Dementia Dementia is an acquired decline in intellectual function resulting in loss of social independence. Patients experience impairment of memory and at least one other area of cortical function, such as language, calculation, spatial orientation, decision making, judgment, or abstract reasoning. In contrast to patients with confusional states, symptoms progress over months to years, and alertness is preserved until the very late stages of disease. Dementia affects 5–20% of persons over age 65, and, although not part of normal aging, its incidence increases with age. The most common causes, listed in Table 7–4, account for almost 90% of cases. Treatable causes are important to recognize and include hypothyroidism, vitamin B12 deficiency, neurosyphilis, brain tumor, normal pressure (communicating) hydrocephalus, and chronic subdural hematoma. In addition, although not curable, dementia associated with HIV infection may be slowed by antiretroviral treatment. About 10–15% of patients referred for evaluation of dementia suffer from depression (“pseudodementia”), which may also respond to treatment.

TABLE 7–4 Major causes of dementia.

Cerebrovascular disease is the second most common cause of dementia (after Alzheimer disease). In such cases, dementia results from either multiple infarctions in the territory of major cerebral vessels (multi-infarct dementia) or from subcortical infarctions in the distributions of deep penetrating arterioles (lacunar state, Binswanger disease, subcortical arteriosclerotic encephalopathy). There is usually a history of stepwise progression of neurologic deficits, focal signs on neurologic examination, and multiple infarctions on brain imaging studies. Patients generally have a history of hypertension or other risk factors for atherosclerosis.

Chronic drug intoxication is often listed as a cause of dementia but actually produces a confusional state. The existence of alcohol-induced dementia is controversial. Although animal and cell culture studies provide evidence for a direct neurotoxic effect of alcohol, dementia in patients with alcoholism also results from associated nutritional deficiency, recurrent head trauma, and (rarely) acquired hepatocerebral degeneration, a complication of chronic hepatic insufficiency caused by alcoholic cirrhosis.

2. Alzheimer Disease Clinical Features Alzheimer disease is the most common cause of dementia and accounts for more than 50% of cases. It is a slowly progressive disorder that runs a course of 5–10 years, typically beginning with impairment of learning and recent memory. Anomia, aphasia, and acalculia eventually develop, causing loss of employability and inability to manage finances. Spatial disorientation causes patients to become lost easily, and apraxias lead to difficulty with cooking, cleaning, and self-care. A frontal lobe gait disorder may appear, with short, shuffling steps, flexed posture, difficulty turning, and a tendency to fall backward (retropulsion) similar to that seen in Parkinson disease. In later stages, social graces are lost, and psychiatric symptoms such as paranoia, hallucinations, and delusions may appear. Cholinesterase treatments such as donepezil, rivastigmine, and galantamine may help for a couple of years to improve memory, but eventually the neuronal degeneration progresses and these medications are no longer effective. Terminally ill patients are bedridden, mute, and incontinent.

Pathology

The pathology of Alzheimer disease is characterized by extracellular neuritic plaques in the cerebral cortex and in the walls of meningeal and cerebral blood vessels (Figure 7–32). These plaques contain a dense core of amyloid material surrounded by dystrophic neurites (axons and dendrites), reactive astrocytes, and microglia. Other structural changes include the formation of intraneuronal neurofibrillary tangles, neuronal and synaptic loss, reactive astrocytosis, and microglial proliferation. Controversy exists as to which features are most related to the pathogenesis of the disease. The formation of neuritic plaques is particularly characteristic of Alzheimer disease, but there is little evidence that the course or onset of disease correlates with plaque number. Neurofibrillary

tangles are paired helical filaments composed of a hyperphosphorylated form of the microtubule protein tau. They are not specific for Alzheimer disease and occur in several other neurodegenerative disorders. In general, all pathologic changes are most prominent in the hippocampus, entorhinal cortex, association cortex, and basal forebrain. This accounts for the early symptoms of memory loss and disturbance of higher cortical functions, with preservation of primary sensory and motor function until later in the course of the disease.

FIGURE 7–32 Amyloid plaques in the cerebral cortex in Alzheimer disease.

Pathophysiology

A. Amyloid β-Peptide—The major protein in neuritic plaques is amyloid β- peptide (Aβ), which is proteolytically derived from a membrane protein, the β- amyloid precursor protein (APP) encoded by a gene on chromosome 21q21.3- 22.05. APP interacts with extracellular matrix and supports the growth of neurites in neuronal cultures. Genetic evidence implicates Aβ in the pathogenesis of Alzheimer disease. Almost all patients with trisomy 21 (Down syndrome) develop pathologic changes indistinguishable from those seen in Alzheimer disease, suggesting that having an increased copy of the APP gene increases the metabolism of APP to Aβ. About 10% of cases of Alzheimer

disease are familial, with early onset (before age 65 years) and autosomal dominant inheritance. In approximately 5% of these families, Alzheimer disease is strongly linked to missense mutations immediately flanking the Aβ sequence in the APP gene. Transgenic mice expressing human APP with these mutations show elevated levels of Aβ, behavioral abnormalities, and neuritic plaques. The APP mutations result in either increased production of all forms of Aβ or mainly in the long 42-amino-acid form, Aβ42, which self-aggregates and promotes plaque formation. Aβ is toxic to cultured neurons and stimulates production of cytokines from microglial cells. Aβ also triggers the release of glutamate from glial cells and may injure neurons through excitotoxicity. This evidence links an increased production of Aβ, particularly Aβ42, to Alzheimer disease and suggests that Aβ causes the neurodegeneration. Transgenic mice that express mutant forms of familial human APP develop synaptic dysfunction before plaque deposition, indicating that diffusible forms of Aβ are neurotoxic. This may explain why plaque number and disease severity correlate poorly.

B. Presenilins—The enzymatic pathways that regulate Aβ formation are critical areas of current research that may lead to new treatments. Some clues have come from analysis of additional families with Alzheimer disease. APP is cleaved at the amino terminal of the Aβ sequence by the membrane-anchored protease beta-amyloid precursor protein cleaving enzyme (BACE), also known as beta- secretase. This cleavage generates a 99–amino acid carboxyl terminal fragment. A second enzymatic activity termed γ-secretase cleaves this fragment to yield Aβ. Almost 70% of familial cases of Alzheimer disease have been linked to missense mutations in the gene PS-1/S182, on chromosome 14q24.3, which encodes a seven-trans-membrane protein (presenilin 1). Another 20% of cases have been linked to mutations in another gene, STM2 (presenilin 2), on chromosome 1q31-42. The proteins encoded by these genes are 67% identical in amino acid sequence and presumably have similar functions. Current evidence indicates that the presenilins are subunits of γ-secretase, because mutant mice lacking either presenilin show reduced γ-secretase function, and mutations designed to inhibit the predicted aspartyl protease function of presenilins eliminate γ-secretase activity. Mutant variants of presenilins associated with familial Alzheimer disease increase the production of Aβ42. This suggests that these mutations produce Alzheimer disease by selectively altering γ-secretase activity to favor production of the longer, amyloid-producing form of Aβ. In addition, γ-secretase is important for processing Notch proteins and other substrates critical for neuronal function, and mice deficient in presenilins show

deficiencies in spatial memory and synaptic plasticity. Thus, γ-secretase deficiency may contribute to neurodegeneration in patients with presenilin mutations.

C. Apolipoprotein E—The majority of patients with Alzheimer disease are older than 60 years, and in about 50% of these patients, the e4 isoform of apolipoprotein E (apoE4) has been identified as a risk factor. ApoE is a 34-kDa protein that mediates the binding of lipoproteins to the low-density lipoprotein (LDL) receptor and the LDL receptor-related protein (LRP). It is synthesized and secreted by astrocytes and macrophages and is thought to be important for mobilizing lipids during normal development of the nervous system and during regeneration of peripheral nerves after injury. There are three major isoforms (apoE2, apoE3, and apoE4), which arise from different alleles (e2, e3, and e4) of a single gene on chromosome 19q13.2. The e3 allele is the most common, accounting for about 75% of all alleles, whereas e2 and e4 account for roughly 10% and 15%, respectively. The e4 allele is associated with increased risk and earlier onset of both familial and sporadic late-onset Alzheimer disease. In contrast, inheritance of e2 is associated with decreased risk and later onset. It is important to note that Alzheimer disease develops in the absence of e4 and also that many persons with e4 escape disease. Therefore, genotyping is not currently recommended as a useful genetic test.

The mechanism by which apoE alleles alter disease risk is uncertain. In cultured neurons, apoE3 increases neurite outgrowth in the presence of very low- density lipoproteins, whereas apoE4 inhibits outgrowth. Alzheimer patients homozygous for the e4 allele have larger and denser senile plaques than patients homozygous for the e3 allele. ApoE is found in neuritic plaques, and apoE4 binds Aβ more readily than does apoE3. Therefore, apoE4 may facilitate plaque formation or reduce the clearance of Aβ from brain tissue. In addition, apoE enters neurons and binds the microtubule-associated protein tau, which is the major constituent of neurofibrillary tangles. ApoE3 binds tau much more avidly than apoE4. Binding of apoE3 to tau may prevent the formation of neurofibrillary tangles and support the normal microtubule assembly required for neurite outgrowth.

CHECKPOINT

44. What are the treatable causes of dementia?

45. What are the clinical features of Alzheimer disease? 46. In which proteins are there mutations associated with familial forms of

Alzheimer disease? 47. What is the association between apolipoprotein E and Alzheimer

disease?

STROKE

Clinical Presentation Stroke is a clinical syndrome characterized by the sudden onset of a focal neurologic deficit that persists for at least 24 hours and results from an abnormality of the cerebral circulation. It is the third leading cause of death in the United States. The incidence of stroke increases with age and is higher in men than in women. Significant risk factors include hypertension, hypercholesterolemia, diabetes, smoking, heavy alcohol consumption, and oral contraceptive use. Advances in neuroimaging have had a great impact on treatment and outcomes.

Pathophysiology A. Vascular Supply The focal symptoms and signs that result from stroke correlate with the brain area supplied by the affected blood vessel. There are two major categories of stroke, based on pathogenesis: ischemic and hemorrhagic (Table 7–5). In ischemic stroke, vascular occlusion interrupts blood flow to a specific brain region, producing a fairly characteristic pattern of neurologic deficits resulting from loss of functions controlled by that region. The pattern of deficits resulting from hemorrhage is less predictable because it depends on the location of the bleed and also on factors that affect the function of brain regions distant from the hemorrhage (eg, increased intracranial pressure, brain edema, compression of neighboring brain tissue, rupture of blood into ventricles or subarachnoid space).

TABLE 7–5 Classification of stroke.

B. Ischemic Stroke

Ischemic strokes result from thrombotic or embolic occlusion of cerebral vessels. Neurologic deficits caused by the occlusion of large arteries (Figure 7– 33) result from focal ischemia to the area of brain supplied by the affected vessel (Figure 7–34) and produce recognizable clinical syndromes (Table 7–6). Not all signs are present in every patient because the extent of the deficit depends on the presence of collateral blood flow, individual variations in vascular anatomy, blood pressure, and the exact location of the occlusion. Thrombosis usually involves the internal carotid, middle cerebral, or basilar arteries. Symptoms typically evolve over several minutes and may be preceded by brief episodes of reversible focal deficits known as transient ischemic attacks. Emboli from the heart, aortic arch, or carotid arteries usually occlude the middle cerebral artery because it carries more than 80% of blood flow to the cerebral hemisphere. Emboli that travel in the vertebral and basilar arteries commonly lodge at the

apex of the basilar artery or in one or both posterior cerebral arteries.

FIGURE 7–33 Major cerebral arteries. A: Anterior view. B: Inferior view showing the circle of Willis and principal arteries of the brainstem. (Redrawn, with permission, from Waxman SG. Clinical Neuroanatomy, 28th ed. McGraw-Hill, 2017.)

FIGURE 7–34 Vascular territories of the major cerebral arteries. A: Coronal section through the cerebrum. B: Horizontal section through the cerebrum. C: Vascular supply to the cerebral cortex. (Redrawn, with permission, from Chusid JG. Correlative Neuroanatomy and Functional Neurology, 19th ed. Originally published by Appleton & Lange. Copyright © 1985 by The McGraw-Hill Companies, Inc.)

TABLE 7–6 Vascular territories and clinical features in ischemic stroke.

Ischemic strokes involving occlusion of small arteries occur at select locations, where perfusion depends on small vessels that are end arteries. Most

result from a degenerative change in the vessel, described pathologically as lipohyalinosis, which is caused by chronic hypertension and predisposes to occlusion. The most common vessels involved are the lenticulostriate arteries, which arise from the proximal middle cerebral artery and perfuse the basal ganglia and internal capsule. Also commonly affected are small branches of the basilar and posterior cerebral arteries that penetrate the brainstem and thalamus. Occlusion of these vessels causes small areas of tissue damage known as lacunar infarctions. These typically occur in the putamen, caudate, thalamus, pons, and internal capsule and less commonly in subcortical white matter and the cerebellum. Lacunar infarctions produce several fairly stereotyped clinical syndromes. The two most common are pure motor stroke and pure sensory stroke. In pure motor stroke, the infarction is usually within the internal capsule or pons contralateral to the weak side. In pure sensory stroke, the infarction is usually in the contralateral thalamus.

Several vascular, cardiac, and hematologic disorders can cause focal cerebral ischemia (Table 7–7). The most common is atherosclerosis of the large arteries of the neck and base of the brain (Figure 7–35). Atherosclerosis is thought to arise from injury to vascular endothelial cells by mechanical, biochemical, or inflammatory insults (see Chapter 11). Endothelial injury stimulates the attachment of circulating monocytes and lymphocytes, which migrate into the vessel wall and stimulate proliferation of smooth muscle cells and fibroblasts. This leads to the formation of a fibrous plaque. Damaged endothelial cells also provide a nidus for platelet aggregation and activation. Activated platelets secrete growth factors that encourage further proliferation of smooth muscle and fibroblasts. The plaque may eventually enlarge to occlude the vessel or may rupture, releasing emboli.

TABLE 7–7 Conditions associated with focal cerebral ischemia.

FIGURE 7–35 Sites of predilection (dark red areas) for atherosclerosis in the intracranial arterial circulation. (Redrawn, with permission, from Aminoff MJ et al, eds. Clinical Neurology, 9th ed. McGraw-Hill, 2015.)

C. Hemorrhage Stroke

Epidural and subdural hematomas typically occur as sequelae of head injury. Epidural hematomas arise from damage to an artery, typically the middle meningeal artery, which can be ruptured by a blow to the temporal bone. Blood dissects the dura from the skull and compresses the hemisphere lying below. Initial loss of consciousness from the injury results from concussion and may be transient. Neurologic symptoms return a few hours later as the hematoma exerts a mass effect that may be severe enough to cause brain herniation (see Figure 7– 27). Subdural hematomas usually arise from venous blood that leaks from torn cortical veins bridging the subdural space. These may be ruptured by relatively minor trauma, particularly in the elderly. The blood is under low pressure, and symptoms resulting from mass effect may not appear for several days.

Subarachnoid hemorrhage may occur from head trauma, extension of blood from another compartment into the subarachnoid space, or rupture of an arterial aneurysm. Cerebral dysfunction occurs because of increased intracranial pressure and from poorly understood toxic effects of subarachnoid blood on brain tissue and cerebral vessels. The most common cause of spontaneous

(nontraumatic) subarachnoid hemorrhage is rupture of a berry aneurysm, which is thought to arise from a congenital weakness in the walls of large vessels at the base of the brain. The aneurysms become symptomatic in adulthood, usually after the third decade. Rupture suddenly elevates intracranial pressure, which can interrupt cerebral blood flow and cause a generalized concussive injury. This results in loss of consciousness in about half of patients. With very large hemorrhages, global cerebral ischemia can cause severe brain damage and prolonged coma. Focal ischemia may later result from vasospasm of arteries at or near the site of rupture. Recurrence of hemorrhage within the first few days is a common and often fatal complication.

Intraparenchymal hemorrhage may result from acute elevations in blood pressure or from a variety of disorders that weaken vessels. The resultant hematoma causes a focal neurologic deficit by compressing adjacent structures. In addition, metabolic effects of extravasated blood disturb the function of surrounding brain tissue, and nearby vessels are compressed, causing local ischemia. Chronic hypertension is the most common predisposing factor. In hypertensive patients, small Charcot–Bouchard aneurysms appear in the walls of small penetrating arteries and are thought to be the major sites of rupture. Most vulnerable are the small vessels also involved in lacunar infarction. Hypertensive hemorrhages occur mainly in the basal ganglia, thalamus (Figure 7–36), pons, and cerebellum and less commonly in subcortical white matter. Other causes of intraparenchymal hemorrhage include vascular malformations, which contain abnormally fragile vessels susceptible to rupture at normal arterial pressures, and certain brain tumors, such as glioblastoma multiforme, which induce proliferation of fragile vessels within the tumor. Certain platelet and coagulation disorders may predispose to intracerebral hemorrhage by inhibiting coagulation. Cocaine and amphetamines cause rapid elevation of blood pressure and are common causes of intraparenchymal hemorrhage in young adults. Hemorrhage may be related to spontaneous bleeding from the acute elevation in blood pressure, rupture of an occult vascular abnormality, or drug- induced vasculitis. Cerebral amyloid angiopathy is a disorder that occurs mainly in the elderly and may be associated with Alzheimer disease. Deposition of amyloid weakens the walls of small cortical vessels and causes lobar hemorrhage, often at several sites.

FIGURE 7–36 CT scan in hypertensive intracerebral hemorrhage. Blood is seen as a high-density signal at the site of hemorrhage in the thalamus (left arrow) and in its extension into the third ventricle (top arrow) and the occipital horns of the ipsilateral (bottom arrow) and contralateral (right arrow) lateral ventricles. (Reproduced, with permission, from Greenberg DA et al, eds. Clinical Neurology, 8th ed. McGraw-Hill, 2012.)

D. Excitotoxicity

Most efforts to intervene in stroke have focused on the vasculature. In ischemic stroke, these efforts include restoring circulation through surgical endarterectomy and reducing thrombosis with anticoagulant, antiplatelet, and thrombolytic drugs. A complementary approach is to attempt to reduce the vulnerability of brain tissue to ischemic damage. This is based on observations that CNS glutamate homeostasis is markedly altered during ischemia, leading to increased and toxic levels of extracellular glutamate.

Neurons deep within an ischemic focus die from energy deprivation. However, at the edge of the ischemic region, neurons appear to die because of excessive stimulation of glutamate receptors (Figure 7–37). As noted, glutamate is released at excitatory synapses, and glutamate levels in the extracellular space are normally tightly regulated by sodium-dependent reuptake systems in neurons

and glia. In glia, glutamate is further detoxified by conversion to glutamine via the ATP-dependent enzyme glutamine synthetase. Glutamine is then released by glia and taken up by neurons, where it is repackaged into synaptic vesicles for subsequent release. Ischemia deprives the brain of oxygen and glucose, and the resultant disruption in cellular metabolism depletes neurons and glia of energy reserves required to maintain normal transmembrane ion gradients. This leads to the accumulation of intracellular Na+ and the collapse of the transmembrane Na+ gradient, which in turn inhibits glutamate uptake. Declining energy reserves also reduce conversion of glutamate to glutamine in glia. Both events promote accumulation of extracellular glutamate, which stimulates glutamate receptors on surrounding neurons, causing entry of Ca2+ and Na+. The influx of cations depolarizes these neurons, stimulating additional Ca2+ influx through voltage- gated channels.

FIGURE 7–37 Excitotoxicity in neuronal ischemia. Depletion of energy supplies inhibits Na+-K+ ATPase, leading to accumulation of extracellular K+ and a decline in extracellular Na+. The increase in extracellular K+ depolarizes nerve terminals, causing release of glutamate. The reduction in extracellular Na+ reduces Na+-dependent glutamate uptake, potentiating synaptic effects of released glutamate. This

generates a sustained increase in intracellular Ca2+ in the postsynaptic cell, leading to cell death. The bold, red “X” denotes inhibition of Na+-K+ ATPase (left), glutamate transporters (right), and glutamine synthetase (bottom). Other abbreviations are defined in the legend to Figure 7–28.

Ischemia also disrupts K+ homeostasis, leading to an increase in the concentration of extracellular K+ ([K+]o). Neuronal activity can rapidly increase [K+]o, and one major function of glial cells is to keep [K

+]o at about 3 mmol/L to help neurons maintain their resting membrane potential. Two energy-dependent transporters are particularly important for removal of extracellular K+ by glia: an Na+-K+ ATPase and an anion transporter that cotransports K+ and Na+ with Cl–. In ischemia, these energy-dependent mechanisms fail, and K+ released into the extracellular space can no longer be taken up by glia. This depolarizes neurons because the gradient of K+ across neuronal membranes determines the level of the resting membrane potential. Depolarization activates the release of neurotransmitters, increasing the accumulation of glutamate at excitatory synapses and in the extracellular space.

The net effect of these events is a tremendous influx of Na+ and Ca2+ into neurons through glutamate- and voltage-gated ion channels. The resultant overload in intracellular Ca2+ appears to be especially toxic and may exceed the ability of the neuron to extrude or sequester the cation. This results in sustained activation of a variety of calcium-sensitive enzymes, including proteases, phospholipases, and endonucleases, leading to cell death. In support of an excitotoxic mechanism of cell death in stroke are animal studies that demonstrate a reduction in the size of ischemic lesions after treatment with glutamate receptor antagonists.

CHECKPOINT

48. What are the differences between the clinical presenta-tion of stroke resulting from ischemia and stroke caused by spontaneous hemorrhage?

49. What are the most common causes of stroke? 50. What role does glutamate play in neuronal injury during ischemia?

CASE STUDIES

Yeong Kwok, MD

(See Chapter 25, p. 755–58 for Answers)

CASE 31

A 70-year-old woman is brought to the emergency department after a fall. She lives alone and was found by a neighbor who overheard her screams. Her neighbor helped her get up, but she found that she could not walk and had trouble maintaining her balance. Her neighbor also noted that there were many empty bottles of vodka in her apartment. On examination, the patient is alert and cooperative. She has an unsteady, wide-based gait and a positive Romberg test. She has obvious dysmetria on finger-to-nose and heel-to-shin testing. Her x-rays show osteopenia, and her head magnetic resonance imaging shows mild cerebral and cerebellar atrophy. Her blood alcohol level is 0.08%; other laboratory test results are normal.

Questions

A. What is the likely cause of the patient’s fall and neurologic findings? B. What are the common clinical features of this condition? C. If many members of her family, including individuals who never used

alcohol, had similar symptoms, what neurologic disorder might you suspect?

CASE 32

A 43-year-old right-handed man presents to the clinic with gradual onset of right hand and arm weakness. He had been in good health and an avid golfer until a few weeks ago when he noted that he was having trouble keeping his club steady during his swing. His driving distance had markedly decreased, and he had begun to drop things that he was holding with his right hand. He had no numbness or other sensory symptoms. On

physical examination, he appears well and has normal vital signs. He has mild wasting and fasciculations along his right brachioradialis muscle. His grip strength is 4 out of 5 on the right and 5 out of 5 on the left. He has absent reflexes in his right arm and 1+ reflexes on the left. An electromyelogram shows features of denervation, including increased numbers of spontaneous discharges in resting muscle, and a reduction in the number of motor units detected during voluntary contraction. A diagnosis of amyotrophic lateral sclerosis (ALS) is entertained.

Questions

A. What are the presenting clinical symptoms and progression of the clinical course in ALS?

B. Which cells are affected in ALS? C. What are some possible molecular mechanisms responsible for the

pathologic changes?

CASE 33

A 63-year-old man comes to the clinic with a several-month history of difficulty with his gait and coordination. He finds walking difficult and has almost fallen on a number of occasions, especially when trying to change directions. He has also found that using his hands is difficult, and other people have noticed that his hands shake. Physical examination is notable for a resting tremor in the hands that disappears with intentional movement. He has a shuffling gait with difficulty turning. There is so-called cogwheeling rigidity in his arms, a jerky sensation with passive flexion and extension of the arms.

Questions

A. What is the likely diagnosis? What clinical features make this diagnosis likely?

B. What are the underlying pathologic changes responsible for the clinical presentation?

C. What are some possible molecular mechanisms responsible for the

pathologic changes?

CASE 34

A 35-year-old woman presents to the clinic with a chief complaint of double vision. She reports intermittent and progressively worsening double vision for approximately 2 months, rarely at first but now every day. She works as a computer programmer, and the symptoms increase the longer she stares at the computer screen. She has also noted a drooping of her eyelids, which seems to worsen with prolonged working at the screen. Both symptoms subside with rest. She is generally fatigued but has noted no other weakness or neurologic symptoms. Her medical history is unremarkable. Physical examination is notable only for the neurologic findings. Cranial nerve examination discloses impaired lateral movement of the right eye and bilateral ptosis, which worsen with repetitive eye movements. Motor, sensory, and reflex examinations are otherwise unremarkable.

Questions

A. What is the likely diagnosis? What is the pathogenesis of this disease? B. What other neurologic manifestations might you expect to see? C. What is the mechanism by which this patient’s ocular muscle weakness

increases with prolonged activity? D. What associated conditions should be investigated in this patient? E. What treatments should be considered?

CASE 35

A 73-year-old man is brought in by his wife with concerns about his worsening memory. He is a retired engineer who has recently been getting lost in the neighborhood where he has lived for 30 years. He has been found wandering and has often been brought home by neighbors. When asked

about this, he becomes upset and defensive and states that he was just trying to get some exercise. He has also had trouble dressing himself and balancing his checkbook. A physical examination is unremarkable, except that he scores 12 points out of 30 on the Mini-Mental State Examination, a test of cognitive function. A metabolic workup is normal. A computed tomography scan of the head shows generalized brain atrophy, though perhaps only what would be expected for his age. He is diagnosed with dementia, likely from Alzheimer disease.

Questions

A. If a brain biopsy is done, what is likely to be found? B. Where in the brain are the changes characteristic of Alzheimer disease

most prominent, and how does that explain the progression of symptoms?

C. What is the role of the amyloid peptide in Alzheimer disease? D. Is there a role for genetic testing to determine risk for developing

Alzheimer disease at this time?

CASE 36

A middle-aged man is transported to the emergency department unconscious and accompanied by a nurse from the medical floor. The nurse states that the patient was in line in front of her in the hospital cafeteria when he suddenly fell to the floor. He then had a “generalized tonic-clonic seizure.” She called for assistance and accompanied him to the emergency department. No other historical information is available. On physical examination, the patient is confused and unresponsive to commands. He is breathing adequately and has oxygen in place via nasal prongs. His vital signs are as follows: temperature, 38°C; blood pressure, 170/90 mm Hg; heart rate, 105 bpm; respiratory rate, 18/min. Oxygen saturation is 99% on 2 L of oxygen. Neurologic examination is notable for reactive pupils of 3 mm, intact gag reflex, decreased movement of the left side of the body, and Babinski reflexes bilaterally. Examination is otherwise unremarkable.

Questions

A. Describe what is meant by a generalized tonic-clonic seizure. B. What are some of the underlying causes of seizure disorders? Which

cause might you be most concerned about in this patient? C. What is the likely pathophysiology of the seizure in this patient?

CASE 37

A 72-year-old man presents to the emergency department with acute onset of right-sided weakness. The patient was eating breakfast when he suddenly lost strength on the right side of his body such that he was unable to move his right arm or leg. He also noted a loss of sensation in the right arm and leg and difficulty speaking. His wife called 911, and he was brought to the emergency department. His medical history is remarkable for long-standing hypertension, hypercholesterolemia, and recently diagnosed coronary artery disease. On physical examination, his blood pressure is 190/100 mm Hg. Neurologic examination is notable for right facial droop and a dense right hemiparesis. A Babinski reflex is present on the right. CT scan of the brain shows no evidence of hemorrhage. The patient is admitted to the neurologic ICU.

Questions

A. What is the diagnosis? Which artery or vascular territory is apt to be involved?

B. What are some risk factors for this condition? C. What are the possible mechanisms by which this man developed these

focal neurologic deficits? Which are most likely in this patient? Why? D. What underlying disorder may be responsible? How does it result in

stroke?

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CHAPTER

8

Diseases of the Skin Laura B. Pincus, MD, & Timothy H. McCalmont, MD

NORMAL SKIN

The skin is the most accessible organ of the human body. Its most basic function is simply a protective one. As a barrier, the skin holds off desiccation and disease by keeping moisture in and pathogens out. Nevertheless, characterization of the skin as mere “plastic wrap” is a gross underestimation of the anatomic and physiologic complexity of this vital structure.

Unlike parenchymal organs, end-organ dysfunction or failure is not a prerequisite for the diagnosis of a skin disease, because all skin diseases can be observed clinically irrespective of their functional effects. Among the spectacular array of neoplastic, inflammatory, infectious, and genetic cutaneous disorders, some elicit only trivial aberrations in skin structure or function, whereas others lead to profound and morbid consequences.

ANATOMY

The integumentary system consists of a layer of tissue, 1–4 mm thick, that covers all exposed surfaces of the body. The skin merges uninterruptedly with the structurally similar envelope of the mucous membranes, but skin is distinct from mucosa in that it contains adnexal structures such as the eccrine units that exude sweat and the folliculosebaceous apocrine units that produce hairs and oils. There is considerable variation in skin thickness and composition,

depending on the requirements of a particular body site. For example, the thinnest skin overlies the eyelids, where delicacy and mobility are essential. The thickest skin is present on the upper trunk, where sturdiness exceeds mobility in importance. The surfaces of the palms and soles are characterized by a high density of eccrine sweat units, reflecting the importance of this region in temperature regulation; an absence of hairs, which would interfere with sensation; and an accentuation of the cornified layer (see later discussion), contributing to the tackiness needed to handle objects deftly. The size of the structures between sites can also vary greatly, best illustrated by the contrast between the large terminal hair follicles found on the scalp, bearded areas, and genital skin and the small vellus hair follicles found at most other sites.

HISTOLOGY

Using a light microscope, two important skin layers are easily identifiable: a stratified squamous epithelium, the epidermis; and a layer of connective tissue, the dermis. The subjacent adipose tissue is considered a third layer by some and is referred to as the subcutis.

The epidermis consists of keratinocytes arrayed in four distinct substrata: the basal, spinous, granular, and cornified layers (Figure 8–1). Basal keratinocytes include the proliferative pool of keratinocytes. These cells divide, giving rise to progeny that are displaced toward the skin surface. As the keratinocytes move outwardly, they progressively flatten and accumulate keratin filaments within their cytoplasm. Individual keratinocytes are tightly bound together by intracellular junctions called desmosomes (Figure 8–2). The desmosomal junctions appear as delicate “spines” between cells in conventional microscopic sections and are most conspicuous in the epidermal spinous layer (Figure 8–3). Keratin filaments are linked intracellularly and are also attached to the desmosomes, forming a network that is vital to structural integrity.

FIGURE 8–1 Although the epidermis biologically displays a gradient of differentiation, four distinct layers are recognized on the basis of microscopic appearance. Cuboidal germinative keratinocytes serve as a foundation in the basal layer; cells with ample cytoplasm and prominent desmosomes constitute the spinous layer; cells with cytoplasmic granularity resulting from an accumulation of keratin complexes and other structural proteins are found in the granular layer; and anucleate, flattened keratinocytes compose the tough, membrane-like cornified layer. (Redrawn, with permission, from Orkin M et al, eds. Dermatology. Originally published by Appleton & Lange. Copyright © 1991 by The McGraw-Hill Companies, Inc.)

FIGURE 8–2 In an ultrastructural view of a human keratinocyte (A), numerous desmosomes (arrows) (B) appear as plaques that tightly bind two cell membranes together. With very high magnification (C), the attachment of cytoplasmic keratin filaments (F) to the desmosomes can be appreciated. (Reproduced, with permission, from Junqueira LC et al. Basic Histology, 10th ed. McGraw-Hill, 2003.)

FIGURE 8–3 With conventional light microscopy, the numerous desmosomes of the spinous layer appear as delicate attachments (“spines”) between individual keratinocytes.

Melanocytes and Langerhans cells are dendritic cells that are intercalated among the keratinocytes of the epidermis. Melanocytes, which are positioned in the basal layer, synthesize a reddish-brown biochrome, melanin, and dispense it to numerous adjacent keratinocytes through their dendrites (Figure 8–4). This distribution system permits melanin to provide a dispersed screen against the potentially harmful ultraviolet rays of the sun. Langerhans cells share a similar arborized morphology but are positioned in the midspinous layer. Langerhans cells are bone marrow–derived antigen-presenting cells (see also Chapter 3).

FIGURE 8–4 The human melanocyte displays a branching morphology, and the dendrites of the cell maintain contact with 35–40 adjacent keratinocytes in a multicellular structure termed the epidermal melanin unit. The function of the unit is the effective dispersion of melanin pigment, packaged in organelles known as melanosomes, across a broad surface area. (Redrawn, with permission, from Junqueira LC et al. Basic Histology, 10th ed. McGraw-Hill, 2003.)

The epidermal–dermal junction, or basement membrane zone, is a structure that welds the epidermis to the dermis and contributes to the skin barrier. The juncture of the epidermis and dermis is arrayed in an undulating fashion to increase the binding surface area between the two structures and to resist shearing forces. The downward projections of the epidermis are referred to as rete ridges, and the upward projections of the superficial dermis are called dermal papillae (Figure 8–5). Although the basement membrane comprises a thin eosinophilic (pink) band beneath the basal cells in conventional microscopic

sections, it has a sophisticated multilayered structure that reaches from the hemidesmosomes of the basal keratinocytes to the collagen bundles of the superficial dermis (Figure 8–6). The lamina densa and lamina lucida are two of the layers of the basement membrane zone and are so named because of their electron-dense and electron-lucid appearance when viewed ultrastructurally.

FIGURE 8–5 The undulating configuration of the epidermal–dermal junction consists of downward extensions of the epidermis, known as rete ridges, and upward extensions of the dermis, known as dermal papillae.

FIGURE 8–6 Schematic diagram of the basement membrane zone of human epidermis. (Redrawn, with permission, from Orkin M et al, eds. Dermatology. Originally published by Appleton & Lange. Copyright © 1991 by The McGraw- Hill Companies, Inc.)

The dermis consists of a connective tissue gel composed largely of proteins and mucopolysaccharides (so-called ground substance). This matrix serves as the scaffolding that supports the complex neurovascular networks, which course through the skin, and also supports the eccrine (sweat gland) and follicular (hair) adnexal structures. The vast majority of the fibrous structural proteins of the dermis are composed of collagen types I and III, and a network of elastic microfibrils is also woven throughout the full dermal thickness. Fibrocytes, the synthetic units of the structural proteins, are ubiquitous, and there are also mast cells and dendritic immune cells arrayed throughout the dermis. A discussion of the fine structure of the dermis—the dermal vascular and neural networks and the adnexal structures—is beyond the scope of this chapter.

OVERVIEW OF SKIN DISEASES

In the broadest and simplest sense, there are two types of skin diseases: growths and rashes. A skin growth is a cyst, malformation, or benign or malignant neoplasm, something that usually presents clinically as a bump on the skin. A rash is, with rare exception, a nonneoplastic skin disease; it is more precisely referred to as an inflammatory skin condition or a dermatitis. The pathophysiologic aspects of the huge number of described growths and rashes exceed the scope of this chapter. Therefore, our discussion in this chapter will be limited to nine prototypical rashes.

TYPES OF SKIN LESIONS

Physicians interested in the skin learned decades ago that the accurate diagnosis and classification of the many patterns of dermatitis depended on a standardized nomenclature for the description and documentation of rashes. When used in association with a few well-chosen adjectives, the terms used to describe the prototypical types of inflammatory skin lesions (so-called primary lesions) permit vivid description of a rash. To illustrate the importance of terminology, imagine trying to describe a patient’s condition over the telephone to another physician. Talking about a red, raised rash may truthfully describe the eruption in some sense, but the mental image evoked could be any one of dozens of skin diseases. The only way to accurately characterize an eruption is by the use of precisely defined terms.

The most important types of primary lesions include macules and patches, papules and plaques, vesicles and bullae, pustules, and nodules. The terms macule and patch denote flat areas of discoloration without any discernible change in texture. Macules are 1 cm or less in diameter, whereas patches exceed 1 cm in size. Papules and plaques are elevated, palpable skin lesions in which the breadth of the lesion exceeds its thickness. A papule is small, 1 cm or less in diameter, whereas a plaque exceeds 1 cm in size. Vesicles and bullae are fluid- filled spaces within the skin. Vesicles are less than 1 cm in diameter, whereas bullae exceed 1 cm in size. A vesicle or bulla containing purulent fluid is known as a pustule. A nodule is a solid, rounded skin lesion in which diameter and thickness are roughly equal.

TYPES OF INFLAMMATORY SKIN DISEASES

Different inflammatory processes involve different structures within the skin and display different microscopic patterns. Experience has shown that pattern analysis can serve as a useful means of diagnosis and classification. Pattern analysis depends on accurate recognition of the distribution of inflammation within the skin as well as recognition of the specific structures affected by the inflammatory reaction. There are nine distinct patterns of dermatitis (Table 8–1; Figure 8–7). All nine of these patterns and some of the diseases that produce them are discussed in detail next.

TABLE 8–1 Patterns of inflammatory skin disease.

FIGURE 8–7 Nine patterns of inflammatory skin disease. (See also Table 8–1.)

CHECKPOINT

1. What are the two most basic barrier functions of skin? 2. How is skin distinct from mucosa? Why are these terms important? 3. What are the main primary lesions? Why are these terms important? 4. What are the main patterns of inflammatory skin disease? 5. What is the value of knowing the microscopic pattern of inflammation of

a skin lesion? What additional informa-tion is needed for this information to be most useful?

PATHOPHYSIOLOGY OF SELECTED SKIN DISEASES

PATTERN: PSORIASIFORM DERMATITIS

Example: Psoriasis Overview Psoriasis is a common chronic, persistent or relapsing, scaling skin condition. Individual lesions are distinctive in their classic form: sharply marginated and erythematous and surmounted by silvery scales (Figure 8–8). Most patients with psoriasis have a limited number of fixed plaques, but there is great variation in clinical presentation.

FIGURE 8–8 Classic plaque-type psoriasis (psoriasis vulgaris) consisting of sharply marginated, scaling plaques. (Image used with permission from Dr. Timothy Berger.)

Epidemiology and Etiology

Psoriasis affects between 1% and 2% of individuals of both sexes in most ethnic groups. The most common age at onset is the third decade, but psoriasis can develop soon after birth, and psoriasis of new onset has been documented in a centenarian.

Several lines of evidence have established that genetic factors contribute to the development of psoriasis. There is a high rate of concordance for psoriasis in monozygotic twins and an increased incidence in relatives of affected individuals. The gene products of specific class I alleles of the major histocompatibility complex (MHC) are overexpressed in patients with psoriasis. Psoriasis is not merely a genetic disorder, however, because some susceptible individuals never develop characteristic lesions. In other predisposed individuals, a number of environmental factors, including infection, physical injury, stress, and drugs, can serve as triggers for the development of psoriasis (Table 8–2).

TABLE 8–2 Factors that induce or exacerbate psoriasis.

Histopathology and Pathogenesis

Psoriasis is the prototypical form of psoriasiform dermatitis, a pattern of inflammatory skin disease in which the epidermis is thickened as a result of rete

ridge elongation (Figures 8–7 and 8–9). In psoriatic lesions, epidermal thickening reflects excessive epidermopoiesis (epidermal proliferation). The increase in epidermopoiesis is reflected in shortening of the duration of the keratinocyte cell cycle and doubling of the proliferative cell population. Because of these alterations, lesional skin contains up to 30 times as many keratinocytes per unit area as normal skin. Evidence of excessive proliferation is also manifest microscopically as numerous intraepidermal mitotic figures.

FIGURE 8–9 Histopathologic features of psoriasis at low magnification. The rete ridges are strikingly and evenly elongated, and the overlying cornified layer contains cells with retained nuclei (parakeratosis), a pattern that reflects the increased epidermal turnover.

During normal keratinocyte maturation, nuclei are eliminated as cells enter the cornified layer and condense to form a semipermeable envelope. In psoriasis, the truncation of the cell cycle leads to an accumulation of cells within the cornified layer with retained nuclei, a pattern known as parakeratosis. As parakeratotic cells accumulate, neutrophils migrate to the cornified layer. Histopathologically, the silvery scale of psoriatic plaques consists of a thick layer of parakeratotic keratinocytes with numerous intercalated neutrophils. At times, the number of neutrophils in the stratum corneum is so great that lesions assume a pustular appearance.

Psoriasis also induces endothelial cell hyperproliferation, which yields pronounced dilation, tortuosity, and increased capillary permeability in the superficial dermis (Figure 8–10). The vascular alterations contribute to the bright erythema seen clinically. The capillary changes are most pronounced at the

advancing margins of psoriatic plaques.

FIGURE 8–10 In a psoriatic plaque at high magnification, dilated capillaries are evident in an edematous portion of the superficial dermis.

After years of research, a large number of immunologic abnormalities that involve both innate and adaptive immunity have been documented in psoriatic skin. Antigenic stimuli are thought to activate the innate immune response, leading to the production of cytokines, such as interferon, tumor necrosis factor (TNF), interleukin-23 (IL-23), and IL-12, by macrophages, dendritic cells, and neutrophils. This leads to attraction, activation, and differentiation of T cells. These T cells, most importantly T-helper 1 and T-helper 17 cells, produce cytokines that lead to epidermal hyperplasia, recruitment of inflammatory cells, and ultimately a positive feedback loop that perpetuates the pathologic process. Recent studies suggest that microRNAs probably play a role in the pathogenesis of psoriasis as well.

Clinical Manifestations

The cardinal features of psoriasis plaques include sharp margination, bright erythema, and nonconfluent, whitish or silvery scales. Lesions can occur at any site, but the scalp, the extensor surfaces of the extremities, and the flexural

surfaces are often involved. Psoriasis commonly affects the nail bed and matrix, yielding pitted or markedly thickened dystrophic nails. Mucosal surfaces are often spared, although the tongue can sometimes be involved (referred to as geographic tongue).

The only extracutaneous manifestation of psoriasis is psoriatic arthritis, a deforming, asymmetric, oligoarticular arthritis that can involve small or large joints. The distal interphalangeal joints of the fingers and toes are characteristically involved. Psoriatic arthritis is classified as one of the seronegative spondyloarthropathies, distinguishable from rheumatoid arthritis by a lack of circulating autoantibodies (so-called rheumatoid factors) or circulating immune complexes and by linkage with specific MHC class I alleles, including HLA-B27.

There are many variants of psoriasis, all of which are histopathologically similar but which differ greatly in clinical distribution (Table 8–3).

TABLE 8–3 Variants of psoriasis.

CHECKPOINT

6. What evidence supports a genetic role in the develop-ment of psoriasis? An environmental role?

7. Which cell types hyperproliferate in psoriasis? 8. What immunologic defects have been identified in psoriasis?

PATTERN: INTERFACE DERMATITIS

Example: Lichen Planus Overview Lichen planus is a distinctive pruritic eruption that usually presents with numerous small papules. Individual lesions have angulated borders, flat tops, and a violaceous hue, attributes that form the basis of their alliterative description as pruritic, polygonal, purple papules (Figure 8–11). The individual papules of lichen planus sometimes coalesce to form larger plaques. Minute whitish streaks, barely visible to the naked eye and known as Wickham striae, are often found on the surfaces of lesions.

FIGURE 8–11 Pruritic, polygonal, flat-topped papules of lichen planus.

Epidemiology and Etiology

Lichen planus generally develops in adulthood and affects women slightly more commonly than men. Although the factors that trigger lichen planus remain obscure in many patients, it is clear that the rash represents a cell-mediated immune reaction that directly or indirectly damages basal keratinocytes of the epidermis. Observations that suggest a cell-mediated mechanism include the occurrence of lichen planus–like eruptions as a manifestation of graft-versus- host disease after bone marrow transplantation and the development of a lichen

planus–like eruption in mice after injection of sensitized, autoreactive T cells. Although most lichen planus is idiopathic, drugs are one established cause of lichen planus or lichen planus–like reactions. Therapeutic gold and antimalarial agents are the medications most closely linked to the development of lichenoid eruptions, but a long list of other agents has accumulated (Table 8–4).

TABLE 8–4 Medications that induce lichenoid (lichen planus–like) reactions.

Histopathology and Pathogenesis

Lichen planus is a form of lichenoid interface dermatitis, a type of inflammatory skin disease in which a dense infiltrate of lymphocytes occupies the perijunctional dermis immediately subjacent to the epidermis, in association with vacuolization of the lower epidermis (Figure 8–12). The dermal infiltrate is composed largely, if not entirely, of T lymphocytes. Some of the T cells are also found within the epidermis, where adjacent vacuolated, injured keratinocytes are found. Dense eosinophilic (pink) globules, known as colloid bodies, are also identifiable within the epidermis and the infiltrate (Figure 8–13). Colloid bodies are condensed, anucleate keratinocytes that have succumbed to the inflammatory reaction. Although the keratinocytes bear the brunt of the lymphocyte attack, melanocytes may be coincidentally destroyed in the reaction as “innocent bystanders.” Free melanin pigment is released as melanocytes are damaged, and the pigment is phagocytosed by dermal macrophages known as melanophages.

FIGURE 8–12 Histopathologic features of lichen planus at low magnification. There is a bandlike infiltrate of lymphocytes that impinges on the epidermal–dermal junction, and some keratinocytes adjacent to the infiltrate show cytoplasmic vacuolization.

FIGURE 8–13 Necrotic keratinocytes (so-called colloid bodies) in a lesion of lichen planus appear as rounded and eosinophilic globules along the epidermal–dermal junction (arrows).

In incipient lesions of lichen planus, CD4 helper T lymphocytes predominate, and some of the cells have been found in proximity to macrophages and Langerhans cells (see also Chapter 3). In contrast, CD8 cytotoxic T cells comprise the bulk of the infiltrate in mature lesions. This shift in infiltrating T- cell composition is thought to reflect the afferent and efferent aspects of lesional development. In the afferent phase, causative antigens are processed and presented to helper T cells, probably in the context of specific HLA determinants. The stimulated CD4 lymphocytes then elaborate specific cytokines that lead to the recruitment of cytotoxic lymphocytes. Cell-mediated cytotoxicity and cytokines such as interferon-γ and TNF are then thought to contribute to the vacuolization and necrosis of keratinocytes as a secondary event. Emerging data reveal that microRNAs may also play a role in the pathogenesis of the oral form of lichen planus.

The clinical appearance of lichen planus lesions reflects several synchronous alterations in the skin. The dense array of lymphocytes in the superficial dermis yields the elevated, flat-topped appearance of each papule or plaque. The chronic inflammatory reaction induces accentuation of the cornified layer (hyperkeratosis) of the epidermis, which contributes to the superficial whitish coloration perceived as Wickham striae. Although the many melanophages that

accumulate in the papillary dermis hold a brownish-black pigment, the fact that the pigmented cells are embedded in a colloidal matrix such as the skin permits extensive scattering of light, a phenomenon known as the Tyndall effect. Thus, the human eye interprets a lesion of lichen planus as dusky or violaceous despite the fact that the pigment that serves as the basis for the coloration is melanin.

Clinical Manifestations

Lichen planus affects both skin and mucous membranes. Papules are generally distributed bilaterally and symmetrically. The sites most commonly involved include the flexor surfaces of the extremities, the genital skin, and the mucous membranes. Rarely, lichen planus may involve the mucosa of internal organs, such as the esophagus. Cutaneous lesions are virtually never seen on the palms, soles, or face.

In general, lichen planus variants can be grouped into three categories.

A. Lichen Planus Papules Arrayed in an Unusual Configuration—In these variants, typical individual papules of lichen planus are grouped in a distinctive larger pattern. In annular lichen planus, small lichenoid papules coalesce to form a larger ring. Linear and zosteriform patterns of lichen planus have also been observed. When lichen planus presents in an unusual configuration, it is prone to being underdiagnosed or misdiagnosed.

B. Lichen Planus Papules Arrayed at Distinctive Sites—Although most lichen planus is widespread, at times papules are restricted to a specific body site, such as the mouth (oral lichen planus) or genitalia. Nearly 25% of all lichen planus patients have disease limited to the mucous membranes.

C. Lichen Planus Papules with Unusual Clinical Morphology—Some examples of lichen planus defy clinical recognition because the appearance of the individual lesions is atypical. Erosive, vesiculobullous, atrophic, and hypertrophic lesions can be seen. In erosive lichen planus, the interface reaction directed against the epidermis is so profound that the entire epidermis becomes necrotic and ulceration ensues. The closely related entity vesiculobullous lichen planus is also characterized by an intense interface reaction that yields necrosis of the epidermal junctional zone across a broad front. As a result of basal layer necrosis, the epidermis becomes detached from its dermal attachments and a blister develops. In atrophic lichen planus, the rate of destruction of keratinocytes by the lichenoid interface reaction exceeds the rate of epidermal

regeneration, and the epidermis becomes attenuated as a result. In contrast, in hypertrophic lichen planus, the rate of epidermal regeneration triggered by the interface reaction exceeds the rate of destruction, and thick, verrucous, hyperkeratotic lesions develop. All of these variants are histopathologically similar with the exception of the foci of ulceration seen in erosive lichen planus.

CHECKPOINT

9. Which skin cells are damaged by cell-mediated immune reactions in lichen planus?

10. Which drugs have been most commonly implicated in licheniform eruptions?

11. What synchronous alterations in the skin are reflected in the clinical appearance of lichen planus?

Example: Erythema Multiforme Overview Erythema multiforme is an acute cutaneous eruption that presents with a wide spectrum of clinical severity. The eruption is commonly brief and self-limited, but repetitive or generalized attacks can be disabling or even life threatening. As the name implies, variation in lesional morphology can be seen, but most patients present with a monomorphous pattern in a given bout. The prototypical lesion is a red macule or thin papule that expands centrifugally and develops a dusky or necrotic center, creating a target-like pattern (Figure 8–14).

FIGURE 8–14 Target lesions—a characteristic pattern seen in erythema multiforme—consist of a papule or plaque with a central zone of epidermal necrosis surrounded by a rim of erythema. (Image used with permission from Dr. Timothy Berger.)

Epidemiology and Etiology

Erythema multiforme is an uncommon but distinctive skin disease that afflicts men and women in nearly equal numbers. The peak incidence is in the second to fourth decades of life, and onset during infancy or early childhood is a rarity. Like lichen planus, erythema multiforme represents a cell-mediated immune reaction that eventuates in necrosis of epidermal keratinocytes. Herpes simplex

viral infection and reactions to medications have been established as the most common causes of erythema multiforme. Other known causes include Mycoplasma infection, contact dermatitis, drugs, and radiation.

Histopathology and Pathogenesis

Erythema multiforme is a prototypical form of vacuolar interface dermatitis. In contrast to lichen planus, which typically presents with a dense, obscuring lichenoid infiltrate within the superficial dermis, in erythema multiforme the inflammatory infiltrate is sparse. Thus, the vacuolated keratinocytes that are widely distributed within the epidermal basal layer are conspicuous in the face of a sparse infiltrate, and the damaged keratinocytes serve as the basis for the name of this pattern of inflammatory skin disease.

The dermal infiltrate in erythema multiforme is composed of a mixture of CD4 and CD8 T lymphocytes. CD8 cytotoxic cells are also found within the epidermis, in proximity to vacuolated and necrotic keratinocytes. Keratinocytes killed in the course of the inflammatory reaction become anucleate and are manifest microscopically as round, dense, eosinophilic bodies similar to the colloid bodies of lichen planus (Figure 8–15).

FIGURE 8–15 Histopathologic features of erythema multiforme, a type of vacuolar interface dermatitis. There is a modest infiltrate of lymphocytes in the vicinity of the epidermal–dermal junction

where vacuolated and necrotic keratinocytes are conspicuous.

Although lichen planus and erythema multiforme are clinically, microscopically, and etiologically distinct, both appear to share a common pathogenetic pathway in which specific inciting agents recruit effector lymphocytes into the epidermis and papillary dermis. After this recruitment, keratinocytes are injured and killed by the combined negative influences of cytotoxicity and cytokines such as interferon-γ and TNF.

Many cases of so-called erythema multiforme minor are triggered by herpes simplex viral infection. A relationship between erythema multiforme and herpetic infection had long been suspected based on the documentation of preceding herpes simplex lesions in patients with erythema multiforme. Support for this relationship was strengthened after antiherpetic drug therapy, in the form of oral acyclovir, was shown to suppress the development of erythema multiforme lesions in some individuals. Molecular studies have substantiated the relationship by confirming the presence of herpes simplex DNA within skin from erythema multiforme lesions. Herpesvirus DNA is also demonstrable within peripheral blood lymphocytes and within lesional skin after resolution but not within nonlesional skin. These findings suggest that viral DNA is disseminated from the primary infection in the peripheral blood and becomes integrated into the skin at specific target sites. The herpetic genomic fragments then contribute to the development of a cytotoxic effector response in their chosen target tissue, the skin.

The target-like clinical appearance of many erythema multiforme lesions reflects zonal differences in the intensity of the inflammatory reaction and its deleterious effects. At the periphery of an erythema multiforme lesion, only sparse inflammation, slight edema, and subtle vacuolization of the epidermis are apparent in the outer erythematous halo. In contrast, the dusky “bull’s eye” often shows pronounced epidermal vacuolization, with areas of near-complete epidermal necrosis.

Clinical Manifestations

Erythema multiforme is generally limited to the skin and mucous membranes. The lesions develop rapidly in crops and are initially distributed on acral surfaces, although proximal spread to the trunk and face commonly occurs. Mucosal erosions and ulcers are seen in roughly 25% of cases, and mucositis can be the sole presenting feature of the disease. Although erythema multiforme is an epithelial disorder, nonspecific constitutional symptoms such as malaise can also occur.

Although the spectrum of erythema multiforme exists as a continuum, a given patient is usually classified as having minor or major disease. The disorder is referred to as erythema multiforme minor when there are scattered lesions confined to the skin or when skin lesions are observed in association with limited mucosal involvement. A diagnosis of erythema multiforme major is based on the presence of prominent involvement of at least two of three mucosal sites: oral, anogenital, or conjunctival. Many examples of erythema multiforme major also display severe, widespread cutaneous involvement. Although Stevens–Johnson syndrome had classically been used to describe severe cases of erythema multiforme, consensus classification has separated Stevens–Johnson syndrome from erythema multiforme and added it to the spectrum of toxic epidermal necrolysis. These two entities, Stevens–Johnson syndrome and toxic epidermal necrolysis, are now considered to represent variant dermatologic manifestations of severe idiosyncratic reactions. Most often the result of medications, these entities involve vast regions of skin and mucosal necrosis (Figure 8–16) with secondary vesiculation. Pathologically, the findings are similar to those of a severe burn in that the integrity of a patient’s skin fails, resulting in an increased risk of infectious and metabolic sequelae.

FIGURE 8–16 Toxic epidermal necrolysis. Generalized maculopapular erythema of the trunk and extremities is followed by extensive desquamation, as seen on this patient’s trunk, resulting from epidermal necrosis. Patients are often admitted to a burn unit for acute care. (Image used with permission from Dr. Timothy Berger.)

CHECKPOINT

12. What is the prototypical lesion in erythema multiforme? 13. In what ways is erythema multiforme similar to and dif-ferent from

lichen planus? 14. What are some complications of toxic epidermal necrolysis?

PATTERN: VESICULOBULLOUS DERMATITIS

Example: Bullous Pemphigoid Overview Bullous pemphigoid is a blistering disease in which tense, fluid-filled spaces develop within erythematous, inflamed skin. The blisters in bullous pemphigoid develop because of detachment of the epidermis from the dermis (subepidermal vesiculation) as the result of a specific inflammatory reaction directed against structural proteins. The term “pemphigoid” reflects the clinical similarity of bullous pemphigoid to pemphigus vulgaris, another form of blistering skin disease characterized by intraepidermal, rather than subepidermal, vesiculation. The distinction between bullous pemphigoid and pemphigus vulgaris is an important one, because bullous pemphigoid has a more favorable prognosis.

Epidemiology and Etiology

Bullous pemphigoid is generally a disorder of the elderly. There are rare reports of bullous pemphigoid in children and young adults, but the vast majority of patients are older than 60 years. There is no sex predilection.

It has been known for years that immunoglobulins and complement are deposited along the epidermal–dermal junction in bullous pemphigoid. The deposited antibodies are specific for antigens within the basement membrane zone (BP180 and BP230), and bullous pemphigoid thus represents a form of autoimmune skin disease. The specific factors that induce autoantibody production have not been identified.

Histopathology and Pathogenesis

Microscopically, biopsies from fully developed bullous pemphigoid lesions show a subepidermal cleft containing lymphocytes, eosinophils, and neutrophils, as well as eosinophilic (pink) material that represents extravasated macromolecules such as fibrin (Figure 8–17). An inflammatory infiltrate of eosinophils, neutrophils, and lymphocytes is also evident in the dermis beneath the cleft. These findings represent the aftermath of an inflammatory reaction

centered on the basement membrane zone.

FIGURE 8–17 Histopathologic features of bullous pemphigoid. There is a subepidermal cleft that contains numerous eosinophils and lymphocytes, and a similar infiltrate is present in the superficial dermis. Ultrastructurally, the separation is within the lamina lucida of the basement membrane zone, at the level of the bullous pemphigoid antigen (see Figure 8–6).

Insights into this reaction can be obtained from direct immunofluorescence microscopy, in which fluorochrome-labeled anti-immunoglobulin G (IgG), anti- IgA, anti-IgM, and anticomplement antibodies are incubated with lesional skin. Using an ultraviolet microscope to localize the fluorochrome, tagged antibodies specific for IgG and complement component C3 are found in a linear distribution along the epidermal–dermal junction (Figure 8–18). Circulating IgG that binds to the basement membrane zone of human epidermis is also identifiable in bullous pemphigoid patients. These antibodies are capable of complement fixation, and pathogenicity has been confirmed by injection into laboratory animals, in whom the antibodies bind to the junctional zone and induce blisters.

FIGURE 8–18 Direct immunofluorescence findings in lesional skin from a bullous pemphigoid patient. When fluorochrome-stained sections are viewed through an ultraviolet microscope, a bright linear band, signifying deposition of immunoglobulin G, is evident along the epidermal–dermal junction. (Image used with permission from Dr. Kari Connolly.)

The autoantibodies (IgG) in bullous pemphigoid are directed against hemidesmosomal proteins, namely bullous pemphigoid antigen 180 and bullous pemphigoid antigen 230. The binding of these autoantibodies to the basement membrane zone leads to an inflammatory cascade with activation of the classic complement cascade (see Chapter 3). Complement fragments induce mast cell degranulation and attract neutrophils. The presence of eosinophils in the infiltrate of bullous pemphigoid is probably a reflection of mast cell degranulation, because mast cell granules contain eosinophil chemotactic factors. Numerous enzymes are released by granulocytes and mast cells during the reaction, and enzymatic digestion is thought to be the primary mechanism behind the separation of the epidermis from the dermis with formation of tense bullae. It is also possible that the bullous pemphigoid antigen plays a vital structural role that is compromised by autoantibody binding, leading to cleavage. Quantifiable titers of bullous pemphigoid antigen correlate to disease activity.

Clinical Manifestations

Patients with bullous pemphigoid present with large, tense blisters on an

erythematous base (Figure 8–19). Lesions are most commonly distributed on the extremities and lower trunk, but blisters can develop at any site. Most patients experience considerable pruritus in association with their blisters, possibly triggered by the many eosinophils in the dermal infiltrate. Mucous membrane lesions develop in up to one-third of patients and are usually clinically innocuous.

FIGURE 8–19 Large tense bullae on erythematous bases are distributed over the lower trunk. (Image used with permission from Dr. Timothy Berger.)

Some patients with bullous pemphigoid present with itchy, erythematous

plaques, with no blistering for an extended period of time, but blisters eventually develop in most patients. This pattern is known as pre-eruptive or urticarial bullous pemphigoid. Immunofluorescence and histopathologic examination of biopsies from such patients reveals junctional deposition of autoantibodies and complement in association with an eosinophil-rich infiltrate, implying that the inflammatory reaction is identical to that of conventional bullous pemphigoid. The explanation for the delayed blistering seen in these patients is not presently known.

Bullous pemphigoid is a disease of the skin and mucous membranes only, and systemic involvement has never been documented. Some patients with bullous pemphigoid have developed skin lesions synchronously with a diagnosis of malignancy, but careful studies with age-matched controls have not demonstrated an increased incidence of bullous pemphigoid in cancer patients.

CHECKPOINT

15. How do pemphigus and pemphigoid differ, and why is the distinction important?

16. How does immunoglobulin binding to the bullous pemphigoid antigen cause blistering in lesions of bul-lous pemphigoid?

17. Is there a connection between bullous pemphigoid and cancer?

PATTERN: VASCULITIS

Example: Leukocytoclastic Vasculitis Overview Leukocytoclastic vasculitis is an inflammatory disorder affecting small blood vessels of the skin that typically presents as an eruption of reddish or violaceous papules, a pattern known as palpable purpura (Figure 8–20). The lesions develop in crops, and individual papules persist for a few days or weeks and generally less than a month. Although each individual lesion is transient, the duration of the eruption can vary from weeks to months, and in exceptional cases crops can develop over a period of years.

FIGURE 8–20 Purpuric papules are scattered on the lower extremity in leukocytoclastic vasculitis. (Image used with permission from Dr. Timothy Berger.)

Epidemiology and Etiology

Leukocytoclastic vasculitis can develop at any age, and the incidence is equal in both sexes. The most common precipitants include infections and medications. Bacterial, mycobacterial, and viral infections can all trigger bouts, but poststreptococcal and poststaphylococcal eruptions are most common.

A wide variety of drugs have been established as leukocytoclastic vasculitis elicitors, including antibiotics, thiazide diuretics, and nonsteroidal anti-

inflammatory agents. Among antibiotics, penicillin derivatives are the foremost offenders.

Histopathology and Pathogenesis

The name of this disorder conveys its chief pathological attributes, namely an inflammatory reaction involving blood vessels in association with an accumulation of necrotic nuclear (leukocytoclastic) debris. The key steps that contribute to this pattern include the accumulation of triggering molecules within the walls of small blood vessels, subsequent stimulation of the complement cascade with the elaboration of chemoattractants, and entry of neutrophils with oxidative enzyme release, eventuating in cellular destruction and nuclear fragmentation. The molecules that trigger leukocytoclastic vasculitis are immune complexes, consisting of antibodies bound to exogenous antigens usually derived from microbial proteins or medications. Circulating immune complexes have been documented by laboratory assays of serum from patients with active leukocytoclastic vasculitis, and the presence of circulating complexes can also be deduced based on the finding of low serum complement levels during exacerbations. The exact factors that lead to preferential deposition of immune complexes within small cutaneous vessels (venules) remain unknown, but the fact that venules exhibit relatively high permeability in the face of a relatively low flow rate is probably contributory. The deposited complexes are detectable within vessel walls by direct immunofluorescence testing (Figure 8–21).

FIGURE 8–21 Direct immunofluorescence microscopy localizes complement component C3 within the walls of small cutaneous vessels. The complement fragments are present after activation of the complement cascade by immune complexes. Immunoglobulin deposition within vessel walls is detectable by the same method. (Image used with permission from Dr. Kari Connolly.)

After becoming trapped in tissue, immune complexes activate the complement cascade, and localized production of chemotactic fragments (such as C5a) and vasoactive molecules ensues (see Chapter 3). Chemoattractants draw neutrophils out of vascular lumens and into vascular walls, where release of neutrophilic enzymes results in destruction of the immune complexes, the neutrophils, and the vessel. Microscopically, this stage is characterized by an infiltrate of neutrophils, neutrophilic nuclear dust, and protein (fibrin) in the vessel wall, a pattern that has historically been called “fibrinoid necrosis” (Figure 8–22). Throughout the inflammatory reaction, the integrity of the channel is progressively compromised. As cellular interstices widen, erythrocytes and fibrin exude through the vessel wall and enter the surrounding dermis.

FIGURE 8–22 Histopathologic features of leukocytoclastic vasculitis, a form of small-vessel vasculitis. Neutrophils, neutrophilic nuclear debris, and amorphous protein deposits are present within the expanded wall of a cutaneous venule.

Leukocytoclastic vasculitis lesions are raised and papular because lesional skin is altered and expanded by an intense vasocentric infiltrate containing numerous neutrophils. The erythematous or purpuric quality of leukocytoclastic vasculitis is attributable to the numerous extravasated erythrocytes that accumulate in the dermis of fully developed lesions. In patients with repetitive or persistent leukocytoclastic vasculitis, extravasated erythrocyte debris is metabolized into hemosiderin, which accumulates within macrophages (siderophages) in the deep dermis. The dermal hemosiderin can contribute to a dusky, violaceous clinical appearance, clinically similar to, but pathologically distinct from, the pigmentary changes seen in lichen planus. After resolution of the eruption, the hyperpigmentation resolves slowly over a period of weeks to months as the hemosiderin is resorbed.

Clinical Manifestations

Lesions of leukocytoclastic vasculitis can develop at any site but are usually distributed on the lower extremities or in dependent areas. Although purpuric lesions comprise the most common clinical pattern, a variety of other morphologic patterns, including vesicopustules, necrotic papules, and ulcers, can

develop. These patterns often reflect secondary ischemic changes superimposed on the primary vasculitic papule. Vesicopustules develop after ischemic necrosis of the epidermis results in subepidermal separation or after massive dermal accumulation of neutrophils secondary to immune complex deposition. Necrotic papules, eschars, and ulcers are end-stage lesions that develop after total necrosis of the epidermis and superficial dermis. In essence, these lesions represent vasculitic infarcts.

Leukocytoclastic vasculitis is not merely a dermatitis but often part of a systemic vasculitis involving small vessels. In such cases, the vascular eruption is accompanied by arthralgias, myalgias, and malaise. Arthralgias and myalgias are probably attributable to vasculitic changes in small vessels in joint capsules and soft tissue. Vasculitic involvement of the kidneys, liver, and gastrointestinal tract can also occur. Such involvement of abdominal organ systems often presents clinically as abdominal pain. Laboratory studies are important to evaluate possible renal or hepatic impairment.

CHECKPOINT

18. Why are leukocytoclastic vasculitis lesions papular? 19. What are the most common precipitants of leukocyto-clastic vasculitis? 20. When leukocytoclastic vasculitis is part of a systemic vasculitis, what

additional symptoms are typically observed?

PATTERN: SPONGIOTIC DERMATITIS

Example: Allergic Contact Dermatitis Overview Allergic contact dermatitis is an eruption, usually pruritic, caused by a specific immune-mediated reaction to a substance that has touched the skin. The acute phase is characterized by erythematous papules, vesicles, and bullae confined to the area of primary contact of the “allergen” (Figure 8–23). Often the blisters break down and result in weeping and formation of a yellowish crust.

FIGURE 8–23 Allergic contact dermatitis, characterized by confluent, linear, eruptive vesicles with surrounding erythema. (Image used with permission from Dr. Timothy Berger.)

Epidemiology and Etiology

Reliable data on the incidence of allergic contact dermatitis are impossible to gather because of the vast number of people affected, including those with mild disease who do not come to medical attention. However, the disorder has been estimated to cost millions annually in occupation-related direct medical costs and lost productivity.

The factors that determine which individuals will react to which substances

are not known, although HLA types are thought to play a role. Some animal models of allergic contact dermatitis demonstrate autosomal inheritance patterns.

Histopathology and Pathogenesis

As the term “spongiotic dermatitis” implies, spongiosis is the pathologic hallmark of this category of skin disease. The term “spongiosis” refers to edema of the epidermis, which separates keratinocytes from one another. Microscopically, edema makes visible the normally indiscernible “spines,” or desmosomes, that interconnect the keratinocytes (Figures 8–7 and 8–24). Spongiosis may be slight and barely perceptible microscopically or so massive that it is evident clinically as a blister. Spongiotic dermatitis is accompanied by a variable amount of perivascular inflammation that may be around the superficial vascular plexus or the superficial and deep vascular plexuses, or perivascular and interstitial in distribution (Figure 8–25). The infiltrate is typically composed of lymphocytes, but eosinophils are often concurrently present in significant numbers in spongiotic dermatitis.

FIGURE 8–24 Allergic contact dermatitis. Intercellular edema has made the “spines” (desmosomes) between keratinocytes visible.

FIGURE 8–25 Histopathologic features of allergic contact dermatitis, a type of acute spongiotic dermatitis. There is a perivascular and interstitial infiltrate of inflammatory cells. The paleness of the papillary dermis is a result of edema.

The series of events leading to the development of allergic contact dermatitis has been and continues to be intensively studied, because the mechanism of development of contact hypersensitivity in the skin is analogous to the cell- mediated rejection of organs used for transplantation. Delayed-type (type IV) hypersensitivity reactions consist of two phases: induction (sensitization/afferent) and elicitation (efferent). In the induction phase, the allergen that has come into contact with an individual naive to that allergen binds to an endogenous protein and alters it to make it appear foreign. This protein– allergen complex is then intercepted by the immunosurveillance cells of the skin: the Langerhans cells. Langerhans cells are bone marrow–derived dendritic cells that reside in the epidermis and form a network at the interface of the immune system with the environment. They engulf the complex, partially degrade (“process”) it, migrate to the lymph nodes, and present antigenic fragments on the cell surface in conjunction with an MHC-II molecule. The Langerhans cells with antigen–MHC-II complexes on the surface contact naive T cells possessing T-cell receptors that specifically recognize the MHC-II–allergen complex. The binding of the T-cell receptors to the MHC-II–allergen complex in the context of important co-stimulatory molecules on the surface of the Langerhans cells

stimulates clonal expansion of reactive T cells. This process progresses over a period of days. If the allergen exposure is transient, the first exposure often does not result in a reaction at the exposure site. However, a contingent of “armed and ready” memory T cells is now policing the skin, waiting for the allergen to reappear. The individual is now said to be sensitized.

The elicitation phase begins once the sensitized individual encounters the antigen again. Memory T cells from the prior exposure have been policing the skin constantly. The Langerhans cells again process antigen and migrate to lymph nodes, but presentation and T-cell proliferation also occur at the site of contact with the allergen. Nonspecific T cells in the vicinity are recruited and stimulated by the inflammatory cytokines released by the specifically reactive T cells, and an amplification loop ensues, eventuating in clinically recognizable dermatitis. This complex series of events takes time to develop, resulting in the 24- to 48-hour delay between re-exposure and eruption. Many individuals have experienced this delay in their own experience with poison ivy or poison oak. The onset of these disorders never occurs while the yardwork is being completed or during the hike but always a day or two later.

Delayed-type hypersensitivity serves the organism’s need for defense against noxious invaders such as viruses; responding T cells recognize virally infected cells and selectively kill them. The development of contact allergy represents an aberrance of this protective mechanism, and the allergen invokes a somewhat nonselective onslaught of T cells that damage the epidermis and result in spongiotic dermatitis histopathologically and a pruritic erythematous blistering eruption clinically.

Clinical Manifestations

Few skin diseases are as well embedded in the lay lexicon as poison ivy and poison oak, which are among the most common causes of allergic contact dermatitis. Although there are many causes of allergic contact dermatitis, a number of airborne allergens are frequently identifiable in occupational settings. For those who have been unfortunate enough to experience a full-blown case of poison ivy or oak (so-called Rhus dermatitis, after the genus of plant involved), the salient features of the eruption are well known, manifesting as an extremely pruritic erythematous eruption on areas of skin exposed directly to the allergenic plant leaves. The eruption consists of erythematous papules, papulovesicles, vesicles, or bullae, often in a linear pattern where the offending leaf was drawn across the skin. Linear streaks, although characteristic, are not always noted because the eruption will assume the pattern of the exposure: a hand covered in

allergen that then touches the face may result in a rash in a nonlinear configuration.

A common misconception regarding Rhus dermatitis is that blister fluid from broken blisters (or even touching the blistered area) causes the eruption to spread. In fact, once the eruption has developed, the allergen has been irreversibly bound to other proteins or has been so degraded that it cannot be transferred to other sites. Apparent spread of the eruption to other sites can be accounted for by several possible scenarios. First, the Rhus allergen is tremendously stable and can persist on unwashed clothing and remain capable of inducing allergic contact dermatitis for up to 1 year. Inadvertent contact with contaminated clothes or other surfaces may induce new areas of dermatitis often thought to represent spread and not additional contact. (Washing the skin with soap and water soon after contact with the offending sap will usually abort development of the eruption.) Second, intense allergic contact dermatitis can induce an eruption on skin that was never contacted by allergen. This poorly understood phenomenon is termed “autosensitization.” The autosensitization eruption consists of erythematous papules or papulovesicles that are often confined to the hands and feet but may be generalized. The pattern of individual lesions is not linear or geometric, as it is at the original site of allergic contact dermatitis.

Importantly, Rhus is but one cause of allergic contact dermatitis. The list of known antigens numbers in the thousands, and there are countless ways for these substances to come into contact with the skin. Often an unnatural geometric pattern of an eruption is the clue to an “outside-in” disease, caused by a contactant. Importantly, a contact eruption does not develop immediately on contact but only after a delay of 24–48 hours. This sometimes makes identification of the offending agent difficult, as the connection between exposure and eruption is obscured by the time delay. Patch testing is a useful clinical technique for helping to pinpoint a possible cause when an unknown contactant is suspected as the origin of a persistent or recurrent eruption. In patch testing, a panel of small amounts of standardized antigens are applied in an array to unaffected skin (typically on the back) and left in place for 48 hours. The patches are then removed and the skin is inspected for development of erythema or vesiculation; wherever a reaction is present, the substance that induced the reaction is noted. Readings are performed again at 96 hours to detect long- delayed reactions. To be useful clinically, positive patch test reactions must be correlated with the pattern of the original eruption and the overall clinical context.

CHECKPOINT

21. What is spongiosis? 22. What are the two phases of development of allergic contact dermatitis?

What steps are involved in each? 23. What is the role of patch testing in patients with sus-pected allergic

contact dermatitis?

PATTERN: PANNICULITIS

Example: Erythema Nodosum Overview Panniculitis is an inflammatory process that occurs in the fat of the subcutis. Erythema nodosum is the most common form of panniculitis, presenting most often with tender red nodules on the anterior lower legs (Figure 8–26). The number of lesions is variable, but typically a dozen or more lesions may be present at onset.

FIGURE 8–26 Erythema nodosum on the lower legs of a woman. The lesions are firm, painful, red or red-brown plaques and nodules. Lesion borders are indistinct. (Image used with permission from Dr. Timothy Berger.)

Because the infiltrate in panniculitis occurs deeply in the skin, demarcation of individual lesions is often indistinct. Fever and constitutional symptoms—in particular arthralgias—may accompany the onset of erythema nodosum. The duration of the eruption is typically a few weeks to a few months.

Epidemiology and Etiology

Erythema nodosum is a common condition, although precise data regarding its prevalence are not available. Women seem especially susceptible to its

development, and there is an adult female/male predominance of 3:1. This is not true in childhood cases, in which boys and girls are equally affected. Erythema nodosum represents a final common pathway of inflammation that may develop in response to any one of a number of general causes, including infection, medication, hormones (including pregnancy), and inflammatory disease. Streptococcal pharyngitis, sulfonamide-containing drugs, estrogen-containing oral contraceptives, and inflammatory bowel disease are well-known inducers of the disorder.

Histopathology and Pathogenesis

Panniculitis can be separated into two broad categories based on the distribution of inflammation: mostly lobular panniculitis and mostly septal panniculitis (see Figure 8–7). The septa are the fibrous divisions between fat compartments and contain the neurovascular bundles. The lobules are the conglomerations of adipocytes demarcated by septa. The modifier “mostly” is meant to convey that the inflammatory process is not strictly confined to a single compartment but, in fact, will frequently spill over from one to the other. An important step in making a specific histopathologic diagnosis is deciding where the majority of the inflammatory response is located.

In the case of erythema nodosum, the inflammatory response occurs in the septal compartment and consists of lymphocytes, histiocytes, and granulocytes (neutrophils and eosinophils) (Figure 8–27). Multinucleated histiocytes within the septa are a finding of considerable diagnostic value (Figure 8–28). The septa are thickened and may become fibrotic depending on the density of the infiltrate and the duration of the reaction. Even though the infiltrate is largely confined to subcutaneous septa, there is commonly an element of fat necrosis at the edges of the subcutaneous lobules in erythema nodosum. Evidence of fat necrosis may be seen in the form of an infiltrate of foamy (lipid-laden) macrophages at the periphery of subcutaneous lobules or in the form of small stellate clefts within multinucleate macrophages, indicating an element of lipomembranous fat necrosis.

FIGURE 8–27 Histopathologic features of erythema nodosum, a form of septal panniculitis. The septa are thickened and inflamed. There is little inflammation of the fat lobules.

FIGURE 8–28 Erythema nodosum. There are multiple large multinucleated giant cells in this septum. Note the prominent fibrous background with increased cellularity.

The favored hypothesis regarding the mechanism of development of erythema nodosum is that of a delayed-type hypersensitivity reaction occurring in the septal fat. Immune complex deposition has not been found in the lesions. It is not yet known why systemic hypersensitivity is localized to the fat in such a microscopically distinctive fashion.

Clinical Manifestations

As mentioned, erythema nodosum presents as tender, deep-seated, red to red- brown nodules. As the lesions age, they evolve to more “bruise-like” patches or thin plaques. Erythema nodosum tends to occur on the anterior shins but may involve the thighs, the extensor forearms, and, rarely, the trunk. Because the lesions represent a hypersensitivity response to some inciting stimulus, they may persist or continue to develop in crops for as long as the stimulus is present. In the case of streptococci-associated erythema nodosum, the lesions will probably resolve within a few weeks after successful antibiotic treatment of the primary infection. A prolonged course of erythema nodosum should prompt a search for persistent infection and possible other causes. Erythema nodosum may also be the presenting sign of sarcoidosis (see the following discussion).

CHECKPOINT

24. What are the two general categories of panniculitis? 25. Which category of panniculitis does erythema nodo-sum fit into? What

are the clinical and histopathological features of erythema nodosum? 26. What are some common precipitators of erythema nodosum?

PATTERN: NODULAR DERMATITIS

Example: Cutaneous Sarcoidosis Overview Sarcoidosis is an enigmatic systemic disease with a hugely variable clinical spectrum ranging from mild asymptomatic skin papules to life-threatening lung disease. Lesions are often red-brown dermal papules or nodules that may occur anywhere on the cutaneous surface but have a special predilection for the face

(Figure 8–29). Similar nodular granulomas can occur in the pulmonary tree and other viscera.

FIGURE 8–29 Reddish-brown papule near the nose, typical of sarcoidosis. (Image used with permission from Dr. Timothy Berger.)

Epidemiology and Etiology

Sarcoidosis can affect patients of any age or ethnic background but does occur more frequently in young adults and, in the United States, is more common in people of black African descent. Among this population, estimates of disease incidence range from 35.5 to 64 cases per 100,000 compared with 10–14 cases per 100,000 in whites. In Europe, Irish and Scandinavian populations are at increased risk.

Numerous causes of sarcoidosis have been proposed, including infectious agents. Among these, Mycobacterium species (especially M tuberculosis) have been favored suspects, although investigation has yielded contradictory results. Other proposed etiologic agents include Histoplasma, viruses, and minute systematized foreign particles (which may incite a reactive process in susceptible

individuals), although no solid evidence supporting these suspected causes exists. One report found polarizable foreign material in diseased skin of patients with sarcoidosis, but the authors emphasized that this finding probably reflects the propensity of sarcoidal lesions to develop around a nidus of foreign material in affected patients and does not imply that sarcoidosis is directly caused by foreign detritus. The extent to which genetic heritage determines the susceptibility of an individual to sarcoidosis is unclear, although a higher than expected incidence of sarcoidosis among siblings of affected patients suggests a genetic role. Although both HLA and non-HLA genes (such as the TNF gene) have been implicated in sarcoidosis, alterations in these genes and their interaction with environmental factors continue to be areas of investigation.

Histopathology and Pathogenesis

Sarcoidosis is manifest microscopically as collections of tissue macrophages (ie, histiocytes), known as granulomas, situated within the dermis (Figures 8–30 and 8–31). Unlike the tuberculoid granulomas of tuberculosis, sarcoidal granulomas are noncaseating and do not show central coagulation necrosis. Multinucleated histiocytes formed by the fusion of individual cells are a common finding (Figure 8–32). The characteristic microscopic appearance of sarcoidal granulomas is of small numbers of lymphocytes around the granulomas (“naked granulomas”). This appearance contrasts with the dense lymphocytic infiltrate that blankets the granulomas in many other granulomatous disorders, including tuberculosis. Sarcoidal granulomas can occupy almost the entire dermis in affected skin or may occur only in relatively small foci that are widely spaced. Histochemical stains for infectious organisms are generally negative.

FIGURE 8–30 Histopathologic features of sarcoidosis, a nodular dermatitis. Note the nodular collections of histiocytes scattered throughout the dermis.

FIGURE 8–31 Sarcoidosis. Pale-staining histiocytes form nodular aggregates among the collagen of the dermis.

FIGURE 8–32 Sarcoidosis. Multinucleated giant cells such as the one seen here in the center of the field are common in sarcoidal granulomas.

Just as the cause of sarcoidosis remains unknown, the mechanisms of granuloma formation in sarcoidosis are not completely understood. In general, certain antigenic stimuli elicit a T-cell reaction (see prior discussion regarding the pathogenesis of allergic contact dermatitis). Antigens presented in the proper context induce the responding T cells to release various cytokines. The specific cytokines monocyte chemotactic factor and migration inhibitory factor, along with a host of others, recruit macrophages to the site and direct the cells to remain there. Even though lymphocytes are a small component of sarcoidal granulomas microscopically, they are believed to be crucial to the pathogenesis of the disease.

Studies of the organization of sarcoidal granulomas suggest a pattern of lymphocyte arrangement similar to that of tuberculoid leprosy, a condition in which a potent immune response keeps the M leprae organisms in relative check. In these conditions, the lymphocytes present within the centers of the granulomas are CD4 positive, whereas CD8-positive cells are arranged at the periphery. This structure may allow the CD4 helper cells to direct the immune response to center around an offending antigen while the CD8 suppressor cells limit the extent of the response. Granulomas are not organized in this fashion in lepromatous leprosy, and the lack of an effective suppressive reaction permits

uncontrolled proliferation of M leprae bacilli.

Clinical Manifestations

The clinical picture in sarcoidosis is quite broad. The spectrum of symptoms in an individual patient depends on which tissues are involved and to what extent. There are several prototypical presentations. One consists of bilateral pulmonary hilar lymphadenopathy (resulting from sarcoidal granulomas in perihilar lymph nodes) and acute erythema nodosum, a combination known as Löfgren syndrome. Fever, arthralgias, uveitis, and lung parenchymal involvement are common in Löfgren syndrome. Another variant of sarcoidosis involves the nose, with beadlike papules at the rims of the nares (see Figure 8–29). This presentation is known as lupus pernio, a term of some antiquity that still enjoys widespread use in dermatology. More recently, the designation “nasal rim sarcoidosis” has been proposed for this variant. This cutaneous finding usually indicates significant involvement of the tracheobronchial tree or lung parenchyma.

Skin disease occurs in systemic sarcoidosis in only one-third of cases, although about 80% of patients with sarcoidosis of the skin have concurrent systemic disease. The lungs are commonly involved, and the possibility of lung involvement should always be investigated in any case of sarcoidosis. Cutaneous sarcoidosis has been termed “the great imitator,” as the clinical morphology can be variable, including skin-colored to red-brown papules, plaques, and nodules; hair loss (alopecia) on the scalp or other sites; pigmentary alteration; ulcers; and numerous other patterns. New dermal papules or nodules arising within tattoos that have been present even for many years are a well-recognized phenomenon in sarcoidosis. This should not be surprising because tattoo pigment is a foreign body that is phagocytosed by tissue macrophages and probably serves as a nidus for the development of sarcoidosis lesions. New dermal papules have also been described in association with scars.

Diagnosing sarcoidosis may be difficult. It is often a diagnosis of exclusion. Only when the clinical spectrum is consistent with sarcoidosis and standard investigations have failed to uncover a clear origin (infectious or otherwise) can a diagnosis of sarcoidosis be issued with confidence. Helpful studies include chest x-ray and bone radiographs with findings suggestive of sarcoidosis or a biopsy of skin or other involved tissue showing the noncaseating granulomas characteristic of the disease.

CHECKPOINT

27. Who gets sarcoidosis? How common is it? 28. What pattern of inflammatory skin disease does sar-coidosis exhibit? 29. How does the pathology of sarcoidosis skin lesions cor-respond to

clinical lesions?

PATTERN: FOLLICULITIS & PERIFOLLICULITIS

Example: Acne Overview Acne most commonly presents as follicle-based comedones, inflammatory papules, or pustules on the face, neck, chest, and back (Figure 8–33). Teenagers are stereotypically afflicted, but neonatal acne and adult acne are also common. Disfiguring nodulocystic acne with resulting severe scarring does not occur before puberty.

FIGURE 8–33 Acne vulgaris. There are numerous inflamed pustules and papules with central black plugs termed open comedones or “blackheads.” (Image used with permission from Dr. Timothy Berger.)

Epidemiology

Acne vulgaris is so common that it is said by some authors to affect practically everyone at some point in their lives. The peak incidence is at 18 years of age, although adults can also have acne. There are studies showing that 3% of men and 5% of women have acne between 40 and 49 years of age.

Histopathology and Pathogenesis

Histopathologically, comedonal acne is manifest as a widened follicle with a dense keratin plug within its infundibulum. If the follicular orifice is patulous, the acne lesion is said to be an open comedone. If the orifice is normal and the follicle is expanded below the skin surface, the lesion is termed a closed comedone. Secondary inflammatory changes occur commonly within plugged follicular units. Neutrophils may accompany the keratinous plug within the follicular canal, creating a pustular lesion. Inflammatory acne lesions are a consequence of follicles that have ruptured with resultant spillage of keratinous debris into the perifollicular dermis, evoking a dense inflammatory reaction with a mixture of neutrophils, lymphocytes, and histiocytes (Figure 8–34).

FIGURE 8–34 Histopathologic features of acne. There is a follicle with a central keratin plug. Surrounding the follicle, there is lymphocytic inflammatory infiltrate. This lesion would correspond to an erythematous papule seen in inflammatory acne (see Figure 8–33).

An understanding of the evolution of acne lesions has led to therapies that are effective for the vast majority of cases. There are four essential components to the development of acne lesions: (1) plugging of the folliculosebaceous unit; (2) sebum production; (3) overgrowth of the bacterium Propionibacterium acnes within the plugged follicle; and (4) a secondary inflammatory response. The formation of keratin plugs within follicles is a complex process thought to be genetically controlled on a cellular level. Keratinocytes become sticky and fail to

slough appropriately, yielding follicular plugging. Contrary to a commonly held belief, being “dirty” does not cause acne, and vigorous or frequent cleansing does not improve the condition. However, some exogenous substances such as oily cosmetics or petrolatum-based hair care products may promote comedone formation and thus exacerbate acne.

Plugged follicles alone would never become more than comedones, however, if it were not for sebum production and P acnes overgrowth. P acnes is a commensal organism of the skin. However, with ample sebum as a food source within the well-protected environment of a plugged follicle, P acnes overgrowth occurs. The sebum is broken down to constituent lipids and free fatty acids. The failure of keratinous debris and sebum to exit the follicle freely expands the follicular canal. The bacteria release factors chemotactic for neutrophils, and their infiltration of the follicle results in pustule formation. Neutrophilic enzymes weaken the follicle wall and follicular rupture occurs, releasing large amounts of inflammatory reactants into the dermis. Lymphocytes, macrophages, and more neutrophils respond, and the comedonal lesion is transformed into an inflamed papule, pustule, or nodule of acne. Follicular rupture and an intense secondary inflammatory reaction may eventuate with profound scarring in some hosts.

Clinical Manifestations

The spectrum of acne severity is quite broad. In the neonate, maternal androgens stimulate enlargement of and sebum overproduction from sebaceous glands. The presence of sebum promotes P acnes overgrowth, and acne ensues until the maternal androgens have cleared and the sebaceous glands atrophy to a normal neonatal size. Significant sebum production does not begin again until puberty. Under the stimulation of androgens at puberty, sebaceous glands enlarge once again and produce sebum in the sebaceous areas of the body, namely the face, neck, chest, and back (the same areas affected most by acne). Onset may be gradual or rapid, and severity may range from primarily comedonal to inflammatory papules and pustules to highly inflammatory, painful nodules. Severe scarring variants may be explosive in onset and present with systemic symptoms of fever and arthralgias. Age at onset and family history are predictors of the severity of acne.

Acne may present as a component of a syndrome, as in polycystic ovary disease (ie, Stein–Leventhal syndrome) or so-called SAPHO syndrome (synovitis, acne, palmoplantar pustulosis, hyperostosis, and osteitis). At least in polycystic ovary disease, there may be hormonal influences that predispose to

the acne lesion development.

CHECKPOINT

30. Why do some infants develop acne? What factors explain its spontaneous resolution?

31. What is the pathophysiology of lesion development in acne? 32. What are some broad treatment categories for acne, and which aspect of

acne pathogenesis does each address?

PATTERN: PERIVASCULAR DERMATITIS WITHOUT EPIDERMAL CHANGE

Example: Urticaria Clinical Presentation Urticaria is a common condition that manifests clinically as relatively transient papules and/or plaques, colloquially called “hives” or “wheals.” In some patients, the disease is short-lived, whereas in others it persists for decades. Urticaria is at one end of a spectrum with angioedema at the other: Urticaria involves the superficial dermis, whereas angioedema involves the deep dermis and subcutaneous tissue. Both urticaria and angioedema are sometimes accompanied by anaphylaxis, a potentially life-threating condition (see Chapter 3).

Epidemiology and Etiology

Urticaria affects approximately 15–25% of the population and affects people of all ages. Urticaria is associated with angioedema in approximately 50% of patients, whereas about 40% of patients have urticaria alone and 10% have angioedema alone. Urticaria is classified as acute if it lasts less than 6 weeks and chronic if it lasts more than 6 weeks.

In some patients, a specific cause of urticaria can be identified, such as sunlight, water, medication, pressure, vibration, heat, cold, exercise, or emotional stress. In other patients, no specific trigger can be pinpointed. Cold

urticaria is usually acquired, although it can be inherited in the familial cold autoinflammatory syndrome owing to a mutation in the CIAS1 gene on chromosome 1q44.

Histopathology and Pathogenesis

Microscopically, urticaria is characterized by a sparse, mixed, perivascular infiltrate without any alterations to the epidermis. The infiltrate is composed mostly of lymphocytes and eosinophils. A few admixed neutrophils are present in some cases. The infiltrate is sparse in intensity (Figures 8–35 and 8–36). Slight dermal edema is often present, although this may be difficult to appreciate microscopically. Most changes are located in the upper dermis.

FIGURE 8–35 Histopathologic features of urticaria. Low-power microscopic features of urticaria include a relatively sparse perivascular and interstitial infiltrate of lymphocytes and eosinophils without epidermal change.

FIGURE 8–36 Histopathologic features of urticaria. A high-power view demonstrates numerous eosinophils (arrow) within the inflammatory infiltrate.

Urticaria is the result of mast cell degranulation that causes the release of histamine and other pro-inflammatory cytokines, such as prostaglandins, leukotrienes, and platelet activating factor. While the type I hypersensitivity reaction mediated by IgE is the classic cause of mast cell degranulation, there are many other mediators of mast cell degranulation, including complement activation, physical stimuli, viral infections, and autoantibodies. The release of histamine causes capillary vasodilation in the superficial dermis with subsequent extravasation of protein-rich fluid into the superficial aspects of the skin and the development of the urticarial papules and/or plaques. The lesions resolve when the fluid gets resorbed. Angioedema is a result of the same process, although it involves the deep dermis and subcutaneous tissue and typically manifests as diffuse swelling rather than as discrete papules or plaques, owing to the deeper location of the changes.

Clinical Manifestations

The primary lesion of urticaria is a papule or plaque without epidermal change, ranging in morphologic shape from round to annular to arcuate (Figure 8–37). The lesions range from erythematous to white and can range in size from a few

millimeters to many centimeters. A surrounding rim of erythema is sometimes present. There is no change to the surface of the skin since the pathology is located in the dermis. Individual urticarial lesions are often pruritic and typically last for less than 1 day. The lesions resolve without any change to the skin. Any part of the body can be affected, although the trunk and extremities are the most common sites of involvement. Urticaria that can be elicited by physical stroking or scratching of the skin is called dermotographism. Angioedema lesions present as diffuse swelling, most often of mucous membranes and/or the hands and feet. These lesions can persist for up to 3 days. If the respiratory tract is involved, the swelling can be life-threatening.

FIGURE 8–37 Urticaria skin lesions with typical erythematous to white pruritic papules and plaques with surrounding rims of erythema. (Used, with permission, from Richard P. Usatine, MD in Usatine RP et al, eds. Color Atlas of Family Medicine, 2nd ed. McGraw-Hill, 2013.)

CHECKPOINT

33. What type of cell causes urticaria upon degranulation? 34. What are some causes of mast cell degranulation? 35. What are the typical clinical and microscopic findings of urticaria and

angioedema?

CASE STUDIES Yeong Kwok, MD

(See Chapter 25, p. 758–62 for answers)

CASE 38

A 25-year-old woman presents with a complaint of rash that has developed over the past several weeks and seems to be progressing. On examination, she is noted to have several plaque-like lesions over the extensor surfaces of both upper and lower extremities as well as similar lesions on her scalp. The plaques are erythematous, with silvery scales, and are sharply marginated.

Questions

A. What is the likely diagnosis? Is this skin disease genetic, environmental, or both? Based on what evidence?

B. What are the pathophysiologic mechanisms behind the development of the plaques, scale, and erythema characteristic of this disorder?

C. What immunologic defects have been implicated in patients with this skin disease?

CASE 39

A 35-year-old woman who recently returned from Africa presents to the clinic with complaints of a rash. During her trip, she developed an itchy rash on both arms. She has an unremarkable medical history. Medications recently taken include chloroquine for malaria prophylaxis. Examination

discloses multiple small violaceous papules on the flexor surfaces of the arms. The lesions have angular borders and flat tops. Some of the lesions have minute white streaks on the surface, barely visible to the naked eye.

Questions

A. What is the likely diagnosis? What is the possible underlying cause? B. What is the pathophysiologic mechanism by which these skin lesions

are formed? C. What histopathologic changes in the skin are responsible for the

appearance of these lesions as violaceous papules with minute white striae?

CASE 40

A 27-year-old woman presents to the urgent care clinic complaining of a red, itchy rash developing suddenly the day before on her arms and legs and spreading to the trunk. She denies ulcers in the mouth or genital area. Her medical history is unremarkable except for occasional episodes of genital herpes. The most recent outbreak was approximately 2 weeks ago. She generally takes oral acyclovir on such occasions, but her prescription has run out and so she did not take any with her last bout. On physical examination, she has multiple erythematous papules over the arms, legs, and trunk. Many of the papules have a central area of duskiness or clearing, such that the lesions resemble targets. There is no evidence of mucosal involvement.

Questions

A. What is the likely diagnosis? B. What is the pathophysiologic mechanism by which these skin lesions

are formed? In what ways is this disease similar to and different from lichen planus?

C. What factors may have triggered this rash? What evidence supports this link?

D. What is responsible for the target-like appearance of these lesions, and

what would the histopathology show?

CASE 41

A 65-year-old man presents to the dermatology clinic with a complaint of blisters developing on his abdomen and extremities over the past week. The lesions consisted initially of red patches followed by blister formation. They are pruritic but not painful. The patient has no other complaints and denies mucous membrane involvement. Examination shows only multiple large, tense blisters with an erythematous base over the lower trunk and extremities. The clinical picture is felt to be most consistent with bullous pemphigoid.

Questions

A. Why is this condition called bullous pemphigoid? B. What would one expect histologic examination to show? C. What would one expect to find on direct immunofluorescence

microscopy? D. What is the presumed mechanism by which blister formation occurs in

bullous pemphigoid?

CASE 42

A 60-year-old man presents to the clinic with complaints of a recurring rash. He states that for the past 2–3 months, he has had several episodes of a painless, nonpruritic rash over his distal lower extremities. The lesions are described as purple and raised. His medical history is remarkable for hepatitis C—with no history of cirrhosis—and peripheral neuropathy. The patient has recently been treated for otitis media with amoxicillin. He has taken no other medications. Physical examination is notable only for multiple reddish-purple papules over the distal lower extremities (palpable purpura). The underlying skin is hyperpigmented. Biopsy reveals

neutrophils, neutrophilic debris, and amorphous protein deposits involving the small blood vessels, consistent with fibrinoid necrosis.

Questions

A. What is the likely dermatologic diagnosis? What are some possible precipitants of that disease in this patient?

B. What is the underlying pathogenetic mechanism by which the lesions are formed?

C. What histologic characteristics are responsible for the appearance of the lesions as papular and purpuric?

D. What additional symptoms should this patient be asked about? Should any laboratory tests be ordered?

CASE 43

A 30-year-old woman presents to the clinic complaining that she has “an itchy rash all over the place.” She noticed that her legs became red, itchy, and blistered about 2 days after she had been hiking in a heavily wooded area. She says that scratching broke the blisters, and afterward the rash became much worse and spread all over. She is convinced that the rash could not be poison ivy because once before she was exposed to that plant and did not develop a rash. On examination, there are erythematous vesicles and bullae in linear streaks on both of her legs. Some areas are weepy, with a yellowish crust. There are ill-defined erythematous plaques studded with papulovesicles on the trunk and arms.

Questions

A. What is the likely diagnosis? What feature on the physical examination is the cardinal sign?

B. What made the eruption spread? C. How do you explain the diagnosis to the patient in light of the fact that

she did not develop a rash after known exposure to poison ivy in the past? Why didn’t the rash appear until 2 days after the apparent exposure?

CASE 44

A 45-year-old woman presents to clinic with a rash on her legs for 2 months. She notes that the rash started soon after babysitting her niece, who had “strep throat.” She initially had a sore throat herself, but it stopped hurting after she took 2 days’ worth of antibiotics she had left over from a previous prescription. On examination, on the anterior lower legs, she has several scattered ill-defined erythematous nodules, which are tender to palpation.

Questions

A. What is the likely diagnosis? What is the probable cause? What might explain why the eruption has persisted?

B. What are some other common causes of this condition? C. What is the pathophysiologic mechanism of skin lesion formation? D. What are the histopathologic findings of this disease?

CASE 45

A 52-year-old African American man presents to the clinic with a rash that has been worsening for several months. Review of systems is notable for a chronic cough. Examination reveals multiple red-brown, dermis-based papules on the trunk, arms, and face. Several lesions are clustered near the nares. The examination is otherwise unremarkable.

Questions

A. What is the likely diagnosis? What information is necessary to confirm the diagnosis?

B. Which organ system (in addition to the skin) is at risk for disease involvement based on the clinical examination?

C. What are the histopathologic features of this disease? D. How does the disease present clinically?

CASE 46

A 15-year-old girl presents to the clinic complaining of “pimples” for 6 months. She has been using an over-the-counter face wash four times a day to keep the oil and dirt off, but it has not helped. Examination reveals several dozen erythematous papules and pustules over the forehead and central face with scattered open and closed comedones. A diagnosis of moderate inflammatory acne is entertained.

Questions

A. Why has this patient’s meticulous cleansing practice not helped her condition? What advice would you give her regarding facial cleansing?

B. What is the life cycle of an inflammatory acne papule? C. What are some general categories of acne treatment, and what

component of lesion development does each address?

CASE 47

A 12-year-old boy is brought to the pediatrician’s office for evaluation of hives. He has no significant past medical history and no history of allergies. He has just joined the middle school cross-country team and noticed that he gets hives about 10 minutes into each training run. The hives are very itchy, irregular blotches on his legs and trunk, about 10–20 cm in size, and they persist for about 30 minutes. He does not experience swelling of the lips or oropharynx and denies any wheezing or shortness of breath. His physical examination is normal without skin lesions or oral swelling at that moment, and his lungs are clear. He does have mild dermatographism with wheals that form on the back after stroking with a tongue depressor.

Questions

A. What is the likely cause of the patient’s hives (urticaria)? B. What is the cellular mechanism of the urticaria? C. What more serious complication is often associated with urticaria?

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CHAPTER

9

Pulmonary Disease Thomas H. Sisson, MD, Dru Claar, MD, Mark S. Chesnutt, MD, & Thomas J. Prendergast, MD

The principal physiologic role of the lungs is to make oxygen available to tissues for metabolism and to remove the main byproduct of that metabolism, carbon dioxide. The lungs perform this function by moving inspired air into close proximity to the pulmonary capillary bed to enable gas exchange by simple diffusion. This is accomplished at a minimal workload, is regulated efficiently over a wide range of metabolic demand, and takes place with close matching of ventilation to lung perfusion. The extensive surface area of the respiratory system must also be protected from a broad variety of infectious or noxious environmental insults.

Humans possess a complex and efficient respiratory system that satisfies these diverse requirements. When injury to components of the respiratory system occurs, the integrated function of the whole is disrupted. The consequences can be profound. Airway injury or dysfunction results in obstructive lung diseases, including asthma and chronic obstructive pulmonary disease (COPD). Injury to the pulmonary parenchyma can produce restrictive lung diseases, such as idiopathic pulmonary fibrosis, acute respiratory distress syndrome, and pulmonary vascular disease. To understand the clinical presentations of lung disease, it is necessary first to understand the anatomic and functional organization of the lungs that determines normal function.

CHECKPOINT

1. What are the two principal physiologic roles of the lungs? 2. What are the requirements for successful lung function?

NORMAL STRUCTURE & FUNCTION OF THE LUNGS

ANATOMY

The mature respiratory system consists of visceral pleura-covered lungs contained by the chest wall and diaphragm, the latter serving under normal conditions as the principal bellows muscle for ventilation. The lungs are divided into lobes, each demarcated by intervening visceral pleura. Each lung possesses an upper and lower lobe; the middle lobe and lingula are the third lobes in the right and left lungs, respectively. At end expiration, most of the volume of the lungs is air (Table 9–1), whereas almost half of the mass of the lungs is accounted for by blood volume. It is a testament to the delicate structure of the gas-exchanging region of the lungs that alveolar tissue has a total weight of only 250 g but a total surface area of 75 m2.

TABLE 9–1 Components of a normal human lung.

Connective tissue fibers and surfactant serve to maintain the anatomic integrity of this large and complex surface area. The connective tissue fibers are highly organized collagen and elastic structures that radiate into the interstitium. These fibers divide segments, invest airways and vessels, and support alveolar walls with a delicate, elastic fibrous network. The multidirectional elastic support provided by this network allows the lung, from alveoli to conducting airways, to support itself and retain airway patency despite large changes in volume.

Surfactant is a complex material produced by type II alveolar cells; it is composed of multiple phospholipids and specific associated proteins. The physiologic function of a subset of surfactant lipoproteins is to enhance the anatomic stability of the alveoli. The presence of surfactant covering the alveolar epithelial surface reduces surface tension, allowing expansion of alveoli with a transpulmonary distending pressure of less than 5 cm H2O. In the absence of this surface-active layer, increasing surface tension associated with a reduction of alveolar volume during expiration would result in the collapse of alveoli. The distending pressure required to re-expand these collapsed alveoli would be greater than normal ventilatory effort could produce.

Airway & Epithelial Anatomy Further anatomic division of the lungs is based primarily on the separation of the tracheobronchial tree into conducting airways, which provide for movement of air from the external environment to areas of gas exchange, and terminal respiratory units, or acini, the airways and associated alveolar structures participating directly in gas exchange (Figure 9–1). The proximal conducting airways are lined with ciliated pseudostratified columnar epithelial cells, are supported by a cartilaginous skeleton in their walls, and contain secretory glands in the epithelial wall. The ciliated epithelium has a uniform orientation of cilia that beat in unison toward the pharynx. This ciliary action, together with the mucus layer produced by submucosal mucous secretory glands, provides a mechanism for the continuous transport of contaminating or excess material out of the lungs. Circumferential airway smooth muscle is also present but, as with secretory glands, is reduced and then lost as the airways branch farther into the lung and diminish in caliber. The smallest conducting airways are nonrespiratory bronchioles. These are characterized by a loss of smooth muscle and cartilage but a retention of a cuboidal epithelium that may be ciliated and is not a site of gas exchange. The lobes of the lung are divided into less distinct lobules, defined as collections of terminal respiratory units incompletely bounded by connective tissue septa. Terminal respiratory units are the final physiologic and anatomic unit of the lung, with walls of thin alveolar epithelial cells that provide gas exchange with the alveolar–capillary network.

FIGURE 9–1 Subdivision of conducting airways and terminal respiratory units. This schematic illustration demonstrates the subdivisions of both the conducting airways and the respiratory airways. Successive branching produces increasing generations of airways, beginning with the trachea. Note that gas-exchanging segments of the lung are encountered only after extensive branching, with a concomitant decrease in airway caliber and an increase in total cross-sectional area (see Figures 9–2 and 9–3). (Redrawn, with permission, from Weibel ER. Morphometry of the Human Lung. Academic Press, 2013.)

The principal site of resistance to airflow in the lungs is in medium-sized bronchi (Figure 9–2). This seems counterintuitive because one would expect smaller-caliber airways to be the major site of resistance, but repetitive branching of the small airways leads to a profound increase in cross-sectional area that does not contribute significantly to airway resistance in healthy individuals (Figure 9–3). Under pathologic conditions such as asthma, in which smaller bronchi and bronchioles become narrowed, airway resistance can increase dramatically.

FIGURE 9–2 Location of the principal site of airflow resistance. The second- through fifth- generation airways include the segmental bronchi and larger bronchioles. They present the greatest resistance to airflow in normal individuals. The smaller airways contribute relatively little despite their smaller caliber because of the enormous number arranged in parallel. Compare with Figure 9–3. (Adapted, with permission of Elsevier, from Pedley TJ et al. The prediction of pressure drop and variation of resistance within the human bronchial airways. Respir Physiol. 1970;9(3):387.)

FIGURE 9–3 Airway generation and total airway cross-sectional area. Note the extremely rapid increase in total cross-sectional area in the respiratory zone (compare with Figure 9–1) and the fall in resistance as a consequence of the increase in cross-sectional area (compare with Figure 9–2). As a result, the forward velocity of gas during inspiration becomes very low at the level of the respiratory bronchioles, and gas diffusion becomes the chief mode of ventilation. (Redrawn, with permission, from West JB et al. West’s Respiratory Physiology: The Essentials, 10th ed. Lippincott Williams & Wilkins, 2015.)

The pulmonary arteries are found in close association with the branching bronchial tree in the lungs (Figure 9–4). Both arterial blood flow and bronchial airflow are actively regulated by changing vessel or airway caliber. The anatomic relationship between arteries and bronchi provides an ideal setting for the continuous matching of ventilation and perfusion to different lung segments.

FIGURE 9–4 Airway, vascular, and lymphatic anatomy of the lung. This schematic diagram demonstrates the general anatomic relationships of the airways and terminal respiratory units with the vascular and lymphatic systems of the lung. Important points are as follows: (1) The pulmonary arterial system runs adjacent to the bronchial tree, while the draining pulmonary veins are found distant from the airways; (2) the bronchial wall blood supply is provided by bronchial arteries, branches of systemic arterial origin; (3) lymphatics are found adjacent to both the arterial and venous systems and are very abundant in the lung; and (4) lymphatics are found as far distally as the terminal respiratory bronchioles, but they do not penetrate to the alveolar wall. (A, alveolus; AD, alveolar duct; RB, respiratory bronchiole; TB, terminal bronchiole.) (Redrawn, with permission, from Staub NC. The physiology of pulmonary edema. Hum Pathol. 1970;1:419. Copyright © Elsevier.)

Vascular & Lymphatic Anatomy The pulmonary vascular system includes two distinct circuits that distribute blood through the lungs: the pulmonary and bronchial circulations. The right ventricle pumps its entire output of mixed venous blood through pulmonary arteries toward alveolar capillaries. Pulmonary arteries and arterioles are smooth

muscle–invested vessels located adjacent to bronchi within the pulmonary bronchovascular bundle. Pulmonary arterioles are very sensitive to alveolar PO2, with a prominent vasoconstrictor response to hypoxia. Hypoxic pulmonary vasoconstriction allows matching of alveolar perfusion to ventilation (see below). Pulmonary veins arise from alveolar capillaries to form vessels that traverse the intralobular septa to return oxygenated blood to the left atrium.

Bronchial arteries that arise from the aorta and intercostal arteries deliver oxygenated blood at systemic pressures to nearly all the intrapulmonary structures proximal to the terminal bronchioles, including the bronchial tree, hilar structures, pulmonary arteries and veins, pulmonary nervous system and lymphatics, connective tissue septa, and visceral pleura. Most lung tumors receive their blood supply from the bronchial circulation. There are abundant bronchopulmonary anastomoses at the arteriolar and capillary levels that are silent in health but which may enlarge in disease to contribute to hemoptysis. Drainage of the bronchial circulation occurs both to the right atrium via the azygos vein and to the left atrium via the pulmonary veins. The latter represents an anatomic shunt of deoxygenated blood, typically representing less than 5% of cardiac output.

Pulmonary lymphatics arise in connective tissue spaces beneath the visceral pleura and in deep plexuses at the junction of the terminal bronchioles and alveoli. Lymphatics do not enter the alveolar peri-interstitial space (see Figure 9– 4). As a result, fluid in the alveolar interstitium must move to the region of terminal bronchioles to gain access to draining lymphatics. Lymphatic ducts travel principally in the peribronchovascular sheath back to hilar and mediastinal lymph nodes before entering the left thoracic duct or right lymphatic duct. Lymphatic drainage of the pleural space occurs through plexuses investing the costal, diaphragmatic, and mediastinal parietal pleura that are anatomically separate from pulmonary lymphatics.

Pulmonary Nervous System The lungs are richly innervated with neural fibers from parasympathetic (vagal), sympathetic, and the so-called nonadrenergic, noncholinergic (NANC) systems. Efferent fibers include the following: (1) parasympathetic fibers, with muscarinic cholinergic efferents that mediate bronchoconstriction, pulmonary vasodilation, and mucous gland secretion; (2) sympathetic fibers, whose stimulation produces bronchial smooth muscle relaxation, pulmonary vasoconstriction, and secretory gland activity inhibition; and (3) the NANC system, with multiple transmitters implicated, including adenosine triphosphate

(ATP), nitric oxide (NO), and peptide neurotransmitters such as substance P and vasoactive intestinal peptide (VIP). The NANC system participates in inhibitory events, including bronchodilation, and may function as the predominant reciprocal balance to the excitatory cholinergic system.

Pulmonary afferents consist principally of the vagal sensory fibers. These include the following:

1. Fibers from bronchopulmonary stretch receptors, located in the trachea and proximal bronchi. Stimulation of these fibers by lung inflation results in bronchodilation and an increased heart rate.

2. Fibers from irritant receptors, also found in proximal airways. Stimulation of these fibers by diverse nonspecific stimuli elicits efferent responses, including cough, bronchoconstriction, and mucus secretion.

3. C fibers, or fibers from juxtacapillary (J) receptors, which are unmyelinated fibers ending in lung parenchyma and bronchial walls and respond to mechanical and chemical stimuli. The reflex responses associated with C fiber stimulation include a rapid shallow breathing pattern, mucus secretion, cough, and heart rate slowing with inspiration.

Immune Structure & Function Of all the body’s organs, the lungs have a unique exposure to environmental insults. Nonexertional ventilation in an adult totals about 7500 L of air per day, an amount increased substantially with activity. This exposure to an open, nonsterile environment imposes an ongoing risk of toxic, infectious, and inflammatory insults. Furthermore, the pulmonary circulation contains the only capillary bed through which the entire circulating blood volume must flow in each cardiac cycle. As a consequence, the lung is an obligatory vascular sieve and functions as a principal site of defense against the hematogenous spread of infection and other noxious influences. Protecting the lungs from environmental and infectious injury involves a set of complex responses capable of providing timely and successful defense against attack via the airways or vascular bed. As outlined in Table 9–2, it is convenient for discussion purposes to separate these responses into two major categories—nonspecific physical and chemical protections and specific immune structures and actions—all functioning to prevent injury to or microbial invasion of the very large epithelial and vascular area of the lung.

TABLE 9–2 Lung defenses.

LUNG VOLUMES, CAPACITIES & THE NORMAL SPIROGRAM

The volume of gas in the lungs is divided into volumes and capacities as shown by the bars at the left of the figure below. Lung volumes are primary: They do not overlap each other. Tidal volume (VT) is the amount of gas inhaled and exhaled with each resting breath. A normal tidal volume in a 70 kg person is approximately 350–400 mL. Residual volume (RV) is the amount of gas remaining in the lungs at the end of a maximal exhalation. Lung capacities are composed of two or more lung volumes. The vital capacity (VC) is the total amount of gas that can be exhaled after a maximal inhalation. The vital capacity and the residual volume together constitute the total lung capacity (TLC), or the total amount of gas in the lungs at the end of a maximal inhalation. The functional residual capacity (FRC) is the amount of gas in the lungs at the end of a resting tidal breath. (ERV, expiratory reserve volume; IC, inspiratory capacity; IRV, inspiratory reserve volume.)

The spirogram at the right in the figure is drawn in real time. The first tidal breath shown takes 5 seconds, indicating a respiratory rate of 12 breaths/min. The forced vital capacity (FVC) maneuver begins with an inhalation from FRC to TLC (lasting about 1 second) followed by a forceful exhalation from TLC to RV (lasting about 5 seconds). The amount of gas exhaled during the first second of this maneuver is the forced expiratory volume in 1 second (FEV1). Normal subjects expel approximately 80% of the FVC in the first second. The ratio of the FEV1 to FVC (referred to as the FEV1/FVC or FEV1%) is diminished in patients with obstructive lung disease and increased in patients with restrictive lung disease.

(Redrawn, with permission, from Staub NC. Basic Respiratory Physiology. Churchill Livingstone, 1991. Copyright © Elsevier.)

CHECKPOINT

3. What are the roles of the connective tissue and sur-factant systems in lung function?

4. What is the role of the ciliary action of the respiratory epithelium? 5. Why are medium-sized bronchi, rather than small air-ways, the major

site of resistance to airflow in the lungs? 6. What are the physiologic functions of the efferent para-sympathetic,

sympathetic, and NANC neural systems of the lung? 7. What are the categories of afferent vagal sensory receptors? 8. What are the different roles of the pulmonary and bron-chial arteries? 9. What sensitive mechanism do the pulmonary arteries have for matching

alveolar perfusion with ventilation? 10. What are the components of the nonspecific defense system of the

lungs? 11. What are the humoral and cellular components of the specific immune

defense system of the lungs?

PHYSIOLOGY

At rest, the lungs take 4 L/min of air and 5 L/min of blood, direct them within 0.2 μm of each other, and then return both to their respective pools. With maximal exercise, flow may increase to 100 L/min of ventilation and 25 L/min of cardiac output. The lungs thereby perform their primary physiologic function of making oxygen available to the tissues for metabolism and removing the major byproduct of that metabolism, carbon dioxide. The lungs perform this task largely free of conscious control while maintaining PaCO2 within a 5% tolerance. It is a magnificent feat of evolutionary plumbing and neurochemical control.

Static Properties: Compliance & Elastic Recoil The lung maintains its extremely thin parenchyma over an enormous surface area by means of an intricate supporting architecture of collagen and elastin fibers. Anatomically, as well as physiologically and functionally, the lung is an elastic organ.

The lungs inflate and deflate in response to changes in the volume of the semi-rigid thoracic cage in which they are suspended. An analogy is the inflation of a blacksmith’s bellows by pulling the handles apart, thus increasing the volume of the bellows, lowering pressure, and causing inflow of air. Air enters the lungs when pressure in the pleural space is reduced by the expansion of the chest wall. The volume of air entering the lungs depends on the change in pleural pressure and the compliance of the respiratory system. Compliance is an intrinsic elastic property that relates a change in volume to a change in pressure. The compliance of both the chest wall and the lungs contributes to the compliance of the respiratory system (Figure 9–5). The compliance of the chest wall does not change significantly with thoracic volume, at least within the physiologic range. The compliance of the lungs varies inversely with lung volume. At functional residual capacity (FRC), the lungs are normally very compliant, approximately 200 mL/cm H2O. Thus, a reduction of only 5 cm H2O pressure in the pleural space will draw a breath of 1 L.

FIGURE 9–5 Interaction of the pressure–volume properties of the lungs and the chest wall. Resting lung volume (FRC) represents the equilibrium point at which the elastic recoil of the lung (tendency to collapse inward) and the chest wall (tendency to spring outward) are exactly balanced. Other lung volumes can also be defined by reference to this diagram. Total lung capacity (TLC) is the point at which the inspiratory muscles cannot generate sufficient force to overcome the elastic recoil of the lungs and chest wall. Residual volume (RV) is the point at which the expiratory muscles cannot generate sufficient force to overcome the elastic recoil of the chest wall. Compliance is calculated by taking the slope of these pressure–volume relationships at a specific volume. Note that the compliance of the lungs is greater at low lung volumes but falls considerably above two-thirds of vital capacity. (Modified, with permission, from Staub NC. Basic Respiratory Physiology. Churchill Livingstone, 1991. Copyright © Elsevier.)

The tendency of a deformable body to return to its baseline shape is its elastic recoil. The elastic recoil of the chest wall is determined by the shape and structure of the thoracic cage. Lung elastic recoil is determined by two factors: tissue elasticity and the forces needed to change the shape of the air–liquid interface of the alveolus (Figure 9–6). Expanding the lungs requires overcoming local surface forces that are directly proportionate to alveolar surface tension. Surface tension is a physical property that reflects the greater attraction between molecules of a liquid than between molecules of that liquid and adjacent gas. At the air–liquid interface of the lung, molecules of water at the interface are more strongly attracted to each other than they are to the air above. This creates a net force drawing water molecules together in the plane of the interface. If the interface is stretched over a curved surface, that force acts to collapse the curve. The law of Laplace quantifies this force: The pressure needed to keep the curve open (in this case represented by a sphere) is directly proportionate to the surface tension at the interface and inversely proportionate to the radius of the sphere (Figure 9–7).

FIGURE 9–6 The effect of surface forces on lung compliance: a simple experiment demonstrating the effect of surface tension at the air–liquid interface of excised cat lungs. When inflated with saline, there are no surface forces to overcome, and the lungs are both more compliant and show no difference (hysteresis) between the inflation and deflation curves. When inflated with air, the pressure required to distend the lung is greater at every volume. The difference between the two curves represents the contribution of surface forces. There is also a pronounced hysteresis to lungs inflated with air that reflects surfactant recruited into the alveolar liquid during inflation (upward arrow), where it further reduces surface forces during deflation (downward arrow). (Reproduced, with permission, from Clements JA, Tierney DF. Alveolar instability associated with altered surface tension. In: Handbook of Physiology, Respiration. Sect. 3, Vol. II, Chapter 69. Washington, DC: American Physiological Society; 1965:1565–84.)

FIGURE 9–7 The importance of surface tension. If two connected alveoli have the same surface tension, then the smaller the radius, the greater the pressure tending to collapse the sphere. This could lead to alveolar instability, with smaller units emptying into larger ones. Alveoli typically do not have the same surface tension, however, because surface forces vary according to surface area as a result of the presence of surfactant: The relative concentration of surfactant in the surface layer of the sphere increases as the radius of the sphere falls, augmenting the effect of surfactant at low lung volumes. This tends to counterbalance the increase in pressure needed to keep alveoli open at diminished lung volume and adds stability to the alveoli, which might otherwise tend to collapse into one another. Surfactant thus protects against the regional collapse of lung units, a condition known as atelectasis, in addition to its other functions. (r, radius of alveolus; P, gas pressure; T, surface tension.)

Surfactant is a mixture of phospholipid (predominantly dipalmitoylphosphatidylcholine [DPPC]) and specific surfactant proteins. These hydrophobic molecules displace water molecules from the air–liquid interface, thereby reducing surface tension. This reduction has three physiologic implications. First, it reduces the elastic recoil pressure of the lungs, thereby reducing the pressure needed to inflate them. This results in reduced work of breathing. Second, it allows surface forces to vary with alveolar surface area, thereby promoting alveolar stability and protecting against collapse and atelectasis (see Figure 9–7). Third, it limits the reduction of hydrostatic pressure in the pericapillary interstitium caused by surface tension. This reduces the forces promoting fluid transudation and the tendency to accumulate interstitial edema.

Pathologic states may result from reduced lung elastic recoil causing an increase in compliance (emphysema), increased interstitial connective tissue leading to decreased compliance (pulmonary fibrosis), or disruption of surfactant with an increase in surface forces (infant respiratory distress syndrome [IRDS]) (Figure 9–8).

FIGURE 9–8 Static expiratory pressure–volume curves in normal subjects and patients with emphysema and pulmonary fibrosis. The underlying physiologic abnormality in emphysema is a dramatic increase in lung compliance. Such patients tend to breathe at very high lung volumes. Patients with pulmonary fibrosis have very noncompliant lungs and breathe at low lung volumes. (Redrawn, with permission, from Pride NB et al. Lung mechanics in disease. In: Fishman AP, ed. Vol III, Part 2, of Handbook of Physiology.Section 3. Respiratory. American Physiological Society, 1986.)

Dynamic Properties: Flow & Resistance Inflation of the lungs must overcome three opposing forces: elastic recoil, including surface forces; inertia of the respiratory system; and resistance to airflow. Since inertia is negligible, the work of breathing can be divided into work to overcome elastic forces and work to overcome flow resistance.

Increased elastic forces predominate in two common disorders: diffuse parenchymal fibrosis and obesity. The reduction in lung compliance in fibrotic lung disease, and in chest wall and respiratory system compliance in obesity,

increases the work of breathing. Obese subjects also experience increased airflow resistance, largely though not entirely because of their tendency to breathe at lower lung volumes.

Flow resistance depends on the nature of the flow. Under conditions of laminar or streamlined flow, resistance is described by the Poiseuille equation: Resistance is directly proportionate to the length of the airway and the viscosity of the gas, and inversely proportionate to the fourth power of the radius. A reduction by one-half of the airway radius leads to a 16-fold increase in airway resistance. Airway caliber is, therefore, the principal determinant of airway resistance under laminar flow conditions. Under conditions of turbulent flow, the driving pressure needed to achieve a given flow rate is proportionate to the square of the flow rate. Turbulent flow also depends on gas density and not on gas viscosity.

Most of the resistance to normal breathing arises in medium-sized bronchi and not in smaller bronchioles (see Figure 9–2). There are three main reasons for this counterintuitive finding. First, airflow in the normal lung is not laminar but turbulent, at least from the mouth to the small peripheral airways. Thus, where flow is highest (in segmental and subsegmental bronchi), resistance depends chiefly on flow rates. Second, in small peripheral airways, where airway caliber is the principal determinant of resistance, repetitive branching creates a very large number of small airways arranged in parallel. Their resistance is reciprocally additive, making their contribution to total airway resistance minor under normal conditions. Third, there is a transition to laminar flow approaching the terminal bronchioles as a consequence of increased cross-sectional area and decreased flow rates (see Figure 9–3). In the respiratory bronchioles and alveoli, bulk flow of gas ceases and gas movement occurs by diffusion.

Airway resistance is determined by several factors. Many disease states affect bronchial smooth muscle tone and cause bronchoconstriction, producing an abnormal narrowing of the airways. Airways may also be narrowed by hypertrophy (chronic bronchitis) or infiltration (sarcoidosis) of the airway mucosa. Physiologically, the radial traction of the lung interstitium supports the airways and increases their caliber as lung volume increases. Conversely, as lung volume decreases, airway caliber also decreases and resistance to airflow increases. Patients with airflow obstruction often breathe at large lung volumes because higher volumes tend to increase elastic lung recoil, maximize airway caliber, and minimize flow resistance.

Analysis in terms of laminar and turbulent flow assumes that airways are rigid tubes. In fact, they are highly compressible. The compressibility of the

airways underlies the important phenomenon of effort-independent flow; airflow rates during expiration can be increased with effort only up to a certain point. Beyond that point, further increases in effort do not increase flow rates. The explanation for this phenomenon relies on the concept of an equal pressure point. Pleural pressure is generally negative (sub-atmospheric) throughout quiet breathing. Peribronchiolar pressure, the pressure surrounding small, noncartilaginous conducting airways, is closely related to pleural pressure. Hence, during quiet breathing, conducting airways are surrounded by negative pressure that helps keep them open. Pleural and peribronchiolar pressure becomes positive during forced expiration, subjecting distensible conducting airways to positive pressure. The equal pressure point occurs where the surrounding peribronchiolar pressure equals or exceeds pressure inside the airway, causing dynamic compression of the airways, which leads to instability and potential airway collapse (Figure 9–9).

FIGURE 9–9 The concept of the equal pressure point. For air to flow through a tube, there must be a pressure difference between the two ends. In the case of forced expiration with an open glottis, this driving pressure is the difference between alveolar pressure (the sum of pleural pressure and lung elastic recoil pressure) and atmospheric pressure (assumed to be zero). Frictional resistance causes a fall in this driving pressure along the length of the conducting airways. At some point, the driving pressure may equal the surrounding peribronchial pressure; in this event, the net transmural pressure is zero. This defines the equal pressure point. Downstream (toward the mouth) from the equal pressure point, pressure outside the airway is greater than the driving pressure inside the airway. This net negative pressure tends to collapse the

airway, resulting in dynamic compression. The more forcefully one expires, the more the pressure surrounding collapsible airways increases. Flow becomes effort independent. (Palv, alveolar pressure; Patm, atmospheric pressure; PL, lung elastic recoil pressure; Ppl, pleural pressure.)

The equal pressure point is not an anatomic site but a functional result that helps clarify different mechanisms of airflow obstruction. Because the driving pressure of expiratory airflow is principally lung elastic recoil pressure, a loss of lung elasticity that reduces recoil pressure without changing pleural or peribronchiolar pressure will lead to dynamic compression at higher lung volumes. The resultant air trapping contributes to symptomatic dyspnea in patients with obstructive lung disease. Patients with emphysema lose lung elastic recoil and may have severely impaired expiratory flow even with airways of normal caliber. The presence of airway disease will increase the drop in driving pressure along the airways and may generate an equal pressure point at even higher lung volumes. Conversely, an increase in recoil pressure will oppose dynamic compression. Patients with pulmonary fibrosis may have abnormally high flow rates despite severely reduced lung volumes.

The Work of Breathing A constant minute ventilation can be achieved through multiple combinations of respiratory rate and tidal volume. The two components of the work of breathing —elastic forces and resistance to airflow—are affected in opposite ways by changes in frequency and depth of breathing. Elastic resistance is minimized by frequent shallow breaths; resistive forces are minimized by fewer larger-tidal- volume breaths. Figure 9–10 shows how these two components can be summed to provide a total work of breathing for different frequencies at a fixed minute ventilation. The set point for basal respiration is the point at which the total work of breathing is minimized. In normal humans, this occurs at a frequency of approximately 15 breaths/min. In different diseases, this pattern is altered to compensate for the underlying physiologic abnormality.

FIGURE 9–10 Minimizing the work of breathing. These diagrams divide the total work of breathing at the same minute ventilation into elastic and resistive components. In disease states that increase elastic forces (eg, pulmonary fibrosis), total work is minimized by rapid, shallow breathing; with increased airflow resistance (eg, chronic bronchitis), total work is minimized by slow, deep breathing. (Redrawn, with permission, from Lumb AB. Nunn’s Applied Respiratory Physiology, 8th ed. Elsevier, 2016. Copyright © Elsevier.)

The amount of energy needed to maintain the respiratory muscles during quiet breathing is small, approximately 2% of basal oxygen consumption. In patients with lung disease, the energy requirements are greater at rest and increase dramatically with exercise. Patients with emphysema may not be able to increase their ventilation by more than a factor of 2 because the oxygen cost of breathing exceeds the additional oxygen made available to the body.

Oxygen Transport Oxygen is poorly soluble in blood. At a temperature of 37°C and an oxygen partial pressure of 100 mm Hg (PaO2 = 100), total oxygen dissolved in 100 mL of whole blood is approximately 0.3 mL. Because basal oxygen consumption in the average adult human is approximately 250 mL/min, dissolved oxygen content would be inadequate to meet metabolic demands. Instead, the high oxygen needs of complex internal organs are met by a soluble protein that binds oxygen rapidly, reversibly, and with a high storage capacity, namely, hemoglobin.

Hemoglobin is a complex tetramer of two alpha and two beta polypeptide chains, each of which contains a heme group with an iron atom in the ferrous

form (Fe2+) at its center capable of binding molecular oxygen (O2). Each molecule of hemoglobin can bind four oxygen molecules. Under physiologic conditions, 1 g of fully saturated hemoglobin can carry approximately 1.34 mL of oxygen. Therefore, 100 mL of blood containing 15 g/dL of saturated hemoglobin contains 20.1 mL of O2, nearly 70 times the amount in solution. The conventional way to represent oxygen bound to hemoglobin is the hemoglobin saturation (SO2), the ratio of oxygen bound to hemoglobin divided by the total oxygen-binding capacity, typically expressed as a percentage. Note that SO2 alone does not determine oxygen content. Blood oxygen content is the sum of two terms: dissolved oxygen and oxygen bound to hemoglobin. Dissolved oxygen is a linear function of the oxygen partial pressure (PO2) and solubility, whereas oxygen bound to hemoglobin is the product of three terms: oxygen- carrying capacity, hemoglobin concentration, and hemoglobin saturation (SO2):

CO2 = (0.003 × PO2) + (1.34 × [Hemoglobin] × SO2)

This equation explains why oxygen content and tissue oxygen delivery may be low despite 100% SO2 if hemoglobin concentration is markedly reduced.

Because of its physical chemistry, hemoglobin saturation has a complex relationship with the partial pressure of oxygen. Interactions among the four polypeptide chains in the heme molecule increase overall affinity for oxygen as each oxygen-binding site is filled. If we graph PO2 against SO2 to represent the oxyhemoglobin dissociation curve, we see that the relationship is not linear but S-shaped or sigmoidal (Figure 9–11). The curve is very steep in the physiologic range, (ie, between 10 and 70 mm Hg PO2). Above a partial pressure of 70 mm Hg, the oxyhemoglobin dissociation curve flattens. This relationship explains the suitability of hemoglobin for its primary physiologic role: the reversible binding to oxygen with uptake in the lungs and release in the tissues. Both illness and changes in altitude can cause the PO2 to decrease significantly, but as long as the partial pressure remains above 70 mm Hg, there will be a minimal consequence on the O2 saturation of hemoglobin and, in turn, the oxygen content. From 70 to 40 mm Hg, a fall in PO2 is associated with a proportionally larger increase in the release of oxygen from hemoglobin while retaining a relatively high end- capillary PO2 to promote oxygen diffusion into tissues. Below 40 mm Hg, small changes in PO2 continue to release oxygen to tissues, down to very low PO2 levels encountered in some capillary beds.

FIGURE 9–11 Oxygen–hemoglobin dissociation curve. pH 7.40, temperature 38°C. (Data from Severinghaus JW. Blood gas calculator. J Appl Physiol. 1966;21:1108.)

Distribution of Ventilation & Perfusion Inhaled air and pulmonary arterial blood flow are not distributed equally to all lung regions. In healthy individuals, heterogeneous distribution is due principally to two factors: the effects of gravity and the fractal geometry of repetitive branching of airways and vessels.

Pleural pressure varies from the top to the bottom of the lung by approximately 0.25 cm H2O/cm. It is more negative at the apex and more positive at the base. The effect is shifted to an anteroposterior distribution in the supine position and is greatly diminished (although not abolished) at zero gravity. Regional ventilation depends on regional pleural pressure (Figure 9–12). More negative pleural pressure at the lung apex causes greater expansion of the apical alveoli. Because lung compliance is higher at lower lung volumes, ventilation is preferentially distributed to the lower lobes at FRC.

FIGURE 9–12 Distribution of ventilation at different lung volumes. The effect of gravity and the weight of the lung cause pleural pressure to become more negative toward the apex of the lung. The effect of this change in pressure is to increase the expansion of apical alveoli. A: Total lung capacity. At high lung volumes, the compliance curve of the lung is flat; alveoli are almost equally expanded because pressure differences cause small changes in lung volume. B: Functional residual capacity. During quiet breathing, the lower lobes are on the steep part of the pressure–volume curve. This increased compliance at lower volumes is why ventilation at FRC is preferentially distributed to the lower lobes. C: Residual volume.

Below functional residual capacity (FRC), there may be dependent lung units that are exposed to positive pleural pressures. These units may collapse, leading to areas of lung that are perfused but not ventilated. (Reprinted, with permission, from Hinshaw HC et al. Diseases of the Chest, 4th ed. WB Saunders, 1979. Copyright © Elsevier.)

Pulmonary blood flow is a low-pressure system that functions in a gravitational field across 30 vertical centimeters. In the upright position, there is a nearly linear increase in blood flow from the top to the bottom of the lung. Within any horizontal (isogravitational) plane, however, there is significant heterogeneity of blood flow because of the fractal geometry of repetitive vessel branching, resulting in heterogeneous resistance. Figure 9–13 illustrates the details of this distribution.

FIGURE 9–13 Effect of changing hydrostatic pressure on the distribution of pulmonary blood flow. Capillary blood flow in different regions of the lung is governed by three pressures: pulmonary arterial pressure, pulmonary venous pressure, and alveolar pressure. Pulmonary arterial pressure must be greater than pulmonary venous pressure to maintain forward perfusion; there are, therefore, three potential arrangements of these variables. Zone 1: Palv > Part > Pven. There is no capillary perfusion in areas where alveolar pressure is greater than the capillary perfusion pressure. Because alveolar pressure is normally zero, this occurs only where mean pulmonary arterial pressure is less than the vertical distance from the pulmonary artery. Zone 2: Part > Palv > Pven. Pulmonary arterial pressure exceeds alveolar pressure, but alveolar pressure exceeds pulmonary venous pressure. The driving pressure along the capillary is dissipated by resistance to flow until the transmural pressure is negative and compression occurs. This zone of collapse then regulates flow, which is intermittent and dependent on fluctuating pulmonary venous pressures. Zone 3: Part > Pven > Palv. Flow is independent of alveolar pressure because the pulmonary

venous pressure exceeds atmospheric pressure. Zone 4: Zone of extra-alveolar compression. In dependent lung regions, lung interstitial pressure may exceed pulmonary arterial pressure. In this event, capillary flow is determined by compression of extra-alveolar vessels. The right side of the diagram shows a near- continuous distribution of blood flow from the top of the lung to the bottom, demonstrating that in the normal lung there are no discrete zones. The normal human lung at FRC spans 30 vertical centimeters, half of which distance is above the pulmonary artery and left atrium, and representative pulmonary arterial pressures are 33/11 cm H2O with a mean of 19 cm H2O. There is, therefore, no physiologic zone 1 in upright humans except perhaps in late diastole. Left atrial pressure averages 11 cm H2O and is sufficient to create zone 3 conditions two-thirds of the distance from the heart to the apex. However, in patients undergoing positive-pressure mechanical ventilation, or in patients with airway disease creating lung units that fail to empty during the normal respiratory cycle, alveolar pressure is no longer atmospheric. Under conditions of positive end-expiratory pressure (PEEP), Palv may be as high as 15–20 cm H2O. This potentially shifts the entire distribution of pulmonary blood flow. (Adapted and reprinted, with permission, from Hughes JM et al. Effect of lung volume on the distribution of pulmonary blood flow in man. Respir Physiol. 1968;4(1):58–72. Copyright © Elsevier.)

One additional factor that regulates blood flow is hypoxic pulmonary vasoconstriction. The smooth muscle cells of pulmonary arterioles are sensitive to alveolar PO2 (much more so than to arterial PO2). As alveolar PO2 falls, there is arteriolar constriction, an increase in local resistance to flow, and redistribution of flow to regions of higher alveolar PO2. When regionalized, this is an effective mechanism to diminish local blood flow without a significant increase in mean pulmonary arterial pressure. When it affects more than 20% of the pulmonary circulation, such as in the setting of global alveolar hypoxia, widespread pulmonary vasoconstriction increases mean pulmonary arterial pressure and may result in pulmonary hypertension.

Matching of Ventilation to Perfusion The functional role of the lungs is to place ambient air in close proximity to circulating blood to permit gas exchange by simple diffusion. To accomplish this, air and blood flow must be directed to the same place at the same time. Optimum functioning of the respiratory system requires that ventilation be matched to perfusion.

In the normal individual, typical resting alveolar ventilation is approximately 4 L/min, whereas pulmonary artery blood flow is 5 L/min. This yields an overall ratio of ventilation to perfusion of 0.8. As noted above, ventilation and perfusion are both preferentially distributed to dependent regions at rest, although the increase in gravity-dependent flow is more marked with perfusion than with ventilation. Hence, the ratio of ventilation to perfusion is highest at the apex and lowest at the base (Figure 9–14).

FIGURE 9–14 Changing distribution of ventilation and perfusion down the upright lung. The two straight lines reflect the progressive increases in ventilation and perfusion. The slope is steeper for perfusion. The ratio of ventilation to perfusion is, therefore, lowest at the base and highest at the apex. (Redrawn, with permission, from West JB. Ventilation/Blood Flow and Gas Exchange, 5th ed. Blackwell, 1990.)

Regional alterations from the ideal situation of matched uniform distribution of ventilation and perfusion (Figure 9–15, top left panel) are referred to as mismatch and are an extremely important phenomenon underlying the functional impairment of many disease states. A distribution may favor low ratios, with the extreme case being a shunt (perfusion without ventilation, or

= 0), or it may favor high ratios, with the extreme case being alveolar dead space (ventilation without perfusion, or = ∞). These two types of mismatch affect respiratory function very differently. A shift toward low ratios occurs when regional ventilation is reduced or eliminated but perfusion persists, as might happen with atelectatic lung or in areas of lung consolidation where alveoli are filled with fluid or infected debris. A shunt (Figure 9–15, top right panel) is the extreme case of a low area where ventilation is absent and the ratio goes to zero. Pulmonary arterial (mixed venous) blood will then pass to the systemic arterial circulation without coming into contact with alveolar gas. The primary physiologic effect of such a right-to-left shunt is to reduce arterial PO2. The lung is able to reflexively respond to regional hypoxemia by decreasing blood flow to these areas, thereby restoring ventilation-to-perfusion balance.

In the normal individual, approximately one-third of resting minute ventilation fills the main conducting airways. This is the anatomic dead space; it represents ventilation to areas that do not participate in gas exchange. If

participating gas-exchanging regions of the lung are ventilated but not perfused, as may occur in pulmonary embolism, pulmonary vascular disease, or emphysema, these regions will also fail to function in gas exchange and are referred to as alveolar dead space or wasted ventilation (Figure 9–15, bottom left panel). Functionally, a shift toward high ratios means that more work of breathing supports ventilation that does not participate in gas exchange, reducing the overall efficiency of ventilation and CO2 removal. In the absence of respiratory compensation, the primary effect of increased dead space is a rise in arterial PCO2; PaO2 may fall as a result of increased alveolar PO2. Because the respiratory control center is exquisitely sensitive to small changes in PaCO2, however, the most common integrated response to increasing wasted ventilation is to increase total minute ventilation (which includes ventilation to perfused alveolar units), thereby maintaining PaCO2 nearly constant. In this compensated state (Figure 9–15, bottom right panel), PaO2 remains normal. The A–a ΔPO2 is increased (discussed below). This adaptive response may be unconscious, but it presents a clinical problem when the individual can no longer sustain an increased minute ventilation, as in the patient with advanced emphysema.

FIGURE 9–15 Four models of the relationship of ventilation to perfusion. In this schematic representation, circles represent respiratory units and tubes depict conducting airways. The colored channels represent pulmonary blood flow, which enters the capillary bed as mixed venous blood (blue) and leaves it as arterialized blood (red). Blood is depicted as purple in situations with insufficient oxygenation. Large arrows show the distribution of inspired gas; small arrows show the diffusion of O2 and CO2. In the ideal case (top left panel), the PaO2 and PaCO2 leaving respiratory units A and B are identical. A shunt (top right panel) is a low Eqn04Fig.jpg area where ventilation is absent and the ratio goes to zero, reducing the arterial PO2. If participating gas-exchanging regions of the lung are ventilated but not perfused, these regions also fail to function in gas exchange; referred to as alveolar dead space (bottom left panel), they reduce the overall efficiency of ventilation and CO2 removal (wasted ventilation). However, one response to an increasing wasted ventilation is to increase total minute ventilation (which includes ventilation to perfused alveolar units), thereby maintaining a nearly constant PaCO2 and normal PaO2, a so-called compensated state (bottom right panel). See text for details. (Redrawn, with permission, from Comroe J. Physiology of Respiration, 2nd ed. Year Book, 1974. Copyright © Elsevier.)

Ventilation/perfusion mismatching commonly occurs between the limiting cases of true shunts and alveolar dead space. The effect on arterial blood gases of shifts in the distribution of ratios can be predicted from a discussion of the

limiting cases (Figure 9–16). The top panel of Figure 9–16 illustrates a respiratory unit where on one side (B), ventilation has been reduced but perfusion maintained. This defines an area of low ratio. The physiologic effect of low areas is similar to the effect of shunts: hypoxemia without hypercapnia. The difference between low areas and true shunts can also be seen by comparing this schematic with Figure 9–15. As shown in the top right panel of Figure 9–15, shunted blood does not come into contact with inspired air; therefore, no amount of additional oxygen supplied to inspired air will reverse the fall in systemic arterial PO2. In contrast, a low area (Figure 9– 16, top panel) does come in contact with inspired air, and the fall in systemic arterial PO2 can be reversed with increased inspired oxygen.

FIGURE 9–16 Ventilation/perfusion mismatching. The top panel shows a respiratory unit where on one side (B), ventilation has been reduced but perfusion is maintained. This defines an area of low Eqn04FigB.jpg ratio. The physiologic effect of low Eqn04Fig.jpg areas appears similar to the effect of shunts: hypoxemia without hypercapnia. But since a low Eqn04Fig.jpg area does come in contact with inspired air, the fall in PaO2 can be reversed with increased inhaled FiO2. The bottom panel shows a respiratory unit where on one side (B), blood flow has been decreased but ventilation is maintained. This defines an area of high Eqn04FigB.jpg ratio. The effect on lung function can be understood by dividing the unit into an area with a normal Eqn04Fig.jpg ratio (A) and an area of dead space or wasted ventilation (B′). The physiologic effect of high Eqn04Fig.jpg ratios is to increase PCO2, typically leading to increased respiration to return PaCO2 to normal. (Blue, deoxygenated; red, oxygenated.) See the Figure 9–15 legend and the text for details. (Redrawn, with permission, from Comroe J. Physiology of Respiration, 2nd ed. Year Book, 1974. Copyright © Elsevier.)

The bottom panel of Figure 9–16 illustrates a respiratory unit where on one side (B), blood flow has been decreased but ventilation maintained. This defines an area of high ratio. The effect on lung function can be understood by dividing the unit into an area with a normal ratio (A) and an area of dead space or wasted ventilation (B′). The physiologic effect of high ratios is an increase in PCO2, typically leading to increased respiration to return PaCO2 to normal.

Hyperventilation of unaffected lung regions can compensate for the increase in PCO2 from alveolar dead space but cannot compensate for the decrease in PO2 from areas of shunt. The reason for this is the different ways that oxygen and carbon dioxide are carried in blood, and the different relationships between the content and partial pressure of these gases. Because PCO2 and CO2 content are linearly related within the normal physiologic range, increased ventilation to one respiratory unit will reduce the PCO2 and the CO2 content of blood leaving that unit. Overall CO2 content will be the mean of the affected and unaffected units. Because PCO2 is proportionate to the CO2 content, the reduced CO2 content of hyperventilated lung regions compensates for elevated content from areas of wasted ventilation. The PCO2 and CO2 content of the mixture track together.

Hyperventilation or increasing inspired oxygen to unaffected lung regions does not compensate for the decrease in PO2 from areas of true shunt. The O2 content of blood is not linearly related to PO2 (see Figure 9–11). The sigmoidal shape of the oxyhemoglobin dissociation curve indicates that hemoglobin is nearly maximally saturated at a PO2 of 60. Increasing PO2 from 60 to 600 increases partial pressure 10-fold but O2 content by only 10%. Increasing ventilation or increasing alveolar PO2 to unaffected respiratory units can increase

end-capillary PO2 but will not change the O2 content of blood leaving those units. Overall O2 content will be the mean of normal blood oxygen content and shunted, desaturated blood. The reduced oxygen content of the mixture tends to lie on the steep portion of the oxyhemoglobin dissociation curve, with the result that modest decreases in oxygen content lead to large decreases in the PO2.

Arterial blood gases detect major disturbances in respiratory function. One attempt to assess more subtle abnormalities of gas exchange is to calculate the difference between the alveolar and arterial PO2. This is referred to as the A–a ΔPO2 or A–a DO2. The alveolar–capillary membrane permits full equilibration of alveolar and end-capillary oxygen tension under normal matching. There is nonetheless a small A–a ΔPO2 in normal individuals as a result of right-to-left shunting through the bronchial veins and the thebesian veins of the left heart. This accounts for approximately 2–5% of resting cardiac output and leads to an A–a ΔPO2 of 5–8 mm Hg in healthy young adults breathing ambient air at sea level. Increasing the fractional inspired concentration of oxygen (FiO2) increases this value: A normal A–a ΔPO2 breathing 100% oxygen is approximately 100 mm Hg. Normal values increase with age, presumably as a result of the closure of dependent airways with a consequent shift toward low ratios. Further increases in the A–a ΔPO2 reflect areas of low ratio, including shunting.

Control of Breathing The lungs inflate and deflate passively in response to changes in pleural pressure. Therefore, control over respiration lies in control of the striated muscles—chiefly the diaphragm but also the intercostals and abdominal wall— that change pleural pressure.

These muscles are under both automatic and voluntary control. The rhythm of spontaneous breathing originates in the brainstem, specifically in several groups of interconnected neurons in the medulla. Research into the generation of the respiratory rhythm has identified that it originates in the neurons of the pre- Bötzinger complex. Respiratory neurons are either inspiratory or expiratory and may fire early, late, or in an accelerating fashion during the respiratory cycle. Their integrated output is an efferent signal via the phrenic nerve (diaphragm) and spinal nerves (intercostals and abdominal wall) to generate rhythmic contraction and relaxation of the respiratory musculature. The result is spontaneous breathing without conscious input. However, by attending to breathing, one may alter this pattern. Eating, speaking, singing, swimming, and

defecating all rely on voluntary control over automatic breathing.

A. Sensory Input

The frequency, depth, and timing of spontaneous breathing are modified by information provided to the respiratory center from both chemical and mechanical sensors (Figure 9–17).

FIGURE 9–17 Schematic representation of the respiratory control system. The interrelationships among the CNS controller, effectors, and sensors are shown, as are the connections among these components. (Redrawn, with permission, from Berger AJ et al. Regulation of respiration. N Engl J Med. 1977;297:92. Copyright © 1977 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.)

There are chemoreceptors in the peripheral vasculature and in the brainstem. The peripheral chemoreceptors are the carotid bodies, located at the bifurcation of the common carotid arteries and the aortic bodies near the arch of the aorta. The carotid bodies are particularly important in humans. They function as sensors of arterial oxygenation. There is a graded increase in firing of the carotid body in response to a fall in the PaO2. This response is most marked below 60

mm Hg. An increase in the PaCO2 or a fall in arterial pH potentiates the response of the carotid body to decreases in the PaO2.

In humans, the carotid bodies are solely responsible for the increased ventilation seen in response to hypoxia. Bilateral carotid body resection, which has been performed to treat disabling dyspnea and may happen as an unintended consequence of carotid thromboendarterectomy, results in a complete loss of this hypoxic ventilatory drive while leaving intact the response to changes in PaCO2. Central chemoreceptors mediate the response to changes in PaCO2. There is growing evidence that these chemoreceptors are widely dispersed throughout the brainstem. They are separate from the neurons that generate the respiratory rhythm. The increased ventilatory response to elevation in PaCO2 is mediated through changes in chemoreceptor pH. The blood–brain barrier permits free diffusion of CO2 but not hydrogen ions. CO2 is hydrated to carbonic acid, which ionizes and lowers brain pH. Central chemoreceptors probably respond to these changes in intracellular hydrogen ion concentration.

There are a variety of pulmonary stretch receptors located in airway smooth muscle and mucosa whose afferent fibers are carried in the vagus nerve. They discharge in response to lung distention. Increasing lung volume decreases the rate of respiration by increasing expiratory time. This is known as the Hering– Breuer reflex. There are unmyelinated C fibers located near the pulmonary capillaries (hence juxtacapillary [J] receptors). These fibers are quiet during normal breathing but can be directly stimulated by the intravenous administration of irritant chemicals such as capsaicin. They appear to stimulate the increased respiratory drive in interstitial edema and pulmonary fibrosis. Skeletal movement transmitted by proprioceptors in joints, muscles, and tendons causes an increase in respiration and may have some role in the increased ventilation of exercise. Finally, there are muscle spindle receptors in the diaphragm and intercostals that provide feedback on muscle force. They may be involved in the sensation of dyspnea when the work of breathing is disproportionate to ventilation.

B. Integrated Responses

Under normal conditions in healthy adults, the hydrogen ion concentration in the region of the central chemoreceptors determines the drive to breathe. Changes in chemoreceptor pH are largely determined by the PaCO2. The PaO2 is not an important part of the baseline respiratory drive under normal conditions.

Breathing is stimulated by a fall in the PaO2, a rise in the PaCO2, or an increase in the hydrogen ion concentration of arterial blood (fall in arterial pH).

Ventilation increases approximately 2–3 L/min for every 1 mm Hg rise in PaCO2. This response (Figure 9–18) occurs first through sensitization of the carotid body receptor. The carotid body will increase its firing in response to an increased PaCO2 even in the absence of changes in the PaO2. This accounts for approximately 15% of the ventilatory response to hypercapnia. The majority of the response is mediated through pH changes in the region of the central chemoreceptors. Changes in arterial pH are additive to changes in PaCO2. CO2 response curves under conditions of metabolic acidosis have an identical slope but are shifted to the left. The ventilatory response to an increased PaCO2 falls with age, sleep, aerobic conditioning, and increased work of breathing.

FIGURE 9–18 Ventilatory response to CO2. The curves represent changes in minute ventilation plotted against changes in inspired PCO2 at different values of alveolar PO2. There is a linear increase in ventilation with increasing PCO2. The rate of increase is greater at lower PO2 values, but the curves begin from a common point at which ventilation should cease in response to lowered PCO2. In awake humans, arousal maintains ventilation even when the PCO2 falls below this level; when lightly anesthetized, apnea does occur. In the case of metabolic acidosis, this x-intercept is shifted to the left, but the slope of the lines remains virtually unchanged. This indicates that the effects of metabolic acidosis are separate from and

additive to the effects of respiratory acidosis. (BTPS, body temperature and pressure, saturated with water vapor.) (Redrawn, with permission, from Barrett KE et al. Ganong’s Review of Medical Physiology, 25th ed. McGraw-Hill, 2016.)

The individual response to hypoxemia is extremely variable. Normally, there is little increase in ventilation until the PaO2 falls below 50–60 mm Hg. At this point, there is a rapid increase in ventilation that reaches its maximum at approximately 32 mm Hg. Below this level, further decreases in PaO2 lead to depression of ventilation. The response to hypoxia is affected by the PaCO2. An increase in the alveolar PCO2 will shift the isocapnic O2 response curve upward and to the right (Figure 9–19).

FIGURE 9–19 Isocapnic ventilatory response to hypoxia. These curves represent changes in minute ventilation plotted against changes in alveolar PO2 when the alveolar PCO2 is held constant at 37, 44, or 49 mm Hg. When PCO2 is in the normal range (37–44 mm Hg), there is little increase in ventilation until the PO2 is reduced to between 50 and 60 mm Hg. The ventilatory response to hypoxia is augmented at elevated PCO2 levels. The response to hypoxia is not linear, as it is in response to an increased PCO2, but resembles a rectangular hyperbola asymptotic to infinite ventilation (at a PO2 in the low 30s) and ventilation without tonic stimulation from the carotid bodies (which occurs above a PO2 of 500 mm Hg). Not shown is the fall in minute ventilation that occurs with extreme hypoxia (PO2 values below 30 mm Hg) as a result of depression of the respiratory center. (Redrawn, with permission, from Barrett KE et al. Ganong’s Review of Medical Physiology, 25th ed. McGraw-Hill, 2016.)

A fall in arterial hydrogen ion concentration increases minute ventilation. This response results chiefly from stimulation of the carotid bodies and is independent of changes in PaCO2. There is a response to severe metabolic acidosis in the absence of carotid bodies. It is assumed that this response is mediated by central chemoreceptors; it may represent a breakdown of the blood– brain barrier.

C. Special Situations

1. Chronic hypercapnia—In patients with chronic hypercapnia, brain pH is returned toward normal by compensatory changes in serum and tissue bicarbonate levels. As a result, central chemoreceptors become less sensitive to further changes in PaCO2. In this instance, a patient’s basal minute ventilation may depend on tonic stimuli from the carotid bodies. If such a patient were given high concentrations of inspired oxygen, carotid body output may be depressed, leading to a fall in minute ventilation. The change in minute ventilation does not fully account for the hypercapnia in response to supplemental oxygen, suggesting that ablation of hypoxic pulmonary vasoconstriction also plays a role.

2. Chronic hypoxia—Long-term residence at high altitude—or sleep apnea with repeated episodes of severe oxygen desaturation—may blunt the hypoxic ventilatory response. In such individuals, the development of lung disease and hypercapnia may attenuate all endogenous stimuli to breathing. This pattern is seen in patients with obesity-hypoventilation syndrome.

3. Exercise—Exercise may increase minute ventilation up to 25 times the resting level. Strenuous but submaximal exercise in a healthy individual typically causes no change or only a slight increaes in PaO2 as a result of increased pulmonary blood flow and better matching of ventilation to perfusion, with no change or a slight fall in PaCO2. Changes in arterial oxygenation are, therefore, not a factor behind the increased ventilatory response to exercise. The reason for the increased ventilatory response is not known with certainty. Two contributing factors are the increased production of carbon dioxide and increased afferent discharge from joint and muscle proprioceptors.

CHECKPOINT

12. What are the components of lung elastic recoil? What is the role of surfactant?

13. What three opposing forces must be overcome nor-mally to inflate the lungs?

14. What are four factors affecting airway resistance? 15. What are the components of the work of breathing? 16. What factors regulate ventilation, and what factors reg-ulate perfusion? 17. How are ventilation and perfusion normally matched? 18. What are the effects of changing CO2 and O2 levels on respiratory

control?

PATHOPHYSIOLOGY OF SELECTED LUNG DISEASES

OBSTRUCTIVE LUNG DISEASES: ASTHMA & CHRONIC OBSTRUCTIVE PULMONARY DISEASE (COPD)

The fundamental physiologic problem in obstructive lung diseases is increased resistance to expiratory airflow as a result of caliber reduction of conducting airways. This increased resistance can be caused by processes (1) within the lumen, (2) in the airway wall, or (3) in the supporting structures surrounding the airway. Examples of luminal obstruction include the increased secretions seen in asthma and chronic bronchitis. Airway wall thickening and airway narrowing can result from inflammation, a characteristic feature of both asthma and chronic bronchitis, or from bronchial smooth muscle contraction, as occurs in asthma. Emphysema is the classic example of obstruction caused by loss of surrounding supporting structure, with expiratory airway collapse resulting from the destruction of the elastin-containing alveolar wall attachments. Although the causes and clinical presentations of these diseases are distinct, the common elements of their physiology are instructive.

1. Asthma

Clinical Presentation Asthma is a clinical syndrome with multiple phenotypes. This diversity reflects complex interactions between genetic predisposition and environmental exposure and suggests heterogeneity in the underlying pathophysiology.

Asthma is a disease of airway inflammation and variable airflow obstruction characterized by intermittent symptoms, including wheezing, chest tightness, shortness of breath (dyspnea), and cough, together with demonstrable bronchial hyperresponsiveness. Exposure to defined allergens or to various nonspecific stimuli initiates a cascade of cellular activation events in the airways, resulting in both acute and chronic inflammatory processes mediated by a complex and integrated assortment of locally released cytokines and other mediators. Release of mediators can alter airway smooth muscle tone and responsiveness, induce mucus hypersecretion, and damage airway epithelium. These pathologic events result in chronically abnormal airway architecture and function.

Inherent in the definition of asthma is the possibility of considerable variation in the magnitude and manifestations of the disease within and between individuals over time. For example, whereas many asthmatic patients have infrequent and mild symptoms, others may have persistent or prolonged symptoms of great severity. Similarly, initiating or exacerbating stimuli may be quite different between individual patients.

Epidemiology & Risk Factors

Asthma is a common chronic pulmonary disease, affecting approximately 300 million people worldwide. Globally, asthma is estimated to be responsible for 346,000 deaths per year. Overall U.S. prevalence was 7.8% in 2015, with higher rates in males younger than 18 years (9.9%) and in females older than 18 years (9.7%). Each year, approximately 500,000 hospital admissions and 4500 deaths in the United States are attributed to asthma. Prevalence, hospitalizations, and fatal events have all increased in the United States over the past 30 years. Mortality rates reached a plateau in the late 1990s and have declined slightly over the past two decades. Hospitalization rates have been highest among blacks and children, and death rates are consistently highest among blacks aged 15–24 years.

The strongest identifiable predisposing factor for the development of asthma is atopy, or the production of immunoglobulin E (IgE) antibodies in response to allergen exposure. Fifty-six percent of asthma cases were attributable to atopy in the National Health and Nutrition Examination Survey III (NHANES III) of 12,106 Americans done between 1988 and 1994. Exposure of sensitive patients

to inhaled allergens increases airway inflammation, airway hyperresponsiveness, and asthma symptoms. Symptoms may develop immediately (immediate asthmatic response) or 4–6 hours after allergen exposure (late asthmatic response). Table 9–3 lists common allergens. In multiple studies, obesity is associated with an increased prevalence of asthma.

TABLE 9–3 Asthma: Provocative factors.

Indoor exposure to house dust mite or cockroach antigens is a strong risk

factor for asthma. At the same time, it is well established that children raised on farms have lower prevalences of atopy and asthma, and the diversity of microbial exposure in childhood was shown in one study to be inversely related to risk of asthma. These observations, and the increased incidence of atopy, allergy, and autoimmune diseases in the developed world, have led researchers to pursue the underlying causes of atopy itself. One theory is that early childhood exposure to infections and/or organic antigens may fundamentally alter adaptive immunity. Some exposures may promote a TH1 phenotype (differentiation of CD4+ T-helper cells toward a TH1 response characterized by production of interferon γ), whereas the absence of these exposures may promote a TH2 phenotype (characterized by a primary cytokine response that includes interleukin-4 [IL-4], IL-5, IL-13, and tumor necrosis factor, which together are associated with atopy, allergic diseases, and asthma). The complexity of the immune response and its interactions with the human microbiome preclude any firm conclusions at this time, but this is a rapidly expanding area of research that promises to reshape our basic understanding of the etiology of atopy and asthma.

Genetic factors are also strongly implicated in asthma risk. Twin studies have estimated that asthma is up to 60% heritable. In addition, multiple genome-wide association studies have demonstrated the genetic link between the ORMDL3/GSDMD locus on chromosome 17q21 and childhood asthma. The functional mechanism underlying this association remains unclear. Epigenetic changes via DNA methylation of genes involved in the TH1 and TH2 immune responses have also been found to be associated with asthma.

Pathogenesis

The fundamental abnormality in asthma is an increased reactivity of airways to stimuli. As outlined in Table 9–3, there are many known provocative agents (triggers) for asthma. These can be broadly categorized as follows: (1) physiologic or pharmacologic mediators of asthmatic airway responses; (2) allergens that can induce airway inflammation and reactivity in sensitized individuals; and (3) exogenous physicochemical agents or stimuli that produce airway hyperreactivity. Some of these provocative agents will produce responses in people with asthma only (eg, exercise, adenosine), whereas others produce characteristically magnified responses in people with asthma that can be used to distinguish them from people without asthma under controlled testing conditions (eg, histamine and methacholine; see later discussion). There is no single mechanism that serves to explain the occurrence of asthma in all individuals.

There are, however, common events that characterize the pathologic processes that result in asthma. It is important to recognize the central role of airway inflammation in the evolution of asthma.

The earliest event in asthmatic airway responses is the activation of local inflammatory cells, principally mast cells and eosinophils. Inflammatory cell activation can occur directly by specific IgE-dependent mechanisms or indirectly via other processes (eg, osmotic stimuli or chemical irritant exposure). Acute- acting mediators, including leukotrienes, prostaglandins, and histamine, rapidly induce smooth muscle contraction, mucus hypersecretion, and vasodilation with endothelial leakage and local edema formation. Epithelial cells also appear to be involved in this process, releasing leukotrienes and prostaglandins as well as inflammatory cytokines on activation. Some of these preformed and rapidly acting mediators possess chemotactic activity that attracts additional inflammatory cells, such as eosinophils and neutrophils, to airway mucosa.

A critical process that accompanies these acute events is the recruitment, multiplication, and activation of inflammatory cells through the actions of a network of locally released cytokines and chemokines. Cytokines and chemokines participate in a complex and prolonged series of events that results in the perpetuation of local airway inflammation and hyperresponsiveness (Table 9–4). These events include the promotion of mast cell and eosinophil growth, the influx and proliferation of T lymphocytes, and the differentiation of B lymphocytes to IgE- and IgA-producing plasma cells. An important component of this process now appears to be the differentiation and activation of helper T lymphocytes of the TH2 phenotype. These TH2 lymphocytes, through their production of cytokines, including IL-3, IL-4, IL-5, IL-6, IL-9, IL-10, and IL-13, promote the activation of mast cells, eosinophils, and other effector cells and drive IgE production by B cells, all of which are pathologic components of the asthma phenotype. Thus, through their specific mediators, these multiple cells participate in the many pro-inflammatory processes that are active in the airways of people with asthma. Among these are injury to epithelial cells and denudation of the airway, greater exposure of afferent sensory nerves, and consequent neurally mediated smooth muscle hyperresponsiveness; the upregulation of IgE- mediated mast cell and eosinophil activation, including acute and long-acting mediator release; and submucosal gland hypersecretion with increased mucus volume. Concurrently, the production of growth factors such as TGF-β, TGF-α, and fibroblast growth factor (FGF) by epithelial cells as well as macrophages and other inflammatory cells drives the process of tissue remodeling and submucosal airway fibrosis. This submucosal fibrosis can result in fixed airway

obstruction that may complicate chronic airway inflammation in asthma.

TABLE 9–4 Asthma: Cellular inflammatory events.

Although there is a classic paradigm of allergic asthma (described above), there is an increasing recognition of alternative asthma phenotypes, reinforcing the notion of heterogeneity in the underlying pathogenesis. For example, adult onset non-allergic asthma and asthma in the obese patient are two recognized phenotypes, both of which are typically difficult to treat. The poor response to

standard therapy for allergic asthma has motivated research into alternative mechanisms that underlie these phenotypes. Neutrophilic inflammation and the TH17 immune response are areas of active investigation, but thus far, the precise mechanisms that drive these alternative phenotypes remain unclear.

Pathology

The histopathologic features of asthma reflect the cellular processes at play. Airway mucosa is thickened, edematous, and infiltrated with inflammatory cells, principally lymphocytes, eosinophils, and mast cells. Hypertrophied and contracted airway smooth muscle is also conspicuous. Bronchial and bronchiolar epithelial cells are frequently damaged, in part by secreted cytotoxic molecules from eosinophils, including major basic protein and eosinophil chemotactic protein. Epithelial cell injury and death leave portions of the airway lumen denuded, exposing autonomic and probably noncholinergic, nonadrenergic afferents that can mediate airway hyperreactivity. Secretory gland hyperplasia and mucus hypersecretion are seen, with mucus plugging of airways, a prominent finding in severe asthma. Even in mildly involved asthmatic airways, inflammatory cells are found in increased numbers in the mucosa and submucosa, and subepithelial myofibroblasts are noted to proliferate and produce increased interstitial collagen. In turn, submucosal fibrosis contributes to the relatively fixed airway obstruction that occurs in a subset of people with asthma. The pathologic findings seen in severe fatal asthma parallel the pathologic events described previously but reflect the greater magnitude of the insult. In these patients, severe airway epithelial injury and loss are noted, often with severe and complete obstruction of the airway lumen by mucus plugs.

Pathophysiology

Local cellular events in the airways have important effects on lung function. As a consequence of airway inflammation, mucus hypersecretion, and smooth muscle hyperresponsiveness leading to bronchoconstriction, the airways are narrowed, resulting in an increase in resistance (recall that Raw ∝ 1/radius4). The small-caliber peripheral airways do not contribute significantly to airflow resistance in healthy individuals, but as these airways narrow in patients with asthma, they contribute substantially to airflow obstruction. Bronchial neural function also appears to play a role in the evolution of asthma, although this is probably of secondary importance. Cough and reflex bronchoconstriction mediated by vagal efferents follows stimulation of bronchial irritant receptors.

Peptide neurotransmitters may also play a role. The pro-inflammatory neuropeptide substance P can be released from unmyelinated afferent fibers in the airways and can induce smooth muscle contraction and mediator release from mast cells. VIP is the peptide neurotransmitter of some airway nonadrenergic, noncholinergic neurons and functions as a bronchodilator; interruption of its action by VIP cleavage can promote bronchoconstriction.

Airway obstruction occurs diffusely, although not homogeneously, throughout the lungs. As a result, ventilation of respiratory units becomes nonuniform, and the matching of ventilation to perfusion is altered. Areas of both abnormally low and abnormally high ratios exist, with the low ratio regions contributing to hypoxemia. Pure shunt is unusual in asthma even though mucus plugging is a common finding, particularly in severe, fatal asthma. Arterial CO2 tension is usually normal to low, given the increased ventilation seen with asthma exacerbations. Even mild hypercapnia should be viewed as an ominous sign during a severe asthma attack, indicating progressive airway obstruction, muscle fatigue, and falling alveolar ventilation.

Clinical Manifestations

The manifestations of asthma are readily explained by the presence of airway inflammation and obstruction.

A. Symptoms & Signs—The variability of symptoms and signs is an indication of the tremendous range of disease severity, from mild and intermittent disease to chronic, severe, and sometimes fatal asthma. Importantly, symptoms of asthma do not necessarily correlate with the degree of airway obstruction. 1. Dyspnea and chest tightness—The sensations of dyspnea and chest

tightness result from several concerted physiologic changes. The greater muscular effort required to overcome increased airway resistance is detected by spindle stretch receptors, principally of intercostal muscles and the chest wall. Hyperinflation from airway obstruction results in thoracic distention. Lung compliance falls, and the work of breathing increases, also detected by chest wall sensory nerves and manifested as chest tightness and dyspnea. As obstruction worsens, increased mismatching produces hypoxemia. Rising arterial CO2 tension and, later, evolving arterial hypoxemia (each alone or together as synergistic stimuli) will stimulate respiratory drive through peripheral and central chemoreceptors. This stimulus in the setting of respiratory muscle fatigue produces progressive dyspnea.

2. Wheezing—Smooth muscle contraction, together with mucus hypersecretion and retention, results in airway caliber reduction and prolonged turbulent airflow, producing auscultatory and audible wheezing. The intensity of wheezing does not correlate well with the severity of airway narrowing; as an example, with extreme airway obstruction, airflow may be so reduced that wheezing is barely detectable if at all.

3. Cough—Cough results from the combination of airway narrowing, mucus hypersecretion, and the neural afferent hyperresponsiveness seen with airway inflammation. It can also be a consequence of nonspecific inflammation after superimposed infections, particularly viral, in patients with asthma. By virtue of the compressive narrowing and high velocity of airflow in central airways, cough provides sufficient shear and propulsive force to clear collected mucus and retained particles from narrowed airways.

4. Tachypnea and tachycardia—Tachypnea and tachycardia may be absent in mild disease but are virtually universal in acute exacerbations.

5. Pulsus paradoxus—Pulsus paradoxus is a fall of more than 10 mm Hg in systolic arterial pressure during inspiration. It appears to occur as a consequence of lung hyperinflation, with compromise of left ventricular filling together with augmented venous return to the right ventricle during vigorous inspiration in severe obstruction. With increased right ventricular end-diastolic volume during inspiration, the intraventricular septum is moved to the left, compromising left ventricular filling and output. The consequence of this decreased output is a decrease in systolic pressure during inspiration, or pulsus paradoxus.

6. Hypoxemia—Airway narrowing reduces ventilation to affected lung units, causing mismatching with a shift toward low ratios, resulting in an increase in the A–a ΔPO2 and frank hypoxemia in severe cases. True shunt is unusual except in very severe asthma.

7. Hypercapnia and respiratory acidosis—In mild to moderate asthma, ventilation is normal or increased, and the arterial PCO2 is either normal or decreased. In severe attacks, airway obstruction may worsen and respiratory muscle fatigue supervenes, with the evolution of alveolar hypoventilation, hypercapnia, and respiratory acidosis. Note that this progression can occur despite continued tachypnea. An increased respiratory rate does not reverse alveolar hypoventilation because tidal volumes are reduced secondary to dynamic hyperinflation.

8. Obstructive defects by pulmonary function testing—Patients with mild

asthma may have entirely normal pulmonary function between exacerbations. During active asthma attacks, all indices of expiratory airflow are reduced, including FEV1, FEV1/FVC (FEV1%), and peak expiratory flow rate (Figure 9–20). FVC is often also reduced as a result of air trapping that results from premature airway closure before full expiration. Administration of a bronchodilator improves airflow obstruction. A consequence of airflow obstruction is incomplete emptying of lung units at end expiration resulting in air trapping (increased RV) and hyperinflation (increased total lung capacity). Pulmonary diffusing capacity for carbon monoxide (DLCO) is often increased as a consequence of the increased lung (and lung capillary blood) volume.

FIGURE 9–20 Flow–volume curves (“loops”) from standard spirometry are shown for a normal patient (center), a patient with a severe obstructive ventilatory defect (right), and a patient with a moderate restrictive ventilatory defect (left).

9. Bronchial hyperresponsiveness—Bronchial hyperresponsiveness is

defined as either (1) a 12% or greater increase in the FEV1 in response to an inhaled bronchodilator, or (2) a 20% or greater decrease in FEV1 in response to a provoking factor that, at the same concentration, causes less than a 5% change in a healthy individual. Methacholine and histamine are agents for which standardized provocation testing has been established. Such testing reveals nonspecific hyperresponsiveness in virtually all people with asthma, including those with mild disease and normal spirometry findings. Other agents that have been used to establish specific exposure sensitivities include sulfur dioxide and toluene diisocyanate.

CHECKPOINT

19. What is the fundamental physiologic problem in obstructive lung disease? Give an example of each of its three principal sources.

20. What are the pathologic events that contribute to chronically abnormal airway architecture in asthma?

21. What are the three categories of provocative agents that can trigger asthma?

22. Which acute-acting mediators contribute to asthmatic airway responses? 23. What are some histopathologic features of asthma? 24. Name three reasons for increased airway resistance in asthma. 25. Why is arterial PCO2 usually low in asthma exacerbations? 26. What are some of the common symptoms and signs of acute asthma?

2. COPD: Chronic Bronchitis & Emphysema COPD is defined by the presence of persistent respiratory symptoms and airflow limitation secondary to airway and alveolar abnormalities resulting from exposure to noxious particles or gases. It is common, preventable, and treatable. Pathologic changes in COPD are seen throughout the respiratory tract, from the large airways to the alveoli. Therefore, in COPD, there can be a spectrum of clinical manifestations that share a common underlying etiology.

Clinical Presentation

COPD is often described as two discrete processes, chronic bronchitis and emphysema, both of which can lead to the development of fixed airway

obstruction. From a physiologic perspective, it is useful to separate these two processes, as each has distinct pathologic mechanisms. However, it is important to recognize that both processes can be found within the same individual, with a varying contribution of each to the overall clinical phenotype.

A. Chronic Bronchitis—Chronic bronchitis is defined by a clinical history of productive cough for 3 months of the year for 2 consecutive years. Both dyspnea and airway obstruction, often with an element of reversibility, are intermittently to continuously present. Chronic bronchitis predominantly impacts the airways. Inflammation in the larger airways leads to mucosal thickening and to mucus hypersecretion, which contributes to the productive cough. Extension of the inflammatory changes into smaller bronchioles produces airflow obstruction.

B. Emphysema—Pulmonary emphysema is a condition marked by irreversible enlargement of the airspaces distal to the terminal bronchioles, accompanied by destruction of their walls, most often without obvious fibrosis. In contrast to chronic bronchitis, the primary pathologic defect in emphysema is not in the airways but rather in the respiratory unit walls, where the loss of elastic tissue results in a loss of the recoil tension necessary to support distal airways during expiration. Progressive dyspnea and nonreversible obstruction accompany the airspace destruction without mucus hypersecretion or productive cough. Furthermore, the loss of alveolar surface area and the accompanying capillary bed for gas exchange contribute to progressive hypoxia and dyspnea. Pathologic and etiologic distinctions can be made among various patterns of emphysema, but the clinical presentations of all are similar.

Etiology & Epidemiology

COPD is a major cause of global morbidity and mortality. It is currently the third leading cause of death worldwide. In 2012, more than 3 million people died as a result of COPD, representing 6% of all deaths globally. Furthermore, by 2030 the prevalence of the disease is projected to rise to an expected 4.5 million deaths worldwide. In the United States, COPD is also the third leading cause of death. It is estimated to have direct costs of approximately $32 billion dollars per year. The prevalence of COPD is nearly equal between men and women, but mortality rates are higher in men. Mortality also increases with age and is higher in persons of lower socioeconomic status. Despite its significant global impact, COPD is likely under-diagnosed. Cigarette smoking is the most common risk factor worldwide, representing the principal cause in up to 90% of patients.

Although fewer than 50% of heavy smokers develop COPD during their lifetime, cigarette smokers have a greater mortality rate from COPD than non- smokers. Beyond tobacco smoke exposure, population-based studies suggest that chronic dust (including silica and cotton) and chemical fume exposure are significant contributing risk factors for COPD. In the developing world, indoor exposure to smoke from burning biofuels is a major cause of COPD.

The most important identified genetic risk factor for the evolution of COPD is deficiency of α1-protease (α1-antitrypsin) inhibitor. Reduced circulating and tissue levels can lead to the early onset of severe emphysema. Alpha1-protease inhibitor is capable of inhibiting several types of proteases, including neutrophil elastase, which is implicated in the genesis of emphysema (see Pathophysiology section below). Autosomal dominant mutations, especially in northern Europeans, produce abnormally low serum and tissue levels of this inhibitor, altering the balance of connective tissue synthesis and proteolysis. A homozygous mutation (the ZZ genotype) results in inhibitor levels 10–15% of normal, and the risk of emphysema, particularly in smokers who carry this mutation, is dramatically increased.

Pathophysiology

COPD develops in response to chronic inflammation triggered by the inhalation of noxious particles or gas, such as are found in cigarette smoke. Repeated inhalational exposure leads to persistent inflammation, producing the characteristic pathophysiologic changes seen in patients with COPD. The primary site of inflammatory involvement determines the predominant pathophysiologic process in an individual, with airway-predominant disease causing chronic bronchitis and parenchyma disease causing emphysema.

A. Chronic Bronchitis—The principal pathologic features are inflammation of airways, particularly small airways, and hypertrophy of large airway mucous glands, with increased mucus secretion and accompanying mucus obstruction of airways (Figure 9–21). The airway mucosa is variably infiltrated with inflammatory cells, including polymorphonuclear leukocytes and lymphocytes. Mucosal inflammation can substantially narrow the bronchial lumen. As a consequence of the chronic inflammation, the normal ciliated pseudostratified columnar epithelium is frequently replaced by patchy squamous metaplasia. In the absence of normal ciliated bronchial epithelium, mucociliary clearance function is severely diminished or completely abolished. Hypertrophy and hyperplasia of submucosal glands are prominent features, with the glands often

making up more than 50% of the bronchial wall thickness. Mucus hypersecretion accompanies mucous gland hyperplasia, contributing to luminal narrowing. Bronchial smooth muscle hypertrophy is common, and hyperresponsiveness to nonspecific bronchoconstrictor stimuli (including histamine and methacholine) can be seen. Bronchioles are often infiltrated with inflammatory cells and are distorted, with associated peribronchial fibrosis. Mucus impaction and luminal obstruction of smaller airways are often seen. In the absence of any superimposed process, such as pneumonia, the gas-exchanging lung parenchyma, composed of terminal respiratory units, is largely undamaged. The result of these combined changes is chronic airway obstruction and impaired clearance of airway secretions.

FIGURE 9–21 Bronchial wall anatomy. Structure of a normal bronchial wall. In chronic bronchitis, the thickness of the mucous glands increases and can be expressed as the ratio of (b–c)/(a–d); this is known as the Reid index. (Redrawn, with permission, from Thurlbeck WM. Chronic airflow obstruction in lung disease. In: Bennington JL, ed. Major Problems in Pathology. Saunders, 1976.)

The nonuniform airway obstruction of chronic bronchitis has substantial effects on ventilation and gas exchange. Airway narrowing that leads to prolonged expiratory time produces hyperinflation. Ventilation/perfusion relationships are altered with increased areas of low ratios. These low mismatches are largely responsible for the more significant resting hypoxemia seen in chronic bronchitis, compared with that seen in emphysema. True shunt

(perfusion with no ventilation) is unusual in chronic bronchitis.

B. Emphysema—The principal pathologic event in emphysema is thought to be a continuing destructive process resulting from an imbalance between lung oxidant insults and lung antioxidants and antielastases (particularly neutrophil α1-anti-protease) (Figure 9–22). Oxidants, whether endogenous (superoxide anion) or exogenous (eg, cigarette smoke), can inhibit the normal protective function of protease inhibitors (antiproteases), allowing progressive tissue destruction.

FIGURE 9–22 Schema of the elastase–antielastase hypothesis of emphysema. Activation is represented by solid lines, inhibition by dashed lines. The lung is protected from elastolytic damage by α1- protease inhibitor and α2-macroglobulin. Bronchial mucus inhibitor protects the airways. Elastase is derived primarily from neutrophils, but macrophages secrete an elastase-like metalloprotease and may ingest and later release neutrophil elastase. Oxidants derived from neutrophils and macrophages or from cigarette smoke may inactivate α1-protease inhibitor and may interfere with lung matrix repair. Endogenous antioxidants such as superoxide dismutase, glutathione, and catalase protect the lung against oxidant injury. (MMPs, matrix metalloproteinases.)

In contrast to chronic bronchitis, a disease of the airways, emphysema is a disease of the surrounding lung parenchyma. The physiologic consequences result from three important changes: (1) destruction of terminal respiratory units; (2) loss of alveolar–capillary bed; and (3) loss of the supporting structures of the

lung, including elastin-containing connective tissue. This loss of connective tissue reduces the normal support of noncartilaginous airways, leading to a lung with diminished elastic recoil and increased compliance. A premature expiratory collapse of airways ensues, with characteristic obstructive symptoms and physiologic findings.

The pathologic picture of emphysema is one of progressive destruction of terminal respiratory units or lung parenchyma distal to terminal bronchioles. Airway inflammatory changes are minimal if present, although some mucous gland hyperplasia can be seen in large conducting airways. The interstitium of respiratory units harbors some inflammatory cells, but the chief finding is a loss of alveolar walls and enlargement of airspaces. Alveolar capillaries are also lost, which can result in decreased diffusing capacity and progressive hypoxemia, particularly with exercise.

Alveolar destruction is not uniform in all cases of emphysema. Anatomic variants have been described on the basis of the pattern of destruction of the terminal respiratory unit (or acinus, as it is also known). In centriacinar emphysema, destruction is focused in the center of the terminal respiratory unit, with the respiratory bronchioles and alveolar ducts relatively spared. This pattern is most frequently associated with prolonged smoking. Panacinar emphysema involves destruction of the terminal respiratory unit globally, with diffuse airspace distention. This pattern is typically, although not uniquely, seen in α1- protease inhibitor deficiency. It is important to note that the distinction between these two patterns is largely pathologic; there is no significant difference in the clinical presentation. An additional emphysema pattern of clinical importance is bullous emphysema. Bullae are large confluent airspaces formed by greater local destruction or progressive distention of lung units. They are important because of the compressive effect they can have on surrounding lung and the large physiologic dead space associated with these structures.

Clinical Manifestations

A. Chronic Bronchitis—The clinical manifestations of chronic bronchitis are principally the result of the obstructive and inflammatory airway process. 1. Cough with sputum production—Cough is productive of thick, often

purulent sputum owing to the ongoing local inflammation and the high likelihood of bacterial colonization and infection. Sputum viscosity is increased largely as a result of the presence of free DNA (of high molecular weight and high viscosity) from lysed cells. With increased inflammation

and mucosal injury, hemoptysis can occur but is usually scant. Cough, which is very effective in clearing normal airways, is much less effective because of the narrow airway caliber and the greater volume and viscosity of secretions.

2. Wheezing—Persistent airway narrowing and mucus obstruction can produce localized or more diffuse wheezing. This may be responsive to bronchodilators, representing a reversible component to the obstruction.

3. Inspiratory and expiratory rhonchi—Increased mucus production, together with defective mucociliary escalator function, leaves excessive secretions in the airways, despite increased coughing. Rhonchi are heard prominently in larger airways during tidal breathing or with cough.

4. Cardiac examination—Tachycardia is common, especially with exacerbations of bronchitis or with hypoxemia. If hypoxemia is significant and chronic, pulmonary hypertension can result; cardiac examination reveals a prominent pulmonary valve closing sound (P2) or elevated jugular venous pressure and peripheral edema.

5. Imaging—Typical chest radiographic findings include increased lung volumes with relatively depressed diaphragms consistent with hyperinflation. Prominent parallel linear densities (“tram track lines”) of thickened bronchial walls are common. Cardiac size may be increased, suggesting right heart volume overload. Prominent pulmonary arteries are common and are associated with pulmonary hypertension.

6. Pulmonary function tests—Diffuse airway obstruction is demonstrated on pulmonary function testing as a global reduction in expiratory flows and volumes. FEV1, FVC, and the FEV1/FVC (FEV1%) ratio are all reduced. The expiratory flow–volume curve shows substantial flow limitation (see Figure 9–20). Some patients may respond to bronchodilators. Measurement of lung volumes reveals an increase in the RV and FRC, reflecting air trapped in the lung as a result of diffuse airway obstruction and early airway closure at higher lung volumes. DLCO is typically normal, reflecting a preserved alveolar–capillary bed.

7. Arterial blood gases—Ventilation/perfusion mismatching is common in chronic bronchitis. The A–a ΔPO2 is increased and hypoxemia is common, mainly because of significant areas of low ratios; hypoxemia at rest tends to be more profound than in emphysema. With increasing obstruction, increasing PCO2 (hypercapnia) and respiratory acidosis, with compensatory metabolic alkalosis, are seen.

8. Polycythemia—Chronic hypoxemia is associated with a variable

erythropoietin-mediated increase in hematocrit. With more severe and prolonged hypoxia, the hematocrit may increase to over 50%.

B. Emphysema—Emphysema presents as a noninflammatory disease manifested by dyspnea, progressive nonreversible airway obstruction, and abnormalities of gas exchange, particularly with exercise. 1. Breath sounds—Breath sounds in emphysema are typically decreased in

intensity, reflecting decreased airflow, prolonged expiratory time, and prominent lung hyperinflation. Wheezes, when present, are of diminished intensity. Airway sounds, including crackles and rhonchi, are unusual in the absence of superimposed processes such as infection or pulmonary edema.

2. Cardiac examination—Tachycardia may be present as in chronic bronchitis, especially with exacerbations or hypoxemia. Pulmonary hypertension is a common consequence of pulmonary vascular obliteration. Cardiac examination may reveal prominent pulmonary valve closure (increased P2, the pulmonary component of the second heart sound).

3. Imaging—Hyperinflation is common, with flattened hemidiaphragms and an increased anteroposterior chest diameter. Parenchymal destruction produces attenuated lung peripheral vascular markings, often with proximal pulmonary artery dilation as a result of secondary pulmonary hypertension. Cystic or bullous changes may also be seen.

4. Pulmonary function tests—Lung parenchymal destruction and loss of lung elastic recoil are the fundamental causes of pulmonary function abnormalities. The loss of elastic support in lung tissue surrounding the airways results in the increased dynamic compression of airways (see Figure 9–9), especially during forced expiration. Premature airway collapse reduces all flows, including FEV1, FVC, and the FEV1/FVC (FEV1% ratio). As with chronic bronchitis and asthma, the expiratory flow–volume curve shows substantial limitation in flow (see Figure 9–20). Expiratory time prolongation and early airway closure caused by loss of elastic recoil result in air trapping (increased RV). TLC is also increased, although often a substantial amount of this increase comes from gas trapped in poorly or noncommunicating lung units, including bullae. The DLCO is generally decreased in proportion to the extent of emphysema, reflecting the progressive loss of alveoli and their capillary beds.

5. Arterial blood gases—Emphysema is a disease of alveolar wall destruction. The loss of alveolar capillaries creates mismatches with areas of high ventilation relative to perfusion. Typically, patients with

emphysema adapt to high ratios by increasing their minute ventilation. They may maintain nearly normal PO2 and PCO2 levels despite advanced disease. With greater disease severity and further loss of capillary perfusion, the DLCO falls, leading to exercise-related and, when severe, resting arterial hemoglobin desaturation. Hypercapnia, respiratory acidosis, and a compensatory metabolic alkalosis are common in severe disease.

6. Polycythemia—As in chronic bronchitis, chronic hypoxemia is frequently associated with an elevated hematocrit.

CHECKPOINT

27. What is the leading cause of COPD? 28. Describe the pathophysiologic changes in emphysema versus chronic

bronchitis. 29. Mutations of which protein are strongly correlated with an increased risk

of emphysema? 30. Name eight symptoms and signs of chronic bronchitis. 31. Name six symptoms and signs of emphysema.

RESTRICTIVE LUNG DISEASE: IDIOPATHIC PULMONARY FIBROSIS

Interstitial lung disease, or diffuse parenchymal lung disease, is a descriptive term that encompasses more than 180 different disorders. These disorders are grouped together because of shared pathologic, physiologic, clinical, and radiographic features. Diffuse parenchymal lung diseases are characterized by a thickened alveolar interstitium resulting from the accumulation of inflammatory cells, mesenchymal cells, and collagen-rich extracellular matrix, leading to fibrosis and capillary remodeling (Figure 9–23). Diffuse lung fibrosis leads to increased lung elastic recoil, decreased lung compliance and lung volumes, impaired oxygen diffusion, and alterations in matching, leading to impairment in gas exchange through a pattern known as restrictive lung disease.

FIGURE 9–23 Categories of diffuse parenchymal lung disease that often lead to restrictive lung disease. In the absence of underlying malignancy or a history of chemical or radiation therapy, diffuse parenchymal lung disease can be broadly grouped into the clinical categories shown. (IPF, idiopathic pulmonary fibrosis; IIP, idiopathic interstitial pneumonia; DIP, diffuse interstitial pneumonia; RBILD, respiratory bronchiolitis-associated interstitial lung disease; AIP, acute idiopathic pneumonia; COP, cryptogenic organizing pneumonia; NSIP, nonspecific interstitial pneumonia; LIP, lymphocytic interstitial pneumonia.)

Diffuse parenchymal lung disease is often referred to as interstitial lung disease, but the modifier “interstitial” is an incomplete characterization of the pathologic process. The lung interstitium formally refers to the region of the alveolar wall exclusive of and separating the basement membranes of alveolar epithelial and pulmonary capillary endothelial cells. In the normal lung, this interstitium is a space that may contain a few mesenchymal cells (eg, fibroblasts), extracellular matrix molecules (eg, collagen, elastin, proteoglycans), and tissue leukocytes, including mast cells and lymphocytes. Under pathologic conditions, not only is the interstitium affected, but characteristic changes are also observed in the alveolar epithelium, including denudation and hypertrophy. In addition, the capillary endothelium can be involved with a loss or remodeling of alveolar capillaries. This extensive disruption of normal lung structure by interstitial processes profoundly influences lung function.

The known causes of diffuse parenchymal lung disease include occupational

and environmental exposures to organic and inorganic dusts, collagen vascular diseases (e.g. rheumatoid arthritis, progressive systemic sclerosis, dermatomyostitis/polymyositis), and medication toxicity. There are also diffuse parenchymal lung diseases of unknown etiology, such as idiopathic pulmonary fibrosis (IPF). The physiologic consequences seen in IPF are typical of other causes of diffuse parenchymal lung disease, particularly in their advanced stages. For that reason, the remainder of this section focuses on IPF as a prototypical interstitial lung disease.

Clinical Presentation IPF, previously known as interstitial pulmonary fibrosis or cryptogenic fibrosing alveolitis, is caused by progressive fibrosis leading to destruction of normal lung architecture. This process produces not only a restrictive defect, with altered ventilation and increased work of breathing, but also vascular alterations that can severely impair normal pulmonary perfusion and gas exchange.

The usual presentation of IPF is one of insidious onset. Patients present with dyspnea that has progressed over months to years and often a chronic non- productive cough. Fever and chest pain are generally absent. As the disease worsens, dyspnea may occur even at rest. Digital cyanosis, clubbing, and pulmonary hypertension are common in later stages.

The diagnosis of IPF may be made by high-resolution chest CT in the setting of an appropriate clinical history and pulmonary function tests or by surgical lung biopsy. Typical CT findings are described below. The histopathologic correlate to IPF is usual interstitial pneumonia (UIP), a temporally and spatially heterogeneous pattern of scattered clusters of mature collagen deposition and alveolar wall destruction interspersed with areas of normal alveolar architecture. These clusters of ongoing fibrosis are called fibroblastic foci and contain both fibroblasts and myofibroblasts; inflammatory cells may be present but are usually sparse.

Etiology & Epidemiology Compared with COPD, IPF is an uncommon disorder with an estimated prevalence of 18 per 100,000 people in the United States. This disease has a male predominance and typically presents in the sixth to seventh decades of life (it is rare in individuals under 50 years of age). By definition, the term “idiopathic” indicates that none of the known causes of diffuse parenchymal lung disease are present in an individual patient. Major risk factors include

tobacco smoke exposure, environmental exposures (eg, metal and wood dusts), occupational exposures (in farming and agriculture), and chronic viral infections. Beyond environmental exposures, recent studies clearly implicate both common and rare variant genetic alterations in the development of IPF. The common variants account for approximately one-third of the risk for developing IPF and include the MUC5B and TOLLIP genes. Associations with other disorders, including emphysema, gastroesophageal reflux disease, and obesity, remain undefined. The clinical, imaging, and histopathologic features of IPF can be indistinguishable from diffuse parenchymal lung diseases of known etiology. Therefore, it is critical that a thorough history and physical examination are performed on each patient, since the treatment and prognosis differ between IPF and these other interstitial disorders.

The natural history of IPF is classically one of unremitting progression, and the median survival is approximately 3 years from the time of diagnosis. The clinical course is nonetheless very heterogeneous and may reflect separate phenotypes of the disease rather than variable progression. Risk factors for accelerated progression include gender, older age at diagnosis (>70 years), cumulative tobacco smoke exposure, and severity of disease by symptoms (dyspnea score) or standardized assessment (extent of radiographic disease, severity of pulmonary restriction on pulmonary function tests, presence of pulmonary hypertension). Combining these predictive variables into scoring algorithms such as the GAP index (Gender, Age, Physiology [including FVC and DLCO]) can aid the clinician in estimating the 1-year mortality rate for a patient with IPF. Further attempts to improve such predictions have examined the utility of IPF biomarkers. One example is the serum concentration of matrix metalloproteinase 7, with increasing levels portending a worse outcome. Other less well-established prognostic biomarkers include serum levels of monomeric periostin, YKL-40, and CCL18.

A subset (5–15%) of patients with IPF will also experience an acute exacerbation of their disease characterized by an abrupt deterioration in symptoms, lung function, and radiographic abnormalities. Pathologic specimens from such patients reveal diffuse alveolar damage in addition to findings of UIP. These episodes are associated with a poor prognosis, with an associated median survival of 3–4 months.

Pathophysiology The pathophysiology of IPF and its histopathologic correlate UIP is an active area of research with the promise that a better understanding of the basic

mechanisms of this disease will translate into more effective treatments. Mounting evidence implicates repetitive microinjury to the alveolar epithelium followed by aberrant wound repair as the predominant mechanism of disease pathogenesis. Defects (both genetic and acquired) in type II alveolar epithelial cells increase the susceptibility of these cells to injury and apoptosis and also interfere with their regenerative capacity. For example, genetic mutations in type II alveolar epithelial cell-specific proteins, including surfactant proteins A and C, have been identified in individuals with familial pulmonary fibrosis. These mutations result in protein defects that induce cell stress and promote cell death. Furthermore, mutations in telomere maintenance genes (including TERT and TERC) result in telomere shortening and impair the regenerative capacity of the type II alveolar epithelium. In addition to these genetic studies, histopathologic and biomarker studies of IPF patients consistently identify abnormalities in the alveolar epithelium, including denudation and/or hyperplasia of cells in areas of active fibrosis and elevated systemic levels of surfactant proteins. Finally, animal studies confirm that a targeted injury to the type II alveolar epithelial cell is sufficient to cause fibrosis. It is currently unknown what environmental challenges are responsible for the repetitive microinjury to the susceptible type II alveolar epithelial cell (hence the idiopathic designation). It is likely that multiple different insults contribute. For example, both animal models and human studies implicate chronic herpesvirus infection as an important etiologic factor in a subset of patients. Furthermore, acute viral infections likely contribute to acute exacerbations of the disease.

Following epithelial injury, fibroblasts accumulate in the lung interstitium and typically differentiate into myofibroblasts. Myofibroblasts are highly contractile and contribute to tissue destruction. They are also responsible for the synthesis and deposition of extracellular matrix proteins such as fibronectin and collagen. In normal wound healing, fibroblasts undergo apoptosis following the successful restoration of an intact epithelium. In contrast, (myo)fibroblasts in IPF are resistant to apoptotic stimuli, favoring their persistence and ultimately contributing to the progressive nature of the disease. How epithelial injury initiates (myo)fibroblast activation is an area of ongoing study. However, current evidence suggests that the damaged type II alveolar epithelial cells secrete profibrogenic mediators, including transforming growth factor-β (TGF-β), connective tissue growth factor, and platelet-derived growth factor. Disruption of the epithelium also leads to plasma leak, activation of TGF-β, activation of the clotting cascade, and generation of thrombin, which can activate fibroblasts through cleavage of protease-activated receptors. In turn, the fibrogenic mediators, in conjunction with the development of a stiff, cross-linked, collagen-

rich extracellular matrix, drive myofibroblast differentiation and apoptosis resistance.

In this working model of epithelial injury, myofibroblast activation, and failed wound repair, the inflammatory cell contribution to disease pathogenesis remains controversial. Disappointingly, a large clinical trial of the combination of an anti- inflammatory regimen of azathioprine and prednisone with N-acetylcysteine to suppress inflammation was found to increase the risk of death and hospitalization. And, in fact, histopathologic examination of tissue biopsies from patients with IPF typically reveals a paucity of inflammatory cells. On the other hand, it seems implausible that repetitive injury to the alveolar epithelium would not result in the activation of an immune response. Animal models implicate the interplay between recruited monocytes/macrophages and regulatory T cells in the pathogenesis of pulmonary fibrosis. In addition, many studies have demonstrated that, with fibrotic insult, activated macrophages are capable of secreting pro-fibrotic mediators such as TGF-β (Table 9–5).

TABLE 9–5 Cellular events involved in lung injury and fibrosis.

Ultimately, the disruption of the alveolar epithelium with an associated

perturbation in surfactant homeostasis, the accumulation of extracellular matrix proteins, and the persistence of contractile apoptosis-resistant myofibroblasts result in increased tissue stiffness, decreased lung compliance, and a reduction in lung volume. As this cycle progresses, there is continued alveolar destruction, with large areas of fibrosis and residual airspaces lined by cuboidal epithelium that originates from airway basal cells. This process, known as bronchiolization, leads to the development of honeycomb cysts that can be appreciated both in pathologic specimens and on radiographic imaging. Along with alveolar destruction, pulmonary vascular changes occur. These changes can be precipitated by patchy obliteration of the accompanying vascular bed and by hypoxemic vasoconstriction. The resultant altered physiology includes increased pulmonary vascular resistance, pulmonary arterial hypertension, and right heart strain. Table 9–6 summarizes the pathophysiology of interstitial lung diseases.

TABLE 9–6 Pathophysiology of interstitial lung diseases.

Clinical Manifestations A. Symptoms & Signs 1. Cough—An intermittent, irritating, nonproductive cough is often the first

symptom of IPF. It may be refractory to antitussive therapy. The mechanism is likely multifactorial, with fibrotic damage to terminal respiratory units causing bronchial and bronchiolar distortion, which leads to alterations in both stimulatory and inhibitory nerve fibers involved in cough reflexes. Although epithelial cells may be injured, mucus hypersecretion and a productive cough are not typically seen in early disease.

2. Dyspnea and tachypnea—Multiple factors contribute to dyspnea in patients with IPF. Fibrosis of lung parenchyma decreases lung compliance; in combination with alterations in surfactant turnover, the distending pressure required to inflate the lungs increases, as does the work of breathing. Increased stimuli from C fibers in fibrotic alveolar walls or stretch receptors in the chest wall may sense the increased force necessary to inflate less compliant lungs. Tachypnea results from increased lung sensory receptor stimuli and the attempt to maintain a normal alveolar minute ventilation (and hence normal PaCO2) as lung volumes decrease. A rapid, shallow breathing pattern also reduces ventilatory work in the face of increased lung elastic recoil. The diminished capillary bed and thickened alveolar–capillary membrane contribute to hypoxemia with exercise. In advanced disease, altered gas exchange with severe mismatching can produce hypoxemia at rest.

3. Inspiratory crackles—Diffuse fine, dry inspiratory crackles are common and reflect the successive opening on inspiration of respiratory units that are collapsed owing to fibrosis and the loss of normal surfactant.

4. Digital clubbing—Clubbing of the fingers and toes is a common finding, but the cause is unknown. There is no established link with any specific physiologic variable, including hypoxemia.

5. Cardiac examination—As with hypoxemia from other causes, cardiac examination can reveal evidence of pulmonary hypertension with a prominent pulmonary valve closure sound (P2). This can be accompanied by right heart overload or decompensation, with elevated jugular venous pressure, the murmur of tricuspid regurgitation, or a right-sided third heart sound (S3).

B. Imaging

Characteristic chest radiograph findings include reduced lung volumes with increased reticular opacities prominent in the lung periphery and loss of definition of vascular structures, diaphragms, and cardiac border. Fibrosis surrounding expanded small airspaces is seen as honeycombing. With pulmonary hypertension, central pulmonary arteries may be enlarged. Typical chest CT findings include diffuse septal thickening, increased subpleural reticular opacities, traction bronchiectasis with pleural scalloping, and subpleural, clustered, small (3–10 mm) cystic airspaces (honeycombing). Ground glass opacities are usually absent.

C. Pulmonary Function Tests

Lung fibrosis typically produces a restrictive ventilatory pattern, with reductions in TLC, FEV1, and FVC, while maintaining a preserved or even increased ratio of FEV1/FVC (FEV1%) (see Figure 9–20). The increased elastic recoil produces normal to increased expiratory flow rates when adjusted for lung volume. The DLCO in lung fibrosis is progressively reduced as a function of the fibrotic obliteration of lung capillaries.

D. Arterial Blood Gases

Hypoxemia is common in advanced IPF. It results from the patchy nature of fibrosis, causing extreme variability in regional compliance and ventilation leading to prominent mismatching shifted toward low ratios. Cardiac output tends to be low, which reduces mixed venous PO2 (PmvO2). Diffusion impairment increases with the severity of fibrosis but rarely contributes to resting hypoxemia. Diffusion impairment also contributes to abnormal oxygenation and is a significant contributor to exercise-induced desaturation when the combination of a low PmvO2 and reduced capillary transit time limits the oxygen loading of hemoglobin (Figure 9–24). Arterial PCO2 is typically low because of increased ventilation owing to hypoxia and the irritant stimuli of lung fibrosis. Only in the later stages of disease, when increased lung elastic recoil and work of breathing prevent appropriate ventilation, does the PaCO2 rise above normal. Hypercapnia is a grave sign, implying an inability to maintain adequate alveolar ventilation as a result of excess work of breathing.

FIGURE 9–24 Change in PaO2 along the pulmonary capillary. The typical transit time at rest for an erythrocyte through an alveolar capillary is 0.75 seconds. In the normal lung, the partial pressure difference and rate of diffusion of O2 across the alveolar–capillary barrier assure complete saturation of hemoglobin in 0.25 seconds. Even with the shorter capillary transit time of exercise, the normal lung allows for complete saturation of hemoglobin in the alveolar capillary. If the alveolar–capillary barrier is thickened, as in lung fibrosis, and particularly if the starting point (the mixed venous PO2) is reduced, diminished diffusion in the setting of shortened capillary transit time will cause alveolar end-capillary blood not to be fully saturated with O2. Greater desaturation will occur with progressive exercise as peripheral extraction further decreases the mixed venous PO2. (Redrawn, with permission, from West JB. West’s Pulmonary Pathophysiology: The Essentials, 9th ed. Lippincott Williams & Wilkins, 2017.)

CHECKPOINT

32. How does interstitial lung disease affect lung function? 33. Name five events in the pathophysiology of idiopathic pulmonary

fibrosis. 34. Name eight symptoms and signs of idiopathic pulmo-nary fibrosis.

PULMONARY EDEMA

Clinical Presentation Pulmonary edema is the accumulation of excess fluid in the extravascular compartment of the lungs, principally in the interstitium and alveolar spaces. This accumulation may occur slowly, as in a patient with occult renal failure, or emergently, as in left ventricular failure after an acute myocardial infarction. Pulmonary edema most commonly presents with dyspnea. Dyspnea is breathing perceived by a patient as both uncomfortable or anxiety-provoking and disproportionate to the level of activity. Dyspnea from pulmonary edema may be present only with exertion, or the patient may experience dyspnea at rest. In severe cases, pulmonary edema may be accompanied by edema fluid in the sputum and can cause acute respiratory failure.

Etiology Pulmonary edema is a common problem associated with a variety of medical conditions (Table 9–7). In light of these multiple causes, it is helpful to think about pulmonary edema in terms of underlying physiologic principles.

TABLE 9–7 Causes of pulmonary edema by physiologic principle.

Pathophysiology All blood vessels leak; under normal conditions, fluid moves between blood vessels and the spaces around them while protein flux is minimal. In the adult human, the pulmonary capillaries are the major site of fluid flux from the pulmonary vasculature. The Starling equation—(Jv ≈ K ([Pc − Pi] − σ[πc − πi]) —describes the movement of fluid between the pulmonary capillaries and the pulmonary extravascular compartment. The flux of fluid across a semipermeable membrane (Jv) is related to the inherent permeability of the membrane to fluid (K = fluid filtration coefficient of the capillary endothelium) and macromolecules (σ = protein reflection coefficient of the capillary endothelium), as well as the hydrostatic and colloid oncotic pressure gradients across the membrane (Pc = capillary hydrostatic pressure; Pi = interstitial hydrostatic pressure; πc and πi are the capillary and interstitial colloid oncotic pressures, respectively).

Under normal conditions, there is net flow out of the blood vessels into the interstitial space owing to the following pressure dynamics:

1. Pulmonary capillary hydrostatic pressure exceeds interstitial hydrostatic pressure; therefore, hydrostatic forces favor fluid movement out of the capillaries into the interstitial space.

2. Pulmonary capillary colloid oncotic pressure exceeds interstitial colloid oncotic pressure, favoring fluid movement out of the interstitial space into the capillaries.

3. The effect of hydrostatic forces is greater than that of colloid oncotic forces; thus, there is a net fluid movement out of the capillaries into the interstitial spaces. The net rate of fluid movement out of the pulmonary capillaries under normal conditions is approximately 15–20 mL/h, representing less than 0.01% of pulmonary blood flow.

Alveoli have homeostatic mechanisms to protect against extravascular fluid in the interstitial space. The alveolar epithelium forms a tight cellular barrier that is nearly impermeable to the passage of protein into the alveolar space. Restricting protein from the space maintains higher colloid oncotic pressure in the interstitium. In addition, alveolar epithelial cells actively transport sodium out of the alveoli. Both mechanisms create gradients that favor movement of fluid out of the alveolar space into the interstitium, maintaining alveolar function.

The pulmonary lymphatic system also helps prevent the accumulation of

extravascular fluid in the interstitium under normal conditions. Fluid in the lung interstitial space is removed by pericapillary lymphatic vessels. These vessels do not enter the alveolar wall and are thus termed “juxta-alveolar.” In addition, the pericapillary interstitium is contiguous with the perivascular and peribronchial interstitium. Interstitial pressure in these more central areas is negative relative to the pericapillary interstitium, so fluid tracks centrally away from the airspaces. In effect, the perivascular and peribronchiolar interstitium acts as a sump for fluid, and it can accommodate approximately 500 mL with a negligible increase in interstitial hydrostatic pressure. Because this fluid is protein-depleted relative to blood, osmotic forces favor resorption from the interstitium into blood vessels adjacent to these central areas. This is the major site of resorption of fluid from the perivascular and peribronchiolar interstitium. Edema fluid may track further into the mediastinum, where it is taken up by mediastinal lymphatics. The perivascular and peribronchiolar interstitium is also contiguous with the interlobular septa and the visceral pleura. In some patients, a significant amount of fluid transits out through the visceral pleural into the pleural space, where there is high-capacity resorption through pores on the parietal pleura into parietal pleural lymphatics. The rate of fluid resorption by the lymphatic system is usually sufficient to prevent fluid accumulation in the interstitium and the alveolar spaces.

Pulmonary edema occurs when fluid exiting the pulmonary vascular space exceeds the capacity for fluid clearance. At some undefined critical level after the perivascular and peribronchiolar interstitium has been filled, increased interstitial hydrostatic pressure causes edema fluid to enter the alveolar space (Figure 9–25). The pathway into the alveolar space remains unknown but is thought to occur by bulk flow. Pulmonary edema may occur under a number of different conditions:

FIGURE 9–25 Stages in the accumulation of pulmonary edema fluid. The three columns represent three anatomic views of the progressive accumulation of pulmonary edema fluid. From left to right, the columns represent a cross-section of the bronchovascular bundle showing the loose connective tissue surrounding the pulmonary artery and bronchial wall, a cross-section of alveoli fixed in inflation, and the pulmonary capillary in cross-section. The first stage is an eccentric accumulation of fluid in the pericapillary interstitial space. The limitation of edema fluid to one side of the pulmonary capillary maintains gas transfer better than symmetric accumulation. When the formation of edema fluid exceeds lymphatic removal, it distends the peribronchovascular interstitium. At this stage, there is no alveolar flooding, but there is some crescentic filling of alveoli. The third stage is alveolar flooding. Note that each individual alveolus is either totally flooded or has minimal crescentic filling. This pattern probably occurs because alveolar edema interferes with surfactant, and, above some threshold, there is an increase in surface forces that greatly increases the transmural pressure and causes flooding. (Redrawn, with permission, from Lumb AB. Nunn’s Applied Respiratory Physiology, 8th ed. Copyright Elsevier, 2016.)

1. The hydrostatic pressure gradient increases (elevated pulmonary

capillary hydrostatic pressure). Pulmonary edema that occurs in this setting is termed cardiogenic or hydrostatic pulmonary edema. This is primarily a mechanical process resulting in an ultrafiltrate of plasma. Edema fluid in this setting has a relatively low protein content, generally less than 60% of a patient’s plasma protein content. In normal individuals, pulmonary capillary pressure (ie, pulmonary capillary wedge pressure) must exceed approximately 20 mm Hg before the fluid leaving the vascular space exceeds the rate of resorption. Cardiogenic or hydrostatic pulmonary edema classically results from elevated pulmonary venous and left atrial pressures resulting from left ventricular systolic or diastolic failure as well as aortic or mitral valve disease.

2. Vascular endothelial cell and/or alveolar epithelial cell permeability increases. Pulmonary edema that occurs in this setting is termed noncardiogenic or increased-permeability pulmonary edema. The permeability of the endothelial or epithelial barrier may increase as a result of cellular injury. Local or systemic activation of inflammatory cascades typically initiates the dysfunction of both the endothelial and epithelial barriers. Following injury to these cells, both fluid and protein permeability increase, although there may be little change in hydrostatic pressure. Under conditions of increased membrane permeability, edema fluid has protein content similar to intravascular fluid, generally at least 70% of the plasma protein content. The acute respiratory distress syndrome (ARDS) epitomizes this type of pulmonary edema (detailed discussion below).

3. The oncotic pressure gradient decreases (low plasma colloid oncotic pressure). Edema fluid in this setting has a relatively low protein content. Hypoalbuminemia owing to prolonged illness or nephrotic syndrome can cause this type of pulmonary edema.

4. Lymphatic drainage is impaired. This form of pulmonary edema is rare but may be seen with physical obstruction of the lymphatic system from malignancy (lymphoma) or infection (histoplasmosis, tuberculosis), from obliteration of lymphatics owing to radiation therapy for breast or lung cancer, or from idiopathic causes (yellow nail syndrome).

Hydrostatic and permeability pulmonary edema are not mutually exclusive and in fact may be closely linked. Pulmonary edema occurs when the hydrostatic pressure is excessive for a given capillary permeability and for a given rate of clearance of interstitial fluid. For instance, in the presence of damaged capillary endothelium, small increases in an otherwise normal hydrostatic pressure gradient may cause large increases in edema formation. Similarly, if the

alveolar–epithelial barrier is damaged, even the baseline flux of fluid across an intact capillary endothelium may cause alveolar filling.

ARDS is the archetypal example of increased-permeability edema. While the underlying pathophysiology is complex, the fundamental mechanism is an inflammation-mediated disruption of the alveolar–capillary barrier. Through loss of endothelial and epithelial barrier integrity, the normal homeostatic mechanisms of fluid balance are disrupted, and protein-rich fluid accumulates in the alveolar space. This loss of integrity may result from direct injury to the alveolar epithelium following local activation of inflammation by inhaled toxins or pulmonary infection, or it may occur after primary injury to the pulmonary capillary endothelium following systemic activation of inflammation by circulating toxins, as in sepsis or pancreatitis. This is in contrast to cardiogenic pulmonary edema, in which both the alveolar epithelium and the capillary endothelium are usually intact. There are a wide range of clinical entities that can lead to ARDS, including some with increased pulmonary capillary endothelial permeability and others with increased alveolar epithelial permeability (see Table 9–7). These pathologically diverse processes can be grouped together into the syndrome of ARDS because they share a final common pathway of alveolar–capillary barrier injury, which results in characteristic changes in pulmonary mechanics and lung function.

ARDS is the byproduct of a dysregulated inflammatory response by the lung to an injurious insult. Whether the injury occurs directly or indirectly, the insult activates the innate immune response through resident immune cells such as the alveolar macrophage. These cells recognize both exogenous factors, such as those derived from microorganisms, and endogenous factors, elaborated by local or distant cellular injury, through “pattern recognition” receptors (eg, toll-like receptors). Receptor activation stimulates pro-inflammatory responses. Through the release of cytokines and chemokines, such as IL-1B, TNF, IL-6, and IL-8, circulating inflammatory cells, including neutrophils and monocytes, are recruited to the lung and undergo activation that further potentiates the pro- inflammatory signal. The propagation of this inflammatory cascade results in direct and indirect tissue injury through the release of a variety of factors, including other cytokines and chemokines, proteases, eicosanoids, and reactive oxygen species. Loss of barrier integrity as a result of injury to both the alveolar epithelium and capillary endothelium ultimately leads to the leakage of protein- rich fluid into the alveolar spaces throughout the lung. There, the edema fluid inactivates surfactant, increasing surface tension with resultant alveolar instability and atelectasis. Increased surface tension decreases the interstitial

hydrostatic pressure, further favoring fluid movement into the alveolus. The loss of surfactant activity and the filling of airspaces cause the significant physiologic derangements that characterize ARDS, including decreases in lung compliance and lung volume and severe hypoxemia (secondary to low and shunt).

The histopathology of increased-permeability pulmonary edema reflects the inflammatory disruption of the alveolar–capillary barrier. Grossly, the lungs are edematous and heavy. The surface appears violaceous, and hemorrhagic fluid exudes from the cut pleural surface. Microscopically, there is infiltration of the interalveolar septa and the interstitium by inflammatory cells and erythrocytes. Type I pneumocytes are damaged, leaving a denuded alveolar barrier. Sheets of pink-staining material, known as hyaline membranes, line the denuded basement membrane. Composed of cellular debris and plasma proteins (including fibrin), these hyaline membranes are the hallmark of the diffuse alveolar damage of ARDS. In some patients, the alveolar–capillary barrier injury progresses to fibrosis; in other, surviving patients, complete recovery with regeneration of an intact alveolar epithelium from the residual type II pneumocytes may occur.

Clinical Manifestations Cardiogenic and noncardiogenic pulmonary edema both result in increased extravascular lung water, and both may result in respiratory failure. Given the differences in pathophysiology, it is unsurprising that the clinical manifestations are very different in the two syndromes.

A. Increased Hydrostatic Pulmonary Edema (Cardiogenic Pulmonary Edema)

Early increases in pulmonary venous pressure may be asymptomatic. The patient may notice only mild exertional dyspnea or a nonproductive cough stimulated by activation of irritant receptors coupled with C fibers. Orthopnea and paroxysmal nocturnal dyspnea occur when the patient’s recumbent position allows for the redistribution of blood or edema fluid from the lower extremities into the venous circulation, thereby increasing thoracic blood volume and pulmonary venous pressures.

Clinical signs begin with the accumulation of interstitial fluid. Cardiac examination may reveal a third heart sound, but there is a paucity of lung findings in purely interstitial edema. Early evidence of interstitial edema may be appreciated on a chest radiograph as an increase in the caliber of the upper lobe vessels (“pulmonary vascular redistribution”) and as fluid accumulating in the

perivascular and peribronchial spaces (“cuffing”). The chest radiograph may also show Kerley B lines, which represent fluid in the interlobular septa. With increasing interstitial edema, lung compliance falls, and the patient begins to breathe more rapidly and shallowly to adapt to the increased elastic work of breathing. As alveolar flooding begins, there are further decreases in lung volume and pulmonary compliance. With some alveoli filled with fluid, there is an increase in the fraction of the lung that is perfused but poorly ventilated. This shift toward low ratios causes an increase in A–a ΔPO2, if not frank hypoxemia. Supplemental oxygen corrects the hypoxemia, provided there is an absence of shunt. The PaCO2 is often normal or low, reflecting the increased drive to breathe. The patient may become sweaty and cyanotic. The sputum may become pink, frothy edema fluid. It is pink from capillary hemorrhage resulting from high pulmonary venous pressures. Auscultation reveals inspiratory crackles chiefly at the bases, where the hydrostatic pressure is greatest, but potentially throughout both lungs. Rhonchi and wheezing (“cardiac asthma”) may occur. On the chest radiograph, the accumulation of fluid in the interstitium and alveolar spaces results in the development of bilateral perihilar opacities.

B. Increased-Permeability Pulmonary Edema (Noncardiogenic Pulmonary Edema)

The most common form of increased-permeability pulmonary edema is ARDS. According to the consensus Berlin Definition, ARDS is characterized by acute onset (<7 days) of bilateral radiographic pulmonary infiltrates and respiratory failure not fully explained by heart failure or volume overload, with an associated impairment in oxygenation defined as a PaO2/FiO2 ratio of 300 or less. The severity of ARDS is defined by the severity of impairment in oxygenation. Mild, moderate, and severe ARDS are defined by PaO2/FiO2 ratios of 200–300 mm Hg, 100–200 mm Hg, and less than 100 mm Hg, respectively. ARDS is the final common pathway of a number of different serious medical conditions, all of which lead to increased pulmonary capillary leak. The range of clinical presentations includes all the diagnoses in the adult ICU, including sepsis, pneumonia, pancreatitis, aspiration of gastric contents, shock, lung contusion, nonthoracic trauma, toxic inhalation, near-drowning, and multiple blood transfusions. About one-third of ARDS patients initially have sepsis syndrome.

Although the mechanism of injury varies, damage to capillary endothelial cells and alveolar epithelial cells is common in ARDS regardless of cause. After

the initial insult (eg, an episode of high-grade bacteremia), there is generally a period of stability, reflecting the time it takes for pro-inflammatory mediators released from stimulated inflammatory cells to cause damage. Damage to endothelial and epithelial cells causes increased vascular permeability and reduced surfactant production and activity. These abnormalities lead to interstitial and alveolar pulmonary edema, alveolar collapse, a significant increase in surface forces, markedly reduced pulmonary compliance, and hypoxemia. For the first 24–48 hours after the insult, the patient may experience increased work of breathing, manifested by dyspnea and tachypnea, but without abnormalities on the chest radiograph. At this early stage, the increased A–a ΔPO2 reflects alveolar edema with mismatching shifted to low ratios that can usually be corrected by increased FiO2 and increased minute ventilation. The clinical picture may improve, or there may be a further fall in compliance and disruption of pulmonary capillaries, leading to areas of true shunting and refractory hypoxemia. The combination of increased work of breathing and progressive hypoxemia usually requires mechanical ventilation. Because the underlying process is heterogeneous, there is normal-appearing lung adjacent to atelectatic or consolidated lung. Therefore, ventilating patients at typical tidal volumes may overdistend normal alveoli, reduce blood flow to areas of adequate ventilation, and precipitate further lung injury (“volu-trauma”).

Hypoxemia can be profound in ARDS and is typically followed days later by hypercapnia owing to increasing dead space ventilation. Radiographically, there may be patchy alveolar opacities or “whiteout” of the lungs, representing diffuse confluent alveolar filling. Pathologically, diffuse alveolar damage is seen, characterized by inflammatory cell infiltration and the formation of hyaline membranes. Mortality is 20–40%. Most patients die from some complication of their presenting illness, not from refractory hypoxemia. Of those who survive, a majority will recover near-normal lung function, but their recovery may be prolonged (6–12 months). A subset of survivors will develop new pulmonary fibrosis or reactive airway disease.

CHECKPOINT

35. What four factors are involved in the production of pul-monary edema? How are they affected in cardiogenic versus noncardiogenic causes of pulmonary edema?

36. What are the common causes of noncardiogenic pul-monary edema? 37. Is lung damage from increased-permeability pulmo-nary edema

reversible? If so, how? 38. What are the two major reasons that mechanical venti-lation is often

required in severe pulmonary edema?

PULMONARY EMBOLISM

Clinical Presentation The English word “embolus” derives from a Greek word meaning “plug” or “stopper.” A pulmonary embolus consists of material that gains access to the venous system and then to the pulmonary circulation. Eventually, it reaches a vessel whose caliber is too small to permit free passage where it forms a plug, occluding the lumen and obstructing perfusion. There are many types of pulmonary emboli (Table 9–8). The most common is pulmonary thromboembolism, which occurs when venous thrombi, chiefly from the lower extremities, migrate to the pulmonary circulation. Note that it is a normal function of the pulmonary microcirculation to prevent embolic material from entering the systemic arterial system. The lungs possess both excess functional capacity and a redundant vascular supply, allowing them to filter a significant number of thrombi and platelet aggregates with minimal impact on lung function or hemodynamics. However, large thromboemboli, or a large enough accumulation of smaller ones, can cause substantial impairment of cardiac and respiratory function and death.

TABLE 9–8 Types of pulmonary emboli.

Pulmonary thromboemboli are common and cause significant morbidity. They are found at autopsy in 25–50% of hospitalized patients and are considered a major contributing cause of death in one-third of those. However, the diagnosis is made antemortem in only 10–20% of cases.

Etiology & Epidemiology Pulmonary embolism (PE) and deep venous thrombosis represent a continuum of a single disease that has been coined venous thromboembolic disease (VTE). VTE is a common problem with an annual incidence of approximately 100 new cases per 100,000 persons. The risk of this disorder increases with advancing age and also differs among ethnicities (with the greatest incidence in African- Americans). Furthermore, pulmonary embolism accounts for approximately 5– 10% of in-hospital deaths, making this disorder the leading cause of inpatient mortality.

The thromboemboli that cause PE almost never originate in the pulmonary circulation; they arrive there by dislodging from their site of origin and traveling through the venous circulation. More than 95% of pulmonary thromboemboli arise from thrombi in the deep veins of the lower extremity: the popliteal, femoral, and iliac veins. Venous thrombosis below the popliteal veins or

occurring in the superficial veins of the leg is clinically common but not a risk factor for pulmonary thromboembolism because thrombi in these locations rarely migrate to the pulmonary circulation without first extending above the knee. Because fewer than 20% of calf thrombi will extend into the popliteal veins, isolated calf thrombi may be observed with serial tests to exclude extension into the proximal deep system. Venous thromboses occasionally occur in the upper extremities or in the right side of the heart; this happens most commonly in the presence of intravenous catheters or cardiac pacing wires and may be of increasing clinical importance as the use of long-term intravenous catheters increases.

Risk factors for pulmonary thromboembolism are, therefore, the risk factors for the development of venous thrombosis in the deep veins of the legs (deep venous thrombosis) (Table 9–9). The German pathologist Rudolf Virchow described these risk factors in 1856: venous stasis, vascular wall endothelial injury, and increased activation of the clotting system. His observations remain valid today.

TABLE 9–9 Risk factors for venous thrombosis.

The most prevalent risk factor in hospitalized patients is stasis from immobilization, especially in those undergoing surgical procedures. The incidence of calf vein thrombosis in patients who do not receive appropriate anticoagulant prophylaxis after total knee replacement is reported to be as high as 84%; it is more than 50% after hip surgery or prostatectomy. The risk of fatal pulmonary thromboembolism in these patients may be as high as 5%. Physicians

caring for these patients must, therefore, be aware of the magnitude of the risk and institute appropriate prophylactic therapy (Tables 9–9 and 9–10).

TABLE 9–10 Risk of postoperative deep venous thrombosis or pulmonary embolus in patients who do not receive anticoagulant prophylaxis.

Malignancy and tissue damage at surgery are the two most common causes of increased activation of the coagulation system. Injury to the venous endothelium can result from stasis with local hypoxia, the insertion of intravenous catheters, or mechanical injury during surgery or trauma. Prior thrombosis can also damage venous valves and lead to venous incompetence, which promotes stasis.

Enhanced activation of the clotting cascade can result from both acquired disorders (eg, estrogen-containing medication, malignancy) and genetic predispositions (thrombophilias). Advances now permit identification of genetic disorders in up to one-third of unselected patients with venous thrombosis and in more than half of patients with familial thrombosis (see Table 9–9). It is now clear that these genetic variants may interact with other factors (eg, oral

contraceptive use, dietary deficiencies) to increase thrombosis risk. In addition to these major risk factors, recent studies implicate other risk

factors for the development of VTE: specifically, inflammatory disorders such as Crohn disease, ulcerative colitis, and rheumatoid arthritis; and infections, such as urinary tract infection, respiratory tract infection, and bacteremia. The metabolic syndrome also increases the risk of VTE, especially the presence of abdominal obesity. The mechanisms by which these other risk factors increase the likelihood of VTE is unclear, but it is hypothesized that systemic inflammation upregulates procoagulant factors while concurrently inhibiting fibrinolysis. Abdominal obesity may also cause venous stasis secondary to increased abdominal pressure.

Pathophysiology Venous thrombi are composed of a friable mass of fibrin, with many erythrocytes and a few leukocytes and platelets randomly enmeshed in the matrix. When a venous thrombus travels to the pulmonary circulation, it causes a broad array of pathophysiologic changes (Table 9–11).

TABLE 9–11 Pathophysiologic changes in pulmonary embolism.

A. Hemodynamic Changes

Every patient with a pulmonary embolus has some degree of mechanical obstruction of the pulmonary arterial circulation. The severity of mechanical obstruction depends on the clot burden, the neurohumoral reflexes stimulated by

mediators released from the thrombus, and the presence or absence of pre- existing cardiopulmonary disease. Patients without pre-existing cardiopulmonary disease can accommodate occlusion of up to roughly one-third of the pulmonary circulation with a negligible increase in pulmonary vascular resistance and pulmonary arterial pressure. The normal pulmonary circulation adapts to the diverted blood flow through the recruitment and dilation of compliant pulmonary arterial vessels (see Figure 9–13). These adaptive mechanisms fail when a greater proportion of the pulmonary circulation is compromised by larger emboli and/or by the elaboration of vasoconstricting mediators, at which point pulmonary vascular resistance and pulmonary arterial pressure increase. In patients with pre-existing cardiopulmonary disease, increases in pulmonary artery pressures have not been shown to correlate with extent of embolization. The likely explanation is that normal adaptive mechanisms are ineffective in patients with pre-existing pulmonary hypertension, making them susceptible to significant instability with any subsequent impairment of the pulmonary vasculature.

Large emboli that do not completely occlude vessels, particularly in patients with compromised cardiac function, may cause an acute increase in pulmonary vascular resistance. This leads to acute right ventricular strain and can lead to a fatal decrease in cardiac output. The most devastating and feared complication of acute pulmonary thromboembolism is sudden occlusion of the pulmonary outflow tract (“saddle embolus”), reducing cardiac output to zero and causing immediate cardiovascular collapse and death. Such dramatic presentations occur in less than 5% of cases and are essentially untreatable. They serve to highlight the importance of primary prevention of venous thrombosis.

B. Changes in Ventilation

The immediate effect is the generation of increased “dead space” ventilation (ie, regions of the lung with preserved Eqn02.jpg but absent Eqn03.jpg). This increase in dead space impairs the elimination of carbon dioxide with minimal effect on oxygenation. However, in the setting of thromboembolism, patients typically compensate for the increase in dead space ventilation by increasing their total minute ventilation. In fact, patients classically overcompensate for the increase in dead space, exhibiting a respiratory alkalosis on their arterial blood gas analysis.

C. Changes in Oxygenation

Mild to moderate hypoxemia with a low PaCO2 is the most common finding in acute pulmonary thromboembolism. Mild hypoxemia may be obscured by the tendency to rely on oximetry alone, because more than half of patients will have oxygen saturations (SaO2) above 90% (Figure 9–26). Historically, the A–a ΔPO2 was thought to be a more sensitive indicator of PE because it compensates for the presence of hypocapnia and the amount of inspired FiO2. However, the Prospective Investigation of Pulmonary Embolism Diagnosis II (PIOPED II) study called this thinking into question. An A–a ΔPO2 less than 20, which is normal or near normal depending on patient age, was found in one-third of patients with an acute PE identified by CT scan (see Figure 9–26).

FIGURE 9–26 Arterial PO2 and A–a O2 difference in 74 patients with PE from the PIOPED II study. Blood gases were drawn while patients were breathing room air. (Data from Stein PD et al. Clinical characteristics of patients with acute pulmonary embolism: data from PIOPED II. Am J Med. 2007;120:871.)

No one mechanism fully accounts for hypoxemia in acute PE. At least five mechanisms have been suggested:

1. Local hypoperfusion interfering with surfactant production by alveolar type II cells. Surfactant is subsequently depleted, resulting in alveolar edema, alveolar collapse, and areas of atelectasis creating lung units with little or no ventilation. These changes result in an increase in lung units with low

ratios, including some areas of true shunting, both of which contribute to an increased A–a ΔPO2 and arterial hypoxemia.

2. The diversion of blood flow through poorly ventilated or nonventilated lung zones. Perfusion is normally reduced to hypoventilated lung regions through hypoxic pulmonary vasoconstriction. However, if pulmonary artery pressure increases after thromboembolism, perfusion may increase in

areas of vasoconstriction, resulting in shifts toward low areas causing hypoxemia.

3. True right-to-left shunts. Increased pulmonary artery, right ventricular, and right atrial pressures favor the development of shunts in a small percentage of patients with hypoxemia from acute pulmonary thromboembolism. It has been proposed that these shunts result from the opening of a foramen ovale or from pulmonary artery to pulmonary venous shunting, but their exact location is unknown.

4. Low mixed venous PO2. In some patients with pre-existing impaired cardiac function or with large emboli that cause acute right ventricular strain, cardiac output may fall, with a resultant fall in mixed venous oxygen concentration. This is an important cause of hypoxemia in seriously ill patients.

5. Decreased pulmonary capillary surface area resulting in decreased lung diffusion capacity.

D. Bronchoconstriction

Reflex bronchoconstriction causes wheezing and increased work of breathing in some patients.

E. Pulmonary Infarction

Obstruction of small pulmonary arterial branches that act as end arteries leads to pulmonary infarction in about 10% of cases. It is generally associated with some concomitant abnormality of the bronchial circulation such as that seen in patients with left ventricular failure and chronically elevated left atrial pressures.

Clinical Manifestations A. Symptoms & Signs The classic triad of a sudden onset of dyspnea, pleuritic chest pain, and hemoptysis occurs in a minority of cases. In a large study of patients with PE, dyspnea was present in 73% of cases, and pleuritic chest pain was present 44% of the time. Dyspnea probably results from reflex bronchoconstriction as well as increased pulmonary artery pressure, loss of pulmonary compliance, and stimulation of C fibers. In patients with large emboli, acute right heart strain may contribute to dyspnea. Pleuritic chest pain is more common than pulmonary infarction; one group has suggested that the pain is caused by areas of

pulmonary hemorrhage. Hemoptysis is seen with pulmonary infarction but may also result from transmission of systemic arterial pressures to the microvasculature via bronchopulmonary anastomoses, with subsequent capillary disruption. It may reflect hemorrhagic pulmonary edema from surfactant depletion or neutrophil-associated capillary injury. Syncope may signal a massive embolus.

The most predictive physical finding for pulmonary embolism is not in the chest but the leg: a swollen, tender, warm, and reddened calf that provides evidence for deep venous thrombosis. The absence of such evidence does not exclude the diagnosis, because the clinical examination is insensitive and the absence of signs may indicate that the entire thrombus has embolized. Auscultatory chest findings are common but nonspecific. Atelectasis may lead to inspiratory crackles; infarction may cause a focal pleural friction rub; and the release of mediators may cause bronchoconstriction and wheezing. In large embolization, one may find signs of acute right ventricular strain such as a right ventricular lift and accentuation of the pulmonary component of the second heart sound.

Difficulty in recognizing patients with pulmonary embolism results from the lack of sensitivity and specificity of symptoms and signs. This challenge has motivated the development of clinical probability scoring systems. The Wells and Geneva scores are examples of two well-validated diagnostic aids that allow the clinician to assign disease likelihoods to individual patients. In both scoring systems, points are tabulated based on the presence of specific clinical features that are readily assessed. Patients can then be categorized into different tiers of disease probability to facilitate clinical decisions on the performance and interpretation of diagnostic tests and the initiation of treatments.

B. Electrocardiography

The electrocardiogram is abnormal in 70% of patients with acute PE. However, the most common abnormalities are sinus tachycardia and nonspecific ST and T wave changes, each seen in approximately 40% of patients. The classic finding of an acute right ventricular strain pattern on ECG—a deep S wave in lead I and both a Q wave and an inverted T wave in lead III (S1Q3T3)—was observed in 11% of patients in the Urokinase Pulmonary Embolism Trial.

C. Laboratory Findings

An increase in the A–a ΔPO2 is seen in more than two-thirds of cases, and

hypoxemia is a common yet nonspecific finding. Measurement of the degradation product of cross-linked fibrin, D-dimers, can be used to exclude the diagnosis of acute PE in symptomatic outpatients deemed to have a low pretest probability of PE based on clinical criteria. Depending on the specific assay and patient population, the D-dimer has a high sensitivity (85–99%) and moderate to high specificity (40–93%). Most studies suggest that D-dimer cannot be used to exclude PE in a patient with an intermediate or a high pretest probability for PE.

Brain natriuretic peptide (BNP), an indicator of ventricular stretch, and cardiac troponins, which indicate cardiac myocyte injury, are commonly measured in patients with PE. Because of low sensitivity and specificity, these markers cannot be used to diagnose PE. However, an elevation of BNP or troponins in the setting of known PE has been shown to correlate with the presence of right ventricular overload and a greater risk of adverse outcomes, including respiratory failure and death.

D. Chest Radiography

The chest radiograph was normal in only 12% of patients with confirmed pulmonary thromboembolism in the PIOPED study. The most common findings were atelectasis, parenchymal opacities, and small pleural effusions. However, the prevalence of these findings was the same in hospitalized patients without suspected pulmonary thromboembolism. Local oligemia (Westermark sign) or pleura-based areas of increased opacity that represent intraparenchymal hemorrhage (Hampton hump) are rare. The chest radiograph is necessary to exclude other common lung diseases and to permit interpretation of the ventilation/perfusion scan, but it does not itself establish the diagnosis. Paradoxically, it may be most helpful when normal in the setting of acute severe hypoxemia.

E. Ventilation/Perfusion Scanning

A perfusion scan is obtained by injecting microaggregated radiolabeled albumin with a particle size of 50–100 μm into the venous system and allowing the particles to embolize to the pulmonary capillary bed (approximate diameter 10 μm). The substance is labeled with a gamma-emitting isotope of technetium (Tc- 99m pertechnetate) that permits imaging of the distribution of pulmonary blood flow. A ventilation scan is performed by having the patient breathe xenon (Xe- 133) or a radioactive aerosol and doing sequential scans during inhalation and exhalation. A normal perfusion scan excludes clinically significant pulmonary

thromboembolism. A segmental or larger perfusion defect in a radiographically normal area that shows normal ventilation is diagnostic. This is referred to as a “mismatched” defect and is highly specific (97%) for pulmonary thromboembolism.

Only a minority of ventilation/perfusion scans reveal clearly diagnostic findings, however. The PIOPED study demonstrated that nondiagnostic ventilation/perfusion scans can stratify a patient’s risk of pulmonary thromboembolism. Furthermore, within the categories of high-, medium-, and low-probability studies, the clinician’s pretest assessment of the probability of pulmonary thromboembolism can further stratify patients. When clinical probabilities and scan interpretations are discordant, additional evaluation for VTE should be performed.

F. Computed Tomography and Pulmonary Angiography

Computed tomography scanning with intravenous contrast (CT pulmonary angiography) has widely supplanted ventilation/perfusion scanning as the initial test of choice to diagnose PE. The diagnostic strength of this imaging modality lies in its high negative predictive value and its ability to identify other conditions that cause dyspnea and chest pain (eg, pneumonia, aortic dissection). Multiple trials have shown a high sensitivity and specificity of this imaging technique, although the diagnostic test characteristics depend on patient selection, expertise of the technician performing the contrast injection, and the experience of the interpreting radiologist. The PIOPED II trial evaluated CT angiography for the diagnosis of PE and found a sensitivity of 83% and a specificity of 96% (Table 9–12). Several other studies indicate that the risk of PE after a negative CT scan in patients with a low or intermediate clinical probability of PE is less than 2%. Consistent with the first PIOPED trial comparing ventilation/perfusion scanning and traditional pulmonary angiography, pretest probability based on clinical risk scores must be taken into account when interpreting CT pulmonary angiography. As is true with the ventilation/perfusion scan, if test results and clinical probabilities are discordant, further testing, such as ventilation/perfusion scanning, lower extremity Doppler ultrasonography, or pulmonary angiography, must be considered.

TABLE 9–12 Positive and negative predictive values of CT angiography for acute pulmonary embolism (PE).

Pulmonary angiography is a safe but invasive procedure with well-defined morbidity and mortality data. Minor complications occur in approximately 5% of patients. Most are allergic contrast reactions or transient kidney injury or are related to percutaneous catheter insertion; cardiac perforation and arrhythmias are reported but rare. Among the PIOPED I patients who underwent angiography, five deaths (0.7%) were directly related to the procedure. Pulmonary angiography remains the reference standard for the diagnosis of PE, but its role compared with CT angiography is a subject of ongoing debate. There is general agreement that angiography is indicated when the diagnosis is in doubt but there is a high clinical pretest probability of PE, or when the diagnosis of PE must be established with certainty, as when anticoagulation is contraindicated or placement of an inferior vena cava filter is contemplated. An intraluminal filling defect in more than one projection establishes a definitive diagnosis. Secondary findings highly suggestive of PE include an abrupt arterial cutoff, asymmetry of blood flow (especially segmental oligemia), and a prolonged arterial phase with slow filling. Pulmonary angiography was performed in 755 patients in the PIOPED I study. A definitive diagnosis was established in 97%; in 3%, the studies were nondiagnostic. Four patients (0.8%) with negative angiograms subsequently had pulmonary thromboemboli at autopsy. Serial angiography has demonstrated minimal resolution of thrombus prior to day 7 following presentation. Thus, negative angiography within 7 days of presentation excludes the diagnosis.

G. Resolution

The variability among patients is so great that generalizations are difficult. The largest number of patients monitored serially with quantitative assessments was in the Urokinase Pulmonary Embolism Trial. In that study, serial perfusion scans showed a resolution of 35–56% of perfusion defects at 9–14 days. More recent studies, some involving quantitative angiography, have tended to support the time course of these findings.

In a few patients, pulmonary emboli do not resolve completely but become organized and incorporated into the pulmonary arterial wall as an epithelialized fibrous mass, producing what is termed chronic pulmonary thromboembolism. This entity presents with stenosis of the central pulmonary arteries, with associated pulmonary hypertension and right ventricular failure (cor pulmonale). Treatment is surgical.

CHECKPOINT

39. Where do 95% of pulmonary thromboemboli originate? 40. What are the risk factors for pulmonary thromboemboli? 41. What hemodynamic changes are brought about by sig-nificant

pulmonary thromboemboli? 42. What changes in ventilation/perfusion relation-ships are brought about

by significant pulmonary thromboemboli? 43. Suggest some possible explanations for hypoxemia in pulmonary

thromboembolism. 44. What are the clinical manifestations of pulmonary thromboembolism?

CASE STUDIES Yeong Kwok, MD

(See Chapter 25, p. 762–65 for answers)

CASE 48

A 25-year-old previously well woman presents to your office with complaints of episodic shortness of breath and chest tightness. She has had the symptoms on and off for about 2 years but states that they have worsened lately, occurring two or three times a month. She notes that the symptoms are worse during the spring months. She has no exercise-induced or nocturnal symptoms. The family history is notable for a father with

asthma. She is single and works as an administrative assistant in a high-tech firm. She lives with a roommate, who moved in approximately 2 months ago. The roommate has a cat. The patient smokes occasionally when out with friends and drinks socially, but has no history of illicit drug use. Examination is notable for mild end-expiratory wheezing. The history and physical examination are consistent with a diagnosis of asthma. Pulmonary function tests are ordered to confirm the diagnosis.

Questions

A. What are the three categories of provocative agents that can trigger asthma? What are some possible triggers in this patient?

B. Describe the early events responsible for the pathogenesis of asthma. How does this result in chronic airway inflammation and airway hyperresponsiveness?

C. What pathogenetic mechanisms are responsible for this patient's symptoms of wheezing, shortness of breath, and chest tightness?

D. What might you expect the results of her pulmonary function tests to be? Why?

CASE 49

A 67-year-old man presents to your office with worsening cough, sputum production, and shortness of breath. He has been a cigarette smoker for the past 50 years, smoking approximately 1 pack a day. He has a chronic AM cough productive of some yellow sputum but generally feels okay during the day. He was in his usual state of health until two weeks ago when he developed a cold. Since then, he has had a hacking cough and increased thick sputum production. He also has had difficulty walking more than a block without stopping due to shortness of breath. Physical examination reveals prolonged expiration, audible wheezing, and diffuse rhonchi throughout both lung fields. Chest x-ray shows hyperinflation of both lungs with a flattened diaphragm.

Questions

A. What are the two major clinical syndromes classified as chronic obstructive pulmonary disease? How do they differ?

B. Of the two syndromes above, which is predominant in this patient? What are the epidemiology and predisposing factors for this condition?

C. What might the pulmonary function tests show in this patient? D. How do arterial blood gases differ in chronic bronchitis and

emphysema?

CASE 50

A 68-year-old man presents to the clinic with a complaint of shortness of breath. He states that he has become progressively more short of breath over the last 2 months, such that he is now short of breath with walking one block. In addition, he has noted a nonproductive cough. He denies fever, chills, night sweats, chest pain, orthopnea, and paroxysmal nocturnal dyspnea. He has noted no lower extremity edema. The medical history is unremarkable. Physical examination reveals a respiratory rate of 19/min and fine, dry inspiratory crackles heard throughout both lung fields. Digital clubbing is present. A diagnosis of idiopathic pulmonary fibrosis is made.

Questions

A. What are the cellular events involved in lung injury and fibrosis in idiopathic pulmonary fibrosis?

B. What pathophysiologic mechanisms are responsible for this patient’s symptoms of dyspnea and cough? What pathogenetic mechanisms are responsible for his physical findings of tachypnea, inspiratory crackles, and digital clubbing?

C. What might you expect the chest x-ray film and pulmonary function tests to show?

CASE 51

A 72-year-old man presents to the emergency department complaining of severe shortness of breath. He has long-standing poorly controlled hypertension and a history of coronary artery disease and two myocardial infarctions. About 1 week before admission, he had an episode of substernal chest pain lasting approximately 30 minutes. Since then he has noted progressive shortness of breath to the point that he is now dyspneic on minimal exertion such as walking across the room. He notes a new onset of shortness of breath while lying down. He is only comfortable when propped up by three pillows. He is occasionally awakened from sleep acutely short of breath. On examination he is afebrile, with a blood pressure of 160/100 mm Hg, heart rate of 108/min, respiratory rate of 22/min, and oxygen saturation of 88% on room air. He is pale, cool, and diaphoretic. Jugular venous pressure is 10 cm H2O. Chest auscultation reveals rales in both lungs to the mid-lung fields. Cardiac examination reveals tachycardia, with an audible S3 and S4. No murmurs or rubs are heard. Extremities are without edema. The ECG shows left ventricular hypertrophy and Q waves in the anterior and lateral leads, consistent with this patient’s history of hypertension and myocardial infarction. Chest x-ray film reveals bilateral fluffy infiltrates consistent with pulmonary edema. He is admitted to the ICU with a diagnosis of heart failure and possible myocardial infarction.

Questions

A. What are the four factors that account for almost all cases of pulmonary edema? Which are probably responsible for this patient’s pulmonary edema?

B. How does poor cardiac function cause pulmonary edema?

CASE 52

A 57-year-old man undergoes total knee replacement for severe degenerative joint disease. Four days after surgery, he develops an acute onset of shortness of breath and right-sided pleuritic chest pain. He is now in moderate respiratory distress with a respiratory rate of 28/min, heart rate

of 120 bpm, and blood pressure of 110/70 mm Hg. Oxygen saturation is 90% on room air. Lung examination is normal. Cardiac examination reveals tachycardia but is otherwise unremarkable. The right lower extremity is postsurgical, healing well, with 2+ pitting edema, calf tenderness, erythema, and warmth; the left leg is normal. He has a positive Homan sign on the right. Acute pulmonary embolism is suspected.

Questions

A. Where did the pulmonary embolism probably arise from? B. What are this patient’s risk factors for thromboembolism? C. What are the hemodynamic changes seen in acute pulmonary

embolism? D. What changes might be expected in ventilation/perfusion relationships

and why?

CASE 53

A 46-year-old man presents to the hospital with a 5-day history of worsening cough, high fever, and shortness of breath. On physical examination, he is noted to be tachypneic (respiratory rate of 30 breaths/min), hypoxic with a low oxygen saturation (89%), and febrile (39°C). Chest x-ray film reveals infiltrates in both lower lobes. A complete blood count reveals a high white blood cell count. He is admitted to the hospital. Despite treatment with oxygen and antibiotics, he becomes more hypoxic and requires endotracheal intubation and mechanical ventilation. Blood cultures grow Streptococcus pneumoniae. Despite mechanical ventilation using high oxygen concentrations, his arterial blood oxygen level remains low. His chest x-ray film shows progression of infiltrates throughout both lung fields. He is diagnosed with acute respiratory distress syndrome (ARDS).

Questions

A. What are the main pathophysiologic factors in ARDS that cause an accumulation of extravascular fluid in the lungs?

B. What are the common causes of ARDS? C. What accounts for the severe hypoxia often found in ARDS, despite the

use of mechanical ventilation and high concentrations of oxygen?

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CHAPTER

10

Cardiovascular Disorders: Heart Disease Fred M. Kusumoto, MD, FACC

Diseases of the cardiovascular system frequently confront the physician involved in the day-to-day care of patients. Knowledge of the underlying pathophysiologic processes associated with diseases of the heart and blood vessels provides a critical framework for patient management. This chapter deals with diseases of the heart, with the following chapter focusing on diseases of the blood vessels. Normal cardiac structure and function are summarized here, and pathophysiologic mechanisms for commonly encountered cardiac problems are then discussed, with emphasis on arrhythmias, heart failure, valvular heart disease, coronary artery disease, and pericardial disease.

NORMAL STRUCTURE & FUNCTION OF THE HEART

ANATOMY

The heart is a complex organ whose primary function is to pump blood through the pulmonary and systemic circulations. It is composed of four muscular chambers: the main pumping chambers, the left and right ventricles, and the left and right atria, which act like “priming pumps” responsible for the final 20–30%

of ventricular filling (Figure 10–1A). Peripheral venous return from the inferior and superior venae cavae fills the right atrium and ventricle (through the open tricuspid valve) (Figure 10–1B). With atrial contraction, additional blood flows through the tricuspid valve and completes the filling of the right ventricle. Unoxygenated blood is then pumped to the pulmonary artery and lung by the right ventricle through the pulmonary valve (Figure 10–1C). Oxygenated blood returns from the lung to the left atrium via four pulmonary veins (Figure 10–1D). Sequential left atrial and ventricular contraction pumps blood back to the peripheral tissues. The mitral valve separates the left atrium and ventricle, and the aortic valve separates the left ventricle from the aorta (Figures 10–1D and 10–1E).

FIGURE 10–1 Anatomy of the heart. A: Anterior view of the heart. B: View of the right heart with the right atrial wall reflected to show the right atrium. C: Anterior view of the heart with the anterior wall removed to show the right ventricular cavity. D: View of the left heart with the left ventricular wall turned back to show the mitral valve. E: View of the left heart from the left side with the left ventricular free wall and mitral valve cut away to reveal the aortic valve. (Redrawn, with permission, from Cheitlin MD et al, eds. Clinical Cardiology, 6th ed. Originally published by Appleton & Lange. Copyright © 1993 by The McGraw-Hill Companies, Inc.)

The heart lies free in the pericardial sac, attached to mediastinal structures only at the great vessels. During embryologic development, the heart invaginates into the pericardial sac like a fist pushing into a partially inflated balloon. The pericardial sac is composed of a serous inner layer (visceral pericardium) directly apposed to the myocardium and a fibrous outer layer called the parietal

pericardium. Under normal conditions, approximately 40–50 mL of clear fluid, which probably is an ultrafiltrate of plasma, fills the space between the layers of the pericardial sac.

The left main and right coronary arteries arise from the root of the aorta and provide the principal blood supply to the heart (Figure 10–2). The large left main coronary artery usually branches into the left anterior descending artery and the circumflex coronary artery. The left anterior descending coronary artery gives off diagonal and septal branches that supply blood to the anterior wall and septum of the heart, respectively. The circumflex coronary artery continues around the heart in the left atrioventricular groove and gives off large obtuse marginal arteries that supply blood to the left ventricular free wall. The right coronary artery travels in the right atrioventricular groove and supplies blood to the right ventricle via acute marginal branches. The posterior descending artery, which supplies blood to the posterior and inferior walls of the left ventricle, arises from the right coronary artery in 80% of people (right-dominant circulation) and from the circumflex artery in the remainder (left-dominant circulation).

FIGURE 10–2 Coronary arteries and their principal branches in humans. (Redrawn, with permission, from Ross G. The cardiovascular system. In: Ross G, ed. Essentials of Human Physiology. Copyright © 1978 by Year Book Medical Publishers, Inc., Chicago. Copyright © Elsevier.)

Contraction of the heart chambers is coordinated by several regions in the

heart composed of myocytes with specialized automaticity (pacemaker) and conduction properties (Figure 10–3). Cells in the sinoatrial (SA) node and the atrioventricular (AV) node have fast pacemaker rates (SA node: 60–100 bpm; AV node: 40–70 bpm), and the His bundle and Purkinje fibers are characterized by rapid rates of conduction. Because it has the fastest intrinsic pacemaker rhythm, the SA node is usually the site of initiation of the cardiac electrical impulse during a normal heartbeat. The impulse then rapidly depolarizes both the left and right atria as it travels to the AV node. Conduction velocity slows from 1 m/s in atrial tissue to 0.05 m/s in nodal tissue. After the delay in the AV node, the impulse moves rapidly down the His bundle (1 m/s) and Purkinje fibers (4 m/s) to simultaneously depolarize the right and left ventricles. The atria and ventricles are separated by a fibrous framework that is electrically inert, so under normal conditions the AV node and the contiguous His bundle form the only electrical connection between the atria and ventricles. This arrangement allows the atria and ventricles to beat in a synchronized fashion and minimizes the chance of electrical feedback between the chambers.

FIGURE 10–3 Conducting system of the heart. Typical transmembrane action potentials for the SA and AV nodes, other parts of the conduction system, and the atrial and ventricular muscles are shown along with the correlation to the extracellularly recorded electrical activity (ie, the electrocardiogram [ECG]). The action potentials and ECG are plotted on the same time axis but with different zero points on the vertical scale. The PR interval is measured from the beginning of the P wave to the beginning of the QRS. (LAF, left anterior fascicle.) (Redrawn, with permission, from Barrett KE et al, eds. Ganong’s Review of Medical Physiology, 25th ed. McGraw-Hill, 2016.)

The electrical activity of the heart can be measured from the body surface at standardized positions by electrocardiography. On the electrocardiogram (ECG), the P wave represents depolarization of atrial tissue; the electrocardiographic wave (QRS) interval, ventricular depolarization; and the T wave, ventricular repolarization (see Figure 10–3). Because normal ventricular depolarization occurs almost simultaneously in the right and left ventricles—usually within 60– 100 ms—the QRS complex is narrow. Although the electrical activity of the small specialized conduction tissues cannot be measured directly from the surface, the interval between the P wave and the start of the QRS complex (PR interval) primarily represents the conduction time of the AV node and His bundle.

HISTOLOGY

Ventricular myocytes are normally 50–100 mm long and 10–25 mm wide. Atrial and nodal myocytes are smaller, whereas myocytes of the Purkinje system are larger in both dimensions. Myocytes are filled with hundreds of parallel striated bundles termed myofibrils. Myofibrils are composed of repeating units, termed sarcomeres, that form the major contractile unit of the myocyte (Figure 10–4). Sarcomeres are complex structures composed of contractile proteins, myosin and actin, which are connected by cross-bridges, and a regulatory protein complex, tropomyosin. (See Cellular Physiology section later.)

FIGURE 10–4 A: Electron photomicrograph of cardiac muscle. The fuzzy thick lines are intercalated disks (×12,000). (Reproduced, with permission, from Bloom W et al. A Textbook of Histology, 10th ed. Saunders, 1975. Copyright © Elsevier.) B: Diagram of cardiac muscle as seen under the light microscope (top) and the electron microscope (bottom). (N, nucleus.) (Redrawn, with permission, from Braunwald E et al. Mechanisms of contraction of the normal and failing heart. N Engl J Med. 1967;277:794. Copyright © 1967 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.)

PHYSIOLOGY

Physiology of the Whole Heart Because the ventricles are the primary physiologic pumps of the heart, analysis has focused on these chambers, particularly the left ventricle. The function of intact ventricles is traditionally studied by evaluating pressure–time and pressure–volume relationships.

In pressure–time analysis (Figure 10–5), pressures in the heart chambers and great vessels are measured during the cardiac cycle and plotted as a function of time. At the beginning of the cardiac cycle, the left atrium contracts, forcing additional blood into the left ventricle and giving rise to an a wave on the left atrial pressure tracing. At end diastole, the mitral valve closes, producing the first heart sound (S1), and a brief period of isovolumic contraction follows, during which both the aortic and mitral valves are closed but the left ventricle is actively contracting. When intraventricular pressure rises to the level of aortic pressure, the aortic valve opens and blood flows into the aorta. Beyond this point, the aorta and left ventricle form a contiguous chamber with equal pressures, but left ventricular volume decreases as blood is expelled. Left ventricular contraction stops and ventricular relaxation begins, and end systole is reached when intraventricular pressure falls below aortic pressure. The aortic valve then closes, producing the second heart sound (S2). Throughout systole, blood has slowly accumulated in the left atrium (because the mitral valve is closed), giving rise to the v wave on the left atrial pressure tracing. During the first phase of diastole—isovolumic relaxation—no change in ventricular volume occurs, but continued relaxation of the ventricle leads to an exponential fall in left ventricular pressure. Left ventricular filling begins when left ventricular pressure falls below left atrial pressure and the mitral valve opens. Ventricular relaxation is a relatively long process that begins before the aortic valve closes and extends past the mitral valve opening. The rate and extent of ventricular relaxation depend on multiple factors: heart rate, wall thickness, chamber volume and shape, aortic pressure, sympathetic tone, and presence or absence of myocardial ischemia. Once the mitral valve opens, there is an initial period of rapid filling of the ventricle that contributes 70–80% of blood volume to the ventricle and occurs largely because of the atrioventricular pressure gradient. By mid-diastole, flow into the left ventricle has slowed, and the cardiac cycle begins again with the next atrial contraction. Right ventricular pressure–time analysis is similar but with lower pressures because the impedance to flow in the pulmonary

vascular system is much lower than in the systemic circulation.

FIGURE 10–5 Diagram of events in the cardiac cycle. From top downward: pressure (mm Hg) in

aorta, left ventricle, left atrium, pulmonary artery, right ventricle, right atrium; blood flow (mL/s) in ascending aorta and pulmonary artery; ECG. Abscissa, time in seconds. (Valvular opening and closing are indicated by AO and AC, respectively, for the aortic valve; MO and MC for the mitral valve; PO and PC for the pulmonary valve; TO and TC for the tricuspid valve.) Events of the cardiac cycle at a heart rate of 75 bpm. The phases of the cardiac cycle identified by the numbers at the bottom are as follows: 1, atrial systole; 2, isovolumetric ventricular contraction; 3, ventricular ejection; 4, isovolumetric ventricular relaxation; 5, ventricular filling. Note that late in systole, aortic pressure actually exceeds left ventricular pressure. However, the momentum of the blood keeps it flowing out of the ventricle for a short period. The pressure relationships in the right ventricle and pulmonary artery are similar. (Redrawn, with permission, from Milnor WR. The circulation. In: Mountcastle VB, ed. Medical Physiology, 2 vols. Mosby, 1980. Copyright © Elsevier.)

In pressure–volume analysis (Figure 10–6), pressure during the cardiac cycle is plotted as a function of volume rather than time. During diastole, as ventricular volume increases during both the initial rapid filling period and atrial contraction, ventricular pressure increases (curve da). The shape and position of this curve, the diastolic pressure–volume relationship, depend on relaxation properties of the ventricle, the elastic recoil of the ventricle, and the distensibility of the ventricle. The curve shifts to the left (higher pressure for a given volume) if relaxation of the ventricle is decreased, the ventricle loses elastic recoil, or the ventricle becomes stiffer. At the beginning of systole, active ventricular contraction begins and volume remains unchanged (isovolumic contraction period) (ab). When left ventricular pressure reaches aortic pressure, the aortic valve opens, and ventricular volume decreases as the ventricle expels its blood (curve bc). At end systole (c), the aortic valve closes and isovolumic relaxation begins (cd). When the mitral valve opens, the ventricle begins filling for the next cardiac cycle, repeating the entire process. The area encompassed by this loop represents the amount of work done by the ventricle during a cardiac cycle. The position of point c depends on the isovolumic systolic pressure–volume curve. If the ventricle is filled with variable amounts of blood (preloads) and allowed to contract, but the aortic valve is prevented from opening, a relatively linear relationship exists, termed the isovolumic systolic pressure–volume curve (Figure 10–6B). The slope and position of this line describe the inherent contractile state of the ventricle. If contractility is increased by catecholamines or other positive inotropes, the line will shift to the left.

FIGURE 10–6 A: Pressure–volume loop for the left ventricle. During diastole, the left ventricle fills and pressure increases along the diastolic pressure–volume curve from d to a. Line ab represents isometric contraction, and bc the ejection phase of systole. The aortic valve closes at point c, and pressure drops along cd (isovolumic relaxation), until the mitral valve opens at point d and the cycle repeats. The distance from b to c represents the stroke volume ejected by that beat. Point a represents end-diastole, and point c represent end-systole. B: If the left ventricle is filled by varying amounts (a, a′, a″) and allowed to undergo isovolumic contraction, a relatively linear relationship, the isovolumic pressure–volume relation, can be defined.

Pressure–volume relationships help illustrate the effects of different stresses on cardiac output. Cardiac output of the ventricle is the product of the heart rate and the volume of blood pumped with each beat (stroke volume). The width of the pressure–volume loop is the difference between end-diastolic volume and end-systolic volume, or the stroke volume (see Figure 10–6). The stroke volume depends on three parameters: contractility, afterload, and preload (Figure 10–7). Changing the contractile state of the heart will change the width of the pressure– volume loop by changing the position of the isovolumic systolic pressure curve. The impedance against which the heart must work (aortic pressure for the left ventricle) is termed afterload; increased afterload will cause a decrease in stroke volume. Preload is the amount of filling of the ventricle at end-diastole. Up to a point, the more a myocyte or ventricular chamber is stretched, the more it will contract (Frank–Starling relationship), so that increased preload will lead to an increase in stroke volume.

FIGURE 10–7 A: Increasing afterload from b to b′ decreases stroke volume from bc to b′c′. B: Increasing preload from a to a′ increases stroke volume from bc to b′c′, but at the expense of increased end- diastolic pressure. C: An increasingly contractile state shifts the isovolumic pressure–volume relationship leftward, increasing stroke volume from bc to b′c′.

Pressure–time and pressure–volume relationships are critical for understanding the pathophysiologic mechanisms of diseases that affect the entire ventricular chamber function, such as heart failure and valvular abnormalities.

Cellular Physiology A. Ventricular and Atrial Myocytes The cellular mechanism of myocyte contraction after electrical stimulation is too complex to fully address in this section, but excellent discussions of electromechanical coupling can be found. Briefly, when the myocyte is stimulated, sodium channels on the cell surface membrane (sarcolemma) open, and sodium ions (Na+) flow down their electrochemical gradient into the cell. This sudden inward surge of ions is responsible for the sharp upstroke of the myocyte action potential (phase 0) (Figure 10–8). A plateau phase follows, during which the cell membrane potential remains relatively unchanged owing to the inward flow of calcium ions (Ca2+) and the outward flow of potassium ions (K+) through several different specialized potassium channels. Repolarization occurs because of the continued outward flow of K+ after the inward flux of Ca2+ has stopped.

FIGURE 10–8 Changes in ionic conductances responsible for generating action potentials for ventricular or atrial tissue (right) and a sinus or AV node cell (left). In nodal cells, rapid Na+ channels are absent, so that the action potential upstroke is much slower. Diastolic depolarization observed in nodal cells is a result of decreased K+ efflux and slow Na+ and Ca2+ influx. Ca2+ (T): influx via Ca2+ (T) channels; Ca2+ (L): influx via Ca2+ (L) channels.

Within the cell, the change in membrane potential from the sudden influx of Na+ and the subsequent increase in intracellular Ca2+ causes the sarcoplasmic reticulum to release large numbers of calcium ions via specialized Ca2+ release channels, although the exact signaling mechanism is unknown. Once in the cytoplasm, however, Ca2+ released from the sarcoplasmic reticulum binds with the regulatory proteins troponin and tropomyosin. Myosin and actin are then allowed to interact and the cross-bridges between them bend, giving rise to contraction (Figure 10–9). The process of relaxation is also poorly understood but appears to involve return of Ca2+ to the sarcoplasmic reticulum via two transmembrane sarcoplasmic reticulum-embedded proteins: Ca2+-ATPase and phospholamban. Reuptake of Ca2+ is an active process that requires adenosine triphosphate (ATP).

FIGURE 10–9 Initiation of muscle contraction by Ca2+. When Ca2+ binds to troponin C, tropomyosin is displaced laterally, exposing the binding site for myosin on actin (dark area). ATP hydrolysis then changes the conformation of the myosin head and fosters its binding to the exposed site. For simplicity, only one of the two heads of the myosin-II molecule is shown. (Redrawn, with permission, from Ganong WF. Review of Medical Physiology, 22nd ed. McGraw-Hill, 2005.)

B. Pacemaker Cells

The action potential of pacemaker cells is different from that described for ventricular and atrial myocytes (see Figure 10–8). Fast sodium channels are absent, so that a rapid phase 0 depolarization is not observed in SA nodal and AV nodal cells. In addition, these cells are characterized by increased automaticity from a relatively rapid spontaneous phase 4 depolarization. A combination of a reduced outward flow of K+ and an inward flow of Na+ and Ca2+ via specialized channels appears to be responsible for this dynamic change in membrane

potential. Myofibrils are sparse, although present, in the specialized pacemaker cells.

CHECKPOINT

1. What are the differences in pacemaker and conduction properties in different regions of the heart, and why do these differences explain the observation that cardiac electrical impulses normally arise in the SA node?

2. Describe pressure–time analysis through the cardiac cycle. 3. Describe pressure–volume analysis through the cardiac cycle. 4. What are preload and afterload? 5. Briefly describe the molecular mechanism of electrome-chanical

coupling in cardiac myocyte contraction.

PATHOPHYSIOLOGY OF SELECTED CARDIOVASCULAR DISORDERS

ARRHYTHMIAS

At rest, the heart is normally activated at a rate of 50–100 bpm. Abnormal rhythms of the heart (arrhythmias) can be classified as either too slow (bradycardias) or too fast (tachycardias).

Bradycardia Bradycardia can arise from two basic mechanisms. First, reduced automaticity of the sinus node can result in slow heart rates or pauses. As shown in Figure 10– 10, if sinus node pacemaker activity ceases, the heart will usually be activated at a slower rate by other cardiac tissues with pacemaker activity. Reduced sinus node automaticity can occur during periods of increased vagal tone (sleep, carotid sinus massage, “common faint”), with increasing age, and secondary to drugs (beta blockers, calcium channel blockers).

FIGURE 10–10 Rhythm strip showing bradycardia resulting from sinus node pause. Atrial activity (arrows) suddenly ceases, and after approximately 3 s a junctional escape beat is observed (J).

Second, slow heart rates can occur if the cardiac impulse is prevented from activating the ventricles normally because of blocked conduction (Figure 10–11). Because the fibrous valvular annulus is electrically inert, the AV node and His bundle normally form the only electrically active connection between the atria and the ventricles. Although this arrangement is useful for preventing feedback between the two chambers, it also makes the AV node and His bundle vulnerable sites for blocked conduction between the atria and ventricles. Although block can be observed in either the left or right bundle branches, bradycardia does not necessarily occur, because the ventricles can still be activated by the contralateral bundle. Atrioventricular block has been classified as first degree when there is an abnormally long atrioventricular conduction time (PR interval >0.22 s) but activation of the atria and ventricles still demonstrates a 1:1 association. In second-degree atrioventricular block, some but not all atrial impulses are conducted to the ventricles. Finally, in third-degree block, there is no association between atrial and ventricular activity. Atrioventricular block can occur with increasing age, with increased vagal input, and as a side effect of certain drugs. Atrioventricular block can sometimes also be observed in congenital disorders, such as muscular dystrophy, tuberous sclerosis, and maternal systemic lupus erythematosus, and in acquired disorders, such as sarcoidosis, gout, Lyme disease, systemic lupus erythematosus, ankylosing spondylitis, and coronary artery disease.

FIGURE 10–11 Rhythm strip demonstrating third-degree (complete) heart block with no association between atrial activity (arrows) and ventricular activity (dots).

Bradycardia resulting from either decreased automaticity or blocked conduction requires evaluation to search for reversible causes. However, implantation of a permanent pacemaker is often required.

Tachycardia Tachycardias can arise from three basic cellular mechanisms (Figure 10–12). First, increased automaticity resulting from more rapid phase 4 depolarization can cause rapid heart rate. Second, if repolarization is delayed (longer plateau period), spontaneous depolarizations (caused by reactivation of sodium or calcium channels) can sometimes occur in phase 3 or phase 4 of the action potential. These depolarizations are called triggered activity because they depend on the existence of a preceding action potential. If these depolarizations reach threshold, tachycardia can occur in certain pathologic conditions. Third, and most commonly, tachycardias can arise from a re-entrant circuit. Any condition that gives rise to parallel but electrically separate regions with different conduction velocities (such as the border zone of a myocardial infarction or an accessory atrioventricular connection) can serve as the substrate for a re-entrant circuit.

FIGURE 10–12 Tachyarrhythmias can arise from three different mechanisms. First, increased automaticity from more rapid phase 4 depolarization can cause arrhythmias. Second, in certain conditions, spontaneous depolarizations during phase 3 (early after-depolarizations [EAD]) or phase 4 (delayed after- depolarizations [DAD]) can repetitively reach threshold and cause tachycardia. This appears to be the mechanism of the polymorphic ventricular tachycardia (torsades de pointes) observed in some patients taking procainamide or quinidine and the arrhythmias associated with digoxin toxicity. Third, the most common mechanism for tachyarrhythmia is re-entry. In re-entry, two parallel pathways with different conduction properties exist (perhaps at the border zone of a myocardial infarction or a region of myocardial ischemia). The electrical impulse normally travels down the fast pathway and the slow pathway (shaded region), but at the point where the two pathways converge, the impulse traveling down the slow pathway is blocked since the tissue is refractory from the recent depolarization via the fast pathway (a). However, when a premature beat reaches the circuit, block can occur in the fast pathway, and the impulse will travel down the slow pathway (shaded region) (b). After traveling through the slow pathway, the impulse can then enter the fast pathway in retrograde fashion (which, because of the delay, has recovered excitability) and then re-enter the slow pathway to start a continuous loop of activation, or re-entrant circuit (c).

The best studied example of re-entrant tachyarrhythmias is Wolff–Parkinson– White syndrome (Figure 10–13). As mentioned, the AV node normally forms the only electrical connection between the atria and the ventricles. Perhaps because of incomplete formation of the annulus, an accessory atrioventricular connection

is found in approximately 1 in 1000 persons. This accessory pathway is usually composed of normal atrial or ventricular tissue. Because part of the ventricle is “pre-excited” over the accessory pathway rather than via the AV node, the surface ECG shows a short PR interval and a relatively wide QRS with a slurred upstroke, termed a delta wave. Because the atria and ventricles are linked by two parallel connections, re-entrant tachycardias are readily initiated. For example, a premature atrial contraction could be blocked in the accessory pathway but still conduct to the ventricles via the AV node. If enough time has elapsed so that the accessory pathway has recovered excitability, the cardiac impulse can travel in retrograde fashion to the atria over the accessory pathway and initiate a re-entrant tachycardia.

FIGURE 10–13 Re-entrant tachyarrhythmia resulting from Wolff–Parkinson–White syndrome. A: The first two beats demonstrate sinus rhythm with pre-excitation of the ventricles over an accessory pathway. The large arrows show the delta wave. An atrial premature contraction (APC) blocks in the accessory pathway, which leads to normalization of the QRS, and the atria are activated in retrograde fashion via the accessory pathway (small arrows), and supraventricular tachycardia ensues. B: The left panel schematically depicts the first two beats of the rhythm strip. The QRS is wide owing to activation of the ventricles over both the AV node and the accessory pathway. The middle panel depicts the atrial premature contraction, which is blocked in the accessory pathway but conducts over the AV node. In the right panel, the atria are activated in retrograde fashion over the accessory pathway, and a re-entrant circuit is initiated.

The best example of tachycardias from triggered activity is long QT syndrome. More than 40 years ago, investigators described several clusters of patients with a congenital syndrome associated with a long QT interval and ventricular arrhythmias. Data have shown that the long QT interval can be a result of several specific ion channel defects. For example, a reduction in potassium channel function leads to a prolonged plateau period (Figure 10–14). The prolonged plateau phase in ventricular tissue leads to a prolonged QT interval. These patients are prone to triggered activity because of the reactivation of sodium and calcium channels (early after depolarizations). Triggered activity in the ventricles can lead to life-threatening ventricular arrhythmias.

FIGURE 10–14 In certain patients with long QT syndrome, potassium channel function is reduced (diagonal arrows), which leads to prolongation of the action potential of ventricular myocytes and prolongation of the QT interval. In some cases, reactivation of sodium and calcium channels can lead to triggered activity that can initiate life-threatening ventricular arrhythmias.

Regardless of the mechanism, the approach to the immediate clinical management of tachycardias depends on whether the QRS complex is narrow or wide. If the QRS complex is narrow, ventricular depolarization must be occurring normally over the specialized conduction tissues of the heart, and the arrhythmia must be originating at or above the AV node (supraventricular)

(Figure 10–15).

FIGURE 10–15 In supraventricular tachycardia, the QRS is narrow because the ventricles are depolarized over the normal specialized conduction tissues (light blue region). Five possible arrhythmias are commonly encountered. First, in atrial fibrillation, multiple microreentrant circuits can lead to chaotic activation of the atrium. Because impulses are reaching the AV node at irregular intervals, ventricular depolarization is irregular. Second, in atrial flutter, a macroreentrant circuit, traveling up the interatrial septum and down the lateral walls, can activate the atria in a regular fashion at approximately 300 bpm. The AV node can conduct only every other or every third beat, so that the ventricles are depolarized at 150 or

100 bpm. In AV nodal re-entrant tachycardia, slow and fast pathways exist in the region of the AV node and a microreentrant circuit can be formed. Fourth, in atrioventricular re-entry, an abnormal connection between the atrium and ventricle exists so that a macroreentrant circuit can be formed with the AV node forming the slow pathway, and the abnormal atrioventricular connection forming the fast pathway. Finally, in atrial tachycardia, an abnormal focus of atrial activity as a result of either re-entry, triggered activity, or abnormal automaticity can activate the atria in a regular fashion.

A wide QRS complex suggests that ventricular activation is not occurring normally over the specialized conduction tissues of the heart. The tachycardia is arising from ventricular tissue or is a supraventricular tachycardia with aberrant conduction over the His–Purkinje system or an accessory pathway. Criteria have been developed to distinguish between ventricular and supraventricular tachycardia with aberrance.

HEART FAILURE

Inadequate pump function of the heart, which leads to congestion resulting from fluid in the lungs and peripheral tissues, is a common end result of many cardiac disease processes. Heart failure (HF) is present in approximately 3 million people in the United States; more than 400,000 new cases are reported annually. The clinical presentation is highly variable; for an individual patient, symptoms and signs depend on how quickly heart failure develops and whether it involves the left, right, or both ventricles.

1. Left Ventricular Failure Clinical Presentation Patients with left ventricular failure most commonly present with a sensation of breathlessness (dyspnea), particularly when lying down (orthopnea) or at night (paroxysmal nocturnal dyspnea). In addition, the patient may complain of blood- tinged sputum (hemoptysis) and occasionally chest pain. Fatigue, nocturia, and confusion can also be caused by heart failure.

On physical examination, the patient usually has elevated respiratory and heart rates. The skin may be pale, cold, and sweaty. In severe heart failure, palpation of the peripheral pulse may reveal alternating strong and weak beats (pulsus alternans). Auscultation of the lungs reveals abnormal sounds, called rales, that have been described as “crackling leaves.” In addition, the bases of the lung fields may be dull to percussion. On cardiac examination, the apical impulse is often displaced laterally and sustained. Third and fourth heart sounds

can be heard on auscultation of the heart. Because many patients with left ventricular failure also have accompanying failure of the right ventricle, signs of right ventricular failure may also be present (see next section).

Etiology

Heart failure is a pathophysiologic complex associated with dysfunction of the heart and is a common end point for many diseases of the cardiovascular system. There are many possible causes (Table 10–1), and the specific reason for heart failure in a given patient must always be sought. In general, heart failure can be caused by (1) inappropriate workloads placed on the heart, such as volume overload or pressure overload; (2) restricted filling of the heart; (3) myocyte loss; or (4) decreased myocyte contractility. Any one of these causes can initiate an evolving sequence of events that are described next. Each of these four causes can have several possible underlying mechanisms. For example, in developed countries, the most common cause of myocyte loss is cell death owing to an obstructed artery (see the discussions of atherosclerosis and myocardial infarction later in the chapter). However, myocyte loss can also arise from genetic disorders (often of the intracellular proteins responsible for myocyte architecture; eg, dystrophin) or as an inflammatory response (eg, after a viral infection or other insult).

TABLE 10–1 Causes of left ventricular failure.

Pathophysiology

The pathophysiology of heart failure is complex and must be understood at multiple levels. Traditionally, research has focused on the hemodynamic changes of the failing heart, considering the heart as an isolated organ. However, studies of the failing heart have emphasized the importance of understanding changes at the cellular level and the neuro-hormonal interactions between the heart and other organs of the body (Table 10–2).

TABLE 10–2 Pathophysiologic changes associated with heart failure.

A. Hemodynamic Changes— From a hemodynamic standpoint, heart failure can arise from worsening systolic or diastolic function or, more frequently, a combination of both. In systolic dysfunction, the isovolumic systolic pressure curve of the pressure–volume relationship is shifted downward (Figure 10–16A). This reduces the stroke volume of the heart with a concomitant decrease in cardiac output. To maintain cardiac output, the heart can respond with three compensatory mechanisms: First, increased return of blood to the heart (preload) can lead to increased sarcomere contraction (Frank–Starling relationship). In the pressure–volume relationship, the heart operates at a′ instead of a, and stroke volume increases, but at the cost of increased end-diastolic pressure (Figure 10– 16D). Second, an increase in catecholamine release can increase cardiac output both by increasing the heart rate and shifting the systolic isovolumetric curve to the left (Figure 10–16C). Finally, the cardiac muscle can hypertrophy and ventricular volume can increase, which shifts the diastolic curve to the right (Figure 10–16B). Although each of these compensatory mechanisms can temporarily maintain cardiac output, each is limited in its ability to do so, and if the underlying reason for systolic dysfunction remains untreated, the heart ultimately fails.

FIGURE 10–16 A: Systolic dysfunction is represented by a shift of the isovolumic pressure–volume curve to the right (dashed line), thus decreasing stroke volume. The ventricle can compensate by (B) shifting the diastolic pressure–volume relationship rightward (dashed line) by increasing left ventricular volume or elasticity, (C) increasing contractile state (dashed line) by activating circulating catecholamines, and (D) increasing filling or preload (a to a′).

In diastolic dysfunction, the position of the systolic isovolumic curve remains unchanged (myocyte contractility is preserved). However, the diastolic pressure–volume curve is shifted to the left, with an accompanying increase in

left ventricular end-diastolic pressure and symptoms of heart failure (Figure 10– 17). Diastolic dysfunction can be present in any disease that causes decreased relaxation, decreased elastic recoil, or increased ventricle stiffness. Hypertension often leads to compensatory increases in left ventricular wall thickness that can cause diastolic dysfunction by changing all three parameters. Lack of sufficient blood to the myocytes (ischemia) can also cause diastolic dysfunction by decreasing relaxation. If ischemia is severe, as in myocardial infarction, irreversible damage to the myocytes can occur, with replacement of contractile cells by fibrosis, which will lead to systolic dysfunction. In most patients, a combination of systolic and diastolic dysfunction is responsible for the symptoms of heart failure.

FIGURE 10–17 In diastolic dysfunction, the diastolic pressure–volume relation is shifted upward and to the left (dashed line), which leads to an elevated left ventricular end-diastolic pressure (a′) and reduced stroke volume.

B. Neuro-hormonal Changes—After an injury to the heart (see Table 10–1), an increased secretion of endogenous neuro-hormones and cytokines is observed. Initially, an increase in the activity of the adrenergic and renin–angiotensin

systems provides a compensatory response that maintains perfusion of vital organs. However, over time these changes can lead to a progressive deterioration of cardiac function.

Increased sympathetic activity occurs early in the development of heart failure. Elevated plasma norepinephrine levels cause increased cardiac contractility and an increased heart rate that initially help maintain cardiac output. However, continued increases lead to increased preload (as a result of venous vasoconstriction) and afterload (from arterial vasoconstriction), which can worsen heart failure. In addition, sympathetic hyperactivity causes deleterious cellular changes, discussed in the next section.

Reduced renal blood pressure stimulates the release of renin and increases the production of angiotensin II. Both angiotensin II and sympathetic activation cause efferent glomerular arteriolar vasoconstriction, which helps maintain the glomerular filtration rate despite a reduced cardiac output. Angiotensin II stimulates aldosterone synthesis, which leads to sodium resorption and potassium excretion by the kidneys. However, a vicious circle is initiated as the continued hyperactivity of the renin–angiotensin system leads to severe vasoconstriction, increased afterload, and a further reduction in cardiac output and the glomerular filtration rate.

Heart failure is associated with an increase in the release of vasopressin from the posterior pituitary gland. Vasopressin is another powerful vasoconstrictor that also promotes reabsorption of water in the renal tubules.

Heart failure is associated with the release of cytokines and other circulating peptides. Cytokines are a heterogeneous family of proteins that are secreted by macrophages, lymphocytes, monocytes, and endothelial cells in response to injury. The interleukins (ILs) and tumor necrosis factor (TNF) are the two major groups of cytokines that may have an important pathophysiologic role in heart failure. Upregulation of the gene responsible for TNF with an accompanying increase in circulating TNF plasma levels has been found in patients with heart failure. TNF appears to have an important role in the cycle of myocyte hypertrophy and cell death (apoptosis), described in the next section. Preliminary in vitro data suggest that IL-1 may accelerate myocyte hypertrophy. Another peptide important for mediating some of the pathophysiologic effects observed in heart failure is the potent vasoconstrictor endothelin, which is released from endothelial cells. Preliminary data have suggested that excessive endothelin release may be responsible for hypertension in the pulmonary arteries observed in patients with left ventricular heart failure. Endothelin is also associated with myocyte growth and collagen deposition in the interstitial

matrix.

C. Cellular Changes—Pathophysiologic changes at the cellular level are very complex and include changes in Ca2+ handling, adrenergic receptors, the contractile apparatus, and myocyte structure.

In heart failure, both delivery of Ca2+ to the contractile apparatus and reuptake of Ca2+ by the sarcoplasmic reticulum are slowed. Decreased levels of messenger ribonucleic acid (mRNA) for the specialized Ca2+ release channels have been reported by some investigators. Similarly, myocytes from failing hearts have reduced levels of mRNA for the two sarcoplasmic reticulum proteins, phospholamban and Ca2+-ATPase.

Two major classes of adrenergic receptor are found in the human heart. Alpha1-adrenergic receptors are important for inducing myocardial hypertrophy; levels of α1 receptors are slightly increased in heart failure. Heart failure is associated with significant β-adrenergic receptor desensitization as a result of chronic sympathetic activation. This effect is mediated by downregulation of β1- adrenergic receptors, downstream uncoupling of the signal transduction pathway, and upregulation of inhibitory G proteins. All these changes lead to a further reduction in myocyte contractility.

Cardiac myocytes cannot proliferate once they have matured to their adult form. However, there is a constant turnover of the contractile proteins that make up the sarcomere. In response to the hemodynamic stresses associated with heart failure, angiotensin II, TNF, norepinephrine, and other molecules induce protein synthesis via intranuclear mediators of gene activity such as c-fos, c-jun, and c- myc. This causes myocyte hypertrophy with an increase in sarcomere numbers and a re-expression of fetal and neonatal forms of myosin and troponin. Activation of this primitive program results in the development of large myocytes that do not contract normally and have decreased ATPase activity.

The heart enlarges in response to continued hemodynamic stress. Changes in myocardial size and shape associated with heart failure are collectively referred to as left ventricular remodeling. Several tissue changes appear to mediate this process. First, heart failure is associated with myocyte loss via a process called apoptosis (programmed cell death). Unlike the process of necrosis, apoptotic cells initially demonstrate decreased cell volume without disruption of the cell membrane. However, as the apoptotic process continues, the myocyte ultimately dies, and “holes” are left in the myocardium. Loss of myocytes places increased stress on the remaining myocytes. The process of apoptosis is accelerated by the proliferative signals that stimulate myocyte hypertrophy such as TNF. Although

apoptosis is a normal process essential in organs made up of proliferating cells, in the heart apoptosis initiates a vicious circle whereby cell death causes increased stress that leads to hypertrophy and the further acceleration of apoptosis.

A second tissue change observed in heart failure is an increased amount of fibrous tissue in the interstitial spaces of the heart. Collagen deposition results from fibroblast activation and myocyte death. Endothelin release leads to interstitial collagen deposition. The increase in connective tissue increases chamber stiffness and shifts the diastolic pressure–volume curve to the left.

Finally, heart failure is associated with gradual ventricle dilation. Myocyte “slippage” as a result of the activation of collagenases that disrupt the collagen network may be responsible for this process.

Clinical Manifestations

A. Symptoms 1. Shortness of breath, orthopnea, paroxysmal nocturnal dyspnea

—Although many details of the physiologic mechanisms for the sensation of breathlessness are unclear, the inciting event is probably increasing pulmonary capillary pressure as a consequence of elevated left ventricular and atrial pressures. The rise in pulmonary capillary pressure relative to plasma oncotic pressure causes fluid to move into the interstitial spaces of the lung (pulmonary edema), which can be seen on chest x-ray film (Figure 10–18). Interstitial edema probably stimulates juxtacapillary J receptors, which in turn causes reflex shallow, rapid breathing. Replacement of air in the lungs by blood or interstitial fluid can cause a reduction of vital capacity, restrictive physiology, and air trapping as a result of closure of small airways. The work of breathing increases as the patient tries to distend stiff lungs, which can lead to respiratory muscle fatigue and the sensation of dyspnea. Alterations in the distribution of ventilation and perfusion result in relative ventilation–perfusion mismatch, with consequent widening of the alveolar–arterial O2 gradient, hypoxemia, and increased dead space. Edema of the bronchial walls can lead to small airway obstruction and produce wheezing (“cardiac asthma”). Shortness of breath occurs in the recumbent position (orthopnea) because of reduced blood pooling in the extremities and abdomen, and, because the patient is operating on the steep portion of the diastolic pressure–volume curve, any increase in blood return leads to marked elevations in ventricular pressures.

Patients usually learn to minimize orthopnea by sleeping with the upper body propped up by two or more pillows. Sudden onset of severe respiratory distress at night—paroxysmal nocturnal dyspnea—probably occurs because of the reduced adrenergic support of ventricular function that occurs with sleep, the increase in blood return as described previously, and normal nocturnal depression of the respiratory center.

FIGURE 10–18 Posteroanterior chest x-ray film in a man with acute pulmonary edema resulting from left ventricular failure. Note the bat’s-wing density, cardiac enlargement, increased size of upper lobe vessels, and pulmonary venous congestion. (Reproduced, with permission, from Cheitlin MD et al, eds. Clinical Cardiology, 6th ed. Originally published by Appleton & Lange. Copyright © 1993 by The McGraw-Hill Companies, Inc.)

2. Fatigue, confusion—Fatigue probably arises because of the inability of the heart to supply appropriate amounts of blood to skeletal muscles. Confusion may arise in advanced heart failure because of under-perfusion of the cerebrum.

3. Nocturia—Heart failure can lead to reduced renal perfusion during the day while the patient is upright, which normalizes only at night while the patient is supine, with consequent diuresis.

4. Chest pain—If the cause of failure is coronary artery disease, patients may have chest pain secondary to ischemia (angina pectoris). In addition, even without ischemia, acute heart failure can cause chest pain by unknown mechanisms.

B. Physical Examination

1. Rales, pleural effusion—Increased fluid in the alveolar spaces from the mechanisms described previously can be heard as rales. Increased capillary pressures can also cause fluid accumulation in the pleural spaces.

2. Displaced and sustained apical impulse—In most people, contraction of the heart can be appreciated by careful palpation of the chest wall (apical impulse). The normal apical impulse is felt in the midclavicular line in the fourth or fifth intercostal space and is palpable only during the first part of systole. When the apical impulse can be felt during the latter part of systole, it is sustained. Sustained impulses suggest that increases in left ventricular volume or mass are present. In addition, when left ventricular volume is increased as a compensatory mechanism of heart failure, the apical impulse is displaced laterally.

3. Third heart sound (S3)—The third heart sound is a low-pitched sound heard during rapid filling of the ventricle in early diastole (Figure 10–19A). The exact mechanism responsible for the genesis of the third heart sound is unknown, but the sound appears to result either from the sudden deceleration of blood as the elastic limits of the ventricular chamber are reached or from the actual impact of the ventricular wall against the chest wall. Although a third heart sound is normal in children and young adults, it is rarely heard in healthy adults older than 40 years. In these individuals, the presence of a third heart sound is almost pathognomonic of ventricular failure. The increased end-systolic volumes and pressures characteristic of the failing heart are probably responsible for the prominent third heart sound. When it arises because of left ventricular failure, the third heart sound is usually heard best at the apex. It can be present in patients with either diastolic or systolic dysfunction.

FIGURE 10–19 A: Phonocardiogram showing typical third heart sound (S3). It follows the second sound (S2) by 0.16 s. B: Phonocardiogram showing a fourth heart sound (S4) and its relation to first sound (S1).

4. Fourth heart sound (S4)—Normally, sounds arising from atrial contraction are not heard. However, if there is increased ventricle stiffness, a low-pitched sound at end-diastole that occurs concomitantly with atrial contraction can sometimes be heard (Figure 10–19B). As with the third heart sound, the exact mechanism for the genesis of the fourth heart sound is unknown. However, it probably arises from the sudden deceleration of blood in a noncompliant ventricle or from the sudden impact of a stiff ventricle against the chest wall. It is best heard laterally over the apex at the point of maximal impulse, particularly when the patient is partially rolled over onto the left side. The fourth heart sound is commonly heard in any patient with heart failure resulting from diastolic dysfunction.

5. Pale, cold, and sweaty skin—Patients with severe heart failure often have peripheral vasoconstriction, which maintains blood flow to the central organs and head. In some cases, the skin appears dusky because of reduced oxygen content in venous blood as a result of increased oxygen extraction from peripheral tissues that are receiving low blood flow. Sweating occurs because body heat cannot be dissipated through the constricted vascular bed of the skin.

2. Right Ventricular Failure Clinical Presentation Symptoms of right ventricular failure include shortness of breath, pedal edema, and abdominal pain.

The findings on physical examination are similar to those of left ventricular failure but in different positions, because the right ventricle is anatomically anterior and to the right of the left ventricle (see Figure 10–1). Patients with right ventricular failure may have a third heart sound heard best at the sternal border or a sustained systolic heave of the sternum. Inspection of the neck reveals elevated jugular venous pressures. Because the most common cause of right ventricular failure is left ventricular failure, signs of left ventricular failure are often also present.

Etiology

Right ventricular failure can have several causes. As mentioned, left ventricular failure can cause right ventricular failure because of the increased afterload placed on the right ventricle. Increased afterload can also be present from abnormalities of the pulmonary arteries or capillaries. For example, increased flow from a congenital shunt can cause reactive pulmonary artery constriction, increased right ventricular afterload, and, ultimately, right ventricular failure. Right ventricular failure can occur as a sequela of pulmonary disease (cor pulmonale) because of destruction of the pulmonary capillary bed or hypoxia- induced vasoconstriction of the pulmonary arterioles. Right ventricular failure can also be caused by right ventricular ischemia, usually in the setting of an inferior wall myocardial infarction (Table 10–3).

TABLE 10–3 Causes of right ventricular failure.

Pathophysiology

The pathophysiology of right ventricular failure is similar to that described for the left ventricle. Both systolic and diastolic abnormalities of the right ventricle can be present and usually occur because of inappropriate loads placed on the ventricle or primary loss of myocyte contractility.

Patients with isolated right ventricular failure (pulmonary hypertension, cor pulmonale) can have a mechanical reason for left ventricular failure. The interventricular septum is usually bowed toward the thinner-walled and lower- pressure right ventricle. When right ventricular pressure increases relative to the left, the interventricular septum can bow to the left and prevent efficient filling of the left ventricle, which may lead to pulmonary congestion. Rarely, the bowing can be so severe that left ventricular outflow is partially obstructed.

Clinical Manifestations

A. Shortness of Breath—In left ventricular failure, patients may be short of breath because of pulmonary edema, as discussed previously. In patients with right-sided failure resulting from pulmonary disease, shortness of breath may be a manifestation of the underlying disease (eg, pulmonary embolus, chronic obstructive pulmonary disease). In some patients with right ventricular failure, congestion of the hepatic veins with formation of ascites can impinge on normal diaphragmatic function and contribute to the sensation of dyspnea. In addition,

reduced right-sided cardiac output alone can cause acidosis, hypoxia, and air hunger. If the cause of right-sided failure is a left-sided defect such as mitral stenosis, the onset of right heart failure can sometimes lessen the symptoms of pulmonary edema because of the decreased load placed on the left ventricle.

B. Elevated Jugular Venous Pressure—The position of venous pulsations of the internal jugular vein can be observed during examination of the neck (Figure 10– 20A). The vertical distance above the heart at which venous pulsations are observed is an estimate of the right atrial or central venous pressure. Because the position of the right atrium cannot be precisely determined, the height of the jugular venous pulsation is measured relative to the angle of Louis on the sternum. Right atrial pressure can then be approximated by adding 5 cm to the height of the venous column (because the right atrium is approximately 5 cm inferior to the angle). Jugular venous pulsations are usually observed less than 7 cm above the right atrium. Elevated atrial pressures are present any time this distance is greater than 10 cm. Elevated atrial pressures indicate that the preload of the ventricle is adequate but ventricular function is decreased and fluid is accumulating in the venous system. Other causes of elevated jugular pressures besides heart failure include pericardial tamponade, constrictive pericarditis, and massive pulmonary embolism.

FIGURE 10–20 A: Examination of jugular venous pulse and estimation of venous pressure. (RA, right atrium; RV, right ventricle.) B: Jugular venous pressure waveforms in relation to the electrocardiogram (P wave, QRS, and T wave) and the first and second heart sounds (S1 and S2). The bottom of the x descent occurs coincident with the first heart sound (S1). The v wave occurs just after the apical impulse is felt at the same time the second heart sound (S2) is heard. See text for further explanation of jugular venous waveforms.

In addition to relative position, individual waveforms of the jugular venous pulse can be assessed. Three positive waves (a, c, and v) and two negative waves (x and y) can be recognized (Figure 10–20B). The a wave is caused by transmitted right atrial pressure from atrial contraction. The c wave is usually not present on bedside examination; it is thought to arise from bulging of the tricuspid valve during isovolumic contraction of the right ventricle. The x descent is thought to result from atrial relaxation and downward displacement of

the tricuspid annulus during systole. The v wave arises from continued filling of the right atrium during the latter part of systole. Once the tricuspid valve opens, blood flows into the right ventricle and the y descent begins. Evaluation of the individual waveforms will become particularly important when pericardial disease is discussed.

C. Anasarca, Ascites, Pedal Edema, Hepatojugular Reflux & Abdominal Pain —Elevated right-sided pressure leads to fluid accumulation in the systemic venous circulation. Venous congestion can be manifested by generalized edema (anasarca), ascites (collection of fluid in the peritoneal space), and dependent edema (swelling of the feet and legs). Pressing on the liver for approximately 5 seconds can lead to displacement of blood into the vena cava; when the right ventricle cannot accommodate this additional volume, an increase in jugular venous pressure (“hepatojugular reflux”) can be observed. Expansion of the liver from fluid accumulation can cause distention of the liver capsule with accompanying right upper quadrant abdominal pain.

CHECKPOINT

6. What are the clinical presentations of left ventricular heart failure (HF) and right ventricular failure?

7. What are the four general categories that account for almost all causes of HF?

8. Explain the differences between the pathophysiology of HF resulting from systolic versus diastolic dysfunction.

9. What are the major clinical manifestations and compli-cations of left- versus right-sided heart failure?

VALVULAR HEART DISEASE

Dysfunctional cardiac valves can be classified as either narrow (stenosis) or leaky (regurgitation). Although the tricuspid and pulmonary valves can become dysfunctional in patients with endocarditis, congenital lesions, or carcinoid syndrome, primary right-sided valvular abnormalities are relatively rare and are not discussed further here. In this section, the pathophysiologic mechanisms of

stenotic and regurgitant aortic and mitral valves are addressed. Figure 10–21 provides a general classification of heart murmurs. Any disease

process that creates turbulent flow in the heart or great vessels can cause a murmur. For instance, ventricular septal defect is associated with a systolic murmur because of the abnormal interventricular connection and the pressure difference between the left and right ventricles; patent ductus arteriosus is associated with a continuous murmur because of a persistent connection between the pulmonary artery and the aorta. However, valvular lesions are the principal cause of heart murmurs. Thus, an understanding of heart murmurs gives insight into the underlying pathophysiologic processes of specific valvular lesions.

FIGURE 10–21 The timing of the principal cardiac murmurs.

Heart murmurs can be either systolic or diastolic. During systole, while the left ventricle is contracting, the aortic valve is open and the mitral valve is closed. Turbulent flow can occur either because of an incompetent mitral valve, leading to regurgitation of blood back into the atrium, or from a narrowed aortic valve. In diastole, the situation is reversed, with filling of the left ventricle through an open mitral valve while the aortic valve is closed. Turbulent flow occurs when there is mitral valve narrowing or aortic valve incompetence. Valve stenosis usually develops slowly over time; lesions that cause valvular regurgitation can be either chronic or acute.

1. Aortic Stenosis Clinical Presentation For all causes of aortic stenosis, there is usually a long latent period of slowly increasing obstruction before symptoms appear. In descending order of frequency, the three characteristic symptoms of aortic stenosis are chest pain (angina pectoris), syncope, and heart failure (see prior discussion). Once symptoms occur, the prognosis is poor if the obstruction is untreated, with average life expectancies of 2, 3, and 5 years for angina pectoris, syncope, and heart failure, respectively.

On physical examination, palpation of the carotid upstroke reveals a pulsation (pulsus) that is both decreased (parvus) and late (tardus) relative to the apical impulse. Palpation of the chest reveals an apical impulse that is laterally displaced and sustained. On auscultation, a midsystolic murmur is heard, loudest at the base of the heart, and often with radiation to the sternal notch and the neck. Depending on the cause of the aortic stenosis, a crisp, relatively high- pitched aortic ejection sound can be heard just after the first heart sound. Finally, a fourth heart sound (S4) is often present.

Etiology

Table 10–4 lists and describes various causes of aortic stenosis.

TABLE 10–4 Causes of aortic stenosis.

Pathophysiology

The normal aortic valve area is approximately 3.5–4.0 cm2. Critical aortic stenosis is usually present when the area is less than 0.8 cm2. At this point, the systolic gradient between the left ventricle and the aorta can exceed 150 mm Hg, and most patients are symptomatic (Figure 10–22A). The fixed outflow obstruction places a large afterload on the ventricle. The compensatory mechanisms of the heart can be understood by examining the Laplace law for a sphere, where wall stress (T) is proportionate to the product of the transmural pressure (P) and cavitary radius (r) and inversely proportionate to wall thickness (W):

FIGURE 10–22 Aortic stenosis. A: Drawing of the left heart in left anterior oblique view showing anatomic features of aortic stenosis. Note structures enlarged: left ventricle (thickened); poststenotic dilation of the aorta. B: Drawing showing auscultatory and hemodynamic features of predominant aortic stenosis. Cardinal features include left ventricular hypertrophy and systolic ejection murmur. (A, aortic valve; EC, ejection click; P, pulmonary valve; SM, systolic murmur.) (Redrawn, with permission, from Cheitlin MD

et al, eds. Clinical Cardiology, 6th ed. Originally published by Appleton & Lange. Copyright © 1993 by The McGraw-Hill Companies, Inc.) C: Pressure–volume loop in aortic stenosis. The left ventricle becomes thickened and less compliant, forcing the diastolic pressure–volume curve upward, which results in elevated left ventricular end-diastolic pressure (a′). Because the left ventricle must pump against a fixed gradient (increased afterload), b increases to b′. Finally, the hypertrophy of the ventricle results in increased inotropic force, which shifts the isovolumic pressure curve leftward.

In response to the pressure overload (increased P), left ventricular wall thickness markedly increases—while the cavitary radius remains relatively unchanged—by parallel sarcomere replication. These compensatory changes, termed “concentric hypertrophy,” reduce the increase in wall tension observed in aortic stenosis (see Aortic Regurgitation). Analysis of pressure–volume loops reveals that, to maintain stroke volume and because of decreases in ventricular compliance, left ventricular end-diastolic pressure increases significantly (Figure 10–22C). The thick ventricle leads to a prominent a wave on left atrial pressure tracings as the ventricle becomes more dependent on atrial contraction to fill the ventricle.

Clinical Manifestations

A. Symptoms 1. Angina pectoris—Angina can occur because of several mechanisms. First,

approximately half of all patients with aortic stenosis have significant concomitant coronary artery disease. Even without significant coronary artery disease, the combination of increased oxygen demands because of ventricular hypertrophy and decreased supply as a result of excessive compression of the vessels can lead to relative ischemia of the myocytes. Finally, coronary artery obstruction from calcium emboli arising from a calcified stenotic aortic valve has been reported, although it is an uncommon cause of angina.

2. Syncope—Syncope in aortic stenosis usually results from decreased cerebral perfusion from the fixed obstruction but may also occur because of transient atrial arrhythmias with loss of effective atrial contribution to ventricular filling. In addition, arrhythmias arising from ventricular tissues are more common in patients with aortic stenosis and can cause syncope.

3. Heart failure—(See Heart Failure.) The progressive increase in left ventricular end-diastolic pressure can cause elevated pulmonary venous pressure and pulmonary edema.

B. Physical Examination—Because there is a fixed obstruction to flow, the

carotid upstroke is decreased and late. Left ventricular hypertrophy causes the apical impulse to be displaced laterally and to become sustained. The increased dependence on atrial contraction is responsible for the prominent S4. Flow through the restricted orifice gives rise to a midsystolic murmur. The murmur is usually heard best at the base of the heart but often radiates to the neck and apex. It usually presents as a crescendo–decrescendo murmur, and, in contrast to mitral regurgitation, the first and second heart sounds are easily heard. As aortic valve narrowing worsens, the murmur peaks later in systole. When calcified leaflets are present, the murmur tends to have a harsher quality. An aortic ejection sound, caused by the sudden checking of the leaflets as they open, is heard only when the leaflets remain fairly mobile, as in congenitally malformed valves.

Although obstruction of blood flow from the left ventricle usually results from valvular disease, obstruction can also occur above or below the valve and can present in somewhat the same way as valvular aortic stenosis. A membranous shelf that partially obstructs flow just above the valve in the aorta can sometimes be present from birth. In this condition, the systolic murmur is usually heard best at the first intercostal space at the right sternal border. Subvalvular stenosis can occur in some patients who develop severe hypertrophy of the heart (Figure 10–23). Hypertrophic cardiomyopathy is a genetic disorder that often involves a mutation of one of the sarcomere proteins such as myosin or actin. This well-recognized clinical entity—hypertrophic cardiomyopathy— can also be manifested by a crescendo–decrescendo systolic murmur noted on physical examination. However, obstruction of the outflow tract in hypertrophic cardiomyopathy is dynamic, with greater obstruction when preload is decreased from decreased intra-ventricular volume. For this reason, having the patient stand or perform the Valsalva maneuver (expiratory effort against a closed glottis), both of which decrease venous return, causes the murmur to increase. Both maneuvers cause a decrease in murmurs caused by valvular stenosis, because less absolute blood volume flows across the stenotic aortic valve.

FIGURE 10–23 Hypertrophic cardiomyopathy (left lateral view). The cardinal features are displayed. (Redrawn, with permission, from Cheitlin MD et al, eds. Clinical Cardiology, 6th ed. Originally published by Appleton & Lange. Copyright © 1993 by The McGraw-Hill Companies, Inc.)

2. Aortic Regurgitation Clinical Presentation Aortic regurgitation can be either chronic or acute. In chronic aortic regurgitation, there is a long latent period during which the patient remains asymptomatic as the heart responds to the volume load. When the compensatory mechanisms fail, symptoms of left-sided failure become manifest. In acute aortic regurgitation, compensatory mechanisms have no time to be activated, so shortness of breath, pulmonary edema, and hypotension—often with cardiovascular collapse—occur suddenly.

Physical examination of patients with chronic aortic regurgitation reveals hyperdynamic (pounding) pulses. The apical impulse is hyperdynamic and displaced laterally. On auscultation, three murmurs may be heard: a high-pitched early diastolic murmur, a diastolic rumble called the Austin Flint murmur, and a systolic murmur. A third heart sound is often present. However, in acute aortic regurgitation, the peripheral signs are often absent, and in many cases the left ventricular impulse is normal. On auscultation, the diastolic murmur is much softer, and the Austin Flint murmur, if present, is short. The first heart sound will be soft and sometimes absent.

Etiology

Chronic and acute aortic regurgitation can result either from valvular or aortic root abnormalities (Table 10–5).

TABLE 10–5 Causes of aortic regurgitation.

Pathophysiology

Aortic regurgitation places a volume load on the left ventricle because during diastole, blood enters the ventricle both from the left atrium and from the aorta. If the regurgitation develops slowly, the heart responds to the increased diastolic pressure by fiber elongation and replication of sarcomeres in series, which leads to increased ventricular volumes. Because systolic pressure remains relatively unchanged, increased wall stress—by the Laplace law—can be compensated for by an additional increase in wall thickness. This response, “eccentric hypertrophy”—so named because the ventricular cavity enlarges laterally in the chest and becomes eccentric to its normal position—explains the different ventricular geometry observed in patients with aortic regurgitation versus those with aortic stenosis (concentric hypertrophy caused by the systolic pressure overload). Ultimately, chronic aortic regurgitation leads to huge ventricular volumes as demonstrated in the pressure–volume loops (Figure 10–24). The left ventricle operates as a low-compliance pump, handling large end-diastolic and

stroke volumes, often with little increase in end-diastolic pressure. In addition, no truly isovolumic period of relaxation or contraction exists because of the persistent flow into the ventricle from the systemic circulation. Aortic pulse pressure is widened. Diastolic pressure decreases because of regurgitant flow back into the left ventricle and increased compliance of the large central vessels (in response to increased stroke volume); elevated stroke volume leads to increased systolic pressures (Figure 10–24C).

FIGURE 10–24 Aortic insufficiency (regurgitation). A: Drawing of the left heart in left anterior oblique view showing the anatomic features of aortic insufficiency. Note structures enlarged: left ventricle, aorta. B: Drawing showing the auscultatory and hemodynamic features of predominant aortic insufficiency. Cardinal features include large hypertrophied left ventricle, large aorta, increased stroke volume, wide pulse pressure, and diastolic murmur. (A, aortic valve; DM, diastolic murmur; P, pulmonary valve; SM, systolic murmur.) (Redrawn, with permission, from Cheitlin MD et al, eds. Clinical Cardiology, 6th ed. Originally published by Appleton & Lange. Copyright © 1993 by The McGraw-Hill Companies, Inc.) C: Pressure–volume loop in chronic aortic insufficiency. A marked enlargement in left ventricular volume shifts the diastolic pressure–volume curve rightward. Hypertrophy of the ventricle shifts the

isovolumic pressure–volume curve leftward (not shown), but ultimately the ventricle dilates, contractility decreases, and the isovolumic pressure–volume curve shifts to the right. Stroke volume is enormous, although effective stroke volume may be minimally changed because much of the increase in stroke volume leaks back into the ventricle. Because the ventricle is constantly being filled from the mitral valve or the incompetent aortic valve, no isovolumic periods exist.

Clinical Manifestations

A. Shortness of Breath—Pulmonary edema can develop, particularly if the aortic regurgitation is acute and the ventricle does not have time to compensate for the sudden increase in volume. In chronic aortic regurgitation, compensatory mechanisms eventually fail and the heart begins to operate on the steeper portion of the diastolic pressure–volume curve.

B. Physical Examination 1. Hyperdynamic pulses—In chronic aortic regurgitation, a widened pulse

pressure is responsible for several characteristic peripheral signs. Palpation of the peripheral pulse reveals a sudden rise and then drop in pressure (water-hammer or Corrigan pulse). Head bobbing (DeMusset sign), rhythmic pulsation of the uvula (Müller sign), and arterial pulsation seen in the nail bed (Quincke pulse) have been described in patients with chronic aortic regurgitation.

2. Murmurs—Three heart murmurs can be heard in patients with aortic regurgitation: First, flow from the regurgitant volume back into the left ventricle can be heard as a high-pitched, blowing, early diastolic murmur usually perceived best along the left sternal border. Second, the rumbling murmur described by Austin Flint can be heard at the apex during any part of diastole. The Austin Flint murmur is thought to result from regurgitant flow from the aortic valve impinging on the anterior leaflet of the mitral valve, producing functional mitral stenosis. Finally, a crescendo– decrescendo systolic murmur, thought to arise from the increased stroke volume flowing across the aortic valve, can be heard at the left sternal border.

In acute, severe aortic regurgitation, the early diastolic murmur may be softer because of rapid diastolic equalization of ventricular and aortic pressures. The first heart sound is soft because of early mitral valve closure from aortic regurgitation and elevated ventricular pressures.

3. Third heart sound—A third heart sound can be heard because of concomitant heart failure or because of the exaggerated early diastolic filling of the left ventricle.

4. Apical impulse—The apical impulse is displaced laterally because of the increased volume of the left ventricle.

3. Mitral Stenosis Clinical Presentation The symptoms of mitral stenosis include dyspnea, fatigue, and hemoptysis. Occasionally, the patient complains of palpitations or a rapid heartbeat. Finally, the patient with mitral stenosis may present with neurologic symptoms such as transient numbness or weakness of the extremities, sudden loss of vision, or difficulty with coordination.

The characteristic murmur of mitral stenosis is a late low-pitched diastolic rumble. In addition, an opening snap may be heard in the first portion of diastole (Figure 10–25). Auscultation of the lungs may reveal rales.

FIGURE 10–25 Mitral stenosis. A: Drawing of the left heart in left anterior oblique view showing the anatomic features of mitral stenosis. Note the enlarged left atrium and small left ventricle. B: Drawing showing the auscultatory and hemodynamic features of mitral stenosis. Cardinal features include thickening and fusion of mitral valve cusps, elevated left atrial pressure, left atrial enlargement, opening snap, and diastolic murmur. (A, aortic; DM, diastolic murmur; M, mitral; OS, opening snap; P, pulmonary; PSM, presystolic murmur; T, tricuspid.) C: Pressure–volume loop in mitral stenosis. Filling of the left ventricle is restricted from a to a′, decreasing stroke volume to b′c′.

Etiology

Mitral stenosis is most commonly a sequela of rheumatic heart disease (Table 10–6). Infrequently, it may be caused by congenital lesions or calcium deposition. Atrial masses (myxomas) can cause intermittent mitral valve obstruction.

TABLE 10–6 Causes of mitral stenosis.

Pathophysiology

The mitral valve is normally bicuspid, with the anterior cusp approximately twice the area of the posterior cusp. The mitral valve area is usually 5–6 cm2; clinically relevant mitral stenosis usually occurs when the valve area decreases to less than 1 cm2. Because obstruction of flow protects the ventricle from pressure and volume loads, the left ventricular pressure–volume relationship shows relatively little abnormality other than decreased volumes. However, analysis of hemodynamic tracings shows the characteristic elevation in left atrial pressures (Figure 10–25B). For this reason, the main pathophysiologic abnormalities in mitral stenosis are elevated pulmonary venous pressure and elevated right-sided pressures (pulmonary artery, right ventricle, and right atrium). Dilation and reduced systolic function of the right ventricle are commonly observed in patients with advanced mitral stenosis.

Clinical Manifestations

A. Symptoms 1. Shortness of breath, hemoptysis, and orthopnea—All these symptoms

occur because of elevated left atrial, pulmonary venous, and pulmonary capillary pressures (the actual mechanisms are described in the section on heart failure).

2. Palpitations—Increased left atrial size predisposes patients with mitral stenosis to atrial arrhythmias. Chaotic atrial activity (ie, atrial fibrillation) is commonly observed. Because ventricular filling is particularly dependent on atrial contraction in patients with mitral stenosis, acute hemodynamic decompensation may occur when organized contraction of the atrium is lost.

3. Neurologic symptoms—Reduced outflow leads to dilation of the left atrium and stasis of blood flow. A thrombus in the left atrium is observed on echocardiography in approximately 20% of patients with mitral stenosis, and the prevalence increases with age, the presence of atrial fibrillation, the severity of stenosis, and any reduction in cardiac output. Embolic events that lead to neurologic symptoms occur in 8% of patients in sinus rhythm and in 32% of patients with chronic or paroxysmal atrial fibrillation. In addition, left atrial enlargement can sometimes impinge on the recurrent laryngeal nerve and lead to hoarseness (Ortner syndrome).

B. Physical Examination—On auscultation of the heart, the diastolic rumble occurs because of turbulent flow across the narrowed mitral valve orifice. An opening snap, analogous to the ejection click described for aortic stenosis, may be heard in early diastole. The opening snap is heard only when the patient has relatively mobile leaflets.

Rales occur because elevated pulmonary capillary pressures lead to an accumulation of intra-alveolar fluid.

4. Mitral Regurgitation Clinical Presentation The presentation of mitral regurgitation depends on how quickly valvular incompetence develops. Patients with chronic mitral regurgitation develop symptoms gradually over time. Common complaints include dyspnea, easy fatigability, and palpitations. Patients with acute mitral regurgitation present with symptoms of left heart failure: shortness of breath, orthopnea, and shock. Chest

pain may be present in patients whose mitral regurgitation is due to coronary artery disease.

On physical examination, patients have a pansystolic regurgitant murmur that is heard best at the apex and often radiates to the axilla. This murmur often obscures the first and second heart sounds. When mitral valve incompetence is severe, a third heart sound is often present. In chronic mitral regurgitation, the apical impulse is often hyperdynamic and displaced laterally.

Etiology

In the past, rheumatic heart disease accounted for most cases of mitral regurgitation. Mitral valve prolapse is now probably the most common cause, followed by coronary artery disease. The tips of the anterior and posterior mitral valve leaflets are held in place during ventricular contraction by the anterolateral and posteromedial papillary muscles. The valves are connected to the papillary muscles via thin fibrous structures called chordae tendineae. In patients with mitral valve prolapse, extra tissue present on the valvular apparatus can undergo myxomatous degeneration by the fifth or sixth decade. Mitral regurgitation follows as a result of either poor coaptation of the valve leaflets or sudden rupture of the chordae tendineae. In coronary artery disease, obstruction of the circumflex coronary artery can lead to ischemia or rupture of the papillary muscles (Table 10–7).

TABLE 10–7 Causes of mitral regurgitation.

Pathophysiology

When the mitral valve fails to close properly, regurgitation of blood into the left atrium from the ventricle occurs during systole. In chronic mitral regurgitation, the compensatory mechanism to this volume load is similar to the changes seen in aortic regurgitation. The left ventricle and atrium dilate, and to normalize wall stress in the ventricle there is also concomitant hypertrophy of the ventricular wall (see prior discussion of the Laplace law). Diastolic filling of the ventricle increases because it is now the sum of right ventricular output and the regurgitant volume from the previous beat. In acute mitral regurgitation, the sudden volume load on the atrium and ventricle is not compensated for by chamber enlargement and hypertrophy. The sudden increase in atrial volume leads to prominent atrial v waves with transmission of this elevated pressure to the pulmonary capillaries and the development of pulmonary edema (Figure 10– 26).

FIGURE 10–26 Mitral insufficiency (regurgitation). A: Drawing of the left heart in left lateral view showing the anatomic features of mitral insufficiency. Note the structures enlarged: left atrium and left ventricle. B: Drawing showing the auscultatory and hemodynamic features of mitral insufficiency. Cardinal features include systolic backflow into left atrium, left atrial enlargement, left ventricular enlargement (hypertrophy in acute lesions), prominent v wave caused by filling from both the pulmonary veins and the regurgitant jet, and holosystolic murmur. (3, third heart sound; A, aortic; P, pulmonary; SM, systolic murmur.) (Redrawn, with permission, from Cheitlin MD et al, eds. Clinical Cardiology, 6th ed. Originally published by Appleton &

Lange. Copyright © 1993 by The McGraw-Hill Companies, Inc.) C: Pressure–volume loop in mitral insufficiency. Increased ventricular volumes shift the diastolic pressure–volume curve rightward. Stroke volume is increased because the ventricle can now eject blood into the low-pressure left atrium. With chronic volume loads, the isovolumic pressure–volume curve eventually shifts to the right.

Clinical Manifestations

A. Symptoms 1. Pulmonary edema—Rapid elevation of pulmonary capillary pressure in

acute mitral regurgitation leads to the sudden onset of pulmonary edema, manifested by shortness of breath, orthopnea, and paroxysmal nocturnal dyspnea. In chronic mitral regurgitation, the symptoms develop gradually, but at some point, the compensatory mechanisms fail and pulmonary edema develops, particularly with exercise.

2. Fatigue—Fatigue can develop because of decreased forward blood flow to the peripheral tissues.

3. Palpitations—Left atrial enlargement may lead to the development of atrial fibrillation and accompanying palpitations. Patients with atrial fibrillation and mitral regurgitation have a 20% incidence of cardioembolic events.

B. Physical Examination 1. Holosystolic murmur—Regurgitant flow into the atrium produces a high-

pitched murmur heard throughout systole. The murmur begins with the first heart sound, continues to the second heart sound, and is of constant intensity throughout systole. It finally ends when left ventricular pressure drops to equal left atrial pressure during isovolumic relaxation. Unlike with the murmur of aortic stenosis, there is little variation in the intensity of the murmur as the heart rate changes. In addition, the murmur does not change in intensity with respiration. It is usually heard best at the apex and often radiates to the axilla. If anterior leaflet rupture has occurred, the mitral regurgitation murmur will sometimes radiate to the back.

2. Third heart sound—A third heart sound is heard if heart failure is present. Because of increased and rapid filling of the ventricle during diastole, it may also be heard in the absence of overt failure in patients with severe mitral regurgitation.

3. Displaced and hyperdynamic apical impulse—The compensatory increase in left ventricular volume and wall thickness in patients with chronic mitral regurgitation is manifested by a laterally displaced apical impulse. Because the ventricle now has a low-pressure chamber (the left

atrium) into which to eject blood, the apical impulse is often hyperdynamic. When mitral regurgitation develops suddenly, the apical impulse is not displaced or hyperdynamic, because the left ventricle has not had enough time for compensatory volume increases to occur.

CHECKPOINT

10. What are the clinical presentations of each of the four major categories of valvular heart disease?

11. What are the most common causes of each category of valvular heart disease?

12. What is the pathogenesis of each category of valvular heart disease? 13. What are the major clinical manifestations and compli-cations of each

category of valvular heart disease?

CORONARY ARTERY DISEASE

Clinical Presentation Chest pain is the most common symptom associated with coronary artery disease. It is usually described as dull and can often radiate down the arm or to the jaw. It does not worsen with a deep breath and can be associated with shortness of breath, diaphoresis, nausea, and vomiting. This entire symptom complex has been termed angina pectoris, or “pain in the chest”; this phrase was first used by Heberden in 1744.

Clinically, angina is classified according to the precipitant and the duration of symptoms. If the pain occurs only with exertion and has been stable over a long period of time, it is termed stable angina. If the pain occurs at rest, it is termed unstable angina. Finally, regardless of the precipitant, if the chest pain persists without interruption for prolonged periods and irreversible myocyte damage has occurred, it is termed myocardial infarction.

On physical examination, the patient with coronary artery disease may have a fourth heart sound or signs of heart failure and shock. However, more than any other cardiovascular problem, the initial diagnosis relies on patient history.

Etiology Atherosclerotic obstruction of the large epicardial vessels is by far the most common cause of coronary artery disease. Spasm of the coronary arteries from various mediators such as serotonin and histamine has been well described and is more common in Japanese individuals. Rarely, congenital abnormalities can cause coronary artery diseases (Table 10–8).

TABLE 10–8 Causes of coronary artery disease.

Pathophysiology Coronary blood flow brings oxygen to myocytes and removes waste products such as carbon dioxide, lactic acid, and hydrogen ions. The heart has a tremendously high metabolic requirement; although it accounts for only 0.3% of body weight, it is responsible for 7% of the body’s resting oxygen consumption. Cellular ischemia occurs when there is either an increased demand for oxygen relative to maximal arterial supply or an absolute reduction in oxygen supply.

Although situations of increased demand such as thyrotoxicosis and aortic stenosis can cause myocardial ischemia, most clinical cases result from decreased oxygen supply. Rarely, reduced oxygen supply can arise from decreased oxygen content in blood—such as occurs in carbon monoxide poisoning and anemia—but more commonly stems from coronary artery abnormalities (see Table 10–8), particularly atherosclerotic disease. Myocardial ischemia may arise from a combination of increased demand and decreased supply; for example, cocaine abuse increases oxygen demand (by inhibiting reuptake of norepinephrine at adrenergic nerve endings in the heart) and can reduce oxygen supply by causing vasospasm.

Atherosclerosis of large coronary arteries remains the predominant cause of angina and myocardial infarction. Raised fatty streaks, which appear as yellow spots or streaks in the vessel walls, are seen in coronary arteries in almost all members of any population by 20 years of age (see Chapter 11). They are found mainly in areas exposed to increased shear stresses, such as bending points and bifurcations, and are thought to arise from isolated macrophage foam cell migration into areas of minimal chronic intimal injury. In many people, this process progresses with additional foam cell migration, smooth muscle cell proliferation, and extracellular fat and collagen deposition (Figure 10–27). The extent and incidence of these advanced lesions vary among persons in different geographic regions and ethnic groups.

FIGURE 10–27 Mechanisms of atheroma production. A: Structure of the normal muscular artery. The adventitia, or outermost layer of the artery, consists principally of recognizable fibroblasts intermixed with smooth muscle cells loosely arranged between bundles of collagen and surrounded by proteoglycans. It is usually separated from the media by a discontinuous sheet of elastic tissue, the external elastic lamina. B: Platelet aggregates, or microthrombi, form as a result of platelet adherence to the exposed subendothelial connective tissue. Platelets that adhere to the connective tissue release granules whose constituents may gain entry into the arterial wall. Platelet factors thus interact with plasma constituents in the artery wall and may stimulate events shown in the next illustration. C: Smooth muscle cells migrate from the media into the intima through fenestrae in the internal elastic lamina and actively multiply within the intima. Endothelial cells regenerate in an attempt to re-cover the exposed intima, which thickens rapidly owing to

smooth muscle proliferation and the formation of new connective tissue. (Redrawn, with permission, from Ross R et al. The pathogenesis of atherosclerosis. [Part 1.] N Engl J Med. 1976;295:369. Copyright © 1976 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.)

The underlying pathophysiologic processes differ for each clinical presentation of coronary artery disease. In patients with stable angina, fixed narrowing of one or several coronary arteries is usually present. Because the large coronary arteries usually function as conduits and do not offer resistance to flow, the arterial lumen must be decreased by 90% to produce cellular ischemia when the patient is at rest. However, with exercise, a 50% reduction in lumen size can lead to symptoms. In patients with unstable angina, fissuring of the atherosclerotic plaque can lead to platelet accumulation and transient episodes of thrombotic occlusion, usually lasting 10–20 minutes. In addition, platelet release of vasoconstrictive factors, such as thromboxane A2 or serotonin, and endothelial dysfunction may cause vasoconstriction and contribute to decreased flow. In myocardial infarction, deep arterial injury from plaque rupture may cause the formation of a relatively fixed and persistent thrombus. Recent research has emphasized that plaque composition mediated by inflammation has an important role in clinical presentation. Loss of the extracellular matrix and cellular necrosis owing to the inflammatory response appear to be the key mediators of plaque rupture.

The heart receives its energy primarily from ATP generated by the oxidative phosphorylation of free fatty acids, although glucose and other carbohydrates can be used. Within 60 seconds after coronary artery occlusion, myocardial oxygen tension in the affected cells falls essentially to zero. Cardiac stores of high-energy phosphates are rapidly depleted, and the cells shift rapidly to anaerobic metabolism with consequent lactic acid production. A dysfunction of myocardial relaxation and contraction occurs within seconds, even before the depletion of high-energy phosphates occurs. The biochemical basis for this abnormality is not well understood. If perfusion is not restored within 40–60 minutes, an irreversible stage of injury characterized by diffuse mitochondrial swelling, damage to the cell membrane, and marked depletion of glycogen begins. The exact mechanism by which irreversible damage occurs is multifactorial, and severe ATP depletion, increased extracellular calcium concentrations, lactic acidosis, and free radicals are all likely mediators of this process.

In experimental preparations, if ischemic myocardium is perfused within 5 minutes, systolic function returns promptly, whereas diastolic abnormalities may take up to 40 minutes to normalize. With prolonged episodes of ischemia—up to 1 hour—it may take up to 1 month to restore ventricular function. When the

heart demonstrates this prolonged period of decreased function despite normal perfusion, the myocardium is said to be “stunned.” The biochemical basis for stunning is poorly understood. If reperfusion occurs later or not at all, systolic function often will not return to the affected area.

Clinical Manifestations A. Chest Pain Chest pain has traditionally been ascribed to ischemia. However, more recent evidence suggests that, in patients with coronary artery disease, 70–80% of episodes of ischemia are actually asymptomatic. When present, the chest pain is thought to be mediated by sympathetic afferent fibers that richly innervate the atrium and ventricle. From the heart, the fibers traverse the upper thoracic sympathetic ganglia and the five upper thoracic dorsal roots of the spinal cord. In the spinal cord, the impulses probably converge with impulses from other structures. This convergence is probably the mechanism for the chest wall, back, and arm pain that sometimes accompanies angina pectoris. The importance of these fibers can be demonstrated in patients who have had a heart transplant. When these patients develop atherosclerosis, they remain completely asymptomatic, without development of angina.

Evidence suggests that the actual trigger for nerve stimulation is adenosine. Adenosine infusion into the coronary arteries can produce the characteristic symptoms of angina without evidence of ischemia. In addition, blocking the adenosine receptor (P1) with aminophylline leads to reduced anginal symptoms despite similar degrees of ischemia.

Three factors probably account for the large proportion of asymptomatic episodes: dysfunction of afferent nerves, transient reduced perfusion, and differing pain thresholds among patients. Dysfunction of afferent nerves may cause silent ischemia. Patients with transplanted hearts do not sense cardiac pain despite significant atherosclerosis. Peripheral neuropathy in patients with diabetes may explain the increased episodes of silent ischemia described in this patient population. Transient reduced perfusion may also be an important mechanism for silent ischemia. Within a few seconds after cessation of perfusion, systolic and diastolic abnormalities can be observed. Angina is a relatively late event, occurring after at least 30 seconds of ischemia. Finally, differing pain thresholds among patients may explain the high prevalence of silent ischemia. The presence of angina is moderately correlated with a decreased pain tolerance. The mechanism for different pain thresholds is

unknown but may result from differences in plasma endorphins.

B. Fourth Heart Sound and Shortness of Breath

Both these findings may occur because of diastolic and systolic dysfunction of the ischemic myocardium. (See Heart Failure.)

C. Shock

The site of coronary artery occlusion determines the clinical presentation of myocardial ischemia or infarction. As a general rule, the more myocardium that is supplied by the occluded vessel, the more significant and severe are the symptoms. For example, obstruction of the left main coronary artery or the proximal left anterior descending coronary artery will usually present as severe cardiac failure, often with associated hypotension (shock). In addition, shock may be associated with coronary artery disease in several special situations. If septum necrosis occurs from left anterior descending artery occlusion, myocardial rupture with the formation of an interventricular septal defect can occur. Rupture of the anterior or lateral free walls from occlusion of the left anterior descending or circumflex coronary arteries, respectively, can lead to the formation of pericardial effusion and tamponade. Rupture of myocardial tissue usually occurs 4–7 days after the acute ischemic event, when the myocardial wall has thinned and is in the process of healing. Sudden hemodynamic decompensation during this period should arouse suspicion of these complications. Finally, circumflex artery occlusion may result in ischemia and dysfunction or overt rupture of the papillary muscles, which can produce severe mitral regurgitation and shock.

D. Bradycardia

Inferior wall myocardial infarctions usually arise from occlusion of the right coronary artery. Because the area of left ventricular tissue supplied by this artery is small, patients usually do not present with heart failure. However, the artery that provides blood supply to the AV node branches off the posterior descending artery, so that inferior wall myocardial infarctions are sometimes associated with slowed or absent conduction in the AV node. Besides ischemia, AV nodal conduction abnormalities can occur because of reflex activation of the vagus nerve, which richly innervates the AV node.

Dysfunction of the sinus node is rarely seen in coronary artery disease, because this area receives blood from both the right and the left coronary

arteries.

E. Nausea and Vomiting

Nausea and vomiting may arise from activation of the vagus nerve in the setting of an inferior wall myocardial infarction.

F. Tachycardia

Levels of catecholamines are usually raised in patients with myocardial infarction. This helps maintain stroke volume but leads to an increased heart rate.

CHECKPOINT

14. What is the clinical presentation of coronary artery dis-ease along the continuum from stable angina to unsta-ble angina to myocardial infarction?

15. What are the most common causes of coronary artery disease? 16. How do the pathophysiologies of stable angina, unsta-ble angina, and

myocardial infarction differ? 17. What are the major clinical manifestations and compli-cations of

coronary artery disease?

PERICARDIAL DISEASE

Pericardial disease may include inflammation of the pericardium (pericarditis) or abnormal amounts of fluid in the space between the visceral and parietal pericardia (pericardial effusion).

Pericarditis Clinical Presentation The patient presents with severe chest pain. Descriptions of the pain are variable, but the usual picture is of a sharp retrosternal onset with radiation to the back and worse with deep breathing or coughing. The pain is often position

dependent: worse when lying flat and improved while sitting up and leaning forward.

On physical examination, the pericardial rub is pathognomonic of pericarditis. It is a high-pitched squeaking sound, often with two or more components.

Occasionally, continual inflammation of the pericardium leads to fibrosis and the development of constrictive pericarditis (Figure 10–28). Examination of the jugular venous pulsation is critical in the patient who may have constrictive pericarditis. The jugular venous pressure is elevated, and the individual waveforms are often quite prominent. In addition, there can be an inappropriate increase in the jugular venous pulsation level with inspiration (Kussmaul sign). Hepatomegaly and ascites may be noted on physical examination. On auscultation of the heart, a high-pitched sound called a pericardial knock can be heard just after the second heart sound, often mimicking a third heart sound.

FIGURE 10–28 Magnetic resonance image of a cross-section of thorax showing pericardial thickening (arrows) in a patient with constrictive pericarditis. (Used, with permission, from Charles Higgins, MD in Cheitlin MD et al, eds. Clinical Cardiology, 6th ed. Appleton & Lange, 1993.)

Etiology

Table 10–9 lists the causes of acute pericarditis. Viruses, particularly the coxsackieviruses, are the most common cause of acute pericarditis. Viruses are also probably responsible for “idiopathic” pericarditis.

TABLE 10–9 Causes of pericarditis.

Pathophysiology

In pericarditis, microscopic examination of pericardial specimens obtained at surgery (eg, stripping or window) or autopsy shows signs of acute inflammation, with increased numbers of polymorphonuclear leukocytes, increased vascularity, and fibrin deposition. If the inflammation is of long duration, the pericardium can become fibrotic and scarred, with calcium deposition.

The heavily fibrotic pericardium can inhibit the filling of the ventricles. At this point, signs of constrictive pericarditis appear (see following discussion).

Clinical Manifestations

A. Chest Pain—Chest pain is probably a result of pericardium inflammation. Inflammation of adjacent pleura may account for the characteristic worsening of pain with deep breathing and coughing.

B. Physical Examination 1. Friction rub—The pericardial friction rub is thought to arise from friction

between the visceral and parietal pericardial surfaces. The rub is traditionally described as having three components, each associated with rapid movement of a cardiac chamber: The systolic component, which is probably related to ventricular contraction, is the most common and most easily heard. During diastole, there are two components: one during early diastole, resulting from rapid filling of the ventricle, and another quieter component that occurs in late diastole, thought to be due to atrial contraction. The diastolic components often merge so that a two- component or “to-and-fro” rub is most commonly heard.

2. Signs of constriction—In the patient with constrictive pericarditis, early diastolic filling of the ventricle occurs normally, but the filling is suddenly stopped by the nonelastic thickened pericardium. This cessation of filling can be observed on the pressure–time curve of the ventricle and is probably responsible for the diastolic knock (Figure 10–29). In addition, the rapid emptying of the atrium leads to a prominent y descent that makes the v wave more noticeable on the atrial pressure tracing (Figure 10–30). Systemic venous pressure is elevated, because flow entering the heart is limited. Usually with inspiration, the decrease in intrathoracic pressure is transmitted to the heart, and filling of the right side of the heart increases with an accompanying fall in systemic venous pressure. In patients with constrictive pericarditis, this normal response is prevented and the patient develops Kussmaul sign (Figure 10–31). Elevated systemic venous pressure can cause fluid accumulation in the liver and intraperitoneal space, leading to hepatomegaly and ascites.

FIGURE 10–29 Phonocardiogram of the typical sharp, early diastolic pericardial knock (K). (Reproduced, with permission, from Cheitlin MD et al, eds. Clinical Cardiology, 6th ed. Originally published by Appleton & Lange. Copyright © 1993 by The McGraw-Hill Companies, Inc.)

FIGURE 10–30 Jugular venous pressure waveforms in various kinds of heart disease. In right ventricular failure, mean jugular venous pressure is elevated, but the waveforms remain relatively unchanged. If right ventricular failure is accompanied by tricuspid regurgitation, the v wave may become more prominent (because the right atrium is receiving blood from both systemic venous return and the right ventricle). In constrictive pericarditis, the y descent becomes prominent because the right ventricle rapidly fills in early diastole. In contrast, in pericardial tamponade, the right ventricle fills only during early systole, so that only an x descent is observed. In both constrictive pericarditis and pericardial tamponade, mean jugular venous pressure is elevated.

FIGURE 10–31 Brachial arterial and right atrial pressures showing pulsus paradoxus in a patient with constrictive pericarditis and an increase in right atrial pressure on inspiration (Kussmaul sign). Both the systolic and diastolic atrial pressures rise with inspiration. (Redrawn, with permission, from Cheitlin MD et al, eds. Clinical Cardiology, 6th ed. Originally published by Appleton & Lange. Copyright © 1993 by The McGraw-Hill Companies, Inc.)

PERICARDIAL EFFUSION & TAMPONADE

Clinical Presentation Pericardial effusion may occur in response to any cause of pericarditis, so the patient may develop chest pain or pericardial rub, as described previously. In addition, pericardial effusion may develop slowly and may be asymptomatic. However, sudden filling of the pericardial space with fluid can have catastrophic consequences by limiting ventricular filling (pericardial tamponade). Patients with pericardial tamponade often complain of shortness of breath, but the diagnosis is most commonly made by noting the characteristic physical examination findings associated with pericardial tamponade.

These characteristic physical signs arise from the limited filling of the ventricle. The three classic signs of pericardial tamponade are called the Beck triad after the surgeon who described them in 1935: (1) hypotension, (2) elevated jugular venous pressure, and (3) muffled heart sounds. In addition, the patient may have a decrease in systemic pressure with inspiration (paradoxical pulse).

Etiology

Almost any cause of pericarditis can cause pericardial effusion.

Pathophysiology The pericardium is normally filled with a small amount of fluid (30–50 mL) with an intrapericardial pressure that is usually about the same as the intrapleural pressure. With the sudden addition of fluid, the pericardial pressure can increase, at times to the level of the right atrial and right ventricular pressures. The transmural distending pressure of the ventricle decreases and the chamber collapses, preventing appropriate filling of the heart from systemic venous return. The four chambers of the heart occupy a relatively fixed volume in the pericardial sac, and hemodynamic evaluation reveals equilibration of ventricular and pulmonary artery diastolic pressures with right atrial and left atrial pressures, all at approximately intrapericardial pressure.

Clinical Manifestations Because the clinical manifestations of pericardial effusion without tamponade are similar to those of pericarditis, they are not described here. Instead, the pathophysiologic mechanisms for the symptoms and signs of pericardial tamponade are described.

A. Shortness of Breath

Dyspnea is the most common symptom of pericardial tamponade. The pathogenesis probably relates to a reduction in cardiac output and, in some patients, the presence of pulmonary edema.

B. Elevated Jugular Venous Pressure

Jugular venous pressure is elevated (see Figure 10–30). In addition, cardiac tamponade alters the dynamics of atrial filling. Normally, atrial filling occurs first during ventricular ejection (y descent) and then later when the tricuspid valve opens (x descent). In cardiac tamponade, the atrium can fill during ventricular contraction so that the x descent can still be seen. However, when the tricuspid valve opens, further filling of the right atrium is prevented because chamber size is limited by the surrounding pericardial fluid. For this reason, the y descent is not seen in the patient with pericardial tamponade. Loss of the y descent in the setting of elevated jugular venous pressure should always arouse suspicion of pericardial tamponade.

C. Hypotension

Hypotension occurs because of reduced cardiac output.

D. Paradoxical Pulse

Arterial systolic blood pressure normally drops 10–12 mm Hg with inspiration. Marked inspiratory drop in systolic blood pressure (>20 mm Hg) is an important physical finding in the diagnosis of cardiac tamponade but can also be seen in severe pulmonary disease and, less commonly, in constrictive pericarditis (see Figure 10–31). Marked inspiratory decline in left ventricular stroke volume occurs because of decreased left ventricular end-diastolic volume. With inspiration, increased blood return augments filling of the right ventricle, which causes the interventricular septum to bow to the left and reduce left ventricular end-diastolic volume (reverse Bernheim effect). Also during inspiration, flow into the left atrium from the pulmonary veins is reduced, further reducing left ventricular preload.

E. Muffled Heart Sounds

Pericardial fluid can cause the heart sounds to become muffled or indistinct.

CHECKPOINT

18. What are the clinical presentations of each form of peri-cardial disease discussed?

19. What are the most common causes of pericarditis and pericardial effusion?

20. What are the major clinical manifestations and com-plications of pericarditis and pericardial effusion with tamponade?

CASE STUDIES

Yeong Kwok, MD

(See Chapter 25, p. 765–68 for answers)

CASE 54

A 25-year-old man presents to the hospital with light-headedness and palpitations for the past 2 hours. He had four or five previous episodes of palpitations in the past, but they had lasted only a few minutes and went away on their own. These episodes were not associated with any specific activity or diet. He denies any chest pain. On physical examination, he is noted to be tachycardic with a heart rate of 180 bpm and a blood pressure of 105/70 mm Hg. An ECG shows a narrow complex tachycardia at 180 bpm. The tachycardia terminates suddenly, and the patient’s heart rate drops to 90 bpm. A repeat ECG shows sinus rhythm with a short PR interval and a wide QRS with a slurred upstroke (delta wave). The patient is diagnosed as having Wolff–Parkinson–White syndrome.

Questions

A. What is the significance of the delta wave on this patient’s ECG? B. How are re-entrant tachycardias initiated in this condition? C. What are two other mechanisms that give rise to tachycardias?

CASE 55

A 66-year-old woman presents to the clinic with shortness of breath, leg swelling, and fatigue. She has a long history of type 2 diabetes and hypertension but until recently had been able to go for daily walks with her friends. In the past month, the walks have become more difficult owing to shortness of breath and fatigue. She also sometimes awakens in the middle of the night owing to shortness of breath and has to prop herself up on three pillows. On physical examination, she is noted to be tachycardic with a heart rate of 110 bpm and a blood pressure of 105/70 mm Hg. Her lung exam is notable for fine crackles on inspiration at both bases. Her cardiac exam is notable for the presence of a third and fourth heart sound and jugular venous distension. She has 2+ pitting edema to the knees bilaterally. An ECG shows sinus rhythm at 110 bpm with Q waves in the anterior

leads. An echocardiogram shows decreased wall motion of the anterior wall of the heart and an estimated ejection fraction of 25%. She is diagnosed with systolic heart failure, likely secondary to a silent myocardial infarction.

Questions

A. What are the four broad mechanisms that can lead to heart failure? Which of these are at work in this case?

B. What are the differences between systolic and diastolic dysfunction? C. What are the causes of this patient’s shortness of breath, awakening in

the middle of the night, and need to prop herself up on three pillows?

CASE 56

A 59-year-old man is brought to the emergency department by ambulance after experiencing a syncopal episode. He states that he was running in the park when he suddenly lost consciousness. He denies any symptoms preceding the event, and he had no deficits or symptoms upon arousing. On review of systems, he does say that he has had substernal chest pressure associated with exercise for the past several weeks. Each episode was relieved with rest. He denies shortness of breath, dyspnea on exertion, orthopnea, and paroxysmal nocturnal dyspnea. His medical history is notable for multiple episodes of pharyngitis as a child. He is otherwise well. He has no significant family history. He was born in Mexico and moved to the United States at age 10 years. He does not smoke, drink alcohol, or use illicit drugs. On examination, his blood pressure is 110/90 mm Hg, heart rate 95 bpm, respiratory rate 15/min, and oxygen saturation 98%. Neck examination reveals both pulsus parvus and pulsus tardus. Cardiac examination reveals a laterally displaced and sustained apical impulse. He has a grade 3/6 midsystolic murmur, loudest at the base of the heart, radiating to the neck, and a grade 1/6 high-pitched, blowing, early diastolic murmur along the left sternal border. An S4 is audible. Lungs are clear to auscultation. Abdominal examination is benign. He has no lower extremity edema. Aortic stenosis is suspected.

Questions

A. What are the most common causes of aortic stenosis? Which is most likely in this patient? Why?

B. How does aortic stenosis cause syncope? C. What is the pathophysiologic mechanism by which aortic stenosis

causes angina pectoris? D. How does aortic stenosis result in the physical findings described? E. Based on the way this patient presented, what is his life expectancy if

left untreated?

CASE 57

A 64-year-old man presents to the clinic with a 3-month history of worsening shortness of breath. He finds that he becomes short of breath after walking one block or one flight of stairs. He awakens at night, gasping for breath, and has to prop himself up with pillows in order to sleep. On physical examination, his blood pressure is 190/60 mm Hg and his pulses are hyperdynamic. His apical impulse is displaced to the left and downward. On physical examination, there are rales over both lower lung fields. On cardiac examination, there are three distinct murmurs: a high- pitched, early diastolic murmur loudest at the left lower sternal border, a diastolic rumble heard at the apex, and a crescendo–decrescendo systolic murmur heard at the left upper sternal border. Chest x-ray film shows cardiomegaly and pulmonary edema, and an echocardiogram shows severe aortic regurgitation with a dilated and hypertrophied left ventricle.

Questions

A. What accounts for the dilation and hypertrophy of the left ventricle in aortic regurgitation?

B. What is the pathophysiology of the wide pulse pressure (difference between the systolic and diastolic blood pressure) and the hyperdynamic pulses?

C. What explains the murmurs heard in this patient? D. What are the underlying mechanisms responsible for the patient’s

shortness of breath with exertion and at night?

CASE 58

A 45-year-old man presents with a history of shortness of breath, irregular heartbeat, and hemoptysis. He notes that over the past 2 weeks, he has become easily “winded” with minor activities. Also, he has coughed up some flecks of blood on a few occasions. He has noted a fast heartbeat and, on occasion, a pounding sensation in his chest. He gives a history of being ill for several weeks after a severe sore throat in childhood. On physical examination, his pulse rate is noted to be 120–130 bpm and his rhythm is irregularly irregular. He has distended jugular venous pulses and rales at the bases of both lung fields. On cardiac examination, there is an irregular heartbeat as well as a soft diastolic decrescendo murmur, loudest at the apex. An ECG shows atrial fibrillation as well as evidence of left atrial enlargement.

Questions

A. What is the likely diagnosis in this patient, and what are the elements in the history, physical examination, and ECG that support the diagnosis?

B. What is the main pathophysiologic mechanism in this condition, and how does it explain the irregular heartbeat, shortness of breath, and hemoptysis?

C. What neurologic complication might this patient develop?

CASE 59

A 59-year-old man presents to the emergency department with a 4-hour history of “crushing” chest pain. His cardiac examination is normal with no murmurs and normal heart sounds. An ECG reveals ST segment elevation in the lateral precordial leads, and cardiac enzymes show evidence of myocardial injury. He undergoes emergent cardiac catheterization that

shows a thrombus in the left circumflex artery. He undergoes successful angioplasty, and a stent is placed. He is monitored in the cardiac intensive care unit. He does well until the next day, when he develops sudden shortness of breath and decreasing oxygen saturations. Physical examination now reveals jugular venous distention, rales at both lung bases, and a blowing holosystolic murmur loudest at the apex, radiating into the axilla.

Questions

A. What likely accounts for this patient’s sudden decompensation? B. What is the main pathophysiologic derangement in this condition? C. What changes in the heart take place if this condition develops slowly

rather than suddenly?

CASE 60

A 55-year-old man presents to the clinic with complaints of chest pain. He states that for the past 5 months he has noted intermittent substernal chest pressure radiating to the left arm. The pain occurs primarily when exercising vigorously and is relieved with rest. He denies associated shortness of breath, nausea, vomiting, or diaphoresis. He has a medical history significant for hypertension and hyperlipidemia. He is taking atenolol for his high blood pressure and is eating a low-cholesterol diet. His family history is notable for a father who died of myocardial infarction at age 56 years. He has a 50-pack-year smoking history and is currently trying to quit. His physical examination is within normal limits with the exception of his blood pressure, which is 145/95 mm Hg, with a heart rate of 75 bpm.

Questions

A. What is the likely diagnosis? How would you classify his diagnosis clinically?

B. What are the most common causes of this disease? Which is the most likely in this patient?

C. What are this patient’s risk factors for coronary artery disease?

D. What is the hypothesized mechanism by which atherosclerotic plaques form?

E. What is the pathogenetic mechanism by which plaque formation results in the symptoms mentioned?

CASE 61

A 35-year-old man presents to the emergency department with complaints of chest pain. The pain is described as 8 on a scale ranging from 1 to 10, retrosternal, and sharp in nature. It radiates to the back, is worse with taking a deep breath, and is improved by leaning forward. On review of systems, he has noted a “flu-like illness” over the past several days, including fever, rhinorrhea, and cough. He has no medical history and is taking no medications. He denies tobacco, alcohol, or drug use. On physical examination, he appears in moderate distress from pain, with a blood pressure of 125/85 mm Hg, heart rate 105 bpm, respiratory rate 18/min, and oxygen saturation of 98% on room air. He is currently afebrile. His head- and-neck examination is notable for clear mucus in the nasal passages and a mildly erythematous oropharynx. The neck is supple, with shotty anterior cervical lymphadenopathy. The chest is clear to auscultation. Jugular veins are not distended. Cardiac examination is tachycardic with a three- component high-pitched squeaking sound. Abdominal and extremity examinations are normal.

Questions

A. What is the likely diagnosis? B. What are the most common causes of this disease, and which is most

likely in this patient? C. What is the pathophysiologic mechanism for his chest pain? D. What is the sound heard on cardiac examination? What is its cause? E. What are two possible complications of this disease? What might you

look for on physical examination to make certain these complications are not present?

CASE 62

A 65-year-old woman is hospitalized with a large anterior myocardial infarction. After 4 days in the hospital, she is doing well and plans are being made for discharge to a rehabilitation facility to help her regain her strength and recover her cardiac function. While going to the bathroom, she passes out suddenly. On examination, her blood pressure is 60/40 mm Hg, her heart rate is 120, and she has distant heart sounds. An emergent echocardiogram shows rupture of the anterior wall and pericardial tamponade.

Questions

A. What are the three classic signs of pericardial tamponade (Beck triad)? B. What is the pathophysiology of pericardial tamponade? C. What is the mechanism of paradoxica pulse?

REFERENCES

General Fuster V et al. Hurst’s The Heart, 14th ed. McGraw-Hill, 2017. Kusumoto FM. Cardiovascular Pathophysiology. Hayes Barton, 2004. Mohrman DE et al. Cardiovascular Physiology, 8th ed. McGraw-Hill, 2014.

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Prystowsky EN et al. Treatment of atrial fibrillation. JAMA. 2015 Jul 21;314(3):278–88. [PMID:

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Brown DA et al. Expert consensus document: mitochondrial function as a therapeutic target in heart failure. Nat Rev Cardiol. 2017 Apr;14(4):238–50. [PMID: 28004807]

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Dharmarajan K et al. Epidemiology, pathophysiology, and prognosis of heart failure in older adults. Heart Fail Clin. 2017 Jul;13(3):417–26. [PMID: 28602363]

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Viau DM et al. The pathophysiology of hypertensive acute heart failure. Heart. 2015 Dec;101(23):1861–7. [PMID: 26123135]

Valvular Heart Disease Abramowitz Y et al. Mitral annulus calcification. J Am Coll Cardiol. 2015 Oct 27;66(17):1934–41. [PMID: 26493666]

Argulian E et al. Misconceptions and facts about mitral regurgitation. Am J Med. 2016 Sep;129(9):919–23. [PMID: 27059381]

Asgar AW et al. Secondary mitral regurgitation in heart failure: pathophysiology, prognosis, and therapeutic considerations. J Am Coll Cardiol. 2015 Mar 31;65(12):1231–48. Erratum in: J Am Coll Cardiol. 2015 May 26;65(20):2265. [PMID: 25814231]

Carabello B. How to follow patients with mitral and aortic valve disease. Med Clin North Am. 2015 Jul;99(4):739–57. [PMID: 26042880]

Coffey S et al. The prevalence, incidence, progression, and risks of aortic valve sclerosis: a systematic review and meta-analysis. J Am Coll Cardiol. 2014 Jul 1;63(25 Pt A):2852–61. [PMID: 24814496]

Joseph J et al. Aortic stenosis: pathophysiology, diagnosis, and therapy. Am J Med. 2017 Mar;130(3):253– 63. [PMID: 27810479]

Nappi F et al. Ischemic mitral valve prolapse. J Thorac Dis. 2016 Dec;8(12):3752–61. [PMID: 28149574] Rader F et al. Left ventricular hypertrophy in valvular aortic stenosis: mechanisms and clinical implications. Am J Med. 2015 Apr;128(4):344–52. [PMID: 25460869]

Small A et al. Biomarkers of calcific aortic valve disease. Arterioscler Thromb Vasc Biol. 2017 Apr;37(4):623–32. [PMID: 28153876]

Sritharen Y et al. Pathophysiology of aortic valve stenosis: is it both fibrocalcific and sex specific? Physiology (Bethesda). 2017 May;32(3):182–96. [PMID: 28404735]

Coronary Artery Disease Boudoulas KD et al. Coronary atherosclerosis: pathophysiologic basis for diagnosis and management. Prog Cardiovasc Dis. 2016 May-Jun;58(6):676–92. [PMID: 27091673]

Brown RA et al. Current understanding of atherogenesis. Am J Med. 2017 Mar;130(3):268–82. [PMID: 27888053].

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Pericardial Disease Dad T et al. Pericarditis and pericardial effusions in end-stage renal disease. Semin Dial. 2016 Sep;29(5):366–73. [PMID: 27228946]

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CHAPTER

11

Cardiovascular Disorders: Vascular Disease Igor Mitrovic, MD

This chapter reviews the normal structure and function of the vascular component of the cardiovascular system and then considers the pathophysiology of three common conditions frequently seen by practicing physicians: atherosclerosis, hypertension, and shock.

NORMAL VASCULAR STRUCTURE & FUNCTION

ANATOMY & HISTOLOGY

The blood vessels are a closed system of conduits that carry blood from the heart to the tissues and back to the heart. All of the blood flows through the lungs, but the systemic circulation is made up of many different circuits in parallel (Figure 11–1). This permits wide variation in regional systemic blood flow without changing the total systemic flow.

FIGURE 11–1 Diagram of circulation in the adult. (GI, gastrointestinal.) (Redrawn, with permission, from Barrett KE et al, eds. Ganong’s Review of Medical Physiology, 25th ed. McGraw-Hill, 2016.)

Figure 11–2 summarizes the characteristics of the various types of blood vessels in humans. Note that as the diameter of the vessels decreases, their number in the body increases so that the total cross-sectional area increases.

FIGURE 11–2 Characteristics of systemic blood vessels. Cross-sections of the vessels are not drawn to scale because of the huge range in size from aorta and vena cava to capillaries. (Redrawn from Burton AC. Relation of structure to function of the tissues of the wall of blood vessels. Physiol Rev. 1954;34:619.)

All blood vessels are lined by a single layer of endothelial cells. Collectively, the endothelial cells constitute a remarkable organ that secretes substances that affect the diameter of the vessels and provide for their growth, their repair when injured, and the formation of new vessels that carry blood to growing tissues.

Arterial Vessels The aorta, the large arteries, and the arterioles are made up of an outer layer of connective tissue, the adventitia; a middle layer of smooth muscle, the media; and an inner layer, the intima, containing the layer of endothelial cells and some subendothelial connective tissue. The walls of the aorta and the large arteries contain abundant elastic tissue, much of it concentrated in the internal elastic lamina, a prominent band between the intima and the media, and another band, the external elastic lamina, between the media and the adventitia (Figure 11–3). The vessels are stretched by the force of cardiac ejection during systole, and the elastic tissue permits them to recoil during diastole. This maintains diastolic pressure and aids the forward motion of the blood. The walls of the arterioles contain less elastic tissue than the arteries but proportionately more smooth

muscle (see Figure 11–2). The muscle is extensively innervated by noradrenergic nerve fibers, which are constrictor in function. In some instances, there is a cholinergic innervation, which is vasodilator in function. The arteries and the arterioles offer considerable resistance to the flow of blood and are known as the resistance vessels.

FIGURE 11–3 Cross-section of a small artery. (Redrawn, with permission, from Ganong WF. Review of Medical Physiology, 22nd ed. McGraw-Hill, 2005.)

Capillaries The terminal portions of the arterioles, sometimes called metarterioles, drain into the capillaries. On the upstream side, the openings of the capillaries are surrounded by smooth muscle precapillary sphincters. There is debate about whether the metarterioles and sphincters are innervated. The capillaries themselves are made up of a single layer of endothelial cells. Outside these cells are occasional pericytes that serve numerous functions, including the mechanical support, signaling (eg, regulation of endothelial cell proliferation and migration), and regulation of protein diffusion, as well as cell extravasation across the capillary walls). Pericytes have also been shown to possess a variety of stem cell properties (Figure 11–4). The capillaries anastomose extensively, and although each capillary is only 5–9 µm in diameter, there are so many of them that the total cross-sectional area of all the capillaries is about 4500 cm2.

FIGURE 11–4 Cross-sections of capillaries. Left: Continuous type of capillary found in skeletal muscle. Right: Fenestrated type of capillary. (Adapted from Fawcett DW. Comparative observations on the fine structure of blood capillaries. In: Orbison JL et al, eds. The Peripheral Blood Vessels. Williams & Wilkins, 1962.)

Some substances cross capillary walls by vesicular transport, a process that involves plasma endocytosis, movement of the vesicles formed in this way across the endothelial cell cytoplasm, and exocytosis on the tissue side. However, relatively little material is moved in this fashion, and most fluid and solute exchange occurs at the junctions between endothelial cells. In the liver, there are large gaps between endothelial cells (Chapter 14). In endocrine tissues, the small intestine, and the kidneys, tissues in which there is bulk flow of material across capillary walls, the cytoplasm of the endothelial cells is attenuated to form gaps called fenestrations. These gaps appear to be closed by a discontinuous membrane, which permits the passage of substances up to approximately 600 nm in diameter. In skeletal muscle, cardiac muscle, and many other tissues, there are no fenestrations, but the junctions between endothelial cells permit the passage of substances up to 10 nm in diameter. Finally, in brain capillaries, there are tight junctions between the endothelial cells. These tight junctions permit very little passive transport and are a key component of the blood–brain barrier. Water and carbon dioxide enter the brain with ease, but movement of most other substances in and out of brain tissue is mainly via transport proteins in the endothelial cells.

Venules & Veins The venules are very similar to capillaries; they are about 20 µm in diameter, and their approximate total cross-sectional area is 4000 cm2. They drain into veins that have modest amounts of smooth muscle and elastic tissue in their relatively

thin walls and average 5 mm in diameter. The veins drain into the superior and inferior vena cavae, which in turn drain into the right atrium of the heart. The walls of the veins, unlike those of the arteries and arterioles, are easily distended and can expand to hold more blood without much increase in intravascular pressure. Therefore, they are known as capacitance vessels. They are innervated, and their smooth muscle can contract in response to noradrenergic stimulation, pushing blood into the heart and the arterial side of the circulation. The intima of the limb veins is folded at intervals to form the venous valves that prevent retrograde flow.

Lymphatics The smallest lymphatic vessels are made up of endothelial tubes. Fluid appears to enter them through loose junctions between the endothelial cells. They drain into larger endothelial tubes that have valves and contractile walls containing smooth muscle, so that the fluid they contain moves centrally. The central lymphatics drain into the right and left subclavian veins. Thus, the lymphatic system drains excess fluid from the tissues back into the vascular system.

CHECKPOINT

1. How does the composition of the wall of an arteriole dif-fer from that of an artery?

2. What are the modes of transport across the capillary wall? In which organ is transport greatest?

3. Why are veins termed capacitance vessels?

PHYSIOLOGY

Biophysical Considerations In any system made up of a pump and a closed system of pipes such as the heart and the blood vessels, the flow of fluid between the two ends of the system depends on the pressure difference generated by the pump and the resistance to flow in the pipes:

In the cardiovascular system, this translates into:

where CO is cardiac output, MAP is mean arterial pressure, and Pra is the pressure in the right atrium. Since Pra is normally close to 0 mm Hg, this expression has the following corollary:

Thus, mean arterial pressure increases when there is an increase in cardiac output or when the diameter of the blood vessels (principally the arterioles, the main resistance vessels) is decreased.

Flow in blood vessels is laminar (ie, an infinitely thin layer of blood next to the vessel wall does not move, the next layer moves slowly, and the next layer moves more rapidly, with the fastest flow in the center). Usually the flow is smooth, and no sound is generated. However, if flow is accelerated, it becomes turbulent when a critical velocity is reached. Constriction of a blood vessel or a heart valve causes faster flow in the constricted region because the kinetic energy of flow is increased and the potential energy is decreased (the Bernoulli principle). Therefore, critical velocity is more often reached. The turbulence causes noise. The examining physician hears this noise through the stethoscope as a bruit or murmur. The two terms are often used interchangeably, although the term “murmur” is more commonly applied to noise heard over the heart and the term “bruit” to noise heard over blood vessels. The sounds of Korotkoff heard over an artery below a blood pressure cuff (discussed later) are an example.

The main factors that determine flow in a blood vessel are the pressure difference between its two ends, the radius of the vessel, and the viscosity of the blood. This relationship can be expressed mathematically by the Poiseuille– Hagen formula:

where

F = flow PA − PB = pressure difference between the two ends of the tube η = viscosity r = radius of tube L = length of tube

Because flow is equal to pressure difference divided by resistance (R),

Note that flow varies directly and pressure inversely with the fourth power of the radius of the vessel. This is why small changes in the diameter of the arterioles, the principal resistance vessels, cause large changes in pressure. For example, when the radius of a vessel is doubled, resistance is decreased to 6% of its previous value. Conversely, a small decrease in arterial diameter produces a relatively marked increase in blood pressure. Viscosity also has an effect, but, except at very high or very low values, the effect is small. Viscosity is high in polycythemia and low in anemia.

Figure 11–5 illustrates the relationship between distending pressure and wall tension. This relationship is called the law of Laplace. It states that the wall tension (T) in a hollow viscus is equal to the product of the transmural pressure (P) and the radius (r) divided by the thickness of the wall (W):

FIGURE 11–5 Law of Laplace. In a hollow object (eg, viscus, blood vessel), the distending pressure (P) equals the wall tension (T). (Redrawn, with permission, from Barrett KE et al, eds. Ganong’s Review of Medical Physiology, 24th ed. McGraw-Hill, 2012.)

In thin-walled structures, wall thickness is negligible, but in structures such as arteries it becomes a significant factor. The transmural pressure is the pressure inside the viscus minus the pressure outside the viscus, but in the body the latter is negligible. Therefore, in a distensible hollow viscus, transmural pressure at equilibrium is equal to wall tension divided by the two principal radii of curvature of the object (r1 and r2):

The operation of this law in the lungs is discussed in Chapter 9. In a cylinder such as a blood vessel, one radius is infinite, so

Thus, the smaller the radius of a vessel, the lower the wall tension necessary to balance the distending pressure. For example, the wall tension in the aorta is about 170,000 dynes/cm, whereas in capillaries it is about 16 dynes/cm. This is why the thin-walled, delicate capillaries do not collapse. The law of Laplace also applies to the heart. When the heart is dilated, it must develop more wall tension to function. Consequently, its work increases.

With these principles and Figure 11–2 in mind, plus the fact that the major sites of vascular resistance are the arterioles, it is possible to understand the pressures in the various parts of the vascular system (Figure 11–6) and the velocity of flow in them. Systolic and diastolic pressures in the aorta and large arteries are stable, and there is a large pulse pressure. Normal pressure is about 120/80 mm Hg in healthy young adults. In the arterioles, there is a sharp drop, so that pressure at the entrances to the capillaries is about 37 mm Hg and pulse pressure has disappeared. At the ends of the capillaries, it is about 17 mm Hg and falls steadily in the venous system to about 5 mm Hg at the entrance of the vena cavae into the right atrium. Velocity falls in the arterioles, is low in the capillaries because of the large total cross-sectional area, and increases again in

the large veins.

FIGURE 11–6 Diagram of the changes in pressure and velocity as blood flows through the systemic circulation. (RR, relative resistance, which is highest in the arterioles; TA, total cross-sectional area of the vessels, which increases from 4.5 cm2 in the aorta to 4500 cm2 in the capillaries [Figure 11–2].) (Redrawn, with permission, from Barrett KE et al, eds. Ganong’s Review of Medical Physiology, 25th ed. McGraw-Hill, 2016.)

The pressures mentioned previously are, of course, those recorded with patients in the supine position.

Because of the weight of the blood, there is a pressure increase in the standing position in both arteries and veins of 0.77 mm Hg for each centimeter below the heart it is measured and a corresponding decrease of 0.77 mm Hg for each centimeter above the heart. Thus, when the mean arterial pressure at the level of the heart is 100 mm Hg, the mean arterial pressure in a large artery in the foot of a standing averaged-sized adult is about 180 mm Hg; in the head, it is about 62 mm Hg.

Measurement of Arterial Pressure

Arterial pressure can be measured directly by inserting a needle into an artery. Alternatively, it can be measured by the auscultatory method. The familiar inflatable cuff attached to a manometer is placed around the upper arm at the level of the heart and a stethoscope is placed over the brachial artery below the cuff. The cuff is inflated to well above the suspected systolic pressure and then deflated slowly. At the systolic pressure, a faint tapping sound is heard as blood first begins to pass beyond the cuff. With further lowering of the pressure, the sound becomes louder and then dull and muffled before finally disappearing. These are the sounds of Korotkoff, which are produced by turbulent flow in the brachial artery. The change from staccato to muffled sound occurs when blood first passes under the cuff continuously, even though the artery is still partially constricted. Continuous flow has a different auditory quality than interrupted flow. Finally, at the diastolic pressure, the sound disappears. Although diastolic pressure measured directly with a catheter in the brachial artery correlates best with the disappearance of sound in normal adults, in children and after exercise it correlates better with the change to a muffled sound.

Normal Arterial Pressure Normal blood pressure in the brachial artery at heart level in healthy young adults is about 120/80 mm Hg. It is affected by many factors, including emotion and anxiety, and in some individuals, blood pressure is higher when taken by a physician in the clinic than it is during normal activities at home (“white-coat hypertension”). Systolic and diastolic pressures normally fall by as much as 20 mm Hg during sleep. Therefore, normal subjects are called “dippers.” In individuals with hypertension, the fall during sleep is reduced or absent (ie, hypertensives are “nondippers”).

There is general agreement that blood pressure rises with advancing age, but there has been uncertainty about the magnitude of this rise because hypertension is a common disease whose incidence increases with advancing age. However, individuals who have systolic blood pressures <120 mm Hg at age 50–60 years and never develop clinical hypertension still have systolic pressures that rise throughout life (Figure 11–7). This rise may be the closest approximation to the rise in normal individuals. Individuals with mild hypertension that is untreated show a significantly more rapid rise in systolic pressure. In both groups, diastolic pressure also rises but then starts to fall in middle age as the stiffness of arteries increases. Consequently, pulse pressure rises with advancing age.

FIGURE 11–7 Effects of age and sex on systolic, diastolic, and pulse pressure in humans. Data are from a large group of individuals who were studied every 2 years throughout their adult lives. Group 1: individuals who had systolic blood pressures <120 mm Hg at age 50–60 years. Group 4: Individuals who had systolic blood pressure ≥160 mm Hg at age 50–60 years and had not received treatment for hypertension (ie, individuals with mild, untreated hypertension). Values for females are shown by the solid black lines, and those for males are shown by the dashed red lines. (Redrawn, with permission, from Franklin SS et al. Hemodynamic patterns of age-related changes in blood pressure: the Framingham Heart Study. Circulation. 1997;96:308.)

It is interesting that systolic and diastolic blood pressures are lower in young women than in young men until the age of 55–65 years, after which they become comparable. Because there is a positive correlation between blood pressure and the incidence of heart attacks and strokes (discussed later), the lower blood pressure before menopause in women may be one reason why, on average, women live longer than men.

Capillary Circulation In the capillaries, the velocity of blood flow is decreased because, although single-vessel diameter is small, there is a large total cross-sectional area. It is in the capillary bed that nutrients leave and wastes enter the circulation. The forces producing movement of solute and solvent across capillary walls are called Starling forces after the physiologist who first described them and analyzed their function. They are the hydrostatic pressure difference across the capillary wall (capillary pressure minus tissue pressure) and the osmotic pressure gradient across the capillary wall (capillary oncotic pressure minus tissue oncotic pressure). The hydrostatic pressure gradient is outward because tissue pressure is low, and the oncotic pressure gradient is inward because large molecules in the blood do not cross the capillary wall. Obviously, most of the net movement of substances out of a typical capillary occurs at its arteriolar end, where the net pressure gradient is outward primarily because hydrostatic pressure in the capillary (about 37 mm Hg; Figure 11–8) is greater than the oncotic pressure. As the capillary resistance and the filtration progressively cause a decrease in the hydrostatic pressure along the length of the vessel, the inwardly directed oncotic pressure gradient becomes greater than the hydrostatic pressure gradient so that, at the venular end, fluid is reabsorbed. Thus, net flow is out of the capillary at the arteriolar end and into the capillary at the venular end. Any excess solute and solvent in the tissues is picked up by the lymph vessels and moved to the venous circulation by the main lymphatic ducts. Flow in the small lymphatics is passive, but in the larger lymphatic ducts there are valves and the walls contract.

FIGURE 11–8 Schematic representation of pressure (P) gradients across the wall of a muscle capillary. The numbers at the arteriolar and venular ends of the capillary are the hydrostatic pressures in millimeters of mercury at these locations. The arrows indicate the approximate magnitude and direction of fluid movement. In this example, the pressure differential at the arteriolar end of the capillary is 11 mm Hg

([37 − 1] − 25) outward; at the opposite end, it is 9 mm Hg (25 − [17 − 1]) inward. (Redrawn, with permission, from Barrett KE et al, eds. Ganong’s Review of Medical Physiology, 25th ed. McGraw-Hill, 2016.)

REGULATION OF THE CARDIOVASCULAR SYSTEM

Given the vital nature of the cardiovascular system in maintaining blood flow to vital organs and adjusting flow so that it is increased in active tissues and decreased in inactive tissues, it is not surprising that multiple cardiovascular regulatory mechanisms have evolved. Cardiovascular adjustments are effected by altering the output of the pump (the heart), changing the diameter of the resistance vessels (chiefly the arterioles), and altering the amount of blood pooled in the capacitance vessels (the veins).

The regulation of cardiac output is discussed in Chapter 10. The caliber of the arterioles is regulated by vasodilator metabolites produced in metabolically active tissues, by the process of autoregulation, by a variety of vasoregulatory substances produced by endothelial cells, by circulating vasoactive hormones, and by a system of vasomotor nerves to the blood vessels and the heart. Discharge in the vasomotor nerves is regulated in feedback fashion by carotid sinus and aortic arch baroreceptors that monitor pressure in the arteries (the high-pressure baroreceptor system) and baroreceptors in the cardiac atria and great veins (the low-pressure baroreceptor system).

Vasodilator Metabolites Various metabolic changes occurring in active tissues produce substances that dilate vessels supplying the tissues. This helps ensure the increased blood flow necessary to support the increased tissue activity. One important vasodilator is CO2. Another is K

+, and adenosine dilates blood vessels in some tissues. In addition, the rise in temperature and the fall in pH that occur in some metabolically active tissues have a vasodilator effect.

Autoregulation Many tissues have the ability to maintain a relatively constant blood flow during changes in perfusion pressure; this process is called autoregulation. The physiologic basis of autoregulation is unsettled. One factor is the myogenic response to stretch of the smooth muscle in arterioles; as pressure inside a vessel

rises, its smooth muscle is stretched, and its response is to contract. Smooth muscle contracts in the absence of extrinsic innervation. Another factor may be the accumulation of vasodilator metabolites; when flow to a tissue is reduced, the metabolites are not washed away, and they accumulate even in the absence of increased activity.

Substances Secreted by the Endothelium The blood vessels are lined by a continuous layer of endothelial cells, and these cells play a vital role in the regulation of vascular function. They respond to flow changes (shear stress), stretch, a variety of circulating substances, and inflammatory mediators. In response to these stimuli, they secrete growth regulators and vasoactive substances. The growth factors regulate vascular development and are important in a number of diseases. The vasoactive substances produced by the endothelium generally act in a paracrine fashion to regulate local vascular tone. They include prostaglandins, such as prostacyclin, and also thromboxanes, nitric oxide, and endothelins.

A. Prostaglandins & Thromboxanes

Prostacyclin is produced by endothelial cells and thromboxane A2 by platelets from their common precursor, arachidonic acid. Thromboxane A2 causes platelet aggregation and vasoconstriction, whereas prostacyclin promotes vasodilation. The balance between the two is one of the mechanisms favoring local vasoconstriction and clot formation at sites of vascular injury while keeping the clot from extending, thereby maintaining normal flow in neighboring uninjured areas. The balance between platelet thromboxane A2 and endothelial prostacyclin can be shifted by the administration of low doses of aspirin. Thromboxane A2 and prostacyclin are both produced from arachidonic acid by the cyclooxygenase pathway. Aspirin produces an irreversible inhibition of cyclooxygenase. However, endothelial cells make more cyclooxygenase within a few hours, whereas circulating platelets do not, and new platelet cyclooxygenase appears only as new platelets enter the circulation over a period of days. Therefore, the chronic administration of small doses of aspirin reduces intravascular clotting for prolonged periods and is of value in preventing myocardial infarctions, unstable angina, transient ischemic attacks, and stroke.

B. Nitric Oxide

The production of a potent vasodilator by endothelial cells was first suspected when it was noted that removal of the endothelium from rings of arterial tissue converted the normal dilator response to acetylcholine into a constrictor response. The responsible agent was first called endothelium-derived relaxing factor, but it is now known to be nitric oxide (NO). NO is produced from arginine (Figure 11–9) in a reaction catalyzed by nitric oxide synthase (NOS). Three forms of NOS have been cloned: NOS1, found in the nervous system; NOS2, found in macrophages and related immune cells; and NOS3, found in endothelial cells. NOS1 and NOS3 are activated by agents that increase intracellular Ca2+, including the vasodilators acetylcholine and bradykinin, whereas NOS2 is activated by cytokines. The NO formed in endothelial cells diffuses to adjacent vascular smooth muscle cells, where it activates soluble guanylyl cyclase, producing cyclic guanosine monophosphate (cGMP; see Figure 11–9). The cGMP mediates the relaxation of vascular smooth muscle.

FIGURE 11–9 Synthesis of nitric oxide (NO) from arginine in endothelial cells and its action via stimulation of soluble guanylyl cyclase and generation of cyclic guanosine monophosphate (cGMP) to produce relaxation in vascular smooth muscle cells. The endothelial form of nitric oxide synthase (NOS) is activated by increased intracellular Ca2+, and an increase in Ca2+ is produced by acetylcholine (ACh), bradykinin, or shear stress acting on the cell membrane. Thiol, tetrahydrobiopterin, flavin adenine dinucleotide (FAD), and flavin mononucleotide (FMN) are requisite cofactors. (GTP, guanosine triphosphate; NAD, nicotinamide adenine dinucleotide; NADP, nicotinamide adenine dinucleotide phosphate.) (Redrawn, with permission, from Barrett KE et al, eds. Ganong’s Review of Medical Physiology, 24th ed. McGraw-Hill, 2012.)

The vasodilators that act by way of NO in vivo include not only acetylcholine and bradykinin but vasoactive intestinal polypeptide (VIP), substance P, and some other polypeptides. In addition, various substances that produce vasoconstriction in vivo would have a much greater constrictor effect if they did not simultaneously release NO. Consequently, NO is a major local regulator of blood flow. Its widespread role in regulation of the vascular system is indicated by the fact that an infusion of amino acid analogs of arginine that inhibit NOS causes blood pressure to rise. Thus, it appears that NOS is acting in a chronic fashion to keep the vascular system dilated.

NO is responsible in large part for reactive hyperemia, the vasodilation and increased blood flow that occur in tissues and organs after a transient obstruction of their blood supply is removed. It can be seen in the forearm after occlusion of the blood supply above the elbow, and it can be quantitated by measuring the increase in forearm volume by plethysmography. NO-dependent vasodilation can also be measured clinically by determining the dilator response to graded doses of acetylcholine injected intra-arterially.

Recent advances in the field of NO research have led to the identification of asymmetric-dimethylarginine (ADMA), an endogenous inhibitor of NOS enzymes. ADMA has been shown to be associated with endothelial dysfunction, cardiovascular mortality, and chronic kidney disease. The plasma ADMA level can serve as a component of a diagnostic tool for assessing cardiovascular health. Experimental evidence indicates that ADMA can be therapeutic target.

NO is present in many tissues in addition to the vascular system. Its function in some of these tissues is discussed in other chapters of this book.

C. Endothelins

Endothelial cells also produce endothelin-1 (ET-1), the most potent vasoconstrictor agent yet discovered. Three closely related endothelins have been identified in mammals: ET-1, endothelin-2 (ET-2), and endothelin-3 (ET-3). All are polypeptides related to the sarafotoxins, polypeptides found in snake venoms. They contain 21 amino acid residues and two disulfide bonds (Figure 11–10). All are cleaved from larger prohormones (big endothelins) by endothelin-converting enzymes. The most widely expressed endothelin, ET-1, is found in vascular endothelial cells, vascular smooth muscle cells, macrophages, fibroblasts, myocardiocytes, brain neurons, and pancreatic and intestinal epithelial cells, among others. Alternatively, ET-2 expression is restricted to intestinal epithelial cells and ovarian cells, whereas ET-3 expression is observed only in vascular endothelial cells and intestinal epithelial cells.

FIGURE 11–10 Structure of human endothelins and one of the snake venom sarafotoxins. The amino acid residues that differ from endothelin-1 are indicated in blue. (Redrawn, with permission, from Ganong WF. Review of Medical Physiology, 22nd ed. McGraw-Hill, 2005.)

Over the past several years, our understanding of endothelin physiology and pathophysiology (particularly related to ET-1) has increased tremendously. Two G protein–coupled receptors—A and B—that mediate endothelin effects have been identified. Endothelin A receptor has the greatest affinity for ET-1, whereas endothelin B receptor has the same affinity for all three polypeptide isoforms. Interestingly, vascular smooth muscle cells express both endothelin receptors, and their activation leads to vasoconstriction. Endothelial cells, however, express

only endothelin B receptor, which stimulates endothelial NOS, leading to NO- dependent smooth muscle relaxation. Recent animal data suggest that the activation of the endothelin B receptor in collecting ducts leads to a similar NO- dependent increase in sodium excretion. Moreover, there are indications that ET- 1 may contribute to the extracellular matrix remodeling in vascular, cardiac, and kidney disease, and endothelin receptor antagonists are emerging as important agents in the treatment of pulmonary hypertension.

Circulating Hormones That Affect Vascular Smooth Muscle Hormones in the circulation that have general effects on the vascular system include vasoconstrictors and vasodilators. The principal vasoconstrictors are norepinephrine and epinephrine (Chapter 12), vasopressin (Chapter 19), and angiotensin II (Chapter 21). The principal vasodilators are vasoactive intestinal peptide (VIP; Chapter 13), kinins, and natriuretic peptides.

A. Kinins

The kinins are two related vasodilator polypeptides called bradykinin and lysyl- bradykinin (Figure 11–11). The decapeptide lysyl-bradykinin can be converted to the nonapeptide bradykinin by aminopeptidase. Both are metabolized to inactive fragments by the carboxypeptidase kininase I or the dipeptidylcarboxypeptidase kininase II. Kininase II and angiotensin-converting enzyme are the same enzyme, so inhibition of angiotensin-converting enzyme for the treatment of hypertension or heart failure increases plasma and tissue kinins.

FIGURE 11–11 Kinins. Lysyl-bradykinin (top) can be converted to bradykinin (bottom) by aminopeptidase. The peptides are inactivated by kininase I (KI) or kininase II (KII) at the sites indicated by the short arrows. (Redrawn, with permission, from Barrett KE et al, eds. Ganong’s Review of Medical Physiology, 24th ed. McGraw-Hill, 2012.)

Kinins are formed from two kininogens: high-molecular-weight (HMW) kininogen and low-molecular-weight (LMW) kininogen. These kinin precursor

proteins are products of a single gene produced by alternative splicing. The proteases responsible for the cleavage of kininogens are the kallikreins, a family of enzymes encoded in humans by three genes situated on chromosome 19.

Lysyl-bradykinin and bradykinin are primarily tissue hormones produced, for example, by the kidneys and actively secreting glands, but small amounts are also found in the circulating blood. They act on two receptors, B1 and B2, both coupled to G proteins. Kinins increase blood flow to actively secreting glands by producing vasodilation, and, when injected systemically, they are relatively potent vasodilators.

B. Natriuretic Hormones

Atrial natriuretic peptide (ANP), a polypeptide containing 28 amino acid residues, is secreted from the atria when atrial myocytes are stretched. Brain natriuretic peptide (BNP) was originally isolated from the brains of experimental animals, but in humans it is secreted by the ventricular myocytes and is also known as β-type natriuretic peptide. CNP, a third type of natriuretic peptide, is also found in humans. These peptides cause natriuresis through at least two mechanisms: (1) direct inhibition of epithelial sodium channels (ENaC) in the renal collecting ducts; and (2) inhibition of renin release from the renal juxtaglomerular cells, which ultimately leads to decreased aldosterone production. This leads to the urinary excretion of sodium and water, a reduction of intravascular volume, and a decreased stretch of the atrial myocytes. The natriuretic peptides also antagonize the pressor effects of angiotensin II and other pressor hormones by increasing intracellular cGMP. All three have vasodilatory activity, but CNP uniquely induces a greater effect on veins than arterioles. While circulating levels of these peptides are increased in heart failure, the elevations are presumed to be a compensatory response to the detrimental changes of volume overload secondary to a decrease in cardiac output. Nevertheless, measurement of the circulating BNP level is used in the differential diagnosis and evaluation of heart failure. As all three natriuretic peptides have cardio- and reno-protective properties, their therapeutic potential in chronic heart failure treatment is currently under study.

An additional uncharacterized natriuretic hormone that acts by inhibiting Na+-K+ adenosine triphosphatase (ATPase) is present in the circulation, but it raises rather than lowers blood pressure (see sections below concerning hypertension and salt sensitivity). There is substantial evidence that this hormone is actually ouabain and that it is secreted by the adrenal glands in response to increased dietary sodium intake.

Neural Control Via the Sympathetic Vasomotor System Table 11–1 summarizes the factors affecting the caliber of the arterioles in the body, and hence peripheral resistance and tissue blood flow. This list includes the factors discussed previously plus a few additional polypeptides that have minor or special effects. It also includes the control of blood pressure by noradrenergic and, in some instances, cholinergic sympathetic vasomotor nerves to the arterioles. In addition to the extensive nerve supply to these resistance vessels, there is a moderate innervation of the capacitance vessels.

TABLE 11–1 Summary of factors affecting the caliber of the arterioles.

Discharge of the noradrenergic vasomotor nerves causes constriction of the arterioles innervated by the nerves, and if the discharge is general rather than local, there is an increase in blood pressure. In addition, the discharge of sympathetic noradrenergic nerves innervating the heart increases blood pressure by increasing the force and rate of cardiac contraction (inotropic and chronotropic effects), increasing stroke volume and cardiac output. Noradrenergic stimulation also inhibits the effect of vagal stimulation, which normally slows the heart and decreases cardiac output.

The main control of vasomotor discharge is feedback regulation via the baroreceptors in the high-pressure and low-pressure portions of the circulatory system (Figure 11–12). The baroreceptors are stretch-sensitive nerve endings located in the carotid sinuses and aortic arch on the arterial side and in the walls of the great veins and the cardiac atria on the venous side. The nerve fibers relay impulses in cranial nerves IX and X to the medulla oblongata, where the fibers end in the nucleus tractus solitarius (Figure 11–13). From the nucleus, second- order neurons pass to the caudal portion of the ventrolateral medulla and environs. From there, third-order inhibitory neurons pass to the rostral ventrolateral medulla, the location of the cell bodies of the neurons that control blood pressure. The axons of these neurons descend into the spinal cord and innervate the cell bodies of the blood pressure–regulating preganglionic sympathetic neurons in the intermediolateral gray column of the spinal cord. The axons of the preganglionic neurons leave the spinal cord and synapse on the postganglionic neurons in the ganglionic chain and collateral ganglia, as well as on the catecholamine-secreting cells in the adrenal medulla. The axons of the postganglionic noradrenergic neurons innervate the blood vessels and the heart. Figure 11–13 illustrates these pathways and the probable synaptic mediator at each synapse in the chain. Note in particular that the increased activity in the baroreceptor afferents produced by increases in blood pressure inhibits sympathetic vasomotor outflow, whereas a decreased baroreceptor afferent discharge stimulates sympathetic vasomotor outflow. This is brought about by the inhibitory γ-aminobutyric acid–secreting neuron link between the caudal portion of the ventrolateral medulla and the rostral ventrolateral medulla. In addition, an increased baroreceptor discharge stimulates afferents from the nucleus tractus solitarius to the dorsal motor nucleus of the vagus and the nucleus ambiguus. This increases the vagal discharge to the heart, slowing the cardiac rate and decreasing cardiac output.

FIGURE 11–12 Feedback regulation of systemic blood pressure by baroreceptors. (Redrawn, with permission, from Barrett KE et al, eds. Ganong’s Review of Medical Physiology, 25th ed. McGraw-Hill, 2016.)

FIGURE 11–13 Basic pathways involved in the medullary control of blood pressure. The vagal efferent pathways to the heart are not shown. The probable neurotransmitters in the pathways are indicated in parentheses. (ACh, acetylcholine; CVLM, IVLM, and RVLM, caudal, intermediate, and rostral ventrolateral medulla, respectively; GABA, γ-aminobutyric acid; Glu, glutamate; IML, intermediolateral gray column; IX, glossopharyngeal nerve; NE, norepinephrine; NTS, nucleus tractus solitarius; X, vagus nerve.) (Reproduced, with permission, from Reis DJ et al. Role of adrenaline neurons of the ventrolateral medulla [the C1 group] in the tonic and phasic control of arterial pressure. Clin Exp Hypertens [A]. 1984;6:221. By permission of the publisher Taylor & Francis

Ltd, http://www.informaworld.com)

There are ancillary reciprocal circuits between the nucleus tractus solitarius and the more dorsal portions of the brainstem and the hypothalamus that smooth and adjust the response of the baroreceptor pathway, but the primary neural regulation of blood pressure is mediated by the baroreceptor pathway in the medulla oblongata.

In addition to direct effects on vasomotor discharge, the baroreceptor pathway causes changes in endocrine function that augment the homeostatic value of baroreceptor responses. Adrenal medullary secretion is increased by the discharge of the sympathetic nervous system, although the contributions of circulating catecholamines to the increase in blood pressure are relatively small. An increased sympathetic discharge also increases renin secretion from the kidneys. The resultant increase in circulating angiotensin II not only acts directly on vascular smooth muscle to cause constriction but also increases aldosterone secretion, which in turn increases Na+ retention, expanding intravascular volume. Associated with an increased vasomotor discharge, there is also an increase in antidiuretic hormone (ADH; also referred to as vasopressin) secretion from the posterior pituitary. ADH expands total body water by increasing free water retention in the kidney (acting through the V2 vasopressin receptor). Although the primary role of ADH is to facilitate the lowering of osmolality, ADH also facilitates the expansion of intravascular volume. Although the volume expansion resulting from ADH is relatively small, ADH release increases with the severity of the effective circulating volume loss. Moreover, activation of the lower affinity V1 vasopressin receptor on vascular smooth muscle results in a marked increase in vascular tone.

Baroreceptor function can be tested in experimental animals and judiciously in humans by infusing the pressor drug phenylephrine at different doses and at each dose measuring the slowing of the heart rate by determining the interval between the R waves (RR interval) of the ECG. Figure 11–14 provides an example of results of this type of testing.

FIGURE 11–14 Baroreflex-mediated lowering of the heart rate during phenylephrine infusion in a human subject. Note that the values for the RR interval of the ECG, which are plotted on the vertical axis, are inversely proportionate to the heart rate. (Redrawn, with permission, from Kotrly K et al. Effects of fentanyl-diazepam- nitrous oxide anaesthesia on arterial baroreflex control of heart rate in man. Br J Anaesth. 1986;58:406.)

CHECKPOINT

4. Why do small changes in the diameter of the arterioles have relatively large effects on blood pressure?

5. Why does the velocity of blood flow decrease greatly in the capillaries and then increase in the veins?

6. What categories of factors are involved in regulating the diameter of arterioles?

7. By what mechanism does NO, produced by endothelial cells, act as a vasodilator?

8. What are the principal hormonal vasoconstrictors and vasodilators? 9. What is the role of baroreceptors in the feedback regula-tion of the high-

and low-pressure portions of the circu-latory system?

PATHOPHYSIOLOGY OF SELECTED VASCULAR DISORDERS

ATHEROSCLEROSIS

Prevalence & Significance A condition that afflicts the large and medium-sized arteries of almost every human, at least in societies in which cholesterol-rich foodstuffs are abundant and cheap, is atherosclerosis. This condition begins in childhood and, in the absence of accelerating factors, develops slowly until it is widespread in old age. However, it is accelerated by a wide variety of genetic and environmental factors (see later discussion). It is characterized by localized fibrous thickenings of the arterial wall associated with lipid-infiltrated plaques that may eventually calcify. Old plaques are also prone to ulceration and rupture, triggering the formation of thrombi that obstruct flow. Therefore, atherosclerosis leads to vascular insufficiency in the limbs, abnormalities of the renal circulation, and dilations (aneurysms) and even rupture of the aorta and other large arteries. It also leads to common severe and life-threatening diseases of the heart and brain because of the formation of intravascular clots at the site of the plaques.

In the United States and most other developed countries, it has been calculated that atherosclerosis is the underlying cause of about 50% of all deaths. Almost all patients with myocardial infarction—and most of those with stroke resulting from cerebral thrombosis—have atherosclerosis. The incidence of ischemic heart disease and strokes has been declining in the United States since 1963, but atherosclerosis is still very common. Thus, atherosclerosis underlies and is fundamentally responsible for a large portion of the clinical problems seen by physicians caring for adult patients.

Pathogenesis The initial event in atherosclerosis is the infiltration of low-density lipoproteins (LDLs) into the subendothelial region. The endothelium is subject to shear stress, the tendency to be pulled along or deformed by flowing blood. This is most marked at points where the arteries branch, which is also where the lipids accumulate to the greatest degree.

The LDLs are oxidized or altered in other ways. Thus, altered LDLs activate various components of the innate immune system, including macrophages, natural antibodies, and innate effector proteins such as C-reactive protein and complement. Altered LDLs are recognized by a family of scavenger receptors expressed on macrophages that cooperate with toll-like receptors to stimulate

inflammation and drive atherogenesis. The scavenger receptors mediate uptake of the oxidized LDL into macrophages and the formation of foam cells (Figure 11–15). The foam cells form fatty streaks. The streaks appear in the aorta in the first decade of life, in the coronary arteries in the second decade, and in the cerebral arteries in the third and fourth decades.

FIGURE 11–15 Formation of a fatty streak in an artery. After vascular injury, monocytes bind to the endothelium, then cross it to the subendothelial space, and become activated tissue microphages. The macrophages take up oxidized low-density lipoproteins (LDLs), becoming foam cells. T cells release cytokines, which also activate macrophages. In addition, the cytokines cause smooth muscle cells to proliferate. Under the influence of growth factors, the smooth muscle cells then move to the subendothelial space, where they produce collagen and take up LDL, adding to the population of foam cells. (Redrawn, with permission, from Hajjar DP et al. Atherosclerosis. Am Scientist. 1995;83:460.)

Oxidized LDLs have a number of deleterious effects, including stimulating the release of pro-inflammatory cytokines (such as macrophage migration

inhibitory factor and type I interferon) and inhibiting NO production. Vascular smooth muscle cells in the vicinity of foam cells are stimulated and move from the media to the intima, where they proliferate, lay down collagen and other matrix molecules, and contribute to the bulk of the lesion. Smooth muscle cells also take up oxidized LDL and become foam cells. Lipids accumulate both intracellularly and extracellularly.

Although an elevated serum LDL level is a major risk factor for, and oxidized LDL is a critical participant in, the development of atherosclerotic plaques, not all circulating LDLs are the same. Small, dense LDL particles are more atherogenic than large LDL particles. The small, dense LDL particles appear to be more rapidly oxidized and may more easily invade the subendothelial layer of arterial walls. Moreover, evidence suggests that the level of small, dense LDL particles also correlates with other atherogenic metabolic abnormalities.

The intercellular “soup” in the plaques contains a variety of cell-damaging substances, including ozone. In addition, the “loading” of macrophages with cholesterol can be lipotoxic to the endoplasmic reticulum, resulting in macrophage apoptosis and plaque necrosis. Cholesterol crystals associated with necrotized macrophages further stimulate inflammation and lead to the recruitment of neutrophils. As the atherosclerotic lesions age, immune system T cells and monocytes are attracted to them, creating a vicious cycle of necrosis and inflammation.

As plaques mature, a fibrous cap forms over them. The plaques with defective or broken caps are most prone to rupture. The lesions alone may distort vessels to the point that they are occluded, but it is usually the rupture or ulceration of plaques that triggers thrombosis, blocking blood flow.

Atherosclerotic lesions have been shown to have many of the characteristics of a low-grade infection. A substantial number of studies support an association of various infections, including Chlamydophila pneumoniae (an organism usually associated with respiratory infection), with the development of atherosclerosis. However, only C pneumoniae (and no other bacteria) has actually been isolated in the atherosclerotic plaque. Clinical trials have been conducted to assess the effects of antibiotics in the secondary prevention of composite cardiovascular events, but results have been disappointing. Although these studies have yet to establish a definitive causal link between infection and atherosclerosis, it is clear that infections, by increasing an organism’s inflammatory state, can at least promote atherosclerosis. A characteristic of atherosclerosis that is currently receiving considerable attention is its association with a deficient release of NO and defective vasodilation. As noted, oxidized

LDLs inhibit NO production. If acetylcholine is infused via catheter into normal coronary arteries, the vessels dilate; however, if it is infused when atherosclerosis is present, the vessels constrict. This indicates that the endothelial secretion of NO is defective.

Interestingly, recent experimental evidence indicates that the activation of the vasculature endothelial receptor for endothelin B both stimulates eNOS and exerts antiproliferative effects on vascular smooth muscle cells. It has been speculated that the disrupted signaling via this receptor can be an additional contributing factor in the pathophysiology of atherosclerosis.

Relation to Dietary Cholesterol & Other Lipids Transforming a monocyte into a lipid-ingesting macrophage involves the appearance on its surface of a unique type of oxidized LDL receptor, the scavenger receptor. Monocytes are stimulated to produce these receptors by the action of macrophage colony-stimulating factor secreted by endothelial cells and vascular smooth muscle cells. When oxidized LDL-receptor complexes are formed, they are internalized and the receptors recycle to the membrane while the lipid is stored.

Obviously, lipid accumulation in foam cells is a key event in the progression of atherosclerotic lesions, and it is well established that lowering plasma cholesterol slows the progress of atherosclerosis. Figure 11–16 summarizes the main pathways of the metabolism of ingested lipids. Because lipids are relatively insoluble, they are transported as special lipoprotein particles that increase their solubility. Dietary cholesterol and triglycerides are packaged in the protein- coated chylomicrons in intestinal epithelial cells. Under the influence of lipoprotein lipase, these particles release triglycerides to fat depots and muscles, and the resulting chylomicron remnants are taken up by the liver. The liver also synthesizes cholesterol and packages it with specific proteins to form very-low- density lipoproteins (VLDLs). These lipoprotein particles enter the circulation, and, under the influence of lipoprotein lipase, donate triglycerides to tissues. In this way, they become cholesterol-rich intermediate-density lipoproteins (IDLs) and low-density lipoproteins (LDLs). The LDLs supply cholesterol to the tissues. They provide all cells with cholesterol for the production of cell membranes and other uses. They also provide most of the cholesterol that is the precursor for all steroid hormones. As noted, oxidized LDLs are taken up by macrophages and smooth muscle cells in atherosclerotic lesions. On the other hand, high-density lipoproteins (HDLs) take cholesterol from peripheral cells and transport it to the liver where it is metabolized, keeping plasma and tissue

cholesterol low. For this reason, it is referred to as “good cholesterol” as opposed to LDL cholesterol, which is “bad cholesterol.” Efforts are being made to increase HDL levels by pharmaceutical means in the treatment of atherosclerosis.

FIGURE 11–16 Simplified diagram of the lipoprotein systems for transporting lipids in humans. In the exogenous system, chylomicrons rich in triglycerides of dietary origin are converted to chylomicron remnants rich in cholesteryl esters by the action of lipoprotein lipase. In the endogenous system, very-low- density lipoproteins (VLDLs) rich in triglycerides are secreted by the liver and converted into intermediate- density lipoproteins (IDLs) and then to low-density lipoproteins (LDLs) rich in cholesteryl esters. Some of the LDLs enter the subendothelial space of arteries, are oxidized, and then are taken up by macrophages, which become foam cells. The letters on the chylomicrons, chylomicron remnants, VLDL, IDL, and LDL, identify the primary apoproteins found in them. (LCAT, lecithin-cholesterol acyltransferase.)

Clinical Manifestations Because atherosclerosis is an abnormality of arterial blood vessels, it can affect almost any organ in the body. Calcified atherosclerotic plaques are occasionally detected on x-ray film, and angiographic visualization of deformed arterial walls is possible. In general, however, atherosclerosis is asymptomatic until one of its complications develops.

In coronary arteries, atherosclerotic narrowing that reduces the lumen of a coronary artery more than 75% causes angina pectoris, the chest pain that results when pain-producing substances accumulate in the myocardium.

Typically, the pain comes on during exertion and disappears with rest, as the substances are washed out by the blood. When atherosclerotic lesions cause clotting and occlusion of a coronary artery, the myocardium supplied by the artery dies (myocardial infarction). (Myocardial infarction is also discussed in Chapter 10.)

In the cerebral circulation, arterial blockage at the site of atherosclerotic plaques causes thrombotic strokes. (Strokes are discussed in Chapter 7.) In the abdominal aorta, extensive atherosclerosis can lead to aneurysmal dilation and rupture of the vessel. In the renal vessels, the localized constriction of one or both renal arteries causes renovascular hypertension (see later discussion). In the circulation to the legs, vascular insufficiency causes intermittent claudication (fatigue and usually pain on walking that is relieved by rest). If the circulation of a limb is severely compromised, the skin can ulcerate, producing lesions that are slow to heal. Frank gangrene of the extremities may also occur. Less frequently, clot formation and obstruction may occur in vessels supplying the intestines or other parts of the body.

Risk Factors As noted, the progression of atherosclerosis is accelerated by a wide variety of genetic and environmental factors (risk factors), which are summarized in Table 11–2. Obviously, treating the accelerating conditions that are treatable and avoiding those that are avoidable should reduce the incidence of myocardial infarctions, strokes, and other complications of atherosclerosis.

TABLE 11–2 Conditions that accelerate the progression of atherosclerosis and the mechanisms responsible.

Estrogen increases cholesterol removal by the liver, and the progression of atherosclerosis is less rapid in premenopausal women than in men. In addition, epidemiologic evidence suggests that estrogen replacement therapy may protect the cardiovascular system in postmenopausal women. On the other hand, large doses of estrogens increase the incidence of blood clots, and even small doses produce a slight increase in clotting. In addition, in several studies, estrogen treatment of postmenopausal women failed to prevent second heart attacks. The reason for the discrepancies between the epidemiologic and experimental data is currently unsettled.

The effect of increased plasma levels of homocysteine and related molecules, such as homocystine and homocysteine thiolactone, a condition sometimes called hyperhomocystinemia, deserves emphasis. These increases are associated with accelerated atherosclerosis, and the magnitude of the plasma elevation is positively correlated with the severity of the atherosclerosis. Markedly elevated levels resulting from documented mutations of relevant genes are rare, but mild elevations occur in 7% of the general population. The mechanism responsible for the accelerated vascular damage is unsettled, but homocysteine is a significant source of H2O2 and other reactive forms of oxygen, and this may accelerate LDL oxidation.

Homocysteine is an intermediate in the synthesis of methionine. It is

metabolized by enzymes dependent on vitamin B6, vitamin B12, and folic acid. Supplementing the diet with these vitamins reduces plasma homocysteine, usually to normal. Determining whether such supplements also reduce the incidence of accelerated atherosclerosis will require prolonged, careful clinical trials, and the results of such studies to date are inconclusive.

Evidence is now overwhelming that lowering plasma cholesterol and triglyceride levels and increasing plasma HDL levels slows, and in some cases reverses, the atherosclerotic process. The desired decrease in lipids can sometimes be achieved with the dietary restriction of cholesterol and saturated and trans fat alone, even though dietary restriction initiates a compensatory increase in cholesterol synthesis in the body. When dietary treatment is not adequate, reducing the conversion of mevalonate to cholesterol with statins, drugs that inhibit hepatic 3-methylglutaryl coenzyme A (HMG-CoA) reductase, the enzyme that catalyzes this reaction, is beneficial. The currently available HMG-CoA reductase inhibitors include atorvastatin, lovastatin, pitavastatin, pravastatin, simvastatin, fluvastatin, and rosuvastatin.

Despite promising preliminary experimental results using gene therapy for severe hypercholesterolemia in animals with congenitally defective LDL receptors, this approach has proven challenging in humans owing to the lack of efficient gene targeting. The development of the CRISPR/Cas9 system, a more precise and efficient gene editing technique, is anticipated to ameliorate this problem. Other molecular biologic approaches to slowing or preventing the development of atherosclerosis are under development.

Antioxidant treatment with agents such as α-tocopherol, vitamin E, and β- carotene has been used to inhibit LDL oxidation, which reduces the incidence of atherosclerotic changes in experimental animals. However, the results of antioxidant treatment in humans have generally been disappointing or negative.

Men who smoke a pack of cigarettes a day have a 70% increase in death rate from ischemic heart disease compared with nonsmokers, and there is also an increase in women. Smoking cessation lessens the risk of death and of myocardial infarction. The deleterious effects of smoking include endothelial damage caused by carbon monoxide–induced hypoxia. Other factors may also be involved. Thus, stopping smoking is a major way to slow the progress of atherosclerosis.

Because of the increased shear stress imposed on the endothelium by an elevated blood pressure, hypertension is another important modifiable risk factor for atherosclerosis. Lowering blood pressure has its greatest effect in reducing the incidence of stroke, but there are beneficial effects on ischemic heart disease

as well. With modern methods of treatment, blood pressure in people with hypertension can generally be reduced to normal or near-normal values, and the decrease in strokes, myocardial infarctions, and renal failure produced by such treatment is clear testimony to the value of reducing or eliminating this risk factor.

In people with diabetes, there are both microvascular and macrovascular complications (see Table 18–6). The latter are primarily related to atherosclerosis. People with diabetes face a twofold increase in the incidence of myocardial infarction compared with those without diabetes; severe circulatory deficiency in the legs with gangrene is relatively common; people with diabetes experience more thrombotic strokes; and renal failure is a serious problem (Chapter 18). While chronic hyperglycemia itself can induce vascular damage, it is becoming more apparent that insulin resistance, such as occurs in type 2 diabetes mellitus, contributes markedly to the development of atherosclerosis. Insulin activation of the PI3K/Akt pathway leads to the increased expression of endothelin B receptor, the consequent activation of nitric oxide synthase in endothelial cells, and a resultant enhancement of local vasodilation and decrease in local inflammation. Insulin resistance downregulates the P13K/Akt pathway, leading to the selective activation of the p-ERK pathway, facilitating the proliferation and migration of vascular smooth muscle cells that contribute to the development of atherosclerosis. Importantly, however, the rigorous control of blood pressure in people with diabetes is more efficacious in reducing cardiovascular complications than is the rigorous control of blood glucose.

Obesity has long been appreciated as an independent risk factor for atherosclerosis. However, the mechanism(s) through which obesity promotes atherosclerosis has remained enigmatic until recently. Despite the overproduction of the satiety-inducing adipose-derived adipokines (eg, leptin and adiponectin) in obese subjects, they exhibit marked resistance to the effects of these adipokines. Moreover, obesity induces a low-grade inflammation of adipose tissue by producing pro-inflammatory cytokines such as TNFα and interleukin 6, as well as by recruiting inflammatory cells. In addition to the enhanced production of adipokines, perivascular fat also produces pro- inflammatory molecules. Taken together, these findings indicate that perivascular adipose tissue may be a major contributor to obesity-induced atherosclerosis.

Nephrotic syndrome and hypothyroidism also accelerate the progression of atherosclerosis and are treatable conditions.

Although local inflammation clearly plays a direct role in the pathogenesis of

atherosclerosis, the possibility that indirect mechanisms associated with autoimmune diseases, infections (including gum disease and gastric infections), or exposure to various pollutants contribute to (or even initiate) atherosclerosis remains controversial.

CHECKPOINT

10. What is the most common cause of death in the United States among individuals older than 45 years?

11. What is the hypothesized mechanism of atherosclerotic plaque formation?

12. What are some ways in which atherosclerotic plaques can cause cardiovascular disease?

13. Name five treatable risk factors that accelerate the pro-gression of atherosclerosis.

HYPERTENSION

Hypertension is not a single disease but rather a syndrome with multiple causes. In most instances, the cause remains unknown, and the cases are lumped together under the term essential hypertension (Table 11–3). However, mechanisms are continuously being discovered that explain hypertension in new subsets of the formerly monolithic category of essential hypertension, and the percentage of cases in the essential category continues to decline. Essential hypertension is often called primary hypertension, and hypertension in which the cause is known is called secondary hypertension, although this separation seems somewhat artificial. This chapter discusses the pathogenesis of hypertension and its complications in general terms and then discusses the specific causes of the currently defined subgroups and the unique features, if any, that each adds to the general findings in patients with high blood pressure.

TABLE 11–3 Primary and secondary causes of hypertension.

Pathogenesis Current guidelines of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure define normal blood pressure as a systolic pressure of less than 120 mm Hg and a diastolic pressure of less than 80 mm Hg. Hypertension is defined as an arterial pressure greater than 140/90 mm Hg in adults on at least three consecutive visits to the doctor’s office. People whose blood pressure is between normal and 140/90 mm Hg are considered to have pre-hypertension, and people whose blood pressure falls in this category should appropriately modify their lifestyle to lower their blood pressure to below 120/80 mm Hg. As noted (see Figure 11–7), systolic pressure normally rises throughout life, and diastolic pressure rises until age 50–60 years but then falls, so that pulse pressure continues to increase. In the past, emphasis has been on treating individuals with elevated diastolic pressure. However, it now appears that, particularly in elderly individuals, treating systolic hypertension is equally important or even more so in reducing the cardiovascular complications of hypertension. Moreover, some studies indicate that overly aggressive treatment (particularly of diastolic hypertension) may be associated with adverse cardiac events (primarily myocardial infarctions) in patients with coronary artery disease or chronic heart failure. The explanation for this may be that because the coronary arteries fill during diastole, in individuals with coronary artery disease or heart failure, adequate cardiac muscle perfusion depends a somewhat higher diastolic blood pressure.

The most common cause of hypertension is increased peripheral vascular resistance. However, because blood pressure equals total peripheral resistance times cardiac output, prolonged increases in cardiac output can also cause hypertension. These are seen, for example, in hyperthyroidism and beriberi. In addition, increased blood volume causes hypertension, especially in individuals with mineralocorticoid excess or renal failure (see later discussion); and increased blood viscosity, if marked, can increase arterial pressure.

Clinical Presentation Hypertension by itself does not cause symptoms. Headaches, fatigue, and dizziness are sometimes ascribed to hypertension, but nonspecific symptoms such as these are no more common in people with hypertension than they are in normotensive controls. Instead, the condition is discovered during routine screening or when patients seek medical advice for its complications. These

complications are serious and potentially fatal. They include myocardial infarction, heart failure, thrombotic and hemorrhagic strokes, hypertensive encephalopathy, and renal failure (Figure 11–17). This is why hypertension is called “the silent killer.”

FIGURE 11–17 Pathogenesis of the complications produced by arterial hypertension. (LVH, left ventricular hypertrophy.) (Redrawn, with permission, from Drago J et al. Hypertension. Chapter 13 in Lilly LS et al, eds. Pathophysiology of Heart Disease: A Collaborative Project of Medical Students and Faculty, 6th ed. Wolters Kluwer, 2016.)

Physical findings are also absent in early hypertension, and observable changes are generally found only in advanced severe cases. These may include hypertensive retinopathy (ie, narrowed arterioles seen on funduscopic examination) and, in more severe cases, retinal hemorrhages and exudates, along with swelling of the optic nerve head (papilledema). Prolonged pumping against an elevated peripheral resistance causes left ventricular hypertrophy, which can be detected by echocardiography, and cardiac enlargement, which can be detected on physical examination. It is important to listen with the stethoscope over the kidneys because in renal hypertension (see later discussion), narrowing of the renal arteries may cause bruits. These bruits are usually continuous throughout the cardiac cycle. It has been recommended that the blood pressure response to rising from the sitting to the standing position be determined. A blood pressure rise on standing sometimes occurs in essential hypertension, presumably because of a hyperactive sympathetic response to the erect posture. This rise is usually absent in other forms of hypertension.

In many patients with hypertension, the condition is benign and progresses slowly; in others, it progresses rapidly. Actuarial data indicate that, on average, untreated hypertension reduces life expectancy by 10–20 years. Atherosclerosis is accelerated, and this in turn leads to ischemic heart disease with angina pectoris and myocardial infarctions (Chapter 10), thrombotic strokes and cerebral hemorrhages (Chapter 7), and renal failure (Chapter 16). Another complication of severe hypertension is hypertensive encephalopathy, in which there is confusion, disordered consciousness, and seizures. This condition, which requires vigorous treatment, is probably due to arteriolar spasm and cerebral edema.

In all forms of hypertension regardless of cause, the condition can suddenly accelerate and enter the malignant phase. In malignant hypertension, there is widespread fibrinoid necrosis of the media with intimal fibrosis in the arterioles, narrowing them and leading to progressive severe retinopathy, heart failure, and renal failure. If untreated, malignant hypertension is usually fatal in 1 year.

MANAGEMENT

A discussion of disease treatment is beyond the scope of this book. However, it should be noted that in all forms of hypertension, modern treatment with β- adrenergic blocking drugs, inhibitors of the renin–angiotensin system, Ca2+ channel inhibitors, and diuretics reduce blood pressure, usually to normal levels. Though these drugs use different signaling and effector cascades, they all lower blood pressure either by decreasing arterial vascular tone and/or by decreasing intravascular volume. In addition, these treatments delay or prevent complications and lengthen life expectancy. Moreover, evidence indicates that aggressive blood pressure lowering in high-risk individuals (including those with a systolic blood pressure below 140 mm Hg) has substantial benefits in preventing major cardiovascular events.

In the case of essential hypertension, these therapies are not curative and must be continued indefinitely. Unfortunately, a significant number of patients with essential hypertension do not achieve adequate blood pressure control despite treatment. Recent evidence suggests that management approaches could be both improved and simplified if plasma renin measurement was added to current diagnostic algorithms. Patients with high and normal plasma renin would be treated primarily with renin-angiotensin-aldosterone inhibitors (such as ACE inhibitors or ARBs), and patients with low renin would be treated primarily with

volume-depleting agents. The principle behind this approach lies in the assumption that hypertension arises as a consequence of either abnormally high activity of endogenous vasoconstrictors or abnormally elevated total body salt and subsequent expanded blood volume. While these two variables are not completely independent, renin measurement can facilitate the identification of the prevailing mechanism: If a patient has an elevated (or even normal) level of renin in the face of hypertension, the renin-angiotensin-aldosterone system (RAAS) is inappropriately reacting in the face of high blood pressure. If renin is low and pressure is high, the RAAS response is appropriate, so the likely culprit is the elevated body fluid volume. In the first situation, a preferred treatment approach is RAAS inhibition. In the second scenario, a therapy that decreases intravascular volume is preferred. While this approach can be effective in the long-term treatment of essential hypertension, if a cause of hypertension can be identified (so-called secondary hypertension), its treatment may result in a cure. Consequently, it is important to identify such cases. For example, an important exception to the above algorithm is the case of primary aldosteronism (autonomous excessive aldosterone secretion) in which pathologic aldosterone- mediated intravascular volume expansion is the cause of the hypertension. In this disease, renin is appropriately physiologically suppressed owing to the pathologic aldosterone excess. Treatment focuses on removing an inciting aldosterone-secreting adrenal tumor or medically treating the aldosterone excess, most often with a mineralocorticoid receptor antagonist.

Etiology A. Coarctation of the Aorta Congenital narrowing of the aorta usually occurs just distal to the origin of the left subclavian artery. Peripheral resistance is increased above the constriction. Therefore, blood pressure is elevated in the arms, head, and chest but lowered in the legs. However, because the constriction is proximal to the renal arteries, renin secretion is increased in most cases of coarctation as a result of the reduction in arterial pressure in the renal arteries. This tends to increase blood pressure throughout the body. Elimination of the constriction by resecting the narrowed segment of the aorta usually cures the condition.

B. Salt Sensitivity

Although salt intake has been recognized as a factor in the pathophysiology of hypertension, not all hypertension is salt sensitive. As shown in Table 11–4,

about 30% of whites with normal renal function and normal blood pressure are salt sensitive compared with 55% of whites with essential hypertension. For unknown reasons, a much larger percentage of black people with hypertension are salt sensitive (73%). These figures have obvious significance in terms of recommendations about salt intake in hypertension.

TABLE 11–4 Salt sensitivity in humans.

It should be emphasized that the figures just cited refer to individuals with normal renal function and normal (or reduced) secretion of mineralocorticoid hormones. When renal function is reduced, mineralocorticoid secretion is increased (or the effects of mineralocorticoids are enhanced), there is an abnormal retention of salt and water, and hypertension is produced on this basis (see later discussion).

The genetic mechanisms responsible for the differences in salt sensitivity are still debated. One leading hypothesis posits that salt-mediated hypertension stems from salt-induced vascular dysfunction leading to increased total peripheral resistance. Salt can activate three pathways that may lead to vascular smooth muscle contraction: (1) salt stimulates a subset of G proteins (G12-13) responsible for the activation of myosin light chain kinase, which phosphorylates

myosin to initiate contraction; (2) salt stimulates the Rho/Rho kinase pathway, which inhibits myosin light chain phosphatase to prevent smooth muscle relaxation; and (3) short-term increases in dietary salt intake stimulate the release of endogenous ouabain (whose effect on the vascular smooth muscle parallels the effects of the cardiac glycoside) to inhibit Na+-K+ ATPase with a consequent decrease in Na+–Ca+ exchanger activity, ultimately elevating intracellular calcium levels and increasing smooth muscle tone. Experimental evidence suggests that individual differences in these signaling pathways may indeed contribute to salt-related hypertension.

A second hypothesis proposes that the kidney of salt-sensitive individuals cannot effectively excrete a salt load, leading to an inappropriate increase in intravascular volume and resultant hypertension. Supporting data include the frequent finding that kidney transplantation often ameliorates hypertension in salt-sensitive patients. Also, animal studies that have defined an aldosterone- independent activation of mineralocorticoid receptors and a sympathetically mediated activation of sodium reabsorption in the distal renal tubule both support the contribution of renal defects in sodium handling to salt-sensitive hypertension. Regardless of the mechanism, however, the salt sensitivity of blood pressure is an influential risk factor of prognostic importance in cardiovascular disease.

C. Renal Abnormalities

Goldblatt’s observation that renal artery constriction increased blood pressure in experimental animals was rapidly followed by demonstration of the same event in humans. However, disappointment followed when it was found that renal hypertension resulting from constriction of one or both renal arteries accounted for only a very small percentage of cases of clinical hypertension. The narrowing can result from atherosclerosis, the fibroelastic overgrowth of the renal artery wall, or external pressure on the vessel. The initial constriction decreases renal arteriolar pressure, which leads to increased renin secretion. (The renin–angiotensin system is discussed in Chapters 16 and 21.) However, in many cases, some other mechanism takes over chronically to maintain the hypertension. The nature of this other mechanism is unknown.

Ureteral obstruction can cause hypertension in animals and probably in humans by increasing renal interstitial pressure, thus decreasing the pressure gradient across the renin-secreting juxtaglomerular cells.

Acute and chronic glomerulonephritis and other forms of diffuse kidney disease can cause hypertension when the loss of the ability to excrete salt is

severe enough that Na+ and water are retained and blood volume is expanded.

D. Hormonal Disorders (Chapters 12 & 21)

A remarkable number of adrenal abnormalities cause hypertension. These mainly include conditions in which mineralocorticoids are secreted in excess, but excess cortisol secretion can also cause hypertension (albeit, through mineralocorticoid receptor activation), as does excess catecholamine secretion by adrenal medulla tumors. (These disorders are covered in detail in Chapters 12 and 21.)

One particular contributing factor to hypertension in women is estrogen. Secretion of angiotensinogen from the liver is under endocrine control and is uniquely stimulated by estrogens. Consequently, it is increased in women taking contraceptive pills containing large amounts of estrogens. When circulating angiotensinogen is increased, more angiotensin II is formed and blood pressure rises. The normal compensation for this response is decreased renin secretion because angiotensin II feeds back directly on the juxtaglomerular cells to reduce renin secretion. However, in some women, the compensation is incomplete and the estrogens cause a significant increase in blood pressure. Some women with this condition have underlying essential hypertension, which is triggered by the estrogens, but in others, the hypertension is cured by stopping estrogen treatment.

In view of the fact that Na+ retention resulting from mineralocorticoid excess causes hypertension, it may seem surprising that a natriuretic hormone is also a suspected cause of hypertension. ANP and other natriuretic peptides of cardiac origin cause sodium loss in the urine and generally lower blood pressure. However, there is also a digitalis-like natriuretic substance in the circulation. Its source seems to be the adrenals, although it has also been claimed that it is secreted by the hypothalamus. This substance, which may be naturally occurring ouabain, inhibits Na+-K+ ATPase. This results in a loss of Na+ in the urine, but an accumulation of Ca2+ in cells because of the Na+ gradient decrease across the cell membrane. The increase in intracellular Ca2+ causes vascular smooth muscle to contract. Consequently, blood pressure is increased. However, the physiologic and pathophysiologic significance of this natriuretic hormone remains unsettled, and its hypersecretion cannot yet be considered a proved cause of clinical hypertension.

E. Neurologic Disorders

The nervous system plays a key role in maintaining blood pressure in normal

individuals (see prior discussion). Clonidine and other drugs lower blood pressure by acting on the brain to decrease sympathetic discharge, and several of the most effective treatments for chronic hypertension act peripherally to reduce the effect of vasomotor sympathetic discharge to the blood vessels and heart. These and other observations suggest that clinical hypertension could be caused by central nervous system (CNS) abnormalities. Interruption of the afferent input from the baroreceptors to the CNS in experimental animals causes increased blood pressure. However, emphasis has been placed on the variability of the blood pressure in such animals rather than on any consistent elevation of mean arterial pressure. There is some evidence that chronic pressure on the rostral ventrolateral medulla (see Figure 11–13) caused by minor anatomic abnormalities can cause hypertension in humans. However, this evidence is controversial, and as yet it cannot be said that this is an established cause of hypertension.

F. Nitric Oxide

An intriguing observation in experimental animals is that the administration of drugs that inhibit the production of NO increase blood pressure. Furthermore, there is a sustained elevation in blood pressure in knockout mice in which the genetic expression of the endothelial form of NOS has been disrupted. These observations suggest that NO has a chronic blood pressure–lowering effect and raise the possibility that inhibiting the production (or the effects) of NO may contribute to the development of hypertension in humans. The corollary of this conclusion is that therapies targeting vascular NO production or responsiveness could be successful in treating hypertension.

G. Facilitation of Na+–H+ Exchange

In approximately 50% of patients with essential hypertension, the function of a ubiquitous pH-regulating Na+–H+ exchanger in cell membranes is enhanced. Evidence indicates that this is associated with a polymorphism in the gene for one of the β subunits of a G protein that facilitates the function of the G protein. However, the overall significance of this abnormality remains to be determined.

H. Obesity

An association between body weight and hypertension has been appreciated for some time now. Moreover, insulin resistance and hyperlipidemia (along with obesity) are more prevalent in patients with essential hypertension and in their

normotensive relatives than in the general population (or in patients with hypertension from known causes). This has led to a speculation that insulin resistance (via the hyperinsulinemia-mediated stimulation of the sympathetic nervous system) contributes to hypertension. However, experimental evidence is limited. It is more likely that the hypertension seen in obesity stems from a number of metabolic, hormonal, and humoral perturbations triggered by the accumulation of visceral (and perivascular) fat tissue. Increased fat tissue either directly (through local synthesis) or indirectly leads to an increase in the number of signaling molecules (eg, leptin, TNFα, VEGF, angiotensin II, and other molecules) that trigger an increased activation of the RAAS and sympathetic systems, endothelial dysfunction, and sodium retention. These changes can lead to an increase in peripheral vascular resistance and/or an increase in fluid retention, resulting in elevated blood pressure.

CHECKPOINT

14. Describe five physical findings in long-standing or severe hypertension. 15. Name 10 known causes of hypertension and a means by which each

could be identified as the cause of hyper-tension in a patient. 16. What is the effect on blood pressure of disrupting the gene for the

endothelial cell form of NOS in mice?

SHOCK

The term “shock” is used to denote various conditions, including the response to the passage of electric current through the body, the state that follows immediately after interruption of the spinal cord, and the stunned reaction to bad news. In the current context, it refers to an abnormality of the circulatory system in which there is inadequate tissue perfusion because of a relatively or absolutely inadequate cardiac output. The causes are divided into four groups: hypovolemic shock, an inadequate volume of blood to fill the vascular system; distributive, vasogenic, or low-resistance shock, an increase in the size of the vascular system produced by vasodilation in the presence of a normal blood volume; cardiogenic shock, inadequate heart output as a result of myocardial abnormalities; and obstructive shock, inadequate cardiac output as a result of

blood flow obstruction in the lungs or heart. Table 11–5 provides examples of the conditions or diseases that can cause each type of shock.

TABLE 11–5 Types of shock, with examples of conditions or diseases that can cause each type.

Hypovolemic Shock Hypovolemic shock is characterized by hypotension; a rapid, thready pulse; cold, pale, clammy skin; intense thirst; rapid respiration; and restlessness or, alternatively, torpor. Urine volume is markedly decreased. However, none of these findings is invariably present. Hypovolemic shock is commonly subdivided into categories on the basis of cause. The use of terms such as “hemorrhagic shock,” “traumatic shock,” “surgical shock,” and “burn shock” is of some benefit because although there are similarities among these various forms of shock, there are important features that are unique to each.

In hypovolemic and other forms of shock, inadequate tissue perfusion leads to increased anaerobic glycolysis, with the production of large amounts of lactic acid. In severe cases, the blood lactate level rises from a normal value of about 1 mmol/L to 9 mmol/L or more. The resulting lactic acidosis depresses the myocardium, decreases peripheral vascular responsiveness to catecholamines, and may be severe enough to cause coma.

Multiple compensatory reactions come into play to defend extracellular fluid volume (Table 11–6). The large number of reactions that have evolved indicates the importance of maintaining blood volume for survival.

TABLE 11–6 Compensatory reactions activated by hypovolemia.

A decrease in pulse pressure or mean arterial pressure decreases the number of impulses ascending to the brain from the arterial baroreceptors, resulting in an increased vasomotor discharge. The resulting vasoconstriction is generalized, sparing only the vessels of the brain and the heart. The coronary vessels are dilated because of the increased myocardial metabolism secondary to an increase in heart rate. Vasoconstriction in the skin accounts for the coolness and pallor, and vasoconstriction in the kidneys accounts for the shutdown in renal function.

The immediate cardiac response to hypovolemia is tachycardia. With more extensive loss of volume, tachycardia can be replaced by bradycardia. With very severe hypovolemia, tachycardia reappears. Bradycardia may be due to the unmasking of a vagally mediated depressor reflex, perhaps related to limiting blood loss.

Vasoconstriction in the kidney reduces glomerular filtration. This reduces water loss, but it reaches a point at which nitrogenous products of metabolism accumulate in the blood (prerenal azotemia). If hypotension is prolonged, there may be severe renal tubular damage, leading to acute kidney injury.

The fall in blood pressure and the decreased O2-carrying power of the blood caused by the loss of red cells results in the stimulation of the carotid and aortic chemoreceptors. This not only stimulates respiration but increases

vasoconstrictor discharge. In severe hypovolemia, the pressure is so low that there is no longer any discharge from the carotid and aortic baroreceptors. This occurs when the mean blood pressure is about 70 mm Hg. Under these circumstances, if the afferent discharge from the chemoreceptors via the carotid sinus and vagus nerves is stopped, there is a paradoxic further fall in blood pressure rather than a rise.

Hypovolemia causes a marked increase in the circulating levels of the pressor hormones angiotensin II, epinephrine, norepinephrine, and vasopressin. ACTH secretion is also increased, and angiotensin II and ACTH both cause an acute increase in aldosterone secretion. The resulting retention of Na+ and water helps re-expand blood volume.

Forms of Hypovolemic Shock

Hemorrhagic shock is probably the most carefully studied form of shock because it is easily produced in experimental animals. With moderate hemorrhage (5–15 mL/kg body weight), pulse pressure is reduced but mean arterial pressure may remain normal. With more severe hemorrhage, blood pressure always falls.

After hemorrhage, the plasma protein lost in shed blood is gradually replaced by hepatic synthesis, and the concentration of plasma proteins returns to normal in 3–4 days. The increase in circulating erythropoietin increases red blood cell formation, but it takes 4–8 weeks to restore red cell counts to normal.

Traumatic shock develops when there is severe damage to muscle and bone. This is the type of shock seen in battle casualties and automobile accident victims. Bleeding into the injured areas is the principal cause of such shock. The amount of blood that can be lost into a site of injury that appears relatively minor is remarkable; the thigh muscles can accommodate 1 L of extravasated blood, for example, with an increase in the diameter of the thigh of only 1 cm.

Breakdown of skeletal muscle is a serious additional problem when shock is accompanied by extensive muscle crushing (crush syndrome). When pressure on tissues is relieved and they are once again perfused with blood, free radicals are generated, which cause further tissue destruction (reperfusion-induced injury). Increased Ca2+ in damaged cells can reach toxic levels. Large amounts of K+ enter the circulation. Myoglobin and other products from reperfused tissue can accumulate in the kidneys, where glomerular filtration is already reduced by hypotension, and the tubules can become clogged, causing anuria.

Surgical shock is a result of combinations, in various proportions, of external

hemorrhage, bleeding into injured tissues, and dehydration. In burn shock, there is a loss of plasma from burn surfaces, and the

hematocrit rises rather than falls, producing severe hemoconcentration. In addition, there are complex metabolic changes. For these reasons, as well as the ease of infection in burned areas and kidney damage, the mortality rate when third-degree burns cover more than 75% of the body is close to 100%.

Distributive Shock In distributive shock, most of the symptoms and signs described previously are present. However, vasodilation causes the skin to be warm rather than cold and clammy. Anaphylactic shock is a good example of distributive shock. In this condition, an accelerated allergic reaction causes the release of large amounts of histamine, producing marked vasodilation. Blood pressure falls because the size of the vascular system exceeds the amount of blood in it even though blood volume is normal.

A second type of distributive shock is neurogenic shock, in which a sudden loss of sympathetic autonomic activity (as seen in head and spinal cord injuries) results in vasodilation and blood pooling in the veins. The resulting decrease in venous return reduces cardiac output and frequently produces fainting, or syncope, a sudden transient loss of consciousness. A more benign and much more common form is postural syncope, which occurs on rising from a sitting or lying position. This is common in patients taking drugs that block sympathetic discharge or its effects on the blood vessels. Falling to the horizontal position restores blood flow to the brain, and consciousness is regained. Pressure on the carotid sinus produced, for example, by a tight collar can cause sufficient bradycardia and hypotension to cause fainting (carotid sinus syncope). Fainting caused by a variety of activities has been given appropriate names, such as micturition syncope, cough syncope, deglutition syncope, and effort syncope.

Syncope resulting from neurogenic shock is usually benign. However, it must be distinguished from syncope resulting from other causes and, therefore, merits investigation.

Another form of distributive shock is septic shock. (This condition is discussed in detail in Chapter 4.) It is now the most common cause of death in ICUs in the United States. It is a complex condition that includes elements of hypovolemic shock resulting from loss of plasma into the tissues (“third spacing”) and cardiogenic shock resulting from toxins that depress the myocardium. It is associated with excess NO production, and therapy with drugs that scavenge NO may be beneficial.

Streptococcal toxic shock syndrome is a particularly severe form of septic shock in which group A streptococci infect deep tissues; the M protein on the surface of those bacteria has an antiphagocytic effect. This M protein is also released into the circulation, where it aggregates with fibrinogen.

Cardiogenic Shock About 25% of syncopal episodes are of cardiac origin and are due either to the transient obstruction of blood flow through the heart or to sudden decreases in cardiac output caused by various cardiac arrhythmias. In addition, fainting is the presenting symptom in 7% of patients with myocardial infarctions.

Cardiogenic shock results whenever the pumping function of the heart is impaired to the point that blood flow to tissues is no longer adequate to meet resting metabolic demands; most commonly, it is due to extensive infarction of the left ventricle. The incidence of shock in patients with myocardial infarction is about 10%, and the mortality rate is 60–90%.

However, cardiogenic shock can also be caused by other diseases (eg, heart failure, arrhythmias) that severely compromise normal ventricular function. The symptoms are those of hypovolemic shock plus congestion of the lungs and viscera resulting from failure of the heart to put out all the venous blood returned to it. Consequently, the condition is sometimes called “congested shock.”

Obstructive Shock The picture of congested shock is also seen in obstructive shock. Causes include massive pulmonary emboli, tension pneumothorax with kinking of the great veins, and bleeding into the pericardium with external pressure on the heart (cardiac tamponade). In the latter two conditions, prompt surgery is required to prevent death. Pulsus paradoxus occurs in cardiac tamponade. Normally, blood pressure falls about 5 mm Hg during inspiration. In pulsus paradoxus, this response is exaggerated, and blood pressure falls 10 mm Hg or more as a result of increased fluid pressure in the pericardial sac on the external surface of the heart. However, pulsus paradoxus also occurs with labored respiration in severe asthma, emphysema, and upper airway obstruction.

Refractory Shock Some patients with hypovolemia or septic shock die soon after the onset of the condition, whereas others recover as compensatory mechanisms gradually

restore the circulation to normal. In an intermediate group of patients, shock persists for hours and gradually progresses. It eventually reaches a state in which there is no longer any response to vasopressor drugs and in which, even if the blood volume is returned to normal, cardiac output remains depressed. This condition, once known as irreversible shock, is now known as refractory shock. Patients often die despite vigorous treatment; however, more and more are now saved as our understanding of the pathophysiologic mechanisms has increased and treatment has improved. Therefore, “refractory shock” seems to be a more appropriate term than “irreversible shock.”

Various factors appear to make shock refractory. Precapillary sphincters are constricted for several hours but then relax, whereas postcapillary venules remain constricted. Therefore, blood flows into the capillaries and remains there. Various positive feedback mechanisms contribute to the refractory state. For example, cerebral ischemia depresses vasomotor and cardiac discharge, causing blood pressure to fall and making the shock worse. This, in turn, causes a further reduction in cerebral blood flow. In addition, myocardial blood flow is reduced in severe shock. Myocardial failure makes the pumping action of the heart less effective and consequently makes the shock worse and further lowers myocardial blood flow.

A complication of shock that has a very high mortality rate is pulmonary damage with the production of acute respiratory distress syndrome. The cause appears to be capillary endothelial cell damage and damage to alveolar epithelial cells with the release of cytokines (Chapter 9).

CHECKPOINT

17. What are the four major pathophysiologic forms of shock? 18. Name three pathophysiologic consequences of lactic acidosis in shock. 19. Describe five specific forms of hypovolemic shock. 20. Name three specific forms of distributive shock, and dis-tinguish them

from hypovolemic shock. 21. Name three factors that tend to make shock refractory.

CASE STUDIES

Yeong Kwok, MD

(See Chapter 25, p. 769–70 for answers)

CASE 63

A 65-year-old woman presents to the clinic to establish care. Her past medical history is notable for type 2 diabetes and hypertension. She has a 45-pack-year smoking history. A few weeks ago, she was shovelling her driveway when she had to stop owing to tightness in her chest. She does not get any regular exercise because her calves become very painful after walking one block.

Questions

A. What is the likely diagnosis? B. What is the pathogenesis of this condition? C. What are this patient’s rick factors, and how do they contribute to the

development of atherosclerosis?

CASE 64

A 56-year-old black man presents to the clinic for a routine physical examination. He has not seen a physician for 10 years. On arrival, he is noted to have a blood pressure of 160/90 mm Hg.

Questions

A. Does this man have hypertension? Why or why not? B. What physical findings might be present if he has had long-standing

hypertension? C. What are some of the important complications of hypertension? D. What are some causes of hypertension?

CASE 65

A young woman is brought to the emergency department by ambulance after a severe motor vehicle accident. She is unconscious. Her blood pressure is 64/40 mm Hg, and her heart rate is 150 bpm. She is intubated and hand-ventilated. There is no evidence of head trauma. The pupils are 2 mm and reactive. She withdraws to pain. Cardiac examination reveals no murmurs, gallops, or rubs. The lungs are clear to auscultation. The abdomen is tense, with decreased bowel sounds. The extremities are cool and clammy, with thready pulses. Despite aggressive blood and fluid resuscitation, the patient dies.

Questions

A. What are the four major pathophysiologic causes of shock? Which was likely in this patient?

B. What pathogenetic mechanism accounts for this patient’s unresponsiveness? For the cool, pale extremities?

C. What forms of hypovolemic shock may have been present in this patient? Why?

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CHAPTER

12

Disorders of the Adrenal Medulla Lauren Fishbein, MD, PhD, & Tobias Else, MD

The adrenal medulla secretes catecholamines (epinephrine, norepinephrine, dopamine). The catecholamines help prepare the body to deal with emergency situations. The major disorder of the adrenal medulla is pheochromocytoma, a neoplasm characterized by excessive catecholamine secretion.

NORMAL STRUCTURE & FUNCTION OF THE ADRENAL MEDULLA

ANATOMY

The adrenal medulla is the reddish-brown central portion of the adrenal gland. Accessory medullary tissue is sometimes located in the retroperitoneum near the sympathetic ganglia or along the abdominal aorta (paraganglia) (Figure 12–1).

FIGURE 12–1 Anatomic distribution of extra-adrenal chromaffin tissue in the newborn. (Redrawn, with permission, from Coupland R. The Natural History of the Chromaffin Cell. Longman, Green, 1965.)

HISTOLOGY

The adrenal medulla is made up of polyhedral cells arranged in cords or clumps. Embryologically, the adrenal medullary cells derive from neural crest cells. Medullary cells are innervated by cholinergic preganglionic nerve fibers that reach the gland via the splanchnic nerves. The adrenal medulla can be regarded as a specialized sympathetic ganglion, in which preganglionic sympathetic nerve fibers (using acetylcholine as a neurotransmitter) directly make contact with postganglionic cells, which secrete catecholamines (mainly epinephrine) directly into the circulation. This relationship is analogous to the other sympathetic ganglia, which connect preganglionic cholinergic sympathetic nerve fibers with

postganglionic fibers using catecholamines (mainly norepinephrine) as neurotransmitters. Medullary parenchymal cells accumulate and store their hormone products in prominent, dense secretory granules, 150–350 nm in diameter. Histologically, these cells and granules have a high affinity for chromium salts (chromaffin reaction) and thus are called chromaffin cells and contain chromaffin granules. The granules contain the catecholamines epinephrine and norepinephrine. Morphologically, two types of medullary cells can be distinguished: epinephrine-secreting cells, which have larger, less dense granules, and norepinephrine-secreting cells, which have smaller, very dense granules. Separate dopamine-secreting cells have not been identified. Ninety percent of medullary cells are the epinephrine-secreting type, and 10% are the norepinephrine-secreting type.

PHYSIOLOGY

The catecholamines help to regulate metabolism, the contractility of cardiac and smooth muscle, and neurotransmission.

Formation, Secretion & Metabolism of Catecholamines The adrenal medulla secretes three catecholamines: epinephrine, norepinephrine, and dopamine. Secretion occurs after the release of acetylcholine from the preganglionic neurons that innervate the medullary cells. Figure 12–2 illustrates the major biosynthetic pathways and hormonal intermediates for the catecholamines. In humans, most (80%) of the catecholamine output of the adrenal medulla is epinephrine. This epinephrine predominance occurs because the enzyme phenylethanolamine N-methyltransferase (PMNT), which converts norepinephrine to epinephrine, is upregulated by cortisol, which is found in high concentration locally from the surrounding adrenal cortex. Norepinephrine is principally found in paraganglial cells associated with the sympathetic nervous system and in the central nervous system, where it functions as a major neurotransmitter.

FIGURE 12–2 Biosynthesis and catabolism of catecholamines. The catecholamines are synthesized from tyrosine (TYR). The enzyme catechol-O-methyltransferase (COMT) generates metanephrine (MN) from epinephrine (E) and normetanephrine (NM) from norepinephrine (NE). COMT is constitutively active in pheochromocytomas and paragangliomas, and the release of these substances is constant rather than episodic. (3MT, 3-methoxytyramine; AADC, aromatic L-amino acid decarboxylase; DA, dopamine; DBH,

dopamine beta-hydroxylase; DOPAC, dihydroxyphenylacetic acid; HVA, homovanillic acid; MAO, monoamine oxidase; PNMT, phenylethanolamine N-methyltransferase; VMA, vanillylmandelic acid.)

Approximately 70% of the epinephrine and norepinephrine and 95% of the dopamine found in plasma are conjugated to sulfate and are inactive. In the supine state, the normal plasma level of free epinephrine is about 30 pg/mL (0.16 nmol/L); there is a 50–100% increase on standing. The normal plasma level of free norepinephrine is about 300 pg/mL (1.8 nmol/L), and the plasma free dopamine level is about 35 pg/mL (0.23 nmol/L).

Most catecholamine metabolism takes place within the same cells in which they are synthesized, mainly because of catecholamines leaking from the vesicular stores into the cytoplasm. These vesicular stores exist in a dynamic equilibrium, with outward passive leakage counterbalanced by an inward active transport controlled by vesicular monoamine transporters. Typically, the catechol–O–methyltransferase (COMT) enzyme methylates norepinephrine and epinephrine to form normetanephrine and metanephrine, respectively. These are then deaminated by monoamine oxidase.

In catecholaminergic neurons, the presence of monoamine oxidase in the cytoplasm leads to the formation of reactive catecholaldehydes. Production of these toxic aldehydes depends on the dynamics of the vesicular–axoplasmic monoamine exchange and an enzyme-catalyzed conversion to nontoxic acids or alcohols. In sympathetic nerves, the aldehyde produced from norepinephrine is converted to 3,4-dihydroxyphenylglycol. Subsequent extraneuronal O- methylation leads to the production of 3-methoxy-4-hydroxyphenylglycol, and its oxidation in the liver, catalyzed by alcohol and aldehyde dehydrogenases, leads to the formation of vanillylmandelic acid (VMA). Compared with intraneuronal deamination, the extraneuronal O-methylation of norepinephrine and epinephrine represents a minor pathway of metabolism.

The single largest source of epinephrine is the adrenal medulla. In the circulation, the catecholamines have a short half-life of about 2 minutes. Normally, only very small quantities of free epinephrine (about 6 µg/day) and norepinephrine (about 30 µg/day) are excreted, but about 700 µg of VMA is excreted daily.

Regulation of Catecholamine Secretion Physiologic stimuli affect medullary secretion through the nervous system. Medullary cells secrete catecholamines after the release of acetylcholine from the preganglionic neurons that innervate them. Catecholamine secretion is low in the basal state and is reduced even further during sleep. Emergency situations

cause an increase in adrenal catecholamine secretion as part of a generalized sympathetic discharge that serves to prepare the individual for stress (“fight-or- flight” response). Physiological stress such as psychological, physical (eg, mechanical, thermal), metabolic (eg, hypoglycemia, exercise), or pathologic (eg, obstructive sleep apnea) stress leads to catecholamine secretion.

Mechanism of Action of Catecholamines The effects of epinephrine and norepinephrine are mediated by their actions on two classes of receptors: α- and β-adrenergic receptors (Table 12–1). Alpha receptors are subdivided into α1 and α2 receptors, and β receptors are subdivided into β1, β2, and β3 receptors. Alpha1 receptors mediate smooth muscle contraction in blood vessels and the genitourinary (GU) tract and increase glycogenolysis. Alpha2 receptors mediate smooth muscle relaxation in the gastrointestinal (GI) tract and the vasoconstriction of some blood vessels. Alpha2 receptors also decrease insulin secretion. Beta1 receptors mediate an increased rate and force of myocardial contraction and stimulate lipolysis and renin release. Beta2 receptors mediate smooth muscle relaxation in the bronchi, blood vessels, GU tract, and GI tract and increase hepatic gluconeogenesis and glycogenolysis, muscle glycogenolysis, and the release of insulin and glucagon.

TABLE 12–1 Physiologic effects of catecholamines on the adrenergic receptors of selected tissues.

Intracellular post-receptor signaling is different for each subclass of

adrenergic receptor. Stimulation of α1-adrenergic receptors results in an increase in intracellular Ca2+ concentrations. First, phospholipase C is activated by the guanine nucleotide binding stimulatory protein, Gs. Phospholipase C hydrolyzes the membrane-bound phospholipid, phosphatidylinositol-4,5-bisphosphate, to generate two second messengers: diacylglycerol and inositol-1,4,5-trisphosphate. Diacylglycerol in turn activates protein kinase C, which phosphorylates various cellular substrates. Inositol-1,4,5-trisphosphate stimulates the release of intracellular Ca2+, which then initiates various cellular responses.

Activation of α2-adrenergic receptors results in a decrease in intracellular cyclic adenosine 3′,5′-monophosphate (cAMP). The mechanism involves receptor interaction with an inhibitory G protein, Gi, leading to inhibition of the adenylyl cyclase enzyme and, therefore, a decrease in cAMP. The fall in cAMP level leads to a decrease in activity of the cAMP-dependent protein kinase A. The Gi protein also stimulates K

+ channels and inhibits voltage-sensitive Ca2+

channels. On the other hand, β-adrenergic receptors stimulate adenylyl cyclase through

the mediation of Gs. Activation of β-adrenergic receptors thus leads to an increase in cAMP, activation of the cAMP-dependent protein kinase A, and the consequent phosphorylation of various cellular proteins. The Gs protein can also directly activate voltage-sensitive Ca2+ channels in the plasma membrane of cardiac and skeletal muscle.

The α1- and β1-adrenergic receptors are generally found in organs and tissues (eg, heart and gut) heavily innervated by—and situated so as to be readily activated by the stimulation of—the sympathetic nerves. The α1- and β1- adrenergic receptors are preferentially stimulated by norepinephrine, especially that released by nerve endings. In contrast, the α2- and β2-adrenergic receptors are generally situated in post-junctional sites in organs and tissues (eg, uterine and bronchial skeletal muscle) remote from sites of norepinephrine release. The α2- and β2-adrenergic receptors are preferentially stimulated by circulating catecholamines, especially epinephrine.

Differences in tissue distribution, accessibility by nerve fibers, preferences for epinephrine versus norepinephrine, and differences in post-receptor signaling are thus responsible for the diverse effects of catecholamines in an organ- and cell- specific manner.

Effects of Catecholamines

The catecholamines have been termed fight-or-flight hormones because their effects on the heart, blood vessels, smooth muscle, and metabolism assist organisms in responding to stress. Table 12–1 presents the principal physiologic effects of the catecholamines.

In the peripheral circulation, norepinephrine produces vasoconstriction in most organs (via α1 receptors). Epinephrine produces vasodilation via β2 receptors in skeletal muscle and the liver and vasoconstriction elsewhere. The former usually outweighs the latter, and for that reason epinephrine usually lowers total peripheral resistance.

Norepinephrine causes both systolic and diastolic blood pressures to rise. The rise in blood pressure stimulates the carotid and aortic baroreceptors, resulting in reflex bradycardia and a fall in cardiac output. Epinephrine causes a widening of pulse pressure but does not stimulate the baroreceptors to the same degree, so the pulse rises and cardiac output increases. Hence, pheochromocytomas and other tumors of the adrenal medulla, which usually secrete norepinephrine, lead to vasoconstriction and an increase in blood pressure.

The effects of catecholamines on metabolism include effects on glycogenolysis, lipolysis, and insulin secretion, mediated by both α- and β- adrenergic receptors. These metabolic effects result primarily from the action of epinephrine on four target tissues: liver, muscle, pancreas, and adipose tissue (see Table 12–1). The result is an increase in the levels of circulating glucose and free fatty acids. The increased levels of these two substances help provide an adequate supply of metabolic fuel to the nervous system and muscle during physiologic stress.

The amount of circulating plasma epinephrine and norepinephrine needed to produce these various physiologic and metabolic effects has been determined by infusing catecholamines into resting subjects. For norepinephrine, the threshold for cardiovascular and metabolic effects is a plasma level of about 1500 pg/mL, or about five times the basal level. In normal individuals, the plasma norepinephrine level rarely exceeds this threshold. However, for epinephrine, the threshold for tachycardia occurs at a plasma level of about 50 pg/mL, or about twice the basal level. The threshold for increasing systolic blood pressure and lipolysis is about 75 pg/mL of epinephrine; for increasing glucose and lactate, about 150 pg/mL; and for increasing insulin secretion, about 40 pg/mL. In healthy individuals, plasma epinephrine levels often exceed these thresholds.

The physiologic effect of circulating dopamine is unknown. Centrally, dopamine acts to inhibit prolactin secretion. Peripherally, in small doses, injected dopamine produces renal vasodilation, probably by binding to a specific

dopaminergic receptor. In moderate doses, it also produces vasodilation of the mesenteric and coronary circulation and vasoconstriction peripherally. It has a positive inotropic effect on the heart, mediated by action on the β1-adrenergic receptors. Moderate to large doses of dopamine increase the systolic blood pressure without affecting diastolic pressure.

Overview of Adrenal Medullary Disorders Pheochromocytoma is an uncommon tumor of adrenal medullary tissue that causes the production of excessive amounts of catecholamines. Patients typically present with sustained or episodic hypertension or with a syndrome characterized by episodic palpitations, tachycardia, chest pain, headache, anxiety, pallor, excessive sweating, and hyperglycemia. Pheochromocytomas can usually be cured if diagnosed and treated properly. Pheochromocytomas are closely related to paragangliomas, which sometimes are termed extra-adrenal pheochromocytomas. Paragangliomas arise from paraganglia, which are cell conglomerates found in close proximity to the sympathetic and parasympathetic ganglia. Most parasympathetic paragangliomas are found in the head and neck area (eg, carotid body, vagal nerve) and often do not secrete any catecholamines. Most sympathetic paragangliomas arise in the abdomen and often secrete norepinephrine.

CHECKPOINT

1. What is the embryologic origin of the cells of the adre-nal medulla? 2. Which nerve fibers innervate the adrenal medulla? 3. Which catecholamines are secreted by the human adre-nal medulla? Of

these, which is the major product? 4. What are the major physiologic stimuli of catechola-mine secretion? 5. What are the subtypes and distribution of catechola-mine receptors? 6. What physiologic processes do each subtype of cat-echolamine receptor

control, and how do catechola-mines bring about each of these physiologic processes?

PATHOPHYSIOLOGY OF SELECTED

DISORDERS OF THE ADRENAL MEDULLA

Pheochromocytomas are the main pathological entity of the adrenal medulla. Other tumors of the adrenal medulla or its embryonic precursors include neuroblastomas and ganglioneuromas. Neuroblastomas are one of the most common tumors of early childhood. In response to therapy (or even spontaneously), neuroblastomas can differentiate into ganglioneuromas. Both of these tumors secrete catecholamines, but symptoms due to catecholamine excess are usually absent because catecholamine levels do not reach those observed with pheochromocytomas. The absence of the adrenal medulla (eg, after bilateral adrenalectomy) is usually well tolerated, though sometimes symptoms such as orthostatic hypotension may be observed.

PHEOCHROMOCYTOMA AND PARAGANGLIOMA

Pheochromocytomas are neoplasms of the chromaffin cells of the adrenal medulla or paraganglial cells of extramedullary sites (and hence termed paragangliomas). These tumors secrete excessive amounts of epinephrine, norepinephrine, or both; dopamine is rarely secreted. Most pheochromocytomas and paragangliomas secrete norepinephrine and cause sustained or episodic hypertension. Tumors that secrete epinephrine cause hypertension less often; more frequently, they produce episodic hyperglycemia, glucosuria, and other metabolic effects.

Table 12–2 summarizes the clinical features of pheochromocytomas. Pheochromocytomas and paragangliomas are uncommon, probably found in about 0.1% of all patients with hypertension and in approximately two to eight individuals per million in the population. They occur in both sexes and in all age groups but are most often diagnosed in the fourth or fifth decade of life. Children with pheochromocytomas are more likely to have a familial cancer syndrome and to present with multifocal and/or extra-adrenal tumors.

TABLE 12–2 Clinical features of pheochromocytoma and paraganglioma.

The diagnosis is important because a sudden release of catecholamines from these tumors during anesthesia, surgery, or obstetric delivery may prove fatal. Pheochromocytoma was classically referred to as the “tumor of tens” because it was previously believed that 10% are extra-adrenal, 10% are outside the abdomen, 10% are multiple, 10% are bilateral, 10% are asymptomatic, 10% are hereditary, and 10% are malignant. Recent research has proved this designation to be untrue. It is now appreciated that about 23% occur at extra-adrenal sites, that multifocal pheochromocytomas or paragangliomas can be found in roughly one-third of pediatric cases, that about 30–40% of cases are hereditary, and that up to 20% of cases are malignant.

Etiology Several genetic syndromes, all transmitted in an autosomal dominant fashion, are associated with an increased risk of pheochromocytoma and sympathetic or parasympathetic nervous system paragangliomas. Most familial cases are caused by one of four syndromes: neurofibromatosis type 1 (NF1; also known as von Recklinghausen disease), von Hippel–Lindau syndrome, multiple endocrine

neoplasia type 2 (MEN 2), and hereditary paraganglioma syndrome (Table 12– 3). The genetic basis of these syndromes is now well defined.

TABLE 12–3 Major genetic syndromes associated with pheochromocytoma and paraganglioma.

Although not part of the diagnostic criteria, patients with NF1 have an increased incidence of pheochromocytoma caused by mutation of the NF1 gene.

Pheochromocytomas associated with NF1 often secrete both epinephrine and norepinephrine.

Pheochromocytoma is a frequent occurrence in patients with von Hippel– Lindau syndrome, a multiple-cancer predisposition syndrome caused by mutations of the VHL tumor suppressor gene. VHL-associated pheochromocytomas often secrete norepinephrine.

MEN 2 has two subtypes. In MEN 2A, patients develop pheochromocytomas as well as calcitonin-producing medullary thyroid carcinoma or C-cell hyperplasia of the thyroid and parathyroid hormone (PTH)–producing adenomas of the parathyroid (primary hyperparathyroidism). In MEN 2B, patients develop pheochromocytomas in association with medullary carcinoma of the thyroid and numerous oral mucosal neuromas, but no parathyroid adenomas. About 50% of patients with MEN 2A and MEN 2B have pheochromocytomas, often bilaterally. MEN 2–associated pheochromocytomas often predominately secrete epinephrine. A mutated RET proto-oncogene localized to chromosome 10q11.2 is responsible for MEN 2A and MEN 2B. The codon position of the RET mutation is related to disease phenotype. For example, mutations in RET at the specific codon 634 are associated with MEN 2A, and mutations at the specific codon 918 are associated with MEN 2B. These germline mutations of the RET proto-oncogene were the first examples of a dominantly acting oncogenic point mutation found to cause a heritable neoplasm in humans. Missense mutations in RET can be detected by DNA analysis, allowing for the identification of patients predisposed to MEN 2 to be screened for early detection and treatment of the associated neoplasms.

The most prevalent mutation in RET involves a cysteine residue at amino acid position 634. RET encodes a plasma membrane–associated tyrosine kinase that associates with a number of different related receptors. When these receptors are activated, they dimerize and bring two molecules of the RET tyrosine kinase close together, which initiates the cellular transmission of the signal. The 634- cysteine residue is part of an intramolecular sulfide bridge between associated cysteine residues. When one cysteine is absent, two RET molecules form intermolecular bridges, resulting in the initiation of intracellular signaling even in the absence of receptor association or ligand activation.

Hereditary paraganglioma syndromes are transmitted in an autosomal dominant fashion and most often are caused by germline mutations in genes coding for subcomponents of the succinate–dehydrogenase complex (SDHD, SDHB, SDHC, and rarely SDHA and SDHAF2) that serves as complex II of the mitochondrial respiratory chain. Extra-adrenal paragangliomas of the abdomen

(SDHB) and head and neck area (SDHD and SDHC) are more common in patients with hereditary paraganglioma. SDHD (and SDHAF2) mutations are influenced by a “parent-of-origin effect.” Affected patients always inherit the defective allele from their father. Individuals who inherit a mutant maternal allele do not exhibit an increased risk for a paraganglioma, but such carriers of a maternally inherited allele can transmit the defect to offspring. Patients with SDHx mutations are at risk for multiple primary paragangliomas in various locations, including adrenal pheochromocytoma, as well as other tumor types such as renal carcinomas and gastrointestinal stromal tumors. Recently, mutations have been identified in other genes, including TMEM127, MAX, FH, and EPAS1, which can more rarely cause hereditary pheochromocytomas and/or paragangliomas. SDHx and VHL mutations cause perturbations in the same intracellular

signaling cascade typically induced by hypoxia, thereby leading to a “pseudo- hypoxia” state. SDHx mutations lead to an accumulation of succinate, which in turn inhibits an enzyme that hydroxylates the transcription factor hypoxia- inducible factor α (HIF1α.) Normally, the hydroxylated HIF1α is recognized by the VHL protein and marked for destruction by the proteosome. Therefore, SDHx mutations and VHL mutations essentially protect HIF1α from degradation. This protection leads to an increase in HIF1α-induced transcription of downstream genes in the hypoxia pathway, which is in part responsible for the neoplastic phenotype.

Germline mutations in NF1, RET, VHL, SDHx, and others account for up to 30–40% of cases of pheochromocytomas and paragangliomas. Given the high frequency of germline mutations, genetic counseling and genetic testing are recommended for all patients with pheochromocytomas or paragangliomas, particularly those with a positive family history, multifocal disease, or a diagnosis before age 50 years. If a patient tests positive for a susceptibility gene mutation, their blood relatives should be offered genetic counseling and testing, and if positive for a gene mutation, those relatives can be screened for early detection of disease. Almost all pheochromocytomas and paragangliomas (at least 80%) occur in the abdomen, and most of these are in the adrenal medulla. Extra-adrenal paragangliomas (including sympathetic and parasympathetic paragangliomas) are found in the para-aortic and perirenal area, the organ of Zuckerkandl, the urinary bladder, the heart, the neck, and the posterior mediastinum (see Figure 12–1). Some of these tumors can lead to very specific symptoms (eg, urinary bladder paraganglioma can cause a hypertensive crisis with voiding). Grossly, pheochromocytomas and paragangliomas are generally

well circumscribed but vary in size, with weights ranging from less than 1 g to several kilograms (Figure 12–3). They are highly vascular tumors and frequently have cystic, necrotic, or hemorrhagic areas. Microscopically, the tumor consists of large pleomorphic cells arranged in sheets separated by a highly vascular stroma. In the cytoplasm, there are catecholamine-containing storage granules similar to those in normal adrenal medullary cells. Mitoses are rare, but tumor invasion of the adrenal capsule and blood vessels is common even in benign pheochromocytomas and paragangliomas. About 20% of pheochromocytomas and paragangliomas are malignant. Malignancy is established only when a metastasis occurs in a site where chromaffin cells are not usually found (eg, liver, lung, bone, brain). Unfavorable prognostic factors suggesting a malignant course include large tumor size, extra-adrenal location, younger age, dopamine secretion, and SDHB mutation.

FIGURE 12–3 Cross-section of adrenal, showing a pheochromocytoma and hyperplasia of the medulla in a patient with multiple endocrine neoplasia type 2a. The patient also had a medullary carcinoma of the thyroid and a large pheochromocytoma in the contralateral adrenal. (Reproduced, with permission, from Chandrasoma P et al, eds. Concise Pathology, 3rd ed. Originally published by Appleton & Lange. Copyright © 1998 by The McGraw- Hill Companies, Inc.)

Pathogenesis Most pheochromocytomas and paragangliomas predominantly release norepinephrine, and many also release epinephrine (Table 12–4). Rarely, a pheochromocytoma releases predominantly or only epinephrine; even more rarely, paragangliomas can predominantly secrete dopamine.

TABLE 12–4 Pathophysiologic and clinical manifestations of catecholamine excess.

In about half of patients with a pheochromocytoma or a paraganglioma, clinical manifestations vary in intensity and occur in an episodic or paroxysmal fashion. The paroxysms are related to sudden catecholamine discharge from the tumor. The sudden catecholamine excess causes hypertension, palpitations, tachycardia, chest pain, headache, anxiety, blanching, and excessive sweating. Such paroxysms usually occur several times a week but may occur only once every few months or up to 25 times daily. Paroxysms typically last for 15 minutes or less but may last for days. As time passes, the paroxysms usually become more frequent but generally do not change in character. A typical paroxysm may be produced by activities that compress the tumor (eg, bending, lifting, exercise, defecation, eating, deep palpation of the abdomen) and by emotional distress or anxiety.

Other patients have persistently secreting tumors and more chronic symptoms, including sustained hypertension. However, such patients also usually experience paroxysms related to transient increases in catecholamine release. The long-term exposure to high levels of circulating catecholamines seems not to produce the classic hemodynamic responses observed after the acute administration of catecholamines. This may be due in part to desensitization of the cardiovascular system to catecholamines and may explain why some patients with a pheochromocytoma or a paraganglioma are entirely asymptomatic.

Clinical Manifestations The clinical manifestations of pheochromocytoma and paraganglioma result from the increased secretion of epinephrine and norepinephrine. Table 12–5 lists commonly reported manifestations.

TABLE 12–5 Clinical findings in pheochromocytoma.

The classical pentad of symptoms in patients with a pheochromocytoma or a paraganglioma consists of headache, palpitation, perspiration, pallor, and orthostasis. The most common presenting feature is hypertension. In about half of cases, hypertension is sustained but the blood pressure shows marked fluctuations, with peak pressures during symptomatic paroxysms. During a hypertensive episode, the systolic blood pressure can rise to as high as 300 mm Hg. In about one-third of cases, hypertension is truly intermittent. In some individuals, hypertension is absent. The blood pressure elevation caused by the catecholamine excess results from two mechanisms: α-adrenergic receptor– mediated arteriolar vasoconstriction, leading to an increase in peripheral resistance; and β1 receptor–mediated increases in cardiac output and in renin release, leading to increased circulating levels of angiotensin II. The increased total peripheral vascular resistance is probably primarily responsible for the maintenance of high arterial pressures.

Hypertensive crisis may be precipitated by a variety of drugs, including tricyclic antidepressants, antidopaminergic agents, metoclopramide, and naloxone. Beta blockers should not be administered until alpha blockade has been established. Otherwise, blockade of β2-adrenergic receptors, which promote vasodilation, will allow unopposed α-adrenergic receptor activation and produce marked vasoconstriction and hypertension.

Peripheral vasoconstriction, mediated by α receptors, causes both facial pallor and cool, moist hands and feet. Chronic vasoconstriction of the arterial and venous beds leads to a reduction in plasma volume and predisposes to postural hypotension. In others, orthostatic hypotension is associated with decreased cardiac stroke volume and an impaired response of total peripheral vascular resistance to changes in posture, perhaps indicative of diminished arteriolar and venous responsiveness. The reduced responsiveness of the vasculature to norepinephrine in patients with a pheochromocytoma or a paraganglioma is probably related to the downregulation of α-adrenergic receptors resulting from persistently elevated norepinephrine levels.

Table 12–6 summarizes the complications of pheochromocytoma and paraganglioma. If unrecognized and untreated, pheochromocytoma and paraganglioma may be complicated by hypertensive retinopathy (retinal hemorrhages or papilledema); nephropathy; myocardial infarction, resulting from either myocarditis or coronary artery vasospasm; pulmonary edema, secondary either to left-sided heart failure or noncardiogenic causes; and stroke from cerebral infarction, intracranial hemorrhage, or embolism. Cerebral

infarction results from hypercoagulability, vasospasm, or both. Hemorrhage occurs secondary to severe arterial hypertension. Emboli can originate in mural thrombi in patients with dilated cardiomyopathy. Cardiomyopathy during a catecholamine surge often resembles takotsubo (stress/catecholamine-induced) cardiomyopathy (the so-called “broken heart” syndrome). Takotsubo cardiomyopathy may be a presenting sign in patients with undiagnosed pheochromocytoma or paraganglioma. In pregnancy, pheochromocytoma may lead to significant maternal morbidity and fetal demise.

TABLE 12–6 Complications of pheochromocytoma and paraganglioma.

The metabolic effects of excessive levels of circulating catecholamines

increase both blood glucose and free fatty acid levels. Increased glycolysis and glycogenolysis, combined with an α-adrenergic receptor–mediated inhibition of insulin release, cause an increase in blood sugar levels. In addition, epinephrine stimulates glucose production by gluconeogenesis and decreases insulin- mediated glucose uptake by peripheral tissues such as skeletal muscle. In pheochromocytoma, impaired glucose homeostasis may also result from β- adrenergic receptor desensitization, which produces relative insulin resistance. Glucose intolerance is common, and diabetes mellitus may occur.

Epinephrine raises blood lactate concentrations by stimulating glycogenolysis and glycolysis. An increase in oxygen consumption from the catecholamine stimulation of metabolism occurs in combination with a decrease in oxygen delivery to tissues from vasoconstriction, leading to lactate accumulation.

Occasionally, pheochromocytomas may also produce peptide hormones leading to specific paraneoplastic phenomena. For example, hypercalcemia may occur, related to an excessive production of PTH-related peptide (PTHrP) in cases of malignant pheochromocytomas (similar to some other malignancies). Hypercalcemia may also occur as a result of the excessive production of PTH itself in cases of MEN 2A in which pheochromocytoma occurs in conjunction with primary hyperparathyroidism. Very rarely, the ectopic production of adrenocorticotropic hormone (ACTH) by pheochromocytoma may lead to “ectopic” Cushing syndrome. Rare cases have been described in which a pheochromocytoma produces vasoactive intestinal peptide (VIP) (causing severe diarrhea), growth hormone–releasing hormone (GHRH) (causing acromegaly), corticotropin-releasing hormone (CRH) (causing Cushing syndrome), insulin (causing hypoglycemia), or other peptide hormones.

An increase in metabolic rate may cause weight loss (or, in children, lack of weight gain), and impaired heat loss from peripheral vasoconstriction may cause a mild elevation of basal body temperature, heat intolerance, flushing, or increased sweating.

During paroxysms, patients may experience marked anxiety, and when episodes are prolonged or severe, there may be visual disturbances, paresthesias, or seizures. A feeling of fatigue or exhaustion usually follows these episodes. Some patients present with psychosis or confusion.

There may be abdominal discomfort resulting from a large adrenal or extra- adrenal mass. Remarkably, some patients with pheochromocytomas or paragangliomas are entirely asymptomatic.

Somewhat different clinical manifestations occur with predominantly epinephrine-releasing pheochromocytomas. Symptoms and signs include

hypotension, prominent tachycardia, widened pulse pressure, cardiac arrhythmias, and noncardiogenic pulmonary edema. Acute hemorrhagic necrosis of the tumor may present initially as acute abdominal pain with marked hypertension, followed by hypotension, shock, and sudden death as a consequence of the sudden cessation of catecholamine production (“fulminant pheochromocytoma crisis”). Death may also result from cardiovascular collapse secondary to prolonged vasoconstriction and loss of blood volume into the interstitium.

Patients with pure epinephrine-producing pheochromocytomas may be hypotensive because of epinephrine-induced peripheral vasodilation. Other patients with severe arterial vasoconstriction may appear to be in shock. In still others, the prolonged vasoconstriction of a hypertensive crisis may lead to shock.

Diagnosis Pheochromocytoma and paraganglioma are diagnosed by demonstrating abnormally high concentrations of catecholamines or their breakdown products in the plasma or urine. Increases in plasma metanephrine and normetanephrine concentrations are greater and more consistent than increases in plasma catecholamines. Pheochromocytoma tumor cells produce large amounts of metanephrines from catecholamines leaking into the cytoplasm from vesicular stores and metabolized by catechol-O-methyltransferase (COMT) present in pheochromocytoma cells. Thus, the elevated plasma levels of free metanephrine and normetanephrine in patients with pheochromocytoma are probably due mostly to metabolism before, and not after, the release of catecholamines into the circulation. These metabolites are particularly useful for detecting pheochromocytomas. There is a long list of medications and drugs that can lead to false positive tests, and patients, especially those with indeterminate levels of metanephrines and catecholamines, should be taken off these if possible and retested before the diagnosis of a pheochromocytoma or a paraganglioma is made.

In addition to these catecholamine metabolites, plasma levels of chromogranin A (a substance found in chromaffin granules) are significantly higher in patients with pheochromocytomas, especially those with malignant tumors. For malignant pheochromocytomas, serum chromogranin A levels can also be monitored during chemotherapy of malignant pheochromocytomas to gauge tumor response and to detect relapse. However, chromogranin A is a nonspecific marker for a pheochromocytoma and a paraganglioma and several

medications, including proton pump inhibitors, can increase chromogranin A levels.

Administration of the antihypertensive agent clonidine can be used to differentiate essential hypertension from hypertension caused by pheochromocytoma or paraganglioma. This is especially useful in cases in which plasma or urine metanephrine/catecholamine levels are indeterminate. Clonidine is a potent α2 agonist that stimulates α2 receptors in the brain, reducing sympathetic outflow and blood pressure. A dose of 0.3 mg of clonidine is given orally, and blood pressure and plasma catecholamine levels are determined periodically over the next 3 hours. Essential hypertension depends partly on centrally mediated catecholamine release. Normally, administering clonidine suppresses sympathetic nervous system activity and substantially lowers plasma norepinephrine levels, reducing blood pressure. However, in patients with a pheochromocytoma or a paraganglioma, the drug has little or no effect on plasma catecholamine levels because the tumor behaves autonomously. Thus, blood pressure remains unchanged.

Once a diagnosis of a pheochromocytoma or paraganglioma is made, the next step is to localize the neoplasm or neoplasms radiographically to permit surgical removal. Computed tomography (CT) or magnetic resonance imaging (MRI) can be used in tumor localization. CT and MRI have good sensitivity but poor specificity for detecting pheochromocytomas. Nuclear imaging studies such as iodine-123–metaiodobenzylguanidine (123I-MIBG) scintigraphy have limited sensitivity but better specificity in diagnosis. For example, the specificity of 123I- MIBG scintigraphy is very good for confirming that a tumor is a pheochromocytoma or a paraganglioma and for ruling out metastases, but not all tumors are MIBG avid, limiting the test’s sensitivity. In addition, 6-[fluorine-18]- fluorodopamine positron emission tomography (PET) can aid in both tumor diagnosis and localization in patients with positive biochemical test results. Some pheochromocytomas and paragangliomas also express somatostatin receptors and can be imaged with an octreotide scan or a Gallium-68- DOTATATE-PET scan, which uses radiolabeled somatostatin receptor agonists.

Treatment Surgical resection is the main treatment for a pheochromocytoma and a paraganglioma. Nevertheless, any surgery in patients with pheochromocytoma or paraganglioma, including resection of the tumor itself, involves the risk of significant complications, including stroke and death. Operative and

postoperative complications are directly associated with preoperative systolic blood pressure, tumor size, excretion of urinary catecholamines and their metabolites, duration of anesthesia, and number of previous surgeries. Patients with a pheochromocytoma or paraganglioma must be treated with the proper adrenergic blockade with medication prior to any surgical or interventional procedure. Understanding the pathophysiology of the hormonal secretion is critically important in preparing the patient for surgery. For example, as noted previously, it is important that hypertension not be treated with β blockers, which could cause a paradoxical worsening of hypertension by allowing unopposed α stimulation. Instead, an α receptor blocker, such as phenoxybenzamine, a noncompetitive antagonist, should be used as treatment of catecholamine- induced hypertension. Other medications that can be used impair catecholamine production, such as methyltyrosine, which blocks the enzyme tyrosine hydroxylase.

CHECKPOINT

7. What genetic mutations are found in patients with pheochromocytoma? 8. What are the symptoms and signs of pheochromocytoma? 9. What are some complications of untreated pheochromocytoma? 10. What are the metabolic and neurologic effects of pheochromocytoma? 11. How is the diagnosis of pheochromocytoma made?

CASE STUDIES Yeong Kwok, MD

(See Chapter 25, p. 770 for answers)

CASE 66

A 39-year-old woman comes to the office complaining of episodic anxiety, headache, and palpitations. She states that without dieting, she has lost 15

pounds over the past 6 months. The physical examination is normal except for a blood pressure of 200/100 mm Hg and a resting pulse rate of 110 bpm. The chart review shows that prior blood pressures have always been normal, including one 6 months ago. A diagnosis of pheochromocytoma is entertained.

Questions

A. What other features of the history should be elicited? Why is family history important?

B. What laboratory tests should be ordered, and what results should be anticipated? If the laboratory tests are nondiagnostic but suspicion is high, what other test can be done?

C. What is the pathogenesis of the symptoms of anxiety, headache, palpitations, and weight loss in pheochromocytomas?

REFERENCES

General Fitzgerald PA. Adrenal medulla and paraganglia. In: Gardner DG et al, eds. Greenspan’s Basic & Clinical Endocrinology, 10th ed. McGraw-Hill, 2017.

Kwatampora L et al. Endocrine malignancies. In: Kantarjian HM et al, eds. The MD Anderson Manual of Medical Oncology, 3rd ed. McGraw-Hill, 2016.

Pheochromocytoma & Paraganglioma Barron J. Phaeochromocytoma: diagnostic challenges for biochemical screening and diagnosis. J Clin Pathol. 2010 Aug;63(8):669–74. [PMID: 20547690]

Chen Y et al. Accuracy of plasma free metanephrines in the diagnosis of pheochromocytoma and paraganglioma: a systematic review and meta-analysis. Endocr Pract. 2017 Oct;23(10):1178–92. [PMID: 28704094]

Eisenhofer G et al. Diagnostic tests and biomarkers for pheochromocytoma and extra-adrenal paraganglioma: from routine laboratory methods to disease stratification. Endocr Pathol. 2012 Mar;23(1):4–14. [PMID: 22180288]

Eisenhofer G et al. Supine or sitting? Economic and other considerations for the use of plasma metanephrines for diagnosis of pheochromocytoma. Clin Endocrinol. 2015 Mar; 82(3):463–4. [PMID: 25186957]

Else T. 15 years of paraganglioma: pheochromocytoma, paraganglioma and genetic syndromes: a historical perspective. Endocr Relat Cancer. 2015 Aug:22(4):T147–59. [PMID: 26273101]

Favier J et al. Paraganglioma and pheochromocytoma: from genetics to personalized medicine. Nat Rev Endocrinol. 2015 Feb;11(2):101–11. [PMID: 25385035]

Fishbein L. Pheochromocytoma and paraganglioma: genetics, diagnosis and treatment. Hematol Oncol Clin North Am. 2016 Feb;30(1):135–50. [PMID: 26614373]

Gagnon N et al. Takotsubo-like cardiomyopathy in a large cohort of patients with pheochromocytoma and paraganglioma. Endocr Pract. 2017 Oct;23(10):1178–92. [PMID: 28704094]

Galetta F et al. Cardiovascular complications in patients with pheochromocytoma: a mini-review. Biomed Pharmacother. 2010 Sep; 64(7):505–9. [PMID: 20580187]

Gimm O et al. Malignant pheochromocytomas and paragangliomas: a diagnostic challenge. Langenbecks Arch Surg. 2012 Feb;397(2):155–77. [PMID: 22124609]

Harari A et al. Malignant pheochromocytoma: a review. Am J Surg. 2011 May;201(5):700–8. [PMID: 20870212]

Lenders J et al. Pheochromocytoma and paraganglioma: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2014 Jun;99(6):1915–42. [PMID: 24893135]

Prejbisz A et al. Cardiovascular manifestations of phaeochromocytoma. J Hypertens. 2011 Nov;29(11):2049–60. [PMID: 21826022]

Roman-Gonzalez A et al. Malignant pheochromocytoma-paraganglioma: pathogenesis and TNM staging and current clinical trials. Curr Opin Endocrinol Diabetes Obes. 2017 Jun:24(3):174–83. [PMID: 28234804]

Sarathi V. Characteristics of pediatric pheochromocytoma/paraganglioma. Indian J Endocrinol Metab. 2017 May-Jun;21(3):470–4. [PMID: 28553607]

Tischler AS et al. 15 years of paraganglioma: pathology of pheochromocytoma and paraganglioma. Endocr Relat Cancer. 2015 Aug;22(4):T123–33. [PMID: 26136457]

CHAPTER

13

Gastrointestinal Disease Jason C. Mills, MD, PhD, AGAF, & Matthew A. Ciorba, MD

Gastrointestinal (GI) diseases most often present with one or more of four common classes of symptoms and signs: (1) abdominal or chest pain; (2) altered ingestion of food (eg, resulting from nausea, vomiting, dysphagia [difficulty swallowing], odynophagia [painful swallowing], or anorexia [lack of appetite]); (3) altered bowel movements (ie, diarrhea or constipation); and (4) GI tract bleeding, either occurring without warning or preceded by one or more of the foregoing (Table 13–1). However, not all cases of a particular GI disease present in the same way. For example, peptic ulcer disease, although typically accompanied by abdominal pain, may be painless.

TABLE 13–1 Common presentations of GI disease.

GI disease may be limited to the GI tract (eg, reflux esophagitis, peptic ulcer, diverticular disease), be a manifestation of a systemic disorder (eg, inflammatory bowel disease), or present as a systemic disease resulting from a primary GI pathologic process (eg, vitamin deficiencies resulting from malabsorption). Because different parts of the GI tract are specialized for certain functions, the most prominent causes, consequences, and manifestations of disease differ from one anatomic site to another.

Acutely, GI disease can be complicated by dehydration, sepsis, or bleeding or by their consequences, such as shock. Dehydration can occur as a consequence of even subtle alterations in fluid input or outflow because the volume of fluid traversing the GI tract daily is enormous (see later discussion). Sepsis can result from disruption of the barrier function against pathogens in the environment, including bacteria resident in the colon. The tendency for bleeding is a reflection

of the tremendous vascularity of the GI tract and the difficulty of applying pressure at the site of bleeding.

Chronically, GI disease can be complicated by malnutrition and deficiency states. These occur because many primary GI diseases result in malabsorption (failure to absorb one or more necessary nutrients in ingested food).

GI tract disease can present as partial or complete obstruction (blockage of movement of contents down the GI tract) caused by adhesions and stenosis resulting from the proliferation of connective tissue in response to inflammation. The symptoms and signs of obstruction can range from mild nausea, abdominal pain, and anorexia to projectile vomiting and rebound tenderness. In severe cases, obstruction can result in perforation, infarction and bleeding, hypotension, shock, sepsis, and death. The severity of symptoms depends on the extent of obstruction, the degree to which the obstruction compromises blood flow to the affected region, and the stage in the natural history of the process at which the patient presents for medical attention.

CHECKPOINT

1. What are the cardinal symptoms and signs of GI disease? 2. What are some acute systemic complications of primary GI disease? 3. What additional systemic manifestations can occur as a result of chronic

GI disease?

STRUCTURE, FUNCTION & CONTROL OF THE GI TRACT

STRUCTURE OF THE GI TRACT

The GI tract is one of the most complex and important organ systems. It comprises the alimentary canal, a hollow structure extending from the mouth to the anus, and associated glandular organs that empty their contents into the canal (Figure 13–1). The alimentary canal is 7–9 m in the adult and includes the mouth, esophagus (23–25 cm), stomach, small intestine (duodenum, jejunum,

ileum; 6–7 m), large intestine (cecum and colon; 1.0–1.5 m), rectum, and anus. The canal is connected to the salivary glands, the pancreas, and the gallbladder, the sources of exocrine secretions that play an essential role in digestion.

FIGURE 13–1 Progress of food along the alimentary canal. Food undergoes both mechanical and

chemical changes to render it suitable for absorption and assimilation. (Redrawn, with permission, from Mackenna BR et al, eds. Illustrated Physiology, 6th ed. Churchill Livingstone, 1997. Copyright © Elsevier.)

The wall of the GI tract is composed of four main layers. From the lumen outward, these include the mucosa, submucosa, muscularis externa, and serosa (Figure 13–2). The precise structure of some of these layers, most notably the mucosa, varies from one region of the GI tract to the next. The mucosa has three components: specialized epithelial cells that line the lumen; the underlying lamina propria, a layer of connective tissue that contains small blood and lymphatic vessels, immune cells, and nerve fibers; and the muscularis mucosa, a thin layer of muscle cells. The muscularis mucosa is an important boundary in determining whether cancer of the GI tract is still localized to its site of origin or is likely to have metastasized (ie, spread to distant regions of the body). The submucosa is a layer of loose connective tissue directly beneath the mucosa containing larger blood and lymphatic vessels and a nerve plexus of the intrinsic or enteric nervous system, termed the submucosal nerve (Meissner) plexus. This nerve plexus is particularly important for secretion control in the GI tract. In some areas, the submucosa also contains glands and lymphoid tissue. The muscularis externa is composed of an inner circular and an outer longitudinal layer of smooth muscle and is responsible for the motility of the GI tract. Between these muscle layers lies the myenteric nerve (Auerbach) plexus, a division of the enteric nervous system that regulates motility. The serosa is an outer sheath of squamous mesothelial cells and connective tissues, where larger nerves and blood vessels travel in a bed of connective and adipose tissue.

FIGURE 13–2 Schematic illustration of the structure of a portion of the digestive tract with various possible components. (Redrawn, with permission, from Bevelander G. Outline of Histology, 7th ed. Mosby, 1971. Copyright © Elsevier.)

FUNCTIONS OF THE GI TRACT

The overall function of the GI tract is to take in nutrients and process them into a form that can be used by the body and to eliminate wastes. The major physiological processes that occur in the GI tract are digestion, secretion, motility, and absorption.

A. Digestion Food is taken into the mouth as large particles containing macromolecules that are not immediately absorbable into the body. Digestion is the process that converts nutrients in food to products that can be absorbed by cells of the mucosa. Digestion includes physical processes (eg, chewing, GI contractions) that break up the food, mix it with digestive secretions, and propel it along the alimentary canal, and chemical processes (involving digestive enzymes) that degrade food components (proteins, fats, polysaccharides) to products that can be absorbed (amino acids, fatty acids, monosaccharides). Digestive enzymes

arise from exocrine glands (salivary gland, pancreas, gallbladder, and liver) and from cells and glands in the mucosa or are found on the apical surface of certain epithelial cells.

B. Secretion During the process of digestion, large volumes of fluid are secreted into the lumen of the GI tract. Secretions arise from exocrine glands (salivary glands, pancreas, gallbladder) and from epithelial cells lining the GI lumen (or glands that connect to the lumen). The daily fluid load in the GI tract is approximately 2 L of oral intake and 7 L of secretions (1.5 L saliva, 2.5 L gastric juice, 0.5 L bile, 1.5 L pancreatic juice, and 1 L intestinal secretions). From this total of 9 L, approximately 100 mL ends up in stool daily; the balance is recycled (Figure 13– 3).

FIGURE 13–3 Approximate flow rates per day and ionic constituents of fluid passing through different levels of the intestine. (Redrawn, with permission, from Schiller et al. Diarrhea. In: Feldman M et al, eds. Sleisenger and Fordtran’s Gastrointestinal and Liver Disease, 10th ed. Elsevier, 2016.)

C. Motility Secretions and luminal contents are moved from mouth to anus and mixed by a process termed motility because of the coordinated contractions of smooth muscle. Smooth muscle cells have a resting membrane potential (small excess of negative charge) in their interior as a result of the activity of pumps in the plasma membrane. When a cell is depolarized, this potential difference is

transiently abolished, generating a signal that (1) causes contraction of actin and myosin filaments and (2) is propagated to neighboring cells, resulting in the coordinated response of muscle contraction. Depolarization of a cell can occur spontaneously or in response to a neural or hormonal stimulus depending on the specific characteristics of different cells. GI smooth muscle displays differences in contractile properties in different regions of the tract. “Slow-wave” oscillating depolarizations occur in some areas, and rapid “spike” depolarizations occur in other areas. Each type occurs with a characteristic intrinsic frequency, but each can also be triggered by specific stimuli such as stretch, neuronal input, or hormones. Short bursts of spikes cause phasic motor activity; longer bursts cause tonic muscle contraction. Tonic contraction occurs at sphincters (“gates” that allow further movement down the GI tract only during relaxation). Phasic electrical activity occurs at the intervening regions of the GI tract (between sphincters).

D. Absorption The products of digestion (amino acids, small peptides, monosaccharides, fatty acids) are taken into the body by the process of absorption. Absorbed molecules can pass across (transcellular route) or between (paracellular route) the epithelial cells lining the intestine to enter the blood or lymphatic systems. In general, this transport can occur by either a passive, energy-independent mechanism that occurs down an electrochemical gradient (of charge or concentration) or by an active, energy-requiring process that occurs against an electrochemical gradient. Passive transport can occur by simple diffusion (random molecular motion) of uncharged molecules that readily pass the lipid layer plasma membrane. In this manner, short-chain fatty acids are absorbed in the small intestine. Charged molecules that cannot cross the plasma membrane diffuse through specialized channels (transmembrane proteins) within the apical and basolateral membrane of epithelial cells. For instance, water is absorbed by diffusion through aquaporins (proteins that form water channels) in the small intestine. Some molecules that are absorbed by diffusion bind to transporter proteins in the plasma membrane that facilitate their transfer into the cell (facilitated diffusion). For example, fructose is absorbed into epithelial cells of the small intestine by facilitated diffusion through the apical membrane GLUT-5 transporter.

Active transport requires metabolic energy. There are two classes of active transport. In primary active transport, the transport molecule itself hydrolyzes adenosine triphosphate (ATP). An example of primary active transport is the

Na+-K+ ATPase found in the basolateral membrane of intestinal epithelial cells, which expels three Na+ ions from cells in exchange for two K+ ions that are pumped into the cell. This unequal transport of ions generates a transmembrane potential (negative inside; ie, transport is electrogenic). In secondary active transport, the transporter itself does not hydrolyze ATP, but transport depends on an electrochemical gradient that has been established by primary active transport. The Na+-K+ ATPase maintains a low intracellular Na+ concentration and an inside negative potential in epithelial cells, thereby providing the electrochemical gradient for the secondary active transport of many absorbed molecules. For example, glucose is absorbed against a concentration gradient across the apical membrane of epithelial cells in the small intestine by secondary active transport with Na+ ions by the SGLT1 transporter. Two Na+ ions are transported down their electrochemical gradient (generated by the Na+-K+ ATPase), dragging with them one glucose molecule. For large molecules such as proteins, transport occurs by pinching off from, and fusion of membrane vesicles with, the plasma membrane. These processes are termed endocytosis (uptake into epithelial cells) and exocytosis (export out of epithelial cells).

In addition to the major roles of the GI tract related to digestion and absorption, the digestive tract has other functions essential for the maintenance of health and homeostasis.

E. Defense The mucosa of the GI tract is the largest surface of the body exposed to the environment, and the gut, like the skin, must protect the body from the external environment. Defense involves protection against ingested toxins, bacteria, and viruses, as well as the bacteria and toxins that normally exist in the large intestine (Table 13–2). The magnitude of the problem is illustrated by the observation that there are more bacterial cells in the human colon than cells in the entire body. Defense involves two mechanisms:

TABLE 13–2 Mechanisms of defense of the GI tract (and features of structure and function involved).

1. Adaptive immune defense—The mucosal immune system or gut- associated lymphoid tissue (GALT) surveys the contents of the intestinal lumen through a variety of mechanisms that use cells of both myeloid and lymphoid lineages. Myeloid-derived cells (specific populations of dendritic cells and macrophages) extend processes that interact with the intestinal epithelial barrier to sense the luminal environment. Aggregates of lymphoid cells include Peyer patches (larger aggregates in the distal small intestine) and isolated lymphoid follicles located throughout the intestine. These lymphoid aggregates also play important roles in immune surveillance (Figure 13–4). GALT protects against pathogenic bacteria, viruses, and toxins and enables tolerance of potentially immunogenic dietary substances and bacteria.

FIGURE 13–4 Systemic and local features of gut immunology. (Redrawn, with permission, from Dotan I et al. Mucosal immunology and inflammation. In: Feldman M et al, eds. Sleisenger and Fordtran’s Gastrointestinal and Liver Disease, 10th ed. Elsevier, 2016. Copyright © Elsevier.)

2. Innate immune defense—These mechanisms include the secretion of fluid (eg, abundant acid secreted by the stomach), electrolytes, and mucus, as well as the tight junctions between epithelial cells. The secretions neutralize and flush away potentially damaging bacteria and macromolecules, and the tight junctions of the gut epithelium prevent their ingress into tissues.

In the intestine, mucus is secreted by specialized goblet cells. The mucus forms a protective layer over the epithelial cells. A group of antimicrobial peptides are secreted into the intestinal lumen. The specialized cells in the small intestine that perform this function are the Paneth cells that produce and secrete lysozyme and alpha-defensins that contribute to defense and healing. Alpha- defensins have broad-spectrum activity and are thought to create holes in bacterial cell walls and prevent them from colonizing the small intestine. Trefoil peptides are secreted into the lumen of the GI tract with mucus. Among their many effects, they promote the healing of mucosal lesions.

F. Regulation of Fluid & Electrolyte Balance The small intestine receives 8–9 L of fluid with electrolytes per day and secretes a further 1 L with electrolytes per day. Most of the fluid is absorbed. Thus, secretion and absorption must be regulated to maintain balance. Increased secretion or diminished absorption causes diarrhea, which can be fatal because of fluid and electrolyte loss.

G. Excretion Undigested food products, bacteria, and certain heavy metals (eg, copper and iron excreted in bile) are excreted in feces.

CHECKPOINT

4. What are the major functions of the GI tract? 5. Describe the four major layers of a cross-section through the GI tract. 6. What volumes of fluid are transferred into and out of the GI tract each

day? 7. Describe the general mechanism of electrolyte trans-port across

epithelial cells.

8. Describe how the GI tract defends itself.

REGULATION MECHANISMS OF THE GI TRACT

The processes of motility, secretion, digestion, and absorption are under close physiologic regulation by nerves, hormones, and paracrine substance (Figure 13–5).

FIGURE 13–5 Neural, endocrine, and paracrine mechanisms of control in the GI tract.

A. Neural Control There are two components of GI innervation:

1. Intrinsic innervation by the enteric nervous system—The enteric nervous system is the third division of the autonomic nervous system (Figure 13–6). An enteric neuron has its cell body within the wall of the GI tract and is thus intrinsic to the gut. The enteric nervous system comprises a series of ganglionated nerve plexuses that extend from the esophagus to the rectum and are organized into two principal components: (1) the myenteric, or Auerbach, plexus, which is sandwiched between the layers of the muscularis externa; and (2) the submucosal or Meissner plexus, which lies in the submucosa. The enteric nervous system is extensive, containing as many neurons as are present in the spinal cord. It contains sensory or afferent neurons (sometimes called intrinsic primary afferent neurons [IPANs]) that sense the environment (eg, intestinal pH, osmolality, wall stretch), interneurons (the connectors), and secretomotor or efferent neurons that control many cell types to stimulate or inhibit the motility, secretion, absorption, and immune functions of the GI tract. In this manner, the enteric nervous system can regulate the GI tract in a reflex manner without input from the central nervous system (CNS). For this reason, it is often called the “little brain.” Enteric neurons use many neurotransmitters, most notably neuropeptides.

FIGURE 13–6 The enteric nervous system. Left: Enteric nervous system of the small intestine. Enteric neurons are organized in two nerve plexuses, the submucosal plexus and myenteric plexus. Additional plexuses, including the deep muscular, periglandular, and villous plexuses are discussed in the text. (Redrawn with permission from Brookes SJH et al. Functional histoanatomy of the enteric nervous system. In: Johnson LR, ed. Physiology of the Gastrointestinal Tract, 4th ed. Elsevier, 2006.) Right: The enteric nervous system includes sensory neurons, interneurons, and motor neurons. Complete reflex arcs exist within the enteric nervous system.

2. The degree to which the CNS regulates the enteric nervous system varies by region. The characteristic functions of structures derived from the

embryonic foregut (eg, esophageal peristalsis, relaxation of the lower esophageal sphincter, gastric accommodation and peristalsis, pyloric sphincter function) depend more on CNS control. However, functions of structures derived from the embryonic midgut and hindgut (eg, intestinal peristalsis, mucosal secretion) can continue without input from the CNS.

3. The clinical importance of the enteric nervous system is seen in clinical syndromes in which its function is lost; this can occur at several levels. In esophageal achalasia, for example, as a result of enteric nervous system defects, the body of the esophagus is quiet and the lower sphincter is tonically contracted, making ingestion of food difficult or impossible. More distally, failure of the enteric nerves to migrate into the colon during development (as in Hirschsprung disease) or the loss of enteric nervous system function (as in pseudo-obstruction of the small bowel) has severe clinical consequences, including abdominal pain, distension, and a risk of catastrophic intestinal perforation.

4. Extrinsic innervation by parasympathetic and sympathetic nerves —Extrinsic neurons that innervate the GI tract have cell bodies outside the gut wall and allow a bidirectional communication between the brain and the gut (the brain–gut axis) (Figure 13–7). This communication can regulate the function of the enteric nervous system or directly control the activity of other cell types.

FIGURE 13–7 The extrinsic innervation of the GI tract by the parasympathetic and sympathetic nerves. Preganglionic parasympathetic nerves from the medulla and the sacral spinal cord project fibers in the vagal and pelvic nerves, respectively, to the wall of the GI tract and innervate enteric neurons that serve as postganglionic parasympathetic nerves. Preganglionic sympathetic nerves project fibers from the thoracolumbar regions of the spinal cord to the prevertebral ganglia, where they innervate postganglionic sympathetic nerves that project to the GI tract. Both the parasympathetic and sympathetic preganglionic nerves release acetylcholine (ACh), which activates nicotinic receptors on postganglionic nerves. Postganglionic parasympathetic nerves release acetylcholine and peptides, whereas postganglionic sympathetic nerves release norepinephrine (NE).

In parasympathetic innervation, the vagus nerve (cranial nerve X) innervates the esophagus, stomach, gallbladder, pancreas, and the first part of intestine, cecum, and proximal colon. The pelvic nerve from the sacral spinal cord innervates the distal colon and the rectum. Preganglionic cell bodies in the medulla (vagus) or sacral spinal cord (pelvic nerve) project fibers to some enteric neurons in the gut wall, which are thus in a sense postganglionic parasympathetic nerves. The preganglionic nerves use acetylcholine as a neurotransmitter, which activates nicotinic receptors on enteric neurons. The postganglionic enteric nerves use acetylcholine (acting on muscarinic receptors) and neuropeptides as neurotransmitters. Parasympathetic stimulation can stimulate and inhibit GI functions.

In sympathetic innervation, preganglionic sympathetic nerves arise from cell bodies in the thoracic spinal cord and project fibers to prevertebral ganglia

(celiac, cranial, and caudal mesenteric ganglion). They release acetylcholine as a neurotransmitter that interacts with nicotinic receptors on the postganglionic nerves. Postganglionic fibers innervate some enteric neurons or directly innervate effector cells in the GI tract, such as vascular smooth muscle cells. Norepinephrine is the major postganglionic neurotransmitter. Sympathetic innervation is often inhibitory to GI functions.

Regarding extrinsic sensory nerves, parasympathetic and sympathetic nerve tracts also carry sensory fibers from the gut to cell bodies located in nodose ganglia and the dorsal root ganglia, respectively. Cell bodies in the nodose and dorsal root ganglia then project fibers to the brainstem (from nodose ganglia) or spinal cord (from dorsal route ganglia). Sensory nerve fibers in the wall of the GI tract detect mucosal pH and osmolality and can respond to amino acids or glucose, temperature, tension, and touch. In this manner, the extrinsic sensory nerves sense changes in the environment of the intestine and trigger central reflexes that initiate secretomotor changes to maintain normal homeostasis. Extrinsic sensory nerves also contribute to GI inflammation and pain. Sensory nerve endings in the wall of the gut detect noxious chemical and mechanical stimuli, including acid, inflammatory agents, and distension. These stimuli trigger the release of the neuropeptides, substance P, and calcitonin gene-related peptide from the endings of sensory nerves within the gut wall, where they induce plasma protein extravasation, granulocyte infiltration, and arteriolar vasodilatation to cause neurogenic inflammation. The same stimuli induce the release of neuropeptides from the central projections of these neurons, where they participate in pain transmission. Additional research is required to define the mechanisms of neurogenic inflammation and GI pain.

B. Hormonal Control Hormones are blood-borne messengers released from endocrine cells or glands into the circulation, which carries them to distant target cells (see Figure 13–5). This mechanism of endocrine regulation was discovered in the GI tract in 1902, when Bayliss and Starling discovered the hormone secretin in the small intestine and showed that it stimulates secretion from the exocrine pancreas. Since then, a large number of hormones have been identified in all regions of the GI tract. In this respect, the GI tract is the largest endocrine organ.

GI hormones have several characteristics in common. They are secreted from endocrine cells scattered throughout the mucosa of the stomach and intestine, rather than being concentrated in specialized glands. GI hormones are invariably peptides, and many of these peptides are present not only in endocrine cells but

also in the nerves of the enteric system and CNS (Table 13–3). Thus, they have dual functions as hormones and neurotransmitters. After feeding, there are elevated levels of many GI hormones in the circulation. These hormones have multiple biological effects, ranging from the stimulation of gastric acid secretion to the suppression of appetite. The physiologic role of some GI hormones has been clearly established by demonstrating that antagonists of hormone receptors block certain physiologic processes. However, in many cases, such antagonists are not available, and the physiologic relevance of hormones that cannot be antagonized remains to be determined.

TABLE 13–3 Secretory products of the GI tract.

C. Paracrine Control

Many substances used for intercellular signaling are rapidly removed from the extracellular fluid by uptake into nearby cells or by enzymatic degradation. Such substances have a short half-life in the extracellular fluid and are consequently capable of regulating only neighboring cells. Paracrine substances are released from non-neuronal sensory cells and neurons and regulate the function of neighboring cells, rather than influencing distant organs by passage through the circulation (see Figure 13–5 and Table 13–3). Examples include histamine and somatostatin, which are released from cells in the stomach to control acid secretion, and serotonin (5-hydroxytryptamine [5-HT]), which is released in the small intestine to control vagus nerve activity.

CHECKPOINT

9. What are the three general mechanisms of control observed in the GI tract?

10. What are the two components of the enteric nervous system? 11. What are the three general types of enteric neuron? 12. Describe the parasympathetic and sympathetic inner-vation of the GI

tract. 13. What is the relationship between the enteric and central nervous

systems?

GI SMOOTH MUSCLE

A. Structure of GI Smooth Muscle The two principal muscle layers that control the motility of the GI tract are the inner circular layer and the outer longitudinal layer of the muscularis externa. They vary in thickness in different regions of the GI tract. For example, the muscles are thickened in the gastric antrum, where strong contractions break up food before it enters the small intestine, and muscle layers are thickened to form sphincters. Most of the GI muscle is smooth muscle, except the pharynx, parts of the esophagus, and the external anal sphincter, which are made up of striated (skeletal) muscle. GI smooth muscle is similar to smooth muscle in other organs: Fusiform cells are packed together in bundles by connective tissue sheaths. Gap

junctions between cells allow signals to readily pass from cell to cell so that the contraction of bundles occurs synchronously. Interstitial cells of Cajal form an extensive network of stellate cells in the muscle layers of the stomach and intestine that are intimately associated with smooth muscle cells and enteric neurons (Figure 13–8). They may have two functions. First, they transmit information from enteric neurons to the smooth muscle cells. Second, they are the pacemaker cells, which have the capacity to generate the basic electrical rhythm or slow waves that are a consistent feature of GI smooth muscle. Animals lacking interstitial cells of Cajal show markedly abnormal GI motility, including defective gastric emptying and intestinal stasis or ileus. Defects in interstitial cells of Cajal may be associated with motility disturbances in patients, and this is an area of active investigation.

FIGURE 13–8 Diagrammatic view of interstitial cells of Cajal (ICC) in the intestine, showing their interaction with enteric nerves and smooth muscle cells.

B. Electrophysiology of GI Smooth Muscle GI smooth muscle cells have a resting membrane potential of –40 to –80 mV as a result of the relative conductances of K+, Na+, and Cl– ions. An electrogenic Na+-K+ ATPase contributes significantly to the resting membrane potential. Less is known about the electrophysiological properties of interstitial cells of Cajal, in part because of difficulties in isolating these cells for study. The resting membrane potential of smooth muscle cells varies characteristically with time

and is called a slow wave or basic electrical rhythm. Slow waves occur at a rate of 3–5/min in the stomach and at a rate of 12–20/min in the intestine. Interstitial cells of Cajal set the frequency of the slow waves, and slow waves are transmitted between cells through gap junctions. Nerves and hormones modulate the amplitude of slow waves. Depending on the amplitude of the slow waves and the excitability of the smooth muscle, slow waves can give rise to action potentials. If the slow-wave depolarization reaches a threshold, a train of action potentials will fire. Action potentials depolarize the membrane of the smooth muscle cells and induce an influx of Ca2+ ions into the cytoplasm through voltage-sensitive Ca2+ channels in the plasma membrane and from intracellular stores, causing contraction. What causes an action potential to occur? The presence of neurotransmitters or hormones released close to the smooth muscle cells alters the resting membrane potentials of the cells, which makes the oscillations in membrane potential (the slow waves) more or less likely to reach threshold and initiate an action potential. However, because inhibitory motor neurons of the GI tract are highly active and can prevent the generation of action potentials, not all slow waves result in active contractions. Action potentials and contractions can only occur when these inhibitory motor neurons are switched off by input from interneurons. Thus, the tonic inhibition serves to control the inherent excitability of the pacemaker cells.

C. Mechanical Properties of GI Smooth Muscle Several characteristic patterns of contraction can be observed in GI smooth muscle. Tonic contractions are best represented by sphincters that act as one- way valves to prevent the retrograde movement of material from distal to more proximal regions and thus to facilitate flow in an aboral direction. The proximal parts of the stomach and the gallbladder also exhibit tonic contractions. Peristaltic contractions are moving waves of contraction that propel digesta along the GI tract. Peristalsis involves the neurally mediated contraction of smooth muscle on the oral side of a bolus of digesta and a neurally mediated relaxation of muscle on the anal side of the digesta. Peristalsis occurs in the pharynx, esophagus, gastric antrum, and small and large intestine. Segmental contractions produce narrow contracted segments between relaxed segments. These movements allow mixing of the luminal contents with GI tract secretions and increase exposure to mucosal surfaces where absorption occurs. Segmentation occurs in the stomach and intestine. Pathologic patterns of motility include spasms, which are very strong and often painful contractions that occur continuously in a dysregulated manner, and ileus, in which contractile

activity is markedly decreased or absent. Ileus often results from irritation of the peritoneum that may occur with surgery, peritonitis, and pancreatitis. Further research is required to understand the mechanisms of these abnormal contractions, which may lead to improved therapies.

CHECKPOINT

14. What are the positive and negative regulators of smooth muscle cell action potentials?

15. What are the functions of interstitial cells of Cajal? 16. What are the general types of contractions observed in the GI tract after

feeding?

OROPHARYNX & ESOPHAGUS

Anatomy & Histology The oropharynx provides entry to the GI tract during swallowing and to the respiratory tract during breathing. It includes the vocal cords, which separate the two tracts and provide the structural basis for speech. Much of the oropharynx is lined with a respiratory-type ciliated pseudocolumnar epithelium.

The esophagus is a hollow tube (25–30 cm long, 2–3 cm wide). The wall of the esophagus consists of a stratified squamous epithelial cell layer, an inner layer of circular muscle, a myenteric nerve plexus, and an outer layer of longitudinal muscle. The first third of the esophagus is composed of striated muscle, the middle third is mixed striated and smooth muscle, and the lower third is purely smooth muscle. The esophagus is delimited by an upper esophageal sphincter (a distinct thickening of striated circular muscle) and a lower esophageal sphincter (a tonically contracted 3–4 cm ring of smooth muscle). The two sphincters generate small luminal zones of high pressure, whereas the rest of the esophageal lumen is at a pressure equal to that of the surrounding body cavities. Between swallows, the two sphincters are closed, preventing entry of air and gastric acid into the esophagus. Regulation of the lower esophageal sphincter is especially important because it controls the passage of digesta into the stomach and prevents the reflux of gastric contents

into the esophagus, where they can damage the mucosa. Between swallows, the lower esophageal sphincter is contracted, in large part by vagal cholinergic mechanisms. During swallowing, vagal inhibitory fibers allow the lower esophageal sphincter to relax, possibly because of a release of inhibitory neurotransmitters from enteric nerves, including nitric oxide and vasoactive intestinal peptide (VIP).

Swallowing Reflex Swallowing begins as a voluntary process that rapidly becomes an involuntary reflex mechanism. During the voluntary oral phase, the tongue pushes a bolus of food to the back of the mouth and into the oropharynx. From there on, the process is involuntary. In the pharyngeal phase, the food bolus stimulates touch receptors in the pharynx. Sensory signals pass by the glossopharyngeal, vagal, and trigeminal nerves to the swallowing center in the medulla and pons. Motor impulses pass through cranial nerves to control an involuntary process that directs food into the esophagus and away from the airway. Breathing is interrupted and the soft palate is elevated, closing the pharyngeal opening of the nasopharynx and preventing food from entering the internal openings of the nostrils. The tongue is pressed against the hard palate, closing the oral opening of the pharynx. The glottis is pulled under the epiglottis, which blocks the laryngeal opening. Cartilages around the larynx are pulled together, further restricting food from entering the respiratory tract. When all openings to the pharynx are closed, a wave of muscular contraction pushes the bolus of food toward the opening of the esophagus. As the food reaches the esophagus, the upper esophageal sphincter relaxes to accept the material and then closes after the bolus has moved through. The esophageal phase of swallowing begins when the bolus passes through the upper esophageal sphincter. Vagal stretch receptors in the wall of the esophagus detect distension by the bolus and induce a vagovagal reflex, during which vagal motor nerves induce a wave of contraction that spreads along the esophagus at 3–5 cm/s. This is termed primary peristalsis (Figure 13–9). As the wave of primary peristalsis reaches the lower esophageal sphincter, the sphincter relaxes to allow the bolus to enter the stomach. Distension of the esophagus by the bolus can initiate another wave of contraction called secondary peristalsis. Often, repetitive waves of secondary peristalsis are required to clear the esophagus of food. Various hormones and neurotransmitters, foods, and drugs can affect the tone of the lower esophageal sphincter pressure.

FIGURE 13–9 Primary peristalsis of the esophagus. The tracings show pressures in the indicated regions of the esophagus at rest and at various times after swallowing. (LES, lower esophageal sphincter; UES, upper esophageal sphincter.) (Redrawn from data in Conklin JL et al. Motor functions of the pharynx and esophagus. In: Johnson LR, ed. Physiology of the Gastrointestinal Tract, 4th ed. Elsevier, 2006.)

The importance of oropharyngeal motility and its control is seen in patients who have had strokes or have dementia. The inability to swallow properly often makes them unable to manage their own oral secretions, resulting in the aspiration of oral contents into the lungs with the development of pneumonia. This is a common cause of death in individuals with these kinds of CNS disorders. Disordered lower esophageal sphincter tone can cause gastroesophageal reflux disease (GERD), presenting as heartburn and potentially

increasing the risk for adenocarcinoma of the esophagus.

CHECKPOINT

17. What is the histologic difference between the proximal one-third and the distal two-thirds of the esophagus?

18. What are the functions of the upper and lower esopha-geal sphincters, and how are they regulated?

19. Describe the three phases of the swallowing reflex.

STOMACH

Anatomy & Histology The stomach is a complex glandular organ guarded by two sphincters: the lower esophageal sphincter and the pyloric sphincter (Figure 13–10). The mucosa is formed by a single layer of epithelial cells that line the lumen of the stomach and descend into funnel-shaped invaginations (ie, broad at the top near the surface and then narrow as they descend deeper). The broad upper portion is called the pit zone. The middle portion, where the invaginations narrow into glands, is the neck, and the deepest zone is the base. The stomach can be divided anatomically into several regions on the basis of structure and function. The cardia is a small region of variable size just distal to the lower esophageal sphincter where the gastric glands are almost entirely composed of mucus-secreting cells. The corpus, or body, is the major part of the stomach and includes the fundus, which is the portion of the body superior to the insertion of the esophagus. Gastric glands in the corpus contain parietal cells (mostly in the neck zone), which secrete hydrochloric acid and intrinsic factor, and chief cells (mostly in the base zone), which secrete the digestive enzyme precursor pepsinogen. The corpus is the principal site of gastric digestion. The pyloric antrum is the distal region of the stomach that secretes the hormone gastrin from G cells. Glands of the antrum, like those of the cardia, secrete mostly mucus. The antrum, a highly muscular portion of the stomach, grinds food and regulates gastric emptying.

FIGURE 13–10 Anatomy and histology of the stomach. (Redrawn, with permission, from Boron WF et al, eds. Medical Physiology, 2nd ed. Saunders Elsevier, 2009.)

Gastric Acid Secretion A number of products are secreted from the stomach. Of these, hydrochloric acid (HCl) is perhaps the most important from a pathophysiologic standpoint. Secretion of acid by the parietal cells of the gastric glands occurs in a basal diurnal pattern but can be stimulated by such diverse factors as the thought of food, distension of the stomach, and protein ingestion. Gastric secretions were the subject of some of the first GI experimentalists. The Italian Lazzaro Spallanzani fed and retrieved from animal stomachs tubes filled with various substances to examine digestion. And Pavlov received his Nobel prize for studying digestive organ secretion long before his work was appreciated for its psychological implications.

A. Molecular Mechanisms of HCl Secretion

The mechanisms by which parietal cells secrete HCl into the stomach have been intensively studied because of the importance of acid secretion to digestion and in disease states. Parietal cells are roughly pyramidal in shape. Their membranes express a H+-K+ ATPase, a primary active transporter responsible for the secretion of HCl. Parietal cells undergo a remarkable change in appearance when stimulated to secrete HCl (Figure 13–11). In the unstimulated state, they harbor an intracellular tubulovesicular network studded with H+-K+ ATPase molecules. On activation, the tubulovesicular membranes fuse with the plasma membrane to form a canalicular membrane with microvilli. The result is an increase in the area of the apical membrane by 50- to 100-fold that allows a markedly increased secretion of HCl by the H+-K+ ATPase pumps directly into the glandular lumen, which in turn squirts the acid into the lumen of the stomach.

FIGURE 13–11 Acid secretion by parietal cells. Top: Upon stimulation, the tubulovesicular network in the parietal cell fuses to form an extensive canalicular membrane with microvilli, which increases the surface area. Bottom: The mechanisms of HCl secretion by parietal cells, stimulated by histamine, acetylcholine, and gastrin, are demonstrated. For abbreviations, see the legend for Figure 13–12.

FIGURE 13–12 Regulation of gastric acid secretion by nerves and hormones. During the cephalic phase of digestion, vagal cholinergic nerves directly stimulate parietal cells and induce the release of histamine from enterochromaffin-like (ECL) cells, which also stimulate parietal cells. Vagal fibers also release gastrin-releasing peptide (GRP) in the antrum to induce gastrin secretion, which is carried in the bloodstream to induce the release of histamine and stimulate parietal cells. During the gastric phase of digestion, food in the stomach triggers vagovagal reflexes and stimulates gastrin secretion. Acidification of the gastric antrum stimulates the release of somatostatin, which inhibits gastrin release and thus acid

secretion; vagal acetylcholine (Ach) inhibits somatostatin release. (CCKB-R, cholecystokinin B receptor; G, gastrin; GRP, gastrin-releasing peptide; GRP-R, GRP receptor; H2-R, histamine 2 receptor; M3-R, muscarinic 3 receptor; S-R, somatostatin receptor.)

The H+-K+ ATPase is a heterodimer of an α-subunit (the catalytically active unit) and a β-subunit (involved in fixing the intracellular location). The H+-K+ ATPase pumps H+ ions from the cell across the apical membrane in exchange for K+ ions (see Figure 13–11). This is an example of primary active transport driven by ATP, which pumps H+ ions against an enormous concentration gradient (1,000,000:1). Tight junctions between cells prevent the re-entry of H+ ions into the mucosa. The K+ ions that have entered the cells then recycle to the lumen or enter interstitial fluid by K+ channels. To maintain electroneutrality, Cl– ions are secreted passively across the apical membrane into the lumen through Cl– channels, forming HCl. The secreted H+ ions are provided by H2O and CO2, which form H2CO3. Carbonic anhydrase generates H

+ ions for secretion and HCO3

– ions, which enter the interstitial fluid by exchange for Cl– ions. Cl– ions enter against their electrochemical gradient, driven by the efflux of HCO3

– down an electrochemical gradient. The secretion of HCO3

– into the blood forms the “alkaline tide,” which can lead to alkalosis when H+ ion secretion is excessive. Water movement maintains the osmotic balance in all regions.

An understanding of the mechanisms of HCl secretion by parietal cells permitted the development of proton pump inhibitors (PPIs), a class of drugs that inhibit the H+-K+ ATPase. Drugs such as omeprazole, a benzimidazole, are inactive at neutral pH levels but, when acidified (in the stomach), bind to sulfhydryl groups of cysteine residues on the external surface of the H+-K+ ATPase, irreversibly inhibiting activity and blocking the hypersecretion of gastric acid. Other experimental drugs, termed acid pump antagonists, competitively interfere with K+ ion binding to block acid secretion. These drugs can be used to inhibit the hypersecretion of gastric acid that causes GERD.

B. Stimulants and Inhibitors of HCl Secretion

The three main stimulants of H+ ion secretion are acetylcholine, gastrin, and histamine, all of which stimulate HCl secretion and induce characteristic shape changes in the stimulated parietal cell. Acetylcholine is released from vagal postganglionic or enteric neurons during feeding. It binds to muscarinic M3-type muscarinic receptors on parietal cells to stimulate H+ ion secretion. Gastrin is a peptide hormone of 17 or 34 amino acids secreted from G cells in the gastric

antrum during feeding. Gastrin binds to cholecystokinin (CCK) type B receptors on parietal cells, which also stimulates H+ ion secretion.

Both acetylcholine and gastrin receptors activate the same signal transduction pathways: activation of phospholipase Cβ, leading to the generation of inositol trisphosphate, which mobilizes Ca2+ from intracellular stores, and diacylglycerol, which activates protein kinase C. Because both acetylcholine and gastrin act through similar intracellular pathways, the combined effects of gastrin and acetylcholine are additive.

Histamine is a paracrine substance secreted by enterochromaffin-like (ECL) and mast cells in the corpus mucosa during feeding. Histamine binds to H2 receptors on parietal cells to activate adenylyl cyclase and increase cAMP. The cAMP activates protein kinase A to stimulate H+ ion secretion. The combination of histamine and acetylcholine or gastrin can increase the rate of acid production by up to 10-fold over basal levels, a much greater effect than simple addition of the effects of the agonists would predict. This effect is known as potentiation. Potentiation requires that two different signal molecules bind to receptors that act through different intracellular mechanisms. Increased intracellular Ca2+ and cAMP activate K+ channels on the apical membrane of parietal cells, thereby promoting K+ ion efflux from the cell. This hyperpolarizes the cell (more negative inside) to promote Cl– ion secretion across the apical membrane. Ca2+ and cAMP also increase insertion of Cl– channels and H+-K+ ATPase into the apical membrane. The combined effects are to stimulate HCl secretion.

Gastrin also regulates growth of the gastric epithelium. Excess gastrin aberrantly produced by certain tumors causes the hyperproliferation of gastric glands and parietal cells and an excess secretion of gastric acid. The excess acid in the small intestine can lead to ulceration of the mucosa, steatorrhea as a result of the inactivation of pancreatic lipases (which are inhibited by low pH), and diarrhea. This condition is termed Zollinger–Ellison syndrome. The excessive administration of PPIs can result in prolonged high luminal pH, which stimulates the hypersecretion of gastrin, which can increase mucosal growth and, in particular, stimulate the hyperplasia (overproliferation) of histamine-secreting ECL cells. Termination of drug treatment then results in an acid production rebound because of the increased content of parietal cells and gastrin-secreting G cells.

In addition to the direct mechanisms by which acetylcholine, gastrin, and histamine stimulate HCl secretion from parietal cells, acetylcholine and gastrin also indirectly stimulate secretion by acting on enterochromaffin-like cells to

promote the release of histamine, which in turn stimulates parietal cells. The importance of histamine to H+ ion secretion is illustrated by studies with histamine H2 receptor antagonists, such as cimetidine. These drugs not only inhibit histamine-stimulated H+ ion secretion but also block the effects of acetylcholine and gastrin. By preventing such potentiation, these agents can be used to effectively treat the hypersecretion of gastric acid.

Somatostatin, a peptide of 14 or 28 amino acids, is an important inhibitor of gastric acid secretion. Somatostatin directly inhibits proton secretion by activating receptors on parietal cells, which couple to inhibit cAMP. Somatostatin also inhibits gastrin and histamine secretion, which indirectly inhibits proton secretion. Somatostatin is secreted by D cells in the gastric antrum and corpus. D cells in the gastric antrum have direct contact with the stomach lumen (open endocrine cells), allowing them to sense the luminal contents. Protons in the antrum stimulate somatostatin secretion, which acts as a paracrine agent to inhibit gastrin secretion from neighboring G cells and to thereby indirectly reduce gastric acid secretion. This is an example of negative- feedback regulation. D cells in the corpus do not have contact with the lumen (closed cells) and thus cannot sense luminal protons. Instead, multiple neurohumoral factors (eg, noradrenalin, CCK, VIP) increase the release of corpus somatostatin, which in turn inhibits acid production indirectly by decreasing histamine release from ECL cells and directly by inhibiting parietal cells. Vagal ACh and the TH1 cytokine interferon-γ inhibit somatostatin release and promote acid secretion. Recent studies indicate that the endocrine cells in the stomach sense luminal nutrient and acid via primary cilia on their apical surfaces.

C. Integrated Regulation of Gastric Acid Secretion

The secretion of gastric acid between meals is low. Three phases of acid secretion occur during feeding (Figure 13–12). The cephalic phase (~30% of response) of secretion, first illustrated by Pavlov’s elegant experiments, is initiated by the sight, smell, taste, and swallowing of food. These stimuli activate the dorsal motor nucleus of the vagal nerve in the medulla, resulting in activation/discharge of the vagus nerve and tributary parasympathetic motor nerves. Stimulation has several consequences. In the corpus, postganglionic nerves release acetylcholine, which directly activates parietal cells by M3 receptors. Acetylcholine also induces histamine release from enterochromaffin cells, which stimulates H+ ion secretion by parietal cells. In the antrum, vagal stimulation induces the release of gastrin-releasing peptide from

postganglionic fibers, which stimulates gastrin release and thus indirectly stimulates H+ ion secretion. Acetylcholine also inhibits somatostatin release from D cells in the corpus and pylorus to stimulate H+ ion secretion.

The gastric phase (~70% of response) of secretion is induced by stimuli within the stomach. Vagal sensory nerves detect gastric distension with food and trigger a vagovagal reflex during which vagal motor nerves release acetylcholine in the stomach to promote acid secretion. Partially digested proteins and amino acids stimulate gastrin release from G cells in the pylorus. G cells, like D cells, are open-type endocrine cells that directly sense the contents of the stomach. Gastrin then further stimulates acid secretion. Acidification of the pylorus stimulates the release of somatostatin, which inhibits acid secretion by a negative-feedback loop as described.

During the intestinal phase, the products of protein digestion, on entering the small intestine, can stimulate gastrin release from G cells in the duodenum. Many substances, most notably fat and acid, stimulate the secretion of hormones from the small intestine that inhibit gastric acid secretion. Examples include secretin and cholecystokinin.

Helicobacter pylori is a bacterium that lives in the mucous layer of the stomach where the enzyme urease is active, converting urea to CO2 and ammonia. Ammonia buffers luminal acid and protects the organism. H pylori also secretes proteins, such as CagA and VacA, that modulate immune responses and directly alter mucosal cell signaling pathways. More than half the world’s population is infected with H pylori. In most cases, the infection, though chronic, is mild and does not cause symptoms. In some individuals, however, the infection remains confined to the antrum but leads to increased acid secretion and symptomatic inflammation that causes ulceration of the stomach or duodenum. Almost all duodenal peptic (ie, acid-associated) ulcers and about half of gastric peptic ulcers have H pylori infection as a root cause; the remaining gastric ulcers are caused by medications (eg, aspirin, nonsteroidal anti- inflammatory drugs). In some patients, chronic H pylori infection can spread to the corpus and lead to chronic inflammation that triggers the death (atrophy) of parietal cells and altered mucosal differentiation patterns (metaplasias) that increase the risk of progression to gastric cancer. In certain geographical regions (eg, East Asia and parts of Central and South America), owing to environmental and/or lifestyle factors that have not been fully elucidated, the risk for progression to gastric cancer is much higher than in other regions (eg, United States, Canada).

Other Gastric Secretions Chief cells in the glands of the gastric corpus secrete pepsinogen, an inactive precursor (zymogen) of the active protease pepsin. Acetylcholine is the main stimulant of pepsinogen secretion, although other factors (eg, gastrin) also stimulate secretion. Once released into the lumen of the stomach, gastric acid and pre-existing pepsin convert pepsinogen to pepsin. Pepsin has a pH optimum of 3.0 and is thus active in the stomach. It is an endopeptidase that begins the degradation of dietary proteins into peptides. However, pepsin accounts for only 10% of the total protein digestion.

Mucins are high-molecular-weight glycoproteins secreted by mucous cells of gastric glands in the corpus and antrum. The peptide backbone of mucins is densely populated with carbohydrate side chains enriched with sulfate groups. Mucins combine with phospholipids, bicarbonate, and water to form the mucous gel layer that adheres to the surface of stomach epithelial cells. This layer acts as physical protection for epithelial cells from damage by contractile food grinding as well as noxious substances such as acid, pepsin, and bile acids. Acetylcholine and mucosal irritation stimulate mucin secretion.

Epithelial cells of the corpus and antrum secrete HCO3 – ions. Although the

secretion of HCO3 – is minor compared with H+ ion secretion, HCO3

– plays a major role in epithelial defense. HCO3

– ions are trapped in the mucous gel to form an “unstirred layer” in proximity to the epithelium, where the pH is 7.0 compared with 1.0–3.0 in the lumen. Acetylcholine and intraluminal acid stimulate HCO3

– secretion. Intrinsic factor is a glycoprotein secreted by parietal cells required for

vitamin B12 absorption. Vitamin B12 (cobalamin) is not made in mammalian cells, and the only source is the diet: meat, fish, and dairy products, but not vegetables or fruit. In the stomach, acid and pepsin release B12 from dietary carrier proteins. The acidic environment permits the binding of B12 to haptocorrin (R factor), a glycoprotein produced by salivary and gastric glands. The B12–haptocorrin complex enters the duodenum, where pancreatic proteases digest the haptocorrin. Free intrinsic factor also enters the duodenum. Intrinsic factor combines with B12 in the less acidic environment of the small intestine, forming a degradation-resistant complex for transport to the ileum. Specific receptors on epithelial cells lining the ileum bind the vitamin B12–intrinsic factor complex, which is taken into cells by endocytosis. The absorbed complex dissociates within the epithelial cells, and then vitamin B12 binds to

transcobalamin II, a protein required for exocytosis and transport to the liver. In autoimmune gastritis, parietal cells are destroyed, leading to a loss of intrinsic factor secretion, which can result in vitamin B12 deficiency and pernicious anemia. This anemia is caused by the impaired synthesis of purines and thymine for which vitamin B12 is required. The only reliable therapy is regular intramuscular injections of vitamin B12.

GASTRIC MOTILITY

A. Patterns of Gastric Motility In terms of motility, the proximal and distal regions of the stomach are distinct. The gastric corpus is a reservoir for gastric digestion. During each swallow, the stretch of the esophagus induces a vagovagal reflex that causes the gastric corpus to relax in preparation to receive the food, a phenomenon known as receptive relaxation. When food enters the stomach, it relaxes further to accommodate a meal of 1.5 L without any increase in pressure, a phenomenon called accommodation, which involves vagovagal and local enteric reflexes. Thus, the stomach is a reservoir for ingested food. The antrum of the stomach is highly muscular, and here contractions serve to break food into smaller pieces, thereby facilitating digestion. The pyloric sphincter controls the rate at which the antral contractions propel partially digested food, or chyme, into the duodenum. During fasting, the antrum is relatively quiescent, with occasional forceful contractions occurring every 75–90 min. These intense contractions, of 5–10 min in duration, are part of a general wave of contractions that sweep the entire length of the GI tract during fasting: the migrating myoelectric complex. Feeding disrupts the migrating myoelectric complex, and now the antrum contracts frequently at a rate of about three contractions per minute. These slow waves of peristaltic contraction originate from spontaneously active interstitial cells of Cajal in the pacemaker zone in the middle of the body of the stomach, and they sweep toward the antrum. When the membrane potential of muscle cells depolarizes to reach threshold, action potentials fire. Contractions occur during the plateau phase of the action potential. Gastrin and acetylcholine stimulate contraction by increasing the magnitude and duration of the action potentials.

B. Gastric Emptying Immediately after a meal, the stomach may contain up to 1 L of material, which empties slowly into the small intestine. Regulation of gastric emptying occurs by alterations in the motility of the proximal and distal stomach, pylorus, and duodenum. Gastric emptying is brought about by an increase in tone (intraluminal pressure) in the proximal stomach, an increase in the strength of antral contractions, the opening of the pylorus, and the inhibition of duodenal segmental contractions.

The rate of gastric emptying depends on the chemical and physical composition of chyme that enters the duodenum through the stimulation of both neural and hormonal pathways. Solids and liquids empty at different rates: Liquids empty rapidly, and solids empty only after a lag phase. Acid, fat, and hyperosmolar solutions entering the duodenum slow gastric emptying through the stimulation of neuronal and hormonal mechanisms. Sensory neurons in the duodenum, both vagal and spinal, respond to nutrients, H+ ions, and the hyperosmolar content of chyme. Vagal motor nerves decrease antral contractions, contract the pylorus, and decrease proximal gastric motility. This results in the intestinal feedback inhibition (slowing) of gastric emptying. The main vagal mediator that stimulates contraction is acetylcholine. VIP and nitric oxide are neuronal mediators that inhibit contraction. Many hormones released by endocrine cells in the small intestine have been implicated in the feedback inhibition of gastric emptying. Secretin, the release of which is stimulated by acid, inhibits antral contractions and stimulates pyloric sphincter contraction to slow emptying. Cholecystokinin, the release of which is stimulated by fat, acts on vagal sensory nerve receptors to produce a vagovagal reflex that decreases gastric emptying.

The importance of nervous system control over gastric motility is reflected in the high incidence of dumping syndrome (nausea, bloating, flushing, and explosive diarrhea) that occurs as a consequence of stomach dysmotility in some patients who have undergone surgical procedures such as partial gastrectomy or nonselective vagotomy.

CHECKPOINT

20. Describe the cell types found in the mucosa of the gas-tric corpus and antrum, and indicate the products of each cell type.

21. What are the roles of the proximal and distal stomach? 22. Describe the ionic basis of HCl secretion from the gastric parietal cells. 23. Name a neurotransmitter, hormone, and paracrine agent that stimulates

acid secretion from parietal cells. 24. Name a peptide that inhibits acid secretion from the parietal cells. 25. Describe the mechanisms of the cephalic, gastric, and intestinal phases

of gastric acid secretion. 26. Name two types of drug with distinct mechanisms of action that can be

used to treat the hypersecretion of gastric acid. 27. What is the role of the parietal cell in the absorption of vitamin B12? 28. Describe two processes by which the gastric mucosa is protected from

acid in the lumen. 29. What are the patterns of motility in the corpus and the antrum? 30. How does the composition of the digesta in the lumen of the small

intestine affect the rate of gastric emptying?

GALLBLADDER

Anatomy & Histology

The gallbladder is a muscular sac with a resting volume of about 50 mL that lies on the inferior surface of the liver. It is connected to the hepatic biliary system by the cystic duct, which leads to the common bile duct whose opening into the proximal duodenum is controlled by the sphincter of Oddi. The common bile duct and the pancreatic duct usually join just proximal to this sphincter.

Physiology A. Bile Secretion Bile, which is produced by the liver, flows down the hepatic duct and into the gallbladder through the cystic duct. It is stored there until gallbladder contraction is stimulated to expel the contents of the gallbladder back through the cystic duct into the common bile duct and through the sphincter of Oddi into the duodenum. Stimuli for gallbladder contraction and sphincter of Oddi relaxation necessary for proper bile flow include both hormones and neural inputs. Fat in the intestine stimulates secretion of the hormone CCK from I-cells. CCK causes the gallbladder to contract and the sphincter of Oddi to relax. Depending on how

long it remains in the gallbladder, bile becomes concentrated. Bile composition is further modified by mucin production under the control of prostaglandins and by the saturation of bile cholesterol controlled in part by estrogens. The most prominent disorders of the gallbladder involve gallstone formation (see later discussion).

SMALL INTESTINE

Anatomy & Histology Three regions can be distinguished along the approximately 6–7 m length of the small intestine. The pyloric sphincter marks the beginning of the duodenum, which is largely retroperitoneal, fixed in its location, and 20–25 cm in length. Because of this sphincter, stomach contents normally enter the duodenum in small spurts containing tiny suspended particles. In the duodenum, gastric contents are mixed with the secretions of the common bile duct and pancreatic duct. Beyond the duodenum, the small intestine is mobile and suspended in the peritoneal cavity by a mesentery. The proximal two-fifths is called the jejunum. The distal three-fifths is called the ileum, which ends at the ileocecal valve at the start of the large intestine.

The most striking gross structural features of the small intestine are the numerous villi (projections of the mucosa into the lumen of the intestine that measure approximately 1 mm in height) (Figure 13–13). Each villus contains a single terminal branch of the arterial, venous, and lymphatic trees. Villi increase the absorptive capacity 5-fold and allow efficient transfer to the circulatory system of substances absorbed from the gut lumen by enterocytes (surface epithelial cells). By electron microscopy, each enterocyte contains 3000–5000 microvilli, plasma membrane evaginations on the apical side of the cell that further increase the absorptive surface area by 200-fold. Many digestive enzymes expressed by intestinal epithelial cells are located at the tips of these microvilli. As a group, these densely packed microvilli make up a “brush border” facing the intestinal lumen.

FIGURE 13–13 Anatomy and histology of the small and large intestine. (Redrawn, with permission, from Boron WF et al, eds. Medical Physiology, 2nd ed. Saunders Elsevier, 2009. Copyright © Elsevier.)

Invaginations of the intestinal epithelium into the wall, called the crypts of Lieberkühn, surround the villi. These structures are the location of epithelial intestinal stem cells and their proliferative daughters that together constantly

produce new differentiated epithelial cells that form the epithelial lining of the intestine. Each small intestinal crypt contains tetrapotential stem cells at or near the crypt base that produces the four mature epithelial cell types: absorptive enterocytes, mucus-secreting goblet cells, hormone-secreting enteroendocrine cells, and antimicrobial peptides and growth factor–secreting Paneth cells. Enterocytes, goblet cells, and enteroendocrine cells migrate out of crypts and onto adjacent villi. These cells then die by apoptosis at the tips of villi and are extruded into the lumen of the intestine; the average life span of these cells is about 4–6 days. On the other hand, Paneth cells are much longer lived (~60 days), and they migrate to the crypt base where they are in close contact with epithelial stem cells.

It has been increasingly appreciated that the external and internal surfaces of the human body are inhabited by commensal microbes that are not mere passengers but actually perform critical functions. In the adult small intestine, a large and diverse population of commensal microbes inhabits the lumen. Most of these microbes are bacteria, and the major phyla represented are Bacteroides and Firmicutes. Most of these microbes are anaerobes (able to live in the absence of oxygen). The density increases dramatically in the lumen of the intestine (from hundreds per milliliter in the duodenum to trillions per milliliter in the colon). These bacteria function to aid in the digestion of complex carbohydrates. Based on studies in germ-free organisms, it is estimated that these intestinal microbiota increase our ability to extract nutrients from food by as much as 30%. The microbiota also play key roles in training the mucosal immune system and developing blood vessels in the intestine. The microbial populations most closely associated with the mucosa appear to be quite distinct from those associated with the lumen and include many members of the Lachnospiraceae and Ruminococcaceae families. Functioning cooperatively, these microbes form a critical barrier to pathogens. Furthermore, bacteria are not the only commensal microorganisms in the intestine: Archaea (single-cell microorganisms without nuclei), fungi, and viruses are normally present in the lumen of the intestine. In most cases, their roles are still undefined.

Digestion & Absorption in the Small Intestine The small intestine is the main site of digestion and the principal site of nutrient absorption. Thus, it is appropriate to review all steps of digestion in the GI tract and then to consider the mechanisms by which nutrients are absorbed.

A. Carbohydrates

Carbohydrates, which are mainly present in the diet as polysaccharides and disaccharides, must be digested to monosaccharides for absorption. Intestinal microbes (in particular, Bacteroides spp) contain a large repertoire of glycoside hydrolases that aid in the breakdown of complex plant polysaccharides. This is an important beneficial function of commensal intestinal microbes. Alpha- amylases in salivary and pancreatic secretions cleave interior α-1,4 glucose linkages in large polymers of starch to form fragments (disaccharides, trisaccharides, and oligosaccharides). Oligosaccharidases and disaccharidases in the brush border of enterocytes digest small fragments to monosaccharides, glucose, galactose, and fructose. Glucose and galactose, along with two Na+ ions, are absorbed across the apical membrane of enterocytes by the same transporter, SGLT1. The passive uptake of water also occurs, maintaining osmolality on both sides of the cell membrane. The extrusion of Na+ out of the basolateral membrane by Na+-K+ ATPase provides an electrochemical Na+ gradient that drives the absorption of glucose and galactose against their concentration gradients. Fructose is absorbed into the cell by facilitated diffusion through the apical membrane by a different transporter, GLUT-5. All three hexoses leave the cell by facilitated diffusion through a common transporter, GLUT-2, located in the basolateral membrane.

Lactose intolerance is the most common problem of carbohydrate digestion. It results mainly from the reduction of lactase activity in adults. Lactase is expressed normally at high levels in the jejunum of neonatal and infant humans. In many parts of the world, lactase levels are gradually reduced after weaning. However, lactase levels do not decrease significantly in populations in which milk products are an important part of the adult diet. Lactase activity is rate limiting for lactose digestion in most adults throughout other regions of the world. If lactase is deficient, nondigested lactose is not absorbed. The nonabsorbed lactose retains water in the lumen to maintain the osmolality of chyme equivalent to that of plasma. This fluid retention causes abdominal pain (cramps), nausea, and diarrhea. Bacterial fermentation of lactose in the distal small intestine and colon further exacerbates these symptoms.

Mutations of the gene encoding SGLT1 impair glucose and galactose absorption in some patients. Affected individuals develop diarrhea when they consume sugars normally absorbed by SGLT1, because of defects in the absorption of Na+, monosaccharides, and water. In contrast, fructose, which is absorbed by GLUT-5, does not cause diarrhea.

B. Proteins

Proteins entering the intestine derive from the diet and also from cells shed from the mucosa. Protein digestion begins in the stomach by the action of pepsin, but most protein digestion occurs in the lumen of the duodenum and the jejunum by the action of pancreatic proteases (trypsin, chymotrypsin, carboxypeptidases), yielding small oligopeptides and free amino acids. Peptidases on the surfaces of intestinal epithelial cells are required for the digestion of larger oligopeptides to yield smaller peptides and additional amino acids. Dipeptides and tripeptides are absorbed into enterocytes by secondary active cotransport with H+ ions by the oligopeptide cotransporter, PepT1. The H+ ions in the lumen are provided by a Na+-K+ transporter in the apical membrane. Amino acid uptake from the lumen occurs through several different transporters. Each transporter is specific for various side chain groups: acidic, basic, neutral, and imino. The uptake of most amino acids into enterocytes is coupled to cotransport with Na+ ions driven by the Na+-K+ ATPase in the basolateral membrane. Absorbed dipeptides and tripeptides are hydrolyzed to amino acids within the enterocytes by independent cytosolic peptidases. Amino acids exit the cell through the basolateral membrane by cation-independent amino acid transporters. Infants can absorb proteins by endocytosis, providing a mechanism for the transfer of immunoglobulins, and thus passive immunity, from mother to child.

C. Lipids

Triglycerides constitute about 90% of dietary lipid; cholesterol, phospholipids, sphingolipids, fatty acids, and fat-soluble vitamins make up the balance. Dietary lipids are first emulsified by mechanical digestion (chewing, antral contractions, segmentation), which produces fine droplets suspended in aqueous fluid. Lipid digestion begins in the stomach by the combined action of swallowed lingual lipase from salivary glands and gastric lipase secreted by gastric gland chief cells in the fundus. These lipases convert triglycerides to fatty acids and diglycerides. Most lipid digestion occurs in the duodenum and jejunum. Lipids in the lumen form micelles as a result of the emulsifying properties of bile salts, phospholipids, and mixing contractions of the stomach and intestine. The most important enzyme in lipid digestion is pancreatic lipase. Lipase is secreted as an active enzyme, but full activity requires an alkaline pH and binding to a cofactor called colipase. Procolipase is also secreted in pancreatic juice and is converted to colipase by trypsin in the intestinal lumen. Lipase is only active at the oil– water interface of the triglyceride droplets. Colipase promotes the binding of lipase to the surface of micelles and thereby facilitates digestion. Lipase cleaves the fatty acid ester linkages at the 1 and 3 positions of the glycerol backbone of

triglycerides to yield free fatty acids and a 2-monoglyceride. The major barrier to lipid absorption is an unstirred layer on the surface of

the enterocytes that is not readily mixed with the bulk fluid in the intestinal lumen because of the highly convoluted surface of the epithelium. The short- and medium-chain fatty acids that are water soluble and the long-chain fatty acids, monoglycerides, lysophospholipids, and cholesterol in the micelles diffuse through the unstirred layer to the surface of the enterocytes. Proton secretion creates an acidic microenvironment at the surface of enterocytes and promotes the protonation of fatty acids. Protonated (uncharged) fatty acids, monoglycerides, lysophospholipids, and cholesterol leave the micelles. Being protonated and thus lipid soluble, they readily diffuse into the cell. Fatty acids of less than 10 carbon atoms in length can pass through cells and enter the blood directly. The uptake of long-chain fatty acids (and some phospholipids) appears to be mediated by a specialized fatty acid transporter protein: microvillous membrane fatty acid–binding protein. Within the enterocyte, long-chain fatty acids bind to fatty acid–binding proteins that transport the newly absorbed long- chain fatty acids to the smooth endoplasmic reticulum for reassembly into triglycerides with absorbed 2-monoglycerides. The triglycerides, cholesterol esters, and phospholipids are combined with specific proteins in the Golgi apparatus of enterocytes and assembled into chylomicrons, which are exported from the basolateral membrane of the cell. They enter the lymphatic system through the large intraendothelial channels and subsequently are delivered to the bloodstream. During a relatively brief circulation, they are partially lipolyzed by cell-surface lipases and acquire more protein components. The liver is the main destination for chylomicron remnants. Note that chylomicrons serve as the primary transporters of fat-soluble vitamins in the circulation.

D. Fluid and Electrolytes

The small intestine is the major site of water absorption. Water moves into and out of the lumen of the intestine to keep its contents iso-osmotic with plasma. Water transport in either direction is thus passive, being secondary and proportional to the movement of ions (especially Na+ and Cl– ions) and nutrients. In the small intestine, water absorption is greatest in mature epithelial cells at villous tips. Water secretion is greatest in immature cells at villous crypts. Most passage of water (and ions) occurs by transcellular transport through aquaporins, a family of water channels. There is also some paracellular transport of water and ions. Epithelial cells lining the GI tract are interconnected by tight junctions. Junctions are somewhat leaky, allowing some water and small

ions to move between the lumen and the mucosa via paracellular transport. The resistance of tight junctions is an important determinant of the relative degree that transcellular transport occurs, and this resistance varies throughout the intestines. Tight junctions are most leaky in the duodenum and jejunum, becoming progressively less leaky (tighter) in the ileum and colon. Larger ions and organic solutes are more restricted in their movement across tight junctions.

The jejunum is the main site of absorption of Na+ ions. Na+ absorption is mainly transcellular, either by cotransport with nutrients (sugars, amino acids) or by Na+–K+ exchange. There is also parallel Na+ and Cl– absorption by a paracellular route. HCO3

– ions are secreted in the proximal duodenum, but in the jejunum, HCO3

– and Cl– ions are absorbed in large amounts. In the ileum, HCO3

– is secreted and Cl– is absorbed. K+ ion absorption from the lumen of the small intestine occurs mainly by passive paracellular transport. The Na+-coupled glucose transporter (SGLT1) in the apical membrane of the small intestine takes up two Na+ ions with each glucose molecule. This property is central to the development of effective therapeutic oral rehydration solutions that contain glucose, Na+, Cl–, and HCO3

– to enhance water and electrolyte uptake during severe diarrhea (eg, cholera).

The absorption of electrolytes and water is regulated by hormones and neurotransmitters. For example, angiotensin II and aldosterone, which are generated and released during dehydration, promote the absorption of NaCl in the intestine.

Secretion in the Small Intestine The cells of the crypts of Lieberkühn are important sites of electrolyte and water secretion. The Na+-K+ ATPase in the basolateral membrane of epithelial cells provides the electrochemical gradients for secondary active transport and the diffusion of other ions. An Na-K-2Cl– transporter in the basolateral membrane mediates the uptake of Na+, Cl–, and K+ ions into the cell (Figure 13–14). This is an example of secondary active transport: With the entry of Na+ ions, an electrochemical gradient drives the uptake of K+ and Cl– ions against electrochemical gradients. Excess K+ ions leave the cell by basolateral K+ channels that can be regulated by Ca2+ and cAMP. Cl– ions diffuse across the apical membrane of the enterocytes and into the intestinal lumen through a Cl– channel that is regulated by cAMP. This electrogenic secretion of Cl– ions provides a small negative charge to the lumen relative to the interstitial fluid,

which drives the secretion of Na+ ions by a paracellular route. Water follows by transcellular and paracellular routes to maintain iso-osmolality with plasma. The net result is thus the secretion of NaCl and water.

FIGURE 13–14 Mechanisms of fluid and electrolyte secretion by epithelial cells of the intestinal crypts. Top: Ionic basis of secretion of Cl– and Na+ ions. Bottom: Regulation of fluid and electrolyte secretion by submucosal neurons and mast cells of the lamina propria. Activated mast cells release histamine, which either directly acts on epithelial cells or acts on submucosal neurons to stimulate the release of acetylcholine, which then acts on epithelial cells.

Fluid and electrolyte secretion flushes bacterial products and toxins away from the surface of the epithelium and thus plays a role in mucosal defense.

Numerous substances, termed secretagogues, stimulate fluid and electrolyte secretion in both health and diseases (see Figure 13–14). Neurotransmitter secretagogues from the submucosal plexus include VIP and acetylcholine. Paracrine secretagogues include bradykinin, serotonin, histamine, and prostaglandins. Some products from immune cells indirectly stimulate secretion by acting on submucosal neurons to induce the release of acetylcholine or VIP, which then acts on enterocytes to stimulate secretion. Luminal secretagogues include bacterial toxins. A toxin from cholera modifies G proteins and thereby permanently activates adenylyl cyclase and increases intracellular levels of cAMP. Strong activation of the apical Cl– channels of crypt cells results in a massive secretion of Cl– ions and, in consequence, of water and Na+ ions. Patients with cholera may excrete up to 20 L of diarrhea per day, leading to rapid dehydration and death. An inexpensive and effective treatment is oral rehydration with glucose-containing solutions. The glucose drives the sodium– glucose cotransporter to transport both molecules into enterocytes, and with them chloride and water, thereby offsetting the fluid efflux mediated by the bacterial toxin. Because these cotransporters are lacking in the colon, its maximum absorptive capacity (5 L/d) is considerably less than that of the small intestine (12 L/d).

One type of Cl– ion channel in the apical membrane is encoded by the gene for cystic fibrosis and is termed the cystic fibrosis conductance regulator (CFTR). The CFTR is expressed in many epithelial cells throughout the body. Mutations in the channel result in improper folding and premature degradation of the channel protein. The secretion of Cl– ions and, in consequence, of Na+ ions and water, is diminished. In the airway, this results in the production of thick secretions that impair ventilation.

Motility of the Small Intestine A. Electrical Activity of Small Intestinal Muscle In the human duodenum, slow waves occur at a frequency of 11–13/min. The slow-wave frequency declines to the ileum. The slow waves may or may not be associated with action potentials. In the intestine, slow waves alone do not cause contractions. However, when action potentials fire, they give rise to strong but highly localized contractions, the magnitude of which depends on the frequency of the action potentials. The slow waves are entirely intrinsic: They are generated within the intestine and probably depend on the unstable membrane potentials of the interstitial cells of Cajal. The frequency with which action

potentials fire depends on the excitability of the muscle cells, which is influenced by circulating hormones, extrinsic nerves, and the enteric nervous system.

B. Mechanical Activity of Small Intestinal Muscle

During periods of fasting, the intestine is quiescent. However, every 90–120 min, there are bursts of action potentials in the muscle that induce waves of contraction lasting about 5 min. These migrating myoelectric complexes take 90 min to traverse the small intestine. By the time the migrating myoelectric complex reaches the ileum, another begins in the stomach. These waves of contraction clear the small intestine of its contents, acting as a “housekeeper” to keep the lumen relatively clean, thereby minimizing bacterial overgrowth (Figure 13–15). The migrating myoelectric complex is associated with cycling levels of motilin, a 22–amino acid peptide hormone secreted by endocrine cells in the duodenum. Motilin may act on the enteric nervous system to regulate the migrating myoelectric complex. Its release appears to be under neural control, although luminal contents can also stimulate motilin release. The effect of motilin is to stimulate the contraction of gastric and intestinal smooth muscle during the interdigestive period between meals.

FIGURE 13–15 Mechanical activity of the small intestine during fasting and after feeding. The recordings are of intraluminal pressures measured at indicated regions of the intestine of a conscious dog. The migrating myoelectric complexes in the fasting state are disrupted by feeding, which induces segmentation and peristaltic contractions. (Redrawn, with permission, from Boron WF et al, eds. Medical Physiology, 2nd ed. Saunders Elsevier, 2009. Copyright © Elsevier.)

During feeding, the migrating myoelectric complexes cease, probably because of the action of the vagus and gut hormones such as gastrin and cholecystokinin (see Figure 13–15). The migrating myoelectric complexes are replaced by phasic contractions that are brief (a few seconds at each site) and restricted to short lengths of intestine (a few centimeters). Phasic contractions serve both to mix and propel food through the small intestine. Rhythmic segmented contractions provide the major local mixing activity in the small intestine. In this process, a short segment contracts while adjacent segments are relaxed. Then, the contracted segment relaxes while previously relaxed adjacent segments contract. As these contractions alternate, chyme is forced in both

directions, mixed with cell secretions, and brought into contact with cells lining the lumen. Short waves of peristalsis propel chyme distally, mixing chyme in successive segments and propelling it through the intestine.

C. Peristaltic Reflex

Localized chemical or mechanical stimulation of the small intestine results in a contraction on the oral side of the stimulus and relaxation on the anal side. These responses are controlled by the enteric nervous system. Sensory neurons that respond to chemicals (eg, acids) or mechanical stimuli (stroking the mucosa or stretching the muscle with a bolus of digesta) activate excitatory ascending interneurons, which then innervate excitatory motor neurons (Figure 13–16). These neurons release excitatory neurotransmitters, acetylcholine, and the neuropeptide substance P, which activates receptors on circular muscle cells to trigger contraction. The sensory neurons also excite descending interneurons that innervate inhibitory motor neurons. They, in turn, release inhibitory neurotransmitters, VIP, and nitric oxide, which relax circular muscle.

FIGURE 13–16 The peristaltic reflex of the small intestine. Enteric sensory nerves detect chemical or mechanical stimulation of the mucosa or stretch of the muscle layer. Signals are transmitted in an oral or anal direction by interneurons. Excitatory motor nerves release acetylcholine (ACh) and substance P (SP), which cause muscle contraction on the oral side of the stimulus. Inhibitory motor nerves release vasoactive intestinal peptide (VIP) and nitric oxide (NO), which cause muscle relaxation on the anal side of the stimulus.

Opioid drugs such as morphine, which are highly effective for the relief of chronic pain (eg, cancer pain), have the detrimental side effect of inhibiting the motility of the small intestine. Opioids act on enteric nerves to inhibit the secretion of excitatory neurotransmitters to thereby inhibit peristalsis. The inhibition of motility slows down intestinal transit, allowing for more complete absorption, so the volume entering the colon is diminished and constipation results.

CHECKPOINT

31. Describe the hormonal reflex by which fat in the intes-tine stimulates the secretion of bile.

32. Describe the mechanism by which glucose is absorbed across the apical and basolateral membranes of an enterocyte.

33. What is the mechanism of absorption of tripeptides across an intestinal epithelial cell?

34. What is the role of bile in lipid absorption in the intestine? 35. List three general mechanisms of Na+ ion absorption in the small

intestine. 36. Describe the mechanism of fluid and electrolyte secre-tion in the crypts

of Lieberkühn. 37. Name two neurotransmitters that are secretagogues. 38. How do certain bacterial toxins stimulate fluid and elec-trolyte secretion

in the crypts of Lieberkühn? 39. Describe the pattern of intestinal motility during fasting and after

feeding. 40. Name one hormone that maintains the fasting pat-tern of motility and

one that induces the fed pattern of motility in the small intestine. 41. Name the neurotransmitters that mediate the ascend-ing and descending

limbs of the peristaltic reflex.

COLON

Anatomy & Histology The adult colon is 1.0–1.5 m in length. Its various segments (cecum; ascending, transverse, descending, and sigmoid colon; and rectum) are involved in absorbing water and electrolytes; secreting mucus; and forming, propelling, and storing unabsorbed material (feces). The colon is also the home of the majority of the intestinal microbes.

The surface of the colon consists of a columnar epithelium with no villi and few folds except in the distal rectum (see Figure 13–13). The epithelial cells include absorptive cells and contain microvilli on their surface as well as mucus- secreting goblet cells. Colonic crypts contain goblet cells, endocrine cells, absorptive cells, and epithelial stem cells. As in the small intestine, the stem cells and their daughter progenitor cells serve to replenish the differentiated cells of the epithelium that continually turn over throughout life.

Digestion & Absorption in the Colon Digestion in the colon occurs as a consequence of the action of the colonic microbiota. Short-chain fatty acids released by microbial action on dietary fiber are an important source of energy for the colon. More importantly, these short- chain fatty acids promote survival of healthy colonic epithelium while inducing apoptosis (programmed cell death) in epithelial cells that are progressing toward malignant transformation.

Fluid and electrolyte absorption has been well studied and is a major function of the colon. Up to 5 L of water can be absorbed per day across the colonic epithelium. Furthermore, the colonic epithelium can also take up sodium against a considerable concentration gradient. Aldosterone, a hormone involved in fluid and electrolyte homeostasis, increases colonic sodium conductance in response to volume depletion, thus playing an important role in maintaining fluid and electrolyte balance.

Secretion in the Colon The major secretory products of the colon are the mucin proteins produced and secreted by goblet cells that reside in the epithelial layer. Mucins are very-high- molecular-weight proteins (mostly due to the extensive glycosylation). In the lumen of the colon, they are hydrated and form a layer overlying the epithelial

cells. They serve to lubricate and prevent the opposing sides of the intestinal tube from sticking together (thus collapsing the tube). In addition, mucins participate in innate immunity. As antimicrobial peptides bound by immunoglobulins that are secreted into the lumen, mucins form a barrier to intestinal microbes and pathogens.

Motility of the Colon Unlike the stomach and small intestine, the colon is rarely inactive, although its activity is less easily characterized than that of the stomach, which has the pattern known as receptive relaxation, or than that of the small intestine, which displays the pattern known as the migrating motor complex and segmental to- and-fro action. Some patterns are discernible, however, such as the gastrocolic reflex (colonic mass peristalsis after a meal). Disorders of colonic motility are common complications of autonomic neuropathy in patients with diabetes mellitus and can cause severe GI complaints. Stool continence requires contraction of the puborectalis muscle and the anal sphincter. Defecation involves relaxation of the puborectalis by the sacral parasympathetic nerves, resulting in a straightening of the anorectal angle. Rectal distension results in reflex sympathetic-mediated internal and external sphincter relaxation.

CHECKPOINT

42. How does colonic motility differ from small intestine motility? 43. What is the major secretory product of the colon? 44. What volume of water is the colon capable of absorbing per day?

OVERVIEW OF GI DISORDERS

DISORDERS OF MOTILITY

Disorders of motility affect all major regions of the GI tract. Because GI tract motility is a complex result of smooth muscle contraction under neural and hormonal control, abnormal motility of the GI tract can occur through damage to GI smooth muscle or to the neural and hormonal mechanisms by which it is

controlled, or both. An example of muscle damage leading to abnormal motility is seen in esophageal stricture as a result of caustic ingestion or acid reflux. Abnormal neural control of motility is seen in esophageal achalasia. Esophageal motility disorders are typically characterized by dysphagia and odynophagia. An example of a neural defect that affects motility is Hirschsprung disease. These patients are typically younger than 2 years and either present after birth with the inability to pass meconium or later with severe constipation. The structural defect is a lack of myenteric neurons in the distal colon owing to a congenital defect whereby the migration of neural crest precursor cells does not occur properly.

Motility disorders of the stomach include gastroparesis as a complication of diabetes mellitus, and dysmotility as a consequence of stomach surgery, from either resection of part of the stomach or vagotomy. Vagotomy entails the surgical transection of the vagus nerve trunks, which prevents vagus-stimulated acid secretion and regulation of gastric motility. Before the availability of histamine H2 receptor antagonists and PPIs, selective vagotomy of the stomach was used as a treatment for the hypersecretion of gastric acid. Vagotomy is still sometimes performed as a treatment for Zollinger–Ellison syndrome (ie, acid hypersecretion and severe peptic ulcer disease caused by a gastrin-secreting tumor). In hypertrophic pyloric stenosis, food cannot pass freely out of the stomach owing to spasmodic narrowing of the pyloric outlet caused by hypertrophy of the pyloric musculature. It is more common in boys and presents shortly after birth with nonbilious vomiting. It is readily managed surgically.

The symptoms and signs of motility disorders in the stomach depend on their cause. Because vagotomy cuts fibers influencing the enteric nervous system as well as the intended fibers that influence acid secretion, a classic complication of vagotomy is disordered gastric motility. This may present clinically as either partial outlet obstruction or as too-rapid emptying of gastric contents into the duodenum, with resulting fluid shifts and vasomotor symptoms (“dumping syndrome”). Sometimes, however, patients may develop symptoms of stomach distension, nausea, early satiety, and vomiting suggestive of partial gastric outlet obstruction. To ameliorate the latter symptoms, pyloroplasty (severing the fibers of the pyloric sphincter) is done to render the sphincter less competent, so that food can pass more easily into the duodenum. Intrinsic neuropathy (eg, in diabetes mellitus) results in delayed gastric emptying, nausea, vomiting, and constipation rather than the classic dumping syndrome. The pathophysiologic basis for these differences is unknown.

In the small intestine and colon, disordered motility occurs in irritable bowel

syndrome. Irritable bowel syndrome is characterized by recurrent episodes of abdominal pain, bloating, and diarrhea alternating with constipation in the absence of detectable organic disease and structural abnormalities. The cause of this condition is still unclear.

DISORDERS OF SECRETION

Clinically recognized disorders of secretion involve the production of acid, intrinsic factor, or mucus by the stomach; digestive enzymes and bicarbonate by the pancreas; bile by the liver; and water and electrolytes by the small intestine in response to secretagogues.

Either elevated gastric acid secretion or diminished mucosal defense can predispose to the development of peptic ulcers. Ulcers represent discrete regions of erosion through the entire mucosa. Acid-induced damage may occur in the form of an ulcer either in the stomach (gastric ulcer) or in the first part of the small intestine (duodenal ulcer). Acid-induced injury may also occur in the form of more diffuse and less clearly demarcated inflammation anywhere along the GI tract from the lower esophagus through the duodenum. It appears that elevated acid secretion, almost always in the setting of H pylori infection, is relatively more important in the development of duodenal ulcer, whereas diminished mucosal defense (eg, from diminished mucus secretion in some cases) is a more crucial factor in the development of gastric ulcer, with H pylori underlying only about half the cases. Disorders of secretion involving the liver and pancreas are discussed in Chapters 14 and 15, respectively. Diarrhea, the major secretory disorder of the small intestine, is discussed later.

DISORDERS OF DIGESTION & ABSORPTION

Physiologically significant digestion and absorption can occur throughout the GI tract. Indeed, the effectiveness of sublingual nitroglycerin therapy for patients with angina is a testimonial to the efficacy of sublingual absorption. Nevertheless, the clinically prominent disorders of digestion and absorption focus on the small intestine and colon and the accessory organs (pancreas and liver) whose secretions (digestive enzymes, bicarbonate, and bile) are necessary for digestion and absorption in the small intestine.

GI MANIFESTATIONS OF SYSTEMIC DISEASE

A wide range of systemic conditions and diseases may produce symptoms and signs in the GI tract. These include endocrine disorders that alter control of GI tract functions or that predispose to pancreatitis or peptic ulcer disease; complications of diabetes mellitus, including autonomic neuropathy and ketoacidosis; pregnancy, deficiency disorders, including deficiency of zinc, and niacin; and neoplastic, hematologic, and rheumatologic conditions (Table 13–4).

TABLE 13–4 GI manifestations of systemic diseases and their pathophysiologic mechanisms.

CHECKPOINT

45. What are some common symptoms of esophageal dysmotility? 46. Why does vagotomy often create motor disorders in the stomach?

PATHOPHYSIOLOGY OF DISORDERS OF THE ESOPHAGUS

The major disorders of the esophagus are related to motor functions: Disordered peristalsis and increased lower esophageal sphincter tone are seen in esophageal achalasia, whereas inappropriate lower esophageal sphincter relaxation results in reflux esophagitis.

ESOPHAGEAL ACHALASIA

Clinical Presentation Esophageal achalasia is a motor disorder in which the lower esophageal sphincter fails to relax properly. As a result, a functional obstruction (ie,

obstruction as a result of abnormal function in the absence of a visible mass or lesion) is created that is manifested as dysphagia (inability to swallow), regurgitation, and chest pain. It is a progressive disease in which severe radiographic distortion of the esophagus develops.

Etiology The underlying cause of esophageal achalasia, which occurs with an incidence of 0.5–1.0 per 100,000 population per year, is unknown. Degeneration of the myenteric plexus and loss of inhibitory neurons that release VIP and nitric oxide, which dilate the lower esophageal sphincter, may contribute. Esophageal involvement in Chagas disease, resulting from damage of the neural plexuses of the esophagus by the parasite Trypanosoma cruzi, bears a striking resemblance to esophageal achalasia. A number of other disorders, including malignancies, may present with manometric pressure characteristics or radiographic features similar to those observed in idiopathic esophageal achalasia.

Pathology & Pathogenesis Although achalasia is manifested as a motor disorder of esophageal smooth muscle, it is actually a result of defective innervation of smooth muscle in the esophageal body and lower esophageal sphincter. Lower esophageal sphincter tone is normally characterized by tonic contraction with intermittent relaxation resulting from a neural reflex arc (see earlier discussion). In achalasia, the lower esophageal sphincter is even more tightly contracted and does not relax properly in response to swallowing because of a partial loss of neurons in the wall of the esophagus. Thus, achalasia can be thought of as a disorder caused by defective inhibitory pathways of the esophageal enteric nervous system. Interestingly, injecting botulinum toxin into the lower esophageal sphincter diminishes the activity of the excitatory pathways, thereby ameliorating symptoms. In addition to dysfunction of the lower esophageal sphincter, loss of normal peristalsis in the esophageal body is often seen in achalasia, consistent with the hypothesis of myenteric plexus degeneration. Variations of achalasia also exist in which normal peristalsis is replaced by simultaneous contractions of large or small amplitude.

Clinical Manifestations Over months and years, lower esophageal sphincter dysfunction results in

tremendous enlargement of the esophagus. Normally intended as a direct conduit to the stomach, the esophagus in advanced cases of achalasia can hold as much as 1 L of putrid, infected material, imposing a high risk of aspiration pneumonia. Without treatment, patients display progressive, severe weight loss with worsening chest pain, mucosal ulceration, lung infection, and occasional esophageal rupture, culminating in death.

REFLUX ESOPHAGITIS

Clinical Presentation The predominant presenting symptom of reflux is burning chest pain (heartburn) resulting from recurrent mucosal injury. Patients also frequently report the sensation of burning in the back of their throat and a sour (acidic) taste from the reflux. Symptoms are often worse at night, when lying supine, or after consuming foods or drugs that diminish lower esophageal sphincter tone, such as caffeinated beverages.

Etiology Common causes of reflux esophagitis are those conditions that result in persistent or repetitive acid exposure to the esophageal mucosa. These include disorders that increase the rate of spontaneous transient lower esophageal sphincter relaxations (Table 13–5) or impair reflexes that normally follow transient lower esophageal sphincter relaxations with a secondary wave of esophageal peristalsis. Conditions that increase gastric volume or pressure (eg, partial or complete gastric outlet obstruction) and conditions that increase acid production also contribute. Occasionally, reflux esophagitis can be caused by alkaline injury (eg, pancreatic juice or bile refluxing through both an incompetent pyloric sphincter and a relaxed lower esophageal sphincter). Hiatal hernia, a disorder in which a portion of the proximal stomach slides into the chest cavity with upward displacement of the lower esophageal sphincter, can also contribute to the development of reflux.

TABLE 13–5 Modulators of lower esophageal sphincter pressure.

Pathology & Pathogenesis Normally, the tonically contracted lower esophageal sphincter provides an effective barrier to reflux of acid from the stomach back into the esophagus. This is reinforced by secondary esophageal peristaltic waves in response to transient lower esophageal sphincter relaxation. Loss of lower esophageal sphincter tone (ie, the opposite of achalasia), increased frequency of transient relaxations, loss of secondary peristalsis after a transient relaxation, increased stomach volume or pressure, or increased acid production, can all make the reflux of acidic stomach contents more likely, causing pain or erosion. Recurrent reflux can damage the mucosa, resulting in inflammation, hence the term “reflux esophagitis.” Recurrent reflux itself predisposes to further reflux because the scarring that occurs with healing of the inflamed epithelium renders the lower esophageal sphincter progressively less competent as a barrier.

Pepsin and bile can be refluxed in addition to acid to cause esophagitis. In most cases of esophageal reflux disease, a common pathophysiologic thread can be identified (Figure 13–17). Recurrent mucosal damage results in the infiltration of granulocytes and eosinophils, basal cell hyperplasia, and eventually the development of friable, bleeding ulcers and exudates over the mucosal surface. These pathologic changes set the stage for scar formation and sphincter incompetence, predisposing to recurrent cycles of inflammation.

FIGURE 13–17 Pathophysiology of esophageal reflux disease. (LES, lower esophageal sphincter.)

An increased frequency of transient lower esophageal sphincter relaxations may be partly in response to increased gastric distension. Normally, transient lower esophageal sphincter relaxations are accompanied by increased esophageal peristalsis. Individuals with defects in excitatory pathways that promote peristalsis may, therefore, be at increased risk for the development of esophageal reflux. Changes in the types of prostaglandin produced by the esophagus have been noted in reflux esophagitis, perhaps contributing to impairment of healing and predisposing to recurrences. In contrast to other forms of acid-mediated injury, H pylori infection does not appear to contribute to the development of reflux or esophagitis.

Clinical Manifestations Heartburn is the usual symptom of reflux esophagitis, typically worsening on lying prone. With recurrent reflux, a range of complications may develop. The most common complication is the development of stricture in the distal esophagus. Progressive obstruction, initially to solid food and later to liquid, presents as dysphagia. Other complications of recurrent reflux include hemorrhage or perforation; hoarseness, coughing, or wheezing; and pneumonia as a result of aspiration of gastric contents into the lungs, particularly during sleep. Chronic recurrent reflux can also result in a change in the esophageal epithelium from squamous to columnar histology (resembling that of the stomach and/or intestine). Termed Barrett esophagus, the disorder is more common in men and in smokers, and it leads to a greatly increased risk of adenocarcinoma. Adenocarcinomas in the distal esophagus and proximal (cardiac) stomach related to Barrett esophagus are among the most rapidly increasing types of cancer in young male patients in the United States.

CHECKPOINT

47. What are the roles of the lower esophageal sphincter structure in achalasia and in reflux esophagitis?

48. What are possible causes of achalasia? 49. What is the relationship of esophageal reflux to Barrett esophagus and

cancer?

PATHOPHYSIOLOGY OF DISORDERS OF THE STOMACH

Common disorders involving the stomach reflect the importance of its role as a secretory organ, in particular of acid and intrinsic factor. Disorders of acid secretion result in acidpeptic disease, whereas loss of intrinsic factor secretion results in the inability to absorb vitamin B12, manifesting as pernicious anemia. The major motility disorder of the stomach is gastroparesis.

ACID-PEPTIC DISEASE

Clinical Presentation Patients with acid-peptic disease typically present with chronic, mild, gnawing or burning abdominal or chest pain resulting from superficial or deep erosion of the GI mucosa. Sudden complications include GI tract bleeding, resulting in hematemesis (vomiting of blood) or melena (tarry stools from the effect of acid on blood), and perforation and infection, resulting in severe abdominal pain and signs of acute abdomen (absence of bowel sounds, guarding, rebound tenderness). The latter presentation reflects the fact that in some cases, acid- peptic disease can be painless in the early stages and detected only when it leads to an intra-abdominal catastrophe.

Classically, duodenal ulcer presents as gnawing or burning epigastric pain occurring 1–3 hours after meals, often waking the patient at night, with antacids or food producing relief. However, many patients later documented to have duodenal ulcer do not fit this symptom profile. Elderly patients in particular often present with a complication of duodenal ulcer but no history of pain.

Etiology Various causes of absolute or relative increased acid production (see Figure 13– 12) or decreased mucosal defenses (see Table 13–2) predispose to acid-peptic disease. As mentioned, the bacterium H pylori is the root cause of a number of forms of acid-peptic disease, including duodenal ulcer, gastric ulcer, and gastritis (Figure 13–18).

FIGURE 13–18 Relation of H pylori infection to upper GI tract conditions. The figure shows that most patients with gastroduodenal ulcers, gastric lymphoma, or adenocarcinoma have also been infected with H pylori. Note, however, that the circles are not to scale, because gastric cancer occurs in less than 1% of those infected with H pylori. Note, too, that relationships among the different conditions are more complex than depicted. Despite the fact that patients with cancer often have a prior history of ulcers, as shown in Figure 13–19, patients with a history of H pylori infection (which causes ulcers) are less likely to develop cancer. (MALT, mucosa-associated lymphoid tissue.) (Redrawn, with permission, from Calam J et al. Pathophysiology of duodenal and gastric ulcer and gastric cancer. BMJ. 2001;323:980. With permission from BMJ Publishing Group Ltd.)

Pathology & Pathogenesis Corrosive agents (acid and pepsin) secreted by the stomach play a key role in gastric ulcer, duodenal ulcer, and acute erosive gastritis. These diseases have distinctive but overlapping pathogeneses with the common themes of either excessive acid secretion or diminished mucosal defense. Exactly why one but not another form of acid-peptic disease should develop in a given individual remains unclear. H pylori infection can cause acid-peptic disease by multiple mechanisms, including direct alteration of signal transduction in mucosal and immune cells, which in turn can increase acid secretion and diminish mucosal defenses. Figure 13–19 illustrates the complex interactions of H pylori infection and its location and virulence, along with its clinical consequences (eg, inflammation, increased or decreased acid secretion).

FIGURE 13–19 Patterns of chronic H pylori infection with respect to acid production and pathology. Left: Acid hyposecretion. H pylori infection spreads into the stomach body and causes the suppression of parietal cells, low acid secretion, atrophic gastritis, intestinal metaplasia, and predisposition to gastric cancer. Right: Acid hypersecretion. H pylori infection primarily of the stomach antrum causes decreased somatostatin and increased gastrin secretion, increasing acid secretion and predisposition to duodenal ulceration. (Redrawn from Calam J et al. Pathophysiology of duodenal and gastric ulcer and gastric cancer. BMJ. 2001;323:980. With permission from BMJ Publishing Group Ltd.)

H pylori is an extremely common pathogen; rates of infection are highest in the world’s poorest countries, where sanitation facilities and standards of personal hygiene are low. The most likely route of spread from person to person is fecal–oral. As many as 90% of infected individuals show signs of inflammation (gastritis or duodenitis) on endoscopy, although many, if not most, of these individuals are clinically asymptomatic. Despite this high rate of association of inflammation with H pylori infection, the important role of other factors is indicated by the fact that only about 15% of infected individuals ever develop a clinically significant ulcer. These other factors (both genetic and environmental, such as cigarette smoking) must account for the individual variations and are pathophysiologically important. Nevertheless, the role of H pylori is of particular clinical importance because, of patients who do develop acid-peptic disease, especially among those with duodenal ulcers, the vast majority have H pylori infection. Furthermore, treatment that does not eradicate H pylori is associated with the rapid recurrence of acid-peptic disease in most patients. There are numerous strains of H pylori that vary in their production of toxins such as CagA and VacA that directly alter cellular signaling pathways. Variations in bacterial strains, natural variation in the balance of inflammatory mediators (eg, TH1 vs. TH2 vs. TH17 cytokines) triggered by infection, and a variety of environmental and lifestyle factors may explain why H pylori infection is asymptomatic in most patients, causes peptic ulcers in some, and increases the risk of developing lymphoma and adenocarcinoma in a few.

1. Gastric Ulcer

Gastric ulcer is distinguished from erosive gastritis by the depth of the lesion, with gastric ulcers penetrating through the mucosa. The actual ulcer crater is often surrounded by an area of intact but inflamed mucosa, suggesting that gastritis is a lesion that predisposes to the development of gastric ulcer. Most gastric ulcers occur on the lesser curvature of the stomach. It is likely that gastric ulcer represents the outcome of a number of different abnormalities summarized next.

Some gastric ulcers are believed to be related to impaired mucosal defenses,

because the acid and pepsin secretory capacity of some affected patients is normal or even below normal.

Motility defects have been proposed to contribute to the development of gastric ulcer in at least three ways. First, they may contribute because of a tendency of duodenal contents to reflux back through an incompetent pyloric sphincter. Bile acids in the duodenal reflux material act as an irritant and may be an important contributor to a diminished mucosal barrier against acid and pepsin. Second, they may contribute as a result of delayed emptying of gastric contents, including reflux material, into the duodenum. Third, they may contribute as a result of delayed gastric emptying and hence food retention, causing increased gastrin secretion and gastric acid production. It is not known whether these motility defects are a cause or a consequence of gastric ulcer formation.

Mucosal ischemia may play a role in the development of gastric ulcer. Prostaglandins are known to increase mucosal blood flow as well as bicarbonate and mucus secretion and to stimulate mucosal cell repair and renewal. Thus, their deficiency, resulting from nonsteroidal anti-inflammatory drug (NSAID) ingestion or other insults, may predispose to gastritis and gastric ulcer, as might diminish bicarbonate or mucus secretion resulting from other causes. Subsets of gastric ulcer patients with each of these defects have been identified. Thus, the risk factors (NSAID ingestion, smoking, psychologic stress, H pylori infection) that have been associated with gastric ulcer probably act by diminishing one or more mucosal defense mechanisms.

Gastritis (inflammation of the gastric mucosa) as a result of aspirin or other NSAID use, bile salts, alcohol use, or other insults may predispose to ulcer formation by (1) attenuating the barrier created by the epithelial cells or the mucus and bicarbonate they secrete; or (2) reducing the quantity of prostaglandins produced by epithelial cells that might otherwise diminish acid secretion.

2. Acute Erosive Gastritis

Acute erosive gastritis includes inflammation resulting from superficial mucosal injury, mucosal erosion, or shallow ulcers caused by a wide variety of insults, most notably alcohol use, drug use, and stress. Ethanol ingestion predisposes to gastritis but not to gastric ulcer. Unlike gastric or duodenal ulcers, in erosive gastritis the submucosa and muscularis mucosa are not penetrated. Acid hypersecretion, gastric anoxia or ischemia (eg, in shock), altered natural defenses (especially diminished mucus secretion), altered epithelial renewal, changes in tissue mediators (eg, prostaglandins), reduced intramucosal pH, and

intramucosal energy deficits have been suggested as factors in the development of superficial gastric mucosal injury.

3. Chronic Atrophic Gastritis

Chronic atrophic gastritis is a heterogeneous group of conditions characterized by inflammatory cell infiltration into the corpus of the stomach with gastric mucosal atrophy that leads to the death of parietal cells and, ultimately, the dropout of gastric glands. In chronic disease, unlike in acute erosive gastritis, endoscopic abnormalities may not be grossly apparent. The capacity to secrete gastric acid is progressively reduced, and the serum levels of gastrin are elevated in an attempt to restore parietal cell activity. Atrophic gastritis can be a purely autoimmune disease associated with the production of autoantibodies to parietal cells, intrinsic factor, and gastrin, but it can also be the result of H pylori infection. Traditionally, it has been thought that autoimmune gastritis is more likely to cause pernicious anemia, whereas atrophic gastritis in the setting of H pylori infection is more associated with the risk of progression to gastric adenocarcinoma. More recent epidemiological studies have disputed this supposition. Specifically, any condition with chronic loss of parietal cell mass or activity can lead to compensatory GI endocrine hyperplasia of reactive G cells. Progression to an autonomous gastrin-producing neuroendocrine tumor of the GI tract (gastrinoma) is a rare cause of ulcer disease. Alternatively, H pylori– mediated atrophic gastritis greatly increases the risk of the inflammatory infiltrate progressing to a lymphoma of mucosa-associated lymphoid tissue type (called a MALToma).

4. Duodenal Ulcer

Even more commonly than gastric ulcers, duodenal ulcers are sequelae of H pylori infection, caused by altered mucosal inflammatory responses and excessive acid secretion. Various other risk factors, including diet, smoking, and excessive alcohol consumption, may influence the development of duodenal ulcers, although specific associations (eg, between coffee or spicy foods and the development of ulcers) have not been demonstrated. Genetic factors also play a role; studies support the existence of a heritable component in duodenal ulcers distinct from that involved in gastric ulcer. Likewise, psychologic stress has been implicated in duodenal ulcer disease, perhaps by an autonomic-mediated influence on acid secretion (see Figure 13–12). Interestingly, duodenal ulcers are associated with a decreased risk of developing gastric adenocarcinoma, perhaps because chronic H pylori infection predisposes to cancer in the setting of

atrophic gastritis, in which parietal cells are lost, whereas duodenal ulcers are caused by acid secretion; thus, patients with duodenal ulcers are not likely to have pronounced parietal cell atrophy (see Figure 13–19).

Clinical Manifestations Those forms of acid-peptic disease characterized by exclusively superficial mucosal lesions (eg, acute erosive gastritis) can result in either acute or chronic GI tract bleeding, accompanied by a significant drop in hematocrit and related complications (eg, precipitating angina in a patient with coronary artery disease). Patients with acute massive bleeding present with hematemesis, rectal bleeding, or melena, depending on the site of origin, the rate of transit of blood through the GI tract, and the extent of hemorrhage. Acute massive hemorrhage (>10% of blood volume over minutes to hours) is manifested by hypotension, tachycardia, and orthostatic blood pressure and heart rate changes on standing, often with dizziness.

In addition to hemorrhage, complications of duodenal ulcer and gastric ulcer include life-threatening perforation and obstruction.

CHECKPOINT

50. How does pernicious anemia result from a secretory dis-order of the stomach?

51. What is the typical acid secretion status of patients with pernicious anemia?

52. In which acid-peptic disorder are diminished mucosal defenses more important than acid hypersecretion?

53. How might motility defects contribute to gastric ulcer? 54. What factors may predispose a patient to duodenal ulcer disease? 55. How do NSAIDs contribute to acid-peptic disease? 56. What is the importance of H pylori infection in acid-peptic disease? 57. What other factors besides H pylori infection contribute to acid-peptic

disease?

GASTROPARESIS

Clinical Presentation A common complication of stomach disorders is delayed gastric emptying (Table 13–6). Known as gastroparesis, it is manifested by nausea, bloating, vomiting, and either constipation or diarrhea. The condition can also occur silently, producing metabolic derangements (eg, of blood glucose in patients with diabetes mellitus) in the absence of somatic symptoms.

TABLE 13–6 Conditions producing symptomatic gastric motor dysfunction.

Etiology Gastroparesis is a common complication of poorly controlled diabetes mellitus, with consequent autonomic neuropathy.

Pathology & Pathogenesis Disorders of gastric motility result from alterations in a number of normal gastric

functions. These include (1) serving as a reservoir for ingested solids and liquids (eg, alteration caused by resection of the stomach); (2) mixing and homogenizing ingested food; and (3) functioning as a barrier that allows only small spurts of well-mixed chyme beyond the pyloric sphincter. The resulting disorders span the range from partial or complete gastric outlet obstruction to excessively rapid emptying and typically result from interference with the normal mechanisms by which these functions are controlled. These include the intrinsic contractility of gastric smooth muscle, the enteric nervous system, the autonomic nervous system’s control over enteric nervous system function, and gut hormones.

Because the pyloric sphincter, like all sphincters, displays tonic contraction with intermittent transient relaxation, loss of vagal control results in excessive tonic contraction and symptoms of various degrees of gastric outlet obstruction. Disorders that affect the enteric nervous system, such as the neuropathy of diabetes mellitus and surgical cutting of the stomach wall or vagal trunk, typically cause delayed emptying. However, it is important to remember that, in some cases, delayed emptying can result in symptoms expected from excessively rapid emptying. For example, an excessively contracted pylorus that can open completely but does so infrequently can result in entry into the duodenum of too large a bolus of chyme from the excessively distended stomach. Such a bolus may not be efficiently handled by the small intestine, resulting in poor absorption and diarrheal symptoms characteristic of dumping syndrome.

Hormones play an ill-defined but important role in regulating GI motility in health and disease. For example, the antibiotic erythromycin is recognized by the receptor for the GI hormone motilin, affecting GI motility. Some patients with gastroparesis are observed to experience substantial improvement with erythromycin analogs, especially when complaints related to partial gastric outlet obstruction, such as bloating, nausea, and constipation, are prominent.

Because different patients have different relative contributions of the intrinsic nervous system, enteric nervous system, autonomic nervous system, higher centers of the CNS, and hormones over control of their GI tract motility, not all treatments for gastroparesis are effective for a majority of patients even with the same initial complaints.

Clinical Manifestations Complications of gastroparesis include the development of bezoars from retained gastric contents, bacterial overgrowth, erratic blood glucose control, and, when nausea and vomiting are profound, weight loss. Elevated blood

glucose can be either a cause or a consequence of delayed gastric emptying. Bacterial overgrowth itself can result in both malabsorption and diarrhea. For unknown reasons, the symptoms of gastroparesis are variable from patient to patient as well as over time in a given patient and often correlated poorly with delayed gastric emptying. In some cases, serotonin antagonists that decrease visceral perception may be more helpful than prokinetic agents in alleviating symptoms.

CHECKPOINT

58. What are the symptoms of delayed versus rapid gastric emptying? 59. What are the complications of gastroparesis? 60. Why might erythromycin improve diabetic gastroparesis?

DISORDERS OF THE GALLBLADDER

Gallbladder disease is most commonly due to gallstones (cholelithiasis).

1. Cholelithiasis Clinical Presentation Gallstones are most often asymptomatic, discovered incidentally at autopsy or during surgery for an unrelated condition. Of patients who do have symptoms referable to cholelithiasis, presentations range from mild nausea or abdominal discomfort after eating fatty or fried foods to severe right upper quadrant or midepigastric abdominal pain and jaundice. A history of chronic mild symptoms with dietary association typically predates an acute episode of abdominal pain. The typical patient with gallstones is female, has a history of high dietary fat intake, has had prior pregnancies (reflecting the role of estrogens in gallstone pathogenesis), and is in her 40s (reflecting the time necessary for progression to symptomatic disease).

Etiology

Gallstones come in several varieties. Most are composed largely of cholesterol with or without calcium deposits. Occasionally, especially in patients with a

chronic hemolytic disease, bilirubin stones may form. Depending on the cause and the pathophysiologic mechanism involved, patients can have one or more of the following: a few large individual stones; many smaller stones; or “sludge,” a thickened viscous gel resulting from a concentration of bile believed to be highly prone to stone formation.

Pathology and Pathogenesis

Cholelithiasis is of multifactorial origin. However, the formation of cholesterol gallstones usually requires the formation of bile whose cholesterol concentration is greater than its percentage solubility. The normal processes that prevent gallstone formation probably include the fact that bile does not normally stay in the gallbladder long enough to become lithogenic (prone to stone formation). Thus, the loss of gallbladder muscular wall motility (resulting from intrinsic disease of the muscle wall, altered levels of hormones such as CCK, or altered neural control) and excessive sphincteric contraction, which impair emptying, are important predisposing factors. One consequence of decreased gallbladder emptying is excessive bile concentration, leading to heightened lithogenicity. This can occur from decreased water absorption or altered bile composition resulting from increased cholesterol content or saturation. Other factors can cause an increased tendency to form stones at any given degree of concentration and saturation, including the presence of nucleating versus antinucleating factors in bile and the size and composition of the bile acid pool. Figure 13–20 summarizes the factors that predispose to gallstone formation, including estrogens, prostaglandins, increased mucus and glycoprotein production by the gallbladder epithelium, and chronic bacterial colonization or infection. Estrogens may play multiple roles, first affecting bile composition (increasing cholesterol and its saturation in bile) but also diminishing gallbladder motility (hence predisposing to stasis, sludge formation, and lithogenicity). Prostaglandins, which are protective in the stomach by increasing mucus production, may actually contribute to lithogenicity by the same mechanism. Thus, NSAIDs that block prostaglandin production are often beneficial for the prevention of gallstones in patients so predisposed, probably by decreasing mucus production.

FIGURE 13–20 Pathophysiology of cholelithiasis.

Clinical Manifestations

The major clinical presentation of gallstones is inflammation of the gallbladder, or cholecystitis. Cholecystitis can be acute, chronic, or acute against a background of chronic disease. An episode of acute cholecystitis can progress to acute pancreatitis if a stone travels down the common bile duct but fails to clear the sphincter of Oddi, thereby blocking the pancreatic duct. Likewise, an inflamed gallbladder can become infected or can undergo infarction and necrosis, setting the stage for systemic sepsis if the patient does not receive systemic broad-spectrum antibiotics and undergo emergency cholecystectomy (Figure 13–21).

FIGURE 13–21 Clinical and pathologic effects of cholelithiasis. (Redrawn, with permission, from Chandrasoma P et al, eds. Concise Pathology, 3rd ed. Originally published by Appleton & Lange. Copyright © 1998 by The McGraw- Hill Companies, Inc.)

PATHOPHYSIOLOGY OF DISORDERS OF THE SMALL INTESTINE & COLON

Diseases of the small and large intestine include diarrhea, inflammatory bowel disease, and diverticular disease. Diarrhea is a symptom that has many causes and diverse pathogenetic mechanisms, including altered motility, secretion, digestion, and absorption. Although intestinal disorders are particularly prominent causes, disease of the stomach, pancreas, and biliary tract can also cause diarrhea. Inflammatory bowel diseases (IBDs) are poorly understood

chronic autoimmune processes in the small intestine, colon, or both. Diarrhea, abdominal pain, and malabsorption are prominent features. Important systemic manifestations may complicate IBDs. Diverticular disease occurs most prominently in the colon, in part as a direct or indirect consequence of altered motor function. Irritable bowel syndrome (IBS) is not a disease per se but a functional disorder manifested by abdominal pain with diarrhea or constipation in the absence of organic disease or gross structural changes of the intestine.

DIARRHEA

Clinical Presentation The symptoms of diarrhea are an increased stool frequency, increased stool volume, and a decrease in stool consistency. Any process that increases the frequency of defecation or volume of stool makes it looser, because time- dependent water absorption is responsible for the normal soft but formed consistency of stool. Infectious diarrheas are discussed in Chapter 4. This chapter focuses on general aspects of diarrhea and diarrheas from other causes.

Patients’ subjective assessments of bowel movements are colored by their baseline bowel habits. An individual with chronic constipation, with bowel movements once every 3 days or so, may regard three soft stools in a day as diarrhea. In contrast, an individual on a high-fiber diet may normally have bowel movements twice or even three times a day.

Diarrhea can be acute (<2 weeks’ duration) or chronic (>4 weeks). Acute diarrhea is usually due to an infectious cause. The most common noninfectious causes are medication side effects.

The simplest idea is that diarrhea is caused by too much secretion or not enough absorption. Osmotic (malabsorptive) diarrhea is caused by malabsorbed nutrients or poorly absorbed electrolytes that retain water in the lumen. Malabsorption occurs when the ability to digest or absorb a particular nutrient is defective and may result from disordered mixing (altered motility), pancreatic insufficiency (altered digestion), or damage to enterocytes or their surface transporters (altered absorption). This type of diarrhea stops when the patient fasts. Secretory diarrhea results when secretagogues maintain elevated rates of fluid transport out of epithelial cells into the GI tract lumen. This type of diarrhea does not stop when the patient fasts. These physiologic distinctions are useful in both the diagnosis and therapy of diarrheal disorders. In terms of

transport capacity, the small intestine far exceeds the colon (owing to the enormous surface area of the brush border). Thus, infectious, toxic, or other causes of heightened secretion in the small intestine can overwhelm absorptive mechanisms in the colon, resulting in diarrhea.

Etiology Flow in the GI tract is a steady state involving massive fluid secretion into and absorption from the GI lumen. Each process is controlled by both extrinsic and intrinsic factors. Subtle aberrations in input or output at any of several levels can result in diarrhea with or without nutrient malabsorption. Thus, an excessive osmotic load, increased secretion, or diminished fluid resorption may result in diarrhea (Table 13–7).

TABLE 13–7 Mechanisms of diarrhea and major specific causes.

An excessive osmotic load in the GI tract may come about in three different ways: by direct oral ingestion of excessive osmoles, by ingestion of a substrate that may be converted into excessive osmoles (eg, when bacterial action on the nondigestible carbohydrate lactulose generates a diarrhea-causing osmotic load in the colon), and as a manifestation of a genetic disease such as an enzyme deficiency in the setting of a particular diet (eg, milk consumption by a lactase- deficient individual).

Secretion is increased by either blood-borne or intraluminal secretagogues. These include endogenous endocrine products (eg, overproduction of VIP by a tumor), exotoxins resulting from direct ingestion (eg, acute food poisoning) or infection (eg, cholera), or GI luminal substances (eg, bile acids) that stimulate secretion.

Absorption of fluid, electrolytes, and nutrients can be diminished by many factors, including the toxic effects of alcohol and mucosal damage from infectious agents and from cytokines and prokinetic agents. Cytokines are released by immune and other cells (eg, in response to infection). Prokinetic agents speed up GI motility, thereby diminishing the time available for the absorption of any given nutrient, fluid, or electrolyte load. Finally, inflammatory and other disorders resulting in loss of mucus, blood, or protein from the GI tract may be manifested as diarrhea. Table 13–8 lists symptoms and signs suggesting specific causes of diarrhea.

TABLE 13–8 Clues to diagnosis of diarrhea from other symptoms and signs.

Pathology & Pathogenesis Recognizing the pathophysiologic subtypes of secretory (Tables 13–9 and 13– 10) and osmotic diarrheas provides a means of approaching the diagnosis and therapy of diarrheal disorders. For example, nonbloody diarrhea that continues in the absence of oral intake must be due to a secretory mechanism, whereas diarrhea that diminishes as oral intake is curtailed (eg, in a patient receiving intravenous hydration) suggests an osmotic/malabsorptive cause. Likewise, the presence of white blood cells in the stool suggests an infectious or inflammatory origin of diarrhea, although their absence does not rule out such causes.

TABLE 13–9 Histologic features of small intestinal diseases causing malabsorption.

TABLE 13–10 Symptoms and signs of malabsorption and relevant pathophysiology.

Of the many causes of diarrhea (Table 13–11), infectious agents are among

the most important because they cause acute, sometimes life-threatening diseases whose pathogenesis is relatively well understood and because they are usually treatable. The symptoms of diarrhea caused by infectious agents are due either to toxins that alter small bowel secretion and absorption or direct mucosal invasion. The noninvasive toxin-producing bacteria are generally small bowel pathogens, whereas the invasive organisms are typically localized to the colon. Diarrheas caused by infectious agents are discussed in Chapter 4.

TABLE 13–11 Most likely causes of diarrhea in seven different clinical categories.

Evidence suggests that infectious causes of diarrhea can interface more

intimately with normal mechanisms of secretory control than had been previously realized. Thus, in addition to its direct effect on the G protein controlling Cl– ion secretion in the crypts of the small intestinal epithelium, cholera activates the enteric nervous system to cause fluid and electrolyte secretion in the colon.

Clinical Manifestations Dehydration, malnutrition, weight loss, and specific vitamin deficiency syndromes (eg, glossitis, cheilosis, and stomatitis) are common signs in diarrhea depending on its cause, severity, and chronicity (see Tables 13–8 and 13–10). In certain circumstances (eg, in young children), viral gastroenteritis is associated with a high mortality rate from dehydration when supportive measures (ie, oral or intravenous rehydration) are not promptly provided. Some individuals with diarrhea from parasitic infections remain relatively asymptomatic, whereas others may develop more severe symptoms and complications, including intestinal perforation.

CHECKPOINT

61. By what mechanisms do infectious agents cause diarrhea? 62. Name three ways in which an excessive osmotic load can occur in the GI

tract.

INFLAMMATORY BOWEL DISEASE

Clinical Presentation Inflammatory bowel disease is distinguished from infectious entities by exclusion and by chronicity (months; not days or weeks). Patients often experience recurrent episodes of mucopurulent (ie, containing mucus and white cells), bloody diarrhea characterized by lack of positive cultures for known microbial pathogens and failure to respond to antibiotics alone. Because inflammatory bowel disease is characterized by exacerbations and remissions, favorable responses to therapy may be difficult to distinguish from spontaneous remissions occurring as part of the natural history of the disease.

Etiology Triggers for the development of IBD remain elusive. However, epidemiologic studies suggest that smoking and gastrointestinal infections are possible contributory factors. There are two forms of chronic inflammatory bowel disease: Crohn disease, which is transmural and granulomatous in character, occurring anywhere along the GI tract; and ulcerative colitis, which is superficial and limited to the colonic mucosa. The exact causes of inflammatory bowel disease are unknown despite progress in understanding its pathogenesis.

Pathology & Pathogenesis Genetic risk and environmental factors are recognized as two key elements in the pathogenesis of inflammatory bowel disease. An explosion of newly recognized susceptibility genes for both Crohn disease and ulcerative colitis have been discovered through genome-wide associations. These studies evaluated thousands of single nucleotide polymorphisms (SNPs) in thousands of patients with inflammatory bowel disease and compared them with SNPs from thousands of people without the disease (controls). These studies found abnormalities in several categories of susceptibility genes in patients with inflammatory bowel disease. These included modulators of immune function, autophagy, and epithelial function that participate in the interaction of host and microorganism. Importantly, the relative risk of most of these susceptibility genes is low (most have a 20–30% increase in relative risk of developing disease). Therefore, most people who harbor risk alleles for inflammatory bowel disease do not develop disease.

Genetic factors are clearly not the sole contributor to inflammatory bowel disease. Many environmental factors have been found to contribute to the development of Crohn disease, including pathogenic microorganisms (bacteria and viruses), the repertoire of indigenous intestinal microbes (the microbiota), dietary factors, smoking, defective immune responses, and psychosocial factors. Moreover, recent studies suggest that patterning the activity of certain aspects of the immune system during the neonatal period strongly influences immune responses in the adult. Because the composition of the intestinal microbiota is in large part transmitted by the mother, maternal effects are thought to be a contributing factor to GI disease as well. Specifically, early exposure to intestinal microbiota may be an important component of the pathogenesis of inflammatory bowel disease.

The normal intestine is able to modulate frank inflammatory responses to its

constant bombardment with dietary and microbial antigens in the lumen. This process may be defective in Crohn disease, resulting in uncontrolled inflammation. There has been considerable interest in the role of cytokines, such as interleukins and tumor necrosis factor, in Crohn disease. Cytokine profiles of TH1 and TH17 categories have been implicated in Crohn disease. Mice lacking the TH1-inhibiting cytokine interleukin-10 have a TH1 cytokine profile and develop spontaneous intestinal inflammation. Monoclonal antibodies to tumor necrosis factor alpha (TNFα) reduce inflammation in these animals and patients. Similar factors may contribute to the pathogenesis of ulcerative colitis, including infections, allergies to dietary components, immune responses to bacteria and self-antigens, and psychosocial factors. In mice, targeted disruption of the genes for the T-cell receptor and the cytokine IL-2 results in GI tract disease resembling ulcerative colitis.

The two forms of inflammatory bowel disease have characteristic differences and in many cases considerable overlap in presentation (Table 13–12). The features common to all forms of inflammatory bowel disease are mucosal ulceration and GI tract inflammation, indistinguishable, in fact, from that which can occur acutely during invasive infectious diarrhea. Other factors besides the presence of key gene products, including infectious agents, altered host immune responses, immune-mediated intestinal damage, psychologic factors, and dietary and environmental factors, may contribute to a final common pathway of disordered immune response.

TABLE 13–12 Similarities and differences between ulcerative colitis and Crohn disease.

Clinical Manifestations

A. Crohn Disease Crohn disease most typically occurs in the distal ileum. However, the distribution of the disease can also involve the colon or less commonly any other region of the GI tract (including the oral cavity, esophagus, stomach, and proximal small intestine). A characteristic feature is that areas of ulceration and inflammation occur in a discontinuous fashion and involve the entire thickness of the bowel wall. Recurrence of disease can occur in previously uninvolved regions of the intestine and can even involve adjacent mesentery and lymph nodes. The combination of deep mucosal ulceration and submucosal thickening gives the involved mucosa a characteristic “cobblestone” appearance.

Perforation, fistula formation, abscess formation, and small intestinal obstruction are frequent complications of Crohn disease, although an indolent course occurs in most patients. The full-thickness involvement of the bowel wall may predispose to these complications. Frank bleeding from the mucosal ulcerations can be either insidious or massive, as can protein-losing enteropathy. Another important complication is an increased incidence of intestinal and colon cancer.

Patients with Crohn disease often manifest symptoms outside the GI tract. Most commonly, inflammatory disorders of the joints (arthritis), skin (erythema nodosum), eye (uveitis, iritis), mucous membranes (aphthous ulcers of the buccal mucosa), bile ducts (sclerosing cholangitis), and liver (autoimmune chronic active hepatitis) are also observed in these patients. Renal disorders, especially nephrolithiasis, are observed in one-third of patients with Crohn disease, probably related to increased oxalate absorption associated with steatorrhea. Amyloidosis is a serious complication of Crohn disease, as is thromboembolic disease. Both complications are probably reflections of the systemic character of the inflammatory process. Patients are often malnourished and show evidence of nutrient deficiency states.

B. Ulcerative Colitis

In contrast to Crohn disease, inflammation in ulcerative colitis is restricted to the mucosa of the colon and rectum. It typically begins at the anorectal junction and extends proximally. Ulcerative colitis and Crohn disease are similar in presentation (eg, bloody diarrhea and malabsorption) and in at least some of the complications (eg, protein-losing enteropathy and malnutrition), reflecting the widespread involvement of the mucosa in both entities. In both conditions, acute inflammatory cells (neutrophils) are located within the crypt epithelium (cryptitis) and crypt lumens (crypt abscesses). There is also a robust infiltration

of chronic inflammatory cells in the mucosa. However, because ulcerative colitis generally is limited to the mucosa, obstruction, perforation, and fistula formation are not typical complications. Most patients have mild disease, and, as with Crohn disease, some patients will have only one or two episodes during their lifetimes. As with Crohn disease, there is an increased risk of colonic adenocarcinoma that increases with the duration of the disease. Chronic disease can also lead to damage of the muscularis propria, leading to toxic megacolon, a thin-walled, dilated, poorly motile area of the colon susceptible to rupture. In the mucosa, chronic damage and ulceration can lead to excess granulation tissue that protrudes into the lumen of the intestine (pseudopolyps).

Both ulcerative colitis and Crohn disease can go into remission after treatment with first-line anti-inflammatory agents such as sulfasalazine and glucocorticoids. Crohn disease and ulcerative colitis also respond to therapy with monoclonal antibodies against the inflammatory cytokine TNFα. These anti- TNFα therapies are now commonly used to treat patients with moderate to severe disease as the benefit-to-risk profile of these agents is superior to that of corticosteroids. Moreover, these steroid-sparing agents can be used long term to maintain clinical remission. Newer and developing therapies for IBD include those that target cytokines (interleukin-23), inflammatory signaling kinases (Janus kinase [JAK]), and cell trafficking (integrins and adhesions molecules).

The natural history of both diseases is of periods of remission interrupted by active disease; medical therapy during exacerbations is directed toward supportive measures and attempts to induce remission. Because these diseases can recur after resecting involved regions of the GI tract, operative management is generally limited to relief of persistently symptomatic intestinal obstruction or bleeding.

CHECKPOINT

63. How is inflammatory bowel disease distinguished from infectious diarrhea?

64. What are the differences between ulcerative colitis and Crohn disease? 65. What are the complications of inflammatory bowel disease?

DIVERTICULAR DISEASE

Clinical Presentation Nearly 80% of patients with diverticula are asymptomatic except for chronic constipation. Of those who develop other symptoms, the most common presentation is an intermittent and unpredictable griping lower abdominal pain (diverticulitis). Additional features of the presentation depend on which of the two major complications of diverticula the patient develops.

A patient who develops diverticulitis (see later discussion) may present with fever and with symptoms and signs of peritoneal irritation (guarding, rebound tenderness, absence of bowel sounds). A patient who develops diverticular bleeding may present with stools that are either frankly bloody or positive for occult blood.

Etiology Diverticulosis results from an acquired deformity of the colon in which the mucosa and submucosa herniate through the underlying muscularis (Figure 13– 22). The incidence of this disease is increased with affluent, Westernized lifestyles. A rarity at the turn of the century, today it afflicts 30% of adults in the U.S. population. Its incidence increases with age, starting from about 40 years. Epidemiologic studies suggest that the consumption of highly refined foods and less fiber, with a resulting increased prevalence of chronic constipation, may be responsible for the increased prevalence of diverticular disease.

FIGURE 13–22 Top: Pathophysiology of diverticular disease. Bottom: Cross-sectional drawing of the colon, showing principal points of diverticula formation between mesenteric and antimesenteric teniae. (Redrawn, with permission, from Goligher JC. Surgery of the Anus, Rectum and Colon, 5th ed. Baillière Tyndall, 1984.)

Pathology & Pathogenesis A. Diverticulosis

Most acquired diverticula occur in the colon; the descending colon and sigmoid (left side) are involved in more than 90% of cases. Both structural and functional factors are believed to contribute to the development of diverticulosis. Acquired abnormalities in colonic wall connective tissue are believed to be the structural basis of diminished resistance to mucosal and submucosal herniation (see Figure 13–22). The functional abnormality is believed to be related to chronic constipation and the development of a transmural pressure gradient from the colonic lumen to the peritoneal space as a result of vigorous muscle contraction in the colonic wall. This functional abnormality is most likely related to changes in dietary habits; decreased dietary fiber makes the forward propulsion of feces at normal transmural pressures more difficult. This increased muscle contraction, which contributes to the development of diverticular disease, is also believed to cause the abdominal pain that is the cardinal symptom of uncomplicated diverticular disease. The pain may last hours to days, with sudden relief on passing flatus or feces. Constipation or diarrhea and flatulence are common findings during such episodes, leading to the suggestion that there is a relationship between irritable bowel syndrome and the development of diverticulosis. Treatment of the pain of diverticular disease with opioids is contraindicated because opioids directly raise intraluminal pressure and hence may increase the risk of perforation.

B. Diverticular Bleeding

Diverticula are a source of bleeding in 3–5% of patients with diverticulosis. Branches of the colonic intramural arteries (vasa recta) are closely associated with the diverticular sac, presumably leading to occasional rupture and bleeding. This is the most common cause of massive lower GI bleeding in the elderly. Diverticular bleeding is typically painless and not believed to be associated with a focus of inflammation.

The differential diagnosis of painless bleeding per rectum also includes internal hemorrhoids (dilated venous channels in the anal canal) and angiodysplasia. The latter consists of small, focal proliferations of dilated blood vessels in the mucosa, typically found in elderly patients.

C. Diverticulitis

This most common complication of diverticulosis develops when a focal area of inflammation occurs in the wall of a diverticulum in response to irritation by fecal material. The patient develops symptoms of abdominal pain and fever with

a risk of progression to abscess with or without perforation. These symptoms mimic acute appendicitis; however, pain localizes typically to the left (rather than the right) lower abdominal quadrant. Perforations usually are self- contained, but the potential for subsequent fistula formation and intestinal obstruction is high. Approximately 15–25% of patients who develop diverticulitis will require surgery.

Clinical Manifestations About one-fifth of all individuals with diverticular disease develop one of the two major complications: diverticular bleeding or diverticulitis. These disorders must be distinguished from carcinoma, inflammatory bowel disease, infection, and ischemic injury. Ischemia can arise from a variety of conditions, including atherosclerosis, vasculitis, hypercoagulable states, heart failure, and shock. Definitive diagnosis is typically made by colonoscopy.

CHECKPOINT

66. Where in the GI tract do most diverticula occur? 67. What predisposing factors contribute to the develop-ment of diverticular

disease? 68. What are the major complications of diverticular disease?

IRRITABLE BOWEL SYNDROME

Irritable bowel syndrome is the most common cause of referral to gastroenterologists. It is characterized by altered bowel habits with abdominal pain in the absence of any detectable organic pathological process or specific motility or structural abnormalities.

A change in bowel habits with increasing diarrhea, constipation, or alternating diarrhea and constipation, is the principal symptom of irritable bowel syndrome. Abdominal pain, which may be caused by intestinal spasms, is also common to all patients with irritable bowel syndrome. Bloating or perceived abdominal distension is another common feature. Intraluminal gas can result from swallowing air, diminished gas absorption, and bacterial fermentation, although

the cause of irritable bowel syndrome is unknown. Stress appears to have a considerable influence on these symptoms. Symptoms of irritable bowel syndrome frequently occur during or after a stressful event, and stressful events in early life may predispose to the development of irritable bowel syndrome.

Much of our understanding of the pathophysiology of irritable bowel syndrome derives from the study of motility. In normal persons, high-amplitude peristaltic contractions occur 6–8 times per day. In constipated patients with irritable bowel syndrome, the frequency of high-amplitude peristaltic contractions of the intestine is diminished compared with normal subjects, suggesting that the constipation may be due to diminished motility. Visceral hyperalgesia may also occur in patients with irritable bowel syndrome. In patients with irritable bowel syndrome, distension of the colon with a balloon, to a degree that is not painful in normal individuals, can induce pain, indicative of visceral hyperalgesia.

Irritable bowel syndrome is a complex disorder, and its cause is poorly understood. Several theories have been proposed to explain the disorder, including alterations in sensitivity of the extrinsic and intrinsic nervous systems of the intestine, which may contribute to exaggerated sensations of pain and to abnormal control of intestinal motility and secretion. An alteration in the balance of secretion and absorption is also a potential cause. Microbial dysbiosis is another important potential etiology. This etiology is supported by the response of the symptoms of some patients to either antibiotic or probiotic therapy. Although there is no gross inflammation of the intestine, there are reports of an increased influx of inflammatory cells (lymphocytes) into the colon of affected individuals as well as of the destruction of enteric neurons. One proposed theory is that irritable bowel syndrome develops as a result of an earlier and resolved bout of interstitial inflammation. In experimental animals, induction of intestinal inflammation induces visceral hyperalgesia and altered intestinal motility and secretion that persists many months after the inflammation is resolved. A similar mechanism may occur in a subset of patients who develop irritable bowel syndrome after an infection causes intestinal inflammation.

CHECKPOINT

69. List three characteristics of irritable bowel syndrome. 70. What are possible factors in the pathogenesis of irrita-ble bowel

syndrome?

CASE STUDIES Yeong Kwok, MD

(See Chapter 25, p. 770–73 for answers)

CASE 67

A 60-year-old man presents to the clinic with a 3-month history of gradually worsening dysphagia (difficulty swallowing). He first noticed the problem when eating solid food such as steak, but now it happens even with drinking water. He has a sensation that whatever he swallows becomes stuck in his chest and does not go into the stomach. He has also developed worsening heartburn, especially upon lying down, and has had to prop himself up at night to lessen the heartburn. He has lost 10 kg as a result of his swallowing difficulties. His physical examination is unremarkable. A barium swallow x-ray reveals a decrease in peristalsis of the body of the esophagus along with dilatation of the lower esophagus and tight closure of the lower esophageal sphincter. The distal esophagus and lower esophageal sphincter have a beaked appearance. There is very little passage of barium into the stomach.

Questions

A. What is the likely diagnosis in this patient, and what is the underlying pathophysiology of this condition?

B. Botulinum toxin can be used to treat this disorder. How does it help ameliorate the symptoms?

C. What are the possible complications of this disorder, and how do they arise?

CASE 68

A 32-year-old woman presents to her primary care provider complaining of a persistent burning sensation in her chest and upper abdomen. The symptoms are worse at night while she is lying down and after meals. She has tried drinking hot cocoa to help her sleep. She is a smoker and frequently relies on benzodiazepines for insomnia. She notes a sour taste in her mouth every morning. Her physical examination is normal.

Questions

A. What is the pathogenetic mechanism of this patient’s GI disorder? B. How may her lifestyle impact her symptoms? C. What are some complications of chronic esophageal reflux disease?

CASE 69

A 74-year-old man with severe osteoarthritis presents to the emergency department reporting two episodes of melena (black stools) without hematochezia (bright red blood in the stools) or hematemesis (bloody vomitus). He takes 600 mg of ibuprofen three times a day to control his arthritis pain. He denies alcohol use. On examination, his blood pressure is 150/70 mm Hg, and his resting pulse is 96/min. His epigastrium is minimally tender to palpation. Rectal examination reveals black tarry stool in the vault, grossly positive for occult blood. Endoscopy demonstrates a 3 cm gastric ulcer. Helicobacter pylori is identified on biopsies of the ulcer site.

Questions

A. What are some of the proposed mechanisms for acid-peptic disease and specifically gastric ulcer disease?

B. How may this patient’s analgesic use predispose him to acid-peptic disease?

C. What role does H pylori infection play in the pathogenesis of ulcer disease? How should this be taken into account when treating this patient?

CASE 70

A 67-year-old man with type 2 diabetes is seen by his primary care provider for frequent nausea, vomiting, bloating, and intermittent diarrhea over the preceding 2 weeks. The vomiting typically occurs approximately 1–2 hours after eating. He states that over the past year, he has become increasingly depressed after the death of his wife and has been less adherent to his oral hypoglycemic regimen and evening insulin. He also reports 6 months of worsening neuropathic pain in his feet. His fasting fingerstick blood glucose level is 253 mg/dL.

Questions

A. How may diabetes contribute to the development of gastroparesis? Is this patient’s poor glucose control a cause or consequence of gastroparesis?

B. How can delayed gastric emptying cause diarrhea?

CASE 71

A 40-year-old woman presents to the emergency department with a history of worsening right upper quadrant pain. The pain started after she had pizza for dinner 2 days ago and is described as a sharp, stabbing sensation under her right ribs. She has also felt ill, developed slight nausea, and had a low- grade fever. There has been no vomiting or diarrhea. The physical examination reveals an obese woman with a low-grade fever and tenderness to palpation of the right upper quadrant of her abdomen. An abdominal ultrasound reveals a 2 cm gallstone lodged in the cystic duct with swelling of the gallbladder and thickening of the gallbladder wall.

Questions

A. What are the mechanisms involved in gallstone formation? B. What factors in the pathogenesis of gallstones may be responsible for

the fact that it is more common in premenopausal women? C. What local complications can ensue from gallstone disease?

CASE 72

A 45-year-old man comes to clinic with a history of excessive bloating, foul-smelling flatus, and loose stools for the past several months. He notes that about 30–60 minutes after breakfast each morning, he experiences cramping, bloating, passage of smelly flatus, and a very loose, watery bowel movement. He has not seen any blood or mucus in the stool and also denies any weight loss. This does not happen with lunch or dinner. Every day for breakfast, he eats a big bowl of cereal with milk and a yogurt smoothie. The physical exam is unremarkable with normal bowel sounds, no organomegaly, and no abdominal tenderness. He was advised to do a dietary trial of stopping dairy intake for 1 week. All his symptoms resolve, and he is diagnosed with lactose intolerance.

Questions

A. Why do people develop lactose intolerance? B. Why does the inability to digest lactose lead to diarrhea?

CASE 73

A 42-year-old man with long-standing Crohn disease presents to the emergency department with a 1-day history of increasing abdominal distension, pain, and obstipation. He is nauseated and has vomited bilious material. He has no history of abdominal surgery and has had two exacerbations of his disease this year. He is febrile with a temperature of 38.5°C. Examination reveals multiple oral aphthous ulcers, hyperactive bowel sounds, and a grossly distended, diffusely tender abdomen without

an appreciable mass. Abdominal radiographs reveal multiple air–fluid levels in the small bowel with minimal colonic gas, findings consistent with a small bowel obstruction.

Questions

A. Describe the significance of the oral aphthous ulcers in the distribution of Crohn disease.

B. What factors are thought to be involved in the pathogenesis of Crohn disease? What is the evidence to support the role of cytokines in the pathogenesis of Crohn disease?

C. What are the GI complications of Crohn disease? D. Describe some of the extraintestinal manifestations of Crohn disease.

CASE 74

A 56-year-old man presents to his doctor with an 8-week history of gradually worsening diarrhea. He has had as many as eight loose watery bowel movements per day, occasionally mixed with blood. He has felt fatigued and has some left lower quadrant discomfort. He denies any travel, recent infections, or recent use of antibiotics. A colonoscopy shows chronic colitis of the entire rectum and sigmoid colon. Stool cultures and other studies for enteric pathogens are negative.

Questions

A. What is the likely diagnosis, and why? B. Anti-inflammatory agents such as sulfasalazine and glucocorticoids

have traditionally been used in the treatment of this condition. What are some of the newer agents that can also be used?

C. Why do patients with this condition often get frequent surveillance colonoscopies? What is another feared complication of this condition?

CASE 75

A 76-year-old woman with chronic constipation reports a 4-day history of “achy” left lower quadrant abdominal pain, graded 7/10, accompanied by low-grade fever and nausea. A colonoscopy performed 2 years ago revealed sigmoid diverticular disease. On examination, she has a temperature of 38.6°C. Her abdomen has a tender 3 × 2 cm mass in the left lower quadrant. Bowel sounds are normal. Her stool is positive for occult blood. An abdominal series shows a bowel gas pattern consistent with ileus and no evidence of free peritoneal air. A CT scan with contrast of the abdomen and pelvis shows pericolonic fat stranding with no evidence of an abscess. She is started on antibiotics and intravenous fluids with significant improvement in her symptoms.

Questions

A. Describe the pathogenesis of diverticular disease. B. Why should opioids be avoided in the treatment of her abdominal pain? C. What are the complications of diverticular disease?

CASE 76

A 32-year-old woman comes to the clinic complaining of a 3-month history of abdominal bloating, crampy abdominal pain, and a change in her bowel habits. Previously she had regular bowel movements, but 4 months ago, she developed gastroenteritis with nausea and vomiting after a cruise. The constant diarrhea and vomiting went away after a week, but since then she has had periods of constipation, lasting up to 3 days, alternating with periods of diarrhea. During the diarrheal episodes, she can have three to four loose bowel movements per day, though without blood or mucus in the stool. She describes diffuse abdominal cramping and bloating that are somewhat relieved by bowel movements. Her symptoms worsen during periods of stress. There has been no weight loss or fever. There is no association with particular foods (eg, wheat or dairy products). Her physical examination is unremarkable except for mild abdominal tenderness with no rebound or guarding. Serologic tests for celiac disease are negative. Stool

cultures and examinations are negative for bacterial or parasitic infections. A colonoscopy is unremarkable.

Questions

A. What is the likely diagnosis? B. What are the theories about the pathophysiology of this condition?

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CHAPTER

14

Liver Disease Nizar A. Mukhtar, MD, & Mandana Khalili, MD, MAS

Although many different pathogenic agents and processes can affect the liver (Table 14–1), they are generally manifested in individual patients in a limited number of ways that can be assessed by evaluating some key parameters. Liver disease can be acute or chronic, focal or diffuse, mild or severe, and reversible or irreversible. Most cases of acute liver disease (eg, caused by viral hepatitis) are so mild that they never come to medical attention. Transient symptoms of fatigue, loss of appetite, and nausea are often ascribed to other causes (eg, flu), and minor biochemical abnormalities referable to the liver that would be identified in blood studies are not discovered. The patient recovers without any lasting medical consequences. In other cases of acute liver injury, symptoms and signs are severe enough to call for medical attention. The entire range of liver functions may be affected or only a few, as is the case with liver injury resulting from certain drugs that cause isolated impairment of the liver’s role in bile formation (cholestasis). Occasionally, viral, drug-induced, and other acute liver injury occurs in an overwhelming manner, resulting in massive liver cell death and progressive multi-organ failure. This syndrome of acute liver failure (also referred to as fulminant hepatic failure) carries a high mortality rate; however, in recent years, use of emergency liver transplantation has significantly improved survival.

TABLE 14–1 Liver diseases.

Liver injury may continue beyond the initial acute episode or may be recurrent (chronic hepatitis). In some cases of chronic hepatitis, liver function remains stable or the disease process ultimately resolves altogether. In other

cases, there is progressive and irreversible deterioration of liver function. Cirrhosis is the ultimate consequence of progressive liver injury. Cirrhosis

can occur in a subset of cases of chronic hepatitis that do not resolve spontaneously or after repeated episodes of acute liver injury, as in the case of chronic alcoholism. In cirrhosis, the liver becomes hard, shrunken, and nodular and displays impaired function and diminished reserve because of a decreased amount of functioning liver tissue. More importantly, the physics of blood flow is altered such that the pressure in the portal vein is elevated. As a result, the blood is diverted around the liver rather than filtered through the liver. This phenomenon, termed portal-to-systemic (or portosystemic) shunting, has profound effects on the function of various organ systems and sets the stage for certain devastating complications of liver disease (described later).

Although liver disease resulting from many different causes may present in common ways, the reverse is also true (ie, liver disease from specific causes may have distinctly different presentations in different patients). For example, consider two patients with acute viral hepatitis: One may present with yellow eyes and skin—a manifestation of impaired liver function—complaining of nothing more than itching, fatigue, and loss of appetite, whereas the other may be brought to the emergency department moribund, with massive gastrointestinal (GI) bleeding and encephalopathy. Such variations in the severity of liver disease are probably due to genetic, immunologic, and environmental (including perhaps nutritional) factors that are currently poorly understood.

The consequences of liver disease can be either reversible or irreversible. Those arising directly from acute damage to the functional cells of the liver, most notably hepatocytes, without destruction of the liver’s capacity for regeneration, are generally reversible. Like many organs of the body, the liver normally has both a huge reserve capacity for the various biochemical reactions it carries out and the ability to regenerate fully differentiated cells and thereby recover completely from acute injury. Thus, only in the most fulminant cases or in end-stage disease are there insufficient residual hepatocytes to maintain minimal essential liver functions. More commonly, patients display transient signs of liver cell necrosis and disordered function followed by full recovery. The symptoms and signs of this sort of acute liver injury can best be understood as an impairment of the normal biochemical functions of the liver.

Other consequences of liver disease are irreversible, typically seen in the patient with cirrhosis. These are best understood as a result of portosystemic shunting of blood flow. They include a heightened sensitivity to noxious substances absorbed from the GI tract (encephalopathy), an increased risk of

massive GI bleeding (development of varices and coagulopathy), and malabsorption of fat in the stool (as a result of decreased bile flow). Nevertheless, some of these consequences are treatable. Commonly, patients with cirrhosis present with superimposed acute liver injury (eg, caused by an alcoholic binge or other drug exposure). Because they have a decreased hepatocyte mass and much less functional reserve, they are more sensitive to acute liver injury than patients with a normal liver.

CHECKPOINT

1. What parameters must you consider in assessing a patient with liver disease?

2. What factors may determine the difference in severity of liver disease between two patients with acute hepatitis resulting from the same cause?

3. In what ways is the patient with underlying cirrhosis who presents with acute hepatitis likely to be differ-ent from the patient with a previously normal liver and acute hepatitis?

STRUCTURE & FUNCTION OF THE LIVER

ANATOMY, HISTOLOGY & CELL BIOLOGY

The liver is located in the right upper quadrant of the abdomen in the peritoneal space just below the right side of the diaphragm and under the rib cage (Figure 14–1). It is anatomically separated into two predominant lobes: right and left. The right lobe has two lesser segments: the posterior caudate lobe and the inferior quadrate lobe. The liver can also be differentiated functionally via the portal blood flow into four sectors, which are further subdivided into eight segments. The liver weighs approximately 1400 g in the adult and is covered by a fibrous capsule. It receives nearly 25% of the cardiac output, approximately 1500 mL of blood flow per minute, via two sources: venous flow from the portal vein, which is crucial to the performance of the liver’s roles in bodily functions, and arterial flow from the hepatic artery, which is important for liver oxygenation and which supplies the biliary system via the cystic artery. These

vessels converge within the liver, and the combined blood flow exits via the so- called central veins (also called terminal veins or hepatic venules) that drain into the hepatic vein and ultimately the inferior vena cava.

FIGURE 14–1 Location of the liver. (Redrawn, with permission, from Wolf DC. Evaluation of the size, shape and consistency of the liver. In: Walker HK et al, eds. Clinical Methods, 3rd ed. Butterworth, 1990.)

The portal vein carries venous blood from the small intestine, rich in freshly absorbed nutrients—as well as drugs and poisons—directly to the liver. Also flowing into the portal vein before its entry into the liver is the pancreatic venous drainage, rich in pancreatic hormones (insulin, glucagon, somatostatin, and pancreatic polypeptide). The portal vein forms a specialized capillary bed that allows individual hepatocytes to be bathed directly in portal blood. In part because of this system of blood supply, the liver is a prime site for the metastatic spread of cancer, especially from the GI tract, breast, and lung.

Concepts of Liver Organization The substance (parenchyma) of the liver is organized into plates of hepatocytes lying in a cage of supporting cells termed reticuloendothelial cells (Figure 14– 2A). The plates of hepatocytes are generally only one cell thick, and individual plates are separated from each other by vascular spaces called sinusoids. It is in these sinusoids that blood from the hepatic artery is mixed with blood from the portal vein on the way to the central vein. The reticuloendothelial cell meshwork

in which the hepatocytes reside includes diverse cell types, most importantly the endothelial cells that make up the walls of the sinusoids; specialized macrophages, termed Kupffer cells, which are anchored in the sinusoidal space; and stellate cells or lipocytes, fat-storing cells involved in vitamin A metabolism, which lie between the hepatocytes and the endothelial cells. Approximately 30% of all cells in the liver are reticuloendothelial cells, and about 33% of these are Kupffer cells. Yet, because reticuloendothelial cells are smaller than hepatocytes, the reticuloendothelial system accounts for only 2– 10% of the total protein in the liver. The reticuloendothelial cells are much more than just a cage for hepatocytes. They perform specific functions, including phagocytosis and cytokine secretion, and communicate with each other as well as with hepatocytes. Their dysfunction contributes both to hepatocyte necrosis in acute liver disease and to hepatic fibrosis in chronic liver disease.

FIGURE 14–2 A: Detailed structure of the liver lobule. (Redrawn, with permission, from Chandrasoma P et al. Concise Pathology, 3rd ed. Originally published by Appleton & Lange. Copyright © 1998 by The McGraw-Hill Companies, Inc.) B: Relationship of lobule to acinus. (CV, central vein; PS, portal space [or triad].) (Redrawn, with permission, from Leeson CR. Histology, 2nd ed. Saunders, 1970. Copyright © Elsevier.) C: Hepatic acinus. (HV, hepatic venule.) (Reproduced, with permission, from Gumucio JJ. Hepatic transport. In: Kelley WN (ed): Textbook of Internal Medicine. Lippincott, 1989.)

A. Lobules

Under the microscope at low-power magnification, liver architecture has been

traditionally described in terms of the lobule (Figure 14–2B). Neat arrays of hepatocyte plates are organized around individual central veins to form hexagons with portal triads or tracts (sheath-like structures containing a portal venule, hepatic arteriole, and bile canaliculus) at their corners. The hepatocytes adjacent to the portal triad are termed the limiting plate. Disruption of the limiting plate is a significant diagnostic marker of some forms of immune-mediated liver disease. This may be seen in liver biopsies from patients with liver disease of unknown cause.

B. Functional Zonation

Physiologically, it is more useful to think of liver architecture in terms of the portal-to-central direction of blood flow: Blood entering the sinusoids from a terminal portal venule or hepatic arteriole flows past hepatocytes closest to those vessels first (termed zone 1 hepatocytes) and then percolates past zone 2 hepatocytes (so called because they are not the first hepatocytes reached by blood entering the hepatic parenchyma). The last hepatocytes reached by the blood before it enters the central vein are termed zone 3 hepatocytes. Thus, the microscopic organization of the liver can be viewed in terms of functional zones. A liver acinus is defined as the unit of liver tissue centered around the portal venule and hepatic arteriole whose hepatocytes can be imagined to form concentric rings of cells in the order in which they come into contact with portal blood, first to last (Figure 14–2C). Hepatocytes at either extreme of the acinus (zones 1 and 3) appear to differ in both enzymatic activity and physiologic functions. Zone 1 hepatocytes, exposed to the highest oxygen concentrations, are particularly active in gluconeogenesis and oxidative energy metabolism. They are also the major site of urea synthesis (because freely diffusible substances such as ammonia absorbed from protein breakdown in the gut are largely extracted in zone 1). Conversely, zone 3 hepatocytes are more active in glycolysis and lipogenesis (processes requiring less oxygen). Zone 2 hepatocytes display attributes of both zone 1 and zone 3 cells.

C. Receptor-Mediated Uptake

Functional zonation applies only to processes driven by the presence of diffusible substances. The liver, however, is also involved in many pathways participating in receptor-mediated uptake and active transport of substances unable to diffuse freely into cells. These substances enter whichever hepatocytes have the appropriate transporters regardless of their zone. Similarly, substances tightly bound to carrier proteins for which the liver does not have receptors are

cleared equally poorly by hepatocytes in all three zones.

Hepatocytes: Polarized Cells with Functional Segregation All surfaces of a hepatocyte are not the same. Hepatocytes have three sides. One side, the apical surface, forms the wall of the bile canaliculus. The second side, the basolateral surface, is in contact with the bloodstream via the sinusoids. The last side, the lateral domain, is bordered by the two other surfaces. Very different activities go forward at these regions of the hepatocyte plasma membrane; tight junctions between hepatocytes serve to maintain segregation of apical and basolateral plasma membrane domains. Processes related to bile transport and excretion act at the apical plasma membrane (Figure 14–3A). Uptake from and secretion into the bloodstream are activities that occur across the basolateral membrane (Figure 14–3B).

FIGURE 14–3 A: Mechanism of bile acid secretion. About 90% of these compounds derive from bile acids absorbed in the intestinal epithelium and recirculated to the liver. The remainder are synthesized in the liver by conjugating cholic acid with the amino acids glycine and taurine. This process occurs in the smooth endoplasmic reticulum (SER). B: Protein synthesis and carbohydrate storage in the liver. Protein synthesis occurs in the rough endoplasmic reticulum, which explains why liver cell lesions or starvation leads to a decrease in the amounts of albumin, fibrinogen, and prothrombin in a patient’s blood. In several diseases, glycogen degradation is depressed, with an abnormal intracellular accumulation of this compound.

(RER, rough endoplasmic reticulum; SER, smooth endoplasmic reticulum.) (Redrawn, with permission, from Junqueira LC et al. Basic Histology, 10th ed. McGraw-Hill, 2003.)

Effects of Hepatocyte Dysfunction In view of this organization, it is perhaps not surprising that hepatocyte dysfunction can sometimes involve disruption of bile flow (cholestasis) with relative preservation of other functions. There is, however, no clear line between the consequences of disturbed apical and basolateral functions: Cholestasis, although initially a disorder of apical bile flow, is ultimately manifested at the basolateral surface. This is because it is at the basolateral surface that bilirubin and other substances to be excreted across the apical plasma membrane into the bile must first be taken up from the bloodstream. Similarly, disruption of energy metabolism or protein synthesis, although initially impinging on the secretory and metabolic processes of the hepatocyte, will ultimately affect the bile transport machinery in the apical plasma membrane as well.

Capacity for Regeneration Although the normal liver contains very few cells in mitosis, when hepatocytes are lost, poorly understood mechanisms stimulate proliferation of the remaining hepatocytes. This is why in most cases of fulminant hepatic failure with massive hepatocellular death, if the patient survives the acute period of hepatic dysfunction (usually with medical therapy in the hospital), recovery will be complete. Similarly, surgical resection of liver tissue is followed by proliferation of the remaining hepatocytes (hyperplasia). Numerous growth factors (eg, HGF, TGF-α) and cytokines (eg, TNF, IL-1, IL-6) are involved in positioning the liver on a continuum between cell proliferation and cell death.

CHECKPOINT

4. From which vascular beds do the hepatic central veins derive their blood flow?

5. Why is the liver a major site for metastasis of malignant neoplasms from other parts of the body?

6. What cell types make up the liver, and what are their dis-tinguishing characteristics?

7. What is the difference between the lobule concept and the acinus

concept of liver subarchitecture? 8. What are some physiologic consequences of functional zonation in the

liver? 9. What activities are found in zone 1 hepatocytes? In zone 3 hepatocytes? 10. Which structures normally maintain the separation of the apical and

basolateral plasma membrane domains of the hepatocyte? 11. What happens to the remaining hepatocytes when part of the liver is

surgically resected?

LIVER BLOOD FLOW & ITS CELLULAR BASIS

The portal blood flow, being venous in nature, is normally under low hydrostatic pressure (about 10 mm Hg). Accordingly, there must be little resistance to its flow within the liver, allowing the blood to percolate through the sinusoids and achieve maximal contact—for exchange of substances—with hepatocytes. Two unique features—fenestrations in the endothelial cells and lack of a typical basement membrane between endothelial cells and hepatocytes—aid in making the liver a low-pressure circuit for the flow of portal blood. These features are altered in cirrhosis, resulting in increased portal pressure and profound changes in liver blood flow, with devastating clinical consequences.

Fenestrations are spaces between the endothelial cells that make up the walls of the portal capillary system that allow plasma and its proteins, but not red blood cells, free and direct access to the surface of the hepatocytes. This feature is crucial to the liver’s function of uptake from and secretion into the bloodstream. This feature also contributes to the efficiency of the liver as a filter of portal blood. Most of the capillary beds in the body lack such fenestrations.

PHYSIOLOGY

Table 14–2 lists the diverse functions of the liver grouped into four broad categories. Although there is considerable overlap between them, systematically considering each category is a useful way to approach the patient with liver disease.

TABLE 14–2 Functions of the normal liver.

Energy Generation & Substrate Interconversion Much of the body’s carbohydrate, protein, and lipid is synthesized, metabolized, and interconverted in the liver; products are removed from or released into the bloodstream in response to the energy and substrate needs of the body.

A. Carbohydrate Metabolism

After a meal, the liver achieves net glucose consumption (eg, to synthesize glycogen and generate metabolic intermediates via glycolysis and the tricarboxylic acid cycle). This occurs as a result of a confluence of several effects. First, the levels of substrates such as glucose increase. Second, the levels of hormones that affect the amount and activity of metabolic enzymes change. Thus, when blood glucose levels increase, the ratio of insulin to glucagon in the bloodstream also increases. The net effect is increased glucose use by the liver. In times of fasting (low blood glucose) or stress (when higher blood glucose is needed), hormone and substrate levels in the bloodstream drive the liver’s metabolic pathways responsible for net glucose production (eg, the pathways of glycogenolysis and gluconeogenesis). As a result, blood glucose levels are raised to, or maintained in, the normal range in spite of wide and sudden changes in the rate of glucose input (eg, ingestion and absorption) and output (eg, use by tissues) from the bloodstream (Figure 14–4).

FIGURE 14–4 Pathways of hepatic carbohydrate and lipid metabolism. (ATP, adenosine triphosphate; CoA, co-enzyme A; HMG, hepatic 3-methylglutaryl; NAD, nicotinamide adenine dinucleotide (oxidized form); NADH, nicotinamide adenine dinucleotide hydride (reduced form); OAA, oxaloacetic acid; VLDL, very-low-density lipoprotein.) (Redrawn, with permission, from Schwartz CC. Hepatic metabolism. In: Kelley WN, ed. Textbook of Internal Medicine. Lippincott, 1989.)

B. Protein Metabolism

Related to its important role in protein metabolism, the liver is a major site for processes of oxidative deamination and transamination (Figure 14–5). These reactions allow amino groups to be shuffled among molecules to generate substrates for both carbohydrate metabolism and amino acid synthesis. Likewise, the urea cycle allows nitrogen to be excreted in the form of urea, which is much less toxic than free amino groups in the form of ammonium ions. Impairment of this function in liver disease is discussed in greater detail later.

FIGURE 14–5 Urea cycle. Dashed lines signify pathways whose extent of involvement varies from patient to patient depending on genetic, dietary, and other factors. (Redrawn, with permission, from Powers-Lee SG et al. Urea synthesis and ammonia metabolism. In: Arias IM et al, eds. The Liver: Biology and Pathology, 3rd ed. Raven Press, 1994.)

C. Lipid Metabolism

The liver is the center of lipid metabolism. The liver manufactures nearly 80% of the cholesterol synthesized in the body from acetyl-CoA via a pathway that connects the metabolism of carbohydrates with that of lipids (see Figure 14–4). Moreover, the liver can synthesize, store, and export triglycerides (see Figure 14–4). The liver is also the site of keto acid production via the pathway of fatty acid oxidation that connects lipid catabolism with activity of the tricarboxylic acid cycle.

In the process of controlling the body’s cholesterol and triglyceride levels, the liver assembles, secretes, and takes up various lipoprotein particles (Figure 14– 6). Dietary fat is first absorbed into the small intestine, then packaged into chylomicrons. Following the removal of triglycerides, the chylomicron remnant is taken up by the liver via low-density lipoprotein (LDL) receptor–mediated

endocytosis. To distribute lipids systemically, very-low-density lipoproteins (VLDLs) are secreted by the liver and transport triglycerides and cholesterol to adipose tissue for storage or to other tissues for immediate use. As triglycerides are removed, the structure of VLDL particles is modified by the loss of lipid and protein components rendering intermediate-density lipoprotein (IDL) and further downstream LDL. LDL particles are then returned to the liver via the LDL receptor. On the other hand, high-density lipoproteins (HDLs), lipoproteins synthesized and secreted from the liver, scavenge excess cholesterol and triglycerides from other tissues and from the bloodstream, returning them to the liver, where they are excreted. Thus, HDL secretion and LDL removal are both mechanisms by which cholesterol in excess of that needed by various tissues is removed from the circulation (Figures 14–6B and 14–6C).

FIGURE 14–6 Lipoprotein metabolism in the liver. A: Exogenous fat transport pathway. B: Endogenous fat transport pathway. C: Pathway of reverse cholesterol transport. In each pathway, lipoprotein particles are used to solubilize cholesteryl esters (and triglyceride), either for the purpose of import from the GI tract (A), distribution to various tissues (B), or transport to the liver for excretion in bile (C). During their circulation, specific lipoprotein particles are transformed by the addition and removal of apoproteins and by the action of enzymes in plasma or in tissues (eg, lipoprotein lipase [LPL], cholesteryl

ester transfer protein [CETP]). Intermediate-density lipoproteins (IDLs) are intermediates in the conversion of very-low-density lipoproteins (VLDLs) to low-density lipoproteins (LDLs). (HDL, high-density lipoprotein.) (Redrawn from Breslow JL. Genetic basis of lipoprotein disorders. J Clin Invest. 1989;84:373.)

Synthesis & Secretion of Plasma Proteins The liver manufactures and secretes many of the proteins found in plasma, including albumin, several clotting factors, a number of binding proteins, and even certain hormones and hormone precursors. By virtue of the actions of these proteins, the liver has important roles in maintaining plasma oncotic pressure (serum albumin), coagulation (clotting factor synthesis and modification), blood pressure (angiotensinogen), growth (insulin-like growth factor-1), and metabolism (steroid and thyroid hormone–binding proteins).

Solubilizing, Transport & Storage Functions The liver plays an important role in solubilizing, transporting, and storing a variety of very different substances that would otherwise be difficult for tissues to obtain or move in and out of cells. Specific cells in the liver perform these functions by manufacturing specialized proteins that serve as receptors, binding proteins, or enzymes.

A. Enterohepatic Circulation of Bile Acids

Bile is a detergent-like substance synthesized by the liver that permits a variety of otherwise insoluble substances to be dissolved in an aqueous environment for transport into or out of the body. Bile acids are a major component of bile and are recycled via the so-called enterohepatic circulation between the liver and the intestines. After synthesis and active transport from hepatocyte cytoplasm into the bile canaliculus (across the apical plasma membrane of the hepatocyte), bile is collected in the biliary tract (and sometimes stored in the gallbladder) and excreted via the common bile duct into the duodenum. While still in the cytoplasm of the hepatocyte, many bile acids are conjugated to sugars, which increases their water solubility. Once in the duodenum, bile acids serve to solubilize lipids, facilitating the digestion and absorption of fats. In the terminal ileum, both conjugated and deconjugated bile acids are taken up and transported from enterocytes to portal blood flow. Portal blood returns them to the liver, where specialized bile acid transporters (predominantly sodium taurocholate cotransporter [Ntcp]) return them to the hepatocyte cytosol via the basolateral plasma membrane facing the space of Disse. There they are subject to

reconjugation and secretion across the apical membrane along with other components (eg, pigment, cholesterol) to form new bile. Thereafter, they engage in another cycle of enterohepatic transport.

B. Drug Metabolism and Excretion

Most of the enzymes that catalyze metabolic processes needed to detoxify and excrete drugs and other substances are located in the smooth endoplasmic reticulum of hepatocytes. These pathways are used to metabolize not only exogenous drugs, but also many endogenous substances that would otherwise be difficult for cells to excrete (eg, bilirubin and cholesterol). In most cases, this metabolism involves the conversion of hydrophobic (lipophilic) substances (which are difficult to excrete from cells because they tend to partition into cellular membranes) into more hydrophilic (polar) substances. This process involves the catalysis of covalent modifications to make the substance more charged, so that it will partition more readily into an aqueous medium or at least be solubilized sufficiently in bile. As a result of these processes, collectively termed biotransformations, some substances that would otherwise be retained in cellular membranes can be excreted directly in the urine or transported into the bile for excretion in feces.

C. Phases of Biotransformation

Biotransformation generally occurs in two phases. Phase I reactions involve oxidation reductions in which an oxygen-containing functional group is added to the substance to be excreted. While oxidation itself does not necessarily have a major effect on water solubility, it usually introduces into the drug a reactive “handle” that makes possible other reactions that do render the modified substance water soluble. These phase II reactions usually involve covalent attachment of the drug to a water-soluble carrier molecule, such as the sugar glucuronic acid or the peptide glutathione. Unfortunately, by making substances more chemically reactive, phase I oxidation reactions often convert mildly toxic drugs into more toxic reactive intermediates. If conjugation by phase II enzymes is impaired for some other reason, the reactive intermediate can sometimes react with and damage other cellular structures. This feature of drug detoxification has important clinical implications.

D. The Role of Apolipoprotein in the Solubilization and Transport of Lipids

The detoxification and bile transport pathways allow hepatocytes to convert a

wide range of hydrophobic low-molecular-weight substances (eg, drugs and bilirubin) into more hydrophilic and hence water-soluble forms in which they can be excreted (eg, in bile or by the kidney). However, these are not the only solubilization challenges facing the body. The body also needs a mechanism that makes lipids available to various tissues (eg, to synthesize membranes) and one that removes any excess lipid the tissues do not use. For these processes to occur, lipid must be solubilized in a dispersed form that can be carried through the bloodstream. For this purpose, hepatocytes synthesize a class of specialized apolipoproteins. Apolipoproteins assemble into a variety of lipoprotein particles that transport lipids to and from various tissues by receptor-mediated endocytosis (see prior discussion of lipid metabolism).

E. The Liver’s Role in Producing Binding Proteins

Various cells in the liver synthesize proteins that bind certain substances very tightly (eg, some vitamins, minerals, and hormones). In some cases, this allows their transport in the bloodstream, where they would otherwise not be soluble (eg, steroids bound to steroid-binding globulin, which is synthesized and secreted by hepatocytes). In other cases, binding proteins made by the liver (eg, thyroid hormone–binding globulin) allow transport of specific substances (eg, thyroxine) in a form not fully accessible to tissues. In this way, the effective concentration of the substance is limited to its free concentration at equilibrium, and the tightly bound fraction forms a reservoir of the substance that is made available slowly as the free fraction is metabolized, thereby prolonging its half- life.

In some cases, binding proteins allow the liver to accumulate specific substances in relatively high concentrations and store them in a nontoxic form. Consider iron, for example, an essential nutrient. Free iron can be quite toxic to cells both directly as an oxidant and indirectly as an essential nutrient needed by infectious agents. Control of body iron occurs at the level of the enterocyte in the duodenum (see Chapter 13). Thus, the primary defect in the iron overload disorder hemochromatosis probably involves the enterocyte. Nevertheless, the liver is responsible for making a variety of proteins crucial for iron binding and metabolism. Through the actions of these proteins, the body gets the iron it needs without allowing excess free iron to cause damage or support pathogens.

Transferrin is an iron-binding protein synthesized and secreted into the bloodstream by the liver. On binding of free iron at normal pH, transferrin undergoes a conformational change that gives it high affinity for a specific membrane receptor of the hepatocyte (transferrin receptor). On receptor

binding, the transferrin–transferrin receptor complex is internalized into the endocytic pathway, a progressively more acidic environment. There, at low pH, iron no longer remains bound to transferrin. However, conformational changes that occur at low pH allow transferrin to maintain high-affinity binding to its receptor even in the absence of bound iron. Thus, when the receptor recycles back to the surface, it brings the “empty” transferrin with it. On presentation to the pH 7.4 environment of the bloodstream, transferrin lacking bound iron is released from the receptor, and the cycle can start over again. In this way, transferrin and its receptor keep the bloodstream free of unbound iron. Meanwhile, the free iron released from transferrin in the acidic environment of the endosome is transported into the cytoplasm of the hepatocyte, where it binds to ferritin, a cytoplasmic iron storage protein. This provides a reservoir that can be mobilized in response to the body’s needs but makes iron inaccessible to pathogens and keeps it from causing direct toxic effects. Similar dynamics of plasma-binding proteins, receptors, or cytosolic storage proteins occur for many other substances, including fat-soluble vitamins and steroid hormones.

Whereas most solubilization functions are performed in hepatocytes, some of the binding and storage functions involve accessory cells. Thus, vitamin A storage occurs in fat droplets seen in the lipocytes of the reticuloendothelial system. Lipocytes have been implicated in the pathogenesis of chronic liver injury and cirrhosis. Injury to other cells releases cytokines, which activate the lipocytes. The lipocytes respond by proliferating and by synthesizing collagen and other basement membrane components, leading to an increase in the extracellular matrix and contributing to hepatic fibrosis.

Protective & Clearance Functions Many of the functions of the liver already discussed (eg, drug detoxification and excretion of excess cholesterol by conversion to and solubilization in bile) can also be considered protective. Nevertheless, it is useful to conceptualize the protective function as a separate category because of its clinical importance in ameliorating the consequences of liver disease.

A. Phagocytic and Endocytic Functions of Kupffer Cells

The liver helps remove bacteria and antigens that breach the defenses of the gut to enter the portal blood and also participates in clearing the circulation of endogenously generated cellular debris. It appears that specialized receptors on the Kupffer cell surface bind to glycoproteins (via carbohydrate receptors), to

material coated with immunoglobulin (via the Fc receptor), or to complement (via the C3 receptor), thus allowing damaged plasma proteins, activated clotting factors, immune complexes, senescent blood cells, and so on to be recognized and removed.

B. Endocytic Functions of Hepatocytes

Hepatocytes have a number of specific receptors for damaged plasma proteins distinct from the receptors present on Kupffer cells (eg, the asialoglycoprotein receptor that specifically binds glycoproteins whose terminal sialic acid sugar residues have been removed). The precise physiologic significance of this metabolic action remains unclear.

C. Ammonia Metabolism

Ammonia generated from the deamination of amino acids is metabolized within hepatocytes into the much less toxic substance urea. Loss of this function results in altered mental status, a common manifestation of severe or end-stage liver disease.

D. Hepatocyte Synthesis of Glutathione

Glutathione is the major intracellular (cytoplasmic) reducing reagent and thus is crucial for preventing oxidative damage to cellular proteins. This molecule is a nonribosomally synthesized tripeptide (γ-glutamyl-cystinyl-glycine) that is also a substrate for many phase II drug detoxification conjugation reactions. The liver may also export glutathione for use by other tissues.

Some additional indirect liver functions (eg, its role in maintaining normal sodium and water balance) are inferred from the derangements observed in patients with liver disease, as discussed in the following section.

Tests to Assess Liver Function A number of blood tests are commonly used to assess liver injury. Serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) are measurements of levels of enzymes normally situated within hepatocytes. Their presence in the serum is thus actually a sign of liver cell necrosis rather than an indication of liver function.

To assess liver function more directly, a number of other tests can be used. The levels of albumin, clotting factors, and bilirubin can be measured in blood

samples. Each of these tests has advantages and disadvantages, and no one of them serves as an ideal sole indicator of liver function. For example, albumin has a relatively long half-life (18–20 days); its synthesis can be stimulated in excess of need, and it can be lost via the kidneys in renal disease. Furthermore, about two-thirds of body albumin is located in the extravascular, extracellular space, so changes in fluid distribution can alter serum albumin concentration. Likewise, the simplest measure of clotting factor levels, the prothrombin time (PT), is a relatively insensitive measure because it does not become abnormal until more than 80% of hepatic synthetic capacity is lost. Furthermore, vitamin K deficiency, occurring in patients with nutritional deprivation, chronic cholestasis, or fat malabsorption, can prolong the PT. Serum bilirubin is a good measure of cholestasis, and determining conjugated (direct) versus unconjugated (indirect) bilirubin provides a good assessment of whether cholestasis is intrinsic to the liver or due solely to obstruction (eg, by a stone in the common bile duct). Furthermore, cholestasis, even when caused by liver disease, often does not reflect the degree to which other hepatic functions are lost, and unconjugated hyperbilirubinemia can occur for other reasons (eg, hemolysis).

For these reasons, an accurate assessment of liver function requires several blood tests (eg, AST, ALT, albumin, PT, bilirubin), as well as a clinical assessment of the patient.

The two most common schemes for grading liver function are the modified Child–Pugh score (Table 14–3) and the Model for End-Stage Liver Disease (MELD) score (MELD score = 3.78[Ln serum bilirubin (mg/dL)] + 11.2[Ln INR] + 9.57[Ln serum creatinine (mg/dL)] + 6.43). The Child–Pugh score predicts 1- and 2-year survival, whereas the MELD score predicts short-term (3- month) survival. In the United States, the MELD score is currently used to prioritize patients for donor allocation and liver transplantation. However, the patient’s serum sodium has been found to predict waitlist mortality independent of the MELD score, and its incorporation into the MELD score to generate the MELD-Na score has been shown to improve its accuracy with respect to predicting waitlist mortality (MELD-Na = MELD + 1.32 × (137-Na) − [0.033 × MELD × (137-Na)]). Accordingly, to improve the allocation of donor livers in the United States, the Organ Procurement and Transplantation Network has changed its policy to adopt the newer MELD-Na score instead of the older MELD score for determining priority for liver transplantation.

TABLE 14–3 Grading liver function using the modified Child–Pugh score.

CHECKPOINT

12. What are the roles of the liver in carbohydrate, protein, and lipid metabolism?

13. What are two physiologic mechanisms by which the body transports cholesterol?

14. Explain phase I and phase II reactions in drug detoxification. 15. Name and explain four of the liver’s clearance or protec-tive functions. 16. What specializations allow the liver normally to be a low-pressure

conduit for blood flow?

OVERVIEW OF LIVER DISEASE

TYPES OF LIVER DYSFUNCTION

Most of the clinical consequences of liver disease can be understood either as a failure of one of the liver’s four broad functions (summarized in Table 14–2) or as a consequence of portal hypertension, the altered hepatic blood flow of cirrhosis.

Hepatocyte Dysfunction One mechanism of liver disease, particularly in acute liver injury, is dysfunction of the individual hepatocytes that make up the liver parenchyma. The pathway and extent of hepatocellular dysfunction determine the specific manifestations of liver disease. The outcomes to anticipate when normal hepatic functions fail are described later.

Portal Hypertension Some consequences of liver disease, particularly of cirrhosis, are best understood in terms of what we know about hepatic blood flow. Of greatest clinical importance are the existence under normal circumstances of a low-pressure portal venous capillary bed throughout the liver parenchyma and the functional zonation of portal blood flow.

When pathologic processes (eg, fibrosis) result in an elevation of the normally low intrahepatic venous pressure, blood backs up and a substantial fraction of it finds alternative routes back to the systemic circulation, bypassing the liver. Thus, blood from the GI tract is, in effect, filtered less efficiently by the liver before entering the systemic circulation. The consequences of this portal- to-systemic shunting are loss of the protective and clearance functions of the liver, functional abnormalities in renal salt and water homeostasis, and a greatly increased risk of GI hemorrhage from the development of engorged blood vessels carrying venous blood bypassing the liver (eg, esophageal, gastric, and umbilical varices).

Even in the absence of any intrinsic parenchymal liver disease, portal-to- systemic blood shunting can produce or contribute to encephalopathy (altered mental status resulting from failure to clear poisons absorbed from the GI tract), GI bleeding (resulting from esophageal varices), and the malabsorption of fats and fat-soluble vitamins (caused by the loss of the enterohepatic recirculation of bile), with associated coagulopathy. Table 14–4 categorizes the syndromes

observed in liver disease as being a consequence of hepatocyte dysfunction, portal-to-systemic shunting, or both.

TABLE 14–4 Pathophysiology of syndromes of aberrant function in liver disease.

Pathophysiology of Functional Zonation The fact that hepatocytes in the different zones of the acinus “see” blood in a particular sequence has great pathophysiologic significance. Because zone 1 hepatocytes receive blood that has just left the portal venule or hepatic arteriole, they have access to the highest concentrations of various substances, both good (eg, oxygen, nutrients) and bad (eg, drugs and toxins absorbed from the GI tract). Zone 2 hepatocytes receive blood containing less of these substances, and zone 3 hepatocytes are bathed in blood largely depleted of them. However, zone 3 hepatocytes see the highest concentrations of products (eg, drug metabolites) released into the bloodstream by the hepatocytes of zones 1 and 2. Thus, direct poisons have their most severe impact on zone 1 hepatocytes, whereas poisons generated as a result of hepatic metabolism cause more damage to zone 3 hepatocytes. Similarly, because sinusoidal blood around zone 3 has the lowest oxygen concentration, hepatocytes of this zone are at the greatest risk of injury under conditions of hypoxia.

MANIFESTATIONS OF LIVER DYSFUNCTION

Whether as a result of hepatocyte dysfunction or portal-to-systemic shunting, failure of normal hepatic function underlies the clinical manifestations of liver disease. An understanding of these mechanisms offers insight into the probable causes of illness in a patient with acute or chronic liver disease.

Diminished Energy Generation & Substrate Interconversion A first category of altered liver function involves the intermediary metabolism of carbohydrates, fats, and proteins.

A. Carbohydrate Metabolism

Severe liver disease can result in either hypoglycemia or hyperglycemia. Hypoglycemia results largely from a decrease in functional hepatocyte mass, whereas hyperglycemia is a result of portal-to-systemic shunting, which decreases the efficiency of postprandial glucose extraction from portal blood by hepatocytes, thus elevating the systemic blood glucose concentration.

B. Lipid Metabolism

Disturbance of lipid metabolism in the liver can result in syndromes of fat accumulation within the liver early in the course of liver injury. Perhaps this is because the complex steps involved in assembling lipoprotein particles for the export of cholesterol and triglycerides from the liver are more sensitive to disruption than are the pathways of lipid synthesis. Such disruption results in a buildup of fat that cannot be exported in the form of VLDL.

In certain chronic liver diseases such as primary biliary cirrhosis, bile flow decreases as a result of the destruction of bile ducts. The decrease in bile flow results in decreased lipid clearance via bile, with consequent hyperlipidemia. These patients often develop subcutaneous accumulations of cholesterol termed xanthomas.

C. Protein Metabolism

Any disturbance of protein metabolism in the liver can result in a syndrome of altered mental status and confusion known as hepatic encephalopathy. As with carbohydrate metabolism, altered protein metabolism can result from either hepatocyte failure or portal-to-systemic shunting, with the net effect of elevated blood concentrations of centrally acting toxins, including ammonia generated by amino acid metabolism.

Loss of Solubilization & Storage Functions A. Disordered Bile Secretion The clinical significance of bile synthesis can be seen in the prominence of cholestasis—failure to secrete bile—in many forms of liver disease. Cholestasis can occur as a result of extrahepatic obstruction (eg, from a gallstone in the common bile duct) or selective dysfunction of the bile synthetic and secretory machinery within the hepatocytes themselves (eg, from a reaction to certain drugs). The mechanisms responsible for cholestatic drug reactions are not well understood. Regardless of the mechanism, however, the clinical consequences of severe cholestasis may be profound: A failure to secrete bile results in a failure to solubilize substances such as dietary lipids and fat-soluble vitamins, resulting in malabsorption and deficiency states, respectively. Retained bile salts are also cytotoxic, but in the setting of cholestasis, hepatocytes adapt to the decrease in bile salt uptake by downregulating Na+–bile acid cotransporter while maintaining bile salt excretion. As a result, hepatic necrosis is minimized in

predominantly cholestatic syndromes, with the typical laboratory findings of minimally elevated levels of AST and ALT in the presence of marked jaundice and high levels of bilirubin. However, prolonged exposure to bile salts in chronic cholestatic diseases, such as primary biliary cirrhosis, leads to portal tract cytotoxic injury and inflammation, leading eventually to fibrosis and cirrhosis.

The solubilization function of bile works both to excrete and absorb substances. Thus, in cholestasis, endogenous substances normally excreted via the biliary tract can accumulate to high levels. One such substance is bilirubin, a product of heme degradation (Figure 14–7). The buildup of bilirubin results in jaundice (icterus), which is a yellow discoloration of the sclera and skin. In the adult, the most significant feature of jaundice is that it serves as a readily monitored index of cholestasis, which may occur alone or with other abnormalities in hepatocyte function (ie, as part of the presentation of acute hepatitis). In the neonate, however, elevated bilirubin concentrations can be toxic to the developing nervous system, producing a syndrome termed kernicterus.

FIGURE 14–7 Bilirubin secretion. This water-insoluble compound is derived from the metabolism of hemoglobin in macrophages of the mononuclear phagocyte system. Glucuronyl transferase activity in the hepatocytes causes bilirubin to be conjugated with glucuronide in the smooth endoplasmic reticulum, forming a water-soluble compound. An accumulation of bilirubin and bilirubin glucuronide in the tissues produces jaundice. Several defective processes in the hepatocytes can cause diseases that produce jaundice: a defect in the capacity of the cell to trap and absorb bilirubin (rectangle 1), the inability of the cell to conjugate bilirubin because of a deficiency in glucuronyl transferase (rectangle 2), or problems in the transfer and excretion of bilirubin glucuronide into the biliary canaliculi (rectangle 3). One of the most frequent causes of jaundice, however—unrelated to hepatocyte activity—is the obstruction of bile flow as a result of gallstones or tumors of the pancreas. This causes jaundice primarily as a result of the accumulation

of bilirubin glucuronide in the tissues. (Redrawn, with permission, from Junqueira LC et al, eds. Basic Histology, 10th ed. McGraw-Hill, 2003.)

Similarly, cholesterol is normally excreted either by conversion into bile acids or by forming complexes, termed micelles, with pre-existing (recycled) bile acids. In cholestasis, the resultant buildup of bile acids can lead to their deposition in the skin. This is believed to cause intense itching, or pruritus. Data suggest that, in at least some patients, cholestasis results in altered levels of endogenous opioids. Instead of skin deposition of bile acids, altered endogenous opioid-mediated neurotransmission may be responsible for pruritus. Disorders of bile production are a basis for the formation of cholesterol gallstones. Nevertheless, as mentioned, other hepatocyte functions are often relatively well preserved in the face of significant cholestasis. Table 14–5 summarizes the syndromes that produce jaundice.

TABLE 14–5 Laboratory findings in the differential diagnosis of jaundice.

Hemolysis causes an unconjugated hyperbilirubinemia because the hepatic capacity to take up and conjugate bilirubin is exceeded. Gilbert syndrome reflects a genetic defect in bilirubin conjugation. Thus, the findings in blood and urine are different from what is observed in hemolytic jaundice even though the pathway of bilirubin metabolism is backed up at a similar initial point. Extrahepatic biliary tract obstruction presents the other extreme, in which the actual pathway of bile formation is entirely intact, at least initially. In

obstruction, the bilirubin level in the urine is high because the backed-up metabolite is conjugated and hence much more water soluble than unconjugated bilirubin, which accumulates in hemolysis. Most forms of jaundice that result from liver dysfunction caused by hepatocellular damage reflect variable degrees of overlap between unconjugated and conjugated hyperbilirubinemia.

B. Impaired Drug Detoxification

Two features of the mechanisms of drug detoxification are of particular clinical importance. One is the phenomenon of enzyme induction. It is observed that the presence in the bloodstream of any of the large class of drugs inactivated by phase I enzymes increases the amount and activity of these enzymes in the liver. This property of enzyme induction makes physiologic sense (as a response to the body’s need for increased biotransformation) but can have undesired effects as well: A patient who chronically consumes large amounts of a substance metabolized by phase I enzymes (eg, ethanol) will induce the formation of these enzymes and thus speed up the metabolism of other substances metabolized by the same detoxifying enzymes (eg, antiseizure or anticoagulant medications, resulting in subtherapeutic blood levels of the drugs).

A second clinically important phenomenon in drug metabolism is that phase I reactions often convert relatively benign compounds into more reactive and hence more toxic ones. Normally, this heightened reactivity of phase I reaction products serves to facilitate phase II reactions, making detoxification more efficient. However, under certain conditions in which phase II reactions are impaired (eg, during glutathione deficiency from inadequate nutrition), continued phase I enzyme activity can cause increased liver injury. This is because the products of many phase I reactions, in the absence of glutathione, react with and damage cellular components. Such damage rapidly kills the hepatocytes.

Thus, the combined effects of certain common conditions can make the individual abnormally sensitive to the toxic effects of drugs. For example, the combination of induced phase I activity (eg, caused by alcoholism) with low phase II activity (eg, caused by low glutathione levels from nutritional deprivation) can result in the heightened generation of reactive intermediates with an inadequate capacity to conjugate and detoxify them. A classic example of this phenomenon is acetaminophen toxicity. As little as 2.5 g of acetaminophen can produce significant liver damage in such susceptible individuals, whereas normal individuals have the capacity to detoxify 10 g/d or more. Table 14–6 lists common drugs and chemicals that cause morphologically

distinctive changes in the liver.

TABLE 14–6 Principal alterations of hepatic morphology produced by some commonly used drugs and chemicals.1

C. Lipoprotein Dynamics and Dyslipidemias

The liver’s role in lipid metabolism is illustrated by the genetic defect causing familial hypercholesterolemia. Lack of a functional LDL receptor in such cases renders the liver unable to clear LDL cholesterol from the bloodstream, resulting in markedly elevated serum cholesterol and accelerated atherosclerosis and coronary artery disease. Heterozygotes with one normal LDL receptor allele can be treated with drugs (eg, HMG-CoA reductase inhibitors) that inhibit endogenous cholesterol synthesis and thus upregulate LDL receptor levels. However, there is no effective drug therapy for homozygotes because they have no normal LDL receptors. Hepatic transplantation is an effective therapy for homozygous familial hypercholesterolemia because it provides a genetically different liver with normal LDL receptors.

In acquired liver diseases, the serum cholesterol is elevated in biliary tract obstruction as a result of a blockage of cholesterol excretion in bile, and it is diminished in severe alcoholic cirrhosis, in which fat malabsorption prevents cholesterol intake.

D. Altered Hepatic Binding and Storage Functions

Liver disease influences the liver’s ability to store various substances. As a result, patients with liver disease are at high risk for certain deficiency states such as folic acid and vitamin B12 deficiency. Because these vitamins are needed for DNA synthesis, their deficiency results in macrocytic anemia (low red blood cell count with large red cells reflecting abnormal nuclear maturation), a common finding in patients with liver disease.

Diminished Synthesis & Secretion of Plasma Proteins The clinical significance of liver protein synthesis and secretion derives from the wide range of functions carried out by these proteins. For example, because albumin is the major contributor to plasma oncotic pressure, hypoalbuminemia as a consequence of liver disease or nutritional deficiency presents with marked edema formation. Other important proteins synthesized and secreted by the liver include clotting factors and hormone-binding proteins.

Loss of Protective & Clearance Functions A crucial protective function of the liver is its role as a filter of blood from the GI tract, by which various substances are removed from portal blood before it re- enters the systemic circulation.

A. Clearance of Bacteria and Endotoxins

The clearance of bacteria by the liver’s Kupffer cells is the final line of defense in keeping gut-derived bacteria and their endotoxins out of the systemic circulation. Loss of this capacity in liver disease as a result of portal-to-systemic shunting may help to explain why, in patients with severe liver disease, infections can rapidly become systemic and result in sepsis and the effects of endotoxins.

B. Altered Metabolism of Ammonia

Impairment of the liver’s ability to detoxify ammonia to urea leads to hepatic encephalopathy that manifests as an altered mental status. While several overlapping mechanisms have been implicated in the pathogenesis of hepatic encephalopathy, ammonia is the most well-characterized neurotoxin that precipitates encephalopathy. Ammonia is primarily produced from deamination of glutamine by glutaminase in the enterocytes of the small bowel and colon but is also produced from urea hydrolysis via the bacterial catabolism of nitrogenous

sources, including dietary protein and urea. The intact liver clears nearly all the ammonia that arrives via the portal vein by converting it to glutamine, which prevents ammonia from entering into the systemic circulation. Increased ammonia in the bloodstream can be the direct consequence of impaired liver function (either acute hepatocellular dysfunction or progressive chronic disease) and/or significant shunting of blood around the liver (portal-to-systemic shunting) with consequent bypass of its clearance mechanisms.

Precipitants of increased ammonia levels in the bloodstream and consequent altered mental status include the following: (1) increased protein intake (urea hydrolysis via bacterial catabolism of nitrogenous sources); (2) GI bleeding (increased levels of ammonia and other nitrogenous substances are produced by the breakdown of blood proteins by GI tract microbes); and (3) the systemic inflammatory response to infection (stimulation of proinflammatory cytokine release and endogenous protein catabolism, leading to elevated ammonia production). Thus, once increased protein intake has been excluded, the development of encephalopathy in a patient with chronic liver disease calls for an investigation of possible acute GI bleeding, as well as a search for a potentially catastrophic infection. Pending the outcome of diagnostic studies (eg, serial hemoglobin and hematocrit measurements and cultures of blood, urine, and ascitic fluid), therapy is aimed at diminishing the absorption of ammonia and other noxious substances from the GI tract. When the patient is given the nonabsorbable carbohydrate lactulose, its metabolism by microbes creates an acidic environment. Ammonia is trapped as charged NH4

+ species in the gut lumen and excreted by the resultant osmotic diarrhea. In this way, the toxin is prevented from ever entering the portal circulation, and the patient’s mental status gradually improves. Lactulose also selects for a gut bacterial flora that produces less ammonia. Antibiotics, and in particular rifaximin, have been used in conjunction with lactulose to treat hepatic encephalopathy. Antibiotics are thought to act by decreasing the intestinal production and absorption of ammonia by modulating the intestinal microbiota and preventing bacterial translocation across the gut mucosal surface.

Furthermore, the resulting elevations in blood ammonia and other nitrogen- containing compounds can upregulate peripheral receptors for endogenous benzodiazepine-like products. These effects may also contribute to altered systemic hemodynamics in liver disease.

C. Altered Hormone Clearance in Liver Disease

Normally, the liver removes from the bloodstream the fraction of steroid

hormones not bound to steroid hormone–binding globulin. On uptake by hepatocytes, these steroids are oxidized, conjugated, and excreted into bile, where a fraction undergoes enterohepatic circulation. In liver disease accompanied by significant portal-to-systemic shunting, steroid hormone clearance is diminished, extraction of the circulated enterohepatic fraction is impaired, and enzymatic conversion of androgens to estrogens (peripheral aromatization) is increased. The net effect is an elevation of blood estrogens, which in turn alters hepatocyte protein synthesis and secretion, together with the activation of the metabolizing P450 enzymes. The synthesis of some hepatic proteins increases, whereas the synthesis of others is diminished. Enzymatic P450 activity increases as the liver attempts to partially compensate for the higher blood estrogen levels by increased metabolism. Thus, male patients with liver disease display both gonadal and pituitary suppression as well as feminization.

Sodium & Water Balance Patients with liver disease often display renal abnormalities and complications, most commonly sodium retention and difficulty excreting water. An intrinsic renal lesion is apparently not involved, because the kidneys of patients with liver disease typically function normally when transplanted into patients whose liver is normal. Instead, renal abnormalities associated with liver disease are functional, occurring because liver disease induces altered intravascular pressures and perhaps because of elevated nitric oxide levels or the loss of as yet poorly understood factors secreted from the liver or the endothelium. By whatever homeostatic mechanisms, intravascular volume is perceived as being inadequate when it is actually only maldistributed. Renal mechanisms of salt and water retention are then stimulated to correct what has been sensed as volume depletion. Table 14–7 summarizes some factors that influence renal sodium retention in liver disease. Patients with severe liver disease are at risk for renal failure related to these hemodynamic and neurohumoral alterations.

TABLE 14–7 Factors influencing renal sodium retention in liver disease.

CHECKPOINT

17. Under what circumstances is hypoglycemia seen in liver disease? 18. Name three clinical consequences of cholestasis. 19. The development of hepatic encephalopathy in a patient with chronic

liver disease should lead you to investigate what possible precipitating factors?

20. By what mechanisms can coagulation defects be a con-sequence of liver disease?

21. What is an explanation for hypogonadism in male patients with liver disease?

PATHOPHYSIOLOGY OF SELECTED LIVER DISEASES

ACUTE HEPATITIS

Acute hepatitis is an inflammatory process causing liver cell death either by necrosis or by triggering apoptosis (programmed cell death). A wide range of clinical entities can cause global hepatocyte injury of sudden onset. Worldwide, acute hepatitis is most commonly caused by infection with one of several types of virus. Although these viral agents can be distinguished by serologic laboratory tests based on their antigenic properties, all produce clinically similar illnesses. Other less common infectious agents can also cause liver injury (see Table 14– 1). Acute hepatitis is also sometimes caused by exposure to drugs (eg, isoniazid) or poisons (eg, ethanol).

Clinical Presentation The severity of illness in acute hepatitis ranges from asymptomatic and clinically unapparent to fulminant and potentially fatal. The clinical presentation of acute hepatitis can also be quite variable. Some patients are relatively asymptomatic, with abnormalities noted only by laboratory studies. Others may have a range of symptoms and signs, including anorexia, fatigue, weight loss, nausea, vomiting, right upper quadrant abdominal pain, jaundice, fever, splenomegaly, and ascites. The extent of hepatic dysfunction can also vary tremendously, correlating roughly with the severity of liver injury. The relative extent of cholestasis versus hepatocyte necrosis is also highly variable. Figure 14–8 illustrates the potential interrelationship of acute hepatitis, chronic hepatitis, and cirrhosis.

FIGURE 14–8 Clinical syndromes associated with hepatitis: acute hepatitis (1) is sometimes associated with intrahepatic cholestasis (and thus termed “cholestatic hepatitis”) (2). Fulminant hepatitis (3) is associated with massive necrosis and has a high mortality rate. Chronic viral hepatitis may lead to a carrier state without (4) or with (5) continuing hepatocyte necrosis. Chronic hepatitis associated with continuing necrosis often progresses to cirrhosis, whereas chronic hepatitis associated simply with a carrier state does not. (Redrawn, with permission, from Chandrasoma P et al, eds. Concise Pathology, 3rd ed. Originally published by Appleton & Lange. Copyright © 1998 by The McGraw-Hill Companies, Inc.)

Etiology A. Acute Viral Hepatitis Acute viral hepatitis is commonly caused by one of five major viruses: hepatitis A virus (HAV), hepatitis B virus (HBV), hepatitis C virus (HCV), hepatitis D

virus (HDV), and hepatitis E virus (HEV). Table 14–8 summarizes important characteristics of these viral agents. Other viral agents that can cause acute hepatitis, though less commonly, include the Epstein–Barr virus (cause of infectious mononucleosis), cytomegalovirus, varicella virus, measles virus, herpes simplex virus, rubella virus, and yellow fever virus. A newly discovered DNA virus, SEN virus, may be associated with transfusion-associated acute hepatitis not attributable to other viruses.

TABLE 14–8 Characteristics of various types of viral hepatitis.

HAV, a small RNA virus, causes liver disease both by directly killing hepatocytes and by stimulating the host’s immune response to infected hepatocytes. It is spread by the fecal–oral route from infected individuals.

Although most cases are mild, hepatitis A occasionally causes fulminant liver failure and massive hepatocellular necrosis, resulting in death. Regardless of the severity, patients who recover do so completely, show no evidence of residual liver disease, and have antibodies that protect them from reinfection.

HBV is a DNA virus transmitted through sexual contact or contact with infected blood or other body fluids. Perinatal and early childhood transmission is the most common mode of HBV acquisition worldwide, whereas sexual transmission is most common among adults in the United States. This virus does not kill the cells it infects. Rather, the infected hepatocytes die almost exclusively as a consequence of attack by the immune system after recognition of viral antigens on the hepatocyte surface. Although most cases of hepatitis B infection are asymptomatic or produce only mild disease, an excessive immune response can result in acute liver injury and even liver failure. In a minority of those infected as adults but a majority of individuals infected at birth, the immune response is inadequate to clear the virus and chronic hepatitis B develops. The incidence of infection has significantly declined in the era of HBV vaccination, although prevalence remains high, in part because of the immigration of infected patients from endemic countries. Although the true burden of chronic hepatitis B infection in the United States is unknown, it is estimated that 1.25 million Americans are infected with HBV, with a likely higher prevalence among those who are foreign born. Indeed, data suggest that the number of foreign-born individuals with chronic HBV living in the United States may be greater than previously reported, and the actual prevalence of chronic HBV infection may be as high as 2.2 million persons. Additionally, complications of HBV-induced liver disease result in 3000–5000 deaths each year in the United States.

HCV is an RNA virus, also transmitted by blood and body fluids; it causes a form of hepatitis similar to HBV infection but with a far greater proportion of cases (60–85%) progressing to chronic hepatitis. Acute infection is characterized by mild to moderate illness but is usually asymptomatic. However, chronic HCV can lead to life-threatening complications, including cirrhosis and hepatocellular carcinoma (HCC), usually after decades of infection. It is estimated that between 2.7 and 3.9 million Americans are infected with HCV, many unaware of their infection, and the rate of attributable mortality is rising, currently at approximately 12,000 deaths each year. The Centers for Disease Control and Prevention (CDC) estimates that persons born during 1945–1965 account for approximately three-fourths of all HCV infections in the United States; this cohort thus represents a high-prevalence group that warrants universal screening.

Based on joint guidelines from the American Association for the Study of Liver Diseases (AASLD) and the Infectious Diseases Society of America (IDSA), additional high-prevalence groups that warrant HCV screening include the following: (1) people with a history of high-risk behaviors, such as injection drug or intranasal drug use; (2) people exposed to risks, such as children born to HCV-infected women; (3) people who have been exposed to HCV-infected blood via long-term hemodialysis, tattoos, or needlestick or mucosal exposure; (4) people who received blood products or organ transplants prior to 1992; (5) people who received clotting factor concentrates prior to 1987; (6) people who have been incarcerated; (7) people with human immunodeficiency virus (HIV) infection; and (8) people with unexplained chronic hepatitis or liver disease.

HDV, also known as delta agent, is a defective RNA virus that requires HBV helper functions to cause infection. Thus, individuals chronically infected with HBV are at high risk for HDV infection, whereas those who have been vaccinated against HBV are at no risk. HDV infection occurs either as coinfection with HBV or superinfection in the setting of chronic HBV. HDV infection causes a much more severe form of hepatitis, both in terms of the proportion of fulminant cases of hepatitis B and in the percentage of cases that progress to chronic hepatitis B. In North America, HDV coinfection primarily occurs in high-risk groups, such as injection drug users and those with hemophilia, and in up to 9% of those high-risk patients who have chronic HBV infection. In the United States, the prevalence of HDV coinfection in the general HBV-infected population is not well known.

HEV is an unclassified RNA virus, and like HAV, it is spread primarily via the fecal–oral route. Worldwide, HEV causes approximately 20 million infections and 70,000 deaths annually. Epidemics and acute illnesses predominantly occur in areas of high endemicity, including Asia, Africa, and Mexico, where transmission is propagated by poor sanitation and hygienic practices. In industrialized countries, the incidence of HEV infections has increased over the past decade, with seroprevalence estimated to range between 6% and 26% in the United States, in part owing to travel to endemic areas, to close environmental contact with farm animals, and to consumption of contaminated produce, as well as to increasing clinical recognition. Acute HEV infection is generally benign and self-limited, but can progress to chronic hepatitis, especially in immunocompromised patients, including organ transplant recipients and HIV-infected individuals. Many (up to 70%) pregnant women infected with HEV during their third trimester progress to acute liver failure. Individuals with chronic liver disease are also at increased risk for hepatic

decompensation and mortality upon acquiring acute HEV. HEV remains an under-recognized clinical entity because the laboratory test for HEV viral load is not routinely available in clinical practice. Indeed, it is suspected that many cases of acute HEV infection are misdiagnosed as cases of drug-induced liver injury.

B. Toxic Hepatitis

Most cases of drug-induced liver injury present as acute hepatitis, although some present as cholestasis or with other patterns (see Table 14–6). The incidence of drug-induced hepatitis has been rising. Although fewer than 10% of drug- induced liver injury cases progress to acute liver failure, acetaminophen is now the most common cause of acute liver failure in the United States and the United Kingdom. Hepatic toxins can be further subdivided into those for which hepatic toxicity is predictable and dose dependent for most individuals (eg, acetaminophen) and those that cause unpredictable (idiosyncratic) reactions without relationship to dose (Table 14–9).

C. Alcoholic Hepatitis

Alcohol consumption is the major cause of the burden of liver disease burden in Western countries. Alcoholic liver disease comprises a spectrum of injury, including simple steatosis, acute alcoholic hepatitis, and cirrhosis, and many patients with advanced liver disease present with “acute-on-chronic” liver injury as a result of ongoing alcohol consumption. Acute alcoholic hepatitis can vary in severity from mild and self-limited to severe and life-threatening. Alcoholic hepatitis is associated with female sex, prolonged heavy alcohol consumption, and binge drinking. There is considerable variation among individuals in the amount of ethanol required to cause acute liver injury; this reflects the complex interplay of genetic, nutritional, and environmental factors that contribute to alcoholic liver disease.

Pathogenesis A. Viral Hepatitis The viral agents responsible for acute hepatitis first infect the hepatocyte. During the incubation period, intense viral replication in the liver cell leads to the appearance of viral components (first antigens, later antibodies) in urine, stool, and body fluids. Liver cell death and an associated inflammatory response then ensue, followed by changes in laboratory tests of liver function and the

appearance of various symptoms and signs of liver disease.

1. Liver damage—Host immunologic response plays an important though incompletely understood role in the pathogenesis of liver damage. In hepatitis B, for example, the virus is probably not directly cytopathic. Indeed, there are asymptomatic HBV carriers who have normal liver function and histologic features. Instead, the host’s cellular immune response has an important role in causing liver cell injury. Patients with defects in cell-mediated immunity are more likely to remain chronically infected with HBV than to clear the infection. Histologic specimens from patients with HBV-related liver injury demonstrate lymphocytes next to necrotic liver cells. It is thought that cytolytic T lymphocytes become sensitized to recognize hepatitis B viral antigens (eg, small quantities of hepatitis B surface antigen [HBsAg]) and host antigens on the surfaces of HBV-infected liver cells.

2. Extrahepatic manifestations—Immune factors may also be important in the pathogenesis of the extrahepatic manifestations of acute viral hepatitis. For example, in hepatitis B, a serum sickness–like prodrome characterized by fever, urticarial rash and angioedema, and arthralgias and arthritis appears to be related to immune complex–mediated tissue damage. During the early prodrome, circulating immune complexes are composed of HBsAg in high titer in association with small quantities of anti-HBs. These circulating immune complexes are deposited in blood vessel walls, leading to activation of the complement cascade. In patients with arthritis, serum complement levels are depressed, and complement can be detected in circulating immune complexes containing HBsAg, anti-HBs, immunoglobulin (Ig) G, IgM, IgA, and fibrin.

Cryoglobulinemia is a common finding in chronic hepatitis C infection. Diabetes mellitus also occurs frequently and is now considered an extrahepatic manifestation of HCV. Although the mechanism is not fully understood, it is thought to be predominantly related to an increase in insulin resistance. Insulin resistance appears to diminish following HCV therapy.

Immune factors are thought to be important in the pathogenesis of some clinical manifestations in patients who become chronic HBsAg carriers after acute hepatitis. For example, in patients developing glomerulonephritis with nephrotic syndrome, histopathologic investigation demonstrates deposition of HBsAg, immunoglobulin, and complement in the glomerular basement

membrane. In patients developing polyarteritis nodosa, similar deposits have been demonstrated in affected small and medium-sized arteries.

Other rarer extrahepatic manifestations include papular acrodermatitis and Guillain–Barré syndrome for HBV, and idiopathic thrombocytopenic purpura, lichen planus, Sjögren syndrome, lymphoproliferative diseases, membranoproliferative glomerulonephritis, and porphyria cutanea tarda for HCV.

B. Toxic Hepatitis

The pathogenesis of drug-induced liver injury is not well understood. Table 14–9 and Figure 14–9 summarize speculations on the mechanisms of idiosyncratic and dose-related drug-induced liver injury. Idiosyncratic drug reactions may be due to genetic predisposition in susceptible individuals to certain pathways of drug metabolism that generate toxic intermediates.

TABLE 14–9 Idiosyncratic drug reactions and the affected cells.

FIGURE 14–9 Potential mechanisms of drug-induced liver injury. The normal hepatocyte may be affected adversely by drugs through (A) disruption of intracellular calcium homeostasis that leads to the disassembly of actin fibrils at the surface of the hepatocyte, resulting in blebbing of the cell membrane, rupture, and cell lysis; (B) disruption of actin filaments next to the canaliculus (the specialized portion of the cell responsible for bile excretion), leading to a loss of villous processes and the interruption of transport pumps such as multidrug resistance–associated protein 3 (MRP3), which in turn prevents the excretion of bilirubin and other organic compounds; (C) covalent binding of heme-containing cytochrome P450– metabolizing enzymes to the drug, thus creating nonfunctioning adducts; (D) migration of these enzyme– drug adducts to the cell surface in vesicles to serve as target immunogens for cytolytic attack by T cells, stimulating an immune response involving cytolytic T cells and cytokines; (E) activation of apoptotic pathways by tumor necrosis factor (TNF) receptor or Fas (“DD” denotes “death domain”), triggering the cascade of intercellular caspases, resulting in programmed cell death; or (F) inhibition of mitochondrial function by a dual effect on both β-oxidation and the respiratory-chain enzymes, leading to failure of free fatty acid metabolism, a lack of aerobic respiration, and an accumulation of lactate and reactive oxygen species (which may disrupt mitochondrial DNA). Toxic metabolites excreted in bile may damage bile-duct epithelium (not shown). (CTLs, cytolytic T lymphocytes.) (Reproduced, with permission, from Lee WM. Drug-induced hepatotoxicity. N Engl J Med. 2003;349:474. Copyright © 2003 Massachusetts Medical Society. Reprinted, with permission, from Massachusetts Medical Society.)

Prominent examples of drugs causing acute liver failure that have been withdrawn from the U.S. market include bromfenac, a nonsteroidal anti- inflammatory drug (NSAID), and troglitazone sulfate, a thiazolidinedione used as an insulin-sensitizing agent in diabetes mellitus. Other thiazolidinediones such as rosiglitazone and pioglitazone do not seem to have the same complication, although routine testing of transaminases has been recommended for those taking the drugs. HMG-CoA reductase inhibitors (eg, “statins”) are associated with elevated levels of transaminases in fewer than 3% of individuals but very rarely result in clinical acute liver failure.

Drug- and toxin-induced hepatitis typically occurs at any time during or shortly after exposure and resolves with discontinuation of the offending agent. This is usually the case for both idiosyncratic and dose-dependent reactions.

C. Alcoholic Hepatitis

Ethanol has both direct and indirect toxic effects on the liver, as well as effects on many other organ systems. Table 14–10 lists the mechanisms thought to be responsible for ethanol-induced liver injury. Its direct effects may result from increasing the fluidity of biologic membranes and thereby disrupting cellular functions. Its indirect effects on the liver are in part a consequence of its metabolism. Ethanol is sequentially oxidized to acetaldehyde and then to acetate, with the generation of nicotinamide adenine dinucleotide hydride (NADH) and adenosine triphosphate (ATP). As a result of the high ratio of reduced to oxidized NAD generated, the pathways of fatty acid oxidation and gluconeogenesis are inhibited, whereas fatty acid synthesis is promoted.

TABLE 14–10 Mechanisms of hepatocyte injury by ethanol.

Ethanol can also quantitatively and qualitatively alter the pattern of gene expression in various tissues but especially in the liver, resulting in impaired homeostasis and greater sensitivity to other toxins. These and other biochemical mechanisms may contribute to the common observation of fat accumulation in the liver and the tendency of hypoglycemia to develop in individuals with alcoholism whose liver glycogen has been depleted by fasting. Ethanol metabolism also affects the liver by generating acetaldehyde, which reacts with primary amino groups to inactivate enzymes, resulting in direct toxicity to the hepatocyte in which it is generated. Proteins so modified may activate the immune system against antigens that were previously tolerated as “self.” Alcohol metabolism results in oxidative stress and hepatocyte death, and damaged hepatocytes release endogenous damage-associated molecular patterns (DAMPs) that recruit innate and adaptive immune cells, propagating further liver injury. Additional molecular mechanisms triggered by ethanol that have been implicated in the development of alcoholic liver disease include the induction of oxidative stress with resultant mitochondrial damage, activation of programmed hepatocyte necrosis, development of pericentral hypoxia, disruption of the gut microbiome, and alteration of intestinal epithelia resulting in increased permeability and influx of lipopolysaccharides into the liver that can cause systemic inflammation and trigger apoptotic pathways (Figure 14–10).

FIGURE 14–10 Alcohol consumption leads to hepatocyte apoptosis and necrosis. Increased intestinal permeability following alcohol use results in a lipopolysaccharide (LPS) influx into the liver. LPS induces inflammation, necroptosis, and apoptosis pathways via tumor necrosis factor-alpha (TNFα) and its receptor, tumor necrosis factor receptor-1 (TNFR1). (TRADD, TNFR-associated death domain; cIAP1/2, cellular inhibitor of apoptosis proteins 1 and 2; LUBAC, linear ubiquitin chain assembly complex; NF-Bκ, nuclear factor κB; TAB2/3, transforming growth factor beta-activated kinase 1/MAP3K7 binding proteins 2 and 3; TAK1, transforming growth factor beta-activated kinase 1; IKK, I-κB kinase; CYLD, CYLD lysine 63 deubiquitinase; RIP1/3, receptor-interacting proteins 1 and 3; FADD, fas-associated protein with a death domain; CFLAR, CASP8 and FADD-like apoptosis regulator; RHIM, RIP homotypic interaction motif;

MLKL, mixed lineage kinase domain-like protein.) (Redrawn, with permission, from Nagy LE et al. Linking pathogenic mechanisms of alcoholic liver disease with clinical phenotypes. Gastroenterology. 2016 Jun;150(8):1756–68. Copyright © Elsevier.)

The histopathologic findings of alcoholic hepatitis differ from those of viral hepatitis and include the accumulation of Mallory’s hyaline within hepatocytes and the infiltration of polymorphonuclear leukocytes.

Pathology In uncomplicated acute hepatitis, the typical histologic findings consist of (1) focal liver cell degeneration and necrosis, with cell dropout, ballooning, and acidophilic degeneration (shrunken cells with eosinophilic cytoplasm and pyknotic nuclei); (2) the inflammation of portal areas, with infiltration by mononuclear cells (small lymphocytes, plasma cells, eosinophils); (3) a prominence of Kupffer cells and bile ducts; and (4) cholestasis (arrested bile flow) with bile plugs. Characteristically, although the regular pattern of hepatocyte cords is disrupted, the reticulin framework is preserved. This reticular framework provides scaffolding for liver cells when the hepatocytes regenerate.

Recovery from acute hepatitis from any cause is characterized histologically by hepatocyte regeneration, with numerous mitotic figures and multinucleated cells, and by an almost complete restoration of normal lobular architecture.

Less commonly in acute hepatitis (1–5% of patients), there is a more severe histologic lesion called bridging hepatic necrosis (also called subacute, submassive, or confluent necrosis). Bridging is said to occur between lobules because necrosis involves contiguous groups of hepatocytes, resulting in large areas of hepatic cell loss and a collapse of the reticulin framework. Necrotic zones (“bridges”), consisting of condensed reticulin, inflammatory debris, and degenerating liver cells, link adjacent portal or central areas or may involve entire lobules.

Rarely, in massive hepatic necrosis or fulminant hepatitis (<1% of patients), the liver becomes small, shrunken, and soft (acute yellow atrophy). Histologic examination reveals massive hepatocyte necrosis in most lobules, leading to extensive collapse and condensation of the reticulin framework and portal structures (bile ducts and vessels).

Clinical Manifestations A. Acute viral hepatitis Usually there are three phases to acute viral hepatitis: the pre-icteric phase (prodrome), the icteric phase, and the convalescent phase (Figure 14–11).

FIGURE 14–11 (A) Serum antibody and antigen levels in hepatitis A and hepatitis B. (AST, aspartate aminotransferase, a marker for hepatocellular injury and necrosis; IgM anti-HAV, early antibody response to hepatitis A infection; IgG anti-HAV, late antibody response to hepatitis A infection; HBsAg, hepatitis B surface antigen, a marker of active viral gene expression; HBeAg, hepatitis B early antigen, a marker of infectivity.) Antibodies to the surface or early antigens (anti-HBs or anti-HBe) indicate immunity. (Redrawn, with permission, from Chandrasoma P et al, eds. Concise Pathology, 3rd ed. Originally published by Appleton & Lange. Copyright © 1998 by The McGraw-Hill Companies, Inc.) (B) Course of acute, resolving HCV infection. (ALT, alanine aminotransferase; HCV RNA, hepatitis C viral load; anti-HCV, HCV antibody.) (Redrawn, with permission, from

Hoofnagle JH. Course and outcome of hepatitis C. Hepatology. 2002 Nov;36(5 Suppl 1):S21–9. Copyright © John Wiley and Sons.)

1. Pre-icteric phase (prodrome)—The pre-icteric phase (prodrome), typically lasting 3 or 4 days, is characterized by three sets of symptoms and signs: (1) nonspecific constitutional symptoms and signs: malaise, fatigue, and mild fever; (2) gastrointestinal symptoms and signs: anorexia, nausea, vomiting, altered senses of olfaction and taste (loss of taste for coffee or cigarettes), and right upper quadrant abdominal discomfort (reflecting the enlarged liver); and (3) extrahepatic symptoms and signs: headache, photophobia, cough, coryza, myalgias, urticarial skin rash, arthralgias or arthritis (10–15% of patients with HBV), and, rarely, hematuria and proteinuria (see Figure 14–11).

2. Icteric phase—The icteric phase typically lasts 1–4 weeks. The constitutional symptoms usually improve, although mild weight loss may occur. Pruritus occurs if cholestasis is severe. Right upper quadrant abdominal pain as a result of the enlarged and tender liver, which was present in the prodromal phase, continues. Splenomegaly is noted in 10– 20% of patients.

Jaundice may be observed as a yellowing of the scleras, skin, or mucous membranes. Jaundice is generally not appreciated on physical examination before the serum bilirubin rises above 2.5 mg/dL (41.75 µmol/L). Direct hyperbilirubinemia is an elevated level of conjugated bilirubin in the bloodstream. Its occurrence indicates an unimpaired ability of the hepatocytes to conjugate bilirubin but a defect in the excretion of bilirubin into the bile as a result of intrahepatic cholestasis or posthepatic obstructive biliary tract disease, with overflow of conjugated bilirubin out of hepatocytes into the bloodstream.

Changes in stool color (lightening) and urine color (darkening) often precede clinically evident jaundice. This reflects a loss of bilirubin metabolites from the stool as a consequence of disrupted bile flow. Water-soluble (conjugated) bilirubin metabolites are excreted in the urine, whereas water-insoluble metabolites accumulate in tissues, giving rise to jaundice. Note that in most cases of acute viral hepatitis, the degree of liver impairment is sufficiently mild that jaundice does not develop.

Ecchymoses suggest coagulopathy, which may be due to a loss of vitamin K absorptive capacity from the intestine (caused by cholestasis) or decreased coagulation factor synthesis. Rarely, loss of clearance of activated clotting factors triggers disseminated intravascular coagulation. A coagulopathy in which the prothrombin time can be corrected by parenteral vitamin K but not by oral

vitamin K suggests cholestatic disease, because vitamin K uptake from the gut depends on bile flow. If the prothrombin time cannot be corrected with either oral or parenteral vitamin K, an inability to synthesize clotting factor polypeptides (eg, as a result of massive hepatocellular dysfunction) should be suspected. Correction of prothrombin time with oral vitamin K alone suggests a nutritional deficiency rather than liver disease as the basis for the coagulopathy.

Tests for serum levels of various enzymes normally localized primarily within hepatocytes provide an indication of the extent of liver cell necrosis. For unclear reasons, perhaps related to liver cell polarity, certain forms of liver disease typically result in disproportionate elevations in some parameters. Thus, in alcoholic hepatitis but not in viral hepatitis, AST is often disproportionately elevated relative to ALT (AST:ALT ratio >2.0). One hypothesis is that this is due to a pyridoxine deficiency in alcoholism. Likewise, in cholestasis, alkaline phosphatase is commonly disproportionately elevated relative to AST or ALT. The time course of acute hepatitis is highly variable. In hepatitis A, jaundice is typically seen 4–8 weeks after exposure, whereas in hepatitis B, jaundice usually occurs 8–20 weeks after exposure (see Figure 14–11).

Measuring antigen and antibody titers is a convenient way to assess whether an episode of acute hepatitis is due to viral infection. Moreover, because IgM antibodies are produced early after exposure to antigens (ie, soon after the onset of illness), the presence of IgM antibodies to either HAV or to HBV core antigen (HBcAg) is strong evidence that an episode of acute hepatitis is due to the corresponding viral infection. Several months after the onset of illness, IgM antibody titers wane and are replaced by antibodies of the IgG class, indicating immunity to recurrence of infection by the same virus. The presence of hepatitis B “e” antigen (HBeAg) correlates well with a high degree of infectivity (Table 14–11). However, more sensitive DNA tests have shown low levels of viral DNA in the blood of many who are HBeAg negative and who are thus still infectious.

Subtle or profound mental status changes are seen in fulminant hepatic necrosis. Encephalopathy is believed to be related in part to a failure to detoxify ammonia, which normally occurs through the urea cycle. Other products such as γ-aminobutyric acid (GABA) may not be metabolized. Although ammonia is a neurotoxin, it remains unclear whether it is the major agent of CNS dysfunction or whether elevated blood levels of GABA (perhaps synergistically with other compounds) may act to alter mental status because of its role as a major inhibitory neurotransmitter. In addition to encephalopathic changes caused by an accumulation of toxins, acute hepatic failure is associated with encephalopathy from cerebral edema caused by increased intracranial pressure, perhaps related

to alterations in the blood–brain barrier. Renal dysfunction may complicate fulminant hepatic failure. Affected

patients may develop prerenal azotemia when the glomerular filtration rate falls secondary to intravascular volume depletion. A state of intravascular volume depletion can be induced by the combination of decreased oral intake, vomiting, and ascites formation. If uncorrected, this process can lead to acute tubular necrosis and acute kidney injury. Other causes of renal dysfunction in fulminant hepatic failure include toxins (eg, acetaminophen or Amanita poisoning) or hepatorenal syndrome. Serum creatinine is a more accurate measure than blood urea nitrogen of renal impairment in fulminant hepatic failure resulting from decreased hepatic urea production. Other complications of fulminant hepatic failure include cardiovascular dysfunction as a result of systemic vasodilation and hypotension, pulmonary edema, coagulopathy, sepsis, and hypoglycemia.

TABLE 14–11 Commonly encountered serologic patterns of hepatitis B infection.

3. Convalescent phase—The convalescent phase is characterized by the complete disappearance of constitutional symptoms but persistent abnormalities in liver function tests. Symptoms and signs gradually improve (see Figure 14–11).

Acute hepatitis typically resolves in 3–6 months. Hepatic injury continuing for more than 6 months is arbitrarily defined as chronic hepatitis and suggests, in the absence of continued exposure to a noxious agent, that immune or other mechanisms are at work.

CHECKPOINT

22. Describe the range of clinical presentations of acute hepatitis. 23. Which viruses can cause hepatitis? 24. What are some extrahepatic manifestations of viral hepatitis? 25. What is the basis for the extrahepatic manifestations of viral hepatitis? 26. Describe the spectrum of liver diseases associated with alcohol

consumption. 27. What are some ways that ethanol causes liver injury?

CHRONIC HEPATITIS

Chronic hepatitis is a category of disorders characterized by the combination of liver cell necrosis and inflammation of varying severity persisting for more than 6 months. It may be due to viral infection; drugs and toxins; genetic, metabolic, or autoimmune factors; or unknown causes. The severity ranges from an asymptomatic stable illness characterized only by laboratory test abnormalities to a severe, gradually progressive illness culminating in cirrhosis, liver failure, and death. Based on clinical, laboratory, and biopsy findings, chronic hepatitis is best assessed with regard to (1) distribution and severity of inflammation; (2) degree of fibrosis; and (3) etiology, which has important prognostic implications. Table 14–12 presents a simplified scoring system for assessing liver biopsies for chronic hepatitis.

TABLE 14–12 Three simple systems for the histologic grading and staging of chronic hepatitis.

Clinical Presentation Patients may present with fatigue, malaise, low-grade fever, anorexia, weight loss, mild intermittent jaundice, and mild hepatosplenomegaly. Others are initially asymptomatic and present late in the course of the disease with complications of cirrhosis, including variceal bleeding, coagulopathy, encephalopathy, jaundice, and ascites. In contrast to chronic persistent hepatitis, some patients with chronic active hepatitis, particularly those without serologic evidence of antecedent HBV infection, present with extrahepatic symptoms such as skin rash, diarrhea, arthritis, and various autoimmune disorders (Table 14–13).

TABLE 14–13 Extrahepatic manifestations of chronic viral hepatitis.

Etiology Either type of chronic hepatitis (persistent or active) can be caused by infection with several hepatitis viruses (eg, hepatitis B, with or without hepatitis D superinfection, and hepatitis C); a variety of drugs and poisons (eg, ethanol,

isoniazid, acetaminophen), often in amounts insufficient to cause symptomatic acute hepatitis; genetic and metabolic disorders (eg, α1-antitrypsin deficiency, Wilson disease); or immune-mediated injury of unknown origin. Table 14–1 summarizes known causes of chronic hepatitis. Less than 5% of otherwise healthy adults with acute hepatitis B remain chronically infected with HBV; the risk is higher in those who are immunocompromised or of young age (ranging from 90% in newborns of HBeAg-positive mothers to 25–30% in infants and children younger than 5 years). Among those chronically infected, about two- thirds develop mild chronic hepatitis and one-third develop severe chronic hepatitis (see later discussion). Those with HDV coinfection progress to chronic hepatitis at higher rates than seen with isolated HBV infection. HDV superinfection is also associated with a high incidence of acute liver failure. Finally, 60–85% of individuals exposed to acute hepatitis C develop chronic hepatitis, and rates are not significantly affected by age, mode of acquisition, or the presence of coinfections.

Pathogenesis Many cases of chronic hepatitis are thought to represent an immune-mediated attack on the liver occurring as a result of the persistence of certain hepatitis viruses or after prolonged exposure to certain drugs or noxious substances (Table 14–14). In some, no mechanism has been recognized. Evidence that the disorder is immune mediated is that liver biopsies reveal inflammation (lymphocyte infiltration) in characteristic regions of the liver architecture (eg, portal versus lobular). Furthermore, a variety of autoimmune disorders occur with high frequency in patients with chronic hepatitis (see Table 14–13).

TABLE 14–14 Drugs implicated in the etiology of chronic hepatitis.

A. Chronic Viral Hepatitis

Viral hepatitis is the most common cause of chronic liver disease in the United States. In approximately 5% of adult cases of HBV infection and 60–85% of hepatitis C infections, the immune response is inadequate to clear the liver of virus, resulting in persistent infection. The individual becomes a chronic carrier, intermittently producing the virus and hence remaining infectious to others. Biochemically, these patients are often found to have viral DNA integrated into their genomes in a manner that results in the abnormal expression of certain viral proteins with or without the production of intact virus. Viral antigens expressed on the hepatocyte cell surface are associated with class I HLA determinants, thus eliciting lymphocyte cytotoxicity and resulting in hepatitis. The severity of chronic hepatitis depends largely on the activity of viral replication and the response of the host’s immune system.

Chronic hepatitis B infection predisposes the patient to the development of hepatocellular carcinoma (HCC). Although in the setting of HBV infection most cases of HCC occur in the presence of cirrhosis, 10–30% of cases occur in the absence of cirrhosis or advanced fibrosis. It remains unclear whether hepatitis B infection is the initiator or simply a promoter of the process of tumorigenesis. In hepatitis C infection, HCC develops exclusively in the setting of cirrhosis.

B. Alcoholic Chronic Hepatitis

Chronic liver disease in response to some poisons or toxins may represent the triggering of an underlying genetic predisposition to immune attack on the liver. In alcoholic hepatitis, however, repeated episodes of acute injury ultimately cause necrosis, fibrosis, and regeneration, leading eventually to cirrhosis (Figure 14–12). As in other forms of liver disease, there is considerable variation in the extent of symptoms before cirrhosis develops.

FIGURE 14-12 Changes in the hepatic subendothelial space during fibrosing liver injury. Cellular and matrix alterations in the space of Disse are critical events in the pathogenesis of hepatic fibrosis. The activation of lipocytes, characterized by proliferation and increased fibrogenesis, is associated with the replacement of the normal low-density matrix with a high-density matrix. These alterations are likely to underlie, at least in part, the loss of both endothelial fenestrations (pores) and hepatocytic microvilli typical of chronic liver injury. (Redrawn, with permission, from Bissell DM. Cell-matrix interaction and hepatic fibrosis. Prog Liver Dis 1990;9:143–55. Copyright © Elsevier.)

C. Nonalcoholic Fatty Liver Disease

In light of increasing obesity in the United States, there has been a significant increase in the prevalence of nonalcoholic fatty liver disease (NAFLD), a form of chronic liver disease associated with the metabolic syndrome. NAFLD refers to the presence of hepatic steatosis, with or without inflammation and fibrosis, when no other causes of secondary hepatic fat accumulation (eg, heavy alcohol

consumption) are present. NAFLD is an umbrella term for a spectrum of liver disease severity, ranging from nonalcoholic fatty liver (NAFL), in which inflammation is minimal, to nonalcoholic steatohepatitis (NASH), in which active inflammation poses a risk for fibrosis and progression to cirrhosis. On biopsy, the inflammation associated with NASH may be histologically indistinguishable from alcoholic steatohepatitis.

NAFLD is prevalent worldwide and is the most common liver disease in industrialized Western countries. In the United States, the estimated prevalence of NAFLD ranges from 10% to 46%, and the prevalence of NASH ranges from 3% to 5%, with variation by age, gender, and ethnicity. NAFLD is strongly associated with metabolic risk factors such as obesity, dyslipidemia, insulin resistance, and type 2 diabetes mellitus, and the rising incidence of NAFLD parallels rising rates of obesity worldwide.

The pathogenesis of NAFLD has not been fully elucidated, but the most widely supported theory implicates insulin resistance as the key mechanism leading to hepatic steatosis and steatohepatitis. Lipid accumulation in hepatocytes results from an increased influx of lipids to the liver or decreased lipid disposal (Figure 14–13).

FIGURE 14-13 Pathogenesis of hepatic steatosis. An increased uptake and/or decreased disposal of lipids (fatty acids) in the liver results in an accumulation of fat within hepatocytes. (FFA, free fatty acid; NEFA, non-esterified fatty acid; VLDL, very-low-density lipoprotein.) (Redrawn, with permission, from Machado MV et al. Pathogenesis of nonalcoholic steatohepatitis. Gastroenterology. 2016 Jun;150(8):1769–77. Copyright © Elsevier.)

Lipid accumulation has been shown to lead to toxicity by diverse mechanisms, including increased oxidative stress as a result of the generation of reactive oxygen species by mitochondrial and peroxisomal fatty acid oxidation. In addition, lipotoxicity can result from changes in cell signaling pathways that regulate metabolism and stress responses, promoting a sterile inflammatory response that can potentiate liver cell injury and death. Dying hepatocytes can induce a wound-healing response via DAMPs, cytokines, and hedgehog, which can promote inflammation, fibrogenesis, and hepatocarcinogenesis. Factors that promote lipotoxicity in patients with NAFLD include iron overload, environmental toxins, medications, viral hepatitis, congenital liver disease, alpha-1 antitrypsin deficiency, and Wilson disease.

In general, patients with simple steatosis are at low risk of histologic progression, but patients with NASH can progress to cirrhosis and end-stage liver disease and are at risk for HCC. Long-term studies of patients with NAFL and NASH have revealed that such patients have increased overall mortality and that the most common cause of death in these patients is cardiovascular disease. Moreover, patients with NASH (but not NAFL) have increased rates of liver- related mortality. Management of NAFLD is centered on modifying risk factors and treating metabolic comorbidities. Vitamin E is the only agent that has been shown to improve liver histology in a subgroup of adults who are nondiabetic with biopsy-proven NASH.

D. Other Chronic Hepatitis

Some patients develop chronic hepatitis in the absence of evidence of preceding viral hepatitis or exposure to noxious agents (Figure 14–14). Autoimmune hepatitis is a chronic inflammatory liver disease characterized by a mainly T cell–mediated immune response to as yet unidentified auto-antigen(s). Patients with autoimmune hepatitis demonstrate serologic evidence of disordered immunoregulation, manifested as hyperglobulinemia and circulating autoantibodies, as well as a typical pattern of injury on liver histology characterized by a plasma cell–rich inflammatory infiltrate, interface hepatitis, hepatic rosette formation, and emperipolesis (presence of an intact cell within the cytoplasm of another cell). Nearly 75% of these patients are women, and many have other autoimmune disorders. A genetic predisposition is strongly

suggested. Most patients with autoimmune hepatitis show histologic improvement in liver biopsies after treatment with systemic corticosteroids. The clinical response, however, can be variable.

FIGURE 14–14 Chronic hepatitis, showing marked lymphocytic infiltration and fibrosis of the portal areas. The lymphocytes extend into the peripheral part of the lobule through the limiting plate. There is ongoing hepatocyte necrosis in the peripheral part of the lobule (piecemeal necrosis). (Reproduced, with permission, from Chandrasoma P et al, eds. Concise Pathology, 3rd ed. Originally published by Appleton & Lange. Copyright © 1998 by The McGraw-Hill Companies, Inc.)

Cholestatic forms of autoimmune liver disease have also been well characterized, including primary biliary cholangitis and primary sclerosing cholangitis. In primary biliary cholangitis, it is postulated that intralobular bile duct injury is triggered by environmental exposure(s) in a genetically susceptible individual. The persistent activation of innate and adaptive immune responses mediates inflammation and bile duct epithelial damage. The resultant bile duct destruction leads to chronic cholestasis and the intrahepatic accumulation of

cytotoxic bile acids. This accumulation causes liver cell injury and a secretion of pro-inflammatory cytokines that induce inflammation and fibrosis in surrounding tissue. The vast majority of individuals with primary biliary cholangitis will have detectable anti-mitochondrial antibody; the diagnosis is also supported by elevated levels of alkaline phosphatase. On liver biopsy, there is histopathological evidence of nonsuppurative cholangitis and destruction of small or medium-sized interlobular bile ducts.

Primary sclerosing cholangitis is characterized by chronic inflammation of the biliary epithelium leading to multifocal intra- and/or extrahepatic biliary strictures and fibrosis; these in turn often lead to biliary cirrhosis and malignancy. The pathogenesis of primary sclerosing cholangitis remains poorly understood, but it is also theorized to occur in genetically susceptible individuals after exposure to an unknown environmental trigger. While immune dysregulation is implicated in its pathogenesis, primary sclerosing cholangitis lacks other classic features of autoimmune disease such as female predominance, pathogenic autoantibodies, and response to immunosuppressive medications. Owing to its definite association with inflammatory bowel disease, a “leaky gut” hypothesis has been postulated, whereby translocation of gastrointestinal flora leads to disruption of biliary epithelial cells and exposes cholangiocytes to bile acid toxicity. Cholangiography showing characteristic bile duct stricturing establishes the diagnosis of primary sclerosing cholangitis.

Pathology All forms of chronic hepatitis share the common histopathologic features of (1) inflammatory infiltration of hepatic portal areas with mononuclear cells, especially lymphocytes and plasma cells; and (2) necrosis of hepatocytes within the parenchyma or immediately adjacent to portal areas (periportal hepatitis, or “piecemeal necrosis”).

In mild chronic hepatitis, the overall architecture of the liver is preserved. Histologically, the liver reveals a characteristic lymphocyte and plasma cell infiltrate confined to the portal triad without disruption of the limiting plate and no evidence of active hepatocyte necrosis. There is little or no fibrosis, and what there is generally is restricted to the portal area; there is no sign of cirrhosis. A “cobblestone” appearance of liver cells is seen, indicating hepatocyte regeneration.

In more severe cases of chronic hepatitis, the portal areas are expanded and densely infiltrated by lymphocytes, histiocytes, and plasma cells. There is necrosis of hepatocytes at the periphery of the lobule, with erosion of the

limiting plate surrounding the portal triads (piecemeal necrosis; see Figure 14– 14). More severe cases also show evidence of necrosis and fibrosis between portal triads. The normal liver architecture is disrupted by bands of scar tissue and inflammatory cells that link portal areas to one another and to central areas (bridging necrosis). These connective tissue bridges are evidence of hepatic architecture remodeling, a crucial step in the development of cirrhosis. Fibrosis may extend from the portal areas into the lobules, isolating hepatocytes into clusters and enveloping bile ducts. Hepatocyte regeneration is seen with mitotic figures, multinucleated cells, rosette formation, and regenerative pseudolobules. Progression to cirrhosis is signaled by extensive fibrosis, loss of zonal architecture, and regenerating nodules.

Clinical Manifestations Some patients with mild chronic hepatitis are entirely asymptomatic and identified only in the course of routine blood testing; others have an insidious onset of nonspecific symptoms such as anorexia, malaise, and fatigue or hepatic symptoms, such as right upper quadrant abdominal discomfort or pain. Fatigue in chronic hepatitis may be related to a change in the hypothalamic–adrenal neuroendocrine axis brought about by altered endogenous opioidergic neurotransmission. Jaundice, if present, is usually mild. There may be mild tender hepatomegaly and occasional splenomegaly. Palmar erythema and spider telangiectases are seen in severe cases. Other extrahepatic manifestations are unusual. By definition, signs of cirrhosis and portal hypertension (eg, ascites, collateral circulation, and encephalopathy) are absent. Laboratory studies show mild to moderate increases in serum aminotransferase, bilirubin, and globulin levels. Serum albumin and the prothrombin time are normal until late in the progression of liver disease.

The clinical manifestations of chronic hepatitis probably reflect the role of a systemic genetically controlled immune disorder in the pathogenesis of severe disease. Acne, hirsutism, and amenorrhea may occur as a reflection of the hormonal effects of chronic liver disease. Laboratory studies in patients with severe chronic hepatitis are invariably abnormal to various degrees. However, these abnormalities do not correlate with clinical severity. Thus, the serum bilirubin, alkaline phosphatase, and globulin levels may be normal and aminotransferase levels only mildly elevated at the same time that a liver biopsy reveals severe chronic hepatitis. However, an elevated prothrombin time usually reflects severe disease.

The natural history and treatment of chronic hepatitis varies depending on its

cause. The complications of severe chronic hepatitis are those of progression to cirrhosis: variceal bleeding, encephalopathy, coagulopathy, hypersplenism, and ascites. These are largely due to portosystemic shunting rather than diminished hepatocyte reserve (see later discussion).

CHECKPOINT

28. What are the categories of chronic hepatitis based on histologic findings on liver biopsy?

29. What are the causes of chronic hepatitis? 30. What are the consequences of chronic hepatitis?

CIRRHOSIS

Clinical Presentation Cirrhosis is an irreversible distortion of normal liver architecture characterized by hepatic injury, fibrosis, and nodular regeneration. The clinical presentations of cirrhosis are a consequence of both progressive hepatocellular dysfunction and portal hypertension (Figure 14–15). As with other presentations of liver disease, not all patients with cirrhosis develop life-threatening complications. Indeed, in nearly 40% of cases of cirrhosis, it is diagnosed at autopsy in patients who did not manifest obvious signs of end-stage liver disease.

FIGURE 14–15 Clinical effects of cirrhosis of the liver. (Redrawn, with permission, from Chandrasoma P et al, eds. Concise Pathology, 3rd ed. Originally published by Appleton & Lange. Copyright © 1998 by The McGraw-Hill Companies, Inc.)

Etiology Table 14–1 lists the causes of cirrhosis. The initial injury can be due to a wide range of processes. A crucial feature is that the liver injury is not acute and self- limited, but rather chronic and progressive. In the United States, alcohol abuse is the most common cause of cirrhosis. In other countries, infectious agents (particularly HBV and HCV) are the most common causes. Other causes include

chronic biliary obstruction, drugs, genetic and metabolic disorders, chronic heart failure, and primary (autoimmune) biliary cholangitis.

Pathogenesis The increased or altered synthesis of collagen and other connective tissue or basement membrane components of the extracellular matrix is implicated in the development of hepatic fibrosis and thus in the pathogenesis of cirrhosis. The role of the extracellular matrix in cellular function is an important area of research, and studies suggest that it is involved in modulating the activities of the cells with which it is in contact. Thus, fibrosis may affect not only the mechanics of blood flow through the liver but also the functions of the cells themselves.

Hepatic fibrosis occurs in three situations: (1) secondary to inflammation and the subsequent activation of immune responses; (2) as part of the process of wound healing; and (3) in response to agents that induce primary fibrogenesis. HBV and Schistosoma species are good examples of agents that lead to hepatic fibrosis by stimulating an immune response. Agents such as carbon tetrachloride that attack and kill hepatocytes directly can produce fibrosis as part of wound healing. In both immune responses and wound healing, the fibrosis is triggered indirectly by the effects of cytokines released from invading inflammatory cells. Finally, certain agents such as ethanol and iron may cause primary fibrogenesis by directly increasing collagen gene transcription and thus also increasing the amount of connective tissue secreted by cells.

The actual culprit in all these mechanisms of increased fibrogenesis may be the fat-storing cells (stellate cells) of the hepatic reticuloendothelial system. In response to cytokines, they differentiate from quiescent stellate cells, in which vitamin A is stored, into myofibroblasts, which lose their vitamin A storage capacity and become actively engaged in extracellular matrix production. In addition to the stellate cells, fibrogenic cells also derive from portal fibroblasts, circulating fibrocytes, bone marrow, and epithelial–mesenchymal cell transition. It appears that hepatic fibrosis occurs in two stages (Figure 14–16). The first stage is characterized by a change in extracellular matrix composition from non– cross-linked, non–fibril-forming collagen to collagen that is more dense and subject to cross-link formation. At this stage, liver injury is still reversible. The second stage involves the formation of subendothelial collagen cross-links, the proliferation of myoepithelial cells, and the distortion of hepatic architecture with the appearance of regenerating nodules. Cirrhosis remains a dynamic state in which certain interventions, even at these advanced stages, may yield benefits such as regression of scar tissue and improvements in clinical outcomes.

FIGURE 14–16 Pathways of hepatic stellate cell activation. Features of stellate cell activation can be distinguished between those that stimulate initiation and those that contribute to perpetuation. Initiation is provoked by soluble stimuli that include oxidant stress signals (reactive oxygen intermediates), apoptotic bodies, lipopolysaccharide (LPS), and paracrine stimuli from neighboring cell types, including hepatic macrophages (Kupffer cells), sinusoidal endothelium, and hepatocytes. Perpetuation follows, characterized by a number of specific phenotypic changes, including proliferation, contractility, fibrogenesis, altered matrix degradation, chemotaxis, and inflammatory signaling. (CTGF, connective tissue growth factor; ET-1, endothelin-1; FGF, fibroblast growth factor; HSC, hepatic stellate cells; MMP 2, 9, metalloproteinase-2 and -9; MT, membrane type; MT-1-MMP, membrane type 1 metalloproteinase; NK, natural killer; NO, nitric oxide; PDGF, platelet-derived growth factor; TGF-beta 1, transforming growth factor-beta 1; TIMP-1, 2, metallopeptidase inhibitor-1 and -2; TLR, Toll-like receptors; TRAIL, TNF-related apoptosis-inducing ligand; VEGF, vascular endothelial growth factor.) (Adapted, with permission, from Friedman SL. Molecular regulation of hepatic fibrosis, an integrated cellular response to tissue injury. J Biol Chem. 2000 Jan 28:275(4):2247–50. Copyright © American Society for Biochemistry and Molecular Biology.)

The manner in which alcohol causes chronic liver disease and cirrhosis is not well understood. However, chronic alcohol abuse is associated with impaired

protein synthesis and secretion, mitochondrial injury, lipid peroxidation, formation of acetaldehyde and its interaction with cellular proteins and membrane lipids, cellular hypoxia, and both cell-mediated and antibody- mediated cytotoxicity. The relative importance of each of these factors in producing cell injury is unknown. Genetic, nutritional, and environmental factors (including simultaneous exposure to other hepatotoxins) also influence the development of liver disease in people with chronic alcoholism. Finally, acute liver injury (eg, from exposure to alcohol or other toxins) from which a person with a normal liver would fully recover may be sufficient to produce irreversible decompensation (eg, hepatorenal syndrome) in a patient with underlying hepatic cirrhosis.

Pathology The liver may be large or small, but it always has a firm and often nodular consistency. Although several noninvasive methods for staging the extent of fibrosis exist, including the use of serum biomarkers and imaging techniques to measure liver stiffness (eg, elastography), these methods are accurate for severe (fibrosis stage F3) and minimal (F1) fibrosis, but not intermediate stages. Liver biopsy remains the only method for definitively diagnosing significant fibrosis (F ≥2) and cirrhosis (F4). Histologically, all forms of cirrhosis are characterized by three findings: (1) marked distortion of hepatic architecture; (2) scarring as a result of increased deposition of fibrous tissue and collagen; and (3) regenerative nodules surrounded by scar tissue. When the nodules are small (<3 mm) and uniform in size, the process is termed micronodular cirrhosis. In macronodular cirrhosis, the nodules are more than 3 mm and variable in size. Cirrhosis from alcohol abuse is usually micronodular but can be macronodular or both micronodular and macronodular. Scarring may be most severe in central regions, or dense bands of connective tissue may join portal and central areas.

More specific histopathologic findings may help establish the cause of cirrhosis. For example, the invasion and destruction of bile ducts by granulomas suggests primary (autoimmune) biliary cirrhosis; extensive iron deposition in hepatocytes and bile ducts suggests hemochromatosis; and alcoholic hyaline and infiltration with polymorphonuclear cells suggest alcoholic cirrhosis.

Clinical Manifestations The clinical manifestations of progressive hepatocellular dysfunction in cirrhosis are similar to those of acute or chronic hepatitis and include constitutional

symptoms and signs: fatigue, loss of vigor, and weight loss; GI symptoms and signs: nausea, vomiting, jaundice, and tender hepatomegaly; and extrahepatic symptoms and signs: palmar erythema, spider angiomas, muscle wasting, parotid and lacrimal gland enlargement, gynecomastia and testicular atrophy in men, menstrual irregularities in women, and coagulopathy (see Figure 14–15).

Clinical manifestations of portal hypertension include ascites, portosystemic shunting, encephalopathy, splenomegaly, and esophageal and gastric varices with intermittent hemorrhage (see Figure 14–15 and Table 14–15).

TABLE 14–15 Manifestations of cirrhosis.

A. Portal Hypertension

Portal hypertension is defined by a portal venous pressure gradient greater than 5 mm Hg. Portal hypertension results from a rise in intrahepatic vascular resistance. The cirrhotic liver loses the physiologic characteristic of a low-

pressure circuit for blood flow seen in the normal liver. The increased blood pressure within the sinusoids is transmitted back to the portal vein. Because the portal vein lacks valves, this elevated pressure is transmitted back to other vascular beds, resulting in splenomegaly, portal-to-systemic shunting, and many of the complications of cirrhosis discussed later.

B. Ascites

Ascites refers to the presence of excess fluid within the peritoneal cavity. Patients with ascites develop physical examination findings of increasing abdominal girth, a fluid wave, a ballotable liver, and shifting dullness. Ascites can develop in patients with conditions other than liver disease, including protein-calorie malnutrition (from hypoalbuminemia) and cancer (from lymphatic obstruction). In patients with liver disease, ascites is due to portal hypertension and can be confirmed by the presence of a serum-to-ascites albumin gradient (SAAG) of 1.1 g/dL (11 g/L) or more. Calculating the SAAG involves measuring on the same day the albumin concentration in serum and ascitic fluid (via abdominal paracentesis) and subtracting the ascitic fluid value from the serum value. Ascites can develop in approximately 50% of patients with compensated cirrhosis over a 10-year follow-up period, and it is associated with significant morbidity and mortality.

It is useful to recognize that liver disease with ascites formation occurs in a wide clinical spectrum. At one end is fully compensated portal hypertension with no ascites present because the volume of ascites generated is less than the approximately 800–1200 mL/d capacity of the peritoneal lymphatic drainage. At the other extreme is the typically fatal hepatorenal syndrome, in which patients with liver disease, usually with massive ascites, succumb to rapidly progressing acute kidney injury. The hepatorenal syndrome seems to be precipitated by intense and inappropriate renal vasoconstriction and is characterized by extreme sodium retention typical of prerenal azotemia, but in the absence of true volume depletion (see Chapter 16). Nonetheless, the presence of clinically apparent ascites in a patient with liver disease is associated with poor long-term survival. Over the years, various mechanisms have been proposed to explain ascites formation. No single hypothesis of pathogenesis easily explains all findings at all points in time during the natural history of portal hypertension. Portal hypertension and inappropriate renal sodium retention are important elements of all theories. The end result of ascites occurs when excess peritoneal fluid exceeds the capacity of lymphatic drainage, leading to increased hydrostatic pressure. The fluid can then be seen to visibly weep from the lymphatics and

pool in the abdominal cavity as ascites. The underfill/vasodilatation hypothesis proposes that the primary event in

ascites formation is vascular, with a reduced effective circulating volume leading to the activation of the renin–angiotensin system and subsequent renal sodium retention. The classic underfill hypothesis postulates that elevated hepatic sinusoidal pressure leads to the sequestration of blood in the splanchnic venous bed. This results in underfilling of the central vein with diversion of intravascular volume to the hepatic lymphatics, which, like the central vein, drain the space of Disse. The peripheral arterial vasodilatation or splanchnic vasodilatation hypothesis adds the idea that, with portal-to-systemic shunting, vasodilatory products (eg, nitric oxide) normally cleared by the liver are instead delivered to the systemic circulation, where they cause peripheral arteriolar vasodilation, particularly in the splanchnic arterial bed. The resultant reduced arterial vascular resistance (Figure 14–17) is associated with decreased central filling pressures, decreased renal arterial perfusion, reflex renal arterial vasoconstriction, and increased renal tubular sodium resorption. Sodium retention expands the intravascular volume, which exacerbates portal venous hypertension. The imbalance between hydrostatic versus oncotic pressure in the portal vein results in ascites formation. Although the splanchnic vasodilatation hypothesis accounts for many of the findings in ascites formation, the use of transhepatic intrajugular portal-to-systemic shunting (TIPS) as a means of decompressing the portal vein in patients with ascites provides a counterargument. As a result of the procedure, peripheral arteriolar vasodilation appears to increase (perhaps as a result of shunting vasodilators such as nitric oxide that are normally cleared by the liver), yet ascites is generally dramatically improved.

FIGURE 14–17 Proposed mechanism for ascites formation in cirrhosis according to the splanchnic vasodilation hypothesis. This hypothesis incorporates elements of the underfill and vasodilation theories. (Redrawn, with permission, from Gines P et al. Management of cirrhosis and ascites. N Engl J Med. 2004;350:1646. Copyright © 2004 Massachusetts Medical Society. Reprinted, with permission, from Massachusetts Medical Society.)

Those who support the overflow hypothesis have proposed that the primary event in the development of ascites is inappropriate renal sodium retention. In this view, ascites is the consequence of fluid overflow from the intravascular volume-expanded portal system into the peritoneal cavity. But what triggers the inappropriate renal sodium retention? One possibility is that there may exist a hepatorenal reflex by which elevated sinusoidal pressure triggers increased sympathetic tone or endothelin-1 secretion. Either of these pathways could cause an inappropriate degree of renal vasoconstriction, a decrease in glomerular filtration rate, and, by tubuloglomerular feedback (see Chapter 16), sodium retention. Note that endothelin-1 is both a renal vasoconstrictor and a stimulant of epinephrine secretion, which in turn stimulates more endothelin-1 secretion. Alternatively, it is possible that an as yet unidentified product from the diseased

liver interferes with atrial natriuretic peptide (ANP) action at the kidney or is in some other way responsible for an inappropriate increase in renal sodium retention. Supporters of the overflow hypothesis point to the fact that many cirrhotic patients have sodium handling defects in the absence of ascites and do not have a measurable increase in renin–angiotensin activity. However, studies have shown that the renal sodium retention in these patients can be reversed by the use of an angiotensin II receptor antagonist.

Most likely, multiple mechanisms contribute to the development of ascites and to its perpetuation, worsening, or improvement in diverse clinical situations. Regardless of the initial events, once fully established, many if not all of the mechanisms described in Figure 14–17 are likely to contribute to ascites formation.

C. Hepatorenal Syndrome

Hepatorenal syndrome refers to a distinct form of kidney injury resulting from renal vasoconstriction that develops in response to the systemic and splanchnic arterial vasodilation in patients with advanced liver disease. The incidence of hepatorenal syndrome in patients who develop decompensated liver disease is 18% within 1 year of diagnosis and up to 40% at 5 years. This disorder generally occurs in patients with cirrhosis and ascites. It is characterized by a progressive rise in serum creatinine (>1.5 mg/dL) with no improvement after 48 hours of withholding diuretics and volume expansion with albumin and in the absence of shock, ingestion of nephrotoxic agents, and underlying parenchymal kidney disease. The urine produced is notable for an extremely low sodium content (<10 mmol/L) and an absence of casts, resembling the findings in prerenal azotemia. Yet, when central venous pressures are measured, the patient does not show intravascular volume depletion, and the disorder does not respond to hydration with normal saline. The renal abnormalities of hepatorenal syndrome appear to be functional because no pathologic changes are identifiable in the kidney. In addition, when a kidney is transplanted from a patient dying of hepatorenal syndrome, it functions well in a recipient without liver disease. While diagnostic criteria for hepatorenal syndrome have been developed (and recently modified), diagnosing and differentiating hepatorenal syndrome from other causes of acute kidney injury in cirrhotic patients can be difficult. Other than hepatorenal syndrome, only acute tubular necrosis and other causes of prerenal azotemia are common in this setting.

Hepatorenal syndrome can be classified into two types, each having different clinical and prognostic characteristics. Type 1 hepatorenal syndrome is rapidly

progressive with a doubling of the serum creatinine concentration to a level greater than 2.5 mg/dL in less than 2 weeks. It is associated with multiorgan failure. Patients with type 2 hepatorenal syndrome have less severe and more slowly progressive renal insufficiency, and usually ascites resistant to diuretics. The onset of hepatorenal syndrome can be dramatic or insidious. It can be precipitated by an acute event, such as infection (notably spontaneous bacterial peritonitis) or hypovolemia from GI bleeding or overdiuresis. The prognosis after developing hepatorenal syndrome is dismal (overall survival of 50% at 1 year). Untreated, type 1 patients survive only weeks, type 2 patients, only 4-6 months.

The pathophysiology of hepatorenal syndrome is related to the distinct hemodynamic and circulatory changes that occur in patients with severe hepatic dysfunction. Portal hypertension triggers arterial vasodilation in the splanchnic circulation and a subsequent reduction in systemic vascular resistance, which can no longer be compensated for by an augmented cardiac output. An increased production or activity of vasodilators within the splanchnic circulation, particularly nitric oxide, carbon monoxide, and endogenous cannabinoids, leads to such arterial vasodilation. In advanced cirrhosis, arterial pressure must be maintained through the activation of vasoconstrictor systems, including the renin–angiotensin system and the sympathetic nervous system, as well as by the excess secretion of antidiuretic hormone (arginine vasopressin). These compensatory mechanisms help maintain effective arterial blood volume and relatively normal arterial pressure but lead to intrarenal vasoconstriction and hypoperfusion, which impairs renal function. By the same mechanisms, affected patients may develop further retention of sodium and free water, worsening edema and ascites.

The best approach to the management of hepatorenal syndrome, based on knowledge of its pathogenesis, is the administration of vasoconstrictor drugs. Use of the vasopressin analog, terlipressin, together with albumin, can be considered as initial therapy for hepatorenal syndrome. Terlipressin is effective in approximately 40–50% of patients with type 1 hepatorenal syndrome; data on the use of vasoconstrictors in type 2 hepatorenal syndrome are limited. Renal replacement therapy in the form of hemodialysis or continuous venovenous hemofiltration has been used, particularly in patients awaiting transplantation or in those with acute, potentially reversible hepatorenal syndrome. There is no evidence, however, that renal replacement therapy improves the prognosis of patients with cirrhosis who are not candidates for a liver transplant. Liver transplantation remains the optimal treatment for patients with hepatorenal

syndrome.

D. Hypoalbuminemia and Peripheral Edema

The progressive worsening of hepatocellular function in cirrhosis can result in a fall in the concentration of albumin and other serum proteins synthesized by the liver. As the concentration of these plasma proteins decreases, the plasma oncotic pressure is lowered, thereby tilting the balance of hemodynamic forces toward the development of both peripheral edema and ascites.

These hemodynamic changes further contribute to an avid sodium-retaining state despite the total body water and sodium overload seen by urinalysis in the cirrhotic patient. Serum sodium may be low as a result of superimposed water retention caused by antidiuretic hormone release triggered by volume stimuli. There are typically no obvious clinical manifestations until the serum sodium concentration falls below 120 mEq/L, at which point neurologic symptoms can occur. Attempts to raise the serum sodium, including fluid restriction and the administration of vasopressin receptor antagonists (eg, tolvaptan and conivaptan) are generally not recommended owing to adverse effects and lack of clear benefit. Hyponatremia is simply a late manifestation of end-stage liver disease and a strong predictor of mortality in patients with cirrhosis.

A low serum potassium and metabolic alkalosis may be observed as a consequence of elevated aldosterone levels responding to renin release (and angiotensin II release) by the kidneys, which sense afferent intravascular depletion.

E. Spontaneous Bacterial Peritonitis

Spontaneous bacterial peritonitis is defined as infection of ascitic fluid in the absence of an intra-abdominal event (such as a bowel perforation or another surgically treatable source) that would account for the entry of pathogenic organisms into the peritoneal space. This complication carries a high mortality rate and is predictive of poor overall prognosis. The presence of infection is confirmed by an elevated ascitic fluid absolute polymorphonuclear leukocyte count of 250 cells/µL or more and definitively confirmed by a positive ascitic fluid bacterial culture. Symptoms and signs include fever, hypotension, abdominal pain or tenderness, decreased or absent bowel sounds, and an abrupt onset of hepatic encephalopathy in a patient with ascites. Alternatively, patients with spontaneous bacterial peritonitis can have subtle or no symptoms, and hence, a high index of suspicion may be required for timely diagnosis.

Patients with advanced liver disease with large-volume ascites or very low ascitic fluid protein levels, a prior history of spontaneous bacterial peritonitis, and episodes of upper GI bleeding are at increased risk for this complication. Ascitic fluid is an excellent culture medium for a variety of pathogens, including Enterobacteriaceae (chiefly Escherichia coli), group D streptococci (enterococci), Streptococcus pneumoniae, and viridans streptococci. The greater risk in patients with low ascitic fluid protein levels may be due to a low level of opsonic activity in the fluid.

While the exact pathogenesis of spontaneous bacterial peritonitis is not known, cirrhosis predisposes to the development of GI bacterial overgrowth and increased intestinal permeability. Peritonitis may occur because of bacterial seeding of the ascitic fluid via the blood or lymph or by bacteria traversing the gut wall. Enteric organisms may also enter the portal venous blood via the portosystemic collaterals, bypassing the reticuloendothelial system of the liver.

F. Gastroesophageal Varices and Bleeding

As blood flow through the liver is progressively impeded, hepatic portal venous pressure rises. In response to the elevated portal venous pressure, there is a decrease in blood vessel wall thickness and an enlargement of blood vessels that anastomose with the portal vein, such as those on the surface of the bowel and lower esophagus. These enlarged vessels are termed gastroesophageal varices. They eventually develop in approximately 50% of patients with cirrhosis, generally when the portal hypertensive gradient exceeds 12 mm Hg. Physical examination may reveal an enlargement of hemorrhoidal and periumbilical vessels. Gastroesophageal varices are of more significance clinically, however, because of their tendency to rupture. Variceal hemorrhage occurs in 25–40% of patients with cirrhosis and is a leading cause of morbidity and mortality in these individuals. Each episode of active variceal bleeding is associated with a 30% mortality risk, and survivors have a 70% risk of recurrent bleeding within 1 year. GI bleeding from varices and other sources (eg, duodenal ulcer, gastritis) in patients with cirrhosis is often exacerbated by concomitant coagulopathy (see later discussion).

G. Hepatic Encephalopathy

Hepatic encephalopathy presents as a range of reversible neuropsychiatric abnormalities that occur as a consequence of advanced decompensated liver disease or portal-to-systemic shunting. Table 14–16 provides a list of common

precipitants. Neuropsychiatric symptoms can be episodic or persistent. Changes in sleep pattern, starting with hypersomnia and progressing to a reversal of the sleep–wake cycle, are often an early sign. Cognitive changes range from mild confusion, apathy, and agitation to marked confusion, obtundation, and even coma. More advanced neurologic features include tremor, bradykinesia, asterixis (flapping motions of outstretched, dorsiflexed hands), hyperactive deep tendon reflexes, and less commonly, transient decerebrate posturing and flaccidity. Cerebral edema, an important accompanying feature in patients with encephalopathy in acute liver disease, is not seen in cirrhotic patients with encephalopathy. Subtle changes of hepatic encephalopathy are present in up to 15% of patients with advanced liver disease and may only be detectable by a number of specialized measures, such as psychometric testing. Such patients have sometimes been referred to as having subclinical or minimal hepatic encephalopathy.

TABLE 14–16 Common precipitants of hepatic encephalopathy.

Hepatic encephalopathy is diagnosed by history and clinical features in the appropriate context and after excluding other causes of altered mental status. Common precipitants of encephalopathy are onset of GI bleeding, increased dietary protein intake, and an increased catabolic rate resulting from infection (including spontaneous bacterial peritonitis). Similarly, because of compromised first-pass clearance of ingested drugs, affected patients are exquisitely sensitive

to sedatives and other drugs normally metabolized in the liver. Other causes include electrolyte imbalance as a result of diuretics, vomiting, alcohol ingestion or withdrawal, or procedures such as TIPS. TIPS also exacerbates hepatic encephalopathy given the direct bypass of portal venous blood flow into the systemic circulation via the hepatic vein, while bypassing the hepatic parenchyma.

The pathogenesis of hepatic encephalopathy is likely multifactorial and complex. One proposed mechanism is related to toxins in the gut such as ammonia, derived from the metabolic degradation of urea or protein; glutamine, derived from the degradation of ammonia; or mercaptans, derived from the degradation of sulfur-containing compounds; and manganese. Because of anatomic or functional portal–systemic shunts, these toxins bypass the liver’s detoxification processes and produce alterations in mental status. Exposure to these toxins can cause astrocyte swelling and structural changes in neurons. In addition, high ammonia levels can result in abnormal cerebral blood flow and glucose metabolism. Increased levels of ammonia, glutamine, and mercaptans can be found in the blood and cerebrospinal fluid. There is also an increase in cerebral manganese deposition in patients with cirrhosis. However, blood ammonia and spinal fluid glutamine levels correlate poorly with the presence and severity of encephalopathy. In addition, the role of manganese in hepatic encephalopathy remains unclear.

Alternatively, there may be impairment in the normal blood–brain barrier, rendering the CNS susceptible to various noxious agents. Increased levels of other substances, including metabolic products such as short-chain fatty acids and endogenous benzodiazepine-like metabolites, have also been found in the blood. Importantly, some patients show improvement in encephalopathy when treated with flumazenil, a benzodiazepine receptor antagonist.

Another proposed mechanism postulates a role for GABA, the principal inhibitory neurotransmitter of the brain. GABA is produced in the gut, and increased levels are found in the blood of patients with liver failure. More recently, cerebral and systematic inflammation has been implicated in the pathogenesis of hepatic encephalopathy. Although the exact mechanisms are unknown, possibilities include cytokine-mediated changes in blood–brain barrier permeability, potential changes in glutamate uptake by astrocytes, and changes in the expression of GABA receptors.

Once the diagnosis is made, it is helpful to grade the severity of hepatic encephalopathy. Stages I through IV are based on degree of behavioral changes, intellectual dysfunction, and alterations in consciousness. Therapy includes the

management of potential precipitants and is directed at reducing intestinal ammonia production or increasing the removal of ammonia from the circulation. Nonabsorbable synthetic disaccharides (eg, lactulose) are catabolized by colonic bacteria to short-chain fatty acids, which lower luminal pH. This change in pH favors the formation of ammonium (NH4

+), which reduces the absorption of ammonia (NH3) into the circulation. Disaccharides such as lactulose are thus the mainstay of therapy. As discussed above in the section Altered Metabolism of Ammonia, the antibiotic rifaximin has been used in conjunction with lactulose to treat hepatic encephalopathy.

H. Coagulopathy

Factors contributing to coagulopathy in cirrhosis include loss of hepatic synthesis of clotting factors, some of which have a half-life of just a few hours. Under these circumstances, a minor or self-limited source of bleeding can become massive.

Hepatocytes are also functionally involved in the maintenance of a normal coagulation cascade through the absorption of vitamin K (a fat-soluble vitamin whose absorption depends on bile flow), which is necessary for the activation of some clotting factors (II, VII, IX, X). An ominous sign of the severity of liver disease is the development of a coagulopathy that does not respond to parenteral vitamin K, suggesting deficient clotting factor synthesis rather than impaired vitamin K absorption because of fat malabsorption. Finally, the loss of the liver’s capacity to remove activated clotting factors and fibrin degradation products may play a role in the increased susceptibility to disseminated intravascular coagulation, a syndrome of coagulation factor consumption that results in uncontrolled simultaneous clotting and bleeding.

I. Splenomegaly and Hypersplenism

Enlargement of the spleen is a consequence of elevated portal venous pressure and the consequent engorgement of the organ. Thrombocytopenia and hemolytic anemia occur because of the sequestering of formed elements of the blood in the spleen, from which they are normally cleared as they age and are damaged.

J. Hepatocellular Carcinoma

The five-year cumulative risk of HCC in patients with cirrhosis ranges from 5% to 30% depending on the patient’s sex, ethnicity, cause of liver disease, and stage

of cirrhosis. In the United States, the incidence of HCC has been rising over the past few decades, with over 20,000 new cases diagnosed each year, attributable to the increased prevalence of NAFLD, HCV cirrhosis, and chronic HBV infections owing to immigration from high-prevalence regions. Several etiologic factors have been identified in the development of this tumor, although cirrhosis is present in the vast majority (80–90%) of patients who develop HCC.

The risk of developing HCC is increased 100-fold in those with chronic hepatitis B infection, and worldwide, HBV accounts for over 50% of all HCC cases and nearly all childhood cases. While HCC can occur in the absence of cirrhosis, over 70% of HBV-related cases occur in those with advanced fibrosis or cirrhosis. Risk factors for HCC in this population include male sex, older age or longer duration of infection, coinfection (HCV, HDV, HIV), mycotoxin aflatoxin exposure, genotype C, and, in particular, high levels of viral replication, as evidenced by a persistent elevation of HBV viral load.

In patients with chronic hepatitis C, the risk of developing HCC is increased 15- to 20-fold, with risk limited to those with advanced fibrosis and cirrhosis. It has been projected that the incidence of HCV-related HCC cases in the United States will continue to rise over the next several decades. Risk factors for developing HCC include male sex, older age and duration of chronic HCV infection, coinfections (HBV, HIV), heavy alcohol use, obesity, and metabolic factors. Importantly, HCV patients with advanced fibrosis remain at risk for HCC despite cure of infection, and ongoing surveillance for HCC is recommended.

Chronic hepatitis B and C account for 60–70% of all HCC cases in the United States. While any cause of cirrhosis can lead to HCC, alcoholic cirrhosis and nonalcoholic steatohepatitis account for most of the remaining U.S. cases. Obesity and metabolic syndrome are increasingly recognized as risk factors for liver cancer, and patients with NASH who have advanced fibrosis (both stage 3 and 4 fibrosis) have also been shown to be at heightened risk for HCC.

K. Pulmonary Complications

Up to one-third of patients with decompensated cirrhosis have problems associated with oxygenation and may present with shortness of breath. There are three main pulmonary complications of cirrhosis to consider: hepatopulmonary syndrome, portopulmonary syndrome, and hepatic hydrothorax. In addition, mild hypoxemia can be caused by massive ascites, with resulting diaphragmatic elevation and ventilation–perfusion mismatch.

Hepatopulmonary syndrome consists of the triad of advanced liver failure,

hypoxemia, and intrapulmonary vascular dilation and shunting. The cause of pulmonary precapillary and capillary vasodilatation is unknown, but substances such as nitric oxide, endothelin, and arachidonic acid are thought to be involved. As a result of ventilation–perfusion mismatch, patients often present with platypnea, dyspnea that worsens in the upright position secondary to preferential perfusion of dilated vessels in the lung bases. Classically, contrast-enhanced echocardiography is used for definitive diagnosis and can reveal opacification of the left heart chambers within three to six cardiac cycles if a right-to-left intrapulmonary shunt is present. Liver transplantation leads to resolution of hepatopulmonary syndrome. However, the development of severe pulmonary hypertension in patients with advanced liver failure may be a contraindication to liver transplantation.

Portopulmonary hypertension refers to the development of pulmonary hypertension in patients with advanced liver disease and advanced portal hypertension. Patients can present with hypoxia, dyspnea on exertion, fatigue, and even signs of right heart failure. Patients have evidence of elevated pulmonary vascular resistance and a transpulmonary gradient in the setting of pulmonary arterial vasoconstriction. Targeted therapy (eg, epoprostenol, vasodilators) and management of right heart failure can delay progression, but prognosis is poor. Liver transplantation is associated with high operative risk when pulmonary hypertension becomes severe.

Individuals with cirrhosis and ascites can develop hepatic hydrothorax and present with shortness of breath, cough, or chest discomfort. In this condition, fluid accumulates in the pleural space owing to small defects in the diaphragm, most commonly on the right side. Negative intrathoracic pressure generated during inspiration favors the passage of fluid from the intra-abdominal cavity to the pleural space. Diagnostic thoracentesis should be performed to exclude alternative causes of pleural effusion, particularly infection. Treatment aims to prevent or reduce fluid accumulation with diuretics, a low-sodium diet, and occasionally therapeutic thoracentesis (or paracentesis to decrease pressure from tense ascites) for highly symptomatic patients refractory to or intolerant of conservative measures. TIPS may benefit selected patients (eg, Child–Pugh class A or B with no encephalopathy) who need repeated thoracenteses. If they are otherwise suitable candidates, patients with cirrhosis and persistent hepatic hydrothorax should be referred for liver transplantation.

L. Miscellaneous Manifestations

Other findings on physical examination of patients with cirrhosis include spider

angiomas (prominent blood vessels with a central arteriole and small vessels radiating from it seen in the skin, particularly on the face and upper trunk), Dupuytren contractures (fibrosis of the palmar fascia), testicular atrophy, gynecomastia (enlargement of breast tissue in men), palmar erythema, lacrimal and parotid gland enlargement, and diminished axillary and pubic hair (see Figure 14–15). These findings are largely a consequence of estrogen excess resulting from the decreased clearance of endogenous estrogens by the diseased liver combined with the decreased hepatic synthesis of steroid hormone–binding globulin. Both mechanisms result in tissues receiving higher than normal concentrations of estrogens. In addition, the longer half-life of androgens may allow a greater degree of peripheral aromatization (conversion to estrogens by, eg, adipose tissue, hair follicles), further increasing estrogen-like effects in patients with cirrhosis. Xanthomas of the eyelids and extensor surfaces of tendons of the wrists and ankles can occur with chronic cholestasis such as occurs in primary biliary cirrhosis. Finally, profound muscle wasting and cachexia in cirrhosis probably reflect the diminution of the liver’s synthesis of carbohydrate, lipid, and amino acids.

CHECKPOINT

31. What are the defining features of cirrhosis? 32. What are the three categories of hepatic fibrosis? Name one agent

causing each. 33. What are the two postulated stages of the development of cirrhosis? 34. What are the major clinical manifestations of cirrhosis? 35. For each major clinical manifestation of cirrhosis, suggest a reasonable

hypothesis to account for its pathogenesis.

CASE STUDIES Yeong Kwok, MD

(See Chapter 25, p. 773–74 for answers)

CASE 77

A 28-year-old man, who recently emigrated from the Philippines, was noted to have a positive tuberculin skin test result in the clinic. His chest radiograph showed no active tuberculosis, and he denied any symptoms of this infection, including weight loss, cough, or night sweats. To prevent future disease, daily dosing with isoniazid was recommended for the next 9 months. Two weeks after initiating therapy, the patient reported progressive fatigue, intermittent bouts of nausea, and abdominal pain. He also noticed darkened urine and light-colored stools. His sister noted a gradual yellowing of his eyes and skin. Blood tests showed a marked increase in serum bilirubin and aminotransferases. The isoniazid was discontinued, and his symptoms subsided with his liver enzymes normalizing.

Questions

A. Describe the subtypes of toxic hepatitis. B. What typical histologic findings are noted during uncomplicated acute

hepatitis? C. What is the pathogenesis of the clinical jaundice seen in this patient?

CASE 78

A 44-year-old man is concerned about abnormal liver tests drawn for his pre-employment physical 6 months ago. His serum aminotransferase levels were two times normal at that time and remain unchanged after repeat testing. On further questioning, he denies regular alcohol use but states that he used to inject heroin. Currently, he reports some fatigue but says he feels well otherwise. His primary care physician orders serologic testing, which reveals HBsAg-positive, anti-HBs-negative, and anti-HBc-positive IgG. Anti-HDV and anti-HCV test results are both negative.

Questions

A. Based on these antigen and antibody test results, what is the patient’s diagnosis?

B. What percentage of patients with acute hepatitis B remain chronically infected with HBV? Of those patients, how many develop chronic active disease? What are the significant complications of chronic active infection?

C. What is the significance of hepatitis D superinfection? D. What evidence exists supporting immune-mediated damage in chronic

active hepatitis?

CASE 79

A 63-year-old man with a long history of alcohol use presents to his new primary care physician with a 6-month history of increasing abdominal girth. He has also noted easy bruisability and worsening fatigue. He denies any history of GI bleeding. He continues to drink three or four cocktails a night but says he is trying to cut down. Physical examination reveals a cachectic man who appears older than his stated age. Blood pressure is 108/70 mm Hg. His scleras are anicteric. His neck veins are flat, and chest examination demonstrates gynecomastia and multiple spider angiomas. Abdominal examination is significant for a protuberant abdomen with a detectable fluid wave, shifting dullness, and an enlarged spleen. The liver edge is difficult to appreciate. He has trace pitting pedal edema. Laboratory evaluation shows anemia, mild thrombocytopenia, and an elevated prothrombin time. Abdominal ultrasonogram confirms a shrunken, heterogeneous liver consistent with cirrhosis, significant ascites, and splenomegaly.

Questions

A. Describe possible mechanisms for alcohol-induced cirrhosis. B. What is the proposed mechanism of portal hypertension, and how does

it affect ascites formation? C. Significant hematologic abnormalities exist in this patient. How might

they be explained?