Alzhemier's Disease

Alzheimer’s & Dementia 13 (2017) 374-380

Alzheimer’s disease: The next frontier—Special Report 2017

Jason Karlawisha, Clifford R. Jack, Jr.b, Walter A. Roccac,d, Heather M. Snydere, Maria C. Carrillo

e,* a Department of Medicine, University of Pennsylvania, Philadelphia, PA, USA

bDepartment of Radiology, Mayo Clinic, Rochester, MN, USA cDepartment of Health Sciences Research, Mayo Clinic, Rochester, MN, USA

d Department of Neurology, Mayo Clinic, Rochester, MN, USA

e Division of Medical & Scientific Relations, Alzheimer’s Association, Chicago, IL, USA

Abstract In the history of medicine, one means to progress is when we make the decision that our assump-

*Corresponding au

E-mail address: m

http://dx.doi.org/10.10

1552-5260/� 2017 th

tions and definitions of disease are no longer consistent with the scientific evidence, and no longer serve our health care needs. The arc of scientific progress is now requiring a change in how we diagnose Alzheimer’s disease. Both the National Institute on Aging—Alzheimer’s Association (NIA-AA) 2011 workgroup and the International Work Group (IWG) have proposed guidelines that use detectable measures of biological changes in the brain, commonly known as biological markers, or biomarkers, as part of the diagnosis. This Special Report examines how the development and validation of Alzheimer’s disease biomarkers—including those detectable in the blood or cere- bral spinal fluid, or through neuroimaging—is a top research priority. This has the potential to mark- edly change how we diagnose Alzheimer’s disease and, as a result, how we count the number of people with this disease. As research advances a biomarker-based method for diagnosis and treatment at the earliest stages of Alzheimer’s disease, we envision a future in which Alzheimer’s disease is placed in the same category as other chronic diseases, such as cardiovascular disease or diabetes, which can be readily identified with biomarkers and treated before irrevocable disability occurs. � 2017 the Alzheimer’s Association. Published by Elsevier Inc. All rights reserved.

1. Introduction

After Dr. Alois Alzheimer’s 1906 case report of the dis- ease that came to bear his name, for much of the 20th century, Alzheimer’s disease was defined as an unusual cause of dementia in adults we now consider middle-aged [1]. “Senile dementia” was the diagnosis for the more common cause of dementia in individuals 65 and older. In 1976, Robert Katzman, M.D., made the case that these definitions should change.

Arguing that an age-based distinction between dementia due to Alzheimer’s disease and senile dementia was neither scientifically nor medically sensible [2], he used scientific data to conclude that the two conditions were in fact one and to call them both Alzheimer’s disease. “Although further studies are clearly indicated, the fact remains that neither the

thor. Tel.: (312) 335-5722; Fax: (866) 875-2553.

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e Alzheimer’s Association. Published by Elsevier Inc. All ri

clinician, the neuropathologist nor the electron microscopist can distinguish between the two disorders [Alzheimer’s dis- ease and senile dementia] except by the age of the patient” [2]. His rationale was pragmatic—dementia at any age causes substantial personal, medical and economic burden.

Dr. Katzman’s contribution that Alzheimer’s disease was a cause of dementia across a wide age span was incorporated into diagnostic criteria published in 1984, known as the Na- tional Institute of Neurological and Communicative Disor- ders and Stroke and the Alzheimer’s Disease and Related Disorders Association (now known as the Alzheimer’s Asso- ciation) Criteria, or NINCDS-ADRDA Criteria [3]. These criteria did not include biomarkers for the diagnosis of Alz- heimer’s disease.

Since Dr. Katzman’s time, Alzheimer’s science has made notable discoveries. Using certain biomarkers, we can now distinguish between Alzheimer’s disease and other causes of dementia. In this sense, the arc of scientific progress is

ghts reserved.

J. Karlawish et al. / Alzheimer’s & Dementia 13 (2017) 374-380 375

now requiring another change in how we diagnose Alz- heimer’s disease. Both the National Institute on Aging— Alzheimer’s Association (NIA-AA) 2011 workgroup [4–6] and the International Work Group (IWG) [7–9] have proposed guidelines that use biomarkers as part of the diagnosis. The guidelines use biomarkers (such as brain imaging of amyloid plaques, changes in brain volume, and measures of tau and amyloid in spinal fluid) and clinical symptoms to define dementia caused by Alzheimer’s disease, and also preclinical Alzheimer’s and mild cognitive impairment (MCI) due to Alzheimer’s [10–19].

The science of Alzheimer’s is the primary driver of this change. Drug interventions in people with Alzheimer’s dis- ease dementia have repeatedly reported negative results. Research shows moments in the course of the disease when an intervention might effectively slow or even stop the disease. The Dominantly Inherited Alzheimer’s Network (DIAN) study findings have shown brain changes starting 10 to 20 years before the onset of dementia symptoms [20]. Ongoing trials in this population are testing interventions at this pre-symptomatic point in an effort to delay or even prevent the onset of dementia symptoms. Other clinical trials (A4 Study, etc.) are testing interventions in people who do not have memory (cognitive) and thinking (functional) changes or high-risk genes but do have measurable Alz- heimer’s biomarkers [21].

The development and validation of biomarkers— including those detectable in the blood or cerebrospinal fluid, or through neuroimaging—may significantly change how we identify Alzheimer’s disease and, as a result, how we estimate the number of people with this disease. This is important because Alzheimer’s disease prevalence and inci- dence estimates are used to calculate other statistics, which are used to describe the scope of the Alzheimer’s problem in the U.S., illustrate the need to combat the disease, and iden- tify and allocate the resources needed to address it.

2. Rethinking our assumptions about Alzheimer’s disease

The U.S. has, since 2011, charted a national plan to address Alzheimer’s disease. The first of the plan’s five goals is to effectively treat and prevent the disease by 2025 [22]. Researchers and those who translate research into clinical practice have reached a consensus. A core strategy to achieve this goal relies on studies testing drugs in persons who have biomarker confirmation of the presence of Alz- heimer’s disease [23]. Studies such as the A4 Study dis- cussed above, as well as trials in persons with Alzheimer’s disease dementia, are enrolling persons who have these bio- markers [24,25].

This strategy aligns with approaches taken with other common diseases of aging, such as cardiovascular disease. Clinicians use measures of biological change, such as elevated levels of blood pressure or cholesterol, to diagnose and treat individuals. Their goal is to prevent the person from

suffering another heart attack or worsening heart failure, or to prevent these problems from happening in the first place. Someday, clinicians may have a similar strategy to diagnose and treat Alzheimer’s disease. They may use biological mea- sures (biomarker-based) to diagnose and then prescribe treatments to these persons, treatments that trials have shown to slow cognitive and functional decline or even prevent the onset of symptoms of dementia.

Alzheimer’s-related brain changes—amyloid plaques and tau tangles among others—contribute to the cognitive impairment observed in dementia due to Alzheimer’s [26–29]. A clinically effective intervention that targets these brain changes will help to validate the disease as a continuum that begins before cognitive decline. This confirmation will change how we identify (and therefore estimate) individuals with Alzheimer’s disease. It will alter the estimated prevalence and incidence of the disease, just as the treatment of vascular disease has altered the estimated prevalence of dementia among individuals with primarily vascular lesions [30,31]. As these events unfold, they compel us to plan for a future when Alzheimer’s disease is defined using biomarkers alone, not symptoms. (See sidebar: “Determining the incidence and prevalence of Alzheimer’s disease.”)

3. The evolving diagnosis of Alzheimer’s disease

Current methods of diagnosis do not conform to what we know about the disease. The 1984 NINCDS-ADRDA criteria for Alzheimer’s disease defined it as a clinical dis- ease caused by underlying brain changes [3]. The assump- tion was that an individual with an amnestic dementia would have Alzheimer’s-related brain changes, namely am- yloid plaques and tau neurofibrillary tangles, if the individ- ual came to autopsy. Conversely, individuals without amnestic dementia would not have plaques or tangles at au- topsy. This definition of Alzheimer’s intertwines the signs and symptoms of dementia and the underlying brain changes [3].

In the years that followed the adoption of those criteria, studies suggested that the clinical symptoms and underlying brain changes do not always align. Autopsy studies found that 10–30% of individuals who met NINCDS-ADRDA criteria for Alzheimer’s disease did not have significant Alzheimer’s-related brain changes (i.e., plaques and/or tan- gles). Instead, they had other (non-Alzheimer’s) brain changes at autopsy [32]. Often Alzheimer’s was mixed with non-Alzheimer’s brain changes, such as cerebral infarc- tions or Lewy body disease, particularly in older individuals [26,27,33]. Furthermore, autopsy studies in individuals who were cognitively normal for their age found that roughly 30% had Alzheimer’s-related brain changes at death [34–36].

Over the past two decades, biomarkers of Alzheimer’s disease-related brain changes continued to be developed. They fit into two classes: (1) brain imaging of amyloid and

What are biomarkers? A biomarker, or biological marker, is a measurable in-

dicator of some biological state or condition in the human body. Clinicians use biomarkers to diagnose the presence or absence of disease, assess the risk of developing a dis- ease, or understand how a patient has responded to a treatment. For example, a high blood glucose level (blood sugar) may indicate the presence of diabetes, and lowering that level can indicate the success of a pre- scribed diet or medication.

Researchers are investigating several promising bio- markers for Alzheimer’s disease. These include, but are not limited to, the amount of accumulation of the proteins beta-amyloid and tau in the brain. These proteins can be measured using brain imaging or the levels in cerebro- spinal fluid. Another kind of biomarker is changes in brain size and activity.

Identifying and then validating biomarkers for Alz- heimer’s is critical. They will facilitate early diagnosis and treatment. Many researchers believe that early inter- vention—either at the mild cognitive impairment (MCI) stage or even before symptoms appear—offers the best chance of slowing or stopping the progression of Alz- heimer’s disease and therefore the best chance of preser- ving brain function.

Biomarkers also have an important role in the discov- ery of treatments. They enable researchers to identify which individuals to enroll in clinical trials to test new therapies. Biomarkers allow researchers to enroll those individuals with the brain changes that treatments target. (It’s important to note that the most effective biomarker test or combination of tests may differ depending on the stage of the disease and other factors.) Biomarkers also allow researchers to monitor the effects of these treatments. The more a change in a biomarker maps onto the health of the patient, the better that biomarker is in assessment of whether a treatment is effective.

Research on new strategies for earlier diagnosis, including ongoing efforts to identify and validate bio- markers for Alzheimer’s disease, is among the most active areas in Alzheimer’s science.

J. Karlawish et al. / Alzheimer’s & Dementia 13 (2017) 374-380376

tau buildup, and of brain volume and brain metabolism changes, and (2) measures of relevant proteins in spinal fluid [10–19]. These biomarkers illustrate or represent the presence of amyloid plaques, tau tangles and brain cell death or injury [37]. Studies have validated that biomarkers are indeed reliable measures of the relevant disease-related changes in the living brain [38–44]. These studies, like autopsy studies, also demonstrated that roughly one-third of individuals who meet NINCDS–ADRDA criteria for Alzheimer’s disease do not have the required brain changes (and thus do not have Alzheimer’s disease) [43–

46]. In addition, studies showed that roughly one-third of clinically normal older individuals do have Alzheimer’s- related brain changes without the clinical symptoms [43– 45,47,48].

Recognizing the potential for biomarkers, both the NIA- AA and the IWG have proposed that, when used alongside clinical criteria, biomarkers can increase the confidence that a diagnosis of dementia is or is not due to Alzheimer’s disease [4–9,49]. Importantly, the NIA-AA also proposed that biomarkers could identify MCI as due either to Alzheimer’s or other diseases [5]. The equivalent term for biomarker-positive individuals with MCI is prodromal Alzheimer’s disease in the IWG criteria.

Further, the NIA-AA proposed that cognitively normal individuals with abnormal Alzheimer’s biomarkers have preclinical Alzheimer’s disease. If this is validated, then individuals who have no cognitive impairment but have Alzheimer’s biomarkers have Alzheimer’s disease [4].

A biomarker-based diagnosis of Alzheimer’s disease— one based on brain changes, not cognitive or functional changes—will change the incidence and prevalence of Alzheimer’s.

4. The prevalence and incidence of Alzheimer’s disease in a new era of research

Today, we understand that Alzheimer’s disease exists as a continuum beginning with a phase that may only be detectable through biomarkers and ending with the de- mentia stage. In the future, a biomarker-based diagnosis of Alzheimer’s disease will impact the estimates of inci- dence and prevalence of Alzheimer’s. It will add a popu- lation of individuals that currently is not included in estimates (people with Alzheimer’s biomarkers but no de- mentia) and remove a population that currently is included (people with dementia but no Alzheimer’s bio- markers).

The Alzheimer’s Association 2017 Alzheimer’s Disease Facts and Figures (DOI: 10.1016/j.jalz.2017.02.001) reports the prevalence and incidence of Alzheimer’s in the U.S. Among individuals age 65 and older, the prevalence in 2017 is estimated to be 5.3 million (one in 10 people age 65 and older, or 10 percent, have Alzheimer’s dementia), and 480,000 people age 65 or older will develop Alzheimer’s dementia in the U.S. in 2017.

Epidemiologists, demographers, and biostatisticians will use these prevalence and incidence estimates to calculate other statistics, such as the numbers of people providing care and support for someone with the disease, the costs of care, and mortality. Clinicians, policy makers, and organiza- tions use these statistics to describe the size of the Alz- heimer’s problem in the U.S., to demonstrate the need to combat the disease, and to identify the resources needed to address it.

Determining the incidence and prevalence of Alzheimer’s disease

Counting the incidence or prevalence of Alzheimer’s disease or dementia due to Alzheimer’s is complex. In the absence of registries akin to cancer registries or routine disease monitoring systems used to track infectious diseases, investigators must make a series of assumptions. These assumptions mean we are not so much counting as we are estimating the prev- alence and incidence of Alzheimer’s disease. Below, we review these assumptions, and why studies have arrived at different estimates.

The process begins with identifying a study population, usually a cohort of individuals in a given region. It could also be a representative sample in various regions. Next, investigators select a strategy to identify the cases of dementia due to Alz- heimer’s disease in that given population. Some studies have used a two-phase strategy that starts with a brief cognitive test administered to the total group of participants to identify potential cases (known as the screening phase of the survey), who are then more fully evaluated using the Alzheimer’s disease diagnostic criteria [52–56]. Other studies fully evaluate a random sub-group from the total participants; still others fully evaluate the entire participating group.

A crucial methodological step to identify the individuals with Alzheimer’s disease is the choice of diagnostic criteria that will be used in the study. Historically, studies have used a clinical diagnosis of the disease—that is, they counted people who had signs and symptoms of dementia. They have not included biomarkers as part of the criteria for the disease, nor have they excluded people with signs and symptoms of dementia but no biomarkers for Alzheimer’s disease.

In most cases, the onset of dementia is gradual. It is therefore difficult in the early stages of the disease to assign a diag- nosis of dementia. Consequently, investigators using brief cognitive tests face the error of mistakenly diagnosing someone as cognitively normal, and therefore without the disease, when in fact, the person is not normal; in other words, the error of false negatives, which can lead to an underestimate of prevalence and incidence. More recent studies, therefore, have abandoned brief screening tests. Instead, they either fully examine all participants in the sample or they fully examine a random sample of the study population [54–56]. Each of the design choices described above creates variability in who is selected for evaluation and, hence, as studies differ in these choices, there is variability in their respective prevalence estimates.

The Alzheimer’s Association uses estimates for the prevalence and incidence of Alzheimer’s disease modeled by the Chi- cago Health and Aging Project at Rush University Medical Center, called CHAP [57,58]. CHAP is a longitudinal, population-based study in a geographically defined area of Chicago with significant population diversity. It began in 1993 with a census of individuals age 65 or older using in-home interviews and random sampling of participants for clinical evaluation for dementia due to Alzheimer’s [57].

CHAP researchers identify an individual living with Alzheimer’s disease by detecting cognitive decline that then triggers a clinical assessment. The clinician uses the 1984 NINCDS-ADRDA criteria for the clinical diagnosis of Alzheimer’s disease to determine if the dementia is caused by Alzheimer’s disease [3]. These criteria focus on dementia assessed by an interview with the participant and an informant, usually their partner or child (if available), and cognitive testing [3].

CHAP uses newly diagnosed cases of Alzheimer’s—incidence—to determine the prevalence. This is a notable feature. It minimizes missing cases of the disease whose symptoms are mild or evenvery mild [57–59]. Evaluation is repeated in 3-year cycles. Calculations of national and state-by-state prevalence figures as well as estimates of future prevalence are extrap- olated from the CHAP data and incorporate age, sex and race: (1) risk of developing dementia due to Alzheimer’s, (2) increased risk of mortality among those with dementia due to Alzheimer’s, (3) U.S. mortality rates, (4) U.S. education levels, and (5) U.S. current and projected total population [30]. Since their first publication in 2003, CHAP produced updated estimates of prevalence in 2013 utilizing 2010 U.S. Census Bureau population information [58]. The Association’s 2017 Alzheimer’s Disease Facts and Figures prevalence estimates are reported from these data for U.S. residents age 65 and older.

Other U.S.-based studies have measured either the prevalence or incidence of dementia. Two of note are the Health and Retirement Study-Alzheimer’s Disease and Memory Study (HRS-ADAMS)—a nationally representative sample [30,60]— and the Framingham Heart Study (FHS)—a study of all-cause dementia over time in Framingham, Massachusetts [61]. HRS-ADAMS and FHS have consistently reported estimates that are lower than CHAP estimates [30,60–62].

At a 2009 conference convened by the NIA and the Alzheimer’s Association, researchers concluded that these discrep- ancies were mainly due to differences in diagnostic criteria, differences that reflect the study’s different goals [59]. HRS- ADAMS defines a case using the Diagnostic and Statistical Manual of Mental Disorders (DSM) criteria for dementia, incorporating impairments in both cognition and function [59,63]. In addition, people exhibiting the symptoms of Alzheimer’s disease are not counted as having Alzheimer’s if they are determined to have vascular dementia. HRS- ADAMS focuses on the severity of disability, not the precision of the diagnosis of Alzheimer’s disease, which is the goal of CHAP [57,60,62]. The Framingham Heart Study uses DSM criteria for dementia and the NINCDS-ADRDA criteria, an approach that achieves the goal of determining if a case of dementia is caused by Alzheimer’s [61].

The estimates from each of these studies are often discussed as different numbers measuring the same thing, a conclusion that destabilizes confidence that we can talk coherently about the prevalence of Alzheimer’s disease. They are in fact different

J. Karlawish et al. / Alzheimer’s & Dementia 13 (2017) 374-380 377

numbers because they are measuring different things in different populations using different means of identifying individuals with all-cause dementia and/or dementia due to Alzheimer’s [59]. None of the studies referenced above used biomarkers in their estimates; inclusion of biomarkers would markedly alter estimates of the prevalence and incidence of Alzheimer’s disease.

As research advances a biomarker-based strategy for detection and treatment at the earliest stages of Alzheimer’s disease, ever more accurate estimates of the number of persons affected will be needed to understand the full extent of that burden. These estimates will very likely be greater than current estimates and will require appropriate, modernized research and public health strategies.

J. Karlawish et al. / Alzheimer’s & Dementia 13 (2017) 374-380378

Validated Alzheimer’s disease biomarkers will transform how study results are interpreted and change the messages and terms professionals and society use to talk about who has Alzheimer’s disease and how big of a problem the dis- ease poses.

To accurately answer the question, “What is the true prevalence and incidence of Alzheimer’s disease?” we have to identify Alzheimer’s disease in a way that is grounded in current science and makes sense to individ- uals, families, clinicians, researchers, and healthcare policy makers. Looking ahead, a biologically-based Alzheimer’s disease diagnosis will yield different prevalence and inci- dence figures than a diagnosis that uses only the severity of cognitive or functional impairment (either using DSM or NINCDS-ADRDA criteria). It will exclude individuals who have dementia but do not have the Alzheimer’s bio- markers and thus do not have Alzheimer’s disease. On the other hand, it will include individuals with MCI who have Alzheimer’s biomarkers and therefore have Alz- heimer’s disease, a proportion that may, according to exist- ing studies, be as high as 56% of persons with a diagnosis of MCI [50,51]. Even further in the future and with more research, it will also include people who do not have cognitive impairment but have Alzheimer’s disease biomarkers.

Epidemiologic and related natural history studies that measure cognition in older adults and that want to estimate the prevalence and incidence of Alzheimer’s disease will need to gather biomarker data from their participants. We should expect that these study results will further disrupt our understanding of the causes and trajectories of cognitive impairment. Studies that do not use these mea- sures will not be able to accurately report the prevalence and incidence of Alzheimer’s disease. (They can report on the clinical severity of cognitive impairment in a popu- lation using constructs such as dementia or mild cognitive impairment.)

It is possible that these biomarker measures will add to the burdens and risks encountered by research participants. This, in turn, may hinder study recruitment, retention, and accessibility. Studies to assess why individuals might refuse to undergo biomarker measures, test interventions to change that decision, and discover messaging that motivates the intention to undergo biomarker testing will be essential to address this problem. Studies will likely benefit from collab- orations among epidemiologists, bioethicists, clinicians,

biomarker scientists, and decision-scientists who interpret data and help make public health recommendations.

5. Conclusion

Even with scientific progress, a common question from the public has been, “What’s the difference between Alz- heimer’s disease and dementia?” The NINCDS-ADRDA diagnostic criteria of 1984 aimed to help answer that ques- tion [3]. Alzheimer’s disease is the most frequent cause of the dementia syndrome.

As dementia science has progressed, biomarker-based data have advanced our understanding of who has Alz- heimer’s disease as well as contributed to a more accurate clinical diagnosis of who has dementia due to Alzheimer’s. Biomarker-based clinical criteria and future clinical trial data will continue to change our understanding of who has Alzheimer’s disease, as improved diagnostic techniques will provide earlier identification of cognitive impairment, and of the brain changes that lead to it.

As with cardiovascular disease, we must care not just about those who have had a disease-manifesting event, such as a heart attack, but everyone who has cardiovascular disease-related biological changes that precede the heart attack. All of these individuals represent the societal burden of cardiovascular disease. Similarly, although we have known for years about the occurrence of dementia due to Alzheimer’s, as a result of the recent use of biomarkers in studies, we have learned that a proportion of people previ- ously thought to have cognitive impairment caused by Alz- heimer’s disease lack those biomarkers. The diagnosis of Alzheimer’s disease will come to include the full spectrum of persons with Alzheimer’s biomarkers, those who are symptomatic—with either dementia or MCI—and those who are still asymptomatic but have preclinical Alzheimer’s disease. All individuals with biomarkers of Alzheimer’s dis- ease, including those with and without dementia symptoms, will represent the full disease burden.

Additional research and development of guidelines for the future use of biomarkers is urgently needed to optimize ther- apeutic strategies for this potentially much larger population of people with Alzheimer’s disease. Successful validation of biomarkers will bring our definition of Alzheimer’s disease in line with the remarkable advances we have seen in Alz- heimer’s research over the past decade. This latest research is now allowing us to envision a future in which Alzheimer’s

J. Karlawish et al. / Alzheimer’s & Dementia 13 (2017) 374-380 379

is no longer a disease leading to irrevocable cognitive and functional decline and death, but rather a chronic condition like cardiovascular disease, AIDS, or some cancers that can often be managed with early intervention.

Acknowledgments

The Alzheimer’s Association wishes to provide a special thank you to Dr. Lenore Launer, Senior Investigator and Chief, Neuroepidemiology Section, at the National Institute on Aging of the National Institutes of Health, and Dr. David Knopman, Professor of Neurology at the Mayo Clinic in Rochester, MN, and chair of the Alzheimer’s Association Medical & Scientific Advisory Council, for their review and comments during the preparation of this manuscript.

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