Summary Article

Research in Autism Spectrum Disorders 7 (2013) 1228–1233

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Research in Autism Spectrum Disorders

J o u r n a l h o m e p a g e : h t t p : / / e e s . e l s e v i e r . c o m / R A S D / d e f a u l t . a s p

Review

Comorbidity and autism: Trends, topics and future directions

Johnny L. Matson, Rachel L. Goldin *

Louisiana State University, United States

A R T I C L E I N F O

Article history:

Received 1 July 2013

Accepted 1 July 2013

Keywords:

Autism spectrum disorders

Comorbidities

Intellectual disabilities

Co-occurring

A B S T R A C T

Autism spectrum disorders (ASD) are common, life-long disorders. Symptoms of ASD often

co-occur with other difficulties that also tend to be chronic and debilitating. Common co-

occurring difficulties include intellectual disabilities, physical conditions, comorbid

psychopathologies and challenging behaviors. However, the idea of comorbidities among

individuals with ASD is still a relatively new notion. Thus, a new field of research is

developing examining comorbidities with ASD. This field is developing rapidly but is still

lacking. This paper highlights the most common forms of comorbid disorders, and the

future direction of research on comorbidities among individuals with ASD is discussed.

� 2013 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1228

2. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1229

3. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1229

3.1. Physical conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1229

3.2. Comorbid psychopathology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1230

3.3. Challenging behaviors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1230

3.4. Intellectual disabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1231

4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1231

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1232

1. Introduction

Autism spectrum disorders (ASD) consist of persons who have deficits in communication, social skills, repetitive and ritualistic behaviors and a host of other difficulties that impede adjustment and quality of life (Fodstad, Matson, Hess, & Neal, 2009; Matson, Gonzalez, Wilkins, & Rivet, 2008; Matson, Dempsey, & Fodstad, 2009; Matson, Fodstad, & Dempsey, 2009). Not only do these symptoms lead to difficulties in adjustment, but they often co-occur with a host of other difficulties many of which are also chronic and debilitating. Among the most common of these problems are intellectual disabilities (ID). Motor difficulties and various forms of psychopathology and behavior problems are also common (LoVullo & Matson, 2009; Matson & Rivet, 2008; Matson, Dempsey, et al., 2009; Medeiros, Kozlowski, Beighley, Rojahn, & Matson, 2012; Smith & Matson, 2010a,b,c). The cumulative effect of these core symptoms and disorders are major obstacles in development and

* Corresponding author at: Department of Psychology, Louisiana State University, Baton Rouge, LA 70803, United States. Tel.: +1 617 966 6076.

E-mail address: rgold3@lsu.edu (R.L. Goldin).

1750-9467/$ – see front matter � 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.rasd.2013.07.003

J.L. Matson, R.L. Goldin / Research in Autism Spectrum Disorders 7 (2013) 1228–1233 1229

everyday adjustment (Matson, Dempsey, LoVullo, & Wilkins, 2008; Matson, Dempsey, et al., 2009). Furthermore, as the number of comorbidities accumulate, the intensity of individual treatments and the total number of interventions increases as well.

This notion of comorbidity among persons with ASD is a relatively new concept. For example, the DSM-IV notes that ASD and ADHD cannot be diagnosed concurrently. This point is interesting since we now know that these two conditions co-occur at very high rates (Tureck, Matson, May, Davis, & Whiting, 2013). The overlap of ADHD with ASD is not the only high rate comorbid condition. Rather, the overlap of other disorders with ASD has been found to occur at high rates, but is also not generally accepted or well researched in the literature (Matson & LoVullo, 2008).

What is occurring presently is the development of an entire new field of study; this field would be characterized best as comorbidity with ASD. Many questions are being addressed. These issues include determining the most common forms of comorbid disorders, how symptoms are expressed, what disorders above and beyond ASD co-occur most with this syndrome, to what extent are there common etiologies, how best to assess and diagnose these disorders when they occur in tandem, what intervention strategies are likely to be most efficacious, and a host of additional theoretical and pragmatic concerns (Matson & LoVullo, 2009; Matson, Wilkins, et al., 2009). Given the increasing attention on comorbid conditions among persons with ASD, a review of research trends and patterns on the topic would seem to be in order. The purpose of this paper was to examine these developments.

2. Methods

The literature search was conducted via SCOPUS. This search engine is the world’s largest scientific abstract and citation database of the peer-reviewed literature. Over 20,000 titles from over 5000 international publishers are included. Topics cover technical, scientific, medical and social science domains. As such, SCOPUS covers the widest range of topics among the major search engines. Having said that, no search engine or combination of search engines is all inclusive. However, identifying articles using the terms autism and comorbid, the search could be characterized as representative of this topic. To further expand our search we cross checked references identified in the aforementioned search as a means of identifying additional references. What follows are our observations based on the studies that we identified.

3. Results and discussion

A total of 1528 articles were identified using the aforementioned search terms. The bulk of the papers were excluded because they did not meet the intent of the study. For example, many studies on autism had nothing to do with comorbidity and many papers on comorbidity had nothing to do with autism. After these studies were removed, 449 papers remained. Of these studies, 348 discussed conditions that occurred conjointly with ASD, while 101 papers reported on ASD with multiple comorbid conditions. The majority of the papers dealt with only autism or autism and PDD-NOS. However, some papers described Asperger’s Syndrome as well. Rarely was Asperger’s included with autism or PDD-NOS.

Our findings parallel what is occurring in research across the mental health field. For example, Friborg, Martinussen, Kaiser, Øvergard and Rosenvinge (2013), reported on the high rates of comorbidity between personality and anxiety disorder. Similarly, ADHD symptoms have been commonly reported among persons with bipolar disorder (Masi et al., 2006; Wildes & Marcus, 2013). A host of other conditions have also been reported as co-occurring. Specifics on what conditions are being most frequently studied comorbid with ASD will be the focus of this paper.

The literature on comorbidity breaks down into four broad categories. These topics include physical conditions, mental health issues, challenging behaviors, and intellectual disabilities. Each of these categories is rapidly evolving into a field unto itself as the research on each topic expands exponentially. These four topics will be reviewed separately.

3.1. Physical conditions

A number of physical conditions co-vary with ASD. Ninety studies were found on this topic. In 75 studies, one comorbidity was studied, while for 15 studies, two or more comorbidities were noted. The most frequently studied topic has been epilepsy. Twenty-six papers were noted. Another popular topic for study has been sleep disturbances which have been researched in 25 papers. Other conditions were studied in fewer than 10 papers. In rank order from the most to least frequently studied topic was gastrointestinal disorder, allergic autoimmune disease, developmental coordination disorder, deafness, general neurological conditions, seizures, obesity, general medical conditions, cerebral palsy, birth defects, Lyme disease, LEOPARD Syndrome, clumsiness, handwriting impairments, motor problems, and bone fractures.

These data confirm that a host of physical conditions co-occur with ASD. Most of this research is of recent origin and thus more conditions are likely to be identified. Additionally, how these behaviors are expressed and identifying, how often the various conditions co-occur and how they cluster together has yet to be determined. Information of this type is very important for clinicians. They point out the importance of medical examinations for children identified with ASD. Conversely, for children with some specific disabilities such as cerebral palsy, screening for ASD should be routine (Table 1).

Table 1

Physical conditions comorbid with ASD.

Numbers of studies Multiple comorbidities

Condition Total One comorbidity

Epilepsy 26 21 5

Sleep disturbance 25 20 5

Gastrointestinal disorders 8 8 0

Allergic and autoimmune disease 5 4 1

Developmental coordination disorder 4 0 4

Deaf 4 4 0

Neurological conditions (general) 3 3 0

Seizures 3 3 0

Obesity 3 3 0

General medical condition 1 1 0

Cerebral palsy 1 1 0

Birth defects 1 1 0

Lyme disease 1 1 0

LEOPARD Syndrome 1 1 0

Clumsiness 1 1 0

Handwriting impairments 1 1 0

Motor problems 1 1 0

Bone fractures 1 1 0

Table 2

Comorbid psychopathology.

Condition Numbers of studies Multiple comorbidities

Total One comorbidity

ASD 81 62 19

General psychopathology 59 47 12

Anxiety 35 22 13

Depression 19 10 9

Schizophrenia/psychosis 18 16 2

OCD 15 15 0

Tics and Tourette’s 15 10 5

Bipolar disorder 9 7 2

Specific language impairment 3 3 0

Catatonia 2 2 0

Eating disorders 1 1 0

Hoarding 1 1 0

Borderline personality disorder 1 1 0

Gender identity disorder 1 1 0

Oppositional defiant disorder 1 1 0

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3.2. Comorbid psychopathology

Comorbid psychopathology is also a frequently researched topic. A total of 261 papers were published on the topic. Of these studies, the vast majority of papers dealt with only one comorbidity (199 studies). Papers which addressed two or more comorbid conditions were reported numbered 62. Prevalence data have been published showing rates of ADHD in ASD at over 50%. This fact may account at least in part for the fact that there are more studies on comorbid psychopathology than on other disorders. This topic is followed in frequency by general psychopathology, anxiety disorders, depression, the combined topics of schizophrenia and psychosis, tics and Tourette’s, bipolar disorder, specific language impairments, catatonia, eating disorders, hoarding, gender identity disorder, and oppositional defiant disorder (Table 2).

The number of co-occurring problems which may occur in a person diagnosed as having ASD are substantial. Fifteen different disorders are reported in a variety of studies. The bulk of the research is on ADHD, general psychopathology and anxiety. However, as with physical conditions, most of this research is of recent origin. Thus, a great deal more research and expansion of current topics is likely in the near future. Adding a measure or measures of general psychopathology should, as a result, accompany measure(s) of core symptoms of ASD.

3.3. Challenging behaviors

Challenging behaviors have received far less attention than the two topics just reviewed. This finding is disappointing given that challenging behaviors are frequent and debilitating among persons with ASD. Only 31 studies were found on this

Table 3

Challenging behaviors.

Condition Numbers of studies Multiple comorbidities

Total One comorbidity

General discussion of challenging behaviors 12 4 8

Stereotypies 6 4 2

Offenders 4 4 0

Aggression 3 3 0

Self-injurious behavior 3 3 0

Disruptive behavior 2 2 0

Selective eating 1 1 0

Table 4

Intellectual disabilities.

Condition Numbers of studies Multiple comorbidities

Total One comorbidity

Intellectual disabilities 65 51 14

Brain injury 2 2 0

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topic. From these papers, 21 reported on comorbid challenging behaviors while 10 papers report on two or more challenging behaviors. Topics covered include a general discussion of challenging behaviors, stereotypies, offenders, aggression, self- injurious behaviors, disruptive behavior, and selective eating. A general discussion of challenging behaviors was twice as frequently studied as the next most frequently studied topic, stereotypies. In the field of intellectual disabilities the topic is and has been front and center for decades. Researchers in the ASD field also need to make this topic a priority (Table 3).

3.4. Intellectual disabilities

Intellectual disabilities are the most common co-occurring problems with ASD. Estimated of up to 70% of ASD cases have been reported as also having intellectual disabilities (Isaksen, Diseth, Schsølberg, & Shjeldal, 2013; Mandell et al., 2012; Matson & Kozlowski, 2011). A total of 65 studies were found dealing with intellectual disabilities comorbid to ASD. Of these, 51 papers addressed one comorbid problem while 14 discussed other comorbid conditions in addition to intellectual disabilities. Brain injury, which may or may not result in intellectual disability, based on severity of the damage and when onset occurred was also studied. Two studies specific to this condition were reported among persons with ASD. In both cases, no additional comorbidities were noted.

Many of these intellectual disabilities studies included an analysis of particular genetic conditions. Among these topics were Down Syndrome, Fragile X Syndrome, tubular sclerosis, Cornelia de Lange, Angelman Syndrome, Klinefelter Syndrome, 22q 11.2 deletion syndrome and Prader-Willi Syndrome. Presumably the attempt to link these known genetic disorders to ASD is an attempt to better understand the cause(s) of ASD. Of course, obtaining an accurate diagnosis of core symptoms of ASD at an early age is also a part of this equation (Table 4).

As can be seen here intellectual disabilities had received a good deal of attention and deservedly so. Both intellectual disabilities and ASD are heterogeneous and tend to be associated with a number of comorbidities. Additionally, these two problems are among the most frequent, serious and debilitating disabilities confronting mental health professionals (Ali, Hassiotis, Strydom, & King, 2012; Call, Findley, & Reavis, 2013; Gardiner & Iarocci, 2012; Lin & Wuang, 2012; Rojahn, Zaja, Turygin, Moore, & van Ingen, 2012). Thus, a considerable amount of research is needed of a descriptive/exploratory nature. Additionally, how best to assess and treat these comorbid problems still requires considerable development. The fact that a good deal of study has already occurred is a promising piece of data with respect to possible future developments. The topic of comorbidities will likely grow exponentially both in the quantity and quality of research published in this new field expands. Additionally, this data will shape new thinking about ASD. The notion that the condition is a free standing disorder of childhood has already been rejected. In its place is the recognition that ASD is life long and multifaceted. Thus, how ASD overlaps with many other disorders has changed the scientific communities thinking regarding etiology, assessment and treatment.

4. Conclusion

Comorbidity with ASD and other conditions is a topic of recent origin. The amount of research on the topic is growing rapidly, but much is yet to be done. For example, what effects do these comorbidities have on the family (Bourke-Taylor, Pallant, Law, & Howie, 2012), what interventions are most effective for various challenging behaviors such as

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stereoptypies (Giles, St Peter, Pence, & Gibson, 2012; Peter-Scheffer et al., 2012) or self-injury (Carr, 1977), and which occurs with ASD? Also, further investigations pertaining to how communication disorders vary by disorder (Esposito & Venuti, 2010; Fodstad, Matson, Hess, & Neal, 2009; Paul, Fuerst, Ramsay, Chawarska, & Klin, 2011), how these comorbidities effect quality of life (Varni, Seid, & Kurtin, 2001), and the impact of having a comorbid condition on the rate of learning, development, and overall adjustment also appear to be warranted. These and many other relevant topics have yet to be adequately addressed.

A number of diagnostic issues are also of interest as well. For example, how are diagnoses being made, and what methods and procedures are being employed to address the growing awareness of comorbid disorders with ASD? Additionally, what gaps need to be filled in the literature and how should they be prioritized?

More also needs to be learned about the course of the disorder and how various comorbid conditions effect this progression. Researchers have found that intellectual disability is a predictor of poorer prognosis among persons with ASD. However, what are the effects of other comorbid conditions, what is the effect of multiple comorbid conditions and how do they interact with one another. These and many other related issues have yet to be adequately addressed in the assessment literature.

Researchers have investigated gender differences in core symptoms of ASD. However, the effects of gender on co- occurring disorders, has yet to be established as a priority. Some have also questioned whether gender gaps in ASD male female ratios may be biased by current thinking about symptom presentation. Comorbidity has a role to play in this debate as well. Researchers know that certain challenging behaviors and types of psychopathology are more prevalent in one gender. Does this hold when ASD also presents, and is symptom expression the same? All of these issues are important in furthering our understanding of the causes of the disorder and how to better assess and treat the condition. Many of these questions will be more fully answered in the years ahead.

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