Comorbidity of Developmental Disorders

Deborah Dewey, Ph.D.

Department of Paediatrics
University of Calgary
Behavioural Research Unit
Alberta Children's Hospital
Calgary, Alberta

Health professionals who study children with developmental problems, and those who educate and treat them, tend to speak of diagnostic categories. Researchers and clinicians have attempted to classify childhood developmental disorders into discrete diagnostic categories such as those found in the Diagnostic and Statistical Manual of Mental Disorders IV (DSM-IV) (American Psychiatric Association, 1994) or the International Classification of Diseases 10 (ICD 10) (World Health Organization, 1992). In many cases, however, children with these disabilities do not display just one discrete disorder but several disorders. For example, children with reading disabilities often have Attention Deficit/Hyperactivity Disorder (ADHD), and children with ADHD frequently meet criteria for some other psychiatric condition. When this occurs, the term comorbidity is used to refer to the fact that few children fit neatly into one single discrete disorder.
"Comorbid" is a term that has been borrowed from medicine. Its original meaning indicated the presence of at least two diseases. An individual with diabetes and asthma, for instance, is said to be comorbid for these two diseases. In contrast, an individual reporting frequent urination and thirstiness is not said to be comorbid for these two conditions, because they are symptoms; their co-occurrence suggests morbidity for a single disease, diabetes. When the term "comorbidity" was transferred to the mental health world, there was one element missing that prevented its accurate application: the precise distinction between symptom and disease (or disorder). For instance, when a child has difficulties in learning, moodiness, behaviour problems, and difficulties in printing/writing, the child could be viewed as displaying a learning disability, attention deficit hyperactivity disorder (ADHD) and/or developmental coordination disorder (DCD). The co-occurrence of these apparently disparate symptoms causes problems in both diagnosis and treatment. In addition, it raises questions about the etiology and mutual interdependence of various developmental disorders.
In this discussion of the issue of comorbidity of developmental disorders, the first question that will be addressed is this: How widespread is comorbidity across various developmental disorders? Research evidence suggests that 50 - 80 % of children with any diagnosis meet criteria for at least one other diagnosis (Biederman, Faraone, Keenan, Knee, & Tsuang, 1990). The co-occurrence of ADHD and dyslexia is well established (Gilger, Pennington, & DeFries, 1992). The rates of overlap are typically estimated from 30-50% (Dykman & Ackerman, 1991; Semrud-Clikeman, et al., 1992). Several studies have also demonstrated that children with ADHD display a high prevalence of language problems (Carte, Nigg, & Hinshaw, 1996; Elbert, 1993). Similarly, a number of studies have reported that many children with learning disabilities also display DCD (Silver, 1992; Sugden & Wann, 1987). The overlap of ADHD with anxiety and/or depression in children, adolescents and adults has also been documented (Biederman, Faraone, Mick, Moore, & Lelon, 1996).
The question remains, however, as to how extensive is the overlap of developmental disabilities. An answer to this question can be found in the results of the Ontario Child Health Study (Offord, et al., 1987). This study reported that 13-22% of their epidemiological sample met the criteria for at least two of the following disorders: conduct disorder, ADHD, emotional disorder, somatization. In our own research carried out in Calgary we have also found very high levels of overlap of developmental disabilities (Dewey, Wilson, Crawford, & Kaplan, 2000; Kaplan, Crawford, Wilson, & Dewey, 2000). For example, 58% of our sample of children with ADHD also displayed reading disabilities and 27% of these children with ADHD had DCD. Further, 82% of our children with DCD displayed some other comorbid disorder. The results of this research suggest then that comorbidity of developmental disorders appears to be the rule, rather than the exception.
Based on the above evidence the question that must be asked is how can we better conceptualize the significant comorbidity among developmental disorders? The enormous overlap found among developmental disorders suggests that they are all reflective of a more general impairment of brain structure or function. This has been referred to as Atypical Brain Development (ABD) (Gilger & Kaplan, in press; Kaplan, Wilson, Dewey, & Crawford, 1997).
What is meant by this concept? Specifically, ABD can be viewed as a unifying concept that may assist researchers and educators to come to terms with the fact that developmental disabilities are typically nonspecific and heterogeneous, and that comorbidity of symptoms in children with developmental disabilities is the rule rather than the exception. ABD may be a function of both genetic and environmental factors, as well as the interaction of the two. It refers to the more generalized dysfunction in structure, activation, etc. underlying these developmental disorders. It is a term that can be used to address the full range of developmental disorders that are found to be overlapping much of the time in any sample of children with developmental disabilities. Further, ABD does not replace the common labels currently used in the field, but it provides a perspective from which we can view and investigate children with developmental problems.


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