ADHD: Mimicry, Comorbidity And Correlates
When assessing children referred for suspected ADHD it is essential to obtain objective information regarding the presence of ADHD symptoms across situations.
It is also important to obtain information regarding the age at which these symptoms developed, how long these symptoms have been present, and to make a determination of the degree of impairment in functioning, across situations, the child experiences as a result of these symptoms. All of these issues must be addressed in attempting to make a formal diagnosis of this extremely common childhood disorder.
While obtaining all of this information is necessary, it is not enough. It is not enough for two reasons.
- First, it is possible for other conditions to mimic symptoms of ADHD such that, while the topography of the child’s behavior may be highly suggestive of ADHD, “symptoms” may arise from causes other than ADHD. Prior to making a diagnosis it is important to rule out these causes so that the focus of treatment is directed toward appropriate targets for intervention.
- Secondly, we now know that children with ADHD often display diagnosable conditions in addition to this primary diagnosis. In such cases it is important to appropriately assess for possible comorbid conditions so that intervention efforts can be directed toward the full range of problems displayed by the child.
The focus here will be on providing a brief overview of some of the factors and conditions that can mimic symptoms of ADHD and discuss, in somewhat more detail, the issue of comorbidity as it relates to assessing problems displayed by children with ADHD
ADHD: THE ISSUE OF MIMICRY
In conducting an assessment of a child suspected of having ADHD it is essential that the clinician to be aware of and be attentive to the fact that problems of activity level, impulsivity and inattention can be caused by a variety of factors other than ADHD. Some of these conditions are of a psychological nature while others result from physical conditions.
Psychological Issues
- Both child anxiety and depressive disorders can result in problems in concentration and symptoms of inattention as well as increased levels of activity, in some instances.
- Children with histories of physical or sexual abuse or who have been repeatedly exposed to parental violence may display post traumatic stress symptoms, that may be reflected in clinically significant problems of inattention. And, the degree of behavioral disorganization that can occur in such cases may result in increased levels of activity, which can be confused with ADHD, in the absence of an adequate assessment.
- With the preschool child, it can sometimes be difficult to distinguish between the noncompliant (and seemingly inattentive) and oppositional and defiant behaviors displayed by the child with early Oppositional Defiant Disorder and the child with ADHD.
- It is also the case that assessing for ADHD in the presence of conditions such as mental retardation and certain pervasive developmental disorders can sometimes pose a challenge due to the problems of inattention and disorganization of behavior that may be seen in each of these conditions.
Appropriate diagnosis in such instances is best accomplished by considering the total clinical picture rather than simply those behaviors commonly seen as core symptoms of ADHD.
Physical Factors
- Sensory deficits such as visual or auditory impairments can result in problems of both inattention and behavioral disorganization, that can be confused with ADHD core symptoms if these impairments are not sufficiently assessed and taken into account.
- Medication side effects from drugs such as Phenobarbital and Dilantin can contribute to problems of inattention and activity level.
- While the findings are less clear, there is clinical evidence that certain asthma medications such as Theophylline can also cause side effects such as inattention and increased activity level.
- Significant problems of inattention, mimicking the symptoms of ADHD Inattentive Type, can also occur in the child with absence seizures.
- Likewise, children with RTH (Resistance to Thyroid Hormone) frequently display a full complement of ADHD symptomatology.
- One might also include developmental and genetic disorders (Fragile X syndrome), and endocrine and autoimmune disorders (PANDAS) among those conditions that may be manifested in such a way as to mimic ADHD symptoms.
Although it is unlikely that any of these conditions perfectly mimic the developmental history or constellation of symptoms of ADHD, children with these disorders are often referred for evaluation of ADHD.
Given that psychological and physical factors, such as the ones cited here, can result in problems of inattention and/or activity level that can mimic the symptoms of ADHD, it is clear that such factors must be ruled out prior to rendering a diagnosis of ADHD.
In attempting to make a differential diagnosis, it is important to evaluate possible causal factors, neuroanatomical and physiological data, and the pattern and severity of symptoms developmentally and across settings.
ADHD: THE ISSUE OF COMORBIDITY
WHAT IS COMORBIDITY?
The term comorbidity first appeared in the psychological and psychiatric literature in the mid 1980's. Since that time there has been a dramatic increase in interest in this topic as reflected in the number of journal articles containing the term comorbidity in the title. In 1986 there were only two such articles; by 1993 the number had increased to 243. Since that time, work in this area has continued unabated. Indeed, it has been suggested that, comorbidity has emerged as perhaps the single most important concept for psychiatric research and practice and that its potential implications for theory and treatment are just beginning to be realized.
A variety of definitions of comorbidity have been offered.
- From a medical epidemiology perspective, Feinstein has defined comorbidity as “any distinct additional entity that has existed or that may occur during the clinical course of a patient who has the index disease under study” (p. 467).
- Blashfield has referred to comorbidity as “the co-occurrence of different diseases in the same individual (p.61)".
- Caron and Rutter have defined comorbidity as “the simultaneous occurrence of two or more unrelated [italics added] conditions (p. 1063)”.
As can be seen from these definitions, the term comorbidity refers to a situation in which an individual, who has been diagnosed with one specific disorder, is also found to meet diagnostic criteria for one or more additional disorders.
Although the use of the concept of comorbidity seems relatively straightforward, there has been considerable controversy regarding the use of this term with reference to most psychiatric disorders.
The primary reason for this concern is highlighted in the definition of comorbidity provided by Caron and Rutter, presented above. Here, comorbidity was defined in terms of the co-occurrence of two or more unrelated conditions. The word unrelated is most relevant. It has been argued that, unlike in the medical arena, where the etiology and pathological processes of specific disease entities are often reasonably well understood (and where the existence of specific disease processes can be assumed) there is seldom a detailed understanding of underlying causal factors in the case of psychiatric disorders.
Without knowledge regarding the etiology of coexisting disorders, one cannot be certain that individuals who meet diagnostic criteria for more than one psychiatric disorder actually have unrelated conditions.
It has been suggested that, what appear to be separate disorders may actually be the result of overlapping diagnostic criteria or the result of arbitrary diagnostic distinctions between different syndromes which may be variations on the same underlying disorder.
Increased estimates of comorbidity may also result from the fact that, especially with younger children, what looks like comorbidity may reflect relatively nonspecific expressions of psychopathology, associated with lowered levels of cognitive development, as opposed to more clearly articulated examples of psychopathology that are more likely to be seen in older children and adolescents.
As a result of these issues it has been suggested that, while the term comorbidity may be appropriate for use in referring to the presence of multiple medical “disease” entities, it’s use is less appropriate in the psychological/psychiatric arena where we are dealing with putative “syndromes”, defined largely in terms of signs and symptoms, without detailed knowledge of underlying etiological factors.
Those who have criticized the use of the term comorbidity in referring to examples of child and adult psychopathology have advocated substitute terms such as “diagnostic co-occurrence” or “diagnostic covariation” as these terms do not imply an association among disease entities, and refer to overlap at the descriptive, rather than pathological/etiological level.
While acknowledging that use of the term comorbidity with reference to psychopathology must be done cautiously for the reasons just outlined, the term comorbidity will be used here for purposes of the present discussion. This term, rather than suggested alternatives, is preferred for the following reasons;
- The term comorbidity has come to be widely used in the psychopathology literature to refer to instances where individuals with one disorder are also found to meet criteria for another disorder.
- There are comorbidity studies of child psychopathology, specifically studies of ADHD, which provide data suggesting that (despite some exceptions) evidence of comorbidity is often found even when one controls for overlapping diagnostic criteria.
- There are recent research findings which provide support for the view that there are distinct subgroups of children, defined in terms of the presence of ADHD and different comorbid features, which suggest specific family-genetic risk factors, varying clinical course and, in some cases, differential responses to pharmacological treatment.
COMORBID CONDITIONS ASSOCIATED WITH ADHD
A number of studies have provided information regarding the type and degree of comorbidity found with children and adolescents diagnosed with ADHD. These findings are considered in the sections to follow.
LEARNING DISABILITIES
Children with ADHD often show problems functioning in the academic environment. These problems include behaviors which impede learning and cause disruptions in the classroom, poor academic performance, lowered levels of school achievement, and specific learning disabilities.
With regard to more general school-related difficulties, research findings clearly suggest that children with attention deficit hyperactivity disorder perform more poorly in school relative to control subjects. They typically show evidence of more grade repetitions and more frequent placement in special classes.
Follow-up studies of children with ADHD have also found that the academic and learning problems of such children often persist into adolescence and are associated with chronic underachievement and school failure.
While academic and school related difficulties seem ubiquitous in children diagnosed with ADHD, specific learning disabilities are somewhat less frequent, although still commonly found.
In reviewing comorbidity findings prior to 1991, Biederman, et al found that the degree of overlap between ADHD and learning disabilities defined in various ways ranged from a low of 10 % to a high of 92%.
Barkley , reviewing this and the more recent literature, has suggested that the best estimate of comorbidity is likely to be in the range of 19 to 26 per cent when learning disability is “conservatively” defined (i.e., significant delay in reading, math, or spelling relative to IQ, with achievement in one of these areas at or below the 7th percentile).
He notes that, if learning disability is defined simply in terms of a significant discrepancy between IQ and achievement, comorbidity estimates are as high as 53%. When a more lax criterion, such as having achievement levels at least two grades below current grade placement are used, then comorbidity estimates as high as 80% are found.
Apart from more general problems of school performance, school achievement and learning disabilities, children with ADHD also frequently show other types of developmentally related difficulties that can impair their functioning in the school environment. Most prominent in this regard are speech and language disorders which have been found to occur in as many as 30 to 64 % of children diagnosed with ADHD.
OPPOSITIONAL DEFIANT AND CONDUCT DISORDER
The finding of high levels of comorbidity with oppositional defiant disorder and with conduct disorder is very common, although relatively few studies have focused specifically on oppositional defiant disorder.
In reviewing research findings up to 1990, Biederman, et al found that the few studies which had looked at oppositional defiant disorder consistently reported comorbidity rates around 35 %.
Studies focusing on conduct disorder suggested comorbidity estimates in the 30 to 50 per cent range. More recent reviews of this literature have yielded somewhat similar findings. Barkley, for example, has suggested that, by the age of seven, 35 to 60 % of clinic-referred children with ADHD meet diagnostic criteria for ODD and that, at some point, 30 to 50% of children/adolescents with ADHD will meet criteria for CD.
Findings in this area suggest that, not only is comorbidity common, but that when oppositional defiant or conduct disorder occur in conjunction with ADHD the clinical picture is one of increased severity compared to that found in children and adolescents with ADHD alone.
Children with combined ADHD and ODD seem to represent an intermediate group with regard to severity of symptoms when compared to ADHD only children, who show less severe problems, and children with ADHD and CD, who show more severe problems.
Commenting on cases where ADHD exists with a comorbid conduct disorder, Biederman et al have noted that “...there is increasing evidence that children with attention deficit hyperactivity disorder plus conduct disorder appear to have a particularly severe form of attention deficit hyperactivity disorder.” These investigators go on to indicate that “...subgrouping based on comorbidity with conduct disorder may be of potential value in determining which children with attention deficit hyperactivity disorder have a more serious prognosis and different family-genetic risk factors and [who may] require specialized comprehensive therapeutic interventions[1] (p.568).”
ANXIETY DISORDERS
ADHD has not only been found to be related to disruptive behavior disorders such as oppositional defiant and conduct disorder but also to internalizing problems such as anxiety disorders and depression.
Biederman, et al, reviewing studies prior to 1991, on the association between ADHD and anxiety disorders found a comorbidity rate of approximately 25 per-cent. More recently, comorbidity rates between 25 and 40% have been found in clinically referred samples, and rates between 23 and 58.8% have been found in general population studies.
These findings, which suggest relatively high rates of comorbidity, must be tempered by the fact that they relate primarily to younger children; this link between ADHD and anxiety disorders has been found to be markedly reduced in adolescents.
In further qualifying the nature of the relationship between ADHD and comorbid anxiety disorders, it should be noted that there is other research which suggests that there are significant differences in comorbidity estimates between children displaying attention deficit disorder with and without hyperactivity. Here, children with DSM III diagnosed attention deficit disorder without hyperactivity (analogous to DSM IV - predominately inattentive type) have been found to show the highest degree of comorbidity with regard to anxiety disorders.
Regarding the effects of anxiety disorder on the clinical picture of children with ADHD, it has been suggested that comorbid anxiety reduces the impulsiveness often associated with ADHD.
MOOD DISORDERS
While not all investigators have found an association between ADHD and mood disorders, early epidemiologic and clinical studies have typically suggested comorbidity estimates between 15 and 75%. Reviewing more recent findings, Barkley has suggested that the overall comorbidity rate for some type of mood disorder is most likely in the 40 to 50% range.
The presence of major depressive disorder in combination with ADHD seems to complicate the usual clinical picture seen in children with ADHD. Follow up studies of children with ADHD and major depressive disorder have suggested that, while both disorders are independently associated with significant psychiatric morbidity, the combination of the two disorders appears to suggest a subgroup of children who show an especially poor long term outcome.
While research findings typically support the notion that children with ADHD often display comorbid depressive disorders, findings concerning comorbidity with bipolar disorders is somewhat more controversial, despite the fact that a small number of studies suggest that the degree of overlap between ADHD and bipolar disorder is in the range of 11 to 22 per-cent.
Despite these findings, it has been suggested that this degree of apparent comorbidity may be, in part, an artifact due to the fact that similar symptoms (e.g., attentional problems, poor judgment, high activity level) are taken as diagnostic indicators of both disorders.
It also seems to be the case that the relationship between ADHD and bipolar disorder is largely unidirectional. That is, the presence of bipolar disorder seems to suggest an increased risk for ADHD, while the presence of ADHD does not seem to suggest an increased risk of developing bipolar disorder.
Additional research is needed to further investigate the precise relationship between ADHD and bipolar disorder. This seems to be especially important since there is at least one study which has suggested that adolescents who commit suicide show higher rates of both bipolarity and ADHD than do adolescents who attempt suicide without success.
TOURETTE’S DISORDER
There is data to suggest that Tourette’s Disorder may also be a comorbid feature of ADHD. Clinical studies suggest that, of those individuals with Tourette’s Disorder, somewhere between 40 and 50 % show features of ADHD, with symptoms of ADHD typically preceding the onset of Tourette’s symptoms.
While such findings are of interest in suggesting overlap between the two disorders, it is possible that the magnitude of overlap is in part related to referral practices. That is, children displaying both disorders may be more likely to be referred for assessment and/or treatment than children who show either of the disorders alone. It is noteworthy that the only published population-based study suggests a much lower rate of ADHD diagnoses (12%) in children with Tourette’s disorder.