Proper differential diagnosis of ADHD, CD and ODD requires not only discrimination among the three disorders, but also from a wide range of other psychiatric, developmental and medical conditions. ADHD can be conceptualized as a cognitive/devel-opmental disorder, with an earlier age of onset than CD. ADHD children more frequently show deficits on measures of attentional and cognitive function, have increased motor activity and greater neurodevelopmental abnormalities. In contrast, CD children tend to be characterized by higher levels of aggression and greater familial dysfunction.
A significant proportion of children present with symp-toms of both ADHD and CD, and both conditions should be diag-nosed when this occurs. Comorbid ADHD and CD is consistently reported to be more disabling with poorer long-term outcome than either disorder alone. These children show persistantly in-creased levels of aggressive behaviors at an early age. This is in contrast to the more typical episodic course seen in children who have CD alone.
It appears that among children with ADHD, those who are most hyperactive/impulsive are at greatest risk for developing ODD. ODD symptoms, such as “loses temper”, “actively defies”, and “swears”, are less characteristic of children with ADHD. In general, the onset of ODD symptoms peaks by age 8 years and shows a declining course thereafter, while hyperactivity and at-tentional problems appear at a much earlier age and often persist, although the levels of inattentiveness and/or hyperactivity often decrease with age.
A diagnosis of CD supersedes ODD since approximately 90% of children with CD would also meet criteria for ODD, and some question whether they represent a spectrum of severity or distinct diagnoses. Although the majority of ODD children will not develop CD, in some cases ODD appears to represent a developmental precursor of CD. In cases where ODD precedes CD, the onset of CD is typically before age 10 years (childhood onset CD). In children who have the onset of CD after age 10 years, symptoms of ODD and ADHD are usually not present dur-ing early childhood. It has been shown that children with ODD demonstrate lower degrees of impairment and are more socially competent as compared with children with CD. Furthermore, children with CD come from less advantaged families, and have greater conflict with school and judicial systems as compared with children with ODD.
Mood and anxiety disorders, learning disorders, mental retardation, pervasive developmental disorders, organic mental disorders and psychotic disorders may all present with impair-ment of attention, as well as hyperactive/impulsive behaviors. The diagnosis of ADHD in DSM-IV requires that the symptoms of inattention/cognitive disorganization and impulsivity/hyper-activity are not better accounted for by one of the above condi-tions. Differentiating ADHD from bipolar disorder in childhood is complicated by the low base rate of bipolar disorder and the variability in clinical presentation. A positive family history of bipolar disorder is especially helpful in diagnosing bipolar disor-der in children. In addition, a variety of medical conditions such as epilepsy, Tourette’s disorder, thyroid disease, postinfectious and/or post traumatic encephalopathy and sensory impairments can present with symptoms similar to ADHD and must also be considered. Finally, many medications which are prescribed to children can mimic ADHD symptomatology. Examples include anticonvulsants (e.g., phenobarbital), antihistamines, deconges-tants, bronchodilators (e.g., theophylline) and systemic steroids.