Nearly 70% of children with attention deficit hyperactivity disorder (ADHD) have at least one other comorbid psychiatric condition. These comorbid conditions can affect treatment decisions and prognosis and pose special challenges to the psychopharmacologist treating these patients. In addition, some of these disorders have symptoms that overlap those of ADHD, which may make diagnosis difficult.

Disruptive behavior disorders

Oppositional defiant disorder (ODD), conduct disorder (CD), and ADHD are part of the spectrum of disorders that DSM-IV designates as disruptive behavior disorders. Up to 50% of youngsters with ADHD also meet criteria for ODD, and between 30% and 50% meet criteria for CD.

Children with ODD have a pattern of negativistic, defiant, disobedient, and hostile behavior toward authority figures.7 They often actively resist and refuse to comply with requests by parents and teachers. Children with ADHD, in contrast, may not comply because of impulsive or distractible symptoms.

Youngsters with CD often violate societal norms or the basic rights of others, displaying such behaviors as:

• aggressiveness
• destruction of property
• deceitfulness or theft
• serious violation of rules (e.g., running away, school truancy)

Although CD and ADHD are independent disorders, in clinic settings one rarely sees a youngster with CD who does not have a history of ADHD. Hinshaw et al.,32 in a review of the literature, concluded that youngsters with combined CD and ADHD have more severe and persistent difficulties. In a study of 84 children recruited for a study of CD, Klein et al.33 found that 69% also met criteria for ADHD.

Several studies have found that the risk of substance abuse disorders in patients with ADHD is increased only if comorbid CD is present.34,35

Learning disorders

Learning disorders or learning disabilities (LDs) are defined by achievement that is substantially below a youngster’s intelligence quotient (IQ). A substantial difference has been defined in DSM-IV as two standard deviations between IQ and achievement. Prevalence rates are affected by the degree of discrepancy used to define the disorder. Conservative estimates suggest that between 20% and 30% of children with ADHD have a comorbid LD. One study examining the overlap between ADHD and LDs found that 50% of children with ADHD also had an LD, with the overwhelming majority having a specific reading disability.36

Anxiety disorders

Some anxiety symptoms such as restlessness, difficulties with concentration, and irritability can also occur in ADHD, so any youngster presenting with ADHD symptoms should be evaluated for anxiety disorders. The comorbidity between ADHD and the anxiety disorders combined is approximately 25%.

The childhood anxiety disorders include several subtypes. Children with separation anxiety disorder have excessive concerns about separating from a close attachment figure from home. They may have morbid concerns about something bad happening to a parent or about being kidnapped. These symptoms can cause the youngster to avoid leaving the home, to refuse to go to school, and to experience nightmares and problems getting to sleep.

Youngsters with generalized anxiety disorder have excessive worries about a wide range of events, including school and competence.

Social phobia may cause a child to avoid any situation in which he or she feels that others may judge him or her.

Obsessive-compulsive disorder may result in a wide variety of ritualistic behaviors or uncomfortable, involuntary thoughts that impair the child’s ability to function.

Mood disorders

Major depressive disorder

Major depressive disorder is characterized by a persistently depressed mood or loss of pleasure and interest. In children, the mood can often be irritable. Other symptoms of depression include appetite or sleep disturbances, psychomotor retardation or agitation, decreased concentration, fatigue, feelings of worthlessness, and suicidal thoughts.

Bipolar disorder

Adult patients in the manic phase of a bipolar disorder often exhibit a euphoric, expansive, or irritable mood. In its severest form, grandiose or paranoid delusions may be present.

In many children, the most prominent symptom may be an explosive, irritable mood. Associated symptoms include:

• grandiosity
• decreased need for sleep
• increased talkativeness
• racing thoughts
• distractibility
• psychomotor agitation
• excessive pleasure-seeking activities

The relationship and comorbidity between ADHD and bipolar disorder in children currently is one of the most debated diagnostic areas in child and adolescent psychiatry.37,38 One controversial issue is symptom overlap, which may confuse the diagnosis. In addition, it has been suggested that the manic disorder in children often does not have the cyclical quality that is seen in classic bipolar disorder and that children are more irritable than euphoric. Biederman et al.39 found that in a sample of 6- to 17-year-old children with ADHD, 11% also met criteria for the diagnosis of bipolar disorder, a finding that is higher than in most other samples.

Gilles de la Tourette’s syndrome

The prevalence of tics in ADHD is unclear. Some studies fail to distinguish between youngsters with and without preexisting tics. Other studies differ on how tics are rated, and some evidence exists of a genetic link between Gilles de la Tourette’s syndrome (Tourette syndrome) and ADHD in a subset of youngsters.40,41 As many as two-thirds of children with this syndrome also have symptoms of ADHD and obsessive-compulsive disorder.42

Pervasive developmental disorders

Children with pervasive developmental disorders (e.g., autism, Asperger’s syndrome) frequently exhibit a wide variety of difficulties, including attentional problems and impulsivity.43 The concern has been raised that the diagnosis of pervasive developmental disorder may be missed and ADHD may be misdiagnosed in patients with severe behavioral problems.45

In a study evaluating the pattern of medication use in a population of higher-functioning patients with pervasive developmental disorders, Martin et al.44 found that 20% were taking stimulants.

Adapted from: Hirsch GS and Koplewicz HS. Attention-deficit hyperactivity disorder. In: Devinsky O and Westbrook LE, eds. Epilepsy and Developmental Disabilities. Boston: Butterworth-Heinemann; 2001;187–204.
With permission from Elsevier (www.elsevier.com).
Reviewed and revised June 2004 by Steven C. Schachter, MD, epilepsy.com Editorial Board.

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