Introduction

Attention-deficit/hyperactivity disorder (ADHD) is a heterogeneous disorder. DSM-IV (Diagnostic and Statistical Manual of Mental Disorders-IV) recognizes three subtypes of ADHD: a predominantly inattentive subtype, a predominantly hyperactive–impulsive subtype, and a combined subtype (American Psychiatric Association, 1994). These categories acknowledge clinical heterogeneity and reflect a change in emphasis from earlier definitions that stressed motoric symptoms to current nosology in which inattention is emphasized. ADHD is one of the major clinical and public health problems because of its associated morbidity and disability in children, adolescents, and adults. Its impact on society is enormous in terms of financial cost, stress to families, impact on academic and vocational activities, as well as negative effects on self-esteem. Data from cross-sectional, retrospective, and follow-up studies indicate that children with ADHD are at risk for developing other psychiatric difficulties in childhood, adolescence, and adulthood including antisocial behaviors, substance use disorders, and mood and anxiety symptoms and disorders.

The pathophysiology of ADHD

ADHD is a heterogeneous disorder of unknown etiology. An emerging neuropsychologic and neuroimaging literature suggests that abnormalities in frontal networks or fronto-striatal dysfunction is the disorder's underlying neural substrate, and that catecholamine dysregulation is its underlying patho-physiologic substrate. The pattern of neuropsychologic deficits found in ADHD children implicates executive functions and working memory; this pattern is similar to that which has been found among adults with frontal lobe damage, which suggests that the frontal cortex or regions projecting to the frontal cortex are dysfunctional in at least some ADHD children.