Clinical phenomenology and epidemiology

Attention-deficit hyperactivity disorder (ADHD) is characterized by persistent inattention and/or situationally excessive motor activity, and impulsive behavior (Barkley, 1990). According to the current Diagnostic and Statistical Manual of Mental Disorder (4th edition (DSM-IV); American Psychiatric Association, 1994), individuals must exhibit symptoms for at least 6 months and must express the symptoms by 7 years of age. The symptoms must be developmentally inappropriate and exhibited in at least two settings. DSM-IV specifies three subtypes: predominantly inattentive (ADHD-I), predominantly hyperactive–impulsive (ADHD-HI), and combined type (ADHD-C). The number and nature of items endorsed within lists of inattentive and hyperactive/impulsive symptoms determines the specific diagnostic subtype.

ADHD is the most prevalent childhood psychiatric disorder and is estimated to affect 3–11% of the school-age population, depending on the source of the sample (American Psychiatric Association, 1994; Wolraich et al., 1996). There is a much higher incidence rate in boys, who are 2.5–9 times more likely than girls to be diagnosed with ADHD (Szatmari et al., 1989; Barkley, 1990; Wolraich et al., 1996). The disorder often has a chronic course, with 30–50% of affected children exhibiting ADHD symptoms into adulthood (Barkley et al., 1990; Weiss and Hechtman, 1993). Numerous problems are associated with childhood and adulthood ADHD, including poor academic performance, learning disabilities, conduct disorders, antisocial personality disorder, lower occupational success, poor social relationships, frequent car accidents, and a higher incidence of anxiety and depression (Barkley et al., 1990, 1996; Biederman et al., 1993; Weiss and Hechtman, 1993; Murphy and Barkley, 1996; Mannuzza et al., 1998).