Attention deficit hyperactivity disorder (ADHD): from a childhood neuropsychiatric disorder to an adult condition.
Behavioural genetic studies indicate that ADHD is significantly familial, and that this familiality is primarily due to genetic influences, and non-shared environmental factors specific to the individual account for significant vari-ance in ADHD.
Abstract
Attention deficit hyperactivity disorder (ADHD) is one of the most common childhood neuropsychiatric disorders and it often persists into adulthood; it is a neurobiological syndrome with an estimated prevalence among children and adolescents of 5%. The psychopathology of this disorder is marked by developmentally inappropriate and pervasive expressions of inattention, overactivity and impulsiveness. ADHD is also associated with functional impairments across multiple academic and social domains and is commonly accompanied by a range of externalising and internal-ising disorders. The most recent definition of ADHD in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders ( DSM-IV ) distinguishes between diagnostic subtypes characterised by maladaptive levels of both inattention and hyperactivity-impulsivity (combined type), maladaptive levels of inattention only (predominantly inat-tentive type), and maladaptive levels of hyperactivity-impulsivity alone (predominantly hyperactive-impulsive type). Behavioural genetic studies indicate that ADHD is significantly familial, and that this familiality is primarily due to genetic influences. In addition, non-shared environmental factors specific to the individual account for significant vari-ance in ADHD. Numerous studies have attempted to identify the specific environmental or genetic factors that increase the likelihood of a child developing ADHD; these studies clearly suggest that multiple genes are involved in the aetiology of ADHD, and that few, if any, of these genes are necessary or sufficient to cause ADHD. The aetiology of ADHD is multifactorial. A genetic cause linked to dopamine deficit is frequent and primary, but various environmental factors, including viral infection, maternal smoking during pregnancy, prematurity, cerebral hypoxic ischaemia, alcohol exposure, and nutritional and endocrine disorders may contribute as secondary the disorder persists into adulthood, while an additional 50% have a partial diagnosis in adulthood with persistence of some symptoms leading to continued impairments. Furthermore, most of these ‘partial ex-tinction processes define an endophenotype in ADHD that can be related dimensionally to inattention, hyperactivity, and impulsivity.