The exact cause of ADHD is unknown. Brain imaging studies suggest that there is smaller total cerebral volume as well as reduced global and local activation of the basal ganglia and the anterior frontal lobe in patients with ADHD.11,17,28 These areas of the brain are involved with executive functions including impulse control, organization and planning, sustained goal directed activities and socially responsive behavior.17
Many possible causes of ADHD have been investigated including biological and social conditions, injuries (infection, prolonged hypoxia, toxic agents) to the central nervous system and genetics.2 Multiple studies indicate genetics may account for approximately 80% of ADHD cases.15,23 For example, children of ADHD adults have a higher risk for ADHD; ADHD is more prevalent in relatives of ADHD children than in the relatives of non-ADHD children and in identical twins, when one twin has the disorder the other twin is highly like to be affected as well.15 Studies support a multiple or interactive gene concept with several genes making small contributions to the total genetic effect.2,15,29 The suspect genes include the dopamine receptor genes DRD4 and DRD5, the dopamine transporter gene DAT, and the b-receptor gene for the thyroid growth hormone GRTH.7,15,29,33 Recent reports are beginning to link genotypes (the internally coded, inheritable genetic information) to specific phenotypes (the outward physical manifestation of the genetic code) of ADHD.29,33 For example, the DRD4 gene is predominantly linked with the combination diagnosis of ADHD and oppositional defiant disorder while the DRD5 gene is predominantly associated with the inattentive and combined subtypes of ADHD.29 Other factors thought to contribute to ADHD include prenatal exposure to smoke, lead or alcohol; prematurity; and intrauterine growth retardation.7,15 Environmental factors such as excessive TV viewing, low parental education and prenatal maternal anxiety have also been investigated with confusing results.29 Comorbid conditions including behavioral conditions (anxiety, depression, oppositional defiant, and conduct disorder and obsessive-compulsive disorder), developmental issues (learning and language disorders, dyslexia or other neurodevelopmental disorders) and physical conditions (tics, Tourette's Syndrome, and sleep apnea) have been reported.1,2
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