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The pathogenesis (cause) of ADHD has not been totally defined. One theory springs from observations regarding variations in functional brain-imaging studies between those with and without symptoms. Similar variations have been shown in studies of the structure of the brain of affected and unaffected individuals. Animal studies have demonstrated differences in the chemistry of brain transmitters involved with judgment, impulse control, alertness, planning, and mental flexibility.
Research has shown that ADHD does seem to cluster in families. A genetic predisposition has been demonstrated in (identical) twin and sibling studies. If one identical twin is diagnosed with ADHD, there is a 92% probability of the same diagnosis in the twin sibling. When comparing nonidentical twin sibling subjects, the probability falls to 33%. Overall population incidence is felt to be 8%-10%.
Several investigations have demonstrated that children who have ADHD usually have a least one close relative (child or adult) who also has ADHD. At least one-third of all fathers who have ADHD will produce a child with ADHD. With the newer realization that adults may also experience ADHD symptoms, it is not unusual to have a parent's "problem at my job" be credited to ADHD -- often at the same time that their child's diagnosis is being established! Lastly, several studies have demonstrated a number of genes that may reflect a role in altered brain neurochemistry that provides a physiologic basis for this disorder and inheritance pattern.
Genes that control the relative levels of chemicals in the brain called neurotransmitters seem to be different in ADHD, and levels of these neurotransmitters are out of normal balance.
- MRI and other imaging studies suggest that these imbalances occur in parts of the brain that control certain types of movement and executive functions.
- These areas of the brain may be smaller and/or less active in people with ADHD.
- Shifting from one mind-set or strategy to another (that is, flexibility)
- Organization (for example, anticipating both needs and problems)
- Planning (for example, goal setting)
- Working memory (that is, receiving, storing, then retrieving information within short-term memory)
- Separating emotions from reason
- Regulating speech and movements appropriately
For more information, see our slideshow on attention deficit hyperactivity disorder symptoms and facts.
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Biederman, J., et al. "Do Stimulants Protect Against Psychiatric Disorders in Youth With ADHD? A 10-Year Follow-up Study." Pediatrics 124.1 July 2009: 71-78.