Medical Health Encyclopedia

Nerve Pathways. Another area of interest is a network of nerves called the basal-ganglia thalamocortical pathways. Abnormalities along this neural route have been associated with ADHD, Tourette's syndrome, and obsessive-compulsive disorders, which all share certain symptoms.

Genetic Factors

Genetic factors may play the most important role in ADHD. The relatives of ADHD children (both boys and girls) have much higher rates of ADHD, antisocial, mood, anxiety, and substance abuse disorders than the families of non-ADHD children. A study reported that 90% of children with a diagnosis of ADHD shared it with their twin.

Genetic Factors Regulating Dopamine and Advantages in Early Man. Most of the research on the underlying genetic mechanisms targets the neurotransmitter dopamine. Variations in genes that regulate specific dopamine receptors have been identified in a high proportion of people with addictions and ADHD. Such genes have been associated with novelty seeking and extroversion. Some experts theorize that the genetic variants may have first appeared thousands of years ago, and affect as many as half of ADHD children. Furthermore, the genetic variations may have offered some benefits to their early carriers. In such people, a genetic predilection for novelty-seeking and risk-taking may have supplied an advantage in reproduction, mating, hunting, and achieving dominance.

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Genetic Resistance to Thyroid Hormone. About 50% of adults and 70% of children with a genetic resistance to thyroid hormone, essential for normal brain development, have ADHD. People who have this condition appear to have a more severe form of ADHD. The thyroid disorder is not a common cause of ADHD. Only those with a family history of thyroid disease are at risk.

Pregnancy and ADHD

ADHD is often associated with problem pregnancies and difficult deliveries. If a women smokes during pregnancy, a genetically susceptible child is at higher risk for ADHD. One study indicated that an increased risk also existed in children of women who were exposed during pregnancy to environmental toxins, including dioxins and polychlorinated biphenyls (PCBs).