Advanced imaging techniques have detected differences in the brains of ADHD children compared to those of non-ADHD children. In some studies, brain scans reveal that the right side of the brain is smaller in ADHD children than in non-ADHD children (ordinarily the right and left sides of the brain are the same size). The right side contains three important areas: the prefrontal cortex; the caudate nucleus; and globus pallidus. The prefrontal cortex, which is located in the front of the brain, is thought to be the brain's command center and regulates the ability to inhibit responses. The caudate nucleus and globus pallidus, located near the center of the brain, speed up or stop orders coming from the prefrontal cortex. Abnormalities in these areas may impair a person's ability to brake actions, resulting in the impulsivity typical of ADHD people. Also located here are important neurotransmitters -- chemical messages in the brain -- including norepinephrine, dopamine, and serotonin, which affect mental and emotional functioning. Dopamine is under particular scrutiny. One recent study reported that adults with ADHD had abnormally low levels of DOPA decarboxylase, the enzyme that produces dopamine.
Problems Surrounding Pregnancy
ADHD is often associated with problem pregnancies and with difficult deliveries. Maternal smoking during pregnancy is also associated with a higher risk for ADHD. One study indicated that an increased risk also existed in children of women who were exposed to environmental toxins, including dioxins and polychlorinated biphenyls (PCBs), during pregnancy.
Evidence that genetic factors increase susceptibility is mounting. In a study of twins, 90% of children with a full diagnosis of ADHD shared it with their twin. Most likely, more than one gene is responsible for inherited cases; this is not surprising, since there is no consensus that ADHD is even a single disorder. Researchers are reporting underlying genetic mechanisms that regulate hyperactivity, particularly those that affect the neurotransmitter dopamine. Studies are finding that a variation of a dopamine D4 receptor gene is common in a high proportion of people with addictions and ADHD, and appears to be associated with novelty-seeking and extroversion. About 50% of adults and 70% of children with a genetic resistance to thyroid hormone have ADHD. The thyroid hormone is essential for normal brain development. People who have this condition appear to have a more severe form of ADHD. The thyroid disorder is not a common cause of ADHD, however, and only those with a family history of thyroid disease are at risk.
A number of studies have suggested that sugar plays no role in hyperactivity. In fact, one study reported that ADHD children had fewer problems after a high-carbohydrate breakfast than after a high-protein one. Another reported that children actually moved more slowly after a high-sugar meal, suggesting that the carbohydrates may have a sedative effect. Studies on the effect of food and food-additive allergies are controversial. For example, one reported that 62% of ADHD children had symptoms provoked by various foods and additives. Another study indicated, however, that less than 5% of children with attention-deficit hyperactivity disorder are affected by food additives and even then, the effect is very slight. Among the additives and foods that parents report as culprits in inciting behavioral changes are any artificial flavors or coloring (particularly red), milk, chocolate, eggs, and wheat. Allergies themselves have recently been associated with a higher risk for behavioral problems; children who respond to allergen-restrictive diets may not have had true ADHD in the first place.
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