Attention-Deficit Hyperactivity Disorder

Description

Alternative Names

Causes

Advanced imaging techniques have detected differences in the brains of ADHD children compared to those of non-ADHD children.

Brain Structures. Increasingly, research using imaging techniques reports differences in volume in certain parts of the brain between ADHD children and those without the syndrome. The areas affected include the following:

• The prefrontal cortex. The prefrontal cortex, which is located in the front of the brain, is thought to be the brain's command center; it regulates the brains ability to inhibit responses. A number of imaging studies has indicated that the prefrontal cortex of the brain in ADHD may be underactive in comparison with those without ADHD.
• The caudate nucleus and the globus pallidus. The caudate nucleus and globus pallidus, located near the center of the brain, speed up or stop orders coming from the prefrontal cortex. A major 2002 study reported that it was smaller than average in young children with ADHD but tended to normalize as children got older. Abnormalities in these areas may impair a person's ability to brake actions, resulting in the impulsivity typical of people with ADHD.
• The cerebellum (the area above the brain stem). This area helps control muscle tone and balance and synchronizes muscle activity. An important 2002 study reported that on average this area is smaller in children with ADHD compared to those without the condition.

It is important to note that such abnormalities are not progressive and are not related to intelligence.

Brain Chemicals. Abnormal activity of certain brain chemicals in the prefrontal cortex may contribute to ADHD. Dopamine and norepinephrine are of special interest. These chemical messengers, called neurotransmitters, affect both mental and emotional functioning. They are also important in the "reward" response, in which a person experiences pleasure in response to certain stimuli (such as food or love). Studies now suggest that increased levels of other brain chemicals (glutamate, glutamine, and GABA), collectively called Glx, interact with the pathways that transport dopamine and norepinephrine.

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. In a twin study, 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.

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 receptor 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.

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, however, and only those with a family history of thyroid disease are at risk.

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 in genetically susceptible children. 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).

Dietary Factors

Infant malnutrition is a strong risk factor of ADHD. Even if children receive enough food later on, infants who suffer from malnutrition may develop behavior problems, the most prevalent being attention-deficit disorder.

A number of dietary factors have been researched in ADHD, including sensitivities to certain food chemicals, deficiencies in fatty acids and zinc, and sensitivity to sugar. No clear evidence has emerged that implicates any of these nutritional factors in ADHD.

Deficiencies in Zinc and Essential Fatty Acids. Some studies have found an association between deficiencies in certain fatty acids (compounds that make up fats and oils) and ADHD. Related to these findings are studies reporting an association between zinc deficiencies and ADHD. (Zinc is important in the metabolism of fatty acids, which in turn affects dopamine, the neurotransmitter likely to be involved with ADHD.)