Attention Deficit Hyperactivi... Health Article

Demographics

ADHD is a common childhood disorder. It is estimated to affect 3–7% of all children in the United States, representing up to two million children. The percentage may in fact be even higher, with up to 15% of boys in grades one through five being afflicted. On average, at least one child in each public and private classroom in the United States has ADHD. In countries such as Canada, New Zealand, and Germany, the prevalence rates are estimated to be 5–10% of the population.

The traditional view of ADHD is that boys are affected more often than girls. Community-based samples have found an incidence rate in boys that is double that of girls. In fact, statistics gathered from patient populations have reported male-to-female ratios of up to 4:1. However, as the understanding of ADHD has grown since the early 1990s and as the symptoms have been better recognized, the actual number of females who are affected by ADHD may be more similar to males than previously thought.

Causes and symptoms

The cause of ADHD is unknown. However, evidence is consistent with a biological cause rather than an environmental cause (e.g., home life). Not all children from dysfunctional homes or families have ADHD.

For many years, it was thought that ADHD developed following a physical blow to the head, or from an early childhood infection, leading to the terms "minimum brain damage" and "minimum brain dysfunction." However, these definitions apply to only a very small number of people diagnosed with ADHD, and so have been rejected as the main cause.

Another once-favored theory was that eating refined sugar or chemical additives in food produced hyperactivity and inattention. While sugar can produce changes in behavior, evidence does not support this proposed association. Indeed, in 1982, the results presented at a conference sponsored by the U.S. National Institutes of Health conclusively demonstrated that a sugar- and additive-restricted diet only benefits about 5% of children with ADHD, mostly young children and those with food allergies.

The biological roots of ADHD may involve certain areas of the brain, specifically the frontal cortex and nearby regions. One explanation is that the executive functions are controlled by the frontal lobes of the brain. Magnetic resonance imaging (MRI) examination of subjects who are exposed to a sensory cue has identified decreased activity of regions of the brain that are involved in tasks that require attention. Another MRI-based study published in November 2003 also implicates a region of the brain that controls impulsive behavior. Finally, a study conducted by the U.S. National Institute of Mental Health (NIMH) documented that the brains of children and adolescents with ADHD are 3–4% smaller than those of their ADHD-free counterparts. Additionally, the decreased brain size is not due to the use of drugs in ADHD treatment, the researchers concluded in a paper published in October 2002.

ADHD symptoms can sometimes be relieved by the use of stimulants that increase a chemical called dopamine. This chemical functions in the transmission of impulses from one neuron to another. Too little dopamine can produce decreased motivation and alertness. These observations led to the popular "dopamine hypothesis" for ADHD, which proposed that ADHD results from the inadequate supply of dopamine in the central nervous system.

The observations that ADHD runs in families (10–35% of children with ADHD have a direct relative with the disorder) point to an underlying genetic origin. Studies with twins have shown that the occurrence of ADHD in one twin is more likely to be mirrored in an identical twin (who has the same genetic make-up) than in a fraternal twin (whose genetic make-up is similar but not identical).

The genetic studies have implicated the binding, transport, and enzymatic conversion of dopamine. Two genes in particular have been implicated: a dopamine receptor (DRD) gene on chromosome 11 and the dopamine transporter gene (DAT1) on chromosome 5.

There may be environmental factors that influence the development of ADHD. Complications during pregnancy and birth, excessive use of marijuana, cocaine, and/or alcohol (especially by pregnant women), ingestion of lead-based paint, family or marital tension, and poverty have been associated with ADHD in some people. However, many other ADHD sufferers do not display any of these associations.

Heavy use of alcohol by a pregnant woman can lead to malformation of developing nerve cells in the fetus, which can result in a baby of lower than normal birth weight with impaired intelligence. This condition, called fetal alcohol syndrome, can also be evident as ADHD-like hyperactivity, inattention, and impulsive behavior.