ADHD: What Causes ADHD?

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Understandably, one of the first questions parents ask when they learn their child has an attention disorder is “Why? What went wrong?”

Health professionals stress that since no one knows what causes ADHD, it doesn’t help parents to look backward to search for possible reasons. There are too many possibilities to pin down the cause with certainty. It is far more important for the family to move forward in finding ways to get the right help.

Scientists, however, do need to study causes in an effort to identify better ways to treat, and perhaps some day, prevent ADHD. They are finding more and more evidence that ADHD does not stem from home environment, but from biological causes. When you think about it, there is no clear relationship between home life and ADHD. Not all children from unstable or dysfunctional homes have ADHD. And not all children with ADHD come from dysfunctional families. Knowing this can remove a huge burden of guilt from parents who might blame themselves for their child’s behavior.

Over the last decades, scientists have come up with possible theories about what causes ADHD. Some of these theories have led to dead ends, some to exciting new avenues of investigation.

One disappointing theory was that all attention disorders and learning disabilities were caused by minor head injuries or undetectable damage to the brain, perhaps from early infection or complications at birth. Based on this theory, for many years both disorders were called “minimal brain damage” or “minimal brain dysfunction.” Although certain types of head injury can explain some cases of attention disorder, the theory was rejected because it could explain only a very small number of cases. Not everyone with ADHD or LD has a history of head trauma or birth complications.

Another theory was that refined sugar and food additives make children hyperactive and inattentive. As a result, parents were encouraged to stop serving children foods containing artificial flavorings, preservatives, and sugars. However, this theory, too, came under question. In 1982, the National Institutes of Health (NIH), the Federal agency responsible for biomedical research, held a major scientific conference to discuss the issue. After studying the data, the scientists concluded that the restricted diet only seemed to help about 5 percent of children with ADHD, mostly either young children or children with food allergies.

ADHD Is Not Usually Caused by:

  • too much TV
  • food allergies
  • excess sugar
  • poor home life
  • poor schools

In recent years, as new tools and techniques for studying the brain have been developed, scientists have been able to test more theories about what causes ADHD.

Using one such technique, NIMH scientists demonstrated a link between a person’s ability to pay continued attention and the level of activity in the brain. Adult subjects were asked to learn a list of words. As they did, scientists used a PET (positron emission tomography) scanner to

observe the brain at work. The researchers measured the level of glucose used by the areas of the brain that inhibit impulses and control attention. Glucose is the brain’s main source of energy, so measuring how much is used is a good indicator of the brain’s activity level. The investigators found important differences between people who have ADHD and those who don’t. In people with ADHD, the brain areas that control attention used less glucose, indicating that they were less active. It appears from this research that a lower level of activity in some parts of the brain may cause inattention.

The next step will be to research WHY there is less activity in these areas of the brain. Scientists at NIMH hope to compare the use of glucose and the activity level in mild and severe cases of ADHD. They will also try to discover why some medications used to treat ADHD work better than others, and if the more effective medications increase activity in certain parts of the brain.

Researchers are also searching for other differences between those who have and do not have ADHD. Research on how the brain normally develops in the fetus offers some clues about what may disrupt the process. Throughout pregnancy and continuing into the first year of life, the brain is constantly developing. It begins its growth from a few all-purpose cells and evolves into a complex organ made of billions of specialized, interconnected nerve cells. By studying brain development in animals and humans, scientists are gaining a better understanding of how the brain works when the nerve cells are connected correctly and incorrectly. Scientists at NIMH and other research institutions are tracking clues to determine what might prevent nerve cells from forming the proper connections. Some of the factors they are studying include drug use during pregnancy, toxins, and genetics.

Research shows that a mother’s use of cigarettes, alcohol, or other drugs during pregnancy may have damaging effects on the unborn child. These substances may be dangerous to the fetus’s developing brain. It appears that alcohol and the nicotine in cigarettes may distort developing nerve cells. For example, heavy alcohol use during pregnancy has been linked to fetal alcohol syndrome (FAS), a condition that can lead to low birth weight, intellectual impairment, and certain physical defects. Many children born with FAS show much the same hyperactivity, inattention, and impulsivity as children with ADHD.

Drugs such as cocaine–including the smokable form known as crack–seem to affect the normal development of brain receptors. These brain cell parts help to transmit incoming signals from our skin, eyes, and ears, and help control our responses to the environment. Current research suggests that drug abuse may harm these receptors. Some scientists believe that such damage may lead to ADHD.

Toxins in the environment may also disrupt brain development or brain processes, which may lead to ADHD. Lead is one such possible toxin. It is found in dust, soil, and flaking paint in areas where leaded gasoline and paint were once used. It is also present in some water pipes. Some animal studies suggest that children exposed to lead may develop symptoms associated with ADHD, but only a few cases have actually been found.

Other research shows that attention disorders tend to run in families, so there are likely to be genetic influences. Children who have ADHD usually have at least one close relative who also has ADHD. And at least one-third of all fathers who had ADHD in their youth bear children who have ADHD. Even more convincing: the majority of identical twins share the trait. At the National Institutes of Health, researchers are also on the trail of a gene that may be involved in transmitting ADHD in a small number of families with a genetic thyroid disorder.

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