What Causes ADHD?
Understandably, one of the first questions parents ask when they
learn their child has an attention disorder is "Why? What went
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--iincluding the smokable form known as
crack--sseem 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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