16. What Causes Attention-Deficit Hyperactivity Disorder?
Physical Factors
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 they 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, chemicals messages
in the brain, including norepinephrine, dopamine, and serotonin,
which affect mental and emotional functioning. Dopamine
is under particular scrutiny. One study, for example, reported
that adults with ADHD had abnormally low levels of DOPA
decarboxylase, the enzyme that produces dopamine. 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.
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 during pregnancy to environmental
toxins, including dioxins and polychlorinated biphenyls
(PCBs).
Genetic
Factors
Evidence is increasing that genetic factors play an important
role in ADHD. 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. 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. Other genes under suspicion are those
that regulate dopamine transportation and one called the
dopamine D2 receptor gene. Not all studies, however, have
confirmed the role of dopamine in ADHD.
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.
Diet
A number of studies have suggested that sugar plays no role
in hyperactivity. One study reported, in fact, 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
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, then, may not have
had true ADHD in the first place.
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© 2003 The Kids Clinic -
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