ADHD Basics:
What Causes ADHD?
One
of the first questions a parent will have is "Why? What went wrong?"
"Did I do something to cause this?" There is little compelling evidence
at this time that ADHD can arise purely from social factors or
child-rearing methods. Most substantiated causes appear to fall in the
realm of neurobiology and genetics. This is not to say that
environmental factors may not influence the severity of the disorder,
and especially the degree of impairment and suffering the child may
experience, but that such factors do not seem to give rise to the
condition by themselves.
The parents' focus should be on looking
forward and finding the best possible way to help their child.
Scientists are studying causes in an effort to identify better ways to
treat, and perhaps someday, to prevent ADHD. They are finding more and
more evidence that ADHD does not stem from the home environment, but
from biological causes. Knowing this can remove a huge burden of guilt
from parents who might blame themselves for their child's behavior.
Over
the last few 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.
Environmental agents
Studies
have shown a possible correlation between the use of cigarettes and
alcohol during pregnancy and risk for ADHD in the offspring of that
pregnancy. As a precaution, it is best during pregnancy to refrain from
both cigarette and alcohol use.
Another environmental agent that
may be associated with a higher risk of ADHD is high levels of lead in
the bodies of young preschool children. Since lead is no longer allowed
in paint and is usually found only in older buildings, exposure to
toxic levels is not as prevalent as it once was. Children who live in
old buildings in which lead still exists in the plumbing or in lead
paint that has been painted over may be at risk.
Brain injury
One
early theory was that attention disorders were caused by brain injury.
Some children who have suffered accidents leading to brain injury may
show some signs of behavior similar to that of ADHD, but only a small
percentage of children with ADHD have been found to have suffered a
traumatic brain injury.
Food additives and sugar
It has been
suggested that attention disorders are caused by refined sugar or food
additives, or that symptoms of ADHD are exacerbated by sugar or food
additives. In 1982, the National Institutes of Health held a scientific
consensus conference to discuss this issue. It was found that diet
restrictions helped about 5 percent of children with ADHD, mostly young
children who had food allergies. A more recent study on the effect of
sugar on children, using sugar one day and a sugar substitute on
alternate days, without parents, staff, or children knowing which
substance was being used, showed no significant effects of the sugar on
behavior or learning.
In another study, children whose mothers
felt they were sugar-sensitive were given aspartame as a substitute for
sugar. Half the mothers were told their children were given sugar, half
that their children were given aspartame. The mothers who thought their
children had received sugar rated them as more hyperactive than the
other children and were more critical of their behavior.
Genetics
Attention
disorders often run in families, so there are likely to be genetic
influences. Studies indicate that 25 percent of the close relatives in
the families of ADHD children also have ADHD, whereas the rate is about
5 percent in the general population. Many studies of twins now show
that a strong genetic influence exists in the disorder.
Researchers
continue to study the genetic contribution to ADHD and to identify the
genes that cause a person to be susceptible to ADHD. Since its
inception in 1999, the Attention-Deficit Hyperactivity Disorder
Molecular Genetics Network has served as a way for researchers to share
findings regarding possible genetic influences on ADHD.
Recent studies on causes of ADHD
Some
knowledge of the structure of the brain is helpful in understanding the
research scientists are doing in searching for a physical basis for
attention deficit hyperactivity disorder. One part of the brain that
scientists have focused on in their search is the frontal lobes of the
cerebrum. The frontal lobes allow us to solve problems, plan ahead,
understand the behavior of others, and restrain our impulses. The two
frontal lobes, the right and the left, communicate with each other
through the corpus callosum, (nerve fibers that connect the right and
left frontal lobes).
The basal ganglia are the interconnected
gray masses deep in the cerebral hemisphere that serve as the
connection between the cerebrum and the cerebellum and, with the
cerebellum, are responsible for motor coordination. The cerebellum is
divided into three parts. The middle part is called the vermis.
All
of these parts of the brain have been studied through the use of
various methods for seeing into or imaging the brain. These methods
include functional magnetic resonance imaging (fMRI) positron emission
tomography (PET), and single photon emission computed tomography
(SPECT). The main or central psychological deficits in those with ADHD
have been linked through these studies. By 2002 the researchers in the
NIMH Child Psychiatry Branch had studied 152 boys and girls with ADHD,
matched with 139 age- and gender-matched controls without ADHD. The
children were scanned at least twice, some as many as four times over a
decade. As a group, the ADHD children showed 3-4 percent smaller brain
volumes in all regions – the frontal lobes, temporal gray matter,
caudate nucleus, and cerebellum.
This study also showed that the
ADHD children who were on medication had a white matter volume that did
not differ from that of controls. Those never-medicated patients had an
abnormally small volume of white matter. The white matter consists of
fibers that establish long-distance connections between brain regions.
It normally thickens as a child grows older and the brain matures.
Although
this long-term study used MRI to scan the children's brains, the
researchers stressed that MRI remains a research tool and cannot be
used to diagnose ADHD in any given child. This is true for other
neurological methods of evaluating the brain, such as PET and SPECT.
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