What
Causes Autism?
It
is generally accepted that autism
is caused by abnormalities in brain
structures or functions. Using a variety
of new research tools to study human
and animal brain growth, scientists
are discovering more about normal
development and how abnormalities
occur.
The
brain of a fetus develops throughout
pregnancy. Starting out with a few
cells, the cells grow and divide until
the brain contains billions of specialized
cells, called neurons. Research
sponsored by NIMH and other components
at the National Institutes of Health
is playing a key role in showing how
cells find their way to a specific
area of the brain and take on special
functions. Once in place, each neuron
sends out long fibers that connect
with other neurons. In this way, lines
of communication are established between
various areas of the brain and between
the brain and the rest of the body.
As each neuron receives a signal it
releases chemicals called neurotransmitters,
which pass the signal to the next
neuron. By birth, the brain has evolved
into a complex organ with several
distinct regions and subregions, each
with a precise set of functions and
responsibilities.
Different
parts of the brain have different
functions
- The
hippocampus makes it possible to
recall recent experience and new
information
- The
amygdala directs our emotional responses
- The
frontal lobes of the cerebrum allow
us to solve problems, plan ahead,
understand the behavior of others,
and restrain our impulses
- The
parietal areas control hearing,
speech, and language
- The
cerebellum regulates balance, body
movements, coordination, and the
muscles used in speaking
- The
corpus callossum passes information
from one side of the brain to the
other
But
brain development does not stop at
birth. The brain continues to change
during the first few years of life,
as new neurotransmitters become activated
and additional lines of communication
are established. Neural networks are
forming and creating a foundation
for processing language, emotions,
and thought.
However,
scientists now know that a number
of problems may interfere with normal
brain development. Cells may migrate
to the wrong place in the brain. Or,
due to problems with the neural pathways
or the neurotransmitters, some parts
of the communication network may fail
to perform. A problem with the communication
network may interfere with the overall
task of coordinating
sensory information, thoughts, feelings,
and actions.
Researchers
supported by NIMH and other NIH Institutes
are scrutinizing the structures and
functions of the brain for clues as
to how a brain with autism differs
from the normal brain. In one line
of study, researchers are investigating
potential defects that occur during
initial brain development. Other researchers
are looking for defects in the brains
of people already known to have autism.
Scientists
are also looking for abnormalities
in the brain structures that make
up the limbic system. Inside the limbic
system, an area called the amygdala
is known to help regulate aspects
of social and emotional behavior.
One study of high-functioning children
with autism found that the amygdala
was indeed impaired but that another
area of the brain, the hippocampus,
was not. In another study, scientists
followed the development of monkeys
whose amygdala was disrupted at birth.
Like children with autism, as the
monkeys grew, they became increasingly
withdrawn and avoided social contact.
Differences
in neurotransmitters, the chemical
messengers of the nervous system,
are also being explored. For example,
high levels of the neurotransmitter
serotonin have been found in a number
of people
with autism. Since neurotransmitters
are responsible for passing nerve
impulses in the brain and nervous
systme, it is possible that they are
involved in the distortion of sensations
that accompanies autism.
NIMH
grantees are also exploring differences
in overall brain function, using a
technology called magnetic resonance
imaging (MRI) to identify which parts
of the brain are energized during
specific mental tasks. In a study
of adolescent boys, NIMH researchers
observed that during problem-solving
and language tasks, teenagers with
autism were not only less successful
than peers without autism, but the
MRI images of their brains showed
less activity. In a study of younger
children, researcers observed low
levels of activity in the parietal
areas and the corpus callosum. Such
research may help scientists determine
whether autism reflects a problem
with specific areas of the brain or
with the transmission of signals from
one part of the brain to another.
Each
of these differences has been seen
in some but not all the people with
autism who were tested. What could
this mean? Perhaps the term autism
actually covers several different
disorders, each caused by a different
problem in the brain. Or perhaps the
various brain differences are themselves
caused by a single underlying disorder
that scientists have not yet identified.
Discovering the physical basis of
autism should someday allow us to
better identify, treat, and possibly
prevent it.
Factors
affecting brain development
But
what causes normal brain development
to go awry? Some NIMH researchers
are investigating genetic causes-the
role that heredity and genes play
in passing the disorder from one generation
to the next. Others are looking at
medical problems related to pregnancy
and other factors.
Heredity.
Several studies of twins suggest that
autism- or at least a higher likelihood
of some brain dysfunction-can be inherited.
For example, identical twins are far
more likely than fraternal twins to
both have autism. Unlike fraternal
twins, which develop from two separate
eggs, identical twins develop from
a single egg and have the same genetic
makeup.
It
appears that parents who have one
child with autism are at slightly
increased risk for having more than
one child with autism. This also suggests
a genetic link. However, autism
does not appear to be due to one particular
gene. If autism, like eye color,
were passed along by a single gene,
more family members would inherit
the disorder. NIMH grantees, using
state-of-the-art gene splicing techniques,
are searching for irregular segments
of genetic code that the autistic
members of a family may have inherited.
Some
scientists believe that what is inherited
is an irregular segment of genetic
code or a small cluster of three to
six unstable genes. In most people,
the faulty code may cause only minor
problems. But under certain conditions,
the unstable genes may interact and
seriously interfere with the brain
development of the unborn child.
A
body of NIMH-sponsored research is
testing this theory. One study is
exploring whether parents and siblings
who do not have autism show minor
symptoms, such as mild
social, language, or reading problems.
If so, such findings would suggest
that several members of a family can
inherit the irregular or unstable
genes, but that other as yet unidentified
conditions must be present for the
full-blown disorder to develop.
Pregnancy
and other problems. Throughout
pregnancy, the fetal brain is growing
larger and more complex, as new cells,
specialized regions, and communication
networks form. During this time, anything
that disrupts normal brain development
may have lifelong effects on the child's
sensory, language, social, and mental
functioning.
For
this reason, researchers are exploring
whether certain conditions, like the
mother's health during pregnancy,
problems during delivery, or other
environmental factors may interfere
with normal brain development. Viral
infections like rubella (also called
German measles), particularly in the
first three months of pregnancy, may
lead to a variety of problems, possibly
including autism
and retardation. Lack of oxygen
to the baby and other complications
of delivery may also increase the
risk of autism. However, there is
no clear link. Such problems occur
in the delivery of many infants who
are not autistic, and most children
with autism are born without such
factors. Find
an Autism Lawyer in Any State:
|
