ARTICLES
What
causes autism? While there’s still much debate,
the evidence
says it’s mainly in the genes
by Peter Szatmari, MD, FRCPC
Our understanding
of autism has changed dramatically over the past
decade. We now know that autism is
not one disorder, but several closely related “disorders”,
including Asperger syndrome, atypical autism,
and disintegrative disorder. They may involve
a range
of behaviors at different ages and degrees of
functioning, thus the term “autism spectrum
disorder”.
In
spite of recent publicity, there is good
epidemiological evidence that the measles,
mumps, and rubella vaccine is not an
environmental risk factor for autism.
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Along with these changes in
classification, there has been a greater understanding
of the causes
of autism. Developmental delay, epilepsy, minor
facial and bodily abnormalities, increased rate
of obstetrical complications, an unequal sex
ratio, and extremes of head size represent subtle,
but
still not clearly defined, signs that autism
is a neuropsychiatric disorder.
Meanwhile, studies
have shown that the concordance rate in identical
twins is very much higher than
in non-identical twins, thereby establishing
that genetic factors play a key role.
But the
genetics of the disorder must be complex, as
the mode of transmission does not follow
any recognizable pattern. Modelling studies
have
shown that multiple genes in interaction
probably account
for the genetic complexity underlying the
disorder.
It is important to point out that
these data do not exclude an environmental risk
factor
as well;
as long as it is understood that "environmental" in
this context can include any event after
fertilization.
The only environmental factors
for which we have preliminary evidence
of such causation
are thalidomide-induced
abnormalities produced during the embryonic
period and anti-convulsants taken during
pregnancy.
In spite of recent publicity,
there is good epidemiological evidence that the
measles,
mumps, and rubella
vaccine is not an environmental risk
factor for autism.
The strong genetic effects observed
in family and twin studies have encouraged
investigators
to conduct
studies that attempt to identify actual
susceptibility genes.
Although several
promising findings are based on candidate gene
studies
(particularly
in
a region
on the long arm of chromosome 15),
these have yet to be replicated consistently.
Several
studies have found that regions on chromosomes
2, 7, and 13 may contain
one
or more genes that
are associated with the development
of autism. But actual susceptibilty
genes
have not yet
been identified. Further progress
may depend on collecting
very large sample sizes.
Another
helpful approach is to identify more immediate
biological
effects
of these presumed
susceptibility
genes. Autopsy examinations and
MRI (magnetic resonance imaging)
studies
have found
larger volumes of white
matter in general and subtle
structural changes in cell density and alignment,
particularly
in the limbic system. Imaging
studies
have also
reported unusual activation of
the amygdala (a brain structure
that deals with emotion and such
things as facial recognition)
and surrounding
structures
in response
to social stimuli.
A minority
of children with autism have a co-existing disorder
of
the central
nervous system that
presumably "causes" the
disorder. In total, these conditions
probably account for only 10-15%
of cases, but they should
be kept
in mind as their diagnosis
will have clinical implications.
In
terms of co-morbid medical
disorders, good evidence
now exists that disturbances
of the
gastrointestinal
system are not more common
in children with autism than
in
the general
population of
children. No
causative factors have been
found to differentiate children
with
autism from children with
other disorders on the spectrum
such as Asperger
syndrome. Good
evidence exists that these
related conditions arise
from a common
familial, presumably
genetic, mechanism.
It is
generally anticipated that with newer technologies
and study
designs
the risk
factors that start
the causal chain that ends
with this profoundly disabling
disorder will soon be identified.
The
great hope is that from this understanding, more
definitive treatments
can be developed
to improve long-term
outcomes for all children with
autism spectrum disorders.
Excerpted from The
Causes of Autism Spectrum Disorders by Peter
Szatmari. Dr. Szatmari has been working
in the field of autism and pervasive developmental
disorders for more than 20 years. He is a professor,
vice-chair of research, and head of the division
of Child Psychiatry at McMaster University,
where he was recently appointed first holder of
the
Chedoke Health Chair in Child Psychiatry.
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