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AUTISM CAUSES
The causes and etiology of autism are
areas of debate and controversy; there is currently no consensus, and
researchers are studying a wide range of possible genetic and
environmental causes. Since autistic individuals are all somewhat
different from one another, there are likely multiple "causes" that
interact with each other in subtle and complex ways, and thus give
slightly differing outcomes in each individual. Two environmental theories
include the impact of vaccines on the immune system (of which a
statistically significant link has never been found despite many attempts;
see the vaccine theory sub-heading in the Causes of autism page for a more
extensive treatment) and a more recent theory relating autism to high
levels of television viewing while young.
Research claims also link autism with abnormal blood vessel function, and
oxidative stress. This line of research may lead to new medical therapies.
Physiology and
Neurology
Autism appears to involve a greater amount of the brain than
previously thought. A study of 112 children (56 with autism and 56
without), published in the Journal of Child Neuropsychology, found those
with autism to have more problems with complex tasks, such as tying their
shoelaces or writing, which suggests that many areas of the brain are
involved. Children with autism performed simple tasks as well as or better
than those without. In tests of visual and spatial skills, autistic
children did well at finding small objects in complex pictures (e.g.,
finding the character Waldo in "Where's Waldo" pictures). However, they
found it difficult to tell the difference between similar-looking people.
Children with autism tended to do well in spelling and grammar, but found
it much more difficult to understand complex speech, such as idioms or
similes when the meaning of the phrase is figurative. They would, for
example, not understand that "He kicked the bucket" meant someone had
died, or were likely to actually hop if told to "hop it".
The inference from this research, according to researchers at the
Pittsburgh School of Medicine, is that "These findings show that you
cannot compartmentalize autism. It's much more complex.”
The research from this perspective has a number of implications:
Autism is more than likely a global disorder which affects how the brain
processes the information it receives, while complex information tends to
make this more readily apparent.
Neurological ‘wiring’ in people with autism manifest abnormalities in the
areas of the brain that communicate with each other.
Observed abnormalities provide a reasonable explanation for why children
with autism have problems with complex tasks which require multiple areas
of the brain to work together; autistic people tend to do better in tasks
that only require one region of the brain.
The causes of autism are possibly more pervasive than previously believed;
for example, more areas of the brain are affected than just those
involving social interaction, communication, interests, and imagination.
Autism may not be primarily a disorder of social interaction; research
must now take into account non-social aspects.
A possible explanation for the characteristics of the syndrome is a
variation in the way the brain itself reacts to sensory input and how
parts of the brain then handle the information. An electroencephalographic
(EEG) study of 36 adults (half of whom had autism) at Washington
University in St. Louis found that adults with autism show differences in
the manner in which neural activity is coordinated. The implication seems
to be that there is poor internal communication between different areas of
the brain. (Electroencephalographs, or EEGs, measure the activity of brain
cells.)
The study indicated that there were abnormal patterns in the way the brain
cells were connected in the temporal lobe of the brain. (The temporal lobe
deals with language.) These abnormal patterns would seem to indicate
inefficient and inconsistent communication inside the brain of autistic
people.
Studies in neuropathology indicate abnormalities in the amygdala,
hippocampus, septum, mamillary bodies, limbic system,and the cerebellum.
Autistic brains are slightly larger and heavier and a larger than normal
head circumference is commonly noted.
In the limbic system, there is an excess of cells and they are too small.
The neurons themselves appear to be underdeveloped. Dendritic trees which
provide the basis for connections between neurons are truncated (i.e.
shortened).
In the cerebellum, purkinje cells are widely affected. The anatomic
differences correlate to the curtailment of development earlier than 30
weeks gestation. In other words, the development of the cells appears to
have stopped at some time before the 30th week in utero
An enlarged third ventricle of the brain appears to accompany autism in
those who are non-mentally retarded, but the reasons for this and its
effects are still unknown.
Research has not yet established exactly what is specific to autism and
what may be seen in other disorders however.
Genetic Component
Main article: Heritability of autism
Genetic influence comprises a significant aspect of research in the causes
of autism. Originally hinting toward this was the observation that there
is about a 60% concordance rate for autism in monozygotic (identical)
twins, while dizygotic (non-identical) twins and other siblings only
exhibit about 4% concordance rates. A theory featuring mirror neurons
states that autism may involve a dysfunction of specialized neurons in the
brain that should activate when observing other people. In
typically-developing people, these mirror neurons are thought to perhaps
play a major part in social learning and general comprehension of the
actions of others.
Researchers from France showed that the gene called SHANK3, also known as
ProSAP2, regulates the structural organization of dendritic spines and is
a binding partner of neuroligins; genes encoding neuroligins are mutated
in autism and Asperger syndrome. A mutation of a single copy of the gene
on chromosome 22q13 can result in language or social communication
disorders (see also 22q13 deletion syndrome). Though not present in all
individuals with autism, the mutations hold potential to illustrate some
of the genetic components of spectrum disorders.
The MET gene, linked to brain development, regulation of the immune
system, and repair of the gastrointestinal system, has been linked through
research to autism. A mutation of the gene, rendering it less active, has
been found to be common amongst children with autism. Researchers, from
the Vanderbilt Kennedy Center for Research on Human Development, reported
in the Proceedings of the National Academy of Sciences that the mutation
in the MET gene raises risk of autism by 2.27 times.
Most recently, the Autism Genome Project, an international research team
composed of 137 scientists in 50 institutions, has implicated the neurexin
1 gene, located on chromosome 11, as a cause of some cases of autism.[70]
DNA from over 1,600 families was analyzed in what was the largest-scale
genome scan conducted in autism research at the time.
A large database showing theoretical links between autism and genetic loci
indicates that the genetic influence may extend to every human chromosome.
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1
Terminology
2
History
3
Characteristics
Key Behaviors
Noted behaviors
Social development
Sensory system
Autism and blindness
Communication difficulties
Repetitive behaviors
Effects in education
4
DSM definition
5
Types of autism
Asperger's and Kanner's syndrome
Autism as a spectrum disorder
6
Epidemiology
7
Treatment
8
Causes
Physiology and Neurology
Genetic Component
9
Sociology
Community and politics
Culture
Autistic adults
Terminology
Autistic savants
References |
