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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 the way autism manifests is different for each person, 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 for a more extensive treatment) and a more recent theory relating autism to high levels of television viewing while young. A correlation has also been found between autism and the mercury emitted from coal power plants, though the implications of this correlation is uncertain. Mercury measurements of hair samples from autistic children's first haircuts were significantly lower than a matched group of normal children, declining as measures of severity increased.
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 to 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.
Individuals with autism are also far more likely to develop epilepsy than would otherwise be expected (estimated 10-30% incidence).
Genetic component
Main article: Heritability of autism
Genetic influence comprises a significant aspect of research in the causes of autism. A large database showing theoretical links between autism and genetic loci summarises research indicating that the genetic influence may extend to every human chromosome. It has been observed in one twin-study in Britain that there was about a 60% concordance rate for autism in monozygotic (identical) twins, while dizygotic (non-identical) twins and other siblings comparatively exhibited about 4% concordance rates. Some research posits that the chances that an identical twin of an autistic person will also be autistic are 85-90%. The increased probabilities of siblings having autism has been calculated at about 35-fold more than normal.
Accompanying impairments are also a common feature of autism. Some people with autism also have gastrointestinal, immunological or neurological symptoms in addition to behavioral impairments. These associated complexes have also lead to the search for specific genetic connections and helped to focus on reasonable genetic implications.
Since genes provide the information for processes and structure at the level of the cell and its components during the growth and development of a human as well as maintenance during life, gene mutations (altered versions) and deletions (complete absence of genetic material) and possibly extra copies of genes would mean that the causes of autism begin very early. If a mutated gene fails to perform properly, then cells, proteins, enzymes and other crucial aspects of normal function may be significantly altered and operate incorrectly. Deletions could mean the complete absence of a sequence of events due to missing proteins or cell components for example. These genetic alterations and deletions will simply bring about a changed structure or process which effects a great many other needed structures and processes.
Another important aspect of research in genetic factors is environmental effects and the incidence of autism. During the lifetime of a person, gene mutations and deletions may be environmentally triggered or exacerbated. Conversely, it may also be that environment will not be a factor and nothing will change the autism characteristics. For autism, the answer to these possible explanations is still being researched and there is evidence that both may be true simply because there is more than one way a person may develop autism.
Deletions and Mutations
Deleted genes have been noted as a probable influence or cause in autism. By locating specific missing genetic material the significance may be that specific genetic sites for autism controlling or causing autism (autism susceptibility alleles) may be located precisely. Another significant aspect of this research is that these deletions of genetic material indicate that autism may be established in some cases during meiosis (error-prone meiosis model) and this places the genesis of autism in some at the very beginning of life.
One very important question in this line of research is whether or not gene deletions are a cause or consequence of autism-susceptibility loci located elsewhere in the chromosomes.
Gene mutations may mean a gene does not function at all or does not function in the normal way. Since genes direct how the body grows and develops, mutations, like deletions, will effect a person at the most basic levels.
Mutation and deletion effects have been delineated in numerous research publications.
Correlated characteristics include global developmental delay, mild to severe delay of speech, social communication disorders and cognitive abilities, autistic like behaviour, high tolerance of pain, and repetitive mannerisms (e.g. chewing or mouthing).
Gene interaction may also complicate the causes leading to multiple genetic origins of autism, In a cascade like effect, when a gene loci is altered or omitted, others are effected due to change in interaction between genes and/or their functions.
Though not present in all individuals with autism, these mutations and deletions hold potential to point the way to more the genetic components of spectrum disorders. The research also advanced basic understanding in the genetic architecture of the genome of autistic individuals and will help in focusing future research.
One practical aspect of this type of research may be the development of a test that would confirm the autism diagnosis in children exhibiting symptoms and identify families who carry genetic defects that could be inherited by their children.
Mirror neurons
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.
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Important notice:
The content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other
qualified health provider with any questions you may have regarding a medical condition.
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