causes
Many causes of autism have been proposed, but understanding of the theory of causation of autism and the other autism spectrum disorders is incomplete. Heritability contributes about 90% of the risk of a child developing autism, but the genetics of autism are complex and typically it is unclear which genes are responsible.In rare cases, autism is strongly associated with agents that cause birth defects. Many other causes have been proposed, such as exposure of children to vaccines; these proposals are controversial and the vaccine hypotheses lacks compelling scientific evidence.
Is Autism Genetic?
Autism has a strong genetic basis, although the genetics of autism are complex and it is unclear whether ASD is explained more by rare mutations with major effects, or by rare multigene interactions of common genetic variants. Complexity arises due to interactions among multiple genes, the environment, and epigenetic factors which do not change DNA but are heritable and influence gene expression. Studies of twins suggest that heritability is 0.7 for autism and as high as 0.9 for ASD, and siblings of those with autism are about 25 times more likely to be autistic than the general population. However, most of the mutations that increase autism risk have not been identified. Typically, autism cannot be traced to a Mendelian (single-gene) mutation or to a single chromosome abnormality like fragile X syndrome, and none of the genetic syndromes associated with ASDs have been shown to selectively cause ASD. Numerous candidate genes have been located, with only small effects attributable to any particular gene. The large number of autistic individuals with unaffected family members may result from copy number variations—spontaneous deletions or duplications in genetic material during meiosis. Hence, a substantial fraction of autism cases may be traceable to genetic causes that are highly heritable but not inherited: that is, the mutation that causes the autism is not present in the parental genome.
Is autism caused by synaptic disfunction?
Several lines of evidence point to synaptic dysfunction as a cause of autism. Some rare mutations may lead to autism by disrupting some synaptic pathways, such as those involved with cell adhesion.Gene replacement studies in mice suggest that autistic symptoms are closely related to later developmental steps that depend on activity in synapses and on activity-dependent changes. All known teratogens (agents that cause birth defects) related to the risk of autism appear to act during the first eight weeks from conception, and though this does not exclude the possibility that autism can be initiated or affected later, it is strong evidence that autism arises very early in development.
Is autism caused by environmental factors?
Although evidence for other environmental causes is anecdotal and has not been confirmed by reliable studies, extensive searches are underway. Environmental factors that have been claimed to contribute to or exacerbate autism, or may be important in future research, include certain foods, infectious disease, heavy metals, solvents, diesel exhaust, PCBs, phthalates and phenols used in plastic products, pesticides, brominated flame retardants, alcohol, smoking, illicit drugs, vaccines,[and prenatal stress,although no links have been found, and some have been completely dis-proven.
Is autism caused by vaccines?
Parents may first become aware of autistic symptoms in their child around the time of a routine vaccination. This has led to theories blaming vaccine "overload", a vaccine preservative or the MMR vaccine for causing autism. The latter theory was supported by litigation-funded study that has since been shown to have been "an elaborate fraud".Although these theories lack convincing scientific evidence and are biologically implausible, parental concern about a potential vaccine link with autism has led to lower rates of childhood immunizations, outbreaks of previously-controlled childhood diseases in some countries, and the preventable deaths of several children
Recent studies show that there is a certain relationship between autism and brain. Following is explained as follows:-
There is a Relationship Between Autism and Brain Structure:-
Recent brain studies show that autistic brains grow at an unusual rate between age 1 and 2, and then slow again to a normal rate of growth. Some imaging studies suggest that certain areas of the brain are larger than is typical. Research is ongoing to determine whether these differences in brain structure cause autism, are caused by autism, or are co morbid with autism and caused by something else.
There Is a Relationship Between Autism and Brain Activity:-
Recent brain imaging studies show that autistic people and typically developing people do not use their brains in the same way. Autistic people do not use their brains to "daydream" in the same way as most people, nor do they process information about faces in the same way. So far, while we know that this information is true, we don't know what causes these differences -- or whether these differences somehow cause autistic symptoms.
There Is a Relationship Between Autism and Brain Chemicals:-
Chemicals in the brain transmit signals which allow the brain to function normally. Sophia Colamarino explains: "Nerve cells communicate using electrochemical signals; there is evidence from many different domains that the ability of the brain to transfer information may be defective." An understanding of which transmitters are problemmatic may lead to effective treatments.
