Possible Causes of Autism
Autism Causes
To date, there is no accepted single cause of Autism although there are numerous theories.
It is becoming apparent that:
- Autism Spectrum Disorder (ASD) is most probably caused by multiple factors interacting in complex ways (i.e. genes, environment and brain development).
- That ASD is not etiologically homogeneous. That is, there are probably numerous sub-types of ASD each with differing etiologies. For example, there is evidence of a sub-group of children diagnosed with ASD (20-30%) who show skill regression between 18 – 24 months after apparently normal initial development (Lainhart et al, 2002) while other children with ASD show consistently delayed development.
Genetics
Genetics have been shown to play a role but do not explain the full picture or the recent increase in reported cases. Studies have shown that if one identical twin has the diagnosis, then there is a 30 – 40% chance that the other twin will develop ASD. This concordance is hardly ever seen with non-identical twins. (Bailey et al, 1995) When a wider definition of ASD is used, the probability rates jump to 90% for identical twins and 10% for non-identical (Bailey et al, 1995). The probability of receiving an ASD diagnosis when another sibling has already been diagnosed is estimated between 2 and 14%, a 10 to 20 fold increase over the general population incidence (Hertz-Picciotto et al, 2006). No single gene has been identified as responsible and most genetic researches believe that multiple genes are involved (International Molecular Genetic Study of Autism Consortium, 1998). Research into genetics suggests that at least 40% of ASD cases may have an environmental cause (Hertz-Picciotto et al, 2006).
Viruses & Vaccinations
A few studies have begun to find some cases of ASD linked to maternal exposure to certain viruses (measles, mumps, rubella, herpes, syphilis, cytomegalovirus and toxoplasmosis) and chemicals (thalidomide and valproic acid). However, these account for a very small proportion of all cases (Hertz-Picciotto et al, 2006). There is also some evidence that the preservative thimerosol in childhood vaccinations may be linked to some cases – again evidence is mixed. Some large-scale studies have been set up to begin to understand the contribution of environmental factors to the etiology of ASD, for instance the CHARGE (Childhood Autism Risk from Genetics and Environment) study at University California-Davis.
Environmental Risk Factors For Autism
A 2003 article in The Journal of Autism and Developmental Disorders reported that the incidence of ASD diagnosis in the United States increased from 4-5 per 10,000 children in the 1980s to 30-60 per 10,000 in the 1990s, a ten-fold increase in as many years.
A portion of this increase is undoubtedly due to greater clinician awareness of the disorder and wider inclusion criteria, but this can not explain such a rapid and dramatic increase. More and more researchers are turning to environmental causes (e.g. heavy metals, PCBs, pesticides and PDBEs) that contribute to autism as a central hypothesis. There is also growing interest in the role of the immune system in ASD etiology, the suggestion that certain chemicals (widely present in our environment) may be responsible for observed inflammation in the brains of those with neurodevelopmental disorder
Brain Development
It is widely accepted that atypical brain development underlies the development of the observable symptoms of ASD (see Autism Symptoms & Characteristics). How genes and the environment interact to cause the brain to begin developing differently is still being debated. What is clear is that these differences in brain development can be traced to either before birth or very soon after birth even though the behavioral and social signs of Autism tend not be observable until after 18 months of life. The exact nature of the brain differences is not clear either.
Studies have shown differential development in many brain areas including the frontal and temporal lobes, the cerebellum, and the sub-cortical amygdala and hippocampus. Scarcity of evidence, methodological challenges and conflicting findings have not yet allowed precise conclusions to be drawn about either the specific brain regions affected or the mechanism of development that lead to observed brain differences. Other studies point to the patterns of connectivity between and within brain areas as the issue (e.g. Courchesne and Pierce, 2005) rather than specific loci while other researchers focus on specific types of neurons (e.g. mirror neurons) or patterns of neuronal activation (e.g. Bernier et al, 2007).