Possible Causes of Autism
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 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.