Fresh from listening to some of my Facebook friends argue about vaccines, a question occurred to me. What DOES cause autism? Not the MMR (measles-mumps-rubella) vaccine (see for example, Jain et al. 2015. Also, the weight of a huge amount of scientific evidence vs. one tiny, discredited study. I will fight you). But SOMETHING causes autism, so what is it? Much of the following is a summary of “Environmental factors in autism” by Andreas Grabrucker (2013).

So, to start, what actually IS autism?

Autism is a developmental brain disorder, generally classified on a spectrum (autism spectrum disorder or ASD), meaning it can vary in severity and symptoms among people. Interestingly, although most people with autism can’t count toothpicks really fast like Dustin Hoffman in Rain Man, his character was actually based on a real person. Among the most common symptoms of ASD are impaired social behavior and repetitive behaviors.

Some autism advocates believe terms like ‘disorder’ and ‘impairment’ are loaded, and don’t like them. I use them here for consistency with medical and research terminology, but it is a good thing to be aware of.

Autism has a genetic component, meaning it can run in families (Abrahams and Geschwind 2010). However, autism risk is strongly affected by the developmental environment as well. Here are the environmental factors most strongly linked to autism. Note that although some of these are likely causal, others may just co-occur with autism often.

Risk factors during pregnancy

1. Prenatal virus, allergy, or auto-immune disease – thought to alter immune function in the placenta or fetus

2. Zinc deficiency – very common in autistic children, may represent a physiological mechanism, may relate to infections

3. Abnormal melatonin synthesis – melatonin is an important hormone, abnormalities may relate to light pollution or zinc

4. Maternal diabetes – linked to a two-fold increase in autism risk, mechanism here is unclear

5. Stress and trauma during pregnancy and birth – may be related to immune function, can cause similar behaviors in other animals

6. Some chemicals that negatively affect humans (‘toxins’) – for example, pesticides, valproic acid, and thalidomide

7. Advanced age in either parent – may be due to increased mutation risk or increased risk of pregnancy complications

Postnatal risk factors

There are not currently well-supported risk factors for autism that occur after birth. This may change, as some are being investigated, but none are strongly supported at this point.

In conclusion, risk factors for autism may be most important during pregnancy, and certainly seem to be linked to immune function, stress, certain toxic chemicals, and pregnancy complications. What we are still missing is a unifying physiological hypothesis tying these together in a meaningful way that incorporates the genetic risk.

References

1. Jain, A., Marshall, J., Buikema, A, Bancroft, T., Kelly, J.P., Newschaffer, C.J. (2015). Autism occurrence by MMR vaccine status among US children with older siblings with and without autism. Journal of the American Medical Association. 313 (15): 1534-1540.

2. Grabrucker, A.M. (2013) Environmental factors in autism. Frontiers in Psychiatry 3 (118).

3. Abrahams, B.S., Geschwind, D.H. (2010) Connecting genes to brain in the autism spectrum disorders. Arch. Neurol. 67, 395-299.