Autism and Environmental Factors

In 2007, the Centers for Disease Control and Prevention’s Autism and Developmental Disabilities Monitoring Network (ADDM) released the data that about 1 in 150 8-year-old children in multiple areas of the United States had an Autism Spectrum Disorder (ASD). Changes in the diagnostic criteria and the awareness of autism will not alone explain this epidemiological explosion. Are there environmental factors that with an underlying genetic susceptibility are responsible for the rise in autism?

Currently, no definitive cause has been identified for autism. Autism is a very complex neurobehavioral disorder that may result from a cocktail of factors possibly comprising of genetic and environmental interactions. Understanding autism presents significant challenges that would require the input of genetic, biological, neurological and psychological researchers working together to comprehend the many facets of this disorder. In 2007, the Institute of Medicine's Forum on Neuroscience and Nervous System Disorders suggested that a complex interplay of environmental stressors, genetic mutations, and other biological factors likely play a significant role in the development and/or progression of autism spectrum disorder (ASD).

The idea that environmental factors are the sole cause of autism has been very controversial. Some studies suggest that individuals exposed to specific heavy metals and other toxic insults are likely to precipitate dysfunctions in the limbic system, and the temporal and frontal lobes, which are observed in autism spectrum disorder. Evidence shows that exposure to the following agents might cause neuro-toxicity and may interfere with the normal childhood neurodevelopment during pregnancy and the first years of development: mercury, aluminum, arsenic, lead, manganese, thallium, clay, coal, soil, uranium, xenobiotic agents (including carcinogens such as nitrosamines, hydrazines, and pesticides) (Exley & Esiri , 2006; Wasserman, et al., 2007; Opler, et al., 2004; Bowler, et al., 2006; Haas, et. al., 2003;).

The neuro-toxicity hypothesis in autism has gained considerable popularity. Heavy metals were considered as possible agents contributing to the etiology of autism. It was hypothesized that some children who had ingested lead through pica and other eating habits, “became” autistic. Fetal exposure to certain chemicals such alcohol, cocaine and sodium valproate have been associated with behavioral deficits seen in autism (Shearer & Larson, 1982; Jackson & Garrod, 1978). Nanson,1992; Harris et al., 1995; Fombonne, 2002; David at al., 1992; Moore et al., 2000; Schneider and Przewlocki, 2005).

Recent attention from the media has centered on Thimerasol as a contributing agent to autism. Thimerasol is a mercury containing preservative in the MMR vaccine (Measles, Mumps and Rubella). In combination with gastrointestinal vulnerability or mitochondrial dysfunction, thimerasol may possibly lead to autism. However, it is critical to point out that larger and controlled studies have not provided consistent support of this hypothesis (Volkmar, 2000; CDC & NIH, 2003).

A number of researchers and institutions are now studying the possible role of a wide range of toxic chemicals, genetic predisposition, and the interplay between these two in altering brain development and social behaviors during early life. A major study, for instance, is underway at The University of California, Davis (UCDavis) and Childhood Autism Risks from Genetic and the Environment (CHARGE) The UC Davis Institute is using a large sample of autistic children to investigate if there is a correlation between the onset of autism and chemical exposures during early childhood.