Toxins Cause Autism? The Jury Is Still

¶ … Toxins Cause Autism?

The Jury is Still Out

Nicholas Kristoff's writes about the issue of environmental toxins and autism, and the link between exposure to these toxins and the rise in autism spectrum disorders. Autism comprises a clinically heterogeneous group of disorders -- collectively referred to as "autism spectrum disorders" (ASD) -- that share common features of impaired social relationships, impaired language and communication, and repetitive behaviors or a narrow range of interests. Autism has a current prevalence of 1% in children.

Kristoff explores the relationship between toxins and autism by referring to expert opinion; that is, public health officials who have found epidemiological links between exposure to such harmful substances as mercury, lead, and thalidomide and neurodevelopmental disorders. Kristoff cites studies in which researchers "have found that disproportionate shares of children develop autism after they are exposed in the womb to medications such as thalidomide (a sedative), misoprostol (ulcer medicine) and valproic acid (anticonvulsant)."

Kristoff calls for more research, increased regulation and consumer protection. Noting that the EPA only regulates 200 of the 80,000 chemicals registered in the U.S. And that the use of dangerous plastic containers is widespread, Kristoff cites the need for more press to be given to environmental toxins and health outcomes for such diseases as cancer and autism. While lamenting the decreased vaccinations as a result of national attention directed toward the now discredited link between vaccines and autism, Kristoff declares the precautionary principle applicable in the case of toxins and autism. Paying heed to this issue would serve Americans well.

Kristoff convincingly argues for his viewpoint that toxins and autism may be linked in ways that deserve more attention from the press, from public health officials and scientists, and from the government, including the White House. His rhetorical strategies are evident in the way in which he shifts from expert pronouncements to political message to editorializing. His readers, presumably other experts as environmental health officials, politicians, and the general public, would probably listen to his warnings and quite possibly take action in finding ways of reducing environmental toxin exposure.

However, review of the scientific literature may reveal a different picture. In fact, the jury is still out regarding the issue of heavy metal toxin exposure and the prevalence of autism. Indeed, most articles that have been published since Kristoff's editorial have been much more cautious about establishing the alleged link between toxins and autism. While many researchers do see at least a correlative association, others are more apt to state that the scientific evidence is inconclusive and that much more research needs to be done.

The question of the occurrence of autism and toxin exposure was asked in early cases including articles by Soden et al., 2007, Barbaresi et al., e2009 and Thompson et al., 2007. In "Sorting out the spinning of autism: heavy metals and the question of incidence," Mary Catherine DeSoto and Robert T. Hitlan of the Department of Psychology in University of Northern Iowa state that the reasons for the rise in autism prevalence are a subject of heated professional debate. Featuring a critical appraisal of some research used to question whether there is a rise in cases and if rising levels of autism are related to environmental exposure to toxins, they aimed to evaluate the actual state of scientific knowledge. In addition, they surveyed the empirical research on the topic of autism and heavy metal toxins. Overall, they concluded that the various causes that have led to the increase in autism diagnosis are likely multi-faceted, and understanding the causes is one of the most important health topics today.

They argue that scientific research does not support rejecting the link between the neurodevelopmental disorder of autism and toxic exposures.

DeSoto and Hitlan analyzed the data reported in some articles that have been, or might be, taken to support the view of no-real-increase or no-environmental-connection.

There have been only three empirical, case control studies of those with and without ASD compared on a measure of actual mercury levels in the body that purportedly fail to find any link. Two of these data sets actually show that those with an ASD appear to have more metals, contrary to what the original authors say about the data. According to DeSoto and Hitlan, a very recent study reported in 2010 offers evidence against an association without any major flaws.

On the other hand, there are numerous scientists that have investigated autistic persons and compared measured levels of heavy metals, reported differences, and concluded a difference does exist. Other researchers have used indices of heavy metal exposures such as the zinc/copper ratio, urinary porphyrins, or measured toxins combined with genetic expression. To summarize, of the 58 empirical reports on autism and heavy metal toxins, 43 suggest some link may be present, while 13 reports found no link. Even with the tendency for null results not to be reported, it cannot be said there is no evidence for a link between heavy metal toxins and autism: although the question may still be open-in sum, the evidence favors a link (DeSoto and Hitlan 2010).

However, S.T. Schultz of the University of Texas Health Science School found a strikingly dissimilar result. His report reviews current literature regarding the association of the pharmaceutical preservative thimerosal and other mercury exposures with the risk for autism. The evidence Schultz presented did not support a causal association between autism and mercury exposure from the preservative thimerosal. While stating that the association between autism and exposure to thimerosal remains controversial, Schultz reported a cohort study by Heron and colleagues (2004) that investigated developmental disorders and did not find an association with thimerosal exposure, and a cross-sectional study by Thompson and colleagues (2007) found no association between neuropsychological functioning and exposure to thimerosal. Schultz continued that the risk for autism from other mercury exposures such as from dental amalgam restorations or environmental mercury release into the atmosphere is ambiguous. Since mercury is a known neurotoxin, Schultz concluded that more research should be done to ensure that mercury exposure from any source does not contribute to autism (Schultz 2010).