Environmental toxins may cause autism

May 17 2007

The increase in the number of diagnosed cases of autism in recent years has sparked concern that environmental toxins may cause this complex disorder.

A new study found, however, that exposure to Rh immune globulin preserved with mercury-containing thimerosal before birth was no higher for children with autism. The study led by Judith Miles, at the University of Missouri-Columbia, will be published online in the American Journal of Medical Genetics Part A.

Although experts anticipate that autism will be the first behavioral/psychiatric disorder for which major genes will be identified, there is still fierce debate that thimerosal, a preservative commonly used in vaccines and is almost 50 percent ethylmercury, is responsible for the rise in the disorder. Autism diagnoses have increased significantly during the last two decades, which coincides temporally with the addition of five pediatric vaccines to the immunization schedule, exposing children to increasing doses of ethylmercury, a known toxin. Though the vast majority of studies indicate no association between vaccines and autism, the FDA, CDC and American Academy of Pediatrics, recommended that thimerosal be removed from all routinely recommended early childhood vaccines; this was accomplished by 2002.

The current study investigated thimerosal exposure during pregnancies that resulted in the birth of a child subsequently diagnosed with autism. Rh negative women are routinely treated with Rh immune globulin (RhIg) during the third trimester to prevent hemolytic disease, in which the mother's immune system attacks fetal blood cells. Like many vaccines, RhIg manufactured in the U.S. contained thimerosal prior to 2001. Since young fetal brains are more susceptible to neurotoxic effects, researchers led by Judith Miles, MD, PhD, of the Thompson Center for Autism and Neurodevelopmental Disorders at the University of Missouri-Columbia, assessed Rh status and thimerosal exposure of mothers of children with autism.

The study included 214 mothers of 230 children diagnosed with an autism spectrum disorder (ASD). Rh status, RhIg with thimerosal exposure, and Rh incompatibility (in which the mother's Rh status is different than the fetus') were established by reviewing medical records. The results showed that in children with autism, Rh negative status was no higher in their mothers than in the general population, that exposure to RhIg (preserved with thimerosal) before birth was no higher and that pregnancies were not more likely to be Rh incompatible.

The authors note that few studies have focused on pregnancies of Rh negative mothers who received RhIg during pregnancy, probably because the thimerosal is diluted before reaching the fetus and has been assumed to be innocuous. Nevertheless, there is a concern that even very small doses delivered when the brain is especially sensitive can be toxic. Numerous internet sites and one research study assert that RhIg causes autism and that a high percentage of mothers of children with autism are Rh negative, neither of which was shown to be true in the current study. In addition, a recent study hypothesized that Rh incompatibility itself could disrupt fetal neurodevelopment thus playing a role in autism, but the current study found no increase in the proportion of Rh incompatibility in mothers of autistic children. In response to the claim that only certain groups of children are at risk, the authors also analyzed specific ASD subgroups and found that none had significant increases in either Rh negativity or thimerosal exposure during pregnancy.

"This study adds to the evidence that there is no causal association between thimerosal and childhood autism," the authors state. They point out that even though RhIg and childhood vaccines are now thimersol free in this country, it is important to analyze questions of safety since thimerosal continues to be used in many places around the world to preserve vaccines, which makes them affordable. They conclude, "We hope this report of no association between autism, Rh negativity and thimerosal exposure during pregnancy will offset some of the decreased compliance with immunization recommendations which is known to increase morbidity and mortality from childhood infectious diseases."