MONDAY, Oct. 19, 2009 (HealthDay News) -- A new study finds no differences in blood mercury levels between children who are developing normally and those diagnosed with autism or autism spectrum disorders.
The findings, appearing online Oct. 19 in the journal Environmental Health Perspectives, may quell some of the debate surrounding mercury and autism.
The researchers did not, for instance, look at whether mercury might play a causal role in this group of developmental disorders.
"This is an excellent study that has addressed a very important question for many families about the role of mercury [in autism]," said Dr. Patricia Manning-Courtney, medical director of The Kelly O'Leary Center for Autism Spectrum Disorders at Cincinnati Children's Hospital Medical Center. "It's a very clear study [in] that no differences in mercury were seen. Is it going to put the questions to rest? I think no. It's another important contribution to the large body of knowledge on this topic, but I think it will still be an area of great controversy."
Previous studies have shown higher blood mercury levels in people with autism, but the results, along with the quality of the studies, have been inconsistent.
One of the biggest controversies claims that vaccines containing the preservative thimerosal, which contains mercury, might cause autism.
But thimerosal is no longer used routinely in children's vaccines and, earlier this year, a U.S. court ruled that there is no scientific evidence that childhood vaccines such as the measles-mumps-rubella vaccine caused autism in children of parents seeking compensation from a federal fund.
The authors of this study wanted to fill one gap in the research: a lack of good evidence comparing mercury levels in children with autism and children without autism.
The authors looked at 452 children aged 2 to 5, who either had autism/autism spectrum disorder, developmental delays other than autism (such as Down syndrome), or who were developing normally.
Blood samples were taken from the children while mothers answered questions about different possible exposures to mercury, such as diet, dental amalgams, thimerosal-containing vaccines and even personal-care products such as earwax removal systems and nasal sprays.
First results showed that children with autism actually had much lower levels of mercury in their blood, but this was explained by the fact that these children ate less fish.
When the results were adjusted for this and other variables, mercury levels came out about the same between the autism group and the control group.
"Not only do we not see differences, but the values are pretty close to national averages," said study author Irva Hertz-Picciotto, chief of environmental and occupational health and a faculty member at the MIND Institute at the University of California, Davis.
Overall, children with developmental delays other than autism had lower blood mercury concentrations.
Children who had mercury-based dental fillings who ground their teeth or chewed gum had blood concentrations of the metal. And those few children who had had a mercury-containing vaccine did not show elevations in their blood levels.
"The relationship between [mercury] intake to blood levels seems to follow the pattern that we expect and it's well known that most of the mercury in the body does come from fish consumption," Hertz-Picciotto said. "There really were very few children who had vaccines that would have or could have contained thimerosal."
Most of the children in the study had received vaccines after thimerosal was removed from vaccines, she said.
Mercury only has a half-life in the circulating blood of a few months, Hertz-Picciotto said, so "clearly this does not tell us anything causal because all of the mercury represents exposure that had happened post-diagnosis."
Sallie Bernard, executive director of SafeMinds (Sensible Action for Ending Mercury-Induced Neurological Disorders), expressed concern about confusing the findings with the interpretation of the findings.
"An interpretation says the study shows no link between mercury and autism, but that's not what the paper is saying," Bernard said. "It says they looked to see if [there] was higher exposure in kids with autism after they got their diagnosis. These are current exposures, not what might have happened at an earlier time point in pregnancy or in the first year of life, so you don't know what affect that might have had."
The study also does not go into whether certain children may be more susceptible to mercury's effects, she said.
Learn more about autism spectrum disorders at the U.S. Centers for Disease Control and Prevention.