Blood Mercury Levels in Children With Autism Similar to Those in Typically Developing Kids

Blood Mercury Levels in Children With Autism Similar to Those in Typically Developing Kids

Caroline Cassels

New findings from the CHildhood Autism Risks from Genetics and the Environment (CHARGE) study show that preschoolers with ASD had neither elevated nor reduced total Hg levels in blood compared with unaffected control children, after adjustment for dietary, medical, pharmaceutical, and dental sources of Hg, and resemble those of nationally representative samples.

“After adjusting for intake [of Hg], by and large there are no differences between children with AU and children with typical development in their circulating levels of Hg,” lead investigator Irva Hertz-Picciotto, PhD, MPH, chief of the Division of Environmental and Occupational Health and M.I.N.D. Institute at the University of California–Davis, told Medscape Psychiatry.

However, she added, this current analysis does not address the issue of whether there is a causal link between Hg and AU/ASD because Hg levels were measured after a diagnosis of AU/ASD was made.

The paper was published online October 19 in Environmental Health Perspectives.

Contradictory Findings

The study’s findings contradict some previous studies suggesting that children with AU have higher blood Hg levels relative to unaffected control children. Hg has drawn particular attention in terms of ASD because of its known neurotoxicity in some forms.

In addition, the inclusion of thimerosal, an Hg-based preservative found in some childhood vaccines, has further fueled this debate, although many vaccines are now thimerosal-free.

Much of the previous research investigating a potential association between Hg and AU have included only single sources of Hg and/or have used subjects from selective populations.

Uniquely, the CHARGE study is a population-based case-control study of 2- to 5-year-olds that examined a wide variety of Hg sources including fish consumption, dental amalgams, vaccines, and other pharmaceutical sources of Hg exposure such as nasal sprays or earwax removal products that may contain Hg. The researchers also looked at whether children with dental amalgams and a habit of grinding their teeth or chewing gum had higher circulating Hg levels.

The study findings were based on data from 452 children. Of these participants, 249 had a diagnosis of AU, 143 were typically developing, and 60 had other developmental delays such as Down syndrome.

Don’t Be Afraid of Fish

Data on possible sources of Hg including fish consumption and dental amalgams were collected via interview with the subjects’ parents. Information on vaccines was obtained from the children’s vaccination and medical records.

At study outset, said Dr. Hertz-Picciotto, the children with AU/ASD appeared to have lower blood Hg levels than typically developing children. However, it turned out those children with AU/ASD tended to eat less fish, and once researchers adjusted for this they found blood Hg concentrations were roughly the same between the 2 study groups. In addition, they found that blood Hg concentrations were very similar to a nationally representative sample of 2- to 5-year-old children.

Interestingly, and somewhat surprising, she added, was the finding that children with dental amalgams who engaged in teeth grinding or gum chewing had higher circulating blood levels of Hg.

Dr. Hertz-Picciotto said her team plans to conduct further research examining prenatal Hg exposure.

She added that although certain types of fish, such as swordfish and shark, tend to be high in Hg, overall it has been shown that fish consumption during pregnancy has a positive effect on children’s cognitive function.

“Pregnant women shouldn’t be afraid of eating fish. They just need to choose the right fish,” said Dr. Hertz-Picciotto.

She added that organizations such as the Environmental Protection Agency have excellent Web sites that provide advice on fish selection she added.

Fred Volkmar, MD, director, Yale University Child Study Center, New Haven, Connecticut, who was not involved in the study, said the findings are reassuring to both clinicians and parents and suggest Hg does not appear to play a role in AU/ASD.

“The trouble with science is that you can never prove the negative — you can only fail to prove the positive. But this study, which is from a well-respected group of researchers and published in a well-respected journal, seems to suggest Hg does not appear to be a contributing factor [in AU],” Dr. Volkmar told Medscape Psychiatry.

He also applauded the study’s population-based design and the fact that the researchers considered multiple sources of Hg.

However, he acknowledged the need for more studies to replicate these findings and to investigate the potential role of prenatal Hg exposure in AU etiology.

The authors have disclosed no relevant financial relationships.

Environ Health Perspect. Published online October 19, 2009.