Dr. David Bellinger and colleagues’ interesting September JADA study, "A Dose-Effect Analysis of Children’s Exposure to Dental Amalgam and Neuropsychological Function: The New England Children’s Amalgam Trial" (
Bellinger DC, Trachtenberg F, Daniel D, Zhang A, Tavares MA, McKinlay S. JADA 2007;138[9]: 1210–6[Abstract/Free Full Text]
), measured neuropsychological function (NF) of children associated with the use of dental amalgam. This well-conducted study presents relevant conclusions indicating that use of dental amalgam was not associated with an increase in children’s risk of experiencing NF. However, given that these U.S. children were born when vaccines were still preserved with thimerosal, it is possible that a substantial proportion of them might have been exposed to ethylmercury early in life.
Given that the youngest children enrolled in our trial were 6 years old, it would be difficult to get accurate information, retrospectively, on the thimerosal levels in the vaccine lots administered to these children years in the past.
Although children are exposed to mercury-amalgam restorations, in the past younger children in the United States were exposed to ethylmercury derived from thimerosal-containing vaccines (TCVs). After 1999, this became a controversial issue, and only now are studies considering this form of early exposure to organic mercury. TCVs such as diphtheria-tetanus-pertussis (DTP) and hepatitis B were among the vaccines given to young infants. The early schedules of these vaccines generated concerns for preventing plausible NF effects, sufficient to stop their use in young children.
Because we do not know the type of vaccines used in these children, there are chances of TCV exposure, and a lively debate over NF effects still persists.1,2 Unfortunately, these studies used retrospective databases such as Vaccine Safety Datalink (VSD)1 and Vaccine Adverse Event Reporting System (VAERS),2 which have the disadvantage of insufficient information that could identify colinearity and track intervening variables.
Because mercury neurotoxicity involves long latencies and atypical responses between low and high doses,3 this issue remains open to investigation. The authors’ well-conducted study has been presented with differential statistical strategies in order to deal with all possible interactions of their data. However, because TCVs still are used in many countries,4 we cannot miss the opportunity to discuss NF studies that consider all forms of mercury exposure (including TCVs).
Although the authors are not being challenged (on the contrary, the study’s active data collection design offers the opportunity to look into crucial issues related to mercury exposure and NF), a post hoc addressing of this theme can enrich the scant knowledge of NF consequences due to early ethyl-mercury exposure.
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