Methylmercury and Neurodevelopment: Reanalysis of the Seychelles Child Development Study Outcomes at 66 Months of Age
- Philip W. Davidson, PhD;
- James Kost, MA;
- Gary J. Myers, MD;
- Christopher Cox, PhD;
- Thomas W. Clarkson, PhD
- KEYWORDS:
- child development
- diet
- fishes
- hazardous substances
- methylmercury compounds
- toxic substances, environmental
To the Editor: There is ongoing controversy about the effects of methylmercury (MeHg) exposure from fish consumption on child development. We previously reported no adverse developmental outcomes in children tested at 5.5 years of age following both prenatal and postnatal exposure from fish consumption in our longitudinal study in the Seychelles Islands.1 A similar study in the Faeroe Islands reported subtle adverse effects associated with prenatal exposure from consumption of both fish and pilot whales.2
A National Academy of Sciences panel recently reviewed the evidence for health risks from dietary MeHg exposure.3 The review included both the Seychelles and Faeroes studies. The panel concluded that both studies were well designed, controlled, and executed, and that a variety of differences, including methodological, might account for the difference in results. The panel made 2 suggestions concerning the analysis of our 5.5-year test scores: (1) the use of raw test scores instead of standardized scores and the inclusion of the child's age at testing as an additional covariate in the analysis would provide better control for the age of the child at testing; and (2) adjustment for which of the 3 staff members administered the test battery would improve the precision of the analysis.
Methods
In the original study, all children were tested at a mean (SD) age of 66 (6) months and standardized test scores afforded inherent control for age, and intertester reliability among the 3 testers was consistently high. We reanalyzed the data from the 6 primary end points at 66 months, using the same statistical procedures reported earlier, except that raw test scores were used, and age at testing and tester were included as additional covariates in the regression analyses.
Results
The reanalyses confirmed our previous findings. Table 1 summarizes the results in the same format as our earlier report.1 The significance and direction of the parameter estimates for prenatal and postnatal exposure are nearly identical to those reported earlier. We did find significant effects of both age at testing (to be expected since scores were no longer scaled) and tester (not unlikely since testers did not have 100% agreement). There continues to be an association among both prenatal and postnatal exposure and the Preschool Language Scale Total Score, as well as an association in males only among postnatal exposure and scores on the Woodcock-Johnson Applied Problems and the Bender Gestalt drawing and copying errors.
Table. Multiple Regression Reanalysis of 66-Month Test Scores*
Comment
These associations continue to suggest beneficial effects with increasing mercury levels that may reflect dietary benefits of fish consumption. In a population exposed to MeHg from consumption of ocean fish, we continue to find no evidence of adverse effects.
