Prenatal Methylmercury Exposure and Developmental Outcomes: Review of the Evidence and Discussion of Future Directions

Abstract

I conducted a review of the published literature to assess the strength of the evidence for an association between prenatal exposure to methylmercury (MeHg) and subsequent child development. I identified 12 studies on this subject published since 1980. Of these, 3 were longitudinal studies—2 conducted in the Seychelle Islands, and 1 in the Faroe Islands. Nine were cross-sectional studies conducted in different countries where seafood, a source of MeHg, constituted a major part of the diet. The ages of the children studied ranged from 2 weeks to 12 years. The results of the longitudinal studies were contradictory. Researchers in the Faroe Islands identified an association between MeHg exposure and developmental effects, whereas those in the Seychelle Islands identified no such association. This inconsistency was mirrored in the results of the cross-sectional studies where there were some positive and some negative findings. It was concluded that it was not possible from currently available data to determine whether there is an association between prenatal MeHg exposure and adverse developmental effects in children. In advance of future research, consideration should be given to resolving the uncertainties surrounding exposure assessment and outcome measurement, as both elements varied between studies. It was suggested that questions of exposure assessment would benefit from the application of an expert review process. Outcome assessment would benefit from the development of theoretically based measures of specific aspects of cognitive functioning to replace the relatively crude measures of attainment and IQ currently employed in most studies. This would assist in the development of classic longitudinal studies by allowing repeated assessment over the full age range and providing data that are more readily interpretable and comparable between studies.

Correlation between blood mercury levels in mothers and newborns in Itaituba, Pará State, Brazil

“…The various forms of organic Hg are particularly neurotoxic. Both prenatal and postnatal exposure to MeHg can adversely affect the central nervous system, but the compound appears to be most neurotoxic during the prenatal period, when the brain is developing rapidly 31,32 . We should also assume that these blood levels only provide accurate information on recent exposure, and that cord blood cannot be expected to reflect exposure earlier in pregnancy.…”

Early developmental effects of separate or combined perinatal exposure to methylmercury (MeHg) and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) in the rat

“…This possibility is supported by data from studies of the victims of the mass MeHg poisonings in Japan [4] and Iraq [14,15]. On the other hand, however, epidemiological studies of children from populations of high fish consumption appear to be inconclusive or contradictory [16]. Particular attention is being paid to the studies performed on two large cohorts of children, the Faroe Island and the Seychelles cohorts.…”

Methylmercury elicits rapid inhibition of cell proliferation in the developing brain and decreases cell cycle regulator, cyclin E

“…Considering the distinct sensitivity of the fetus, recent epidemiological studies have investigated the possible developmental effects of early exposure to dietary levels of MeHg (Myers et al, 2003;Grandjean et al, 1997;Davidson et al, 1998). While these studies have produced ambiguous, often contradictory, results (Spurgeon 2006), animal studies reveal behavioral deficits at MeHg exposures that do not produce histologic changes (Sakamoto et al, 2004), implying that subtle MeHg effects may occur at subtoxic doses.…”