Impact of prenatal methylmercury exposure on neurobehavioral function at age 14 years

Abstract

A cohort of 1022 consecutive singleton births was generated during 1987-1988 in the Faroe Islands, where increased methylmercury exposure occurs from traditional seafood diets that include pilot whale meat. The prenatal exposure level was determined from mercury analyses of cord blood, cord tissue, and maternal hair. At age 14 years, 878 of 1010 living cohort members underwent detailed neurobehavioral examination. Eighteen participants with neurological disorders were excluded. Blood and hair samples obtained from the participants were analyzed for mercury. The neuropsychological test battery was designed based on the same criteria as applied at the examination at age 7 years. Multiple regression analysis was carried out and included adjustment for confounders. Indicators of prenatal methylmercury exposure were significantly associated with deficits in finger tapping speed, reaction time on a continued performance task, and cued naming. Postnatal methylmercury exposure had no discernible effect. These findings are similar to those obtained at age 7 years, and the relative contribution of mercury exposure to the predictive power of the multiple regression models was also similar. An analysis of the test score difference between results at 7 and 14 years suggested that mercury-associated deficits had not changed between the two examinations. In structural equation model analyses, the neuropsychological tests were separated into five groups; methylmercury exposure was significantly associated with deficits in motor, attention, and verbal tests. These findings are supported by independent assessment of neurophysiological outcomes. The effects on brain function associated with prenatal methylmercury exposure therefore appear to be multi-focal and permanent.

Fig. 1

Path diagram of the structural equation model that was used to determine the overall effect of prenatal methylmercury exposure on verbal functions, as expressed by a series of verbally mediated neuropsychological test results, and adjusted for confounders (H-Hg = maternal hair-mercury concentration; B-Hg = cord-blood mercury concentration; Whale = frequency of whale meat dinners during pregnancy; BNT = Boston Naming Test without and with cues; SIMILAR = WISC-R Similarities; and CVLT = California Verbal Learning Test scores for learning, short-term recall, long-term recall, and recognition).

Fig. 2

Association between prenatal methylmercury exposure and the average reaction time (ms) on the NES2 Continuous Performance Test administered to 859 participants from a Faroese birth cohort at age 14 years. Each line at the bottom represents one observation at the exposure level indicated.

Fig. 3

Association between prenatal methylmercury exposure and the total correct score on the Boston Naming Test after cues administered to 859 participants from a Faroese birth cohort at age 14 years. Each line at the bottom represents one observation at the exposure level indicated.