Cognitive deficits at age 22 years associated with prenatal exposure to methylmercury

Abstract

Prenatal exposure to mercury has been associated with adverse effects on child neurodevelopment. The present study aims to determine the extent to which methylmercury-associated cognitive deficits persist into adult age. In a Faroese birth cohort originally formed in 1986-1987 (N = 1,022), prenatal methylmercury exposure was assessed in terms of the mercury concentration in cord blood and maternal hair. Clinical examinations of 847 cohort members at age 22 years were carried out in 2008-2009 using a panel of neuropsychological tests that reflected major functional domains. Subjects with neurological and psychiatric diagnoses were excluded from the data analysis, thus leaving 814 subjects. Multiple regression analysis included covariates previously identified for adjustment. Deficits in Boston Naming Test (BNT) and other tests of verbal performance were significantly associated with the cord-blood mercury concentration. Deficits were also present in all other tests applied, although most were not statistically significant. Structural equation models were developed to ascertain the possible differences in vulnerability of specific functional domains and the overall association with general intelligence. In models for individual domains, all of them showed negative associations, with crystallized intelligence being highly significant. A hierarchical model for general intelligence based on all domains again showed a highly significant negative association with the exposure, with an approximate deficit that corresponds to about 2.2 IQ points at a 10-fold increased prenatal methylmercury exposure. Thus, although the cognitive deficits observed were smaller than at examinations at younger ages, maternal diets with contaminated seafood were associated with adverse effects in this birth cohort at age 22 years. The deficits affected major domains of brain functions as well as general intelligence. Thus, prenatal exposure to this marine contaminant appears to cause permanent adverse effects on cognition.

Keywords: Environmental exposure; Methylmercury compounds; Neuropsychological measures; Prenatal exposure delayed effects; Structural equation modeling.

Fig. 1

A structural equation model showing the standardized negative effect of a latent variable for prenatal exposure to methylmercury on a second-order latent variable for general mental ability in a measurement model with two first-order factors, and with the manifest test variables corrected for a set of covariates. LogWhale = Log10(Maternal Whale Dinners +1); LogHgB = Log10(Hg in Cord Blood + 1); LogHgH = Log10(Hg in Mother Hair + 1); Hg* = Latent Hg-variable; g = Latent variable for general mental ability; Gf = Latent variable for fluid reasoning; Gc = Latent variable for verbal comprehension. Coefficients are standardized values. Double headed arrow indicates correlation of residuals. Numbers at arrows are residual variances. For simplicity, covariates are only shown schematically with no values or intercorrelations. Covariates are: Sex, Maternal fish dinners during pregnancy, Maternal Raven, Mother employed (age 14), Father employed (age 14), Age at examination, Tested in language, School grade (age 14), Lead exposure, and PCB exposure.

Fig. 2

A Structural Equation Model (SEM) showing the standardized negative effect of a latent variable for prenatal exposure to methylmercury on a second-order latent variable for general mental ability in a measurement model with seven first-order factors, and with the manifest test variables corrected for a set of covariates. Parameter names are as in Fig. 1. For Gsm, Glr, Gs, Gt, and Gps, see Table 3 footnote. Coefficients are standardized values. Double headed arrows indicate correlation of residuals. Numbers at arrows are residual variances. As in Figure 1, residual variances for manifest variables, and covariates, are not shown. Covariates are: Sex, Maternal fish dinners during pregnancy, Maternal Raven, Mother employed (age 14), Father employed (age 14), Age at examination, Tested in language, School grade (age 14), Lead exposure, and PCB exposure.