Neurodevelopmental aspects of methylmercury exposure: Neuropsychological consequences and cultural issues

The teratogenic potential of mercury is well established and prenatal exposures can result in significant neurodevelopmental abnormalities. These can include cortical and cerebellar lesions, neuronal malfunctioning, and reduced brain size. Although neuropsychological consequences of methylmercury poisoning in adults are limited to deficits in visuoconstruction, memory and motor function, methylmercury poisoning in utero can show more diffuse cognitive sequelae. Previous studies assessing safe exposure levels of prenatal methylmercury exposures used gross neurological examinations in very young children. However, learning deficits or developmental delays in specific cognitive domains caused by early brain insults may become apparent only when the child reaches primary school age and has developed sufficiently to perform tasks that tap functioning in specific cognitive domains. The goal of the current study was to investigate the effects of in utero methylmercury exposures among primary school-age children from the Brazilian Amazon. Assessment of the specific cognitive domains of visuoconstruction and visual memory (domains known to be affected in methylmercury exposures) were assessed in relation to methylmercury exposure and cultural-saliency of test items.; The study included 258 Brazilian Amazonian children (7–12 years old) representing three communities ranging from low to very high exposure levels of methylmercury. Study results indicate a dose-response relationship between degree of methylmercury exposure and scores on tests assessing visuoconstructional skills and visual memory. The use of a novel scoring scheme adjusting for possible cultural differences slightly improved discrimination of methylmercury effects among at-risk methylmercury-exposed and relatively unexposed Amazonian children. A comparison between qualitative scoring and standard scoring of the Stanford-Binet copying task indicated that both were sensitive to subtle methylmercury induced declines in visuospatial and visual memory domains. Culturally-salient test items were also assessed in this unique group and was found to correspond to American children's pattern of performance. The current results suggest that low-level methylmercury exposures affect the cognitive domains of visuoconstruction and visual memory in a dose-dependent manner. Additionally, results of the current study suggest that the Stanford-Binet Copying task may be a useful task for assessing neurotoxicant exposures in diverse cultural groups.