Background

The neurotoxic effects of methylmercury have been demonstrated in both human and animal studies. Both adult and fetal brains are susceptible to the effects of methylmercury toxicity. However, the specific effects of adult exposures have been less well-documented than those of children with pre-natal exposures. This is largely because few studies of methylmercury exposures in adults that have used sensitive neurological endpoints. The present study reports on the results of neuropsychological testing and hair mercury concentrations in adults (>17 yrs) living in fishing communities of Baixada Cuiabana (Mato Grosso) in the Pantanal region of Brazil.

Methods

A cross-sectional study was conducted in six villages on the Cuiaba River. Participants included 129 men and women older than 17 years of age. They were randomly selected in proportion to the age range and number of inhabitants in each village. Questionnaire information was collected on demographic variables, including education, occupation, and residence history. Mercury exposure was determined by analysis of hair using flameless atomic absorption spectrophotometry. The neurocognitive screening battery included the Wechsler Memory Scale, Digit Symbol, Concentrated Attention Test of the Toulouse-Pierron Factorial Battery, the Manual Ability Subtests of the Tests of Mechanical Ability, and the Profile of Mood States.

Results

Mercury exposures in this population were associated with fish consumption. The hair mercury concentration in the 129 subjects ranged from 0.56 to 13.6 mcg/g; the mean concentration was 4.2 2.4 mcg/g and the median was 3.7 mcg/g. Hair mercury levels were associated with detectable alterations in performance on tests of fine motor speed and dexterity, and concentration. Some aspects of verbal learning and memory were also disrupted by mercury exposure. The magnitude of the effects increased with hair mercury concentration, consistent with a dose-dependent effect.

Conclusions

This study suggests that adults exposed to methylmercury may be at risk for deficits in neurocognitive function. The functional domains disrupted in adults, namely attention, fine-motor function and verbal memory are similar to but not identical with those previously reported in children with prenatal exposures.