The relationship between blood Hg concentrations and auditory function in children and adults was investigated by Counter et al. (1998). The study sample consisted of 75 individuals (36 children and 39 adults) from a gold-mining region in Ecuador (study area) and 34 individuals (15 children and 19 adults) from a control area. Blood Hg concentrations were significantly higher in individuals from the gold-mining area than in individuals from the control region (17.5 µg/L versus 3.0 µg/L). Neuro-otological examinations were carried out on all individuals. Audiological evaluations, consisting of determinations of pure tone air-conduction thresholds in each ear at 0.25, 0.5, 1, 2, 3, 4, 6, and 8 kHz, were carried out on 40 individuals in the study area. Brainstem auditory-evoked-potential studies were carried out on 19 subjects in the study area. The absolute latencies of waves I, II, III, IV, and V and the interpeak latencies of I-III, III-V, and I-V were measured for left and right sides. Blood Hg concentration was significantly associated with hearing threshold at 3 kHz in the right ear only and for children only. A borderline association was found between blood Hg concentration and I-III interpeak transmission time on the left side. The authors concluded that although the end points assessed were generally unaffected at the blood Hg concentrations represented in the cohort of adults and children in the study area, the associations found were consistent with an effect of Hg at the level of the auditory nerve and the cochlear nuclear complex.
The results of nearly 30 years of experimental studies using various animal models have helped characterize the neurotoxic effects of MeHg following in utero or early postnatal exposures (see Table 5-11). Several excellent reviews on the topic have been published over the years (WHO 1976, Chang 1977, Inskip and Piotrowski 1985, IPCS 1990, Burbacher et al. 1990, Gilbert and Grant-Webster 1995, Clarkson 1997) including a recent “Toxicological Profile for Hg” published by the Agency for Toxic Substances and Disease Registry (ATSDR 1999). In general, experimental studies have reported a continuum of neurodevelopmental effects similar to those reported in studies of humans exposed to MeHg (see