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Toxicological Effects of Methylmercury
Sensory, Neurophysiological, and Other End Points in Children
In the Faroe Islands cohort, the 7-year evaluation included, in addition to the neuropsychological tests, assessments of visual acuity, near contrast sensitivity, otoscopy and tympanometry, and neurophysiological tests (pattern-reversal-visual-evoked potentials at 30' and 15', brainstem auditory-evoked potentials at 20 and 40 clicks per second (Hz), and postural sway) (Grandjean et al. 1997). Visual acuity, contrast sensitivity, auditory thresholds, and visual-evoked potentials were not significantly associated with prenatal MeHg exposures. For brainstem auditory-evoked potential, peaks I, III, and V were slightly delayed at increased cord-blood Hg concentrations at both 20 Hz and 40 Hz (p values, 0.01 to 0.10), although interpeak latencies were not associated with Hg at either frequency. In additional analyses reported separately (Murata et al. 1999b), in which data collected during the second year of this phase of the study were excluded due to concerns about the electromyograph used, higher maternal hair and cord-blood Hg concentrations were associated with lower peak III latencies, as well as with longer peak I-III latencies. Of the four conditions under which postural sway was assessed, only when subjects stood on the platform without foam under it with their eyes closed did the results approach significance (p = 0.09). Visual acuity and contrast sensitivity were not related to Hg exposure.
In a cross-sectional study of 149 6- to 7-year-old children living in a fishing village on Madiera, many of the same neurophysiological tests were administered (Murata et al. 1999a). Because patterns of fish consumption were considered to be stable, current maternal-hair Hg concentration was used as a measurement of a child's prenatal Hg exposure (mean, 9.6 ppm; range, 1.1-54.4 ppm). With respect to brainstem auditory evoked potential, maternal-hair Hg was significantly associated with I-III and I-V interpeak latencies at both 20 and 40 Hz, as well as with total latencies for peaks III and V at both frequencies. Those results are similar to the findings in the Faroe Islands cohort, at least among the children who were tested in the first year (see above). With respect to visual-evoked potentials on a pattern-reversal task, maternal-hair Hg concentration was significantly associated with one of the three latencies measured (N145 at 15' ), as well as with the N75-N145 and P100-N145 latencies (15' only). As noted above, VEP latencies were unrelated to Hg concentrations in the Faroe Islands cohort.