Visual Field Losses in Workers Exposed to Mercury (Hg) Vapor

Abstract

Purpose:: Visual fields losses associated with Hg intoxication have only been assessed in exposure to methyl mercury. Here we evaluate visual field sensitivity in a group of former factory workers with mercury intoxication due to occupational exposure to mercury vapor .

Methods:: We tested both eyes of 35 subjects (30 males) previously exposed to mercury in their work environment (mean age = 44.3 ±5.7 yrs; mean exposure duration = 10.1 ±4.9 yrs; mean years away from exposure = 7.7 ±4.6 yrs). All subjects underwent a complete ophthalmological exam; inclusion criteria were normal fundus and Snellen VA 20/25, or better. They presented, in average, 41.1 ±1.7 µg Hg/g urinary creatinine during the period of exposure and 1.4 ±0.9 µg Hg/g urinary creatinine at the time of the examination. Visual fields were analyzed with the Humphrey Field Analyzer II (model 750i) in two tests: standard automated perimetry (SAP, white-on-white) with the Sita Standard 30-2 strategy and short wavelength automated perimetry (SWAP, blue-on-yellow) with the full threshold central 30-2 strategy. Results were analyzed as the mean sensitivity at the central 5º and at eccentricities corresponding to each of five successive concentric 5º rings. The order of the tests and the first eye tested were randomized. The tests were performed in an otherwise dark room, with fixation continuously monitored by the experimenter.

Results:: We found reduction of visual sensitivity in both visual field tests. Compared to controls, visual field sensitivities of mercury exposed subjects were lower for all eccentricity, for both SAP (p<.05) and SWAP (p<.05). The average mean deviation revealed greater sensitivity losses in the SWAP (-5.32 ±4.3 dB for Hg exposed and -1.85 ±2.05 dB for controls) than in the SAP (-3.25 ±2.89 dB for Hg exposed and -1.12 ±1.43 dB for controls). There was no correlation between the results of the visual field tests and either the Hg urinary levels or visual acuity.

Conclusions:: The present results show significant reduction of the central visual sensitivity in mercury vapor intoxication. In addition, the greater loss in SWAP (blue on yellow) compared to SAP (white on white) indicates selectivity of the impairment - a finding that might contribute to understand its mechanisms. Previous studies used the Goldman perimeter to examine subjects with Hg intoxication and reported visual field constriction, but no information on visual field losses due to mercury vapor exposure was available.

Keywords: visual impairment: neuro-ophthalmological disease • visual fields • perimetry