Visual Pathway Alterations after Severe Acute Occupational Elemental Mercury Poisoning: Report of a Series of 29 Cases

Abstract

BACKGROUND: There are almost no available data on retinal involvement after acute exposure to high concentrations mercury and the available reports are on a small number of patients that suffered chronic expposure. The purpose of this paper is to report visual findings in 29 workers exposed to very high concentrations of mercury vapor in a factory in northern Spain in 2012.METHODS: Twenty-nine patients and 16 controls were evaluated in a comparative case series. Fifteen of the 29 workers underwent belated chelation for heavy metal intoxication, only 3 in a prompt way. The mercury levels in blood and urine samples, visual acuity (VA), contrast sensitivity (CS), visual field (VF), color discrimination and optical coherence tomography (OCT) data were recorded. The pattern reversal visual-evoked potentials (PRVEP), full-field and multifocal electroretinography (ffERG/mfERG), pattern electroretinography (PERG), systemic symptoms, presence of erethism, and electromyography (EMG) were also gathered. A descriptive analysis was performed. The evolution of patients who did not undergo chelation (group 1), those who underwent late chelation (group 2), and those with deep VF defects (group 3) were compared with a control group. The correlations between variables also were studied.RESULTS: The VA was affected slightly. The loss of CS in at least one of four spatial frequencies and color vision alterations occurred in 96.5% (n=28) and 44.8% (n=13), respectively, in the entire group. VF alterations were identified in 72.4% (n=21). No morphologic changes were seen in the OCT scans. Latencies over 100 milliseconds and reduced amplitudes of P100 were found in the PRVEP. The ffERG and PERG results suggested that both the outer and inner retinal processes were involved. The mfERG indicated reduced parafoveal retinal function. Twenty-six workers exhibited symptoms of erethism. The EMG showed sensorimotor polyneuropathy and multiple mononeuropathy alterations. Significant negative correlations among blood mercury levels, VA, and ffERG were observed.CONCLUSION: Advanced visual functions were impaired significantly independent of the mercury levels. Delayed chelation was not beneficial. Although neurologic and visual pathway involvement was clearly demonstrated, this study also showed clear functional retinal participation in impaired vision.