Persistence of mercury-induced motor and sensory neurotoxicity: systematic review of workers previously exposed to mercury vapor

Abstract

Elemental mercury (Hg⁰) is a well-recognized neurotoxicant, but it is uncertain whether and for how long its neurotoxicity persists. Among studies that evaluated previously-exposed workers, only one examined workers during and also years after exposure had ceased. The objective of this review is to create a series of ‘synthetic’ longitudinal studies to address the question of persistence of Hg⁰ neurotoxicity in occupationally exposed workers. We systematically reviewed studies describing objective motor and sensory effects in previously-exposed mercury workers. Data from physical examination (PE), neurobehavioral (NB) tests, and electrophysiological studies (EPS) were extracted into structured tables and examined for their consistency and dose-relatedness and then compared with the corresponding results from studies of currently exposed workers. We identified six cohorts that described neurological findings in 1299 workers, examined an average of 4.8–30 years after the cessation of exposure. Historical group mean U_Hg levels ranged from 23 to >500 μg/L, with U_Hg levels >6000 μg/L in some individuals. Overall, few findings were significant; most were inconsistent across the previous-exposure studies, and in comparisons between studies of previously and currently exposed workers. The results of this systematic review indicate that Hg⁰-related neurotoxic effects detectable on PE, NB testing, and EPS are substantially reversed over time. To the extent that such effects do persist, they are reported principally in workers who have had very high-dose exposures. In addition, based on the limited available data, those effects reported to persist have been described as having little or no functional significance.

Keywords:

• Occupational disease
• elemental mercury
• persistence
• neurotoxicity
• tremor
• physical examination
• neurobehavioral function
• motor function
• electrophysiological studies

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Acknowledgements

The authors wish to thank Ms. J. Rivera for bibliographic excellence. The authors are very grateful to the four anonymous reviewers selected by the Editor for the value of their comments in improving the manuscript.

Declaration of interest

Jonathan Borak and Cheryl Fields are employees of Jonathan Borak & Company, which provides consulting services on environmental and occupational health issues (including concerns related to mercury exposure) to government and private clients. Both Ms. Fields and Dr. Borak are part time employees of Yale University. Elan Louis is a full-time employee of Yale University. The authors have not appeared in the last five years in any legal or regulatory proceedings related to the contents of this paper and have no plans to participate in such proceedings in the future. Dr. Borak and Ms. Fields contributed to the research reported in this paper during their normal course of employment without supplementary funding or external support. Dr. Louis was supported by grant R01 NS094607 from NINDS.

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