Manganese in the workplace: Different exposures, different effects?

Introduction:  Neurotoxic effects of excess exposure to airborne Mn were first described in workers in 1837.  Because of the Parkinson-like features of manganism, clinical diagnosis and research have focused almost exclusively on motor deficits resulting from changes in the basal ganglia functions.  

Methods: A scoping review of studies of manganese-exposed workers and relevant animal studies was performed.  

Results: Studies on early neurobehavioral manifestations of occupational exposure to Mn dust have primarily examined motor functions and results from cognitive testing were inconsistent. Motor dysfunction is a diagnostic criteria for workers exposed to manganese. However, recent studies of welders exposed to manganese fumes have shown dose-related cognitive deficits. Non-human primate studies have shown cognitive deficits and corresponding changes in neurotransmitters, in the absence of motor deficits. Imaging studies show that changes in the brain are not limited to the globus pallidus, but are more widespread.  

Discussion: These studies raise new questions regarding Mn neurotoxicity with respect to cognitive processes. Currently, symptoms of motor dysfunction are considered necessary for the diagnosis of manganese toxicity and for compensation claims.  The findings of this review of the literature on manganese exposure in workers suggests that, although there is overlap, there may be differences in the effects of manganese exposure from manganese dust and manganese fumes. This may reflect entry routes to the brain and co-exposures. We consider whether limiting diagnostic criteria to motor disorders adequately reflects neural damage and functional handicaps resulting from excess occupational exposure to manganese.