Although studies suggest that PD may be largely an environmentally mediated disease, the clinical characteristics are unusual for a toxic process—most notably the prominent asymmetry. Several toxins have been implicated in outbreaks of parkinsonism, including carbon disulfide (33), manganese (34,35), 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (36), paraquat (37), and solvent abuse (38). In general, these cases appear to be much more symmetric than sporadic and genetic PD, as would be expected from a toxic etiology (39). It is possible that the type of exposures, including chronicity, mode of entry, and genetic factors, influence neurodegeneration but it remains unclear why environmentally mediated PD would be so strikingly asymmetric.
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