Manganese and Parkinson's disease: a critical review and new findings

Abstract

Background: Excess accumulation of manganese (Mn) in the brain results in a neurological syndrome with cognitive, psychiatric, and movement abnormalities. The highest concentrations of Mn in the brain are achieved in the basal ganglia, which may precipitate a form of parkinsonism with some clinical features that are similar and some that are different to those in Parkinson's disease (PD). Recently, scientists have debated the possibility that Mn may have an etiological role in PD or that it may accelerate the expression of PD.

Objective: The goal of this review was to examine whether chronic Mn exposure produces dopamine neuron degeneration and PD or whether it has a distinct neuropathology and clinical presentation.

Data source: I reviewed available clinical, neuroimaging, and neuropathological studies in humans and nonhuman primates exposed to Mn or other human conditions that result in elevated brain Mn concentrations.

Data extraction: Human and nonhuman primate literature was examined to compare clinical, neuroimaging, and neuropathological changes associated with Mn-induced parkinsonism.

Data synthesis: Clinical, neuroimaging, and neuropathological evidence was used to examine whether Mn-induced parkinsonism involves degeneration of the nigrostriatal dopaminergic system as is the case in PD.

Conclusions: The overwhelming evidence shows that Mn-induced parkinsonism does not involve degeneration of midbrain dopamine neurons and that l-dopa is not an effective therapy. New evidence is presented on a putative mechanism by which Mn may produce movement abnormalities. Confirmation of this hypothesis in humans is essential to make rational decisions about treatment, devise effective therapeutic strategies, and set regulatory guidelines.

Figure 1

(A) Schematic depicting the different brain structures comprising the basal ganglia (labeled in the right hemisphere). The left hemisphere shows the nigrostriatal dopamine fibers whose cell bodies are located in the substantia nigra (SN) and innervate the caudate and putamen. These are the axonal projections that degenerate in Parksinson’s disease. T1-weighted MRI at the level of the globus pallidus of a control nonhuman primate brain (B; boxed areas) and a nonhuman primate brain exposed to Mn (C). Note the increase in signal intensity (white areas) in the Mn-exposed animal (C) relative to the control animal (B). SNT, subthalamic nucleus.