Neuromelanin in parkinsonian disorders: an update.

Neuromelanin (NM) is a dark pigment that accumulates linearly with aging in substantia nigra (SN) and locus coeruleus (LC). The dual protective and toxic role of NM has been hypothesized according to its intraneuronal or extraneuronal deposition. The melanized dopaminergic neurons in SN and LC seem to have special vulnerability to neurodegeneration in Parkinson's disease (PD). The paramagnetic properties of NM due to its association to metals like iron induce T1 prolongation; hence the measurement of SN-sensitive contrast could be a useful diagnostic biomarker in neurodegenerative disease like PD and other atypical parkinsonisms. This paper will review NM histopathology and neurochemistry studies in health and diseases and the role of imaging targeting NM load in parkinsonian disorders.