ABSTRACT
The olfactory system is a complex network whose organization and function depends on both peripheral and central input. It is commonly affected in Parkinson’s disease (PD), parkinsonism-plus syndromes (PPS), other neurodegenerative disorders [e.g., Alzheimer’s disease, (AD)], and in normal aging. Olfactory dysfunction usually appears early in the disease process. In PD, olfactory function is commonly impaired whereas, in PPS, olfactory function is only mildly impaired or preserved. Olfactory function is also impaired in familial forms of parkinsonism associated with a monogenic defect. In contrast to individuals with sporadic PPS, affected members of PPS kindreds do show olfactory impairment. Interestingly, olfactory dysfunction does not appear to be due to a dopamine deficiency. The neuropathological changes in the olfactory system appear to be disease specific. This suggests that olfactory dysfunction in neurodegenerative disorders may reflect a central rather than a peripheral process. The organization of the normal olfactory system is gradually being elucidated at the molecular, cellular, and system levels. The mechanisms underlying olfactory dysfunction in PD and other neurodegenerative diseases remain unknown.
