Olfactory dysfunction in Parkinson's disease.

The olfactory system is one of the nonmotor systems severely affected in Parkinson's disease (PD). Olfactory dysfunction occurs early in the disease process, is independent of disease stage, duration, and treatment. However, olfactory dysfunction appears to be dependent on disease subtype. Olfaction is mildly impaired or preserved in most of the parkinsonism-plus syndromes (PPS). This provides a means of differential diagnosis between typical PD and PPS. Olfactory function is impaired also in familial forms of parkinsonism in which the genetic defect is known. In familial parkinsonism, olfactory function is impaired in both typical PD and PPS phenotypes. Olfactory dysfunction does not appear to be a manifestation of dopamine deficiency. Olfactory dysfunction is also associated with other neurodegenerative diseases such as Alzheimer's disease (AD), Huntington's disease (HD), as well as with normal aging. The neuropathological changes observed in the olfactory system in PD and other neurodegenerative diseases appear to be disease-specific, raising the possibility that olfactory dysfunction may be the result of a central rather than a peripheral process. The cellular and molecular mechanisms underlying olfactory dysfunction in PD and other neurodegenerative diseases remain unknown.