Chapter 9. Olfactory function in Parkinson’s disease

Olfactory dysfunction is an early ‘pre-clinical’ sign of Parkinson’s disease (PD). In this chapter I briefly review what is known about such dysfunction in PD and related diseases. In general, the dysfunction is robust and bilateral, anosmia is not the norm, women exhibit less loss than men, and some monogenetic forms of the disease exhibit the dysfunction. The loss is unrelated to disease stage or magnitude of the motor symptoms, save perhaps in the earliest stages of the disorder. Lewy body pathology, a hallmark of PD, appears to be correlated with the olfactory dysfunction, although it is not known whether this reflects a causal relationship. Dopamine is over-expressed in the olfactory bulbs of PD patients. Interestingly, deficiencies in other neurotransmitters, most notably acetylcholine, are correlated with the olfactory loss. A number of diseases often misdiagnosed as PD have little or no smell loss, including essential tremor, vascular parkinsonism, and progressive supranuclear palsy, making olfactory testing of value in differential diagnosis. The limited data suggest that the physiological basis of PD-related olfactory dysfunction is likely multifactorial, representing a combination of environmentally and genetically determined pathologies that involve multiple transmitter systems.