Olfactory dysfunction in Parkinson disease and REM sleep behavior disorder

Olfactory dysfunction is an appealing biomarker for Parkinson disease (PD) because of the high prevalence (>90%) among PD patients and the relative ease of testing, as compared to other putative biomarkers such as neuroimaging of the dopamine system. Hyposmia develops during the early stages of PD and, therefore, may be one of the most sensitive markers for the early diagnosis of PD. Sniffin' Sticks, the University of Pennsylvania Smell Identification Test, and the Odor Stick Identification test for the Japanese, a short and simple nonlexical, olfactory identification test, can be useful for diagnosing PD and for discriminating between PD and other parkinsonian syndromes. REM sleep behavior disorder (RBD) is indicative of the development of neurodegenerative diseases involving synuclein pathology, including PD, multiple system atrophy, and dementia with Lewy bodies (DLB). Reduced odor identification was comparable in patients with PD, DLB, and RBD, but distinct from that in healthy subjects. Further, insights obtained from a deeper understanding of the pathophysiology of olfactory dysfunction in PD and RBD may lead to a deeper understanding of the mechanism underlying Lewy neurodegenerative diseases.