Odor Identification Test in Idiopathic REM-Behavior Disorder and Parkinson's Disease in China

Background Olfactory dysfunction is common in Parkinson's disease (PD) and idiopathic rapid eye movement sleep behavior disorder (iRBD), which is a risk factor in the development of PD. However, a few studies have conflicting results when comparing dysosmia in the patients with iRBD and PD. There is no study investigating the olfactory function in Chinese patients with iRBD. Additionally, the Sniffin' Sticks screening 12 test (SS-12) contains several odors that are not familiar to people in different cultures. Methods Odor identification was evaluated in iRBD patients (n = 54), PD patients (n = 54) and healthy controls (n = 54). With the identification data, a brief odor identification test was established and then validated in other subjects. Results Odor identification scores in iRBD patients were significantly higher than those in PD patients (P< 0.001) but lower than those in controls (P< 0.001). At the cut-off value of 7.5, the Sniffin' Sticks clearly differentiated iRBD and PD patients from the controls, and the brief test could increase the specificity in diagnosing PD. Neither the Sniffin' Sticks nor the brief test could clearly differentiate PD and iRBD patients from each other. Conclusions Olfaction is more impaired in PD patients than in iRBD patients, possibly due to the heterogeneity of iRBD patients. The Sniffin' Sticks could be a useful tool for differentiating iRBD patients from the healthy population, and it could be useful for screening people at high-risk of PD in China, especially when combined with polysomnography. To reduce the expense and time required for the Sniffin' Sticks test, this study shows that a brief test is feasible.