Olfactory dysfunction in idiopathic REM sleep behavior disorder

Olfactory dysfunction is frequently observed in patients with idiopathic REM sleep behavior disorder (iRBD) such as Parkinson’s disease or dementia with Lewy bodies. Olfactory function tests using Sniffin’ Sticks and Odor Stick Identification Test for Japanese (OSIT-J) were performed in 73 consecutive middle-aged (range, 50–69 years) patients with iRBD, 33 consecutive older-aged (71–82 years) patients with iRBD, and 28 control subjects (55–70 years). Odor identification was more frequently impaired than odor threshold or discrimination among the iRBD group and allowed better discrimination between the middle-aged iRBD group and age-adjusted control subjects. The area under the curve for threshold, discrimination, identification, TDI score and OSIT-J score determined from receiver operating characteristic curves were 0.831 (0.753–0.909), 0.761 (0.666–0.855), 0.938 (0.894–0.982), 0.939 (0.897–0.981), and 0.965 (0.931–0.999), respectively. Discrimination and identification scores were significantly lower in the older-aged iRBD group than in the middle-aged iRBD group. A significant correlation was observed between the identification score on Sniffin’ Sticks and OSIT-J score (r = 0.5910, P < 0.0001, n = 106, Spearman’s rank). Anosmia/hyposmia may be a feature of iRBD. Olfactory dysfunction in iRBD is a consistent, widespread central nervous abnormality of different olfactory modalities with different cognitive complexity.