Olfactory dysfunction in patients with narcolepsy with and without REM sleep behaviour disorder

Patients with idiopathic rapid eye movement sleep behaviour disorder (RBD) frequently develop Parkinson's disease and the majority present with hyposmia, which is a potential preclinical non-motor sign of Parkinson's disease. Accordingly, it has been proposed that the clinical symptoms of hyposmia and RBD in combination have to be considered as very early symptoms of Parkinson's disease. Since not only patients with idiopathic RBD but also patients in whom RBD is associated with narcolepsy present with an olfactory dysfunction we investigated if hyposmia in RBD patients with concomitant narcolepsy is RBD specific or if narcolepsy per se is associated with olfactory dysfunction. We studied olfactory function in 20 narcoleptic patients each with RBD (9 male and 11 female; mean age 45.4 +/- 14.0 years, range 20-75 years) and without associated RBD (8 male and 12 female; mean age 44.4 +/- 13.40 years, range 20-70 years) and 40 age- and gender-matched healthy control subjects using standardized 'Sniffin' Sticks'. Both, narcoleptics with (Narc/+RBD) and without RBD (Narc/-RBD) had a significantly higher olfactory threshold (Narc/+RBD, P = 0.0001; Narc/-RBD, P = 0.0001), lower discrimination scores (P = 0.001; P = 0.014) and lower identification scores (P = 0.057; P = 0.003) than controls. There were no symptoms or signs for early parkinsonism in both patient groups. Our results show for the first time that narcolepsy per se is associated with olfactory dysfunction. In contrast to patients with idiopathic RBD, hyposmia in patients with RBD associated with narcolepsy is unlikely to be a predictor for developing parkinsonism.