Cognitive decline in association with hyposmia in idiopathic rapid eye movement sleep behavior disorder: a prospective 2-year follow-up study

Background and purpose The aim was to analyze the characteristics and progression of cognitive dysfunction in non-demented idiopathic rapid eye movement sleep behavior disorder (iRBD) patients with baseline olfactory function. Methods From a prospective polysomnography-confirmed iRBD cohort, 25 patients (16 patients in 2-year follow-up) and 13 normal controls were included. Initial and 2-year follow-up cognitive functions were analyzed with olfactory function and F-18-fluorinated-N-3-fluoropropyl-2 beta-carboxymethoxy-3 beta-(4-iodophenyl)-nortropane (F-18-FP-CIT) uptake in deep nuclei initially. Results Idiopathic RBD patients had impaired attention, memory and executive function compared to controls. Baseline cognitive tests were comparable between the iRBD subgroups with and without hyposmia. F-18-FP-CIT uptake tended to be lower in the hyposmic group than in the normosmic group. The olfactory test score was positively correlated with amygdala uptake in iRBD patients (P = 0.027). After 2 years, visuospatial and verbal memory dysfunction worsened more in hyposmics than in normosmics. Lower initial olfactory test score was associated with more severe declines in verbal memory function. Conclusions Hyposmia may be a predictive sign of cognitive decline in iRBD patients.