REM sleep absence in patients referred to polysomnography for REM sleep behavior disorder

Detection of REM sleep behavior disorder (RBD) with polysomnography (PSG) is relevant for the diagnosis of alpha-synucleinopathies. However, some patients referred for suspicion of RBD do not present REM sleep at PSG (NoREMSusRBD), rendering the study inconclusive. Our objective was to investigate disorders possibility associated with REM sleep absence in patients referred to PSG for investigation of RBD, in particular alpha-synucleinopathies. A sleep-lab database was revised to select NoREMSusRBD (n = 15) and patients: with no REM sleep referred for suspicion of other sleep disorder (NoREMSusOther, n = 28); referred for RBD suspicion with negative PSG (NegativeRBD, n = 24); alpha-synucleinopathies with no REM sleep (NoREM alpha, n = 23) and idiopathic RBD (iRBD, n = 26). NoREMSusRBD patients were compared with the other groups regarding PSG data and the emergence of prodromal features or established criteria for alpha-synucleinopathy. Severe Obstructive Sleep Apnea (OSA) was significantly more frequent in the NoREMsusRBD compared to the NoREM alpha and iRBD groups. No patient in the NoREMSusRBD developed a alpha-synucleinopathy (2 cases on the iRBD group). The prevalence of prodromal features in NoREMSusRBD (n = 7, 46.7%) was similar to that of iRBD (n = 18, 69.2%) and significantly higher than in the other groups. Apnea-Hypopnea Indices (AHI) were significantly higher in the NoREMSusRBD compared with iRBD and NoREM alpha. Our study suggests that the absence of REM sleep in NoREMSusRBD could be caused by OSA but does not exclude the possibility of underlying alpha- synucleinopathy, suggested by an increased prevalence of prodromal features. These data support the need for excluding OSA in patients suspected for RBD and recommends follow-up of NoREMSusRBD patients to uncover a possible alpha- synucleinopathy.