The inter-night variability of REM sleep without atonia in adult patients with central disorders of hypersomnolence

Study objectives: Rapid eye movement (REM) sleep without atonia (RWA), the neurophysiological hallmark of REM sleep behavior disorder, is a typical feature of nocturnal sleep in patients with narcolepsy type 1 (NT1). Its reproducibility across different nocturnal polysomnographies (PSG) has never been investigated in patients with CDH. We assessed the inter-night variability of RWA. Methods: Two-hundred-fifty adults (45 % males) hospitalized for suspected CDH, including 92 NT1, 29 narcolepsy type 2 (NT2), 36 idiopathic hypersomnia (IH), and 93 subjective hypersomnolence (sHS), undergoing two consecutive night-PSGs, multiple sleep latency test (MSLT) and cerebrospinal fluid hypocretin-1 (CSF-HCRT1) measurement, were recruited. The inter-night variability of REM atonia index (RAI) was assessed with Wilkoxon's test, McNemar's test, Bland-Altman plots, and Cohen's kappa. Results: Compared with the other groups, NT1 patients presented lower RAI both as absolute values and frequency in the pathological range (RAI≤0.8). RAI slightly decreased from night1 to night2 only in NT1 (difference = 0.03, 95 %CI = 0.01–0.05), whereas it did not vary in other groups. In NT1, the frequency of pathological RAI≤0.8 was not different across the nights and showed good agreement (32.5 %vs.42.5 %; p = 0.09, kappa = 0.7). In NT1, after adjustment, RAI≤0.8 was independently associated with older age, lower MSLT sleep latency, more MSLT-sleep onset REM periods, higher wake after sleep onset, non-REM stage-1, and less non-REM stage-3. Conclusions: In CDHs, RWA has low inter-night variability, which makes it a valuable marker in clinical practice and research activity. In adult NT1, RWA is associated with older age and with features suggestive of a poorer sleep quality and higher daytime sleep propensity.