Polysomnographic phenotype of isolated REM sleep without atonia

Isolated REM sleep without atonia (iRSWA) is regarded as a prodromal phase of REM sleep behavior disorder and synucleinopathies. In iRSWA patients, we investigated the polysomnographic characteristics that are known to be altered in (prodromal) Parkinson's disease (PD): periodic limb movements of sleep [PLMS] (increased), REM density (reduced), and heart rate variability ([HRV] (reduced).

We compared video-polysomnographic studies of 49 iRSWA subjects with 41 controls. RSWA and PLMS were scored visually. REM density (REM/hour) and HRV were calculated automatically.

We found a higher median total (15.90 vs 7.20; p = 0.001), REM (21.80 vs 11.0; p < 0.001) and non-REM (11.75 vs 5.72; p = 0.027) PLMS index, and a higher mean REM density (342.45 vs 275.96; p = 0.010) in the iRSWA group, with a significant positive correlation between RSWA severity and these variables (r = 0.39; p < 0.00, r = 0.48; p < 0.001, r = 0.24; p = 0.021, r = 0.28; p = 0.012). We found no significant difference in HRV between groups.

Our results suggest an association between RWSA and REM density and PLMS, but not HRV. The positive correlation between these variabilities may imply overlapping pathophysiological processes.

The evidence of higher REM density and normal HRV weakens the hypothesis that iRWSA is a prodromal PD stage. An alternative interpretation is, however, that REM density and HRV change during caudal-rostral neurodegeneration.