Large muscle movements during sleep across the synucleinopathy spectrum: a polysomnographic study of isolated REM...

To characterize nocturnal motor activity across isolated REM sleep behavior disorder (iRBD), Parkinson's disease with REM sleep behavior disorder (PD+RBD), PD without RBD (PD-RBD), and healthy controls, with particular focus on large muscle group movements (LMMs) during sleep.

We included 109 participants: controls (n = 25; 13F/12M, age 67.6 ± 6.70 years), iRBD (n = 25; 4F/21M, 70.7 ± 8.42 years), PD+RBD (n = 31; 10F/21M, age 71.1 ± 7.30), and PD-RBD (n = 28; 9F/19M, age 67.9 ± 8.05 years). Conventional sleep architecture, REM atonia index (RAI), leg-movement subtypes, and LMM indices and duration across total sleep time (TST), NREM sleep, and REM sleep were analyzed.

As expected, RAI showed the clearest group separation, with reduced values in iRBD and PD+RBD. Sleep instability, reflected by increased stage shifts, was greater in iRBD and PD+RBD than in controls. Among leg-movement measures, only isolated leg movement index remained significantly increased after correction, specifically in PD+RBD. LMM provided the most distinctive additional signal: total LMM index differed across TST, NREM, and REM sleep, with lower values in PD-RBD and higher polysomnography (PSG)-defined REM-related LMM activity in iRBD and PD+RBD. LMM duration was longer in all clinical groups than in controls, whereas LMM associated with arousal or wake did not differ significantly across groups.

Quantitative assessment of LMM, together with refined leg-movement phenotyping and REM atonia measures, may improve characterization of nocturnal motor dysregulation across the synucleinopathy spectrum. LMM may represent a PSG-derived marker of sleep-state instability and REM-related motor activation, with potential relevance for iRBD progression and PD phenotyping. Statement of Significance This study suggests that the nocturnal motor phenotype of synucleinopathy is broader than REM sleep without atonia and conventional PLMS measures alone. By showing that LMM and isolated leg movements help distinguish iRBD, PD+RBD, and PD-RBD, the findings support a more refined polysomnographic framework for capturing disease-related motor disorganization during sleep. The contrasting profile of PD-RBD, with fewer but longer LMM, also points to nocturnal hypokinesia as a potentially distinct motor signature. If confirmed longitudinally, these measures could improve phenotypic stratification and help identify clinically meaningful pathways linked to progression, cognition, autonomic dysfunction, and future targeted treatments.