Periodic oscillatory slowing and aperiodic broadband shifts in isolated REM sleep behavior disorder are associated with phenoconversion.

STUDY OBJECTIVES: Conventional EEG slowing in isolated REM sleep behavior disorder (iRBD) may be confounded by disease-related shifts in the aperiodic background. We separated periodic and aperiodic EEG components to clarify oscillatory slowing and examine associations with phenoconversion and clinical measures. METHODS: Resting-state EEG was recorded from 134 patients with iRBD and 51 healthy controls (HC). During follow-up, 26 converted to Parkinson's disease or dementia with Lewy bodies. Power spectra (2-50 Hz) were parameterized using FOOOF to extract aperiodic offset, low-frequency slope (2-30 Hz), and high-frequency slope (30-50 Hz), and to derive the periodic spectrum after aperiodic removal. Group differences, spatial distributions, survival associations, and clinical correlations were analyzed. RESULTS: After removing the aperiodic component, iRBD showed higher band-averaged periodic 4-8 Hz power than HC (p=0.042), consistent with slowing of the dominant alpha rhythm. In aperiodic analyses, iRBD exhibited higher offset than HC (p<0.001). Converters showed more pronounced oscillatory slowing (p=0.008). Topographically, converters showed higher offset in the occipital region. Survival analyses showed that higher posterior periodic 4-8 Hz power (hazard ratio = 1.342, 95% confidence interval = 1.1-1.6) and offset (hazard ratio = 7.457, 95% confidence interval = 2.6-21.3) were associated with phenoconversion risk. Among converters, MoCA-K correlated with low-frequency slope (r=-0.613), whereas MDS-UPDRS-III correlated with offset (r=0.605). CONCLUSIONS: Separating periodic from aperiodic EEG activity revealed distinct RBD- and phenoconversion-related oscillatory slowing and aperiodic alterations. These findings suggest that aperiodic components may provide additional prognostic and clinical information beyond conventional band power.