Study Objectives: Idiopathic/isolated REM-sleep behavior disorder (iRBD) often precedes the onset of synucleinopathies. Here, we longitudinally investigated whether resting-state EEG relative spectral power, rhythmic and arrhythmic components of the power spectra, and functional connectivity at baseline differ between participants with iRBD who converted towards a synucleinopathy at follow-up and those who did not. 
Methods: Eighty-one participants with iRBD (66.89±6.91 years) underwent in-laboratory polysomnography, resting-state EEG recording, and a neuropsychological evaluation at the moment of the iRBD diagnosis. We estimated the power spectral density using standard analyses and derived a spectral estimate of rhythmic and arrhythmic components. Then, global and pairwise functional connectivity analyses were computed. A pixel-based corrected permutation test was used to compare each metric between groups. 
Results: After a mean follow-up duration of 5.01±2.76 years, 34 participants were diagnosed with a synucleinopathy (converters; 67.81±7.34 years) and 47 remained disease-free (non-converters; 65.53±7.09 years). Among converters, 22 patients were diagnosed with Parkinson’s disease and 12, with dementia with Lewy body. Compared to non-converters, converters exhibited higher relative theta spectral power, accompanied by steeper slopes of the arrhythmic component and higher theta rhythmic power, which were most prominently found in the posterior cortical regions. Furthermore, converters showed a higher global functional connectivity in the beta band but lower alpha band functional connectivity between the left temporal and occipital regions.
Conclusion: Patients with iRBD who developed a synucleinopathy exhibit alterations of resting-state EEG arrhythmic and rhythmic components and functional connectivity, highlighting a perturbation of large-scale networks and imbalanced excitatory-to-inhibitory activity.