Four studies met the criteria, including over 500 participants with iRBD, prodromal DLB, and controls. Across studies, iRBD showed global EEG slowing with increased theta–delta power and reduced posterior alpha activity, most prominently in parieto-occipital regions. Phenoconversion to DLB was associated with a further reduction in dominant alpha frequency, disrupted posterior connectivity, and increased network randomness on functional connectivity analysis. Machine-learning and deep-learning models trained on EEG spectrograms or connectivity matrices achieved AUCs of 0.83–0.89 for predicting conversion.