Objective: This study aimed to characterize intracortical inhibitory and facilitatory circuit alterations in alpha-synucleinopathies using transcranial magnetic stimulation (TMS) to improve early diagnosis and disease monitoring. Methods: We enrolled 45 drug-naïve Parkinson's disease (PD) patients, 20 dementia with Lewy bodies (DLB) patients, 15 idiopathic REM sleep behavior disorder (iRBD) patients, and 20 healthy controls (HC). All participants underwent standardized clinical assessments and TMS paired-pulse protocols to evaluate intracortical inhibition (short-interval intracortical inhibition, SICI; short-latency afferent inhibition, SAI) and facilitation (intracortical facilitation, ICF). Results: SICI was reduced in PD, iRBD, and DLB compared to HC, indicating early GABAergic dysfunction. SAI was impaired in iRBD and DLB but preserved in PD, suggesting cholinergic dysfunction specific to these groups. ICF was significantly altered in DLB, with individual variability in iRBD, indicating heterogeneous glutamatergic involvement. Conclusions: Cortical inhibitory and facilitatory abnormalities are present in alpha-synucleinopathies from the prodromal stage. Cholinergic dysfunction in iRBD and DLB may serve as an early biomarker for cognitive risk, while GABAergic and glutamatergic alterations suggest non-dopaminergic involvement in PD. Significance: These findings support the use of TMS as a potential biomarker tool for early detection, patient stratification, and disease progression monitoring in alpha-synucleinopathies.
