BACKGROUND: Isolated REM sleep behavior disorder (iRBD) indicates an early α-synucleinopathy with approximately 90% of individuals pheno-converting to Parkinson's disease (PD), dementia with Lewy bodies (DLB), or multiple system atrophy (MSA). Recently, prodromal disease definitions (pPD, pDLB, pMSA) have been introduced. OBJECTIVE: To investigate the overlap of prodromal definitions in an established iRBD cohort. METHODS: We applied the current diagnostic criteria for pPD, pDLB, and pMSA to N = 55 individuals from the local iRBD cohort. RESULTS: All except two individuals fulfilled at least one of the prodromal disease definitions
most individuals were classified as pPD (94.5%). 56% of the individuals fulfilled more than one definition: 32.7% pMSA&pPD, 10.9% pDLB&pPD, and 12.7% all three (pPD&pDLB&pMSA). The cognitive screening [(pPD = pMSA) >
pDLB], motor symptoms [pPD <
(pDLB = pMSA)], and olfactory testing [(pPD = pDLB) <
pMSA] significantly differed between groups. CONCLUSIONS: The observed overlap leads to challenges in patient counseling and risk disclosure. Better discrimination will facilitate research in early α-synucleinopathy phases.