[<sup>18</sup>F]Fluorodeoxyglucose positron emission tomography ([<sup>18</sup>F]FDG PET) Characterizes Neurodegeneration Levels Across the α-Synucleinopathy Continuum.

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Background[18F]Fluorodeoxyglucose positron emission tomography ([18F]FDG PET) represents an endorsed neurodegeneration biomarker in neuronal α-synucleinopathies. Idiopathic/isolated rapid eye movement (REM) sleep behavior disorder (iRBD) represents a prodromal stage of such disorders.ObjectivesTo assess [18F]FDG PET as a neurodegeneration biomarker, using published brain metabolic disease-related patterns, and a regional-based approach, across the prodromal to overt α-synucleinopathy continuum.MethodsWe included 83 prodromal subjects with iRBD, comprising non-converters (n = 56) and converters (n = 27) to an overt α-synucleinopathy (either Parkinson's disease [PD] or dementia with Lewy bodies [DLB]) according to the last available follow-up, and 85 subjects with PD (n = 40) and DLB (n = 45). For comparison, we enrolled a group of healthy subjects (n = 41). Participants underwent brain [18F]FDG PET at baseline. Analysis of covariance was used to test the ability of previously published [18F]FDG PET disease-related patterns in characterizing neurodegeneration levels along the prodromal to overt α-synucleinopathy continuum, and across the motor-predominant (parkinsonism-first) and the cognitive-predominant (dementia-first) clinical trajectories. We further assessed metabolic changes using a regional-based approach.ResultsAll disease-related patterns effectively discriminated clinical stages, from prodromal to overt α-synucleinopathies, with comparable performance. [18F]FDG PET significantly distinguished all groups along the cognitive-predominant pathway; whereas in the motor-predominant pathway, converter patients were not significantly discriminated from non-converters. Regionally, the inferior parietal, precuneus, and middle frontal areas exhibited the most prominent decrease in [18F]FDG uptake with progression, alongside relative parallel progressive increases in the cerebellum, pons, parahippocampal areas, putamen, and pallidum.Conclusions[18F]FDG PET disease-related patterns efficiently characterize neurodegeneration from prodromal to overt α-synucleinopathy, best assessing the cognitive-predominant (dementia-first) pathway. © 2026 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

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