In-vivosignatures of neurodegeneration in isolated rapid eye movement sleep behaviour disorder

Background and purpose Isolated rapid eye movement sleep behaviour disorder (iRBD) is a parasomnia, recently recognized as a risk factor for progression to Parkinson's disease, dementia with Lewy body and multiple system atrophy. Biomarker studies in iRBD are relevant due to lack of evidence in this condition. The identification of biomarkers able to predict progression to synucleinopathy diseases is critical for iRBD. Fluorodeoxyglucose-positron emission tomography (FDG-PET) imaging might provide information about ongoing neurodegenerative processes. In the present study, we tested for presence of brain hypometabolism patterns as biomarkers of neurodegeneration in single iRBD individuals. Methods We recruited 37 subjects with polysomnography-confirmed iRBD, with neuropsychological assessment and available FDG-PET scan. Images were analysed with a validated statistical parametric mapping procedure, providing individual hypometabolism maps. Results According to the neuropsychological evaluation, 22 subjects with iRBD had normal cognition and 15 subjects showed impairments, particularly in visuoperceptive/visuospatial and memory domains. One-fifth of the cases were impaired on the Qualitative Scoring of Pentagon Test. In 32 iRBD cases, FDG-PET statistical parametric maps revealed significant cerebral hypometabolism, namely in the occipital lobes (n = 5), occipital and cerebellar regions (n = 13), occipitoparietal regions (n = 13) and a selective cerebellar hypometabolism (n = 1). Five cases had normal FDG-PET scans. Conclusions These imaging findings indicate that brain neurodegenerative processes are present and already detectable in iRBD. The different hypometabolism patterns in the single individuals may reflect specific early pathophysiological events due to the underlying synucleinopathy, with a specific neural vulnerability for the occipital cortex that might pre-date a risk of progression towards dementia with Lewy body.