Circadian alterations in isolated Rapid-Eye-Movement sleep behavior disorder: associations with clinical, glymphatic, and dopaminergic imaging markers

Alterations in sleep-wake patterns are well-recognized features of overt alpha-synucleinopathies. However, their presence in prodromal stages, characterised by isolated REM sleep behavior disorder (iRBD), remains controversial, and their association with clinical symptoms and imaging risk factors has been underexplored. In this study, we used seven-day ambulatory circadian monitoring to characterize circadian and sleep-wake disturbances in a well-characterised sample of forty-two patients with video-polysomnography-confirmed iRBD and twenty-three healthy controls (HC). We investigated the associations between circadian disturbances and non-motor clinical symptoms, assessed in a comprehensive neuropsychological protocol, alongside imaging proxies of glymphatic system functioning, namely the Diffusion Tensor Imaging Along the Perivascular Spaces (DTI-ALPS) index, and volume of perivascular spaces and choroid plexuses. Moreover, we assessed the associations between circadian alterations and striatal dopaminergic uptake in the most affected putamen, a well-established risk factor for conversion to alpha-synucleinopathies, as assessed with presynaptic dopaminergic imaging. Patients with iRBD exhibited reduced diurnal physical activity, along with reduced amplitude and increased fragmentation of the sleep probability and motor activity rhythms. Circadian alterations correlated with greater neuropsychiatric symptoms and reduced psychomotor and mental speed. Patients with iRBD exhibited a decreased DTI-ALPS and larger volume of perivascular spaces, but only the former was associated with sleep impairment. Decreased putaminal dopaminergic uptake was associated with rest-activity rhythm fragmentation. These results show that circadian alterations are observed in early stages of neurodegeneration and are associated with clinical and imaging risk markers of conversion to alpha-synucleinopathies, posing them as possible targets for intervention in prodromal disease stages.