Synucleinopathies include a spectrum of disorders varying in features and severity, including idiopathic/isolated REM sleep behaviour disorder (iRBD), Parkinson's disease (PD), and dementia with Lewy bodies (DLB). Distinct brain atrophy patterns may already be seen in iRBD; however, how brain atrophy begins and progresses remains unclear. A multicentric cohort of 1276 participants (451 polysomnography-confirmed iRBD, 142 PD with probable RBD, 87 DLB, and 596 controls) underwent T1-weighted MRI and longitudinal clinical assessments. Brain atrophy was quantified using vertex-based cortical surface reconstruction and volumetric segmentation. The unsupervised machine learning algorithm, Subtype and Stage Inference (SuStaIn), was used to reconstruct spatiotemporal patterns of brain atrophy progression. SuStaIn identified two distinct subtypes of brain atrophy progression: 1) a "cortical-first" subtype, with atrophy beginning in the frontal lobes and involving the subcortical structures at later stages; and 2) a "subcortical-first" subtype, with atrophy beginning in the limbic areas and involving cortical structures at later stages. Both cortical- and subcortical-first subtypes were associated with a higher rate of increase in MDS-UPDRS-III scores over time, but cognitive decline was subtype-specific, being associated with advancing stages in patients classified as cortical-first but not subcortical-first. Classified patients were more likely to phenoconvert over time compared to stage 0/non-classified patients. Among the 88 patients with iRBD who phenoconverted during follow-up, those classified within the cortical-first subtype had a significantly increased likelihood of developing DLB compared to PD, unlike those classified within the subcortical-first subtype. There are two distinct atrophy progression subtypes in iRBD, with the cortical-first subtype linked to an increased likelihood of developing DLB, while both subtypes were associated with worsening parkinsonian motor features. This underscores the potential utility of subtype identification and staging for monitoring disease progression and patient selection for trials. This study was supported by grants to S.R. from Alzheimer Society Canada (0000000082) and by Parkinson Canada (PPG-2023-0000000122). The work performed in Montreal was supported by the Canadian Institutes of Health Research (CIHR), the Fonds de recherche du Québec - Santé (FRQS), and the W. Garfield Weston Foundation. The work performed in Oxford was funded by Parkinson's UK (J-2101) and the National Institute for Health Research (NIHR) Oxford Biomedical Research Centre (BRC). The work performed in Prague was funded by the Czech Health Research Council (grant NU21-04-00535) and by The National Institute for Neurological Research (project number LX22NPO5107), financed by the European Union - Next Generation EU. The work performed in Newcastle was funded by the NIHR Newcastle BRC based at Newcastle upon Tyne Hospitals NHS Foundation Trust and Newcastle University. The work performed in Paris was funded by grants from the Programme d'investissements d'avenir (ANR-10-IAIHU-06), the Paris Institute of Neurosciences - IHU (IAIHU-06), the Agence Nationale de la Recherche (ANR-11-INBS-0006), Électricité de France (Fondation d'Entreprise EDF), the EU Joint Programme-Neurodegenerative Disease Research (JPND) for the Control-PD Project (Cognitive Propagation in Prodromal Parkinson's disease), the Fondation Thérèse et René Planiol, the Fonds Saint-Michel; by unrestricted support for research on Parkinson's disease from Energipole (M. Mallart) and the Société Française de Médecine Esthétique (M. Legrand); and by a grant from the Institut de France to Isabelle Arnulf (for the ALICE Study). The work performed in Sydney was supported by a Dementia Team Grant from the National Health and Medical Research Council (#1095127). The work performed in Cologne was funded by the Else Kröner-Fresenius-Stiftung (grant number 2019_EKES.02), the Köln Fortune Program, Faculty of Medicine, University of Cologne, and the "Netzwerke 2021 Program (Ministry of Culture and Science of Northrhine Westphalia State). The work performed in Aarhus was supported by funding from the Lundbeck Foundation, Parkinsonforeningen (The Danish Parkinson Association), and the Jascha Foundation.

Glymphatic dysfunction potentially contributes to Parkinson’s disease (PD) via impaired clearance of metabolic waste products. Obstructive sleep apnea (OSA) can disturb sleep, which is necessary for proper glymphatic function, and is frequent in PD. We investigated the glymphatic function in de novo PD and its relation to OSA. Fifty-four PD patients (mean age 58.9 ± 12.2 years) and 32 controls (mean age 59.4 ± 8.3 years) underwent polysomnography and 3 T magnetic resonance imaging of the brain. Diffusion tensor imaging along the perivascular space (DTI-ALPS) was calculated using atlas-based automatic regions of interest selection. In PD, ALPS-index negatively correlated with apnea-hypopnea index (rho = −0.41; p = 0.002), oxygen desaturation index (rho = −0.38; p = 0.006), sleep stage N1 (rho = −0.42; p = 0.002), and arousal index (rho = −0.24; p = 0.018), while in controls, no such correlations were observed. Glymphatic dysfunction is related to OSA severity in de novo PD but not in controls. We suggest that OSA may contribute to neurodegeneration via glymphatic impairment in PD.

Diffusion tensor image analysis along the perivascular space (DTI-ALPS) is a potential non-invasive marker of glymphatic function that typically relies on manual region of interest (ROI) placement. This study compared ALPS indices in treatment-naïve, de novo diagnosed patients with Parkinson’s disease (PD), patients with isolated REM behavior disorder (iRBD), and healthy controls using both manual and automatic approaches to the ROI selection used in ALPS-index calculation. ALPS indices were analyzed bilaterally and correlated with clinical severity (MDS-UPDRS) and nigrostriatal denervation (DAT-SPECT). ANCOVA revealed significant inter-group differences using both manual (p = 0.018) and automatic (p = 0.002) ROI selection methods. The automatic ROI selection approach showed significantly lower ALPS indices in PD compared to controls (p = 0.001) and iRBD (p = 0.009). ALPS indices correlated with symptom severity and nigrostriatal denervation. These findings underscore the reliability of the automatic ROI placement approach for ALPS index calculation and may indicate early glymphatic alterations in Parkinson’s disease.

Striosomes and matrix are two compartments that comprise the striatum, each having its own distinct immunohistochemical properties, function, and connectivity. It is currently not clear whether prodromal or early manifest Parkinson’s disease (PD) is associated with any striatal matrix or striosomal abnormality. Recently, a method of striatal parcellation using probabilistic tractography has been described and validated, using the distinct connectivity of these two compartments to identify voxels with striosome- and matrix-like connectivity. The goal of this study was to use this approach in tandem with DAT-SPECT, a method used to quantify the level of nigrostriatal denervation, to analyze the striatum in populations of de novo diagnosed, treatment-naïve patients with PD, isolated REM behavioral disorder (iRBD) patients, and healthy controls. We discovered a shift in striatal connectivity, which showed correlation with nigrostriatal denervation. Patients with PD exhibited a significantly higher matrix-like volume and associated connectivity than healthy controls and higher matrix-associated connectivity than iRBD patients. In contrast, the side with less pronounced nigrostriatal denervation in PD and iRBD patients showed a decrease in striosome-like volume and associated connectivity indices. These findings could point to a compensatory neuroplastic mechanism in the context of nigrostriatal denervation and open a new avenue in the investigation of the pathophysiology of Parkinson’s disease.

Isolated REM sleep behavior disorder, characterized by dream-enacting movements during REM sleep, is a male-predominant parasomnia and the strongest prodromal marker of synucleinopathies. Individuals with this disorder show cortical atrophy whose regional distribution covaries with gene expression patterns measured in the healthy human brain. However, the effect of sex on these brain changes remains unknown. The study objective is to comprehensively assess sex differences in cortical morphology and to characterize the healthy-brain gene expression correlates of brain abnormalities using the largest international multicentric MRI dataset of polysomnography-confirmed patients. Males have significantly more extensive and severe cortical thinning compared to females, despite similar age and clinical features. Imaging transcriptomics analyses indicate that regions affected in female patients map onto areas with higher expression of estrogen-related receptor genes, particularly ESRRG and ESRRA, in the healthy brain. These findings support potential sex-specific neuroprotection in the prodromal stages of synucleinopathies and may inform personalized and targeted therapeutic strategies.

Isolated rapid eye movement (REM) sleep behavior disorder (iRBD) is characterized by the loss of muscle atonia and abnormal, often violent, movements and vocalizations during REM sleep. It is the strongest prodromal marker for progression to synucleinopathies such as dementia with Lewy bodies (DLB) and Parkinson’s disease (PD). iRBD individuals already show brain changes consistent with manifest synucleinopathies, but their mechanisms remain poorly understood. The glymphatic system is involved in the brain clearance of waste products and its defect has been associated with a higher likelihood of pathological burden and neurodegeneration. The presence of glymphatic system dysfunction revealed by neuroimaging in iRBD and its association with disease progression remain uninvestigated, including its potential to predict conversion toward distinct trajectories of synucleinopathies. We analyzed diffusion-weighted imaging data from a large international multicentric cohort of polysomnography-confirmed iRBD individuals and healthy controls. We used diffusion tensor imaging along the perivascular space to assess glymphatic function and derived a glymphatic index based on the diffusivity occurring within masks placed on associative and projection fibers adjacent to the lateral ventricles. The index was compared between groups and correlated with motor and cognitive features. Cox regression models assessed the relationship between glymphatic index and the likelihood of conversion to PD, DLB or remaining disease-free. Our analyses included 250 polysomnography-confirmed iRBD participants (mean age 66.5 ± 6.8 years; 87% men) and 178 controls (65.7 ± 6.8 years; 81% men). There was a significantly reduced glymphatic index in iRBD individuals compared to controls. Among the 224 iRBD individuals followed longitudinally (6.1 years, 1-16 years), 65 developed a neurodegenerative disease. iRBD converters exhibited lower glymphatic index compared to non-converters. Lower glymphatic index was associated with a higher risk of phenoconversion towards PD over time compared to remaining disease-free (hazard ratio = 2.43, 95% CI = 1.13-5.25, P = 0.012). This study reports glymphatic dysfunction in iRBD and its potential for predicting conversion to PD, underscoring its utility in identifying at-risk iRBD individuals.