Patients with idiopathic REM sleep behavior disorder (RBD) have significantly worse cognitive performance and function compared with non-Parkinson disease (PD) mutation carriers and patients with hyposmia, according to study results published in PLoS One.

Participants in the multicenter international longitudinal Parkinson Progression Markers Initiative (PPMI) were included in the analysis. Patients were divided into several groups based on clinical status, including patients with de novo PD and dopamine transporter binding deficit (n=423), RBD (n=39), and hyposmia (n=26).

In addition, non-PD mutation carriers (NMCs, n=126), including patients with the G2019S mutation in the Leucine-rich repeat kinase 2 (LRRK2) gene (n=88) and patients with glucocerebrosidase (GBA) gene mutations (n=38), were analyzed. Investigators compared participants’ neuropsychological performance using a neuropsychological test battery.

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Participants in the RBD group had significantly worse performances on the Symbol Digit Modality Test compared with participants in the hyposmia (mean [SD] 32.4 [9.16] vs 41.8 [9.98]; P =.002) and NMC (mean [SD] 32.4 [9.16] vs 45.2 [10.9]; P <.001) cohorts. In addition, the RBD cohort performed worse on the Judgment of Line Orientation compared with participants in the hyposmia (11.3 [2.36]vs 12.9 [1.87]; P =.004) and NMC groups (11.3 [2.36] vs 12.9 [1.87]); P <.001). RBD patients also had worse performance on the Montreal Cognitive Assessment vs patients with hyposmia (25.5 [4.13] vs 27.3 [1.71]; P =.02).

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The relatively small number of patients in each cohort may likely limit the generalizability of the findings across the broader patient population.

Findings from this study suggest that there exists “a gradient of prodromalness that is consistent with the proposed Braak staging, such that individuals with manifestations presumably resulting from more rostral neurodegeneration, namely the RBD cohort, have worse cognition than hyposmics or asymptomatic carriers of PD-associated genes.”


Chahine LM, Urbe L, Caspell-Garcia C, et al. Cognition among individuals along a spectrum of increased risk for Parkinson’s disease. PLoS One. 2018;13(8):e0201964.