The fate of patients with REM sleep behavior disorder and mild cognitive impairment

Objective: To investigate clinical and dopaminergic pre-synaptic brain imaging characteristics of subjects with idiopathic rapid eye movement (REM) behavior disorder (iRBD) and mild cognitive impairment (MCI), and to evaluate the combined predictive value of risk factors for short-term conversion to synucleinopathy. Method: In sum, 44 polysomnography (PSG)-confirmed iRBD patients (68.5 ± 7.2 years; 38 males) underwent ¹²³I-FP-CIT-SPECT, comprehensive neuropsychological evaluation, clinical examination and clinical follow-up every six months (30.6 ± 21.5 months). Step-wise logistic regression was applied to identify those features discriminating iRBD patients with (iRBD-MCI; n = 14) and without MCI (normal cognition [NC], iRBD-NC; n = 30). The risk of neurodegeneration was estimated with Kaplan–Meier analysis. Predictors of phenoconversion were assessed with Cox proportional-hazards analysis, adjusting for age, gender and education. A generalized linear model (GLM) was applied to define the best combination of risk factors predicting conversion at follow-up. Results: At baseline, patients with iRBD-MCI showed reduced striatal dopamine transporter (DAT) specific to non-displaceable binding ratio (SBR) and more constipation compared with iRBD-NC patients (p < 0.0001). During the follow-up, 10 patients (22.7%) develop an overt synucleinopathy. GLM analysis showed that patients with orthostatic hypotension, non-motor experiences of daily living, reduced putaminal DAT-SPECT SBR, and cognitive impairment in verbal memory/visuoconstruction abilities were at higher risk of phenoconversion (Hazard Ratio [HR] 26.05; Sensitivity 90%; Specificity 100%; Accuracy 97.73%; Positive Predictive Value 100%; Negative Predictive Value 97.14%). Conclusions: iRBD-MCI patients showed a more severe dopaminergic neuroimaging and clinical phenotype. Combining clinical and neuroimaging markers allowed to achieve excellent ability in identifying iRBD patients at high risk of developing a synucleinopathy within about three years from diagnosis.