ObjectiveWhile isolated rapid eye movement sleep behaviour disorder (iRBD) is known as a prodrome of α-synucleinopathies, the prediction for its future phenoconversion to parkinsonism-first or dementia-first subtype remains a challenge. This study aimed to investigate whether visuospatial dysfunction predicts dementia-first phenoconversion in iRBD.MethodsPatients with iRBD and control subjects were enrolled in this prospective cohort study. Baseline neuropsychological assessment included the Unified Parkinson’s Disease Rating Scale part III, Montreal Cognitive Assessment (MoCA), Rey-Osterrieth complex figure (ROCF), Colour Trails test (CTT), Farnsworth-Munsell 100-hue test and Digit Span test. The anterior and posterior subscores of MoCA as well as their modified versions were explored. A composite score derived from ROCF and CTT was also explored. Regular follow-up was conducted to determine the phenoconversion status of iRBD patients.ResultsThe study included 175 iRBD patients and 98 controls. During a mean follow-up of 5.1 years, 25.7% of patients experienced phenoconversion. Most of the neuropsychological tests could differentiate dementia-first but not parkinsonism-first convertors from non-convertors. The modified posterior subscore of MoCA, by integrating the Alternating Trail Making and Clock Drawing components into original the posterior subscore, which mainly reflects visuospatial function, was the strongest predictor for dementia-first phenoconversion (adjusted HR 5.48, 95% CI 1.67 to 17.98).ConclusionVisuospatial dysfunction, as reflected mainly by the modified posterior subscore of MoCA, is a predictive factor for dementia-first phenoconversion in iRBD, suggesting its potential for being a biomarker for clinical prognostic prediction and potential neuroprotective trials aiming to delay or prevent dementia.