Prospective memory in idiopathic REM sleep behavior disorder with or without mild cognitive impairment: A preliminary study

Objective: The ability to execute delayed intentions, known as prospective memory (PM), is crucial to everyday living. PM failures are reported in mild cognitive impairment (MCI) and Parkinson’s disease, however, no study to date has investigated PM functioning in individuals at high risk of developing these conditions, precisely those diagnosed with idiopathic REM sleep behavior disorder (iRBD). We aimed to assess PM in iRBD according to patients’ cognitive status and to determine the underlying nature of their difficulties. Method: Fifty-eight participants, including 20 healthy controls (HC) and 38 polysomnographic-confirmed iRBD patients with (iRBD-MCI = 13) or without (iRBD-nMCI = 25) MCI participated in this study. Following a neuropsychological assessment, PM was evaluated using a self-administered questionnaire (PRMQ), a simple clinical measure (envelope test), and a laboratory cue salience task. Results: No significant group differences were noted on the PRMQ and envelope test. On the PM laboratory task, non-parametric analyses revealed better detection accuracy in HC than both iRBD groups for all high and low salient cues. While iRBD-nMCI and iRBD-MCI patients performed similarly on the high salient condition, the latter showed significant difficulty in detecting low salient cues. Multiple regression analyses revealed executive dysfunction as the best predictor to significantly account for differences in the low salient condition in iRBD. Conclusion: PM difficulties in iRBD are most important in patients with MCI (vs without MCI) and may be attributed to a gradual alteration in executive mechanisms. PM impairment could act as a promising indicator of early cognitive dysfunction in iRBD.