The clinical relevance of cognitive impairment in REM sleep behavior disorder

REM sleep behavior disorder (RBD) is a REM sleep parasomnia characterized by complex and often violent behaviors, potentially harmful for bed partners and patients themselves.1 RBD without signs of neurologic disorders is conventionally called “idiopathic” RBD (iRBD). However, more than 80% of patients with iRBD eventually develop a neurodegenerative disease, mostly a synucleinopathy.2 According to the largest longitudinal study to date, among the patients with iRBD who develop a neurodegenerative disease, 42% convert to Parkinson disease (PD), 50.5% to dementia with Lewy bodies (DLB), and 7.5% to multiple system atrophy.3 Conversely, 42.3% of patients with PD exhibit RBD,4 and the presence of RBD is associated with cognitive impairment in PD,5 thus identifying a malignant PD subtype as opposed to a more benign “mainly motor” phenotype.6 Moreover, considering its high prevalence in DLB, RBD has been included in the core clinical features of the revised DLB diagnostic criteria.7 Thus, there is an increasingly clear association between cognitive impairment and RBD.