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.