Isolated rapid eye movement sleep behaviour disorder: Clinical and research implications

Rapid eye movement (REM) sleep behaviour disorder (RBD), initially described in 1986 by Schenck et al as an REM sleep parasomnia,1 is characterised by dream enactment behaviours and increased muscle activity during REM sleep.2 Isolated RBD (iRBD, ie, in the absence of associated neurological disorders like narcolepsy, overt synucleinopathy, anti-IgLON5 disease and other less frequent conditions) has gained increasing relevance in the last decade as numerous studies demonstrated that (1) most iRBD patients develop an overt synucleinopathy over time3 and (2) in patients with iRBD (even those with long-standing iRBD, ie, those not phenoconverting ten years or more after iRBD diagnosis) biomarkers of synuclein-related neurodegeneration are common, including pathological synuclein aggregates in several tissues,4 possibly indicating an underlying neurodegenerative process.5 Based on this evidence, iRBD is now recognised as an early-phase synucleinopathy in most cases, with phenoconversion over time into dementia with Lewy bodies (DLB), Parkinson's disease (PD) or, less commonly, multiple system atrophy (MSA). A meta-analysis showed that the risk for developing neurodegenerative diseases was 33.5% at 5 years follow-up, 82.4% at 10.5 years and 96.6% at 14 years.