der requires clinically significant distress or  impairment (Criterion  E); this determination
will depend on a number of factors, including the frequency of events, the potential for vi-
olence or injurious behaviors, embarrassment , and distress in othe r household members. 
Associated Features Supporting Diagnosis
Severity determination is best made based on  the nature or conseq uence of the behavior
rather than simply on frequency. Although the behaviors are typically prominent and vi-
olent, lesser behaviors may also occur. 
Prevalence
The prevalence of REM sleep behavior disord er is approximately 0.38%–0.5% in the gen-
eral population. Prevalence in patients with psychiatric disorders may be greater, possibly
related to medications prescribed for the psychiatric disorder.
Development and Course
The onset of REM sleep behavior disorder may be gradual or rapid, and the course is usu-
ally progressive. REM sleep behavior disorder associated with neurodegenerative disor-
ders may improve as the unde rlying neurodegenerative disorder progresses. Because of
the very high association with the later appe arance of an underlyi ng neurodegenerative
disorder, most notably one of the synucleino pathies (Parkinson’s disease, multiple system
atrophy, or major or mild neurocognitive di sorder with Lewy bodies), the neurological
status of individuals with REM sleep behavior disorder should be closely monitored.
REM sleep behavior disorder overwhelmingly affects males older than 50 years, but in-
creasingly this disorder is being identified in females and in younger individuals. Symp-