What is REM sleep behavior disorder and what condition is it often associated with?

• A. A disorder where REM atonia is lost, leading to dream-enactment behaviors; often associated with synucleinopathies like Parkinson's disease
• B. A disorder of rapid eye movements during non-REM sleep; often associated with diabetes
• C. A disorder causing frequent nightmares; often associated with anxiety disorders
• D. A disorder with insomnia; often associated with hypothyroidism

Answer: (A)

REM sleep behavior disorder is defined by the loss of the normal muscle paralysis that occurs during REM sleep, so people can act out their dreams with movements and vocalizations. This dream-enactment happens because the usual REM atonia is absent. The standout connection is with neurodegenerative diseases that involve α-synuclein, especially Parkinson's disease, but also Lewy body dementia and multiple system atrophy. Often, REM sleep behavior disorder appears years before the motor or cognitive symptoms of these conditions, making it an important early sign. The other descriptions describe different sleep phenomena or unrelated conditions and don’t capture the core feature of REM-atonia loss or its association with synucleinopathies.