REM sleep behavior disorders

**Definition:** Parasomnia in which the normal skeletal muscle paralysis (atonia) of REM sleep is lost, so patients "act out" dreams with vocalizations and complex motor behavior (punching, kicking, falling out of bed), often injuring themselves or bed partners.

**Pathophysiology:** - Normal REM atonia: the sublaterodorsal nucleus (subcoeruleus) in the pons activates glutamatergic neurons that excite medullary (ventromedial medulla) inhibitory GABA/glycine neurons, which hyperpolarize spinal alpha motor neurons. - In RBD, degeneration or dysfunction of these brainstem circuits (pontine and medullary) removes this inhibition, causing REM sleep without atonia (RSWA). - Strongly linked to alpha-synuclein pathology (Lewy bodies, glial cytoplasmic inclusions).

**Epidemiology / Risk factors:** - Classically older men (>50 years), although the sex difference is less pronounced than once thought. - Idiopathic (isolated) RBD is common. Over 70-80% of patients with isolated RBD eventually develop a synucleinopathy (often within about 10-15 years), so it may be the earliest sign. - Secondary causes: narcolepsy (especially type 1), antidepressants (SSRIs, SNRIs, TCAs, mirtazapine), brainstem lesions (stroke, tumor, MS), and alcohol/sedative withdrawal.

**Key associations:** - Alpha-synucleinopathies: Parkinson disease, dementia with Lewy bodies, multiple system atrophy. It is NOT typically associated with tauopathies such as Alzheimer disease or PSP. - Narcolepsy.

**Clinical features:** - Violent or vocal movements during REM, typically in the later part of the night when REM predominates. - Patients can often recall a dream that matches the behavior (e.g., being chased or attacked). - Patients are easily awakened and become oriented and alert, unlike sleepwalking.

**Diagnosis:** - Gold standard: video polysomnography showing REM sleep without atonia (increased tonic or phasic EMG activity in REM) plus history or recorded dream-enactment behavior. - Evaluate for secondary causes (medications, neurologic disease) and monitor for neurodegeneration.

**Differential diagnosis (high-yield contrasts):** - NREM parasomnias (sleepwalking, night terrors): occur in slow-wave (N3) sleep during the first third of the night, usually in children, with no dream recall and amnesia for the event, and the patient is hard to arouse. - Nightmare disorder: patient wakes fully with dream recall but without motor activity. - Nocturnal seizures, obstructive sleep apnea (which can mimic RBD).

**Treatment:** - Safety first: remove sharp or hazardous objects, pad the floor, lower the bed, and have the bed partner sleep separately. - Melatonin (first-line in many centers, with fewer adverse effects, especially in older or cognitively impaired patients). - Clonazepam (a benzodiazepine, effective but risks sedation, falls, confusion, and worsening of sleep apnea). - Treat or avoid triggering medications where possible.

**Contrast with sleep paralysis:** Sleep paralysis is persistence of REM atonia on waking (or onset of sleep), whereas RBD is loss of atonia during REM.

**Boards pearl:** A 65-year-old man whose wife reports he punches and yells during sleep, with a history of constipation, hyposmia, or tremor → think RBD as a prodrome of Parkinson disease or Lewy body dementia. Confirm with polysomnography.