REM sleep behavior disorder

**Definition:** A REM-sleep parasomnia with recurrent episodes of dream enactment (vocalization and/or complex motor behaviors) during REM sleep, due to loss of the normal skeletal muscle atonia of REM. Patients usually recall a dream that matches the behavior (e.g., being chased or attacked) and are easily awakened and oriented, unlike in sleepwalking.

**Normal physiology:** During REM, the sublaterodorsal nucleus (subcoeruleus) in the pons activates glutamatergic neurons that project to the medullary magnocellular nucleus. These neurons then activate inhibitory glycinergic/GABAergic interneurons that hyperpolarize spinal alpha motor neurons, producing atonia. Extraocular muscles and the diaphragm are spared.

**Pathophysiology:** Degeneration or dysfunction of these brainstem REM-atonia circuits (pontine tegmentum and medulla) removes the inhibition, so motor neurons are free to act out dream content. In idiopathic RBD, this is usually an early sign of alpha-synuclein pathology (Lewy bodies/glial cytoplasmic inclusions) in the brainstem.

**Epidemiology:** Typically men over 50 (male predominance in idiopathic RBD). Younger patients and women are more likely to have secondary RBD, such as narcolepsy type 1 or antidepressant-induced RBD.

**Key associations:** - **Alpha-synucleinopathies:** Parkinson disease, dementia with Lewy bodies, and multiple system atrophy. Most patients with idiopathic RBD eventually develop one of these, often 10 or more years later, so RBD is a prodromal marker. The risk is highest for DLB and PD. RBD is a core clinical feature of DLB. - **Not typically tauopathies:** RBD is uncommon in Alzheimer disease, PSP, and CBD. This helps differentiate MSA (RBD common) from PSP (RBD uncommon). - **Narcolepsy** (especially type 1). - **Drugs:** SSRIs, SNRIs, TCAs, and MAOIs (e.g., venlafaxine), and mirtazapine. Also seen with alcohol or sedative withdrawal, and with brainstem lesions (stroke, tumor, MS).

**Clinical features:** Vivid, action-filled, violent dreams; shouting, punching, kicking, and falling out of bed; injury to the patient or bed partner (bruises, lacerations, fractures, subdural hematoma). Episodes occur in the later part of the night, when REM periods are longest. Other prodromal synucleinopathy features include hyposmia, constipation, and orthostatic hypotension.

**Diagnosis:** Video polysomnography is the gold standard. It shows REM sleep without atonia (increased tonic or phasic EMG activity in the chin or limb leads) plus observed dream enactment behavior. Rule out mimics such as obstructive sleep apnea (pseudo-RBD), NREM parasomnias, nocturnal seizures, and PTSD-related nightmares.

**Differentiation from NREM parasomnias (sleepwalking, night terrors):**

| | RBD | NREM parasomnia | |---|---|---| | Sleep stage | REM | N3 (slow-wave) | | Timing | Late night | First third of night | | Age | Older adults | Children | | Recall | Vivid dream recall | Usually no recall | | Arousal | Easily awakened, alert | Difficult to arouse, confused |

**Management:** - **Safety measures:** Remove dangerous objects, pad the floor, place the mattress on the floor, use bed rails or barriers, and have the bed partner sleep separately if needed. - **Pharmacologic:** **Melatonin** (high dose) and **clonazepam** are first-line. Melatonin is often preferred in older adults or those with dementia, OSA, or fall risk, given clonazepam's side effects (sedation, confusion, falls, respiratory depression). - Review and, if possible, stop any offending antidepressants. - Monitor long term for the emergence of neurodegenerative disease.

**Board pearl:** An older man whose wife reports that he punches and yells during sleep, with a normal neurologic exam, has RBD. Next step: polysomnography. Counsel about the risk of future Parkinson disease or Lewy body dementia, and treat with clonazepam or melatonin.