Expected Polysomnographic Finding in REM Sleep Behavior Disorder
The expected polysomnographic finding would be REM sleep without atonia (RSWA), characterized by sustained or intermittent elevation of submental EMG tone or excessive phasic muscle activity in the limb EMG during REM sleep. 1
Diagnostic Criteria and PSG Findings
The International Classification of Sleep Disorders (ICSD-2) establishes that the hallmark diagnostic feature of RBD on polysomnography is the presence of REM sleep without atonia, which manifests as either: 1
- Sustained muscle activity (tonic): Defined as epochs with at least 50% of the REM epoch duration showing submental EMG amplitude greater than the minimum amplitude observed during NREM sleep 2
- Excessive transient muscle activity (phasic): Characterized by excessive phasic bursts of muscle activity in the chin or limb EMG channels 2
In normal REM sleep, there is profound muscle atonia with minimal EMG activity. However, in RBD, this normal atonia is considerably lost, resulting in a higher baseline EMG amplitude throughout REM periods. 2
Additional Polysomnographic Characteristics
Beyond the loss of REM atonia, the polysomnogram may capture: 1
- Actual episodes of complex motor behaviors: The video component of the PSG may document limb jerking, violent movements, or other dream enactment behaviors synchronized with the EMG abnormalities 1, 3
- Increased phasic submental EMG density: Research demonstrates that RBD patients show significantly increased phasic EMG activity during REM sleep compared to controls 4
- Time-synchronized video evidence: Mandatory documentation showing actual behaviors corresponding to the EMG abnormalities during REM sleep 3, 2
Clinical Context
This 60-year-old man's presentation of nocturnal violent behaviors occurring twice monthly is highly consistent with RBD, particularly given his age (RBD typically presents in the sixth or seventh decade). 1, 2 The polysomnographic confirmation of REM sleep without atonia is essential because: 1
- Differential diagnosis: PSG helps distinguish RBD from conditions that can mimic it clinically, such as obstructive sleep apnea with vigorous arousals, non-REM parasomnias, nocturnal seizures, or periodic limb movements 1, 5
- Prognostic significance: Idiopathic RBD carries a 70% risk of developing α-synucleinopathy (Parkinson's disease, dementia with Lewy bodies, or multiple system atrophy) within 12 years of diagnosis 3, 6
Critical Pitfall to Avoid
Do not confuse RBD with NREM parasomnias (such as night terrors or sleepwalking), which occur during deep NREM sleep in the first third of the night and show normal REM atonia on PSG. RBD occurs specifically during REM sleep, typically in the latter half of the night when REM periods are longer, and is characterized by the pathognomonic loss of REM atonia. 3, 2