Post-Anoxic Myoclonus
The jerking movements that occur after anoxic brain injury are called myoclonus, specifically post-anoxic myoclonus (PAM). 1, 2
Clinical Definition and Characteristics
Myoclonus consists of sudden, brief, shock-like jerks that occur in 16%-37% of patients within the first week after cardiac arrest. 2 The American Heart Association describes it as diffuse or multifocal muscle jerks, or more subtle focal motor movements involving the face and limbs, observed in approximately 18%-34% of post-cardiac arrest patients. 1
The clinical presentation can be classified by several features: 2
- Timing: Early onset (within 24 hours) versus late onset (days 3-7)
- Distribution: Focal, multifocal, or generalized
- Phenotype: Whole body jerks, limbs only, or facial/ocular movements
- Neurologic localization: Cortical versus subcortical origin
Important Clinical Terminology
When myoclonus is continuous and lasts 30 minutes or longer, it is termed status myoclonus (SM) or myoclonic status epilepticus—though the use of "status" is based on duration rather than EEG findings. 2
A critical distinction exists between acute post-anoxic myoclonus and Lance-Adams syndrome: 2, 3
- Acute post-anoxic myoclonus occurs in the immediate post-arrest period and typically indicates severe hypoxic-ischemic brain injury
- Lance-Adams syndrome (chronic post-hypoxic myoclonus) is a chronic myoclonic disorder characterized by action-induced myoclonic jerks, predominantly affecting distal limbs, that develops in survivors who have already regained consciousness 3
Relationship to EEG Findings
Up to half of patients with myoclonus have electrographic seizures or status epilepticus on EEG, and clinical examination alone cannot predict which patients are having seizures. 1 The presence or absence of time-locked EEG correlates has important implications:
- Myoclonus with EEG correlate: May represent true seizure activity
- Myoclonus without EEG correlate: Controversial whether this requires treatment, as its role in secondary brain injury is unclear 1
Myoclonus can cause significant muscle artifact on EEG, which may require neuromuscular blockade for accurate interpretation. 1
Prognostic Implications
The presence of status myoclonus generally indicates poor prognosis, but favorable neurologic recovery remains possible in a small proportion of patients. 2 In patients treated with targeted temperature management:
- Status myoclonus predicts poor outcome with a false positive rate of 5% (range 2%-11%) 2
- Early onset myoclonus (days 1-2) had a false positive rate of 0.2% for poor outcomes 2
- Timing, duration, phenotype, and neurologic localization do not reliably differentiate patients with favorable versus poor outcomes 2
Specific EEG and clinical features may help identify patients more likely to recover consciousness: 4
- Multi-focal myoclonus with midline-maximal spikes over a continuous background was associated with recovery in 4 of 5 patients 4
- No patient with suppression burst or low voltage background at any time point regained consciousness 4
- Preserved brainstem reflexes and normal voltage background were more common in those who recovered 4
Common Pitfalls
Detection of myoclonus is often masked by sedatives and neuromuscular blocking agents, making early recognition challenging. 2 Additionally, the substantial heterogeneity in definitions and terminology used across studies makes interpretation of the literature difficult. 1