Objectives: Although myoclonus less than or equal to 72 hours after cardiac arrest (CA) is often viewed as a single entity, there is considerable heterogeneity in its clinical and electrophysiology characteristics, and its strength of association with outcome. We reviewed definitions, electroencephalogram, and outcome of myoclonus post-CA to assess the need for consensus and the potential role of electroencephalogram for further research.

Data sources: PubMed, Embase, and Cochrane databases.

Study selection: English-language adult (≥ 18 yr) studies from 1966 to May 31, 2024, reporting myoclonus, myoclonic status/status myoclonus (MyS/SM), myoclonic status epilepticus (MSE), and/or early Lance-Adams Syndrome (eLAS) less than or equal to 72 hours post-CA. All study designs were independently screened by two authors.

Data extraction: Data on patients presenting myoclonus, MyS/SM, MSE, and eLAS less than or equal to 72 hours post-CA, along with their definitions, electroencephalogram, and outcomes were extracted. The Newcastle-Ottawa Scale and Cochrane-Risk-of-Bias Assessment tool were used to evaluate study quality (PROSPERO n.CRD42023438107).

Data synthesis: Of 585 identified articles, 119 met the inclusion criteria, revealing substantial heterogeneity in definitions, electroencephalogram, and outcomes. Among 3881 patients, myoclonus was reported in 2659, MyS/SM in 883, MSE in 569, and eLAS in 40. Among patients with a defined outcome, a Cerebral Performance Category (CPC) scale of 1-2 was reported in 9.8% of patients with myoclonus, 5.8% with MyS/SM, 5.7% with MSE, and 82.0% with eLAS. Electroencephalogram was recorded in 2714 patients (69.9%). CPC of 1-2 was observed in 1.6% of patients with suppression/suppression burst (SB)/unreactive (U) electroencephalogram, 11.3% with non-SB/U electroencephalogram and status epilepticus (SE), and 22.3% with non-SB/U electroencephalogram without SE.

Conclusions: Heterogeneity in definitions resulted in weak associations with outcomes. We propose to investigate myoclonus by including related electroencephalogram patterns: myoclonus associated with suppression/SB background electroencephalogram, myoclonus with nonsuppression/SB background but SE-electroencephalogram, and myoclonus with nonsuppression/SB background without SE-electroencephalogram. This pragmatic research approach should be validated in future studies.