Post-hypoxic myoclonus: Differentiating benign and malignant etiologies in diagnosis and prognosis

Neurological function following cardiac arrest often determines prognosis. Objective tests, including formal neurological examination and neurophysiological testing, are performed to provide medical providers and decision-makers information to help guide care based on the extent of neurologic injury. The demonstration of post-hypoxic myoclonus on examination has been described to portend poor outcome after cardiac arrest, but recent studies have challenged this idea given that different forms of post-hypoxic myoclonus predict disparate prognoses. The presence of myoclonus status epilepticus (MSE) usually signals a poor outcome, especially if generalized. Lance-Adams syndrome (LAS), another form of post-hypoxic myoclonus, carries a better prognosis. Differentiating subtypes of post-hypoxic myoclonus is therefore critical. This can be difficult in the acute setting with clinical examination alone due to the use of sedation to facilitate mechanical ventilation, and neurophysiological studies may be more reliable. In this review, we describe and compare clinical and neurophysiological features of MSE and LAS. Generalized epileptiform activity and burst suppression on electroencephalography tend to be more common in MSE, and focal epileptiform activity at the vertex may define LAS. Those with multifocal MSE may have better outcomes than those with generalized MSE. We conclude that neurophysiological testing is vital acutely after cardiac arrest when post-hypoxic myoclonus is present to help determine prognostication and guide decision-making.