Posthypoxic Myoclonus

Posthypoxic myoclonus is myoclonic jerking caused by severe cerebral hypoxic ischaemic injury, most commonly after cardiac arrest. It comprises two clinically and prognostically distinct entities.

Acute posthypoxic myoclonic status epilepticus occurs within the first day after the hypoxic insult in a comatose patient. It generally indicates a poor prognosis, reflecting severe underlying cortical injury.

Chronic posthypoxic myoclonus (Lance Adams syndrome) is a substantially more benign, action induced myoclonic syndrome. It develops in patients who survive the acute insult with preserved or recovering consciousness.

Distinguishing the two entities is critical for prognostication and for deciding whether to continue aggressive treatment after cardiac arrest.

Pathophysiology and prognostic significance

• Acute posthypoxic myoclonic status epilepticus reflects severe, diffuse cortical hypoxic ischaemic injury. The myoclonus manifests this widespread cortical damage and is not a treatable seizure disorder in the conventional sense.
• This explains its association with a poor overall neurological prognosis and its typically limited or absent response to conventional antiepileptic treatment.
• Lance Adams syndrome reflects a more selective pattern of injury affecting specific cortical and subcortical motor pathways. These patients have survived the insult with meaningful cognitive recovery.
• In Lance Adams syndrome the myoclonus is a genuine and often treatable movement disorder. It is not a marker of ongoing severe, diffuse brain injury.
• Never distinguish the two entities on the presence of myoclonus alone. Assess the overall clinical trajectory, timing of onset relative to the insult and level of consciousness, and follow established, validated multimodal prognostication protocols after cardiac arrest.
• Prematurely withdrawing treatment on the mistaken assumption that all posthypoxic myoclonus indicates a poor prognosis can be catastrophic.

• Acute posthypoxic myoclonic status epilepticus presents with frequent, often continuous, generalised or multifocal myoclonic jerking. It develops within the first day after cardiac arrest or other severe hypoxic ischaemic insult, in a patient who remains comatose.
• It is typically accompanied by other features of severe cerebral injury: absent brainstem reflexes, absent motor response to pain, and severe abnormalities on electroencephalography and other prognostic testing.
• Lance Adams syndrome presents with action induced or stimulus sensitive myoclonic jerking, typically of the limbs. Voluntary movement, attempted purposeful action or external stimuli trigger or exacerbate it.
• It develops in a patient who has regained consciousness and shows meaningful cognitive recovery after the hypoxic insult. The myoclonus can be significantly disabling and interfere with voluntary movement and rehabilitation, but overall neurological recovery is generally favourable.
• The distinguishing feature is the context in which the myoclonus occurs, not the myoclonus itself.
• Myoclonus in a still comatose patient in the first day after hypoxic injury, particularly alongside other markers of severe injury, suggests the acute, poor prognosis form.
• Myoclonus that emerges or persists in a patient who is regaining consciousness and showing other signs of neurological recovery suggests the more favourable Lance Adams syndrome.

• Continuous electroencephalographic monitoring is essential in any patient with myoclonus after hypoxic ischaemic brain injury. It characterises the electrographic correlate of the myoclonus and identifies other features relevant to prognosis.
• A burst suppression or generalised suppression pattern, or a malignant, highly epileptiform background, supports a poor prognosis in the acute setting. A more preserved, reactive background is more compatible with the favourable Lance Adams presentation.
• Perform full, multimodal prognostication after cardiac arrest according to established protocols. It incorporates clinical examination (pupillary and corneal reflexes, motor response), electroencephalography, somatosensory evoked potentials, neuron specific enolase levels and neuroimaging.
• Bilaterally absent cortical N20 responses on somatosensory evoked potentials are strongly associated with poor outcome.
• Delay prognostication decisions until at least seventy two hours after the hypoxic insult, and longer where sedation or targeted temperature management has been used. Assessing too early risks premature, inaccurate prognostication.
• Never use myoclonus alone as the sole basis for prognostication or treatment limitation decisions, because the two entities differ so markedly in prognosis. Always complete a full, multimodal assessment before concluding on overall prognosis.

• Manage acute posthypoxic myoclonic status epilepticus with standard status epilepticus protocols: benzodiazepines, then escalation to further antiepileptic agents according to standard algorithms.
• Aggressive, escalating anticonvulsant treatment frequently fails to control the myoclonus in this context. It originates in severe, diffuse structural cortical injury and not in a primarily electrical, treatment responsive seizure process.
• Integrate treatment decisions into the broader prognostication and goals of care discussion for the individual patient. Do not pursue control of the myoclonus as an isolated goal in a patient with an established poor overall prognosis.
• Lance Adams syndrome has a generally more favourable prognosis and a genuinely treatable myoclonus. Treat it with antimyoclonic drugs, with levetiracetam and sodium valproate the commonly used first line agents because of reasonable evidence of efficacy and favourable tolerability.
• Clonazepam, piracetam (where available) and, for refractory cases, other antimyoclonic agents used in myoclonic movement disorders may be considered as adjunctive or alternative treatment where first line agents give inadequate control. Seek specialist neurology input.
• Full rehabilitation, including physiotherapy and occupational therapy, is essential. It addresses the functional impact of the myoclonus on voluntary movement and activities of daily living.
• Even with optimal drug treatment, the myoclonus in Lance Adams syndrome frequently persists to some degree. Ongoing functional adaptation and support form part of the neurological rehabilitation programme after the hypoxic ischaemic insult.
• Communicate clearly and accurately with patients and families about the distinction between the two entities and their differing prognoses. Myoclonic jerking after a severe hypoxic ischaemic event understandably generates anxiety.
• Explain the ongoing, full prognostic assessment so that families understand it and can make informed decisions on goals of care throughout the acute and recovery periods.