Synopsis
Describe positive and negative myoclonus accurately, distinguish epileptic from physiological, toxic-metabolic and structural causes, identify encephalopathic emergencies, and choose cause-directed or syndrome-specific treatment with neurophysiological support.
- Myoclonus is a sudden, brief, shock-like movement caused either by active muscle contraction—positive myoclonus—or a momentary interruption of tonic muscle activity—negative myoclonus.
- Classify distribution as focal, segmental, multifocal or generalised and relationship to action, posture, stimulus, sleep and awareness before naming a cause.
- Physiological myoclonus includes hypnic jerks and hiccups; symptoms become pathological when persistent, progressive, functionally disruptive or linked to epilepsy or systemic illness.
- Cortical myoclonus is often action- and stimulus-sensitive, predominantly distal and time-locked to an EEG cortical discharge on specialist back-averaging studies.
- Asterixis is negative myoclonus: irregular lapses of posture seen in hepatic, hypercapnic, uraemic and drug-related encephalopathy, not a conventional oscillatory tremor.
- Epileptic myoclonus may occur alone or in generalised syndromes; morning upper-limb jerks and tonic-clonic seizures after sleep deprivation suggest juvenile myoclonic epilepsy.
- Common provoking medicines include opioids, serotonergic drugs, antipsychotics, lithium, some antiseizure medicines and sedative withdrawal, especially when renal clearance falls.
- Routine EEG can support epileptic myoclonus, but EEG–EMG polygraphy and jerk-locked back-averaging may be needed to define cortical origin.
- Treat the cause first. Levetiracetam or sodium valproate are NICE first-line options for myoclonic seizures, with current MHRA restrictions governing valproate.
- Carbamazepine, gabapentin, oxcarbazepine, phenytoin, pregabalin, tiagabine and vigabatrin can aggravate myoclonic seizures and should not be used for that seizure type.
Key red flags
Brief irregular lapses of sustained posture causing flapping hands, knee buckling or dropping objects suggest asterixis or another negative-myoclonus syndrome.
Acute multifocal jerks or asterixis with delirium in renal, hepatic or respiratory failure or after opioid accumulation requires urgent systemic cause correction.
Myoclonus with inducible or spontaneous clonus, hyperreflexia, agitation, fever and autonomic change after serotonergic exposure suggests serotonin toxicity rather than a primary movement disorder.
Investigation priorities
Define distribution, stimulus sensitivity, action relationship and drug or withdrawal exposure.
Management branches
Jerks begin abruptly with confusion, physiological deterioration or a new medicine exposure.
- 1. Stabilise ABCDE, check glucose, temperature and oxygenation, identify convulsive or non-convulsive status and assess for serotonin or neuroleptic malignant syndromes.
- 2. Review prescriptions, renal clearance, dose changes, substances and withdrawal, withholding plausible culprits safely while antidotal or supportive advice is sought.
- 3. Correct metabolic, respiratory, hepatic, renal and infectious precipitants and use EEG urgently when awareness does not recover or epileptic activity remains possible.
- 4. Reassess jerks after physiology improves before committing the patient to chronic symptomatic antiseizure medication.