Myoclonus

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Myoclonus is a phenomenological endpoint with many generators. Cortical, subcortical, brainstem, spinal and peripheral sources have different timing and stimulus relationships. Positive jerks can be confused with tics, chorea, tremor and dystonia. Negative myoclonus appears as a sudden drop or flapping lapse when the arms are held out, and may cause unexplained falls or dropped objects. Clinical examination should test rest, outstretched posture, action, touch, sound and a functional task without provoking unsafe falls.

Aetiological groups include physiological, essential, epileptic and secondary myoclonus. Secondary causes encompass anoxic injury, neurodegeneration, infection or autoimmune encephalitis, metabolic encephalopathy, structural lesions and medicines. The tempo is crucial: acute generalised myoclonus with confusion is more likely toxic-metabolic or post-hypoxic than an isolated inherited syndrome. Progressive myoclonic epilepsies combine seizures and myoclonus with neurological decline and require tertiary genetic and metabolic evaluation.

Treatment is mechanism-specific. Correcting uraemia, hypercapnia, liver failure, electrolyte disturbance or a causative medicine can be more effective than adding a symptomatic drug. Epileptic cortical myoclonus may respond to broad-spectrum antiseizure treatment, while clonazepam is sometimes used across mechanisms but creates sedation, dependence and falls. Video-neurophysiology, paediatric disease, post-anoxic prognostication and medicine combinations require specialist governance.

Key points

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.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Physiological myoclonus

Hypnic jerks and hiccups occur in healthy physiology and become clinically concerning when persistent, progressive or functionally disruptive.

Epileptic syndromes

Generalised and focal epilepsies can produce cortical myoclonic jerks, often with morning, stimulus or sleep-deprivation patterns and an EEG correlate.

Toxic and metabolic encephalopathy

Opioids, serotonergic medicines, lithium, renal or liver failure, hypercapnia and sedative withdrawal can cause multifocal jerks or asterixis.

Structural and degenerative disease

Cortical lesions, post-hypoxic injury, infection and rapidly progressive neurodegeneration can disrupt motor networks and generate focal or generalised myoclonus.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Cortical hyperexcitability
Abnormal motor-cortical discharge rapidly activates selected muscles and may be time-locked to an EEG potential during cortical myoclonus.
2
Subcortical or spinal generator
Brainstem, basal-ganglia and spinal circuits can produce jerks without a preceding cortical discharge, often with broader synchronous recruitment.
3
Negative motor interruption
Brief lapses in ongoing muscle activation cause negative myoclonus or asterixis, seen particularly during toxic-metabolic encephalopathy.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Positive myoclonus

Lightning-like muscle jerks with abrupt displacement, occurring singly or in clusters and without the rhythmic oscillation of tremor, represent positive myoclonus.

Negative myoclonusRed flag

Brief irregular lapses of sustained posture causing flapping hands, knee buckling or dropping objects suggest asterixis or another negative-myoclonus syndrome.

Epileptic morning jerks

Bilateral upper-limb jerks shortly after waking, worsened by sleep deprivation and associated with generalised tonic-clonic seizures, strongly suggest a generalised epilepsy syndrome.

Toxic-metabolic patternRed flag

Acute multifocal jerks or asterixis with delirium in renal, hepatic or respiratory failure or after opioid accumulation requires urgent systemic cause correction.

Serotonergic syndromeRed flag

Myoclonus with inducible or spontaneous clonus, hyperreflexia, agitation, fever and autonomic change after serotonergic exposure suggests serotonin toxicity rather than a primary movement disorder.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Video phenomenology and medication timelineFirst step
Why
Define distribution, stimulus sensitivity, action relationship and drug or withdrawal exposure.
Interpretation and limitations
A precise frame-by-frame sequence helps separate myoclonus from tics or tremor. Include renal-dose changes, opioid exposure and over-the-counter serotonergic agents.
02
Urgent metabolic and organ-function screen
Why
Find reversible causes of acute generalised or negative myoclonus.
Interpretation and limitations
Check glucose, electrolytes including calcium and magnesium, renal and liver profiles, blood gas, blood count and infection studies as directed; results are interpreted with ventilation, medicines and cognition.
03
EEG with surface EMG polygraphy
Why
Determine whether jerks are epileptic and identify a cortical time relationship.
Interpretation and limitations
Generalised spike or polyspike-wave accompanying a jerk supports epileptic myoclonus. Jerk-locked back-averaging can reveal a cortical potential not obvious on routine visual EEG.
04
MRI brain
Why
Evaluate focal, segmental, progressive or otherwise unexplained myoclonus for structural or neurodegenerative causes.
Interpretation and limitations
Targeted imaging is especially important with focal signs, cognitive decline or new adult onset. Normal MRI does not exclude epilepsy or a toxic-metabolic generator.
05
CSF, autoimmune, metabolic or genomic testing
Why
Investigate encephalitis, rapidly progressive disease or a progressive myoclonic epilepsy when the phenotype supports it.
Interpretation and limitations
Testing is specialist-directed and time-sensitive when infection or autoimmune encephalitis is plausible; broad genetic panels without careful phenotyping can generate uncertain variants.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Tremor

Regular rhythmic oscillation around a joint favours tremor, whereas myoclonus is brief, shock-like and usually irregular.

Chorea or tics

Flowing unpredictable movements suggest chorea, while stereotyped urge-driven suppressible events favour tics rather than lightning-like jerks.

Dystonia

Sustained patterned postures and twisting contractions last longer than myoclonus, though dystonic and myoclonic movements may coexist.

Functional jerks

Marked variability, long reaction latency, entrainment or a Bereitschaftspotential can support functional myoclonus through specialist neurophysiology.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01AcuteTreat encephalopathic myoclonus as a symptomFirst stepJerks begin abruptly with confusion, physiological deterioration or a new medicine exposure.

11. Stabilise ABCDE, check glucose, temperature and oxygenation, identify convulsive or non-convulsive status and assess for serotonin or neuroleptic malignant syndromes.
22. Review prescriptions, renal clearance, dose changes, substances and withdrawal, withholding plausible culprits safely while antidotal or supportive advice is sought.
33. Correct metabolic, respiratory, hepatic, renal and infectious precipitants and use EEG urgently when awareness does not recover or epileptic activity remains possible.
44. Reassess jerks after physiology improves before committing the patient to chronic symptomatic antiseizure medication.

02ClassifyLocalise the jerk mechanismPersistent myoclonus remains after immediate systemic threats are addressed.

11. Record positive versus negative jerks and whether they are focal, segmental, multifocal or generalised at rest, posture and action.
22. Test stimulus sensitivity and timing without creating falls, then compare tics, tremor, chorea, dystonia and functional jerks.
33. Use EEG–EMG polygraphy and targeted MRI, adding CSF, metabolic or genomic studies when progression or encephalitis is suspected.
44. Formulate physiological, epileptic, essential or secondary myoclonus and name the confidence limits before selecting therapy.

03ControlMatch treatment to generatorJerks remain functionally important after cause-directed treatment.

11. For myoclonic seizures, choose NICE-recommended broad-spectrum therapy with epilepsy expertise and avoid known aggravating antiseizure medicines.
22. For cortical non-epileptic myoclonus, consider levetiracetam, clonazepam or other specialist options based on physiology and comorbidity.
33. Titrate one treatment against eating, writing, gait, sleep or injury while measuring sedation, mood, cognition and falls.
44. Refer progressive, refractory or childhood-onset cases to a tertiary service for syndrome, genomic and multidisciplinary review.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

NICE first-line option for myoclonic seizures and a frequently used symptomatic treatment for cortical myoclonus.

Levetiracetam for myoclonic seizures

A common adult regimen begins at 250–500 mg twice daily and is titrated to response, with maintenance reduced for renal impairment under BNF and epilepsy-specialist guidance.

Monitor irritability, aggression, depression, sedation and suicidality. Confirm whether the jerks are epileptic, adjust for renal clearance and withdraw gradually rather than abruptly.

Effective broad-spectrum first-line option for myoclonic seizures, particularly within generalised epilepsy syndromes.

Sodium valproate for myoclonic seizures

Use an individualised divided regimen titrated to the lowest effective dose only when current MHRA initiation, specialist and reproductive-risk conditions are satisfied.

Major teratogenic and neurodevelopmental risks require the Pregnancy Prevention Programme where applicable; counsel males under current rules and monitor hepatic, pancreatic, haematological, weight and reproductive adverse effects.

Can suppress some cortical, subcortical and stimulus-sensitive myoclonus when cause treatment and preferred agents are insufficient.

Clonazepam

A specialist may start a very low night-time or divided dose and increase slowly to the minimum effective exposure, following dependence and controlled-drug guidance.

Sedation, falls, cognitive impairment, tolerance and dependence limit long-term value. Avoid abrupt withdrawal and take extra care with respiratory disease, opioids and frailty.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Falls and traumatic injury

Negative lapses or large generalised jerks cause dropping objects, collapse, fractures and loss of safe mobility.

Loss of speech and self-care

Frequent action myoclonus disrupts feeding, writing, communication and rehabilitation even when strength and sensation remain preserved.

Status or encephalopathic progression

Epileptic myoclonus may accompany status, while toxic or degenerative jerks can signal worsening systemic or cerebral disease.

Treatment-related sedation

Antiseizure and sedating medicines may reduce jerks but worsen cognition, balance, breathing and participation, particularly during metabolic illness.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Track jerk distribution, frequency and triggers with functional measures such as spills, dropped objects, falls, writing and ability to feed independently.
Repeat cognition, respiratory status, renal and hepatic function when acute or drug-related myoclonus is suspected, adjusting accumulating medicines promptly.
Review EEG correlation and all seizure types after treatment, because reducing visible jerks does not prove control of the wider epilepsy syndrome.
Monitor mood and behaviour with levetiracetam, reproductive and organ safeguards with valproate, and sedation or dependence with clonazepam.
Escalate progression, ataxia, cognitive decline or new seizures for tertiary metabolic, autoimmune and genomic reassessment.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Asterixis is a lapse

The apparent flap reflects repeated brief interruptions of tonic contraction, which is why it is classified as negative myoclonus rather than tremor.

Action reveals cortex

Cortical myoclonus often intensifies during precise voluntary movement and with sensory stimulus because hyperexcitable sensorimotor cortex is recruited.

The EEG may need EMG

A tiny cortical potential can be hidden by movement artefact or timing, and averaging EEG backwards from muscle onset may demonstrate the relationship.

One drug can both cause and treat

Antiseizure medicines differ by syndrome: levetiracetam may help myoclonus, whereas carbamazepine and several other agents can aggravate epileptic myoclonic jerks.

Post-hypoxic jerks vary

Acute myoclonic status in coma and chronic action myoclonus after recovery have different prognostic and treatment implications requiring EEG and critical-care context.

11Common pitfallsFrequent interpretation and management errors.

01
Calling asterixis tremor can obscure hypercapnia, uraemia, liver failure or medicine accumulation requiring urgent systemic treatment.
02
Treating all jerks as epilepsy without checking awareness, physiology and EEG risks adding sedating drugs to toxic-metabolic encephalopathy.
03
Prescribing carbamazepine for unrecognised myoclonic epilepsy can exacerbate the very seizure type being treated.
04
Using valproate without current MHRA safeguards disregards substantial reproductive risk even when the medicine is neurologically effective.
05
Judging post-anoxic prognosis from myoclonus alone before multimodal assessment can lead to falsely pessimistic decisions.