Myoclonus jerk
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Myoclonus Jerk
Definition
Myoclonus (or myoclonic jerks) consists of sudden, brief (<100 ms), shock-like, involuntary movements resulting from:
- Positive myoclonus - brief active muscle contraction
- Negative myoclonus - brief inhibition of ongoing muscle activity (e.g., asterixis in hepatic failure is the most common negative myoclonic jerk)
The muscle contraction lasts only 20-50 ms - faster than chorea, with which it may be confused. - Adams and Victor's Principles of Neurology, 12th Ed.
Classification
1. By Etiology
| Type | Description | Examples |
|---|---|---|
| Physiological | Occurs in normal subjects | Hypnagogic (sleep-start) jerks, hiccups (singultus), anxiety-induced, exercise-induced, benign infantile myoclonus with feeding |
| Essential | No known cause, no gross neurological deficit | Hereditary (myoclonus-dystonia), sporadic; often co-occurs with postural tremor |
| Epileptic | Seizures dominate, no encephalopathy initially | Juvenile myoclonic epilepsy (Janz), Lennox-Gastaut, infantile spasms, epilepsia partialis continua, Baltic myoclonus (Unverricht-Lundborg) |
| Symptomatic | Progressive or static encephalopathy dominates | Metabolic (renal/hepatic failure), storage diseases, Lafora body disease, post-anoxic, toxic/drug-induced |
Bradley and Daroff's Neurology in Clinical Practice
2. By Anatomic Origin (Site)
| Origin | Features |
|---|---|
| Cortical | Most common; giant cortical potentials on EEG back-averaging; stimulus-sensitive |
| Subcortical | Thalamic, reticular (brainstem) myoclonus; generalized, often arrhythmic |
| Spinal | Segmental (rhythmic or arrhythmic) or propriospinal (flexion/extension of trunk, aggravated by stretching) |
| Peripheral | Rare; peripheral nerve or root origin; hemifacial spasm is an example |
3. By Distribution
- Focal - one body part
- Segmental - two or more contiguous regions
- Multifocal - non-contiguous regions, may be asynchronous
- Generalized (polymyoclonus) - widespread, lightning-like, arrhythmic; may involve the whole body
Clinical Characteristics
- Duration: < 100 ms (typically 20-50 ms); very fast - the fastest of all movement disorders
- Trigger patterns:
- Spontaneous - at rest, no provocation
- Action myoclonus - occurs during purposeful movement; often very disabling (e.g., Lance-Adams syndrome post-anoxia)
- Reflex myoclonus - triggered by visual, auditory, or tactile stimuli
- Frequency: ranges from single rare jerks to constant repetitive contractions
- Amplitude: from subtle (no joint displacement) to violent (moves the entire body)
- Rhythmicity: usually arrhythmic; rhythmic forms suggest brainstem or spinal cord origin (e.g., palatal myoclonus/palatal tremor)
Key Clinical Examples
| Condition | Myoclonus Features |
|---|---|
| Hypnagogic jerk | Physiological; occurs at sleep onset; benign |
| Asterixis | Negative myoclonus; bilateral flapping tremor; seen in hepatic/renal/respiratory encephalopathy |
| Juvenile Myoclonic Epilepsy (JME) | Action myoclonus on waking; exacerbated by fatigue and alcohol |
| Unverricht-Lundborg disease | Stimulus-sensitive polymyoclonus; progressive; autosomal recessive (CSTB gene) |
| Lafora body disease | Progressive myoclonic epilepsy; EPM2A/NHLRC1 mutations |
| Lance-Adams syndrome | Post-anoxic action myoclonus (after cardiac arrest) |
| Palatal myoclonus/tremor | Rhythmic soft palate movement; audible click; dentate-olivary pathway lesion |
Pathophysiology
Myoclonus generally arises in the CNS, though rare peripheral causes exist. The underlying mechanism is thought to involve overexcitability or impaired inhibition of cortical or subcortical neuronal circuits. In cortical myoclonus, enhanced cortical excitability (reduced GABAergic inhibition) generates brief discharges that spread via corticospinal tracts. The frequent association with epilepsy suggests a shared mechanism of abnormal synchronized neuronal firing. - Harrison's Principles of Internal Medicine, 22nd Ed. (2025)
Hereditary myoclonus can be grouped by phenotype:
- Prominent myoclonus alone
- Myoclonus + another movement disorder (e.g., dystonia, ataxia)
- Myoclonus as a manifestation of a broader syndrome
Diagnosis
- EEG back-averaging (jerk-locked back-averaging): identifies cortical correlate before the myoclonic jerk - essential for distinguishing cortical vs. subcortical origin
- EMG: burst duration < 100 ms; helps confirm true myoclonus vs. fasciculation/tremor
- MRI brain/spine: to identify structural causes (stroke, demyelination, tumor)
- Metabolic workup: renal, hepatic, electrolytes, glucose
- Genetic testing: for hereditary forms
Differential Diagnosis
Myoclonus must be distinguished from:
- Fasciculations - lower motor neuron origin, not CNS
- Chorea - longer duration contractions
- Tremor - rhythmic, oscillatory
- Tics - suppressible, stereotyped, preceded by urge (myoclonic jerks are not suppressible)
- Clonus - rhythmic contractions at a joint due to hyperreflexia/spasticity
- Epilepsia partialis continua - continuous rhythmic focal jerking
Treatment
Treatment is directed at the underlying cause first. Pharmacological options:
| Drug | Dose Range | Notes |
|---|---|---|
| Clonazepam | 1.5-15 mg/day | First-line for most myoclonus |
| Valproic acid | 10-15 mg/kg/day | Especially for epileptic myoclonus |
| Levetiracetam | 1000-4000 mg/day | Often very effective; used when others fail |
| Carbamazepine | 600-1200 mg/day | Select cases |
| Lacosamide | 200-400 mg/day | Select patients |
| 5-Hydroxytryptophan + carbidopa | 400-2800 mg/day + 75-300 mg/day | Specifically for Lance-Adams syndrome (post-anoxic action myoclonus) |
| Acetazolamide | 250-1000 mg/day | Some forms |
Goldman-Cecil Medicine; Harrison's 22nd Ed.
Sources: Goldman-Cecil Medicine | Bradley and Daroff's Neurology in Clinical Practice | Adams and Victor's Principles of Neurology (12th Ed.) | Harrison's Principles of Internal Medicine (22nd Ed., 2025)
Recent literature note: A 2025 systematic review (PMID 39773812) highlights the need for standardized terminology in post-cardiac arrest myoclonus, relevant when distinguishing Lance-Adams syndrome from malignant post-anoxic myoclonus. A 2026 review (PMID 42359663) addresses pathophysiology and prognostic significance of myoclonus in hypoxic-ischemic encephalopathy - an area where understanding continues to evolve.
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