Myoclonus
Myoclonus is a brief, shock-like jerk lasting under 1 second, produced by sudden muscle contraction (positive myoclonus) or sudden loss of tone (negative myoclonus, such as asterixis), arising from the cortex, subcortical structures, brainstem, spinal cord, or peripheral nerve. Toxic-metabolic causes are among the most common, so management starts with stopping the offending drug and correcting the metabolic derangement, then clonazepam or valproic acid for persistent jerks.
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What this article covers
- Overview
Definition: myoclonus is a sudden, brief (under 1 second), shock-like, jerky movement of an isolated body part or the whole body, produced by contraction of an entire muscle strong enough to move a joint.
- Epidemiology
Toxic-metabolic causes predominate: one of the most common causes of generalized myoclonus is a toxic-metabolic encephalopathy.
- Risk factors
Organ failure: kidney, hepatic, or respiratory failure (including hypercapnic CO2 narcosis) produces both positive and negative myoclonus.
- Pathophysiology
Cortical myoclonus: abnormal cortical discharges produce jerks that are often epileptic, with epileptic activity on EEG and giant somatosensory evoked potentials; posthypoxic myoclonus, corticobasal…
- Clinical features
Clinical diagnostic basis: myoclonus is diagnosed at the bedside by a jerk that is sudden, shock-like, lasts under 1 second, is not rhythmic (unlike tremor), is briefer than a tic and lacks a…
- Differential diagnosis
Myoclonic seizures: jerks under 1 second with retained awareness, often on waking, each accompanied by a generalized spike-and-wave discharge on EEG.
- Diagnosis
Established clinically: myoclonus is a clinical diagnosis made from the bedside features above (sudden jerk under 1 second, positive or negative, not rhythmic, not suppressible, not sustained)…
- Management
First, stop or reduce the offending drug: discontinue or lower opioids, tramadol, gabapentin, SSRIs, lithium, quinolones, amiodarone, lamotrigine, phenytoin, or carbamazepine, since drug-induced myoclonus resolves once the agent clears.
- Complications
Falls and injury: repetitive myoclonic jerks and posthypoxic negative myoclonus of the legs cause falls.
- Prognosis
Determined by the cause: physiologic myoclonus is benign, while drug-induced and metabolic myoclonus resolve with removal of the drug or correction of the derangement.
- Prevention
Avoid serotonergic combinations: do not combine two or more serotonergic drugs, and recognize early serotonin toxicity by Hunter criteria (spontaneous clonus, or inducible or ocular clonus with…
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