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by Hector A. Gonzalez-Usigli, MD

Myoclonus is a brief, shocklike contraction of a muscle or group of muscles. Diagnosis is clinical and sometimes confirmed by electromyographic testing. Treatment includes correction of reversible causes and, when necessary, oral drugs to relieve symptoms.

Myoclonus may be

  • Focal

  • Segmental (contiguous areas)

  • Multifocal (noncontiguous areas)

  • Generalized

It may be physiologic or pathologic.

Physiologic myoclonus may occur when a person is falling asleep and during early sleep phases (called hypnic myoclonus). Hypnic myoclonus can be focal, multifocal, segmental, or generalized and may resemble a startle reaction. Another form of physiologic myoclonus is hiccuping (diaphragmatic myoclonus).

Pathologic myoclonus can result from various disorders and drugs (see Table: Causes of Myoclonus). The most common causes are

  • Hypoxia

  • Drug toxicity

  • Metabolic disturbances

Other causes include degenerative disorders affecting the basal ganglia and some dementias.

Causes of Myoclonus



Degeneration of the basal ganglia

Lewy body dementia

Huntington disease

Parkinson disease

Progressive supranuclear palsy


Alzheimer disease

Creutzfeldt-Jakob disease

Progressive myoclonic encephalopathies (eg, mitochondrial disorders, certain types of epilepsy, such as sialidosis, neuronal ceroid lipofuscinosis, and Unverricht-Lundborg disease)

Metabolic disturbances


Hyperglycemia, nonketotic





Liver failure


Physical and hypoxic encephalopathies

Electric shock



Traumatic brain injury

Toxic encephalopathies


Heavy metals (including bismuth)

Methyl bromide

Viral encephalopathies

Encephalitis lethargica

Herpes simplex encephalitis

Postinfectious encephalitis

Subacute sclerosing panencephalitis







MAO inhibitors*

Opioids (usually dose related)



Tricyclic antidepressants*



*At toxic or high doses.

With long-term treatment; dose-related.

DDT = dichlorodiphenyltrichloroethane; MAO = monoamine oxidase.

Symptoms and Signs

Myoclonus can vary in amplitude, frequency, and distribution.

Muscle jerks may occur spontaneously or be induced by a stimulus (eg, sudden noise, movement, light, visual threat). Myoclonus that occurs when patients are suddenly startled (startle myoclonus) may be an early symptom of Creutzfeldt-Jacob disease. Myoclonus due to severe closed head trauma or hypoxic-ischemic brain damage may worsen with purposeful movements (action myoclonus) or may occur spontaneously when movement is limited because of injury.

Myoclonus due to metabolic disturbances may be multifocal, asymmetric, and stimulus-induced; it usually involves facial or proximal limb muscles. If the disturbance persists, generalized myoclonic jerks and, ultimately, seizures may occur.


  • Clinical evaluation

Diagnosis of myoclonus is clinical. Testing is done based on clinically suspected causes.


  • Correcting the metabolic disturbance or other cause if possible

  • Stopping or reducing the dose of the causative drug

  • Drug therapy to relieve symptoms

Treatment of myoclonus begins with correction of underlying metabolic disturbances or other causes if correctable. If a drug is the cause, the drug is stopped, or the dose is reduced.

For symptom relief, clonazepam 0.5 to 2 mg po tid is often effective. Valproate 250 to 500 mg po bid or levetiracetam 250 to 500 mg po once/day to bid may be effective; rarely, other anticonvulsants help. Doses of clonazepam or valproate may need to be lower in the elderly.

Many forms of myoclonus respond to the serotonin precursor 5-hydroxytryptophan (initially, 25 mg po qid, increased to 150 to 250 mg po qid), which must be used with the oral decarboxylase inhibitor carbidopa (50 mg every morning and 25 mg at noon or 50 mg every evening and 25 mg at bedtime).

Key Points

  • Myoclonus is a brief, shocklike muscle contraction that can vary in severity and distribution.

  • Myoclonus can be physiologic (eg, hiccuping, sleep-related muscle contractions) or secondary to various brain disorders, systemic disorders, or drugs.

  • If a metabolic disturbance is the cause, correct it, and when necessary, give drugs (eg, clonazepam, valproate, levetiracetam) to relieve symptoms.

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