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), or 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 2: Movement and Cerebellar Disorders: 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.

Table 2
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Causes of Myoclonus Cause Examples Degeneration of the basal ganglia Lewy body dementia Huntington disease Parkinson disease Progressive supranuclear palsy Dementias Alzheimer disease Creutzfeldt-Jakob disease Progressive myoclonic encephalopathies (eg, sialidosis, neuronal ceroid lipofuscionosis, Unverricht-Lundborg disease) Metabolic disturbances Hypercapnia Hyperglycemia, nonketotic Hypocalcemia Hypoglycemia Hypomagnesemia Hyponatremia Liver failure Uremia Physical and hypoxic encephalopathies Electric shock Heatstroke Hypoxia Traumatic brain injury Toxic encephalopathies DDT Heavy metals (including bismuth) Methyl bromide Viral encephalopathies Encephalitis lethargica Herpes simplex encephalitis Postinfectious encephalitis Subacute sclerosing panencephalitis Drugs Antihistamines* Carbamazepine* Cephalosporins* Levodopa† Lithium* MAO inhibitors* Opioids (usually dose related) Penicillin* Phenytoin* Tricyclic antidepressants* Valproate* *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.

Diagnosis

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

Treatment

• Correction of metabolic disturbance
• Drug therapy to relieve symptoms

Treatment begins with correction of underlying metabolic disturbances or other causes if correctable.

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.

Last full review/revision January 2013 by Hector A. Gonzalez-Usigli, MD; Alberto Espay