Myoclonus is a brief, shocklike contraction of a muscle or group of muscles. Diagnosis is clinical and by selective testing. Treatment includes correction of reversible causes and sometimes oral drugs (eg, clonazepam, valproate).

Physiologic myoclonus may occur as a person falls asleep (nocturnal myoclonus). Myoclonus can result from other disorders and certain drugs (see Table 2: Movement and Cerebellar Disorders: Causes of Myoclonus). The most common causes are hypoxia, drug toxicity, and metabolic disturbances; other causes include degenerative disorders affecting the basal ganglia and some dementias.

Myoclonus may be focal, segmental (contiguous areas), multifocal (noncontiguous areas), or generalized.

Table 2

Causes of Myoclonus

Cause
Examples

Degeneration of the basal ganglia
Lewy body dementia

Huntington’s disease

Parkinson’s disease

Progressive supranuclear palsy

Dementias
Alzheimer’s disease

Creutzfeldt-Jakob 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 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.

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).