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Myoclonus

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 MyoclonusTables). 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*

CarbamazepineSome Trade Names
TEGRETOL
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*

Cephalosporins*

Levodopa

LithiumSome Trade Names
ESKALITH
LITHOBID
LITHONATE
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*

MAO inhibitors*

Opioids (usually dose related)

Penicillin*

PhenytoinSome Trade Names
DILANTIN
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*

Tricyclic antidepressants*

ValproateSome Trade Names
DEPAKENE
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*

*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, clonazepamSome Trade Names
KLONOPIN
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0.5 to 2 mg po tid is often effective. ValproateSome Trade Names
DEPAKENE
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250 to 500 mg po bid or levetiracetamSome Trade Names
KEPPRA
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250 to 500 mg po once/day to bid may be effective; rarely, other anticonvulsants help. Doses of clonazepamSome Trade Names
KLONOPIN
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or valproateSome Trade Names
DEPAKENE
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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).

Last full review/revision August 2007 by Hector A. Gonzalez-Usigli; Alberto Espay

Content last modified August 2007

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