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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 Some Trade Names
KLONOPIN
Click for Drug Monograph
, valproate Some Trade Names
DEPAKENE
Click for Drug Monograph
).

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 Some Trade Names TEGRETOL Click for Drug Monograph * Cephalosporins* Levodopa* Lithium Some Trade Names ESKALITH LITHOBID LITHONATE Click for Drug Monograph * MAO inhibitors* Opioids (usually dose related) Penicillin* Phenytoin Some Trade Names DILANTIN Click for Drug Monograph * Tricyclic antidepressants* Valproate Some Trade Names DEPAKENE Click for Drug Monograph * *At toxic or high doses. †With long-term treatment; dose-related. DDT = dichlorodiphenyltrichloroethane; MAO = monoamine oxidase.

Symptoms

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 Some Trade Names
KLONOPIN
Click for Drug Monograph
0.5 to 2 mg po tid is often effective. Valproate Some Trade Names
DEPAKENE
Click for Drug Monograph
250 to 500 mg po bid or levetiracetam Some Trade Names
KEPPRA
Click for Drug Monograph
250 to 500 mg po once/day to bid may be effective; rarely, other anticonvulsants help. Doses of clonazepam Some Trade Names
KLONOPIN
Click for Drug Monograph
or valproate Some Trade Names
DEPAKENE
Click for Drug Monograph
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 David Eidelberg, MD; Michael Pourfar, MD

Content last modified August 2007