Myoclonus

From: Merck Manual

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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 KLONOPINClick for Drug Monograph , valproate Some Trade Names DEPAKENEClick 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 TEGRETOLClick for Drug Monograph *
Cephalosporins*
Levodopa*
Lithium Some Trade Names ESKALITHLITHOBIDLITHONATEClick for Drug Monograph *
MAO inhibitors*
Opioids (usually dose related)
Penicillin*
Phenytoin Some Trade Names DILANTINClick for Drug Monograph *
Tricyclic antidepressants*
Valproate Some Trade Names DEPAKENEClick 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 KLONOPINClick for Drug Monograph 0.5 to 2 mg po tid is often effective. Valproate Some Trade Names DEPAKENEClick for Drug Monograph 250 to 500 mg po bid or levetiracetam Some Trade Names KEPPRAClick 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 KLONOPINClick for Drug Monograph or valproate Some Trade Names DEPAKENEClick 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
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