# 114 Myoclonus


Author(s): Nicholas DeMonaco and Robert Arnold MD

Background   Myoclonus is a movement disorder described as focal or generalized, sudden, brief, shock-like, involuntary movements caused by muscle contractions. This Fast Fact discusses its causes, evaluation, and therapy.

Characteristics and Differential Diagnosis   Hiccups are an example of normal, physiological myoclonus, while asterixis is an example of the myoclonus seen with generalized encephalopathy. In nocturnal myoclonus or periodic leg movement disorder, there is activity in the flexor muscles of the legs and feet during light sleep. It can be seen in the setting of chronic nervous system diseases or in elderly patients with no other abnormalities. The brief, shock-like movements of myoclonus may be difficult to distinguish from dystonia, which classically has repetitive, patterned, sustained movements. An acute dystonic reaction can occur with use of drugs which block dopamine receptors, including certain antipsychotics (haloperidol), antiemetics (metoclopramide), and calcium-channel blockers

Causes   The etiologies of myoclonus are numerous. Near the end of life, metabolic abnormalities and medication-induced myoclonus predominate. Metabolic causes include liver failure, renal failure, hyponatremia, and hypoglycemia. The medications and toxins associated with myoclonus include opioids, anticonvulsants (gabapentin, phenytoin, valproate, lamotrigine, and phenobarbitol), tricyclic antidepressants and selective serotonin reuptake inhibitors, contrast dye, anesthetics, antibiotics (penicillins, cephalosporins, imipenem, and quinolones), cannabinoids and the chemotherapeutic agent ifosfamide Opioid-induced myoclonus occurs commonly and is often misdiagnosed (See Fast Facts #57, 58). When myoclonus occurs due to toxins or medications, the jerks are usually generalized and may be provoked by a stimulus or voluntary movement. Other causes of myoclonus include focal CNS damage from a tumor or stroke, encephalopathies (viral, metabolic or degenerative), or seizure disorders.

Treatment   Myoclonus can disrupt sleep, make coordinated movements difficult, and be bothersome to patients or families. Treatment consists of correction of the underlying cause and symptomatic treatment of the myoclonus. If the offending agent is a non-essential medication, it should be discontinued. In the case of opioid-induced myoclonus, rotation to a different opioid may help. Benzodiazepines are the primary symptomatic treatment. While any benzodiazepine will work, a continuous infusion of midazolam has been suggested given the drug’s compatibility with morphine and short half-life, allowing rapid dose titration. Sedation is likely. If sedation is to be avoided, the muscle relaxant dantrolene in doses of 50-100 mg per day has also been reported as effective.


  1. Fahn S. Overview, history, and classification of myoclonus. Adv Neurol. 2002; 89:13-17.
  2. Gordon MF. Toxin and drug-induced myoclonus. Adv Neurol. 2002; 89:49-76.
  3. Jankovic J. Hyperkinetic movement disorders. In: Basow DS. UpToDate. Waltham, MA: UpToDate; 2003.
  4. Mercadante S. Pathophysiology and treatment of opioid-related myoclonus in cancer patients. Pain. 1998; 74:5-9.
  5. Rivest J. Myoclonus (Rev). Can J Neurol Sci. 2003; 30(Suppl 1):S53-58.

Fast Facts and Concepts are edited by Drew A Rosielle MD, Palliative Care Center, Medical College of Wisconsin. For more information write to: drosiell@mcw.edu. More information, as well as the complete set of Fast Facts, are available at EPERC: www.eperc.mcw.edu.

Version History: This Fast Fact was originally edited by David E Weissman MD and published in May 2004. Re-copy-edited in April 2009.

Copyright/Referencing Information: Users are free to download and distribute Fast Facts for educational purposes only. DeMonaco D, Arnold R. Myoclonus. Fast Facts and Concepts. April 2004; 114. Available at: http://www.eperc.mcw.edu/EPERC/FastFactsIndex/ff_114.htm.

Disclaimer: Fast Facts and Concepts provide educational information. This information is not medical advice. Health care providers should exercise their own independent clinical judgment. Some Fast Facts cite the use of a product in a dosage, for an indication, or in a manner other than that recommended in the product labeling. Accordingly, the official prescribing information should be consulted before any such product is used.

ACGME Competencies: Medical Knowledge, Patient Care

Keyword(s): Non-Pain Symptoms and Syndromes