Fast Fact and Concept #114: Myoclonus

Author(s): N DeMonaco and R Arnold

Myoclonus is a movement disorder described as focal or generalized, sudden, brief, shock-like, involuntary movements caused by muscle contractions. 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.

The differential diagnosis of myoclonus is extensive. 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 phenobarbitone), tricyclic antidepressants and SSRI's, 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.

Myoclonus may 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. If sedation is to be avoided, the muscle relaxant dantrolene in doses of 50-100 mg per day has also been effective.

References

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

Fast Facts were edited by David Weissman MD, Palliative Care Center, Medical College of Wisconsin until January 2007.  For comments/questions write to the current editor, Drew Rosielle MD: drosiell@mcw.edu. The complete set of Fast Facts is available at EPERC: www.eperc.mcw.edu

Copyright/Referencing Information: Users are free to download and distribute Fast Facts for educational purposes only. Citation for referencing: DeMonaco D and Arnold R. Fast Facts and Concepts #114. Myoclonus. May 2004. End-of-Life/Palliative Education Resource Center www.eperc.mcw.edu.

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Creation Date: 5/2004

Purpose: Instructional Aid, Self-Study Guide, Teaching

Audience(s)

ACGME Competencies: Medical Knowledge, Patient Care

Keyword(s): Non pain symptoms & syndromes, Other neurologic disorders

Specific Disease and Organ System Category(s): Other neurological disorde