NORD gratefully acknowledges John N. Caviness, MD, Professor of Neurology, Mayo Clinic College of Medicine, for assistance in the preparation of this report.
Myoclonus is the term used to describe the sudden, involuntary jerking of a muscle or group of muscles caused by muscle contractions (positive myoclonus) or muscle relaxation (negative myoclonus). The twitching or jerking of muscles cannot be controlled by the person experiencing it. Myoclonic jerks may occur infrequently or many times a minute. They sometimes occur in response to an external event or when a person attempts to make a movement. By itself, myoclonus may be seen as a symptom rather than a disease. To some degree, it may occur occasionally to otherwise healthy people. (For instance, hiccups may be considered a type of myoclonus.) In severe cases, it can interfere with movement control and balance, and limit various everyday activities such as eating or talking.
There are different forms of myoclonus and they are classified in different ways. One way of classifying the different forms is by their cause. Some of the types of myoclonus are:
This occurs in neurologically normal people. The occurrence of myoclonus during sleep and sleep transitions is the most common example.
In this type, the myoclonic jerks or twitches are usually the most prominent or only clinical finding. This type of myoclonus usually progresses slowly or not at all. There are hereditary (autosomal dominant) and non-inherited, random (sporadic) forms.
Progressive myoclonus epilepsy (PME)
This is a group of diseases characterized by myoclonus, epilepsy, and other symptoms such as trouble walking or speaking. These disorders tend to get worse over time (progressive).
This typically occurs just at the moment of dropping off to sleep. In some cases, the affected individual does not find it particularly troublesome. In other cases, it may interfere with the sleep process. Myoclonus may be a symptom in certain sleep disorders such as restless legs syndrome.
Symptomatic (secondary) myoclonus
This is the most common category and usually is found in the setting of an identifiable underlying disorder. Myoclonus may not be the most prominent clinical symptom. Common co-existing problems include ataxia, dementia, and Parkinsonism. Myoclonus may also be a symptom associated with infections, non-neurologic medical illnesses, toxic-metabolic states, and storage diseases.
Myoclonus is caused by an abrupt and brief discharge of motor neurons to affected muscles. In most cases, this results from a disturbance in the central nervous system, although it is believed that in rare cases may be caused by an injury to the nerves outside the central nervous system (peripheral nerves). Several different locations within the brain are thought to be involved in myoclonus. As a result, various types of testing is usually required to define the cause of myoclonus.
The locations that cause myoclonus are reflected in the physiological classification of myoclonus:
– Cortical (Focal or multifocal source)
– Cortical-Subcortical (e.g. Myoclonic Epilepsy)
Chemicals that carry messages from one nerve cell to another (neurotransmitters) may play a role. In some cases, myoclonus may be present because of an imbalance in these chemicals. However, the specific causes are not well understood at this time.
Myoclonus affects males and females in equal numbers. Some forms of myoclonus are common and some forms are rare. In general, the incidence of myoclonus is 1.3 cases per 100,000 person-years, and the prevalence is 8.6 cases per 100,000 populations.
If the cause for the underlying disorder cannot be cured, then the standard treatment for myoclonus is medications that may help reduce symptoms. The first line of therapy depends on where the myoclonus originates within the nervous system (i.e. physiological classification).For cortical myoclonus (most common source), this would include levetiracetam and/or valproic acid, a type of tranquilizer, and other drugs known as benzodiazepine derivatives which include clonazepam. The beneficial effects of certain drugs, including clonazepam, may diminish over time.
Many of the drugs used for myoclonus, such as barbiturates, phenytoin, and primidone, are also used to treat epilepsy. Certain of these drugs may have side effects such as sleepiness, unsteady gait (ataxia), or lethargy, and patients and their families should be aware of these beforehand.
Newer therapies have been suggested for very specific types of myoclonus. Deep brain stimulation may be considered for the inherited Myoclonus-Dystonia Syndrome, a type of Subcortical/Nonsegmental myoclonus. Botulinum toxin has been tried for palatal myoclonus, a type of segmental myoclonus. Extensive physician consultation is needed before these therapies should be performed.
Genetic counseling will be of benefit for patients with the inherited forms of myoclonus and their families.
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Some studies have shown that doses of 5-hydroxytryptophan (5-HTP), a building block of serotonin, leads to improvement in patients with some types of myoclonus. However, other studies indicate that this therapy is not effective in all people with myoclonus and may even cause the jerks and twitches to worsen in some affected individuals.
Because myoclonus is complex in nature it may require a combination of drugs for effective treatment. Some drugs currently being studied in different combinations include levetiracetam, clonazepam, sodium valproate, and primidone. In some people, treatment may also include hormonal therapy.
Caviness J. Myoclonus in NORD Guide to Rare Disorders. Lippincott, Williams & Wilkins. 2003. 627-28.
Caviness JN. Treatment of Myoclonus. Neurotherapeutics. 2014;11:188-200.
Camfield P, Camfield C. Long-term prognosis for symptomatic (secondarily) generalized epilepsies: a population-based study. Epilepsia. 2007;48(6):1128-32.
Abraham A, Elena C, Melamed E, Djaldetti R. Successful treatment of truncal myoclonus. Mov Disord. 2007; 5;22(7):1055-6.
Walters AS. Clinical identification of the simple sleep-related movement disorders. Chest. 2007; 131:1260-66.
National Institute of Neurological Disorders and Stroke Myoclonus Fact Sheet. Last updated February 23, 2015. http://www.ninds.nih.gov/disorders/myoclonus/detail_myoclonus.htm?css=print Accessed March 26, 2015.
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