Myoclonus describes a symptom and not a diagnosis of a disease. It refers to sudden, involuntary jerking of a muscle or group of muscles. Myoclonic twitches or jerks usually are caused by sudden muscle contractions, called positive myoclonus, or by muscle relaxation, called negative myoclonus. Myoclonic jerks may occur alone or in sequence, in a pattern or without pattern. They may occur infrequently or many times each minute. Myoclonus sometimes occurs in response to an external event or when a person attempts to make a movement. The twitching cannot be controlled by the person experiencing it.
In its simplest form, myoclonus consists of a muscle twitch followed by relaxation. A hiccup is an example of this type of myoclonus. Other familiar examples of myoclonus are the jerks or "sleep starts" that some people experience while drifting off to sleep. These simple forms of myoclonus occur in normal, healthy persons and cause no difficulties. When more widespread, myoclonus may involve persistent, shock-like contractions in a group of muscles. In some cases, myoclonus begins in one region of the body and spreads to muscles in other areas. More severe cases of myoclonus can distort movement and severely limit a person's ability to eat, talk, or walk. These types of myoclonus may indicate an underlying disorder in the brain or nerves.
Myoclonus may develop in response to infection, head or spinal cord injury, stroke, brain tumors, kidney or liver failure, lipid storage disease, chemical or drug poisoning, or other disorders. Prolonged oxygen deprivation to the brain, called hypoxia, may result in posthypoxic myoclonus. Myoclonus can occur by itself, but most often it is one of several symptoms associated with a wide variety of nervous system disorders. For example, myoclonic jerking may develop in patients with multiple sclerosis, Parkinson's disease, Alzheimer's disease, or Creutzfeldt-Jakob disease. Myoclonic jerks commonly occur in persons with epilepsy, a disorder in which the electrical activity in the brain becomes disordered leading to seizures.
Classifying the many different forms of myoclonus is difficult because the causes, effects, and responses to therapy vary widely. Listed below are the types most commonly described.
Although rare cases of myoclonus are caused by an injury to the peripheral nerves (defined as the nerves outside the brain and spinal cord, or the central nervous system), most myoclonus is caused by a disturbance of the central nervous system. Studies suggest that several locations in the brain are involved in myoclonus. One such location, for example, is in the brain stem close to structures that are responsible for the startle response, an automatic reaction to an unexpected stimulus involving rapid muscle contraction.
The specific mechanisms underlying myoclonus are not yet fully understood. Scientists believe that some types of stimulus-sensitive myoclonus may involve overexcitability of the parts of the brain that control movement. These parts are interconnected in a series of feedback loops called motor pathways. These pathways facilitate and modulate communication between the brain and muscles. Key elements of this communication are chemicals known as neurotransmitters, which carry messages from one nerve cell, or neuron, to another. Neurotransmitters are released by neurons and attach themselves to receptors on parts of neighboring cells. Some neurotransmitters may make the receiving cell more sensitive, while others tend to make the receiving cell less sensitive. Laboratory studies suggest that an imbalance between these chemicals may underlie myoclonus.
Some researchers speculate that abnormalities or deficiencies in the receptors for certain neurotransmitters may contribute to some forms of myoclonus. Receptors that appear to be related to myoclonus include those for two important inhibitory neurotransmitters: serotonin and gamma-aminobutyric acid (GABA). Other receptors with links to myoclonus include those for opiates and glycine, the latter an inhibitory neurotransmitter that is important for the control of motor and sensory functions in the spinal cord. More research is needed to determine how these receptor abnormalities cause or contribute to myoclonus.
Treatment of myoclonus focuses on medications that may help reduce symptoms. The drug of first choice to treat myoclonus, especially certain types of action myoclonus, is clonazepam, a type of tranquilizer. Dosages of clonazepam usually are increased gradually until the individual improves or side effects become harmful. Drowsiness and loss of coordination are common side effects. The beneficial effects of clonazepam may diminish over time if the individual develops a tolerance for the drug.
Many of the drugs used for myoclonus, such as barbiturates, levetiracetam,phenytoin, and primidone, are also used to treat epilepsy. Barbiturates slow down the central nervous system and cause tranquilizing or antiseizure effects. Phenytoin, levetiracetam, and primidone are effective antiepileptic drugs, although phenytoin can cause liver failure or have other harmful long-term effects in individuals with PME. Sodium valproate is an alternative therapy for myoclonus and can be used either alone or in combination with clonazepam. Although clonazepam and/or sodium valproate are effective in the majority of people with myoclonus, some people have adverse reactions to these drugs.
Some studies have shown that doses of 5-hydroxytryptophan (5-HTP), a building block of serotonin, leads to improvement in people with some types of action myoclonus and PME. However, other studies indicate that 5-HTP therapy is not effective in all people with myoclonus, and, in fact, may worsen the condition in some individuals. These differences in the effect of 5-HTP on individuals with myoclonus have not yet been explained, but they may offer important clues to underlying abnormalities in serotonin receptors.
The complex origins of myoclonus may require the use of multiple drugs for effective treatment. Although some drugs have a limited effect when used individually, they may have a greater effect when used with drugs that act on different pathways or mechanisms in the brain. By combining several of these drugs, scientists hope to achieve greater control of myoclonic symptoms. Some drugs currently being studied in different combinations include clonazepam, sodium valproate, levetiracetam, and primidone. Hormonal therapy also may improve responses to antimyoclonic drugs in some people.
Within the Federal government, the National Institute of Neurological Disorders and Stroke (NINDS), a component of the National Institutes of Health (NIH), has primary responsibility for research on the brain and nervous system. As part of its mission, the NINDS supports research on myoclonus at its laboratories in Bethesda, Maryland, and through grants to universities and major medical institutions across the country.
Scientists are seeking to understand the underlying biochemical basis of involuntary movements and to find the most effective treatment for myoclonus and other movement disorders.
Investigators are evaluating the role of neurotransmitters and receptors in myoclonus. If abnormalities in neurotransmitters or receptors are found to play a causative role in myoclonus, future research can focus on determining the extent to which genetic alterations are responsible for these abnormalities and on identifying the nature of those alterations. Scientists also may be able to develop drug treatments that target specific changes in the receptors to reverse abnormalities, such as the loss of inhibition, and to enhance mechanisms that compensate for these abnormalities. Identifying receptor abnormalities also may help researchers develop diagnostic tests for myoclonus. NINDS-supported scientists at research institutions throughout the country are studying various aspects of PME, including the basic mechanisms and genes involved in this group of diseases.
For more information on neurological disorders or research programs funded by the National Institute of Neurological Disorders and Stroke, contact the Institute's Brain Resources and Information Network (BRAIN) at:
P.O. Box 5801
Bethesda, MD 20824
Information also is available from the following organizations:
National Organization for Rare Disorders (NORD)
55 Kenosia Avenue
Danbury, CT 06810
Tel: 203-744-0100 Voice Mail 800-999-NORD (6673)
"MyoclonusFact Sheet", NINDS, Publication date July 2012.
NIH Publication No. 12-4793
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Last Modified February 23, 2015