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Myoclonus (pronounced /maɪˈɒklənəs/) is brief, involuntary twitching of a muscle or a group of muscles. It describes a medical sign (as opposed to symptom) and, generally, is not a diagnosis of a disease. The myoclonic twitches or jerks are usually caused by sudden muscle contractions; they also can result from brief lapses of contraction. Contractions are called positive myoclonus; relaxations are called negative myoclonus. The most common time for people to encounter them is while falling asleep (hypnic jerk), but myoclonic jerks are also a sign of a number of neurological disorders. Hiccups are also a kind of myoclonic jerk specifically affecting the diaphragm.
Myoclonic jerks may occur alone or in sequence, in a pattern or without pattern. They may occur infrequently or many times each minute. Most often, myoclonus is one of several signs in a wide variety of nervous system disorders such as multiple sclerosis, Parkinson's disease, Alzheimer's disease, Subacute sclerosing panencephalitis and Creutzfeldt-Jakob disease and some forms of epilepsy.
In almost all instances in which myoclonus is caused by CNS disease it is preceded by other symptoms; for instance, in CJD it is generally a late-stage clinical feature that appears after the patient has already started to exhibit gross neurological deficits.
Anatomically, myoclonus may originate from lesions of the cortex, subcortex or spinal cord. The presence of myoclonus above the foramen magnum effectively excludes spinal myoclonus, but further localisation relies on further investigation with electromyography (EMG) and electroencephalography (EEG).
Myoclonic seizures can be described as 'jumps'. They are caused by rapid contraction and relaxation of the muscles. People without epilepsy can suffer small but similar jerks in the form of hiccups or brief twitches; these are perfectly normal.
In someone with epilepsy, myoclonic seizures cause abnormal movements on both sides of the body at the same time. In reflex epilepsies, myoclonic seizures can be brought on by flashing lights or other environmental triggers. (See photosensitive epilepsy.)
Familiar examples of normal myoclonus include hiccups and hypnic jerks that some people experience while drifting off to sleep. A similar but distinguishable type of jerk is often experienced during meditation. Severe cases of pathologic myoclonus can distort movement and severely limit a person's ability to sleep, eat, talk, and walk. Myoclonic jerks commonly occur in individuals with epilepsy. The most common types of myoclonus include action, cortical reflex, essential, palatal, progressive myoclonus epilepsy, reticular reflex, sleep, and stimulus-sensitive.
In juvenile myoclonic epilepsy, seizures usually involve the neck, shoulders, and upper arms. These seizures typically occur shortly after waking up. They normally begin between puberty and early adulthood. They can usually be controlled with medication, but it must be taken for life.
In rare cases, myoclonic seizures can be symptomatic of Lennox-Gastaut syndrome, beginning in early childhood and usually involving the face, neck, shoulders, and upper arms. In these cases, the seizures tend to be strong and difficult to control.
Progressive myoclonic epilepsy includes both myoclonic and tonic-clonic seizures. Treatment is not normally successful for any extended period of time. 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.
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. Benign myoclonic movements are commonly seen during the induction of general anesthesia with intravenous medications such as etomidate and propofol. These are postulated to result from decreased inhibitory signaling from cranial neurons. 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 or Lupus. Myoclonic jerks commonly occur in persons with epilepsy, a disorder in which the electrical activity in the brain becomes disordered leading to seizures.
Although some cases of myoclonus are caused by an injury to the peripheral nerves, 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 brainstem 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, which constricts blood vessels and brings on sleep, and gamma-aminobutyric acid (GABA), which helps the brain maintain muscle control. Other receptors with links to myoclonus include those for opiates, drugs that induce sleep, and for glycine, 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 benzodiazepine. Dosages of clonazepam usually are increased gradually until the patient 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 patient develops a tolerance for the drug.
Many of the drugs used for myoclonus, such as barbiturates, phenytoin and primidone, are also used to treat epilepsy. Barbiturates slow down the central nervous system and cause tranquilizing or antiseizure effects. Phenytoin and primidone are effective antiepileptics drugs, although phenytoin can cause liver failure or have other harmful long-term effects in patients 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 patients with myoclonus, some people have adverse reactions to these drugs.
Some studies have shown that doses of 5-hydroxytryptophan (5-HT) leads to improvement in patients with some types of action myoclonus and PME. However, other studies indicate that 5-HT therapy is not effective in all people with myoclonus, and, in fact, may worsen the condition in some patients. These differences in the effect of 5-HT on patients 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, piracetam, and primidone. Hormonal therapy also may improve responses to antimyoclonic drugs in some people. Alcohol taken before sleep seems to help this condition, long term use is not recommended and it is not a cure.
Although myoclonus is not a life-threatening condition, it may result in serious, debilitating impairments. Action Myoclonus with its positive and negative myoclonus components is generally considered the most serious.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Myoclonus". A list of authors is available in Wikipedia.|