What Is Myoclonus?
Myoclonus is the medical term for sudden, rapid, brief, involuntary jerking of a muscle or group of muscles. These shock-like movements may be caused by sudden muscle contractions (positive myoclonus) or sudden losses of muscle tone (negative myoclonus). Many different neurological disorders can cause myoclonus; therefore, neurologists consider it a symptom of disease rather than a specific diagnosis. Indeed, everyone experiences myoclonus on occasion. For example, "sleep starts," also known as “hypnic jerks,” are the shock-like twitches that some people experience while drifting off to sleep. This is a form of myoclonus that occurs in normal, healthy individuals. Pathological myoclonus, however, may be extremely disabling.
Myoclonus may affect a small region (focal or segmental myoclonus), such as one hand, or may produce violent jerks over the entire body (generalized myoclonus). Myoclonic jerks may occur alone or in sequence, either in a pattern or randomly. When patterned, they sometimes resemble more common forms of tremor. They may occur infrequently or many times each minute. All types of myoclonus are similar insofar as the movement cannot be controlled by the person who is experiencing it and, in contrast to a tic, there is no warning or premonitory sensation. In more severe cases, myoclonus can distort movement and severely limit a person’s ability to walk, talk and eat. These types of myoclonus may indicate that an underlying condition of the brain or nerves is causing the myoclonus.
Classifying the many different kinds of myoclonus is difficult, as the causes, clinical effects, and responses to treatment vary greatly. It can be classified in a number of ways. By distribution (body parts affected), myoclonus can be classified as focal, multifocal, or generalized and by provoking factors as spontaneous and reflex. Myoclonus can also be classified on physiology, meaning, the presumed source of its origin in the nervous system: cortical (brain), subcortical (between the brain and spinal cord), spinal, or peripheral (nerves outside of the central nervous system). It is useful for physicians to classify myoclonus by physiology because the presumed source of myoclonus (cortical, subcortical, spinal, or peripheral) helps guide the physician towards the most effective treatment.
Myoclonus can occur at rest, when maintaining posture, or during action. Stimulus sensitive myoclonus is a type of myoclonus triggered by outside stimuli such as lights, noise or movement. It can be brought on by surprise as well.
Types of Myoclonus
Myoclonus can also be classified by etiology (the underlying cause), which is described in more detail below.
Neurologic Diseases That Can Be Accompanied by Myoclonus
Examples of neurologic diseases that can be accompanied by myoclonus are listed below:
- Multiple sclerosis
- Parkinson's disease
- Dementia with Lewy bodies
- Corticobasal degeneration
- Multiple system atrophy
- Frontotemporal dementia
- Alzheimer's disease
- Creutzfeldt-Jakob disease
- Stiff-person syndrome
- Autoimmune encephalitis
- Celiac disease
Several types of secondary myoclonus are described in further detail below:
The diagnostic approach to a patient with myoclonus has two objectives: (1) identifying what part of the nervous system is producing myoclonus and (2) establishing the cause. Clinicians are able to categorize myoclonus on the basis of its distribution over the body, its electrophysiological characteristics, and its etiology (cause).
Physicians may order certain tests to learn what is causing the myoclonus. These may include:
- Electroencephalogram (EEG): a record of the brain’s electrical activity
- Electromyography (EMG): measures muscle activity and the electoral signals associated with myoclonus
- Imaging tests including MRI, CT scan, or PET scan to look for abnormalities such as lesions or tumors
- Blood tests to see if there any medical conditions that could be causing the myoclonus
Although some cases of myoclonus are caused by an injury to the peripheral nerves (defined as the nerves outside the brain and spinal cord), most forms of myoclonus are caused by a disturbance of the central nervous system (the brain and spinal cord). Studies suggest that several locations in the brain are involved in myoclonus. One such location is in the area of the brainstem located 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 over excitability 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 by binding to a receptor. Laboratory studies suggest that an imbalance between these chemicals may underlie myoclonus. Receptors that appear to be related to myoclonus include those for two important inhibitory neurotransmitters: serotonin, which constricts blood vessels and promotes sleep, and gamma-aminobutyric acid (GABA), which helps the brain maintain muscle control.
If possible, the underlying cause of myoclonus should be corrected, but this is not always possible. If the myoclonus is due to a medication side effect, then the myoclonus usually resolves with discontinuation of that medication. If myoclonus persists despite eliminating treatable causes, then the treatment of myoclonus otherwise focuses on medications help reduce symptoms.
The drugs used to treat myoclonus usually possess anti-seizure properties. Epileptic myoclonus and cortical (arising from the brain’s cerebral hemispheres) myoclonus respond best to clonazepam and sodium valproate, which may be used alone or in combination. Clonazepam is a tranquilizer and is commonly used to treat myoclonus. Dosages of clonazepam are usually increased gradually until the patient improves or side effects become bothersome. 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 medication.
If disability from the myoclonus is not adequately improved on either or both of these medications, then medications such as levetiracetam (Keppra) can be added, which is also very effective in treating myoclonus. Levetiracetam has also been shown to be effective in posthypoxic myoclonus. Primidone may also be of value was an additional drug, as well as clobazam and acetazolamide in severely affected patients.
Often, a single drug is not effective by itself, and combinations of medications are frequently required.
When medications fail, botulinum toxin can be used. In certain types of myoclonus, such as palatal myoclonus and hemifacial spasm, botulinum toxin (BTX) injections can be very effective. This medication is directly injected into the muscles causing the involuntary jerks at relatively low doses, allowing for relaxation of the muscles, thus preventing the spasms.
Rarely, surgical intervention known as deep brain stimulation (DBS) is considered as a last resort for certain forms of myoclonus. Further studies are needed to evaluate the effectiveness of such treatment. Tumors that cause myoclonus in children with opsoclonus-myoclonus may need to be surgically removed and/or treated with chemotherapy and radiation.
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