Myoclonus describes a symptom and generally is 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.
- Action myoclonus is characterized by muscular jerking triggered
or intensified by voluntary movement or even the intention to move. It may
be made worse by attempts at precise, coordinated movements. Action
myoclonus is the most disabling form of myoclonus and can affect the arms,
legs, face, and even the voice. This type of myoclonus often is caused by
brain damage that results from a lack of oxygen and blood flow to the
brain when breathing or heartbeat is temporarily stopped.
- Cortical reflex myoclonus is thought to be a type of epilepsy
that originates in the cerebral cortex - the outer layer, or "gray
matter," of the brain, responsible for much of the information processing
that takes place in the brain. In this type of myoclonus, jerks usually
involve only a few muscles in one part of the body, but jerks involving
many muscles also may occur. Cortical reflex myoclonus can be intensified
when patients attempt to move in a certain way or perceive a particular
sensation.
- Essential myoclonus occurs in the absence of epilepsy or other
apparent abnormalities in the brain or nerves. It can occur randomly in
people with no family history, but it also can appear among members of the
same family, indicating that it sometimes may be an inherited disorder.
Essential myoclonus tends to be stable without increasing in severity over
time. Some scientists speculate that some forms of essential myoclonus may
be a type of epilepsy with no known cause.
- Palatal myoclonus is a regular, rhythmic contraction of one or
both sides of the rear of the roof of the mouth, called the soft palate.
These contractions may be accompanied by myoclonus in other muscles,
including those in the face, tongue, throat, and diaphragm. The
contractions are very rapid, occurring as often as 150 times a minute, and
may persist during sleep. The condition usually appears in adults and can
last indefinitely. People with palatal myoclonus usually regard it as a
minor problem, although some occasionally complain of a "clicking" sound
in the ear, a noise made as the muscles in the soft palate contract.
- Progressive myoclonus epilepsy (PME) is a group of diseases
characterized by myoclonus, epileptic seizures, and other serious symptoms
such as trouble walking or speaking. These rare disorders often get worse
over time and sometimes are fatal. Studies have identified at least three
forms of PME. Lafora body disease is inherited as an autosomal
recessive disorder, meaning that the disease occurs only when a child
inherits two copies of a defective gene, one from each parent. Lafora body
disease is characterized by myoclonus, epileptic seizures, and dementia
(progressive loss of memory and other intellectual functions). A second
group of PME diseases belonging to the class of cerebral storage
diseases usually involves myoclonus, visual problems, dementia, and
dystonia (sustained muscle contractions that cause twisting movements or
abnormal postures). Another group of PME disorders in the class of
system degenerations often is accompanied by action myoclonus,
seizures, and problems with balance and walking. Many of these PME
diseases begin in childhood or adolescence.
- Reticular reflex myoclonus is thought to be a type of
generalized epilepsy that originates in the brainstem, the part of the
brain that connects to the spinal cord and controls vital functions such
as breathing and heartbeat. Myoclonic jerks usually affect the whole body,
with muscles on both sides of the body affected simultaneously. In some
people, myoclonic jerks occur in only a part of the body, such as the
legs, with all the muscles in that part being involved in each jerk.
Reticular reflex myoclonus can be triggered by either a voluntary movement
or an external stimulus.
- Stimulus-sensitive myoclonus is triggered by a variety of
external events, including noise, movement, and light. Surprise may
increase the sensitivity of the patient.
- Sleep myoclonus occurs during the initial phases of sleep,
especially at the moment of dropping off to sleep. Some forms appear to be
stimulus-sensitive. Some persons with sleep myoclonus are rarely troubled
by, or need treatment for, the condition. However, myoclonus may be a
symptom in more complex and disturbing sleep disorders, such as restless
legs syndrome, and may require treatment by a doctor.
Although some 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 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 type of tranquilizer. 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 antiepileptic 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-HTP), a
building block of serotonin, leads to improvement in patients 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 patients. These differences in the effect
of 5-HTP 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.
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. The National Institute of
Neurological Disorders and Stroke (NINDS), a unit of the Federal
Government's National Institutes of Health (NIH), is the agency with
primary responsibility for research on the brain and nervous system.
Investigators at NINDS laboratories 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.
Keeping on Top of Your Condition
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Moving Forward
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Myoclonus Research Foundation, Inc.
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Web: http://www.myoclonus.com/
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