Myoclonus -A brief, involuntary twitching of a muscle or a group of muscles. It
describes a medical sign and, generally, is not a diagnosis of a disease.
These twitches are caused by sudden muscle contractions. However, they can result
from brief lapses (relaxations) of contraction.
Contractions are positive myoclonus.
Relaxations are negative myoclonus.
A common time that these muscle contractions are experienced is:
During the transition from wakefulness to sleep, known as hypnic jerk.
Hiccups are also a kind of myoclonic jerk specifically affecting the diaphragm.
When a spasm is caused by another it is known as a "provoked spasm". Shuddering attacks
seen in babies also fall in this category.
Myoclonic jerks may occur:
Alone.
In a sequence.
In a pattern.
Without pattern.
Infrequently.
Many times each minute.
Myoclonus is one of several signs across a wide variety of nervous system disorders
such as:
Multiple Sclerosis.
Parkinson's Disease.
Alzheimer's Disease.
Subacute Sclerosing Panencephalitis.
Creutzfeldt-Jakob disease (CJD).
Serotonin toxicity.
Some forms of Epilepsy.
Myoclonus may originate from lesions of the cortex, subcortex or spinal cord.
Myoclonus may develop in response to:
Infection.
Head or spinal cord injury.
Stroke.
Brain tumours.
Kidney or liver failure.
Lipid storage disease.
Chemical poisoning.
Drug poisoning.
Prolonged oxygen deprivation to the brain, called hypoxia, may result in posthypoxic
myoclonus.
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
then 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.
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.
Neurotransmitters are released by neurons and attach themselves to receptors on parts
of neighbouring 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.
Types of myoclonus -Classifying the many different forms of myoclonus is difficult
because the causes, effects, and responses to therapy vary widely.
Types most commonly described are:
Action myoclonus - characterised 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 - 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 individuals 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. In some families, there is an association of essential myoclonus,
essential tremor, and even a form of dystonia, called myoclonus dystonia. Other
forms of essential myoclonus may be a type of epilepsy with no known cause.
Palatal myoclonus - 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) - a group of diseases characterised 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 many 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 characterised 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 - thought to be a type of generalized epilepsy that originates
in the brain stem, 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 individual.
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 health professional.
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.
Treatment of myoclonus focuses on medications that may help reduce symptoms. 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.
