Myasthenia Gravis Fact Sheet
Table of Contents
Myasthenia gravis is a chronic autoimmune neuromuscular disease characterized
by varying degrees of weakness of the skeletal (voluntary) muscles of the body.
The name myasthenia gravis, which is Latin and Greek in origin, literally means
"grave muscle weakness." With current therapies, however, most cases of
myasthenia gravis are not as "grave" as the name implies. In fact, for the
majority of individuals with myasthenia gravis, life expectancy is not lessened
by the disorder.
The hallmark of myasthenia gravis is muscle weakness that increases during
periods of activity and improves after periods of rest. Certain muscles such as
those that control eye and eyelid movement, facial expression, chewing, talking,
and swallowing are often, but not always, involved in the disorder. The muscles
that control breathing and neck and limb movements may also be affected.
Myasthenia gravis is caused by a defect in the transmission of nerve impulses
to muscles. It occurs when normal communication between the nerve and muscle is
interrupted at the neuromuscular junction - the place where nerve cells connect
with the muscles they control. Normally when impulses travel down the nerve, the
nerve endings release a neurotransmitter substance called acetylcholine.
Acetylcholine travels through the neuromuscular junction and binds to
acetylcholine receptors which are activated and generate a muscle contraction.
In myasthenia gravis, antibodies block, alter, or destroy the receptors for
acetylcholine at the neuromuscular junction which prevents the muscle
contraction from occurring. These antibodies are produced by the body's own
immune system. Thus, myasthenia gravis is an autoimmune disease because the
immune system - which normally protects the body from foreign organisms -
mistakenly attacks itself.
The thymus gland, which lies in the upper chest area beneath the breastbone,
plays an important role in the development of the immune system in early life.
Its cells form a part of the body's normal immune system. The gland is somewhat
large in infants, grows gradually until puberty, and then gets smaller and is
replaced by fat with age. In adults with myasthenia gravis, the thymus gland is
abnormal. It contains certain clusters of immune cells indicative of lymphoid
hyperplasia - a condition usually found only in the spleen and lymph nodes
during an active immune response. Some individuals with myasthenia gravis
develop thymomas or tumors on the thymus gland. Generally thymomas are benign,
but they can become malignant.
The relationship between the thymus gland and myasthenia gravis is not yet
fully understood. Scientists believe the thymus gland may give incorrect
instructions about the production of the acetylcholine receptor antibodies,
thereby setting the stage for the attack on neuromuscular transmission.
Although myasthenia gravis may affect any voluntary muscle, muscles that
control eye and eyelid movement, facial expression, and swallowing are most
frequently affected. The onset of the disorder may be sudden. Symptoms often are
not immediately recognized as myasthenia gravis.
In most cases, the first noticeable symptom is weakness of the eye muscles.
In others, difficulty in swallowing and slurred speech may be the first signs.
The degree of muscle weakness involved in myasthenia gravis varies greatly among
patients, ranging from a localized form, limited to eye muscles (ocular
myasthenia), to a severe or generalized form in which many muscles - sometimes
including those that control breathing - are affected. Symptoms, which vary in
type and severity, may include a drooping of one or both eyelids (ptosis),
blurred or double vision (diplopia) due to weakness of the muscles that control
eye movements, unstable or waddling gait, weakness in arms, hands, fingers,
legs, and neck, a change in facial expression, difficulty in swallowing and
shortness of breath, and impaired speech (dysarthria).
Myasthenia gravis occurs in all ethnic groups and both genders. It most
commonly affects young adult women (under 40) and older men (over 60), but it
can occur at any age.
In neonatal myasthenia, the fetus may acquire immune proteins (antibodies)
from a mother affected with myasthenia gravis. Generally, cases of neonatal
myasthenia gravis are transient (temporary) and the child's symptoms usually
disappear within few weeks after birth. Other children develop myasthenia gravis
indistinguishable from adults. Myasthenia gravis in juveniles is common.
Myasthenia gravis is not directly inherited nor is it contagious.
Occasionally, the disease may occur in more than one member of the same family.
Rarely, children may show signs of congenital myasthenia or congenital
myasthenic syndrome. These are not autoimmune disorders, but are caused by
defective genes that control proteins in the acetylcholine receptor or in
acetylcholineterase.
Unfortunately, a delay in diagnosis of one or two years is not unusual in
cases of myasthenia gravis. Because weakness is a common symptom of many other
disorders, the diagnosis is often missed in people who experience mild weakness
or in those individuals whose weakness is restricted to only a few muscles.
The first steps of diagnosing myasthenia gravis include a review of the
individual's medical history, and physical and neurological examinations. The
signs a physician must look for are impairment of eye movements or muscle
weakness without any changes in the individual's ability to feel things. If the
doctor suspects myasthenia gravis, several tests are available to confirm the
diagnosis.
A special blood test can detect the presence of immune molecules or
acetylcholine receptor antibodies. Most patients with myasthenia gravis have
abnormally elevated levels of these antibodies. However, antibodies may not be
detected in patients with only ocular forms of the disease.
Another test is called the edrophonium test. This approach requires the
intravenous administration of edrophonium chloride or Tensilon(r), a drug that
blocks the degradation (breakdown) of acetylcholine and temporarily increases
the levels of acetylcholine at the neuromuscular junction. In people with
myasthenia gravis involving the eye muscles, edrophonium chloride will briefly
relieve weakness. Other methods to confirm the diagnosis include a version of
nerve conduction study which tests for specific muscle fatigue by repetitive
nerve stimulation. This test records weakening muscle responses when the nerves
are repetitively stimulated, and helps to differentiate nerve disorders from
muscle disorders. Repetitive stimulation of a nerve during a nerve conduction
study may demonstrate decrements of the muscle action potential due to impaired
nerve-to-muscle transmission.
A different test called single fiber electromyography (EMG), in which single
muscle fibers are stimulated by electrical impulses, can also detect impaired
nerve-to-muscle transmission. EMG measures the electrical potential of muscle
cells. Muscle fibers in myasthenia gravis, as well as other neuromuscular
disorders, do not respond as well to repeated electrical stimulation compared to
muscles from normal individuals.
Computed tomography (CT) or magnetic resonance imaging (MRI) may be used to
identify an abnormal thymus gland or the presence of a thymoma.
A special examination called pulmonary function testing - which measures
breathing strength - helps to predict whether respiration may fail and lead to a
myasthenic crisis.
Today, myasthenia gravis can be controlled. There are several therapies
available to help reduce and improve muscle weakness. Medications used to treat
the disorder include anticholinesterase agents such as neostigmine and
pyridostigmine, which help improve neuromuscular transmission and increase
muscle strength. Immunosuppressive drugs such as prednisone, cyclosporine, and
azathioprine may also be used. These medications improve muscle strength by
suppressing the production of abnormal antibodies. They must be used with
careful medical followup because they may cause major side effects.
Thymectomy, the surgical removal of the thymus gland (which is abnormal in
myasthenia gravis patients), improves symptoms in more than 50 percent of
patients without thymoma and may cure some individuals, possibly by re-balancing
the immune system. Other therapies used to treat myasthenia gravis include
plasmapheresis, a procedure in which abnormal antibodies are removed from the
blood, and high-dose intravenous immune globulin, which temporarily modifies the
immune system and provides the body with normal antibodies from donated blood.
These therapies may be used to help individuals during especially difficult
periods of weakness. A neurologist, along with the primary care physician, will
determine which treatment option is best for each individual depending on the
severity of the weakness, which muscles are affected, and the individual's age
and other associated medical problems.
A myasthenic crisis occurs when weakness affects the muscles that control
breathing, creating a medical emergency and requiring a respirator for assisted
ventilation. In patients whose respiratory muscles are weak, crises - which
generally call for immediate medical attention - may be triggered by infection,
fever, an adverse reaction to medication, or emotional stress.
With treatment, the outlook for most patients with myasthenia gravis is
bright: they will have significant improvement of their muscle weakness and they
can expect to lead normal or nearly normal lives. Some cases of myasthenia
gravis may go into remission temporarily and muscle weakness may disappear
completely so that medications can be discontinued. Stable, long-lasting
complete remissions are the goal of thymectomy. In a few cases, the severe
weakness of myasthenia gravis may cause a crisis (respiratory failure), which
requires immediate emergency medical care. (see above)
Within the Federal Government, the National Institute of Neurological
Disorders and Stroke (NINDS), one of the Federal Government's National
Institutes of Health (NIH), has primary responsibility for conducting and
supporting research on myasthenia gravis.
Much has been learned about myasthenia gravis in recent years. Technological
advances have led to more timely and accurate diagnosis, and new and enhanced
therapies have improved management of the disorder. Much knowledge has been
gained about the structure and function of the neuromuscular junction, the
fundamental aspects of the thymus gland and of autoimmunity, and the disorder
itself. Despite these advances, however, there is still much to learn. The
ultimate goal of myasthenia gravis research is to increase scientific
understanding of the disorder. Researchers are seeking to learn what causes the
autoimmune response in myasthenia gravis, and to better define the relationship
between the thymus gland and myasthenia gravis.
Today's myasthenia gravis research includes a broad spectrum of studies
conducted and supported by NINDS. NINDS scientists are evaluating new and
improving current treatments for the disorder. One such study is testing the
efficacy of intravenous immune globlin in patients with myasthenia gravis. The
goal of the study is to determine whether this treatment safely improves muscle
strength. Another study seeks to understand the molecular basis of synaptic
transmission in the nervous system. The objective of this study is to expand
current knowledge of the function of receptors and to apply this knowledge to
the treatment of myasthenia gravis.
For more information on myasthenia gravis, you may wish to contact:
Myasthenia Gravis Foundation of America, Inc. 5841 Cedar Lake Road
Suite 204 Minneapolis, MN 55416 myastheniagravis@msn.com http://www.myasthenia.org Tel:
800-541-5454 952-545-9438 Fax: 952-646-2028
For more information about stroke research funded or supported by the NINDS,
please contact the institute's Brain Resources and Information Network (BRAIN)
at:
BRAIN P.O. Box 5801 Bethesda, Maryland 20824 (301) 496-5751 (800)
352-9424 http://www.ninds.nih.gov
Prepared by: Office of Communications and Public Liaison National
Institute of Neurological Disorders and Stroke National Institutes of
Health Bethesda, MD 20892
NINDS health-related material is provided for information purposes only and
does not necessarily represent endorsement by or an official position of the
National Institute of Neurological Disorders and Stroke or any other Federal
agency. Advice on the treatment or care of an individual patient should be
obtained through consultation with a physician who has examined that patient or
is familiar with that patient's medical history.
All NINDS-prepared information is in the public domain and may be freely
copied. Credit to the NINDS or the NIH is appreciated.
Reviewed April 21,
2003
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