HISTORY
CLINICAL FEATURES AND SYMPTOMS
MG occurs in all races, both sexes, and at any age. It is most common in young adult women
and older men. MG is not directly inherited nor is it contagious. It does occasionally
occur in more than one member of the same family. MG may affect any muscle that is under
voluntary control. Certain muscles are more often involved and these include the ones that
control eye movements, eye lids, (chewing, swallowing, coughing, and facial expression.
Muscles that control breathing and movements of the arms and legs may also be affected.
Weakness of the muscles needed for breathing may cause shortness of breath and difficulty
taking a deep breath and coughing. The muscle weakness of MG increases with continued
activity and improves after periods of rest. The muscles involved by MG vary greatly from
one patient to the next. Weakness may be limited to the muscles controlling eye movements
and the eye lids. This is the mildest form of the illness and is called ocular MG. In its
severest form, MG involves many of the voluntary muscles of the body including those
needed for breathing. The degree and distribution of muscle weakness for many patients
falls in between these two extremes. When the muscle weakness is severe and involves
breathing, hospitalization is usually necessary.
CAUSES
The voluntary muscles of the entire body are controlled by nerve impulses arising in the
brain. These nerve impulses travel down the nerves to the place where the nerves meet the
muscle fibers. Nerve fibers do not actually connect with muscle fibers. There is a space
between the nerve ending and muscle fiber and this area is called the neuromuscular
junction. When the nerve impulse, originating in the brain, arrives at the nerve ending it
releases a chemical called acetylcholine. The acetylcholine travels across the space to
the muscle fiber side of the neuromuscular junction where it attaches to receptor sites.
There are many receptor sites on the muscle side of the neuromuscular junction. The muscle
contracts when enough of the receptor sites have been activated by the acetylcholine. In
MG there is a reduction in the number of these receptor sites, as much as 80%. This is
responsible for the muscle weakness. The reduction of the receptor sites is caused by an
antibody that destroys or blocks the receptor site at the neuromuscular junction.
Antibodies are proteins that play an important role in our immune system. They are
normally directed at foreign proteins called antigens that attack the body. Such foreign
proteins include viruses and bacteria The antibodies help the body to protect itself from
these foreign proteins. For reasons which we do not understand the immune system of the
patient with MG makes antibodies against the receptor sites of the neuromuscular junction.
These abnormal antibodies can be measured in the blood of patients with MG. The antibodies
destroy the receptor sites more rapidly than they can be replaced by the body. The muscle
weakness occurs when acetylcholine cannot activate enough receptor sites at the
neuromuscular junction.
DIAGNOSIS
There are many disorders that may cause muscle weakness. In evaluating the problem the
doctor will carry out a neurological evaluation that includes testing muscles and checking
reflexes. There are several tests that may be used in establishing a diagnosis of MG. The
edrophonium (Tensilon) test is performed by injecting this chemical into a vein.
Improvement of strength immediately after the injection provides strong support for the
diagnosis of MG. EMG studies including repetitive electrical stimulation of nerves can
also provide support for the diagnosis of MG when characteristic patterns are present. A
blood test for the abnormal antibodies can be performed to see if they are present.
SPECIAL ISSUES
Most MG women experience no change or an improvement in their weakness during pregnancy. A
few may note worsening in weakness during some point in the pregnancy or following the
delivery. This is usually temporary. In approximately 10-15% of the newborn infants of MG
mothers there is a temporary type of MG weakness that lasts from several days to weeks.
The weakness is due to the transfer of the abnormal antibodies from the mother to the
fetus before the baby is born. Why the weakness occurs in only a minority of the infants
is not known. This weakness in the newborn requires proper treatment and, once it clears,
does not recur in later life. Certain drugs may make the weakness in the MG patient worse.
Some antibiotics (especially the aminoglycosides), narcotics, penicillamine, magnesium,
muscle relaxants, cardiac antiarrhythmia agents, and anesthetics may increase the
weakness. Prednisone, when used in the treatment of MG, may temporarily worsen the
patient's weakness before improvement occurs. It is important that the patient inform any
physician or dentist who is consulted for other problems that he/she has MG and help the
doctor avoid the drugs that may aggravate the weakness. Sometimes it is still very
important to use such drugs. In that case, they should be used with care. Occasionally the
weakness in an MG patient may become rapidly worse. When the patient has difficulty
breathing or maintaining an open wind pipe the patient and doctor are faced with a
critical situation. This is referred to as MG crisis. This may occur because of
insufficient medication but more often occurs after a respiratory tract infection or an
infection elsewhere Immediate efforts need to be made to establish a clear airway and this
may include the insertion of an endotracheal tube through the nose or mouth into the
windpipe or creating an opening into the windpipe (tracheostomy). If necessary a
respirator can then be used to help the patient breathe.
TREATMENT
There have been major advances in the treatment of MG in recent years. Although there is
no known cure for MG, the available treatments are sufficiently effective that most
patients will show excellent improvement and can lead normal lives. The various forms of
treatment include medications. thymectomy and plasmapheresis. Medications are most
frequently used in treatment. Anticholinesterase agents (such as Mestinon) allow the
acetylcholine to remain at the neuromuscular junction a little longer than usual so that
more receptor sites can be activated. Prednisone a cortisone-like drug, azathioprine
(Imuran) and cyclosporine may be used to suppress the abnormal action of the immune system
that occurs in MG. Intravenous immunoglobulin (IVIG) has also been used to transiently
suppress the immune system and produce improvement in myasthenic weakness. Thymectomy (the
surgical removal of the thymus gland) is another treatment used. The thymus gland lies
behind the breast bone and is an important part of the immune system. When there is a
tumor in the thymus gland (in 10-15% of patients) the thymus gland is also removed because
of the risk of malignancy. The thymectomy usually, but not always, lessens the severity of
the muscle weakness after some months. In some patients the muscle weakness may completely
disappear and this is called a remission. The degree to which the thymectomy helps varies
with each patient. Plasma exchange (plasmapheresis) may also be useful in the treatment of
MG. This procedure removes the abnormal antibodies from the plasma of the blood in
patients with MG. The improvement in muscle strength may be striking but is usually
short-lived since the abnormal antibodies continue to be formed. When plasma exchange is
used it may, therefore require repeated use. Plasma exchange may be especially helpful
during the period of MG crisis or before thymectomy. Which of these various treatments are
used in a MG patent depends upon the seventy of the weakness, which muscles are weak, the
patient's age, and other associated medical problems. The doctor will determine which of
these treatments is best for each patient.
PROGNOSIS
The current treatments for MG are sufficiently effective that the outlook for most
patients is bright. Although they do not cure MG, most patients will show significant
improvement in their muscle weakness. Most patients can expect to lead normal or nearly
normal lives. In some cases, MG may go into remission, in which case the muscle weakness
disappears. Remission may occur spontaneously, but is usually induced with the help of
immunosuppressive medications. Remission should not be equated with cure. A patient who is
in remission should take the same precautions as prior to your remission, and physicians
who treat MG patients should be made aware that the patient has MG -- even if in
remission. Even though there is much we can do for our patents with MG, there is still
much that we do not understand about this disease. New drugs to improve the treatment of
MG are needed. Research plays an important role in finding new answers and new treatments
for MG.
Prepared by the Public Information Committee of the National Medical Advisory Board of
the Myasthenia Gravis Foundation, Inc.
Ludwig Gutmann, M.D. (Chairman); Robert Griggs, M.D.; Earl R. Hackett, M.D.; Jack H.
Petajan, M.D.; Katharine Donohoe, R.N.
The Myasthenia Gravis pages are intended to provide information - not to advocate particular treatment options. Thus, patients should not change treatment without first consulting their physician.
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Copyright � 1995 by Myasthenia Gravis Foundation of America. All rights
reserved.
Most recent revision Saturday, April 26, 1997.