Myasthenia gravis (MG) is a chronic disease characterized by weakness and
rapid fatigue of the voluntary muscles. The weakness is due to a breakdown in
communication between a nerve ending and its adjoining muscle fiber. The onset
of MG can be sudden, with severe and generalized muscle weakness, but more often
its symptoms in the early stages are subtle and variable, making it difficult to
diagnose correctly. MG affects people of all ages and either sex, but it more
affects young women and old men.
Symptoms of MG
The first noticeable symptom of MG is frequently weakness of the eye
muscles. The disease may remain confined to this area of the body, or it may
progress to muscles involving swallowing, chewing, speech or limb movement.
Symptoms vary among patients, but can include:
- A drooping of one or both the eyelids (ptosis)
- Blurred or double vision
- Unstable or "waddling" gait
- Weakness in the arms or legs
- Difficulty swallowing
- Difficulty breathing
- Difficulty speaking clearly
Once symptoms begin, muscle weakness may worsen over days or weeks. Symptoms
may remain constant, may progress or may fluctuate from hour to hour or day to
day. Weakness tends to worsen with exercise and at the end of the day. When MG
affects a patient’s ability to swallow or breathe, the disease can become
life-threatening if not treated immediately. However, the disease is seldom
fatal if managed properly.
Causes of MG
MG develops when antibodies are produced against the region of the muscle
that connects with a nerve ending. For muscle to contract, the neurotransmitter
chemical acetylcholine must attach to the muscle membrane after its release from
the nerve ending. But this is prevented by the antibody’s attack on the muscle’s
acetylcholine receptors. The result is muscle weakness.
Doctors have found that the thymus gland appears to trigger this autoimmune
response by encouraging production of acetylcholine receptor antibodies.
Located behind the breastbone, the thymus gland is part of the body’s
immune system, producing white blood cells called lymphocytes, which are
responsible for immune responses to bacteria, toxins and viruses. Normally large
in infancy, the thymus gland shrinks in size so that in the normal adult it is
hardly functional. For patients who have MG, however, the thymus gland can have
an excess number of cells (hyperplasia), which may contribute to the autoimmune response.
Diagnosing MG
When MG is suspected, the doctor will perform a thorough physical exam,
looking for muscle weakness or fatigue. Several tests may be performed.
- Tensilon test: Tensilon is a drug that blocks the
breakdown of acetylcholine, helping the neurotransmitter activate the
muscle. For patients who do not have MG, the drug will do nothing to improve
tired muscles. However, for patients with MG, the test is said to be
positive when muscle strength improves. This improvement in muscle strength
lasts only several minutes, however.
- Electromyogram (EMG): An EMG procedure assesses nerve
and muscle function electrically and can support the diagnosis of MG.
- Blood tests: For patients with possible MG, a blood test
may be ordered to measure the presence of acetylcholine receptor antibodies.
About 75 percent of patients with MG show an elevated amount of these
antibodies in the blood. In one-third of MG patients with generalized
weakness and without elevated acetylcholine receptor antibodies, MuSK
antibody is increased, which can help with the diagnosis. The test for MuSK
antibody is also commercially available.
Treating and managing MG
If diagnosed promptly, some patients may be cured of MG by removal of the
thymus gland (thymectomy) or aggressive immunosuppressant therapy. The
effectiveness of treating MG depends on many factors, such as the severity of
the disease, the duration of the disease, the patient’s age and the patient’s
overall health. For the most part, however, MG can be well-controlled with the
following treatment approaches:
- Cholinesterase inhibitors:
These are commonly used drugs to treat MG, but they are most useful in mild
forms of the condition. These drugs work by preventing the breakdown of
acetylcholine, thus increasing the muscle’s ability to contract. The most
commonly prescribed form of this drug is pyridostigmine (Mestinon).
- Immunosuppressants:
Corticosteroids and other drugs such as cyclosporine and azathioprine help
patients with MG by suppressing the activity of the immune system. These
treatments are generally for more severely ill patients. Although they are
often very effective, these drugs can have serious associated side effects.
The patient should consult with his or her doctor regarding the potential
value of these drugs in treating MG.
- Thymectomy: A thymectomy is a
surgical removal of the thymus gland. There is evidence that thymectomy can
lead to remission or reduced drug dependency, but it is more likely to work
if undertaken within 6-12 months of the first onset of symptoms. A
controlled clinical study of the efficacy of thymectomy in MG is currently
underway.
- Plasmapheresis: Plasmapheresis
is a blood plasma exchange process. It "filters" the blood of
acetylcholine receptor antibodies by replacing the patient’s plasma with
donor plasma. Plasmapheresis can be a life-saving tool in the treatment of
MG; however, it is expensive, time-consuming, and can be associated with
side effects such as low blood pressure, infection, and blood clots.
- IVIg: Intravenous infusion of
immunoglobulin can modulate the immune system and reduce the effects of
causative autoantibodies in MG. It is used to treat MG with acute worsening,
especially with breathing involvement.
For patients with MG, a healthy lifestyle and education about one’s disease
can maximize management. Some of the best ways to accomplish this include:
- Eating foods that are high in
potassium such as bananas, tomatoes, apricots, and broccoli
- Avoiding overexertion
- Advising physicians of the
diagnosis prior to taking prescribed drugs that may aggravate MG
- Avoiding excessive heat
For more information on MG, contact the following organizations:
Myasthenia Gravis Foundation of America
1821 University Ave. W., Suite S256
St. Paul, MN 55104
The National Institute of Neurological Disorders and Stroke
National Institutes of Health
Bethesda, MD 20892
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This information is provided by the Cleveland Clinic and is not intended to replace the medical advice of your doctor or health care provider. Please consult your health care provider for advice about a specific medical condition. This document was last reviewed on: 6/15/2006...#9235
