Myasthenia gravis is a chronic (long-lasting) and rare disease that affects the way muscles respond to signals from nerves, leading to muscle weakness. The disease can occur at any age, but it mainly affects women between ages 20 and 40. After age 50, men are more likely to get the disease.
Normal muscle movement relies on chemical signals from the nerves. Nerve signals cause the nerve endings to release a chemical called acetylcholine into the small space between the nerve and the muscle. This chemical binds to special acetylcholine receptors on the muscle cells and causes the muscle to contract.
Myasthenia gravis is an autoimmune disease, which means the body's defense mechanism, the immune system, begins to attack the body's own tissues instead of foreign invaders, such as viruses. In myasthenia gravis, the immune system attacks the acetylcholine receptors with specific antibodies. Some of the receptors are destroyed or blocked, which means that the chemical message cannot be received. Therefore, muscles do not contract properly and become weak. It has been estimated that up to 80% of the receptors can be damaged in this disease.
Normally, the amount of acetylcholine available to be released to muscle cells diminishes with repeated, prolonged or strenuous activity. In a person with myasthenia gravis, that normal decrease combined with fewer working receptors causes increasing weakness, or myasthenic fatigue. Muscles that may seem normal at first become weak with continued use. This is why a person with myasthenia gravis will feel strongest immediately after waking up or resting and weakest at the end of the day when muscles have been in continual use. Myasthenia gravis typically strikes muscles in the face first. Muscles that control the eyelids and eye movements usually are affected early in the disease. Later, facial weakness can make smiling, chewing, swallowing and talking difficult. In most people, the disease eventually spreads to other areas and can affect the muscles of the arms and legs. Sometimes, the disease affects the muscles that control breathing. In some situations, such as during a respiratory infection, this weakness could lead to a myasthenic crisis during which the person may need help to breathe.
No one knows what causes the body to attack its own cells. The thymus, a gland located in the chest just above the heart, is involved in immunity early in life and seems to play an important part in myasthenia gravis, although its precise role is not yet understood. The thymus is abnormal in most people with myasthenia gravis, and about 10% to 15% of people with the disease have a benign (noncancerous) tumor of the thymus.
Although myasthenia gravis is not believed to be inherited or contagious, babies born to mothers who have myasthenia gravis have an increased risk of having the disease at birth. This probably is because the baby acquires the antibodies that attack the body's cells from the mother during gestation. Usually the baby's symptoms go away within a few weeks after birth.
The primary symptoms of myasthenia gravis are that muscles are weak and tire quickly. These symptoms can take many forms, and the symptoms may be more or less severe for different people. Symptoms may include some or all of the following:
Sudden, severe weakness (myasthenic crisis) may develop without warning, sometimes triggered by an infection or other stress, causing breathing and swallowing problems. Ocular myasthenia gravis is limited to the eye while generalized myasthenia gravis may affect muscles throughout the body.
Your doctor will ask detailed questions about your symptoms and will examine you. He or she will test your muscle strength and reflexes, eyelid strength and eye movements. If the results suggest myasthenia gravis, some specific tests may be done to confirm the diagnosis. These include:
In addition, magnetic resonance imaging (MRI) scans or computed tomography (CT) scans may be done to look for abnormalities of the thymus. Pulmonary function tests, which measure breathing strength, also may be recommended, because they can alert doctors to any potential breathing problems. Additional testing, including blood tests or other imaging tests, often is suggested to exclude other possible explanations for a person's symptoms.
Myasthenia gravis is a chronic (long-lasting) disease that usually worsens slowly over time. Although symptoms may fluctuate in severity over the years, the disease rarely goes away on its own. It is likely to return even after periods when the disease has not been active.
Because the cause of myasthenia gravis is unknown, there is no way to prevent it. However, once the disease has developed, there may be ways to prevent episodes of worsening symptoms or flare-ups:
There are many possible treatments for myasthenia gravis. You and your doctor should work together to determine the best treatment for you.
See your doctor if you experience any unexplained muscle weakness or fatigue.
If you have been diagnosed with myasthenia gravis, it is critically important to be prepared to seek help in case of a myasthenic crisis. If the muscles that control your breathing are affected, call your doctor if you develop a respiratory infection or even a small breathing difficulty. Seek immediate help if you have significant difficulty with breathing or swallowing.
Myasthenia gravis is a chronic disease, although there may be periods of remission, when symptoms disappear. Current treatments, however, can allow most people with myasthenia gravis to live normal or near-normal lives. People with ocular myasthenia gravis (without progression over several years) have a better prognosis than those with generalized disease. Other autoimmune disorders, such as systemic lupus erythematosus (SLE or lupus) or rheumatoid arthritis, may be associated with myasthenia gravis and may develop either before or after the weakness is noticed.
National Institute of Neurological Disorders and Stroke
P.O. Box 5801
Bethesda, MD 20824
Myasthenia Gravis Foundation of America
355 Lexington Ave.
New York, NY 10017
American Academy of Neurology (AAN)
1080 Montreal Ave.
St. Paul, MN 55116