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.