Myasthenia gravis (MG) comes from the Greek and Latin words meaning "grave muscular weakness."
MG is a chronic autoimmune disease that causes muscles to tire and weaken easily. The disease is characterized by 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. Under normal conditions, nerves direct muscles to work by sending a message through an area called a receptor. The chemical that delivers the message is called acetylcholine. When acetylcholine binds to a nerve receptor, muscles know to contract. In myasthenia gravis, antibodies block, alter, or destroy the receptors for acetylcholine, which prevents muscle contraction from occurring. These antibodies are produced by the body's own immune system. MG is an autoimmune disease because the immune system—which normally protects the body from foreign organisms—mistakenly attacks itself.
No one knows exactly what causes a body to begin producing the antibodies that destroy acetylcholine receptors. In some cases, the process seems to be related to the thymus gland, which helps produce antibodies. The thymus gland, which lies in the chest area beneath the breastbone, plays an important role in the development of the immune system. In adults with myasthenia gravis, the thymus gland is large and appears abnormal. Scientists believe the enlarged thymus gland may give incorrect instructions to developing immune cells, ultimately resulting in autoimmunity and the production of the acetylcholine receptor antibodies.
About 15% of all myasthenia gravis patients are found to have a thymoma, a tumor of the thymus. Although most thymomas are benign, the thymus is usually removed to prevent the potential spread of cancer. Also, removal seems to improve symptoms of MS in some patients, even if no tumor is present.
Though anyone can develop MS, those most likely to do so are women between age 20 and 40 or men between 50 and 70. Estimates of the number of people affected by MG vary, ranging from five to 14 people per 100,000.