What is myasthenia gravis?
Myasthenia gravis (MG) is a chronic autoimmune disease that affects skeletal muscle strength by impeding the communication between nerves and muscles. MG is typically first noticed when it causes weakness in eye muscles and symptoms such as a drooping eyelid and/or double vision. This is often referred to as ocular MG. From the eye muscles, it can spread over time to facial and neck muscles, causing weakness, slurred speech, difficulty chewing and swallowing, and/or difficulty breathing, and from the head and neck to other parts of the body, resulting in generalized MG. Muscle weakness will vary over time; it tends to worsen with sustained effort and improves with rest.
Body movements, including those as small as keeping the head upright and eyes open, are normally carried out by a coordinated series of muscle contractions. These muscle contractions are initiated by chemical nerve signals. On a microscopic level, a nerve impulse travels to a nerve ending and acetylcholine, a neurotransmitter, is released. This chemical travels from the nerve ending to a muscle fiber across a tiny gap that exists at the "neuromuscular junction" and it binds to one of many acetylcholine receptors on the muscle fiber. This binding activates the receptor and initiates muscle contraction.
With MG, the body's immune system produces proteins (autoantibodies) that target a person's own acetylcholine receptors and block or destroy them. This inhibits the receipt of acetylcholine signals and causes weakness and rapid muscle fatigue.
In the United States, MG is estimated to affect about 20 people in 100,000. Anyone can develop MG, but it is most frequently diagnosed in men over 60 years of age and in women under 40. During pregnancy, a woman with MG may pass acetylcholine receptor antibodies to her fetus. This can cause a newborn to have symptoms of MG, but the symptoms typically resolve within 2-3 months of birth.
The cause of MG is not known, but about 75% of those affected have an abnormally large thymus gland and some develop thymomas (generally benign tumors of the thymus). The thymus is a small gland located behind the upper breastbone at the base of the neck. It plays an active role in the immune system through young adulthood, but normally begins to shrink after puberty and is essentially nonfunctional in adults. The relationship between MG and the thymus is not fully understood, but some think that the thymus plays an indirect role in triggering acetylcholine receptor antibody production.
A small percentage of those with MG have an affected family member, but most do not. Congenital myasthenic syndrome is an inherited condition that causes symptoms similar to MG, but it is not an autoimmune disorder. Another condition, Lambert-Eaton Myasthenic Syndrome, can also cause similar symptoms, but it involves interference with the release of acetylcholine by the nerve ending, not acetylcholine receptor activity.