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Myasthenia gravis is an autoimmune disease where antibodies block the acetylcholine receptors in the synapses of the nerves that control muscles. It is very rare, occurring in only about one out of every 1 to 10 million people, and can easily be mistaken for other diseases that affect nerves in muscles. It is very much an "equal opportunity" disease and has the same incidence in either sex, all ethnic groups, and all age groups.

The disease presents as muscles that grow progressively weaker with use, but improve in function after a period of rest. It often affects the eyes first, usually causing the eylids to droop noticeable, and often resulting in double vision as the patient becomes progressively unable to control fine eye movement. However, in other patients, the disease affects the muscles of the mouth and throat, resulting in difficulty swallowing and slurred speech.

The symptoms of the disease may be intermittent and, in many cases, the disease will resolve itself after a few years even without treatment.

In rare cases, the disease can affect the muscles that control breathing and becomes immediately life-threatening. Infection, fever and medicine interactions can also bring on far more severe symptoms.

The causes of MG are not well understood. It is often found in combination with thyroid diseases, diabetes mellitus, rheumatoid arthritis, and lupus. The majority of patients also show a pathology of the thymus gland and a substantial percentage of those have a thymoma.

MG is difficult to diagnose as it's symptoms are also common to other diseases, including those that have no autoimmune component and need completely different treatment. Even tests for the antibodies that cause for the disease result in a false negative half the time.

Treatment is with cholinesterase inhibitors to improve muscle function and immunosuppresants to reduce the action of the antibodies. Surgery may be useful if the thymus gland is involved. In acute cases, plasmapheresis and immunoglobulin can be used.

MG patients without a thymoma usually have a normal life expectancy, although their quality of life may be affected. The symptoms are not progressive as is the case with Parkinson's disease or Huntington's disease and instead tend to wax and wane over time.