Myasthenia gravis is a chronic autoimmune neuromuscular disease characterized by fatigue and varying degrees of episodic weakness of the voluntary muscles. The hallmark of myasthenia gravis is muscle weakness that increases during periods of activity and improves after periods of rest. Facial muscles and those involved in chewing, talking, and swallowing are often, but not always, involved. The muscles that 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. Normally when impulses travel down the nerve, the nerve endings release a neurotransmitter substance called acetylcholine. In myasthenia gravis, antibodies produced by the body's own immune system block, alter, or destroy the receptors for acetylcholine. The first noticeable symptoms of myasthenia gravis may be weakness of the eye muscles, difficulty in swallowing, or slurred speech.

The onset of the disease involves only eye symptoms in 40% of patients, is generalized in 40%, and affects only the extremities in 10% (muscles close to joints are more severely affected than those further away). As the disease progresses, the weakness often spreads over the entire body. After the first year with myasthenia gravis, almost all patients have eye involvement, and in cases where the disease is destined to become generalized 90% will have become so involved. In 16% of patients only the eyes will ever be involved. During the first three years the progression of the disease is rapid and most deaths occur during this time. Myasthenia gravis occurs at all ages, usually between the ages of 20 and 40. If thymoma is associated, older men are more commonly affected.

Symptoms vary in type and severity. Myasthenia gravis is not directly inherited nor is it contagious. Onset is usually slow, but the disorder is sometimes unmasked by a coincidental infection that leads to a worsening of symptoms. Exacerbations may also occur before the menstrual period and during or shortly after pregnancy. Several diagnostic tests are available to confirm the diagnosis, including a special blood test that can detect the presence of immune molecules or acetylcholine receptor antibodies.

With conventional treatment, the outlook for most patients with myasthenia is bright: they can expect to lead nearly normal lives. Some cases of myasthenia gravis may go into remission temporarily, and muscle weakness may disappear so that medications can be discontinued. In a few cases, the severe weakness of myasthenia gravis may cause respiratory failure, which requires immediate emergency medical care.