Myasthenia gravis is an acquired autoimmune disorder of neuromuscular function marked by fatigue and exhaustion of the muscular system. Fluctuations in severity of muscular atrophy accompany periods of increased or decreased activity.

Myasthenia gravis is not very common. It is estimated to occur at a rate of 2:1,000,000.

Myasthenia gravis is caused by the presence of antibodies against acetylcholine receptors at the neuromuscular junction of skeletal muscles; 80% to 90% of myasthenia gravis patients have antibodies against acetylcholine receptors. Tolerance to acetylcholine receptors by the immune system is somehow lost, but the mechanism is not clear. Myasthenia gravis is considered to be a B-cell–mediated disease process; however, T-cells’ contribution to the process is becoming increasingly more evident. T-cell immunity is dominated by the thymus, and myasthenia gravis patients frequently have thymic hyperplasia, thymoma, or other thymus abnormalities.