Myasthenia gravis (MG) is an autoimmune disorder in which patients produce antibodies to acetylcholine receptors at the neuromuscular junction of striated muscle. The antibodies block the receptors so that they fail to respond to acetylcholine (Figure 5.6). This results in intermittent but progressive weakness of skeletal muscles, including those for breathing and the facial muscles involved in chewing, swallowing, talking and eye movements. The latter can lead to double vision and an inability to raise the eyelids, a condition known as ptosis. Difficulty with respiration may lead to inadequate intake of air and an inability to clear secretions from the respiratory tract. The incidence of pneumonia is increased in these patients. Approximately 75% of patients with MG also have thymic abnormalities such as hyperplasia and thymoma. The incidence of MG has been quoted as up to 14 in 100 000, with a female to male ratio of about 3 : 2.
The problems associated with movement of eye muscles are often the first sign of MG. The presence of autoantibodies may be confirmed by indirect immunofluorescence tests and the levels are a useful measure of disease progression. However, autoantibodies may not be detected in patients where the disease is confined to the facial muscles. Patients are treated with immunosuppressive drugs and cholinesterase inhibitors and with plasmapheresis to remove the autoantibodies. The condition is improved in the majority of patients by thymectomy. The mortality rate in MG patients is currently around 10%, which is a significant improvement on previous decades.