Chapter: Biology of Disease: Disorders of the Immune System
Myasthenia Gravis - Autoimmune Disorders
MYASTHENIA GRAVIS
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
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