Myasthenia gravis (MG) is manifested primarily by muscle weakness. Clinically, there is especially frequent involvement of cranial nerves, most commonly manifested by diplopia and ptosis. In more serious cases there is difficulty in swallowing. Peripheral nerve involvement tends to affect proximal muscles more severely than distal ones. In the most severe cases there is paralysis of chest respiratory muscles. The disease affects only the eyes in about 20% of cases. Whenever muscles are involved, the degree of muscle weakness may fluctuate over short or long periods of time. The basic problem is located at the neuromuscular junction area of striated muscle. In normal persons, acetylcholine is released at nerve terminals on the proximal side of the neuromuscular junction, and the acetylcholine crosses the nerve-muscle junction and acts on acetylcholine receptor sites in the muscle membrane to set off muscle contraction. In patients with MG, acetylcholine receptor antibodies are present that interfere with the binding of acetylcholine to the receptor sites. The classic test for MG used to be a “therapeutic trial” of drugs such as edrophonium (Tensilon). Assays are now available in reference laboratories for serum acetylcholine receptor antibodies. Current assays are positive in 85%-90% of acute MG patients. The test is less sensitive in congenital MG and MG localized to the eyes; also, test results may become negative with therapy. Definitely elevated levels of acetylcholine receptor antibody are fairly specific for MG, with only a few conditions known to produce false positive results (such as D-penicillamine therapy for rheumatoid arthritis).

It is of interest that about 10% (range, 8.5%-15%) of MG patients have an associated thymoma, and about 50%-60% have hyperplasia of the thymus. Antistriated muscle (antistriational) antibodies have been reported in about 95%-99% of patients with MG plus thymoma and about 30% of patients with thymoma but without MG. Since the incidence of MG in patients with thymoma is about 35% (range, 7%-59%), the test results would be expected to be elevated in about 55% of all thymoma patients. Unfortunately, the test results are also elevated in about 25% of MG patients without thymoma. Therefore, absence of this antibody in a patient with MG is strong evidence against thymoma, but presence of antistriated muscle antibody does not prove that the patient has a thymoma.