Agatha M. Thrash, M.D.
Myasthenia gravis is a disease characterized by episodes of muscle weakness, chiefly in muscles innervated by cranial nerves. It is caused by an autoimmune attack on the acetylcholine receptor of the neuromuscular junction, resulting in loss or dysfunction of acetylcholine receptors and jeopardizing normal neuromuscular transmission. Repeated or persistent activity of a muscle group exhausts its contractile power, leading to a progressive partial or complete paralysis. Rest restores strength, at least partially. The demonstration of these two attributes, assuming the patient cooperates fully, is enough to establish the diagnosis.
The onset is usually insidious, but there are instances of fairly rapid development. It is sometimes initiated by an emotional upset or infection (usually respiratory). Once started, a slow progression follows. Usually the muscles of the eyes, and somewhat less often the face, jaws, throat, and neck, are the first to be affected, but in rare cases the initial complaint may be referable to the limbs. However, as the disease advances it spreads to other muscles.
The course of the illness is extremely variable. Rapid spread from one muscle group to another occurs in some, but in others the disease remains unchanged for months before progressing. Remissions may take place without explanation, but these happen in half of the cases and are seldom longer than a month or two. If the disease remits for a year or longer and then recurs, it tends to be progressive. Remission is more likely to occur in the early years of the disease than later. Relapse may follow remission and is occasioned by the same conditions as the initial attack.
The evidence that an autoimmune mechanism is responsible for the functional disorder of the muscle in myasthenia gravis is incontrovertible. What is not known is what stimulates the production of these antibodies and where they are formed. Lennon offers an attractive hypothesis. She proposes that the site of the disease is in the thymus, where there are known to be "myoid" cells (thymic cells resembling striated muscle) that make the receptor antigen. One suggestion, unconfirmed, is that a virus with a tropism for thymic cells that have acetylcholine receptors might injure such cells and induce antibody formation. It might at the same time have a potential for oncogensis, accounting for the 10% of myasthenic patients with thymic tumors.
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