Myasthenia: from the Greek words, myelos, meaning muscle, and astheneia, meaning weakness
Gravis: from the Latin word, gravidus, meaning heavy (serious)
Myasthenia gravis (MG) is an autoimmune disease that is characterized by impairment of motor nerve impulses causing episodic muscle weakness and fatigue, especially in the face, tongue, neck, and respiratory muscles. MG occurs at all ages, usually between the ages of 20 and 40, sometimes in association with a thymic tumor or thyrotoxicosis, as well as in rheumatoid arthritis and lupus erythematosus. It is most common in young women with HLA-DR3; if thymoma (tumor) is associated, older men are more commonly affected. Onset is usually insidious, but the disorder is sometimes unmasked by a coincidental infection that leads to exacerbation of symptoms. Exacerbations may also occur before the menstrual period and during or shortly after pregnancy.
Symptoms are due to a variable degree of neuromuscular transmission blockage caused by autoantibodies binding to acetylcholine receptors. These autoantibodies are found in most patients with the disease and have a primary role in reducing the number of functioning acetylcholine receptors. Additionally, cellular immune activity against the receptor is found. Clinically, this leads to weakness, and initially powerful movements fatigue readily. The external ocular muscles and certain other cranial muscles, including the masticatory, facial, and pharyngeal muscles, are especially likely to be affected, and the respiratory and limb muscles may also be involved. In the ocular variety, there is difficulty with movements of the lids and the eyeball itself. When the pharyngeal muscles are involved, difficulty in swallowing ensues.
Only voluntary (or striated) muscles are affected; involuntary heart muscle and smooth muscle of the gut, blood vessels, and uterus are not involved. Muscles of the limbs may also be affected in some MG patients. Asymmetrical weakness may occur, with one side of the body more affected than the other. Difficulty may be encountered with simple tasks such as combing one's hair, shaving, and putting on makeup. Climbing stairs or walking distances may cause the legs to easily tire. In 10% of the cases, people develop a weakness of the muscles needed for breathing, a condition known as myasthenia crisis. Hospitalization and mechanical breathing assistance may be necessary in such cases. The disease is painless but may become painful if the patient goes into spasm as a result of early fatigue. Skin sensation is preserved.
The effect of pregnancy on MG varies from patient to patient. Symptoms of the disease may disappear, worsen, or remain the same during the course of pregnancy. Obstetrical problems are usually not present because the smooth muscle of the uterus is unaffected by the disease. During second-stage labor, when voluntary striated abdominal muscles are used, weakness becomes noticeable. Pregnant women with MG may pass affected antibodies through the placenta to their unborn child. This results in temporary neonatal myasthenia, in which the infant has muscle weakness that disappears several days to a few weeks after birth.