MYASTHENIA GRAVIS

an autoimmune disorder characterized by muscle weakness and chronic fatigue that are caused by antibodies that block the transmission of nerve impulses from nerve endings to muscles. Myasthenia gravis can occur at any age, but it rarely arises in people younger than 10 or older than 70 years of age. It generally affects females much earlier in life than it does men. Persons with the disease often have a higher incidence of other autoimmune disorders. Myasthenia gravis primarily affects the muscles of the face, neck, throat, and limbs. The onset of symptoms is usually gradual, with initial manifestations of the disease seen in the muscles governing eye movements and facial expressions. Weakness may remain confined to these areas, or it may extend to other muscles, such as those involved in respiration. Muscular exertion seems to exacerbate symptoms, but rest helps restore strength. Death can result from sudden respiratory arrest, but in most cases the disease is not fatal. The autoimmune reaction underlying myasthenia gravis results from a malfunction in the immune system in which the body produces autoantibodies that attack specific receptor molecules located on the surface of muscle cells. These receptors are found at the neuromuscular junction, the spot where nerve cells interact with muscle cells. Under normal circumstances a nerve cell, stimulated by a nerve impulse, releases acetylcholine molecules, which cross the neuromuscular junction and bind to receptor molecules on the muscle cell, thus triggering a muscular contraction. In myasthenia gravis autoantibodies bind to the receptors, preventing acetylcholine molecules from binding to them and thus preventing the muscle from responding to the nerve signal. Treatments for myasthenia gravis include anticholinesterase drugs, which stimulate the transmission of nerve impulses, and corticosteroids, such as prednisone, which dampen the immune response. Removal of the thymus often improves the clinical course of the disease.