multiple sclerosis (MS), chronic, slowly progressive autoimmune disease in which the body's immune system attacks the protective myelin sheaths that surround the nerve cells of the brain and spinal cord (a process called demyelination), resulting in damaged areas that are unable to transmit nerve impulses. The disease also gradually damages the nerves themselves. There are elevated numbers of lymphocytes in the cerebral spinal fluid and of T cells in the blood (see immunity).

The onset of MS is usually at age 20 to 40 years, and its many symptoms affect almost every system of the body. There may be visual difficulties, emotional disturbances, speech disorders, convulsions, paralysis or numbness of various regions of the body, bladder disturbances, and muscular weakness. The course of the disease varies greatly from person to person. In some patients, the symptoms remit and return, sometimes at frequent intervals and sometimes after several years. In others the disease progresses steadily.

There is a genetic predisposition to MS, and environmental factors also seem to play some role. The disease is more common in temperate climates (1:2,000) than in the tropics (1:10,000).

There is no cure for MS, but a number of drugs—the first of which became available in the 1990s—can slow its underlying progress and reduce the frequency of attacks. These are Avonex, Rebif, and Betaseron, forms of beta interferon (a synthetic version of a natural substance produced by the immune system); Copaxone (glatiramer acetate); and Tysabri (natalizumab).

Disease of the brain and spinal cord in which gradual, patchy destruction of the myelin sheath of nerve fibres causes interruption or disordered transmission of nerve impulses. Its early symptoms may include limb weakness or trembling, visual problems, sensory disturbances, unsteady walking, and defective bladder control, which come and go irregularly. Attacks grow more severe, and some symptoms become permanent, sometimes with eventual complete paralysis. Average survival from onset is about 25 years, but a rare acute form progresses over months. The cause remains uncertain and treatment unsatisfactory. Corticosteroids may ease symptoms. MS may be due to a delayed immune response that attacks the myelin sheaths; suggested causes include various common viruses. Dietary causes have also been suggested.

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