CAUSES

MS results from a misdirected immune-system attack leveled primarily against myelin, a white, fatty material that coats wirelike nerve fibers in the central nervous system. Myelin provides insulation that speeds the transmission of nerve signals relaying information into, within, and out of the brain and spinal cord. When myelin is attacked and destroyed in MS, a process known as demyelination, nerve transmission flow is interrupted, resulting in a variety of neurological problems.

The site of myelin attack often dictates the nerve circuitry affected, and therefore the symptoms that are experienced. For example, if demyelination occurs in the nerve circuitry that carries messages between the muscles and the brain, then it may cause problems with movement. If demyelination occurs in the nerves that carry sensory information to the brain, MS may cause problems with sensory functions, such as vision. There is also evidence that, in addition to the myelin sheath surrounding the nerves, the nerve fibers themselves may be attacked at some point during the course of the disease.

The cause of MS is unknown, but genetics, an infectious agent, a faulty immune system, or a combination of these factors appears to play a role in why a person contracts the disease. MS is thought to occur in people who have a genetic susceptibility for the disease, demonstrated by cases of MS occurring within the same family too frequently to be accounted for simply by chance. However, studies of identical twins imply that a genetic predisposition is not the sole cause. A genetically susceptible person must encounter a second factor that will later trigger the development of MS.

Epidemiological studies suggest this second factor is encountered early in life—probably before reaching puberty. Studies have shown that MS is more prevalent in people who spend their first 15 years of life in a temperate climate than in those who live their first 15 years in a tropical climate. Many believe that the second factor is an infectious agent, such as a virus or bacterium. But after years of study, no single infectious agent has been shown to trigger MS. Some experts propose that MS is not triggered by a single infectious culprit, but instead by the way a person’s immune system reacts to an infection.

Studies suggest that MS is an autoimmune disease, in which the immune system attacks the body’s own tissue. New information about the way the immune system may be fooled into attacking its own tissue enforces this possibility. Certain bacteria or viruses have been found to contain proteins that make them resemble the cells of body tissues, including those of the nervous system. In MS, the immune system may be activated to attack both the invaders and the tissues they resemble.