Multiple Sclerosis: What Causes It?

Doctors still don't understand what causes MS, but there are interesting data that suggest that genetics, a person's environment, and possibly even a virus may play a role.

How Does the Environment Affect a Person's Risk of MS?

Epidemiological data show several interesting trends: Different populations and ethnic groups have a markedly different prevalence of MS. The disease is especially common in Scotland, Scandinavia, and throughout northern Europe. In the U.S. the prevalence of MS is higher in whites than in other racial groups.

Studies show that MS is more common in certain parts of the world, but if you move from an area with higher risk to one of lower risk, you acquire the risk of your new home if the move occurs prior to adolescence. Such data suggest that exposure to some environmental agent encountered before puberty may predispose a person to MS.

Moreover, MS is a disease of temperate climates. In both hemispheres, its prevalence increases with distance from the equator.

Also there have been "epidemics" of MS -- for example, the group of people living off the coast of Denmark after WWII, suggesting an environmental cause.

What Role Do Genetics Play in MS?

Researchers believe that MS may be inherited (passed on from parents to children). First, second and third degree relatives of people with MS are at increased risk of developing the disease. Siblings of an affected person have a 2%-5% risk of developing MS.

Researchers believe that there is more than one gene that makes a person more likely to get MS. Some scientists theorize that MS develops because a person is born with a genetic predisposition to react to some environmental agent, which, upon exposure, triggers an autoimmune response.

Sophisticated new techniques for identifying genes may help answer questions about the role of genetics in the development of MS.

What Viruses Are Linked to MS?

Some studies have suggested that many viruses such as measles, herpes, and the flu viruses may be associated with MS. To date, however, this belief has not been proven.

Are There Other Potential Factors That Cause MS?

There is growing evidence suggesting that hormones, including sex hormones, can affect and be affected by the immune system. For example, both estrogen and progesterone, two important female sex hormones, may suppress some immune activity. Testosterone, the primary male hormone, may also act as an immune response suppressor. During pregnancy, estrogen and progesterone levels are very high, which may help explain why pregnant women with MS usually have less disease activity. The higher levels of testosterone in men may partially account for the fact that women with MS outnumber men with MS by 2-3 to 1.

Reviewed by the doctors at the Mellen Center for Multiple Sclerosis Research at The Cleveland Clinic.