While the cause (etiology) of MS is still not known, scientists believe that a combination of several factors may be involved. Studies are ongoing in the areas of immunology (the science of the body’s immune system), epidemiology (that looks at patterns of disease in the population), and genetics in an effort to answer this important question. Understanding what causes MS will be an important step toward finding more effective ways to treat it and—ultimately—cure it, or even prevent it from occurring in the first place.
The major scientific theories about the causes of MS include the following:
It is now generally accepted that MS involves an immune-mediated process—an abnormal response of the body’s immune system that is directed against the myelin (the fatty sheath that surrounds and insulates the nerve fibers) in the central nervous system (CNS—the brain, spinal cord and optic nerves). The exact antigen, or target that the immune cells are sensitized to attack, remains unknown — which is why MS is considered by most experts to be immune-mediated rather than autoimmune. In recent years, however, researchers have been able to identify which immune cells are mounting the attack, some of the factors that cause them to attack, and some of the sites, or receptors, on the attacking cells that appear to be attracted to the myelin to begin the destructive process. Ongoing efforts to learn more about the immune-mediated process in MS—what sets it in motion, how it works, and how to slow or stop it—are bringing us closer to understanding the cause of MS.
MS is known to occur more frequently in areas that are farther from the equator. Epidemiologists—scientists who study disease patterns—are looking at many factors, including variations in geography, demographics (age, gender, and ethnic background), genetics, infectious causes, and migration patterns, in an effort to understand why. Studies of migration patterns have shown that people born in an area of the world with a high risk of MS who then move to an area with a lower risk before the age of 15, acquire the risk of their new area. Such data suggest that exposure to some environmental agent that occurs before puberty may predispose a person to develop MS later on.
Some scientists think the reason may have something to do with vitamin D, which the human body produces naturally when the skin is exposed to sunlight. People who live closer to the equator are exposed to greater amounts of sunlight year-round. As a result, they tend to have higher levels of naturally-produced vitamin D, which is thought to have a beneficial impact on immune function and may help protect against autoimmune diseases like MS. The possible relationship between MS and sunlight exposure is currently being looked at in a Society-funded epidemiological study in Australia.
Other scientists study MS clusters—which are defined as higher-than-expected numbers of cases of MS that have occurred over a specific time period and/or in a certain area. These clusters are of interest because they may provide clues to environmental (such as environmental and industrial toxins, diet, or trace metal exposures) factors that might cause or trigger the disease. So far, cluster studies have not produced clear evidence for the existence of any triggering factor or factors in MS.
Since initial exposure to numerous viruses, bacteria and other microbes occurs during childhood, and since viruses are well recognized as causes of demyelination and inflammation, it is possible that a virus or other infectious agent is the triggering factor in MS. More than a dozen viruses and bacteria, including measles, canine distemper, human herpes virus-6, Epstein-Barr, and Chlamydia pneumonia have been or are being investigated to determine if they are involved in the development of MS, but none have been definitively proven to trigger MS.
While MS is not hereditary in a strict sense, having a first-degree relative such as a parent or sibling with MS increases an individual’s risk of developing the disease several-fold above the risk for the general population. Studies have shown that there is a higher prevalence of certain genes in populations with higher rates of MS. Common genetic factors have also been found in some families where there is more than one person with MS. Some researchers theorize that MS develops because a person is born with a genetic predisposition to react to some environmental agent that, upon exposure, triggers an autoimmune response. Sophisticated new techniques for identifying genes may help answer questions about the role of genes in the development of MS.