The cause of multiple sclerosis (MS) is not definitely understood. The general theory is that MS results from an abnormal autoimmune reaction in genetically susceptible people. The immune system evidently attacks the outer layer of its own nerve fibers in response to an infection or some other environmental trigger. (This layer, known as myelin, is the fatty substance that insulates nerve fibers in the brain, spinal cord and optic nerves.) MS destroys or damages myelin, interrupting the electrical impulses carried by the nerves. It also damages the nerves themselves.

Most of the research done so far has produced evidence that implicates a virus, or viruses, in some way. A recent study found that patients with MS are much more likely than other people to have active infections of Human Herpesvirus Six (HHV-6). Active HHV-6 infections were found in 67 percent of the patients with MS but in none of the control subjects. HHV-6 causes roseola, which most Americans (90 percent ) contract during infancy. Other viral suspects include: herpes viruses 1 and 2, the varicella-zoster virus (which causes chicken pox), the Epstein-Barr virus (which causes mononucleosis), the measles virus, any of the retroviruses, including HIV, and several others. Bacteria have also been implicated, particularly Chlamydia pneumoniae.

Viruses are considered the most likely suspects for several reasons. First, abnormal levels of viral antibodies are consistently found in the cerebrospinal fluid of patients with MS. Fifty years of studies have shown that more than 10 types of viral antibodies are present in patients. Today's researchers now know that children living in northern latitudes accumulate more viruses over the years than those living closer to the equator. This may be because sunlight, which is more intense near the equator, can destroy or inactivate viruses (a virucide). MS is more prevalent among those living in northern latitudesThere has been some public concern over a possible link between vaccinations and MS. But two recent studies have helped allay these fears. Evidence strongly suggests that vaccinations for hepatitis B, tetanus, and influenza do not increase the risk of getting MS. The studies also suggest that vaccinations don't cause relapses in people who already have MS.

Although it is not known which infectious agent is involved, researchers believe that an environmental pathogen acts as a "trigger" in a series of events that results in MS. It is thought that the infectious "trigger" activates white blood cells in a way that enables them to penetrate the brain's defenses (the blood/brain barrier). Once inside the brain, these cells stimulate the other parts of the immune system, which respond by attacking the myelin and also damaging nerve fibers.

It is not clear exactly how an infectious agent becomes a trigger for this sequence, but extensive research continues. Experts think that perhaps one of the following could be responsible for MS:

A Dormant Pathogen. Some agents, such as the various herpes viruses, can lay dormant in a person's system for years, and then be reactivated by various factors.

An Allergic Reaction. Patients with MS may experience an allergic reaction to a common infectious agent.

Molecular Mimicry. The immune system may mistake parts of the myelin protein for an infectious agent that it is already programmed to attack.

Auto-immunization. A small amount of myelin may be released into the blood stream following a viral infection, causing the body to "auto-immunize" itself. The body would then mistake myelin for an invader and attack it as it would any other foreign substance.

It is likely that genetic factors work in combination with infectious agents to cause MS. Researchers think that people with MS have probably inherited a particular combination of genes. They suspect that people with this genetic predisposition get MS in response to some environmental trigger, such as a virus.

New research suggests that the various forms of MS may actually be different diseases with different causes. A study of brain tissue from patients with MS revealed four distinct patterns of lesions. Researchers found that while lesions differed between patients, each patient's own lesions fit only one pattern. MS experts think that each lesion pattern may result from a different cause. More studies are needed to determine if the different lesion patterns call for different treatments.

References

First published October 1, 1999
Last updated May 2, 2003
Copyright © 1999 Accordant Health Services, Inc. All Rights Reserved.

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