Causes

A combination of environmental and genetic factors likely plays a role in causing MS. A current theory is that the disease occurs in people with a genetic susceptibility who are exposed to some environmental assault (a virus or a toxin) that disrupts the blood-brain barrier. Immune factors converge in the nerve cells, triggering inflammation and an autoimmune attack (a self-attack) on myelin and axons. Similar genetic variations have been found in patients with MS and those with other autoimmune disorders. Still, a number of disease patterns have been observed in patients with MS, and some scientists believe that MS may prove to be not a single disorder but the representation of several diseases with different causes.

Genetic Factors

Genetic factors play a role in making some people susceptible to the disease process leading to multiple sclerosis. The most significant genetic link to MS has been identified in the major histocompatability complex (MHC), a cluster of genes on chromosome 6 that are essential for immune system function. A much smaller percentage of MS cases may be due to variations in interleukin-7 (IL-7) and interleukin-2 (IL-2) gene receptors, which are also related to immune system regulation.

Infectious Organisms

Infectious organisms, most likely viruses, have long been a suspect for triggering the autoimmune response in people genetically susceptible to MS. Blood serum samples or cerebral spinal fluid samples often show high levels of antibodies to a variety of organisms. However, no direct cause or effect has been observed between multiple sclerosis and these infections.

Infectious Organisms Under Suspicion. Although many infectious microorganisms have been investigated, no one organism has emerged as a proven trigger. It is possible that different patients may be affected by different organisms, and that infections cause some cases of MS. Organisms at the top of the suspect list are those that can affect the central nervous system, including these three primary suspects:

• HHV-6. Herpesvirus 6 (a form of herpesvirus that causes roseola, a benign disease in children) is also known to cause encephalitis (brain inflammation) in patients with impaired immune systems.
• Epstein-Barr virus (EBV). Evidence suggests an association between EBV, the cause of mononucleosis, and MS. EBV is an extremely common virus and another member of the herpes virus family. Nearly all people have been exposed to EBV.
• Chlamydia Pneumoniae. This atypical bacterium has been associated with persistent inflammation. A few studies have reported significantly higher rates of previous Chlamydia infection in patients with MS than in individuals without MS. However, there is no strong evidence linking the microbe to MS.
• A variety of other organisms have been investigated including adenovirus, canine distemper virus, herpes simplex virus (HSV), influenza virus, measles, mumps, parainfluenza, rubella, vaccinia, and varicella zoster virus (VZV), respiratory syncytial virus, cytomegalovirus, and coronavirus.