While the precise causes of MS are not yet known, much scientific research indicates that a number of factors in combination are probably involved.
Epidemiology - In South Carolina, approximately 25-60 people for every 100,000 will develop MS. However, the prevalence in northern climates is higher. In Canada it is 125/100,000 or greater.
- The peak age of disease onset is approximately 30 years.
- Two thirds of MS cases have onset between 20 and 40 years of age.
- It is rare for MS to begin before 15 or after 50.
- Women are affected about twice as often as men.
Immunologic
Its now generally accepted that MS involves an autoimmune process – an abnormal immune response directed against the CNS. Leukocytes or white blood cells enter the brain from the bloodstream during an MS attack. These white blood cells consist of lymphocytes and monocytes. They are the main constituents of the immune system. Lymphocytes include T cells which direct the immune system to attack myelin. B cells are also lymphocytes. They can make antibody which can contribute to myelin destruction. Monocytes are responsible for myelin destruction as well. Brain cells such as microglia and possibly astrocytes can also contribute to myelin destruction. Plaques can expand, shrink or disappear. This depends on whether there is further inflammation and destruction in the area, whether the inflammation resolves and allows the brain to repair itself, and to what extent the brain is either “scarred“ or can completely repair the injured tissue. The mechanisms involved in shutting the immune system down, resolution of brain inflammation, and repair are just the beginning to be understood. Environmental
Epidemiological studies have demonstrated that people who move from a high risk area, after the age of about 15, to a low risk area carry higher risk of getting MS with them. Conversely, those who move from a low risk area to a high risk area, after age 15, carry the lower risk with them. This suggests that exposure to some environmental agent encountered before puberty may predispose a person to develop MS later on. Trauma
The role of trauma in causing MS or in triggering subsequent MS exacerbations (also known as attacks, relapses or flares) has been the subject of controversy for many years. Until recently, opinion on this issue was based upon anecdotal reports or retrospective information that relied on the memories of patients. A Major Review Shows No Link with Onset or Exacerbation of MS A 1999 report of the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology (Goodin, DS et al., Neurology 52:1737-1745, 1999), based on a comprehensive study of the literature on the subject, concluded that "the evidence supports no association between physical trauma and either MS onset or MS exacerbation." A prospective study, conducted by researchers at the University of Arizona, followed 170 MS patients and 134 controls, over a period of eight years. A prospective study is one that follows a group of people with a given disorder over a specific period of time, beginning before the occurrence of the events being studied. The results of this study, published in 1991 (Sibley, WA et al, J Neurology, Neurosurgery, and Psychiatry 54:584-9), concluded that except for electrical injuries, there was no evidence of a direct relationship between traumatic injury and an MS exacerbation. A second study, performed at the Mayo Clinic, where there is an MS patient population for whom complete clinical records have been maintained for years, supported the Sibley group's findings that traumatic injury is not related to exacerbation of MS. The Mayo study also indicated there is no relationship between traumatic injury and MS onset. Although this study, published in 1993 (Siva, A et al, Neurology 43:1878-82) was retrospective, it was based upon the detailed clinical records of 164 long term patients with definite MS, actively followed at the Mayo Clinic. Traumatic Events Often Due to Symptoms of MS Both studies showed that there are more traumatic events among people with MS than in the healthy control group. Many traumas were caused by MS symptoms such as incoordination, impaired balance, or abnormalities of gait or vision. These events, however, were not precipitating factors in the onset or exacerbation of the disease. Viruses and Infectious Agents
Numerous viruses and infectious agents have been investigated as a possible cause or triggering factor for MS. These have included Rabies, Herpes simplex virus (HSV), scrapie, measles, parainfluenza, coronavirus, canine distemper virus, and HTLV (human leukemia T cell virus), among others. Although none of these infectious agents have been shown to cause MS directly, some of them, like measles and other viral infections can trigger MS inflammatory events. Most recently the herpes viruses (Herpes zoster virus and Human Herpes virus type 6) and chlamydia have been studied as possible causes of MS. However further studies are needed to provide a definitive answer. Genetic
The chance of inheriting MS is low. The chance of acquiring the disease varies depending on sex and one’s relationship to the affected member. The daughter of a parent with MS has approximately a 9-10% chance of acquiring MS. A sibling has approximately a 4-5% risk , a first cousin about 1%. The concordance rate between identical ( monozygotic ) twins is about 30%. In general almost 15% of MS patients have 1 or more relatives with the disease. There is no evidence that MS is transmissible between spouses. Some genes have been identified but they have no practical value in identifying potentially affected individuals.
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