The protective myelin covering of the
nerve fibers in the central nervous system is damaged in people with multiple sclerosis.
Myelin is a fatty material that forms a sheath around a nerve and permits electrical
impulses to be conducted along the nerve fiber with speed and accuracy, much like the
insulation around an electrical wire does. When myelin is damaged, the nerves do not
conduct the nerve impulses properly, which may lead to various bodily dysfunction.
The precise causes of multiple sclerosis
(MS) are not yet known; however, scientific research indicates that that a number of
factors in combination are probably involved:
Immunologic:
It is now generally accepted that MS involves an auto-immune process—an abnormal
response by the immune system directed against the central nervous system (CNS). In patients with MS, the body's immune system
cannot differentiate between virus proteins and its own myelin and therefore launches an attack against itself. The exact
target (antigen) the immune cells are sensitized to attack remains unknown. However,
researchers have recently been able to identify which immune cells are mounting the
attack; how they are activated 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. The destruction of myelin (the fatty sheath that surrounds and insulates the
nerve fibers) causes the nerve impulses to be slowed or halted and produces the symptoms
of MS. Researchers are looking for highly specific immune modulating therapies to stop
this abnormal immune response without harming normal immune cells.
Environmental:
Studies of MS have taken into account variations in geography,
socioeconomics, genetics, and other factors (epidemiology) and migration patterns. These
studies have shown that people who are born in an area of the world with a high risk of MS
and move to an area with a lower risk, acquire the risk of their new home, if the move
occurs before the age of 15 years. Such data suggest that exposure to some environmental
agent that occurs before puberty may predispose a person to develop MS in later years.
In the U.S., multiple sclerosis occurs more frequently in states that are above the 37th parallel
than in states below it. From east to west, the 37th parallel extends from Newport News, Virginia, to
Santa Cruz, California and runs along the northern border of North Carolina to the northern border of
Arizona and including most of California. The MS prevalence rate for the region below the 37th parallel
is 57 to 78 cases per 100,000 people. The prevalence rate for those above the 37th parallel is almost
double that of those below the 37th parallel: 110 to 140 cases per 100,000 people.
Viral:
Initial exposure to numerous viruses occurs during childhood. Since viruses are well
recognized as causes of demyelination and inflammation, it is possible that a virus is the
triggering factor in MS. More than a dozen viruses including measles, canine distemper,
and herpes (HHV-6) have been investigated to determine if they are involved in the
development of MS. Researchers have also discovered epidemics of MS. For example, four epidemics of MS
have occurred in the Faroe Islands (between Iceland and Scandinavia) between 1943 and 1989. This region was
occupied by British troops during World War II. Since the incidence of MS has increased each year for 20 years
after World War II in this region, researchers believe the British troops unknowingly brought a disease-causing
agent with them during their occupation. However, it has not yet been definitively proven that any one virus
triggers MS.
Genetic:
Multiple sclerosis is not directly hereditary. However, having a first-degree relative such as a
parent or sibling with MS increases a person's risk of developing the disease many times
above the risk for the general population. Studies show there is a higher prevalence of
certain genes in populations with high rates of MS. Common genetic factors have also been
found in some families where there is more than one person with MS. Some neurologists
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 and mapping genes may help answer questions
about the role of genetics in the development of multiple sclerosis.
Trauma:
The role of trauma in causing multiple sclerosis or in triggering subsequent MS
exacerbation (also referred to as attacks, relapses or flares) has been a controversial
subject 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 prospective study, conducted by
researchers at the University of Arizona, followed 170 MS patients and 134 control
subjects, over a period of eight years. Prospective studies follow 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, Journal
of 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, 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 the onset of MS. Although the Mayo 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.
Both studies showed that there are more
traumatic events among people with MS than in the healthy control group. Many traumas were
caused by symptoms such as lack of coordination, impaired balance, or abnormalities of
gait or vision. These events, however, were not precipitating factors in the disease.
For additional information on what causes multiple sclerosis, please visit the National Multiple
Sclerosis website at http://www.nmss.org/
Updated: July 23, 2007
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