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Older Siblings May Not Have Greater MS Risk

Environmental Factors That Increase Sibling Risk of the Disease Still Unknown
By Patti Connor
WebMD Health News

Aug. 22, 2005 -- A new study contradicts previous findings that when multiple sclerosis occurs it strikes older siblings most often.

Multiple sclerosis is a disease of the central nervous system (brain and spinal cord). It occurs when myelin, a substance that forms a sheath around nerve fibers, is destroyed by the immune system.

Worldwide, multiple sclerosis is thought to affect as many as 2.5 million people. Although genetics play a role in making a person more susceptible to the disease, environmental factors are also thought to increase the risk of the disease.

"The data presented here cast no doubt on the importance of environmental factor to MS risk," say researchers Dessa Sadovnick of the University of British Columbia, Canada, and George Ebers of Oxford University, England.

In the U.S., approximately 400,000 people have MS, according to the National Multiple Sclerosis Society. On a weekly basis, about 200 people in the nation are diagnosed as having the disease, which affects two to three times as many women as men. According to the National Multiple Sclerosis Society, an individual's risk of developing MS increases several fold if a close family member has MS.

Young Adults Most Often Affected

In general, multiple sclerosis strikes adults between the ages of 20 and 50. Occasionally, however, it is diagnosed in children.

Symptoms include:

  • Stiffness
  • Weakness
  • Numbness
  • Vision problems
  • Sexual problems
  • Bladder and bowel control problems
  • Problems swallowing

    Multiple sclerosis can also include emotional and intellectual problems.

    Environmental factors such as exposure to germs and viruses may play a role in developing multiple sclerosis. Previous findings on the risk of developing multiple sclerosis were reported and interpreted in the context of the hygiene hypothesis, which held that siblings born later were protected from infection through exposure early in life -- usually from infection in their older siblings.

    The new study, published by The Lancet Neurology, appeared online today. The researchers analyzed data from the Canadian Collaborative Project on Genetic Susceptibility to multiple sclerosis.

No Connection Between Birth Order, Incidence of MS

Researchers compared data taken from 10,995 individuals with multiple sclerosis with that of 26,336 healthy siblings.

The data were divided into two groups:

  • Sibling groups where one sibling had MS
  • Sibling groups where at least two siblings had MS

The researchers looked at birth order. The researchers found no association between birth order and the risk of developing MS.

In groups of at least seven siblings where one suffered from MS, more siblings who were born late in the birth order had MS. In groups where more than one sibling suffered from MS the same was found -- late birth order had a higher frequency of MS.

Siblings with MS tended to be slightly younger than siblings without MS.

"This study does not support the prediction of the hygiene hypothesis, which suggests that people with MS would be born earlier than expected within their sibships," writes Sodovnick, in a news release.

"It also suggests that environmental risks for MS must be accounted for by factors that do not affect birth-order position," he writes.

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