Having an older brother or sister with multiple sclerosis does not protect siblings lower on the birth-order ladder from developing the neurological disease, a Canadian study suggests.
A genetic predisposition and other factors - not exposure or lack of exposure to germs from an older sibling - are more likely behind the onset of multiple sclerosis, said lead author Dr. Dessa Sadovnick, a geneticist at the University of British Columbia in Canada.
Previous small studies have suggested that children in compact families and those early on the birth-order list are at higher risk for MS. The theory is that children whose immune systems are exposed to infections introduced by older siblings are less likely to develop the potentially debilitating disease.
But Sadovnick's study of almost 11,000 Canadians with MS and more than 26,000 of their healthy siblings found no link between elevated risk for the disease and birth order. In fact, in families with a large number of siblings, those with MS tended not to be the eldest but to be born "later on in the sib-ship."
"They weren't usually the first-born or second-born, meaning that they usually did have this exposure to other brothers and sisters who were going to school, bringing home germs and different diseases," she said from Vancouver. "And they weren't building up sufficient immunity to prevent them from getting MS."
Dr. Paul O'Connor, head of the multiple sclerosis clinic at Toronto's St. Michael's Hospital, called the study "definitive" and said health professionals and families dealing with MS can have a high degree of confidence in its conclusions.
"I think this study will be definitive because this is one of the largest samples of MS (patients), if not the largest sample, ever studied with respect to this question," he said.
That's because the researchers had a treasure trove in the 12-year-old Canadian Collaborative Project on Genetic Susceptibility to MS - with more than 20,000 MS patients, the largest such database in the world.
Their study, published in the latest online issue of Lancet Neurology, shows that MS is "not something that you could consciously prevent or that you could consciously avoid within your family," Sadovnick said.
"So it's not that you're inoculating or not inoculating or being overly protective of your children's exposure to germs or going overboard and exposing your children to germs. One way or the other, you can't alter the risk for MS."
Multiple sclerosis is an unpredictable disease caused by inflammation and patchy destruction of the protective myelin covering the central nervous system. Those with the progressive disease, which affects almost twice as many women as men, tend to go through intermittent periods of attack followed by recovery.
The cause is not known, but most researchers believe MS is an autoimmune disease (something triggers the body's immune system to malfunction and start attacking the myelin), says the Multiple Sclerosis Society of Canada. Age of onset is usually between 20 and 40, but MS has also been diagnosed in children.
Multiple sclerosis initially may cause numbness and tingling in the extremities and temporary loss of vision in one eye. As the disease progresses, patients may experience a range of symptoms, including extreme fatigue, balance and co-ordination problems, muscle stiffness and weakness, and speech and cognitive difficulties.
About 100 in 100,000 Canadians have MS, said Sadovnick, noting that the prevalence of the disease is on the rise.
TORONTO, Canada