Smokers face twice the risk of developing multiple sclerosis (MS), a crippling disease that destroys the lining of nerve cells, new research shows.

Norwegian and American scientists have found people smoked -- including those who've quit the habit -- have nearly double the risk of MS as those who never used tobacco. The effect wasn't quite as great as the long-established impact of smoking on the risk of cardiovascular trouble, such as heart attacks and chest pain, but it was larger than the increased risk of asthma associated with tobacco use. "We found this rather strong relationship between smoking and MS. It's almost comparable with cardiovascular disease" and smoking, says study leader Trond Riise, an epidemiologist at the University of Bergen in Norway. Multiple sclerosis involves the destruction of a protein called myelin that covers nerve fibers -- probably by the body's own immune system. Myelin both protects nerves and lets them function properly. People with MS suffer a wide range of symptoms with varying severity. These include difficulty walking and swallowing, fatigue and vision problems. The disease, which affects 400,000 Americans, usually sets in between the 20s and 50s. Scientists aren't sure why smoking might heighten the risk of developing MS. One explanation, Riise says, is that smoking saps the immune system and makes people vulnerable to infections, particularly of the throat, that trigger the nervous system disease. "We believe that it's most likely that the factor that initiates this disease is an infection," Riise says, though he admits that the mechanism remains "speculation." A report on the findings appears in the Oct. 28 issue of Neurology. A 2001 study by Harvard University study turned up a similar link between smoking and MS in female nurses. That study, reported in the American Journal of Epidemiology in 2001, found the risk of MS diminished, though not entirely, in people who'd quit smoking compared with those who still smoked. However, the risk increased the longer a woman smoked. Riise's group looked at the risk of MS and smoking in 22,240 Norwegian men and women living in Hordaland County in 1997. Of those, 8,239 never smoked, while 7,892 smoked and 6,109 had given up the habit. The researchers identified 86 cases of MS. The risk of developing the disease was 80 percent greater among smokers or former smokers than among people who'd never smoked. The association between smoking and the nerve disorder was greater for men than women. It didn't appear to wane significantly in former smokers, although most people had quit relatively recently before being diagnosed with MS, Riise says. The average duration of smoking before diagnosis was about 15 years. In addition to raising the odds of developing MS and perhaps other autoimmune diseases, previous research suggests smoking also appears to worsen flare-ups of symptoms. However, Stephen Reingold, vice president of research programs at the National Multiple Sclerosis Society, says the newest findings don't address that effect. "It does not tell us anything about the impact of smoking for people who have MS," he says. Reingold says smoking, infections and other environmental exposures or behaviors probably interact with a genetic susceptibility to MS.

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