Cigarette smoking has already been identified as a risk factor for multiple sclerosis (MS). Now new research for the first time also implicates smoking in the progression of the degenerative disease.

Researchers from the Harvard School of Public Health reported that current and past smokers with multiple sclerosis were more than three times as likely as patients who had never smoked to have more rapid progression of their disease.

While the finding must be confirmed, a multiple sclerosis expert tells WebMD that it gives smokers with multiple sclerosis another good reason to stop lighting up. It could also give future researchers important clues about the disease.

"If indeed smoking does contribute to the progression of MS then that is something else for scientists to look at to better understand the disease and its progression," says Nicholas LaRocca, PhD, who is director of health care delivery and policy research for the National Multiple Sclerosis Society.

Smoking Hastens MS Progression

Between 300,000 and 500,000 Americans have multiple sclerosis, with about twice as many women afflicted as men.

Early in the course of the disease most patients have what is known as relapsing-remitting MS, meaning that their symptoms may come and go at random. This can last for many years. As the disease advances patients may develop secondary progressive MS, in which symptoms occur with increasing frequency due to steady deterioration of the brain and spinal cord.

In the newly published Harvard study, researcher Miguel A. Hernan, MD, and colleagues were able to follow a group of MS patients from the time of their first symptoms, sometimes years before multiple sclerosis was confirmed.

They did this by accessing a national health database from the U.K. to identify 179 patients originally diagnosed with relapsing-remitting MS. Medical records for the patients for the years prior to diagnosis were then examined, and information on smoking status was obtained from computer records.

The researchers found that the risk of progressing from relapsing-remitting MS to secondary progressive disease during roughly five years of follow-up was 3.6 times higher for current and past smokers than for patients who never smoked.

In an extension of the study comparing multiple sclerosis patients to people without the disease, cigarette smoking was associated with a 30% increase in the likelihood of getting MS. However, the finding was not significant and the researchers could not rule out the possibility of this being a chance finding.

Can Quitting Make a Difference?

The findings identify smoking as possibly the first modifiable risk factor for multiple sclerosis progression. But Hernan tells WebMD that the study does not address the question of whether patients who stop smoking can really alter the course of their disease.

"Our data can't tell us if quitting makes a difference," he says. "It may be that by the time MS is diagnosed the damage has been done."

LaRocca agrees, although he says multiple sclerosis patients, like everyone else, should not smoke.

"I would like to see everyone who is smoking stop," he says. "But I think it is premature to tell patients that they can slow their MS course by stopping."

Explaining the MS, Smoking Link

The researchers offered several theories about the mechanism that could be driving the smoking-MS connection. A number of recent studies have suggested a link between nitric oxide, one of many chemicals present in cigarette smoke, and MS.

Nitric oxide and other chemicals that smokers breathe in could also damage the cells that protect myelin, a protective nerve coating that is eventually destroyed by multiple sclerosis.

"At this stage, this is all speculation," Hernan says. "More study is needed to answer these questions."

By Salynn Boyles, reviewed by Michael W. Smith, MD

SOURCES: Hernan, M. Brain, March 9, 2005. Miguel A. Hernan, MD, DrPH, department of epidemiology, Harvard School of Public Health, Boston. Nicholas LaRocca, PhD, director of health care delivery and policy research, National MS Society.