GRONINGEN, Netherlands -- Smoking has been overrated as an important factor in spurring multiple sclerosis progression and disability, researchers here said.
GRONINGEN, Netherlands, Oct. 9 -- Smoking has been overrated as an important factor in spurring multiple sclerosis progression and disability, researchers here said.
Smoking was not associated with primary or secondary MS progression on any measures except for some weak associations with disability, found Marcus Koch, M.D., of the University Medical Center Groningen here, and colleagues, in a large cohort study in the Oct. 9 issue of Neurology.
"Our data suggest that cigarette smoking has no important influence on disease progression or the development of a progressive disease course in MS," they wrote.
This conflicts with the only previous study to link smoking and MS progression, although a number of studies have linked the MS occurrence rate to tobacco exposure.
The earlier study, published in the journal Brain in 2005, showed that smoking increased the likelihood of progression more than threefold (hazard ratio 3.6, 95% confidence interval 1.3 to 9.9).
"We believe that the greater sample size and the smaller proportion of censored patients make our estimate of the hazard ratio more accurate," Dr. Koch and colleagues said.
The researchers analyzed smoking questionnaire responses from 364 patients in the Groningen MS database, which contains prospectively collected data on disease course and disability accumulation with three- to 12-month follow visits at an MS clinic since 1985.
The study included 247 women and 117 men. The disease course was benign relapsing-remitting MS for 75 patients, relapsing-remitting MS for 79, secondary progressive MS for 117, and primary progressive MS for 93.
The researchers calculated average number of cigarettes smoked per day and number of pack-years smoked up to the time of progression and up to the time of participation in the study.
MS may become clinically apparent only when axonal degeneration reaches a threshold level, the researchers said.
"If smoking had an important influence on the pathophysiologic mechanisms underlying progression in MS, one would expect smokers to reach that threshold earlier than nonsmokers," they wrote.
But, smokers did not have a shorter time in the relapsing remitting phase of the disease before secondary progression (P=0.51). Nor was there a difference among nonsmokers and those who averaged one to 10 cigarettes or more than 10 cigarettes a day (P=0.26).
Likewise, age at progression was similar between smokers and nonsmokers (P=0.59 for secondary progression and P=0.49 for primary progressive MS) and among nonsmokers and those who smoked one to 10 or more cigarettes (P=0.74 for secondary progression and P=0.72 for primary progressive MS).
The speed with which patients reached "landmark" disability scores on the Expanded Disability Status Scale (4.0 for functional but not ambulatory disability and 6.0 for functional and ambulatory disability) was not different between smokers and nonsmokers.
Disability scores and Multiple Sclerosis Severity Scores were not correlated with total pack-years in the overall cohort.
However, MS severity was significantly but weakly associated with pack-years smoked after MS onset in the overall group (P=0.03).
Among women, there were also weak but significant correlations between total pack-years smoked and both MS severity (P=0.03) and disability (P=0.02) and between disability scores and pack-years smoked before MS onset (P=0.01).
"All of these correlations were significant but the correlation coefficients were very weak," the researchers said. "This could be an area of further investigation."