LYON, FRANCE—Smoking may be associated with an increased risk of multiple sclerosis (MS) among young people, according to research presented at the 28th Congress of the European Committee for Treatment and Research in Multiple Sclerosis (ECTRIMS).
Levels of cotinine, a nicotine metabolite recognized as a biochemical marker for tobacco use, of 10 ng/mL or higher were associated with an odds ratio of developing MS of 1.5, said Jonatan Salzer, a postgraduate student, and colleagues at Umeå University in Sverige, Sweden. After users of smokeless tobacco were excluded in accordance with questionnaire data, patients with cotinine levels of 10 ng/mL or higher had an odds ratio of developing MS of 1.6.
The effect of elevated cotinine levels on the risk of MS was only evident among individuals younger than 26.4. The investigators did not observe a dose–response effect. The study findings on cotinine were replicated in the results of a retrospective smoking questionnaire.
An Analysis of Prospectively Collected Blood Samples
To investigate the effects of laboratory-defined tobacco use on the risk of MS, Mr. Salzer and his colleagues conducted a nested case–control study. The group searched for diagnoses of MS in northern Sweden in two population-based biobank cohorts to identify blood samples taken before MS onset. Of the 2,887 patients initially identified, 192 had donated blood to the biobanks before disease onset and were eligible for inclusion in the study.
Blood samples were drawn a median of nine years before disease onset. The researchers measured levels of cotinine using an immunoassay. In addition, the investigators collected data on tobacco use retrospectively using a questionnaire in a subset of the subjects. This step enabled the exclusion of subjects who had used smokeless tobacco. The risk for MS was estimated using matched logistic regression.
Smoking Early in Life May Trigger Immunologic Events Leading to MS
Smoking increased the risk for MS by 50% among all subjects. Among patients younger than 26 with cotinine levels of 10 ng/mL or higher, the odds ratio of MS was 2.2. Excluding patients who used smokeless tobacco, individuals younger than 26 with cotinine levels of 10 ng/mL or higher had an odds ratio of MS of 2.4. Conversely, no effect on MS risk by cotinine levels was observed when analyzing only subjects above the age of 26.
"This study confirms that smoking is associated with an increased risk for MS. It has the advantage of using analyses of cotinine levels in samples collected several years before disease onset, thus excluding any risk for recall bias and minimizing the risk for reversed causation," said Mr. Salzer. The fact that the increased risk for MS was only observed in young subjects "indicates that smoking-related immunologic events contributing to the development of MS may occur early in life," he added.