Tobacco smoking and multiple sclerosis: a systematic review of systematic and narrative reviews of observational studies

Aim The aim of this review was to systematically review systematic and narrative reviews of observational studies published on the relation between tobacco smoking and multiple sclerosis (MS). Subjects and methods A systematic search of the literature in PubMed and Scopus was conducted in June 2016. Two authors independently reviewed the search results and extracted key points. Review studies (i.e., review, systematic, narrative, or meta-analysis) that reported on smoking and MS were selected. The methodological quality of included systematic reviews and meta-analyses was assessed using the AMSTAR checklist. The methodological quality of narrative reviews was evaluated using a seven-item measure developed by the authors. Results Seventeen reviews met the inclusion criteria and were categorized as either systematic reviews or narrative reviews. Smoking was associated with increased risk of MS for evercompared with never-smokers and current compared with non-smokers. The summary odds ratio of MS for eversmokers versus never-smokers varied between 1.40 (95% CI: 1.29–1.52) and 1.46 (95% CI: 1.33–1.59). Conclusion This review of reviews identifies smoking as a risk factor for MS susceptibility. However, more research is needed to conclusively establish the mechanisms driving this association.