Smoking and worsening disability in multiple sclerosis: A meta-analysis

Multiple sclerosis (MS) is a chronic demyelinating disorder affecting young adults. Environmental factors and lifestyle behaviors are pivotal in MS pathophysiology. Smoking has been considered as an important risk factor in MS. Various recent studies have been conducted to measure the role of smoking on worsening disability in patients with MS, thus we intended to systematically assess effect of smoking on evolution of disability in this study.

We queried MEDLINE, EMBASE and Cochrane Library with following keywords "Multiple Sclerosis, Smoking, Tobacco Use, Disability" on December 1st 2016. Original articles were included when smoking history was mentioned, disability was measured via expanded disability status scale (EDSS) or multiple sclerosis severity score (MSSS). Studies with insufficient outcome data, non-human, or in other languages than English were excluded.

Through literature review after duplicate removals, 268 articles were retrieved. A total of 56 articles were screened and 15 articles were assessed for eligibility, finally, eleven articles were included in this systematic review and meta-analysis. Ever smoking was significantly associated with increased EDSS (standardized mean difference (SMD) = 0.15, 95% CI = 0.01-0.28), but had no significant association with risk of reaching EDSS 4 (HR = 1.24, 95% CI = 0.89-1.72) or EDSS 6 (HR = 1.17, 95% CI = 0.88-1.57). Smoking had no effect on MSSS (SMD = 0.14, 95% CI = -0.04-0.32) or T2 lesion volume (SMD = 0.07, 95% CI = -0.08-0.22).

This meta-analysis showed smoking increased EDSS, insignificant findings were possibly due to the small number of studies, significant differences in methodologies, and variations in reporting of disability outcomes.