Smoking is a risk factor for early conversion to clinically definite multiple sclerosis

Background: Cigarette smoking increases the risk for development of multiple sclerosis and modifies the clinical course of the disease. In this study, we determined whether smoking is a risk factor for early conversion to clinically definite multiple sclerosis after a clinically isolated syndrome. Methods: We included 129 patients with a clinically isolated syndrome, disseminated white-matter lesions on brain magnetic resonance imaging, and positive oligoclonal bands in the cerebrospinal fluid. The patients' smoking status was obtained at the time of the clinically isolated syndrome. Results: During a follow-up time of 36 months, 75% of smokers but only 51% of non-smokers developed clinically definite multiple sclerosis, and smokers had a significantly shorter time interval to their first relapse. The hazard ratio for progression to clinically definite multiple sclerosis was 1.8 (95% confidence interval, 1.2-2.8) for smokers compared with non-smokers (P = 0.008). Conclusions: Smoking is associated with an increased risk for early conversion to clinically definite multiple sclerosis after a clinically isolated syndrome, and our results suggest that smoking is an independent but modifiable risk factor for disease progression of multiple sclerosis. Therefore, it should be considered in the counseling of patients with a clinically isolated syndrome.