Smoking is associated with progressive disease course and increased progression in clinical disability in a prospective cohort of people with multiple sclerosis

Multiple sclerosis has a variable disease course. The contribution of modifiable lifestyle factors to disease course has not been well studied, although one cohort has reported that smoking is associated with conversion to secondary progressive MS course and another that smoking is not. We conducted a prospective cohort study of people with MS in Southern Tasmania from 2002 to 2004 with 78 % (203/259) of eligible participating and 198 with one or more reviews and confirmed MS. The cohort had a high retention rate (90 % (183/203)). The median follow- up time was 909 days. Smoking data were collected at baseline and six-monthly reviews. Clinical disability assessments were conducted annually in conjunction with a real time clinical notification system for relapses. A repeated measures analysis and other statistical methods were used. Cumulative pack-years (p-y) smoked after cohort entry was associated with an increase in longitudinal MSSS (p < 0.001). Relative to the 0 pack years (p-y) category (in the year prior to the MSSS measure) those in the 0 to 1 p-y category had an adjusted mean difference in MSSS of 0.34 (95 % CI 0.28, 0.66); those in the 1 to 2 p-y category had a 0.41 (95 % CI -0.03, 0.85) increase; and those in the 2 or more p-y category had a 0.99 (95 % CI 0.41, 1.58) increase in MSSS. Similar results were found using a variety of statistical approaches or EDSS as a clinical outcome. Smoking during the cohort period was not associated with relapse (cumulative pack years smoked after cohort entry, HR 0.94 (0.69, 1.26) per pack year). A better understanding of the mechanisms underlying smoking and multiple sclerosis, particularly progressive forms of the disease, may provide new insights for the eventual goal of better treatment and prevention of multiple sclerosis.