Article
Patients who continue smoking after they have been diagnosed with multiple sclerosis (MS) may progress to secondary progressive MS earlier than patients who quit smoking, according to research published online ahead of print September 8 in JAMA Neurology. The findings suggest that physicians should advise patients to stop smoking to avoid aggravating MS-related disability, said Jan Hillert, MD, PhD, of the Karolinska Institutet at Karolinska University Hospital Solna in Stockholm.
“Evidence clearly supports advising patients with MS who smoke to quit,” said Dr. Hillert and colleagues. “Health care services for patients with MS should be organized to support such a lifestyle change.”
A Cross-Sectional Analysis
Cigarette smoking is a known risk factor for MS, with an odds ratio between 1.2 and 1.5, but investigators had not assessed whether quitting smoking after diagnosis affects the course of the disease. To clarify the impact of smoking continuation and cessation on time to conversion from relapsing-remitting MS to secondary progressive MS, Dr. Hillert and colleagues performed a cross-sectional study of patients in the Genes and Environment in Multiple Sclerosis (GEMS) Study in Sweden, a population-based case–control study that includes patients with prevalent MS from the Swedish National MS Registry. Researchers included in their main analysis 728 patients who smoked at diagnosis and who completed questionnaires from November 2009 to March 2011 about their smoking habits.
Researchers categorized 332 of these patients as continuous smokers (ie, they averaged at least one cigarette per day every year from the year after diagnosis) and 118 as quitters. Intermittent smokers were not considered in the primary outcome. An optimized accelerated failure time survival model showed that each additional year of smoking after diagnosis accelerated the time to conversion to secondary progressive MS by 4.7%. Uncorrected Kaplan–Meier plots showed that those who continued to smoke each year after diagnosis converted to secondary progressive MS at a median age of 48, compared with a median age of 56 for those who quit smoking, said the researchers. Smoking measured by pack-years had similar associations with time to conversion as years of active smoking, suggesting that these measures may be approximate proxies of each other.
Potential Confounder
Confounders can exist in any association study, and there might be variables associated with smoking that were not captured in this study, the researchers said.
While the groups were well balanced across most measures, those who quit smoking had shorter time to MS treatment than those who continued smoking, which is a potential confounder in the difference in time of secondary progressive MS onset, said Myla D. Goldman, MD, of the University of Virginia in Charlottesville, and Olaf Stüve, MD, PhD, of the University of Texas Southwestern Medical Center at Dallas, in an editorial commentary published with the study.
In addition, the broad categories of patient smoking levels “unfortunately prohibited any granularity about any dose effect of smoking on progression,” Drs. Goldman and Stüve said. “Thus, it remains unclear whether simply cutting back on the amount one is smoking could provide any benefit.”
A Risk Factor Worth Modifying
About 60% of Swedish patients with MS are smokers, reflecting a potentially large overall health benefit to smoking cessation efforts, Dr. Hillert and colleagues said.
“Most importantly, [this study] provides the first evidence, to our knowledge, that quitting smoking appears to delay onset of secondary progressive MS and provide protective benefit,” said Drs. Goldman and Stüve. “Therefore, even after MS diagnosis, smoking is a risk factor worth modifying.”
—Jake Remaly