Cigarette smoking represents an important modifiable environmental risk factor for multiple sclerosis (MS), and a new review article suggests that smoking increases the risk for developing MS through increased inflammation and exposure to free radicals, cyanates, and carbon monoxide. This review was published in JAMA Neurology.
In this narrative review, Harvard researchers explored the relationship between cigarette smoking and MS risk and progression by examining reports in English-language studies. According to the literature, the prevalence of both MS and cigarette smoking jointly and substantially increased in the 20th century, particularly in women. Studies consistently report dose-response associations between smoking and MS, suggesting causality between smoking and increased MS risk.
The association between the pathogenesis of MS and cigarette smoking appears to be modulated by the immune system, in addition to smoking’s neurotoxic effects. A proinflammatory cascade is induced by cigarette smoke and continues throughout the respiratory system, subsequently culminating in the lungs. Ultimately, macrophages release cytokines and free radicals that damage the surrounding alveolar epithelium. Inflammation and immune cells within the lungs also contribute to autoimmunity, driving the risk for MS in people who smoke.
Free radicals and cyanide present in cigarette smoke lead to damage to the mitochondria, which may result in severe damage to myelin. Smoke’s neurotoxic effects may also contribute to worsening disease prognosis in patients with MS who smoke. Additionally, studies have linked cigarette smoking with greater lesion loads on MRI scans in patients with MS as well as clinically isolated syndrome. Concerning findings in other studies suggest that cigarette smoke may adversely affect the efficacy of disease-modifying therapies, such as natalizumab.
While electronic cigarettes have not been extensively studied in regard to their effects on MS risk, researchers have alluded that heated vapor in these devices may be just as harmful as tobacco combustion and may also contribute to the pathogenesis of the autoimmune disorder.
The authors of the review added that “the pathways involved in smoking may be implicated in other environmental exposures (eg, organic solvents) and health habits (eg, waterpipe smoking),” in addition to tobacco smoke.
Researchers note that in terms of MS risk, “increased risk persists after smoking cessation. However, smoking cessation decreases future risk of MS progression and accrual of disability burden.” As such counseling is key in supporting patients with all patients, especially those with MS, before and during smoking cessation.
Disclosure: Several study authors declared affiliations with the pharmaceutical industry. Please see the original reference for a full list of authors’ disclosures.
Rosso M, Chitnis T. Association between cigarette smoking and multiple sclerosis: a review [published online December 16, 2019]. JAMA Neurol. doi: 10.1001/jamaneurol.2019.4271