Environmental Tobacco Smoke Exposure and Risk of Stroke in Never Smokers: An Updated Review with Meta-Analysis.

OBJECTIVES The study aimed to review the epidemiological evidence relating environmental tobacco smoke exposure to stroke in never smokers. METHODS The study is similar to our review in 2006, with searches extended to March 2016. RESULTS Twelve further studies were identified. A total of 28 studies varied considerably in design, exposure indices used, and disease definition. Based on 39 sex-specific estimates and the exposure index current spousal exposure (or nearest equivalent), the meta-analysis gave an overall fixed-effect relative risk estimate of 1.23 (95% confidence interval: 1.16-1.31), with significant (P < .05) heterogeneity. There was no significant heterogeneity by sex, continent, fatality, disease end point, or degree of adjustment for potential confounding factors. Relative risks were less elevated in prospective studies (1.15, 1.06-1.24) than in case-control studies (1.44, 1.22-1.60) or cross-sectional studies (1.40, 1.21-1.61). They also varied by publication year, but with no trend. A significant increase was not seen in studies that excluded smokers of any tobacco (1.07, .97-1.17), but was seen for studies that included pipe- or cigar-only smokers, occasional smokers, or long-term former smokers. No elevation was seen for hemorrhagic stroke. Relative risk estimates were similar using ever rather than current exposure, or total rather than spousal exposure. Eleven studies provided dose-response estimates, the combined relative risk for the highest exposure level being 1.56 (1.37-1.79). Many studies have evident weaknesses, recall bias, and particularly publication bias being major concerns. CONCLUSIONS Although other reviewers inferred a causal relationship, we consider the evidence does not conclusively demonstrate this. We repeat our call for publication of data from existing large prospective studies.