Study of Twins Sheds Light on Smoking and Risk of Fatal Subarachnoid Hemorrhage

A study assessing the effects of smoking on health from a population of more than 16k pairs of twins details the effects of smoking on risk of fatal bleeding events in the brain.

Results of the study, which included more than 800k person-years of follow-up data, suggest those who identified as heavy or moderate smokers had a 3 times greater risk of subarachnoid hemorrhage (SAH) and even those considered light smokers found themselves at 2.8 times greater risk of fatal brain bleeds than their twins who did not smoke.

“Our study provides further evidence about the link between smoking and bleeding in the brain,” said corresponding researcher Ilari Rautalin, BM, a sixth-year medical and PhD student at the University of Helsinki in Finland, in a statement.

After a recent study of twins found SAH was mainly of nongenetic origin, a team of investigators from the University of Helsinki sought to further their understanding of the effect environmental factors have on SAH. With a hypothesis that smoking causes SAH, investigators designed their study using data from the Finnish Twin Cohort study.

From the Finnish Twin Cohort Study, which was initiated in 1974, investigators obtained information related to 16,282 same-sex twin pairs enrolled between 1976-2018. Participants identified for inclusion completed baseline surveys that collected data related to smoking status, hypertension, leisure-time physical activity, body max index, education level, and alcohol use. For the purpose of the current analysis, smokers were categorized as smokers and non-smokers—smokers were further categorized based on number of cigarettes smokers per day defined as light (less than 10), moderate (10-19), or heavy smokers (20 or more).

Participants were followed through the end of 2018 or until death, migration out of Finland, or death. Investigators ascertained deaths from SAH through the use of a nationwide cause-of-death registry. Additionally, pairs of twins were classified as monozygotic, dizygotic, or of unknown zygosity.

In total, 869,469 person-years of follow-up data were included in the investigators’ analyses. During this period, they were able to identify 116 discordant and 2 concordant twin pairs for fatal SAH. Of these, 25 of the discordant pairs were monozygotic. Upon analysis, investigators found smoking was associated with increased risk of SAH death when compared to patients who were considered never smokers (HR, 3.33; 95% CI, 2.24-4.95). Additionally, both heavy (HR, 3.01; 95% CI, 1.58-5.73) and moderate smokers (HR, 3.98; 95% CI, 2.44-6.49) appeared to have a greater risk for fatal SAH than light smokers (HR, 2.83; 95% CI, 1.64-4.9).

In pairwise analyses, investigators found the twin who smoked was at more than a 6 times greater risk for fatal SAH (HR, 6.33; 95% CI, 1.87-21.4) than their nonsmoking twin when assessing discordant pairs. Further analysis indicated this apparent association was consistent regardless of the twin pairs’ zygosity or sex.

“This long-term study in twins helps to confirm the link between subarachnoid hemorrhage and smoking,” said Rose Marie Robertson, MD, FAHA, the American Heart Association (AHA)’s deputy chief science and medical officer and co-director of the AHA Tobacco Center for Regulatory Science, in a statement from the AHA. “Not smoking or quitting if you've already started, is an essential component of primary prevention.”

This study, “Smoking Causes Fatal Subarachnoid Hemorrhage,” was published in Stroke.