According to a recently published study of Finnish twins, smoking most likely causes a significant share of all cases of subarachnoid haemorrhage, the most fatal type of cerebrovascular disturbances. In the study, smoking was identified as the explanation as to why only one twin in pairs of twins develops a fatal brain haemorrhage. The finding is the first proof of an actual causality between smoking and subarachnoid haemorrhage.
In spite of advances in the treatment of subarachnoid haemorrhage (SAH), it remains the most fatal among cerebrovascular disturbances, with as many as half of the patients dying in the first few months after the haemorrhage.
Due to the high mortality rate, causes leading to this life-threatening disorder that are preventable have been feverishly investigated in recent decades all over the world. Several prior studies have reported a link between smoking and a heightened SAH risk, but no studies have so far been able to demonstrate that smoking directly causes these types of brain haemorrhages. In other words, the problem was similar to the situation with lung cancer several decades ago. It took a long time before evidence was found that smoking causes lung cancer, instead of only being associated with a heightened risk of developing lung cancer in an unknown way.
A study of Finnish twins recently published in Stroke, the world's foremost scientific journal on cerebral circulation, aimed to determine why only one twin in pairs of twins died of a fatal subarachnoid haemorrhage. The follow-up study included specific life-style data on as many as over 13,000 pairs of twins. The study indicates that it is precisely differences in smoking among the twins that appear to explain why only one twin in the pairs, namely the twin who smoked, developed a fatal cerebral haemorrhage, while their non-smoking twin brother or sister did not.
Ilari Rautalin, the principal investigator, says that the finding is historic and enormously important.
"Our findings confirm, for the first time, previous suspicions of an actual causal relation between smoking and fatal brain haemorrhage," says doctoral student Rautalin, Bachelor of Medicine, from the University of Helsinki.
The findings make it likely that smoking causes a significant share of subarachnoid haemorrhages in both men and women, which, in turn, markedly highlights the previously known adverse effects of smoking associated with many other diseases.
The two other members of the research group and authors of the article are Miikka Korja, a neurosurgeon at Helsinki University Hospital, and Jaakko Kaprio, professor of genetic epidemiology at the University of Helsinki.
"It's practically impossible to carry out a conventional clinical trial proving causality with regard to smoking, as it would require exposing healthy individuals to smoking and, subsequently, to many adverse health effects associated with smoking. This is why twin studies are an effective way of demonstrating direct causalities related to smoking," Docent Korja and Professor Kaprio point out.
Rautalin, Korja and Kaprio conclude by emphasising the major importance of quitting smoking or never starting the habit in the first place to minimise the risk of fatal cerebral haemorrhage.
Original article: Rautalin I, Korja M, Kaprio J: Smoking causes fatal subarachnoid hemorrhage - a case-control study of Finnish twins. Stroke, 2020. DOI: 10.1161/STROKEAHA.120.031231