Subarachnoid haemorrhage is a severe stroke subtype that is caused by a rupture of a brain aneurysm, an enlargement in brain vessel wall.

Up to one in four subarachnoid haemorrhage patients die quickly after the bleed at home, on the road to a hospital, or in an emergency room. These patients never reach hospital wards and are often incorrectly diagnosed. In many countries, these sudden deaths are classified as sudden cardiac deaths since routine autopsies are rarely conducted outside Nordic countries.

If only patients who survive the initial bleed and reach hospital wards are included in studies on risk factors of subarachnoid haemorrhage, such studies are very likely biased, because they are unable to study risk factors relating to the most devastating outcome, namely sudden death.

Two recent hospital-based studies, that excluded patients who died before reaching hospital wards, reported that smokers and hypertensive individuals have better chances to survive from subarachnoid haemorrhage than non-smokers and those with normal blood pressures.

These findings puzzled researchers and clinicians, because they contradicted earlier reports. For decades, the researchers and clinicians have known that smoking and high blood pressure are the two most important risk factors for subarachnoid haemorrhage, but now the studies suggest that the same factors would paradoxically protect from subarachnoid haemorrhage-related death.

A Finnish study, published in the distinguished Neurology journal, shows that when researches are able to include those people who die before reaching hospital wards, the paradoxical protective effect of smoking and high blood pressure is reversed. Specifically, the study showed that smokers and hypertensive individuals die more frequently before they reach hospital wards than non-smokers and those with normal blood pressure. When these heavy smokers and hypertensive people are left out from statistical analyses, the results are misleading.

"Due to the mandatory autopsies for all sudden deaths in Finland, we were able to identify and include data on those individuals who died before reaching hospital wards. This in turn allowed us to show how results change when all people with subarachnoid haemorrhage, not only those surviving to hospitals, are included in the analyses." says physician Joni Lindbohm, the principal author of the research article.

"In practise, our results show that risk factor and survival estimates of subarachnoid haemorrhage from studies that exclude people dying quickly after the bleed are unreliable. This is important to recognize because most subarachnoid haemorrhage studies are still hospital-based and do not include data on sudden deaths," neurosurgeon, Dr. Miikka Korja states.

According to Lindbohm and Korja, the best way to minimize the risk to sudden death from subarachnoid haemorrhage is to quit smoking and lower elevated blood pressure values.

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The researchers followed 65,521 population-based FINRISK participants during 1.52 million person-years and identified 445 first-ever hospitalized aSAHs and 98 sudden-death aSAHs occurring between 1974 and 2014 (FINRISK / Finnish institute for health and welfare)