From the article:
During the past few decades, the genetic makeup has been regarded as playing a significant role in the development of SAH [subarachnoid haemorrhage]. Contrary to this belief, however, a twin study recently published in the journal Stroke showed that environmental factors account for most of the susceptibility to develop SAH Conducted in Finland, Sweden and Denmark, the study is the largest population level twin study in the world.
This means that instead of screening the close family members of SAH patients, the focus of preventive treatment may now be increasingly shifted to the efficient management of hypertension and smoking cessation intervention. This is what we do with other cardiovascular diseases as well."
The Nordic study combined data on almost 80,000 pairs of twins over several decades. All in all, the follow-up time of all of the twin pairs corresponds to a staggering 6 million person-years.
The researchers nevertheless emphasize that there are rare cases of families among whose members SAH is significantly more common than in the overall population. In these cases genetic factors are the principal cause underlying the development of the disease.