Epidemiology
According to a review of 51 studies from 21 countries, the average incidence of subarachnoid hemorrhage is 9.1 per 100,000 annually. Studies from Japan and Finland show higher rates in those countries (22.7 and 19.7, respectively), for reasons that are not entirely understood. South and Central America, in contrast, have a rate of 4.2 per 100,000 on average.
Although the group of people at risk for SAH is younger than the population usually affected by stroke, the risk still increases with age. Young people are much less likely than middle-aged people (risk ratio 0.1, or 10%) to suffer a subarachnoid hemorrhage. The risk continues to rise with age and is 60% higher in the very elderly (over 85) than in those between 45 and 55. Risk of SAH is about 25% higher in women over 55 compared to men the same age, probably reflecting the hormonal changes that result from the menopause, such as a decrease in estrogen levels.
Genetics may play a role in a person's disposition to SAH; risk is increased three- to fivefold in first-degree relatives of people who have suffered a subarachnoid hemorrhage. However, lifestyle factors are more important in determining overall risk. These risk factors are smoking, hypertension (high blood pressure) and excessive alcohol intake. Having smoked in the past confers a doubled risk of SAH compared to those who have never smoked. Some protection of uncertain significance is conferred by Caucasian ethnicity, hormone replacement therapy, diabetes mellitus and higher than normal levels of cholesterol. Approximately 4% of aneurysmal bleeds occur after sexual intercourse and 10% of people with SAH are bending over or lifting heavy objects at the onset of their symptoms.
Overall, about 1% of all people have one or more cerebral aneurysms. Most of these, however, are small and unlikely to rupture.
Read more about this topic: Subarachnoid Hemorrhage