Epidemiology

Worldwide, almost 500,000 individuals develop a subarachnoid haemorrhage (SAH) caused by an aneurysm each year, with almost two-thirds of these in low- and middle-income countries.[5] The incidence of SAH in most populations is between 6 and 8 per 100,000 people per year.[6] In the UK, more than 15,000 diagnoses were reported in 2021-2022.[7]​ A large variation in SAH incidence exists according to region, age, and sex.

The global incidence of SAH declined between 1980 and 2010 which may parallel the global decreases in blood pressure and smoking prevalence. However, some countries have seen increases in the incidence of SAH. In Japan, the incidence increased between 1980 and 2010, especially in women aged over 55 years.[8] A higher incidence in Hispanic populations compared with in non-Hispanic populations has also been noted in some areas of the US.[9] Incidence also increases with age. The average age at onset is between 50 and 55 years.[1][10][11] SAH is 1.6 times more common in women than in men.[6] It is 2.1 times more common in black people than in white people.[12]

SAH accounts for about 5% of all strokes.[13]

Risk factors

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hypertension

Hypertension is an important risk factor (relative risk is 2.8).[28] It is potentially modifiable.[24][29][30][31][32][33]

smoking

Smoking is one of the most important potentially modifiable risk factors.[24][29][30][31][32][33]​ Relative risk is 1.9.[8][10]​​

family history

Family history is a rare but important risk factor.[34]​ Between 9% and 14% of patients with a SAH have a family history of SAH in a first-degree relative.[35] In patients with two or more first-degree relatives with known cerebral aneurysms, there is a 12% prevalence of harbouring a cerebral aneurysm.[34][36]​ Patients with SAH have a 3-fold to 7-fold increased risk of having SAH than the general population.[14] The risk is highest when the affected relative is a sibling.[28]

Having two or more first-degree relatives with SAH has a relative risk of SAH of 6.6.[14] Patients who have two or more first-degree relatives with SAH are potential candidates for aneurysm screening.[28][35]​​​​[37]

autosomal dominant polycystic kidney disease (ADPKD)

ADPKD is an important risk factor (relative risk is 4.4).[28] One-quarter of patients with ADPKD have aneurysms at autopsy, and 2% to 8% of patients with aneurysms have ADPKD.[38]

Individuals with ADPKD are potential candidates for aneurysm screening.​[28]​​

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alcohol use

The relationship of SAH to excessive alcohol use is less robust than that of hypertension or smoking.[29][30][33]

cocaine use

The relationship of SAH to cocaine use is less robust than that of hypertension or smoking.

Marfan syndrome

Connective tissue disorder with an increased risk for aneurysmal formation and SAH.[38]

Ehlers-Danlos syndrome

Connective tissue disorder with an increased risk for aneurysmal formation and SAH.[38]

pseudoxanthoma elasticum

Connective tissue disorder with an increased risk for aneurysmal formation and SAH.[38]

neurofibromatosis type I

Connective tissue disorder with an increased risk for aneurysmal formation and SAH.[38]

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