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The incidence of SAH is 7.9/100 000 person-years, with average age of onset of 62 (1). Non-traumatic subarachnoid haemorrhage (SAH) is most commonly caused by rupture of a weakness of an intra-cranial blood vessel (aneurysm). Although it represents only 5% of all strokes, SAH accounts for a similar number of years of life lost as the more common types of stroke (2-4). Despite improvements in clinical management of patients, outcomes after SAH remain poor, with an estimated 26% mortality rate and only 55% of patients regaining independent function (4). Knowledge of modifiable risk factors for SAH is important in terms of prevention. Prospective population-based studies suggest that women have a 1.24 times greater risk of SAH than men. This becomes apparent after 50 years of age, and increases with each decade (5). An explanation for this observation is lacking, however. Loss of the protective effects of oestrogen in the post-menopausal state has been proposed, but studies comparing hormone replacement therapy (HRT) exposure vs. no HRT exposure on SAH incidence draw conflicting conclusions (6, 7). Hypertension is an established modifiable risk factor for SAH, approximately doubling the risk of SAH (8, 9). A rise of 10mmHg in systolic blood pressure increases risk of SAH by 21% (10). A recent meta-analysis demonstrates a global decline in blood pressure which parallels a decrease in the global incidence of SAH (1) - highlighting the importance of primary prevention. Whilst calcium channel blockers are routinely used to prevent secondary ischaemia following SAH, there is no evidence to support the preferential use of specific anti-hypertensives in patients at risk. There is some evidence that certain classes of antihypertensive may offer greater protection against stroke compared to other antihypertensives. One meta-analysis found that calcium channel blockers (CCBs) were associated with a reduced risk ratio (RR) of stroke incidence, compared to other antihypertensive classes (RR 0.83, 95% confidence interval 0.79-0.89)(11). In addition, angiotensin 2 inhibitors (ARBs) may reduce stroke risk compared to angiotensin-converting enzyme inhibitors (ACEis)(12), but the literature is inconsistent (11, 13). These findings are often based on meta-analyses of randomised controlled trials that recorded stroke as a secondary outcome – often not specifying if the stroke was haemorrhagic or ischaemic. This protocol describes: 1) a descriptive study of the antecedent demographics and characteristics of SAH patients; 2) a comparative study investigating sex-differences and antihypertensive use on the risk of SAH.



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Subarachnoid haemorrhage, Electonic health records, Protocol