Risk factors for non-aneurysmal subarachnoid haemorrhage: A cross-sectional study

Introduction: Intracranial aneurysms account for the majority of spontaneous subarachnoid haemorrhage (SAH), but up to 15% may be non-aneurysmal. There is limited previous evidence as to risk factors for non-aneurysmal SAH (NASAH).

Methods: This cross-sectional study enrolled all patients with NASAH presenting within a catchment of 2.7 million adults between 01/01/2016 and 31/12/2020. Patients with NASAH were prospectively enrolled in a database, while the prevalence of risk factors was estimated from randomly sampled, stratified population surveys. Crude and standardised incidences (SIs) and incidence rate ratios (IRRs) with 95% confidence intervals (95%CIs) were estimated from quasi-Poisson regression models.

Results: We included 148 patients with NASAH over 11,666,807 patient-years of follow-up (1.07/100,000 patient-years, 95%CI 0.89-1.2), of whom 65 experienced perimesencephalic NASAH (PM-NASAH), corresponding to an SI of 0.70/100,000 patient-years (95%CI 0.54-0.86). Unlike in aneurysmal SAH, female sex appeared to decrease the risk of NASAH (IRR 0.72, 95%CI 0.53-0.98) and PM-NASAH (0.66, 95%CI 0.43-1.04). Smoking was associated with a high incidence of NASAH (SI 2.58/100,000 patient-years, 95%CI 1.9-3.3) and strongly increased the risk of NASAH overall (IRR 3.33, 95%CI 2.31-4.81) and PM-NASAH specifically (IRR 2.81, 95%CI 1.67-4.73). Hypertension increased the risk of NASAH overall (IRR 1.82, 95%CI 1.42-2.33) but did not appear to be a strong risk factor for PM-NASAH (IRR 1.52, 95%CI 0.97-2.38). Ischaemic heart disease (IHD) increased the risk of NASAH (IRR 2.9, 95%CI 1.51-5.57) and PM-NASAH (IRR 2.73, 95%CI 1.22-6.09).

Conclusion : Smoking, hypertension and ischaemic heart disease increase the risk of NASAH. The risk factor profiles of PM-NASAH is similar to that of other aetiologies of NASAH. Contrary to aneurysmal SAH, female sex decreases the risk of NASAH.