Epidemiology of Non-Aneurysmal Subarachnoid Hemorrhage: An Observational Study

Introduction: Spontaneous subarachnoid haemorrhage (SAH) is a devastating neurological event, the most common cause of which is rupture of a cerebral aneurysm. However, in approximately 15% of cases no cause is identified. Non-aneurysmal SAH (NASAH) is poorly defined epidemiologically, with limited data regarding demographic distribution, risk factors and overall incidence. In this study, we present data from a large cross-sectional national study examining the epidemiology of NASAH.

Methods: We performed a cross-sectional study prospectively enrolling NASAH cases from catchment of 2.7 million adults between 01/01/2017 and 31/12/2020. Population demographics were obtained from door-to-door census and risk factors from randomly-sampled surveys. NASAH was defined as atraumatic SAH without a cerebral aneurysm on digital subtraction angiography (DSA), categorised into perimesencephalic (PMH) and non-perimesencephalic (NH) based on the distribution of haemorrhage. We fitted age-adjusted Poisson regression model, and calculated the incidence rate ratio (IRR) associated with various risk factors. Standardised incidences (SIs) were calculated using the WHO World Standard Population.

Results: We identified 148 cases of NASAH over 11,666,807 patient-years of follow-up, corresponding to an incidence of 1.27 (95%CI 1.07-1.49) cases per 100,000 patient-years. PMH had an SI of 0.7 (95%CI 0.54-0.86) per 100,000 compared to 1.1 (95%CI 0.89-1.2) in NH. Female sex was associated with lower risk of both PMH (IRR 0.66, 95%CI 0.43-1) and NH (IRR 0.72, 95%CI 0.53-0.98). Smoking was associated with increased risk for both PMH (IRR 2.8, 95%CI 1.7-4.7) and NH (IRR 3.3, 95%CI 2.3-4.8). Hypertension was more strongly associated with NH (IRR 1.8, 95%CI 1.4-2.3) than PMH (IRR 1.5, 95%CI 0.97-2.4). Ischaemic heart disease was a risk factor for both PMH (IRR 2.7, 95%CI 1.2-6.1) and NH (IRR 2.9, 95%CI 1.5-5.6).

Conclusion : The risk factor profile of NASAH is similar to aneurysmal SAH. However, unlike aneurysmal SAH, there is a male predominance observed in NASAH.