Swedish nationwide study of 377 patents with non-aneurysmal subarachnoid haemorrhage: a disease with distinct demographics and risk factors

Background Non-aneurysmal subarachnoid haemorrhage (NASAH) accounts for less than 20% of spontaneous subarachnoid haemorrhage (SAH). However, its epidemiological characteristics, risk factors and aetiology remain poorly defined. Methods All patients with spontaneous SAH admitted to a neurosurgical centre in Sweden over a 3.5-year period were prospectively enrolled in a database. Epidemiological data, risk factors, Fisher grade and follow-up radiological findings were analysed, comparing NASAH cases to aneurysmal SAH (aSAH). Results A total of 1532 patients with SAH were included, of whom 377 (24.6%) were diagnosed with NASAH. Five NASAH patients exhibited microaneurysms in the perforating arteries (MAPAs) of the vertebrobasilar circulation, identified on follow-up cone-beam CT angiography. Gender distribution and Fisher grade presentation differed significantly between the NASAH and aSAH groups (p<0.001). Risk factors, such as smoking, hypertension and alcohol overuse, were significantly more common in aSAH than NASAH. Conversely, diabetes mellitus (DM) was more prevalent in NASAH than in aSAH (p<0.001). Conclusions This is the largest epidemiological study of NASAH to date. The observed incidence of NASAH was higher than in the previous reports, suggesting either underdiagnosis in earlier studies or a changing proportion of aSAH to NASAH cases. The distinct differences in population characteristics and risk factors suggest that NASAH and aSAH arise from fundamentally different pathophysiological mechanisms. DM emerged as a risk factor for NASAH, and MAPAs were identified as one of the underlying sources of haemorrhage in this subgroup.