BACKGROUND: In patients with spontaneous subarachnoid haemorrhage (SAH), a vascular cause for the bleed is not always found on initial investigations. This study aimed to systematically evaluate the delayed investigation strategies and clinical outcomes in these cases, often described as "non-aneurysmal" SAH (naSAH). METHODS: A systematic review was performed in concordance with the PRISMA checklist. Pooled proportions of primary outcome measures were estimated using a random-effects model. RESULTS: Fifty-eight studies were included (4473 patients). The cohort was split into perimesencephalic naSAH (PnaSAH) (49.9%), non-PnaSAH (44.7%) and radiologically negative SAH identified on lumbar puncture (5.4%). The commonest initial vascular imaging modality was digital subtraction angiography. A vascular abnormality was identified during delayed investigation in 3.9% [95% CI 1.9-6.6]. There was no uniform strategy for the timing or modality of delayed investigations. The pooled proportion of a favourable modified Rankin scale outcome (0-2) at 3-6 months following diagnosis was 92.0% [95% CI 86.0-96.5]. Complications included re-bleeding (3.1% [95% CI 1.5-5.2]), hydrocephalus (16.0% [95% CI 11.2-21.4]), vasospasm (9.6% [95% CI 6.5-13.3]) and seizure (3.5% [95% CI 1.7-5.8]). Stratified by bleeding pattern, we demonstrate a higher rate of delayed diagnoses (13.6% [95% CI 7.4-21.3]), lower proportion of favourable functional outcome (87.2% [95% CI 80.1-92.9]) and higher risk of complications for non-PnaSAH patients. CONCLUSION: This study highlights the heterogeneity in delayed investigations and outcomes for patients with naSAH, which may be influenced by the initial pattern of bleeding. Further multi-centre prospective studies are required to clarify optimal tailored management strategies for this heterogeneous group of patients.