BACKGROUND/OBJECTIVE: Perimesencephalic subarachnoid haemorrhage (pmSAH) has traditionally been considered benign and of venous origin. However, advanced imaging increasingly identifies basilar artery perforator aneurysms (BAPAs) as a subset of cases historically labelled as non-aneurysmal, atraumatic (NAA) pmSAH. The objective was to compare clinical characteristics and outcomes of patients with NAA, BAPA and ruptured posterior circulation aneurysms (r-pc-AN), assessing the impact of pmSAH aetiology on patient outcomes.

METHODS: This retrospective, multicentre, observational cohort study included BAPA cases from the international PERForator Aneurysm registry (2013-2025, 60 centres, 19 countries). Comparison cohorts were from a single high-volume tertiary care centre (2004-2025). The study included 444 patients (n=167 NAA, n=157 BAPA, n=120 r-pc-AN). Excellent outcome was defined as a modified Rankin Scale score of 0-1 at 3-6 months.

RESULTS: Excellent outcomes were achieved in 137/167 (82%) of NAA, 96/140 (69%) of BAPA and 56/102 (55%) of r-pc-AN cohorts (p<0.001). Mortality rates were 1% (NAA), 11% (BAPA) and 18% (r-pc-AN). cCompared with BAPA, NAA patients had significantly higher odds of excellent outcome (adjusted OR, aOR 2.0, 95% CI 1.2 to 3.4, p=0.01), while r-pc-AN were associated with significantly lower odds of excellent outcome (aOR 0.5, 95% CI 0.3 to 0.9, p=0.01). Hydrocephalus and external ventricular drain rates were highest in r-pc-AN (83% and 87%), followed by BAPA (48% and 44%) and NAA (28% and 16%) (p<0.001).

CONCLUSIONS: While pmSAH has been considered benign, our findings challenge this assumption. Patients with BAPA-related pmSAH demonstrated significantly worse outcomes than NAA but better outcomes than r-pc-AN. Further research is needed to distinguish BAPA-pmSAH from NAA-related pmSAH and to establish diagnostic and therapeutic guidelines.

TRIAL REGISTRATION NUMBER: NCT06189014.