Differentiating aetiologies in perimesencephalic SAH: clinical insights into basilar artery perforator aneurysms

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 .