Asymmetrical Basilar Tip Fusion and Selected Posterior Circulation Variants: A Comparative 3D Angiography Study in Aneurysm and Clinical Reference Cohorts.

BACKGROUND AND PURPOSE: The configuration of the basilar tip varies from symmetric to markedly asymmetric fusion of the P1 segment of the posterior cerebral artery-superior cerebellar artery complexes. Prior studies have focused mainly on aneurysm cohorts, limiting assessment of whether these anatomical patterns are enriched relative to background anatomy. We compared selected basilar-tip and posterior circulation variants between patients with basilar apex or basilar artery-superior cerebellar artery aneurysms and a clinical reference angiographic cohort without these aneurysms.

METHODS

We retrospectively reviewed three-dimensional bilateral vertebral angiography performed between April 2012 and March 2024 at a single center. The aneurysm cohort comprised patients with basilar apex or basilar artery-superior cerebellar artery aneurysms; the clinical reference cohort comprised patients imaged for other clinical indications, including non-aneurysmal vascular and tumorous pathology. Two independent raters classified right and left basilar-tip contributions as cranial or caudal, from which symmetry was derived. The artery of Percheron, bilateral thalamoperforator supply, and PICA-origin variants were independently assessed. The posterior communicating artery-to-P1 diameter quotient was calculated per side.

RESULTS

Asymmetrical fusion was more frequent in the aneurysm cohort than in the clinical reference cohort (69.0% vs 44.4%, P = 0.01). Within the aneurysm cohort, asymmetrical fusion was associated with higher prevalences of the artery of Percheron (32.5% vs 5.6%, P = 0.04), bilateral thalamoperforator supply (52.5% vs 11.1%, P = 0.003), PICA-origin asymmetry (75.0% vs 16.7%, P < 0.001), and AICA-PICA/PICA-AICA variants (80.0% vs 27.8%, P < 0.001). In asymmetrical aneurysm cases, the posterior communicating artery-to-P1 diameter quotient was lower in the artery of Percheron group on both the cranial and caudal limbs. Exploratory adjusted analyses suggested that the crude cohort difference in asymmetrical fusion attenuated after accounting for demographic and vascular-risk differences.

CONCLUSIONS

Asymmetrical basilar-tip fusion and related perforator/PICA variants were more common in the aneurysm cohort than in a clinical reference cohort, but these between-cohort differences should be interpreted cautiously because the reference cohort included non-aneurysmal pathology and differed in vascular risk profile. The findings support a broader pattern of posterior circulation developmental asymmetry and may help inform anatomical interpretation and procedural planning.