Introduction The stratification of unruptured cerebral aneurysm rupture risk is largely based on 1998 ISUIA trial results which suggest that aneurysm diameters less than 7 mm have a low risk of rupture. The PHASES score,based on a systematical review of six prospective cohort studies with subarachnoid hemorrhage as outcome, evaluates genetic background, age, hypertension, earlier subarachnoid hemorrhage from another aneurysm, and size and location of the aneurysm. The present study aims to determine whether aneurysm size thresholds used in these scales should be calibrated according to the anatomic location of the aneurysm.

Methods In this IRB-approved ten-year retrospective cohort study, consecutive adult patients presenting to an urban tertiary care academic comprehensive stroke center with subarachnoid hemorrhage were evaluated for cerebral aneurysm anatomic location and dome diameter. The ruptured aneurysm sizes were organized into groups of 3 mm, > 3 mm and 7 mm, and > 7 mm. Classifications of anatomic location were based on conventional categorization and rupture-likelihood such as anterior and posterior communicating arteries. All calculated group percentages and mean aneurysm sizes were compared to evaluate for a correlation between ruptured aneurysm size and anatomic location.

Results From 16 August 2012 and 3 April 2022, 643 of 1040 subarachnoid hemorrhage patients had ruptured aneurysms. Most patients had small aneurysms > 3 mm and 7 mm (55.20%) with a mean size of 5.02 mm or smaller aneurysms 3 mm (26.01%) with a mean size of 2.47 mm. Fewer had large aneurysms > 7 mm (18.79%) with a mean size of 10.26 mm. For all patients, the anatomic distribution varied based on size. Small, ruptured aneurysms defined as 3 mm (mean: 2.44 mm) or > 3 mm and 7 mm (mean: 4.84 mm) were commonly located at ACOM (27.27% and 58.82%, respectively). Large, ruptured aneurysms > 7 mm (mean: 9.74 mm) were often found at PCOM (23.64%). Aneurysms at the MCA and ICA had roughly equivalent size category distributions. 73.33% of aneurysms at the pericallosal artery and 41.18% at PICA were notably small (2.45 mm and 2.07 mm, respectively) with all remaining aneurysm sizes ranging 7 mm.

Conclusion This study shows that the average size of ruptured aneurysms does vary based on its anatomic location, suggesting a more nuanced approach to interpreting the risk of intracranial aneurysm based on size. Smaller aneurysms are more likely to be located at ACOM while large intracranial aneurysms are more likely to be located at PCOM. This may be helpful in calibrating dome size thresholds used to determine future rupture risk and treatment decision-making based on location. This retrospective cohort study is limited in defining a causal relationship between intracranial aneurysm size and location. The observed trend justifies future larger scale studies to more clearly define this correlation.

Disclosures S. Pathak: None. J. Sim: None. G. Cross: None. Y. Radaideh: None. M. Chen: None.