Objective The optimal consensus concerning treatment of incidental small paraclinoid unruptured intracranial aneurysms (UIAs) remains controversial. The aim of this retrospective study was to reveal the natural history of small paraclinoid UIAs with the goal of informing the treatment plan.

Methods 524 patients harbouring 568 paraclinoid UIAs (≤5 mm) were retrospectively evaluated during the mean follow-up of 35.4 months. The aneurysms were divided into two groups with respect to arterial branch: related (ophthalmic and superior hypophyseal artery), and non-related. Medical records were reviewed concerning multiple variables, such as sex, age, hypertension (HTN), diabetes mellitus, smoking and aneurysmal factors (size, arterial relationship, multiplicity and the occurrence of rupture and growth). The cumulative risk and the risk factors of aneurysmal rupture and growth were analysed.

Results Two aneurysmal (0.35%) ruptures and 17 growths (3.0%) were observed during the follow-up of 1675.5 aneurysm-years with an annual rupture of 0.12% and an annual growth of 1.01%. The cumulative survival without aneurysmal growth reached a significant difference in aneurysms ≥4 mm (p=0.001), HTN (p=0.002), and arterial branch-related location (p=0.001). Multivariate analysis disclosed that aneurysm ≥4 mm (HR, 4.41; p=0.003), HTN (HR, 5.74; p=0.003), arterial branch-related location (HR, 6.04; p=0.002), and multiplicity (HR, 0.27; p=0.042) were significant predictive factors for aneurysm growth.

Conclusions Although incidental small paraclinoid UIAs have a relatively lower rupture and growth risk, patients with high-risk factors, including aneurysm ≥4 mm, HTN, arterial branch-related aneurysms, and multiple aneurysms must be monitored closely. The limitation of the retrospective nature of this study should be taken into consideration.