The natural history of untreated small (< 5 mm) intracranial aneurysms: a five-year angiographic follow-up study

Abstract The clinical management of small, unruptured intracranial aneurysms, particularly those measuring less than 5 mm, represents a significant area of debate in neurovascular surgery. The decision to pursue invasive treatment versus conservative surveillance is often guided by limited and sometimes conflicting evidence, leaving a critical gap in clinical decision-making. This study aimed to meticulously characterize the five-year natural history and morphological evolution of a cohort of patients with untreated small intracranial aneurysms (defined as < 5 mm), managed conservatively with serial angiographic surveillance. We conducted a longitudinal cohort study at a single tertiary academic center, enrolling 113 patients (101 female, 89.4%; 12 male, 10.6%; mean age 59 years) with at least one unruptured intracranial aneurysm measuring < 5 mm, diagnosed by digital subtraction angiography (DSA). All patients were managed with a conservative “watch-and-wait” protocol, which included a follow-up DSA at five years. The primary endpoints were morphological changes, including dimensional growth, shrinkage, complete resolution, de novo formation of new aneurysms, and the incidence of aneurysmal subarachnoid hemorrhage (SAH). Over the cumulative 565 patient-years of follow-up, no instances of aneurysmal rupture or SAH were observed. Most aneurysms were located in the anterior circulation (92.2%). The vast majority of aneurysms (n = 87; 77.0%) demonstrated complete morphological stability. A small subset of aneurysms exhibited dynamic changes: 12 (10.6%) demonstrated minor growth but remained within the < 5 mm size category, 6 (5.3%) decreased in size, and 5 (4.4%) underwent complete spontaneous resolution. De novo aneurysm formation was identified in 3 patients (2.7%). While traditional vascular risk factors like hypertension and oral contraceptive use showed no significant correlation with aneurysm behavior, a paradoxical and statistically significant association was found with smoking, which correlated with a higher likelihood of aneurysm resolution (p = 0.016). This study supports that a conservative management strategy with planned imaging surveillance is a safe and viable option for patients with unruptured intracranial aneurysms smaller than 5 mm. The absence of rupture over five years, coupled with the low rate of significant growth, suggests that these lesions have a very low observed risk of rupture and morphological progression over five years. These findings support conservative management for most patients with UIAs < 5 mm, although treatment decisions should remain individualized according to patient- and aneurysm-specific risk factors.