Rupture risk of small unruptured cerebral aneurysms

Abstract

OBJECTIVEThe annual rupture rate of small (3–4 mm) unruptured cerebral aneurysms (UCAs) is 0.36% per year, however, the proportion of small ruptured aneurysms < 5 mm is 35%. This discrepancy is explained by the hypothesis that most acute subarachnoid hemorrhage (SAH) is from recently formed, unscreened aneurysms, but this hypothesis is without definitive proof. The authors aimed to clarify the actual number of screened, ruptured small aneurysms and risk factors for rupture.METHODSThe Unruptured Cerebral Aneurysm Study Japan, a project of the Japan Neurosurgical Society, was designed to clarify the natural course of UCAs. From January 2001 through March 2004, 6697 UCAs among 5720 patients were prospectively registered. At registration, 2839 patients (49.6%) had 3132 (46.8%) small UCAs of 3–4 mm. The registered, treated, and rupture numbers of these small aneurysms and the annual rupture rate were investigated. The rate was assessed per aneurysm. The characteristics of patients and aneurysms were compared to those of larger unruptured aneurysms (≥ 5 mm). Cumulative rates of SAH were estimated per aneurysm. Risk factors underwent univariate and multivariate analysis.RESULTSTreatment and rupture numbers of small UCAs were 1132 (37.1% of all treated aneurysms) and 23 (20.7% of all ruptured aneurysms), respectively. The registered, treated, rupture number, and annual rupture rates were 1658 (24.8%), 495 (16.2%), 11 (9.9%), and 0.30%, respectively, among 3-mm aneurysms, and 1474 (22.0%), 637 (20.9%), 12 (10.8%), and 0.45%, respectively, among 4-mm aneurysms. Multivariate risk-factor analysis revealed that a screening brain checkup (hazard ratio [HR] 4.1, 95% confidence interval [CI] 1.2–14.4), history of SAH (HR 10.8, 95% CI 2.3–51.1), uncontrolled hypertension (HR 5.2, 95% CI 1.8–15.3), and location on the anterior communicating artery (ACoA; HR 5.0, 95% CI 1.6–15.5) were independent predictors of rupture.CONCLUSIONSAlthough the annual rupture rate of small aneurysms was low, the actual number of ruptures was not low. Small aneurysms that ruptured during follow-up could be detected, screened, and managed based on each risk factor. Possible selection criteria for treating small UCAs include a history of SAH, uncontrolled hypertension, location on the ACoA, and young patients. Further large prospective and longitudinal trials are needed.Clinical trial registration no.: C000000418 (https://www.umin.ac.jp/ctr)

Characteristics of patients with ruptured very small intracranial aneurysm sized less than 3 mm

“…In a previous study, however, the proportion of ruptured small intracranial aneurysms sized less than 5 mm was 20.7% among all ruptured intracranial aneurysm. 4) It is known that there is other factors predicting the rupture of VSIA, 6) So the risk of rupture in the VSIA group cannot be ignored even if the aneurysms are very small. Ruptured VSIAs are associated with a lower inflow angle, higher aspect ratio, and higher height-width ratio.…”

“…Hence, patients with VSIAs are considered to be at a lower risk of rupture than non-VSIA patients. In a previous study, however, the proportion of ruptured small intracranial aneurysms sized less than 5 mm was 20.7% among all ruptured intracranial aneurysm [ 4 ]. It is known that there is other factors predicting the rupture of VSIA [ 6 ], So the risk of rupture in the VSIA group cannot be ignored even if the aneurysms are very small.…”

“…If the rupture risk of an intracranial aneurysm is high, sac obliteration using aneurysmal clipping or endovascular coil embolization is recommended. The reported annual rupture rate of intracranial aneurysms is 0.95% per year [ 4 , 8 ], and the rupture risk is reported to be 0.36% in intracranial aneurysms sized 3-4 mm, below 0.5% in aneurysms sized less than 5 mm [ 4 , 7 ]. Therefore, if the size of an intracranial aneurysm is below 3 mm, clinicians rarely treat them and opt for intermittent imaging follow-up evaluations [ 1 ].…”

Characteristics of Cerebral Aneurysms in Japan

“…Recently, we investigated the rupture risk for small UCA using the data of UCAS Japan. 47) During registration, 2839 patients (49.6%) had 3132 (46.8%) small UCAs of 3–4 mm in diameter. We investigated the registered, treated, and rupture numbers of these small UCAs and the annual rupture rate, and compared the characteristics of patients and aneurysms to those of larger unruptured aneurysms (≥5 mm) to identify the risk factors using univariate and multivariate analyses.…”

A register-based SAH study in Japan: high incidence rate and recent decline trend based on lifestyle

“…11 In contrast, the number of treatments for unruptured cerebral aneurysms (UCAs) has been increasing in Japan because UCAs are frequently discovered at screening checkups. 12 Furthermore, the high rates of total UCA treatments (45.5%) and small UCA (< 5 mm in diameter) treatments (37.1%) 13,14 are attributable to a 2.8 times higher rupture rate of UCAs in Japan than in Western countries. 15 However, there is no evidence that the increasing trend in the number of treatments for UCAs or changing lifestyle habits would contribute to reducing the incidence of SAH in Japan.…”