Is Aneurysm Growth a Surrogate of Risk of Rupture?

Background: The natural history of small unruptured incidentally discovered intracranial aneurysms (SUIA) remains unpredictable. Therefore, when asymptomatic, the strategy is not consensual. The aim of this study is to assess the safety of the selection and follow-up protocol used in Geneva, measure the rate of morphological changes of SUIA according to their location and differences in case characteristics between stable and unstable aneurysms. Methods: A prospective consecutive cohort of 292 patients (2006–2014) and 368 SUIA (anterior circulation aneurysms below 7 mm and posterior circulation aneurysms smaller than 4 mm, with absence of previous subarachnoid hemorrhage [SAH]) were observed (mean follow-up time of 3.2 years and 1,177.6 aneurysm years). Baseline epidemiological data, clinical information, risk factors, and aneurysm characteristics were recorded. Results: The mean age of the cohort is 55.1 ± 15.4 years and the women to men ratio is high (3.4:1). Most patients (86.3%) were not significantly disabled, half of the cohort was populated by smokers (52.7%) and 48% suffered high blood pressure. The most frequent site of SUIA were: ophthalmic segment (Oph IC: n = 84, 22.8%)), followed by sylvian bifurcation (MCA bif: n = 82, 22.2%), anterior communicating artery (AcoA: n = 54, 14.7%) M1 segment of the MCA (M1MCA: n = 41, 11.2%). Mean aneurysm size is 3.6 ± 1.9 mm, with a 230 lesions less than 4mm (62.5%). The rate of aneurysm growing is 2.1% per year. Two aneurysm ruptures were observed (M1 and AcoA). Lesions between 4 to 6.9 mm of largest dimension were more likely to grow than lesions smaller than 4mm aneurysms (OR = 3.5, 95% CI 1.6–7.5, p < 0.001).Odds of aneurysms growth per location in the population of untreated patients has a similar distribution to the odds of aneurysm rupture per location in the overall population. Nevertheless and surprisingly, M1MCA site was the location with most growing aneurysms (n = 10; 27%; p = 0.004). Conclusions: It is safe to observe small anterior circulation aneurysms, treating them after a morphological change. However, lesions greater than 4 mm should be screened regularly. Odds of rupture in the general population and odds of aneurysms' growth in the population of patients followed-up categorized according to aneurysm location or size are similar suggesting that aneurysm growth is a good surrogate of aneurysm rupture.