Morphology and interval growth are predictive factors of intracranial aneurysm rupture

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In a study conducted by William Mehan et al, it was found that for patients with unruptured intracranial aneurysms, multilobulated morphology and interval growth of the unruptured aneurysms are “characteristics predictive of a higher risk of subsequent rupture during conservative CT angiography (CTA) follow-up.”

Mehan and colleagues (Massachusetts General Hospital, Harvard Medical School, Boston, USA) say that, although there have been other multicentre prospective studies (ISUIA, UCAS and SUAVE) demonstrating characteristics associated with rupture, the “natural history of unruptured intracranial aneurysms remain uncertain.”

Therefore, the Massachusetts group aimed to identify factors predictive of rupture, or confirm the characteristics found in the aforementioned studies. They say that this is important as aneurysm rupture is associated with “significant morbidity and mortality”.

The single centre study, published in the Journal of NeuroInterventional Surgery was a retrospective review of patients with unruptured intracranial aneurysms followed with serial CT angiography studies between January 1999 and December 2010. The following features for each aneurysm were catalogued from the official radiologic reports and CTA images: maximum diameter, growth between follow-up studies, location, multiplicity, wall calcification, intraluminal thrombus and morphology. Univariate logistic regression analysis of the potential independent risk factors for aneurysm rupture was performed. Statistically significant risk factors from the univariate analysis were then entered into a multivariate logistic regression analysis.  

During the study period, a total of 3,033 cerebral aneurysms were referred to the institution, of which 614 ruptured. A total of six aneurysms in six different patients ruptured during the CT angiography follow-up period, yielding an overall rupture rate of 3.3% and an annual rupture rate of 0.97%. The investigators initially performed a univariate logistic regression analysis of potential risk factors for aneurysm rupture. All six ruptured aneurysms were at least 9mm in maximum diameter compared with a mean of 5.4mm for the unruptured aneurysms (p=0.003).

 
Four of six ruptured aneurysms demonstrated interval growth by the time of rupture compared with four of 174 unruptured aneurysms (p<0.0001). In one of the four ruptured aneurysms that enlarged between serial CT angiography follow-up studies, interval growth was demonstrated on CT angiography studies obtained prior to the study performed at the time of rupture. The mean growth for the aneurysms that ruptured was 4.25mm. Four of six ruptured aneurysms demonstrated multilobulated morphology compared with 16 of 174 of unruptured aneurysms. In three of the four multilobulated aneurysms that ruptured, multilobulated morphology was demonstrated on CT angiography studies obtained prior to the study performed at the time of rupture. A single ruptured aneurysm demonstrated neither interval growth nor multilobulated morphology on CT angiography.  

According to Mehan et al, the non-invasive nature of CT angiography coupled with its relatively high spatial resolution, speed and availability make CT angiography an excellent modality for the serial follow-up of known unruptured intracranial aneurysms. Based on the study findings, unruptured intracranial aneurysms with a multilobulated morphology and/or interval growth between follow-up CT angiography studies are at increased risk of rupture. Further studies of conservative CT angiography follow-up of unruptured intracranial aneurysms with longer follow-up periods and larger numbers of ruptured aneurysms are necessary.

The authors conclude that this study demonstrates that multilobulated morphology and interval growth of unruptured intracranial aneurysms are characteristics predictive of a higher risk of subsequent rupture during conservative CT angiography follow-up. The presence of these features could help guide clinical decisions as to whether these aneurysms are followed conservatively or treated.