Risk of aneurysm rupture at intracranial arterial bifurcations

Aneurysms on the posterior circulation, most commonly located at the basilar top, have a higher risk of rupture than aneurysms on the anterior circulation. If hemodynamic shear stress, which has its maximum impact at the distal carina of bifurcations, explains the higher rupture rate of basilar top aneurysms, aneurysms at the top of the carotid artery should have similar rupture rates given their geometrical similarities.

Our purpose was to compare rupture risks of carotid and basilar artery bifurcation aneurysms.

We included studies from Medline and Embase searches and compared proportions of ruptured and unruptured aneurysms at the basilar and carotid bifurcation with the assumption that carotid aneurysms are twice as prevalent based on the presence of 2 carotid and 1 basilar artery bifurcation on the circle of Willis.

Of all unruptured aneurysms, 8.3% were located on the basilar and 6.0% on the carotid bifurcation; 8.0% of all ruptured aneurysms were located on the basilar and 4.3% on the carotid bifurcation. Subsequently the ratios of carotid versus basilar aneurysms were 0.72 for unruptured and 0.55 for ruptured aneurysms, instead of the expected 2.0.

Aneurysms are less frequently located on the carotid than on the basilar artery bifurcation. The proportion of ruptured carotid aneurysms is smaller than that of unruptured carotid aneurysms, suggesting a lower rupture risk for aneurysms at the carotid artery bifurcation. The anatomical geometry of the bifurcations and concomitant hemodynamic stress are considered an unlikely explanation for the higher risk of posterior circulation aneurysms.