Complex and continuous change in hypothetic risk of rupture of intracranial cerebral aneurysms – Bleb mandala –

Background: This study of the intraoperative microscopic findings indicated complex behavior of aneurysmal bleb affects the rupture risk. Methods: A total of 300 consecutive relatively small (mean 6.2 mm) unruptured ICAs were clipped from July 2005 to January 2019. Microscopically identified blebs were divided into four types based on external appearances. Type A bleb was a thick-walled bleb. Type B bleb was a thin-walled reddish bleb. Type C bleb was a healing organized bleb. Type D was a hemosiderin-laden bleb. Type B and D blebs were considered to be risky blebs for rupture. ICAs with bleb(s) were further categorized into two types, stable aneurysms and unstable aneurysms with Type B and/or D bleb(s). The number and nature of the blebs were compared with the demographic data of the patients and radiological findings of the ICAs to evaluate possible predictors of the hypothetic rupture risk. Results: Aneurysms tended to enlarge as the number of blebs increased (p = 0.073). High risk Type B bleb had significantly larger number of blebs (p < 0.05). Healing Type C bleb was significantly more common in the group with two blebs than one bleb (p < 0.01). No significant differences were found between stable and unstable aneurysms in aneurysm multiplicity, size, and radiological bleb. Conclusions: Aneurysm state continuously changes due to the dynamic change of bleb number and nature. Our data suggests the necessity of new repeatable vessel-wall imaging technology to select risky unstable ICAs for treatment.