Abstract Body: Background: Cerebral aneurysm rupture has a poor prognosis, and a growing aneurysm is prone to rupture. Although hemodynamics is thought to play an important role in aneurysm growth, it is not well understood what hemodynamic environments are involved. We therefore conducted the NHO CFD ABO Study, a prospective observational study to clarify the aneurysm growth risk associated with hemodynamics.
Methods: Computational fluid dynamics analysis was performed using the unique arterial geometry and flow velocities of the enrolled patients. Hemodynamic metrics were compared by multivariate analysis between aneurysms that grew more than 1 mm and those that did not during the 3-year observation period, using known growth risks as confounding factors.
Results: A total of 481 patients were enrolled, and 26 growth aneurysms and 176 non-growth aneurysms met the criteria. For aneurysms <4 mm in size, the time-averaged wall shear stress (WSS) and time-averaged WSS gradient were significantly higher in growth aneurysms for the whole aneurysm, neck, body and parent artery, and transverse WSS was significantly higher on the neck and parent artery. In aneurysms ≥4 mm, the normalized transverse WSS, which represents the degree of multi-directional WSS disturbance, was significantly higher in growth aneurysms for the whole aneurysm and the aneurysm dome. Aneurysms <4 mm were more likely to show whole-aneurysm growth, while aneurysms ≥4 mm were often enlarged at the dome.
Conclusions: These results suggest that there exist two distinct hemodynamic mechanisms for cerebral aneurysm growth depending on size. Aneurysms <4 mm may grow near the aneurysm neck in environments with high magnitudes and multi-directional disturbances of WSS, similar to the environments present at aneurysm development, while aneurysms ≥4 mm may grow in the dome in environments having enhanced multi-directional WSS disturbance. The present evidence may help resolve two conflicting hemodynamic theories of rupture; the "high stress rupture theory" may apply to rupture of aneurysms <4 mm, and the "low stress rupture theory" to rupture of aneurysms ≥4 mm since aneurysm growth is considered a precursor to rupture. Since growing aneurysms, even if not particularly large, are prone to rupture, it would be useful to consider indications for surgical treatment with reference to the hemodynamic predictors of growth obtained in this study.