Cerebral aneurysms arising at nonbranching sites - An experimental study

Background and Purpose The origin and pathogenesis of cerebral aneurysms arising at nonbranching sites are not clear. Using our animal model to induce cerebral aneurysms in rats, we examined induced aneurysms that developed at nonbranching sites. Methods In 35 Sprague-Dawley rats, the left common carotid artery was ligated and renal hypertension was produced to induce cerebral aneurysms. Twelve months later, the circle of Willis was carefully examined under a dissecting microscope. Results Other than cerebral aneurysms at branching sites of the circle of Willis, aneurysmal bulges developing at nonbranching sites were found in the proximal portion of the posterior cerebral artery (P1) on the side of carotid ligation, which supposedly acted as a major collateral pathway after the ligation, in 19 of 35 treated rats, A total of 30 lesions were found in these 19 rats, and they were classified into fusiform aneurysms (22 lesions) involving the entire vessel wall fora short distance and saccular aneurysms (8 lesions) involving only a part of the wall and expanding laterally from the vessel wall. These P1s became larger in caliber and more tortuous after ligation. Aneurysms developed more frequently in proportion to these changes in these vessels. Moreover, most aneurysms in these vessels developed at or near the curvatures. All of the lateral aneurysms were found on the lateral wall of the curvatures of the vessels. Conclusions The present findings indicate that cerebral aneurysms at nonbranching sites and saccular aneurysms at branching sites can occur under the same etiologic conditions. The site of origin is strongly related to hemodynamic stress.