Do cerebral aneurysms originate at the site of medial defects? Microscopic examinations of experimental aneurysms at the fenestration of the anterior cerebral artery in rats.

BACKGROUND: The role of medial defects on cerebral aneurysm formation remains obscure. To examine whether cerebral aneurysms originate at the site of medial defects, we determined the exact locations of both the origin of experimental aneurysms and medial defects at the apex of fenestrations of the anterior cerebral artery (ACA) in rats.
METHODS: Cerebral aneurysms were induced in rats by means of the combination of ligation of the unilateral common carotid artery and induced hypertension. The ACA opposite the ligation was examined under a light microscope.
RESULTS: Medial defects, which could be observed in approximately one-half of the fenestrations, existed always around the lateral margin of the intimal pads. On the contrary, early aneurysmal lesions exclusively occurred at the portion just medial to the intimal pads. Subsequently, both pads and defects seemed to be involved in aneurysm walls.
CONCLUSION: These results clearly indicate that cerebral aneurysms do not originate at the site of medial defects.