This huge aneurysm of the basilar artery is not congenital, but atherosclerotic. Severe atherosclerosis (deposition of fats in the walls of arteries) may gradually weaken the wall of an artery, promoting their aneurysm. The most common example is the abdominal aorta. The aneurysm was not broken and was found at autopsy. It caused noticeable deformation of the anterior surface of the brainstem. The aneurysm was filled with mixed thrombus arranged in several concentric layers.However, there was a light and maintaining blood flow (otherwise there would have been massive brainstem infarction, incompatible with life). Intracranial atherosclerotic aneurysms are much rarer than congenital aneurysms.