Cerebral aneurysms unrelated to arterial bifurcations.

The majority of saccular cerebral aneurysms arise at arterial branchings; those arising elsewhere are rare. Among 557 saccular cerebral aneurysms surgically treated between 1983 and 1994, 29 (5.2%) were unrelated to arterial divisions. These cases were retrospectively analysed. The ages of the subjects ranged from 6 to 80 years (mean 57); 17 (59%) were female, and 12 (41%) male. Twenty-four (83%) presented with subarachnoid haemorrhage. Seventeen (59%) had a history of hypertension, and ten (34%) had multiple cerebral aneurysms. The most common site was the internal carotid artery (45%), followed by the middle cerebral artery (28%), the vertebrobasilar arteries (17%), and the anterior cerebral artery (10%). The aneurysms were classified in relation to arterial curvature into three groups: dorsal curvature (41%), ventral curvature (10%) and non-curvature (49%). During surgery, sclerotic changes were noted in the arterial wall adjacent to the aneurysms in 14 patients (48%). Five cases (17%) had small thin walled (blister-like) aneurysms, four of which ruptured during surgery resulting in poor clinical outcomes. We suggest that arteriosclerotic changes in the arterial wall, carotid siphon, and/or local haemodynamic forces are important factors in the development of this type of aneurysms. At surgery, one should bear in mind the possibility of sclerotic or blister-like aneurysms which require special attention to deal with them.