Basilar tip aneurysms and basilar tip anatomy.

SUMMARY: Five hundred and eighty one cerebral aneurysms seen from 1986-1997 were reviewed. Fifty five basilar tip aneurysms were demonstrated. The analysis of the anatomic disposition of the distal basilar artery in 47 of them showed the prevalence of caudal fusion in 91% of cases. The fusion is either symmetrical or asymmetrical. The neck of the aneurysm is preferentially implanted on the caudal part of the fusion. Recognition of the distal basilar artery anatomy allows the origin and extent of the perforating branches territory arising from the PI segment to be predicted in each type of basilar tip disposition. The caudal variant supplies a small ipsilateral territory, whereas the cranial one often provides a bilateral supply. In the cranial arrangement one can observe interpeduncular anastomoses between the perforators across the midline prior to penetrating the mesencephalo-diencephalic structures. The risks involved in the sacrifice of the P1 segment can then be anticipated.