The lower basilar artery aneurysm is one of the most difficult to approach surgically because of its anatomy. The microsurgical anatomy which is encountered while dissecting cadaver specimens with a transpetrosal approah is presented in this paper. The routes used for this approach are 1) subtemporal approach, 2) retrolabyrinthine approach and 3) supra- and infratentorial approach which combines a retrolabyrinthine approach with a retromastoid-posterior subtemporal approach. With this approach, direct visualization of the lower basilar artery and its vicinity are possible with minimum distance and with minimum brain retraction. However, the surgical field is limited and there exists a risk of disturbing the vestibulocochlear function. So, a thorough knowledge of the anatomy of the petrous portion of the temporal bone and skilful drilling of this bone are indispensable. The landmarks which are the most useful for these approaches are the greater petrosal nerve for a subtemporal approach, and anterior margin of the sigmoid sinus, the superior petrosal sinus and the vestibular aqueduct for a retrolabyrinthine approach.
