A, The inset shows the right temporo-occipital craniotomy and the mastoid exposure. The mastoidectomy has been completed and the otic capsule, composed of the dense cortical bone around the labyrinth, has been exposed. The tympanic segment of the facial nerve and the lateral canal are situated deep to the spine of Henle. Trautmann’s triangle, the patch of dura in front of the sigmoid sinus, faces the cerebellopontine angle. B, The presigmoid dura has been opened and the superior petrosal sinus and tentorium divided, with care taken to preserve both the vein of Labbé that joins the transverse sinus and the trochlear nerve that enters the anterior edge of the tentorium. The abducens and facial nerves are exposed medially to the vestibulocochlear nerve. The posteroinferior cerebellar artery courses in the lower margin of the exposure with the glossopharyngeal and vagus nerves. The superior cerebellar artery passes below the oculomotor and trochlear nerves and above the trigeminal nerve. C, The labyrinthectomy has been completed to expose the internal acoustic meatus. A marginal branch of the superior cerebellar artery loops downward on the cerebellum. D, The dura lining the meatus has been opened and the facial nerve has been trans-posed posteriorly. The cochlear nerve has been divided and bone removed to expose and remove the cochlea. The transcochlear exposure, completed by removing the cochlea and surrounding petrous apex, provides access to the front of the brainstem and vertebrobasilar junction, but at the cost of loss of hearing caused by the labyrinthectomy and almost certain temporary or permanent facial weakness associated with the transposition of the facial nerve. (Images courtesy of AL Rhoton, Jr.)