A, AICA relationships in the right CPA by retrosigmoid approach. The AICA passes laterally between the facial and vestibulocochlear nerves and turns medially to course along the middle cerebellar peduncle and cerebellopontine fissure. A large superior petrosal vein with multiple tributaries, including the pontotrigeminal and transverse pontine veins and the vein of the cerebellopontine fissure, passes behind the trigeminal nerve. The flocculus hides the junction of the facial and vestibulocochlear nerves with the brainstem. B, The flocculus and choroid plexus, which protrudes from the foramen of Luschka, have been elevated to expose the junction of the facial and vestibulocochlear nerves with the brainstem, where the facial nerve is seen below the vestibulocochlear nerve. An AICA branch gives rise to both the subarcuate and labyrinthine arteries. C, A dissector elevates the vestibulocochlear nerve to more clearly define the junction of the facial nerve with the brainstem. The junction of the facial nerve with the brainstem is easier to expose below rather than above the vestibulocochlear nerve. D, The posterior meatal wall has been removed to expose the dura lining the meatus. E, The meatal dura has been opened and the vestibulocochlear nerve displaced downward to expose the facial nerve coursing anterior and superior within the meatus. The nervus intermedius, which arises on the anterior surface of the vestibulocochlear nerve and passes laterally to join the facial nerve, is composed of several rootlets, as is common. F, The cleavage plane between the superior and inferior vestibular nerves has been developed. The cochlear nerve is located anterior to the inferior vestibular nerve. (Images courtesy of AL Rhoton, Jr.)