Middle Fossa Approach to the Internal Auditory Meatus. A, The dura has been removed from the middle fossa floor and cavernous sinus wall to expose the greater petrosal nerve, middle meningeal artery, and the nerves in the sinus wall. The tentorium has been removed preserving the edge adjacent to the superior petrosal sinus. The arachnoid over the Meckel's cave has been elevated with a stitch to show the trigeminal ganglion. We used the angle between the IAM and the long axis of the GSPN, approximately 60°, to precisely localize the IAM on the middle fossa floor (shown in blue). The distance between the geniculate ganglion and the point where the GSPN passes under V3 is approximately 15 mm, and locating the line of medial to posterolateral drilling at this point provides satisfactory exposure of the IAM (blue arrows). B, The bone over the IAM has been removed medially to posterolaterally, and the posterior fossa dura has been exposed to the level of the Bill's bar. The bone at the angle between the greater petrosal nerve and the IAM has been drilled to show the cochlea. The petrous apex has been preserved. As the MMA, V3, greater petrosal nerve, and arcuate eminence are the main anatomical landmarks exposed through the middle fossa approach, once the dura has been elevated, the most easily appreciated and reliable angle to be used in localizing the internal auditory canal seems to be between the IAM and the long axis of the GSPN. C, Intraoperative photograph of the middle fossa approach to the IAM. The insert on the lower left shows the position of the head and the skin incision and the insert on the lower right demonstrates the anatomical structures in a similar view. After a low temporal craniotomy, the middle fossa dura has been elevated. We perform the peeling of the middle fossa floor in 3 steps, most of which involves an anterior-to-posterior peeling. Initially, we move anteriorly as soon as we coagulate and cut the MMA to identify the foramen ovale (route of dissection is marked with a yellow arrow) to begin elevating the dura over the V3 medially (marked with a green arrow). The GSPN can be identified within a distance of approximately 5 mm from the foramen ovale, where the nerve passes under the V3. Early identification of the GSPN as it passes under V3 permits an anterior-to-posterior peeling on the sphenopetrosal groove under direct vision, which will eventually result in safe identification of the geniculate area (marked with a blue arrow). The peeling of the middle fossa dura is easier medial to the GSPN and should extend to the superior petrosal sinus. The angle between the IAM and the long axis of the GSPN has been used for localization, and the bone over the IAM has been drilled medially to posterolaterally. The bone removal has been extended laterally to the Bill's bar. The posterior fossa dura has been opened and the superior vestibular nerve and the cisternal, meatal, and labyrinthine segments of the facial nerve have been exposed. The lesser petrosal has been preserved by using the anterior-to-posterior peeling of the middle fossa dura. The lesser petrosal nerve arises from the geniculate a few millimeters lateral to the greater petrosal nerve and traverses in a small canal below that for tensor tympani toward the foramen ovale. (Images courtesy of AL Rhoton, Jr.)