Figure 1: The progressive level of ventrolateral and ventral brainstem exposure via more radical transpetrosal osteotomies is illustrated. The retrolabyrinthine, translabyrinthine, and transcochlear routes each afford a less obstructed view of the anterior brainstem but also carry higher complication rates and extended operative times as compared to the retrosigmoid approach. 

Figure 2: The anatomy of the left mastoid bone is detailed. The exposure of the antrum highlights the location of the lateral canal. The spine of Henle at the posterosuperior margin of the external meatus is a superficial landmark that approximates the deeper lateral semicircular canal and tympanic segment of the facial nerve (upper image). Additional bone drilling and mastoidectomy exposes the capsule of the posterior and lateral canals and the tympanic and mastoid facial segments. This is the extent of bony removal required for an extended posterior petrosectomy (lower image). Skeletonization of the facial nerve is not necessary (images courtesy of AL Rhoton, Jr).

Figure 3: Skeletonization of the left semicircular canals provides an expanded trajectory along the presigmoid space. Note the markings for dural incisions. The endolymphatic sac is protected during the presigmoid dural incision (image courtesy of AL Rhoton, Jr).

Figure 4: Upon dural opening and gentle retraction of the sigmoid sinus, one can see the operative route toward the cerebellopontine angle (upper image). The expanded view of the relevant cranial nerves is shown (images courtesy of AL Rhoton, Jr).

Figure 5: Once the right sigmoid sinus is fully uncovered through mastoidectomy, additional air cells anterior to the sigmoid sinus are removed to expose the dura of the middle and posterior fossae. Small islands of thin adherent cortical bone are left on the sigmoid sinus to minimize injury to this important venous structure.

Figure 6: The next step is to identify the antrum by following the middle fossa plate and posterior internal auditory canal wall through the remaining superior mastoid air cells. The antrum is located about 15 mm deep to the spine of Henle; the lateral semicircular canal is located deep to the antrum. In the depth of the antrum, the short process of the incus is identified in the fossa incudis. The burr should not contact the incus because the vibrations can be transmitted to the patient’s inner ear structures and cause hearing loss.

Figure 7: The posterior fossa dura is opened first anterior and parallel to the sigmoid sinus, extending from the jugular bulb to the superior petrosal sinus. The endolymphatic sac will be visible inferior to the posterior semicircular canal as a white, thick area of the dura. This structure is the anterior limit of the posterior fossa dural exposure and should be preserved to avoid hearing loss. If the dural incision violates the endolymphatic sac, hearing may still remain intact if the endolymphatic duct is spared. The endolymphatic duct is located posterior to the inferior portion of the posterior semicircular canal.

Figure 8: The second segment of the dural incision follows the floor of the temporal fossa and is continued toward the transverse sinus. This opening must be performed carefully and the subdural space should be examined to prevent injury to the wandering vein of Labbé.

Figure 9: The superior petrosal sinus is now coagulated or ligated with Weck clips. The point where the sinus is divided must be anterior to the insertion point of the main temporal draining veins including the vein of Labbé to preserve their outflow into the transverse-sigmoid sinuses. If the vein of Labbé crosses the area of exposure, it can be partially untethered, mobilized, and protected away from the surgical corridor and retraction vector.

Preoperative assessment of the temporal lobe’s venous drainage patterns and tentorial traversing veins using CT/catheter angiography or magnetic resonance (MR) venography is beneficial in tailoring the safe zones for tentorial transection. If the superior petrosal sinus is intimately involved with the venous drainage of the region, an incision along the tentorium medial to the superior petrosal sinus will spare this sinus and allow an operative corridor, albeit limited, toward the petroclival region around the sinus.

Figure 10: After the superior petrosal sinus is divided, the dural incision is continued on the tentorium parallel to the edge of the petrous pyramid toward the tentorial incisura. Completing the dural incision more posteriorly will lead to a tentorial flap obstructing the surgical corridor. Note the location of the retractor blades and dural retention sutures.

Figure 11: Before the medial edge of the tentorium is transected, CN IV is identified to the point where it enters the dura. To protect the nerve, I cut the tentorium posterior to the location where the nerve enters the tentorial edge.

Figure 12: Next, the posterior tentorial flap is elevated with the temporal lobe and held under a retractor blade. Another retractor blade holds the cerebellum in the posterior direction. The presigmoid dura, temporal dura, and tentorium are mobilized using retention sutures. Note that the sutures are placed as deep as possible close to the bony edge to maximize the exposure.

Figure 13: Gentle posterior retraction of the sigmoid sinus allows a direct panoramic view of the ventrolateral brainstem and the origin of cranial nerves along the petroclival space.