Middle Fossa Approach to Vestibular Schwannomas

Since the technical description by William House in 1961, the middle cranial fossa approach to the internal acoustic canal has become one of the quintessential surgical approaches for accessing the seventh and eighth cranial nerves and the upper cerebellopontine angle. In this chapter the MCF approach comprises an extradural, subtemporal exposure and removal of a portion of the superior surface of the temporal bone to expose the posterior cranial fossa. House and Doyle performed the first vestibular schwannoma removal via the MCF approach in 1961. The MCF approach and its extensions can be used to treat a wide variety of lesions including schwannomas of the 5th through 8th cranial nerves, meningiomas of the IAC, CPA, and petroclival region, and cholesterol granulomas and cholesteatomas, petrous apex infections, chondrosarcomas of foremen lacerum, vascular lesions of the ventral skull base, and nerve decompressions for inflammatory neuropathies. When considering patient’s historical and exam findings, it is important to note that patients who are considered good surgical candidates for an MCF approach will have small to medium-sized tumors, typically <1.5 cm in length and thus, symptoms and findings are often subtle. There are several anatomic considerations unique to the MCF approach that warrant special consideration: the lateral skull and temporal bone including the temporal and infratemporal fossa contents, location and pathway of the frontal branch of the facial nerve, the relationships of the middle cranial fossa floor, otic capsule and IAC relationships, and strategies for finding the IAC. The 3-dimensional boundaries of the anterior petrous apex are: the bony skull base superiorly, the carotid artery laterally, the posterior fossa dura medially, the IAC dura posteriorly and the inferior petrosal sinus inferiorly. Microsurgical removal via the MCF approach is the preferred management strategy patients preferring surgical treatment, who have lesions that do not impact the brainstem and who have preserved hearing of a least AAO-HNS CHE Class B or WRS II. If the preoperative hearing is at the AAO-HNS CHE class A or WRS I limits or better, we quote a 70% chance to preserve at least a WRS I postoperative hearing level if the tumor is 10 mm in length or smaller, which includes the IAC and CPA portions. The preoperative evaluation consists of diagnostic testing to establish the identity and location of the lesion, pure-tone and speech audiometry to determine residual hearing ability and classification, auditory evoked electrical potential testing if intraoperative evoked potential monitoring is to be utilized, and the testing to ensure that the patient is an appropriate surgical candidate. There are three main methods for identification of the internal acoustic canal: (1) identification of the geniculate ganglion and then labyrinthine segment of the facial nerve (House method), (2) identification of the lumen of the SSCC (Fisch method), and (3) identification of the anterior and medial surface of the IAC dura by opening the anterior petrous apex (Garcia-Ibanez method). The main downside to this approach is that the facial nerve is at risk for injury as exposure of the ganglion does not provide any exposure to the IAC and only serves as a landmark for which to base further dissection.