The Middle Cranial Fossa Approach to Vestibular Schwannomas

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The Middle Cranial Fossa Approach to Vestibular Schwannomas


Rick A. Friedman and Jose N. Fayad


 


For decades, gaining access to the cerebellopontine angle and the prepontine cisterns has presented a formidable challenge. Several conventional neurosurgical approaches to this region have been described, including the subtemporal and transsylvian, and a combined approach incorporating both techniques. Modern skull base approaches, including the middle cranial fossa approach and the middle fossa transpetrous approach, have been instrumental in removing the petrous barrier. This chapter describes the utility of the middle cranial fossa approach for removing vestibular schwannomas of less than 2 cm in patients with useful hearing.


image Overview


Approaches designed to expose vestibular schwannomas, petroclival meningiomas, chondromas, chondrosarcomas, and chordomas involving the petrous apex and clivus must take into consideration the vital neighboring neurovascular structures. The risks encountered in the region of the prepontine cistern during the management of aneurysms of the posterior circulation were best described by Drake in 1961: “The upper clival region is to be considered no-man’s land.” Several conventional neurosurgical approaches to this region have been described, including the subtemporal and transsylvian and a combination half-and-half approach incorporating both techniques. Despite advances in microsurgical techniques and neuroanesthesia, the petrous bone has previously been an impediment to satisfactory exposure in this anatomically complex region. Modern skull base approaches, including the middle fossa and the middle fossa transpetrous, have been instrumental in removing the petrous apex barrier, minimizing temporal lobe retraction, and improving the line of sight to this region.


The middle fossa approach was first described in the literature in 1904. The sentinel work of William F. House in 1961 led to the refinement of this approach to the internal auditory canal (IAC) and the cerebellopontine angle (CPA). The approach was used initially for the decompression of the IAC in cases of extensive otosclerosis involving the labyrinthine bone. That indication was abandoned, but the middle fossa approach has become a workhorse in our approach to small vestibular schwannomas, petroclival meningiomas, and chordoma/chondrosarcoma (Table 3.1).


image Surgical Anatomy


The surgical anatomy of the temporal bone from the middle fossa approach is complex (Figs. 3.1 and 3.2). Landmarks are not as apparent as with other approaches through the temporal bone. Laboratory dissection is essential so that the surgeon may become familiar with the anatomy from above.



Anteriorly, the limit of the dissection is the middle meningeal artery, which is anterior and lateral to the greater superficial petrosal nerve. Excessive anterior retraction can lead to postoperative paresthesias in V3. The arcuate eminence roughly marks the position of the superior semicircular canal. The relationship between the arcuate eminence and the superior semicircular canal is inconstant. The superior semicircular canal tends to be perpendicular to the petrous ridge. Medially, the superior petrosal sinus runs along the petrous ridge.



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Fig. 3.1 Anatomical landmarks of the middle cranial fossa (small vestibular schwannoma in relief). EAC, external auditory canal; IAC, internal auditory canal.




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Fig. 3.2 Anatomical landmarks of the middle cranial fossa (small vestibular schwannoma in relief). EAC, external auditory canal; IAC, internal auditory canal.


Surgical tolerances are tight in the area of the lateral IAC. The labyrinthine portion of the facial nerve lies immediately posterior to the basal turn of the cochlea. Bill’s bar separates the facial and superior vestibular nerves. Slightly posterior and lateral to this area is the vestibule and ampullated end of the superior semicircular canal.


Identification of the geniculate ganglion can be accomplished by tracing the greater superficial petrosal nerve posteriorly. The ganglion is dehiscent in approximately 16% of patients.


The IAC lies roughly on the same axis as the external auditory canal; this relationship is useful in orienting the surgical field. The area around the porus acusticus medially is a “safe zone” in comparison to the lateral or fundal region and provides an excellent place to begin IAC dissection. We begin our dissection medially, by drilling in the meatal plane in the area of the bisection of the angle formed by the superior semicircular canal and the greater superficial petrosal nerve. The IAC can be located initially in this medial area of the temporal bone and followed laterally.


image Surgical Technique


Administer preoperative antibiotics and continue them for 24 hours postoperatively. Intraoperative furosemide and mannitol are given to allow easier temporal lobe retraction. The authors administer dexamethasone intravenously during the procedure and continue this for 24 hours postoperatively. Long-acting muscle relaxants are avoided during surgery so as not to interfere with facial nerve monitoring.




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Fig. 3.3 Preauricular/temporal scalp incision. Note position of the frontal branch of cranial nerve VII.


The patient is placed in the supine position with the head turned to the side opposite the lesion. The surgeon is seated at the head of the table and the anesthesiologist at the foot. An incision is made in the preauricular area and extended superiorly in a gently curving fashion. Care must be taken near the anterior extension of the incision to avoid injury to the frontal branch of the facial nerve (Fig. 3.3). The temporalis muscle is incised, beginning at the zygomatic root, along the linea temporalis, and the muscle is elevated from the temporal fossa and reflected anteroinferiorly. This exposes the temporal squama.


Utilizing cutting and diamond burs, a temporal craniotomy is performed. The craniotomy measures approximately 5 × 5 cm and is two-thirds anterior and one-third posterior to the zygomatic root (Fig. 3.4). The inferior limit of the flap should be at the level of the zygoma, approximating the floor of the middle cranial fossa. Care must be taken to avoid laceration of the underlying dura. The bone flap is set aside for later replacement.


The dura is elevated from the floor of the middle fossa. The initial landmark is the middle meningeal artery, which marks the anterior extent of the dissection. Frequently, venous bleeding is encountered from this area and can be controlled with oxidized cellulose (Surgicel). Elevation of the dura proceeds in a posterior-to-anterior fashion. As stated above, in approximately 16% of cases the geniculate ganglion of the facial nerve is dehiscent, and injury can be avoided with posterior-to-anterior dural elevation. The petrous ridge is identified, and care is taken not to lacerate the superior petrosal sinus, as it is elevated from its sulcus. The arcuate eminence and greater superficial petrosal nerve are identified and the House-Urban retractor is placed (Fig. 3.5). These are the major landmarks for the subsequent intratemporal dissection.


When the dura has been elevated, typically with a suction irrigator and a blunt dural elevator, the House-Urban retractor is placed to support the temporal lobe. To maintain a secure position, the teeth of the retaining retractor should be locked against the bone margins of the craniotomy window and the retractor blade must be placed on the true petrous ridge. Using a large diamond bur and continuous suction irrigation, the superior semicircular canal is identified, but not blue-lined, at the arcuate eminence. The superior semicircular canal makes a 45-to 60-degree angle with the IAC (Fig. 3.6).



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Fig. 3.4 Skin flap and temporalis muscle reflected demonstrating the outline of the temporal craniotomy.

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Apr 14, 2018 | Posted by in NEUROSURGERY | Comments Off on The Middle Cranial Fossa Approach to Vestibular Schwannomas
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