Surgery for Acoustic Neuroma: Middle Fossa Approach
For general considerations, clinical presentation, and evaluation of vestibular schwannomas (VSs), please refer to the Retrosigmoid Approach for Acoustic Neuroma chapter.
Indications for the Middle Cranial Fossa Approach
The main indications for the middle cranial fossa (MCF) approach include removal of a small predominately intracanalicular VS, exposure of the labyrinthine and upper tympanic segments of the facial nerve for decompression, vestibular nerve section, and repair of superior semicircular canal dehiscence.
Historically, the MCF approach offers some of the highest hearing preservation rates, but it can also place the facial nerve between the surgeon and the tumor, potentially leading to a higher risk of postoperative facial weakness. In some cases, this configuration results in the need for blind dissection.
This route also requires some retraction on the temporal lobe, with the ensuing potential risk of postoperative seizures and speech disturbances, while providing a limited view of the cerebellopontine (CP) angle.
The MCF approach is poorly tolerated by elderly patients because extradural dissection of the adherent dura in this specific population may be difficult. This approach is suggested for younger patients with smaller tumors that harbor the predominant component of their growth within the internal auditory canal (IAC). Specifically, tumors that involve the fundus of the IAC, a location to which access and visualization are restricted during the retrosigmoid trajectory, are good candidates for the MCF route.
Overall, the MCF approach provides a limited working window into the posterior fossa. This limitation is complicated by the presence of the facial nerve within the surgeon’s view and restricts the surgeon’s ability to resect large tumors. The retrosigmoid option, on the other hand, provides a more panoramic view of the tumor in the CP angle cisterns and its relationship to the surrounding neurovascular structures.
The MCF approach is not favored for tumors with significant extracanalicular extension (>1−2 cm) because of the associated poor visualization of the CP angle and brainstem. Intraoperative navigation based on a high-resolution CT of the temporal bone is used for localization of the IAC and the adjacent temporal bone structures.
Neurophysiologic monitoring of the facial nerve using electromyography and brainstem auditory evoked responses (BAERs) is considered routine.
The IAC is approached through the floor of the MCF. A subtemporal craniotomy provides the corridor to elevate the dura from the floor of the MCF, and the IAC is initially localized based on anatomic landmarks.
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RESECTION OF VESTIBULAR SCHWANNOMA VIA THE MIDDLE CRANIAL FOSSA APPROACH
Before final positioning of the patient on the operating room table, I place a lumbar drain. Early cerebrospinal fluid (CSF) drainage is necessary to avoid temporal lobe injury during the lobe’s extradural retraction. Often 40 to 60 cc of CSF is drained gradually (in 10 cc aliquots) until the lobe is relaxed and safely mobilized for adequate exposure of the petrous ridge.
I prepare the lower quadrant of the patient's abdomen for harvesting adipose grafts used for filling the dural defect at the end of tumor removal.
The patient is positioned supine on the operating room table. A shoulder roll is placed under the patient's contralateral shoulder and the head rotated until the sagittal suture is parallel to or minimally angled from the floor. If the patient has a supple neck, he or she can tolerate up to 70 degrees of rotation. If the patient has a rigid neck, the size of the shoulder roll may be increased to compensate for the limitation in neck rotation. For obese patients with relatively immobile necks, the lateral position is appropriate to avoid interruption of adequate venous return and nonphysiologic neck postures that can lead to postoperative neck pain.
A line connecting the single pin with the midpoint between the opposite two pins (swivel rocker arm) must always cross the equator of the patient's head to prevent skull clamp fixation failure during surgery.
I use a linear incision similar to the one used for the subtemporal approach.
Adequate muscle dissection will expose the posterior root of the zygomatic arch. One must not dissect so low as to violate the mandibular joint or the external auditory canal.
More burr holes may be used, if necessary, to preserve the integrity of the dura during the elevation of the bone flap. Alternatively, the entire set of bone cuts may be completed using a diamond drill. Preservation of the dura’s integrity is crucial to avoid injury to the lobe during the later stages of extradural dissection along the middle fossa floor.
Next, the footplate of the craniotome is used to complete the craniotomy. The bone flap is elevated with its inferior edge parallel and as close as possible to the floor of the middle fossa. The zygomatic arch defines the level of the middle fossa floor and may be used as a reference; this floor slopes slightly upward from an anterior-to-posterior direction.
The origin of the GSPN can be identified posteriorly at the hiatus; the dura is then stripped anteriorly in line with the nerve to prevent dislocation of the nerve out of its groove. The second caveat to the epidural exposure is that the surgeon must find the “true edge” of the petrous ridge. The superior petrosal sinus forms the petrosal groove along the superior aspect of the petrous ridge. The upper edge of this groove is often mistaken for the petrous ridge, resulting in an inadequate elevation of the dura. Proper elevation of the dura along the ridge allows the retractor’s blade to readily rest against the petrous ridge and optimally retract the dura without slippage.
As bone resection proceeds laterally, a 0.5-mm to 1.0-mm diamond burr is used, and the labyrinthine segment of the facial nerve and the vertical crest (Bill’s bar) become visible through the thinned bone. The remaining eggshell of bone over the IAC is removed using an angled microcurette.
The affected vestibular nerve may be sectioned at the fundus to create some slack for tumor manipulation. The facial nerve is stimulated prior to this section to ensure its preservation. When the tumor is more thoroughly dissected from the facial and cochlear nerves, the vestibular nerve is sectioned proximally and the tumor is removed. Any alternation in the BAERs should alert the surgeon to adjust his or her maneuvers, because there could be too much traction on the cochlear nerve or stretch on the labyrinthine artery.
Bipolar coagulation is avoided and irrigation fluid is used to keep the operative field visible. These maneuvers can improve the outcomes for hearing preservation.
Primary dural closure is not possible in this area and alternative methods are needed. Adipose tissue with its globular texture is one of the best barriers against CSF leakage. Strips of adipose tissue are placed across the dural opening to seal the defect. Before placement of the adipose grafts, all air cells must be meticulously waxed. Alternatively, a piece of temporalis muscle in conjunction with temporalis fascia may be used to cover the dural defect over the IAC.
The bone flap is replaced and secured using miniplates, and the rest of closure is conducted in standard fashion.
A lumbar drain is used to drain 8 cc/hour of CSF for 48 hours after surgery. A computed tomography scan the morning after surgery should exclude significant pneumocephalus before the drain is utilized. Patients are mobilized as soon as possible.
Contributor: Andrew R. Conger, MD, MS
For additional illustrations of combined transpetrosal-middle fossa approaches, please refer to the Jackler Atlas by clicking on the image below:
For additional illustrations of the extended middle fossa approach to the cerebellopontine angle, please refer to the Jackler Atlas by clicking on the image below:
For additional illustrations of facial nerve repair, please refer to the Jackler Atlas by clicking on the image below:
For additional illustrations of hypoglossal-facial anastomosis, please refer to the Jackler Atlas by clicking on the image below:
For additional illustrations of the middle fossa approach to the internal auditory canal, please refer to the Jackler Atlas by clicking on the image below:
For additional illustrations of tumor growth patterns, please refer to the Jackler Atlas by clicking on the image below:
Some of the materials included in this chapter have also previously been described in the following articles:
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