Computed Tomography and Magnetic Resonance Imaging

Preoperative computed tomography (CT) and magnetic resonance imaging (MRI) are indispensable to modern skull base surgery (see Chapter 135). Hirsch and Curtin²⁶ noted that MRI and CT are “complementary and not competitive in the initial evaluation of skull base lesions.” Modern CT affords exquisite bony detail and fairly good soft tissue information. The strength of CT lies in delineating the effect of the tumor on the bone of the skull base. Soft tissue–soft tissue discrimination is still limited because of the narrow range of electron densities in soft tissues of interest. More soft tissue discrimination is available with CT if iodinated contrast material is administered intravenously. The advent of MRI with its superb soft tissue sensitivity has resulted in fewer indications for use of iodinated contrast CT techniques. Although MRI is able to evaluate alterations in bony structure, it does so only indirectly, because cortical bone has few mobile water protons and hence nearly no intrinsic signal. For this reason, MRI is inferior to CT in the evaluation of subtle bony changes. Such features are important findings in temporal bone masses. The fine soft tissue discrimination of MRI is improved by the intravenous infusion of the MRI contrast medium gadolinium. Another advantage of MRI is its ability to display numerous imaging planes without repositioning the patient, which makes it easier to acquire coronal, sagittal, and even off-axis oblique images. For these reasons, MRI is superior to CT in defining brain or meningeal effects from temporal bone lesions. Because of potentially dangerous magnetic effects, MRI cannot be used in patients with certain cardiac pacemakers or valves, ferromagnetic cerebral aneurysm clips, some vena cava filters, and ferromagnetic intraocular foreign bodies.

The usual evaluation of a potential temporal bone mass begins with noncontrast temporal bone CT. This study will immediately show large lesions and usually suggests the presence of smaller lesions. Even though results of temporal bone CT may be negative, follow-up MRI may be indicated if clinical findings strongly suggest a mass. Like CT, MRI is focused to the region of interest instead of just providing thick-slice imaging of the entire head. Temporal bone MRI almost always is performed without and with gadolinium. Many practitioners choose to bypass CT if they believe the disease is retrocochlear. CT then can be added after the MRI study if precise bone detail is required.

Both CT and MRI are computerized imaging techniques. For this reason, either can be reformatted in any desired digital display. A popular type of reformatted display is a three-dimensional image. Three-dimensional displays may assist the surgeon by making it easier to understand the extent of the lesion, but they almost never provide diagnostic information that was not available on the standard axial or coronal slices. Moreover, three-dimensional displays are heavily postprocessed. Significant image information can be lost in such processing. At present, three-dimensional images are used almost exclusively as supplemental displays, as dictated by the preference of the practitioner.

Preoperative Carotid Artery Assessment

In extirpation of skull base tumors, appropriate management of the internal carotid artery remains paramount (Fig. 176-1). The morbidity associated with carotid artery occlusion is well known. The first documented carotid ligation, for a sword injury, performed in 1585 by Paré, resulted in stroke.²⁷ Literature review reveals that unselected abrupt sacrifice of this vessel resulted in 26% (65 of 254) cerebral infarctions, with a 46% fatality rate (30 of 65).²⁸ Gradual occlusion, as with a Silverstone clamp, does not seem to alter morbidity. Berenstein and associates²⁹ noted that a significant reduction of blood flow or pressure does not occur until a cross-sectional area less than 2 mm² is attained, regardless of the diameter of the vessel. Late-onset complications from cervical internal carotid artery ligation may result after several hours, as a result of stasis with distal thrombosis and embolic phenomena.

Figure 176-1. Algorithm for preoperative internal carotid artery (ICA)/cerebral blood flow (CBF) evaluation. CT, computed tomography; MRI, magnetic resonance imaging.

Four-Vessel Arteriography

All patients considered for extensive lateral skull base surgery should undergo a four-vessel carotid angiogram with venous phase (see Chapters 135 and 136). Angiography is only qualitative but may provide critical information (Fig. 176-2).

Figure 176-2. Two right-sided paragangliomas in a 61-year-old woman. A, Axial view, computed tomography after administration of intravenous contrast. A dense, pathologically contrast-enhancing mass (m) with tortuous dilated vessels is demonstrated. This lesion has ascended from the glomus jugulare up into the cerebellopontine angle cistern. It covers the internal auditory canal. B, Innominate artery angiogram, lateral subtraction view centered at the craniocervical junction. The wildly hypervascular nature of the paraganglioma is demonstrated. The tumor has parasitized the vertebral artery (va) and is ascending the deep cervical arteries (adc). These vessels have dilated and become tortuous because of the increased flow demanded from them. Besides the large glomus jugulare paraganglioma (m), the patient has a small carotid body paraganglioma (cb).

According to Fisch and colleagues, angiographic evidence of ICA contour irregularity or stenosis at the level of lesion should prompt consideration of elective preoperative ipsilateral carotid balloon occlusion.³⁰ Intraoperative carotid occlusion is associated with a higher rate of permanent neurologic sequelae.

The venous phase provides critical information regarding collateral venous drainage, should ligation of the transverse sinus or the vein of Labbé be required.

Cerebral Blood Flow Assessment

Some understanding of cerebral blood flow (CBF) is essential to carotid artery management. Normal CBF approximates 50 mL/100 g per minute.³¹ In alert humans, CBF less than 20 mL/100 g per minute produces failure of brain function, resulting in at least temporary hemiparesis or hemiplegia.³² At this point, electroencephalography (EEG) usually reveals changes reflecting synaptic transmission failure.³¹ As CBF decreases below 15 mL/100 g per minute, electrical activity is lost.³³ Jones and coworkers³⁴ found that long-lasting ischemia in monkeys from local CBF less than 17 to 18 mL/100 g per minute resulted in infarction. The time course of irreversible neuronal damage at various CBF levels in humans is unknown.

Functional assessment of CBF is required in the following clinical circumstances:

Carotid Management Algorithms

A decision regarding management of the internal carotid artery is best made preoperatively. Intraoperative carotid occlusion usually is performed to control massive bleeding under suboptimal circumstances, resulting in a higher neurologic complication rate. Intraoperative balloon occlusion or intra-arterial shunt placement may be required if the carotid is lacerated and control is difficult.^60–62

After preoperative functional evaluation of the internal carotid artery, either qualitative or quantitative, decisions about management of the carotid are dependent on the following factors:

Vein graft reconstruction, instead of sacrifice of the artery, may be a consideration regardless of the BTO result, especially in young patients and in cases in which contralateral involvement is a possibility, to restore physiologic circulation and reduce risk of late intracranial aneurysms.^33,42 The rationale for preservation of an ICA encased by benign tumor when this is possible, or revascularization when preservation is not possible, is valid.⁶³

Opinion differs further regarding algorithms for management of the internal carotid artery based on functional evaluations, just as opinion differs regarding what constitutes an adequate evaluation in the first place. The results of temporary BTO can be classified as either “pass” or “fail,” as follows:

For patients with BTO results in the “fail” category, definitive resection may not be possible; alternatively, extracranial-intracranial bypass antecedent to the resection procedure should be considered.^42,64–66

Patients who “pass” are further subclassified depending on whether qualitative or quantitative testing has been done. Fisch³⁰ using qualitative BTO, recommends permanent occlusion of the internal carotid artery preoperatively if the neuroradiologist confirms adequate cross-circulation and no neurologic deterioration occurs during the test. In a series reported by Zane and colleagues, only 5% (2 of 46 patients, 1 preoperatively and 1 postoperatively) had permanent neurologic sequelae after occlusion when this algorithm was used.³⁰ If quantitative testing has been performed, transarterial permanent balloon occlusion may be considered for operative candidates in groups 1 and 2 (CBF greater than 30 mL/100 g per minute) (see Fig. 176-1) for the same indications as those that occasioned functional CBF testing. If the decision for occlusion of the internal carotid artery is reached preoperatively, permanent balloon occlusion should be performed 1 to 4 weeks before the resection to lessen thromboembolic sequelae. Placement proximally at the level of the carotid bifurcation and distally just before the ophthalmic artery minimizes late thromboembolic sequelae.⁶⁷ The long-term sequelae of carotid occlusion are unknown, although xenon CT blood flow mapping after BTO correlates well with medium-term neurologic outcome.²⁸ Direct vein graft reconstruction is indicated to allow complete tumor resection (see Fig. 176-1) in patients assigned to group 3 by quantitative CBF assessment (CBF less than 30 mL/100 g per minute), those with bilateral lesions, and young patients with benign tumors, according to Sen and Sekhar.⁶⁴ Intraoperative revascularization is associated with increased operative time, graft occlusion, need for transfusion, and potential for postoperative blowout.³⁰ Risk of graft occlusion is especially high among patients with long grafts.

Disregarding the surgeon’s preference for revascularization or permanent balloon occlusion, the decision optimally is made under controlled circumstances preoperatively.

Sleeve Resection of External Auditory Canal

A sleeve resection of the external auditory canal removes the cartilaginous portion and some or all of the bony canal wall skin circumferentially without bone removal. Split-thickness skin grafts are used in the conchal bowl and external auditory canal. Some surgeons include the tympanic membrane as a medial margin to a sleeve resection, although this type of procedure generally is reserved for malignancies localized to the cartilaginous portion of the canal. Malignancies localized to or involving the osseous portion require at least a lateral temporal bone resection.

Lateral (Partial) Temporal Bone Resection

A lateral temporal bone resection^68–71 removes en bloc the entire osseous and cartilaginous portions of the external auditory canal and tympanic membrane using the extended facial recess approach (Fig. 176-3). This procedure is indicated for patients with malignant neoplasms localized to the osseous canal with no encroachment on the medial mesotympanum. Parotidectomy, neck dissection, and mandibular condylectomy are performed as adjunctive surgical procedures if indicated by preoperative radiologic or intraoperative findings. Intraoperative facial nerve monitoring is useful in this procedure.

Figure 176-3. Margins of resection for lateral temporal bone, subtotal temporal bone, and total temporal bone resections.

Incisions

A soft tissue cylinder is designed to include the tragus and conchal bowl. The incision is taken vertically downward, posterior and superior to the temporal bone. The external auditory canal is sutured shut. A postauricular incision is created approximately a fingerbreadth behind the postauricular sulcus (Fig. 176-4A). It may be extended inferiorly into a neck crease for parotidectomy as required. Retractors then are placed.

Figure 176-4. Lateral temporal bone resection: incision and initial exposure. A, Incisions include a central external auditory canal core, which is sutured closed. Extend incision as needed for neck dissection or temporal muscle rotation flap. Note that the tragus can be preserved for better cosmesis. B, Canal closure, mastoidectomy, and epitympanic dissection have been completed.

A modified neck dissection, total parotidectomy, and partial mandibulectomy may be performed if indicated by perioperative radiologic studies or intraoperative findings.

Mastoidectomy and Facial Recess Approach

A complete mastoidectomy is performed. The superior mastoid dissection is carried to the root of the zygoma, with care taken not to penetrate the osseous portion of the external auditory canal or the tegmen. Epitympanic dissection in an anterior direction should extend into the temporomandibular joint, as depicted in Figure 176-4B. An extended facial recess approach is then performed by “eggshelling” the vertical segment of the facial nerve from the second genu to the stylomastoid foramen (Fig. 176-5A). The incudostapedial joint is separated, and the pillar of bone between the facial recess approach and the fossa incudis also is removed. The tensor tympani tendon is cut with Bellucci scissors. Hypotympanic dissection is performed by extending the facial recess approach through the chorda tympani nerve. Drilling with either a diamond or cutting burr is continued along the inferior aspect of the hypotympanum medial to the annulus and lateral to the jugular bulb and carotid canal until soft tissue is reached, with care taken not to injure the intraparotid facial nerve (see Fig. 176-5B).

Figure 176-5. Lateral temporal bone resection: specimen delivery. A, Extended facial recess dissection. B, Hypotympanic dissection. C, Specimen fractured with osteotome. D, Specimen separated from soft tissue.

Removal of Specimen

Continuity between the hypotympanic dissection and the anterior epitympanic dissection is established by drilling along the anterior mesotympanum into the temporomandibular joint or by fracturing with a curved osteotome (see Fig. 176-5C). Either cautery or sharp dissection is used to separate the bony anterior canal wall from soft tissue of the temporomandibular joint and the superficial lobe of the parotid. The specimen then is sent en bloc for histopathological examination (see Fig. 176-5D).

Closure

If ancillary procedures such as parotidectomy or mandibular resection are not performed and if postoperative radiotherapy is not anticipated, creation of a large mastoid cavity is acceptable. The eustachian tube orifice should be obliterated with temporalis fascia as in a radical mastoidectomy. The mesotympanum and facial ridge may be covered with temporalis fascia graft to facilitate healing. Further conchal bowl cartilage may need to be removed to create a large meatoplasty. A split-thickness skin graft is then sutured to the edges of the conchal bowl and anterior facial skin or tragal skin and allowed to drape into the mastoid cavity and anterior cavity soft tissue. The skin graft then can be tacked into place with absorbable sutures along the soft tissue areas. Use of a graft will minimize postoperative meatal stenosis.⁷² Absorbable gelatin then can be placed on the medial aspect of the mastoid cavity, followed by packing with gauze strips.

If postoperative irradiation is anticipated, the mastoid cavity should be obliterated to avoid osteoradionecrosis.⁷³ This may be achieved with use of abdominal fat and rotation of a temporalis graft to cover the defect. Alternately, a superiorly based sternocleidomastoid muscle flap based on the occipital artery may be rotated by incising the sternocleidomastoid muscle at the clavicle. A skin graft then is sutured to the conchal bowl margins and tacked down to underlying muscle.

Subtotal Temporal Bone and Total Temporal Bone Resections

A subtotal temporal bone resection^70,74–84 essentially allows en bloc resection of the medial surfaces of the mesotympanum, leaving the air cells of the petrous apex and portions of the bony labyrinth (see Fig. 176-3). This type of resection is indicated for patients with malignancies involving the middle ear.

A total temporal bone resection involves an en bloc resection of the temporal bone, including the petrous apex and the sigmoid sinus (see Fig. 176-3). The petrous internal carotid artery may be included in this resection. Total temporal bone resection is of questionable added benefit if a high-grade tumor involves the petrous apex or internal carotid artery or extends outside of the petrous bone intradurally or toward the cavernous sinus. Total resection may be indicated for low-grade malignancies.⁸³

Incision

A large C-shaped postauricular incision or Y-shaped incision can be designed with a central island removed (Fig. 176-6). Alternately, a preauricular incision may be used for cases with involvement of the external auditory canal. In performing the incision, care should be taken to preserve the greater auricular nerve, because this nerve may be used for facial nerve interposition grafting. Also, care should be taken in elevating the flap off the zygoma, so as to not injure the peripheral frontal branch of the facial nerve.

Figure 176-6. Subtotal temporal bone resection. Incisions include a central external auditory canal core, which is sutured closed. Note the tragus can be preserved for better cosmesis. Temporal craniotomy for subtotal temporal bone resection is smaller than for a total temporal bone resection. Parotid gland with main trunk of facial nerve has been elevated from masseter muscle.

Facial Nerve Management

With parotid involvement or involvement of the facial nerve, the distal branches or main trunks of the facial nerve may be identified and transected and marked for future interposition grafting. If no such involvement is present, the facial nerve is identified at the stylomastoid foramen and transected. The parotid is carefully dissected off the masseter. Proximal facial nerve management is considered later.

Proximal Vascular Control, Neck Dissection

If preoperative imaging indicates lymphadenopathy, a supraomohyoid modified neck dissection can be performed. Regardless of lymph node involvement, proximal vascular control of the carotid artery and internal jugular vein can be obtained, and vascular loops are placed around these vessels. The lower cranial nerve, internal carotid artery, and internal jugular vein can be followed superiorly. The sternocleidomastoid muscle is separated from the mastoid tip, as is the digastric muscle.

Infratemporal Fossa Dissection

The masseter attachment to the zygoma is detached. A mandibular osteotomy is performed from the mandibular notch to the angle of the mandible. The mandibular condyle is separated from the glenoid fossa, detaching the lateral and medial pterygoid musculature, with care taken not to injure the internal maxillary artery. Should the internal maxillary artery be injured, brisk back-bleeding would indicate good blood flow from the pterygoid plexus, presaging a viable temporalis muscle useful for reconstruction. The temporalis muscle is elevated off the skull in a subperiosteal fashion and reflected inferiorly, still attached to the coronoid process. A superior infratemporal fossa dissection should allow identification of the foramen spinosum and foramen ovale.

Temporal Craniotomy

A temporal craniotomy is performed to allow exposure of the superior surface of the petrous bone. A larger craniotomy is performed for a total temporal bone resection. The dura is elevated off the petrous bone, proceeding from posterior to anterior.

Subtotal Temporal Bone Resection

The vertical petrous portion of the internal carotid artery is identified (Fig. 176-7). This resection is performed by drilling with diamond burrs through the glenoid fossa and across the eustachian tube orifice. Mobilization of the carotid is just distal to the first genu. The distal eustachian tube orifice should be either sutured or closed with autologous tissue.

Figure 176-7. Subtotal temporal bone resection. A, Bird’s-eye view of proposed resection. Note the facial nerve can be exposed for clean sectioning in the labyrinthine segment. B, Osteotome at carotid canal first genu directed at internal auditory canal fundus. C, Specimen removed showing remaining petrous apex and otic capsule. Roman numerals denote cranial nerves.

Delineation of the Sigmoid Sinus and Jugular Bulb

Presumably, if a subtotal temporal bone resection has been selected, the mastoid air system is not involved in patients with carcinoma. A mastoidectomy is performed, removing the mastoid cortex with cutting burrs. The sigmoid sinus and jugular bulb are carefully delineated with diamond burrs, and the jugular bulb is carefully dissected from the temporal bone, with recognition that the inferior petrosal veins enter it inferomedially. The sigmoid sinus may require ligation or extradural packing for vascular control later in the procedure.

Proximal Facial Nerve Management

Although an osteotomy may be performed at this point, with sectioning of the proximal facial nerve during an often bloody dissection, it probably would cause further stretch injury to the facial nerve. If grafting of the facial nerve is desired, further stretch injury is best avoided. The facial nerve may be delineated from the labyrinthine segment to the internal auditory canal as with routine middle cranial fossa nerve decompression or may be identified by a translabyrinthine approach (see Fig. 176-7A). Nonetheless, clean sectioning of the facial nerve facilitates interposition grafting and later facial nerve function.

Osteotomy

A groove may be drilled along the floor of the middle fossa connecting the glenoid fossa area, lateral internal auditory canal, and posterosuperior mastoid. An osteotome is inserted into the carotid canal just distal to the first genu and pointed toward the internal auditory canal (see Fig. 176-7B). The temporal bone specimen is brought out (see Fig. 176-7C), which often will result in inferior petrosal sinus bleeding, requiring intraluminal packing of the inferior petrosal sinus with oxidized cellulose or control of the jugular bulb and sigmoid sinus.

Total Temporal Bone Resection

The internal carotid artery is dissected to the foramen lacerum (Fig. 176-8). The vessel may have been permanently occluded perioperatively in accordance with the surgical plan. The internal jugular vein is ligated and cut. A suboccipital craniotomy is performed, extending over the transverse sinus and lateral posterior fossa. The dura is divided over the cerebellum, and the transverse sinus is ligated between the vein of Labbé and the superior petrosal sinus. The tentorium is divided from the lateral sinus across the superior petrosal sinus to the inferior edge of V₃ with continued dural incision along the floor of the middle cranial fossa laterally. Cranial nerves VII and VIII and the labyrinthine artery are divided at the internal carotid artery. Cranial nerves IX, X, and XI are divided intradurally. The inferior dural incision along the clivus extends from Meckel’s cave caudal to the lower cranial nerve to the postsigmoid dural edge. An osteotome is inserted just posterior to the foramen ovale and directed slightly posteriorly to avoid entering the foramen lacerum. This will release the temporal bone specimen, with the occasional need for osteotomy just above the jugular bulb to separate the petro-occipital synchondrosis.^84,85 Alternatively, a groove is drilled under direct vision through the posterior fossa to connect to an externally drilled groove posterior to the petrous internal carotid artery.⁸³

Figure 176-8. Schematic depiction of total temporal bone resection. Roman numerals denote cranial nerves. A, The frontotemporal-retroauricular-cervical skin flap is rotated forward. The external ear canal has been transected. The facial nerve has been transected just beyond the stylomastoid foramen. The temporomandibular joint has been opened and the mandibular ramus resected. Cranial nerves IX through XII and the jugular and carotid vessels are exposed to the neck. B, A temporal and suboccipital craniotomy has been extended over the transverse sinus and lateral posterior fossa. A zygomatic osteotomy and a partial mastoidectomy have been performed. The osseous middle fossa floor has been partially resected to expose second (V₂) and third (V₃) divisions of the cranial nerve V and the greater superficial petrosal nerve (GSPN). The middle meningeal artery has been transected. C, Continuing resection of the osseous middle fossa floor; the eustachian tube and the tensor tympani muscle have been transected. The ascending and transverse segments of the petrous ICA have been unroofed, and the ICA has become mobilized anteriorly. D, The temporal and retrosigmoid portions of the dura have been opened. The lateral sinus has been ligated at its junction with the sigmoid; the tentorium has been opened along the inferior edge of the mandibular nerve, trigeminal ganglion, and roof, with the interruption of the superior petrosal sinus. If the superior petrosal sinus is uninvolved, its drainage into the transverse sinus may be preserved by changing the dural incision lines. The dura has been cut over the cerebellum; the transverse sinus has been ligated and transected, and a dural flap has been brought down from over the temporal lobe. The tentorium has been divided from the transverse sinus transection, alongside and behind the petrous ridge, and anteromedially across the sinus. This cut has been connected to an incision in the dura on the middle cranial fossa floor anterior to the petrous base. The outline of the trigeminal ganglion and roots can be seen through the middle fossa dura. E, With the cerebellum retracted medially, cranial nerves V and VII through XI can be seen. The labyrinthine (internal auditory) artery has been transected at its origin from the anterior inferior cerebellar artery (AICA). F, Cranial nerves VII through XI have been transected, and the underlying dura has been incised as the posterior border of the en bloc resection of the petrous bone. The petrous bone specimen then is disconnected from any remaining attachments and removed. ICA, internal carotid artery; PICA, posterior inferior cerebellar artery.

(Used with permission from Sekhar LN, Pomeranz S, Janecka IP, Hirsch B, Ramasastry S. Temporal bone neoplasms: a report on 20 surgically treated cases. J Neurosurg. 1992;76:578-587.)

Reconstruction

The facial nerve and other appropriate cranial nerves can be cable grafted. Dural defects are closed as needed with fascia or pericranium. The temporalis muscle can be rotated and sutured to surrounding soft tissue. Alternately, regional or free flaps are used. Craniotomy bone and zygoma are secured with wire or miniplates. A closed suction drain then is placed before skin closure.

Infratemporal Fossa Approaches

Attacking various problems of the skull base through a lateral approach transgresses the temporal bone and infratemporal fossa. The postauricular infratemporal fossa approaches as described by Fisch^61,86–88 and the preauricular^89–92 approaches offer the best access to the jugular bulb, internal carotid artery, petrous apex, clivus, pterygomaxillary fossa, and nasopharynx. Proponents of these approaches have written extensively on the subject, and the techniques described here have been adapted from their work. Anatomic maneuvers may be added as needed to facilitate tumor exposure and extirpation.^93,94

Postauricular Approaches to the Infratemporal Fossa

Fisch⁶¹ has been the primary innovator of these techniques and should be credited for his work. He has divided these techniques into three basic approaches: Type A dissection entails radical mastoidectomy, anterior transposition of the facial nerve, exploration of the posterior infratemporal fossa, and cervical dissection permitting access to the jugular bulb, vertical petrous carotid, and posterior infratemporal fossa. Type B dissection explores the petrous apex, clivus, and superior infratemporal fossa. Type C allows exposure of the nasopharynx, peritubal space, rostral clivus, parasellar area, pterygopalatine fossa, and anterosuperior infratemporal fossa.

Incisions and Skin Flaps

The planned incision should allow for further extensions without devascularization of the elevated skin flap (Fig. 176-9A). Cervical exposure to a variable extent and anterior extension for dissection toward the clivus and parasellar area should be permitted. Keeping the inferior limb of the incision in a posterior location over the mastoid tip allows extension into the cervical area for dissection while protecting the marginal mandibular nerve. Similarly, extending the incision anterosuperiorly into the frontal area permits elevation over the zygoma to the orbital rim.

Figure 176-9. A, Skin incision for Fisch type A infratemporal fossa dissection. B, Anteriorly based periosteal flap elevation. C to E, Cartilaginous canal skin elevated, everted, and blind sac closure of external canal. F, Blind sac closure reinforced with periosteal flap.

The flap should be elevated superficially to the temporalis and postauricular muscles, leaving a periosteal flap based on the external auditory canal (see Fig. 176-9B). The canal is transected with care to protect the branches of facial nerve anterior to the canal.

The plane of dissection in an anterior direction is in the subcuticular tissue over the temporal, parotid, and cervical areas. In the type A approach, the tertiary branches of the facial nerve are identified and protected, whereas in the type B and type C approaches, the frontal branch is exposed and protected near the lateral orbital rim.

Closure of the External Auditory Canal

The external auditory canal is closed in a watertight blind sac. The cartilaginous canal skin is undermined to approximately the level of the conchal bowl with tenotomy scissors. The cartilaginous canal skin then is everted and sutured with absorbable sutures and reinforced medially with the periosteal flap elevated off the mastoid cortex (see Fig. 176-9C to F).⁹⁶

Removal of External Auditory Canal Wall Skin and Tympanic Membrane

The skin of the osseous portion of the external auditory canal wall is elevated circumferentially down to the annulus. With an operating microscope, the tympanic annulus is elevated, the incudostapedial joint is separated, the tensor tympani tendon is cut, and the neck of the malleus is nipped, allowing total removal of the canal wall skin, tympanic membrane, and attached manubrium (Fig. 176-10).

Figure 176-10. A, Skin of osseous external auditory canal elevated to annulus (note that lateral skin is already removed). B, Removal of canal skin, tympanic membrane, and attached manubrium.

Cervical Dissection

Cervical dissection is done as needed to expose the inferior margins of the tumor. This part of the procedure is not required in most type B and type C approaches, except to gain cervical control of the vascular structures if needed. The greater auricular nerve should be sectioned as distally as possible in the parotid for potential use as an interposition graft if needed. Major structures, including the common, external, and internal carotid arteries, the internal jugular vein, and cranial nerves IX to XII, are identified (Fig. 176-11). Division of the posterior belly of the digastric near the mastoid facilitates identification of these structures up to their entrance in the skull base. Ligation of the occipital artery and ascending pharyngeal artery is indicated when these vessels prove to be the vascular supply to the tumor on arteriography. Transection of the glossopharyngeal nerve often is necessary to follow the carotid artery into the skull base. Vascular loops are placed around the internal carotid artery and jugular vein for quick identification and control in the event of bleeding (see Fig. 176-11).

Figure 176-11. Preparatory dissection with flap elevation and proximal vascular control.

Extratemporal Facial Nerve Dissection

The extratemporal facial nerve may be located deep to the midpoint of a line between the tragal pointer cartilage and the mastoid tip. After release of parotid tissue from the sternocleidomastoid muscle, the tragal cartilage is dissected medially, identifying the facial nerve just inferior to the pointer. The facial nerve is dissected out to tertiary branches by cutting overlying the parotid gland and freeing it from the underlying parotid tissues. This exposure is required for anterior transposition in the type A approach (see Fig. 176-11). In the type B and type C approaches, facial nerve transposition is not required; only the frontal branch is followed distally to allow its preservation when the zygoma is transected. The parotid gland is bluntly dissected off the masseteric fascia to reduce traction on the facial nerve when the mandible is retracted.

Radical Mastoidectomy

The radical mastoidectomy removes the air cell tracts lateral and adjacent to the otic capsule (Fig. 176-12A). Exenteration of all tracts is important to prevent long-term complications after cavity obliteration.⁹⁶ The stapes suprastructure is removed to prevent inner ear trauma. The facial nerve is skeletonized in preparation for transposition. The eustachian tube is obliterated with bone wax impregnated with bone dust and a muscle plug. Removal of the external auditory canal, tympanic membrane, ossicles, and air cells of the temporal bone lateral to the otic capsule constitute the radical mastoidectomy.

Figure 176-12. A, Radical mastoidectomy with facial nerve skeletonization. B, Anterior transposition of the facial nerve.

Type A Approach

FACIAL NERVE TRANSPOSITION

The facial nerve should be skeletonized and freed of its bony canal 180 degrees circumferentially from the geniculate ganglion distal to the stylomastoid foramen. The horizontal semicircular canal is in jeopardy at the second genu, and bone should be removed only on the tympanic side of the facial nerve. The air cells of the mastoid tip are exenterated lateral to the digastric ridge; the cortical shell is removed with a rongeur after incising the attached digastric muscle. At the stylomastoid foramen, the facial nerve is densely adherent to the surrounding fibrous tissue, and these tissues are elevated as a unit to prevent devascularization and stretch injury to the nerve. A new bony canal is drilled in the anterior wall of the epitympanum to receive the nerve. The facial nerve is elevated carefully from the geniculate ganglion to the pes anserinus and is transposed into the new groove and secured within the parotid soft tissue (see Fig. 176-12B).

Meticulous handling of the facial nerve decreases the risk of permanent facial nerve dysfunction after transposition. Use of the operating microscope, diamond burr, and constant irrigation minimizes direct trauma. The epineurium is kept intact. The stapedius nerve, the soft tissue of the stylomastoid foramen including periosteum and enclosed facial nerve, and all medial attachments to the nerve should be sharply dissected to prevent stretch injury.

Intraoperative facial nerve monitoring with EMG is of great benefit during transposition in preventing injury (see Chapter 178).⁸⁰ Immediate feedback is available to the surgeon during manual manipulations, helping prevent direct trauma and stretch injury, the latter being the most common reason for postoperative dysfunction. With feedback monitoring, the surgeon can remove bone over the facial nerve more aggressively.

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