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Current Otolaryngology  > XIV. Skull Base > Chapter 66. Neurotologic Skull Base Surgery >

Neurotologic Skull Base Surgery: Introduction

Although in widespread use, the term "skull base surgery" is somewhat of a misnomer. Only a minority of such procedures are undertaken to expose lesions actually located primarily within the skull base. Most procedures are conducted to expose deep-seated intracranial lesions situated either adjacent to the brainstem (eg, midbrain, pons, or medulla) or beneath the cerebral cortex . Previously, many such tumors were approached via simple openings in the calvaria, which require vigorous and often injurious degrees of brain retraction.

The fundamental principle in transbasal craniotomy is removal of the skull base bone to minimize the need for brain retraction. Although current techniques represent a major enhancement in our ability to control inaccessible tumors while minimizing morbidity, they are not panaceas. For example, experience has shown that these procedures are far more suitable for benign lesions (eg, meningiomas, schwannomas, and paragangliomas) and even for low-grade malignant growths (eg, chordomas and chondrosarcomas) than for high-grade malignant lesions (eg, squamous cell carcinoma, adenocystic carcinoma , and soft tissue sarcomas). Currently, more emphasis is placed on the preservation of function, especially cranial nerves, than on the necessity for radical resection in every case. The value of neurophysiologic nerve monitoring for motor nerves within the surgical field has become well established. In the developmental years of skull base surgery, two-stage procedures were common. More recently, single-stage procedures have become preferred in most centers, even for tumors with sizable intra- and extracranial components , as well as those involving multiple cranial fossae. Computerized imaging modalities provide localizing information that guides the surgeon around vital structures and helps to enable thorough tumor removal.

Jung HW, Yoo H, Paek SH, Choi KS. Long-term outcome and growth rate of subtotally resected petroclival meningiomas: experience with 38 cases. Neurosurgery. 2000;46(3):567. [PMID: 10719852]

Kurtsoy A, Menku A, Tucer B, Oktem IS, Akdemir H. Neuronavigation in skull base tumors. Min Invas Neurosurg. 2005;48(1):7. [PMID: 15747210]

Temporal Bone

Temporal bone resection is a fairly radical operation conducted for malignant disease, particularly squamous cell carcinoma originating in the external auditory canal. Some other indications include adenomatous tumors, such as the aggressive papillary adenocarcinoma of the endolymphatic sac and those arising in salivary tissue (eg, adenocystic carcinoma). In most cases, the lateral portion of the temporal bone housing the ear canal is removed en bloc (Figure 661). The posterior margin consists of the dural lining of the petrous pyramid, which is exposed via mastoidectomy. The anterior margin often includes some or all of the parotid gland and, at times, the mandibular condyle and the temporomandibular joint (Figure 662).

Figure 661.

The degrees of temporal bone resection. The solid lines demarcate the so-called sleeve resection of the soft tissue of the canal. This is an insufficient approach to malignant tumors of the region. The dotted lines depict subtotal temporal bone resection. The dashed lines illustrate total temporal bone resection.

(Reprinted with permission of Jackler RK.)

Figure 662.

Temporal bone resection with a specimen, en bloc, including the external auditory canal, the mandibular condyle, and a portion of the parotid gland.

(Reprinted with permission of Jackler RK.)

Most surgeons remove more deeply involved regions (eg, the cochlea, semicircular canal, and internal auditory canal) piecemeal, using a high-speed drill as resection en bloc risks injury to the internal carotid artery. In advanced lesions, the resection can be carried medially to the internal carotid artery, but its resection is seldom justified. After resection of the condyle, exenteration of the pterygoid muscles , including the third division of the trigeminal nerve to the level of the pterygoid plates, may be accomplished in deeply penetrating lesions. As a general rule, if the facial nerve works preoperatively, a diligent effort should be made to preserve it, although this is not always feasible and engraftment may be needed.

Reconstruction of the defect needs to anticipate the need for radiation therapy . Leaving an open cavity increases the risk of osteoradionecrosis. For this reason, the external auditory meatus is typically sewn shut. A rotation flap of temporalis muscle is often desirable to reinforce the closure with well-vascularized tissue. Regional (eg, pectoralis or trapezius) or even free ( rectus abdominis) flaps may be needed for closure in cases where auriculectomy has been required.

Jackler RK, Driscoll C. Tumors of the Ear and Temporal Bone. Philadelphia: Lippincott Williams & Wilkins, 2000.

Nyrop M, Grontved A. Cancer of the external auditory canal. Arch Otolaryngol Head Neck Surg. 2002;128(7):834. [PMID: 12117346]

Petrous Apex, Petroclival Junction, & Foramen Lacerum

Petrous Apicotomy

The majority of procedures conducted for disease in the petrous apex involves creation of a narrow drainage pathway that circumnavigates the inner ear. Such procedures, which are usually carried out to drain petrositis or cholesterol granulomas, are best termed petrous apicotomy (Figure 663). In the subcochlear route, a channel is excavated along the floor of the external auditory canal and the hypotympanum, which traverses the narrow window between the cochlea, the carotid genu, and the dome of the jugular bulb.

Figure 663.

Petrous apicotomy is a narrow drainage opening created circumventing the inner ear to drain an apical fluid collection (cholesterol granuloma or infection).

(Reprinted with permission of Jackler RK.)

An alternate pathway is the infralabyrinthine approach, conducted between the posterior semicircular canal and the jugular bulb, immediately behind the descending portion of the facial nerve. However, because most apical cysts are located anteriorly medial to the cochlea, the infralabyrinthine route is deeper, more difficult, and creates a less adequate drainage portal.

Brackmann DE, Toh EH. Surgical management of petrous apex cholesterol granulomas. Otol Neurotol. 2002;23(4):529. [PMID: 12170157]

Jacob CE, Rupa V. Infralabyrinthine approach to the petrous apex. Clin Anat. 2005;18(6):423. [PMID: 1601561]

Telischi FF, Luntz M, Whiteman ML. Supracochlear approach to the petrous apex: case report and anatomic study. Am J Otol. 1999;20(4):500. [PMID: 10431893]

Petrous Apicectomy

Petrous apicectomy, the formal removal of the petrous apex, is conducted for neoplasms of the apex and petroclival junction. It is conducted via a low subtemporal craniotomy, which exposes the anterior face of the petrous pyramid (Figure 664). Anatomically, the resection is limited inferiorly by the horizontal portion of the internal carotid artery, laterally by the cochlea and internal auditory canal, and medially by Meckel cave and the trigeminal nerve. Exposing the infratemporal fossa beneath the internal carotid artery requires downfracture and subsequent repair of the zygomatic arch. The characteristic tumor of this region is the chondrosarcoma of the petroclival junction, which arises in the cartilaginous section of the foramen lacerum (Figure 665). Although it is not often necessary, apicectomy is sometimes used for the resection of cholesterol granulomas that have proven recalcitrant to drainage procedures.

Figure 664.

Petrous apicectomy is the surgical resection of the petrous apex and is carried out through a subtemporal exposure of the ventral surface of the petrous pyramid. Note the transapical view of the superior aspect of the cerebellopontine angle. Downward displacement of the zygomatic arch is optional. BA, basilar artery; Co, cochlear.

(Reprinted with permission of Jackler RK.)

Figure 665.

Chondrosarcoma of the petroclival junction arising from the cartilage of foramen lacerum.

(Reprinted with permission of Jackler RK.)

Oghalai JS, Buxbaum JL, Jackler RK, McDermott MW. Skull base chondrosarcoma originating from the petroclival junction. Otol Neurotol. 2005;26(5):1052. [PMID: 16151358]

Clivus

The clivus is not a bone in and of itself, but is rather a region composed of the dorsal part of the sphenoid bone and the portion of the occipital bone located anterior to the foramen magnum. The clivus, which, in Latin, means slope , spans from the posterior clinoid to the anterior margin of the foramen magnum. Adjacent to its dorsal surface is the entire brainstem and the vertebrobasilar system. The subject of clival tumors falls into two categories: (1) intrinsic tumors (especially chordomas) and (2) meningiomas arising from the dural lining of its dorsal surface.

Chordomas arise from notochordal remnants in the midline of the skull base (Figure 666). Initially, they grow to fill the clival marrow compartment but later erode its cortical plate to spread intradurally. This brings them into contact with the brainstem, which may be compressed posteriorly. Intrinsic clival lesions, which remain extradural, are approached anteriorly via either a transsphenoethmoidal or transoral approach. The transsphenoethmoidal approach is well suited for lesions of the mid- and upper clivus, whereas the transoral approach is preferred when lower clival and craniovertebral junction exposure is needed. Recently, endoscopic techniques are increasingly used in surgery of clival tumors.

Figure 666.

Chordoma of the clivus with intracranial involvement due to breaching of the dorsal clival surface.

(Reprinted with permission of Jackler RK.)

Enepekides DJ, Donald PJ. Transoral approaches to the clivus and nasopharynx. Otolaryngol Clin N Am. 2001;34(6):1105. [PMID: 11728936]

Solares CA, Fakhri S, Batra PS, Lee J, Lanza DC. Transnasal endoscopic resection of lesions of the clivus: a preliminary report. Laryngoscope. 2005;115(11):1917. [PMID: 16319599]

Jugular Foramen

The jugular foramen is traversed by the jugular vein and the three lower cranial nerves (CN IX, the glossopharyngeal nerve; CN X, the vagus nerve; and CN XI, the accessory nerve). The vertical segment of the facial nerve lies immediately lateral to the jugular foramen, presenting one of the classic challenges in cranial base surgery. The dome of the jugular bulb approaches the hypotympanic portion of the middle ear. Three tumor types predominate in tumors of this region: (1) glomus jugulare tumors, (2) meningiomas, and (3) lower cranial nerve schwannomas. These may remain confined to the cranial base, but most often possess a component in the upper neck, posterior cranial fossa, or both (Figure 667).

Figure 667.

Jugular foramen tumor with intracranial, foraminal, and extracranial component.

(Reprinted with permission of Jackler RK.)

The jugular foramen approach begins control of the great vessels in the upper neck (Figure 668). Exposure of the foramen itself commences with a mastoidectomy and decompression of the bony covering of the sigmoid sinus. After skeletonization of the descending fallopian canal, the lateral aspect of the jugular foramen is exposed. Tumor resection commences after connecting the skull base and neck dissection followed by proximal and distal occlusion of the jugular vein (Figure 669).

Figure 668.

Surgical exposure of the jugular foramen region after mastoidectomy, anterior rerouting of the facial nerve, and upper neck dissection. MC, mandibular condyle; EAM, closed external auditory meatus; FN, facial nerve; SM, styloid muscle; IX, glossopharyngeal nerve; DM, digastric muscle; C1, the transverse process of C1; XI, accessory nerve; XII, hypoglossal nerve; AP, ascending pharyngeal artery; EC, external carotid artery; IC, internal carotid artery; X, vagus nerve; JV, jugular vein; SCM, sternocleidomastoid muscle.

(Reprinted with permission of Jackler RK.)

Figure 669.

Large glomus jugulare tumor with retrograde spread into the sigmoid sinus and distal involvement of the lumen of the jugular vein. The hypotympanum is extensively eroded.

(Reprinted with permission of Jackler RK.)

Traditionally, many surgeons rerouted the facial nerve anterior to obtain unobstructed access to the jugular foramen. However, this frequently leads to transient palsy, which does not always recover to normal. More recently, a fallopian bridge technique has gained popularity. In this procedure, the facial nerve remains in situ, and microdissection is carried out around it (Figure 6610). Some surgeons use facial nerve rerouting selectively when encasement of the carotid artery necessitates obtaining augmented anterior exposure.

Figure 6610.

Fallopian bridge approach to the jugular foramen, leaving the descending facial nerve in situ.

(Reprinted with permission of Jackler RK.)

Meningiomas and glomus tumors both have a proclivity for growing proximally into the sigmoid sinus and distally into the jugular view. Meningiomas and schwannomas are also more likely to involve the neural plane containing cranial nerves IXXI, although these structures may certainly become involved with larger paragangliomas as well. To reduce blood loss and facilitate orderly microdissection, preoperative embolization is usually conducted. Tumor removal is conducted piecemeal, with resection of involved segments of the sigmoid-jugular system (typically occluded from disease) as required. Although preservation of the stout cranial nerves in the neck is usually readily accomplished, the multiple fine neural branches of the jugular foramen region can be a challenge to preserve when infiltrated by tumor. In such cases, meticulous microdissection, guided by neurophysiologic monitoring, can sometimes be rewarded by preservation of part or all of the lower nerve branches.

The removal of jugular foramen tumors can often be accomplished with preservation of the auditory apparatus. Resection of the middle ear and ear canal with closure of the meatus is necessary under two circumstances: (1) extensive destruction of the ear canal and (2) substantial involvement of the carotid genu (Figure 6611). Intradural penetration of jugular foramen tumors will be discussed with transjugular craniotomy.

Figure 6611.

Ear canal closure is carried out with a meticulous three-layer technique to withstand cerebrospinal fluid pressure, if necessary. 1, everted canal skin; 2, subcutaneous tissue; 3, periosteum.

(Reprinted with permission of Jackler RK.)

Gilbert ME, Shelton C, McDonald A et al. Meningioma of the jugular foramen: glomus jugulare mimic and surgical challenge. Laryngoscope. 2004;114(1):25. [PMID: 14709990]

Jackson CG. Glomus tympanicum and glomus jugulare tumors. Otolaryngol Clin North Am. 2001;34(5):941. [PMID: 11557448]

Pensak ML, Jackler RK. Removal of jugular foramen tumors: the fallopian bridge technique. Otolaryngol Head Neck Surg. 1997; 117(6):586. [PMID: 9419083]

Tekdemir I, Tuccar E, Aslan A, Elhan A, Ersoy M, Deda H. Comprehensive microsurgical anatomy of the jugular foramen and review of terminology. J Clin Neurosci. 2001;8(4):351. [PMID: 11437579]

Wilson MA, Hillman TA, Wiggins RH, Shelton C. Jugular foramen schwannomas: diagnosis, management, and outcomes . Laryngoscope. 2005;115(8):1486. [PMID: 16094130]

Infratemporal Fossa

The infratemporal fossa is not a well-demarcated anatomic compartment but is rather the region of the upper neck that lies beneath the temporal bone and the sphenoid wing. Within it are the jugular vein, the carotid artery, the styloid process, the third division of the trigeminal nerve, the eustachian tube, the pterygoid muscles and their associated bony plates, and a rather impressive venous plexus. Laterally, the infratemporal fossa is defended by the mandible (condyle and ramus) and the zygomatic arch. Medially, it is bounded by the nasopharynx and the lateral wall of the sphenoid sinus. As previously mentioned, jugular foramen tumors often involve the superficial portion of the infratemporal fossa in proximity to the great vessels. Tumors involving the deeper regions include trigeminal schwannomas in the vicinity of the foramen ovale and penetrating malignant neoplasms such as those from the deep lobe of the parotid gland and ear. The most common tumor involving the deep aspect of the infratemporal fossa is nasopharyngeal carcinoma.

Lesions involving the lateral portion of the infratemporal fossa, such as glomus jugulare tumors, are approached via a postauricular incision and include some degree of temporal bone surgery. More anteriorly situated lesions are approached preauricularly, often with access gained through downfracture of the zygomatic arch and either downward displacement or resection of the condyle. When necessitated by penetration of the skull base, the exposure can be combined with middle fossa craniotomy. Resection of the glenoid fossa and division of V3 is needed to expose Meckel cave and the cavernous sinus from this perspective. A reasonably functional pseudoarthrosis usually forms after resection of the glenoid. In one commonly used system of nomenclature , the various depths of infratemporal fossa dissection are referred to as approaches A, B, and C (Figures 6612 and 6613).

Figure 6612.

Infratemporal fossa approaches types A, B, and C (after Fisch).

(Reprinted with permission of Jackler RK.)

Figure 6613.

Neurophysiologic monitoring of the lower cranial nerves in jugular foramen surgery. 10, vagus nerve; 11, accessory nerve; 12, hypoglossal nerve; PP, pharyngeal plexus.

(Reprinted with permission of Jackler RK.)

Internal Auditory Canal & Cerebellopontine Angle

Surgery of tumors of the internal auditory canal (IAC) and cerebellopontine angle (CPA) is a central issue to neurotology. Some of the complex issues in this involved subject are considered , in greater depth, in the chapter that discusses vestibular schwannomas (see Chapter 56, Vestibular Disorders). A decision among the three approaches in widespread use (translabyrinthine, retrosigmoidal, and middle fossa) depends on a number of variables such as size , shape, anatomic location, and pathologic type of tumor, as well as status of hearing (Figure 6614).

Figure 6614.

Overview of posterior fossa approaches to the cerebellopontine angle: retrosigmoidal, retrolabyrinthine, translabyrinthine, and transcochlear.

(Reprinted with permission of Jackler RK.)

Retrosigmoidal Approach

The retrosigmoidal approach is a classic means of exposing the CPA (Figures 6615, 6616, and 6617). It provides wide access to the CPA from the tentorium to the foramen magnum. Although not relevant in vestibular schwannoma surgery, the retrosigmoidal approach provides enhanced access to the inferior region of the CPA when compared with the translabyrinthine approach. The opening is created by removing the calvaria immediately behind the sigmoid and below the transverse sinus. Retraction of the cerebellar hemisphere brings the CPA into view. Access to the IAC is obtained by drilling off its posterior bony lip. Approximately the medial two thirds of the IAC can be exposed without violating a portion of the inner ear. Thus, when the fundus of the canal is involved, direct exposure of the deepest portion of the tumor in the IAC precludes an attempt at hearing conservation. The primary disadvantage of the retrosigmoidal approach is a higher incidence of persistent headache when compared with the translabyrinthine or middle fossa approaches. Although still used routinely as a hearing conservation method in many centers, comparison data show that the middle fossa approach appears to be more successful in this regard, at least for tumors with modest- sized components within the CPA.

Figure 6615.

Overview of the retrosigmoidal approach from an axial perspective. Note the removal of the posterior lip of the internal auditory canal.

(Reprinted with permission of Jackler RK.)

Figure 6616.

Operative view of the cerebellopontine angle via the retrosigmoidal approach. JV, jugular vein; JB, jugular bulb; 11S, spinal root of accessory nerve; 11C, cranial root of accessory nerve; 10, vagus nerve; 9, glossopharyngeal nerve; ES, endolymphatic sac; VA, vestibular aqueduct; PSCC, posterior semicircular canal; CC, common crus; SSCC, superior semicircular canal; Co, cochlea; IV, inferior vestibular nerve; SV, superior vestibular nerve; PA, porus acusticus; Ch, choroids plexus; Fl, flocculus; BS, brainstem; 7, facial nerve; 8, audiovestibular nerve; 5, trigeminal nerve.

(Reprinted with permission of Jackler RK.)

Figure 6617.

Retrosigmoidal approach to a small vestibular schwannoma that extends only partly down the internal auditory canal. Note that the inner ear overlies the lateral one third of the internal auditory canal. C, cochlear nerve; 7, facial nerve; 8, audiovestibular nerve.

(Reprinted with permission of Jackler RK.)

Translabyrinthine Approach

The translabyrinthine approach provides direct exposure of the CPA through the petrous pyramid, the shortest route from the surface (Figures 6618 and 6619). When properly performed, it provides excellent exposure of the lateral aspect of the pons and upper medulla. Exposure of the CPA is bounded superiorly by the tentorium cerebelli and inferiorly by the limitation imposed by the sigmoid sinus and jugular bulb. Because the opening provided by petrosectomy alone is fairly narrow, the exposure is augmented by removing the bone overlying the sigmoid sinus and a variable degree of retrosigmoidal posterior fossa dura (depending on the amount of posterior fossa exposure required). After mastoidectomy and decompression of the sigmoid sinus, removal of the semicircular canals brings into view the bone surrounding the IAC. Excavations around the IAC place it into high relief so that it is fully accessible for microsurgical dissection.

Figure 6618.

Overview of the translabyrinthine approach from an axial perspective. Note that the craniotomy extends from the posterior edge of the external auditory canal to a distance behind the sigmoid sinus, which is posteriorly displaced.

(Reprinted with permission of Jackler RK.)

Figure 6619.

Translabyrinthine approach to a medium-sized vestibular schwannoma. Note the deviation and splaying of the facial nerve on the anterior surface of the tumor.

(Reprinted with permission of Jackler RK.)

The translabyrinthine approach is primarily used for vestibular schwannomas, although it has some role in posterior fossa meningioma surgery as well. Since a portion of the inner ear is removed during the craniotomy, in most centers, this approach is used either for CPA tumors associated with poor hearing or for patients in whom hearing preservation is not a realistic option.

Middle Fossa Approach

The middle fossa approach provides exposure to the IAC from above and limited access to the CPA (Figure 6620). Most centers do not use this approach for tumors exceeding approximately 1.5 cm in CPA diameter. After removal of an approximately 3.0-cm by 3.0-cm plate of calvaria above the ear, the temporal lobe is elevated extradurally off the petrous floor. With the exposure maintained through a specially designed retractor, the bone is removed from the superior aspect of the IAC. Wide excavation of the petrous apex and the region of the porus acusticus provides limited access to the CPA from above. The primary advantage of the middle fossa approach is its superior ability to preserve hearing. The primary disadvantage is the inconvenient location of the facial nerve on the superior surface of the tumor that must be manipulated to a greater degree and thus has a higher rate of temporary postoperative dysfunction.

Figure 6620.

Middle fossa approach to the internal auditory canal and cerebellopontine angle.

(Reprinted with permission of Jackler RK.)

Baumann I, Polligkeit J, Blumenstock G, Mauz PS, Zalaman IM, Maassen MM. Quality of life after unilateral acoustic neuroma surgery via middle cranial fossa approach. Acta Otolaryngol. 2005;125(6):585. [PMID: 16076706]

Ciric I, Zhao JC, Rosenblatt S, Wiet R, O'Shaughnessy B. Suboccipital retrosigmoid approach for removal of vestibular schwannomas: facial nerve function and hearing preservation. Neurosurgery. 2005;56(3):560; discussion 560. [PMID: 15730582]

Day JD, Chen DA, Arriaga M. Translabyrinthine approach for acoustic neuroma. Neurosurgery. 2004;54(2):391. [PMID: 14744286]

Sharp MC, MacFarlane R, Hardy DG, Jones SE, Baguley DM, Moffat DA. Team working to improve outcome in vestibular schwannoma surgery. Br J Neurosurg. 2005;19(2):122. [PMID: 16120514]

Intracranial Aspects of the Jugular Foramen

Meningiomas, schwannomas, and paragangliomas frequently extend intracranially. Meningiomas and schwannomas originate in the intracranial compartment, whereas glomus tumors spread posteriorly from their skull base component via either the neural portion of the foramen or by penetrating the posterior surface of the jugular bulb or sigmoid sinus. In the past, these tumors were commonly approached with a multistage procedure in which the skull base and neck component were removed separately from the intracranial portion. The current trend is toward a single-stage removal via a transjugular craniotomy . This procedure involves the creation of a posterior fossa craniotomy through resection of the sigmoid sinus and jugular bulb, both of which have usually been occluded by tumor growth (Figures 6621 and 6622). This maneuver provides direct visualization of the intracranial aspect of the jugular foramen, including the lower nerve roots emanating from the lateral aspect of the medulla.

Figure 6621.

Transjugular craniotomy is conducted through resection of the sigmoid-jugular system (usually occluded preoperatively by tumor growth). This affords an excellent view of the intracranial aspect of the jugular foramen nerves as well as the lateral aspect of the pons and upper medulla. Note that although the facial nerve is rerouted in this illustration, this is not necessary in most cases.

(Reprinted with permission of Jackler RK.)

Figure 6622.

A jugular foramen schwannoma as visualized through a transjugular craniotomy.

(Reprinted with permission of Jackler RK.)

Some tumors, especially meningiomas, but occasionally lower cranial nerve schwannomas as well, are largely intracranial with little or no foraminal involvement. In such cases, a retrosigmoidal approach is appropriate. It is possible, from this perspective, to drill open the introitus of the intracranial aspect of the jugular foramen from above.

George B, Tran PB. Surgical resection of jugulare foramen tumors by juxtacondylar approach without facial nerve transposition. Acta Neurochir. 2000;142(6):613. [PMID: 10949434]

Oghalai JS, Leung MK, Jackler RK, McDermott MW. Transjugular craniotomy for the management of jugular foramen tumors with intracranial extension. Otol Neurotol. 2004;25(4):570; discussion 579. [PMID: 15241237]

The Ventral Surface of the Brainstem

Transcochlear Approach

In the past, lesions situated anterior to the brainstem were considered to be unresectable. Typical tumors of the region include clival meningiomas and chordomas that had broken through the posterior surface of the clivus to become intradural. Radical petrosectomy, also known as the transcochlear approach, is one method used to expose this inaccessible region (Figure 6623). This procedure entails complete rerouting of the facial nerve, which results in a complete paralysis that recovers only partially and with synkinesis; sacrifice of the entire inner ear; closure of the external auditory meatus and eustachian tube; and skeletonization of the intrapetrous carotid artery. After removal of the apical petrous bone, petroclival junction, and even the lateral aspect of the clivus, an excellent view of the ventral surface of the pons and upper medulla is obtained with minimal brain retraction. Although this method affords excellent exposure, it is associated with high morbidity, including ipsilateral deafness and permanent facial nerve dysfunction. In recent years, this aggressive technique has become increasingly supplanted by the so-called combined approach craniotomies.

Figure 6623.

Transcochlear approach to a prepontine tumor. The anterior limits are the eustachian tube (ET), which has been obliterated with bone wax and the carotid artery (CA). Note that the facial nerve has been rerouted.

(Reprinted with permission of Jackler RK.)

Retrolabyrinthine-Subtemporal Approach

In the combined retrolabyrinthine-subtemporal approach (also called simply the petrosal approach ), a limited presigmoid petrosectomy is combined with a subtemporal opening (Figures 6624 and 6625). The two fossae are connected by division of the tentorium. This affords a wide exposure of the lateral aspect of the midbrain, pons, and medulla. This versatile approach has become the heavily used option in modern neurotology for a wide range of tumors in and around the brainstem. Although exposure is limited in the inferior reaches of the CPA by the sigmoid sinus and jugular bulb, superiorly it readily exposes the cavernous sinus. A partial (retrolabyrinthine) petrosectomy is usually chosen as this approach allows for hearing preservation. In lesions predominantly involving the anterior midline, a greater degree of petrosectomy (eg, translabyrinthine or even transcochlear) may be needed. During a combined-approach craniotomy, care must be taken in elevation and retraction of the posterior temporal lobe to avoid injury to the vein of Labb. Injury to this bridging vein may result in a venous infarct of the temporal-parietal cortex.

Figure 6624.

Combined retrolabyrinthine-subtemporal craniotomy with access to the lateral aspect of the pons and midbrain. Note the preservation of the semicircular canals and endolymphatic sac. The posterior and middle cranial fossae have been made confluent by division of the tentorium. The vein of Labb (VL) is noted on the temporal lobe and joining the transverse sinus.

(Reprinted with permission of Jackler RK.)

Figure 6625.

Combined retrolabyrinthine-subtemporal craniotomy for a meningioma with substantial components in both the middle and posterior cranial fossae. Note the trochlear nerve (4) on the superior surface of the tumor.

(Reprinted with permission of Jackler RK.)

Abdel Aziz KM, Sanan A, van Loveren HR, Tew JM Jr, Keller JT, Pensak ML. Petroclival meningiomas: predictive parameters for transpetrosal approaches. Neurosurgery. 2000;47(1):139. [PMID: 10917357]

Angeli SI, de la Cruz A, Hitselberger W. The transcochlear approach revisited. Otol Neurotol. 2001;22(5):690. [PMID: 11568681]

Bambakidis NC, Gonzalez LF et al. Combined skull base approaches to the posterior fossa. Neurosurg Focus. 2005;19:1. [PMID: 16122215]

Horgan MA, Anderson GJ, Kellogg JX, et al. Classification and quantification of the petrosal approach to the petroclival region. J Neurosurg. 2000;93(1):108. [PMID: 10883912]

Magliulo G. Modified retrolabyrinthine approach with partial labyrinthectomy: anatomic study. Otolaryngol Head Neck Surg. 2001;124(3):287. [PMID: 11240993]

Mortini P, Mandelli C, Franzin A, Giugni E, Giovanelli M. Surgical excision of clival tumors via the enlarged transcochlear approach: indications and results. J Neurosurg Sci . 2001;45(3):127. [PMID: 11731737]

Meckel Cave

Meckel cave, also known as the cavum trigeminale, overlies the petroclival junction. Traversing it is the semilunar ganglion of the fifth cranial nerve (the trigeminal nerve). In close relationship anteromedially is the cavernous sinus. Posteriorly, its mouth opens into the superior aspect of the CPA. The oculomotor nerves IV and VI are in the immediate vicinity of the roof of Meckel cave. Because of the rich representation of arachnoid granulations in this region, meningiomas are especially prevalent . The second most common lesion is trigeminal schwannoma.

The optimal surgical exposure of Meckel cave depends upon whether the tumor is dominantly in the middle fossa, posterior fossa, or bilobed (Figure 6626AC). Middle fossa lesions are approached via a subtemporal craniotomy. Posterior fossa lesions are exposed via a standard retrosigmoidal approach, modified, when necessary, by drilling open the posterior aspect of Meckel cave. In current practice, a bilobed lesion with substantial components in both posterior and middle cranial fossae is addressed by a single opening that connects both fossa (retrolabyrinthine-subtemporal).

Figure 6626.

Surgery for tumors of Meckel cave. (A)  Middle fossa predominant lesions are approached subtemporally. (B) Posterior fossa predominant lesions are approached retrosigmoidally. (C) Bilobed lesions are exposed via a combined craniotomy.

(Reprinted with permission of Jackler RK.)

Al-Mefty O, Ayoubi S, Gaber E. Trigeminal schwannomas: removal of dumbbell-shaped tumors through the expanded Meckel's cave and outcomes of cranial nerve function. J Neurosurg. 2002;96(3):453. [PMID: 11883829]

Danner C, Cueva RA. Extended middle fossa approach to the petroclival junction and anterior cerebellopontine angle. Otol Neurotol. 2004;25(5):762. [PMID: 15354008]

Samii M, Tatagiba M, Carvalho GA. Retrosigmoidal intradural suprameatal approach to Meckel's cave and the middle fossa: surgical technique and outcome. J Neurosurg. 2000;92(2):235. [PMID: 10659009]

Foramen Magnum & Craniovertebral Junction

The exposure of posterior lesions of foramen magnum presents relatively little challenge. Extradural lesions located ventral to the craniovertebral junction are usually approached transorally (eg, lower clival chordoma or odontoid displacement upward). Intradural lesions in this ventral location, such as meningiomas, require a sterile approach from a lateral perspective. The far lateral (transcondylar) approach has been devised for just such cases (Figure 6627; see also Figure 6626). After a retrosigmoidal craniotomy and removal of the posterior ring of the foramen magnum, the posterior margin of the jugular foramen is skeletonized. Working beneath the jugular foramen, the cerebellum is elevated extradurally while a variable portion of the occipital condyle is removed. Usually, adequate exposure is obtained following the removal of approximately half of the condyle. Additional condylar resection may lead to instability that requires the insertion of hardware for stabilization. Resection of the tumor ventral to the medulla and upper spinal cord is carried out between the lower cranial nerve roots and the upper spinal roots.

Figure 6627.

Bone removed via the far lateral (transcondylar) approach to the foramen magnum as seen from above (A) and below (B) . HC, hypoglossal canal; C, condyle; JF, jugular foramen; FN, facial nerve; EC, ear canal.

(Reprinted with permission of Jackler RK.)

Wanebo JE, Chicoine MR. Quantitative analysis of the transcondylar approach to the foramen magnum. Neurosurgery. 2001;49 (4):934. [PMID: 11564256]

Suhardja A, Agur AM, Cusimano MD. Anatomical basis of approaches to foramen magnum and lower clival meningiomas: comparison of retrosigmoid and transcondylar approaches. Neurosurg Focus. 200315;14(6):9. [PMID: 15669794]

Vertebrobasilar Lesions

Neurotologic cranial base approaches have a role in the exposure of aneurysms of the posterior circulation. The subtemporal transapical approach , similar to that used for an apical petrosectomy (noted previously), was initially devised as a means of both exposing basilar tip aneurysms and establishing proximal control of the intrapetrous carotid (see Figure 664). Through the window created in the petrous apex, access is provided to the upper basilar artery. The transcochlear approach is capable of providing access for midbasilar artery aneurysms. Similarly, the transcondylar approach has use in approaching vascular lesions in the region of the vertebrobasilar junction.

Gonzalez LF, Amin-Hanjani S et al. Skull base approaches to the basilar artery. Neurosurg Focus. 2002;19:1. [PMID: 16122212]

Ng PY, Yeo TT. Petrosal approach for a large right posterior cerebral artery (P2) aneurysm. J Clin Neurosci. 2000;7(5):445. [PMID: 10942668]

Meningoceles & Encephaloceles

Defects in the dura of the roof of the petrous pyramid may occur spontaneously or following trauma or may arise as a consequence of long-standing elevated intracranial pressure. The thin bone of the tegmen overlying the mastoid or middle ear is most frequently breached. The petroclival junction is a site where congenital meningoceles may occur. Postsurgical defects after mastoid surgery commonly involve the herniation of brain tissue (encephaloceles). The operative approach to such defects is generally from above, with repair of the temporal floor defect with fascia and, when a substantial defect exists, reinforcement with a plate of bone (Figures 6628 and 6629). In extensive lateral defects, the temporalis muscle may be rotated to augment the repair (Figure 6630).

Figure 6628.

Surgical view of the far lateral approach to a ventrally situated meningioma in the foramen magnum region. Note that tumor resection must be conducted through a veil of lower cranial nerves.

(Reprinted with permission of Jackler RK.)

Figure 6629.

Encephalocele of the tegmen tympani.

(Reprinted with permission of Jackler RK.)

Figure 6630.

Multilayer repair of a large tegmen defect with fascia (intradural), bone (spanning the skull base defect), and inward rotation of the temporalis muscle.

(Reprinted with permission of Jackler RK.)

Mayeno JK, Korol HW, Nutik SL. Spontaneous meningoencephalic herniation of the temporal bone: case series with recommended treatment. Otolaryngol Head Neck Surg . 2004;130(4):486. [PMID: 15100650]

Closure of Defects

Free adipose tissue, usually harvested from the anterior abdominal wall or iliac crest region, is the mainstay of skull base defect obliteration. Local rotation flaps, such as those fashioned using the temporalis muscle or pericranium, are useful supplements for minor soft tissue deficits. More substantial deficits require the use of either regional rotation flaps, such as the pectoralis major or trapezius myocutaneous flaps, or microvascular free flaps, such as the rectus abdominis.

Cerebrospinal Fluid Rhinorrhea & Otorrhea

Aside from cranial nerve neuropathy, cerebrospinal fluid (CSF) leakage is the most prevalent morbidity in cranial base surgery. These surgeries frequently violate pneumatic tracts that ultimately connect to the middle ear and, from there, via the eustachian tube to the nasopharynx. Methods in common use to discourage CSF leak include packing the craniotomy defect with adipose tissue and sealing transected cell tracts with bone wax or other obliterative material. Despite diligent efforts at preventing this complication, CSF rhinorrhea occurs in about 10% of CPA surgeries, regardless of operative technique used, and an even higher percentage of more major skull base resections. Jugular foramen tumors that possess both intracranial and upper neck components are particularly prone to formation of large pseudomeningoceles. This risk can be minimized by avoiding both the opening of unnecessary tissue planes and multilayer closure of the neck tissues. Meticulous hemostasis is important to avoid the need for a cervical drain.

One reason for this persistent incidence is the frequency of transient postoperative CSF hypertension brought about by the impaired resorptive function of arachnoid granulations. Management of CSF leakage includes fluid restriction, medication to reduce CSF production (eg, acetazolamide [Diamox] at 250 mg qid), and CSF diversion via a lumbar subarachnoid drain. Although most CSF leaks halt with such conservative management, a small percentage of skull base surgeries require secondary operative intervention. The most common remedial procedure is obliteration of the eustachian tube.

Becker SS, Jackler RK, Pitts LP. Cerebrospinal fluid leak after acoustic neuroma surgery: a comparison of the translabyrinthine, middle fossa, and retrosigmoidal approaches. Otol Neurotol. 2003;24:107. [PMID: 12544038]

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