Acoustic Neuroma

Acoustic neuroma refers to a benign schwannoma of the eighth cranial nerve. These lesions are relatively common and account for 8% to 10% of all intracranial tumors. Acoustic neuromas arise principally from the vestibular division of the nerve. The superior and inferior divisions are equally affected; intraoperatively, the nerve of origin usually is difficult to determine. The 1992 National Institutes of Health Consensus Conference made vestibular schwannoma the official nomenclature term for these lesions. Acoustic neuromas are slowly growing neoplasms that originate in the nerve sheath and consist of schwannoma cells in a collagenous matrix. They typically are circumscribed, grossly encroaching on and displacing neural structures without direct invasion. Consistency varies, ranging from firm and dense to soft with large cystic spaces.

In the past it was taught that acoustic neuromas usually arise near the myelin-glia junction near the porus acusticus. However, no published histopathologic study has confirmed this assertion. These lesions usually arise within the internal auditory canal; however, they occasionally develop in the CPA medial to the porus. Because acoustic neuromas produce symptoms by exerting pressure on surrounding neurovascular structures, auditory and vestibular symptoms develop earlier with tumors of the internal auditory canal than with tumors of the CPA.

Of the 2000 to 3000 acoustic neuromas diagnosed annually in the United States, more than 95% occur as unilateral and nonhereditary lesions. The remaining acoustic neuromas are manifestations of the neurofibromatoses, which consist of at least two distinct genetic disorders.

Neurofibromatosis type 1 (NF1), or von Recklinghausen’s disease, is a relatively common disorder of autosomal dominant inheritance with variable penetrance and an incidence of 1 in 4000 live births. The gene responsible for NF1 has been localized to chromosome 17. NF1 neuromas occur throughout the body, arising both intracranially and extracranially from the Schwann cells of any nerve; however, acoustic neuromas develop in less than 5% of patients, and bilateral acoustic neuroma is not a part of the syndrome.

Type 2 neurofibromatosis, NF2, is the central form of the disease, characterized by bilateral acoustic neuromas in up to 96% of patients. Schwannomas of the other cranial nerves, meningiomas, and ependymomas also are observed much more commonly in patients with NF2.³ The precise frequency of NF2 is unknown, but it is far less frequent than NF1. The gene responsible for the condition encodes a membrane protein (merlin or schwannomin) and is located on chromosome 22. Acoustic neuromas in NF2 are characterized by an onset early in life, often before the age of 21 years, as opposed to unilateral lesions, most of which are diagnosed between the ages of 40 and 60 years. Thus, acoustic neuromas appearing before the age of 30 years mandate particularly close evaluation of the contralateral ear. Although the acoustic neuromas in NF1 and NF2 resemble the lesions in nonhereditary acoustic neuromas, they are technically more challenging to remove because of a tendency to adhere to nearby structures. The clinical presentation of acoustic neuroma in neurofibromatosis is identical to that of unilateral acoustic neuroma.

Malignant schwannomas may rarely occur. They often are associated with neurofibromatosis, but they also may occur with solitary schwannomas. Another very uncommon variant is a pigmented schwannoma.⁴

Natural History

The growth rate of acoustic neuromas is extremely variable. These tumors generally are slow-growing, with average reported growth rates of 0.2 cm per year; however, growth rates in excess of 2 cm per year have been documented. Acoustic neuromas that are not treated are potentially lethal. Gradual enlargement can lead to indentation of the brainstem, increased intracranial pressure, and death during a course of 5 to 15 years.⁵

Growth of acoustic neuromas generally occurs in three phases: internal auditory canal, cisternal, and brainstem compression (Figs. 177-1, 177-2, and 177-3). Growth within the internal auditory canal results in acoustic and facial nerve compression and attenuation. Displacement of the seventh cranial nerve, the acoustic (eighth) nerve, and the anterior inferior cerebellar artery occurs just medial to the porus acusticus (cisternal portion). Medial tumor growth augments the blood supply of the tumor with bridging vessels from the brainstem surface. Fourth ventricle shift often occurs when the CPA component reaches 2 to 3 cm and total ventricle obstruction with resulting hydrocephalus occurs with continued tumor growth. Trigeminal compression occurs at approximately the 3-cm stage, which permits the superior portion of the tumor to abut the fifth cranial nerve. Massive tumor growth with hydrocephalus, brainstem compression, and cerebellar tonsil herniation may occur even with modern imaging. The otolaryngologist should remain vigilant for signs and symptoms suggestive of retrocochlear pathology.

Figure 177-1. On T2-weighted fast spin echo magnetic resonance imaging, this axial reconstruction shows a 5-mm intracanalicular tumor on the left.

Figure 177-2. A medium-sized vestibular schwannoma, demonstrated by magnetic resonance imaging (A to D) and computed tomography (E). A and B, T1-weighted images before and after intravenous administration of gadolinium contrast The tumor in the right internal auditory canal (IAC), which is mushrooming into the cerebellopontine angle and enlarging the canal, is isointense to mildly hypointense to the cerebellum and enhances markedly with contrast. C and D, T2-weighted and constructive interference in steady-state (CISS) images. The tumor is moderately hyperintense on the T2-weighted images. CISS images precisely demonstrate the cochlea, cystic, and calcified components of the tumor and surrounding vascular structures. E, CT bone windows. Note the enlargement of the IAC.

(Adapted from Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003:245.)

Figure 177-3. Noncontrast coronal T1 magnetic resonance image of a large acoustic neuroma with hydrocephalus, brainstem shift, and cerebellar tonsil herniation.

Meningiomas

Meningiomas represent up to 18% of all intracranial tumors and approximately 3% of CPA tumors.²⁵ The cells lining the arachnoid villi are the cells of origin. These cells are distributed throughout the intracranial space predominantly in relation to veins and dural sinuses. Meningiomas are benign but locally aggressive tumors, which occur at different anatomic sites in the following order of frequency: parasagittal region, falx, convexity, olfactory groove, tuberculum sellae, sphenoid ridge, petrous face (CPA), tentorium, lateral ventricle, clivus, and others.²⁶

The gross appearance typically is that of a globular mass firmly adherent to the dura mater, with characteristic speckles scattered throughout the tumor that correspond to the microscopic psammoma bodies. The tumor displaces but does not invade adjacent neural tissue and has a thin investing capsule. Meningiomas can invade bone without destruction by extension along haversion canals. Adjacent bone is hyperostotic in 25% of cases.²⁶

Many histopathologic classifications have been proposed, but a single, widely used system distinguishes among syncytial, transitional, fibrous, angioblastic, and sarcomatous types.²⁷ The specific histopathologic subtypes and growth characteristics are reviewed elsewhere.²⁸ In the posterior fossa, meningiomas usually arise on the posterior surface of the petrous bone, away from or at the edge of the internal auditory canal, or along the sigmoid sinus. Because they usually arise outside of the canal, these tumors may become large before producing signs and symptoms of cranial nerve VIII compression. Most meningiomas eventually involve cranial nerve VIII.

Diagnostic Studies

Audiovestibular Testing

It is impossible to distinguish acoustic neuromas from meningiomas on the basis of audiovestibular testing. Because meningiomas cause cranial nerve VIII compression, auditory and vestibular test findings become abnormal in a pattern similar to that with acoustic neuromas. Because meningiomas usually do not arise within the internal auditory canal, the sensitivity of audiometric and vestibular testing is lower with meningiomas than with acoustic neuromas. For instance, only 75% of patients with meningiomas exhibit an abnormal ABR.

Imaging

Table 177-2 summarizes features of meningiomas that assist in differentiating them from acoustic neuromas. Unlike acoustic neuromas, meningiomas usually are eccentric to the porus. Whereas acoustic neuromas seldom herniate into the middle fossa, approximately 60% of meningiomas extend to the middle fossa.¹⁹ Meningiomas usually are hemispheric because of their broad-based attachment to the posterior petrous wall, accounting for the obtuse bone-tumor angles in 75% of meningiomas. Unlike the site of origin of acoustic neuromas, that of posterior fossa meningiomas is varied (Figs. 177-4 and 177-5).

Figure 177-4. Cerebellopontine angle meningioma with classic features on magnetic resonance imaging (MRI) and computed tomography (CT). A, Post-gadolinium T1-weighted MRI: The tumor is eccentric to the internal auditory canal (IAC) and broadly dura-based, with obtuse angles at the bone-tumor interface. This hemispherical tumor shows uniform enhancement. Note the lack of IAC involvement. B, T2-weighted MRI: The tumor is homogeneously isointense to gray matter. C, CT bone windows. The tumor shows extensive hyperostosis underlying the dural base.

(From Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003:245-250.)

Figure 177-5. Cerebellopontine angle meningioma with intracanalicular extension, demonstrated by magnetic resonance imaging. A, Post-gadolinium T1-weighted image. Typical hemispherical tumor with intracanalicular extension. The dural tail of the tumor extending posteriorly is best seen on post-gadolinium images. B, T2-weighted image demonstrates the cerebrospinal fluid spaces and clefts surrounding the tumor. C, Constructive interference in steady-state (CISS) image. The internal auditory canal extension and margins of the tumor are better delineated on CISS images than on T2-weighted images.

(From Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003:245-250.)

On CT, approximately two thirds of meningiomas are hyperintense relative to the brain. Unlike acoustic neuromas, meningiomas are homogeneous and occasionally calcified. They show homogeneous enhancement with iodine infusion, which usually permits differentiation from acoustic neuromas. Hyperostosis of adjacent bone is infrequent but characteristic of meningiomas when present.

On MRI, meningiomas are of extremely variable intensity on T2-weighted images and isointense or slightly hypointense to brain on T1-weighted images. The different signal intensities among meningiomas correspond to different histopathologic subtypes.²⁷ Surface flow voids on MRI correspond to marginal pial blood vessels, and arborizing flow voids represent active feeders to the tumor. Calcification and cystic foci cause heterogeneity on MRI images of meningiomas.

Primary Cholesteatomas

Primary cholesteatomas (epidermoids) consist of a stratified squamous epithelial lining surrounding desquamated keratin, which originates from epithelial rests within the temporal bone or CPA. These lesions usually are slow-growing, and symptoms often do not become apparent until the second to fourth decades of life. As the lesions expand, compression and irritation of surrounding structures produce the signs and symptoms.

Primary cholesteatomas arise adjacent to the brainstem. Because they expand into the area of least resistance, primary cholesteatomas have variable shapes with irregular surfaces. They may burrow into crevices on the surface of the brain or extend in a dumbbell pattern into the middle fossa.

Diagnostic Studies

Auditory Testing

Auditory testing does not show any distinguishing features. As with other retrocochlear lesions, speech discrimination is poorer than would be expected from the degree of pure-tone loss. ABR also is frequently abnormal in primary cholesteatomas.³⁰

Imaging

CT and MRI are useful to differentiate primary cholesteatomas from other lesions. Epidermoids originating in the CPA should be distinguished from arachnoid cysts, and epidermoids in the petrous apex should be distinguished from the much more common cholesterol granulomas. This distinction affects therapy because adequate management for primary cholesteatoma requires excision, whereas drainage is sufficient for cholesterol granulomas and arachnoid cysts.

On CT, primary cholesteatomas are less dense than brain (having a density approximating that of cerebrospinal fluid [CSF]) and exhibit no enhancement with intravenous contrast. These lesions have irregular margins and are eccentric to the porus acusticus. Enhancing components suggest an associated malignancy.³¹ With MRI, primary cholesteatomas are inhomogeneous and hypointense to brain on T1-weighted images, and homogeneous and isointense or hyperintense to brain on T2-weighted images (Fig. 177-6). Schwannomas, meningiomas, and chondromas are similar to primary cholesteatomas by intensity criteria; however, epidermoids are differentiated because they are nonenhancing.

Figure 177-6. Cerebellopontine angle (CPA) epidermoid, demonstrated by magnetic resonance imaging. A and B, T1-weighted axial and sagittal images after gadolinium administration. The CPA mass extending to the prepontine cistern is isointense to cerebrospinal fluid (CSF) and shows no enhancement. C, T2-weighted image. The tumor, with irregular, cauliflower-like margins, is isointense to CSF, as in T1-weighted images, and is compressing the fourth ventricle. The internal auditory canal is normal. D, On this fluid-attenuated inversion recovery (FLAIR) sequence image, the epidermoid has heterogeneous hyperintense foci, enabling differentiation of an epidermoid from an arachnoid cyst in this case.

(From Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003:245-250.)

Arachnoid cysts of the CPA are difficult to distinguish from epidermoids on CT and MRI. Both lesions are of CSF density and nonenhancing; compared with primary cholesteatomas, however, arachnoid cysts have smoother surfaces, and special MRI sequences such as diffusion-weighted images (DWIs) may differentiate the intensity of the epidermoid from the CSF intensity of the arachnoid cyst (Fig. 177-7).

Figure 177-7. Cerebellopontine angle arachnoid cyst, demonstrated by magnetic resonance imaging. A, T1-weighted image. B, T2-weighted image. C and D, T1-weighted coronal and axial images, respectively. The cyst is isointense to CSF on all sequences. It is homogeneous, with a smooth surface, which can be helpful in differentiating it from an epidermoid.

(From Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003:245-250.)

Facial Nerve Neuroma

Facial nerve neuromas (schwannomas) are uncommon benign neoplasms of Schwann cells that may arise anywhere along the course of the facial nerve.

Diagnostic Studies

Auditory Testing

The mechanisms for conductive hearing impairment and SNHL have already been described earlier in this chapter. Impedance testing may reflect motor fiber impairment on ipsilateral reflex testing or cranial nerve VIII involvement on contralateral reflex testing. ABR testing of tumors arising in the internal auditory canal shows abnormalities similar to those seen with acoustic neuroma.¹³

Electroneurography

Electroneurography, also known as electroneuronography (ENoG), measures the muscle response to a maximal bipolar stimulation of the facial nerve near the stylomastoid foramen. ENoG potentials may be reduced in facial nerve neuroma even when no facial weakness or tic is present, whereas ENoG findings remain normal in acoustic neuroma until the tumor becomes very large. Some reports suggest that the value of preoperative ENoG in patients with acoustic neuroma lies in predicting whether the lesion actually represents a facial nerve tumor.^33,34

Imaging

Intratemporal facial nerve lesions may produce bone destruction. Because the clinical presentation and audiometric study findings in facial neuroma are not distinct, CT and MRI constitute the mainstays of diagnosis.

Because facial nerve neuromas are histologically identical to acoustic neuromas, they have the same enhancement characteristics. It usually is impossible to identify these lesions using CT. Anterosuperior erosion of the internal auditory canal or erosion of the labyrinthine facial nerve canal if present may be the only diagnostic clue.¹⁹ More distal tumors enlarge the geniculate ganglion and fallopian canal.

As with CT, MRI of facial neuroma produces imaging characteristics identical to those of acoustic neuromas (Fig. 177-8). The preoperative diagnosis of intracranial facial neuroma is difficult. Early facial nerve symptoms are an obvious warning that a CPA lesion may rarely be a facial nerve neuroma. Patients diagnosed with CPA neuromas should be warned preoperatively of a 1% risk of facial nerve neuroma. This possibility is emphasized if findings on preoperative ENoG are abnormal on the tumor side. In some tumors, the lesion may extend from the CPA through the temporal bone course of the nerve to the extratemporal nerve in the parotid gland.

Figure 177-8. Facial schwannoma of internal auditory canal, demonstrated by magnetic resonance imaging. A and B, Post-gadolinium T1-weighted and T2-weighted images. The patient presented with progressive sensorineural hearing loss, with later development of facial palsy. The enhancing intracanalicular tumor is indistinguishable from an acoustic schwannoma. Note also the extension of the tumor to the geniculate ganglion.

(From Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003:245-250.)

Other Cranial Nerve Neuromas

Neuromas may arise on any other cranial nerve in the posterior fossa. On imaging studies, nonacoustic neuromas have the same characteristics as those of acoustic neuromas except for their location. Overall, acoustic neuromas represent 95% of intracranial schwannomas, and trigeminal neuromas are the next most common; however, schwannomas also have been reported on cranial nerves IX, X, XI, and XII. These lesions are distinguished by their different location and by symptoms of dysfunction of the cranial nerve of origin.

Trigeminal neuromas arise intradurally, from the nerve root in the CPA and Meckel’s cave, and extradurally, from the gasserian ganglion in the middle cranial fossa.³⁵ Typically these lesions enlarge Meckel’s cave and produce hypesthesia of the face (Fig. 177-9).

Figure 177-9. Cystic trigeminal schwannoma, demonstrated by magnetic resonance imaging. A, T1-weighted image. B, Gadolinium T1-weighted image. C, T2-weighted image. The bulk of the tumor lies in the posterior fossa, with a small middle cranial fossa component enlarging the left Meckel’s cave (arrow). Note the similarity of the tumor to an arachnoid cyst (see Figure 177-7) on the noncontrast images (A and C). The tumor is nearly of CSF intensity because of the predominance of the intratumoral cystic components.

(From Jackler RK, Brackmann DE, eds. Neurotology. St Louis: Mosby; 1994.)

Neuromas of cranial nerves IX, X, and XI produce smooth enlargement of the jugular foramen and classically cause hypesthesia and weakness of the palate, vocal cord, and shoulder, respectively (Fig. 177-10). If they arise in the posterior fossa, these tumors may grow to a large size before producing predominantly acoustic or cerebellar signs. Accurate preoperative differentiation of these tumors from acoustic neuromas is important because residual hearing is more likely to be preserved with lower cranial nerve neuromas. Hypoglossal neuromas produce motor hemiatrophy of the tongue and enlargement of the hypoglossal canal on radiography.

Figure 177-10. Jugular foramen schwannoma, demonstrated by magnetic resonance imaging (MRI) and computed tomography (CT). A and B, Axial and coronal T1-weighted post-gadolinium images. The large tumor in the left jugular fossa is extending inferiorly. It shows avid enhancement. C, Coronal T2-weighted image depicts the extracranial portion of the tumor, which is hyperintense to gray matter. D, CT bone windows, coronal image. Expansion of the jugular foramen is symmetric, and the margins are well defined.

(From Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003:245-250.)

Glomus Tumors

Glomus tumors (paragangliomas) are discussed in detail in Chapter 176. Because glomus jugulare tumors and glomus vagale tumors may extend into the posterior fossa, they constitute an important consideration in the differential diagnosis of skull base neoplasms.

The first symptom often is pulsatile tinnitus, after which a conductive hearing loss develops. Involvement of the nerves of the jugular foramen and the hypoglossal nerve causes progressive neurologic deficits related to those nerves.

The characteristic appearance on CT with bone windows is irregular destruction of the jugular foramen, as compared with jugular foramen schwannomas, which produce smooth enlargement (Fig. 177-11). The vascular pattern of paragangliomas on angiography is characteristic (Fig. 177-12), and biopsy of these lesions is not indicated.³⁶ Diagnostic angiography should be performed concomitantly with preoperative embolization when surgical resection is planned.

Figure 177-11. Glomus jugulare tumor, demonstrated by computed tomography. A, Axial image shows demineralization of the bony margins of the jugular foramen on the right side (black arrows). There is demineralization of the fine cortical line usually demonstrated along the lateral aspect of the vascular portion of the jugular foramen. Compare with intact cortical plate on the opposite side (white arrow). Note the demineralization of the bony wall separating the carotid canal from the jugular foramen. B, Enlarged axial image shows the demineralization of the lateral cortical plate (arrowhead), as well as slight demineralization of the bony plate between the carotid canal and the jugular foramen (white arrow). Note the proximity of the demineralized bone to the vertical section of the facial nerve canal (black arrow). C, Axial image enlarged through the normal side shows the intact white cortical line (arrow) along the lateral aspect of the jugular foramen. The integrity of this line essentially excludes a glomus jugulotympanic tumor. D, Coronal image shows demineralization of the margins (arrowheads), as well as the soft tissue of the intratympanic portion of the tumor (arrow).

(From Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003.)

Figure 177-12. Glomus jugulare tumor. As seen on this selective external carotid angiogram, the characteristically hypervascular tumor is supplied by the hypertrophic ascending pharyngeal artery (arrow). Note the early draining vein (arrowhead) seen during the midarterial phase. Considerations in the differential diagnosis include other hypervascular tumors such as metastatic hypernephroma, metastatic pheochromocytoma, and myeloma.

(From Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003.)

MRI produces a unique salt-and-pepper mixture of intensities on T1- and T2-weighted images. Arborizing flow voids reveal the prominent tumor vessels of this lesion. Lo¹⁹ described two limitations of MRI in evaluating paragangliomas: (1) bone changes and the relation of tumor to bone landmarks are not visualized; and (2) distinguishing tumor from bone marrow is difficult, especially on gadolinium-enhanced T1-weighted images. Thus, MRI may provide complementary information about infralabyrinthine and intracranial tumor extensions, but bone algorithm CT is the cornerstone of imaging evaluation in paragangliomas.

The combination of CT, MRI, and angiography can provide extensive information about involvement of the internal carotid artery. Magnetic resonance angiography (MRA) is an adjunct to intra-arterial catheterization and formal angiography (see Fig. 177-12). Angiography is a necessary step in preoperative embolization of glomus tumors; however, it is unlikely that MRA can replace direct intravascular angiography in glomus tumor management. For resection of skull base neoplasms involving the posterior fossa, if surgical resection requires manipulation of the artery, preoperative assessment of the adequacy of collateral flow by way of the circle of Willis is necessary. Temporary balloon occlusion of the carotid artery in combination with radioisotope imaging or xenon-enhanced CT can accurately quantify collateral blood flow.³⁷

Arachnoid Cysts

Arachnoid cysts are thin-walled sacs that contain yellow, entrapped CSF. The current theory is that these lesions represent congenital developmental anomalies.³⁸ Symptoms are produced by the mass effect of the cyst on surrounding structures and may be similar to the symptoms of acoustic neuromas. These patients may experience mild to profound hearing loss with a retrocochlear pattern.³⁹

These lesions have CT and MRI characteristics similar to those of epidermoids. Enlargement of the internal auditory canal often is noted but is not characteristic of these lesions. The typical appearance is that of a smooth-surfaced lesion, which on CT approximates CSF and is nonenhancing and on MRI exhibits isointensity or hypointensity to brain on T1-weighted images and hyperintensity to brain on T2-weighted images (see Fig. 177-7). In most patients, no direct intervention is necessary. When treatment is necessary to control symptoms, management of these lesions is not total resection. Instead, surgical drainage by way of retrolabyrinthine exposure is the usual recommended therapy; however, diuretic therapy provides symptomatic relief in a few patients.

Hemangiomas

Hemangiomas are hamartomatous neoplasms originating from blood vessels. Although benign, they produce symptoms by compression of adjacent structures. Hemangiomas are capillary or cavernous.

Capillary hemangiomas typically arise in the area of the geniculate ganglion in association with a perigeniculate capillary plexus.⁴⁰ The lesion is characterized by a progressive facial weakness despite being much smaller than facial nerve neuromas. The expanding lesion with the production of pulsatile tinnitus may expose the upper basal turn of the cochlea. CT shows a smooth enlargement of the geniculate ganglion and enlargement of the labyrinthine portion of the fallopian canal by a soft tissue mass. Although capillary hemangiomas are enhancing, they produce facial weakness at such an early stage that only subtle findings may be apparent on CT, and a very small enhancement in the area of the labyrinthine segment may be the only finding. Other CT findings include honeycomb bone, irregular and indistinct bone margins, and intratumoral bone spicules (Fig. 177-13). These findings contrast with those in facial nerve neuromas, which are larger, more obvious lesions with sharp bone margins.

Figure 177-13. Intratemporal vascular tumor in the region of the geniculate ganglion, demonstrated by computed tomography (CT) and magnetic resonance imaging. A, CT image through the region of the geniculate ganglion shows expansion of the facial nerve (arrow). Irregular bone is seen extending medially toward the petrous apex (arrowhead). B, In this CT image, at a level slightly superior to A, demineralization is evident in the region of the labyrinthine segment of the canal (black arrow). Note that the lesion (arrowheads) contains small spicules of bone. C, Coronal CT image shows the demineralized bone (arrows). The abnormality extends inferiorly to enlarge the FNC (arrowhead) and involve the facial nerve. D, Coronal T2-weighted image shows the relatively high signal (arrows) of the vascular lesion. The fluid within the cochlea (arrowhead) is noted for comparison and orientation. E, Coronal postcontrast gadolinium T1-weighted magnetic resonance image shows the enhancement (arrows) of the lesion.

(From Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003.)

Cavernous hemangiomas are the second type of hemangioma. These lesions arise in the internal auditory canal and produce symptoms typical for an acoustic neuroma. Although they tend to produce symptoms more rapidly than do acoustic neuromas, these two tumor types are identical on CT. However, on MRI, cavernous hemangiomas tend to be slightly more hyperintense than the typical acoustic neuroma.

Cholesterol Granulomas

A cholesterol granuloma arises in the pneumatized spaces of the temporal bone presumably as a result of occlusion of the air cell system. Hemorrhage into the air cells results in a foreign body reaction and progressive granuloma formation. An expansile lesion of the temporal bone results, with extension into the CPA and resultant signs and symptoms of cranial nerve VIII dysfunction. A recent theory suggests that exposed bone marrow may hemorrhage into the pneumatized petrous apex air cells, producing the foreign body reaction and granuloma formation.⁴¹ This newer concept more adequately explains the presence of cholesterol granuloma in well-pneumatized temporal bones.

The CT and MRI findings with this lesion are distinguishable from those with the other common lesions of the petrous apex (Table 177-3). On CT, the lesions result in a punched-out appearance of the temporal bone with an isodense mass of the petrous apex that does not enhance; however, rim enhancement is evident with intravenous contrast. On MRI, T1- and T2-weighted images are hyperintense with respect to brain (Fig. 177-14). Primary cholesteatomas are the main lesions from which cholesterol granulomas should be distinguished. Cholesterol granulomas are much more common and occur 20 times more frequently than epidermoids. Overall, cholesterol granulomas are relatively uncommon compared with acoustic neuromas. Only 1 cholesterol granuloma is identified for every 35 acoustic neuromas diagnosed. Imaging distinguishes petrous apex epidermoids from petrous apex cholesterol granulomas. On CT, epidermoids show rim enhancement. With MRI, cholesterol granulomas are hyperintense on T1- and T2-weighted images, whereas epidermoids are hyperintense only on T2-weighted images.

Figure 177-14. Petrous apex cholesterol granuloma, demonstrated by magnetic resonance imaging. A to C, T1-weighted, proton density, and T2-weighted images, respectively. The lesion is hyperintense on all three images. Note the hypointense rim medial to the lesion accentuated on B and C, attributable to a hemosiderin deposit or chemical shift artifact.

(From Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003.)

Total excision of cholesterol granulomas usually is unnecessary. Drainage may be achieved by a transmastoid or transcanal infralabyrinthine approach with preservation of cranial nerve function. The transcanal, infracochlear, hypotympanotomy approach is preferred because it affords dependent drainage and the possibility of revision, if necessary, through a myringotomy.⁴² In cases with unfavorable anatomy for infracochlear or infralabyrinthine drainage, middle fossa drainage is an option. Suprisingly, long-term follow-up imaging has demonstrated that middle fossa catheter drainage to the mastoid remains patent.⁴³

Asymmetric Petrous Apex Pneumatization

Although asymmetric pneumatization of the petrous apex does not represent a true neoplasm, this condition can be confused with and should be distinguished from true neoplasms. The fat content of bone marrow in a nonpneumatized petrous apex can produce a hyperintense appearance on T1-weighted noncontrast MRI (Fig. 177-15). The lack of bone destruction or expansion on CT, absence of contrast enhancement with gadolinium, and hypointensity on T2-weighted images distinguish this condition from a neoplasm.

Figure 177-15. Asymmetric pneumatization of the petrous apex, demonstrated on an axial T1-weighted image (A) and an axial computed tomography (CT) scan (B). Note that the area of hyperintensity on the T1-weighted image could be confused with a neoplasm. The CT scan reveals extensive pneumatization on the contralateral side and limited pneumatization on the side with hyperintense signal on magnetic resonance imaging.

Mucocele, Mucus Retention Cysts, and Petrous Apex Effusion

Petrous apex air cells may become obstructed, resulting in retained secretions as a mucus retention cyst in the petrous apex (Fig. 177-16) or an expansile mucocele (Fig. 177-17). CT reveals a nonenhancing lesion limited to the petrous apex air cell system. The MRI appearance is consistent with a mucus-filled lesion (hypointense on T1-weighted and hyperintense on T2-weighted images). A symptomatic mucocele behaves like chronic mastoiditis, with pressure symptoms that require drainage for relief. Occasionally, retained petrous apex fluid also can cause symptoms even without mucocele. Medical treatment as for a middle ear effusion or surgical drainage usually is therapeutic.⁴³

Figure 177-16. Retained mucus in petrous apex air cells, demonstrated on an axial T2-weighted magnetic resonance image (A) and an axial computed tomography (CT) scan (B). Note that the mucus is hyperintense on the T2-weighted image, and the CT scan identifies fluid density within the petrous apex air cells.

Figure 177-17. A to C, Axial magnetic resonance images of a petrous apex mucocele: T1-weighted without gadolinium (A), T1-weighted with gadolinium (B), and T2-weighted (C). The lesion is hypointense on the T1-weighted image and shows only rim enhancement with the addition of gadolinium. Note the hyperintensity of the mucocele on the T2-weighted image. D, Axial computed tomography scan of the same lesion. Note that the expansile lesion has eroded the petrous apex.

Petrous Carotid Artery Aneurysms

Although aneurysms of the horizontal carotid artery are rare, they may appear as expansile, well-defined masses (Fig. 177-18). Their preoperative identification is critical for obvious reasons. Carotid aneurysms may be confused with chondrosarcomas on the basis of radiologic appearance.

Figure 177-18. Axial T2-weighted magnetic resonance image of a petrous carotid aneurysm. The lesion is heterogeneous and shows decreased signal intensity within the thrombus. The circumscribing areas of low signal intensity (arrowheads) probably represents hemosiderin. The lumen (long thin arrow) has intermediate signal intensity as a result of slow flow as noted on the arteriogram (not shown) and is difficult to distinguish from thrombus. High-signal-intensity fluid is seen in multiple mastoid air cells (short wide arrow).

(From Tsuruda JS, Halbach VV, Higashida RT, Mark AS, Hieshima GB, Norman D. MRI evaluation of large intracranial aneurysms using cine low flip angle gradient-refocused imaging. Am J Neuroradiol. 1988;9:418.)

Giant Cell Tumors

Giant cell tumors are extremely rare primary neoplasms of the temporal bone. They originate from undifferentiated cells of the supporting connective tissue and consist of multinucleated giant cells in a background of spindle-shaped stromal cells. Patients with this unusual condition demonstrate retrocochlear signs and symptoms. CT typically shows a diffuse lesion of the temporal bone compressing the contents of the internal auditory canal.

Metastatic Tumors

Tumors may metastasize to the CPA from other sites, including lung, breast, prostate, oropharynx, and skin (i.e., cutaneous melanomas).⁴⁴ Tumors from these sites are distinguished by their rapid progression of symptoms and associated neurologic signs, in addition to hearing loss and dizziness. Associated lytic lesions in the petrous apex constitute another distinguishing feature of metastatic tumors. Rapid impairment of hearing, other cranial neuropathies, and brainstem dysfunction suggest malignant neoplasms of the posterior fossa, especially in a patient with a history of another malignancy.

Chordomas

Chordomas are dysontogenetic neoplasms that arise in remnants of the embryonic notochord. Although more than one half of these tumors arise in the sacrococcygeal region, more than one third occur at the skull base in the region of the clivus or, less commonly, the upper cervical vertebrae.⁴⁵ The prominent clinical characteristics are extensive bone destruction and progressive cranial nerve palsies. Extension of clivus chordomas to the petrous apex, sphenoid, or CPA is not unusual. Although fronto-orbital headache and vision complaints (e.g., limited visual fields, diplopia, loss of acuity) are more common, occasionally the initial symptoms reflect extension into the CPA. On CT, the bone destruction is readily apparent, and the masses are homogeneous with moderate enhancement and a greater density than that of bone (Fig. 177-19). MRI reveals isointensity on T1-weighted images and hyperintensity on T2-weighted images (Fig. 177-20).

Figure 177-19. Axial computed tomography scan shows a large midline chordoma involving the petrous apex bilaterally.

Figure 177-20. Noncontrast T1-weighted axial magnetic resonance image of the same chordoma as in Figure 177-19.

Chondrosarcomas

Chondrosarcomas of the skull base also may arise in the CPA. They are clinically indistinguishable from chordomas except that they are centered more laterally. CT shows characteristic bone destruction and invasiveness. MRI shows this lesion well, which is hyperintense on T2-weighted images in the area of bone destruction in the skull base (Figs. 177-21 and 177-22).

Figure 177-21. Right petrous apex chondrosarcoma. A to C, On gadolinium T1-weighted magnetic resonance imaging, the mass is markedly but inhomogeneously enhancing, with components containing small, poorly enhancing or nonenhancing foci in Meckel’s cave (arrow in A) and in the petrous apex (arrows in B), as well as involvement of the longus colli muscle and jugular fossa (arrows in C). Note the markedly enhancing transdural components in the posterior fossa and the dural “tail” in A and B.

(From Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003.)

Figure 177-22. Skull base chondrosarcoma, demonstrated by magnetic resonance imaging. A and B, Axial T1-weighted pre- and postgadolinium images. C, Axial T2-weighted images. D, Coronal T1-weighted image after gadolinium enhancement.

(From Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003.)

Lipomas

Lipomas are hamartomas that are thinly encapsulated and poorly delineated. They appear as soft, multilobular masses of typical adult adipose tissue. Lipomas within the internal auditory canal can produce symptoms typical of an acoustic neuroma. Although on CT these lesions are less dense than neuromas, MRI is diagnostic. The lesions are hyperintense on T1-weighted images, nonenhancing with gadolinium, and hypointense on T2-weighted images (Fig. 177-23). Fat-suppression techniques with T1-weighted sequences are confirmatory because the previously identified lesion that was hyperintense on nonenhanced T1-weighted images is rendered hypointense with fat suppression (Fig. 177-24).

Figure 177-23. Lipoma of the right internal auditory canal, demonstrated by magnetic resonance imaging. A, T1-weighted image without gadolinium enhancement. B, Postgadolinium T1-weighted image. The lipoma has high signal intensity without gadolinium, which does not change after administration of contrast.

(From Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003.)

Figure 177-24. Cerebellopontine angle lipoma, demonstrated by magnetic resonance imaging. A and B, T1-weighted and fat-saturated post-gadolinium images. The signal intensity of the tumor is similar to that of bone marrow. Note the signal loss of the tumor and fatty bone marrow on the fat-saturated images. No enhancement of the lesion is seen. C, T2-weighted image. The lipoma typically is moderately hypointense and is seen with surrounding signal void vessels.

(From Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003.)

Dermoid Tumors

A dermoid is a skin-lined cystic tumor containing dermal and adnexal structures. The lining of the cyst is mature, stratified squamous epithelium. A dermoid is a slowly expanding lesion that produces symptoms similar to those of a primary cholesteatoma. It may be differentiated on CT as a nonhomogeneous cystic mass that contains calcium but that is otherwise less dense than brain.

Teratomas

Teratomas arise from multipotential cells that differentiate into various tissues, representing more than one germ layer. They contain ectodermal, mesodermal, and endodermal tissues. Carcinomatous or sarcomatous changes occur in 10% to 35% of these tumors. When malignant degeneration occurs, symptoms progress rapidly; otherwise, slow progression is the rule, making the lesions indistinguishable from other benign CPA tumors. CT shows a nonhomogeneous lesion of less density than brain without enhancing characteristics.

Hemangioblastomas

Hemangioblastomas are tumors of blood vessel origin occurring primarily in the cerebellum. They also may occur in the cerebral hemispheres and in association with a similar retinal tumor and may be multicentric. Although histologically benign, hemangioblastomas may produce major neurologic dysfunction by compression of the brainstem.

Rapidly progressive signs and symptoms of cerebellar dysfunction are characteristic of this tumor. Hearing and balance are likely to remain normal. Imaging studies identify a tumor intrinsic to the cerebellum that may extend into the CPA.

Medulloblastomas

Medulloblastomas arise from the cells of the external granular layer of the cerebellar folia. They may appear as exophytic masses on the cerebellum with extension into the CPA. Symptoms are produced by destruction of cerebellar tissue and mass effect of the tumor on the CPA’s adjacent structures.

Medulloblastomas are characterized by a rapid development of symptoms. In addition to hearing loss and dizziness, associated neurologic findings include facial weakness, dysmetria of speech and hand motion, perverted nystagmus, and abnormal peripheral reflexes. Temporal bone imaging findings are normal, but CT and MRI show a lesion intrinsic to the cerebellum.

Brainstem Gliomas

Exophytic gliomas may arise on the surface of the pons and grow into the CPA. Gliomas also may be intrinsic to the brainstem. Exophytic gliomas may produce signs and symptoms similar to those of acoustic neuromas, whereas the intrinsic variety produces predominantly long tract signs.

Patients with exophytic lesions may be diagnosed preoperatively to have acoustic neuromas. Long tract signs in association with characteristic brainstem distortion on imaging make preoperative diagnosis possible in the patients with intrinsic lesions.

Tumors of the Fourth Ventricle

Malignant Choroid Plexus Papillomas and Ependymomas

Choroid plexus papillomas and ependymomas arise from the fourth ventricle and cause CPA symptoms by growing through the foramen of Luschka. In this location, they produce early signs of cranial nerve VIII dysfunction. In malignant choroid plexus papillomas, CT shows a mass with enhancing characteristics of a schwannoma (Fig. 177-25). These tumors are distinguishable from an acoustic schwannoma by being separate from the internal auditory canal. Ependymomas may calcify, and those portions have variable imaging characteristics. At MRI, both lesions are isointense to brain on T1-weighted images and mildly hyperintense to brain on T2-weighted images (Fig. 177-26). Malignant ependymomas require multimodality therapy. The role of surgery is for biopsy, brainstem decompression, and management of hydrocephalus.

Figure 177-25. CPA choroid plexus papilloma, demonstrated by contrast computed tomography. The tumor is located at the foramen of Luschka. It is more sharply defined than an ependymoma. Like a giant aneurysm, it is detached from the petrous bone, in contrast with schwannomas and meningiomas.

(From Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003.)

Figure 177-26. CPA ependymoma, demonstrated by magnetic resonance imaging. A and B, Pre- and postgadolinium images. The mass protruding into the right CPA from the foramen of Luschka shows clear demarcation and strong enhancement. C, T2-weighted image. The tumor is hyperintense and does not extend into the IAC.

(From Maya MM, Lo WWM, Kovanlikaya I. Temporal bone tumors and cerebellopontine angle lesions. In: Som PM, Curtin HD, eds. Head and Neck Imaging. 4th ed. Philadelphia: Mosby; 2003:240-250.)

Translabyrinthine Approach

Basic Approach

A transmastoid labyrinthectomy and skeletonization of the sigmoid sinus and posterior fossa dura permit identification of the facial nerve with wide exposure of the internal auditory canal and CPA (Fig. 177-27).

Figure 177-27. Surgical exposure in the translabyrinthine approach to the cerebellopontine angle (CPA). This approach permits direct access to the CPA without retraction of the cerebellum.

Technique

The following discussion details patient positioning, preparation, monitoring, and tumor removal for the translabyrinthine approach. The same techniques are used for the other approaches except when otherwise specified.

The patient is positioned supine with the head turned to the opposite side. Head-holding pins or supports are not used. Electromyography (EMG) electrodes for intraoperative facial nerve monitoring are inserted into the orbicularis oris and orbicularis oculi. Intravenous antibiotic is administered when the patient arrives in the operating room. Antibiotic also is used in the irrigation solution. Hyperventilation usually is sufficient for brain relaxation.

The curved postauricular incision is made with the apex 3 cm posterior to the postauricular crease. The soft tissue and periosteum are elevated from the mastoid and adjacent occipital bone, and self-retaining retractors are inserted.

The bone removal is accomplished in four stages and is performed using the operating microscope, a high-speed drill, and continuous suction-irrigation.⁵ Most of the drilling is performed with cutting burrs; however, diamond burrs are used when drilling is performed on the dura or venous sinuses. The first stage is a complete mastoidectomy. Posteriorly, the bone is removed beyond the sigmoid sinus, and the sinus is skeletonized. The extent of bone removal posterior to the sinus and decompression of the posterior fossa depends on the size of the tumor. Greater posterior removal provides greater exposure of the CPA. Superiorly, the middle fossa plate is identified and thinned. Anteriorly, the facial nerve is identified in its vertical segment but left covered with bone for protection against inadvertent burr trauma.

The second stage is complete labyrinthectomy. The horizontal, superior, and posterior semicircular canals are systematically removed. Particular care is required in working along the inferior border of the horizontal canal because of the proximity of the facial nerve, and along the ampulated end of the posterior canal because this end lies medial to the facial nerve.

The third stage of bone removal consists of actual decompression of the middle and posterior fossa dura and removal of bone around the internal auditory canal. The bone overlying the posterior fossa is removed with cutting and diamond burrs, except for an island of bone over the sigmoid sinus (“Bill’s island”). The mastoid emissary vein should be transected to permit retraction of the sinus and posterior fossa dura. Accordingly, bleeding from the emissary should be controlled with sutures, cautery, or bone wax. The petrous ridge, which is the junction between the middle fossa dura and the posterior fossa dura, should be removed. The superior petrosal sinus, which lies under the petrous ridge, is sometimes adherent to the bone. The resulting bleeding is controlled with bipolar cautery or extradural packing with oxidized cellulose strips (Surgicel). The entire middle fossa dura and posterior fossa dura adjacent to the mastoid are exposed.

The final stage of bone removal consists of skeletonization of the internal auditory canal. The orientation of this structure is roughly parallel to the external auditory canal. Thus, the fundus of the canal is just medial to the floor of the vestibule, which was exposed during the labyrinthectomy. By contrast, identification of the porus requires extensive bone removal as the internal auditory canal is dissected medially. The basic principle of exposing the internal auditory canal is that all bone removal should be accomplished before the dura is opened, and complete bone removal requires decompression of 270 degrees around the circumference of the canal to avoid obscuring any ledges of bone. By completing bone removal before opening the dura, the risk of accidental injury to the nerves of the internal auditory canal is minimized. The inferior border of the canal is skeletonized first. This edge of the canal is identified by gradually enlarging a trough between the inferior edge of the vestibule and the jugular bulb. The cochlear aqueduct usually is identified as the dissection proceeds anteromedially. This structure becomes the inferior limit of the dissection, thereby protecting the lower cranial nerves from injury. After the inferior border of the internal auditory canal has been identified, the bone overlying the porus acusticus is thinned with cutting and diamond burrs. The superior border of the canal is identified last, because the facial nerve is more susceptible to injury in this area. A burr size that will fit between the middle fossa dura and the superior border of the internal auditory canal is used to expose the superior border. The entrance of the facial nerve into the canal can be positively identified just medial to a vertical crest of bone, “Bill’s bar,” along the superior aspect of the fundus. At this point, the bony exposure has been completed, and tumor removal can begin (Fig. 177-28).

Figure 177-28. Surgical exposure in the translabyrinthine approach to the cerebellopontine angle (CPA). This approach permits positive identification of the facial nerve in its bony canal at the fundus of the internal auditory canal and at the brainstem. Inset shows skin incision in relation to the CPA.

Retrosigmoid (Suboccipital) Approach

Basic Approach

Transmastoid decompression of the sigmoid sinus and a retrosigmoid craniotomy permit access to CPA tumors without disturbing the labyrinth. Intradural removal of the posterior wall of the internal auditory canal permits direct access to the medial two thirds of the canal (Fig. 177-29).

Figure 177-29. Surgical exposure in the retrosigmoid approach to the cerebellopontine angle (CPA). This approach permits wide access to the CPA; however, the cerebellum is between the surgeon and the CPA. Thus, cerebellar support (retraction) is required to permit complete visualization of the CPA.

Technique

The soft tissues of the ear are reflected anteriorly and the muscles are widely reflected from the retromastoid region. Complete mastoidectomy is performed, and the sigmoid sinus is skeletonized and decompressed. The posterior semicircular canal also is skeletonized, and the posterior fossa dura up to the posterior semicircular canal is exposed. This area of exposed dura identifies the location of the posterior semicircular canal if the posterior lip of the internal auditory canal needs to be removed later in the procedure. A bone flap is removed posterior to the sigmoid sinus with the otologic drill. This flap can be preserved for replacement at the conclusion of the procedure. Alternatively, in cases with favorable venous anatomy, a craniectomy with skeletonization of the dura and sinus can be performed without entering the presigmoid mastoid air cells.

The dura is incised posterior to the sigmoid sinus, with care taken to avoid injury to the underlying vessels that may be adherent to the dura. Adequate drainage of CSF is necessary to minimize the need for cerebellar retraction. The cerebellum is exposed and supported with retractors to allow visualization of the tumor in the CPA. The neurosurgical techniques for tumor removal at the brainstem are the same regardless of the approach chosen. If the tumor is too large to permit proximal identification of the facial nerve, debulking of the tumor is performed. Once the debulking has been accomplished, the tumor is dissected from the facial nerve toward the porus acusticus (Fig. 177-30).

Figure 177-30. Surgical exposure in the retrosigmoid approach to the cerebellopontine angle (CPA). The mastoidectomy in conjunction with the retrosigmoid exposure permits anterior retraction of the sigmoid sinus, which improves anterior exposure. The mastoidectomy also allows positive identification of the posterior semicircular canal, which facilitates removal of the posterior lip of the internal auditory canal. Inset shows skin incision in relation to the CPA.

If the tumor extends into the internal auditory canal, the posterior lip of the canal is removed with diamond burrs. A dural flap is created lateral to the porus and reflected medially. Drilling of the posterior lip of the canal is continued until the lateral extent of the tumor is visualized. The anatomic limit for exposure of this posterior lip that will preserve hearing is the posterior semicircular canal. Attention to the area of dura previously exposed just posterior to the posterior semicircular canal will direct the surgeon to the precise location of the posterior canal when the posterior lip of the internal auditory canal is removed. Once the lateral limit of the tumor has been identified, tumor removal is accomplished in the same manner as with the translabyrinthine approach.

Unlike in the translabyrinthine approach, in which Bill’s bar permits definitive identification of the facial nerve at the fundus, in the retrosigmoid approach the surgeon relies primarily on the facial nerve monitor for identifying the nerve laterally in the internal auditory canal. Once the location of the nerves is known medially and laterally, the tumor usually can be removed with preservation of the facial nerve. With surgery attempting hearing preservation, the cochlear nerve also should be preserved. Particular care is taken to preserve the blood supply to the internal auditory canal, because hearing preservation depends on an intact blood supply.

After tumor removal, the edges of the dura are reapproximated, and the mastoid and the internal auditory canal are obliterated with abdominal fat. The bone flap is replaced and the wound is closed in separate layers. A compressive dressing is applied. Alternatively, fat with hydroxyapatite cement or titanium can be used.

Middle Fossa Approach

Basic Approach

A temporal craniotomy permits exposure of the internal auditory canal after identification of the superior semicircular canal and geniculate ganglion (Fig. 177-31).

Figure 177-31. A, Coronal view: surgical exposure in the middle fossa approach to the cerebellopontine angle (CPA). A temporal craniotomy permits retraction of the temporal lobe and access to the floor of the middle fossa and internal auditory canal (IAC). B, Axial view: surgical exposure in the middle fossa approach to the CPA. The shaded area indicates the area of bone removal on the floor of the middle fossa. Note that although wider exposure is possible at the porus, limited space is available between the cochlea and the superior semicircular canal as the surgeon approaches the fundus of the IAC.

Technique

The basic surgical exposure is illustrated in Figure 177-32A and B. Mannitol and furosemide are used in addition to hyperventilation for brain relaxation. A lumbar drain placed at the beginning of the procedure can facilitate dural elevation before open drainage of the cerebrospinal fluid. A preauricular incision is extended along the temporal region. The temporalis muscle may be reflected inferiorly or split and retracted to expose the squamosa of the temporal bone. A 5 cm × 5 cm craniotomy is centered over the zygomatic root.

Figure 177-32. A, Skin incision for the middle fossa approach. Although the traditional vertical incision is used in middle fossa facial nerve decompression, the larger flap facilitates bony exposure for tumor removal. B, Surgeon’s view of the internal auditory canal (IAC) via the middle fossa approach. Note the temporal lobe retractor in position. The proximity of the cochlea, labyrinth, and labyrinthine portion of the facial nerve at the fundus of the IAC requires precise identification of anatomic landmarks in this approach. C, Surgeon’s view of the extended middle fossa approach. The anterior limit of bone removal is the carotid artery and the posterior limit is the labyrinth. In the illustration, the middle meningeal artery at the level of the foramen spinosum has been transected, and the horizontal carotid artery is identified coursing parallel to the greater superficial petrosal nerve and medial to the foramen ovale. Continued dissection anteromedially permits access to the cavernous sinus.

The floor of the middle fossa is exposed by dissecting and elevating the dura from posterior to anterior to avoid injury to the geniculate ganglion, which can be dehiscent in up to 15% of patients. The principal landmarks for orientation are the greater superficial petrosal nerve and the arcuate eminence; however, these can be deceptive. Antidromic stimulation of the greater superficial petrosal nerve with the facial nerve stimulator can be helpful for orientation.⁵⁸ In addition, preoperative coronal CT can reveal the extent of pneumatization over the internal auditory canal and labyrinth.

Surgeons should be familiar with all techniques for identifying the internal auditory canal in middle fossa surgery, but some prefer to begin drilling medially near the porus acusticus. In this fashion, the cochlea and superior semicircular canal are not approached until the internal auditory canal has been identified. Wide skeletonization of this canal can be performed medially; however, the proximity of the cochlea and superior semicircular canal only permit unroofing the internal auditory canal laterally.

The dura is opened along the posterior aspect of the internal auditory canal. The facial nerve is freed from the tumor, and dissection proceeds in a medial to lateral direction. The canal is sealed with abdominal fat or temporalis muscle and the bone flap is replaced.

Retrolabyrinthine Approach

Basic Approach

A wide mastoidectomy with skeletonization of the posterior semicircular canal permits access to the CPA with preservation of the seventh and eighth cranial nerves (Fig. 177-33).

Figure 177-33. The retrolabyrinthine approach to the cerebellopontine angle (CPA). Although the exposure is limited anteriorly, removal of the bone posterior to the sigmoid sinus permits wide access to the CPA.

Technique

The incision and soft tissue exposure for the retrolabyrinthine approach are identical to those for the translabyrinthine approach. Mannitol and furosemide are used for brain relaxation. A complete mastoidectomy is performed. The fossa incudis and the lateral and posterior semicircular canals are identified. The facial nerve is skeletonized but kept covered with bone.

Because exposure with this approach depends on collapsing the sigmoid sinus posteriorly, bone is completely removed from the sigmoid sinus and the subocciput posterior to the sinus. Extensive exposure of the retrosigmoid dura is necessary for adequate exposure with this approach. The middle fossa dura is exposed and the petrous ridge is removed to increase the exposure.

The dura is opened as a flap that begins just medial to the sigmoid sinus and preserves the endolymphatic sac. The flap is held anteriorly with sutures and tumor dissection can be performed in the posterior fossa (Fig. 177-34). Once the CPA is entered and CSF is released, the cerebellum falls away. Accordingly, cerebellar retractors are not necessary to expose the cranial nerves in the CPA. Instead, the sigmoid is collapsed posteriorly with fenestrated suction.

Figure 177-34. Surgeon’s view of the cerebellopontine angle (CPA) with the retrolabyrinthine approach to the internal auditory canal. The dura is incised around the endolymphatic sac and reflected anteriorly. Wide posterior bone removal permits access to the CPA without a brain retractor, although suction is used to maintain the exposure. Inset shows skin incision in relation to the CPA.

When tumor removal is completed, the dura is closed with sutures. The defect is obliterated with abdominal fat with the optional use of hydroxyapatite cement or titanium mesh. Closure is performed in layers, and a compressive dressing is applied.

Transcochlear Approach

Basic Approach

As an extension of the translabyrinthine approach, the transcochlear approach affords additional exposure of the skull base. This is obtained by displacement of the facial nerve and removal of the cochlea to provide surgical access to lesions of the petrous tip and clivus (Fig. 177-35).

Figure 177-35. The transcochlear approach to the cerebellopontine angle petrous tip and clivus. Removal of the cochlea and anterior portion of the petrous bone permits wide visualization of the clivus and skull base.

Technique

The incision, soft tissue exposure, complete mastoidectomy, labyrinthectomy, posterior and middle fossa dura decompression, and skeletonization of the internal auditory canal are performed as in the translabyrinthine approach.

The facial nerve is decompressed from the stylomastoid foramen to the geniculate ganglion. After sectioning of the chorda tympani nerve using an extended facial recess approach and transection of the greater superficial petrosal nerve, the facial nerve can be transposed posteriorly. Transposition of the nerve removes the obstacle to extending the exposure anteriorly. The transcochlear approach has been modified to include transection of the external auditory canal and two-layered closure of the meatus. This modification permits removal of the posterior wall of the external auditory canal, thus allowing wider anterior exposure in the areas of the jugular bulb and internal carotid artery. Next, the incus and stapes are removed. The cochlea is removed, thereby exposing the carotid artery anteriorly, the jugular bulb and inferior petrosal sinus inferiorly, and the superior petrosal sinus superiorly. The bony exposure leaves a dura-covered window extending from the superior petrosal sinus to the inferior petrosal sinus and reaching the clivus medially (Fig. 177-36).

Figure 177-36. Surgeon’s view of the skull base with the transcochlear approach. The external auditory canal is transected and sutured closed. Posterior transposition of the facial nerve permits unobstructed visualization from the superior petrosal sinus to the inferior petrosal sinus, anteriorly to the carotid artery, and medially to the clivus. Inset shows skin incision in relation to the cerebellopontine angle.

The same principles of tumor removal are used as with the other approaches to the CPA. However, the position of the facial nerve in relation to the tumor and the proximity of the basilar artery make certain aspects of tumor removal with the transcochlear approach different from other approaches to the CPA. Unlike acoustic neuromas, tumors of the petrous apex and clivus often have the facial nerve on their posterior surface. Thus, the nerve should be dissected free of the tumor and protected. The position of the vertebrobasilar arterial system should be considered. As tumor dissection proceeds medially, the basilar artery will be anterosuperior and the vertebral artery will be posteroinferior. Extensive tumors that cross the midline are removed by reflecting the vessels posteriorly from the tumor capsule. Particular care is taken to preseve small perforating vessels that supply the brainstem, because their damage could be fatal. The surgical defect is filled with abdominal fat (often with hydroxyapatite or titanium mesh, or both), the wound is closed in layers, and a compressive dressing is applied.

Transotic Approach

Technique

Through a postauricular incision, the ear is reflected anteriorly, and the external auditory meatus is transected and closed in two layers. Essentially the same procedure as that described for the transcochlear approach is performed, with the exception that the facial nerve is not completely freed from the fallopian canal. Rather, the nerve is skeletonized and left in its canal. At the conclusion of the exposure, the facial nerve is suspended, traversing the surgical field with a bony cover. The tympanic ring is removed, with systematic exposure of the jugular bulb, jugular foramen, and carotid canal. By working around the facial nerve, the surgeon has access to lesions of the internal auditory canal, CPA, clivus, and jugular foramen (Fig. 177-37). Closure is performed with obliteration of the defect with abdominal fat and optional placement of hydroxyapatite cement or titanium mesh.

Figure 177-37. Surgeon’s view of the transotic approach to the skull base. Note that the approach is identical to the transcochlear approach except that the facial nerve is not transposed. Instead, the nerve is left in its bony canal suspended across the surgical field. The in situ facial nerve limits the exposure in its internal auditory canal and mastoid segments (compare with Fig. 177-36). Inset shows skin incision in relation to the cerebellopontine angle.

Extended Middle Fossa Approach

Basic Approach

For the extended middle fossa approach, the skull base surrounding the otic capsule is carefully removed through a wide temporal craniotomy. This approach exposes lesions extending from the posterior fossa through the tentorium and incisura and anteriorly up to the region of foramen lacerum and the posterolateral aspect of the cavernous sinus (Fig. 177-38).

Figure 177-38. Schematic view (axial) of the extended middle fossa approach to the skull base. By removing the bone posterior and medial to the labyrinth and medial to the carotid artery, the exposure of the middle fossa approach is extended to lesions of the cerebellopontine angle that have spread beyond the tentorium or even into the cavernous sinus.

Combined Techniques (Petrosal Approach)

Technique

The usual procedures in the petrosal approach combine extended middle fossa exposure with full retrosigmoid and retrolabyrinthine exposure⁶³ (Fig. 177-39). The middle cranial fossa, internal auditory canal, posterior fossa, and clivus all are exposed while the otic capsule is preserved. By contrast, the translabyrinthine or transcochlear approach also can be combined with the extended medial fossa approach to provide the widest possible exposure.

Figure 177-39. Combined petrosal approach uses the extended middle fossa, retrolabyrinthine, and retrosigmoid exposure while preserving the labyrinth.

Experimental Approaches

Attempted hearing preservation with partial or complete labyrinthine resection has been reported. McElveen⁶⁴ and Molony⁶⁵ and their colleagues reported systematic sealing of the vestibule during labyrinthectomy with successful hearing preservation after translabyrinthine tumor removal. To expose the lateral internal auditory canal with the retrosigmoid approach, Arriaga and Gorum⁶⁶ reported posterior semicircular canal resection with successful hearing preservation and total tumor removal in two of three enhanced retrosigmoid procedures. In extensive skull base lesions, labyrinthine procedures can be converted to partial labyrinthine resection procedures with concomitant opportunity for preserved hearing.⁶⁷

Although these findings are promising, such techniques are currently experimental, and further investigation is necessary for such approaches to be offered routinely in clinical practice.

Approaches for Skull Base Lesions Requiring Drainage

As described earlier in this chapter, cholesterol granulomas are nonneoplastic lesions that may expand the petrous apex and produce symptoms related to the nerves of the posterior fossa. Similarly, mucoceles of the petrous apex may produce symptoms by their expansion. Both lesions are adequately managed by surgical drainage.

Infralabyrinthine Approach

After a complete mastoidectomy, the sigmoid sinus is decompressed, the posterior semicircular canal and jugular bulb are identified, and the infralabyrinthine air cells are followed into the petrous apex. The cyst is incised, drained, and stented open into the mastoid⁶⁸ (Fig. 177-40). This approach may be technically impossible in a patient with a high jugular bulb. Furthermore, although reexploration is possible in the face of an occluded stent, revision surgery requires formal reexploration of the transmastoid approach.

Figure 177-40. Surgeon’s view of the completed transmastoid infralabyrinthine drainage of a cholesterol granuloma. The dissection is performed medial to the facial nerve and inferior to the level of the labyrinth. The drainage tube permits aeration of the cholesterol granuloma.

Transcanal Infracochlear Approach

The external auditory canal is transected and a superiorly based tympanomeatal flap is elevated. The bony external meatus is widened, and the air cell tract between the jugular bulb and carotid artery is followed into the petrous apex (Fig. 177-41). This approach permits dependent drainage of the petrous apex into the middle ear (see Fig. 177-14). If the drainage tube becomes obstructed, it may be revised through an office myringotomy. The principal disadvantages are the prolonged healing time required for the external auditory canal and the need for direct exposure of the petrous carotid artery.⁴²

Figure 177-41. Surgeon’s view of the transcanal infracochlear approach to the petrous apex. After transection of the ear canal, a superiorly based tympanomeatal flap is elevated. Diamond burrs are used to expose widely the hypotympanum, jugular bulb, and petrous carotid artery. The infracochlear cell tract is followed between the carotid artery and jugular bulb.

(From Giddings NA, Brackmann DE, Kwartler JA. Transcanal infracochlear approach to the petrous apex. Otolaryngol Head Neck Surg. 1991;104:29.)

Preoperative Management

Preoperative antibiotics are routinely used even in uncomplicated cases. Osmotic agents and diuretics for brain relaxation usually are not used in the translabyrinthine approach; however, the retrolabyrinthine, retrosigmoid, middle fossa, and extended middle fossa approaches require such techniques to minimize the need for retraction.

Intraoperative Monitoring

Facial nerve monitoring is used routinely in all cases of posterior fossa neoplasm surgery. Continuous monitoring is performed with EMG electrodes and monitoring equipment with instantaneous visual and loudspeaker output. An audiologist assists with interpreting monitoring signals and stimulating the facial nerve when required to identify nerves intraoperatively.

Although ABR monitoring often is used in hearing preservation cases, this modality provides only delayed feedback to the surgeon and cannot affect intraoperative decision-making or tissue manipulation in a meaningful way. The advent of direct eighth nerve recording systems and electrocochleography (ECoG) modifications may make a practical real-time feedback system available to the surgeon in hearing preservation cases.⁷¹

Postoperative Care

The compression mastoid dressing applied in the operating room is left in place until the fourth postoperative morning if hydroxyapatite or titanium mesh was not employed. The patient is observed in the neurologic intensive care unit for 1 day and then transferred to the general ward. Limited activity is begun on the first postoperative morning, and ambulation usually resumes on the second postoperative day, with discharge home on postoperative day 3 or 4.

Complications