Imaging of the Facial Nerve

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Outline Technique Congenital Anomalies Normal Variants Congenital Ear Deformities Tumors Schwannomas Vascular Tumors Epidermoid Cysts Cholesterol Cysts

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Imaging of the Facial Nerve

Carcinomas Rare Tumors Trauma Inflammatory Disease Intrinsic Inflammation Extrinsic Inflammation Hemifacial Spasm Summary

maging of the facial nerve has evolved into a precise procedure that employs both high-resolution computerized tomography (HRCT) and magnetic resonance imaging (MRI). It has gone from the era of plain films, followed by conventional polytomography,1,2 popularized in the 1960s, through cisternographic tomography,3 performed primarily through the mid-1970s, to the present standards of HRCT and MRI.4–9 The advantages of CT over the techniques in use previously are (1) high-contrast resolution; (2) simultaneous display of densities of air, soft tissue, and bone; (3) ease of examination; and (4) half of the radiation dose of polytomography. The advantages of MRI include (1) easy multiplanar projection without patient repositioning, (2) superior soft tissue resolution, and (3) lack of exposure to ionizing radiation. Facial nerve imaging is today accomplished by using MRI with gadolinium diethylenetriamine pentaacetic acid (DTPA) enhancement and HRCT, either jointly or alone. The facial nerve has a complex, multiplanar course, both intracranially and extracranially (Figs. 23-1, 23-2, and 23-3).10–12 The intracranial portion extends from the brainstem to the internal auditory canal (IAC), for a length of 23 to 24 mm, and includes the premeatal segment, which lies in the cerebellopontine angle (CPA). The intratemporal portion is subdivided into three segments. A length of 5 to 12 mm (average = 10 mm) passes through the IAC and then passes anteriorly and slightly inferiorly in its labyrinthine segment for 3 to 5 mm to reach the geniculate fossa. The nerve has its first surgical genu at the geniculate ganglion, where it turns at an acute angle of approximately 75 degrees to run posteriorly and slightly laterally and from which emerges anteromedially the greater superficial petrosal nerve. The tympanic portion of the facial canal is straight, 10 to 12 mm in length, and, in 65% of cases, covered by a thin bony lamella over its external wall. The mastoid, or second surgical, genu occurring

Sujana S. Chandrasekhar, MD Antonio De la Cruz, MD William W. M. Lo, MD Fred F. Telischi, MD

at the posterosuperior region of the tympanum, subtends an angle of 95 to 125 degrees and results in a nearly vertical descent of the nerve. The vertical (or mastoid) portion of the facial nerve canal descends 13 mm to the stylomastoid foramen. The extratemporal facial nerve, as it emerges from the stylomastoid foramen, runs anteriorly in the substance of the parotid gland and divides into two primary branches: the temporofacial, or superior, division and the cervicofacial, or inferior, division. These two divisions in turn elaborate main branches and break up into a plexus to supply the facial muscles. Accurate clinical history and physical examination are necessary when choosing the imaging modality that will best evaluate facial nerve lesions. Different sites of neural injury must be imaged differently. MRI and HRCT often yield complementary information and at times both are required for optimal demonstration of facial nerve pathology. MRI is the method of choice when (1) the site of involvement is clinically unlocalized because it is the only imaging modality that demonstrates the facial nerve comprehensively from the pons to the parotid gland; (2) the site of lesion is clinically localized to either the intracranial or the extratemporal portion of the nerve because it provides excellent soft tissue contrast; or (3) the onset of symptoms is acute because, with gadolinium enhancement, MRI is capable of showing changes of inflammation not seen on CT.13–15 Thin-section HRCT that makes use of a bone or edgeenhanced algorithm renders exquisite bony detail. It is the preferred initial imaging modality when a lesion is clinically localized in the middle ear or mastoid and is the method of choice in cases of temporal bone trauma.16–18 CT with a standard soft tissue algorithm with and without an intravenous iodinated contrast medium may be used to evaluate the CPA and the parotid gland, but results are 419

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Figure 23-1. Normal HRCT anatomy of the facial nerve-axial views, left ear. A, Arrowheads demonstrate IAC segment, labyrinthine segment, and geniculate ganglion. Note Bill’s bar (vertical crest) separating the facial and superior vestibular nerves. B, Arrowheads demonstrate the tympanic segment. C, The mastoid segment (arrowhead) lies at about 3 o’clock to the jugular fossa. D, Facial nerve (arrowhead) sitting in fat as it exits the stylomastoid foramen.

inferior to MRI in soft tissue contrast, and without intrathecal gas injection this modality does not exclude small IAC tumors.3 This is used only as a secondary option when MRI is not available or when the patient cannot be imaged on the MR scanner, for example, if the patient is too claustrophobic or too large.

TECHNIQUE The technical considerations for MRI of the facial nerve are nearly identical to those for MRI of the CPA and the IAC. These considerations are fully discussed in detail in Chapter 21, Imaging the Cerebellopontine Angle, and are not repeated here. When necessary, similar techniques are extended to cover the course of the extratemporal facial nerve through the parotid gland see (Fig. 23-3F). For detection and demonstration of facial nerve lesions, a well-focused MRI is indispensable because many of these are only a few millimeters in size. The technique employs imaging in transverse and coronal planes, with contiguous or overlapping thin sections of 3 mm each in thickness,

covering the pons to the parotid gland, before and after intravenous injection of a paramagnetic contrast agent. Sagittal or oblique sagittal images can be helpful in displaying lesions in the facial nerve canal (FNC) (see Fig. 23-3E) and do not require patient repositioning.9,14,19,20 Segments of the nerve in the facial canal enhance in the majority of normal subjects after the administration of intravenous gadolinium.21 This enhancement is due to the presence of a perineural vascular plexus in the FNC and may be asymmetrical in normal individuals. Compared with the findings in Bell’s palsy or Ramsay Hunt syndrome, the enhancement in normal subjects is less intense and does not extend into the premeatal segment of the nerve in the IAC (see Figs. 23-3A, B). HRCT for facial nerve evaluation is done in contiguous thin sections, no thicker than 2 mm, with a bone or edgeenhanced algorithm for maximal spatial resolution.4,22 Transverse sections of the temporal bone are obtained at either a 0- or 30-degree plane with respect to the infraorbital-meatal line. Scanning in either plane avoids direct radiation to the ocular lenses. The 30-degree plane is additionally advantageous in that it lies nearly parallel to

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Figure 23-2. Normal HRCT anatomy of the facial nerve-coronal views, left ear. A, Geniculate fossa (arrow). Note semicanal of tensor tympani (arrowhead). B, The labyrinthine and proximal tympanic segments (arrowheads) lie superior to the cochlear capsule, across from the malleus. C, The tympanic segment (arrowhead) is seen as it lies under the lateral semicircular canal, medial to the short process of the incus, and superolateral to the oval window niche. D, Mastoid segment (arrow), medial to the mastoid process, exiting the temporal bone at the stylomastoid foramen.

the tympanic or horizontal segment of the FNC and thereby avoids sectioning of this segment in a crosssectional or “salami” fashion.23 Additional images may be obtained in a modified coronal plane (by tilting the gantry) with the patient either supine or prone. With multirowdetector CT, which is now commonly available, coronal and sagittal images can now be reformatted from axially acquired data with ease and with little loss of resolution.

CONGENITAL ANOMALIES Normal Variants The labyrinthine FNC may originate from the midportion rather than the fundus of the IAC.24 The geniculate fossae may vary in size, although there is generally symmetry between the right and left sides.17 The most common variations in the course of the FNC involve the distal portion of the nerve.25–28 A “drooping,” or protruding, tympanic segment that overlies the oval window and

compromises surgical access may be seen on HRCT and should be recognized (Fig. 23-4A).29 The bony wall of the FNC may be developmentally dehiscent in 35% to 55% of the population, most commonly in the midtympanic segment over the oval window niche.26–28,30,31 The most common tympanic segment aberrations that may be encountered in surgery are: a course over the oval window; bifurcation proximal to the oval window; a course posteriorly either between the oval and round windows or inferior to the round window; a course through the stapedial arch; a course along the superior aspect of the lateral semicircular canal; or a course from the geniculate ganglion straight downward over the promontory.27 Imaging in the coronal plane is most helpful in excluding an aberrant tympanic segment.6 The anterior portion of the tympanic FNC may be enlarged by a persistent stapedial artery in its course from the tympanic cavity to the middle cranial fossa to terminate as the middle meningeal artery.32,33 Most individuals with this finding have few or no symptoms, and such a nonpathologic enlargement must not be mistaken for a

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Figure 23-3. Normal MRI of the left facial nerve-T1-weighted images. A, Axial, precontrast: arrows show normal geniculate ganglion and tympanic segment. B, Axial, postcontrast: same structures. C, Axial, precontrast: mastoid segment (arrowhead). D, Axial, postcontrast: same structure. E, Sagittal, precontrast: mastoid segment (arrow). F, Axial, precontrast: facial nerve (arrow) entering the substance of the parotid gland, posterolateral to the retromandibular vein (arrowhead).

tumor. A persistent stapedial artery can be suspected, even when the artery itself is too small to be visible on CT, in the absence of the ipsilateral foramen spinosum (Fig. 23-5).32 HRCT is diagnostic. MR angiography may be used for confirmation; conventional angiography is not indicated in

these cases. The marrow of the styloid process may mimic the mastoid segment of the facial nerve (see Fig. 23-4B) on HRCT; a distinction between the two structures can be made by following the course of the facial nerve on serial images.

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Congenital Ear Deformities A high incidence of aberrance of the tympanic segment of the facial nerve is seen in patients with other congenital middle ear abnormalities and should be sought.34 The vertical segment may be displaced anteriorly. The motor facial nerve may be congenitally absent, as in Möbius’ syndrome or aplasia of the facial motor nucleus, in which the FNC is very small because it needs to accommodate only sensory and parasympathetic fibers (see Fig. 23-5C).35 Abnormal enlargement of the geniculate fossa should be considered in the evaluation of spontaneous CSF otorrhea, as this condition has been described.36

TUMORS A

Although most neoplasms are extrinsic and involve the facial nerve only secondarily, facial nerve schwannomas and hemangiomas arise from facial nerve structures. The more common extrinsic tumors affecting the facial nerve are epidermoids, cholesterol granulomas, jugulotympanic paragangliomas, and squamous cell carcinomas. Rarer extrinsic tumors are primary fallopian canal paragangliomas, papillary adenomatous endolymphatic sac tumors, metastases, histiocytosis X, embryonal rhabdomyosarcoma, and choristoma.

Schwannomas

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C Figure 23-4. Congenital variant: protruding tympanic segment–coronal HRCT, left ear. A, Tympanic segment of facial nerve (arrowhead) covers the oval window. B, Mastoid segment of facial nerve (arrow) should not be confused with the marrow of the styloid process (arrowhead). Both anomalies are associated with external auditory canal atresias. C, Lateral subcutaneous exit of the mastoid segment of the facial nerve, a rare but highly treacherous anomaly of the facial nerve canal.

Facial nerve schwannomas (FNSs) may involve any segment of the nerve or may involve more than one segment, not always in continuity. They are often sausage-shaped, expanding long segments of the FNC. Latack and colleagues37 have represented eight examples of FNS in diagrammatic form (Fig. 23-6). The clinical presentation and imaging findings depend on the segment(s) of the nerve involved. Because they involve the nerve by compression rather than invasion, facial palsy is generally a late finding. Intratemporal-segment schwannomas demonstrate facial nerve symptomatology earlier than do those at the CPA or the IAC. On MRI, FNSs are heterogeneous lesions hypointense to brain on T1-weighted images (T1WI), isointense on proton-density, and hyperintense on T2weighted images (T2WI) (Fig. 23-7). These tumors enhance briskly with gadolinium (Figs. 23-7B and 23-8). They are isodense to brain with enhancement after iodinated contrast on CT; however, tumors within the IAC or bony FNC can be missed; therefore, MRI is preferable for identifying small lesions.38,39 Lesions involving the distal tympanic and mastoid segments image more characteristically than do those in the perigeniculate, IAC, and parotid portions. An imaging distinction between facial and vestibular schwannomas in the CPA or the IAC is nearly impossible to make (see Fig. 23-7), although histologically they appear quite different.40 Certain imaging “clues” may aid in differentiating these lesions. Anterosuperior erosion of the IAC or erosion of the labyrinthine FNC on HRCT has been suggested as a diagnostic clue, but it is not reliable.41 Eccentric placement of the tumor in the IAC may help in making a preoperative diagnosis of facial schwannoma.42

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Figure 23-5. A and B, Congenital variant: persistent stapedial artery. A, Coronal HRCT, left ear: Arrowhead points to enlarged proximal tympanic segment of FNC caused by persistent stapedial artery. B, Axial CT of skull base: Arrowhead at normal foramen spinosum on right, immediately posterolateral to the foramen ovale. This structure is absent on the side of the persistent stapedial artery. (Courtesy of David F. Sobel, MD) C, Congenital variant: hypoplastic FNC in a child with congenital facial palsy. (left, abnormal; right, normal)

Figure 23-6. Eight types of FNS. The shaded areas represent segments of the nerve involved by tumor. Most FNSs involve long segments of the nerve. (Reproduced with permission from Latack JT, et al: Facial nerve neuromas. Radiologic evaluation. Radiology 149:731–739, 1983.)

There may be more than one component: one in the IAC/posterior cranial fossa and one in the middle cranial fossa connected via a narrow waist through the labyrinthine FNC (see Fig. 23-8). The dumbbell-shaped FNS from the posterior to the middle cranial fossa in the midpetrous region is highly characteristic. Large geniculate ganglion schwannomas may be mistaken for meningiomas, gliomas, or temporal lobe metastases. Coronal images are helpful in demonstrating the extradural origin of schwannomas; also smooth enlargement of the FNC favors the diagnosis of FNS (Fig. 23-9). Tumors arising from the tympanic segment may cause conductive hearing loss as the only symptom (Fig. 23-10). A pathognomonic finding is an enhancing enlargement of varying thickness along a significant length of the nerve (Figs. 23-11 and 23-12).37,38 During radiography of proximal tympanic segment schwannomas, one should not be misled by a persistent stapedial artery or developmental dehiscence of the FNC, as discussed previously. Differential diagnosis for geniculate ganglion lesions includes vascular tumors such as hemangiomas, epidermoid

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Figure 23-8. FNS in posterior and middle cranial fossae. A, Precontrast T1WI axial MRI. B, Postcontrast T1WI axial MRI.

C Figure 23-7. Axial MRI of an FNS. A, The tumor (arrow) is heterogeneous and mildly hypointense to brain on T1WI, precontrast. B, The mass enhances briskly with gadolinium–DPTA on postcontrast T1WI. C, On T2WI, the mass is mildly hyperintense to brain.

cysts, and meningiomas. Distinguishing among these lesions can be done with MRI and HRCT, based on the sharpness of their borders and on enhancement characteristics. On HRCT, the borders of hemangiomas are not sharp (Figs. 23-13 and 23-14), the borders of schwannomas are moderately sharp (see Fig. 23-12B), and the

borders of epidermoid cysts are extremely sharp (see Figs. 23-18A, and 23-20A). Hemangiomas are heterogeneously hyperintense on MRI and enhance strongly with gadolinium-DTPA (Figs. 23-15 and 23-16). Epidermoid cysts are isodense or hypodense on CT and are nonenhancing after contrast administration (see Figs. 23-19B and 23-20B). On MRI, epidermoid cysts are hypointense on T1WI and hyperintense on T2WI (see Figs. 23-18B, C and 23-19C, D), and they do not enhance with gadolinium (see Fig. 23-20D). The CT and MRI features of a meningioma at the geniculate ganglion resemble those in the CPA.

Vascular Tumors Intratemporal vascular tumors include hemangiomas, composed of thin-walled vascular spaces, and vascular malformations, composed of thick-walled vascular spaces lined with a layer of epithelium surrounded by fibroblasts and collagen. The two lesions may coexist in a single mass. The most common site of occurrence is the geniculate ganglion, followed by the IAC and then the mastoid genu. They are usually less than 1 cm in diameter. The nerve is involved by invasion and these tumors cause hemifacial spasm and facial palsy early. If located in the IAC, they

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Figure 23-9. Geniculate ganglion schwannoma, left. Presenting complaint: conductive hearing loss. A, Axial HRCT. Arrowhead points to small soft tissue mass between cochlear promontory and ossicles. B, Coronal T1WI MRI. Arrowhead to small component of tumor corresponds to soft tissue in the middle ear seen on CT. The large component protruding into the temporal lobe was clinically silent.

Figure 23-10. Tympanic FNS, left. A, Axial HRCT. Arrowhead points to soft tissue mass in middle ear, medial to the ossicles, lying along the tympanic FNC. B, Axial T1WI, postcontrast MR. Arrowhead shows enhancing mass along tympanic FN.

also cause a greater degree of sensorineural hearing loss than would be expected based solely on size.43 Early detection may allow complete removal with preservation of facial nerve function, avoiding facial nerve resection and grafting.44 Intratumoral bone spicules may be seen (see Fig. 23-13). This bone formation is a reaction to the hemangioma itself.45 Labyrinthine and geniculate ganglion lesions show subtle CT findings that include irregular and indistinct bone margins and reticular or “honeycomb” bone (see Fig. 23-14). Geniculate ganglion vascular tumors seen with MRI demonstrate nonhomogeneous intensities, which are the MRI correlate of the “honeycomb” seen on CT (see Fig. 23-15). They are difficult to distinguish from schwannomas when they occur at the mastoid genu. Their enhancement with contrast is not helpful on CT because density changes in such small lesions interspersed among bone are difficult to discern. HRCT with gas cisternography is usually required for lesions of the IAC, but MRI with gadolinium-DTPA enhancement demonstrates these lesions well and is preferred over CT (see Fig. 23-16).

Many of these tumors are similar to schwannomas in signal intensity on T1WI and T2WI, but some are most hyperintense than typical schwannomas on T2WI (Fig. 23-17).46,47

Epidermoid Cysts Petrous apex epidermoid cysts attain considerable size before involving the facial and acoustic nerves in the CPA or IAC. More common facial nerve involvement is seen with congenital epidermoid cysts of the supralabyrinthine region of the temporal bone.48 From this point of origin they readily erode the proximal FNC and can either reach around the superior semicircular canal and extend medially superior to the IAC or laterally into the epitympanum, or they can erode the cochlea or superior semicircular canal, which causes fistulization. CT demonstrates an expansile lesion-eroding bone, with sharp bone margins. Calcifications may be seen. T1WI MRI demonstrates low to intermediate signal, and T2WI shows high signal intensity (Figs. 23-18 and 23-19).

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Carcinomas

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Survival in patients with squamous cell carcinoma of the temporal bone is related directly to the depth and extent of tumor involvement.49 Physical examination is unreliable, because the medial external auditory canal and tympanic membrane are visible in only 50% of patients.50 Plain film radiographs and tomograms are likewise inadequate. HRCT, however, is very accurate in assessing the depth and extent of tumor invasion.51 The technique uses a standardized protocol of bone and soft tissue algorithms with thin slices in both axial and coronal planes. Facial nerve involvement in these tumors occurs as a result of tumor extension through the fissures of Santorini into the extratemporal facial nerve, or in the middle ear/mastoid. Adenoid cystic carcinoma causes facial nerve palsy directly as it spreads along the perineurium. Both MR and HRCT are used in imaging this tumor.

Rare Tumors

B Figure 23-11. FNS–sagittal T1WI MRI. Note multifocal lesions from the geniculate ganglion to the stylomastoid foramen (arrowheads). Compare this to the normal sagittal view seen in Figure 23-3E. A, Precontrast. B, Postcontrast.

Acquired epidermoid cysts of the temporal bone may also cause facial nerve symptomatology. Radiologically, they are similar to congenital epidermoid cysts except that they originate from the middle ear and mastoid and may extend around the bony labyrinth to the petrous apex. A common location of FN involvement is in the geniculate ganglion region. Changes of chronic inflammation are seen in the middle ear and mastoid (Fig. 23-20).

Cholesterol Cysts Cholesterol granulomas form from obstruction of drainage of petrous apical air cells, followed by repetitive cycles of hemorrhage. They may cause irritation and initiate a foreign body reaction from their cholesterol crystals. They grow silently in the petrous apex until they exert pressure on cranial nerve V, VI, VII, VIII, IX, X, XI, or XII. These lesions are best imaged by combining HRCT and MRI. CT shows a sharply marginated, expansile lesion, without calcifications. MRI demonstrates high signal intensity on both T1WI and T2WI.

Fewer than 20 cases of paraganglioma of the facial canal (glomus faciale) without involvement of the jugular bulb have been reported in the literature.52–58 Two more cases have been diagnosed at our institution. Arnold’s nerve, the auricular branch of the vagus nerve, passes via one or two mastoid canaliculi from the jugular bulb to the facial canal and ascends in the vertical portion of the FNC.59 Paraganglia (glomus bodies) can be found along Arnold’s nerve within the vertical FNC. HRCT demonstrates expansion of the vertical FNC (Fig. 23-21A) and, with larger tumors, may show destruction in the mastoid (Fig. 23-21B). To demonstrate that the jugular foramen is not involved, catheter or MR venography is helpful. Jugulotympanic paragangliomas may grow to affect the facial nerve secondarily, which is demonstrated on imaging of the tumor. Other tumors of the temporal bone secondarily affecting the facial nerve are endolymphatic sac tumors (ELST), metastatic lesions, Langerhans’ cell histiocytosis, embryonal rhabdomyosarcomas, and non-neoplastic choristomas. Papillary adenomatous tumors of endolymphatic sac origin (ELST) are rare, locally invasive, and often extend into the medial mastoid to cause facial palsy (Fig. 23-22).60 Bilateral ELSTs have been found in patients with von HippelLindau disease.61 Of metastatic lesions to the temporal bone, 34% present with facial nerve palsy.62 The primary tumors are usually in the prostate, breast, or kidney. CT demonstrates bony destruction with tumor encroachment on the FNC; MR with gadolinium-DTPA is useful in equivocal cases.63 Langerhans’ cell histiocytosis causing facial palsy has been reported in 15 cases.39 The pathophysiology is histiocytic infiltration of the temporal bone leading to compression of the nerve within the eroded fallopian canal. CT shows an expansile soft tissue mass with bony labyrinthine and ossicular erosion. Embryonal rhabdomyosarcoma, an early childhood malignancy, involves the temporal bone in 7% of cases. Of 12 cases reported by Wiatrak and Pensak,64 6 had seventh cranial nerve paralysis as a presenting manifestation, and all of these 6 patients had middle ear tumors. The usual path of spread of

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Figure 23-12. FNS, left, extending from the IAC through the descending (mastoid) segment. A, Axial HRCT. Arrowheads at FNS in geniculate and tympanic segments. B, Coronal HRCT. Tumor in mastoid segment (arrowhead). C, Axial T1WI, postcontrast MRI. Tumor in IAC, geniculate and tympanic segments (arrowheads). D, Sagittal T1WI, postcontrast MRI. Tumor in tympanic and mastoid segments (arrowheads).

Figure 23-13. Hemangioma–axial HRCT, left side. Demonstrates intratumoral bone (arrowhead) within IAC hemangioma.

Figure 23-14. Hemangioma–axial HRCT, right side. Note irregular bony margins (arrow), honeycombing, and larger bone spicules (arrowhead).

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A Figure 23-15. Hemangioma of geniculate ganglion–axial T1WI, postcontrast MRI, right. Heterogeneous hyperintensity of the hemangioma (arrowheads) corresponds to the honeycomb appearance seen on HRCT (see Fig. 23-14).

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Figure 23-16. Hemangioma of IAC–axial T1WI, postcontrast MRI, left. Note intense enhancement with gadolinium (arrows). This lesion cannot be differentiated radiologically from schwannoma. The enhancement seen at the geniculate ganglion (arrowhead) was not tumor at surgery.

C Figure 23-18. Suprageniculate congenital epidermoid cyst, left ear, axial projections. A, HRCT. Arrow demonstrates sharp margin of bony erosion. Note involvement of the ampullated end of the superior semicircular canal (arrowhead). B, This lesion has intermediate signal intensity on T1WI MRI. C, The same lesion has high signal intensity on T2WI MRI.

Figure 23-17. Hemangioma–axial T2WI MRI, right. Note hyperintensity of tumor on T2WI in IAC and CPA, distinguishing it from the typical schwannoma.

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Figure 23-19. Petrous apex congenital epidermoid cyst, left ear, axial projections. A, HRCT with bone algorithm. The large lesion is expanding the petrous apex (arrows). B, CT with soft tissue algorithm, postcontrast. The mass is isodense or mildly hypodense to brain and nonenhancing (arrows). C, T1WI MRI. The mass is mildly hypointense to brain with an isointense capsule (arrows). D, T2WI MRI. The mass is markedly hyperintense.

rhabdomyosarcoma of the middle ear cleft is by invasion and destruction of the FNC with infiltration of the facial nerve, extension to the IAC, and from there on to the leptomeninges. The imaging modality of choice in these cases is HRCT because it assesses bony destruction and can evaluate response to therapy. Middle ear salivary gland choristomas, which are tumors composed of normal cells not normally found at the site of occurrence, usually affect the tympanic segment of the facial nerve and the ossicles.65

TRAUMA Specific thin-section HRCT is necessary in the radiologic evaluation of temporal bone trauma, because approximately 60% of temporal bone fractures are not apparent on routine head CT examinations.66 It is preferable to obtain both axial and coronal sections; however, if the status of the patient’s cervical spine precludes positioning for direct coronal images, coronal reconstructions from direct axial images can suffice.6 Accurate preoperative

localization with HRCT is vital because the surgical approach to decompression varies with the exact site of facial nerve injury.67 Gadolinium-enhanced MRI has been advocated for identifying focal enhancement as a method for localizing traumatic injury to the facial nerve.68 Classically, temporal bone fractures have been divided into two types: longitudinal (70% to 90%) and transverse (10% to 30%), defined by the orientation of the fracture relative to the long axis of the petrous bone. Ten percent to 20% of longitudinal fractures result in facial nerve palsy, which is usually delayed in onset and incomplete. The fracture line is oriented along the long axis of the temporal bone and, in palsy cases transgresses the nerve in the perigeniculate region (Figs. 23-23A, B). Facial paralysis occurs in 50% of transverse fractures, and is more frequently of immediate onset and complete. The facial nerve is usually involved in the labyrinthine or tympanic segment (Fig. 23-24). More recent literature indicates that a large percentage of temporal bone fractures are “mixed,” in that the fracture line is both along and across the long axis of the petrous bone. Audiometric data associated with

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Figure 23-20. Acquired epidermoid cyst of the temporal bone, right ear. A, Axial HRCT shows a sharply and smoothly marginated lobular mass extending from the middle ear cavity, anterior to the cochlea to expand the petrous apex, eroding portions of the cochlear capsule. B, Axial CT with soft tissue algorithm shows the mass to be mildly hypodense to brain. The patient’s facial weakness was due to erosion in the geniculate ganglion region (arrow). C, Axial T1WI MRI shows that the mass is mildly hypointense, with a thin isointense capsule (arrowheads), similar to that seen in Figure 23-19, C. D, Coronal T1WI MRI, postcontrast. The cholesteatomatous material shows no enhancement; there is enhancement of tissue in the middle ear and surrounding the capsule (arrowheads) as a result of reactive inflammation.

differently oriented fracture lines are available.69 Other large series have looked at fractures as either otic capsule “sparing” or “involving,” with capsule-involving fractures having a higher incidence of facial palsy.70

INFLAMMATORY DISEASE Intrinsic Inflammation Bell’s palsy, or idiopathic isolated peripheral facial palsy, represents 50% to 85% of all cases of facial nerve palsy.71 The majority of cases have a typical clinical presentation and do not require imaging; however, in 15% there is an atypical presentation or a prolonged course.72,73 In these cases, gadolinium-enhanced MRI is useful and can help the clinician avoid missing a CPA/IAC tumor. The presumed accumulation of gadolinium in areas of inflammation and disruption of vessel integrity has provided the basis for evaluation by means of this modality.14 A number of studies have demonstrated significant enhancement of the facial

nerve in these individuals.74–77 The pathologic enhancement seen on gadolinium-enhanced MRI is much more intense than seen in normal subjects, usually involves the perigeniculate and labyrinthine segments, and can extend into the premeatal segment (Fig. 23-25). These findings are consistent with the theory of meatal nerve entrapment popularized by Fisch and Esslen.78 Nerve enhancement may persist 4 months or longer following the onset of paralysis. No difference is seen radiographically between acute and chronic cases, and there is no prognostic significance attributable to the presence or the degree of facial nerve enhancement in patients with viral inflammatory facial paralysis.79 Enhancement with enlargement of the nerve suggests tumor rather than inflammation. Herpes zoster oticus, or Ramsay Hunt syndrome, manifests as auricular vesicles, ear pain, and facial paralysis. Abnormal gadolinium enhancement of the facial nerve is seen, similar to that seen in Bell’s palsy. If the inflammation spreads to involve the eighth nerve and membranous labyrinth, variable enhancement of these structures is also noted on MRI.75,80,81

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B Figure 23-21. Facial canal paraganglioma (glomus faciale), left ear, axial HRCT. A, Expansion of vertical FNC (arrowhead). B, Arrows show extension of tumor beyond FNC to adjacent mastoid and external auditory canal, encroaching on the jugular fossa (arrowhead).

Lyme disease is a treponemal multisystemic infection that is transmitted by a deer tick (Ixodes dammini). Ten percent of all patients with Lyme disease and 50% of those patients with central nervous system infection will have unilateral or bilateral facial nerve palsies.82 Enhancement of the facial nerve on MRI is seen and is identical to that seen with herpes zoster oticus and Bell’s palsy.

Extrinsic Inflammation Acute otitis media, chronic otomastoiditis, and cholesteatoma have been associated with facial paralysis. Cholesteatoma is present in more than half of the cases of chronic otitis media and facial palsy.83 Imaging is indicated in patients with facial palsy and chronic otomastoiditis or cholesteatoma and is directed toward preoperative surgical planning. HRCT is the imaging modality of choice because it can clearly identify areas of the fallopian canal violated by surrounding inflammatory soft tissue.84 Other temporal bone infections secondarily causing facial nerve palsy include mucormycosis, tuberculosis, and syphilis.85–87 Malignant (necrotizing) otitis externa, a skull base osteomyelitis, can be complicated by facial palsy through spread of infection via the fissures of Santorini in the external auditory canal, which involves the nerve at the

B Figure 23-22. Papillary adenomatous tumor of the endolymphatic sac, left ear, axial projections. A, HRCT. The tumor is destroying bone in the retrolabyrinthine region with extension to the medial mastoid (arrowheads) at the area of the mastoid facial nerve genu. B, T1WI MRI. This tumor (arrowheads) is heterogeneous, containing hypointense, isointense and in particular, hyperintense foci, precontrast.

stylomastoid foramen.88 Nuclear medicine evaluation with technetium scanning to detect the extent of skull base osteomyelitis and gallium scanning for early detection of recurrence are invaluable in this disease. Facial nerve imaging per se is not indicated in these cases; however, CT and MRI are helpful in assessing the extent of disease because they can demonstrate evidence of inflammatory and soft tissue changes around the stylomastoid foramen and mastoid segments of the facial nerve.89

HEMIFACIAL SPASM Hemifacial spasm (Fig. 23-26) is a hyperactive facial nerve dysfunction characterized by painless paroxysmal spasms of the ipsilateral mimetic musculature. It is frequently the

Imaging of the Facial Nerve

A

A

B

B

Figure 23-23. A, Longitudinal temporal bone fracture, left ear, axial HRCT. Comminuted longitudinal fracture (arrows), with large fragment (arrowhead) involving the geniculate fossa, and extension of the fracture laterally through the EAC (twin arrows). B, Longitudinal temporal bone fracture, left ear, axial HRCT. This is a similar but more subtle comminuted fracture compared with the fracture in A. The fracture lines involve the geniculate fossa and extend laterally through the mastoid cortex (arrowheads). Note blood in the mastoid cavity.

Figure 23-24. Transverse temporal bone fracture, left ear, axial HRCT. This patient presented with a complete ipsilateral sensorineural hearing loss and abrupt-onset facial paralysis. The fracture line courses through the ampullary limb of the lateral semicircular canal (arrowhead), transecting the underlying tympanic segment of the facial nerve.

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Figure 23-25. Bell’s palsy, left, postcontrast T1WI MRI. A, Axial. Arrowheads show marked enhancement in the premeatal segment in the fundus of the IAC, the geniculate ganglion, and the tympanic segment. Compare this to the mild to moderate enhancement seen on the asymptomatic right side. B, Coronal. Arrowheads demonstrate marked enhancement of the premeatal segment and the geniculate ganglion. The opposite (normal) side demonstrates mild enhancement in the geniculate ganglion only (line).

Figure 23-26. Hemifacial spasm. This heavily T2-weighted CISS image illustrates the neurovascular cross-compression that can be seen in hemifacial spasm.

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result of compression of the facial nerve at its root exit zone from the brainstem by vascular loops or aneurysms of the posterior or anterior inferior cerebellar artery, the vertebral artery, or the internal auditory artery. Both dynamic HRCT with contrast and MRI have been used to demonstrate the vascular aberrance. Angiography for diagnosis was reserved for equivocal cases in which CT failed to demonstrate the pathology; however, the use of conventional angiography has declined with the advent of MRI and MR angiography.90–93

SUMMARY Effective use of imaging for evaluation of facial nerve disorders requires knowledge of the multiplanar anatomic course of the nerve and an understanding of the clinical disease. Properly oriented, thin-section multiplanar imaging is necessary to evaluate the different facial nerve segments and their disorders. The predicted site and type of onset of facial nerve pathology determine the type of initial imaging modality used. Lesions suspected of involving the intracranial portion of the facial nerve are best seen with MRI with gadolinium-DTPA enhancement. These include brainstem, CPA, and IAC tumors, as well as intracranial vascular aberrancies. When involvement of the intratemporal portion of the facial nerve is suspected, as with cholesteatoma, chronic otitis media, congenital temporal bone abnormality, carcinoma, or trauma, HRCT is the imaging modality of choice. In Bell’s palsy, Ramsay Hunt syndrome, and Lyme disease, facial nerve inflammation is identified most readily on MRI, if indicated. For lesions of the extratemporal facial nerve, soft tissue resolution is best obtained with MRI. Acute palsies are best imaged with MRI. Most importantly, detailed communication between the clinician and the radiologist will result in the optimal combination of imaging techniques for maximal patient benefit.

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