Anatomy and Pathology of the

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1 Residents Section Pattern of the Month Ho et al. natomy and Pathology of the Facial Nerve Residents Section Pattern of the Month Downloaded from by on 01/30/18 from IP address Copyright RRS. For personal use only; all rights reserved Residents inradiology Mai-Lan Ho 1,2 my Juliano 3 Ronald L. Eisenberg 2 Gul Moonis 4 Ho ML, Juliano, Eisenberg RL, Moonis G Keywords: cranial nerve VII, facial nerve DOI: /JR Received July 16, 2014; accepted after revision November 27, Present address: Department of Radiology and iomedical Imaging, University of California, San Francisco, San Francisco, C. 2 Department of Radiology, eth Israel Deaconess Medical Center, Harvard Medical School, 330 rookline ve, oston, M ddress correspondence to R. L. Eisenberg (rleisenb@bidmc.harvard.edu). 3 Department of Radiology, Massachusetts Eye and Ear Infirmary, Harvard Medical School, oston, M. 4 Department of Radiology, Columbia University Medical Center, oston, M. WE This is a web exclusive article. JR 2015; 204:W612 W X/15/2046 W612 merican Roentgen Ray Society natomy and Pathology of the Facial Nerve T he facial nerve (cranial nerve [CN] VII) is a frequently overlooked structure on imaging examinations. Knowledge of the normal anatomy, embryology, and spectrum of abnormality will aid radiologists in correctly diagnosing lesions of the facial nerve. In this article, we review normal facial nerve anatomy, clinical presentations, imaging approaches, and selected cases of facial nerve abnormality (Table 1). Normal natomy The facial nerve is a mixed cranial nerve with motor, parasympathetic, and sensory branches. The intraaxial segment is located within the brainstem and consists of the motor nucleus, superior salivary nucleus (parasympathetic), and nucleus of tractus solitarius (sensory). The cisternal segment consists of the motor root and nervus intermedius (nerve of Wrisberg), which emerge from the brainstem and course into the internal auditory canal. These two portions merge at the internal auditory canal to form the canalicular (intracanalicular, meatal) segment. The labyrinthine segment courses between the cochlea and vestibule of the inner ear and connects to the geniculate ganglion. rising from the geniculate ganglion are the greater superficial petrosal and lesser petrosal nerves, which carry parasympathetic fibers to the lacrimal and parotid glands, and the external petrosal nerve, which sends sympathetic fibers to the middle meningeal artery. The tympanic segment courses backward from the geniculate ganglion inferior to the lateral semicircular canal to the second genu before turning downward to become the mastoid segment, with branches to the stapedius and chorda tympani. Then, the nerve emerges from the stylomastoid foramen and becomes the extracranial segment, which runs through the parotid gland and gives rise to the superior temporozygomatic, inferior cervicofacial, temporal, zygomatic, buccal, mandibular, and cervical branches (Fig. 1). Clinical Presentation Symptoms of facial nerve palsy depend on the location of injury. The motor nucleus of CN VII has dorsal and ventral divisions that supply the upper and lower face, respectively. There is bilateral innervation of the dorsal division but only contralateral innervation of the ventral division. Supranuclear lesions affect upper motor neurons proximal to the motor nucleus in such locations as the cerebral cortex, internal capsule, and cerebral peduncle. This results in central facial palsy that affects the contralateral lower face but spares the forehead and brow muscles. Infranuclear lesions occur distal to the facial nerve nucleus and produce peripheral facial palsy affecting the ipsilateral upper and lower face. Within the infranuclear pathway, the presence of additional symptoms can help further localize lesions. Involvement of CN VI presents with lateral rectus palsy, or inability to abduct the eye. This suggests a lesion of the facial colliculus in the pons, in which efferent CN VII fibers encircle the CN VI motor nucleus. Involvement of CN VIII causes auditory or vestibular symptoms. This suggests a cisternal or canalicular lesion in which CN VII and VIII nerve roots course together after emerging from the cerebellopontine angle. Finally, certain branches of the facial nerve control specialized functions. The greater superficial petrosal nerve controls lacrimation, the stapedius dampens sound transmission in the inner ear, and the chorda tympani is responsible for taste. Lesions proximal to the geniculate ganglion lose all three functions, lesions between the geniculate ganglion and stylomastoid foramen have preserved lacrimation, and extracranial lesions spare all three functions. W612 JR:204, June 2015

2 natomy and Pathology of the Facial Nerve Downloaded from by on 01/30/18 from IP address Copyright RRS. For personal use only; all rights reserved TLE 1: Facial Nerve bnormalities Congenital External auditory canal atresia Oval window atresia Facial nerve dehiscence Inflammation ell palsy Sarcoidosis Langerhans cell histiocytosis Infection Viral acterial Fungal Demyelinating Multiple sclerosis Guillain-arré syndrome Chronic inflammatory demyelinating polyneuropathy Neoplastic enign Schwannoma Paraganglioma Malignant CNS lymphoma Hematogenous metastases Perineural spread of tumor Vascular Venous malformation Neurovascular compression Persistent stapedial artery nomalous vessels Cholesteatoma cquired Congenital Trauma and iatrogenic Facial canal injury Fallopian canal meningocele Malpositioned hardware Radiation neuritis Imaging pproaches CT and MRI have complementary roles in evaluation of the facial nerve. CT provides high spatial resolution for evaluation of osseous details, including fractures, bony destruction, and periosteal reaction. Supranuclear lesions can be evaluated with standard brain MRI, whereas nuclear lesions may require addition of a high-resolution T2- weighted sequence through the brainstem. ssessment of the cisternal, canalicular, and labyrinthine segments requires high-resolution T2-weighted and contrast-enhanced T1- weighted images through the internal auditory canals. oth temporal bone CT and MRI can be used to assess the geniculate ganglion, tympanic and mastoid segments of the facial nerve, and stylomastoid foramen. eyond this level, the extracranial segment and its branches are best evaluated on MRI with coverage of the parotid glands. Congenital The first and second branchial arches and first pharyngeal pouch develop concurrently in utero. The pharyngeal pouch gives rise to the external auditory canal (EC), and the first and second branchial arches form the auditory ossicles and muscles of the middle ear. ecause the facial nerve also develops from the second pharyngeal arch, all congenital aural dysplasias should be evaluated for the presence, morphology, and course of the facial nerve. External auditory canal (EC) atresia, also known as congenital aural dysplasia or microtia, presents with a constellation of findings, including hypoplastic auricle, stenotic or aplastic EC with bony or membranous covering, hypoplastic middle ear with fused and malrotated ossicles, and anteriorly displaced tympanic and mastoid segments of CN VII (Fig. 2). The oval window arises from the otic capsule, but its development is induced by the footplate of the stapes. Oval window atresia is postulated to be related to anterior and inferior displacement of the tympanic and descending portions of the facial nerve, which places the nerve between the stapes anlage and oval window and prevents induction of the oval window. nother theory relates oval window atresia to underdevelopment of the second branchial arch. The incus and stapes may be malformed, and the tympanic segment of CN VII can be inferomedially positioned, overhanging the obliterated oval window (Fig. 3). Congenital facial nerve dehiscence is typically a sporadic anomaly in which there is deficiency of bone overlying the facial nerve canal. In rare cases, the nerve can prolapse through the defect and mimic a mass (Fig. 4). Infection and Inflammation Within the temporal bone, there is a rich circumneural arteriovenous plexus that surrounds CN VII. s a result, the geniculate ganglion, tympanic segment, and mastoid segment normally enhance. However, enhancement of the cisternal, canalicular, or labyrinthine segment is generally pathologic. In the absence of a discrete mass, an infectious or inflammatory cause is likely. The most common cause of acute facial nerve paralysis is known as ell palsy. The cause is unknown, although a possible association with herpes simplex virus (HSV-1) has been described. The majority of cases are selflimited, with symptoms peaking after several days (up to 2 weeks) (Fig. 5). Viral causes include Ramsay-Hunt syndrome (varicella zoster virus), HIV, and polio (Figs. 6 and 7). acterial organisms include orrelia burgdorferi (Lyme disease), Staphylococcus, Streptococcus, and Mycobacterium. Common fungal infections include spergillus and mucormycosis (Fig. 8). Inflammatory causes include sarcoidosis and Langerhans cell histiocytosis, which may show a more nodular and masslike appearance (Fig. 9). Demyelinating Demyelinating disorders that involve the white matter of the brain or spinal cord can also affect the cranial nerves. In active disease, the nerve may show abnormal enhancement, edema, or reduced diffusion indicative of ongoing inflammation. Facial nerve abnormality has been described in multiple sclerosis, Guillain-arré syndrome, and chronic inflammatory demyelinating polyneuropathy (Figs. 10 and 11). When patients with demyelinating disease present with facial palsy, the imaging protocol should include contrast-enhanced MRI of the brain and internal auditory canals. enign Neoplasms enign neoplasms of the facial nerve include schwannoma and paraganglioma. Schwannoma is a tumor of the Schwann cells lining the nerve. This rarely involves CN VII in the geniculate, tympanic, or mastoid segments. There is characteristic fusiform nerve enlargement and smooth bone erosion (Fig. 12). Lesions can be bilateral in patients with neurofibromatosis type II. Paraganglioma is a rare neuroendocrine tumor that can arise anywhere along the sympathetic chain, with common locations including the jugular fossa (glomus jugulare), middle ear (glomus tympanicum), vagal ganglia (glomus vagale), and carotid body (glomus caroticum). Paraganglioma of JR:204, June 2015 W613

3 Ho et al. Downloaded from by on 01/30/18 from IP address Copyright RRS. For personal use only; all rights reserved the fallopian canal of the facial nerve (glomus faciale) is even rarer, with only a handful of cases in the literature. It is theorized that these arise from glomus formations associated with branches of the rnold nerve that enter the mastoid segment of the facial canal via the mastoid canaliculi. Lesions show circumferential expansion of the mastoid segment with surrounding permeative bony destruction. These lesions enhance avidly, with a salt-and-pepper appearance in larger lesions due to flow voids, hemorrhage, or calcification (Fig. 13). Malignant Neoplasms Malignancies involving the facial nerve include CNS lymphoma, hematogenous metastases, and perineural spread of tumor. The imaging appearance is nonspecific, but lesions are often heterogeneous, nodular, and multifocal (Fig. 14). Perineural spread of tumor to the facial nerve has been described with parotid malignancies (particularly adenoid cystic carcinoma) and skin cancers (such as squamous cell carcinoma) (Figs. 15 and 16). Cholesteatoma Cholesteatomas are focal collections of keratinizing squamous epithelium within the middle ear or mastoid process. The acquired variant is most common and results from eustachian tube dysfunction, creating a retraction pocket in the tympanic membrane that continues to grow and accumulate skin cells to form a lobulated mass. The outer wall of the cholesteatoma is enzymatically active, which can result in bony erosion. Pars flaccida cholesteatoma is most common and is centered in the posterosuperior third of the tympanic membrane, lateral to the auditory ossicles. The characteristic appearance on CT is a soft-tissue mass with erosion of the lateral wall of the epitympanum, scutum, and ossicles. In advanced cases, the mass can also erode into the tegmen tympani or facial nerve canal, particularly the tympanic and mastoid segments (Fig. 17). Pars tensa cholesteatoma is seen in 10 20% of cases and is centered in the lower two thirds of the tympanic membrane, medial to the auditory ossicles. Lesions can expand to involve the sinus tympani, facial nerve recess, aditus ad antrum, and mastoid air cells. Congenital cholesteatoma occurs in children and is believed to arise from congenital epithelial rests, typically located at the cochlear promontory (Fig. 18). t MRI, cholesteatomas do not enhance and show characteristic reduced diffusion. Vascular Geniculate fossa venous malformation (previously known as facial nerve hemangioma) is a vascular malformation that usually affects the bone surrounding the geniculate ganglion and occasionally involves the canalicular segment of CN VII. On imaging, lesions are hypervascular with avid contrast enhancement and irregular margins. On CT, there is surrounding bony reaction with coarsening of residual trabeculae, producing a honeycomb pattern (Fig. 19). Patients may present with early peripheral facial nerve paralysis, known as atypical ell palsy. Neurovascular compression syndrome refers to symptomatic mass effect of vascular structures on cranial nerves. The anterior inferior cerebellar artery arises from the mid basilar artery and courses toward the internal auditory canal, giving rise to the labyrinthine artery. nterior inferior cerebellar artery tortuosity and aneurysms can produce mass effect on the root entry zone and cisternal-canalicular segments of CN VII (Fig. 20). Persistent stapedial artery is a rare congenital variant in which the embryonic stapedial artery fails to regress. This commonly occurs in association with an aberrant internal carotid artery. The stapedial artery emerges from the carotid canal, crosses the middle ear over the cochlear promontory, travels through the obturator foramen of the stapes, and enters and enlarges the tympanic segment of CN VII. Typically, the foramen spinosum is absent and the persistent stapedial artery gives rise to the middle meningeal artery (Fig. 21). Rarely, congenital venous abnormalities can affect the facial nerve canal. Causes include enlarged emissary veins, aberrant venous sinuses, venous lakes, and jugular bulb abnormalities (Fig. 22). Trauma and Iatrogenic Causes Transverse temporal bone fractures are more likely to violate the otic capsule with involvement of CN VII and inner ear structures. The perigeniculate area is more susceptible to injury due to traction from the greater superficial petrosal nerve. On CT, a fracture line can be seen coursing through the facial nerve, and there may be bone fragments, auditory ossicles, or hematoma impinging on the facial canal (Fig. 23). Temporal bone CT is used to evaluate patients with cochlear implants or ossicular prostheses. Malpositioned hardware can be evaluated for encroachment on the facial nerve canal. Patients treated with radiation therapy may suffer from radiation neuritis, with nonspecific cranial nerve enhancement on MRI. In addition, fatty marrow changes can be seen in a distribution corresponding to the radiation portal. Miscellaneous Facial (fallopian) canal meningocele refers to a defect in the facial nerve canal, allowing spontaneous herniation of meninges into the fallopian canal. This is usually acquired and is associated with idiopathic intracranial hypertension but in rare cases can result from a congenital defect. Clinical presentations include facial weakness, CSF otorrhea, recurrent meningitis, and spontaneous intracranial hypotension. CT shows a widened labyrinthine canal and lobulated enlargement of the geniculate fossa, which is usually bilateral. High-resolution T2-weighted MRI is recommended to confirm CSF signal in this region. CT cisternography can be performed to confirm communication with the subarachnoid space (Fig. 24). Summary CN VII is a mixed cranial nerve with a complex anatomic course that consists of intraaxial, cisternal, canalicular, labyrinthine, tympanic, mastoid, and extracranial segments. Clinical patterns of facial palsy and associated symptoms help to localize lesions as supranuclear, nuclear, or infranuclear. CT and MRI have complementary roles in imaging evaluation, depending on the segment or segments of interest. Causes of disease include congenital, inflammatory, infectious, demyelinating, neoplastic, cholesteatoma, vascular, traumatic, and iatrogenic. Knowledge of the normal anatomy, embryology, and spectrum of pathology will aid radiologists in correctly diagnosing lesions of the facial nerve. Suggested Reading 1. Toulgoat F, Sarrazin JL, enoudiba F, et al. Facial nerve: from anatomy to pathology. Diagn Interv Imaging 2013; 94: Veillona F, Ramos-Taboada L, bu-eid M, Charpiot, Riehm S. Imaging of the facial nerve. Eur J Radiol 2010; 74: Raghavan P, Mukherjee S, Phillips CD. Imaging of the facial nerve. Neuroimaging Clin N m 2009; 19: Gupta S, Mends F, Hagiwara M, Fatterpekar G. Roehm PC. Imaging the facial nerve: a contemporary review. Rad Res Prac 2013; 2013: May M, Schaitkin M. The facial nerve. New York, NY: Thieme, 2000 (Figures start on next page) W614 JR:204, June 2015

4 natomy and Pathology of the Facial Nerve Downloaded from by on 01/30/18 from IP address Copyright RRS. For personal use only; all rights reserved C Fig. 1 Normal facial nerve anatomy., xial high-resolution T2-weighted MR image shows cisternal segment (black arrow) emerging from root exit zone in cerebellopontine angle and entering internal auditory canal to become canalicular segment (white arrows)., xial high-resolution T2-weighted MR image shows labyrinthine segment (thin arrow) coursing between cochlea and vestibule of inner ear and connecting to geniculate ganglion (thick arrow). C, xial temporal bone CT image shows labyrinthine segment (black arrow) and geniculate ganglion (white arrow). D, xial temporal bone CT image shows tympanic segment (arrows) coursing backward from geniculate ganglion through middle ear cavity. E, Coronal temporal bone CT image shows mastoid segment coursing vertically within temporal bone and exiting via stylomastoid foramen (arrow). Fig. 2 External auditory canal atresia. xial temporal bone CT image shows absence of external auditory canal with atretic bony plate (asterisk). There is anterior displacement of mastoid segment of cranial nerve VII (arrow). D E Fig. 3 Oval window stenosis. Coronal temporal bone CT image shows stenosis of oval window (thick arrow). Tympanic segment of cranial nerve VII (thin arrow) is inferomedially positioned and overhangs this region. Fig. 4 Facial nerve dehiscence. and, xial () and coronal () temporal bone CT images show absence of bony covering of facial nerve canal (arrows), with dehiscence of nerve inferiorly into stapes footplate. JR:204, June 2015 W615

5 Ho et al. Downloaded from by on 01/30/18 from IP address Copyright RRS. For personal use only; all rights reserved Fig. 5 ell palsy. xial T1-weighted contrast-enhanced MR image with fat saturation shows abnormal enhancement of labyrinthine segment (arrow) of cranial nerve VII. This segment is contiguous distally with normally enhancing geniculate ganglion and tympanic segment. Fig. 6 Ramsay-Hunt syndrome. xial T1-weighted contrastenhanced MR image with fat saturation shows avid enhancement of distal canalicular (asterisk) and labyrinthine (arrow) segments of cranial nerve VII, contiguous distally with normally enhancing geniculate ganglion and tympanic segment. Fig. 7 Lyme disease. xial T1-weighted contrast-enhanced MR image shows avid bilateral enhancement of canalicular (thick arrows) and labyrinthine (thin arrows) segments of cranial nerve VII. There is normal enhancement of geniculate ganglia and tympanic segments. Fig. 8 Mucormycosis fungal infection. and, xial () and coronal () temporal bone CT images show irregular soft-tissue expanding mastoid segment of facial nerve (asterisks). Surrounding bone destruction extends into jugular foramen (arrows). Mastoid air cells are completely opacified. Fig. 9 Langerhans cell histiocytosis. and, xial T2-weighted MR image with fat saturation () and T1-weighted contrast-enhanced MR image () show expansile avidly enhancing soft-tissue mass (asterisks) within mastoid portion of temporal bone. There is surrounding bone destruction with extension into tympanic and mastoid segments of facial nerve (arrows). W616 JR:204, June 2015

6 natomy and Pathology of the Facial Nerve Downloaded from by on 01/30/18 from IP address Copyright RRS. For personal use only; all rights reserved Fig. 10 Multiple sclerosis., xial FLIR MR image shows multiple foci of hyperintense signal intensity radiating from lateral ventricles., xial T1-weighted contrast-enhanced MR image shows avid enhancement of distal canalicular (thick arrow) and labyrinthine (thin arrow) segments of cranial nerve VII, contiguous distally with normally enhancing geniculate ganglion and tympanic segment. Fig. 11 Chronic inflammatory demyelinating polyneuropathy. xial T1-weighted contrastenhanced MR image shows abnormal enhancement of left distal canalicular and bilateral labyrinthine segments (arrows) of cranial nerve VII. There is contiguous enhancement of bilateral geniculate ganglia and tympanic segments, which is greater than expected for healthy patient. Fig. 12 Facial nerve schwannoma., xial T2-weighted MR image shows ovoid hyperintense mass smoothly expanding right geniculate fossa (arrow)., xial T1-weighted contrast-enhanced MR image shows homogeneous contrast enhancement (arrow). Fig. 13 Glomus faciale., xial T2-weighted MR image with fat saturation shows lobulated expansile lesion enlarging stylomastoid foramen, with internal flow voids (arrows)., Coronal T1-weighted contrast-enhanced MR image shows tumor extending along mastoid segment of facial nerve (arrows), with heterogeneous enhancement. JR:204, June 2015 W617

7 Ho et al. Fig. 14 CNS lymphoma., xial T1-weighted contrast-enhanced MR image shows rounded enhancing lesion in left centrum semiovale (arrow)., xial T1-weighted contrast-enhanced MR image shows streaky enhancement in right internal auditory canal (arrow). Downloaded from by on 01/30/18 from IP address Copyright RRS. For personal use only; all rights reserved Fig. 17 cquired cholesteatoma. xial temporal bone CT image shows expansile soft-tissue mass with complete opacification of middle ear (asterisk). There is destruction of surrounding bone, including bony facial nerve canal in its tympanic segment (arrow). Mastoid air cells are completely opacified. Fig. 15 Perineural spread of adenoid cystic carcinoma of parotid gland., Coronal temporal bone CT image shows soft-tissue enlargement of stylomastoid foramen (arrow) and mastoid segment of cranial nerve VII., Coronal T1-weighted contrast-enhanced MR image with fat saturation shows enhancing soft tissue along extratemporal and mastoid segments of cranial nerve VII (arrow). Fig. 16 Perineural spread of cutaneous squamous cell carcinoma., Coronal T1-weighted contrast-enhanced MR image with fat saturation shows nodular enhancing tissue extending superiorly through stylomastoid foramen (arrow) and into mastoid segment of central nerve (CN) VII., xial T1-weighted contrast-enhanced MR image shows proximal extension of tumor with enlarged and enhancing tympanic segment and geniculate ganglion (arrow) of CN VII. Fig. 18 Congenital cholesteatoma. xial temporal bone CT image shows wellcircumscribed, rounded mass in epitympanum (asterisk). There is focal erosion along tympanic segment of cranial nerve VII (arrow). W618 JR:204, June 2015

8 natomy and Pathology of the Facial Nerve Fig. 19 Geniculate fossa venous malformation., xial T1-weighted contrast-enhanced MR image shows avidly enhancing geniculate fossa lesion (arrow) with irregular margins., xial temporal bone CT image shows honeycomb pattern of bone change (arrow) in geniculate fossa. Downloaded from by on 01/30/18 from IP address Copyright RRS. For personal use only; all rights reserved Fig. 20 Neurovascular compression. xial highresolution T2-weighted MR image shows tortuous anterior inferior cerebellar artery (arrow), which contacts cranial nerves VII and VIII. Fig. 21 Persistent stapedial artery., Coronal temporal bone CT image shows anomalous vessel arising from carotid canal (asterisk) and coursing over cochlear promontory., xial temporal bone CT image shows anomalous vessel merging distally with tympanic segment of cranial nerve VII (arrow), which appears abnormally thickened as result. Fig. 22 Enlarged emissary veins in facial canal. xial temporal bone CT image shows enlarged petrosal and superior tympanic veins coursing within and enlarging bony canal for tympanic segment of cranial nerve VII (arrow). Fig. 23 Transverse temporal bone fracture. xial temporal bone CT image shows linear fracture coursing perpendicular to long axis of temporal bone. long anterior margin, fracture involves tympanic segment of cranial nerve VII (thick arrow). Fracture continues through vestibule into petrous ridge (thin arrow). Fig. 24 Fallopian canal meningocele., xial temporal bone CT image shows enlargement of labyrinthine segment of facial nerve and geniculate ganglion (arrow)., xial high-resolution T2-weighted MR image shows CSF signal intensity in lesion (arrow). JR:204, June 2015 W619