Imaging of the Facial Nerve

Imaging of the Facial Nerve Prashant Raghavan, MDa,*, Sugoto Mukherjee, MDa, C. Douglas Phillips, MDb KEYWORDS  Facial nerve  CT  MR imaging

ANATOMY The facial nerve is a complex nerve with motor, sensory, and parasympathetic fibers. The motor division is dominant, accounting for approximately 70% of the total axons, with the remainder composed of the sensory division and the nervus intermedius (nerve of Wrisberg). A very important clinical observation in the evaluation of the facial nerve is in distinguishing central and peripheral facial weakness. To a careful clinician, the location of pathologic factors can be deduced based on specific signs and symptoms. A well-understood concept used in clinical evaluation of the facial nerve is that a central palsy spares the forehead and brow, whereas a peripheral palsy involves the entirety of the facial musculature along with loss of emotional or involuntary facial motion. This is the result of the typical bilateral supranuclear innervation of the upper facial musculature. The motor division supplies somatic motor fibers to the muscles of the face, scalp, and auricle, the

buccinator and platysma, the stapedius, the stylohyoideus, and the posterior belly of the digastric; it also contains some sympathetic motor fibers, which constitute the vasodilator nerves of the submandibular and sublingual glands and are conveyed through the chorda tympani nerve. The facial nerve also demonstrates clinically relevant communications with other cranial and cervical spinal nerves (Table 1).1–4

Supranuclear Control: Upper Motor Neuron The supranuclear contribution from the motor cortex originates from pyramidal neurons located in the lower third of the precentral gyrus of the frontal motor cortex. The cortical representation of the face, from the uppermost cortex to the lower, begins with the forehead, followed by the periorbital, midface, and perioral muscles, with the motor cortex for the tongue following them. The cortical projection fibers join the corticobulbar tract and pass within the posterior aspect of the genu of the internal capsule into the medial third of the cerebral peduncle within the midbrain. Coursing inferiorly through the pontine pyramidal tract, most fibers decussate in the caudal pons to reach the contralateral facial motor nucleus. Some fibers do not decussate, and synapse in the ipsilateral facial nucleus. The cortical projection fibers for the upper face demonstrate incomplete decussation, and project to the ipsilateral and the contralateral facial nuclei. However, the supranuclear fibers for the lower facial muscles completely decussate to the contralateral facial

a Division of Neuroradiology, University of Virginia Health System, PO Box 800170, 1215 Lee St, Charlottesville, VA 22908, USA b Department of Neuroradiology, Weill Cornell Medical College, Starr 633A, 510 E. 70th Street, New York, NY 10065, USA * Corresponding author. E-mail address: [email protected] (P. Raghavan).

Neuroimag Clin N Am 19 (2009) 407–425 doi:10.1016/j.nic.2009.06.008 1052-5149/09/$ – see front matter ª 2009 Elsevier Inc. All rights reserved.

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Imaging plays an important role in the evaluation of the complex anatomy of the facial nerve. The nerve is affected by a wide variety of primary pathologic processes and may also be secondarily involved in several congenital, inflammatory, and traumatic and neoplastic disorders of the temporal bone and parotid gland. Conceptually, the facial nerve is best divided along its course into several segments. Some disease processes may involve one segment, whereas others may involve multiple segments.

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Table 1 Communications of the facial nerve Site

Communication

Internal acoustic meatus Geniculate ganglion

Vestibulocochlear nerve Sphenopalatine ganglion by the GSPN Otic ganglion by a branch that joins the lesser superficial petrosal nerve Sympathetic nerves around the middle meningeal artery Auricular branch of the vagus nerve. With the glossopharyngeal, vagus, great auricular (C2, C3), and the auriculotemporal (V3) nerves With the lesser occipital nerve With the trigeminal nerve With the cutaneous cervical nerve

Facial canal Stylomastoid foramen Behind the ear On the face In the neck

nucleus. Hence, unilateral supranuclear lesions often somewhat spare the functions of the upper face.1,4,5 A neglected aspect is the emotional control of facial motion provided by extrapyramidal input, which travels within the reticular formation from the frontal areas, thalamus, and globus pallidus to the facial motor nucleus. When a supranuclear lesion spares this input, emotional facial expressions are generally preserved, despite the presence of facial palsy. Hence, lesions of the midbrain or thalamus can cause contralateral palsy of emotional facial expressions with intact voluntary motion. Also, lesions of the globus pallidus or its projections to the facial nucleus can cause facial bradykinesia.2,4

Motor Nucleus The motor nucleus of the facial nerve lies within the reticular formation of the lower third of the pons, situated above the nucleus ambiguus, behind the superior olivary nucleus, and medial to the spinal tract of the trigeminal nerve. From this origin, the fibers first pass backward and medially toward the rhomboid fossa, then reach the posterior end of the nucleus of the abducens nerve, run upward beneath the facial colliculi and under the floor of the fourth ventricle. After looping around the abducens nucleus, the fibers travel laterally and caudally between the medial border of the trigeminal nucleus and the lateral edge of the facial motor nucleus. This anatomic relationship explains how certain disorders of facial nerve motor function can be associated with abducens palsy (Fig. 1). The motor fibers finally exit the caudolateral border of the pons in the cerebellopontine angle medial to cranial nerve VIII (CN; vestibulocochlear nerve). The medial-to-lateral loop around the abducens

nucleus is called the internal genu of the facial nerve.1,4,5

Nervus Intermedius and Sensory Nucleus The nervus intermedius (also called the nerve of Wrisberg) contains specialized fibers that convey sensation from the posterior region of the external auditory canal and concha; taste from the anterior two thirds of the tongue; and secretomotor function for the lacrimal glands, submandibular and sublingual glands, and minor salivary glands in the nasal cavity, paranasal sinuses, and palate. In the cerebellopontine angle (CPA), the nervus intermedius travels from the pons to the internal auditory meatus between the motor root and CN VIII. The chorda tympani and the greater superficial petrosal nerve (GSPN) are subdivisions of this nerve (Fig. 2).The nervus intermedius also carries the somatic sensations from the posterior aspect of the external auditory canal, pinna, and mastoid region (within the posterior auricular nerve to the dorsal part of the primary sensory nucleus of trigeminal nerve within the pons).1,4,5

Parasympathetic Nucleus The parasympathetic component of the facial nerve originates from the superior salivatory nucleus in the dorsal pons. The facial nerve provides presynaptic parasympathetic fibers to the sphenopalatine and pterygopalatine ganglion for innervation of the lacrimal glands and to the submandibular ganglion for innervation of the sublingual and submandibular salivary glands. The pterygopalatine ganglion is associated with the maxillary nerve (CN V2), whose postsynaptic fibers it distributes, whereas the submandibular ganglion is associated with the mandibular nerve (CN V3). Nerve fibers for these nerves are carried

Imaging of the Facial Nerve

Fig.1. Axial graphic image (A) and heavily T2-weighted MR image (B) through the pons at the level of the facial colliculi demonstrate the course of the facial nerve within the brainstem. Note the loop of the facial nerve (internal genu) around the abducens nerve nucleus. FN, facial nerve; NTS, nucleus tractus solitarius; SSN, superior salivatory nucleus. (From Harnsberger H, Osborn A, Macdonald A, et al. Diagnostic and surgical imaging anatomy: brain, head and neck, spine. Salt Lake City, Utah: Amirsys; 2006. p. 224–31; with permission.)

distally from the pons to the geniculate ganglion within the nervus intermedius. The supply to the lacrimal gland is through the GSPN, which arises at the geniculate ganglion and travels anteromedially through the middle cranial fossa within the pterygoid canal. The GSPN then joins with the deep petrosal nerve (sympathetic) at the foramen lacerum to form the nerve of the pterygoid canal (vidian nerve), which travels through the pterygoid canal to terminate in the pterygopalatine ganglion in its fossa. Postganglionic parasympathetic fibers from there innervate the lacrimal gland by way of the zygomatic branch of CN V2, which passes these fibers on to the lacrimal nerve, which is a branch of CN V1.1,4,5 The preganglionic fibers for the submandibular and sublingual salivary glands pass through the geniculate ganglion and then leave the mastoid segment of the facial nerve within the chorda tympani. The chorda tympani travels through the temporal bone, medial to the handle of malleus, and enters the infratemporal fossa by way of the petrotympanic fissure, where it joins the lingual nerve (CN V3). The parasympathetic fibers synapse with postganglionic cells within the submandibular ganglion, which then innervate the submandibular and sublingual salivary glands.1

Nucleus Tractus Solitarius This nucleus receives taste sensation from the anterior two thirds of the tongue, traveling within

the chorda tympani (see Fig. 2). The cell bodies for taste fibers are located within the geniculate ganglion, with afferent fibers being carried

Fig. 2. Sagittal oblique graphic image highlights the components of the facial nerve. The motor component (yellow), the secretomotor fibers in the chorda tympani (blue), and the parasympathetic fibers that exit through the GSPN (purple) are shown. (From Harnsberger H, Osborn A, Macdonald A, et al. Diagnostic and surgical imaging anatomy: brain, head and neck, spine. Salt Lake City, Utah: Amirsys; 2006. p. 224–31; with permission.)

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Raghavan et al centrally by the nervus intermedius to cells in the nucleus tractus solitarius in the medulla. 1,4,5

PERIPHERAL COURSE OF FACIAL NERVE The peripheral course is subdivided into the cisternal segment in the CPA, the intracanalicular segment in the internal auditory canal (IAC), the labyrinthine segment, the tympanic segment, the mastoid segment, and the extracranial segment.1,5,6

Cisternal Segment in the Cerebellopontine Angle The motor root and the nervus intermedius emerge from the brain stem ventrolaterally near the dorsal pons and travel anterolaterally into the IAC (see Fig. 1), with the nervus intermedius between the motor root of the facial nerve and the vestibulocochlear nerve (hence its name). The vestibulocochlear nerve travels laterally and slightly inferiorly to the facial nerve. The trigeminal nerve is located anteriorly. The anterior inferior cerebellar artery is associated with the nerves and usually is found ventrally, between the nerves and the pons. In the CPA, the facial nerve has no epineurium and is covered only by the pia mater.1,5,6

Internal Auditory Meatus of the Temporal Bone At the internal auditory meatus of the temporal bone, the nervus intermedius joins the motor root to form a common trunk that occupies the area known as the nervi facialis, which lies within the anterosuperior segment of the meatus (Fig. 3). The motor fibers are more anterior, whereas the nervus intermedius fibers remain posterior. At the lateral end of the meatus, a horizontal partition of dura, and occasionally of bone, which is called the transverse or falciform crest, separates the facial nerve from the cochlear nerve inferiorly. Additionally, an incomplete vertical dural and osseous crest (Bill’s bar) separates the facial nerve from the superior vestibular nerve, which is

located posteriorly. The blood supply to this region is from the labyrinthine artery, which is a branch of the anterior inferior cerebellar artery.1,5,6

Facial/Fallopian Canal The facial/fallopian canal consists essentially of three segments, the labyrinthine, tympanic, and mastoid, and it extends from Bill’s bar to the stylomastoid foramen.1,5,6 The labyrinthine segment (Fig. 4) extends from the anterosuperior region of the fundus to the geniculate ganglion. It passes between the ampulla of the superior semicircular canal and the cochlea to travel forward and downward. At the geniculate ganglion, the nerve makes an abrupt sharp turn posteriorly, thus creating the external genu. This marks the beginning of the tympanic segment. Exiting anteriorly from the geniculate ganglion is the first branch of CN VII, the greater superficial petrosal nerve (GSPN). The tympanic segment continues along the medial wall of the tympanic cavity, medial to the incus. It courses superior and posterior to the cochleariform process, along the upper edge of the oval window, and inferior to the lateral semicircular canal (see Fig. 4). At the origin of the stapedius tendon from the pyramidal process, the nerve turns inferiorly to become the mastoid segment (see Fig. 4). The mastoid segment has two branches: the nerve to the stapedius and the chorda tympani. During its course through the mastoid along the posterior aspect of the external auditory canal, the nerve travels slightly posteriorly and usually passes lateral to the inferior aspect of the tympanic annulus. The major blood supply for the facial nerve within the facial canal is the superficial petrosal branch of the middle meningeal artery (proximally) and the stylomastoid artery (distally).1,5,6

Stylomastoid Foramen and Extracranial Segment The facial nerve exits the temporal bone by way of the stylomastoid foramen, deep in the posterior Fig. 3. Sagittal reformatted, high-resolution, three-dimensional, T2-weighted image through the IAC. The facial nerve (thick arrow) is seen in the anterosuperior quadrant of the IAC, above the cochlear nerve (thin arrow) and anterior to the vestibular nerves. (From Harnsberger H, Osborn A, Macdonald A, et al. Diagnostic and surgical imaging anatomy: brain, head and neck, spine. Salt Lake City, Utah: Amirsys; 2006. p. 224–31; with permission.)

Imaging of the Facial Nerve

Fig. 4. Axial (A), coronal (B), and oblique sagittal images demonstrate the segments of the fallopian canal. (A) The labyrinthine segment (thick arrow) and the geniculate fossa (thin arrow) are evident. (B) The tympanic segment is observed to lie inferior to the lateral semicircular canal. (C) The tympanic segment (black solid arrow), the posterior genu (dashed arrow), and the mastoid segment (white arrow) are thrown into profile.

belly of the digastric, and immediately enters the parotid gland. Shortly afterward, the nerve divides into its two terminal branches: the upper temporofacial nerve and a lower cervicofacial nerve at the posterior border of the ramus of the mandible. In the parotid gland, these nerves further divide to form the parotid plexus, which has five major branches: the temporal, zygomatic, buccal, marginal mandibular, and cervical.1,5,6 Imaging procedures are not routinely able to visualize these branches of the facial nerve in their normal state.

Vascular Supply of the Facial Nerve The cortical motor area of the face is supplied by the branches of the middle cerebral artery. The

facial nucleus within the pons receives its blood supply primarily from the anterior inferior cerebellar artery (AICA). The AICA, which is a branch of the basilar artery, enters the IAC with the facial nerve. The AICA branches into the labyrinthine and cochlear arteries. The superficial petrosal branch of the middle meningeal artery also supplies the intrapetrosal facial nerve. The posterior auricular artery supplies the facial nerve at and distal to the stylomastoid foramen. Venous drainage parallels the arterial blood supply. Also, mild enhancement of the labyrinthine segment, geniculate ganglion, and proximal tympanic segments can be normal on postcontrast MR, presumably secondary to the presence of a circumneural venous plexus in these segments.1,4,5

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Raghavan et al FACIAL NERVE PARALYSISçCLINICAL EVALUATION AND IMAGING STRATEGIES Facial paralysis may be consequent to a wide variety of central and peripheral disorders (Table 2). Evaluation of the patient who has facial palsy begins with a detailed history and thorough clinical examination. The severity of paralysis is commonly judged based on the House-Brackmann Facial Nerve Grading System, although many other grading systems exist (Table 3).7–10 It is usually

possible to distinguish between upper and lower motor neuron causes of facial paralysis based on the observation that an upper motor neuron process tends to spare the upper face because of the bilaterality of supranuclear control, whereas a lower motor neuron process affects the upper and lower facial musculature. It may also be possible to localize, in some instances, the precise level of lower motor neuron facial palsy based on affliction of the GSPN (lacrimation), nerve to stapedius (hyperacusis), and the chorda tympani

Table 2 Causes of facial nerve paralysis Type of Paralysis

Cause

Supranuclear

Cerebral infarction, hemorrhage, vascular lesions, demyelinating diseases, brain tumors, trauma, cardiofacial (opercular) syndrome - Inflammation and infection: cerebritis, abscess, multiple sclerosis - Vascular: arteriovenous and cavernous malformations, stroke, Millard-Gubler syndrome - Tumor: lymphoma, brainstem glioma, metastasis - Congenital: absent facial nerve, Mobius syndrome - Trauma: brainstem injuries. - Inflammation and infection: viral infections, sarcoidosis, tubercular and bacterial infections, Ramsay Hunt Syndrome (herpes simplex) - Vascular: vertebrobasilar dolichoectasia, vertebrobasilar and PICA or AICA aneurysms and loops, superficial siderosis - Tumor: neurofibroma, schwannoma, meningioma, epidermoid, carcinomatous meningitis, leukemia, lymphoma - Trauma: basilar skull fractures - Inflammation and infection: Bell’s palsy, otitis media, mastoiditis, meningo-encephalitis, Ramsay Hunt Syndrome (herpes zoster oticus), poliomyelitis, leprosy, Lyme disease, tuberculosis, syphilis, HIV - Vascular: intratemporal ICA aneurysm, anomalous sigmoid sinus - Tumor: glomus tympanicum or jugulare, hemangiomas and other IBVTs, facial nerve schwannoma, choristoma, perineural tumor spread - Congenital: abnormalities of facial nerve canal and course. - Trauma: surgery, fractures through facial nerve canal - Inflammation and infection: malignant otitis externa, Ramsay Hunt syndrome(herpes zoster oticus) - Tumor: parotid tumors, metastatic tumors, perineural spread - Trauma: forceps delivery; penetrating traumas; mandibular block anesthesia; otologic, neurotologic, skull base, and parotid surgery Myasthenia gravis, diabetes, hypertension, hyperthyroidism, alcoholism, exposure to thalidomide or misoprostol Mobius, Di George, Poland, Goldenhar, CHARGE, and trisomy 13 and 18 syndromes Melkersson-Rosenthal syndrome, hereditary hypertrophic neuropathy, temporal arteritis, polyarteritis nodosa and other vasculitides, Landry-Guillain-Barre´ syndrome (ascending paralysis)

Brainstem

Cisternal and intracanilicular

Intratemporal

Extratemporal

Miscellaneous Congenital Idiopathic

Abbreviation: PICA, posterior inferior cerebellar artery.

Imaging of the Facial Nerve

Table 3 House-Brackmann facial nerve grading system Grade

Description

Characteristics

I II

Normal Mild dysfunction

III

Moderate dysfunction

IV

Moderately severe dysfunction

V VI

Severe dysfunction Total paralysis

Normal facial function in all areas Slight weakness noticeable on close inspection; may have very slight synkinesis Obvious, but not disfiguring, difference between two sides; noticeable, but not severe, synkinesis, contracture, or hemifacial spasm; complete eye closure with effort Obvious weakness or disfiguring asymmetry; normal symmetry and tone at rest; incomplete eye closure Only barely perceptible motion; asymmetry at rest No movement

(impaired taste and salivation). Lesions may therefore be classified as suprageniculate, suprastapedial, and suprachordal, respectively. In many instances, such localization is not possible because lesions may span several segments of the nerve.11,12 Central facial palsy is initially investigated using MR imaging. Peripheral facial nerve disorders may be evaluated using high-resolution CT (HRCT), MR imaging, or both. At the institution at which the authors work, HRCT is performed using a multidetector CT scanner in the axial plane 30 superior to the anthropological baseline or parallel to the hard palate. Images are acquired using an edge-enhancing algorithm, with a slice thickness of 0.625 mm at 0.3-mm intervals. The images are viewed at a wide window level, and width and are displayed with a small field of view. Because isotropic voxels can be obtained using helical scanners, these images can be reconstructed in any plane at a workstation, without loss of resolution, to evaluate the entire course of the facial nerve comprehensively. MR imaging incorporates precontrast, 3-mm axial and coronal T1-weighted images; axial fast spin echo images through the brainstem; an axial volumetric, heavily T2-weighted sequence; and postcontrast, fat-suppressed, 3-mm T1-weighted images through the temporal bones. The examination can be tailored to provide answers to specific clinical questions, such as the addition of diffusion-weighted imaging for cases of acute central facial paralysis and MR-angiography for hemifacial spasm. A heavily T2-weighted image provides excellent contrast between the facial and vestibulocochlear nerves and surrounding vascular structures in the CPA cistern and the IAC. Source data from this sequence may be reconstructed in any plane.13

FACIAL NERVE PATHOLOGY Congenital Anomalies of the Facial Nerve The facial nerve begins to develop at 3 weeks of life from the facio-acoustic primordium. Its neural connections are completely established by the 16th week of fetal life. The bony facial canal continues to develop after the 16th week into late fetal life. It is closed by bone in most areas, except at the facial hiatus in the floor of the middle cranial fossa. The canal is, however, also commonly dehiscent, especially at the oval window. The development of the facial nerve is intimately related to that of the structures of the middle ear, external ear, parotid gland, and facial muscles. The facial nerve separates from the acoustic nerve at 5 to 6 weeks of fetal life, at which time the nervus intermedius develops. An in utero insult before this point in time therefore can affect both nerves. Congenital malformations of the facial nerve may range from those that are asymptomatic to those that manifest with profound facial paralysis. Variation in the course of the facial nerve may be observed as an isolated anomaly, with the segment of the nerve distal to the geniculate ganglion being most often affected, or may be evident as part of a wider congenital aural anomaly. The nerve has been observed to divide into two or, rarely, three subunits, with the branches either coursing parallel to each other or diverging. Although a bifurcation anomaly is most often observed affecting the nerve in its distal intratemporal course, rare labyrinthine segment bifurcations have also been reported.14 Alteration in the course of the facial nerve is an important factor influencing surgical planning in patients who have aural atresia (Fig. 5). The descending segment, the second genu, and the pyramidal

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Fig. 5. Aural atresia. Anomalous facial nerve course in aural atresia. (A) Axial and coronal HRCT images show an absence of the tympanic segment and of the right anterior genu. The nerve is not seen beneath the lateral semicircular canal (white arrow) and exits the petrous temporal bone from it anterior aspect. (B) Coronal images at a plane posterior to the anterior genu from the same patient reveal the absence of the geniculate fossa and of the tympanic segment. Only the labyrinthine segment is demonstrated. The normal left facial nerve is shown for comparison.

eminence are often displaced anteriorly and laterally. The nerve may also pass directly through the atresia plate. The angle of the anterior genu may be excessively obtuse. The tympanic segment may be displaced inferiorly to lie over the oval window. The stylomastoid foramen may also be displaced anterolaterally, with the facial nerve exiting at the level of the round window. This may result in increased susceptibility of the nerve to injury at surgery when the auricle is elevated. Anomalies of the GSPN and the chorda tympani have also been reported. Anteromedial displacement of the facial nerve has also been described in cases of congenital non–Mondini-type cochlear malformations.15–18 Bony dehiscence of the facial nerve canal is observed in 30% to 50% of the population. This condition is presumed to arise from a failure of closure of the fallopian canal caused by a malformation of Reichert’s cartilage. It most often occurs at the level of the oval window, but may also be encountered in the mastoid facial nerve canal and in the region of the geniculate ganglion. The nerve is covered by a delicate fibrous membrane and can protrude into the middle ear cleft. Although most patients who have this condition are asymptomatic, a few manifest conductive hearing loss, especially in cases in which the crura of the stapes are attached to the nerve. It has also been suggested that a dehiscence may predispose the nerve to inflammatory processes. The CT appearance of canal dehiscence with facial nerve protrusion is that of a smoothly marginated soft tissue density at the oval window, with inferior convexity related to the undersurface of the lateral semicircular canal, on coronal sections (Fig. 6). This condition, if unrecognized, may result in

inadvertent injury at surgery. A pure dehiscence without protrusion cannot be reliably diagnosed using CT because the wall of the normal facial nerve canal is extremely thin.18–20 Abnormalities of the facial nerve nucleus may occur as a result of several syndromes that can present with congenital facial paralysis. These include the Mobius, Di George, Poland, Goldenhar, CHARGE, and trisomy 13 and 18 syndromes, among others. Mobius syndrome is characterized by congenital, bilateral sixth and seventh cranial palsy associated with a variety of extremity, facial, and thoracic anomalies. Both ischemic injury to the brainstem and genetic abnormalities have been postulated as etiologic factors. Thalidomide

Fig. 6. Coronal HRCT image shows dehiscence of the tympanic segment of the facial canal. The nerve is not seen in its expected location inferior to the lateral semicircular canal, but instead lies inferior to the oval window (black arrow).

Imaging of the Facial Nerve and misoprostol exposure in utero have also been implicated. Several imaging findings have been reported. These include brain stem and cerebellar hypoplasia, calcification in CN nuclei, absence of the facial and hypoglossal eminences in the dorsal brainstem, and nonvisualization of the sixth and seventh nerves on MR images. From a clinical standpoint, absence of the facial nerve, demonstrated on MR is very useful for making an early diagnosis and to avoid unnecessary additional tests.19–21 Abnormalities associated with CHARGE syndrome include colobomas, heart defects, choanal atresia, mental retardation, and genitourinary and ear anomalies. Abnormalities specific to the facial nerve include alterations of the course of the nerve if associated with aural atresia, tympanic segment prolapse, and more commonly, posterior displacement of the labyrinthine segment. These anomalies may be associated with oval window atresia (Fig. 7). Given the high percentage of patients who have CHARGE syndrome and an anomalous facial nerve course, it is imperative to evaluate the entire course of the nerve to avoid iatrogenic surgical trauma.22 Congenital hypoplasia and aplasia of the facial nerve may also occur as an isolated abnormality (Fig. 8). In hypoplasia and aplasia of the eighth nerve, the facial nerve may demonstrate an anomalous course. It may exit the IAC in its middle third in a separate canal, reside in a completely separate canal, or even pass between the temporal bone and temporal lobe (Figs. 9 and 10).16

Facial Nerve Trauma Fractures of the temporal bone may be associated with facial paralysis in up to 50% of cases. Facial paralysis is usually delayed, incomplete, and transient. Although any fracture of the temporal bone can result in facial nerve injury, it is more likely to occur with transverse (38%–50% of cases) rather than with longitudinal (20%) fractures. Immediate facial paralysis usually indicates severe nerve injury (transection), whereas paralysis of delayed onset is usually due to an intramural hematoma. An intraneural hematoma may result from traction exerted by the GSPN, with the resultant edema causing compressive ischemia of the nerve, usually at the meatal foramen, which is the tightest portion of the facial nerve canal. Longitudinal fractures are likely to injure the geniculate ganglion, the GSPN, and the distal tympanic and mastoid segments, in descending order of frequency. With transverse fractures, the sites of injury are the labyrinthine segment (greater than 80% of cases) and, less commonly, the geniculate ganglion and the nerve in the IAC. Fractures are demonstrated in exquisite detail on HRCT (Fig. 11). Involvement of the facial nerve canal can easily be determined. Because injury to the facial nerve itself can only be inferred indirectly based on CT, it has been suggested that MR imaging may have a role to play in the evaluation of facial nerve trauma. It is possible with MR imaging to visualize intraneural hematoma and to demonstrate nerve fiber degeneration and

Fig. 7. CHARGE association. Axial T2-weighted MR image (A) through the orbits shows a right retro-ocular colobomatous cyst (star) associated with microphthalmia. A smaller optic-nerve-head coloboma is seen on the left (black arrow). (B) A coronal reformatted CT image through the left temporal bone demonstrates an absence of the facial nerve in its expected location under the lateral semicircular canal (white arrow). It exits the middle ear cavity anteriorly (not shown). Note the atretic oval window (black arrow).

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Fig. 8. Congenital right facial paralysis due to birth trauma. Sagittal reformatted, three-dimensional CISS images demonstrate the absence of the right facial nerve. Note the empty anterosuperior quadrant in the fundus of the right IAC.

regeneration after trauma as segments of intense enhancement.13,23,24

Inflammatory Disorders of the Facial Nerve Bell’s palsy, the most common cause of facial paralysis, is characterized by an abrupt onset of facial weakness, which peaks by 48 hours, as a general rule. It affects approximately 25 people per 100,000. It has been associated with the presence of HSV-1 DNA in endoneurial fluid and postauricular muscle, suggesting that a reactivation of the virus in the geniculate ganglion may be responsible. This association has, however, yet to be proved. Bell’s palsy is believed to result from inflammatory edema of the facial nerve, with resultant compression of the vascular supply within the tight fallopian canal. The labyrinthine segment resides in the narrowest portion of the canal and

hence is most susceptible to ischemia. The diagnosis of Bell’s palsy is one of exclusion. Weakness is observed to improve in all cases, with up to 80% of patients demonstrating recovery at 6 months. Progressive weakness is distinctly unusual and warrants investigation to rule out other possible causes of facial paralysis, the most worrisome of which is a tumor. MR imaging using gadolinium demonstrates enhancement of the geniculate ganglion, the labyrinthine segment, and proximal tympanic segments without significant enlargement of the nerve (Fig. 12). Enlargement of the intratemporal segments of the facial nerve should raise suspicion for tumor. However, the canalicular segment of the nerve at the fundus of the IAC often enhances and may actually be thickened. MR imaging in the setting of classic Bell’s palsy is probably of limited value. However, any atypicality

Fig. 9. Anomalous course of the facial nerve in isolated cochlear nerve absence and cochlear aplasia. Axial (A) and coronal (B) HRCT images. Note that the facial nerve (black arrows) exits the anterior wall of the IAC in its middle third and follows an anomalous course in a separate canal. The vestibular nerve occupies a separate canal (dashed arrows).

Imaging of the Facial Nerve

Fig. 10. Anomalous course of the facial nerve in absence of the eighth CN. The facial nerve courses in an independent canal with an abnormally obtuse anterior genu (black arrow). Severe labyrinthine dysplasia is present (dashed arrow).

in presentation (eg, insidious onset, progressive course, delayed recovery) may warrant investigation using MR imaging. A wide variety of inflammatory processes may cause facial palsy. These include Lyme disease (up to 10% of facial paralysis in endemic areas), Ramsay Hunt syndrome, sarcoidosis, Guillain-Barre´ syndrome, and diabetes mellitus; connective tissue disorders such as Sjogren’s disease, leprosy, and rare entities such as the Melkersson-Rosenthal syndrome (recurrent facial paralysis, lip edema, and tongue

plication) may also cause facial paralysis. People who have infarcts and demyelination caused by multiple sclerosis may also present with acute facial paralysis.25–28 Herpes zoster oticus (Ramsay Hunt syndrome) results from reactivation of latent varicella zoster virus in the geniculate ganglion that is triggered by stress, aging, or immunosuppression. Burning pain in the external ear is followed by a vesicular eruption associated with facial palsy, vertigo, tinnitus, sensorineural hearing loss, and nystagmus. MR imaging demonstrates enhancement of the facial nerve in any or all of its segments, the vestibulocochlear nerve, the labyrinth, and also the lesions of the external auditory canal (Fig. 13). Enhancement of the intrapontine facial nerve and the pontine facial nucleus has also been described. These findings suggest that the syndrome may be a more widespread inflammatory process of the seventh and eighth nerves rather than an entity centered principally at the geniculate ganglion.29,30 The facial nerve may also be secondarily involved in inflammatory disorders of the temporal bone. About 5% of patients who have acute otitis media and 1% of patients who have cholesteatoma may present with facial nerve paralysis (Fig. 14). Malignant otitis externa may also be associated with facial palsy. The nerve at its tympanic segment is most vulnerable to such involvement. Subtle erosion of the facial nerve canal may be impossible to detect, especially because the canal wall is inherently thin. However, gross invasion of the facial nerve canal is usually demonstrable using HRCT. In equivocal cases, contrastenhanced MR imaging may provide additional information.31

Vascular Disorders of the Facial Nerve

Fig. 11. Transverse temporal bone fracture (arrows) involving the labyrinthine segment of the facial nerve.

Hemifacial spasm is characterized by unilateral involuntary twitching or spasms of some or all of the muscles that are innervated by the facial nerve. It has been accepted that neurovascular contact (NVC) at the root exit zone (REZ, defined as the junctional zone between the central oligodendroglial and peripheral Schwann cell myelin) accounts for hemifacial spasm. The REZ is located approximately 5 to 10 mm from the brainstem surface. It is believed that there is increased vulnerability of the nerves in this region because the myelin, which is located in the transition area between the oligodendroglial and Schwann calls, is thinner than elsewhere. This is of particular concern anteriorly, where the protective buffering effect of CN VIII does not exist. Contact with an adjacent vascular structure may result in the production of

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Fig.12. Bell’s palsy. Axial and coronal contrast-enhanced, fat-suppressed MR images demonstrate enhancement of the intratemporal segment of the facial nerve (arrows), from the distal canalicular segment to the level of the stylomastoid foramen.

orthodromic and antidromic action potentials. This may result in a kindling defect on the motor nucleus of the facial nerve, making it hyperexcitable. The extent of NVC required to produce such an effect is unclear. Hemifacial spasm may be observed despite the presence of significant nerve deformation. Also, hemifacial spasm has been reported to occur with NVC with the distal facial nerve. NVC is usually a result of tortuosity of the anterior inferior cerebellar, posterior inferior cerebellar, vertebral, or basilar arteries, or rarely, of vascular structures associated with arteriovenous malformations or draining veins of large developmental venous anomalies. High-resolution MR imaging offers the advantage of being able to

demonstrate both the nerve and the compressing vascular structures in exquisite detail simultaneously (see Fig. 14). Source images from such sequences may also be manipulated to generate volume-rendered images depicting NVC or compression. This is of great benefit in preoperative planning for microvascular decompression, a procedure that in more than 90% of cases of simple compression provides excellent results. In more complex patterns of compression, such as when the meatal branch of the AICA dorsally or a second vascular structure ventrally sandwiches the REZ, or when a larger, more pulsatile vertebral artery is the culprit vessel, the results of microvascular decompression may be variable. Three-dimensional

Fig.13. Ramsay Hunt syndrome. Axial (A) and coronal (B) fat-suppressed, contrast-enhanced, T1-weighted images through the temporal bones of a patient who had a painful external ear vesicular eruption and acute facial paralysis. (A) Note the intense enhancement of the right facial nerve in its canalicular and labyrinthine segments and of the geniculate ganglion (solid white arrow) compared with the normal left side (dashed arrow). (B) Intense enhancement of the soft tissues of the left EAC, afflicted with the herpetic eruption (arrowheads).

Imaging of the Facial Nerve

Fig. 14. Erosion of the lateral wall of the anterior tympanic facial canal by cholesteatoma (black arrow). Note erosion of the body of the incus.

constructive interference in steady state (Siemens, Erlangen, Germany) imaging has the capability to demonstrate such complex patterns of compression in great detail.32–34

Neoplasms of the Facial Nerve Tumors of the facial nerve usually present with progressive facial paresis or paralysis. Facial paralysis that does not evolve in a pattern such as that typical of Bell’s palsy (eg, insidious onset, progression for more than 3 weeks, persistence for longer than 6 months) must be investigated for the presence of a tumor. However, tumors have been known to precipitate facial paralysis acutely. The facial nerve may be afflicted by both primary neoplasms and also be secondarily involved by tumors of the temporal bone or by perineural spread of malignancy. Facial schwannomas Schwannomas may occur along any segment of the facial nerve. The site of their occurrence largely determines the clinical presentation and treatment approach. Facial paresis is common but not universal with these tumors. When paresis or paralysis does occur, it is usually consequent to pressure exerted on the nerve by the tumor, and even so, it is believed to occur only when a certain percentage of neurons are destroyed. Facial schwannomas in the IAC are more likely to present with sensorineural hearing loss. Those that affect the geniculate ganglion are clinically silent for long periods of time and may be bulky when discovered using imaging procedures. Tympanic

segment schwannomas may demonstrate a lobulated growth pattern and project into the middle ear cavity, displace the ossicular chain, and result in conductive hearing loss. These schwannomas present as avascular retrotympanic masses on otoscopic images. Tumors arising from the peripheral rami of the nerve in the parotid space present as palpable masses.11,35 CT and MR imaging play an important role in the preoperative evaluation of these tumors (Fig. 15). On MR images, they may present as lobulated masses (when in the CPA cistern, IAC, tympanic segment, and parotid space) or segments of fusiform expansion (when in the labyrinthine and mastoid segments).Tumors in the IAC are usually indistinguishable from acoustic or vestibular schwannomas. They may demonstrate a dumbbell-like configuration when they extend into the geniculate ganglion. Such extensions are important to recognize preoperatively, and CT and MR imaging may be useful in doing so. Tumors arising from the geniculate ganglion may grow along the GSPN and project into the middle cranial fossa, occasionally mimicking other extra-axial tumors of the middle fossa, such as a meningioma. Facial nerve schwannomas that span the posterior and middle cranial fossae do so by straddling the midportion of the petrous bone, as opposed to trigeminal schwannomas, whose epicenter is at the petrous apex. CT best depicts the expansion of the bony facial canal produced by these tumors. The dense bone that surrounds the labyrinthine

Fig.15. Left hemifacial spasm. Axial volumetric, heavily T2-weighted image reveals compression of the left facial nerve REZ (arrows) by a tortuous left vertebral artery.

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Raghavan et al segment permits only fusiform expansion, whereas masses in other segments may grow relatively unrestrained to develop lobulations. An apparently aggressive pattern of bone destruction may be seen around the mastoid segment when the tumor erodes into surrounding pneumatized bone (Fig. 16).11,35,36 Facial nerve schwannomas confined to the CPA and IAC are rarely distinguishable from vestibular or acoustic schwannomas. Differentiation of schwannomas from meningiomas in these locations is usually possible because the epicenter of the latter tumors is not in the IAC. Epidermoid tumors of the CPA are readily recognized by their characteristic signal intensities on MR images. Geniculate ganglion schwannomas may resemble hemangiomas, but they produce smooth expansion of the fallopian canal, unlike the rarefied margins encountered with hemangiomas, which may also contain ossific spicules. In rare cases, an expanded geniculate fossa may be the result of a persistent stapedial artery. Smooth expansion of the geniculate fossa has also been described to result from the ingress of cerebrospinal fluid through the meatal foramen, resulting in cerebrospinal fluid otorrhea.37,38 Fusiform thickening and enhancement of the nerve must always raise the specter of perineural tumor spread of malignancy. Surgical management of these tumors depends on their size, location, and the patient’s hearing status. Tumors proximal to the geniculate ganglion with preserved hearing and a CPA component of less than 1 cm are resected using a middle cranial fossa approach. Those with a larger CPA component require a retrosigmoid approach. In patients who have no serviceable hearing, a translabyrinthine approach is employed. A transmastoid approach is used for mastoid segment masses.

Primary facial nerve preservation may be feasible if the tumor is located eccentrically in the nerve (approximately 25% of cases) or if it arises from a nonmotor portion of the nerve. More commonly, however, sacrifice of the nerve is necessary, a procedure that is combined with an interposition cable graft to restore at least partial function.35,36 Intratemporal benign vascular tumors Intratemporal benign vascular tumors (IBVTs) may be subdivided into hemangiomas and vascular malformations. The former are composed of thin-walled vascular spaces and the latter of thick-walled spaces lined by a single endothelial layer surrounded by fibrosis. The distinction between the two types is not rigid, and both these lesions may be encountered in the same specimen. Capillary and cavernous hemangiomas have been described in the temporal bone. IBVTs may grow between bony trabeculae, form new bone, and tend to directly invade the facial nerve, in contrast to schwannomas, which compress the nerve. They are most often encountered in the geniculate ganglion, in the fundus of the IAC, and less often, in the posterior genu. Lesions in the IAC cause early sensorineural hearing loss in conjunction with hemifacial spasm and facial palsy, a constellation of clinical findings that is distinctly unusual with facial schwannomas. Geniculate ganglion tumors present with progressive or recurrent facial palsy. Intratumoral bone spicules on CT images may be the best clue to the diagnosis of these tumors (Fig. 17). IBVTs may also demonstrate subtle expansion of the fallopian canal, with rarefaction of the bony margins. Focal as opposed to fusiform expansion may suggest the presence of an IBVT rather than a schwannoma. IBVTs are

Fig. 16. Facial nerve schwannoma. (A) Axial and coronal HRCT images show a lobulated, expansile mass (black solid arrow) centered at the geniculate ganglion. The meatus for the GSPN is also expanded (black dashed arrow). The mass protrudes into the middle ear cavity and displaces the ossicular chain (white arrow). Fluid is seen in the mastoid antrum (star). (B) Axial contrast-enhanced MR image. The mass enhances fairly homogeneously (arrow), whereas the secretions in the antrum do not (star).

Imaging of the Facial Nerve

Fig.17. Axial contrast-enhanced (A) and bone window (B) CT images of a large mastoid-segment facial schwannoma. Note the apparently aggressive nature of the lesion as it erodes through thin-walled mastoid air cells.

markedly hyperintense on T2-weighted MR images and enhance brightly with contrast. Internal heterogeneity secondary to the bony matrix may be evident (Fig. 18). These tumors, being extraneural in origin, may be resected if detected early, with preservation of the facial nerve. In many cases, however, nerve resection and grafting or anastomosis is required.39,40 Epidermoid cysts and other benign tumors Extradural epidermoid cysts (congenital cholesteatomas) usually occur in the supralabyrinthine portion of the temporal bone and at the petrous apex. They are true cysts that are lined by stratified squamous epithelium and contain keratinous debris. They are believed to arise from aberrant epithelial remnants left at the time of closure of the neural groove at 3 to 5 weeks of fetal life. They are distinguished from more common, acquired cholesteatomas by the preservation of the tympanic membrane integrity and a lack of a history of ear infection. They secondarily involve the facial nerve canal and are more likely to do so in a supralabyrinthine location. Involvement of the geniculate ganglion was seen in three out of seven cases in an article by Robert and colleagues41 On CT images, the cysts appear as sharply marginated, hypodense lesions that do not enhance (Fig. 19). Cholesterol granulomas and mucoceles may appear identical on CT images. Differentiation of these entities is important because epidermoid cysts, being true cysts, need to be resected in their entirety, whereas marsupialization may suffice for mucoceles and cholesterol granulomas. Differentiation may be possible using MR imaging because epidermoid cysts are typically low in intensity on T1-weighted MR images and hyperintense on T2-weighted MR images, whereas cholesterol granulomas are hyperintense on T1-weighted MR

images. Differentiation of these cysts from mucoceles containing clear fluid may be difficult.11,41 Choristomas of the facial nerve are rare, benign entities containing varying amounts of neural and fibrous tissue and smooth muscle cells. These have been reported to occur at the fundus of the IAC and in the geniculate ganglion and are indistinguishable from schwannomas when using imaging techniques.42 Paragangliomas of the facial nerve are rare neoplasms that arise from the paraganglia along the course of the descending facial nerve canal. They may present with pulsatile tinnitus. Motheaten osteolysis of surrounding bone and hypervascularity similar to that of jugulotympanic paragangliomas may be observed on imaging. Hypervascularity has been demonstrated secondary to supply from the posterior auricular and occipital arteries, in contradistinction to an ascending pharyngeal supply in cases of jugulotympanic paragangliomas.43

Fig. 18. Hemangioma of the canalicular segment of the right facial nerve. Axial HRCT shows the characteristic intralesional spicules of calcification.

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Fig. 19. Petrous apex cholesteatoma. This 21-year-old presented with gradual facial paresis. Axial CT image shows a multiloculated, smoothly marginated lesion at the petrous apex eroding into the IAC (star).

Perineural spread of malignancy Several malignancies demonstrate a tendency to spread along nerve fascicles. These include adenoid cystic carcinoma, squamous cell carcinoma, desmoplastic melanoma, and lymphoma.

Large-nerve perineural spread (PNS) of malignancy is an important risk factor for local recurrence, lymph node involvement, and distant metastatic disease. PNS is often clinically silent, a fact that serves to emphasize the importance for radiologists to evaluate potential pathways of PNS when analyzing studies of patients who have head and neck cancer. Detection of PNS may mandate the use of wider margins of resection, prophylactic lymph node dissection, and adjuvant chemotherapy or radiation therapy. PNS may be suspected clinically when patients present with pain, parasthesia, formication (ie, the sensation of insects crawling under the skin), or motor weakness in a specific CN distribution location. The development of new cranial neuropathy in a patient who is being treated for head and neck malignancy is strongly suggestive of PNS. Occasionally, the detection of PNS may precede the detection of the primary tumor.44,45 The fact that certain tumors exhibit a high degree of neurotropism, whereas others with a similar degree of differentiation and proximity to neural tissue do not, has prompted some investigators to propose a molecular basis for this phenomenon to replace the theory of spread into pathways of least resistance, as was believed

Fig. 20. Squamous cell carcinoma of the preauricular skin involving the parotid gland. Axial unenhanced T1 (A), axial (B), and coronal (C) contrast-enhanced, fat-suppressed, T1-weighted images demonstrate an irregular enhancing left-parotid-space neoplasm with PNS into the mastoid segment of the left facial nerve.

Imaging of the Facial Nerve previously. It has been postulated that neural-cell adhesion molecules, which are membrane glycoproteins that mediate cell-to-cell adhesion, may be expressed to a greater degree in tumors that demonstrate PNS. Higher expressivity of neuralcell adhesion molecules has been demonstrated in adenoid cystic carcinoma, squamous cell cancers, and desmoplastic melanomas.45,46 PNS involving the facial nerve classically results from the spread of primary malignant tumors of the parotid space, but it may also occur in other ways. The facial nerve may be involved in two distinct ways. The spread of a tumor in a retrograde fashion from the peripheral rami into the facial canal is the more common of these ways (Fig. 20). Alternatively, a tumor may reach the facial nerve by way of the GSPN (Fig. 21). The GSPN carries the preganglionic parasympathetic nerve fibers, which are part of the nervus intermedius, the lacrimal gland, and the glands of the palate and sinonasal cavities by way of the pterygopalatine ganglion, as discussed earlier. The sphenopalatine ganglion is also the site where the maxillary division of the fifth nerve, traveling in the foramen rotundum, synapses with the facial nerve. The pterygopalatine fossa may therefore receive and transmit tumors from diverse sites in the craniofacial region. PNS is best evaluated using MR imaging. It is of vital importance to obtain precontrast, T1-weighted

images through the pterygopalatine fossa in the evaluation of PNS. Loss of fat signal in the PPF or the various neural canals in the skull base is an important clue in the diagnosis of PNS (see Fig. 21). Although considerable controversy exists concerning the use of fat suppression with postcontrast imaging, the general consensus is that fat suppression should be used to increase sensitivity for PNS. On MR findings that suggest PNS, radiologists should also include increased enhancement and thickening of the nerves in question. Denervation atrophy of the facial musculature may be seen when the facial nerve is affected. CT is of limited value, but may demonstrate fat in the PPF and widening or destruction of the walls of the skullbase canals and foramina.44,47

SUMMARY A variety of congenital, traumatic, vascular, inflammatory, and neoplastic processes may affect the facial nerve. Prudent use of CT and MR imaging combined with a complete understanding of facial nerve anatomy helps in narrowing the differential diagnosis. The precise anatomic course of the facial nerve must be charted in patients who undergo middle ear surgery. Also of great importance is recognition of the fact that the facial nerve may be affected by PNS in patients who have cancers of the head and neck.

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Fig. 21. Perineural spread of an adenoid cystic carcinoma of the soft palate to the facial nerve. The tumor has spread from the left pterygopalatine fossa (arrowheads) to the cavernous sinus (star) and by way of the GSPN (solid white arrow) to the geniculate ganglion and tympanic segment of the facial nerve (dashed white arrow).

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