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Address reprint requests and correspondence: Romy Chung, MD, Diagnostic Radiology, University of California San Diego Health System, 300 W Arbor Dr, #8756, San Diego, California 92103.
The facial nerve is structurally complex, both functionally and anatomically. In this article, we review the functional and structural anatomy of the facial nerve, using computed tomography (CT) and magnetic resonance (MR) imaging to emphasize important details that comprise thorough radiological assessment of the facial nerve. Additionally, CT and MR studies represent important tools for the diagnosis of facial nerve pathology, and the structural information that aids in preoperative planning and postoperative evaluation and CT and MR imaging of varied pathology of the facial nerve have been presented.
The facial nerve is a highly complex structure, both functionally and anatomically. Knowledge of the functional and structural anatomy of the facial nerve anatomy is critical before any diagnosis, clinical management, or surgical treatment that may follow.
Firstly, we review the functional components of the facial nerve, which include the functional components comprising somatomotor, parasympathetic, special sense (taste buds of the tongue and palate), and sensory innervation (skin of the outer ear). Subsequent review of the structural anatomy from the brainstem through the temporal bone to the soft tissues of the face has been illustrated with computed tomography (CT) and magnetic resonance imaging (MRI). Secondly, important pathology has been reviewed in the light of how CT and MRI can help characterize lesions further. Briefly, high-resolution CT is ideal for evaluating the bony abnormalities associated with facial nerve disorders and characterization of the calcifications within facial nerve lesions. MRI is superior for the evaluation of intrinsic properties of the facial nerve and facial nerve lesions.
Major functions of the facial nerve
Functionally, cranial nerve (CN) VII, also known as the facial nerve, contains motor components as well as sensory and parasympathetic nervous system components.
Motor component
The motor nucleus is the principal nucleus of the facial nerve, situated in the lateral pontine tegmentum. The motor functions of the facial nerve include the following: (1) innervation of the occipitalis and the posterior auricular muscles, which control movements of part of the scalp; (2) innervation of the stylohyoid muscle, which draws the hyoid muscle posteriorly and elevates the tongue, and posterior belly of the digastric muscle, which elevates the hyoid bone; (3) innervation of the stapedius muscle, which stabilizes the stapes ossicle; and (4) ultimately, innervation of the muscles of facial expression via the terminal extracranial branches of the facial nerve.
The parasympathetic efferent nucleus is the superior salivatory nucleus (SSN), located lateral and slightly anterior to the motor nucleus. The preganglionic efferent fibers branch off the facial nerve as part of the chorda tympani, a distinct facial nerve branch within the facial canal carrying the preganglionic efferent fibers to the salivary glands. The greater petrosal nerve, also branching off the facial nerve, innervates the lacrimal glands and the mucosal membranes of the nasal cavity and palate.
The chorda tympani also conveys the sensory components of the facial nerve containing afferent fibers carrying the sensation of taste from the anterior two-thirds of the tongue, ultimately reaching the nucleus solitarius in the brainstem, which is located in the pons between the motor nucleus and the SSN. The nervus intermedius (NI), which contains the parasympathetic root and special sensory root of the facial nerve, intervenes between the main facial nerve and the vestibulocochlear nerve in the internal auditory canal (IAC) and converges to form the facial nerve proximal to the genu.
Before delving into finer detail, it is helpful to consider the overall path of the facial nerve (Figure 1). Grossly, the components of the facial nerve arise as nuclei from the brainstem (intra-axial segment) and traverse the cerebrospinal fluid (CSF) cisternal segment at the cerebellopontine angle (CPA) to enter the temporal bone through the IAC (intratemporal segment). The facial nerve then takes a circuitous, Z-shaped path, also known as the fallopian canal, through the temporal bone, ultimately exiting through the stylomastoid foramen into the extracranial soft tissues (extracranial segment). Each of these segments (the intra-axial, cisternal, intratemporal, and extracranial segments) have been discussed individually later (Figure 1). Of these 4 segments, the intratemporal segment is further subdivided, which is also be discussed later.
Figure 1Schematic of the facial nerve. The facial nerve is divided into the following 4 anatomical components: intra-axial, cisternal, intratemporal, and extracranial. Of these, the intratemporal component of the facial nerve is further subdivided into the following 4 anatomical components: the IAC (meatal), labyrinthine, geniculate, tympanic, and mastoid components.
The following nuclei ultimately become components of the facial nerve: the trigeminal motor nucleus (spinal tract of CN V), the superior salivary nucleus, and the nucleus of the tractus solitarius. Although the facial nerve nuclei are characteristically located within the caudal aspect of the ventrolateral pontine tegmentum, for the purposes of radiological evaluation, these nuclei are not radiologically distinguishable from the rest of the brainstem on CT or on state-of-the-art clinical MRI.
Cisternal facial nerve
The facial nerve emerges from the lateral aspect of the lower pons and traverses the CPA cistern. (Figure 2) shows the facial nerve, anterior and close to the superior vestibular nerve, as it crosses the CPA cistern on its way to the IAC. It is worth noting that the facial nerve here comprises 2 substructures, the facial nerve proper (containing the motor fibers) and the NI (containing the parasympathetic and sensory fibers). However, the 2 nerve bundles are poorly visualized and distinguished on CT. However, occasionally, the NI can be recognized as a thin, wispy structure on high-resolution, thin-slice T2-weighted MRI (Figure 3), often situated more closely to the cochleovestibular nerve fibers than the facial nerve proper (Figure 3), before it converges with the facial nerve proper.
Figure 2The cisternal and meatal facial nerve. MR T2-weighted image at the level of the IAC. This image shows the facial nerve as it courses across the left cerebellopontine cistern (the cisternal segment of the facial nerve) and enters the internal auditory canal. This segment within the IAC represents the meatal facial nerve (arrow), anterior to the superior vestibular nerve (arrowhead). Of note, the facial nerve here is seen as a single nerve, though it comprises both the facial nerve proper and the nervus intermedius.
Figure 3The nervus intermedius. MR T2-weighted image of the right brainstem at the level of the IAC. This image shows the facial nerve proper (arrow) anterior to the superior vestibular nerve. The resolution in this image is sufficient to view the occasionally seen nervus intermedius (arrowhead) closely apposed to the superior vestibular nerve before joining with the facial nerve proper.
The intratemporal facial nerve traverses the temporal bone through the IAC (the meatal segment) and then distally into the more narrow bony canal (fallopian canal). The fallopian canal is the bony canal that houses the rest of the intratemporal facial nerve. This fallopian canal segment is structurally the most convoluted, starting at the fundus at the lateral aspect of the IAC and taking a Z-shaped path of nearly 36-38 mm through the temporal bone to the stylomastoid foramen.
Thus, the intratemporal facial nerve is further divided into 5 subsections based on the location within the temporal bone and proximity to nearby structures: the meatal segment through the IAC, the labyrinthine segment, the geniculate segment, the tympanic segment, and the mastoid segment. These segments and the smaller nerves that are given off in each segment are discussed in further detail as follows.
The meatal facial nerve is the portion of the facial nerve within the IAC. This portion of the facial nerve still comprises the NI and the facial nerve proper in this segment (a division that is often, though not always, poorly characterized radiologically) as it enters the IAC alongside the subcomponents of CN VIII, the cochlear and superior and inferior vestibular nerves. The facial nerve is consistently oriented anterior to the superior and inferior vestibular nerves and superior to the cochlear nerve (Figure 4). To best evaluate the presence of all 4 of these nerves, typically a high-resolution T2-weighted thin-section MRI in the sagittal plane with a cross-sectional view of the IAC (Figure 4) is necessary
as axial views show at most only 2 of the 4 nerves in a single axial plane.
Figure 4Orientation of the meatal facial nerve with respect to the cochlear and vestibular nerves. The schematic for the orientation of the facial nerve (CN VII) with respect to the vestibulocochlear nerve (left) and the annotated sagitally T2-weighted, high-resolution, thin-slice MR image of the internal auditory canal (right) showing the facial nerve situated at the anterior superior quadrant of the internal auditory canal. The singular nerve is seen traversing the inferior aspect of the canal and joining with the inferior vestibular nerve.
At the end of the IAC, the facial nerve enters the fallopian canal on the anterior aspect of the Bill bar. This comprises the labyrinthine segment of the facial nerve, a relatively short segment extending between the IAC and the geniculate ganglion (Figure 5A). The Bill bar is a tiny triangular bone separating the facial nerve from the superior vestibular nerve. The labyrinthine segment of the facial nerve courses superior to the cochlea (Figure 5B) and anterior to the vestibule (reflecting its orientation described earlier in the IAC with regard to the superior and inferior vestibular and cochlear nerves).
Figure 5The labyrinthine segment of the facial nerve. (A) Axial CT view at the level of the IAC and ossicles, demonstrating the short labyrinthine segment of the facial nerve (long arrow) between the IAC and the fossa for the geniculate ganglion. It also demonstrates the hairpin turn (“kneelike”) of the geniculate portion of the facial nerve (short arrow). (B) Coronal T2-weighted image demonstrating the labyrinthine portion of the facial nerve (arrow) superior to the cochlea (arrowhead).
The term geniculate (“kneelike”) refers to the bent, hairpin shape of the facial canal and the indwelling geniculate segment of the facial nerve, which is well-appreciated on thin-slice axial CT images (Figure 5A). The geniculate ganglion, situated within the geniculate fossa of the facial canal, receives the nerve fibers from all components of the facial nerve: motor, sensory, and parasympathetic. Please note, however, that only the special sense root of the NI has its unipolar cell bodies in the geniculate ganglion of the facial nerve, also known as the sensory neuronal cell bodies, for taste from the anterior two-thirds of the tongue via the chorda tympani, which branches from the facial nerve later anatomically (refer to the section infratemporal facial nerve for discussion of the mastoid portion of the facial nerve) and is carried to the taste buds by the lingual nerve. The greater petrosal nerve branches at the geniculate portion of the facial nerve to innervate the lacrimal glands and the mucosal membranes of the nasal cavity and palate. Although branching from this geniculate segment of the facial nerve, the greater petrosal nerve cell bodies are located in the SSN, as previously mentioned, not the geniculate ganglion. The peripheral processes of the geniculate ganglion also enter the greater petrosal nerve to supply the palatal taste buds. Additionally, a few cells of the geniculate ganglion send their peripheral processes to the skin of the external auditory canal. Their central processes terminate in the spinal nucleus of CN V.
The tympanic portion of the facial nerve represents the segment of the facial nerve after the geniculate ganglion. It courses medially past the incus (Figure 6) and extends to the “second genu” at the summit of the pyramidal eminence (pyramidal turn). At this point, there is an abrupt change in direction when the facial nerve turns inferiorly and becomes the mastoid portion of the facial nerve (Figure 7). This mastoid portion gives off 2 nerves before exiting the stylomastoid foramen: the chorda tympani and the nerve to the stapedius muscle.
Figure 6Axial T2-weighted MR image demonstrating the anterior tympanic portion of the facial nerve (arrow) anterior to the vestibule (arrowhead).
Figure 7The mastoid segment of the facial nerve. Reformatted oblique coronal CT image demonstrating the near-vertical segment of the mastoid portion of the facial nerve (arrows). The inferior-most aspect of this bony canal represents the stylomastoid foramen (arrowheads).
After the facial nerve exits the stylomastoid foramen, it becomes extracranial. It gives off the posterior auricular nerve and then gives off 2 small motor nerves to the stylohyoid muscle and the posterior belly of the digastric muscle, which are typically below the resolution of MRI. After the facial nerve crosses lateral to the styloid process, it dives into the parotid gland, where it follows a virtual plane between the deep and superficial lobes of the parotid gland and splits into a nerve plexus called the pes anserinus (“goose foot”), which represents a convenient surgical landmark. At the pes anserinus, the facial nerve subdivides into 5 terminal branches: temporal nerve, zygomatic nerve, buccal nerve, marginal mandibular nerve, and cervical nerve.
Imaging considerations for facial nerve pathology
Facial nerve pathology encompasses neoplastic, vascular, infectious, inflammatory, and congenital etiologies. CT and MR studies represent the primary imaging modalities for evaluating facial nerve pathology. CT imaging gives the best evaluation of the associated bony structures and the facial canal in the imaging workup for facial nerve lesions and is better than MRI at characterizing calcifications within lesions.
However, MRI is significantly better at evaluation of the intrinsic tissue and enhancement properties of the facial nerve and most of its lesions.
Neoplastic disease of the facial nerve
Neoplastic disease of the facial nerve can include primary or secondary involvement of the facial nerve. Many of the primary neoplasms of the facial nerve, including facial schwannomas and meningiomas, demonstrate secondary expansion and remodeling of the facial canal, easily seen on noncontrast CT images (Figure 8A)
Figure 8Facial schwannoma. (A) Axial CT at the level of the IAC demonstrates marked expansion of the geniculate fossa (arrows) compatible with a schwannoma in the geniculate portion of the facial canal. (B) Axial T2-weighted MR image at the level of the IAC demonstrates a T1-isointense, expansile mass in the geniculate portion of the facial nerve (arrows). (C) Axial T1-weighted MR image at the level of the IAC demonstrates avid, homogenous enhancement of this mass (arrows) in the geniculate portion of the facial nerve compatible with a schwannoma.
Although rare, schwannomas represent the most common primary neoplasms of the facial nerve. Classically, facial nerve schwannomas demonstrate uniform, avid enhancement, best seen on MRI. When facial nerve schwannomas are large but limited to the CPA cistern and the IAC, they are impossible to distinguish from vestibular schwannomas (Figure 9)
; only very small facial schwannomas may be distinguished from vestibular schwannomas based on their origin in the facial nerve on very thin-slice, high-resolution, contrast-enhanced MRI. It is worth noting that though vestibular schwannomas are far more common, bilateral facial nerve schwannomas are pathognomonic for neurofibromatosis type 2 as well (Figure 9)
Figure 9Bilateral IAC schwannomas. Axial T1-weighted postcontrast MR image at the level of the IACs demonstrates bilateral avidly and homogeneously enhancing smooth masses (arrows) extending into the IACS representing bilateral schwannomas. Given this size, it is impossible to determine radiologically whether these represent facial or vestibular schwannomas without visualizing extension into the facial canal.
Facial nerve schwannomas can occur unilaterally or bilaterally anywhere along the facial nerve (Figure 8, Figure 9), though 83% of confirmed facial schwannomas occur in the geniculate fossa of the facial canal, followed by labyrinthine and tympanic segments of the facial nerve.
Occasionally, facial schwannomas can extend beyond the geniculate ganglion along the greater petrosal nerve through the greater petrosal foramen or arise from the intracranial greater petrosal nerve and grow in the middle cranial fossa
(Figure 10). In this case, the schwannoma can in fact resemble a small avidly enhancing intracranial rounded mass (Figure 10), which can mimic a meningioma or another mass arising from the middle cranial fossa.
Figure 10Schwannoma of the greater petrosal nerve extending into the middle cranial fossa. Axial T1-weighted postcontrast MR image of the middle cranial fossa demonstrates an avidly enhancing rounded mass in the posterior aspect of the right middle cranial fossa (arrow) arising from the greater petrosal nerve (arrowhead) and compatible with a schwannoma of the greater petrosal nerve.
They also demonstrate avid enhancement on both CT and MRI (Figure 14), and when large enough, demonstrate expansion of the bony canal. MRI can demonstrate the presence of a “dural tail,” which is characteristic of a meningioma. CT can also help distinguish meningiomas from schwannomas in 2 ways: the permeative sclerotic changes often seen in the bony structures abutting meningiomas are seen best on CT
and additionally, CT can also demonstrate the presence of small petechial calcifications, which further support the diagnosis of meningiomas.
Meningiomas and schwannomas of the facial nerve often present with slow-onset facial paresis. However, metastatic disease to the facial nerve, though exceedingly rare, should be considered in any patient with a history of cancer and rapidly progressive facial nerve palsy. Primary cancers that most commonly metastasize to the CNs include breast, lung, and melanoma. The use of contrast in MRI is key to the diagnosis of metastatic lesions in the CPA; metastatic disease to the nerve would demonstrate marked enhancement and moderate thickening of the nerve (Figure 11).
Figure 11Metastatic breast carcinoma to the IAC. T1-weighted postcontrast MR image at the level of the IAC. The partially visualized tympanic segment of the facial nerve (long arrow) appears thickened and enhancing, as do visualized cochlear nerve (short arrow) and inferior vestibular nerve (arrowhead) within the IAC compatible with pathology-proven metastatic breast adenocarcinoma.
Glomus tumors of the facial nerve (glomus faciale) are exceptionally rare tumors, but they can be considered in an avidly enhancing facial nerve lesion with flow voids
owing to their exceptional vascularity (Figure 12). The clinical presentation of slow, progressive facial nerve paresis is reflective of the slow growth of glomus tumors.
Figure 12Glomus faciale. The preoperative study demonstrates a markedly enhancing, smoothly marginated expansile lesion in the tympanic segment of the facial nerve (arrow). This was a pathology-proven case of glomus faciale. Though not seen on this image, glomus tumors can demonstrate flow voids because of their exceptional vascularity.
Therefore, correct identification of hemangiomas is critical. Hemangiomas in the IAC usually present with gradually progressive sensorineural hearing loss, whereas those in the geniculate fossa usually present with pure facial nerve symptoms.
and can be seen on contrast-enhanced CT and MR images. When equivocal masses with facial canal expansion are seen on MRI, high-resolution CT should performed for further characterization as noncontrast CT can help identify the characteristic chunky, trabecular calcifications (Figure 13) that help differentiate hemangiomas from schwannomas and meningiomas.
Additionally, the pattern of enhancement of hemangiomas can be helpful. A study reported that hemangiomas of the facial nerve have a specific pattern of contrast enhancement: intensely enhancing anterior core with a moderately enhancing posterior portion.
Figure 13Perifacial nerve hemangioma. Axial CT image at the level of the cochlea demonstrates an expansile mass (long arrows) extending from the geniculate segment to the tympanic segment of the facial canal with trabecular calcifications compatible with a perifacial nerve hemangioma.
Figure 14Meningioma of the geniculate portion of the facial nerve. Axial T1-weighted postcontrast MR image demonstrates a small rounded mass in the geniculate portion of the facial canal (arrow). This represents a pathology-proven meningioma of the geniculate facial nerve.
Infectious and inflammatory conditions of the facial nerve
Bell palsy represents an acute, rapid facial nerve paralysis in a previously asymptomatic, older adult. Acute facial neuritis is typically not an indication for imaging. CT imaging specifically has no role in the evaluation of Bell palsy, though it is worth noting that if CT inadvertently shows facial canal expansion, Bell palsy can be safely excluded from the differential diagnosis. However, if the symptoms are atypical, MRI is indicated for further evaluation
MRI diagnosis of facial neuritis: Diagnostic performance of contrast-enhanced 3D-FLAIR technique compared with contrast-enhanced 3D-T1 fast field echo with fat suppression.
; atypical symptoms of Bell palsy include slowly progressive facial nerve palsy, episodic palsies, unusual degree of ear pain, other associated cranial neuropathies, or facial nerve palsy that extends beyond 2 months.
Reactivation herpes simplex infection along the facial nerve is the most common cause of Bell palsy, and in severe cases, if MRI is performed, swelling of the facial nerve and enhanced labyrinthine segments is seen.
Imaging for Ramsay Hunt syndrome, varicella zoster infection of the facial, vestibulocochlear, and posterior auricular nerves, is also typically not indicated, particularly if the patient demonstrates the characteristic symptoms of facial palsy, vestibular symptoms, and a periauricular rash. However, if imaging is performed in atypical cases, the CNs demonstrate swelling
with increased enhancement of the CNs (Figure 15) involved.
Figure 15Ramsay Hunt syndrome involving the left facial and superior vestibular nerves. Axial T1-weighted postcontrast MR image at the level of the IACs demonstrates an asymmetric, markedly swollen, enhancing facial nerve and superior vestibular nerve in the IAC (arrow). Please note enhancement of the tympanic segments of the facial nerve (arrowhead). This was in the clinical context of a vesicular rash erupting around the left ear with facial nerve palsy and vestibular symptoms, consistent with Ramsay Hunt syndrome.
Another infectious cause of acute facial neuritis is neuroborreliosis, also known as Lyme disease. It is estimated that 10%-15% of patients with untreated Lyme disease develop neurologic involvement and symptomatology
and should be considered in cases of patients in endemic areas with constitutional symptoms and neurologic findings. Lyme disease of the facial nerve demonstrates small, masslike, nodular enhancement of the CNs (Figure 16).
Figure 16Lyme disease with involvement of the facial nerve. Axial T1-weighted postcontrast MR image at the level of the IACs demonstrates small, masslike, nodular enhancement of the distal cranial nerves in the IACs (arrows) in this patient with a history of recent camping trip and increasing facial nerve paresis compatible with Lyme disease of the facial nerve.
though it is only symptomatic in an estimated 3%-5% of patients. In a patient with known sarcoidosis with facial nerve symptoms, MRI is an excellent modality for assessing neurosarcoidosis of the facial nerve. The findings of irregular thickening and enhancement of the facial nerve are nonspecific, but neurosarcoidosis should be considered in the context of dural thickening and enhancement (Figure 17). Imaging can be helpful clinically, as resolution of these imaging findings after treatment has a strong correlation with clinical improvement.
Figure 17Neurosarcoidosis with involvement of the facial nerves. Coronal T1-weighted postcontrast MR image at the level of the IAC demonstrates bilateral thickening and irregular enhancement of the facial nerves bilaterally (upward arrows) in the context of thickened enhancing dura (downward arrows) noted just superiorly in this case of neurosarcoidosis.
Knowledge of the complex functional and structural anatomy of the facial nerve is critical in the radiological evaluation of the facial nerve. CT and MRI of the facial nerve represent important tools for the diagnosis of facial nerve pathology and the structural information that aids in preoperative and postoperative evaluation and represent important tools for the head and neck radiologist and surgeon alike.
References
Hiatt J.L.
Gartner L.P.
Textbook of Head and Neck Anatomy. Lippincott Williams and Wilkins,
Philadelphia2001
MRI diagnosis of facial neuritis: Diagnostic performance of contrast-enhanced 3D-FLAIR technique compared with contrast-enhanced 3D-T1 fast field echo with fat suppression.
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