Anatomy of the Facial Nerve and Associated Structures
The anatomy of the facial nerve beautifully illustrates the complexities of the human peripheral, central, and autonomic nervous systems. The facial nerve, the seventh cranial nerve, contains motor, general sensory, special sensory, and autonomic (visceral) components, all of which are reviewed in this chapter.
The course of the facial nerve as it runs intracranially, intratemporally, and extratemporally is perhaps one of the most intricate routes of any cranial nerve. The physician (otologic/neurotologic surgeon, head and neck surgeon, general otolaryngologist, neurosurgeon, and neurologist) must have comprehensive knowledge of the anatomy, physiology, and course of the facial nerve to diagnose and treat lesions and disorders of the nerve, to avoid surgical complications such as facial nerve injury, and to rehabilitate patients with facial nerve disorders. This chapter will details these elements as well as the anatomic path of the facial nerve and its branches as it emerges from the brainstem and takes its long, circuitous route to its targeted structures.
This section describes an overview of the embryology of the facial nerve. Sataloff has written an entire textbook describing in depth the embryology of the facial nerve.1 The facial nerve, the nerve of the second branchial arch, develops from a collection of neural crest cells in close caudal proximity to the otic placode, the facioacoustic primordium, first seen around the third week of gestation.2 The primordium splits into a caudal main trunk and a rostral sensory (chorda tympani) trunk, which enters the first mandibular arch. By the sixth week, ganglia, including the geniculate ganglion, for the sensory cranial nerves are visible, the facial nerve is distinct from the vestibulocochlear nerve, and the nervus intermedius (see subsequent discussion) can be identified. The peripheral motor divisions appear by the eighth week when development of the second arch progresses to individual muscle groups. At this time, the horizontal and vertical portions of the facial nerve run anterior to the external auditory canal and have started to demarcate. The weeks following the eighth week are characterized by extensive arborization of the peripheral facial nerve and the corresponding development of the muscles of facial expression. By the 16th week, all connections are established, but the nerve itself still runs superficial and anterior to the ear canal with respect to its final position. The nerve finally exits the stylomastoid foramen in its usual position by week 30, but it continues to lie superficial until the development of the mastoid tip between 1 and 3 years of age.
Gerhardt and Otto suggest that the final course of the facial nerve was influenced by two processes of motion of global development of the head with relation to the temporal bone.3 The first is a shifting of the meninges, the labyrinth, and the proximal parts of the first (mandibular) branchial arch and the second (hyoidal) branchial arch in a rostral direction. The proximal facial nerve around the geniculate is tethered to the labyrinth and shifts anteriorly to form the first genu relative to the more distal portion of the nerve, which keeps its dorsoventral pathway along with the second branchial arch, whose most dorsal aspect is the stapes blastema. The second global movement is a rotation of the lateral temporal bone in the dorsal direction due to differentiation of the first branchial arch. As the mandible and mastoid develop, the nerve is pulled posteriorly and inferiorly. Gerhardt and Otto further suggest that when there is hypoplasia of the first mandibular arch, possibly due to early stapedial artery involution, the nerve becomes foreshortened. This foreshortening leads to a relative rostral overshifting of the hyoidal arch structures. The distal portion of the facial nerve as it makes its second genu is more anterior and inferior than normal. This anterior and inferior displacement can put the nerve at the level of the oval window thus interfering with the normal development of the stapes and its interaction with the otic capsule, as well as with the development of the incudostapedial joint.3
In congenital aural atresia, the facial nerve typically takes a slightly more anterior position at the second genu and mastoid segments. The tympanic segment is often dehiscent of bone. Children with aural atresia in the setting of Goldenhar or hemifacial microsomia (hypoplasia of first arch structures) can also have facial nerve weakness, most commonly in the marginal mandibular division. The greatest risk to the facial nerve during the surgical repair of aural atresia is during the canalplasty: drilling the new external auditory canal ( Fig. 1.1 ).
The facial nerve can course laterally as the drilling proceeds inferiorly, interfering with access to and exposure of the stapes bone and mesotympanum. Hugging the tegmen superiorly and the glenoid fossa anteriorly to open into the middle ear at the level of the epitympanum will keep the atresia surgeon away from the facial nerve. Risk of injury to the facial nerve in atresia surgery has been estimated at 0.1%.4 In congenital malformations of the middle ear with a patent ear canal and normal tympanic membrane, an aberrant facial nerve was found in 24% of patients.5 While transposition of the facial nerve with drilling of the endochondral bone of the otic capsule to create an oval window has been described,6,7 enthusiasm for the oval window drillout has waned in recent years due to lack of stability of early hearing gains.8,9
The facial motor nucleus, located in the pontine tegmentum, receives input for voluntary facial expression from the motor cortex located in the precentral gyrus of the cerebral hemispheres. Fibers from the precentral gyrus project to the contralateral facial motor nucleus in the pons via the corticobulbar tract through the thalamus along the posterior limb of the internal capsule. Axons carrying motor output to the frontalis and upper part of the orbicularis oculi extend both to ipsilateral and contralateral facial motor nuclei. Thus, a facial paralysis that spares the upper face (frontalis) is caused by a central lesion, such as a tumor or stroke in the contralateral precentral gyrus. A complete facial paralysis, affecting all muscles of facial expression, is caused by a lesion of the lower motor neuron, anywhere from the facial motor nucleus to the stylomastoid foramen ( Fig. 1.2 ).
These fibers from the precentral gyrus synapse in the facial motor nucleus; postsynaptic fibers initially travel dorsally toward the fourth ventricle where they become myelinated and loop around the motor nucleus of the abducens nerve. This loop, the internal genu, causes a slight bulge in the floor of the fourth ventricle called the facial colliculus. These fibers turn laterally and ventrally, and exit the ventrolateral aspect of the brainstem at the caudal border of the pons at the level of the middle cerebellar peduncle between the sixth and eighth cranial nerves, medial to the nervus intermedius ( Fig. 1.3 ).
Sensory fibers of the facial nerve, traveling in the nervus intermedius, end in the nucleus of the tractus solitarius in the brainstem.
The extramedullary portion of the facial nerve exits the inferior border of the pons in the recess between the olive and the inferior cerebellar peduncle, with the motor root medial, the vestibulocochlear nerve lateral, and the nervus intermedius between.10 The facial nerve and nervus intermedius run in the cerebellopontine angle and join the vestibulocochlear nerve to enter the internal auditory canal at the porus acousticus.
The intratemporal facial nerve refers to the portion of the facial nerve that runs in the temporal bone in the fallopian canal (∼30 cm in total length) and has been classically divided into segments based on location within the temporal bone: meatal, labyrinthine, tympanic (horizontal), and mastoid (vertical) before it exits the temporal bone through the stylomastoid foramen ( Fig. 1.4 ).
The meatal segment of the facial nerve, measuring ∼1.5 cm—the length of the internal auditory canal (IAC)—enters the porus acousticus in an anterior, superior position. The nervus intermedius runs between the facial and vestibulocochlear nerves as they enter the porus. The facial nerve and nervus intermedius travel in the IAC adjacent to the superior vestibular nerve posteriorly and the cochlear nerve inferiorly ( Fig. 1.5 ). The transverse (falciform) crest separates the facial nerve from the cochlear nerve inferiorly, and a small lateral ridge of bone, Bill bar, separates the facial nerve from the superior vestibular nerve posteriorly. The nerves of the IAC are surrounded by dura all the way to the fundus—a “meningeal glove finger.”11 A magnetic resonance imaging and cadaveric study showed the facial nerve to remain superior and anterior to the vestibulocochlear nerve and to maintain a tubular shape throughout its course in the IAC.12
As the facial nerve exits the IAC at the fundus, it turns gently anteriorly and runs in the otic capsule bone for ∼3 to 6 mm between the cochlea and superior semicircular canal ( Fig. 1.6 ). The portion of the nerve as it exits the fundus of the IAC is the thinnest part of the facial nerve, and any decompression of the nerve (e.g., for idiopathic facial paralysis) should encompass this portion of the nerve (see Chapter 11).13
At the lateral end of the labyrinthine segment is a swelling of the nerve, the geniculate ganglion, containing the cell bodies of the pseudounipolar neurons carrying taste sensation to the anterior two-thirds of the tongue via the chorda tympani nerve, and some sensory fibers from the soft palate via the greater superficial petrosal nerve. The greater superficial petrosal nerve, carrying preganglionic parasympathetic innervation to the lacrimal gland and the glands of the nasal mucosa, is joined by the deep petrosal nerve from the carotid plexus carrying postganglionic sympathetic innervation (from the cervical sympathetic ganglion) to become the nerve of the pterygoid (Vidian) canal. This nerve enters the pterygopalatine (sphenopalatine) fossa where the parasympathetic fibers synapse in the pterygopalatine (sphenopalatine) ganglion and both postganglionic sympathetic and parasympathetic (visceral efferent; secretomotor) innervation continues to the minor salivary glands of the soft palate, mucus membrane of the nose, as well as to the lacrimal gland via branches of the trigeminal nerve (greater and lesser palatine branches to the soft palate, pharyngeal branch to the pharynx, posterior superior nasal branches to the nose, auriculotemporal to zygomaticotemporal then to lacrimal nerve to the lacrimal gland).