A Review of Facial Nerve Anatomy

An intimate knowledge of facial nerve anatomy is critical to avoid its inadvertent injury during rhytidectomy, parotidectomy, maxillofacial fracture reduction, and almost any surgery of the head and neck. Injury to the frontal and marginal mandibular branches of the facial nerve in particular can lead to obvious clinical deficits, and areas where these nerves are particularly susceptible to injury have been designated danger zones by previous authors. Assessment of facial nerve function is not limited to its extratemporal anatomy, however, as many clinical deficits originate within its intratemporal and intracranial components. Similarly, the facial nerve cannot be considered an exclusively motor nerve given its contributions to taste, auricular sensation, sympathetic input to the middle meningeal artery, and parasympathetic innervation to the lacrimal, submandibular, and sublingual glands.

The constellation of deficits resulting from facial nerve injury is correlated with its complex anatomy to help establish the level of injury, predict recovery, and guide surgical management. KEYWORDS: Extratemporal, intratemporal, facial nerve, frontal nerve, marginal mandibular nerve he anatomy of the facial nerve is among the most complex of the cranial nerves. In his initial description of the cranial nerves, Galen described the facial nerve as part of a distinct facial-vestibulocochlear nerve complex.1,2 Although the anatomy of the other cranial nerves was accurately described shortly after Galen’s initial descriptions, it was not until the early 1800s that Charles Bell distinguished the motor and sensory components of the facial nerve.

Facial nerve anatomy is categorized in terms of its relationship to the cranium or temporal bone (intracranial, intratemporal, and extratemporal) or its four distinct components (branchial motor, visceral motor, general sensory, and special sensory). The plastic surgeon benefits from a basic knowledge of the intracranial and intratemporal components of the facial nerve to help localize facial nerve pathology and distinguish extratemporal from facial nerve lesions at other anatomic locations. Similarly, a knowledge of the four distinct components of the facial nerve reminds the surgeon that the facial nerve is composed not exclusively of voluntary motor fibers but also of parasympathetics to the lacrimal, submandibular, and sublingual glands; sensory innervation to part of the external ear; and contributions to taste at the anterior two thirds of the tongue.

INTRACRANIAL ANATOMY OF THE FACIAL NERVE Voluntary control of the branchial branch of the facial nerve is initiated intracranially by supranuclear inputs arising from the cerebral cortex projecting to the facial nucleus. These cortical inputs are arranged with forehead representation most rostral and eyelids, midface, and lips sequentially caudal to this.5 The pyramidal system is composed of corticobulbar tracts that project voluntary, ipsilateral cortical inputs via the genu of the internal capsule to the seventh cranial nerve nuclei of the pontine tegmentum. Cell bodies of the upper facial motor nerves giving rise to the frontal branch receive bilateral cortical inputs, and neurons to the remainder of the facial nucleus receive contralateral cortical innervation. Spontaneous facial movements are centrally transmitted via the extrapyramidal system, which involves diffuse axonal connections between multiple regions including the basal ganglia, amygdala, hypothalamus, and motor cortex. The extrapyramidal system regulates resting facial tone and stabilizes the voluntary motor response; hypothalamic inputs modulate the emotional response.

The facial nuclei contain the cell bodies of facial nerve lower motor neurons. These cell bodies receive supranuclear inputs via synapse formation with axons traveling through both the pyramidal and extrapyramidal systems. The confluence of these postsynaptic lower motor neurons round the abducens nucleus and form the facial colliculus at the floor of the fourth ventricle (Fig. 1). The branchial motor branch of the facial nerve exits the brainstem at the cerebellopontine angle, where it is joined by the less robust nervus intermedius. These nerves resemble the nerve roots of the spinal cord in that they are devoid of epineurium but covered in pia mater and bathed in cerebrospinal fluid. The branchial motor nerve–nervus intermedius complex travels about 15.8 mm from the cerebellopontine angle before it begins its course within the temporal bone.6 The parasympathetic component of the facial nerve is composed of visceral motor fibers whose originating cell bodies are scattered within the pontine tegmentum and collectively known as the superior salivatory nucleus.

These nuclei are influenced by involuntary hypothalamic inputs. Cell bodies mediating the general sensory function of the facial nerve reside in the general sensory trigeminal nucleus of the rostral medulla and receive afferent inputs from projections of the geniculate ganglion within the temporal bone. The gustatory nucleus within the pontine tegmentum also receives special sensory inputs from the geniculate ganglion. These impulses, however, were initially generated by taste receptors in the anterior two thirds of the tongue. Ascending sensory inputs from the trigeminal and gustatory nuclei are influenced by the thalamic nuclei prior to their reception within the sensory cortex. Patients with supranuclear lesions involving the motor cortex or internal capsule present clinically with loss of volitional control of the lower facial musculature but persistent facial tone and spontaneous facial movements.

Voluntary control of the forehead musculature is retained because the upper halves of the facial nuclei, which are populated by frontal nerve branch cell bodies, receive bilateral cortical innervation and so not all input is lost after a unilateral supranuclear lesion. Voluntary lip, nose, and cheek movements, however, are lost. It should also be noted that facial muscle dysfunction caused by central injury is frequently accompanied by motor dysfunction of the tongue and hand given the proximity of these cortical control centers within the motor cortex and internal capsule. Reflex arcs involving the facial nuclei, such as the corneal blink (trigeminalfacial), are preserved following supranuclear lesions.

INTRATEMPORAL FACIAL NERVE

The intratemporal anatomy of the facial nerve has been extensively studied to minimize morbidity in skull base surgery while maximizing exposure. In addition, its intraneural topography has been investigated in cadavers and animal models.7–9 Whereas the topography in certain animal models, such as the cat, is shown to be consistent, the topography of the intratemporal facial nerve in the human is highly variable and spatial relationships to other intratemporal structures such as the carotid artery and sigmoid sinus are also variable.10–13 The branching pattern of the intratemporal facial nerve, however, is reasonably consistent. The branchial motor and nervus intermedius components of the facial nerve are loosely associated as they enter the internal auditory meatus of the temporal bone. Both the facial and acoustic nerves enter the temporal bone simultaneously with the facial nerve located superior to the acoustic nerve. The facial nerve, along with the acoustic and vestibular nerves, travel 8 to 10 mm within the internal auditory canal before only the facial nerve enters the fallopian canal. The fallopian canal consists of labyrinthine, tympanic, and mastoid segments.

The labyrinthine segment is the narrowest segment and extends 3 to 5 mm from the edge of the internal auditory canal. The geniculate ganglion resides within the distal part of the labyrinthine segment of the facial nerve and gives rise to the first branch of the facial nerve—the greater petrosal nerve—which carries visceral motor parasympathetic fibers to the lacrimal gland (Fig. 2). The external petrosal nerve is a second, threadlike branch that is occasionally present and provides sympathetic innervation to the middle meningeal artery. The lesser petrosal nerve is the third branch extending from the geniculate ganglion. This branch typically carries parasympathetic fibers associated with the glossopharyngeal nerve (ninth cranial nerve) to the parotid gland. Salivary flow from the parotid gland may not, however, be interrupted by lesions to the glossopharyngeal nerve.

In fact, parasympathetic fibers traveling along the nervus intermedius of the facial nerve can bypass the glossopharyngeal branch to the parotid and provide an alternative source of parasympathetic innervation to maintain salivary flow. Compression of the facial nerve within the labyrinthine segment is particularly common given the canal’s narrow dimensions. The facial nerve occupies up to 83% of the labyrinthine canal cross-sectional area compared with only 64% of the more distal mastoid area.14 The junction of the labyrinthine and tympanic components of the fallopian canal is formed by an acute angle, and shearing of the facial nerve commonly occurs as the nerve traverses this genu.8 The tympanic or horizontal segment extends 8 to 11 mm through the temporal bone. The midtympanic canal represents a second region of fallopian canal narrowing and is a less common point of nerve compression compared with the narrow labyrinthine segment.15 The tympanic segment connects with the mastoid segment at a second genu.

The voluntary motor component of the facial nerve exits the cerebellopontine angle with the nervus intermedius before entering the porous acusticus. The facial nerve traverses the labyrinthine segment before entering the geniculate ganglion. The greater petrosal, external petrosal, and lesser petrosal nerves are given off at this level. The temporal or horizontal segment forms the second component of the intratemporal facial nerve and is located just distal to a sharp genu formed at the distal geniculate ganglion. A second genu separates the temporal and mastoid segments of the intratemporal facial nerve. The general sensory branch of the facial nerve is given off at this level and frequently travels with the general sensory branch of the vagus nerve (Arnold’s nerve) and gives sensation to the external ear. The nerve to stapedius is a motor nerve that helps deaden loud sounds. The chorda tympani is the last branch of the intratemporal facial nerve and is the terminal branch of the nervus intermedius.

Wider cross-sectional area than the other segments, and the facial nerve gives off three branches within this region. The nerve to the stapedius is the first branch and innervates the stapes muscle of the inner ear. Because the cell bodies of this motor nerve are not located in the facial nuclei, patients with congenital ¨ facial palsies such as Mobius syndrome retain innervation to the stapes when the other facial mimetics are paralyzed.8 The sensory branch of the facial nerve is typically the second branch. Ramsay Hunt first noted this general sensory nerve in 1907 when patients presenting with facial paralysis related to herpes zoster also demonstrated a vesicular eruption limited to parts of the external ear.16 Ten cadaveric temporal bone dissections revealed a small branch off the vertical component of the intratemporal facial nerve that arced laterally and inferiorly to supply the posterior and inferior external auditory canal. Tumor encroachment upon this sensory nerve, which is thought to comprise 10 to 15% of the neurons within the intratemporal facial nerve,17 results in hypesthesia of the external ear canal and is known as Hitselberger’s sign, after the physician who described it.

The general sensory branch of the facial nerve travels with Arnold’s nerve, a sensory branch of the vagus nerve that exits the jugular foramen and then joins the course of the facial nerve just distal to the nerve to the stapedius branch.8 The chorda tympani is the terminal extension of the nervus intermedius. It branches off the facial nerve in the distal third of the mastoid segment and runs between the ossicles of the middle ear before exiting the tympanic cavity through the temporal bone at the petrotympanic fissure. It joins the lingual branch of the trigeminal nerve to provide parasympathetic innervation to the submandibular and sublingual glands. Special sensory afferents from the anterior two thirds of the tongue also travel with the chorda tympani, and on occasion the sensory branch of the facial nerve travels with the chorda tympani instead of posteriorly to the main facial nerve trunk. Advocates of this technique note that damage to a small branch of the facial nerve during the initial exploration is far less devastating than an inadvertent injury to the entire motor trunk. However, these peripheral branches are more difficult to identify because of their smaller size and a lack of consistent landmarks.

The arborization of the extratemporal facial nerve typically begins within the substance of the parotid gland and ultimately gives rise to the cervical, marginal mandibular, buccal, zygomatic, and frontal (or temporal) nerve branches. Davis et al dissected 350 cadaveric facial halves and were the first to categorize the branching pattern of the facial nerve into six distinct patterns.20 The facial nerve trunk typically gave rise to superior and inferior divisions. The marginal mandibular and cervical branches of the facial nerve were exclusively derived from the inferior division, whereas the buccal branch always received some contribution from the inferior division and either no or a variable contribution from the superior division (Fig. 3). The frontal branch consistently represented a terminal branch of the superior division of the facial nerve trunk. Baker and Conley reviewed the extratemporal facial nerve anatomy in 2000 parotidectomy cases.21 Their findings suggested that the facial nerve branching pattern was more variable than that noted in Davis’ cadaveric studies, including the presence of a facial nerve trunk trifurcation with a direct buccal branch in a few instances.

EXTRATEMPORAL FACIAL NERVE

The extratemporal component of the facial nerve starts when the facial nerve exits the stylomastoid foramen. In the adult, it is protected laterally by the mastoid tip, tympanic ring, and mandibular ramus, whereas in children younger than 2 years it is relatively superficial. Postauricular incisions in this younger population must be carefully planned because the trunk of the facial nerve is a subcutaneous structure at this level. After exiting the stylomastoid foramen, the facial nerve gives off motor branches to the posterior belly of digastric, stylohyoid, and the superior auricular, posterior auricular, and occipitalis muscles. The facial nerve then travels along a course anterior to the posterior belly of the digastric and lateral to the external carotid artery and styloid process before dividing into its main motor branches at the posterior edge of the parotid gland. The facial nerve trunk is usually identified approximately 1 cm deep and just inferior and medial to the tragal pointer. The parotid and superficial musculoaponeurotic system (SMAS) can then be carefully divided to expose the facial nerve for facial nerve reconstruction.

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A REVIEW OF FACIAL NERVE ANATOMY/MYCKATYN, MACKINNON

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