Facial Nerve

The facial nerve, cranial nerve VII, is a complex nerve that carries several different fiber bundles, including the following:

The facial nerve is formed by the conjoining of the facial motor root and the adjacent nervus intermedius. The facial motor root supplies the muscles of facial expression and arises from the facial motor nucleus located in the ventral lateral tegmentum of the lower pons. The nervus intermedius carries taste, sensory, and parasympathetic fibers and arises from the solitary nucleus/tract (medulla), trigeminal sensory nuclei (medulla–pons), and superior salivatory nucleus (pons), respectively.

The facial nerve, including the motor root and nervus intermedius, emerges from the brainstem at the cerebellopontine angle and enters the internal auditory meatus, next passing through the geniculate ganglion before traversing the facial canal. Within the bony facial canal, several branches arise from and leave the main facial nerve (Figure 25–1). First, parasympathetic fibers are given off to the greater and lesser petrosal nerves, bound for the pterygopalatine and otic ganglia. A small motor branch arises next, to innervate the stapedius muscle in the inner ear. The chorda tympani then arises to carry taste fibers to the anterior two thirds of the tongue and parasympathetic fibers to the submandibular and sublingual salivary glands.

FIGURE 25–1 Course of the facial motor root and nervus intermedius branches of the facial nerve in the facial canal.

The facial nerve is formed by the merging of the facial motor root and the adjacent nervus intermedius. The motor root supplies the muscles of facial expression. The nervus intermedius carries taste, sensory, and parasympathetic fibers. Within the bony facial canal, several branches arise from and leave the main facial nerve. Parasympathetic fibers are given off to the greater and lesser petrosal nerves, bound for the pterygopalatine and otic ganglia. A small motor branch arises next to innervate the stapedius muscle in the inner ear. The chorda tympani then arises to carry taste fibers to the anterior two thirds of the tongue and parasympathetic fibers to the submandibular and sublingual salivary glands.

The facial nerve exits the skull at the stylomastoid foramen before coursing through the parotid gland. After the stylomastoid foramen, the nerve supplies the stylohyoid and the posterior belly of the digastric muscles, then gives off a cutaneous posterior auricular branch before dividing into its five major peripheral branches: temporal (a.k.a., frontal), zygomatic, buccal, mandibular, and cervical branches, which innervate the muscles of facial expression (Figure 25–2).

FIGURE 25–2 Major peripheral branches of the facial nerve.

After exiting the stylomastoid foramen, the facial nerve bifurcates into five major peripheral branches: temporal, zygomatic, buccal, mandibular, and cervical to supply the muscles of facial expression.

(From Oh, S.J. 1993. Clinical electromyography: nerve conduction studies, 2nd ed. Williams & Wilkins, Baltimore, with permission.)

Trigeminal Nerve

The trigeminal nerve, cranial nerve V, carries sensory fibers to the face and motor fibers to the muscles of mastication. It arises from several different nuclei in the brainstem, including one motor nucleus (mid-upper pons) and three separate sensory nuclei. The sensory nuclei include the main sensory nucleus (mid-upper pons), which mediates light touch; the nucleus of the spinal tract of V (pons to upper cervical cord), which mediates pain and temperature; and the mesencephalic nucleus of V (lower midbrain), which mediates proprioception from facial muscles. Exiting from the lateral mid-pons, the nerve is called trigeminal because it branches into three major peripheral nerves that arise from the trigeminal ganglion (also known as the semilunar or gasserian ganglion), located just outside the brainstem on the petrous bone in the middle cranial fossa (Figure 25–3). The cavity formed by the folds of dura that contain the trigeminal ganglion, surrounded by cerebrospinal fluid, is known as Meckel’s Cave. Whereas the trigeminal ganglion contains cell bodies of the sensory fibers from both the main sensory nucleus and the nucleus of the spinal tract of V, the cell bodies of proprioceptive sensory fibers from muscle spindles of trigeminal motor fibers are contained within the mesencephalic nucleus of V in the midbrain.

FIGURE 25–3 Trigeminal ganglion and origin of the three major peripheral nerve branches.

Exiting from the lateral mid-pons, the trigeminal nerve divides into three major peripheral nerves – ophthalmic (V₁), maxillary (V₂), and mandibular (V₃) – which arise from the trigeminal ganglion, located just outside the brainstem on the petrous bone in the middle cranial fossa.

(Adapted with permission from Montgomery, E.B., Wall, M., Henderson, V.W., 1986. Principles of neurologic diagnosis. Little, Brown, Boston.)

The three major peripheral nerve divisions of the trigeminal nerve are the ophthalmic (V₁), maxillary (V₂), and mandibular (V₃) nerves. Each nerve exits the skull through a distinct opening: (1) the ophthalmic nerve through the superior orbital fissure, (2) the maxillary nerve through the foramen rotundum, and (3) the mandibular nerve through the foramen ovale. Each of the three major nerve branches contains sensory fibers, whereas motor fibers are carried solely in the mandibular nerve branches that supply innervation to the muscles of mastication (masseter, temporalis, medial, and lateral pterygoid muscles) and to the anterior belly of the digastric muscle, the mylohyoid, tensor veli palatini, and tensor tympani muscles. Branches of the trigeminal nerve supply light touch, pain, and temperature sensation to the skin of the face, the anterior half of the scalp, most of the oral and nasal mucosa, the anterior two thirds of the tongue, and the dura mater of the anterior and middle cranial fossae (Figure 25–4).

FIGURE 25–4 Trigeminal sensory distribution.

The three branches of the trigeminal nerve – ophthalmic nerve (V₁), maxillary nerve (V₂), and mandibular nerve (V₃) – supply sensation to the face and anterior scalp.

(Adapted with permission from Haymaker, W., Woodhall, B., 1953. Peripheral nerve injuries. WB Saunders, Philadelphia.)

Facial Neuropathy

The most common cranial mononeuropathy is facial nerve palsy, which usually presents as idiopathic Bell’s palsy. Some cases are post-infectious, although a growing amount of evidence suggests that Bell’s palsy is a viral-induced cranial neuritis caused by herpes simplex virus-1 in many cases. In addition, the risk of Bell’s palsy is increased in patients with hypertension or diabetes and in pregnant women (the latter especially late in the pregnancy or in the early postpartum period).

Unilateral facial nerve dysfunction can also be seen in association with several disorders, most commonly in the setting of diabetes. In addition, facial palsy occurs with herpes zoster involving the geniculate ganglion (Ramsay Hunt syndrome), lymphoma, leprosy, cerebellopontine angle tumors such as acoustic neuroma, multiple sclerosis, stroke, and a host of other disorders (Box 25–1). Bilateral facial weakness is less common; it may be seen in Guillain–Barré syndrome, Lyme disease, sarcoid, Melkersson–Rosenthal syndrome, tuberculous meningitis, and leptomeningeal lymphomatosis/carcinomatosis. Bifacial weakness also is noted in some neuromuscular junction disorders and in various muscular dystrophies.

The clinical presentation of facial nerve palsy depends on the location, pathophysiology, and severity of the lesion. A central lesion (proximal to the facial nerve nuclei) causes contralateral weakness primarily of the lower facial musculature, with relative sparing of the orbicularis oculi and frontalis muscles, which are bilaterally innervated. Furthermore, with central lesions there may be facial movement during laughing or crying because the pathways that mediate responses to emotional stimuli are different from those that mediate voluntary facial movement. Peripheral lesions (at or distal to the facial nerve nuclei) cause ipsilateral facial paralysis that affects both the upper and lower facial musculature, resulting in an inability to wrinkle the forehead, close the eye, or smile. In addition, there may be dysfunction and absent taste sensation over the anterior two thirds of the tongue, depending on which branches are involved as the nerve courses through the facial canal.

In patients with idiopathic Bell’s palsy, complete facial paralysis involving the upper and lower face generally occurs within 24 hours and inevitably is accompanied by pain behind the ipsilateral ear. The etiology is thought to be inflammation of the facial nerve, which causes swelling and compression of the nerve in the facial canal.

In most patients, the prognosis is excellent, with full recovery of function over several weeks to months. However, in more severe cases, usually those associated with significant axonal loss, some permanent facial weakness remains, or aberrant reinnervation may occur as the nerve regenerates. Aberrant reinnervation can take one of two forms: (1) an axon that previously innervated a particular muscle grows down a different fascicle and innervates a different muscle than the original one, or (2) a single axon branches into two or more axons that go to two or more different muscles. Either type of aberrant reinnervation can result in synkinesis of facial movements. For example, closing the eye (orbicularis oculi) may be accompanied by movement of the lips (orbicularis oris). Clinically, these reinnervation abnormalities may vary from being very subtle to very severe. In the most extreme case, synkinesis may lead to massive contractions on one side of the face. As most people blink spontaneously every few seconds, synkinesis involving the orbicularis oculi and other facial muscles can clinically appear very similar to hemifacial spasm (see below), although the etiology is quite different.

Aberrant reinnervation may also occur between the motor axons of the facial nerve and the parasympathetic axons (i.e., nerve fibers derived from the facial motor root and nerve fibers derived from the nervus intermedius). Thus, parasympathetic axons may innervate motor endplates, and, conversely, motor axons may innervate the parasympathetic endplates. This may result in lacrimation, salivation, and/or hemifacial sweating when the facial muscles are activated. One can imagine the embarrassing situation wherein tears rather than saliva are produced while eating.

Hemifacial Spasm

Hemifacial spasm is a chronic and often progressive disorder usually associated with chronic compression of or injury to the facial nerve. The disorder is characterized by involuntary contractions that affect one or multiple muscles on one side of the face. The spasms typically occur initially around the eye, and later spread to involve other ipsilateral facial muscles. The contractions are often irregular and persist during sleep. Although several unusual causes of chronic irritation have been reported in the literature, the most common etiology is an aberrant blood vessel lying in contact with the facial nerve near its exit zone from the brainstem. The spasms are thought to be generated by damage to some axons of the facial nerve with ephaptic transmission to other nearby axons. Surgical decompression of the blood vessel away from the facial nerve often results in complete recovery.

As noted above, massive reinnervation and subsequent synkinesis of the facial muscles may occur following an idiopathic facial palsy, leading to a pattern nearly identical clinically to hemifacial spasm. However, the underlying pathophysiology of hemifacial spasm (damage to the facial nerve with ephaptic transmission) differs from that of post-paralytic facial syndrome (massive synkinesis that occurs with spontaneous blinking, due to aberrant reinnervation of muscles following idiopathic facial palsy).