The Facial Nerve




CHAPTER OUTLINE



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  • NEUROANATOMY




    • Supranuclear (upper motor neuron) pathways



    • Facial nucleus and fascicles (lower motor neuron)



    • Subarachnoid space



    • Temporal bone and peripheral course



  • ASSESSMENT OF FACIAL NERVE FUNCTION




    • Facial motor function



    • Taste



    • Tear function



    • Hearing



  • FACIAL NERVE PALSIES




    • Bell palsy



    • Herpes zoster oticus



    • Other infectious diseases



    • Acute inflammatory demyelinating polyradiculopathy



    • Trauma



    • Other causes



  • EXPOSURE KERATOPATHY




    • Risk factors



    • Management



  • FACIAL NERVE DISORDERS: HYPERACTIVITY




    • Blepharospasm



    • Hemifacial spasm



    • Facial myokymia



    • Benign eyelid myokymia



  • KEY POINTS





INTRODUCTION



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The facial nerve is the seventh cranial nerve (CN VII). CN VII is of major importance to ophthalmologists for at least two reasons. First, the facial motor pathways are in close anatomic proximity to the ocular motor pathways; therefore, the evaluation of facial motor function may offer important clues in the diagnosis and localization of lesions that cause ocular motility disorders. Second, CN VII controls eye closure (orbicularis oculi). Weakness of eye closure can lead to exposure keratopathy and visual loss.




NEUROANATOMY



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SUPRANUCLEAR (UPPER MOTOR NEURON) PATHWAYS



Corticobulbar Pathway


Volitional (voluntary) facial movements originate in the precentral gyrus of the frontal lobe cortex. Fibers descend within the corticobulbar tract through the internal capsule and cerebral peduncles to synapse in the facial motor nuclei located in the pons. Cortical fibers controlling the lower face decussate to innervate the contralateral facial nucleus. Fibers innervating the upper face (forehead muscles and orbicularis oculi) originate from both the ipsilateral and contralateral facial nuclei (Figure 12–1). Thus, a unilateral hemispheric lesion that affects the supranuclear pathway (upper motor neuron) produces a contralateral lower facial paresis; voluntary eye closure and forehead movement are relatively spared because the upper face can be controlled by either hemisphere.




Figure 12–1.


Corticobulbar (supranuclear) facial pathway.

Observe that the upper face has bilateral supranuclear input. Thus, a left supranuclear lesion (1) produces a right facial palsy that spares the upper face. However, a left facial nerve lesion (2) would likely affect both the upper and lower left face. (Modified, with permission, from Sibony PA, Evinger C: Anatomy and physiology of normal and abnormal eyelid position and movement. In Miller NR, Newman NJ, editors. Walsh and Hoyt’s clinical neuro-ophthalmology, ed 5. Vol 1, Baltimore, 1998, Lippincott, Williams & Wilkins.)





Other neurological signs and symptoms frequently accompany a facial palsy from upper motor neuron disease. The portion of the precentral motor gyrus that serves the hand and fingers lies immediately superior to the facial motor area, and motor control for the tongue lies adjacent inferiorly (see Figure 12–1). Therefore, hemispheric lesions in this area may produce contralateral weakness of the lower face, hand, and tongue, as well as other hemispheric signs.



Limbic (Extrapyramidal) Pathway


Patients with upper motor neuron disease involving the corticobulbar pathway may be unable to smile voluntarily or close their eyes on command, but periodic blinking or emotional facial responses (such as a smile in response to a funny story) may still occur. This preservation of emotional facial expression occurs because another supranuclear motor pathway with input to the facial nucleus—the limbic (or extrapyramidal) pathway—controls involuntary and automatic facial movements. This pathway involves the basal ganglia, thalamus, and brainstem. Selective damage to the limbic supranuclear input to the facial nucleus leaves volitional facial movement intact but affects emotional facial expression. For example, Parkinson disease interferes with the limbic pathway, and affected patients have a relatively expressionless face with infrequent blinking (“reptilian stare”). Because the corticobulbar pathway is intact in Parkinson disease, affected patients can still produce facial movements on command. Hyperactivity of the limbic facial pathway produces Meige syndrome consisting of blepharospasm with dystonic buccal and nuchal facial movements.



Because both supranuclear systems require an intact lower motor neuron, lesions of the facial nucleus, its fascicles, or the facial nerve affect both voluntary and involuntary facial expression. Lesions involving the supranuclear pathways have less effect on baseline facial tone; therefore, asymmetry of the face at rest is not nearly as noticeable than with lower motor neuron disease.



FACIAL NUCLEUS AND FASCICLES (LOWER MOTOR NEURON)



The facial nucleus is located in the pons, inferior and lateral to the abducens nucleus (see Figure 9–3A). The motor axons exit the nucleus and travel dorsally and medially, where they loop over the abducens nucleus before taking a more ventral course to exit the lateral aspect of the caudal pons. As the CN VII fascicles sweep over the abducens nucleus (the facial genu), they form a bump on the floor of the fourth ventricle called the facial colliculus. The superior salivatory nucleus is located just rostral to the facial nucleus. This parasympathetic motor nucleus controls the sublingual, submandibular, and lacrimal glands via the nervus intermedius, which joins CN VII in the subarachnoid space (Figure 12–2).




Figure 12–2.


Facial nerve anatomy.

Origin and distribution of the seventh cranial nerve.





Important adjacent structures at this level of the brainstem include the cochlear nuclei, the spinal tract and the trigeminal (CN V) nucleus, the abducens (CN VI) nucleus, and descending sympathetic fibers. Thus, potential associated symptoms with a nuclear or fascicular CN VII palsy include ipsilateral facial numbness, Horner syndrome, deafness, or CN VI palsy (see Figure 9–3A). Lesions in this area are frequently the result of demyelination, ischemic infarct, or intrinsic brainstem tumors (Table 12–1).




TABLE 12–1.

CAUSES OF FACIAL NERVE PALSY BY LOCATION





SUBARACHNOID SPACE



CN VII and the nervus intermedius exit the lateral aspect of the pons close together, and close to the point that the eighth cranial nerve (CN VIII) enters the brainstem, in a region of the subarachnoid space known as the cerebellopontine angle. The nervus intermedius merges with CN VII in the subarachnoid space shortly after exiting the brainstem. Tumors in the area can affect CN VIII (hearing and balance), CN VII, and the nervus intermedius. Large tumors in this area can also affect CN V and CN VI, as well as the brainstem and cerebellum. Cere-bellopontine angle tumors include meningiomas, acoustic neuromas, facial neuromas, and epidermoid tumors (see Table 12–1).



Within the subarachnoid space, CN VIII and CN VII are separated by the anterior inferior cerebellar artery. Irritation of CN VII by the anterior inferior cerebellar artery or other vascular structures in this area can cause hemifacial spasm (discussed below).



TEMPORAL BONE AND PERIPHERAL COURSE



CN VII and associated nervus intermedius enter the temporal bone with the vestibulocochlear nerve (CN VIII) through the internal auditory meatus. On entering this bony foramen, CN VII/nervus intermedius separates from the vestibuloacoustic nerve and enters the fallopian canal. The fallopian (or facial) canal carries CN VII nearly 3 cm through a complex, descending Z-shaped course within the temporal bone. Trauma involving the temporal bone is a common cause of CN VII injury. Bony metastasis can also affect CN VII as it courses in the fallopian canal.



The first segment of the fallopian canal contains the geniculate ganglion and the first branch of CN VII, the greater superficial petrosal nerve. This nerve contains fibers that will travel via the vidian nerve to synapse in the sphenopalatine ganglion, eventually supplying the lacrimal gland. Much of the course of the greater superficial petrosal nerve is in the middle cranial fossa, and tumors or inflammatory processes in this area may impair reflex tear secretion (often with accompanying CN V and CN VI dysfunction). Additional branches of CN VII include the nerve to the stapedius muscle and the chorda tympani. The chorda tympani (so called because it courses across the inferior border of the tympanic membrane) is the terminal branch of fibers that began as the nervus intermedius, supplying parasympathetic motor input to the submandibular and sublingual glands. The chorda tympani also carries afferent taste fibers from the anterior two-thirds of the tongue (see Figure 12–2).



CN VII exits its bony course through the stylomastoid foramen and immediately gives off branches to innervate the digastric, stylohyoid, and posterior auricular muscles. The facial nerve then enters the substance of the parotid gland, where it spreads out like a goose’s foot (the pes anserinus), dividing into a superior temporofacial and inferior cervicofacial trunk. Although some variability exists, the superior trunk provides temporal, malar, and infraorbital nerves, and the inferior trunk divides into buccal, mandibular, and cervical portions. Parotid gland tumors (mucoepidermoid and adenoid cystic carcinoma), trauma to CN VII during parotid gland surgery, and parotid infiltration/inflammation (from disorders such as sarcoidosis) can affect CN VII as it courses within the parotid gland (see Table 12–1).



Because CN VII has multiple functions and many branches, a variety of aberrant regeneration syndromes can result from the misdirection of regenerating axons following injury. Crocodile tears result when parasympathetic fibers that were originally destined for the salivary gland are misdirected to the lacrimal gland, causing tearing associated with eating. Lesions that cause this syndrome must involve CN VII at or above the geniculate ganglion, before the greater superficial petrosal nerve arises. Other aspects of aberrant regeneration involving CN VII motor function include eye closure (orbicularis activation) on attempted smiling or puckering. A common form of aberrant regeneration (often following Bell palsy) is lower face movement or dimpling over the chin with eye closure.



The vascular supply of CN VII is abundant. Within the fallopian canal, at least three sources are present. The anterior inferior cerebellar artery enters the internal auditory meatus; the petrosal branch of the middle meningeal artery accompanies the greater petrosal nerve; and the stylomastoid branch of the posterior auricular artery enters the facial canal at the stylomastoid foramen. Despite this rich blood supply, ischemia may be a common factor in CN VII palsies because the narrow confines of the bony facial canal can severely restrict perfusion in the presence of edema. The narrowest portion of the interosseous canal is at the level of the geniculate ganglion; the facial nerve is especially vulnerable at this point.




ASSESSMENT OF FACIAL NERVE FUNCTION



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Clinical testing of the multiple functions of CN VII makes localization possible, although such testing is tedious. For example, lesions of CN VII in the subarachnoid space tend to impair all functions of CN VII. Distal lesions may involve only a few CN VII functions because more proximal branches may be spared. However, pontine lesions can affect the motor, sensory, or autonomic functions in isolation because the superior salivatory nucleus and CN VII nucleus exist as separate nuclei in the brainstem with axons that converge after exiting the brainstem (see Figure 12–2).



FACIAL MOTOR FUNCTION



The evaluation of facial motor function begins as soon as the examiner encounters the patient. Asymmetries of the face may be evident during the history, before a formal examination. In acute upper or lower motor neuron disease, the nasolabial fold may be flattened, the palpebral fissure may be wider, and spontaneous blinks may be incomplete on the affected side. However, following recovery of a lower motor neuron facial palsy, facial tone actually is increased and the face looks “tight” on the affected side, even though on formal testing the muscles may not be as strong as normal.



Formal examination of CN VII functions should be carried out by a systematic fractionation of facial muscle function. Motor function of the upper face is evaluated by asking the patient to raise his or her eyebrows to wrinkle the forehead. Orbicularis oculi function is tested by asking the patient to close the eyes as tightly as possible and noting the degree to which the eyelashes are buried (see Figure 7–14). In addition, the examiner can attempt to open tightly closed eyelids to note weakness or asymmetry between eyes. Using a cotton wisp to check the corneal reflex also evaluates efferent innervation of the orbicularis muscle through the corneal-blink reflex arc (see Box 7–4). The examiner can ask the patient to pucker in order to evaluate the orbicularis oris. Asking the patient to smile and to show his or her teeth are effective methods to evaluate the lower face (Figure 12–3). By asking the patient to show a “million dollar smile,” the examiner invariably brings out both motor and limbic smiles.




Figure 12–3.


Examining facial nerve function.

The patient pictured sustained a peripheral left seventh cranial nerve palsy from trauma. (A) Elevation of the brow is weak on the left side and manifests as poor wrinkling of the left forehead. (B) Lack of left orbicularis strength is evident. Observe that the patient has a good Bell phenomenon. (C) The command, “Smile and show your teeth,” reveals lower face weakness.

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Jan 2, 2019 | Posted by in OPHTHALMOLOGY | Comments Off on The Facial Nerve
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