Cranial Nerve VII

Published on 03/03/2015 by admin

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7 Cranial Nerve VII

Facial

Clinical Vignette

A 62-year-old judge became aware of subtle weakness of his left lower face that he first noted while shaving. Two months later, he noted that he could no longer close his left eye lid fully and he was having increasing weakness of the remainder of his left face. He was referred to a neurologist, who reassured him that he had a “benign” Bell palsy. He sought a second opinion when his facial weakness continued to worsen over another month with an inability to close the eye and to form a symmetrical smile.

Neurologic examination demonstrated weakness in all divisions of the left CN-VII with total inability to close his eye or form a left-sided smile. Palpation of the cheek demonstrated some fullness in the left parotid gland, with the remainder of his head and neck examination being normal. Complete neurologic and otoscopic examinations were unremarkable. Audiologic test results were normal, including the left acoustic/stapedius reflex. A corneal reflex was sluggish on the left but present bilaterally.

A left parotid gland biopsy demonstrated a malignant adenocarcinoma with extension beyond the capsule at surgery. He died of metastatic cancer 20 months later.

Comment: Fortunately, this case represents a relatively rare occurrence. However, it emphasizes that what may initially look routine and benign indeed may have a much more serious pathophysiologic mechanism. The issue in this case is to appreciate the history of a gradual evolution of the neurologic deficits in contrast to the relatively acute onset of idiopathic Bell palsy. Furthermore, Bell palsy is typically preceded by retroauricular pain and often associated with hyperacusis and loss of taste on the anterior two thirds of the tongue. When these symptoms are lacking, as in this instance, the pathoanatomic site is distal to the styloid foramen and potentially within the parotid gland as the facial nerve passes through its body. In addition, the gradual progression of this patient’s symptoms provided a strong suspicion of a neoplasm.

Facial nerve (CN-VII) lesions are the most common cranial mononeuropathy. This is one of the most complex cranial nerves having multiple functions (Fig. 7-1). It has a long and somewhat circuitous course with four primary components. (1) Motor fibers, which constitute the major division and serve the primary function of CN-VII: innervating the muscles of facial expression (unilateral, complete facial weakness is the hallmark of almost all facial neuropathies); (2) autonomic fibers, which are responsible for lacrimal, salivary, and mucous secretions; (3) special sensory fibers, which provide taste from the anterior two thirds of the tongue; and (4) general sensory fibers, which innervate the external auditory canal and a small area behind the ear.

When a patient presents with facial weakness, differentiation should be made between peripheral facial nerve lesions and CNS processes. With the latter, when the patient is relaxed, subtle suggestions of a facial nerve lesion may be appreciated by nasolabial fold flattening on the affected side. Brain lesions such as cerebral infarction, tumor, inflammation, or demyelination are often associated with other findings that can help with localization. For example, a small lesion near the Broca area may result in motor aphasia and facial weakness. Larger lesions affecting a significant portion of a hemisphere, as with large hemispheric strokes, cause a constellation of symptoms, including face, arm, and leg weakness and sensory loss; gaze deviation; and neglect or aphasia. Posterior limb lesions of the internal capsule result in face, arm, and leg weakness without sensory, visual, or cognitive changes. While peripheral facial weakness involves the upper and lower part of the face to the same degree, upper motor neuron lesions typically present with a gradient of weakness (Fig. 7-2), with relative preservation of movement in the brow and forehead (orbicularis oculi and frontalis muscles). This is due to presumed dual hemispheric innervation of the forehead muscles. In addition, corticobulbar tract involvement, as in various suprabulbar palsies, leads to absence of voluntary facial movement but retained reflexive movements such as in response to emotional stimuli.

Anatomy

Intrapontine Portion

CN-VII consists of two primary roots (Fig. 7-1). The larger division carries somatic motor fibers and has its origin within the facial nucleus in the caudal pons, where it lies adjacent to the spinal tract of the trigeminal nerve (CN-V). It then passes dorsally and rostrally to curve around the abducens nerve (CN-VI) nucleus (internal genu) and exits the brainstem at the bulbopontine angle between CN-VI and CN-VIII. Its smaller component, the nervus intermedius (intermediate nerve of Wrisberg), contains a combination of autonomic, special sensory (taste), and general sensory fibers. Its preganglionic parasympathetic fibers arise from the superior salivatory nucleus, relay through the pterygopalatine and submandibular ganglions, and eventually provide efferent function for lacrimation and salivation. The remaining intermediate nerve fibers carry taste and general somatic sensation and have their primary cell bodies in the geniculate ganglion and ultimately terminate within the nucleus solitarius and the spinal tract of CN-V, respectively.

Peripheral CN-VII

Both roots of CN-VII leave the brainstem to enter the temporal bone via the internal auditory meatus, where they accompany the auditory nerve (CN-VIII) passing through the internal auditory canal (Fig. 7-1, bottom). CN-VII continues to the periphery through the facial canal; this segment has five parts, based on their relation to surrounding anatomic structures. (1) The labyrinthine segment passes above the labyrinth and leads anterolaterally to the geniculate ganglions that contain the cell bodies of CN-VII afferents. (2) At this site, the canal abruptly turns posteriorly and forms the external genu of CN-VII. (3) The greater petrosal nerve originates here; it carries preganglionic parasympathetic fibers to the pterygopalatine ganglion, where they synapse and subsequently direct postganglionic fibers to the lacrimal gland. (4) The tympanic segment of CN-VII travels posteriorly and laterally along the medial wall of the middle ear. At the posterior wall of the middle ear, the facial canal changes its course and travels inferiorly toward its exit at the stylomastoid foramen. (5) The vertical portion is named the mastoid segment and has two important branches: proximally, the stapedius nerve arises to innervate the stapedius muscle; more distally, the chorda tympani branches and exits the facial canal and, after traversing the middle ear, joins the lingual nerve belonging to the third division of CN-V. The chorda tympani contains preganglionic parasympathetic fibers that synapse within the submandibular ganglion to innervate the submandibular and sublingual glands. The chorda tympani also carries taste fibers. Their cell bodies originate within the geniculate ganglion, mediating taste sensation from the anterior two thirds of the tongue.

Soon after leaving the skull at the stylomastoid foramen, the distal CN-VII gives rise to several small motor branches innervating the posterior auricular, occipital, digastric, and stylohyoid muscles (Fig. 7-1, top). The main motor trunk of CN-VII then passes through the parotid gland to terminate as the temporal, zygomatic, buccal, mandibular, and cervical branches. The first two innervate the muscles involved in moving the forehead, closing the eyes, and wrinkling the nose. Muscles of the lower face and neck are primarily innervated by the latter two branches. CN-VII subserves all muscles of facial expression except the levator palpebrae superioris and, therefore, CN-VII impairment, with a resultant asymmetric facies, is a major social and cosmetic impediment.

Clinical Correlations and Entities

The facial nerve can be damaged at any level along its complex course. Paralysis of the facial musculature is the hallmark of seventh cranial nerve lesions no matter what the lesion’s anatomic site. The clinical presence or absence of symptoms related to the various other components of the facial nerve is very important in identifying the lesion site.

The patient with a peripheral seventh (facial) nerve palsy in most instances, with the exception of an early very distal branch lesion within the parotid gland, loses function of the entire ipsilateral side of their face and cannot smile, close their eyelid (orbicularis oculi), or wrinkle (frontalis) their forehead.

When intrapontine (Fig. 7-3, #1) lesions affect the facial motor nucleus per se, as well as its exiting fibers, involvement of neighboring brainstem structures is typically seen. The association of peripheral facial paralysis with ipsilateral conjugate gaze palsy (paramedian pontine reticular formation lesion), ipsilateral lateral rectus palsy (sixth cranial nerve lesion), or paresis of the opposite arm and leg (corticospinal tract lesion) usually indicates a pontine localization.

Extramedullary lesions (Fig. 7-3, #2) affecting the seventh nerve as it enters its intracranial course primarily occur within the cerebellopontine (CP) angle. Most commonly these are benign, relatively large acoustic neuromas that initially involve the eighth nerve and later extend to produce a seventh-nerve lesion. Thus, diminished hearing, sometimes initially presenting with tinnitus, usually precedes the onset of this type of peripheral facial paresis (see Fig. 7-4). Occasionally, with very large CP angle tumors there is concomitant involvement of the ipsilateral fifth cranial nerve (trigeminal nerve V) with unilateral facial numbness or initially only loss of the corneal reflex.

A relatively proximal pregeniculate, intracanicular facial nerve lesion (Fig. 7-3, #3) characteristically leads to diminished lacrimation from greater petrosal nerve involvement as well as hyperacusis (an increased sensitivity to sound that is particularly noticeable while using a telephone), due to associated stapedius muscle paresis. These lesions also lead to diminished salivation, absent or altered taste sensation for the anterior two thirds of the tongue, and affected somatic sensation for the external auditory canal.

When a facial nerve lesion is more distally situated, between the geniculate ganglion and the stapedius nerve all of the above findings occur, but lacrimation is spared as the greater petrosal nerve has already exited the geniculate ganglion. If damage occurs in the facial canal, involvement of the stapedius nerve and the chorda tympani (Fig. 7-3, #4) leads to hyperacusis and impaired salivation and taste but no change in lacrimation. When the seventh-nerve lesion is distal to the chorda tympani, it is characterized by a pure ipsilateral facial weakness (Fig. 7-3, # 5). Very rarely, a lesion of this type occurs after the facial nerve exits the skull through the stylomastoid foramen. On occasion, this can cause diagnostic difficulty early on as it may initially involve just individual motor branches, with limited weakness of individual facial muscles before a complete palsy develops. Facial trauma is the most common cause for acute pure motor CN-VII lesions; however, an insidious progressive course suggests that a parotid adenocarcinoma, as illustrated in the vignette on p. 98, is the most likely cause.

Clinical Vignette

A vigorous 18-year-old woman awakened with a mild dull ache behind her left ear. While washing her face she noted an inability to smile on that side and that her left eye lid could not close. As a grandparent had recently had a stroke presenting with facial weakness, she rushed to the local emergency room for immediate physician evaluation. Her clinical examination demonstrated that she was unable to smile, close her eyelid, or wrinkle the forehead on the left. Her left eye was slightly injected and dry secondary to diminished tearing. She had no taste sensation on the anterior of the left tongue. The remainder of her neurologic examination was normal. No imaging studies were indicated.

A diagnosis of idiopathic Bell palsy was made; this patient was most relieved to have not had a stroke. As she lived in an endemic area for Lyme disease, specific antibodies were obtained before discharge on oral prednisone treatment. Over the next 2 months, she experienced a gradual and total return of her facial muscle function.

Comment: This is a classic case of idiopathic Bell palsy with no associated neurologic dysfunction or any specific evidence of systemic disorders predisposing to a facial nerve lesion. However, when the patient lives in an endemic Lyme disease region it is reasonable to check Lyme-specific antibodies before corticosteroids are prescribed.