62 Cranial Nerve VII :The Facial Nerve and TasteH. KENNETH WALKER

Definition

The motor portion, or the facial nerve proper, supplies allthe facial musculature . The principal muscles are the fron-talis, orbicularis oculi, buccinator, orbicularis oris, platysma,the posterior belly of the digastric, and the stapedius muscle .In nuclear or infranuclear ("peripheral") lesions, there is apartial to complete facial paralysis with smoothing of thebrow, open eye, flat nasolabial fold, and drooping of themouth ipsilateral to the lesion . Supranuclear ("central") le-sions spare the brow and eyelid musculature ; there is flat-tening of the nasolabial fold and drooping of the mouthcontralateral to the lesion .

The sensory portion, or intermediate nerve, has the fol-lowing components : (1) taste to the anterior two-thirds ofthe tongue; (2) secretory and vasomotor fibers to the lac-rimal gland, the mucous membranes of the nose and mouth,and the submandibular and sublingual salivary glands ;(3) cutaneous sensory impulses from the external auditorymeatus and region back of the ear. Abnormalities of tasteinclude ageusia (lack of taste) ; hypogeusia (diminished tasteacuity) ; dysgeusia (unpleasant, obnoxious, or perverted taste) .

Technique

Motor

Careful and thoughtful observation is the key to discerningsubtle signs of weakness of muscles supplied by the motorportion. Note especially the blink, nasolabial folds, and cor-ners of the mouth . Asymmetry is the clue to unilateral weak-ness and is best perceived during conversation when thepatient is unaware of being observed .

1 . Blink : The eyelid on the affected side closes just atrace later than the opposite eyelid .

2. Nasolabial folds: The weak one is flatter .3. Mouth : The affected side droops and participates

manifestly less in speaking .

Ask the patient to look up or wrinkle the forehead ; inspectfor asymmetry . Ask him or her to close the eyes tightly .Look for incomplete closure or incomplete "burying" of theeyelashes on the affected side . Observe the nasolabial foldsand mouth while the patient is concentrating on the eyes .As the orbicularis oculi contract tightly, there are milderassociated contractions of muscles about the mouth andnose; these milder contractions are better suited to display-ing slight weakness than when these muscles are tested di-rectly .

Ask the patient to smile, show you teeth, or pull backthe corners of the mouth . Look for asymmetry about themouth .

The most subtle signs of mild facial weakness are theblink reflex and incomplete lid closure . Observe the blink

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reflex during conversation, or tap gently on the glabellawith your index finger or reflex hammer in an attempt tobring out a mild asymmetry of blink . If you strongly suspectbut are having difficulty confirming a mild facial weakness,ask the patient to lie flat on the examining table with faceup. Slide the patient's head off the examining table so thehead is below the body. This forces the eyelids to workagainst gravity. Now ask the patient to close both eyes andinspect for incomplete closure . Tap on the glabella and noteasymmetry of blink .

The facial nerve participates in a number of reflexes(Dejong, 1979) . Assessment of these reflexes provides valu-able additional information about facial nerve function . Ta-ble 62 .1 lists these reflexes, the method of eliciting them,and their clinical interpretation . Some of these reflexes arediscussed in more detail in Chapter 71 (Suck, Snout, Pal-momental, and Grasp Reflexes) .

Taste

The four primary tastes are bitter, sweet, sour, and salty .Screen for disorders of sweet or salty taste with salt andsugar. With the patient's eyes closed and tongue protruded,take a tongue blade and smear a small amount of salt orsugar on the lateral surface and side of the tongue . Instructthe patient to tell you the identity of the substance . Rinsethe mouth thoroughly and repeat the test on the other side,using a different substance .

Basic Science

The cortical fibers of the facial nerve proper originate fromthe lower third of the motor strip. They course in the genuof the internal capsule and the middle third of the cerebralpeduncle, supplying the seventh nucleus in the lower pons .The supranuclear innervation is bilateral to the muscles ofthe forehead and eyes but only contralateral to the musclesof the lower part of the face . This accounts for the sparingof the upper facial muscles with a contralateral cortical le-sion. Figure 62.1 shows the anatomy of the facial nerve .

The facial nucleus participates in the corneal reflex . Cor-neal pain and temperature fibers go through the ophthalmicdivision of the fifth cranial nerve to the spinal nucleus ofthe fifth and thence to the ipsilateral seventh nucleus, caus-ing the eyelid to blink . There are also central connectionsbetween the facial nucleus and the nuclei or projection sys-tems of the second, third, fourth, sixth, and eighth cranialnerves. These connections coordinate movements amongthe eyelids and eyeballs and set up certain reflexes such asthe blink reflex on exposure to strong light or a loud sound .Table 62 .1 lists these reflexes .

The motor fibers for the facial muscles exit from themotor nucleus, curl up and around the sixth nucleus, anddescend laterally from the lower pons .

62. CRANIAL NERVE VII : THE FACIAL NERVE AND TASTE

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Figure 62 .1Origin and distribution of various components of the facial nerve . Symptomatology of lesions at various levels, 1 to 5, of this nerve is listedin Table 62 .4 . The facial, intermedius, and acoustic nerves illustrated to the left continue in the drawing on the right . CR, corpus resiforme ;MLF, medial longitudinal fasciculus ; ML, medial lemniscus, coursing vertically through corpus trapezoideum ; PYR, pyramidal bundles inpars basilaris pontis ; SO, superior olivary nucleus ; VS, nucleus of spinal tract of the fifth nerve. From Haymaker W, Kuhlenbeck H . Disordersof the brainstem and its cranial nerves . In: Baker AB, Joynt RF, eds . Clinical neurology . Philadelphia : Lippincott, 1979 ;3 :chap . 40. Bypermission .

The intermediate nerve joins the motor segment at thepoint where it exits from the pons . The intermediate nerveis composed of contributions from three areas :

1 . The superior salivary nucleus in the pons suppliessecretory fibers. They go to (a) the lacrimal, nasal,and palatine glands (via the greater superficial pe-trosal nerve) and (b) the submandibular and sublin-gual salivary glands (via the chorda tympani nerve) .

2 . The gustatory (solitary) nucleus in the medulla sup-plies sensory fibers . These fibers go to taste buds onthe anterior two-thirds of the tongue (via the chordatympani nerve) .

3 . The dorsal part of the trigeminal tract supplies fibersthat convey cutaneous sensation from the externalauditory meatus and skin behind the ear (distributedwith the facial nerve proper) .

First-order neurons are in the geniculate ganglion . Table62.2 summarizes the brainstem nuclei of the facial and in-termediate nerves .

The facial nerve proper and intermediate nerve lie inthe cerebellopontine angle with the sixth and eighth cranialnerves. The seventh, intermediate, and eighth nerves enterthe internal auditory meatus . The facial and intermediatenerves then enter the facial canal of the petrous portion ofthe temporal bone . The geniculate ganglion of the inter-mediate nerve is in this canal . The greater superficial pe-trosal nerve is destined for the lacrimal, nasal, and palatineglands. It leaves just distal to the geniculate ganglion . Thenerve to the stapedius muscle is given off next . The facial

and intermediate nerves then descend to the stylomastoidforamen, giving off the chorda tympani at either the sty-lomastoid foramen or varying distances proximal to it. Thechorda tympani supplies the anterior two-thirds of the tongueand the submandibular and sublingual glands. The motorpart of the facial nerve leaves the stylomastoid foramen andsupplies the facial musculature . A major part of the nerveforms a plexus within the parotid gland . Table 62 .3 lists thebranches of the facial nerve, beginning centrally and pro-ceeding distally .

Taste sensibility is composed of four qualities : sweet, salt,sour, and bitter. Taste receptors are located on the tongue,palate, pharynx, and larynx . Although all four qualities canbe perceived throughout, there is considerable localization .The tongue, especially the tip and edges, is most sensitiveto sweet and salt; the palate, to sour and bitter . The recep-tors are taste buds. Up to 8 are on each of the fungiformpapillae on the anterior two-thirds of the tongue, and upto 100 on each of the circumvallate papillae on the posteriorpart of the tongue. The taste buds are barrel shaped witha pore opening . Chemoreceptive taste hairs project into thebarrel from neuroepithelial sensory cells . Impulses fromthese cells are transmitted to the brainstem . Afferent fibersfrom the anterior two-thirds of the tongue travel via thelingual nerve to the chorda tympani and then as describedabove to the gustatory nucleus . Taste fibers from the pos-terior third of the tongue, the palate, and the palatal archestravel via the glossopharyngeal nerve and the nodosal gan-glion, also ending in the gustatory nucleus . There are twoascending pathways from the gustatory nucleus. One goesto the hypothalamus . The other goes to the thalamus and

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then to the gustatory center of the cortex, which is probablyarea 43 in the parietal operculum .

Clinical Significance

Motor

A lesion involving the nuclear or infranuclear portion ofthe facial nerve will produce a peripheral facial palsy . If allmotor components are involved, there is complete paralysisof all facial muscles on the involved side . The brow is smooth,

the eye does not close, the nasolabial fold is flat, and thatside of the mouth droops. There is no movement at all .The paradigm of this type of involvement is Bell's palsy .Idiopathic Bell's palsy may strike at any age, often after amild viral illness. Recovery is over a period of weeks tomonths and is variable. The cause of the idiopathic varietyis unknown. Sequelae to Bell's palsy include the following :

1 . Interfacial synkinesis : When the eyes close, the mouthwill twitch. This occurs when the regenerating nervefibers do not grow back into the proper muscles . Thesynkinetic movements are almost always present onthe involved side .

2. Because of the contractures, the face at rest may bemore deeply etched on the side ofthe previous palsy .This can give a false impression of weakness on theopposite side .

Table 62 .1Reflexes Involving the Facial Nerve

Orbicularis oculi reflexPercussion causes reflex contraction of the eye muscle . Thereflex is known as the supraorbital, glabellar, or nasopalpebralreflex, depending upon the site of the stimulus . Both eyesusually close, with the contralateral response being weaker . Thetrigeminal nerve is the afferent side and the facial nerve theefferent side of the reflex . Light and sound can also produce thereflex, with the optic and acoustic nerves providing the afferentside .

The response is weak or abolished in nuclear and peripherallesions, and present or exaggerated in supranuclear lesions . It isexaggerated in Parkinsonism and cannot be voluntarily inhibited .

Palpebral-oculogyric reflexThe eyeballs deviate upward when the eyes are closed, bothwhen awake and asleep . The afferent arc is proprioceptiveimpulses carried through the facial nerve to the mediallongitudinal fasciculus. The oculomotor nerve to the superiorrectus muscles forms the efferent side .

In peripheral and nuclear lesions an exaggeration of thisreflex is known as Bell's phenomenon .Orbicularis oris reflexPercussion on the side of the nose or the upper lip causesipsilateral elevation of the angle of the mouth and upper lip.The reflex arc is composed of the fifth and seventh nerves .Synonyms : nasomental, buccal, oral, or perioral reflex .

This reflex disappears after about the first year of life,recurring with supranuclear facial nerve lesions and with extra-pyramidal diseases, such as parkinsonism .

Snout reflexTapping the upper lip lightly with a reflex hammer, tongueblade, or finger causes bilateral contraction of the musclesaround the mouth and base of the nose . The mouth resembles asnout .

This is an exaggeration of the orbicularis oris reflex . It ispresent with bilateral supranuclear lesions and in diffusecerebral diseases, such as various causes of dementia .

Suck reflexSucking movements of lips, tongue, and mouth are broughtabout by lightly touching or tapping on the lips . At times merelybringing an object near the lips produces the reflex .

Occurs in patients with diffuse cerebral lesions . The snoutreflex occurs in similar circumstances .

Palmomental reflexA stimulus of the thenar area of the hand causes a reflexcontraction ipsilaterally of the orbicularis oris and mentalismuscles .

A number of normal individuals have this reflex, and alsopatients with diffuse cerebral disease . It is significant when othersimilar reflexes are also present .Corneal reflexStimulation of the cornea with a wisp of cotton produces reflexclosure of both ipsilateral (strongest) and contralateral eyelids .The fifth nerve carries the afferent impulses, and the facialnerve the efferent impulses .

Table 62 .2Brainstem Nuclei and Functions of the Facial Nerve

Table 62 .3The Branches of the Facial Nerve Beginning Centrally

Brainstem structure Functions and comments

MotorFacial nucleus Motor to facial muscles : frontalis,

Lacrimal nucleus

orbicularis oculi, platysma, buccinator,posterior belly of digastric, stapedius,stylohyoid, soft palate . Brachial motornucleus .

Visceral motor (parasympathetic) to

Superior salivatorylacrimal gland for tear secretion .

Visceral motor (parasympathetic) tonucleus salivary glands for saliva secretion ; also

SensoryNucleus solitarus

nasal glands .

Special visceral sensory . From taste buds

Nucleus of spinal

of anterior two-thirds of tongue.Geniculate ganglion .

General somatic sensory . Sensation fromtract and main anterior nasopharynx, including uppersensory nucleus part of hard and soft palate . Also fromV tympanic membrane, part of external

auditory canal, lateral surface of ear,and area behind the ear and over themastoid process .

Branch Comments

Greater superficial Leaves just distal to geniculate ganglion,petrosal nerve which lies in facial canal of the petrous

Nerve to stapedius

portion of the temporal bone . Supplieslacrimal, nasal, and palatine glands .

Next branch to be given off, still withinmuscle facial canal.

Chorda tympani Leaves about 5 mm or less before the

Motor branches

stylomastoid foramen . Taste to anteriortwo-thirds of tongue, submaxillary andsublingual salivary glands .

At exit from stylomastoid foramen theposterior auricular, digastric, andstylohoid branches are given off. In theparotid gland the nerve divides into twobranches, the temporofacial andcervicofacial, which supply the remainingmuscles.

Other causes of peripheral seventh nerve palsy include :neoplasm, trauma, middle ear infections, parotid gland sur-gery, granulomatous or carcinomatous meningitis, and di-abetes. The disturbances of function produced by theselesions need not be complete. An important clinical pointis that the clinical manifestations of these disorders are in-distinguishable from idiopathic Bell's palsy .

Supranuclear involvement produces contralateral paral-ysis of the lower facial muscles and sparing of the uppermuscles due to the bilateral supranuclear innervation of thelatter. Subtle weakness is often difficult to confirm. Many,perhaps even most, normal individuals have mild lower fa-cial asymmetries, making interpretation difficult .

Anatomic localization of lesions is made by the charac-teristics of the dysfunction and associated structures in-volved. Table 62 .4 gives the location of lesions and the clinicalmanifestations. Refer to Figure 62 .1 to see exactly wherethe lesions occur .

Table 62.4Localization of Facial Nerve Lesions

Taste

Patients with Addison's disease, pituitary insufficiency, orcystic fibrosis have an increased ability to detect the fourprimary tastes. Taste acuity returns to normal with gluco-corticoid therapy in the cases of adrenal hypofunction . Con-versely, penicillamine therapy may be associated with adecreased acuity for the four primary tastes. A wide varietyof conditions may cause decreased or absent taste . They arelisted in Table 62.5 .

Gustatory sweating is sweating on the face associatedeating. It is seen in the following circumstances :

1 . Diabetes mellitus, presumably due to the diabeticautonomic neuropathy

2. Frey's syndrome : gustatory sweating after injury, in-fection, or surgery of the parotid gland

3. After upper dorsal sympathectomy4. Physiological, occurring after eating highly spiced food .

62 . CRANIAL NERVE VII: THE FACIAL NERVE AND TASTE

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Lesion location Manifestations

Above the facial nucleus Contralateral paralysis of lower fa-(supranuclear lesion) cial muscles with relative preser-

Pons (nuclear or fascicular

vation of upper muscles . Lesionlocated either in brainstem orcortex .

Ventral pontine lesion (of Millard-lesion) Gubler): ipsilateral facial mono-

Cerebellopontine angle

plegia, lateral rectus palsy (VI),contralateral hemiplegia (corti-cospinal fibers). Pontine tegmen-turn lesion (of Foville) : ipsilateralfacial monoplegia ; contralateralhemiplegia (corticospinal fibers) ;paralysis of conjugate gaze toside of lesion (pontine parame-dian reticular formation) .

Ipsilateral facial monoplegia, loss(peripheral nerve le- of taste to anterior two-thirds ofsion). 1 in Figure 62 .1 tongue, impairment of salivary

Facial canal between in-

and tear secretion, hyperacusis(if VIII is not affected) . Addi-tional cranial nerves may be in-volved : deafness, tinnitus, vertigo(VII) : sensory loss over face andabsence of corneal reflex (V) ; ip-silateral ataxia (cerebellarpeduncle) .

Same as above except cranialternal auditory meatus nerves other than VII are notand geniculate ganglion involved .(peripheral nerve typelesion here and subse-quently) . 2 in Figure62 .1

Facial canal between ge- Facial monoplegia ; impaired sali-niculate ganglion and vary secretion ; loss of taste ;nerve to stapedius mus- hyperacusis.cle . 3 in Figure 62 .1

Facial canal between Facial monoplegia ; impaired sali-nerve to stapedius and vary secretion ; loss of taste.leaving of chorda tym-pani . 4 in Figure 62.1

After branching of chorda Facial paralysis, distribution relatedtympani. 5 in Figure to site of lesion .62 .1

Numbers refer to Figure 62 .1 .

Table 62.5Examples of Disorders Reported to Be Associated withGustatory Dysfunction

EndocrineAdrenal cortical insufficiencyCongenital adrenal hyperplasiaPanhypopituitarismCushing's syndromeHypothyroidismDiabetes mellitusTurner's syndromePseudohypoparathyroidism

InflammatoryInfections :CandidaGingivitisHerpes simplexPeriodontitisSialadenitis

Autoimmune :PemphigusSjogren's syndrome

Local alterations of taste buds or papillaeChemicals, drugsXerostomia

NeurologicBell's palsyFamilial dysautonomiaHead traumaMiddle ear operations with Cretinism manipulation or damageto chorda tympani

Multiple sclerosisRaeder's paratrigeminal syndrome

NutritionalCachexiaChronic renal failureCirrhosis of the liverNiacin (vitamin B s deficiency)Zinc deficiency

PsychiatricDepressionSchizophrenia

TumorsOral cavity cancerBase of skull neoplasia

Source: From Doty RL, Kimmelman CP . Smell and taste and their dis-orders . In : Asbury AK, McKhann GM, McDonald WI, eds. Diseases of thenervous system . Philadelphia : W.B . Saunders 1986 ;1 :466-78 .

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No proposed mechanism explains the occurrence of thisphenomenon in all cases .

References

Brodal A. Neurological anatomy in relation to clinical medicine .3rd ed. New York : Oxford University Press, 1981 .

Dejong RN. The neurologic examination . 4th ed. New York : Har-per& Row, 1979;178-98 .

Doty RL . A review of olfactory dysfunctions in man . Am J Otol aryngol 1979;1:57-79.

Doty RL, Kimmelman CP . Smell and taste and their disorders . In :Asbury AK, McKhann GM, McDonald WI, eds. Diseases of thenervous system. Philadelphia : W .B. Saunders, 1986 ;1 :466-78 .

Haymaker W, Kuhlenbeck H . Disorders of the brainstem and itscranial nerves . In : Baker AB, Joynt RF, eds . Clinical neurology .Philadelphia : Lippincott, 1985 ;3 :chap . 40 .

Karnes WE. Diseases of the seventh cranial nerve. In : Dyck PJ,Thomas PK, Lambert EH, Bunge R, eds . Peripheral neur-opathy . 2nd ed . Philadelphia : W.B. Saunders, 1984 ;2 :1266-99 .

Schiffman SS. Taste and smell in disease. N Engl J Med 1983 ;308:1275-79, 1337-43 .