61 Cranial Nerve V:The Trigeminal NerveH. KENNETH WALKER

Definition

Sensory

The sensory portion of the trigeminal supplies touch-pain-temperature to the face . The nerve has three divisions : theophthalmic, maxillary, and mandibular nerves (Figure 61 .1) .The innervation includes the cornea and conjunctiva of theeye; mucosa of the sinuses, nasal and oral cavities ; and duraof the middle, anterior, and part of the posterior cranialfossae. The mandibular division carries the motor portion .The motor portion conveys proprioceptive impulses fromthe temporomandibular joint .

A lesion of the sensory fibers produces hypesthesia oranesthesia of the area supplied . The corneal reflex is absentwhen the area of supply is the eye . Proprioception for thetemporomandibular joint is absent when there is a lesion ofthe mandibular division .

Motor

The motor division of the nerve supplies the muscles ofmastication : masseter, temporal, pterygoid, mylohyoid, anddigastric. These muscles produce elevation, depression,protrusion, retraction, and the side-to-side movements ofthe mandible . The motor division also supplies the tensortympani and tensor palati muscles .

The mandible upon opening deviates toward the para-lyzed side when there is unilateral paralysis of the masti-catory muscles . This direction of the mandible is due to theaction of normal pterygoids on the opposite side . The man-dible droops, and no jaw movement is possible with bilateralparalysis. The involved muscles atrophy in nuclear or infra-nuclear lesions.

The jaw jerk is one of the deep tendon or stretch reflexes .When it is normal, tapping the mandible produces a brisk

contraction . When abnormal, with upper motor neuron le-sions, there is a hyperactive or repeating reflex (clonus) .With nuclear or infranuclear lesions, the reflex is absent .

Technique

Sensory

Tell the patient you are going to test the ability to feel touchor pain on the face. The eyes should be shut . Take a pieceof cotton or the ball of your finger . Lightly touch eitherone or both sides of each of the three divisions of the tri-geminal . Ask the patient to show or say whether you touchedone or both sides of the face .

Next, take a safety pin and gently prick first one side ofeach division and then the other, asking the patient if thereis any difference in the sensation on one side compared tothe other. With the patient's eyes closed, touch sometimeswith the sharp point of the pin and at other times with thedull guard . Ask the patient to describe the sensation .

Test the corneal reflex . Begin by telling the patient youare going to touch the eye gently in order to check the reflex .Take a wisp of cotton and twist it into a point . Ask thepatient to look in the other direction, so you will not betesting the blink reflex . Then gently but firmly touch thecornea at its junction with the sclera . Sensitivity to painincreases medially from this point and decreases laterally .The junction of the cornea and sclera is a good compromisebetween causing pain to the patient and obtaining the reflex .There is a rapid blink of the eye being tested and a con-sensual blink of the other eye . If there is seventh nerveweakness on the side being tested, then observe the con-sensual reflex .

Motor

Test for motor abnormalities as follows :

1 . Observe the skin over the temporal masseter muscles .Concavity or asymmetry suggests atrophy . The tip ofthe mandible should be in the midline .

2. Ask the patient to clench his or her jaws . Palpate themasseter and temporal muscles for asymmetry of vol-ume and for tone .

3. Observe for deviation of the tip of the mandible asthe jaws are opened . Line up a tongue blade with thetip of the nose and the center of the mouth if thereis seventh nerve weakness . This maneuver makes iteasier to perceive deviation . Deviation is to the weakside .

4. Ask the patient to move the jaw from side to sideagainst the resistance of your palm . The paralyzedside will not move laterally .

For the stretch reflex, demonstrate to the patient whatyou are going to do . Have the jaws half open and relaxed .

Figure 61 .1Areas supplied by the three sensory divisions of the trigeminalnerve .

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Then place your index finger on the tip of the mandibleand tap your finger gently but briskly with a reflex hammer .

The trigeminal nerve is the afferent portion of a numberof valuable reflexes involving the facial nerve (see Chapter62 and Table 61 .1) .

Basic Science

Sensory

The three divisions of the nerve carry pain, temperature,and touch modalities from the skin of the face ; the mucosaof sinuses, nose, and mouth ; the teeth ; and portions of thedura. They convey proprioceptive sensation from the teeth,hard palate, temporomandibular joint, and muscles of mas-tication . The three divisions are as follows :

1 . Ophthalmic. Upper division. Innervates forehead, up-per eyelid, cornea (thus the corneal reflex), conjunc-tiva, dorsum of the nose, and dura of some of theanterior cranial fossa. Leaves orbit through the su-perior orbital fissure . Proceeds through the lateralwall of the cavernous sinus in close relation to thethird, fourth, and sixth cranial nerves. Joins other twodivisions to form the trigeminal (semilunar, Gasser-ian) ganglion .

2 . Maxillary . Supplies upper lip, lateral and posteriorportions of nose, upper cheek, anterior temple, mu-cosa of nose, upper jaw, upper teeth, roof of mouth,and dura of part of the middle cranial fossa . Thenerve leaves the pterygopalatine fossa, passes throughthe foramen rotundum, traverses the inferior part ofthe cavernous sinus, and enters the trigeminal gan-glion .

3. Mandibular . Supplies lower lip, chin, posterior cheek,temple, external ear, mucosa of lower part of mouth,anterior two-thirds of the tongue, and portions of thedura of anterior and middle cranial fossae . Propri-oceptive impulses are carried largely in the motornerve, which is incorporated into the mandibular di-vision. It enters the cranium through the foramenovale and goes to the trigeminal ganglion .

Sympathetic and parasympathetic fibers join the threedivisions and are distributed to the pupil, to the nasal mu-

Table 61 .1Location of Central Trigeminal Lesions

cosa causing mucus secretion, to the lacrimal, submaxillary,and sublingual glands, and to the arterioles of the face .

The trigeminal ganglion rests in Meckel's cave, a cavityon the apex of the petrous bone . In this position the gan-glion is lateral to the internal carotid artery and the pos-terior portion of the cavernous sinus . The trigeminalganglion contains pseudounipolar ganglion cells whose in-ternal branches pass into the pons . These internal branchesform the sensory root of the trigeminal, which is analogousto the posterior root of a spinal nerve . The root enters thelateral portion of the middle third of the pons . The brancheseither bifurcate into ascending and descending arms or as-cend or descend without bifurcating . The central processesare distributed to three sensory nuclei (Figure 61 .2) . Be-ginning with the lowest or most caudal they are (1) the spinaltract nucleus, (2) the main sensory nucleus, and (3) themesencephalic nucleus . They are considered below in thatorder.

The spinal tract nucleus (homologous to the most dorsallaminae of the dorsal horn of the spinal cord) : pain andtemperature . The descending central processes of the sen-sory root are gathered as a bundle, the spinal tract of thetrigeminal nerve . This tract descends to the caudal medullawhere it begins to fuse with the dorsolateral tract of Lissauerin the spinal cord . The tract gives off fibers to its nucleus,which lies medial . The nucleus is continuous caudally withthe substantia gelatinosa of the spinal cord and rostrallywith the main trigeminal sensory nucleus (see below) . Thereis probably a topographical localization of fibers in both thetract and in the nucleus .

The spinal tract and nucleus : pain and temperature . Theimportant clinical principle is that lesions in any of the fol-lowing locations will give ipsilateral loss of pain and tem-perature on the face (and of course other findings dependingupon location of the lesion) : peripheral, nuclear, and sen-sory root of the trigeminal ; pontine or medullary locationsthat involve the spinal tract and nucleus. The spinothalamictract from the contralateral half of the body is near thetrigeminal tract and nucleus . Therefore it follows that atthese levels there can be contralateral loss of body pain andtemperature associated with an ipsilateral loss of facial painand temperature if the lesion is sufficiently large . Vascularlesions of the lower medulla are a frequent cause of thisclinical syndrome .

Secondary trigeminal fibers arise from the spinal tractnucleus and cross to the other side. They form the ventraltrigeminal tract (VTT) . The VTT ascends in close relation-ship with the contralateral medial lemniscus terminating inthe ventral posteromedial (VPM) nucleus of the thalamus .

Cortical projections from the VPM go to the somatosen-sory areas of the cortex, principally the postcentral gyrus .There is a somatotopical localization .

The main sensory nucleus (homologous to the dorsal fun-iculus of the spinal cord) : tactile sensation . The ascendingbranches of the sensory root end in this nucleus, which liesin the pons adjacent to the entering root fibers . There is asomatotopical organization .

The large majority of ascending fibers from this nucleuscross the brainstem, travel in association with the contra-lateral medial lemniscus, and end upon the VPM of thethalamus. A smaller group of fibers does not cross but as-cends near the periaqueductal gray as the dorsal trigeminaltract. This tract terminates on the ipsilateral VPM . Thethalamocortical projections go to the somatosensory areasof the cortex .

The mesencephalic nucleus : proprioceptive sensibility . Thisnucleus is located in the lateral dorsal margin of the central

Functional loss Location Structure(s)

Pain, temperature, Lateral rostral Spinothalamictouch over entire pons and above ventral trigeminalbody, including faceipsilaterally

tracts contralaterally

Masticatory muscle Midpons Main sensoryparalysis and pain, nucleus, motortemperature, touch nucleus, andover face ipsilaterally entering root fibers

ipsilaterallyPain, temperature Lateral inferior Spinal tract andover face pons or lateral spinal tract nucleusipsilaterally ; pain, medulla ipsilaterally ;temperature over spinothalamic tractbody (and and occasionallyoccasionally face) ventral trigeminalcontralaterally tract contralaterally

Figure 61 .2The trigeminal nerve and its connections . From Dejong RN . The neurologic examination . 4thed. New York: Harper & Row, 1979. Used with permission .

gray matter lying next to the fourth ventricle. Afferent im-pulses arise in masticatory muscles, teeth, periodontium,hard palate, and the temporomandibular joint . Most affer-ent fibers destined for this nucleus appear to travel withthe motor root, although some fibers may go with all threedivisions of the nerve . The cells of origin of these fibers,unlike those described above, are not in the trigeminal gan-glion; they are in the nucleus itself within the brain . Thisapparently represents an example of a dorsal root ganglionthat exists within the substance of the central nervous sys-tem .

Most central processes of the cells of this nucleus descendas the mesencephalic tract to the motor nucleus of the fifthnerve. There are, however, other and more complex con-nections . This nucleus may be concerned with the force ofthe bite when the source of these proprioceptive impulsesand the distribution of the central processes to the motornucleus are taken into consideration .

Motor

The supranuclear innervation originates in the lower pre-central gyrus, with the contralateral contribution larger thanthe ipsilateral . These fibers, as part of the corticobulbartract, descend in the genu of the internal capsule, coursethrough the cerebral peduncles, and are distributed to themotor nuclei of the trigeminal nerve .

The motor nucleus lies in the middle of the pons medialto the main sensory nucleus . The fibers leave the pons ven-tral to the sensory fibers. The motor root lies against thetrigeminal ganglion and is incorporated into the mandibu-lar division of the sensory nerve .

When the jaw jerk is performed, muscle spindles areactivated as the masticatory muscles are stretched suddenlyby tapping on the mandible . These afferent proprioceptiveimpulses are carried largely in the motor portion of thenerve and end in the mesencephalic nucleus . Collaterals

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from this nucleus terminate on the trigeminal motor nu-cleus thereby setting up a two-neuron reflex arc . (See Chap-ter 72, Deep Tendon Reflexes .)

Clinical Significance

Peripheral lesions involving the sensory portion of the tri-geminal at any point distal to the pontine exit can produceipsilateral pain and/or varying degrees of anesthesia . Thedistribution of the lesion will, of course, determine thesymptoms and findings. Some of the etiologies are (fromSelby, 1984) :

1 . Peripheral lesions: craniofacial trauma, basilar skullfeatures, dental trauma, maxillary sinusitis, primaryor metastatic tumors, aneurysm of the internal carotidartery, cavernous sinus thrombosis, stilbamidine, tri-chlorethylene, lupus, scleroderma, Sjogren's syn-drome, sarcoidosis, probably amyloidosis, and a fairlycommon idiopathic benign sensory neuropathy . Hor-ner's syndrome can be produced by lesions of thenasociliary nerve as it runs with the ophthalmic di-vision .

2. Lesions of the ganglion : herpes zoster infection, pri-mary and metastatic tumors .

3. Trigeminal root lesions : adjacent tumors and vascularmalformations, especially acoustic neurinoma andcholesteatomas . These lesions are prone to producefacial pain that is often misdiagnosed as tic doulou-reux or tooth pain .

Vascular lesions, tumors, and congenital malformations(syringobulbia and syringomyelia) are the common causesof central lesions . Lesions of the sensory cortex will producea raised threshold (but not anesthesia) to pain and temper-ature on the opposite side of the face . Thalamic lesions canproduce contralateral hypesthesia and hyperpathia of theface . Midpontine lesions, when unilateral, produce ipsilat-eral decrease in tactile sensation of the face due to involve-ment of the main sensory nucleus, and ipsilateral paralysisof the masticatory muscles when the motor nucleus is in-volved . Anesthesia or hypesthesia ipsilaterally is seen if thepontine lesion involves entering sensory root fibers carryingpain and temperature modalities . Below the pons, ipsilateralpain and temperature is lost if the spinal tract and nucleusare involved. When the ventral trigeminal tract carryingcrossed pain-temperature fibers is involved, loss of thesemodalities occurs on the opposite side of the face .

Some helpful generalizations about central trigeminallesions are given in Table 61 .1 .

Tic douloureux, or trigeminal neuralgia, is a relativelyfrequent cause of facial pain . There is a paroxysmal painof great intensity that involves any or all of the trigeminaldivision. The pain is more frequent in the maxillary ormandibular division . The pain lasts seconds to minutes andcan be set off by a blast of cold or hot air, shaving, combingthe hair, or similar stimulus . There are no objective findingssuch as anesthesia . The etiology is unknown . For years therewas disagreement about whether the pain originated cen-trally or peripherally. Electron microscopic observations ofpathologic changes in the trigeminal ganglion in recent yearsare suggestive of ganglionic origin . One current suggestionis that adjacent arteries compress the ganglion (Haines etal., 1980) .

The motor nerve as it runs with the mandibular divisioncan be damaged by the lesions listed above . Clinically thereis atrophy and flaccid paralysis of the muscles of mastication .In unilateral paralysis, as the mandible opens it will swingto the paralyzed side due to the action of the normal op-posite external pterygoid . Bilateral paralysis with droppingof the mandible is rare . Spasm of the masticatory musclesis seen with tetanus and strychnine poisoning .

References

Bishop B, Hickenbottom RS, Moriarty TM . Identification and as-sessment of factors contributing to variability of the jaw jerk .Exp Neurol 1984 ;84:549-64 .

Brodal A . Neurological anatomy . 3rd ed . New York : Oxford Uni-versity Press, 1981 ;508-32 .

Dejong RN. The neurologic examination . 4th ed. New York : Har-per& Row, 1979;163-77 .

Haines SJ, Jannetta PJ, Zorub DS . Microvascular relations of thetrigeminal nerve. An anatomical study with clinical correlation .J Neurosurg 1980 ;52:381-86.

Lund JP, Lamarre Y, Lavigne G . Duquet G: Human jaw reflexes.Adv Neurol 1983 ;39:739-75 .

Ongerboer de Visser BW. Anatomical and functional organizationof the reflexes involving the trigeminal system in man : jaw re-flex, blink reflex, corneal reflex, and exteroceptive suppression .Adv Neurol 1983 ;39:727-38 .

Selby G. Diseases of the fifth cranial nerve . In: Dyck PF, ThomasPK, Lambert EH, eds . Peripheral neuropathy . 2nd ed . Phila-delphia: W.B. Saunders, 1984;1224-65 .