As for their visual function, the optic nerves originate in visual receptors in the retina and project posteriorly to the optic chiasm. At the chiasm, nasal fibers of the nerves cross, but temporal fibers continue uncrossed (Fig. 4-1). Temporal fibers of one optic nerve join the nasal fibers of the other to form the optic tracts. The tracts pass through the temporal and parietal lobes to terminate in the calcarine cortex of the occipital lobe. Thus, each occipital lobe receives its visual information from the contralateral visual field. Further projections convey the visual information to other areas of the cerebral cortex for decoding work, such as reading, and tracking moving objects.

FIGURE 4-1 Left, The optic nerves originate in the retinas. Their medial fibers cross at the optic chiasm while the temporal portions continue uncrossed. The recombinations form the optic tracts that synapse at the lateral geniculate bodies (LGB). The optic tracts sweep through the posterior cerebral hemispheres to terminate at the occipital (“visual”) cortex. This system projects the impulses from each visual field to the contralateral occipital lobe cortex. For example, as in this sketch, impulses conveying the shaded half of the bar, which is in the patient’s left visual field, project to the right occipital cortex. Right, As in this case, medical illustrations typically show a patient’s visual fields from the patient’s perspective. This illustration portrays the shaded portion of the bar in the left visual field of each eye.

Visual field abnormalities are considered among the most important findings in neurology. Many of them point to specific neurologic disorders, such as optic neuritis, pituitary adenomas, and migraines (see Fig. 12-9). In addition, several visual field abnormalities are integral parts of frequently occurring neuropsychiatric conditions, such as left homonymous hemianopsia associated with anosognosia, right homonymous hemianopsia with aphasia or alexia, and incongruous deficits with psychogenic disturbances.

As for their role in regulating the size of the pupil, the optic nerves and tracts form the afferent limb of the pupillary light reflex by sending small branches containing information about light intensity to the midbrain. After a single synapse, the oculomotor nerves (the third cranial nerves) form the efferent limb. The oculomotor nerves, which contain some parasympathetic fibers, innervate the pupils’ constrictor fibers. Overall, the light reflex – optic nerves to midbrain and midbrain to oculomotor nerves – constricts pupil size in response to the intensity of light striking the retina. Simply put, when a neurologist shines a bright light into one or both eyes, the light reflex constricts both pupils (Fig. 4-2).

FIGURE 4-2 The light reflex, which is more complex than a deep tendon reflex, begins with its afferent limb in the optic nerve (cranial nerve [CN] II). The optic nerve transmits light impulses from the retinas to two neighboring midbrain structures: (1) In conveying vision, axons synapse on the lateral geniculate body. Then postsynaptic tracts convey visual information to the occipital lobe’s visual cortex. (2) In conveying the light reflex, optic nerve axons also synapse in the pretectal area. Postsynaptic neurons travel a short distance to both the ipsilateral and contralateral Edinger–Westphal nuclei, which are the parasympathetic divisions of the oculomotor (third cranial nerve) nuclei. Those nuclei give rise to parasympathetic oculomotor nerve fibers, which constitute the reflex’s efferent limb (see Fig. 12-17, top). Their fibers synapse in the ciliary ganglia and postsynaptic fibers terminate in the iris constrictor (sphincter) muscles. Thus, light shone in one eye constricts the pupil of that eye (the “direct” [ipsilateral] light reflex) and the contralateral eye (the “consensual” [indirect or contralateral] light reflex). This figure also indicates how oculomotor nerve injuries, because they usually include damage to the parasympathetic component, dilate the pupil. Similarly, it indicates how damaged sympathetic nervous innervation with unopposed parasympathetic innervation, as in the lateral medullary and Horner syndromes, produces pupil constriction (miosis). Finally, it shows how ciliary ganglion damage produces a dilated but extremely sensitive “Adie pupil” (see Fig. 12-17, bottom).

Because of the pupillary light reflex, shining light into one eye will normally provoke bilateral pupillary constriction. In an example of an abnormality detectable by testing the light reflex, if neurologists shine light into the right eye and neither pupil constricts, and then into the left eye and both pupils constrict, the right optic nerve (afferent limb) is impaired. In a different example, if neurologists shine light into the right eye and it produces no constriction of the right pupil but succeeds in provoking left eye pupil constriction, the right oculomotor nerve (efferent limb) is impaired.

In contrasting the optic nerves’ two functions, the visual system constitutes a high-level cortical system, but the light reflex remains a basic brainstem function. Thus, devastating cerebral cortex injuries – from trauma, anoxia, or degenerative illnesses – produce blindness (“cortical blindness”). However, no matter how terrible the cerebral cortex damage – even to the point of patients being severely demented, bedridden, and blind – the pupils continue to react normally to light. In the absence of ocular trauma, if a patient’s pupils no longer react to light, neurologists take it as a sign of brain death.

Routine testing of the optic nerve includes examination of: (1) visual acuity (Fig. 12-2); (2) visual fields (Fig. 4-3); and (3) the ocular fundi (Fig. 4-4). Because the visual system is important, complex, and subject to numerous ocular, neurologic, iatrogenic, and psychogenic disturbances, this book dedicates an entire chapter to visual disturbances particularly relevant to psychiatry (see Chapter 12).

FIGURE 4-3 In testing visual fields by the confrontation method, this neurologist wiggles her index finger as the patient points to it without diverting his eyes from her nose. She tests the four quadrants of each eye’s visual field. (Only by testing each eye individually will she detect a bitemporal quadrantanopia, which is the visual field defect characteristic of pituitary adenomas.) Neurologists test young children and others unable to comply with this method in an abbreviated but still meaningful manner. In this case, the neurologist might assess the response to an attention-catching object introduced to each visual field. For example, a stuffed toy animal, dollar bill, or glass of water should capture a patient’s attention. If the patient does not respond, the neurologist should primarily consider visual field deficit(s). In some cases, a psychogenic visual loss (see later) or inattention (neglect, see Chapter 8) may explain the patient’s failure to respond.

FIGURE 4-4 On fundoscopy, the normal optic fundus or disk appears yellow, flat, and well demarcated from the surrounding red retina. The retinal veins, as everywhere else in the body, are broader than their corresponding arteries. A neurologist will usually see retinal veins pulsate except when intracranial pressure is elevated.

The origin of the optic nerves explains their involvement in certain illnesses and not in others. Unique among the cranial nerves, the optic nerves (and a small proximal portion of the acoustic nerves) are actually projections of the brain coated by myelin derived from oligodendrocytes. In other words, these cranial nerves are extensions of the central nervous system (CNS). Thus, CNS illnesses, particularly childhood-onset metabolic storage diseases and multiple sclerosis (MS)-induced optic neuritis (see Chapter 15), are apt to attack the optic nerves. On the other hand, the optic nerves remain relatively immune from diseases that exclusively attack the peripheral nervous system (PNS) myelin, such as the Guillain–Barré syndrome. Also, by way of contrast, Schwann cells produce the myelin coat of both the remaining cranial nerves and all PNS nerves. They are susceptible to diseases that strike PNS myelin.

Oculomotor, Trochlear, Abducens Nerves (Third, Fourth, Sixth)

The oculomotor, trochlear, and abducens nerves constitute the “extraocular muscle system” because, acting in unison, they move the eyes in parallel to provide normal conjugate gaze. Damage of any of these nerves or the muscle they innervate causes dysconjugate gaze, which results in characteristic patterns of diplopia (double vision). In addition, with oculomotor nerve damage, patients lose their pupillary constriction to light and strength of the eyelid muscle.

The oculomotor nerves (third cranial nerves) originate in the midbrain (Fig. 4-5) and supply the pupil constrictor, eyelid, and adductor and elevator muscles of each eye (medial rectus, inferior oblique, inferior rectus, and superior rectus). Oculomotor nerve impairment, a common condition, thus leads to a distinctive constellation: a dilated pupil, ptosis, and outward deviation (abduction) of the eye (Fig. 4-6). As just discussed, oculomotor nerve injury also impairs the efferent limb of the light reflex. In addition, it impairs the efferent limb of the accommodation reflex, in which the visual system adjusts the shape of the lens to focus on either near or distant objects. (Impaired focusing ability in older individuals, presbyopia [Greek, presbys, old man; opia, eye] results from the aging lens losing its flexibility, not from oculomotor nerve impairment.)

FIGURE 4-5 The oculomotor (third cranial) nerves arise from nuclei in the dorsal portion of the midbrain (see Fig. 2-9). Each descends through the red nucleus, which carries cerebellar outflow fibers to the contralateral limbs. Then each oculomotor nerve passes through the cerebral peduncle, which carries the corticospinal tract destined to innervate the contralateral limbs.

FIGURE 4-6 A, This patient, who is looking ahead, has paresis of the left oculomotor nerve with typical findings: the eye is deviated laterally; the pupil is dilated and unreactive to light; and the upper eyelid covers a portion of the pupil (ptosis). B, In a milder case, with the patient looking ahead, close inspection reveals subtle ptosis, lateral deviation of the eye, and dilation of the pupil. In both cases, patients have diplopia that increases when looking to the right because this movement requires adducting the left eye, but the paretic left medial rectus muscle cannot participate and the patient’s gaze becomes dysconjugate (see Fig. 12-13).

The trochlear nerves (fourth cranial nerves) also originate in the midbrain. They supply only the superior oblique muscle, which is responsible for depression of the eye when it is adducted (turned inward). To compensate for an injured trochlear nerve, patients tilt their head away from the affected side. Unless the neurologist observes a patient with diplopia perform this telltale maneuver, a trochlear nerve injury is difficult to diagnose.

Unlike the third and fourth cranial nerves, the abducens nerves (sixth cranial nerves) originate in the pons (Fig. 4-7 and see Fig. 2-9). Like the fourth cranial nerves, the abducens nerves perform only a single function and innervate only a single muscle. Each abducens nerve innervates its ipsilateral lateral rectus muscle, which abducts the eyes. Abducens nerve impairment, which is relatively common, leads to inward deviation (adduction) of the eye from the unopposed medial pull of the oculomotor nerve, but no ptosis or pupil changes (Fig. 4-8). To review: the lateral rectus muscle is innervated by the sixth cranial (abducens) nerve and the superior oblique by the fourth (trochlear), but all the others by the third (oculomotor). A mnemonic device, “LR₆SO₄,” captures this relationship.

FIGURE 4-7 The abducens (sixth cranial) nerves arise from nuclei located in the dorsal portion of the pons. These nuclei are adjacent to the medial longitudinal fasciculus (MLF; see Fig. 15-3). As the abducens nerves descend, they pass medial to the facial nerves, and then between the upper motor neurons of the corticospinal tract.

FIGURE 4-8 This patient with paresis of the left abducens nerve has medial deviation of the left eye. He will have diplopia on looking ahead and toward the left, but not when looking to the right (see Fig. 12-14).

To produce conjugate eye movements, the oculomotor nerve on one side works in tandem with the abducens nerve on the other. For example, when an individual looks to the left, the left sixth nerve and right third nerve simultaneously activate their respective muscles to produce conjugate leftward eye movement. Such complementary innervation is essential for conjugate gaze. If both third nerves were simultaneously active, the eyes would look toward the nose; if both sixth nerves were simultaneously active, the eyes would look toward opposite walls.

Neurologists most often attribute diplopia to a lesion in the oculomotor nerve on one side or the abducens nerve on the other. For example, if a patient has diplopia when looking to the left, then either the left abducens nerve or the right oculomotor nerve is paretic. Diplopia on right gaze, of course, suggests a paresis of either the right abducens or left oculomotor nerve. As a clue, the presence or absence of other signs of oculomotor nerve palsy (a dilated pupil and ptosis, for example) usually indicates whether that nerve is responsible.

The ocular cranial nerves may be damaged by lesions in the brainstem, in the nerves’ course from the brainstem to the ocular muscles, or in their neuromuscular junctions, but not in the cerebral hemispheres (the cerebrum). Because cerebral damage does not injure these cranial nerves, patients’ eyes remain conjugate despite cerebral infarctions and tumor. Even patients with advanced Alzheimer disease, ones who have sustained cerebral anoxia, and those lingering in a persistent vegetative state retain conjugate eye movement.

For learning purposes, neurologists might best consider ocular cranial nerve lesions according to their brainstem level (midbrain and pons) and correlate clinical features with the admittedly complex anatomy. Because the anatomy is so compact, brainstem lesions that damage cranial nerves typically produce classic combinations of injuries of the ocular nerves and the adjacent corticospinal (pyramidal) tract or cerebellar outflow tracts. These lesions cause diplopia accompanied by contralateral hemiparesis or ataxia. The pattern of the diplopia is the signature of the lesion’s location. The etiology in almost all cases is an occlusion of a small branch of the basilar artery causing a small brainstem infarction (see Chapter 11).

Most importantly, despite producing complex neurologic deficits, brainstem lesions generally do not impair cognitive function. Nevertheless, certain exceptions to this dictum bear mentioning. Wernicke encephalopathy, for example, consists of memory impairment (amnesia) accompanied by nystagmus and oculomotor or abducens nerve impairment (see Chapter 7). Another exception is transtentorial herniation, in which a cerebral mass lesion, such as a subdural hematoma, squeezes the anterior tip of the temporal lobe through the tentorial notch. In this situation, the mass compresses the oculomotor nerve and brainstem to cause coma, decerebrate posturing, and a dilated pupil (see Fig. 19-3).

The following frequently occurring, classic brainstem syndromes, despite their pronounced deficits, typically spare cognitive function. With a right-sided midbrain infarction a patient would have a right oculomotor nerve palsy, which would cause right ptosis, a dilated pupil, and diplopia, accompanied by left hemiparesis (Fig. 4-9). With a slightly different right-sided midbrain infarction, a patient might have right oculomotor nerve palsy and left tremor (Fig. 4-10).

FIGURE 4-9 A, A right midbrain infarction damages the oculomotor nerve that supplies the ipsilateral eye and the adjacent cerebral peduncle, which contains the corticospinal tract that subsequently crosses in the medulla and ultimately supplies the contralateral arm and leg. B, This patient has right-sided ptosis from the right oculomotor nerve palsy and left hemiparesis from the corticospinal tract injury. Also note that the ptosis elicits a compensatory unconscious elevation of the eyebrow to uncover the eye. Neurologists also see eyebrow elevation in other conditions that cause ptosis, such as the lateral medullary syndrome (Fig. 2-10), myasthenia gravis (Fig. 6-3), and cluster headache (Fig. 9-4).

FIGURE 4-10 A, The red nucleus is the intermediate step in conveying cerebellar outflow from the cerebellum to the ipsilateral arm and leg. Each cerebellar hemisphere innervates the contralateral red nucleus that, in turn, innervates the contralateral arm and leg. Because this pattern involves two contralateral steps, neurologists often call it the “double cross.” In this case, a right midbrain infarction damages the oculomotor nerve and adjacent red nucleus, which innervates the left arm and leg. B, This patient has right ptosis from the oculomotor nerve palsy and left arm ataxia from the damage to the cerebellar outflow tract.

A right-sided pons lesion typically translates into a right abducens nerve paresis and left hemiparesis (Fig. 4-11). Notably, in each of these brainstem injuries, mental status remains normal because the cerebrum is unscathed.

FIGURE 4-11 A, A right pontine infarction damages the abducens nerve, which supplies the ipsilateral eye, and the adjacent corticospinal tract, which supplies the contralateral limbs. (This situation is analogous to midbrain infarctions: Fig. 4-8.) B, This patient has inward deviation of the right eye from paresis of the right abducens nerve, and left hemiparesis from right corticospinal tract damage. MLF, medial longitudinal fasciculus.

Another common site of brainstem injury that affects ocular motility is the medial longitudinal fasciculus (MLF). This structure is the heavily myelinated midline tract between the pons and the midbrain that links the nuclei of the abducens and oculomotor nerves (see Figs 2-9, 4-11, 15-3, and 15-4). Its interruption produces the MLF syndrome, also called internuclear ophthalmoplegia, which consists of nystagmus of the abducting eye and failure of the adducting eye to cross the midline. This disorder is best known as a characteristic sign of MS.

The oculomotor and abducens nerves are particularly vulnerable to injury in their long paths between their brainstem nuclei and ocular muscles. Lesions in those nerves produce simple, readily identifiable clinical pictures: extraocular muscle impairment without hemiparesis, ataxia, or mental status impairment. Diabetic infarction, the most frequent lesion of the oculomotor nerves, produces a sharp headache and paresis of the affected muscles. Although otherwise typical of oculomotor nerve infarctions, diabetic infarctions characteristically spare the pupil. In other words, diabetic infarctions cause ptosis and ocular abduction, but the pupil remains normal in size, equal to its counterpart, and reactive to light.

Ruptured or expanding aneurysms of the posterior communicating artery may compress the oculomotor nerve, just as it exits from the midbrain. In this case, oculomotor nerve palsy – which would be the least of the patient’s problems – is just one manifestation of a life-threatening subarachnoid hemorrhage that usually renders patients prostrate from a headache. Children occasionally have migraine headaches accompanied by temporary oculomotor nerve paresis (see Chapter 9). By way of contrast, in motor neuron diseases, amyotrophic lateral sclerosis (ALS) and poliomyelitis, the oculomotor and abducens nerves retain normal function despite destruction of large numbers of motor neurons. Patients may have full, conjugate eye movements despite being unable to breathe, lift their limbs, or move their head.

Disorders of the neuromuscular junction – the cranial and peripheral nerve’s furthest extent – also produce oculomotor or abducens nerve paresis. In myasthenia gravis (see Fig. 6-3) and botulism, for example, impaired acetylcholine neuromuscular transmission leads to combinations of ocular and other cranial nerve paresis. These deficits may puzzle neurologists because the muscle weakness is often subtle and variable in severity and pattern. Neurologists may overlook mild cases or misdiagnose them as a psychogenic disorder. Nevertheless, they illustrate neuroanatomic relationships and are clinically important, especially in their extremes. For example, severe cases may lead to respiratory impairment.

People can usually feign ocular muscle weakness only by staring inward, as if looking at the tip of their nose. Children often do this playfully; however, neurologists diagnose adults with their eyes in such a position as displaying voluntary, bizarre activity. Another disturbance, found mostly in health care workers, comes from their surreptitiously instilling eye drops that dilate the pupil to mimic ophthalmologic or neurologic disorders.

Trigeminal (Fifth)

In contrast to the exclusively sensory function of cranial nerves I and II, and the exclusively motor functions of cranial nerves III, IV, VI, and XII, the trigeminal nerves have both sensory and motor functions. The trigeminal (Latin, threefold) nerves convey sensation from the face and innervate the large, powerful muscles that protrude and close the jaw. Because these muscles’ main function is to chew, neurologists often call them “muscles of mastication.”

The trigeminal nerves’ motor nucleus is situated in the pons, but the sensory nucleus extends from the midbrain through the medulla. The trigeminal nerves leave the brainstem at the side of the pons, together with the facial and acoustic nerves, to become the three cranial nerves – V, VII, and VIII – that pass through the cerebellopontine angle.

Examination of the trigeminal nerve begins by testing sensation in its three sensory divisions (Fig. 4-12). Neurologists touch the side of the patient’s forehead, cheek, and jaw. Areas of reduced sensation, hypalgesia, should conform to anatomic outlines.

FIGURE 4-12 The three divisions of the trigeminal nerve convey sensory innervation of the face. The first division (V₁) supplies the forehead, the cornea, and the scalp up to the vertex; the second (V₂) supplies the malar area; and the third (V₃) supplies the lower jaw, except for the angle. These dermatomes hold more than academic interest. Herpes zoster infections (shingles), trigeminal neuralgia (see Chapter 9), and facial angioma in the Sturge–Weber syndrome (see Fig. 13-13) each typically affect one or another dermatome. In contrast, psychogenic disturbances do not confine themselves to a single dermatome.

Assessing the corneal reflex is useful, especially in examining patients whose sensory loss does not conform to neurologic expectations. The corneal reflex is a “superficial reflex” that is essentially independent of upper motor neuron (UMN) status. Its testing begins with stimulation of the cornea by a wisp of cotton or a breath of air that triggers the trigeminal nerve’s V₁ division, which forms the corneal reflex’s afferent limb. A brainstem synapse innervates both facial (seventh cranial) nerves, which form the efferent limb of the reflex arc. The facial nerves go on to innervate both sets of orbicularis oculi muscles.

The corneal reflex – trigeminal nerves to pons and pons to facial nerves – is analogous to the light reflex. Stimulating one cornea normally will provoke bilateral blinking. If neurologists apply the cotton tip to the right cornea and neither eye blinks, but then applying the cotton tip to the left cornea prompts both eyes to blink, the right trigeminal nerve (afferent limb) is impaired. In a different scenario, if cotton stimulation on the right cornea fails to provide a right eye blink, but it succeeds in provoking a left eye blink, the right facial nerve (efferent limb) is impaired.

In testing the trigeminal nerve’s motor component, neurologists assess jaw muscle strength by asking the patient to clench and then protrude the jaw. The jaw jerk reflex consists of a prompt but not overly forceful closing after a tap (Fig. 4-13). Alterations in the response follow the rules of a deep tendon reflex (DTR). A hyperactive response indicates an UMN (corticobulbar tract) lesion, and a hypoactive response indicates a lower motor neuron (LMN) or cranial nerve lesion. The neurologist should include testing of the jaw jerk reflex in patients with dysarthria, dysphagia, and emotional lability – mostly to assess them for the likelihood of pseudobulbar palsy (see later).

FIGURE 4-13 Tapping the normal, open, relaxed jaw will move it slightly downward. The jaw jerk reflex is the soft rebound. Abnormalities are mostly a matter of the rebound’s rapidity and strength. In a hypoactive reflex, as found in bulbar palsy and other lower motor neuron injuries, patients show little or no rebound. In a hyperactive reflex, as in pseudobulbar palsy and other upper motor neuron (corticobulbar tract) lesions, patients show a quick and forceful rebound.

Injury of a trigeminal nerve causes facial hypalgesia, afferent corneal reflex impairment, jaw jerk hypoactivity, and deviation of the jaw toward the side of the lesion. A variety of conditions – nasopharyngeal tumors, gunshot wounds, and tumors of the cerebellopontine angle, such as acoustic neuromas (see Fig. 20-27) – may cause trigeminal nerve injury.

In a frequently occurring situation, an aberrant vessel or other lesion in the cerebellopontine angle, MS plaques in the pons, or unknown disorder irritates the trigeminal nerve. The irritation causes a terribly painful condition, trigeminal neuralgia, in which patients suffer lancinating jabs in the distribution of a single division of one nerve (see Chapter 9). Similarly, when herpes zoster infects the trigeminal nerve it causes a rash followed by excruciating pain (postherpetic neuralgia) in the distribution of a single division of one trigeminal nerve (see Chapter 14).

Finally, a psychogenic sensory loss involving the face will usually encompass the entire face or be included in a sensory loss of one-half of the body, i.e., a hemisensory loss. In almost all cases, the following three nonanatomic features will be present: (1) the sensory loss will not involve the scalp (although the portion anterior to the vertex is supplied by the trigeminal nerve); (2) the corneal reflex will remain intact; and (3) when only one-half of the face is affected, sensation will be lost sharply rather than gradually at the midline (i.e., the patient will “split the midline”) (see Fig. 3-5).

Facial (Seventh)

The facial nerves’ major functions, like the trigeminal nerves’ major functions, are both sensory and motor: to convey taste sensation and innervate the facial muscles. Also like the trigeminal nerves, the facial nerves’ motor and sensory nuclei are situated in the pons and the nerves exit at the cerebellopontine angle.

Just as the trigeminal nerves supply the muscles of mastication, the facial nerves supply the “muscles of facial expression.” In a unique and potentially confusing arrangement in their neuroanatomy, cerebral impulses innervate both the contralateral and ipsilateral facial nerve motor nuclei. Each facial nerve supplies its ipsilateral temporalis, orbicularis oculi, and orbicularis oris muscles – muscles responsible for a frown, raised eyebrows, wink, smile, and grimace. In the classic explanation, because of their crossed and uncrossed supply, the upper facial muscles are essentially innervated by both cerebral hemispheres, whereas the lower facial muscles are innervated by only the contralateral cerebral hemisphere (Fig. 4-14). A newer explanation proposes that interneurons in the brainstem link the facial nerve nuclei.

FIGURE 4-14 In the classic portrayal, corticobulbar tracts originating in the ipsilateral, as well as in the contralateral, cerebral hemisphere supply each facial nerve nucleus. Each facial nerve supplies the ipsilateral muscles of facial expression. Because the upper half of the face receives cortical innervation from both hemispheres, cerebral injuries lead to paresis only of the lower half of the contralateral face. In contrast, facial nerve injuries lead to paresis of both the upper and lower halves of the ipsilateral side of the face.

Whatever the actual underlying neuroanatomy, facial nerve injuries cause ipsilateral paresis of both upper and lower face muscles. Neurologists term this pattern a “peripheral facial” or “lower motor neuron weakness.” Injuries of the cerebral cortex or upper brainstem, which interrupt the corticobulbar tract, cause paresis of only the lower contralateral face. Neurologists term that pattern a “central” or “upper motor neuron weakness.”

Taste sensation is more straightforward. The facial nerves convey impulses from taste receptors of the anterior two-thirds of the tongue to the brainstem. The glossopharyngeal nerves (the ninth cranial nerve) convey those impulses from the posterior third. A remarkable aspect of taste sensation is that, despite the extraordinary variety of foods, taste perceptions are limited. According to conventional wisdom, taste receptors detect only four fundamental sensations: bitter, sweet, sour, and salty. However, reconsideration of an idea proposed one hundred years ago by a Japanese researcher confirmed that people were able to perceive a fifth taste sensation, originally labeled umami (Japanese, delicious flavor), that people often describe as “richness” or “savory.” This taste is based on detecting L-glutamate, which is an amino acid abundant in high-protein foods and a major constituent of the flavoring monosodium glutamate (MSG). Further research showed that H⁺, K⁺, and Na⁺ trigger salty and sour tastes, and G protein receptors trigger the other ones.

Food actually derives most of its flavor from the aroma that the olfactory nerve detects. The olfactory nerve, not the facial nerve, conveys sensations to the frontal lobe and limbic system.

Routine facial nerve testing involves examining the strength of the facial muscles and, at certain times, assessing taste. The neurologist observes the patient’s face, first at rest and then during a succession of maneuvers that employ various facial muscles: looking upward to furrow the forehead, closing the eyes, and smiling. When weakness is present, the neurologist determines whether it involves both the upper and lower, or only the lower facial muscles. Upper and lower face paresis suggests a lesion of the facial nerve itself. (As previously mentioned, this pattern of paresis may be termed a peripheral or LMN weakness.) In this case, the lesion also probably impairs taste sensation on the same side of the tongue.

With unilateral or even bilateral facial nerve injuries, patients have no cognitive impairment. In contrast, paresis of only the lower facial muscles suggests a lesion of the contralateral cerebral hemisphere, which may also cause hemiparesis, aphasia, or hemisensory loss. For example, aphasia often accompanies weakness of the right lower face. As a general rule, cerebral lesions that cause lower face weakness spare taste sensation.

To test taste, neurologists apply either a dilute salt or sugar solution to the anterior portion of each side of the tongue, which must remain protruded to prevent the solution from spreading. A patient will normally be able to identify the fundamental taste sensations, but not those “tastes” that depend on aroma, such as onion and garlic.

Facial nerve damage typically produces paresis of the ipsilateral upper and lower face muscles with or without loss of taste sensation. Sudden onset, idiopathic facial paralysis, usually with loss of taste sensation, generically labeled Bell’s palsy, has traditionally been attributed to an inflammation or infection of the nerve (Fig. 4-15). In many of these cases, herpes simplex virus or, less often, Borrelia burgdorferi, a tick-borne spirochete that causes Lyme disease (see Chapters 5 and 7), has been the culprit. Destructive injuries, including lacerations, cerebellopontine angle tumors, and carcinomatous meningitis, damage not only the facial nerve, but usually also its neighboring cerebellopontine angle nerves.

FIGURE 4-15 The man on the left has weakness of his right lower face from thrombosis of the left middle cerebral artery: Neurologists might say that he has a “central” (central nervous system: CNS) facial paralysis. In contrast, the man on the right has right-sided weakness of both his upper and lower face from a right facial nerve injury (Bell’s palsy): Neurologists might say that he has a “peripheral” facial (cranial nerve) paralysis. In the center boxed sketches, the man with the central palsy (left) has flattening of the right nasolabial fold and sagging of the mouth downward to the right. This pattern of weakness indicates paresis of only the lower facial muscles. The man with the peripheral palsy (right), however, has right-sided loss of the normal forehead furrows in addition to flattening of his nasolabial fold. This pattern of weakness indicates paresis of the upper as well as the lower facial muscles. The neurologist has asked the men in the circled sketches at the top to look upward – a maneuver that would exaggerate upper facial weakness. The man with central weakness has normal upward movement of the eyebrows and furrowing of the forehead. The man with peripheral weakness has no eyebrow or forehead movement, and the forehead skin remains flat. The neurologist has asked the men in the circled sketches second from the top to close their eyes – a maneuver that also would exaggerate upper facial weakness. The man with the central weakness has widening of the palpebral fissure, but he is able to close his eyelids and cover the eyeball. The man with the peripheral weakness is unable to close the affected eyelid, although his genuine effort is made apparent by the retroversion of the eyeball (Bell’s phenomenon). The neurologist has asked the men in the lowest circled sketches to smile – a maneuver that would exaggerate lower facial weakness. Both men have strength only of the left side of the mouth, and thus it deviates to the left. If tested, the man with Bell’s palsy would have loss of taste on the anterior two-thirds of his tongue on the affected side. The neurologist in the bottom sketches has asked both men to elevate their arms. The man with the central facial weakness also has paresis of the adjacent arm, but the man with the peripheral weakness has no arm paresis. In summary, the man on the left with the left middle cerebral artery occlusion has paresis of his right lower face and arm. The man on the right with right Bell’s palsy has paresis of his right upper and lower face and loss of taste on the anterior two-thirds of his tongue.

Lesions that stimulate the nerve have the opposite effect. For example, aberrant vessels in the cerebellopontine angle can irritate the facial nerve and produce intermittent, completely involuntary, prolonged contractions of the muscles of the ipsilateral side of the face. This disorder, hemifacial spasm (see Chapter 18), which casual observers might misdiagnose as a “nervous tic,” represents the facial nerve counterpart of trigeminal neuralgia.

Facial nerve motor functions are essentially free of psychologic influence. People cannot mimic unilateral facial paresis. Some people, particularly children, who refuse to undergo an examination might forcefully close their eyelids and mouth. The willful nature of this maneuver becomes evident when the neurologist finds resistance on opening the eyelids and jaw, and observes, when the eyelids are pried open, that the eyeballs retrovert (Bell’s phenomenon).

Although impairment of taste, dysgeusia (Greek, geusis, taste), usually occurs along with facial muscle weakness, as in Bell’s palsy and other facial nerve injuries, it might occur with large brainstem lesions, such as MS plaques. However, lesions of this size would also produce problems that would overshadow impaired taste. On the other hand, dysgeusia might develop in isolation. It might be medication-induced. For example, tricyclic antidepressants, acetazolamide (for treatment of pseudotumor cerebri), and levodopa (for Parkinson disease) can diminish or distort taste. Radiotherapy directed at the head and neck, another iatrogenic dysgeusia, causes a combination of salivary secretion loss and tongue damage.

Acoustic (Eighth)

Each acoustic nerve is composed of two divisions with separate courses and functions: hearing and balance. The cochlear nerve, one of the two divisions, transmits auditory impulses from each middle and inner ear mechanism to the superior temporal gyri of both cerebral hemispheres (Fig. 4-16). This bilateral cortical representation of sound explains the everyday observation that damage to the ear or acoustic nerve may cause deafness in that ear, but the patient will still hear sound and speech because it passes through the other acoustic nerve or ear. The bilateral cortical representation of sound also explains why unilateral lesions of the brainstem or cerebral hemisphere – CNS damage – will not cause it. For example, cerebral lesions, such as tumors or strokes, that involve the temporal lobes may cause aphasia and hemiparesis, but they do not impair hearing.

FIGURE 4-16 The cochlear division of the acoustic nerve synapses extensively in the pons. Crossed and uncrossed fibers pass upward through the brainstem to terminate in the ipsilateral and contralateral auditory (Heschl gyrus) cortex of each temporal lobe; however, Heschl gyri, which sit in the planum temporale, receive auditory stimuli predominantly from the contralateral ear. In addition, the dominant-hemisphere Heschl gyrus almost abuts Wernicke language area (see Fig. 8-1) and predictably has a major role in language function.

The neurologist simply tests hearing by whispering into one of the patient’s ears while covering the other. Detailed testing requires audiologic devices.

Acoustic nerve injury may result from medications, such as aspirin or streptomycin, skull fractures severing the nerve, or cerebellopontine angle tumors, particularly acoustic neuromas associated with neurofibromatosis (see Chapter 13). Although cognitive impairment does not generally accompany deafness, in utero rubella infections or kernicterus (see Chapter 13) commonly cause syndromes of mental retardation and congenital deafness. In a related situation, children with congenital hearing impairment, deprived of proper intervention, may grow up to appear mentally retarded and have some features of autism. Cochlear implants, a unique, life-improving innovation, have allowed hearing-impaired infants and children to develop hearing and speaking abilities, such that most of them can enter mainstream education.

Hearing loss associated with older age, presbycusis (Greek, presbys, old man; acusis, hearing), which affects about 25% of people older than 65 years, typically begins with loss of high frequency and eventually progresses to involve all frequencies. Early in its course, presbycusis impairs the ability to distinguish between consonants, e.g. “b” and “v.” One of the first and generally the most troublesome problem for individuals with presbycusis is impaired speech discrimination, especially in rooms crowded with people talking simultaneously, such as restaurants and cocktail parties. Characteristically, older individuals’ inability to hear conversational speech is disproportionately greater than their hearing loss.

As with many age-related impairments, presbycusis results more from degeneration of the special sensory organ than the cranial nerve itself (Box 4-2). In this case, the cochlear mechanism, rather than the acoustic nerve itself, withers. Presbycusis potentially leads to inattention and social isolation. In addition, when hearing impairment accompanies visual impairments, the resulting sensory deprivation may precipitate hallucinations. Such impairments may also overwhelm someone with mild cognitive impairment (see Chapter 7) and lead to misdiagnoses of dementia and psychosis. For the limited problem of age-related hearing impairment in the elderly, physicians should as a general rule dispense hearing aids readily and even on a trial basis. In some cases, cochlear implants have allowed adults with hearing loss uncorrected by hearing aids to regain useful hearing; however, unlike infants and young children, adults usually cannot learn to translate the electric impulses into comprehensible speech.

Box 4-2

Age-Related Special Sense Impairments

Smell	Some degree of anosmia in 75% of individuals older than 80 years
Vision	Presbyopia: mostly inability to accommodate to see closely held or small objects; cataracts (see Chapter 12)
Taste	Loss of taste sensitivity and discrimination, as well as anosmia for aroma
Hearing	Presbyacusis: loss of speech discrimination, especially for consonants; poor high-pitched sound detection, tinnitus