Several of the cranial nerves are multifunctional; their motor, sensory, and autonomic functions must be assessed separately. The optic nerve is actually a central nervous system tract, and the accessory nerve is anatomically an aberrant spinal nerve (the motor neurons reside in the upper spinal cord).
Olfactory Nerve (Cranial N. I)
When patients complain that foods no longer taste right, the first step is to look into the nose for possible obstruction of airflow. Each nostril is then tested separately, using non-noxious odorants such as coffee or soap. (Pungent substances such as ammonia will stimulate trigeminal nociceptors.) Failure to smell anything is termed anosmia. Unpleasant distortion of an innocuous odorant is termed parosmia.
There are several cerebral representations for olfaction in the brain. As a result anosmia is most often secondary to local nasal disease, or to lesions affecting olfactory fibers as they pass through the cribriform plate (see Figure 32–1).
Optic Nerve (Cranial N. II)
Visual acuity is tested with Snellen's chart (at 20 feet) or a hand-held card (at 14 inches). The eyes are tested separately; if acuity is severely reduced, finger counting, detection of hand movement, or light perception should be assessed. Refractive errors are identified by having patients wear their glasses or look through a pinhole. Inspection of the eyes and funduscopic examination will often identify ocular lesions impairing acuity, such as corneal scarring, cataracts, glaucoma, diabetic retinopathy, or macular degeneration.
The eyes are tested separately. The examiner faces the patient and holds an object equidistant between the patient's and the examiner's eyes to compare his or her own experience with that of the patient. A test object is moved slowly inward from the periphery, and the patient is asked to indicate when it is first seen; or the patient can be told to count fingers in different visual quadrants. Stimuli are presented simultaneously to the right and left fields to identify spatial neglect (extinction).
Visual field testing provides very accurate localization of structural lesions (see Figure 25–5). Monocular visual impairment, including either field defect or scotoma (an area of visual loss surrounded by preserved vision), localizes a lesion to the optic nerve, the retina, or other ocular structures. Bitemporal hemianopia, if caused by a single lesion, places that lesion at the optic chiasm. Homonymous hemianopia, quadrantanopia, or bilateral congruent scotomas indicate a lesion behind the chiasm in the contralateral optic tract, lateral geniculate nucleus of the thalamus, optic radiation, or primary visual cortex.
Using an ophthalmoscope, the examiner focuses successively on the cornea, anterior chamber, lens, and vitreous body, and then surveys the optic disk, retinal vessels, and the retina itself. Optic atrophy refers to disk pallor; its many causes include glaucoma, optic nerve compression, infarction, and multiple sclerosis.
In papilledema (optic disk swelling) the disk margins become blurred and elevated. Papilledema can be the result of local pathology, for example the inflammatory demyelination of optic neuritis, in which case visual acuity is acutely impaired by swelling of the optic nerve head. When papilledema is the result of increased intracranial pressure, the normal blind spot becomes enlarged but visual acuity is not initially affected; over time, however, the visual fields become constricted and visual acuity is impaired. In addition, the ratio of the diameter of retinal veins to arteries (normally approximately 3:2) increases, and there may be retinal hemorrhages and whitish exudates.
Other abnormalities identified by funduscopy include arterial narrowing (hypertension), exudates (diabetes mellitus, blood dyscrasias), microaneurysms (diabetes mellitus), subhyaloid hemorrhages (located between the retina and the vitreous membrane and associated with subarachnoid hemorrhage), tubercles and other granulomas, phakomas (glial collections, associated with the hereditary diseases neurofibromatosis and tuberous sclerosis), pigmentary changes (retinitis pigmentosa), and emboli (seen within arteriolar branches of the central retinal artery).
Oculomotor, Trochlear, and Abducens Nerves (Cranial N. III, IV, VI)
The pupillary light reflex is tested by directing a bright light into each eye and observing the bilateral response. Both pupils should constrict to bright light in either eye. The accommodation reflex and pupillary near response are tested by having the patient converge onto an object held close to the eyes; the pupils should constrict.
Anisocoria (unequal pupils) signifies either a parasympathetic lesion affecting the larger pupil or a sympathetic lesion affecting the smaller pupil. A parasympathetic lesion is indicated by marked pupillary dilatation, loss of the light reflex, or both, as well as involvement of extraocular muscles innervated by the oculomotor nerve. A sympathetic lesion is indicated by pupillary constriction, preservation of the light reflex, and signs of Horner syndrome (ptosis and, in some cases, loss of sweating over the ipsilateral face). A pupil with both parasympathetic and sympathetic denervation will be mid-position and unreactive to light.
A unilateral lesion involving the optic nerve or retina is indicated when neither pupil constricts in response to light directed into the affected eye, but both pupils constrict when light is directed into the unaffected eye (afferent pupillary defect). The pupils react equally because the pupillary light reflex is consensual (Figure B–2A; and see Figure 45–7).
Multiple sclerosis produces protean symptoms that wax and wane.
A. A common early symptom of multiple sclerosis (MS) is transient blurred vision in one eye as a result of optic neuritis, inflammation of the optic nerve that occurs during the acute phase of an MS lesion. 1. Shining a light in the normal eye produces both direct and consensual pupillary constriction, but when the light is swung to the affected eye, both pupils dilate because the patient perceives a relative dimming of light intensity. This is referred to as a de-afferented pupil. At no time with a de-afferented pupil is there anisocoria. A positive "swinging flashlight test" indicates a de-afferented pupil. 2. To determine the site of the lesion the examiner must use an ophthalmoscope to evaluate the possibility of a corneal or lenticular opacity, vitreous hemorrhage, retinal detachment, or ischemic retinopathy. 3. If the funduscopic examination is normal, or reveals a slight elevation or blurring of the optic nerve head, the lesion is localized to the visual pathway behind the eye. Lack of a homonymous visual field defect involving the other eye localizes the lesion proximal to the optic chiasm, therefore placing the lesion in the retrobulbar segment of the optic nerve consistent with the diagnosis of optic neuritis.
B. The condition known as internuclear ophthalmoplegia (INO) is frequently the result of MS. The patient complains of double vision. 1. The patient attempts to visually follow a finger moving from side to side. The abducting eye follows but the adducting eye fails to track past the midline. The abducting eye may demonstrate nystagmus. The possibility of a lesion involving the oculomotor nucleus (N. III) is tested by examining convergence. The patient looks at the target finger placed directly in front and follows as the finger is brought toward the patient's nose. Normal adduction during convergence rules out a lesion of the nerve or nucleus. Additionally, the pupils will be observed to constrict as a result of the near response. 2. INO indicates a lesion of the medial longitudinal fasciculus (MLF), the white matter tract that links the third nerve (oculomotor) nucleus in the midbrain with the sixth nerve (abducens) nucleus in the pons to coordinate lateral gaze. Because these tracts are crossed and near the midline, most cases of INO are bilateral.
C. As the result of an MS plaque involving the posterior columns of the cervical cord, patients may experience an electric shock-like sensation traveling down the spine and possibly into the limbs when the examiner flexes the neck (L'hermitte sign). Because MS is a disease of central white matter, lesions may cause upper motor neuron signs, such as brisk reflexes or a Babinski sign.
The afferent end of the accommodation reflex pathway is in visual areas of the occipital lobe, which communicate with the parasympathetic component (the Edinger-Westphal nucleus) of the oculomotor nucleus by a route separate from the light reflex pathway. Lesions that selectively interrupt the light reflex pathway (for example, Argyll-Robertson pupils, seen most often with neurosyphilis) destroy the light reflex but do not impair the accommodation reflex.
The examiner looks for (1) paresis producing conjugate limitation of gaze in a particular direction (both eyes are equally affected so there is no diplopia); (2) paresis affecting one or more extraocular muscles (producing disconjugate eye movements with diplopia); and (3) spontaneous involuntary eye movements (eg, nystagmus).
The patient is asked to look to the right, to the left, and up and down in each horizontal direction. When the eyes follow a moving target they move more slowly (pursuit velocity) than when shifting from one static object to another (saccadic velocity). The two types of movement are controlled by separate anatomical and physiological mechanisms that can be selectively affected by lesions of the cerebrum, brain stem, or cerebellum (see Figures 39–2, 39–9, and 39–14).
Horizontal gaze palsy usually indicates a lesion of the frontal eye field (FEF) or the pontine paramedian reticular formation (PPRF) (see Figures 39–6, 39–9, and 39–10). If the FEF is affected the eyes will be deviated toward the side of the lesion. If the PPRF is affected the eyes will be deviated away from the side of the lesion.
Vertical gaze paresis—limitation of conjugate upward and downward gaze—indicates damage to the midbrain, either the rostral interstitial nucleus of the median longitudinal fasciculus or the posterior commissure.
Monocular limitation of adduction with preserved convergence (internuclear ophthalmoplegia) indicates damage to the ipsilateral median longitudinal fasciculus. There is often horizontal nystagmus in the contra-lateral abducting eye (see Figure B–2).
Disconjugate eye movements indicate impairment of particular cranial nerves or extraocular muscles (see Figures 39–4, 39–5, 39–6, and 39–7). If the eye cannot move outward there is a lesion involving either the abducens nerve (N. VI) or the lateral rectus muscle. If the eye cannot move downward when deviated inward there is a lesion of the trochlear nerve (N. IV) or the superior oblique muscle. Any other monocular limitations in movement are caused by a lesion of the oculomotor nerve (N. III) or one of its muscles of innervation (an exception is internuclear ophthalmoplegia).
The oculomotor nerve controls the levator palpebrae muscle, and ptosis is a common sign of lesions of this nerve. Unlike the mild Horner syndrome, oculomotor denervation can result in total eye closure.
The oblique muscles mediate intorsion and extorsion when the eye is either in mid-position or abducted. A compensatory head tilt (away from the side of the lesion) is therefore a feature of trochlear nerve palsy.
Involuntary repetitive eye movements, or nystagmus, can be either unilateral or bilateral. Pendular nystagmus, with roughly equal velocity in either direction, is most often the result of severe visual impairment during early childhood. Rhythmical nystagmus, a slow drift in one direction followed by a rapid corrective movement in the other, can be present with the eyes at rest and the gaze fixed; it tends to be accentuated by ocular deviation, with the fast component in the direction of gaze.
Horizontal or rotatory nystagmus on primary gaze is most often associated with vestibular lesions, peripheral or central. Vertical nystagmus (the fast component directed upward or downward) suggests a brain stem lesion. Horizontally directed nystagmus on lateral gaze is often a benign side effect of certain drugs, particularly sedatives and anticonvulsants.
Trigeminal Nerve (Cranial N. V)
The three divisions of the trigeminal nerve carry sensation from the face, anterior scalp, eye, and much of the nasal and oral cavities. Fibers in the mandibular division innervate the muscles of mastication (see Figures 45–2 and 45–3).
The examiner initially checks sensations at the forehead, the malar region, and the chin, defining the outer borders of any deficit found. Decreased sensation confined to the entire trigeminal area indicates a peripheral lesion involving the nerve root or the trigeminal ganglion. Decreased sensation confined to one division suggests a more distal lesion. If pain and temperature are decreased, but touch is preserved, the lesion involves the spinal trigeminal tract and nucleus in the lower brain stem or upper cervical cord. If touch is also impaired, the lesion involves the principal trigeminal nucleus in the pons. If impaired sensation extends beyond the borders of the trigeminal area, the lesion is suprasegmental in the upper brain stem, thalamus, or parietal lobe.
Because the corneal reflex is consensual (the efferent end of the reflex pathway is in the facial nerve), a purely trigeminal lesion results in a decreased response in both eyes when the affected side is stimulated and a normal response in both eyes when the unaffected side is stimulated. Unilateral weakness of eye closure occurs in the affected eye when either cornea is stimulated.
Unilateral lesions of the trigeminal nerve or its third division cause the opened jaw to deviate toward the ipsilateral side because of ipsilateral pterygoid muscle weakness. Unilateral suprasegmental lesions are unlikely to cause jaw deviation because motor neurons of the trigeminal motor nucleus receive bilateral innervation from the primary motor cortex.
The jaw jerk is produced by tapping downward on the chin when the jaw is slightly open. Like other tendon reflexes it is decreased or absent with lesions of the nuclear or peripheral nerve (lower motor neurons) and brisk with lesions of the corticobulbar tract or motor cortex (upper motor neurons).
Facial Nerve (Cranial N. VII)
The facial nerve supplies facial muscles, including the frontalis ("wrinkle your forehead"), orbicularis oculi ("close your eyes tightly"), orbicularis oris ("close your lips tightly"), levator anguli oris ("show your teeth"), and platysma ("show your teeth and grimace"). With mild lesions there may not be frank weakness but rather a wider opening between the upper and lower eyelids and a flattening of the nasolabial fold ipsilaterally.
More proximal lesions can involve (1) a branch to the stapedius muscle of the middle ear (resulting in an increased sensitivity to loud sounds, or hyperacusis), (2) the chorda tympani branch to the sublingual and submandibular salivary glands and the tongue (resulting in decreased taste over the tongue's anterior two-thirds, usually tested with sugar or salt), or (3) the greater petrosal branch to the lacrimal gland and nasal mucosa (resulting in decreased lacrimation).
In most people motor neurons innervating the upper facial muscles, particularly the frontal muscle, receive projections from the motor cortex of both hemispheres, whereas those innervating the lower facial muscles receive only contralateral projections. Thus suprasegmental lesions tend to spare the frontal muscle and sometimes eye closure (upper-motor-neuron facial weakness), whereas lower brain stem or nerve damage tends to involve all the facial muscles (lower-motor-neuron facial weakness).
Following peripheral nerve injury such as Bell's palsy or trauma, aberrant reinnervation may produce synkinesis of the eye and mouth. Eye closure results in involuntary elevation of the angle of the mouth on the affected side, whereas baring the teeth results in involuntary closure of the ipsilateral eye.
Vestibulocochlear Nerve (Cranial N. VIII)
The auditory and vestibular nerves run together as the eighth cranial nerve. Both carry information from the labyrinths of the inner ear. The auditory (cochlear) nerve conveys sound information from the cochlea, whereas the vestibular nerve conveys equilibrium information from the utricle, saccule, and semicircular canals (see Figure 40–1).
Neurons in the cochlear nucleus project both ipsilaterally and contralaterally. Thus unilateral deafness signifies a lesion of the cochlear nucleus, the auditory nerve, or the ear. Deafness from peripheral lesions is of two types. Conduction deafness is the result of obstruction or disease of the external auditory canal, the tympanic membrane, or the middle ear. Sensorineural deafness is the result of damage to the cochlea, the cochlear nerve, or the cochlear nuclei. Conduction deafness preferentially affects low tones; sensorineural deafness preferentially affects high tones. Patients with cochlear nerve or cochlear nucleus lesions may have only mildly impaired hearing for pure tones yet severe difficulty in discriminating speech because of its tonal complexity. Both peripheral and central lesions cause tinnitus.
The auditory examination begins with a simple screening test in which the ability of each ear to detect a watch ticking or two fingers rubbed together is compared. The Weber and Rinne tests help to distinguish conduction from sensorineural deafness. In the Weber test a 512 Hz tuning fork is placed midline on the forehead. With conduction deafness the sound will be heard best on the hearing-impaired side, whereas with sensorineural deafness the sound will be heard best on the normal side. In the Rinne test the tuning fork is placed over the mastoid; when the patient reports that the sound is no longer heard, it is held close to the external auditory meatus. In normal subjects and in those with nerve deafness air conduction will outlast bone conduction. With conduction deafness bone conduction will outlast air conduction. The Weber test is most useful when deafness is unilateral and the Rinne test when deafness is bilateral.
The vestibular nerve carries impulses from hair cells in the semicircular canals to the vestibular nuclei in the brain stem. Widespread projections from the vestibular nuclei communicate with the spinal cord, the cerebellum, eye movement control centers, and the forebrain.
Tests of vestibular function involve labyrinthine stimulation, either by head movement, head positioning, or temperature. In some patients vertigo, nystagmus, and sometimes nausea and vomiting are precipitated by any rapid movement of the head. In others, particularly those with benign positional vertigo, vertigo is triggered (after a latency of up to half a minute) by lying on the affected ear.
In the caloric test the ear is irrigated alternately with water several degrees centigrade above and below body temperature. With the head elevated 30 degrees the normal response consists of nystagmus (and vertigo), with the fast component directed away from the cold stimulus but toward the warm stimulus. There are two main patterns of abnormal response. Nystagmus may be absent or briefer in response to a cold or warm stimulus in the same ear; this canal paresis is associated with peripheral vestibular lesions. Conversely, a reduced response may occur when cold water is instilled into one ear or warm water is instilled into the other ear; this directional preponderance is associated with lesions of central vestibular pathways.
When pointing to objects, vertiginous patients whose eyes are closed tend to deviate toward the side of the lesion (past-pointing). Similarly, when walking in place with closed eyes, they tend to rotate toward the affected side.
Glossopharyngeal and Vagus Nerves (Cranial N. IX, X)
Assessment of the ninth and tenth cranial nerves is usually limited to examination of the palate and pharynx. Nasal speech suggests palatal weakness. Hoarseness or a reduced cough suggests laryngeal weakness.
Choking on saliva while talking suggests pharyngeal weakness. Difficulty swallowing (dysphagia) solid food suggests mechanical obstruction such as esophageal carcinoma; dysphagia for liquids as well as solids or for only liquids suggests neurological dysfunction. Dysphagia can be checked by asking the patient to swallow a small amount of water.
When the patient says, "ah" with the mouth open and the tongue relaxed, the palate should rise symmetrically, the uvula should remain in the midline, and the pharyngeal walls should contract symmetrically. With unilateral palatal or pharyngeal weakness, phonation causes the uvula to deviate toward the normal side. The gag reflex is tested by gently touching each side of the pharynx with a cotton-tipped applicator. As with the pupillary and corneal reflexes, the response is bilateral. The response of the palatal and pharyngeal muscles to phonation and tactile stimulation therefore reveals whether a lesion is unilateral or bilateral, and whether the damaged nerves are efferent, afferent, or both.
With unilateral laryngeal lesions the abnormal vocal cord may be positioned in adduction, in which case there may be no symptoms; if it is positioned in abduction there will be hoarseness or aphonia but normal breathing. With bilateral lesions the vocal cords may be positioned in abduction, in which case there will be hoarseness or aphonia but normal breathing; if they are positioned in adduction there will be inspiratory phonation (stridor) and life-threatening respiratory obstruction.
The sternocleidomastoid is the only major striated muscle with ipsilateral cortical representation. Following destructive cerebral lesions weakness in the sternocleidomastoid is unusual but may occur ipsilaterally; contralateral trapezius weakness may also occur. The sternocleidomastoid is tested by having patients press their chin against resistance in the direction of the contralateral shoulder. Bilateral damage causes weakness of forward head flexion. Trapezius weakness is demonstrated by assessing shoulder elevation or shrugging or by observing winging of the upper scapula.
Hypoglossal Nerve (Cranial N. XII)
The hypoglossal nerve innervates the muscles of the tongue. It arises near the midline of the medulla oblongata and exits the posterior fossa through the hypoglossal foramen. Each hypoglossal nucleus receives bilateral projections from the motor cortex.
Inspection of the tongue may reveal atrophy or fasciculations, either of which indicates damage to lower motor neurons, either peripherally or in the medulla. Tongue atrophy, if unilateral, causes reduction in size on one side with excessive ridging and wrinkling of the affected side. Fasciculations in the tongue can be difficult to tell from normal tongue movements or tremor. These small local contractions are nonrhythmic and make the tongue resemble a bag of worms. They should be present when the tongue is completely at rest.
With unilateral weakness because of lesions of upper or lower motor neurons the tongue deviates toward the weak side. If there is no deviation, weakness is assessed by having the patient push the tongue into each cheek while the examiner pushes against the cheek. With unilateral lesions of upper motor neurons there may be no deviation and little evident weakness, yet patients may experience dysarthria (imperfect articulation), particularly with the lingual consonants (tay for anterior tongue, kay for the posterior tongue). Bilateral tongue weakness causes dysarthria, dysphagia, and sometimes even difficulty breathing.
Bilateral lesions of the lower brain stem can result in bulbar palsy. The tongue is paralyzed, atrophic, and fasciculating. There is no movement of the palate or pharynx with phonation, and the gag reflex is absent. With unilateral supranuclear lesions these muscles are often spared or only mildly impaired because of the bilateral cortical projections to lower brain stem motor neurons.
Bilateral lesions of the cerebrum or upper brain stem, however, can result in severe dysarthria and dysphagia because of effects on the corticobulbar pathway. The tongue is paralyzed but neither atrophic nor fasciculating. The palate and pharynx do not move with phonation, but the gag reflex is hyperactive. Such a patient is said to have pseudobulbar palsy. An interesting and unexplained feature of this syndrome is lability or hyperreflexia of emotional response. A remark that would normally produce a mild chuckle precipitates embarrassed peals of laughter, and asking a question such as "How are you feeling?" may result in explosive weeping.