DR. NISHTHA JAINSENIOR RESIDENT

DEPARTMENT OF NEUROLOGYGMC, KOTA.

Vestibulocochlear Nerve

The eighth cranial nerve consists of two separate functional components: the auditory (cochlear) nerve concerned with hearing and the vestibular nerve concerned with equilibrium.

The auditory nerve receives information from the tonotopically organized cochlea, the organ of hearing.

The vestibular nerve derives its input from the saccular and utricular macules (which sense linear acceleration) and the cristae of the semicircular canals (which sense angular acceleration of the head).

COCHLEAR NERVE

Anatomy

The labyrinth is a complex of interconnecting cavities, tunnels, ducts, and canals that lies in the petrous portion of the temporal bone.

The bony labyrinth is filled with perilymph, a thin watery fluid similar to cerebrospinal fluid (CSF).

The membranous labyrinth is an arrangement of sacs and ducts that lies within the bony labyrinth, generally follows its outline, and is filled with endolymph (Scarpa's fluid).

The membranous labyrinth has two major components: the vestibular apparatus and the cochlear duct.

The ossicles span the middle ear cavity and transmit the oscillations of the tympanic membrane to the footplate of the stapes, which sits in the oval window (fenestra vestibuli).

The oval window opens into the vestibule of the inner ear, which connects on one side to the cochlea and on the other to the semicircular canals.

The base of the cochlea faces the internal acoustic meatus and contains myriad fenestrations that admit the filaments of the cochlear nerve.

The scala media, or cochlear duct, is part of the membranous labyrinth.

It lies in the center of the spirals of the cochlea, completing the partition between the scala tympani and scala vestibuli.

The spiral ganglion of the cochlear nerve lies in the spiral canal of the modiolus (Rosenthal's canal).

The organ of Corti rests on the basilar membrane and contains inner and outer hair cells.

The inner hair cells are the receptors, or end-organs, of the cochlear nerve.

Sound waves induce vibrations in the cochlea, which cause movement of the basilar and tectorial membranes.

This movement flexes the stereocilia, which activates the hair cell, causing impulses in the spiral ganglion.

Because of the varying width of the basilar membrane, sound of a certain frequency induces oscillations maximal at a certain point along the cochlear duct, which focally activates hair cells and encodes the frequency.

The spiral ganglion consists of type I and type II bipolar neurons that lie in the modiolus.

Inner hair cells synapse on type I neurons, which make up 95% of the ganglion.

Axons of the spiral ganglion cells form the cochlear nerve, which contains some 30,000 fibers.

Axons from type I cells are myelinated and form the bulk of the nerve.

The type II cells connect with the outer hair cells and modulate the activity of the inner hair cells.

The acoustic nerve traverses the internal auditory canal, where it lies lateral and inferior to the facial nerve.

It crosses the cerebellopontine angle, passes around the inferior cerebellar peduncle, and enters the upper medulla at its junction with the pons near the lateral recess of the fourth ventricle.

Each entering fiber bifurcates to synapse in both the dorsal (posterior) and ventral (anterior) cochlear nuclei.

In the cochlear nuclei, low frequency tones are processed ventrally, high frequencies dorsally.

Second order neurons in the cochlear nuclei give rise to the dorsal, ventral, and intermediate acoustic stria.

Fibers in the dorsal and intermediate acoustic stria run to the contralateral inferior colliculus, most directly, some after a relay in the nucleus of the lateral lemniscus.

This crossed, monaural auditory pathway primarily carries information about sound frequency.

Fibers of the ventral acoustic stria are both crossed and uncrossed and may synapse in the nuclei of the trapezoid body, superior olive, or lateral lemniscus.

The binaural pathway, especially the superior olivary complex component, can determine the time difference between the two ears and aid in the localization of sound.

Fibers from the lateral lemnisci ascend to synapse in the central nucleus of the inferior colliculus.

Axons from the inferior colliculus pass through the brachium of the inferior colliculus to the medial geniculate body (MGB), a special sensory nucleus of the thalamus that is the final relay station in the auditory pathway.

From the MGB, auditory fibers pass through the posterior limb of the internal capsule as the geniculotemporal tract, or auditory radiations, which runs through the sublenticular portion of the internal capsule.

The fibers terminate in the cortex of the transverse temporal convolutions (Heschl's gyrus) and the adjacent planum temporale portion of the superior temporal gyrus which make up the primary and secondary auditory cortex (Brodman's areas 41 and 42).

The primary auditory cortex is tonotopically organized with high frequencies medial and low frequencies lateral.

The auditory association cortex (Wernicke's area in the dominant hemisphere) lies just posterior to the primary auditory cortex.

Blood supply

The blood supply to the cochlea and auditory brainstem nuclei arises from the internal auditory (labyrinthine) artery, usually a branch of the anterior inferior cerebellar artery.

The superior olivary complex and lateral lemniscus are supplied by circumferential branches of the basilar artery

the inferior colliculus is vascularized by branches of the superior cerebellar and quadrigeminal arteries

whereas the medial geniculate bodies receive their blood supply from the thalamogeniculate arteries.

Branches of the middle cerebral artery supply the primary auditory and associated cortices.

Clinical Examination

Before testing hearing, otoscopic examination should be done to ensure the tympanic membrane is intact, and to exclude the presence of wax, pus, blood, foreign bodies, and exudate.

The mastoid region should be examined for swelling and tenderness.

Audiometry

An audiometer is an instrument by which sounds of varying intensity and frequency are presented to a patient.

There are many different audiologic techniques; those used most commonly for neurologic purposes are pure tone and speech audiometry.

The pure tone audiogram displays the severity of any hearing loss in relation to established reference values, and the pattern may suggest the etiology.

As with tuning fork testing, a decrease in AC with normal BC, an air-bone gap, indicates conductive hearing loss, and a decrease in both AC and BC indicates sensorineural loss.

The pure tone audiogram is usually normal with lesions involving the central auditory pathways.

Speech audiometry uses spoken words and sentences instead of pure tones.

The speech reception threshold (SRT) is considered the intensity level at which the patient can correctly understand 50% of the material presented.

Speech discrimination, or intelligibility, is the proportion of the material the patient can understand when presented at a level that should be easily heard.

The loss of discrimination is proportional to the severity of the hearing loss in patients with cochlear lesions.

Poor speech discrimination, out of proportion to pure tone hearing loss, is characteristic of a retrocochlear lesion, such as cerebellopontine angle tumor.

In CN VIII lesions, discrimination may even paradoxically decline as intensity is raised.

Impedance audiometry uses an electroacoustic device, which measures the impedance, or compliance, of the conductive hearing mechanism, like measuring the tightness of a drumhead.

A very stiff drumhead has high impedance, or low compliance, and reflects sound back to the source.

Low impedance allows for greater transmission of sound through the system and less reflection.

A tympanogram measures the impedance of the tympanic membrane.

An abnormal tympanogram is seen in such conditions as otitis media, tympanic membrane perforation, ossicular dislocation, otosclerosis, cerumen impaction, and eustachian tube dysfunction.

The stapedius reflex, or acoustic reflex, measures the change in compliance in response to loud sounds to assess the function of the stapedial muscle.

The reflex arc is via CN VIII, brainstem interneurons, and CN VII.

In the absence of severe hearing loss, an abnormal stapedius reflex may suggest a lesion of CN VII or VIII or the brainstem.

The auditory evoked potential (AEP), also known as the auditory evoked response, or brainstem auditory evoked potential/response (BAEP/BAER), is a minuscule potential produced by auditory stimuli and recorded using electroencephalogram (EEG) electrodes.

The waves that occur in the first 10 milliseconds after an auditory stimulus are short latency far field potentials due to electrical activity at various points along the auditory pathway.

BAERs are used primarily for evaluating suspected CN VIII and brainstem lesions.

There are five to seven waves in the AEP. Wave I is the auditory nerve action potential. Wave II reflect activity in the cochlear nuclei, although it

may be generated by the intracranial segment of the auditory nerve.

Wave III is thought to come from the superior olive. Waves IV and V the inferior colliculus. The wave VI may come from the medial geniculate body. Wave VII from the auditory radiations.

A delay between waves I and III suggests a lesion between the eighth nerve near the cochlea and the lower pons.

An interpeak latency delay between waves III and V suggests a lesion between the lower pons and midbrain.

The primary clinical applications of BAERs have been in the evaluation of patients with cerebellopontine angle tumors and demyelinating disease, and in coma and brain death.

also useful in newborn and infant hearing assessment.

Auditory reflex responses are occasionally useful in evaluating hearing in children, patients with altered mental status, and in hysteria or malingering.

The auditory-palpebral reflex (auro- or acousticopalpebral, cochleopalpebral, or cochleoorbicularis reflex) is a blink or reflex eye closure in response to a loud, sudden noise.

The cochleopupillary reflex is pupillary dilation, or contraction followed by dilation, in response to a loud noise.

The auditory-oculogyric reflex is eye deviation toward a sound.

The general acoustic muscle reflex is a general jerking of the body in response to a loud, sudden noise.

In bedside qualitative assessment of hearing loss, a tuning fork (256 or preferably 512 Hz) is used to distinguish between two types of hearing loss.

Three major tuning fork tests are used for the evaluation of hearing loss:

Weber's testRinne's testSchwabach's test.

The Weber's Test

The purpose of the Weber's test is to help differentiate a conductive from a sensorineural hearing loss in a unilateral hearing loss.

This test is conducted by placing a vibrating tuning fork over the midline of the skull or forehead, over the nasal bone, or over the anterior upper incisors.

Normally, the vibrations are perceived equally in both ears (no lateralization) because bone conduction is equal bilaterally.

In conductive hearing loss, the vibrations are louder in the deaf ear (lateralized to the diseased ear).

In sensorineural hearing loss, the sound is louder in the normal ear (lateralized to the normal ear).

The Rinne's Test

The Rinne's test compares the patient's air and bone conduction.

The stem of the vibrating tuning fork is applied against the mastoid process.

When the patient no longer hears the vibration, the fork is placed next to the ear (approximately 1 cm from the external auditory meatus).

In normal individuals, because air conduction is better than bone conduction, the vibrations are perceived in the ear after they are no longer perceived at the mastoid.

The Rinne's test is said to be normal, or positive, when the tuning fork is heard approximately twice as long by air conduction as by bone conduction.

In cases of conduction deafness, bone conduction is better than air conduction, and therefore the tuning fork cannot be heard when it is placed next to the ear.

With sensorineural hearing loss, both air and bone conduction are diminished to a similar extent, and air conduction remains greater than bone conduction.

The Schwabach's Test

As in the Rinne's test, the tuning fork is held against the mastoid process until the patient is unable to perceive any sound.

The examiner then places the tuning fork over his or her own mastoid bone and compares the bone conduction to that of the patient.

If the examiner hears the tuning fork after the patient no longer hears it, a sensorineural hearing loss is suspected.

Localization of Lesions Causing Sensorineural Deafness

Unilateral dominant posterior temporal lesions or bilateral temporal lesions affecting Heschl's gyri may cause pure word deafness.

Left hemispheric lesions predominantly impair speech discrimination, whereas right hemispheric lesions predominantly impair complex-pitch discrimination.

Irritative lesions of the temporal cortex may result in subjective auditory hallucinations. Auditory hallucinations may be simple (e.g., tinnitus) or complex (e.g., voices, music).

These auditory sensations are most often referred to the contralateral ear

occur more frequently with irritative lesions of Brodmann's areas 42 and 22 than with lesions of Brodmann's area 41.

Bilateral hearing loss may occur with severe bilateral brainstem lesions (e.g., hemorrhage or infarction).

Pineal and midbrain tumors may also cause sudden and complete bilateral deafness (central stem deafness of Brunner) presumably because the auditory pathways in this region are closely packed together.

Peripheral cochlear nerve lesions account for partial or complete deafness, often associated with ipsilateral tinnitus.

Deafness is most prominent for high-frequency tones and may be secondary to

trauma (e.g., basal skull fracture) infections (e.g., syphilis, bacterial infections) drugs (e.g., streptomycin, neomycin) aneurysms of the anterior inferior cerebellar artery tumors of the cerebellopontine angle (e.g., vestibular

schwannomas, epidermoids, meningiomas, arachnoid cysts).

VESTIBULAR NERVE

Anatomy

The peripheral vestibular system is composed of three semicircular canals, the utricle and saccule, and the vestibular component of the eighth cranial nerve.

Each semicircular canal has a sensory epithelium called the crista; the sensory epithelium of the utricle and saccule is called the macule.

The semicircular canals sense angular movements, and the utricle and saccule sense linear movements.

Two of the semicircular canals (anterior and posterior) are oriented in the vertical plane nearly orthogonal to each other; the third canal is oriented in the horizontal plane (horizontal canal).

The crista of each canal is primarily activated by movement occurring in the plane of that canal.

When the hair cells of these organs are stimulated, the signal is transferred to the vestibular nuclei via the vestibular portion of cranial nerve VIII.

Because of its orientation, the macula of the utricle responds maximally to head movement in the sagittal plane, whereas the macula of the saccule responds maximally to head movement in the coronal plane.

The horizontal canal best detects rotational head movement in the side-to-side (“no-no”) direction (with the chin tucked to bring the canal fully horizontal).

The posterior canal best detects movement in the anteroposterior plane (“yes-yes”), and the anterior canal is oriented to detect lateral tilting movement.

The nerve divide into ascending and descending branches that end primarily in the four vestibular nuclei: lateral, medial, superior, and inferior.

Some fibers form the vestibulocerebellar tract and pass directly to the cerebellum, without synapsing in the vestibular nuclei, in the juxtarestiform body.

The medial (Schwalbe's) vestibular nucleus is the largest subdivision of the vestibular nuclear complex, extending from the medulla into the pons.

Vestibular afferents to the superior and medial subnuclei arise predominantly from the semicircular canals.

Afferents to the lateral and inferior subnuclei arise predominantly from the otolith organs.

The vestibular nuclei make connections with four primary areas: cerebellum, spinal cord, oculomotor system, and cortex.

Fibers from the lateral vestibular nucleus go down the ipsilateral spinal cord as the lateral vestibulospinal tract, which is important in the regulation of muscle tone and posture by increasing extensor muscle tone.

Impulses from the medial vestibular nuclei descend to the cervical and upper thoracic spinal cord through the crossed medial vestibulospinal tract.

Ascending vestibular connections extend rostrally to the ventrolateral and ventral posterior thalamic nuclei, and from the thalamus to the somatosensory cortex to provide conscious perception of head position and movement.

The blood supply to the membranous labyrinth is from the internal auditory or labyrinthine artery.

After giving off a branch to the eighth nerve in the cerebellopontine angle, the internal auditory artery transverses the internal auditory meatus and, at the labyrinth, branches into

(a) the anterior vestibular artery to the anterior and lateral semicircular canals and the utricular macula,

(b) the posterior vestibular artery to the posterior semicircular canal, the saccular macula, and part of the cochlea, and

(c) the cochlear artery.

Clinical Examination

Vestibulospinal Reflexes

Past pointing is a deviation of the extremities caused by either cerebellar or vestibular disease.

With acute vestibular imbalance, the normal labyrinth will push the limb toward the abnormal side, and the patient will miss the target.

The past pointing will always be to the same side of the target and will occur with either limb.

With a cerebellar hemispheric lesion, the ipsilateral limbs have ataxia and incoordination; past pointing occurs only with the involved arm and may be to the side of the lesion or erratically to either side of the target.

In vestibulopathy, after a period of compensation the past pointing disappears and may even begin to occur in the opposite direction.

In rombergs test, in unilateral vestibulopathy if balance is lost with eyes closed the patient will tend to fall toward the side of the lesion, as the normal vestibular system pushes him over.

If the patient has spontaneous nystagmus due to a vestibular lesion, the fall will be in the direction of the slow phase.

The Fukuda stepping test is analogous. The patient, eyes closed, marches in place for one minute. A normal individual will continue to face in the same

direction, but a patient with acute vestibulopathy will slowly pivot toward the lesion.

In the star walking test, the patient, eyes closed, takes several steps forward then several steps backward, over and over.

A normal individual will begin and end oriented approximately along the same line.

A patient with acute vestibulopathy will drift toward the involved side walking forward, and continue to drift during the backward phase.

The resulting path traces out a multipointed star pattern.

Vestibulo-Ocular Reflexes

The vestibulo-ocular reflex (VOR) serves to move the eyes at an equal velocity but in the direction opposite the head movement.

This keeps the eyes still in space and maintains visual fixation while the head is in motion.

There are several ways to examine the VOR includingDoll's eye test Head thrust testDynamic visual acuity Calorics.

Oculocephalic Reflex (Doll's Eye Test)

The oculocephalic response is primarily useful in the evaluation of comatose patients.

Turning the head in one direction causes the eyes to turn in the opposite direction.

This response indicates that the pathways connecting the vestibular nuclei in the medulla to the extraocular nuclei in the pons and midbrain are functioning and that the brainstem is intact.

Head Thrust

The head thrust test is done in an awake patient.

Abrupt, rapid movements are made in each direction while the patient attempts to maintain fixation straight ahead, as on the examiner's nose.

Normally the VOR will maintain fixation and the eyes will hold on target.

When the VOR is impaired, the compensatory eye movement velocity is less than the head movement velocity; the eyes lag behind the head movement and a corrective “catch-up” saccade must be made to resume fixation in the eccentric position.

Dynamic Visual Acuity

The ability of the VOR to maintain ocular fixation means that a patient can read even while shaking the head to and fro.

The dynamic visual acuity test is performed by obtaining a baseline acuity, and then determining the acuity during rapid head shaking.

Degradation by more than three lines on the Snellen chart suggests impaired vestibular function.

Caloric Tests

Cold calorics in a comatose patient with an intact brainstem causes tonic deviation of the eyes toward the side of irrigation as the normally active labyrinth pushes the eyes toward the hypoactive, irrigated labyrinth.

In an awake patient, cold calorics cause nystagmus with the fast component away from the irrigated side because the cerebral cortex produces a compensatory saccade that jerks in the direction opposite the tonic deviation.

Nystagmus is seen only when the cortex is functioning normally.

Warm water irrigation has opposite effects.

Bilateral simultaneous cold calorics induce tonic downgaze, warm calorics upgaze.

Localization of Lesions Causing Vertigo

Localizing lesions causing vertigo may be approached by considering three general categories:

peripheral causes (vestibular labyrinthine

disease)

central causes (dysfunction of the vestibular connections)

Peripheral vestibular syndromes are usually of short duration and characterized by severe, often paroxysmal vertigo accompanied by auditory dysfunction (tinnitus and hearing loss).

Nystagmus is often present and is characteristically unidirectional (fast phase away from the side of the lesion), �horizontal rotatory (never vertical or exclusively rotatory), and inhibited by visual fixation.

The subjective environmental twirl, past-pointing, deviation of the outstretched hands, and fall associated with the Romberg's maneuver are toward the slow phase of the nystagmus (toward the side of the lesion).

The peripheral vestibular syndrome is therefore complete (has all of the clinical elements of vestibular dysfunction, e.g., vertigo, nystagmus, deviation of the outstretched hands, Romberg's sign, and so on) and congruent (all the slow deviations are toward the same side, i.e., ipsilateral to �the responsible lesion).

Benign Paroxysmal Positioning Vertigo

very common mechanical disorder of the inner ear in which brief attacks of acute and severe vertigo with concomitant nystagmus and autonomic symptoms is precipitated by certain head movements (often while patients turn in bed).

Commonly, BPPV involves the posterior semicircular canal.

BPPV may follow head trauma, viral labyrinthitis, meniere's disease, migraines, or inner ear surgery, but most cases (50%-70%) are primary or idiopathic and best explained by the canalithiasis or cupulolithiasis theory.

Most cases of posterior canal BPPV are due to canalithiasis.

Other patients (10%-30%) display the lateral or horizontal semicircular canal BPPV variant (HC-BPPV) in which there is a strong linear horizontal nystagmus beating toward the lowermost ear induced by rapid turning of the head from side to side around the longitudinal axis.

The nystagmus exhibits short latency without fatigability, and often reverses its direction on the pathologic side.

The vertigo can be induced by turning the head to either side in the supine position, and is always more prominent on the pathologic side.

The horizontal variant of BPPV tends to resolve more quickly than the posterior canal BPPV.

The provocative positioning maneuver (Dix-Hallpike maneuvers) (patient is briskly moved from the seated position to a position where the head is hanging 45 degrees below the horizontal and rotated 45 degrees to one side)allows for a differentiation between a peripheral or a central origin for positional vertigo.

In normal individuals, these maneuvers do not induce nystagmus.

With peripheral lesions, vertigo, nausea, vomiting, and nystagmus appear several seconds (1 to 15 seconds) after the head position is changed (latency of response due to the period of time for the otoconial mass to be displaced).

The nystagmus is usually torsional, with the upper pole of the eye beating toward the ground.

Fatigue with repeated positioning is seen.

The nystagmus fatigues and abates within 10 seconds of appearance (fatigability due to dispersion of particles in the endolymph), and when the patient is rapidly brought back to a sitting position, the nystagmus beats maximally in the opposite direction (rebound).

With repetition of the maneuver, the nystagmus becomes progressively less severe (habituation).

A central lesion should be suspected and further investigations initiated when

(a) the positioning testing maneuver is positive with the head turned to either side

(b) the nystagmus changes direction immediately after the shift in position and remains for as long as the head is down

(c) the nystagmus is unaccompanied by nausea or vomiting, and if present, vertigo is mild and lasts <60 seconds and

(d) the nystagmus does not display features of adaptability or fatigability.

Peripheral Vestibulopathy

This term refers to conditions characterized by acute or recurrent attacks of episodic vertigo caused by extramedullary disorders of the vestibular system.

These conditions encompass such entities as acute vestibular neuronitis, acute labyrinthitis, epidemic vertigo, and viral labyrinthitis.

Acute vestibular neuronitis, also known as acute vestibular neuritis, neurolabyrinthitis, or unilateral vestibulopathy of unknown cause, is characterized by sudden attacks of severe and prolonged vertigo, nausea, vomiting, and abnormal vestibular function on caloric testing in otherwise healthy patients.

It is unrelated to positional changes of the head and may be recurrent.

Some patients exhibit residual dizziness and imbalance lasting for months.

Cochlear symptoms are absent.

Vestibular neuronitis has been attributed to viral upper respiratory infections.

Vestibular neuronitis affects only a part of the vestibular nerve trunk, usually the superior division (horizontal saccular canal paresis), which travels separately and has its own ganglion, whereas the inferior part (the posterior semicircular canal) is spared.

Patients may also have a combined superior and inferior vestibular neuronitis.

Acute labyrinthitis resembles vestibular neuronitis except that there is associated tinnitus and sensorineural hearing loss.

This syndrome may follow systemic, acute, or chronic middle ear infections (i.e., viral or bacterial labyrinthitis) or occur in association with ototoxic drugs such as aminoglycosides and diuretics (i.e., toxic labyrinthitis).

Viral labyrinthitis have been reported in association with measles, mumps, and rubella.

Episodic vertigo followed by gait imbalance and oscillopsia may be familial (autosomal dominant).

Patients with this familial vestibulopathy have profound bilateral vestibular loss despite normal hearing, sometimes in combination with a spinocerebellar ataxia.

These episodes occurred regularly at 2-minute intervals, each attack lasting for 15 seconds.

This repetitive paroxysmal nystagmus and vertigo was thought to be due to pathologic brief bursts of hyperactivity of the vestibular nuclei.

Meniere's Disease

progressive disorder characterized by episodic acute and severe attacks of vertigo, fluctuating sensorineural hearing loss, and tinnitus.

Typically, the hearing loss in the early stages of Meniere's disease affects only low frequencies, fluctuates, and increases during the acute attack.

Hearing returns to normal after each attack in the beginning of the disease, but as the disease progresses, residual hearing loss after each attack accumulates and the hearing loss spreads to higher frequencies.

There is also often a sensation of uncomfortable pressure or fullness in and around the affected ear.

more often unilateral, although it may be bilateral in approximately 20%-45% of cases.

When bilateral, the disease is usually asynchronous.Pathologically, there is an increased volume of

endolymphatic fluid leading to distention of the semicircular canals: endolymphatic hydrops.

Patients with meniere's disease complain initially of distressing, fluctuating, and episodic vertigo of variable duration and intensity.

Nonpulsatile, low-pitched, continuous tinnitus, usually described as roaring and sensorineural hearing loss may precede the onset of vertigo by months or years.

Individual attacks last several minutes to several hours.

Between attacks, patients are initially symptom free.

Two main variants are recognized:

cochlear Meniere's, in which vertigo and imbalance are absent, and

vestibular Meniere's, in which vertigo is prominent, but hearing loss, tinnitus, and fullness, or ear pressure are absent in the early stages.

ReferrencesDeJong’s The Neurologic examination, sixth edition.

Localization in clinical neurology, sixth edition.

Bradley’s Neurology in Clinical Practice, sixth edition.