The Acoustic (Vestibulocochlear) Nerve



The Acoustic (Vestibulocochlear) Nerve





The vestibulocochlear, acoustic, or eighth cranial nerve (CN VIII) has two components, the vestibular and the cochlear, blended into a single trunk. The cochlear portion subserves hearing; the vestibular nerve subserves equilibration, coordination, and orientation in space.


THE COCHLEAR NERVE


Clinical Examination

Some information about hearing may be obtained simply by observation and gauging the patient’s ability to understand soft and loud tones and low and high pitches; note signs of deafness, such as a tendency to turn the head when listening, lip reading, or speaking with a loud voice. Any history of hearing difficulty, such as trouble using the telephone or hearing conversation in noisy environments, or complaints from family members, should prompt a careful evaluation. 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.

Conductive hearing loss is that due to impaired conduction of sound to the cochlea and may be due to occlusion of the external auditory canal, middle ear disease (e.g., otitis), or abnormality of the ossicular chain (e.g., otosclerosis). Sensorineural hearing loss is that due to disease of the cochlea (e.g., Ménière disease) or eighth cranial nerve (e.g., acoustic neuroma). Central hearing loss is that due to disease of the central pathways. Central hearing loss is very rare because of the bilaterality and redundancy of the auditory system; unilateral lesions of the central auditory pathways typically do not cause any deficit detectable by routine clinical testing.

There are many ways to assess hearing at the bedside. All are crude compared to the information that can be obtained with a formal audiogram. Bedside clinical testing of hearing may theoretically use any available instrument that is capable of making a sound. Because the ability to hear and understand speech is the most important functional aspect of audition, whispered voice is useful; clinically significant hearing loss is usually detectable using this simple modality. In certain types of deafness, loss of speech discrimination is of clinical significance, even though pure tone and even speech thresholds are normal. One key to the effective use of whisper is unpredictability of the stimulus, for example, the numbers, “1, 2, 3” in one ear and “7, 8, 9” in the other. Monosyllables are preferable to common stock questions such as “How are you?” in which hearing a small part may enable the patient to “hear” the rest in context. Alternating words and numbers is a challenging test of hearing. Other useful sounds for bedside testing include finger rub and pure tones created by a tuning fork.


Detailed testing of hearing is done monaurally, ideally while occluding the opposite ear, as by pressing the tragus over the canal, and the patient is asked to compare the sound intensity between the two ears. The examiner may also compare the distance from each ear at which a sound of the same intensity can be heard.

Tuning forks—typically 128, 256, or 512 Hz—are sometimes used to give more specific information and to assess air conduction (AC) and bone conduction (BC). The patient may be asked to compare the loudness of the vibrating fork in the two ears, or the examiner may compare the distance on each side at which the fork begins or ceases to be heard. The examiner with good hearing may compare the patient’s air and bone conduction with his own. In evaluating BC, be certain the patient hears rather than feels the tuning fork.

The Rinne test compares the patient’s AC and BC; it can be done in at least two ways. An activated fork may be placed first on the mastoid process, then immediately beside the ear (or vice versa), and the patient asked which is louder; it should always be louder by the ear. The more time-consuming, traditional method is to place the tuning fork on the mastoid and when no longer heard there move it beside the ear, where it should still be audible. The fork should be heard twice as long by AC as by BC. The Rinne test is normal or positive when AC is better than BC. In conductive hearing loss, AC is impaired but BC is preserved; sound is not conducted normally through the canal or from the tympanic membrane through the ossicular chain to the cochlea, but the sensorineural mechanisms are intact. In sensorineural hearing loss (SNHL), both AC and BC are impaired while retaining their normal relationship of AC better than BC

In the Weber test, a vibrating tuning fork is placed in the midline on the vertex of the skull. It may be placed anywhere in the midline, over the nasal bridge, forehead, or maxilla, but works best over the vertex. Normally, the sound is heard equally in both ears or seems to resonate somewhere in the center of the head; it is “not lateralized.” In conductive hearing loss, the sound is heard better on (“lateralized to”) the involved side. In sensorineural deafness, the sound is heard best in the normal ear.

In summary, with unilateral conductive hearing loss (CHL) there is primarily loss of AC; BC is preserved or even exaggerated and the Weber lateralizes to the involved side (Table 13.1). With unilateral SNHL, AC and BC are both diminished, but AC remains better than BC, and the Weber lateralizes to the normal ear. With CHL, low tones are lost, as are some of the broad or flat consonants and vowels such as m, n, l, r, o, and u. Impairment of speech discrimination parallels the loss for pure tones. There is no recruitment, and tone decay is normal. Patients with CHL tend to hear speech better in a noisy background than in a quiet setting. In SNHL, the hearing loss is worse for higher frequencies, and there is greater difficulty with sibilants, sharp consonants, and short vowels (e.g., in the words sister, fish, twenty, water, and date). Auditory reflex responses that
produce a blink or reflex eye closure in response to a loud, sudden noise are occasionally useful in evaluating hearing in children, patients with altered mental status, and in hysteria or malingering. Mixed hearing loss, with elements of both CHL and SNHL, is not uncommon.








TABLE 13.1 Rinne and Weber Tests




















Normally the auditory acuity is equal in both ears, air conduction is greater than bone conduction (Rinne test normal or positive) bilaterally, and the Weber test is nonlateralizing (midline). The table depicts the pattern on the involved side with unilateral conductive or sensorineural hearing loss



Auditory Acuity


Rinne Test


Weber Test


Conductive hearing loss


Decreased


BC > AC (Rinne negative or abnormal)


Lateralizes to abnormal side


Sensorineural hearing loss


Decreased


AC > BC (Rinne positive or normal)


Lateralizes to normal side



Disorders of Function

Dysfunction of the cochlear nerve and its connections usually causes either diminution or loss of hearing (hypacusis or anacusis), with or without tinnitus. Conductive hearing loss is due to interference with the transmission of sound to the cochlea. Sensorineural hearing loss is due to disease of the cochlea or its central connections. In essence, CHL is due to disease external to the oval window, and SNHL is due to disease central to the oval window. Some causes of hearing loss are listed in Table 13.2.

Sensorineural hearing loss may be due to disease of the cochlea (end-organ deafness), such as in Ménière disease, or to disease of CN VIII or more central structures (retrocochlear), as in acoustic neuroma. Audiographic findings typical of cochlear disease are loss of acuity for pure tones with a parallel impairment of speech discrimination, recruitment, and tone decay. Recruitment is an abnormal loudness of sounds due to cochlear dysfunction, which can cause a paradoxical increase in the perception of louder sounds, sometimes accompanied by sound distortion. Retrocochlear lesions tend to cause a loss of speech discrimination out of proportion to the loss for pure tones, no recruitment, and abnormal auditory adaptation by tone decay.

The cochlear and vestibular nerves run together in a common sheath from the brainstem to their respective end organs, and disorders of the eighth nerve between the cochlea and brainstem may cause hearing loss. Some disease processes affect both divisions peripherally (e.g., labyrinthitis) or centrally (e.g., brainstem neoplasm). In its course across the cerebellopontine angle (CPA), the most important disorder to affect both divisions is a neoplasm. Acoustic neuroma (acoustic
neurinoma, acoustic schwannoma) is the most common, but neurofibroma, meningioma, facial nerve schwannoma, cholesteatoma, epidermoid cyst, and other tumors may arise in the CPA as well. Other conditions that may cause both hearing loss and vertigo include Ménière disease, labyrinthitis, viral infection (especially herpes), trauma, meningitis, vascular occlusion (internal auditory or anterior inferior cerebellar), Susac syndrome, Cogan syndrome, Fabry disease, perilymphatic fistula, toxins, and drugs.








TABLE 13.2 Causes of Hearing Loss







































































Conductive hearing loss



External auditory canal obstruction (e.g., cerumen, foreign bodies, water, blood)



Perforation of the tympanic membrane



Disease of the middle ear



Disease of the nasopharynx with obstruction of the eustachian tube


Sensorineural hearing loss



Disease of the cochlea




Acoustic trauma




Ménière disease




Infections




Congenital conditions (e.g., congenital rubella)




Presbycusis



Disease of the cochlear nerve or nuclei




Tumors (e.g., acoustic neuroma)




Trauma (e.g., skull fracture)




Infection (meningitis, syphilis)




Toxins or drugs




Presbycusis




Nuclear lesions (e.g., vascular, inflammatory, or neoplastic)


Lesions of the central auditory pathways


Nonorganic hearing loss refers to hearing loss in the absence of any organic disease, or hearing loss that is exaggerated. It is more common for real hearing loss to be exaggerated in severity than for it to be feigned with entirely normal hearing. In most instances nonorganic hearing loss is a transient symptom related to acute emotional stress. It may be partial or total, unilateral or bilateral. It is often bilateral and total, and the patient makes no attempt to hear what is said or to read the speaker’s lips. The mainstay of diagnosis is inconsistency in the performance on hearing tests and the absence of verifiable abnormalities on objective tests. Psychogenic hearing loss may be associated with other nonorganic symptoms, such as mutism and blindness. Patients simulating bilateral deafness do not behave as a deaf person does. Deaf individuals usually raise their voices during conversation and keep their eyes fixed on the speaker’s face and lips, watching for any gesture that may help understanding. A deaf man eager to hear will automatically turn his best ear toward the speaker. Experienced lip readers have difficulty with sound-alike words; the dissembler may do better than expected because the words are actually heard.

Many tests have been devised for detection of unilateral nonorganic deafness. The diagnosis is best made audiometrically; discrepancies and inconsistencies on repeated audiometric examinations are typical. With some trickery, a stethoscope—with one earpiece occluded—can be put with the occluded earpiece in the good ear and the open earpiece into the bad ear, to demonstrate that the “deaf” ear can hear. In the yes-no test, the examiner whispers into the patient’s deaf ear after instructions to “say yes if you hear it and no if you don’t.”

Tinnitus is spontaneous noise in the ears originating inside the head. There are many types, and the causes are protean. In many cases no precise etiology can be established. The most common identifiable cause is noise exposure. Objective tinnitus refers to noise audible to both the patient and the examiner, as occurs in carotid stenosis. Most tinnitus is subjective tinnitus. It may vary in pitch and intensity, and may be continuous or intermittent. It may be described in many ways, such as ringing, buzzing, blowing, whistling, swishing, or roaring. Tinnitus is commonly associated with deafness. It is common in presbycusis and in other types of SNHL, and is a fairly constant feature of otosclerosis. Most cases are due to disease of the cochlea or eighth nerve; some are due to CNS disease. Tinnitus is often more noticeable at night when environmental noises are diminished, and it may interfere with sleep. To the patient, tinnitus may be more distressing than the accompanying deafness, and it may cause depression in elderly individuals.

Pulsatile tinnitus is synchronous with the pulse; it is in reality a bruit. Causes include carotid stenosis, arteriovenous malformations, particularly of the dura, glomus tumors, venous hums, and hypertension. Pulsatile tinnitus is fairly common in pseudotumor cerebri, and it occurs occasionally in increased intracranial pressure of other origins. The perilymphatic duct connects the perilymph-filled spaces of the cochlea and an extension of the subarachnoid space in the region of the jugular foramen. Through this channel, pulsations in the subarachnoid space are transmitted to the cochlea. Vascular tinnitus may occasionally be affected by carotid artery compression. Rhythmic tinnitus not synchronous with the pulse may occur with palatal myoclonus (palatal microtremor).

Other causes of tinnitus include cerumen impaction, medications (particularly ototoxic drugs), Ménière disease, acoustic neuroma, and Arnold-Chiari malformation. Muscle spasm, contraction of the tensor tympani, nasopharyngeal sounds, and temporomandibular joint clicking may also simulate tinnitus. Tinnitus may be psychogenic. Bizarre types of tinnitus may occur with pontine and cerebral lesions. Auditory hallucinations may occur in lesions of the temporal lobe; these are frequently epileptic auras. More bizarre hallucinations occur in psychotic and drug-induced states.



THE VESTIBULAR NERVE


Clinical Examination

The conditions that may present as dizziness range from trivial to life threatening, and are often difficult to evaluate and manage. The nebulousness of the patient’s description of dizziness often produces frustration on the part of the clinician, yet in few other conditions are the historical details so pivotal in correct diagnosis. Fortunately, the truly serious conditions that present as dizziness are rare. The first step in understanding the symptom is to have the patient describe what he means by “dizziness.” Patients use the word dizzy to describe vertigo, as well as a number of other sensations, such as lightheadedness or giddiness, sometimes referred to as pseudovertigo. Concomitant dysfunction in several systems may cause dizziness. Conflicting sensory information may certainly cause dizziness; the sensory mismatch from watching a motion picture with dramatic movement in the visual panorama while sitting in a stationary seat illustrates the effect.

In a classic study of 100 dizzy patients in an ambulatory setting, the causes were as follows: vestibulopathy (54), psychiatric disorders (16), multifactorial (13), unknown (8), presyncope (6), dysequilibrium (2), and hyperventilation (1) (Drachman DA, Hart CW. An approach to the dizzy patient. Neurology 1972;22:323-334). The most common treatable conditions were benign positional, or benign paroxysmal positional, vertigo (BPV or BPPV), and psychiatric disorders. Other studies have shown a similar distribution. Some of the causes of dizziness are listed in Table 13.3. Table 13.4 lists causes of dizziness due to labyrinthine or vestibular pathway dysfunction. Before discussing vestibular disease, some discussion of nonspecific dizziness is warranted, since patients with such complaints make up a large proportion of the dizzy population.

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Jun 19, 2016 | Posted by in NEUROLOGY | Comments Off on The Acoustic (Vestibulocochlear) Nerve
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