CHAPTER 14 Neurotology

P. Ashley Wackym, Christina L. Runge-Samuelson

The otolaryngologic subspecialty of otology, neurotology, and cranial base surgery has a focus on the anatomy, physiology, and abnormalities of the three cranial nerves that traverse the temporal bone—the cochlear, vestibular, and facial nerves—as well as specific tests of their functions.¹ Surgical management of intractable vertigo is reviewed in Chapter 92; however, the indications for vestibular neurectomy are briefly introduced in this chapter in the context of vestibular assessment and discussion of various vestibular disorders. Facial nerve injury secondary to temporal bone fracture is reviewed in Chapters 338 and 339 and is included in this chapter only in the context of the cochlear and vestibular nerves, although more extensive reviews are offered elsewhere.²^–⁴

Anatomy of the Inner Ear

The eighth cranial, or vestibulocochlear, nerve consists of two distinct components: the cochlear and the vestibular nerves. The neurons of these primary afferent nerves are bipolar, with their dendrites synapsing on the inner and outer hair cells within the cochlea and on the type I and type II hair cells within the cristae and maculae of the vestibular end-organs. Understanding the functions of the primary afferent neurons constituting the eighth cranial nerve requires knowledge of the complex structures of the peripheral vestibular and auditory systems.

Embryologically, the neurosensory portions of the vestibulocochlear system are derived from ectoderm. With growth and development, these neurosensory epithelial structures become incorporated within the petrous portion of the temporal bone and are encased in the otic capsule. Concurrently, the middle ear space develops as an invagination from the first pharyngeal pouch and comes to lie lateral to the otic capsule. Meanwhile, the external ear develops from the overlying epithelium, and the fundus of its canal abuts the middle ear space at the tympanic membrane.

In postnatal infants and adults, the vestibulocochlear labyrinth consists of a continuous complex of bony spaces encased in the otic capsule and within the petrous bone, called the bony labyrinth (Figs. 14-1 to 14-4). It is filled with perilymph (biochemically similar to cerebrospinal fluid), and suspended within it is an almost true, miniature replica of the bony spaces—the membranous labyrinth. The labyrinth is filled with endolymph (see Fig. 14-2). The cochlea is located anteriorly and is divided from the vestibular labyrinth by the vestibular, cochlear, and facial nerves within the internal auditory canal (IAC). The vestibular portion of the labyrinth lies posterior to the cochlea and the IAC (see Figs. 14-3 and 14-4).

FIGURE 14-1 Coronal section through the right temporal bone and ear showing the relationships of the temporal lobe, inner ear, middle ear, eustachian tube, and pinna.

FIGURE 14-2 Sound waves contact the tympanic membrane, and the acoustic energy is transformed and transmitted to the inner ear through the ossicular chain in the middle ear. Displacement of the oval window by the stapes footplate results in instantaneous transmission of the pressure through perilymph in the scala vestibuli, Reissner’s membrane, endolymph, basilar membrane, and perilymph in the scala tympani and in outward displacement of the round window membrane. Hair cell transduction occurs as the basilar membrane containing the organ of Corti moves relative to the tectorial membrane. CN, cranial nerve.

FIGURE 14-3 Course and relationships of the facial, vestibular, and cochlear nerves from the pontomedullary junction through the temporal bone. AICA, anterior inferior cerebellar artery.

FIGURE 14-4 Anatomy of the inner ear and innervation of the cochlea and vestibular end-organs. Upper left inset, The relationship between the posterior crista and the cupula is demonstrated. Lower left inset, The basal turn of the cochlea is shown, along with the relationships of the scala vestibuli, scala media, scala tympani, and the organ of Corti.

The Cochlear System

The cochlea (from the Greek word for snail) is made up of a hollow tube that spirals 2.5 times over a distance of 33 mm. The bony center, around which the spiral is coiled, assumes the shape of a tapered screw (from the Latin word modiolus) if the outer parts of the bony spiral are removed. The long axis of the modiolus has a sloping anterolateral orientation in the head, with its base abutting the anterior part of the fundus of the IAC. Fine canals (habenula perforata) within the modiolus house the dendrites of the cochlear nerve and their bipolar somata, which constitute the spiral ganglion.

The basal turn of the cochlea forms a distinct bony promontory on the medial wall of the mesotympanum (i.e., the middle ear) and is the only portion of the cochlea that is visible during middle ear operations. The remaining turns of the cochlea are enclosed within the petrous bone. At the posterior aspect of the promontory are two fibrous barriers to communication with the inner ear: the oval window superiorly, which faces laterally and houses the footplate of the stapes, and the round window inferiorly, which faces posteriorly and is contained within the bony round window niche (see Figs. 14-1 and 14-2).

A transverse section through the cochlea shows that it is composed of three compartments: the scala vestibuli, scala media, and scala tympani (see Figs. 14-2 and 14-4). The scala media, which contains endolymph, is separated from the scala vestibuli and tympani by Reissner’s membrane and the basilar membrane, respectively. The scala vestibuli and scala tympani contain perilymph, and these two compartments communicate with each other at the cochlear apex through an opening at the tip of the basilar membrane called the helicotrema. At the base of the upper compartment, called the scala vestibuli, is the oval window, and at the base of the lower compartment, called the scala tympani, is the round window.

The basilar membrane has a complex structure resting on it that is best appreciated in cross section. The basilar membrane and the thin sloping membrane, called Reissner’s membrane, form a tube that ends blindly and is sealed at the helicotrema. This is the scala media, or cochlear duct, and it contains endolymph (see Figs. 14-2 and 14-4). Extending along the entire basilar membrane and spiraling with the cochlea is the organ of Corti. It contains the structurally complex sensory epithelium innervated by the cochlear nerve. As viewed from above, the basilar membrane is widest near the helicotrema and narrowest at the base. Maximal high-frequency vibration of the basilar membrane occurs at the base, and maximal low-frequency vibration occurs at the apex, thereby resulting in hair cell transduction of high frequencies at the base and low frequencies at the apex.

The organ of Corti consists of a single row of inner hair cells and three rows of outer hair cells. The inner and outer hair cells slope toward each other to form a triangular canal between them called the tunnel of Corti. The tectorial membrane extends from a medial-to-lateral direction within the scala media and above the hair cells where the hair cell stereocilia are embedded.

Physiology of Hearing

For an understanding of the various types of hearing loss, it is important to review the mechanism of transmission of sound to the neural receptors (see Fig. 14-2). Air vibrations impinge on the tympanic membrane and cause it and the malleus to vibrate. This physical vibration is transmitted through the incus to the stapes and by way of its footplate to the fluids of the labyrinth. The fluid vibrations produce a stimulus along the basilar membrane that activates the organ of Corti. Impulses are transmitted through the cochlear nerve endings to the cochlear nuclei in the pontomedullary junction. The impulses then continue to the auditory areas of the cortex.

Transformer Mechanism of the Tympanum

The pars tensa of the tympanic membrane and its attached malleus are set in motion by sound; the motion is then continued to the oval window. Because the effective vibratory area of the tympanic membrane is about 17 times as large as the area of the footplate of the stapes and because the manubrium of the malleus is 1.3 times as long as the long process of the incus, the ratio of amplification from the tympanic membrane to the stapedial footplate is about 22 : 1.

Because fluid is incompressible, the round window membrane acts as a compensating membrane to accommodate the vibrations of the stapes footplate (see Fig. 14-2). If sound vibrations were to reach the oval and round windows at the same time, a certain amount of cancellation of the sound would take place. This does not normally take place because of the phase difference between the windows, which is facilitated by several factors. The intact tympanic membrane protects the round window from direct sound impingement; the tympanic membrane is connected to the oval window through the ossicular chain, thus making direct transmission by this route faster; and the round window membrane faces backward, at right angles to the plane of the tympanic membrane, and is recessed within the niche. These factors delay the impingement of sound onto the round window membrane, which produces the phase difference.

As a consequence of these phenomena, a sizable perforation of the tympanic membrane results in a hearing loss of 30 to 35 dB by air conduction, whereas dislocation of the ossicular chain with an intact tympanic membrane produces an air conduction loss of about 55 to 60 dB (i.e., maximal conductive hearing loss).

Transmission in the Labyrinth

Cochlear signal transduction of high frequencies occurs near the basal portion, and transduction of low frequencies occurs near the apical regions of the basilar membrane and its organ of Corti. Detailed mechanisms of sound transduction and perception are beyond the scope of this chapter but can be found elsewhere.¹

Measures of Auditory System Function

There are two major reasons for conducting measures of auditory system function: to provide information leading to determination of the anatomic site of a lesion, from which inferences can be made regarding the underlying pathology resulting in the patient’s symptoms, and to assess the receptive communication capabilities of the patient. Because reduced auditory sensitivity is common to most auditory disorders of peripheral origin, an estimate of the magnitude of the loss of sensitivity provides the basis of any differential diagnostic auditory test battery.

The purpose of this section is to describe the current, primary measures of auditory system function. Some of these measures depend on the subjective response of the patient and are referred to as subjective measures of hearing. Other measures require no subjective response from the patient but require a quiescent, cooperative subject. Such objective measures of auditory system function can be conducted with an alert, cooperative patient or with a sedated or anesthetized patient.

Subjective Measures of Hearing

Tuning Forks

An initial question in the diagnosis of auditory system dysfunction is whether the patient’s hearing deviates in sensitivity from normal-hearing individuals. Before development of the clinical audiometer, assessment of hearing was typically conducted with tuning fork tests. Tuning forks continue to be an integral part of the initial assessment of patients with hearing loss, particularly at the bedside or in the clinic.

Each tuning fork emits a pure tone of a particular frequency, depending on the physical characteristics (i.e., mass and inertia) of the fork. An experienced practitioner can activate the fork by striking it with a “standard” blow. By comparing the length of time that the patient can hear the slowly damping intensity of the fork relative to that of the practitioner (assuming that the practitioner has normal hearing), it is possible to determine the relative magnitude of hearing loss that the patient has at a particular frequency. Because of problems in reliably striking a standard blow and the noise levels in most clinical examination rooms, tuning forks are more often used in a qualitative manner to assess the type of hearing loss. A calibrated audiometer is the instrument of choice for more precisely determining the magnitude and configuration of hearing loss as a function of frequency.

The Rinne test compares the patient’s response to air conduction and bone conduction sensitivity. For normal-hearing persons and those with sensorineural hearing loss, the tuning fork is heard longer by air conduction than by bone conduction because of the advantage provided to the air conduction signal by the normal middle ear system. However, when the patient hears bone conduction longer than air conduction, a conductive (middle ear) hearing loss is suggested. This ensues when the air conduction route of transmission is no longer the more efficient route to the cochlea.

The Weber test assesses the ear to which the auditory signal is referred when the tuning fork is placed at the center of the forehead and the patient is asked in which ear the signal is heard. When both ears are normal or symmetrically abnormal, the auditory signal is localized to the center of the head. If there is unilateral middle ear (conductive) loss, the sound is lateralized to the ear with the conductive loss. If there is unilateral sensorineural loss, the sound is lateralized to the ear with the better sensorineural sensitivity.

The tuning fork tests provide the examining physician with an initial impression of the probability of hearing loss and the possible site of the auditory lesion affecting hearing sensitivity. However, tuning forks do not provide definitive information on the magnitude and configuration of the hearing loss, and they provide virtually no information on potential differential sites of lesions in patients with sensorineural hearing loss. Such information is available from more formal measures of auditory system function.

Pure-Tone Audiometry

Speech Recognition Threshold

Traditionally, the SRT is measured with the use of spondaic words, that is, two-syllable words in which equal stress is placed on each syllable, such as hot-dog, baseball, cowboy, and sidewalk. No standardized method for presentation of the words has been accepted, although practical means for standardization have been suggested.¹¹ The SRT is reported as the decibel HL below which the patient cannot successfully recognize the two-syllable words. It is expected that the SRT will approximately equal the average hearing loss for pure tones in the midfrequency region (500 to 2000 Hz), regardless of the type of hearing loss (i.e., conductive or sensorineural). The SRT has little differential diagnostic significance, except in cases of pseudohypacusis, but it is used to provide a descriptive measure of hearing loss for speech and to confirm the pure-tone air conduction sensitivity measures.

Speech Recognition Measures

Measurement of speech recognition at suprathreshold levels is conducted with standardized lists of words or sentences. Standardized material has been chosen to meet specific criteria that enable comparison with everyday speech. The material available for use includes monosyllabic word lists, nonsense syllables, and sentences. The results are reported as the percent correct scores at a specified level above the SRT.

Persons with conductive hearing loss typically score high with these materials, whereas those with sensorineural hearing loss show decreased discrimination, depending on the magnitude and configuration of the sensorineural hearing loss and the site of the auditory lesion (i.e., cochlear or retrocochlear). Recognition of isolated speech segments is unaffected by conductive hearing loss (if the materials are presented at suprathreshold levels) because the encoding mechanisms of the cochlea and cranial nerve VIII are normal. When the presentation level overcomes the threshold sensitivity loss, the ability to understand speech segments is excellent; however, when the conductive mechanism is normal but lesions of the auditory system affect the cochlear or retrocochlear structures, the ability to understand the consonant elements of speech is affected. When the cochlear structures are normal but cranial nerve VIII or low-brainstem structures are affected by a space-occupying lesion, speech recognition can be severely affected. One of the early diagnostic signs of lesions of cranial nerve VIII or the low brainstem is severely reduced speech recognition scores in the presence of mild or moderate pure-tone hearing loss.

Figures 14-7 and 14-8 provide examples of the effects of conductive and sensorineural hearing loss on the SRT and speech recognition scores of two patients. Figure 14-7 is an example of conductive hearing loss secondary to otosclerosis. Notice the elevated pure-tone air conduction threshold and SRT, together with normal bone conduction sensitivity and excellent speech recognition score. Figure 14-8 is an example of sensorineural hearing loss secondary to Meniere’s disease, which is discussed later. Notice the low-frequency sensorineural hearing loss, the mildly elevated SRT, and the diminished speech recognition score (64%). This example reveals the potential effect of a cochlear lesion site on speech recognition ability. Early in the course of Meniere’s disease, it is unusual for speech discrimination ability to be abnormal. Review of Figures 14-5 and 14-6 reveals other examples in which conductive hearing loss from chronic otitis media (see Fig. 14-5) and sensorineural hearing loss from industrial noise trauma (see Fig. 14-6) affect speech and pure-tone results.

FIGURE 14-7 Air conduction (AC, squares) and bone conduction (BC, square open brackets) pure-tone threshold sensitivities are given as a function of frequency in conductive hearing loss caused by otosclerosis. The speech reception threshold (SRT) is expressed in hearing level (HL) and the speech recognition score is given as the percentage of correct responses. ANSI, American National Standards Institute.

FIGURE 14-8 Air conduction (AC, squares) and bone conduction (BC, square open brackets) pure-tone threshold sensitivities as a function of frequency in sensorineural hearing loss caused by Meniere’s disease. The speech reception threshold (SRT) is expressed as hearing level (HL) and the speech recognition score is given as the percentage of correct responses. ANSI, American National Standards Institute.

Objective Measures of Auditory System Function

Immittance Studies

Among the most significant advancements in the differential diagnosis of middle ear impairments and one that provides definitive information on cranial nerve VIII and low-brainstem function is measurement of acoustic immittance.¹² This procedure requires no active participation by the patient but provides objective evidence of middle ear function and a means of testing the integrity of the acoustic reflex arc. The two procedures included in immittance studies are tympanometry and acoustic reflex measures.

Tympanometry

Tympanometry provides evidence of the relative change in impedance (or its reciprocal, admittance) with a change in ear canal air pressure at the plane of the tympanic membrane. The tympanogram provides indirect evidence of the mechanical integrity of middle ear structures when changes in ear canal air pressure are introduced. When pathologic conditions such as middle ear effusion, ossicular chain fixation, or ossicular chain discontinuity occur, concomitant changes in admittance at the plane of the tympanic membrane take place. Such changes in admittance affect the efficient transmission of acoustic energy across the middle ear space to the cochlea and introduce hearing loss. The changes in transmission characteristics can also be measured objectively by direct measures of changes in relative admittance. Clinical instruments are available by which changes in ear canal air pressure can be introduced while simultaneously measuring the effects of the changes in air pressure on transmission of energy through the middle ear to the cochlea. In a normal middle ear system, negative and positive (relative to atmospheric pressure) changes in air pressure produce predictable decreases in the relative transmission of energy through the middle ear space. When pathologic conditions such as middle ear fluid and ossicular chain fixation occur, the relative changes in admittance decrease, a finding indicative of a high-impedance (low-admittance) middle ear. When ossicular chain discontinuity and some disorders of the tympanic membrane occur, the effect is decreased impedance (increased admittance) of the middle ear system. These measures of relative change in impedance with alterations in ear canal air pressure can also provide evidence of tympanic membrane perforations and the functional integrity of pressure equalization tubes that might have been placed in the tympanic membrane. The tympanogram provides objective evidence of the integrity of the middle ear system and differential diagnostic information on the underlying middle ear source of any resulting conductive hearing loss that might have been demonstrated on the pure-tone audiogram.

Acoustic Reflex

The acoustic reflex refers to the reflexive contraction of the stapedius muscle on delivery of an acoustic stimulus. The stapedius muscle contracts reflexively and bilaterally on presentation of an acoustic stimulus.¹³ The muscle contraction results in a concomitant increase in impedance in the middle ear when measured at the plane of the tympanic membrane. Unfortunately, when middle ear pathologies introduce changes in middle ear impedance, it is not possible to measure evidence of further changes in impedance that might be introduced by contraction of the stapedius muscle. Only when tympanometry reveals the middle ear system to be functioning normally is it possible to test the integrity of the acoustic reflex arc.

When tympanometry has revealed the middle ear system to be functionally normal, two types of acoustic reflex measurements can be made: acoustic reflex threshold measures and acoustic reflex adaptation measures. The same equipment used to obtain the tympanogram can be used to measure the integrity of the acoustic reflex.

Constant air pressure is maintained in the external auditory canal, and impedance or admittance is monitored over time. The intensity of a reflex-inducing acoustic stimulus is increased until a change in impedance or admittance is observed. The lowest intensity at which the reflex-inducing acoustic stimulus results in a change in acoustic impedance or admittance is specified as the acoustic reflex threshold. Typically, lesions in cochlear sites produce a change in the threshold of the acoustic reflex only for wide-band noise stimuli, not for pure-tone stimuli, until the hearing loss exceeds approximately the 60-dB HL. When the hearing loss is of cochlear origin and the loss exceeds 60 dB, there may be an increase in the threshold of the acoustic reflex even for pure-tone stimuli. The acoustic reflex threshold measure can be used in cases of sensorineural hearing loss to provide differential diagnostic information on the site of the sensorineural hearing loss.¹⁴

In patients with a retrocochlear site of the lesion (cranial nerve VIII and low brainstem), the acoustic reflex may be elevated or absent. An abnormally elevated or absent acoustic reflex in the presence of hearing loss at less than a 60-dB HL is audiologic evidence supporting a retrocochlear site of the lesion.¹⁵^,¹⁶

Acoustic reflex adaptation is measured by introducing (into the ear contralateral to the reflex-measuring tip) a 10-second, pure-tone stimulus at 10 dB above the acoustic reflex threshold for that particular stimulus. Acoustic reflex adaptation is defined as a decrease in impedance or admittance that exceeds 50% of the nominal impedance or admittance observed at the onset of the 10-second stimulus. This adaptation is not a function of the inability of the stapedius muscle to maintain contraction throughout a 10-second period, but rather an adaptation to the 10-second, continuous pure tone presented to the contralateral (test) ear. The test ear in acoustic reflex adaptation is the ear receiving the acoustic stimulation, not the ear in which the acoustic impedance or admittance is being measured. Acoustic reflex adaptation is typically observed only in ears in which there is a retrocochlear (cranial nerve VIII or low brainstem) disorder.

Figure 14-9 is an example of audiometric data from a patient with a left acoustic neuroma within the cerebellopontine angle. The pure-tone results reveal mild, left ear sensorineural hearing loss with a very poor speech recognition score (24%). The tympanograms were normal bilaterally, but there was no acoustic reflex identifiable with acoustic stimulation of the left ear. When the measuring tip was in the right ear, evidence of stapedius muscle contraction was observed only with ipsilateral stimulation. When the measuring tip was in the left ear, the stapedius muscle contracted only when the acoustic stimulus was presented contralaterally. As evidenced by acoustic reflex measures, this is the classic audiometric result seen in a patient with a left acoustic neuroma.

FIGURE 14-9 Bilateral auditory test results from a patient with unilateral (left ear) acoustic neuroma extending into the cerebellopontine angle. A, Air conduction (AC, squares [right ear] circles [left ear]) and bone conduction (BC, square open brackets [left ear only]) pure-tone threshold sensitivities as a function of frequency are shown. Measures of pure tone and speech recognition show left sensorineural hearing loss and poor speech discrimination on the left. The speech reception threshold (SRT) is expressed in hearing level (HL) and the speech recognition score is given as the percentage of correct responses. B, Tympanograms and acoustic reflex threshold measures show normal middle ear impedance and absent acoustic reflexes on the left. C, Auditory brainstem evoked responses at a 75-dB normalized hearing level (nHL) show poor wave morphology and delay of wave V on the left. D, Measures of otoacoustic emissions show normal cochlear function despite the sensorineural hearing loss and poor speech discrimination in the left ear.

Auditory Brainstem Evoked Response Measures

Possibly the most powerful audiologic test available today in differentiating between cochlear and retrocochlear lesions is measurement of the auditory brainstem evoked response (ABR). The ABR is one of several clinically useful evoked auditory potentials and the one most often applied in site-of-lesion testing.¹⁷ The ABR is typically evoked with a short-duration pulse delivered to the ear at a predetermined intensity. At high-intensity levels, the acoustic stimulus evokes as many as five amplitude peaks. The peaks were first identified and categorized by Jewett and Williston.¹⁸ Three of these peaks (waves I, III, and V) are the major peaks and are generally accepted as corresponding to firing of the first-order neurons of cranial nerve VIII (wave I), the superior olivary complex (wave III), and the inferior colliculus (wave V). These three major waves are present at approximately 2-msec intervals (in normal-hearing children and adults) after the onset of the acoustic stimulus at high-intensity levels (Fig. 14-10). As revealed in Figure 14-10, as the intensity of the stimulus is decreased, the amplitude of all peaks decreases, the latency of each peak increases, and the replicability of the early waves (I and III) decreases. Only wave V is identifiable at threshold levels.

FIGURE 14-10 Wave latency results for auditory brainstem evoked responses are given as a function of decreasing stimulus level. The time window is 10 msec. Arrows indicate identification of wave V latency.

Starr and Achor were among the earliest to use ABR results to describe a diverse set of patients with neurological disorders.¹⁹ Patients with cortical problems had normal ABR values, whereas patients with acoustic nerve and low-brainstem disorders had abnormal ABR results. This early evidence has been corroborated by numerous other reports in which ABR values were abnormal in patients with cranial nerve VIII and low-brainstem disorders.²⁰^,²¹

Auditory Neuropathy

Results from auditory evoked potential recordings combined with otoacoustic emission (OAE) testing provide objective measures that can identify patients with auditory neuropathy. The term auditory neuropathy, coined by Starr and colleagues,²² has been used to describe a group of patients with abnormal neural function demonstrated by absent or abnormal ABR results and absent middle ear reflexes but with normal outer hair cell function determined by normal OAE testing and cochlear microphonics. These patients also show evidence of poor speech discrimination, particularly in the presence of noise. Pure-tone thresholds vary widely in severity from normal to severe to profound and may be asymmetric or have a variety of configurations. Doyle and coworkers described the audiometric and electrophysiologic findings associated with auditory neuropathy.²³ The results obtained after performing ABR and OAE testing suggest an abnormality of the auditory system at the level of the inner hair cells, at the synapse between hair cells and the cochlear nerve, at the level of the cochlear nerve itself, or a combination of these sites.

The ABR results in patients with cochlear hearing loss typically reveal normal ABR replicability and latencies at high-intensity levels, but with an elevated ABR “threshold” (a function of the elevated hearing thresholds secondary to cochlear pathology). In patients with a retrocochlear site of a lesion, the site of the auditory lesion affects the results. If a space-occupying lesion is present in the brainstem but after the first-order neurons of cranial nerve VIII, wave I may be normal, but all subsequent waves may be absent or significantly delayed in latency. If the lesion affects function of the first-order neurons of cranial nerve VIII, there may be no replicable waveforms evoked by the acoustic stimulus. Figure 14-9 presents the ABR results in a patient with an acoustic neuroma within the left cerebellopontine angle. The ABR result at a 75-dB normalized HL in the right ear reveals a replicable waveform with normal absolute and interwave latencies. The response from the left ear reveals poor replicability of waves III and V, increased latency of waves III and V, and consequently, abnormal I-III and I-V interwave latencies.

The ABR is often used in the preoperative evaluation of patients suspected of having cranial nerve VIII and low-brainstem disorders. An additional application is use of the procedure for monitoring changes in auditory system function intraoperatively. Specifically, under conditions in which a replicable ABR can be evoked preoperatively, it is common to use the procedure intraoperatively to monitor the integrity of the cochlea and cranial nerve VIII (wave I) and evaluate more central auditory structures (waves III and V) as surgery progresses. Intraoperative changes in ABR wave latencies can be used to quantify the effects of surgical procedures on transmission characteristics within the auditory system, but these are not real-time measurements. Immediate postoperative ABR monitoring also provides objective evidence of the integrity of the peripheral auditory system before final closure of the surgical field.

Electrically Evoked Auditory Potentials with Cochlear Implant Users

Electrically evoked auditory potentials have been studied for a number of purposes, including assessment of neural integrity, evaluation of cochlear implant function, and estimation of the psychophysical measures needed to program the cochlear implant speech processor, as well as an indication of performance after cochlear implantation.²⁴^–²⁶ Preimplantation electrical recordings have included placement of a transtympanic needle electrode on the promontory and subsequent delivery of electrical current. In general, these results have been variable, and there has not been a clear relationship between these measures and postimplantation performance.²⁷ Electrical auditory brainstem response (EABR) has been reliably recorded through a variety of cochlear implant devices and arrays. In general, four peaks can be identified, with the most robust being wave V. Wave I is usually obliterated by the stimulus artifact that occurs at the beginning of the recording. The EABR is affected by stimulus intensity, with a decrease in wave V amplitude and a slight increase in latency as the electrical current is decreased. On average, the latency of wave V is approximately 1.0 to 1.5 msec earlier than after the corresponding acoustic signal. Because the age at cochlear implantation has steadily decreased (now as young as 6 months of age), there is increased interest in the use of EABR to determine the threshold for each electrode and thereby assist in programming and fitting of the external speech processor.²⁸ Several studies have compared the EABR wave V thresholds and behavior levels used in fitting of the speech processor. EABR wave V thresholds generally fall within the dynamic range for a given tested electrode and represent levels that are at least audible.²⁴ Thresholds, amplitudes, and waveform morphologies differ across subjects and within individuals for different electrodes.²⁴^,²⁵ The variations may be caused by differences in the population and pattern of surviving primary afferent neurons and dendrites within and across subjects.

Otoacoustic Emission Measures

Kemp was the first to report the presence of audiofrequency energy in the ear canal of subjects with normal hearing who were stimulated with a short-duration, broadband acoustic signal.²⁹ Kemp identified these “emissions” as energy leakage from normal stimulation of cochlear structures. Since the early reports, evidence has accumulated confirming that such acoustic energy leakage is a biochemical property of the healthy, functioning cochlea.

Several methods are available for evoking and recording OAE. Each procedure entails the presentation of an acoustic stimulus to the ear and monitoring of energy in the ear canal. Distortion product, transient emissions, and steady-state measurements each provide differentiation of the energy in the driving stimulus from energy in the evoked emission.

Since identification of the phenomenon in 1978, research interests have focused on sources of the OAE ³⁰ and its clinical applications.³¹ OAE has been determined to be a product of the outer hair cells of the cochlea. Clinical evidence suggests that patients with thresholds of greater than 30-dB HL because of a cochlear-site lesion have no identifiable OAE to transient auditory stimulation. Any middle ear lesion typically precludes measurement of OAE. However, in the case of a retrocochlear lesion, when there has been no retrograde degeneration of the outer hair cells, normal OAE can be evoked in the presence of significant sensorineural hearing loss.³¹ OAE represents the first available auditory function test with which it may be possible to differentiate neural from cochlear sites of a lesion when the potential exists for each site to be implicated in sensorineural loss. As a consequence, OAE results may contribute information leading to more informed decisions by surgeons who need to address the potential for “hearing preservation” after excision of space-occupying lesions affecting cranial nerve VIII.

Figure 14-9D includes results from OAE measurements for the patient with an acoustic neuroma within the cerebellopontine angle presented earlier in this chapter. The results were obtained with a click delivered at an 80-dB sound pressure level. Results of the average OAE can be analyzed in terms of the replicability of the emission as a function of time after delivery of the emission-evoking stimulus (see Fig. 14-9D, lower section). The OAE response can be collapsed over time, and Fourier analysis of the energy provides a measure of the relative amplitude of the energy in the OAE as a function of the frequency (see Fig. 14-9D, upper right section). Noise in the external ear canal is represented by the dark spectrum; OAE energy is represented by the light spectrum. Despite left ear sensorineural hearing loss exceeding a 35-dB HL throughout the frequency range of 250 to 8000 Hz, there is OAE energy throughout the frequency range of 1000 to 4000 Hz. This observation reveals that the outer hair cells of the cochlea are functioning normally and that the source of the sensorineural hearing loss is neural, not cochlear. Postoperative measurements of OAE can reveal any changes in cochlear function caused by surgical intervention to remove the acoustic neuroma.