Brainstem auditory evoked potentials (BAEPs) are used in pediatrics to detect and measure hearing loss in children who cannot be tested behaviorally and to evaluate the auditory brainstem pathways in children who may have neurologic problems. Recording pediatric BAEPs requires close cooperation between audiologists and neurologists because it is impossible to interpret these responses correctly without paying careful attention to both the ear and the brain.
Special technical considerations in childhood
BAEPs in children are smaller than in adults, and the background electrical noise from the electroencephalogram (EEG) and scalp muscles is often higher. Whenever possible, infants and young children should be tested while asleep because sleep reduces both EEG and muscle artifact. Newborn infants quickly fall asleep after feeding. Most older infants will sleep through a 1-hour recording session if they are awakened early on the day of the test. After the electrodes are applied, the infant is left with the mother in a darkened, sound-attenuated room to feed and fall asleep. Children older than 18 months of age usually will be quiet for the procedure, provided that it is introduced to them slowly and gently. If necessary, sleep may be assisted by oral diazepam (0.2 to 0.3 mg/kg) or chloral hydrate (30 to 50 mg/kg), but only if adequate facilities for resuscitation are immediately available. General anesthesia is occasionally necessary in extremely disturbed children and those with severe involuntary movement disorders. Most anesthetics cause small dose-related changes in the latency and amplitude of the responses but do not affect the detection of the different waves or the assessment of thresholds.
BAEPs commonly are evoked by clicks. Each recording should be made with clicks of only one polarity. In patients with high-frequency hearing loss, latency differences between the responses to condensation and rarefaction clicks may distort the responses when using clicks with alternating polarity. Recording separate responses to condensation and rarefaction clicks allows brainstem responses to be distinguished from stimulus artifacts and cochlear microphonics.
For most purposes, responses are recorded using monaural stimuli. If binaural stimuli are used, the examiner should be aware that the binaural response approximates the sum of the responses from each ear. A unilateral hearing loss thus may not be noticed. If the threshold for one ear differs by more than 40 dB from the other ear, contralateral masking to prevent cross-hearing is necessary. Some systems automatically present masking in the contralateral ear.
The intensity of the click is calibrated relative to normal hearing thresholds (nHL) for adults. The acoustic intensity of 0 dB nHL is approximately 30 dB peak-to-peak equivalent sound pressure level (peSPL). , The earphones should be placed or held so that they do not slip off the ear or occlude the ear canal. Some BAEP systems use an earphone with a light circumaural seal and a transparent cover that allows the examiner to observe the ear canal. Insert earphones are perhaps the best way of attenuating the stimulus artifact and preventing collapse of the ear canal.
The stimuli are presented at rates between 10 and 100 Hz. Wave I is best recorded at rates between 10 and 20 Hz. Wave V is recorded most efficiently at 50 to 100 Hz because its amplitude is relatively insensitive to increasing rates. Maximum length sequences provide an effective way of presenting stimuli at rates greater than 100 Hz without running into problems of overlap. ,
Clicks contain energy at all audiometric frequencies, and simple click-evoked BAEPs cannot assess auditory thresholds at different frequencies. Thresholds for the click-evoked BAEP are related most closely to behavioral thresholds between 2,000 and 4,000 Hz. However, children with high-frequency hearing losses and normal hearing at 1,000 Hz can still show BAEPs (albeit delayed) down to nearly normal thresholds. Furthermore, although absent click-evoked potentials usually indicate severe high-frequency loss, they provide very little information about the extent of low-frequency hearing. Procedures for assessing frequency-specific thresholds use two main techniques: (1) limiting the responsiveness of the auditory system by masking; and (2) concentrating the acoustic energy in the stimuli by using tones.
The derived-response technique records the responses to clicks in high-pass masking noise. Different high-pass cut-off frequencies are used for the masking noise, and sequential subtraction of the recordings yields the derived responses to the frequencies between the filter settings. This technique has been used to evaluate the hearing of infants and the development of the cochlea.
Brief tones are another way of evaluating frequency-specific thresholds. The most commonly used tones have linear rise and fall times of 2 cycles each and a plateau of 1 cycle. Brief tones have significant energy in frequencies other than their nominal frequency, and BAEPs to brief tones may be evoked by this spectral splatter. Notched noise or broad-band masking may be used to limit responsiveness to the frequencies of the tone. Determining the thresholds for BAEPs evoked by tones may provide important information about hearing in infancy and childhood. ,
Continuous tones with sinusoidal amplitude modulation have energy limited to the carrier frequency and two side bands separated from the carrier by the frequency of modulation. These frequency-specific stimuli can evoke steady-state responses at the frequency of the modulation. In adults and older children, prominent steady-state responses can be recorded at frequencies near 40 Hz. Unfortunately, these responses are attenuated by sleep and are difficult to record in infants and young children. Responses at frequencies of 70 to 100 Hz are probably the steady-state version of the transient BAEP and can be recorded reliably in sleeping infants.
BAEP thresholds to bone-conducted stimuli are important when assessing middle ear function. The bone vibrator should be positioned over the temporal bone posterior and superior to the ear and held in place by an elastic headband or the examiner’s hand. , The skull bones of infants are not meshed tightly together, and bone-conducted stimuli are not distributed equally through the skull. BAEPs recorded between the vertex and the ipsilateral mastoid in infants are larger and have slightly lower thresholds than those recorded between the vertex and the contralateral mastoid. This asymmetry may aid in the evaluation of whether inner-ear function differs between the ears in infants with bilateral conductive hearing loss. Steady-state responses recorded using bone-conducted stimuli show a similar asymmetry.
BAEPs are recorded between surface electrodes placed on the vertex and the ipsilateral mastoid or ear lobe. If the scalp electrode is located on the forehead, wave V will be smaller but still recognizable. In young infants the vertex electrode is best located in the midline anterior to the fontanelle. In newborn infants, the electrodes should be attached to the scalp with nonirritating tape and saline jelly. In older children, an adhesive paste or collodion and gauze may be used. Collodion should not be used in neonates because of skin sensitivity.
The authors recommend recording simultaneously from two channels: (1) vertex to ipsilateral mastoid; and (2) vertex to contralateral mastoid. It is then unnecessary to switch electrodes when the baby rolls over during sleep and the examiner decides to stimulate the upper ear rather than risk wakefulness by rolling the baby back. A mastoid-to-mastoid recording, which can sometimes make wave I easier to recognize, can be obtained by subtracting one vertex recording from the other.
Because the infant’s response is slower than that of the adult, the recording sweep should be longer (15 or 20 msec) and the low-frequency cut-off of the filters should be lower (20 to 30 Hz) than in adults. This adjustment is particularly important when responses to low-intensity stimuli are being examined. Settings similar to those for adults (10-msec sweeps and 100- to 3,000-Hz filter bandpass) are used in much of the literature and are acceptable for older children. Normative data for latencies and amplitudes are specific to the filter settings.
The BAEP of infants probably is best introduced through the response of the normal newborn ( Fig. 25-1 ). In general, the neonatal BAEP is about one-half the size of the adult BAEP. Wave V is particularly small, and the average V/I amplitude ratio in the normal newborn is about 1.5, whereas in adults it is over 2. These numbers are based on a low-frequency cut-off of 20 or 30 Hz; raising this setting will decrease the V/I ratio. The V/I ratio may decrease with increasing stimulus intensity because wave I increases more than wave V. The general morphology of the response differs in several other ways: in the neonate, wave I is often double-peaked, a prominent negative wave follows wave I, and the negative wave after wave III is small.
As in adults, the peak latencies of the infant’s response increase with decreasing intensity, and the I–V latency increases with increasing rates of stimulation. A rarefaction click usually evokes an earlier wave I than a condensation click. Because wave I of newborns may be double-peaked, latency values involving this wave will differ with the rules used for its measurement. Benchmark latencies for the BAEP in full-term neonates are a I–V latency of 5 msec and a wave V latency of 7 msec at 70 dB nHL.
The scalp distribution of the neonatal BAEP is very different from that of the adult response. This difference may be related to the different orientations of the neonatal auditory pathways or to incomplete myelination in parts of the pathway. In the neonate, wave I is often larger on a mastoid-to-mastoid recording than on a vertex-to-mastoid recording. The dipoles underlying waves III and V are also oriented more laterally. On a recording between the vertex and the contralateral mastoid, waves III and V are small and appear to have opposite polarity to those recorded on the ipsilateral montage ( Fig. 25-2 ). The small size of the contralateral recording in the neonate makes the response very difficult to recognize at low intensities, and when assessing thresholds care must be taken to ensure that the electrode montage is correct. The scalp distribution of the BAEP becomes similar to that of the adult BAEP by the end of the first year of life.
The BAEP can be recorded in normal newborn infants with stimulus intensities as low as 30 dB nHL, provided that the infant is asleep, sufficient averaging is performed, and the acoustic noise in the environment is low. If the recording parameters are optimized, responses can be recorded with stimuli as low as 10 dB nHL. Figure 25-3 shows BAEPs recorded near threshold in a normal newborn infant.
BAEPs can be recorded in premature infants as early as a gestational age of 26 weeks. In these infants they are evoked only by stimuli presented at high intensity and slow rates. The amplitude of the response, particularly wave V, is smaller than that in full-term neonates. The latencies of all the components of the response decrease with increasing conceptional age. Wave V shows a greater change with age than does wave I, and thus the I–V interpeak latency also decreases with age. From 36 to 40 weeks gestational age the latency of wave I decreases by about 0.1 msec, whereas the latency of wave V decreases by 0.4 msec. The increase in latency and decrease in amplitude of wave V that occur with increasing stimulus rate are more marked in premature infants than in full-term infants. Figure 25-4 shows the BAEPs from a normal premature infant and a term infant.
The BAEPs of a premature infant and a full-term infant who have reached the same conceptional age are similar in morphology and interpeak latencies. However, the absolute latencies of specific peaks in the BAEP of premature infants are longer by about 0.3 msec, and this difference persists over the first 2 years of life. This might be partly related to the greater incidence of otitis media in babies born prematurely. In premature infants who are small for gestational age, wave V is shorter in latency than in normal premature infants, but the latency does not change as much with increasing age.
BAEPs recorded from infants within the first few hours after birth differ significantly from those recorded a day after birth. Wave I is delayed just after birth by about 0.8 msec, probably because of residual fluid in the middle ear. However, there is also a significant difference in the I–V interval of 0.2 msec, which indicates some central changes over the first day of life.
In normal children the BAEP to monaural stimulation matures to the adult pattern by the age of about 3 years. After this age, the child’s BAEP has latencies that are similar to those of the adult, although some subtle differences remain. Table 25-1 presents normative developmental data from the authors’ laboratories in Ottawa and Toronto.
|Latency (msec)||Amplitude (μV)|
|Premature (36-wk gestational age)||2.1 (0.3)||5.0 (0.4)||7.4 (0.4)||5.3 (0.3)||0.4||1.5|
|Full-term neonate||2.0 (0.3)||4.8 (0.3)||7.0 (0.3)||5.0 (0.3)||0.5||1.6|
|6 wk||1.8 (0.2)||4.4 (0.3)||6.6 (0.3)||4.9 (0.3)||0.5||1.6|
|3 mo||1.7 (0.2)||4.3 (0.3)||6.4 (0.3)||4.7 (0.3)||0.6||1.6|
|6 mo||1.7 (0.2)||4.1 (0.3)||6.2 (0.3)||4.6 (0.3)||0.6||1.8|
|12 mo||1.7 (0.2)||4.0 (0.3)||6.0 (0.3)||4.3 (0.2)||0.6||2.0|
|2 yr||1.7 (0.2)||3.8 (0.2)||5.7 (0.2)||4.0 (0.2)||0.6||2.2|
* These values are for BAEPs elicited by 70-dB nHL rarefaction clicks presented at a rate of 11 Hz. Standard deviations are given in parentheses for latencies but not for amplitudes because the latter are not distributed normally.
The different components of the BAEP mature differently. Wave I latency reaches adult values by 2 months. Waves III and V show a rapid decrease in latency over the first several months and then a slower decrease to reach adult normal values at the end of the third year. The amplitude of wave V shows a marked increase after 6 months but does not reach adult values until about 5 years. The developmental changes in the latencies of the BAEP can be interpreted in terms of the differential maturation of axonal conduction time and synaptic transmission, and by the changes in the length of the auditory pathways. ,
The threshold for detecting the BAEP decreases by about 10 dB in the first 3 months of life and by a further 5 dB by the end of the first year. These changes probably are related to several factors, including the resolution of neonatal conductive hearing loss and maturation of the cochlea and brainstem. Figure 25-3 illustrates this improvement in threshold.
Gender causes significant differences in BAEPs, but there is some controversy in the literature about when these differences become significant. Some studies of neonates have found a significantly shorter latency of wave V in female babies, whereas other studies have found no differences. More consistent latency differences between males and females show up somewhere between a few months of age and puberty. , The major differences involve the I–III and I–V interpeak latencies. The amplitude of wave V is larger in girls, with this difference becoming apparent by 3 years of age. These latency and amplitude differences probably are caused by gender-related differences in the length of the basilar membrane.
Evaluation of hearing
Hearing Impairment in Infancy
Between 1 and 3 per 1,000 children are born with a sensorineural hearing loss that requires treatment. The cost of hearing impairment to the individual and to society is a result of the decreased communication ability of the hearing-impaired individual. Even mild hearing loss can impede the normal development of speech and language. Conductive hearing losses can be treated medically or surgically; sensorineural hearing losses can be treated with hearing aids, cochlear implants, and communication development training (e.g., “aural habilitation”). A major determinant of how well a hearing-impaired individual ultimately communicates is the age at which the impairment is detected and treatment instituted. It is therefore essential to identify and assess hearing-impaired infants as soon as possible.
There are clearly defined risk factors for hearing loss in the newborn period ( Table 25-2 ). Most of the risk factors for hearing loss (except family history) are present in infants treated in a neonatal intensive care unit (NICU), and many children with significant hearing loss are admitted to an NICU. One early approach to the detection of hearing impairment was therefore to record BAEPs in babies at risk for hearing impairment and babies being discharged from an NICU.
Unfortunately, about 40 percent of infants with significant hearing loss do not show any risk factors. A consensus panel of the National Institutes of Health in the United States therefore recommended universal screening of all newborn infants for hearing impairment. Behavioral testing is not an effective approach to the evaluation of hearing in the first few months of life. Some babies have reflex responses to sound, but sleepy or sick babies may not. Furthermore, because behavioral responses are elicited mainly by loud sounds, hearing losses of mild or moderate degree may not show any abnormality on behavioral testing. BAEPs and otoacoustic emissions are objective techniques for evaluating auditory function in infants. Two main approaches are now used for newborn hearing screening. One approach is to screen all infants with otoacoustic emissions and then to use BAEPs to assess infants who lack otoacoustic emissions. Another approach is to use automated techniques for recording BAEPs to low-intensity sounds in the initial screening test.
Otoacoustic emissions are sounds that originate in the cochlea and can be recorded in the external ear canal using a sensitive microphone. They can be evoked either by presenting a brief stimulus (click or tone) and recording the acoustic energy in the external ear canal over the subsequent 20 msec, or by presenting two tones of different frequency and recording the acoustic energy at the distortion products of these frequencies. Because they take less time than BAEPs, automated procedures using otoacoustic emissions are used widely to identify children who need to be evaluated further for possible hearing impairment. , As otoacoustic emissions depend on the normal activity of hair cells, they are not recorded when cochlear hearing loss exceeds about 40 dB. If emissions are present, cochlear function is sufficiently normal that treatment is probably not indicated. If they are absent, hearing loss is present, but the severity of this loss is not known. Infants who fail this test therefore should be assessed with BAEPs to confirm a persistent hearing loss and to assess its severity. Screening with otoacoustic emissions will miss infants with auditory neuropathy, who usually have normal cochlear function. BAEP screening therefore is recommended in the NICU, where there is a greater incidence of auditory neuropathy than in well-baby nurseries.
Although most significant childhood hearing losses are present at birth, a hearing impairment can develop during the first few years of life. Any parental concern about the hearing of a child should lead to an audiologic evaluation. Infants with certain congenital disorders such as rubella or cytomegalovirus infection may show a deterioration in hearing after birth. All children exposed to postnatal risk factors (e.g., meningitis, encephalitis, skull fracture, and ototoxic medication) should have their hearing tested. It is also important to monitor the auditory status of infants who have risk factors for chronic middle ear effusions (e.g., prematurity, Down syndrome, cleft palate, unilateral atresia of the external auditory canal, or other craniofacial malformations). All children with delayed intellectual development should also be tested, because hearing loss could explain or exacerbate their cognitive problems.
Newborn Hearing Assessment with BAEPs
The click-evoked BAEPs provide a quick assessment of the general hearing threshold and evaluate the neurologic integrity of the brainstem auditory pathways. This evaluation can be used as an initial hearing test (especially for infants in an NICU), as a follow-up test after screening with otoacoustic emissions, and as the initial step in a full objective audiometric evaluation.
Hearing is assessed by using 30-dB nHL monaural clicks presented at a rate of 61 Hz; 4,000 responses are averaged. If replicated responses are not recognizable, the testing is continued at higher intensities until an auditory threshold is obtained. Replicate BAEPs are also obtained for each ear by using 70-dB nHL clicks presented monaurally at a rate of 11 Hz. These responses allow assessment of neurologic function as well as hearing. Any infant not showing responses at 30 dB is retested at the age of 3 to 5 months. By that time, normal thresholds have developed in many of these infants, perhaps because a perinatal conductive loss has resolved. The authors initially hesitated between using 30 and 40 dB nHL as the screening level. Screening at 40 dB greatly reduces the number of follow-up tests and should not miss an infant with a sensorineural hearing loss requiring amplification. However, approximately 40 percent of the babies with a 40-dB threshold in the newborn period showed persistent conductive losses on follow-up. Because we considered it important to monitor these infants, we decided to screen at 30 dB nHL. Screening at 3 months is more accurate than screening in the neonatal period because transient neonatal conductive losses have resolved. Nevertheless, it is important to assess infants while they are in the hospital in case they do not return for follow-up. Figure 25-5 shows the BAEPs obtained on screening and at follow-up for an infant with bilateral sensorineural hearing loss.
A large study has evaluated a similar protocol in over 7,000 infants. This study used accurate ways to calibrate ear-canal sound levels and precise measures of the signal-to-noise ratio to determine when a response was present. More than 99 percent of newborn infants could be tested, and more than 90 percent of these passed the test (at 30 dB nHL). The rest were followed up with more extensive audiometric testing, leading to the identification of sensorineural hearing loss in about 2 percent of the tested population.
BAEP thresholds in the neonatal period closely predict the high-frequency thresholds of the behavioral audiogram obtained when the child is old enough to provide accurate behavioral thresholds. One study defined significant hearing loss on follow-up as a sensorineural loss with thresholds above 40 dB nHL and found that abnormal BAEP results (neonatal BAEP thresholds above 40 dB nHL) correctly detected 98 percent of the hearing-impaired babies (sensitivity), with only 4 percent false-positive results (96 percent specificity). Another defined the target population as those requiring hearing aids and found a sensitivity of 86 percent and specificity of 100 percent.
Between 10 and 30 percent of babies in an NICU have a transient conductive hearing loss that resolves within the first few months of life. , Furthermore, conductive hearing losses may occur by the time of follow-up in a baby who was normal in the neonatal period. Infants who have conductive losses in the neonatal period are more likely to have repeat otitis later than are those with normal neonatal hearing.
Some infants may show transient abnormalities of the BAEP in the newborn period with normal recordings several months later. When the otoacoustic emissions are normal, these findings suggest a transient auditory neuropathy.
Some children with normal thresholds for clicks in the newborn period show significant hearing loss at follow-up. Passing a neonatal screening test is no guarantee that the child’s hearing is normal or that it will continue to be normal. In some infants, sensorineural hearing loss may develop after birth. However, most of the hearing-impaired children who showed normal click-evoked BAEPs in infancy have an audiometric pattern with normal thresholds somewhere between 1 and 4 kHz and significant hearing losses at other frequencies ( Fig. 25-6 ). Normal neonatal BAEPs occurred because the broad-band click elicited a response from the frequency region with normal thresholds.
Auditory steady-state responses may also be used to screen for hearing loss in newborn infants. , Either clicks or modulated noise may be used to evoke responses that can be recorded accurately and quickly in the frequency domain.
Evoked Potential Audiometry
The click-evoked BAEP gives a general indication of hearing but does not provide the frequency-specific thresholds of an “audiogram.” This information is essential to fitting hearing aids. By 6 months of age a normal child will turn to look for a faint sound, and this response can be reinforced by such stimuli as an animated toy on the correctly localized speaker. This approach can provide an accurate assessment of hearing in most children from 6 months to 2 years. As the child gets older, increasingly refined tests become available. Objective testing is necessary in all children before the age of 6 months and in those older children who cannot give reliable thresholds with behavioral testing (e.g., those with cognitive disturbances, emotional disturbance, or multiple handicaps).
The main goal of evoked potential audiometry is to determine thresholds at frequencies between 500 and 4,000 Hz in each ear. Evoked potential audiometry in infants and children usually begins with click stimuli. Thresholds for the BAEP (wave V) are estimated to within 10 dB using rapid stimulus rates (50 to 80 Hz). Under reasonable recording conditions (i.e., a quiet child in a quiet room), the BAEP in a child older than 6 months of age should be detected to within 20 dB of the normal adult behavioral threshold for the click. The threshold for the click-evoked BAEP does not give any information about the response to specific frequencies but can provide a general estimate of hearing, particularly at high frequencies. The next step estimates thresholds at different frequencies by using techniques such as derived responses, tone pips in noise, or steady-state responses.
The most widely studied technique for estimating the audiogram in infants and young children involves recording the BAEPs to brief tones. , A meta-analysis of many such studies indicates that thresholds are estimated between 10 and 20 dB above behavioral thresholds for frequencies between 500 and 4,000 Hz. The variability of the threshold estimation is such that any individual threshold will be within 10 to 20 dB of this mean difference between the physiologic and behavioral thresholds. The basic protocol presents brief tones with alternating onset polarity at a rate of 40 to 50 per second. BAEPs are recorded with a low-frequency cut-off no greater than 30 Hz to obtain two replicate averages of 2,000 trials each. Ipsilateral notch-noise masking will make the thresholds frequency specific. If notch-noise masking is not used, the estimated thresholds may be inaccurate if the slope between adjacent frequencies on the audiogram is steep. Bone conduction thresholds for clicks or tones should be obtained if air conduction thresholds are elevated. Alternating polarity is essential to attenuate the stimulus artifact when recording bone conduction responses.
Auditory steady-state responses can also be used to estimate the audiogram in infants. Because multiple responses can be obtained simultaneously, the time taken to estimate the audiogram is significantly less than that required when using BAEPs to brief tones.
BAEP Evaluation of Specific Hearing Disorders
Bacterial meningitis is the most common cause of acquired sensorineural hearing loss in childhood. All children with meningitis should be assessed audiometrically before leaving the hospital. Because the incidence of this disease is high in the first year of life, the BAEP is often a necessary part of this assessment. The BAEP is probably more accurate than behavioral testing, particularly if the hearing impairment is transient, unilateral, or mild. The incidence of hearing loss in children recovering from meningitis is between 25 and 60 percent when BAEP testing is performed. The hearing loss is conductive in about one-quarter, cochlear in one-half, and retrocochlear in the remaining one-quarter of the children.
The BAEP is essential in the evaluation of infants born with atresia of the external auditory meatus ( Fig. 25-7 ). In a child with unilateral atresia, the status of the normal ear can be assessed easily by BAEPs. During the first few years of life, the BAEP may also be helpful if middle ear disease is suspected in the good ear. In a child with bilateral atresia, the BAEP can demonstrate function of the inner ear (see Fig. 25-7 ) and may suggest (if the scalp topography of the response is asymmetric) that only one inner ear is functioning.
Otitis media and middle ear effusions are very common in the first 3 years of life. It is important to monitor the hearing of children who experience recurrent middle ear disease. Because of the age of the children, monitoring will often require objective tests of auditory function, such as those provided by the BAEP. The conductive hearing losses associated with otitis media show up in the BAEP as elevated thresholds and a delay in all of the waves. The degree of delay can provide an estimate of the amount of hearing loss, although variability in the response may obscure small changes in threshold. BAEPs are very helpful in monitoring infants with cleft palate, who are susceptible to recurrent middle ear effusions.
Ototoxic medications are often used in infancy and childhood, particularly in intensive care nurseries. Because infants and young children are unable to complain of auditory deterioration during the course of treatment, the BAEP may provide an objective means of monitoring hearing and detecting neurotoxicity, particularly if the treatment is prolonged.
Treatment of Sensorineural Hearing Loss
Once sensorineural hearing loss is detected, it is essential to provide the patient with amplification. Fitting hearing aids normally requires frequency-specific and ear-specific estimates of hearing thresholds, acoustic measurements of infants’ external ear (real-ear to coupler differences), and the gain characteristics of the aid. The amplification and compression of the aid are adjusted so that the frequencies and intensities of the normal speech spectrum become audible. A final confirmation of a satisfactory hearing aid is the subject’s ability to discriminate speech with the aid. The most important use of the BAEPs or auditory steady-state responses in the fitting of hearing aids is to provide an accurate audiogram when this cannot be obtained behaviorally. Other possible uses of physiologic testing are to determine maximum levels of amplification and to demonstrate the discriminability of the aided sounds. As they can be evoked by amplitude-modulated tones, which are frequency specific and stable over time, auditory steady-state responses are unlikely to be distorted by amplification in either a sound-field speaker or a hearing aid. These responses may be used to assess aided thresholds ( Fig. 25-8 ). A more important use of the auditory steady-state responses would be to demonstrate the ability of the brain to discriminate changes in the frequency and intensity of amplified sounds.
Cochlear implants can provide stimulation of auditory nerve fibers in children who are so severely hearing impaired that they get little or no benefit from hearing aids. It is customary to record the BAEPs in response to very loud clicks in children being considered for cochlear implants to determine any residual cochlear function. Some of these children show a vertex negative wave at about 3 msec that probably represents activation of vestibular nerve or nucleus rather than any residual function in the hair cells and auditory nerve fibers. Figure 25-9 shows a waveform that may represent this type of response. Electrically elicited BAEPs provide essential information for preoperative evaluation of surviving neural elements in the auditory pathways and for intraoperative assessment of the implant’s function. , After the operation, BAEPs may be used to check the continuing function of the implanted device, to adjust the intensity levels in the external processor, and to monitor the development of the auditory pathways. The waveform of the electrically evoked response is similar to that of the acoustically evoked BAEP. The latencies of the waves are shorter, and wave I usually is obscured by stimulus artifact. Wave V latency is usually about 4.0 msec and changes by only about 0.5 msec from high to low intensity.