Auditory Brainstem Response Testing
For the very young child, physiologic assessment procedures using auditory evoked potentials, such as ABR, are particularly important. The ABR can be used for both identification and assessment of hearing and can provide accurate estimates of threshold sensitivity (26
). Under good recording conditions, and using frequency-specific stimuli, the ABR can provide reliable estimates of sensitivity across the frequency range of hearing (27
) in children of all ages. Although this is the test of choice in children under 6 months of age, the ABR is used for older children who cannot be tested behaviorally. Typical recommendations for the ABR include failure to obtain behavioral audiologic thresholds after two attempts, or a high suspicion that a hearing loss exists and that continued testing would only delay diagnosis.
The ABR follows neuromaturation of the central auditory nervous system and shows wave latency changes until about 18 months of age. Wave I is mature early on in life and show little if any latency change with age, whereas the later waves continue to decrease in latency with increase in age to about 18 months of age, with the most rapid changes occurring in the first 3 months after birth. The interwave latency difference between wave I and V therefore goes from about 5.0 at birth to about 4.0 milliseconds when mature. Thresholds do not change over time if hearing is stable. The ABR is best conducted while the infant is sleeping to eliminate background myogenic noise, which can overshadow the neural response. Young infants can often be tested during a natural sleep, following a feeding. Older infants may need to have sedation. ABR done under sedation is now more routinely done in procedure centers where there is a physician and nursing support for sedation monitoring throughout the test session.
The ABR may be used to estimate an audiogram, but some drawbacks exist. The major limitation is the choice of stimuli available that can be used to elicit a response. Synchronous neural firing of multiple neurons is essential to record a response. The best stimulus to elicit a response is a rapid or abrupt onset stimulus such as a click that stimulates a broad area of the basilar membrane and therefore generates synchronous neural discharge in a large number of neurons. The ABR to click stimuli will provide an overall assessment of the integrity of the auditory pathway and provide a basis on which to start investigating thresholds at specific frequencies.
The click stimulus, however, is not frequency specific but contains energy in a broad frequency range and responses obtained to click stimuli cannot detect frequency-specific impairment. The responses to click stimuli have been found to correlate best with audiometric findings in the higher frequency range from about 1,000 to 4,000 Hz frequency range (28
). Use of this stimulus solely can either underestimate or miss a hearing loss at a particular frequency or frequencies depending on the degree and configuration of the hearing loss (33
). While the use of age-appropriate latency-intensity functions together with the threshold search will help to identify impairments, exact quantification of the impairment at each frequency cannot be done using the click stimulus. Rather more frequency specific or tonal stimuli need to be used.
The stimulus that is commonly used to obtain frequency-specific information is a brief duration tone burst referred to as a tone pip. With the use of this stimulus, fairly good synchronous neural firing can occur. The trade off in becoming frequency specific or tonal by increasing the rise time of the stimulus is the reduction in the synchronous neural discharge. The aim is to achieve a balance of tonality with enough synchronous neural firing to elicit a response. It is necessary to maintain a fast enough rise time to elicit a response yet reduce the acoustic splatter to frequencies above and below the nominal frequency of the stimulus. While producing a stimulus that does not have contributions from other frequencies is important, it will not ensure a place-specific region of excitation on the basilar membrane. Physiologically there is an upward spread of excitation on the basilar membrane as the intensity level of the stimulus is increased beyond 70-dB sound pressure level (SPL) (35
). Spread of energy to frequencies with better hearing will result in an underestimation of threshold level. For children with severe to profound hearing losses, the intensity level of the stimulus will be sufficiently high that there is to be expected some spread of energy on the basilar membrane.
ABRs should be obtained to both air and bone conducted stimuli and thus can provide information about type of hearing loss. Bone conduction should be elicited to at least a click stimulus to assess presence or absence of air-bone gaps. While there are intensity output limitations in bone conduction testing, such testing often confirms the type of auditory impairment. Threshold differences of greater than 15 dB with better bone conduction thresholds than air conduction thresholds are indicative of conductive involvement and warrant further otologic evaluation.
The ABR measures the integrity of a portion of the auditory system through approximately the level of the midbrain and does not measure “hearing” in the true sense of the word. Although it is well established that agreement exists between behavioral thresholds and ABR thresholds for children with typical sensory hearing loss, there
are instances where they will not agree and further investigation is necessary. Examples include cases of normal ABR with no ability to recognize or use sound to “hear.” Conversely, the absence of an identifiable waveform on an ABR test does not necessarily equate to thresholds in the severe to profound range or to “no hearing.” The limits of the ABR equipment should be recognized. Output is more limited than most audiometers used to test behaviorally. Consequently, an absent response on ABR should not be interpreted as the absence of residual hearing. Additionally, the ABR will be affected by the neurologic status of the child. If the auditory system is damaged and neurons cannot fire synchronously, or if there are disruptions of the auditory pathway due to an insult, then the integrity of the ABR may be compromised resulting in no identifiable ABR waveform or a partial waveform with later waves absent, despite the fact that the end organ of hearing, the cochlea, may be functioning normally. With the availability of technology to monitor OAE from the cochlea, discrepancies between behavioral audiologic findings and the ABR, where behavioral audiologic findings show better responsivity to sound than can be predicted by the ABR, are being substantiated by the presence of OAEs. These hearing losses are either auditory neuropathy or neural hearing loss. Auditory neuropathy represents good outer hair cell function but a disruption in the peripheral nerve, whereas neural hearing loss is a breakdown in nerve transmission that occurs more centrally.
Auditory Neuropathy Spectrum Disorder is a term that found more and more frequently in the literature and is being used indiscriminately in cases where OAEs are present and the ABR is absent or abnormal. Rapin and Gravel (36
) delineate clearly the distinction between neural, sensory, sensorineural, central hearing losses, and auditory neuropathy. They point out that neuropathy means that there is peripheral nerve pathology, yet much of the literature contains cases where this was not shown. For example, neonates with hyperbilirubinemia and resulting hearing losses show abnormal ABRs and normal OAEs, yet the primary site of pathology is the central auditory pathway, that is, vestibular and cochlear nuclei and basal ganglia and not the VIII nerve, yet it is often said that hyperbilirubinemia is a risk factor for auditory neuropathy. Combining all children with abnormal ABRs and present OAEs into one category will present challenges for managing the hearing losses effectively. Furthermore, until there is clearer differentiation, we will be unable to help families understand their child’s hearing loss and expected outcomes. This is clearly an area where the audiologist and otolaryngologist need to work closely together to ensure proper diagnosis of hearing loss.
An emerging physiologic evoked potential test, the auditory steady-state response (ASSR), holds promise for predicting frequency-specific thresholds in individuals who cannot provide reliable or valid behavioral thresholds, for example, in young infants and children (38
). The advantage of this measure is that it is truly an objective measure because the response presence or absence is based on statistical analysis and not on visual detection methods as with the ABR.
The generation of the 80-Hz ASSR, like the ABR, is believed to be primarily in the brainstem (42
). ASSR uses relatively tonal stimuli (carriers) that are amplitude- and/or frequency-modulated to evoke a response and can provide accurate frequency-specific estimates of air conduction hearing levels. For pediatric applications, the modulation occurs at a frequency appropriate for infants and children (about 80 to 100 Hz). Single or multiple stimuli presented simultaneously have been used to elicit the ASSR (43
). Furthermore, the 80 Hz ASSR can be obtained in infants and young children who are asleep (44
). The test has the ability to monitor both ears simultaneously, making it attractive for possible reduction in test time compared to the ABR, although more research is needed in this area before conclusions can be drawn as to whether there is truly a time savings. Threshold prediction using the ASSR conducted on adults and children with hearing loss has been shown to provide fairly accurate estimates of the behavioral audiogram (41
Hearing thresholds have been estimated within about 10 to 15 dB in adults with normal hearing and hearing loss using the multifrequency ASSR (39
). Fewer data, however, exist regarding the use of the ASSR for measurement of hearing in infants and children. Perez-Abalo et al. (53
) found that although they were able to determine hearing loss in the severe and profound range, only fair agreement was seen between ASSR thresholds and hearing levels in children with mild hearing loss or normal hearing.
Few data exist regarding the use of ASSR employing bone conduction stimulation. Small and Stapells (54
) used multiple bone conduction stimuli that were both frequency and amplitude modulated in a group of preterm infants (32 to 43 weeks) and a group of full term infants (0 to 8 months). The results obtained in infants were different from those obtained in adults; in infants, the threshold estimates were better in the lower frequencies and poorer in the higher frequencies when compared to adult data. Swanepoel et al. (55
) evaluated bone conduction stimulation in a group of older children and concluded that the stimulus artifact prevents determination of type of hearing loss in most cases of sensorineural hearing loss but interferes less when conductive hearing losses are present. However, results vary with frequency in both cases.
The influence of age on the ASSR suggests that maturation has some bearing on threshold determination in very young infants. Rance and Rickards (56
) found that the prediction of hearing thresholds was similar between young (1 to 8 months of age) infants and older subjects with hearing loss. However, results obtained from infants with normal hearing have suggested that maturational factors, which are sufficient to affect the differentiation
between normal hearing and mild-to-moderate hearing loss, may influence the findings of ASSR assessments carried out in the first few weeks of life (38