Pediatric Audiology and Implantable Hearing Devices
David H. Chi
Diane L. Sabo
Universal newborn hearing screening (UNHS), the lower age of identification of hearing loss, and the need to move rapidly through the evaluation and management process have resulted in a change in pediatric audiology. The population of children who are referred for audiologic evaluations now has a higher percentage of young infants. This has resulted in a greater reliance on physiologic measures for hearing assessment and changed the counseling that needs to be done when hearing loss is identified. Prior to newborn hearing screening, most parents were referred or sought audiologic evaluations for their child who had delays in speech development. Now, with newborn hearing screening, the child presents soon after birth, and families have not had time to think about the possibility of hearing loss or the implications of their child having a hearing loss. The clinician has to consider the impact on the family that a child with a hearing loss can have. In addition, all members of the health care team need to work cohesively to unify terminology, results, and recommendations and to lessen any confusion the families may have when they hear differences. It is important, therefore, for the otolaryngologist and audiologist to have a close working relationship with respect to the diagnosis and ongoing treatment of children with hearing loss.
Congenital hearing loss occurs commonly and is more frequent than other conditions for which newborns are routinely screened (1). Data from newborn hearing screening programs show that the incidence of hearing loss is twice as high as all other diseases screened at birth, occurring in about 2 to 4 per 1,000 births compared to about 1.2 per 1,000 births for the combined totals for all other diseases (2, 3, 4, 5). In the United States, about 8,000 to 12,000 babies are born each year with permanent hearing loss (compiled by the National Institute on Deafness and Other Communication Disorders [NIDCD]). When combined with progressive, unilateral, and conductive hearing loss, as many as 15 per 1,000 will have some degree of hearing dysfunction (6).
Hearing loss in infants, when left undetected, can affect speech and language acquisition leading to poor language and literacy skills. In addition, other areas can be impacted, including academic achievement and social and emotional development (7, 8, 9, 10, 11). If hearing loss is detected early, however, poor outcomes can be diminished, and even eliminated, through early intervention. Studies have shown that communication skills comparable to age-matched hearing peers can be achieved when hearing loss is identified early, and children receive early intervention services (12, 13, 14, 15).
NEWBORN HEARING SCREENING
Identification of hearing loss in infants and children has gone through tremendous changes over the past 10 years. The single most influential reason for this change is UNHS. With implementation of UNHS, hearing loss is being identified at earlier ages compared to prior to newborn hearing screening. A recent study demonstrated that for infants screened for hearing loss compared to those unscreened there was a 24.8-month difference in median age at diagnosis of hearing loss, a 23.6-month difference for fitting of amplification, and a 19.9-month difference for enrollment in intervention (16).
Early hearing loss detection and intervention has been a goal for almost 40 years. Previously, risk factor screening was the only means available to try to identify infants with hearing loss. A 1972 statement by the Joint Committee on Infant Hearing (JCIH) (17) delineated the first high-risk factors for hearing loss and recommended following infants with these high-risk factors: history of hereditary childhood hearing impairment; congenital infection acquired in utero, such as rubella or other nonbacterial fetal infection like cytomegalovirus (CMV), syphilis, and herpes; craniofacial anomalies including those with morphologic abnormalities of the pinna and ear canal; birth weight less than 1,500 g; and a bilirubin level greater than 20. In 1982, bacterial meningitis and severe asphyxia were included with
additional risk indicators added between 1982 and 1994. Although this list was expanded and the name changed to risk indicators, risk indicator screening identified only 50% of infants with significant hearing loss.
additional risk indicators added between 1982 and 1994. Although this list was expanded and the name changed to risk indicators, risk indicator screening identified only 50% of infants with significant hearing loss.
Because hearing loss affects a large number of children and risk indicator screening is ineffective, and age of identification and habilitation of hearing loss is important, the National Institutes of Health’s (NIH) Consensus Development Conference on Early Identification of Hearing Loss (18) concluded that all infants should be screened for hearing impairment, preferably prior to hospital discharge. The following year, JCIH endorsed the goal of universal detection of hearing loss in infants and recommended that physiologic tests be used. The JCIH (19) position statement became a comprehensive guide for early hearing loss detection and intervention for hearing loss and was subsequently revised in 2007. Some of the changes in the 2007 position statement were the expansion in the definition of hearing loss to include auditory neuropathy/dyssynchrony for infants admitted to the neonatal intensive care unit (NICU). Prior to this, the definition included only permanent bilateral, unilateral sensory, or permanent conductive hearing loss. The 2007 position statement also included separate protocols for NICU and regular-care nurseries to address neural hearing loss that occurs more commonly in the NICU. Infants in the NICU for more than 5 days are to have auditory brainstem response (ABR) included as part of their screening so that neural hearing loss will not be missed. For NICU infants who do not pass the ABR, the recommendation is that they are referred directly to an audiologist for rescreening and/or comprehensive evaluation including ABR testing. In addition, if any infant is readmitted during the first month of life for conditions associated with potential hearing loss (e.g., hyperbilirubinemia that requires exchange transfusion or culture-positive sepsis), a repeat hearing screening is recommended before discharge.
TABLE 100.1 JCIH RISK INDICATORS ASSOCIATED WITH PROGRESSIVE OR DELAYED ONSET OF HEARING LOSS IN CHILDHOOD | ||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ||||||||||||||||||||||||
The 2007 Position Statement by JCIH (9) also recommended that all newborns be screened for hearing loss by 1 month of age. If the child fails the screening process, an audiologic diagnostic evaluation should occur before 3 months of age, and when a hearing loss is confirmed, the child should be enrolled in early intervention before 6 months of age. These guidelines, commonly referred to as the 1-3-6 model, have been widely adopted by such organizations as NIH/NIDCD, Health Resources and Services Administration/Maternal and Child Health Bureau, American Speech-Language Hearing Association (ASHA), American Academy of Audiology (AAA), and American Academy of Pediatrics (AAP). The JCIH 2007 (9) also contains a section on the role of otolaryngologist in the care and evaluation of newly diagnosed hearing losses in infants.
Because hearing loss can occur after the child leaves the hospital, the JCIH also recommends that periodic audiologic examination of infants be conducted to detect delayedonset hearing loss. Children with indicators of progressive or delayed-onset hearing loss should be referred for an audiologic assessment at least once by 24 to 30 months of age. Children with risk indicators that are highly associated with delayed-onset hearing loss, such as having received extracorporeal membrane oxygenation (ECMO) or having CMV infection, should have more frequent audiologic assessments.
JCIH risk indicators that are associated with progressive or delayed onset of hearing loss in childhood are listed in Table 100.1.
SCREENING BEYOND THE NEWBORN PERIOD
Given that hearing loss can occur at any time or be of a progressive nature, screening for hearing loss beyond the newborn period is important. In fact, screening for hearing loss in children and middle-ear disorders has been around for many years in schools, clinics, and health departments. Although there is this long history, variability in procedures remains. Routine hearing screening often starts with the preschool-age child who can perform developmentally appropriate pure-tone procedures. These screenings are conducted in daycares, preschools, and in some primary care physician offices. There is currently an initiative to capture the 0- to 3-year-old population who attend early head start programs using otoacoustic emissions (OAE) as the screening tool (20).
Hearing screening in the school-age population has had a variety of policies and procedures as not all states have legislative mandates, and policies can be written by individual school districts or health departments. The AAA (21) and the ASHA (22) recommend pure-tone screening at the child’s initial entry into school, annually in kindergarten through third grade, seventh grade, and eleventh grade. Screening should be conducted at 20-dB HL for frequencies of 1,000, 2,000, and 4,000 Hz. Failure to respond at any frequency in either ear necessitates rescreening. Failure on rescreening requires a referral for audiologic evaluation within 1 month of rescreening and no later than 3 months after the initial screening.
Routine screening of middle-ear disease is not recommended as a stand-alone screening but may be done as part of a comprehensive hearing screening protocol in children who are at risk for otitis media (23). Because of the transient nature of middle-ear disease and resolution of the middle-ear disease without treatment, routine tympanometric screening can lead to too many children being referred to their physicians without middle-ear effusion at the time of the visit.
DEVELOPMENT OF AUDITORY BEHAVIOR
Although infants are born with the ability to hear, some developmental changes occur with respect to their perception of sound, and neural processing ability continues to develop well into childhood and adolescence. The auditory system is intact early and becomes functional during the third trimester at around 25 weeks gestation (24). The central pathway, however, continues to develop well into adolescence with increase in myelinization and neural pathway development. Brainstem myelinization is complete by 1 year of age but the central structures complete myelinization at 10 years or older. These changes can be reflected in evoked potentials that undergo changes in amplitude and latency of the responses, although not in threshold.
Early on in life, infant’s responses to sounds are often of a reflexive nature until they develop the motor control for a more organized response. With increasing age, the intensity level of the auditory stimulus that is needed to elicit a response from the infant decreases. Beginning about 5 to 6 months of age, when formal audiologic techniques can be utilized, adult-like values can be obtained to auditory stimuli. These changes in response levels are not indicative of sensitivity differences but in the minimal stimulus level needed to elicit a response.
Infants begin to localize sounds first in horizontal and then vertical planes. Between 3 and 6 months of age, infants search for the sound, respond to the sound of their name, and respond differentially depending on the tone of voice the parent is using. By about 5 to 6 months, formal and reliable behavioral-audiologic testing can begin. Between 6 and 10 months, infants can seek out the source of sound and respond to common sounds in their environment. They respond to both loud and soft sounds and pay attention when the parent talks to them. Between 10 and 15 months of age, babbling increases and begins to more closely resemble speech. Between 15 to 18 months, a baby is able to directly localize to most sounds, understand simple phrases, identify familiar objects such as body parts, and follow simple directions. A child at 18 months should have an expressive vocabulary of 20 or more words and short phrases.
TEST BATTERY
For pediatric assessment, the cross-check principle, originally developed by Jerger and Hayes (25), is utilized. Use of the cross-check principle helps to minimize mistakes in diagnosis and develop an audiometric profile that best represents the child’s auditory abilities. This principle relies on a combination of behavioral and physiologic tests to provide a comprehensive picture of hearing sensitivity and auditory status. Behavioral tests, although usually considered more subjective because of their reliance on patient participation, are combined with the results of objective physiologic tests that do not require active participation by the child/infant. At very young ages or in the case of significant developmental delay, the electrophysiologic test (e.g., ABR) findings often predominate in the initial decision making about the management of the child with a hearing loss. As the child develops and can participate in behavioral testing, the behavioral findings will serve as validation of electrophysiologic findings, as well as document sequential development of auditory skills. For older children with developmental skills adequate for behavioral testing, audiologic findings with or without ABR supporting data are used to determine management of the hearing loss.
The goal of behavioral audiologic assessment is to establish the degree and nature of any hearing loss for each ear so that management of hearing loss, including
hearing aid fitting, can be done. The evaluation includes a comprehensive history, observation of the child’s awareness of or use of sound, as well as multiple tests to assess various aspects of the auditory system, including cursory otoscopic examination, tympanometry, assessment of acoustic reflexes (ARs), and behavioral audiometric testing (air conduction and bone conduction) to both speech- and frequency-specific stimuli.
hearing aid fitting, can be done. The evaluation includes a comprehensive history, observation of the child’s awareness of or use of sound, as well as multiple tests to assess various aspects of the auditory system, including cursory otoscopic examination, tympanometry, assessment of acoustic reflexes (ARs), and behavioral audiometric testing (air conduction and bone conduction) to both speech- and frequency-specific stimuli.
SPECIAL TESTS
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, 29, 30, 31, 32). 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, 34). 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.
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, 37) 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, 39, 40, 41). 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, 45, 46, 47, 48, 49, 50, 51).
Hearing thresholds have been estimated within about 10 to 15 dB in adults with normal hearing and hearing loss using the multifrequency ASSR (39, 52). 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, 47, 57, 58, 59, 60).
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, 47, 57, 58, 59, 60).
Recently, Rance and Tomlin (61
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
