Infant hearing loss is being diagnosed at an ever-decreasing age due to universal newborn hearing screening programs. It is important to have a careful plan for rapid, accurate, and comprehensive audiological diagnosis. The essential elements of the assessment plan are relatively unvarying; however, there are patient variables that require the need for flexibility and modification of the diagnostic approach. Included in these variables are whether the baby was in a neonatal intensive care unit (NICU) and for what reason(s), other high-risk factors (see Table 1), presence of parent concern regarding hearing loss, additional handicapping conditions, and age (gestational as well as developmental) at the time of evaluation. The following discussion will serve to provide general information and guidelines regarding the diagnosis and amplification of pediatric patients.
- Caregiver concern regarding hearing, speech, language or developmental delay
- Family history of permanent childhood hearing loss
- NICU of greater than 5 days' duration
- In-utero infections (CMV, herpes, rubella, syphilis, toxoplasmosis)
- Craniofacial anomalies
- Physical findings associated with a syndrome known to include hearing loss
- Syndromes associated with hearing loss
- Neurodegenerative disorders
- Infection associated with sensorineural hearing loss
- Head trauma
- Chemotherapy
Diagnostic Assessment
Audiologic diagnosis can be grouped into two general categories: objective measures and behavioral measures. Both play an important role in the diagnosis of hearing loss: the objective measures play the predominant role in early weeks and months, while in later years, the tests of choice generally involve behavioral measures.
Objective Measures
Auditory Brainstem Response (ABR)
In the child who is under 6 months developmental age, the single-most important objective measure is the Auditory Brainstem Response (ABR) using air- and bone-conducted tone burst stimuli. Toneburst stimuli are preferred due to their greater frequency sensitivity compared to a click stimulus; particularly critical is obtaining information about the 2000 Hz frequency region due to its importance for speech understanding. The ABR should be accompanied by assessment with Otoacoustic Emissions, (OAEs) either distortion product or transient evoked, and measurement of acoustic immittance (tympanometry and acoustic reflexes).
Knowing that the baby may awaken at any moment during assessment, it is important to prioritize the test sequence to obtain critical information from each ear first, i.e., a 2000 Hz tone burst ABR. If normal sensitivity can be determined at this frequency, the decision about the necessity for hearing aids will be clearer.
Use of a click stimulus alone does not provide sufficient information for the fitting of hearing aids, due to lack of frequency specificity. While the click generally reflects average thresholds in the high frequencies, a low-frequency, mid-frequency or high-frequency hearing loss can be missed. A recent study, however, suggests that use of a 250 Hz tone burst plus a click stimulus is a reasonable hybrid protocol (Gorga, Johnson, Kaminski, Beauchaine, Garner, & Neely, 2006) that allows good estimation of low- and high-frequency hearing sensitivity.
It is also possible, and in some cases preferred, to begin with bone-conduction ABR testing, accompanied by measures of acoustic immittance. This test, coupled with tympanometry and otoacoustic emissions, may be sufficient to suggest that a failed newborn hearing screening represented middle ear effusion or outer ear debris, and that otolaryngologic, but not necessarily audiological follow-up, is warranted.
This ambitious, but necessary, test battery will typically require more than one visit. ABR testing must be accomplished while the baby sleeps, or is sedated, while the baby must be alert for Behavioral Observation Audiometry (BOA) or Visual Reinforcement Audiometry (VRA) testing. Sleep-deprived and hungry infants can be persuaded to sleep in the clinic long enough to obtain necessary information in one to two test sessions. At least one ABR should be conducted on every child who is to be fitted with hearing aids, to ensure that there is independent, objective confirmation of hearing loss in each ear separately across the frequency range. It is important that a carefully-designed protocol for infant assessment, including clearly defined stimulus and recording parameters, be utilized to ensure reliable and valid information is obtained.
If an ABR test battery results in "no response", what can be concluded? Severe to profound hearing loss would be a likely diagnosis; however, auditory neuropathy (AN) must also be ruled out. While most cases of AN occur in NICU graduates, it is possible for a well baby to have AN as well (Rance, et al., 1999)
In cases where there is no response to a tone burst ABR, assessment for AN is indicated. This is accomplished by presenting a click stimulus and assessing the response to the stimulus in each of the two polarities (condensation and rarefaction) separately. In an ear exhibiting AN, there will be an early response that reverses with reversing stimulus polarity, such that, when summed, will result in an essentially flat line (Kraus et al., 2000). This reversal to polarity represents a pre-neural response classified as the cochlear microphonic; the ABR is a neural response that does not significantly change with change in the polarity of the stimulus. In AN, the OAE is typically present; however, if the baby also has middle ear effusion, the OAE may be obscured. There could also be a concomitant sensory loss.
Auditory Steady-State Response (ASSR)
The Auditory Steady-State Response (ASSR) is another important tool in the objective test battery. The ASSR yields an estimate of auditory thresholds across the frequency range with good frequency specificity using a tone, rather than a tone burst, as the stimulus (Kuwada, Batra, & Maher, 1986). The ASSR, however, is not an appropriate tool for newborn screening, nor should it be used as the sole diagnostic tool when there is a possibility of AN, when hearing is normal, or when the baby is less than one month of age.
When AN is present, neither the ABR nor the ASSR will provide threshold information. Because the neural response is abnormal, peripheral hearing may be "normal", yet the response from ABR evaluation, which reflects neural synchrony beyond the cochlea will, by definition, be abnormal (Kraus et al., 2000). In such cases, fitting of amplification must be made based on other information, such as behavioral responses to sound.
Immittance
Acoustic immittance measures (tympanometry and acoustic reflex thresholds) provide important information about the presence of a conductive component. Immittance measures must also be recorded using parameters suitable for infants. Use of a high-frequency probe tone if the baby is less than 5-7 months old is mandatory; tympanograms and reflexes recorded with conventional low frequency probe tones can provide inaccurate results (Kei et al., 2003). While abnormal tympanometric results signify the need for medical assessment, the presence of normal tympanometry does not rule out a conductive component, as normal tympanograms can be recorded in ears with ossicular abnormality.
OAEs
OAEs must be assessed to ascertain whether a cochlear response exists from the outer hair cells. This is particularly important when there is an ABR that indicates severe-profound hearing loss, or a "no-response" ABR, which may be consistent with AN. Babies with AN will typically exhibit cochlear activity (otoacoustic emissions and/or the cochlear microphonic) in the presence of a severely abnormal ABR. It is important to emphasize, however, that the presence of OAEs does not rule out mild cochlear impairment, nor does it rule out auditory neural impairment.
Behavioral Measures
Finally, in the assessment of the pediatric patient, BOA is important for at least two reasons: 1) to serve as a cross-check of objective measures (what about the baby who consistently startles to a moderately-high intensity stimulus when there is a "no response" ABR? Or the baby with a bilateral, normal ABR who shows no awareness of sound?); and 2) to demonstrate to parents that their child does not respond to sounds that they find to be loud, even uncomfortable, in the test booth. It should be noted that BOA, however, is known to result in an unacceptably high level of false positive and false negative responses, and as such, its use as a primary diagnostic tool is not appropriate (Northern & Downs, 1991).
Diagnosis and Counseling
One of the primary reasons for the advent of newborn hearing screening was the abysmally late average age of diagnosis of pediatric hearing impairment. One of the primary reasons why deaf infants were formerly diagnosed as "autistic" or "retarded" was that imperfect test procedures were routinely utilized. Now that there is excellent evidence of the sensitivity and specificity of objective measures of hearing in infants, there is no reason to delay diagnosis and treatment.
Perhaps the biggest challenge in infant diagnosis is the skill to communicate the diagnosis to parents. Professional knowledge and skills, a calm and serious demeanor, accompanied by empathetic listening and sensitivity to parental concerns constitute just some of the skills necessary to communicate this important and potentially life-changing information to parents. Parents have many questions; it is the audiologist's responsibility to answer initial questions and concerns and to make appropriate referrals (e.g., genetic counseling, educational intervention). It is important that audiologists know the requirements for referrals in their states (in California, a referral to the Department of Education is required within 48 hours of the diagnosis). Information about communication mode (oral, signed, Total Communication) should be presented in such a fashion that parents have necessary information and resources to make their own determination regarding communication preferences. Being prepared to deliver diagnostic information in a confident and professional fashion is an essential aspect of pediatric audiology.
Of equal, perhaps greater, importance is the role that the audiologist plays in counseling the parents regarding treatment options. While there is substantial literature supporting the early fitting of amplification for best language outcomes, there is some controversy over the role of the audiologist in making treatment recommendations. Some parents and other stakeholders hold the belief that educators and/or other parents of hearing impaired or deaf children are the appropriate individuals to counsel parents of newly-diagnosed children about communication options. Additionally, in the quest for "parent-centered" treatment, there are some who would argue that parents have the right to delay or deny the fitting of amplification for their infants. This choice, while perhaps understandable coming from grieving parents or parents who wish their child to be part of the Deaf Community, places the audiologist in a difficult dilemma: providing the known benefits of early amplification vs. respecting the rights of parental choice. There is undoubtedly no "right" answer to this quandary, but awareness of the issues surrounding treatment planning for deaf children is an important issue of which the audiologist must be cognizant. Related counseling regarding speech, language, auditory and academic development may be warranted, as well as information regarding communication options and early intervention programs.
Fitting and Verification of Hearing Aids in Children
The primary intervention for children with hearing impairment is hearing aids, regardless of the type of loss (conductive, cochlear, auditory neuropathy, or mixed). Unless there is clear contraindication, hearing aid fitting should be accomplished as early as possible in the child's life (JCIH, 2000). The FDA requires medical clearance to dispense hearing aids in children, regardless of the fact that cochlear hearing loss is not medically or surgically treatable; however, taking ear impressions and fitting loaner hearing aids should be promptly accomplished. Infants and children with hearing impairment should be fitted without delay even when there is middle ear effusion in addition to permanent hearing loss, when the audiogram is "incomplete" or when the child is too immature to yield valid and reliable behavioral responses.
What information is minimally necessary to begin the fitting process? Ear-specific thresholds by ABR or ASSR for at least 500 Hz and 2000 Hz, with an estimate of conductive component if any, provides reasonable data upon which to provide an initial fitting. Even if ABR indicates minimal, unilateral or "no response" hearing loss, and even if ABR suggests auditory neuropathy, hearing aids are the first line of treatment unless it can be shown that amplification is detrimental to auditory skills development. Because AN is a heterogeneous condition, it is difficult to predict which children will show benefit from conventionally fitted hearing aids, and which will not (Rance, Cone-Wesson, Wunderlich & Dowell, 2002). It is prudent to embark on a conservative, carefully monitored hearing aid fitting, while obtaining regular feedback from parents, teachers and speech-language pathologists about observable differences in auditory behavior with and without amplification.
Unless medically contraindicated, binaural hearing aids are always indicated for bilateral hearing losses, and behind-the-ear style devices will typically be most appropriate. Gain and output requirements, as well as selection of necessary and optional features, can be determined based on the infant's hearing loss and ear anatomy, parent preferences, and the requirements of the third-party payer, if any.
In general, hearing aids for children with mild to severe hearing loss should employ wide-dynamic-range compression (Palmer and Grimes, 2005), multiple compression bands for optimal frequency shaping, and a wide frequency bandwidth. Automatic feedback suppression is helpful, and while a directional microphone can be beneficial in some settings for an older child, its use should be avoided in the infant. Other options, such as volume control locks and locking battery doors, as well as retention cords, well-fitted earmolds, and accessibility for direct audio input are important.
Verification of the appropriateness of the hearing aids must be accomplished relative to the child's individual hearing thresholds for air- and bone-conducted stimuli, and relative to the child's individual ear canal acoustics as measured by the real-ear-to-coupler difference (RECD). Use of an evidence-based pediatric fitting protocol, such as the Desired Sensation Level (DSL) (Seewald, Zelisko, Ramji, Jamieson, 1991, Scollie, et al. 2005) is important to ensure audibility across the frequency spectrum, and a safe output level.
In view of the fact that decisions about cochlear implantation are best made early in the child's life, it is important that appropriate hearing aid fitting and consistent hearing aid use be accomplished without delay. Should the child show little or no gain in auditory development with hearing aids, and should the parents desire, cochlear implantation is optimally performed earlier rather than later in the child's life. Nicholas & Geers (2006) looked at the spoken language skills of 76 children with severe to profound hearing loss. They had been identified and fitted with hearing aids between 1 and 30 months of age and implanted between 12 and 38 months of age and had used a cochlear implant for at least 7 months. The results indicated that early identification and intervention may not be sufficient for optimizing spoken language for children who are profoundly deaf, unless it leads to cochlear implantation. Additionally, the earlier the age of implantation, the better the language skills of the child.
In addition to young age, children who receive cochlear implants show the best outcomes in terms of auditory and speech development when enrolled in an Auditory/oral or Auditory-Verbal communication program (Geers, A.E., Nicholas, J.G. & Sedey, A.L., 2003). It is important that parents recognize that significant post-implant therapy is required for optimal outcomes (this statement could be equally applied to children wearing hearing aids and/or using American Sign Language as their primary language!).
On-going diagnostic evaluations, hearing aid adjustments and/or cochlear implant mappings, and regular communication with the educational or early intervention program are critically important for best outcomes. While the initial hearing aid fitting is an important milestone in audiologic treatment, on-going validation of the hearing aid fitting, the educational plan, the communication mode, and other factors deserve equal emphasis. As hearing loss in children may not be stable, the educational plan or communication mode initially determined to be the best option may prove to need modification over time; this can only be determined by regular monitoring and on-going validation. The audiologist plays an important role in providing care for the child amongst other team members: teachers, other early interventionists, physicians, psychologists, and most critically, parents.
Conclusion
In summary, pediatric diagnosis is best accomplished early, swiftly, and with adherence to protocols that have been shown to be optimal for infants and young children. Amplification must be provided as soon as possible, acknowledging the important role that parents play in the process. Some infants require otolaryngologic treatment, and this should be sought as soon as there is question about medically-treatable conditions. It is recommended that the fitting of amplification should not be unduly prolonged when there is a minimal degree of middle ear effusion. Medical clearance is currently a legal requirement for pediatric amplification; however, as the skills of pediatric audiologists are increasingly recognized by state licensing boards and the FDA, this antiquated requirement will, it is hoped, eventually be abandoned.
While initial amplification should not be delayed, it is important to continue the diagnostic process to refine test results. This is particularly important as an infant's hearing loss can progress over time, and the child can also experience intermittent middle ear effusion. Regular diagnostic re-evaluations, regular hearing aid verification and validation, and long-term rehabilitation are all necessary aspects of pediatric audiology.
Parent counseling is of great importance both when the diagnosis is made, and in setting up and conducting the treatment program. Parents require counseling about benefits and limitations of hearing aids and cochlear implants, reasonable expectations, and troubleshooting malfunctioning equipment. In addition, audiologists should be prepared to provide information to parents regarding potential communication options and educational programs.
It is always important to bear in mind the ultimate reason for newborn hearing screening, diagnosis and treatment: to allow the child to develop language and a communication system that permits parent-child bonding, psychosocial development, sound peer relationships and academic achievement.
References
Geers, A.E., Nicholas, J.G. & Sedey, A.L. (2003). Language skills of children with early cochlear implantation. Ear Hear, 24(1 Suppl), 46S-58S.
Gorga, M.P., Johnson, T.A., Kaminski, J.R., Beauchaine, K.L., Garner, C,A, & Neely, S.T. (2006). Using a combination of click- and tone burst-evoked auditory brain stem response measurements to estimate pure-tone thresholds. Ear Hear, 27(1), 60-74.
JCIH (2000). Joint Committee on Infant Hearing Year 2000 Position Statement - Principles and Guidelines for Early Hearing Detection and Intervention Programs. Audiology Today, Special Issue August. Retrieved on February 6, 2006 from www.audiology.org/NR/rdonlyres/FCBA163D-8A22-4854-A73C-ED694EF8ED23/0/jcihearly.pdf
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Palmer, C., Grimes, A. (2005). Effectiveness of signal processing strategies for the pediatric population: a systematic review of the evidence. Journal of the American Academy of Audiology, 16, 505-514.
Rance, G., Beer, D.E., Cone-Wesson, B., Shepherd, R.K., Dowell, R.C., King, A.M., Rickards, F.W., Clark, G.M. (1999). Clinical findings for a group of infants and young children with auditory neuropathy. Ear Hear, 20, 238-52.
Rance, G., Cone-Wesson, B., Wunderlich, J. & Dowell, R. (2002). Speech perception and cortical event related potentials in children with auditory neuropathy. Ear Hear, 23, 239-253.
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