Integrating Physiologic Technologies for Hearing Evaluation in Infants and Small Children: An Overview.

Everyday in the United States, 33 babies are born with severe to profound hearing loss (1-3 in 1,000 births). It is estimated that another 33 babies are born everyday with mild to moderate hearing loss. Fortunately, universal hearing screening is becoming mandatory across most of the United States, allowing early detection of hearing status and appropriate early (less than three months) intervention.

When hearing loss is detected later, portions of the critical period for stimulating and developing auditory pathways of the brain are lost. Additionally, speech and language development is often delayed in children in whom hearing loss is not detected and managed earlier.

As of July 2000, 31 states have adopted legislation and infant hearing bills are pending in 8 states. This leaves only 11 states (and the District of Columbia) without current provisions addressing or mandating universal screening programs. These programs are successfully providing the first step in early intervention in children.

However, with more and more babies being identified within days and weeks of birth, new problems are arising. Among them, how do we accurately estimate the degree of hearing loss? Of course, determining the type and degree of hearing loss, as best we can, is necessary before effective and appropriate intervention can be initiated.

Three clinical tools available to the audiologist; auditory evoked responses, (AERs), otoacoustic emissions (OAEs) and middle ear analysis (MEA), can be combined to provide reasonably specific information regarding the type and degree of hearing loss present in infants and small children.

Many audiologists are already familiar with these technologies. Nonetheless, for some, these technologies will need to be reviewed to help develop more effective identification and habilitation programs for children identified via universal screening programs.

I. Pediatric protocol - combining tests:

When evaluating infants and small children, it is useful to adopt a battery of tests rather than one specific test. We all remember the famous 'cross-check' principle by Jerger and Hayes. This principle states the results of any single audiometric test cannot be considered valid without verification from another test (Jerger & Hayes, 1976). Techniques such as visual reinforcement audiometry (VRA) are available and are usually successful tools for children 6 months of age and older. However, during the first few months of life, behavioral audiometry tests are difficult to perform and the results are difficult to interpret. Therefore, behavioral tests used for this population are somewhat unreliable. This is partially why audiologists are relying more on physiologic tests for hearing evaluation with newborns. Physiologic tests can provide more objective estimates of hearing.

The most frequently used tests included in the pediatric battery are; auditory evoked responses (AERs), otoacoustic emissions (OAEs), and middle ear analysis (MEA). Clearly, whenever possible, behavioral audiometry should be attempted. Even if the only response obtained is a 'startle response', that is useful information and is encouraged. Physiologic tests do not, nor will they ever, replace the traditional, behaviorally-based hearing test. With presently available equipment, audiologists can usually obtain AERs, OAEs and MEA with relative ease, on most newborns and small children.

AERs:

Auditory evoked responses (AERs) are of interest to audiologists because they provide information regarding how the ear functions and how the brain responds to auditory stimuli.

Early responses, occurring within the first 10 ms post-stimulation include the cochlear microphonic, summating potential, action potential and the auditory brainstem response (ABR, from the auditory nerve and lower brainstem). Air Conduction and bone conduction click-evoked ABRs and/or tone burst ABRs can be recorded at birth, at or near threshold levels equivalent to 30 dB nHL. The air conduction ABR is highly repeatable and is a very powerful tool when used in auditory screenings.

Three events must occur for an ABR to be obtained:

1- Sound (stimuli) must reach the cochlea.
2- The auditory brainstem (and related structures) must respond to cochlear activation and,
3- related auditory nerves must fire in a synchronous fashion.

Although the air conduction ABR is the most popular of the AERs used to evaluate audition in infants and small children, there are many other AERs available to the audiologist for more in-depth analysis of the auditory system.

Electrocohleography (ECochG) can be useful to help determine whether a Wave I response from the ABR is truly present on a particular child's ear. Of course, ECochG has other clinical applications as well and is rarely used in newborn and small children applications.

Middle latency responses (MLR, from the primary auditory cortex and the auditory thalamus) occur between 15 and 50 msec and late latency responses (LLR, from the primary auditory and association cortex) occur between 75-200 msec. The MLR and the LLR help provide information relating to higher levels of auditory/CNS functioning. Rather than simply measuring a response to an acoustic stimuli, these AERs may help measure some aspect of auditory processing of sounds.

In essence, the earlier the evoked response, the more distal its origin. For example, the cochlear microphonic, the summating potential and the action potential all occur within a few milliseconds of the stimulus and are intimately related to the condition of the cochlea, the tympanic membrane and the middle ear structures. The MLR and the LLR are thought to involve more neural activity from the brain itself, and therefore represent higher levels of auditory response.

OAEs:

Otoacoustic emissions (OAEs) provide useful clinical information about outer hair cell function specifically and hearing in general. One would expect to see OAEs present in ears with hearing sensitivity better than 30-35 dB. OAEs can be used as a screening test for hearing loss and can be used in tandem with ABR in the diagnosis of auditory neuropathy. Additionally, OAEs can provide a useful and powerful early warning system regarding hearing loss associated with ototoxicity.

The presence of an OAE demonstrates functioning outer hair cells but does not necessarily indicate hearing thresholds. If the outer hair cells are functioning normally, OAEs may sometimes be recorded at intensities below the threshold of hearing.

Transient OAEs are typically absent when hearing is loss is greater than 30 dB HL and distortion product OAEs (DPOAEs) are usually absent when the hearing loss is greater that 35 to 40 dB HL ( Hall & Mueller, 1997).

Additionally, middle ear effusion negatively impacts OAEs (Sutton, Gleadle & Rowe, 1996). Middle-ear effusion accounted for half of the OAE failures in the special care baby unit, as reported by Sutton et. al. It is shown that negative pressure in the middle ear can (but does not necessarily) cause abnormal OAE responses.

Middle Ear Analysis:

In this article, the term 'Middle Ear Analysis (MEA)' is used to refer to all versions of tympanometry and acoustic reflex testing. For screening purposes, the most common and arguably the most valuable MEA test is the tympanogram. Nonetheless, complete MEA provides important information regarding the integrity of the external auditory canal, the tympanic membrane, the middle ear system and the neural acoustic reflex arc responsible for contraction of the stapedius muscle.

Tympanometric patterns in infants are not typical of those observed in older children and adults. Normal tympanometric responses highly correlate with intact and mobile tympanic membranes in older children and adults. Further, normal tympanograms are unlikely to be found in ears with significant middle ear disease, negative pressure or significant fluid in the middle ear space.

Complex tympanometric patterns can be observed when using a low frequency probe tone, to differentiate factors relating to mass, stiffness and resistance (McKinley, Grose & Roush, 1997). According to Sutton et. al. (1996), a 678 Hz probe tone used with tympanometry is a useful indicator of middle ear function in young babies.

Infants with normal middle ear reflexes (i.e. acoustic reflexes) are not likely to have severe or profound hearing loss, although reflexes can be present with a mild to moderate loss. Additionally, acoustic reflexes obtained via ipsilateral and contralateral pathways provide further diagnostic evidence regarding the integrity of the auditory and related neural systems.

II. Hearing Loss.

Conductive loss:

Conductive hearing loss attenuates sound reaching the cochlea. Infants with conductive hearing loss typically demonstrate positive responses (i.e., they fail the test) on infant screenings. Conductive hearing loss in infants result from a variety of etiologies including; vernix present in the ear canals at the moment of birth, otitis media, congenital otologic anomalies, to name a few. Conductive hearing loss may be medically/surgically treatable, or may be transient. Regardless of etiology, research has shown conductive losses can cause permanent speech and language developmental problems (Zargo & Boltezar, 1992).

Infants who fail initial auditory screenings should be screened a second time. If failure persists, the child is followed with a diagnostic ABR, including tone burst and bone conduction ABR. To determine if a conductive loss is present, bone conduction ABR combined with air conduction ABR is recommended. Parameters to be evaluated include latency/intensity functions and threshold responses. Bone conduction responses can be repeatably recorded down to 20 dB nHL. When performing bone conduction ABR testing, it is critically important that your air conduction (AC) and bone conduction (BC) equipment be calibrated (and preferably dedicated) for ABR use. Both AC and BC systems should be calibrated to the same dB scale. A good approach is to use the dB nHL scale, based on normative values accumulated in your office using children known to have normal hearing.

Conductive losses obliterate the OAE response. This problem is more complex than those associated with ABRs. The OAE problem is two-fold. (1) The conductive loss reduces the intensity of the stimuli reaching the cochlea, and hence reduces the cochlear response. (2) The conductive loss reduces the intensity of the echo traveling back towards the eardrum and hence the OAE microphone picks up a much softer signal or no signal at all.

MEA can provide additional information regarding the status of middle ear structures in cases with ABR and/or OAE screening failures. Tympanograms help us to determine if fluid is present, if the tympanic membrane is intact and mobile, or if a perforation exists. Acoustic reflexes can provide us with a gross estimate of the possible degree of hearing loss, as well as an analysis of the reflex arc involved in the response.

In the future, MEA may become even more popular in the infant evaluation process. For example, we may see screening tympanometers with higher frequency probe tones, allowing pressure to be held at maximum compliance (maximum peak) during measurement of the acoustic reflexes. This same technology might allow OAEs to be recorded at optimal middle ear pressure.

Sensorineural Loss

If a failed ABR or OAE screening is followed-up using bone conducted ABR, and if the failure continues with the BC-ABR, this indicates a possible sensorineural loss. A complete diagnostic ABR should be performed using frequency specific tone bursts to generate a tone burst evoked audiogram. This is necessary to determine the degree of the hearing loss as well as the configuration of the hearing loss. If a sensorineural hearing loss is present, hearing aids will likely be considered as the primary intervention strategy. In all cases of suspected sensorineural hearing loss, based on a failed diagnostic ABR evaluation, OAEs should be administered. Transient Evoked (TEOAEs) and Distortion Product (DPOAEs) are diminished or absent in cases of sensorineural loss. If OAE responses are present in the absence of normal ABRs, this is an indication that the outer hair cells are functioning and the damage lies in the inner hair cells or synapse regions, possibly consistent with auditory neuropathy.

Auditory neuropathy requires alternative intervention strategies. In cases of auditory neuropathy, amplification and cochlear implantation are sometimes inappropriate. Each case must be followed closely and appropriate management techniques should be developed based on the 'team approach' and should be individualized for each patient.

III. Sample Outcomes (not a comprehensive list):

Normal ABR, normal OAE and normal MEA.

The child likely has normal hearing sensitivity and intervention is not required at this time. However, if the child is from a high-risk registry, the audiologist may elect to monitor the child until it can be proven that normal speech and language skills are being developed.

Abnormal ABR, abnormal OAE and absent reflexes with normal tympanogram.

These results indicate a severe to profound hearing loss and intervention is required. Typically, otologic referral and the complete diagnostic audiometric battery are recommended. Usually by age 6 months, frequency specific and ear specific information can be obtained using visual reinforcement audiometry. In the meantime, assuming all indications are consistent with severe to profound hearing loss, appropriate amplification and implant options, as well as habilitation/rehabilitation strategies and educational and cultural issues should be addressed by the audiologist with the parents or guardians and amplification should be initiated as soon as possible.

Normal ABR, abnormal OAE, abnormal tympanogram.

The results are consistent with middle ear pathology, such as a conductive hearing loss caused by otitis media. This child requires a referral to an otolaryngologist and further audiometric evaluation. Tympanometric patterns vary considerably across young children (De Chicchis, Todd and Nozza, 2000)

Normal OAE, abnormal ABR, normal tympanogram.

These test results are consistent with auditory neuropathy (Hall J.W., 2000). This child should not be fit with amplification or implanted immediately. Rather, the child should be closely monitored for a period of time to see if the audiometric findings remain stable or vary, and to determine if other clinical signs and symptoms develop leading to a neurologic diagnosis. Outcomes data regarding management of children with auditory neuropathy are as yet unavailable. Therefore, careful audiologic monitoring and non-invasive, non-aggressive therapies are the starting point for these children.

IV. Intervention

Deciding which type of intervention is best for an infant or small child is always a challenging process. Included in the range of alternatives are hearing aid amplification, cochlear implants, sign language, FM devices, special schooling, speech and language therapy, and many other therapies. No single test will provide all the information required for developing an intervention strategy. Armed with the information from the battery of tests described above, it is possible to offer a significantly enhanced opportunity for normal or near normal development for this challenged population.

Most babies for whom hearing aids are the intervention method of choice will be fit with high technology programmable hearing aids. Ideally, we would have a complete audiogram before fitting a hearing aid to a patient. But, as we know, this is often unobtainable in infants.

Therefore, we typically estimate hearing levels, as best we are able, at as many frequencies as possible and we rely on the help of hearing aid fitting programs such as the Desired Sensation Level (DSL) and hearing aid manufacturer's algorithms to initially program the hearing aid for the baby (Seewald, R.C., Moodie, K.S., Sinclair S.T. and Scollie S.D.(1999).

Combined tone burst and click-evoked ABR thresholds offer opportunities to help design frequency-specific hearing aid programs. While these are not as reliable as behavioral audiograms, they offer the best alternative at this time for children or patients unable or unwilling to provide behavioral threshold information.

The process by which we address the needs of hearing impaired infants continues to evolve. The entire audiologic community is watching to insure that appropriate steps toward early identification and improved care are taken. The process will require revision and refinement, but early detection and diagnosis via integrated physiologic technologies, coupled with appropriate rehabilitation measures such as those described above promise to significantly improve the quality of life for early identified children.
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Author: Dr. Petrak is a clinical audiologist and product manager for evoked potential technologies at ICS Medical Corporation in Schaumburg, IL. Readers can find out more about these technologies by calling 1-800-289-2150 and requesting information or they can visit ICS's website at www.icsmedical.com

References:

De Chicchis, A.R., Todd, N.W., and Nozza, R.J. (2000). Developmental Changes in Aural Acoustic Admittance Measurements. Journal of the American Academy of Audiology. 11:97-102.

Hall, J.W., (2000). Handbook of Otoacoustic Emissions. Published by Singular Publishing, San Diego, California. Pages 439-451.

Hall, J.W., and Mueller, H.G. (1997). Audiologists' Desk Reference. Diagnostic Audiology Principles, Procedures, and Practices. Singular Publishing. San Diego, California.

Jerger, J., and Hayes, D. (1976). The cross-check principal in pediatric audiometry. Archives of Otolaryngology, 102, 614-620.

McKinley AM, Grose JH, and Roush J. (1997). Multifrequency tympanometry and evoked otoacoustic emissions in neonates during the first 24 hours of life. Journal American Academy of Audiology. June; 8(3), 218-223.

Seewald, R.C., Moodie, K.S., Sinclair, S.T., Scollie S.D. (1999): Predictive validity of a procedure for pediatric hearing aid fitting. American Journal of Audiology 8:143-152.

Sutton, GJ, Gleadle P, and Rowe, SJ (1996). Tympanometry and otacoustic emissions in a cohort of special care neonates. British Journal of Audiology, Feb; 30(1), 9-17.

Zargo M; and Boltezar IH (1992). Effects of recurrent otitis media in infancy on auditory perception and speech. American Journal of Otolaryngology. Nov-Dec; 13(6), 366-372.


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