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Selection and evaluation of amplification for infants

Beauchaine, Kathryn Laudin

doi: 10.1097/01.HJ.0000324171.51360.ca
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When providing amplification for infants and very young children, the audiologist should use an approach that takes into account the unique needs of this population. Research has shown that the listening needs of infants are different from those of older children and adults.1–3

For example, infants cannot use contextual cues to grasp the meaning of what they hear in the same way that older children and adults do. Furthermore, young listeners need a better signal-to-noise ratio than adults do to detect phonemes. Infants cannot adjust their hearing aids or FM systems, and they cannot control their listening environments. Thus, it is incumbent upon clinicians to employ a proven approach to fitting amplification. It is also necessary to train the infant's caregivers to be astute amplification troubleshooters.

Although fitting amplification in a timely manner is a well-established goal, it is important to recognize that sometimes there will be barriers to this process. These barriers may include: (1) parents not returning for immediate follow-up, (2) parents denying the presence of hearing loss or the need for remediation, (3) parents seeking a second opinion, and (4) infants with medical needs that supersede the fitting of amplification.4,5 Other possible reasons for delays in amplification include the need for additional audiologic tests and lack of financial resources.6

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STEPS IN FITTING AND EVALUATION

The steps outlined below are undertaken after hearing loss has been confirmed, all appropriate referrals have been made, and parents have given consent to proceed. The steps in the hearing aid fitting and evaluation process include:

  1. obtaining ear impressions to fabricate earmolds,
  2. measuring the real-ear-to-coupler differences (RECD),
  3. determining targets for gain and output,
  4. choosing hearing instruments,
  5. verifying gain and output with selected devices, and
  6. providing orientation to the devices and planning a follow-up schedule.
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Step 1: Ear impressions and earmolds

Before placing otoblocks, it is important to verify that ear canals are free of debris and unoccluded. Also, the shape of the ear canal and its approximate length should be visualized. This visualization, in conjunction with available data about typical infant ear canal length, will ensure appropriate placement of the otoblocks. Data on average ear canal lengths during the first year of life are shown in Table 1.

Table 1

Table 1

Using these data as a guide, the audiologist can mark the otoprobe and insert the otoblock an appropriate distance. Infants who were born prematurely, are small for their age, or have ear anomalies may not have average ear canal lengths. This should be considered in taking the ear impressions.

Because of size limitations, standard tubing may be too wide for some infants' ear canals. In those cases, the tubing may be inserted into the earmold only a short distance before it is glued in place. It is important to use an earmold material that will accept glue and be firm enough to stay open in an infant's narrow ear canal. Finally, the earmold should be relatively soft to ensure comfort. With these considerations in mind, a vinyl earmold material is most appropriate for this population.

After describing the protocol steps to parents and proceeding with ear impressions, it may be useful for the audiologist to show them examples of infant earmolds and hearing instruments to help them better visualize the final product. The appearance of hearing instruments has been identified as a concern of parents.5 Therefore, it may be useful for parents to see an instrument before their child's fitting.

Acoustic feedback can be a challenge. The infant's ear and ear canal size in combination with the growth that occurs during infancy and early childhood will necessitate frequent earmold remakes. To reduce or minimize feedback temporarily, water-based creams can be used on the earmold. Care must be taken to keep the sound bore free of the cream. Comply Wrap™ is an adhesive wrap designed for use with custom instruments, but it can be used to alleviate feedback for an infant's earmolds as well. It can also be directly applied to earmold tubing for very small ear canals (see Figure 1).

Figure 1

Figure 1

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Step 2: Real-ear-to-coupler difference

For the same input, a greater sound pressure level (SPL) develops in the smaller volume of the infant ear as compared to the adult ear.7 Therefore, when fitting amplification to infants, the audiologist needs to take individual measurements to predict the SPL in the ear canal and account for the effect on estimates of thresholds, subsequent targets for hearing aid gain and output, and estimates of hearing aid performance.

For infants, the application of traditional probe-microphone measures is limited because infants lack head control, and their movements and vocalizations compromise the quality of the recordings. The real-ear-to-coupler difference (RECD) allows increased accuracy by accounting for the unique characteristics of an infant's ear canal and earmold. The RECD can also be applied to estimates of threshold for conversion from dB HL to real-ear dB SPL, and can be applied to coupler values to estimate real-ear aided response (REAR) and the real-ear saturation response (RESR).

The RECD cannot be estimated from equivalent ear canal volumes.7 Although age-average RECDs have been measured, there is significant individual variability. Average values are not good predictors for very small birth-weight babies or for ears with anomalies, otitis media with effusion, and tympanic membrane perforations. For typical ears, one ear's RECD is a good predictor of the other ear.8

Measurement of an infant's RECD takes only a few minutes to complete. Descriptions of the RECD process can be found in Moodie, Seewald, and Sinclair9 and in Bagatto.10 For RECD measures, the insertion depth of the probe microphone in the infant ear canal is typically 15 mm in from the intertragal notch or 10 mm past the ear canal entrance. Again, factors such as infant size, prematurity, and presence of ear anomalies should be considered when planning insertion depth. The RECD should be recalculated whenever new earmolds are obtained.

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Step 3: Gain and output targets using a prescriptive method

One of the primary goals of amplification is to provide audibility of speech. Obtaining targets for gain, output, and compression ratio (CR) to meet this goal necessitates use of a prescriptive method. Using older methods, such as a half-gain rule, will not inform the audiologist of the audibility of speech-level inputs or suggest a maximum output. The prescriptive method used should employ RECD values and/or provide age-related transforms. Because infants and young children do not sit quietly while conventional probe-microphone measures are made, gain and output must be measured in a coupler.

The Desired Sensation Level (DSL) 4.1 i/o is a prescriptive method designed specifically for use with children.11 Furthermore, the DSL provides estimates of audibility of the amplified long-term average speech spectrum (LTASS). The NAL-N1 is another prescriptive method that can be used in a similar manner.12 For purposes of this discussion, use of the DSL method will be described.

At the beginning of the fitting process, the DSL is used to obtain the gain, output, and CR targets for the hearing instruments. The DSL is also used in verification. An example of the DSL targets for the amplified LTASS (+) and for the RESR (*) is shown in Figure 2. These targets illustrate real-ear SPL levels for the REAR and RESR needed to make speech audible while remaining below a predicted upper limit of comfort (UL). The LTASS may not be representative of the actual conversational partners that the infant encounters. However, it is a good first estimate. Similarly, the child's actual UL may be greater or less than predicted by the program. The predicted ULs in the DSL are conservative estimates based on the work of Pascoe.13

Figure 2

Figure 2

The DSL program is accessed at the beginning of the fitting process to establish targets for the hearing instruments. It is also accessed later for verification of the fitting. As the child is followed over time, the program is re-activated when new information becomes available, such as new threshold information, or new RECD values. Changes in the device's electroacoustic characteristics should also be entered into the program to evaluate amplification relative to target values.

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Step 4: Choosing hearing aids

As the range of hearing instrument possibilities continues to expand, the audiologist is faced with a number of choices, including the level of technology and circuitry options to select. Because programmable devices often provide more flexible electroacoustic parameters, they may be considered a good option for pediatric fittings. Some analog, non-programmable devices are highly flexible as well. Cost and electroacoustic flexibility must be weighed.

In choosing between digital and analog instruments, the audiologist again must balance cost and effectiveness (proven or presumed). Research data have not demonstrated that one manufacturer's proprietary processing scheme is superior to another. Although it seems likely that all hearing aids will eventually have digital circuitry, at this time, digital is one option to be considered with the other choices.

General circuitry options include peak clipping, compression limiting, and wide dynamic range compression (WDRC). The fitting trend is in the direction of WDRC circuitry, except for profound hearing losses. The specific parameters of compression threshold/knee, compression ratio, and attack and release times continue to be debated. It has been suggested that the lowest tolerated compression threshold might be best for young children to ensure audibility of soft speech sounds.14

Once the clinician has made the above choices and identified targets for gain and output, the field of possible hearing instruments is narrowed. Again, cost will be a consideration as the audiologist weighs one device against another. The audiologist may assist families with identification of funding sources for purchasing the hearing instruments.

Other considerations in hearing instrument selection are size and compatibility with frequency-modulated (FM) systems. Because children frequently encounter noisy listening backgrounds and are often at some distance from their parents, FM use is encouraged to improve the signal-to-noise ratio. All parents should be informed of the applications of FM devices for use at home and at school.

Directional microphones may be used to enhance the signal-to-noise ratio, even for young children.15,16 However, directional microphones are not typically recommended for infants or toddlers. This is because young listeners, who are able to crawl or walk, are often in situations where they are not face-to-face with their parents/caregivers. Directional microphones may hamper incidental learning by preventing children from overhearing words that are not directly intended for them. With those points in mind, it may be prudent to defer directional microphone use until children are slightly older.

Standard options for behind-the-ear hearing instruments used by infants and young children include tamper-resistant battery compartments and tamper-resistant controls. Pediatric-sized tone hooks can be used to improve the retention of the instruments.

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Step 5: Verification

The purpose of verification is to ensure that the amplified speech spectrum closely approximates the prescription. Electroacoustic verification of amplification targets should be completed for both real-ear aided gain (REAG) or REAR and RESR. Programming hearing aids or adjusting potentiometers without the use of electroacoustic verification is unacceptable, particularly when one is working with infants and young children who are unable to provide meaningful feedback. Verification methods that use computer-driven programs, such as the DSL and the Situational Hearing Aid Response Profile (SHARP), 17 will be discussed here. An overview of validation of aided auditory function through outcome measures is beyond the scope of this article, but the reader is referred to Stelmachowicz.18

Verification in the DSL program can be completed for soft, average, and loud speech input levels and for maximum output levels. If the prescribed targets have been met for gain/output and RESR, verification goals are met. For maximum output, verification should be done using the appropriate input level. Although linear peak-clipping and compression-limiting hearing instruments tend to be in saturation for an input level of 90 dB SPL, non-linear devices require a 100-dB SPL input to saturate. The results obtained with a hearing instrument set to match targets are shown in Figure 3.

Figure 3

Figure 3

The SHARP program is a computer-driven verification method that can be used to demonstrate speech audibility for a variety of speech spectra in unamplified and amplified conditions. The SHARP program has two speech spectra that are specific to infants, a cradle and a hip position. Other spectra include: average conversation at 1 meter, raised voice at 1 meter, average conversation at 4 meters, own voice, shout, head shadow at 1 meter, and classroom teacher at 1, 3, 4, and 7 meters.

To use the SHARP program, the audiologist enters audiometric data (e.g., thresholds and RECDs) and electroacoustic information (e.g., gain, output, CR, and compression threshold). Audibility for the specified spectra and the RESR are calculated and displayed. Comparisons between unamplified and amplified conditions can assist in counseling and in decision making. Figure 4 shows SHARP results for two amplified conditions.

Figure 4

Figure 4

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Step 6: Orientation and follow-up planning

Before the family leaves the clinic with hearing instruments, they must be adept at placing the instruments in the infant's ears and trained in care and maintenance of the devices. During the hearing instrument orientation, parents and caregivers should learn how to insert, remove, and operate the devices. Their skill and comfort in this area will affect the child's adjustment to the instruments. The potential for accidental battery ingestion should be minimized through use of tamper-resistant battery compartments. Cautions about storage and disposal of batteries warrant discussion with the family.

There are no secret methods for keeping hearing instruments on an infant. At various ages, infants and young children present different challenges in hearing instrument retention. Often, very young infants are the most compliant; in part, this is because of their limited fine motor skills. Their small size often requires a method to keep the hearing aid seated behind the ear. Toupee tape, liquid adhesives (ItStays!™), and Huggies™ can assist with retention.

When infants are old enough to grasp objects purposefully, they will often locate their hearing instruments, pull them out, and put them into their mouths. The retention devices mentioned above may not prevent this from occurring. Some parents have opted to use a thin bonnet or cap to limit access to the devices. If the latter option is chosen, it is important to verify that the fabric is electroacoustically invisible and does not cause feedback.

If an infant removes the hearing instruments, there is a risk of damage or loss. Thus, it is important to use a retention cord to affix hearing instruments to the child. Several manufacturers offer products for this purpose, including clips and cords. Parents can also make their own retention devices using pacifier clips or diaper pins attached to cord or fishing line that is tied to the hearing instruments or around the tone hook. Despite all efforts, some hearing aids will be lost or damaged beyond repair, so the audiologist should provide parents with warranty and hearing aid insurance information.

Parents should also be provided with tools to care for the devices. A hearing instrument care kit should include: extra batteries, a battery tester, a listening tube for the parents, an earmold blower, and a hearing aid desiccant. In addition to showing parents how to use this equipment, it is critical for the clinician to provide written information to support oral instructions. The importance of reviewing information that was provided in the initial session cannot be overemphasized. Within a few days of the first fitting appointment, a follow-up phone call should be made to answer any questions and to provide reassurance.

Audiologic follow-up schedules will vary depending on the age of the child, the known risk factors for progressive hearing loss, the presence of middle ear dysfunction, and the child's adjustment to amplification. An audiologic visit every 3 months up to 3 years of age is suggested to obtain and monitor audiologic thresholds, to ensure that the hearing instruments are functioning, and that the earmolds are fitting well. Infants who were fitted prior to 6 months of age are likely to require new ear impressions well before 3 months have elapsed.

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SUMMARY

The early identification of hearing loss has led to earlier hearing aid fitting. Audiologists working with infants and very young children should be aware of the special needs of this population. It is important to recognize that children's listening needs are not the same as those of adults and that children require more frequent monitoring of their hearing and hearing instruments than do adults.

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ACKNOWLEDGMENT

The author acknowledges the assistance of Leisha Eiten, Dawna Lewis, and Jodi Winfrey in preparation of this article.

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