Pediatric Hearing Aid Verification: Innovative Trends

Introduction

As the sophistication and adaptivity of hearing aid technology increases, audiologists must use verification strategies that accurately and efficiently assess those aspects of amplification. Research regarding the efficacy of advanced features such as adaptive directional microphones and single-microphone noise reduction algorithms is limited, especially in the pediatric population. Children's listening needs are different from those of adults, and their needs should form the basis of our decisions regarding which advanced technologies are appropriate. Fortunately, audiologists have a variety of verification strategies at their disposal to evaluate the impact of different hearing aid technologies on their pediatric patients.

Evidence-Based Practice

The call for evidence-based approaches for assessment and intervention has grown over the past decade. Although few would argue the need to base clinical decisions on research, translational studies to guide practitioners' decisions, particularly related to hearing aid technology for children, have been limited. The pace of technological innovation in the hearing aid industry has exceeded that of supporting, independent research. By the time studies regarding efficacy are published, devices may have undergone significant modification or may no longer be on the market. Studies of the effects of advanced hearing aid technologies, such as noise reduction and directional microphones, in children are not widespread in the literature.

Additionally, evidence-based practice has often been regarded as including only the evaluation of peer-reviewed research published in scholarly journals, sometimes referred to as external evidence. As Dollaghan (2007) points out, evidence-based practice should encompass not only external resources, but should also be based on the clinician's experience and expertise, as well as the preferences of an informed patient or family. A broader view of evidence-based practice is critical when evaluating treatment options and approaches for children who use hearing aids or cochlear implants. The limited availability of external evidence combined with the heterogeneity of hearing-impaired individuals requires a flexible approach that takes advantage of data from a variety of sources. Further difficulty in evaluating research arises from the use of marketing and testimonials for the promotion of hearing aids by manufacturers, which are not valid sources of external evidence.

A special issue of the Journal of the American Academy of Audiology in 2005 highlighted the state of available research related to hearing aid fittings at that time. In one article in this issue, Palmer and Grimes (2005) specifically addressed the effectiveness of hearing aid signal processing technology for pediatric patients. Based on their systematic review of the literature (Marriage & Moore, 2003; Jenstad et al., 1999), the authors concluded that wide dynamic range compression (WDRC) results in better speech recognition in quiet and in noise for pediatric hearing aid users. Additionally, children with hearing loss require greater audibility (Stelmachowicz et al., 2000) and frequency bandwidth (Stelmachowicz, et al., 2002) to achieve equivalent levels of speech recognition as their normal-hearing peers in quiet. Out of 226 potentially relevant studies, the authors found only 8 that achieved sufficient levels of evidence to be included in the review.

Although these studies provide an important basis for clinicians, evidence is currently limited regarding the use of other hearing aid features and signal processing strategies for children. Features such as directional microphones, single-microphone noise reduction strategies, and feedback suppression are widely available in even the least expensive hearing aid products and, as a result, are likely to be available for pediatric patients. Further developments such as open-fit and receiver-in-the-ear (RITE) devices also continue to grow in popularity and are being used with children. Many audiologists have questioned if these technological advances are appropriate to use with children, and, if so, what methods should be used to verify their function. Unfortunately, guidelines or other forms of external validation are not readily available for clinicians making these important clinical decisions.

Given the limited availability of data regarding the use of these advanced features with children, each audiologist has a responsibility to verify pediatric hearing aid fittings in an effort to collect evidence for efficacy that may not be available from external sources. The purpose of this article is to propose an evidence-based approach to verifying advanced hearing aid technologies for use with children. Any hearing aid verification protocol should be practical and informative, demonstrate that all advanced hearing aid features are functioning appropriately, and assess the degree to which those features affect a child's access to speech.

Desired Sensation Level

The primary goal of early intervention and provision of amplification to young children with hearing loss is to enhance the development of speech and language skills. Therefore, the cornerstone of pediatric hearing aid verification should be to ensure that the speech signal is maximally audible through the hearing aids. A comprehensive approach should take into account the variability of hearing losses among children and the changes expected as they grow and develop. The limited audiological information associated with hearing aid fittings for children should also be considered. Verification strategies should ensure that the maximum output of the hearing aids is safe and comfortable based on the child's degree of hearing loss.

The Desired Sensation Level (DSL; Seewald et al., 2005) method provides a comprehensive approach to pediatric hearing aid selection and verification. The most recent update to the DSL method (Version 5.0) was released in 2005 based on data from a variety of studies completed with pediatric hearing aid patients. Updates incorporated into the most recent version of the DSL method include targets for multiple input levels, different prescriptive approaches for adults and children, updated real-ear-to-coupler-difference (RECD) data, and maximum output limiting based on broadband (speech) inputs. A complete review of the DSL method is beyond the scope of this discussion; however, readers interested in a more comprehensive discussion of DSL are encouraged to review the series of articles published on this topic in Trends in Amplification (Seewald et al., 2005; Bagatto et al., 2005; Scollie et al., 2005).

Another notable update in the most recent version of the DSL method is the ability to generate targets based on auditory brainstem response (ABR) data. Because a significant number of hearing aid fittings on infants and toddlers are based on ABR data, DSL accounts for the differences between thresholds obtained during standard audiometric procedures and those obtained electrophysiologically. The stimuli used to obtain ABR responses are brief in duration compared to the pure tones used in behavioral audiometry. Thresholds are lower for longer duration pure tones than for brief stimuli, and these differences vary as a function of frequency. As a result, the new version of the DSL method incorporates several options to account for these differences in the hearing aid fitting. The DSL method has an included correction, nHL, which is based on the ABR calibration method described by Stapells (1990). However, some clinics account for the duration difference by measuring behavioral thresholds to brief stimuli using a jury of normal hearing subjects as described by Gorga et al. (2006), therefore, the DSL method includes the term eHL to represent ABR threshold where no additional correction is required. In this case, because differences in stimuli are taken into account during the calibration process, additional compensation for brief stimuli will result in significant inaccuracies.

Although either approach is acceptable to account for the differences in stimulus duration between ABR and behavioral audiometry, audiologists should be familiar with the method used to calibrate their ABR equipment. Knowledge about which method is applied will ensure that subsequent hearing aid fittings are accurate. Audiologists who perform ABR testing should also take their calibration method into consideration when reporting results and communicating with audiologists outside of their clinic.

Selection of Hearing Aids

Once a diagnosis of hearing loss has been confirmed and the family is ready to proceed with amplification, hearing aid selection can occur. Audiologists must consider a variety of factors when choosing amplification for children. An ideal device should have a fitting range that provides amplification for the child's current degree of hearing loss and flexibility to allow for potential changes in hearing sensitivity in the future. Durability and compatibility with assistive technologies, such as frequency modulation (FM) systems, are also critical considerations. Hearing aids must also adapt with children as they grow. Based on these requirements, behind-the-ear (BTE) devices are preferable to smaller custom hearing aids that fit in the ear.

The number of available standard features to consider has increased significantly in recent years. Even the least expensive hearing aids may have directional microphones, noise reduction, and some feedback-management strategy. The clinician will more likely need to choose which features to activate rather than which features to order. Advanced hearing-aid features are developed based on the listening needs of adults. If advanced features are selected for children, the effects of each feature on speech audibility should be verified and thoughtfully evaluated.

Directional Microphones

Directional microphones have been shown to improve speech understanding in noise in multiple laboratory studies and real-world environments with adult listeners (see Bentler, 2005 for review). Laboratory studies of directional-microphone benefit in children have also been completed. Gravel et al. (1999) demonstrated improved listening performance in noise with fixed directional microphones for two groups of children with hearing loss who were 4 to 6 years and 7 to 11 years of age. In this study, the source of noise was presented directly behind (180 degrees) the child with speech presented directly in front (0 degrees). The level of the competing multi-talker babble was varied in an adaptive paradigm until each child reached 50% correct performance. Directional microphones resulted in better performance at more difficult signal-to-noise ratios (SNRs) when compared to the omnidirectional listening condition.

More recently, Ricketts and colleagues (2007) evaluated speech recognition in school-age children using directional microphones in a variety of simulated classroom environments. Twenty-six participants from 10 to 17 years of age were included in the study. Speech recognition scores were obtained in both omnidirectional and directional microphone settings. Although directional microphones resulted in better speech recognition in noise for listening situations where the desired speaker was in front of the child, the researchers found speech recognition was either the same or decreased for listening situations where the desired speaker was on the sides or behind the student. Directional benefit in settings where the speaker was not directly in front of the listener was dependent on the ability of the child to orient his or her head towards the sound source.

Results from these studies demonstrate that children can achieve improved speech understanding in noise with directional microphones when the signal of interest is in front of the child. In addition, school-age children with normal hearing and those with hearing loss have demonstrated the ability to orient towards a speaker who is not located in front of them (Ricketts & Galster, 2008). With younger children and infants, however, their attention and orientation in space related to an individual talker could vary significantly from one moment to the next. Furthermore, emerging evidence has suggested that children as young as two years of age access language through overhearing in addition to direct communication (Akhtar, 2005). The effects of directional microphones on overhearing and incidental learning in children with hearing loss have not been investigated, nor has the impact of directional microphones on localization, which has important implications for safety.

Based on the current research literature, directional microphones may be appropriate for children in specific situations where noise is present and the sound source of interest is consistently located in front of the child. Although FM systems provide larger improvements in SNR than directional microphones, FM systems are not always practical, such as in situations where there may be multiple talkers. For listening situations where the location of the signal of interest or the child may vary, an omnidirectional setting may provide more access to sounds from a variety of potential orientations. Given that a child's need for directional microphones will vary based on the listening environment, a hearing aid that allows the parent or caregiver to switch easily between microphone modes is preferable to an auto-switching or adaptive directional microphone. The decision about whether or not to activate directional microphones should be discussed with parents, and counseling regarding the situations where directional microphones might be most beneficial should be provided. Determining the age at which to activate directional microphones is difficult and may be based on factors such as a child's environment, speech and language development, behavior, and the willingness of the parent or caregiver to switch the device appropriately as the listening environment changes.

Single-Microphone Noise Reduction

Reducing the negative perceptual consequences of noise for individuals with hearing loss has been a goal of hearing aid signal processing schemes for several decades (Bentler & Chiou, 2006). A review of the literature conducted by Bentler in 2005 indicated that speech recognition performance in noise is neither improved nor degraded by single-microphone noise reduction algorithms. Evidence to suggest that speech perception is improved by these strategies is not apparent outside of laboratory environments. However, evidence demonstrating the efficacy of noise reduction strategies for increased comfort and ease of listening in noise has begun to emerge. Data from Mueller and Bentler (2005) suggest that noise reduction strategies may improve the acceptable noise level (ANL) for adult hearing aid users and that speech recognition ability does not improve or decrease with the implementation of noise reduction. Increased comfort in noise, even without enhancement of speech understanding, is a beneficial outcome that could increase the acceptance of hearing aids among consumers.

Research characterizing children's performance using single-microphone noise reduction strategies is limited. Furthermore, the variability of noise reduction parameters across manufacturers is substantial (Bentler & Chiou, 2006). The amount of gain reduction, frequencies where gain is reduced, input level of activation, and onset/offset times are only a few of the aspects of noise reduction algorithms that vary among devices. A critical issue is the extent to which noise reduction limits the audibility of speech for pediatric listeners. Recall that children require greater audibility (Stelmachowicz et al., 2000) and bandwidth (Stelmachowicz, et al. 2002) than adult listeners to achieve the same level of speech understanding. If noise reduction strategies are implemented, a necessary component of hearing aid verification is to assess gain and frequency response using a stimulus that will activate that feature.

Open Fit and Receiver in the Ear (RITE) Devices for Children

Recent advances in feedback suppression algorithms combined with the miniaturization of hearing aid components has led to the introduction of small BTE devices that can be fit with minimal ear canal occlusion, known collectively as open-fit hearing aids. A similar implementation uses a miniature BTE coupled to a receiver in the ear canal (RITE). Clinicians who have had successful fittings with open-fit and RITE devices with adults have logically considered using these devices for pediatric patients. Unfortunately, few peer-reviewed studies have been completed with either type of hearing aid, even with adults. Small size and ability to fit without a custom earmold are advantages open fit and RITE devices have over traditional hearing aids, but a number of additional factors must be considered prior to selecting these devices for children.

Because of the small size, both open-fit and RITE hearing aids may not include a telecoil or direct audio input (DAI); therefore, they may not be compatible with FM systems and other hearing assistance technology. In addition, slim tubing and receiver wiring kits include a range of sizes appropriate for most adults; however, the range of available tubing and domes are not adequate to account for the small size of most children's ears. The cost of replacing the ear coupling mechanism as the child grows, particularly with RITE receivers, can be substantial. Additionally, a lack of durability for certain devices and short battery life, related to the smaller battery size, are further limitations for both types of hearing aids.

Although open-fit and RITE devices are similar in size and appearance, each has characteristics that limit their appropriateness for children. RITE hearing aids fit deeply in the ear canal and may occlude the ear canal. The amount of occlusion and the resulting volume between the receiver and eardrum are important considerations. Most RITE fitting algorithms are derived using BTE microphone-location effects. However, the ear canal volumes on which estimates of in-situ gain are based vary between manufacturers and from BTE to deeply inserted completely-in-the-canal (CIC) hearing aids. These differences, as well as potential changes in residual volume (secondary to insertion depth), can change the amount of ear-canal gain significantly. Variations in the size and acoustics of children's ear canals further complicate the estimation of gain for RITE fittings in children and highlight the importance of verification with probe-microphone measures.

Conversely, open-fit hearing aids provide minimal occlusion of the ear canal, which is a definite advantage in terms of sound quality for adults with high-frequency sensorineural hearing loss. However, the open ear canal, combined with gain algorithms that do not provide amplification below 1000 Hz, limit the effectiveness of open-fit devices for children. Even in cases with sloping audiometric configurations that may not require substantial low frequency amplification, the potential for transient middle ear problems and progression of hearing loss are situations that could result in inadequate amplification for a child's hearing loss. Furthermore, fitting ranges for open-fit hearing aids are frequently based on measurements made in a closed coupler, which does not accurately approximate gain in an open ear canal, particularly in the low frequencies. Consequently, gain should be verified in the ear using a probe-microphone system that disables the reference microphone for open-fit measurements. If the reference microphone is not disabled, amplified sound from the ear canal may be measured at the reference microphone as part of the input signal to the hearing aid, which leads to an underestimation of the amount of gain in the patient's ear.

Hearing Aid Verification

Once the clinician selects appropriate devices, the gain and frequency response of the hearing aids should be set to ensure audibility of speech over a range of input levels, and the hearing aid gain, maximum output, and additional features should be verified. Probe microphone measures completed with speech signals at multiple intensity levels provide the most accurate assessment of hearing aid gain characteristics. Maximum output should be verified to ensure that the hearing aid is adequately limited at high intensity levels. When real-ear measurements are not practical, RECD measures are an acceptable alternative. For a complete review of RECD measurements, the interested reader can refer to a comprehensive discussion by Bagatto and colleagues (2005). The DSL method provides targets for gain and output, in addition to age-specific average RECD values, to provide a basis for pediatric hearing aid fittings.

Using a manufacturer's default settings without independent verification is not appropriate for a variety of reasons. Gain and compression characteristics of hearing aids vary substantially among manufacturers, even for individuals with the same audiometric thresholds. Displays of gain and output within a manufacturer's fitting software do not provide accurate estimates of hearing aid gain in the patient's ear. One study found differences of up to 20 dB, particularly at 2000 and 4000 Hz, between insertion gain predicted by the software and measured in the patient's ear (Hawkins & Cook, 2003). In addition, hearing aid maximum output levels for equivalent degrees of hearing loss are similarly variable across manufacturers (Mueller et al., 2008). Without probe-microphone verification, significant errors are likely, and because acoustic access to speech and language for infants and children is essential for development, individualized verification is essential for this population.

In addition to confirmation of gain and output, verification of other hearing aid features should be completed as well. The following approaches can be used with most hearing aid test units; in addition, recent developments in verification equipment have simplified this process in some cases. As with gain and maximum output, assessment of advanced features in the patient's ear is the most accurate method. Given that multiple real-ear measurements are not always practical for young children, a more reasonable tactic is to complete advanced-feature verification in the hearing aid coupler prior to the patient's arrival with an understanding of the differences that exist between real-ear and coupler measures. Any real-ear measurements obtained should be completed with feedback suppression enabled to ensure that high frequency gain reduction does not alter speech audibility.

Most clinicians lack the time to comprehensively assess directionality, but the function of directional microphones can be quickly evaluated in the test box. With noise reduction disabled, measurements are taken in both the omnidirectional and fixed-directional settings. This method does not evaluate all aspects of directionality but does provide the clinician with evidence that the directional microphones are functioning appropriately and how the directional settings may alter the frequency response of the hearing aid. Using multiple input levels will also demonstrate if the directional microphone is level-dependent. Some test systems, such as the Audioscan Verifit, have multiple speakers in the hearing aid test chamber, which allows simultaneous assessment of directional microphones. Systems with single speakers can be used to make sequential measures of directionality by altering the orientation of the hearing aid in relation to the speaker across multiple measurements.

Various parameters of single-microphone noise reduction should be quantified if this feature is activated. The amount of gain reduction, frequencies where gain is reduced, and onset time of noise reduction settings can be verified in a 2 cc coupler using a steady-state signal such as broadband noise or pink noise. Measurements with noise reduction disabled and enabled can help to quantify the impact of noise reduction strategies on audibility and can guide decisions about whether or not to enable noise reduction for the fitting. It is important to note that many noise reduction systems require up to five seconds to fully engage, and measurements should be taken once the gain curve has stabilized.

Limitations of Aided Sound Field Testing

Real ear or RECD measurements, along with verification of advanced hearing aid technologies, provide a solid foundation for pediatric hearing aid fittings. Hearing aid verification is an on-going process and does not end after the initial appointment. Aided sound field testing and functional gain continue to be used by some clinicians despite evidence dating back nearly three decades (Macrae & Frazier, 1980) outlining the limitations of this method. Warbled tones presented at low intensity levels are not processed by the hearing aid in the same way as speech at a more moderate intensity level. As a result, aided audiograms can overestimate the amount of gain provided by the hearing aid. In addition, aided sound-field testing does not assess the gain, maximum output, or advanced hearing aid features. The process of obtaining aided thresholds at discrete frequencies is also time consuming and often not possible with infants and young children. If possible, a more useful tool for behavioral verification is the assessment of aided and unaided speech recognition, which provides evidence for how speech is processed at more typical input levels.

Future Directions

Despite multiple advances in research related to pediatric amplification, further work needs to be completed. Translational studies evaluating directional microphones, noise reduction, and other signal-processing aspects of hearing aids with children will improve audiologists' ability to make decisions about what features are appropriate for young listeners. Efforts should also be made to expand the bandwidth of hearing aids to higher frequencies, which may be important for fostering speech and language development. In order to expand the bandwidth of hearing aids, multiple areas of research are needed, including assessing behavioral measurements beyond 8000 Hz, developing methods to assess hearing aid function at high frequencies, and addressing the mechanical limitations of hearing aid microphones and receivers. Although independent research efforts may have difficulty maintaining the same pace as technological innovations in the hearing aid industry, manufacturers should promote independent validation of features to the greatest extent possible to ensure that further developments benefit hearing aid users of all ages.

Conclusion

Hearing aid verification for young children is critical to demonstrate the appropriateness of hearing aid settings and features and to provide the best possible starting point to enhance communication development. Rapid advances in technology and limited external evidence to help guide clinical decision making requires clinicians to use their clinical expertise, verification data, and feedback from children and their families to make evidence-based decisions. The process of hearing aid verification should be an on-going effort using data from a variety of sources to maximize outcomes for our patients.

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