Fitting Digital Non-Linear Power Aids on Children with Severe-to-Profound Losses

The success of early identification of hearing loss in infants, combined with appropriate early amplification intervention, has resulted in better speech and language skills for infants compared to their late identified peers.^1,2 Such enthusiastic commitment to early identification has also prompted hearing care professionals to consider strategies that would ensure an accurate and timely diagnosis of hearing loss, selection of optimal amplification technology, and employment of accurate and efficient verification strategies.³ Assessing amplification on school-age children will help audiologists understand the unique needs of the pediatric population and will assist them in their role of selecting appropriate amplification for children of all ages.

In recent articles, Kuk and colleagues described some of the key features in hearing aids for pediatric applications4 and those for people with a severe-to-profound degree of hearing loss.⁵ The need to use binaural nonlinear wide dynamic range compression (WDRC) hearing aids with a long time constant was proposed based on the observation that the input levels in a child’s listening environments change constantly.⁶ Wide dynamic range compression hearing aids provide more gain at low inputs and less gain at high inputs and are better suited than linear amplification to ensure audibility of soft speech and comfort for loud sounds.

Despite the theoretical advantages of WDRC, the use of nonlinear hearing aids in children is still scarce. Indeed, a recent survey by Tharpe et al.⁷ showed that less than 50% of audiologists who worked primarily with children used nonlinear hearing aids for their pediatric clients with severe-to-profound degrees of hearing loss, and only about 15% of audiologists indicated that they recommended digital devices for children. An analog linear hearing aid with either compression limiting or peak clipping was the primary circuit option recommended by respondents of the survey.

There may be economic as well as audiologic reasons for the infrequent use of digital or nonlinear hearing aids in children. Low reimbursement rates by states and other agencies may deter audiologists from making such recommendations. This may be a result of the scarcity of definitive evidence presented to such agencies to support the hypothetical advantage of nonlinear hearing aids. On the other hand, a decrease in the perception of loudness in nonlinear hearing aids (compared to linear aids) may be a reason for children with a severe-to-profound hearing loss and their audiologists to continue favoring the use of linear peak-clipping or compression-limiting hearing aids.

On a hypothetical basis, if adequate gain is ensured at low and medium input levels, the reduction of output from hearing aids at a high input level should not be disconcerting to the wearers. Generally speaking, linear circuits provide more output when presented with high inputs compared to a compression circuit. If adequate gain for low and medium inputs is ensured, audibility for the important speech signals will be preserved. As Ching et al.⁸ demonstrated, an increase in output at a high input level does not improve intelligibility; it primarily increases the perception of loudness. Hearing aid wearers may learn to accept this reduction in output from the nonlinear hearing aids if they are given the chance to acclimatize to the hearing aid processing. Kuk⁹ demonstrated that satisfaction and performance with hearing aids improved over time in adult hearing aid wearers.

Reasons for the infrequent recommendation of WDRC hearing aids for children with a severe-to-profound hearing loss might also include a limitation in the availability of hearing aids with such output capability. Digital signal processing in hearing aids now allows for WDRC capabilities in high-gain, high-output power hearing aids with low compression thresholds. These devices have the advantage of providing extra audibility for soft sounds while preserving (or enhancing) intelligibility and comfort at higher input levels.¹⁰

The current study was undertaken to compare the objective and subjective performance of digital power compression hearing aids over analog (and programmable) power hearing aids in school-age children who have a severe-to-profound degree of hearing loss.

Method
Subjects: Thirteen children with a symmetrical (within 10 dB) severe-to-profound hearing loss participated in the study. Eight of the children were recruited from the University of Colorado Health Sciences Center (UC) and 5 were recruited from the University of Kansas Medical Center (KUMC). Of the 8 children from the University of Colorado, 6 had a puretone average (PTA) between 70-80 dB HL and 2 had a PTA around 95 dB HL. Of the 5 subjects from the University of Kansas, 2 had a PTA of 90 dB HL, 2 had a PTA of 100 dB HL, and one had a PTA of 110 dB HL. With the exception of one child who was 6 years old, the other children ranged from 9-14 years of age with a mean age of 12.2 years (SD = 2.6 years).

All the children used oral communication as their primary mode of communication and were mainstreamed into regular classrooms. Most of the children wore binaural analog linear power hearing aids since they were 2-3 years of age. Informed consent was obtained from all the children (and their parents), and they were compensated for their participation. The clinicians did not make any claims of superior performance for the digital hearing aids to the study subjects, their parents, or teachers.

Study hearing aids: The hearing aid used in this study was the Widex Senso P38 (binaurally fitted). This is a digital 3-channel hearing aid that utilizes adaptive, slow-acting Enhanced Dynamic Range Compression (EDRC) as the processing algorithm. A low compression threshold at 20 dB HL is used in each of its three channels. High-level compression (HLC) is used above a conversational input level in each channel to further minimize the risk from a high output (ie, over-amplification, saturation). Ringdahl et al.¹¹ provided a detailed description of the characteristics of this hearing aid.

Procedure: All audiometric testing was performed at the speech and hearing clinic of the respective sites. Typically, each student was seen for a minimum of 4-5 sessions. Subject candidacy for inclusion into the study was determined during the first visit. The adequacy of the child’s own hearing aids and earmolds was also evaluated. Adjustment of the personal hearing aids was made and new earmolds were ordered when necessary.

Speech recognition ability with the child’s own hearing aids (and earmolds) was evaluated during the second visit. In addition, the aided thresholds with the child’s own hearing aids were also determined. For some children whose hearing aids and earmolds were judged to be appropriate, this measure was determined during the first session instead. All testing was done in a sound booth using commercial audiometers. The W-22 word lists were presented in live-voice at 35 dB HL in quiet and at 50 dB HL in a speech-shaped noise (SNR=+10) background. Auditory-alone and auditory-plus-visual modes were used. Mode of presentation was counterbalanced across subjects. Fitting of the study hearing aids followed completion of the speech test under strict guidelines. Aided thresholds with the study hearing aids were then determined. Children either completed speech testing with the study hearing aids on the same day or returned the next day to complete speech testing. The Student Inventory of the LIFE questionnaire¹² was also completed during this session.

The children returned at 1 month and at 3 months post-fitting to complete another round of speech recognition testing with the W-22 words. At the 3-month visit, the children also returned the LIFE questionnaire, the Widex Hearing Aid Questionnaire for Children, and the Widex Pediatric Questionnaire for Parents.

Results
Aided Soundfield Thresholds: The mean binaural aided soundfield thresholds for each group of subjects are separately reported in Figures 1a-1b. Soundfield aided thresholds for subjects’ own hearing aids (O) and for the study hearing aids (S) are included for comparison. For the UC subjects (n=8, Fig. 1a), the aided thresholds obtained with the study hearing aids were about 10-15 dB better than those with the children’s own hearing aids. This improved the aided thresholds from around 30-50 dB HL to around 10-35 dB HL across frequencies. The differences were statistically significant at the 0.05 level at all frequencies.

Figures 1a-1b. Mean binaural sound-field thresholds for subjects at the UC site (1a) and subjects at the KUMC site (1b). "U" represents unaided thresholds; "O" represents aided thresholds with subjects’ own hearing aids; "S" represents aided thresholds with the study hearing aids. Asterisks represent significant aided threshold difference between hearing aids.

Figure 1b shows a 15-20 dB improvement in aided thresholds below 500 Hz for KUMC subjects. The magnitude of the improvement decreased toward the higher frequencies such that the aided thresholds at 1000 Hz and 2000 Hz were only about 5-10 dB better than those obtained with their own hearing aids. The aided thresholds with the study hearing aids at 3000 Hz and 4000 Hz were around 60 dB HL and were poorer than those with the subjects’ own hearing aids. Individual two-tailed t-tests revealed a statistically significant difference at 250 Hz and 500 Hz, but non-significant difference at higher frequencies.

One reason for the high aided thresholds may be the higher unaided thresholds of the KUMC children (many had thresholds beyond the limits of the audiometer). A possible reason for the poorer aided thresholds obtained with the study hearing aids than with the children’s own hearing aids may be related to the absence of (or minimal) saturation distortion in the study hearing aids. The children may have responded to saturation distortion products when they were tested with their own hearing aids.

Speech Recognition Scores: Figures 2a-2b summarize the average speech recognition scores at the UC site and the KUMC site respectively. For the UC children (Figure 2a) at the 35 dB HL auditory-alone condition, the average subject scored 11% improvement with the study hearing aids compared to their own aids at the initial visit. An additional 5% improvement was seen at the 1-month follow-up. The scores at the 1-month and 3-month visits were significantly different from the subjects’ own hearing aids (p < 0.05). A similar trend, but with a greater magnitude of improvement, was observed in the 35 dB HL auditory-plus-visual condition. A small (6%) non-significant improvement of the study hearing aids over subjects’ own hearing aids was noted in the 50 dB HL test condition.

Figures 2a-2b. Average speech recognition scores on the W-22 word lists measured with the subjects’ own hearing aids and with the study hearing aids at different times (initial, 1-month, and 3-months post fitting) and across listening conditions (A- auditory only; AV-auditory plus visual) at the UC site (2a) and the KUMC site (2b).

Figure 2b shows that the effect of the study hearing aids was also evident in the KUMC subjects. At the initial visit for the 35 dB HL condition, an 8% improvement (2% versus 10%) was seen in the auditory-alone condition, and this improvement continued to 18% (2% versus 20%) at the 3-months interval. The same was true for the auditory-plus-visual condition where a 17% (29% versus 46%) improvement was seen initially with the study hearing aids, and improvement continued to 24% (29% versus 53%) at the 3-month visit. At the 50 dB HL auditory-alone condition, no improvement in speech score was seen until the 1-month and 3-months visits in both the auditory and auditory plus visual conditions.

Student’s LIFE Questionnaire: The mean ratings for items on the “LIFE Questionnaire: Student Form” are summarized in Figures 3a-3b for UC and KUMC children respectively. At the UC site (Figure 3a), the average child rated the study hearing aids higher than his/her own hearing aids on all test items, suggesting less difficulty across all classroom and school situations with the study hearing aids than with their own hearing aids. Items that were statistically different at the 0.05 level were identified with an asterisk. These items included: “teacher moving,” “small/large group,” “school assembly,” and “lunchroom.”

Figures 3a-3b. Mean ease of listening rating for each item on the “LIFE questionnaire: Student Form” at the UC site (3a) and the KUMC site (3b). The maximum rating for each of the first 10 items was "10" and the maximum rating for each of the next 5 items was "20." Items that showed significant difference between hearing aids were marked with an asterisk (*).

Similar improvement was also seen with the KUMC children (Figure 3b) across the same listening situations. The majority of items reached statistical significance (p < 0.05) and was identified with an asterisk. Some items showing the most change included #3 (teacher back), #4 (hallway noise), #11 (small group learning), #12 (gymnasium), #14 (lunchroom), and #15 (hanging coats). These items typically represented mild-to-moderately noisy environments and required listening without many visual cues.

Parent’s LIFE Questionnaire: Because the responses from the parents to their children’s hearing aids were similar between the two sites, their responses were collapsed for presentation. Figure 4 shows the percentage of parents who reported that their children performed a specific auditory behavior more frequently with the study hearing aids than with their own hearing aids. At least 60% of parents reported to the first five listening behaviors (to speech and sounds). As many as 33% of parents reported that their children were more socially active while wearing the study hearing aids and 23% reported that their children were less bothered by loud sounds. Over 80% of the parents were satisfied with the study hearing aids.

Figure 4. Percentage of parents who reported (on the Widex Parent Questionnaire) that their children exhibited the indicated listening behaviors more frequently with the study hearing aids than their own hearing aids.

Figure 5 summarizes the percentage of parents who reported that their children could understand better with the study hearing aids than with their own hearing aids in specific listening environments “in the home” and “outside the home.” About 60%-80%+ of parents reported that their children understand speech better with the study hearing aids across listening environments. The most significant situations were the telephone (85% better) and the grocery store (85% better).

Figure 5. Percentage of parents who reported (on the Widex Parent Questionnaire) that their children performed better with the study hearing aids than their own hearing aids in listening situations inside and outside of the home.

Of the 12 parents who reported, 4 noticed a change in their children’s speech production ability since wearing the study hearing aids. All these parents reported a change in voice quality and an increase in their accuracy of articulation. Two reported a decrease in voice loudness and 2 an increase in voice loudness. Two reported an increase in self-correction and 3 reported more accurate imitation of speech sounds (ie, easier simulation). Three parents also reported an increase in the complexity of their children’s sentence structure.

Children’s Questionnaire: Figure 6 shows the percentage of children who rated the study hearing aids as “better than” their own hearing aids in the list of listening situations. In almost all cases, at least 60% (and as many as 90%) of the children rated the study hearing aids as better than their own hearing aids in allowing them to hear and understand. Upon closer examination, many of these items that were rated higher involved listening from a distance (eg, “call from another room,” “call from behind”) or listening in moderately noisy situations (eg, “school bus,” “playground”). Interestingly, only 61% of the children rated the study hearing aids as better than their own hearing aids on the telephone (whereas 85% of their parents reported so).

Figure 6. Percentage of children who reported that they performed better with the study hearing aids than their own hearing aids on a list of listening situations on the Widex Hearing Aid Questionnaire for Children.

Figure 7 summarizes the number of children who preferred the study hearing aids over their own hearing aids at different times (initial, 1-month, and 3-months visits). Although 9 of the 13 children preferred the study hearing aids initially, 11 preferred the study hearing aids at the conclusion of the study (the other 2 reported same performance between hearing aids). Reasons for the preference included “better hearing at more places” (6); “hearing sounds not heard before” (6); “speak better” (6); and “better grades” (6). Other comments included “hearing more soft sounds,” “better on the telephone,” “less feedback,” and “less banging sounds (as in discomfort).”

Figure 7. Number of children who preferred their own hearing aids, the study hearing aids, or who indicated the same preference at different evaluation times.

Discussion
The results of the study showed that a 3-channel digital nonlinear power hearing aid yielded higher satisfaction/performance over analog power hearing aids for children with a severe-to-profound degree of hearing loss. Better aided soundfield thresholds, improved speech understanding for low input sounds, enhanced speech-reading skills, and a general improvement in communication in various listening situations, as assessed on the LIFE and the Widex questionnaires, were reported. Some children and their parents/teachers also reported an improvement in speech production ability. On the other hand, the ability to understand speech in noise at a moderate input level was not affected (although slightly improved for some children). It is worthwhile to speculate which hearing aid feature(s) may have contributed to the improved performance.

Performance at low input levels: The availability of more gain (and output) at low input levels through the use of a low compression threshold at 20 dB HL could have accounted for the improved speech recognition at 35 dB HL (auditory-alone and auditory-plus-visual), the subjective reports of hearing more sounds at a distance, and the improved speech production in some children. Kuk10 has pointed out that, for the same input-output curve above the compression threshold (CT), lowering the CT would yield higher gain for low input sounds below the CT. This allows for greater audibility (and thus intelligibility). Similar observations were also made for adults¹³ and children¹⁴ in other studies.

Performance at moderately high input levels: The current study did not show an immediate improvement in speech recognition ability at the 50 dB HL condition with the study hearing aids, although a slight improvement was seen at later times in some children. Likewise, many children reported a clearer sound perception and better performance in mild-to-moderately noisy environments (as shown in the questionnaires). Such observations may have resulted from the improved specificity of multichannel processing and the extra gain for soft sounds provided by the study hearing aids.

One may also expect some improvements to have resulted from the action of the high-level compression (HLC) used in the study hearing aids. Recall that HLC further reduces gain above a conversational input in order to minimize excessive output and preserve the temporal waveform cues. Such cues could be important in decoding supra-segmental information in the speech input. Van Tasell et al.¹⁵ demonstrated an increased reliance on such cues as one’s hearing loss increased.

The concept of HLC may create a negative impression at the initial fitting for some children who are accustomed to wearing linear hearing aids. Almost half of the children in this study indicated at the initial visit that sounds were “too soft,” “not clear,” “not loud enough,” or that “they cannot hear” with the study hearing aids. This is not surprising given that all of these children had used linear hearing aids since they were 2-3 years of age. This initial reaction was anticipated; therefore, gain settings on the study hearing aids were not increased if the aided thresholds were appropriate. Consequently, it is possible that some children may have been tested at a loudness level below what they were accustomed to.

From that standpoint, it is just as important to note that speech recognition scores at the input level of 50 dB HL were not affected negatively. Speech recognition improved over time for some children. In this study, those children who initially complained of insufficient loudness returned in a week with positive comments about the study hearing aids. This confirms the belief that, minimally, HLC does not negatively affect speech intelligibility. Indeed, it may improve speech perception over time for those whose perception may have been compromised by saturation distortion (ie, peak clipping) and/or reduction of temporal contrasts (ie, compression limiting).

Speech production ability: Although not universal, an improvement in speech production ability was reported in 4 of the 8 children at the UC site. Interestingly, none of the children at the KUMC site reported an improvement in speech production ability. The poorer unaided and aided thresholds and the older age of the children at the KUMC site may explain their lack of improvement in speech production abilities.

The number of channels in a nonlinear hearing aid may affect a child’s progress in speech production. During vocalization, speech produced by a speaker at a normal level would measure 15 dB-20 dB higher at the level of the ear than at a conversational distance of 1 meter.¹⁶ If the child wears a single-channel linear hearing aid (assume no VC adjustment), the high input would result in a high output. This extra output (especially in the lows) could mask the higher frequencies and render them inaudible (ie, upward spread of masking), or it may saturate the hearing aid to result in distortion. In both cases, the higher frequency sounds may not be audible to the child. If the child wears a single-channel compression hearing aid, the high input would result in gain reduction across all frequencies and render the high frequencies inaudible. It would be difficult for children to improve their speech production skills if they cannot hear and/or monitor their own production consistently.

In a multiple-channel hearing aid, the high-level low frequency input at the microphone would activate compression in the low frequency channel only and reduce its output. This spares the high frequencies and may preserve audibility in this region, allowing children to hear/monitor their own high frequency speech more consistently. Such consistent input may be the reason for improved speech articulation seen in some of the UC children. Such an observation was also noted in another study of a digital multichannel hearing aid in children with up to a moderately severe hearing loss.¹⁴

Is Digital Necessary?
A frequently asked question is whether digital hearing aids are necessary for children. From a practical point of view, it is difficult to design an analog multichannel nonlinear hearing aids with a low compression threshold without problems such as high current drain, increased circuit noise, and increased likelihood of feedback.¹⁰

Digital techniques offer the potential to reduce these problems. Specifically, their use could: 1) Result in more flexible and specific filter designs; 2) Implement an expansion circuit below the CT to minimize the perception of circuit noise, and 3) Employ feedback management techniques that allow the availability of high gain with less likelihood of feedback. Indeed, several of the subjects in this study indicated that one of the reasons for their preference is the reduction of feedback. The minimization of artifacts while allowing the use of available gain is an important advantage of applying DSP techniques in today’s hearing aids.

Conclusion
Results from this study support the use of multiple channel nonlinear EDRC hearing aids with a low CT for children with a severe-to-profound degree of hearing loss. Indeed, it may be even more desirable than the use of linear hearing aids which use compression limiting or peak clipping to limit output.The benefits seen in this study include:

• Improved aided thresholds by 10-20 dB across frequencies;
• Improved speech recognition at a low input level (35 dB HL) in quiet, with and without visual cues;
• No decrease (or a slight increase over time) in speech recognition in noise at a moderate input level (50 dB HL), with and without visual cues;
• Improved subjective satisfaction with amplification across a range of daily listening situations as reflected on the LIFE and the Widex questionnaires;
• Improved parental report on hearing aid use;
• Improved speech production ability in four children.

References
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