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Benefits of amplification for unaided speech discrimination in age-related hearing loss with flat type audiogram

      Abstract

      Objective

      The present study aimed to investigate whether hearing aid use can induce improvement as acclimatization effect in unaided speech perception in patients with age-related hearing loss.

      Methods

      Fifty ears in 41 patients (age range: 65-91 years) diagnosed as age-related hearing loss were enrolled in this study. They used hearing aids for more than 8 hours per day. Unaided speech audiometry using 67-S Japanese monosyllabic word list was performed one or two years after the commencement of hearing aid use. The changes in the unaided speech discrimination score before and after the commencement of hearing aid use were analyzed. To investigate factors for improvement, the patients’ backgrounds in terms of age, sex, pure tone average, unaided maximum speech discrimination score, fitting period (one year/two years), fitting ear (bilateral/unilateral), audiogram type (flat-type/other-type), and the level of amplification were also analyzed.

      Results

      Significant improvement in the unaided speech discrimination score after hearing aid use was seen only in the flat-type audiogram group. More than half of older patients in the flat-type audiogram group improved their unaided maximum speech discrimination score 10 % or more. The analysis of aided hearing thresholds revealed that the flat-type audiogram group had significantly lower thresholds of 3kHz and 4kHz than the other-type audiogram group. The age, sex, pure tone average, fitting period, fitting ear, functional gain were not influential factors for improvement. On the other hand, unaided maximum speech discrimination score before using hearing aid and aided hearing threshold at 4kHz had a negative correlation with improvement.

      Conclusion

      The findings suggested that older patients with age-related hearing loss whose audiogram is a flat type can benefit from amplification as means of improving their unaided speech perception since flat-type audiogram can be more easily adjusted to sufficiently amplify speech sound at high frequencies. It should be considered that the potential for experience-dependent plasticity is retained even in older adults.

      Keywords

      1. Introduction

      In recent years, hearing loss has come to be recognized as a long-term risk factor in cognitive decline and dementia [
      • Livingston G
      • Sommerlad A
      • Orgeta V
      • Costafreda SG
      • Huntley J
      • Ames D
      • et al.
      Dementia prevention, intervention, and care.
      ]. Some studies have suggested that hearing aid use could alleviate the negative effects of hearing loss on cognitive decline [
      • Mulrow CD
      • Aguilar C
      • Endicott JE
      • Tuley MR
      • Velez R
      • Charlip WS
      • et al.
      Quality-of-life changes and hearing impairment. A randomized trial.
      ,
      • Amieva H
      • Ouvrard C
      • Giulioli C
      • Meillon C
      • Rullier L
      • Dartigues JF.
      Self-Reported hearing loss, hearing aids, and cognitive decline in elderly adults: A 25-year study.
      . There has been much interest in the time course of changes in performance associated with the use of hearing aids. Neural plasticity of the auditory cortex causes a phenomenon popularly termed ‘acclimatization,’ which is thought to indicate alterations in the nervous system allowing better adaptation to environmental changes through hearing aid use.
      Speech intelligibility decreases with progression of sensorineural hearing loss, but the extent of these two is not always commensurate. At present, the possibility of a change in the speech discrimination score following hearing aid fitting is not generally recognized. Seminal work by Lavie et al. has shown that in older adults, four weeks of hearing aid use induced improvements in speech identification [
      • Lavie L
      • Banai K
      • Karni A
      • Attias J.
      Hearing aid-induced plasticity in the auditory system of older adults: Evidence from speech perception.
      ]. Reber et al. also reported improved speech discrimination after six months of hearing aid use [
      • Reber MB
      • Kompis M.
      Acclimatization in first-time hearing aid users using three different fitting protocols.
      ]. On the other hand, some studies failed to find favorable evidence for acclimatization in speech perception with hearing aids. [
      • Humes LE
      • Wilson DL
      An examination of changes in hearing-aid performance and benefit in the elderly over a 3-year period of hearing-aid use.
      ,
      • Gelfand SA.
      Long-term recovery and no recovery from the auditory deprivation effect with binaural amplification: six cases.
      Gelfand et al. reported individual differences in acclimatization with amplification.[
      • Gelfand SA.
      Long-term recovery and no recovery from the auditory deprivation effect with binaural amplification: six cases.
      ]. Among the factors that may account for the individual variability are different experimental designs, differences in the aided auditory input, the severity and type of hearing loss, cognitive factors, and the circumstances of hearing aid use.
      It is important to investigate whether hearing aids can improve unaided speech intelligibility. Maintaining a high, unaided, maximum speech discrimination score before hearing aid fitting is one of the most important factors for higher efficacy and satisfaction with hearing aid use [
      • Wu X
      • Ren Y
      • Wang Q
      • Li B
      • Wu H
      • Huang Z
      • et al.
      Factors associated with the efficiency of hearing aids for patients with age-related hearing loss.
      ]. The present study aimed to investigate whether hearing aid use can induce plastic changes in unaided speech discrimination in older adults with hearing impairment as well as which cases can obtain auditory acclimatization. Conducting this test without amplification can help assess perceptual plasticity beyond the acclimatization effect itself. In addition, the individual maximum speech discrimination scores used to analyze speech intelligibility in the present study were obtained from patients with different levels of hearing ability and not from data pertaining only to a constant sound pressure level.

      2. Materials and methods

      2.1 Patients

      The present study was performed in accordance with the Declaration of Helsinki and its later amendments and was approved by the ethics review board of Tama-Hokubu Medical Center (No. 30-2). Patients visiting the otolaryngology department at Tama-Hokubu Medical Center between April 2018 and March 2019 for a periodic examination one or two years after hearing aid fitting were enrolled. Six patients with conductive hearing loss were excluded. The remaining 47 patients had a diagnosis of age-related hearing loss. Moreover, six patients who were using their hearing aids less than eight hours per day were also excluded. The remaining 50 ears in 41 patients were analyzed. The patients’ backgrounds in terms of age, sex, pure tone average and unaided maximum speech discrimination score at the start of hearing aid use, fitting period (one year/two years), fitting ear (bilateral/unilateral), audiogram type (flat type/other type), and the level of amplification were investigated. We defined the flat type as audiogram whose hearing gap between 500Hz and 4000Hz was -14dB to +14dB, while the other audiogram was classified into the other-type audiogram.

      2.2 Hearing aid fitting

      All the patients chose a basic model with two to four channels and were able to choose from among three types, namely the completely-in-the-canal, in-the-canal, and in-the-ear types. All the hearing aids were manufactured by RION (Tokyo, Japan). All the participants were fitted by the same two audiologists. We recommended bilateral fitting, but most of the patients chose unilateral fitting for the better ear due to the imbalance between their ears or for financial reasons. As a result, nine of the patients chose bilateral fitting, and the remaining 32 chose unilateral fitting. The first follow-up assessment and fine tuning were undertaken one month after the initial fitting. After the first assessment, follow-up assessments were undertaken every three months for the first year. After the first one-year follow up, the patients continued to undergo an annual assessment. In the present study, all the hearing aids were adjusted to meet the following criteria: 1) functional gain of about one-half of the hearing threshold; and 2) an aided threshold of 35 dB HL at 1000 Hz as required by the Oto-Rhino-Laryngological Society of Japan guidelines [
      • Kodera K
      • Hosoi H
      • Okamoto M
      • Toshiki M
      • Yukihiko K
      • Kimio S
      • et al.
      Guidelines for the evaluation of hearing aid fitting (2010).
      ].

      2.3 Audiological examinations

      Audiological examinations were carried out in a soundproof room using an AA-84 audiometer (RION, Tokyo, Japan) in accordance with the Japan Audiological Society guidelines. The pure tone average was calculated using audiometric thresholds at four frequencies ((500Hz + 1kHz + 2kHz + 4kHz) / 4). The maximum speech discrimination scores for both ears were examined using a native monosyllabic word list (67-S) consisting of 20 words developed by the Japan Audiological Society before hearing aid fitting [

      Japan Audiological Society. Speech audiometry (2003). Audiol Jpn. 2003;46(6):621-37, 10.4295/audiology.46.621

      ]. At the start of the examination, the sound pressure level was set to 40-50 dB above the threshold. The speech recognition curve was determined by assessing speech discrimination three to five times with the sound pressure levels decreased or increased by 10-15dB. The maximum speech discrimination score was defined as the highest score on the curve. For comparison with the scores before fitting, the maximum speech discrimination score under unaided conditions, such as pure tone audiometry, speech audiometry under aided conditions, and measurement of functional gain, was added to the regular hearing aid checkups. As there are no reports of any clinically significant differences in speech discrimination scores using the 67-S Japanese monosyllabic word list, clinical improvement was defined as an increase of 10 % or more based on a report using the57-S Japanese monosyllabic word list compiled by Hirota et al., which consists of 50 words [
      • Hirota E
      • Kodera K
      • Kudo T.
      Application of speech discrimination test for the fitting of hearing aids.
      ].

      2.4 Statistical analysis

      Differences between the groups were analyzed using Pearson's chi-square test or Student's t-test. A statistical correlation between any two variables was evaluated using Pearson's correlation coefficient. Statistical analysis was performed using SPSS Statistics software (version 22.0.0, IBM, Armonk, NY, USA). P<0.05 was considered to indicate statistical significance.

      3. Results

      In total, 50 ears in 41 patients comprising 19 women and 22 men, aged 65-91 years, were enrolled. The patients’ mean age was 78.1, the pure tone average at the start of hearing aid use was 51.4±7.0 dBHL and the average unaided maximum speech discrimination score was 76.3±13.2%. The details of each classification of sex, fitting period, fitting ear, and audiogram type were shown in Table 1. Fig. 1 shows cascade audiograms of the flat-type audiogram group and the other-type audiogram group. Fig. 2 shows the results of correlation analysis between each factor (age, pure tone average before hearing aid use, unaided maximum speech discrimination score before hearing aid use) and changes in the unaided maximum speech discrimination score in all the ears. The vertical axis represents changes in the unaided maximum speech discrimination score before and after hearing aid use. The elements above the horizontal line at 0 show increases, and those below show decreases. In Fig. 2a, there was no correlation between the degree of improvement and age (r= 0.051; p=0.73). Fig. 2b shows no correlation between pure tone average and changes in the unaided maximum speech discrimination score (r=0.06, p=0.41). In Fig. 2c, correlation analysis between the unaided maximum speech discrimination score before hearing aid use and changes in the unaided maximum speech discrimination score indicated a moderate negative correlation (r=-0.59; p=0.000006). A lower starting score led to larger improvement. Fig. 3 shows the results of correlation analysis between hearing thresholds with hearing aids at each frequency (1kHz, 2kHz, 3kHz and 4kHz) and changes in unaided maximum speech discrimination score. As to 4kHz, weak negative correlation was indicated (r= -0.30, p= 0.038). On the other hand, Fig. 4 shows the results of correlation analysis between functional gain at each frequency and changes in unaided maximum speech discrimination score, indicating no significant correlation. Table 2 shows the number of improved ears in all classified groups. There was no significant difference in sex, fitting period, and fitting ear. However, the flat-type audiogram group had a significantly greater proportion of improved ears than the other-type audiogram group. Fig. 5 shows the unaided maximum speech discrimination score before and after the commencement of hearing aid use in individual cases of the flat-type audiogram group and the other-type audiogram group. The elements above the diagonal line show increases, and the elements below show decreases. An increase of 10% or more was considered clinically significant improvement. Table 3 shows the mean value and standard deviation of the pure tone average and the unaided maximum speech discrimination score before and after the commencement of hearing aid use of all the ears, the flat-type audiogram group and the other-type audiogram group. None of the groups demonstrated a significant change in pure tone average between the two periods. In all the ears, the unaided maximum speech discrimination score ranged from 40 to 100% (mean±SD:76.3±13.2%) before the commencement of hearing aid use and from 60 to 100% (mean±SD:78.0±11.1%) after the commencement of hearing aid use, indicating no change. In the flat-type audiogram group, the unaided maximum speech discrimination score ranged from 40 to 100% (mean±SD:76.1±14.0%) before the commencement of hearing aid use and from 60 to 100% (mean±SD:81.5±11.3%) after the commencement of hearing aid use, indicating significant improvement. In the other-type audiogram group, the unaided maximum speech discrimination score ranged from 40 to 100% (mean±SD: 76.5±12.7%) before the commencement of hearing aid use and from 60 to 95% (mean±SD:75.0±10.1 %) after the commencement of hearing aid use, indicating an absence of any change. Table 4 shows the backgrounds of the flat-type audiogram group and the other-type audiogram group. There was no significant difference in age, pure tone average, unaided maximum speech discrimination score before hearing aid use, fitting ear and fitting period. However, the flat-type audiogram group included significantly more women. To investigate differences in the effect of hearing aids on the two groups, the aided hearing threshold at each frequency (1kHz, 2kHz, 3kHz, 4kHz) was analyzed in both groups. Fig. 6 shows the results. At 3kHz and 4kHz, the flat-type audiogram group showed significantly lower aided hearing thresholds than the other-type audiogram group.
      Table 1Backgrounds at the beginning of hearing aid use of all patients (SD: standard deviation, PTA: pure tone average, SDS: speech discrimination score)
      CharacteristicsAll patients
      n=50 (41 patients)
      Age (year, average±SD)78.12±6.0
      Sex
       Male24
       Female26
      PTA (dB HL, average±SD)51.4 ±7.0
      Maximum SDS (%, average±SD)76.3±13.2
      Years
       one year31
       two years19
      Fitting ear
       Bilateral18
       Unilateral32
      Audiogram
       Flat type23
       Others type27
      Fig 1
      Fig. 1Cascade audiogram of the flat-type audiogram group and the other-type audiogram group before hearing aid use.
      Fig 2
      Fig. 2Analysis of the correlation between age and changes in unaided maximum SDS in all the ears (a); Analysis of the correlation between pure tone average before hearing aid use and changes in the unaided maximum SDS in all the ears (b); Analysis of the correlation between unaided maximum SDS before hearing aid use and changes in the unaided maximum SDS in all the ears (c). The vertical axis indicates changes in the unaided maximum SDS before and after hearing aid use. The elements above the horizontal line at 0 show increases, and those below show decreases. SDS: speech discrimination score
      a. Improvement in the unaided maximum SDS showed no significant correlation with age.
      b. Improvement in the unaided maximum SDS showed no significant correlation with pure tone average.
      c. Improvement in the unaided maximum SDS showed a moderate, negative correlation with the unaided maximum SDS before hearing aid use.
      Fig 3
      Fig. 3Analysis of the correlation between changes in unaided maximum SDS and hearing thresholds with hearing aids at 1kHz, 2kHz, 3kHz and 4kHz in all the ears. The vertical axis indicates changes in the unaided maximum SDS before and after hearing aid use. The elements above the horizontal line at 0 show increases, and those below show decreases. At 4kHz improvement in the maximum SDS showed a weak, negative correlation with the hearing thresholds with hearing aids. SDS: speech discrimination score
      Fig 4
      Fig. 4Analysis of the correlation between changes in unaided maximum SDS and functional gain at 1kHz, 2kHz, 3kHz and 4kHz in all the ears. The vertical axis indicates changes in the unaided maximum SDS before and after hearing aid use. The elements above the horizontal line at 0 show increases, and those below show decreases. There was no significant correlation at all the frequencies. SDS: speech discrimination score
      Table 2Changes in unaided maximum SDS of each group (SDS: speech discrimination score)
      CharacteristicsAll10% or more increaseNo change10% or more decreasep value
      No.No.(%)No.(%)No.(%)
      All ears5018 (36%)21(42%)11(22%)
      Sex
       Male249(37.5%)8(33.3%)7(29.2%)0.38
       Female269(34.6%)13(50%)4(15.4%)
      Years
       one year3111(35.5%)14(45.2%)6(19.3%)0.79
       two years197(36.8%)7(36.8%)5(26.4%)
      Fitting ear
       Bilateral186(33.3%)9(50%)3(16.4%)0.65
       Unilateral3212(37.5%)12(37.5%)8(25%)
      Audiogram
       Flat type2313(56.5%)7(30.4%)3(13.1%)0.019
       Other type275(18.5%)14(51.9%)8(29.6%)
      Fig 5
      Fig. 5Unaided maximum SDS in flat-type audiogram group and other-type audiogram group before and after the commencement of hearing aid use. Patients above the diagonal line showed improvement in their unaided maximum SDS after hearing aid use. The red dots indicate patients with 10% or more improvement. The blue dots indicate patients with a decrease of 10% or more. SDS: speech discrimination score
      Table 3Mean value with standard deviation (PTA: pure tone audiometry, SDS: speech discrimination score).
      Before using hearing aidsAfter using hearing aidsp value
      All ears
       PTA (dB HL)51.4 ±7.051.4±7.40.95
       Maximum SDS (%)76.3±13.278.0±11.10.28
      Flat-type audiogram group
       PTA (dB HL)51.4±6.050.8±7.30.33
       Maximum SDS (%)76.1±14.081.5±11.30.016
      Other-type audiogram group
       PTA (dB HL)51.4±7.851.9±7.60.53
       Maximum SDS (%)76.5±12.775.0±10.10.48
      Table 4Backgrounds at the beginning of hearing aid use of each group (SD: standard deviation, PTA: pure tone average, SDS: speech discrimination score).
      CharacteristicsFlat-type audiogramOther-type audiogramp value
      n=23 (18 patients)n=27 (23 patients)
      Age (year, average±SD)77.6±7.078.6±5.20.56
      Sex
       Male7170.0218
       Female1610
      PTA (dB HL, average±SD)51.4±6.051.4±7.81.00
      Maximum SDS (%, average±SD)76.1±14.076.5±12.70.92
      Years
       one year12190.186
       two years118
      Fitting ear
       Bilateral1080.309
       Unilateral1319
      Fig 6
      Fig. 6Box plot showing hearing threshold with hearing aid use at each frequency in the flat-type audiogram group and the other-type audiogram group. The largest value was less than 1.5 times the interquartile range above the 75th percentile while the smallest value was within 1.5 times the interquartile range below the 25th percentile. The outside value was > 1.5 times and < 3 times the interquartile range beyond either end of the box.

      4. Discussion

      In the present study, more than half of older patients in the flat-type audiogram group improved their unaided maximum speech discrimination score 10 % or more, which is regarded as clinically significant improvement. On the other hand, there was no significant improvement in the unaided speech discrimination score in older patients in the other-type audiogram group. Our analysis of the aided hearing threshold at each frequency in each group revealed that the flat-type audiogram group had a significantly lower threshold at 3kHz and 4kHz than the other-type audiogram group. A correlation analysis of all ears between changes in unaided maximum speech discrimination score and hearing threshold with hearing aids showed negative weak correlation at 4kHz. Although the analysis did not indicate significant correlation at 3kHz, consonants fall into the high-frequency area, and that the Speech Intelligibility Index (SII) has established the critical contribution of higher frequencies to speech understanding, therefore, achieving small hearing thresholds at high frequencies is important for improving speech perception. It is generally more difficult for patients with a sloping-type audiogram to achieve full high-frequency amplification than patients with a flat-type audiogram. It is also possible that patients with a non-flat-type audiogram did not see any improvement in speech intelligibility because of sound distortions and/or severe recruitment hearing when their functional gain is big enough to hear low sounds. This may be the factor for the results of correlation analysis between changes in unaided maximum speech discrimination score and functional gain, indicating no significant correlation. Therefore, our results suggested that the flat-type audiogram is an important factor in increasing the benefits of amplification and inducing significant improvement in the unaided speech discrimination score because hearing aids with flat-type audiogram can be more easily adjusted to sufficiently amplify speech sound especially at high frequencies.
      It should be mentioned here that the sex ratio differed significantly between the groups. However, the flat-type audiogram group showed no significant difference in sex because 13 ears, including five male ears and eight female ears, improved by 10% or more while 11 ears, including two male ears and nine female ears, improved less than 10% (p=0.39). Therefore, the present study found no difference in terms of sex.
      The fact that age was not a factor in improving speech intelligibility proved that improvement of speech intelligibility by amplification is possible despite old age. In fact, a lower starting score led to larger improvement. Further, there was no significant correlation between the pure tone average and changes in the unaided maximum speech discrimination score. In any case, the findings suggested that the potential for improvement in unaided speech intelligibility, an important reason for effective use of hearing aids, is retained even in older individuals with advanced hearing difficulties and a low unaided speech discrimination score. Kim et al. also reported that hearing aids benefited patients with a low speech discrimination score as demonstrated by the higher speech discrimination scores achieved with hearing aid use in their study [
      • Baer T
      • Moore BCJ
      • Kluk K.
      Effects of low pass filtering on the intelligibility of speech in noise for people with and without dead regions at high frequencies.
      ].
      There were several cases in which the speech discrimination score decreased by 10% or more despite proper fitting and use of hearing aids. This study covered only patients who reported using hearing aids for eight hours or more per day. Although this wearing time was adequate, differences in family life and talking time may have affected speech intelligibility. More detailed studies of daily life and talking time may demonstrate different findings. Individual differences in aging influencing various aspects of sound recognition ability and physical functions also need to be considered.
      The present study has some limitations. First, the small number of patients may have introduced a selection bias. In addition, several previous studies have reported that patients whose threshold exceeds a certain level often suffer damage to their inner hair cells, which leads to diminished or absent afferent input, limiting the effect of high-frequency amplification [
      • Nathan EA
      • Humes LE
      Contribution of high frequencies to speech recognition in quiet and noise in listeners with varying degrees of high-frequency sensorineural hearing loss.
      ,
      • Kim H
      • Choo OK
      • Park K
      • Gu GY
      • Park SH
      • Jang JH
      • et al.
      Hearing aids are still beneficial to patients, even if they have a low speech discrimination.
      . While Turner et al. found that amplification of speech in a background noise always provided some benefit for listeners with hearing loss, regardless of the degree of hearing loss and/or the frequency region of speech [
      • Hornsby BWY
      • Ricketts TA.
      The effects of hearing loss on the contribution of high- and low-frequency speech information to speech understanding.
      ]. Hornsby et al. made the similar report [
      • Turner CW
      • Henty BA.
      Benefits of amplification for speech recognition in background noise.
      ]. Although the concept of the threshold breakpoint at high frequencies was not considered in our analysis, this study was the first to demonstrate acclimatization brought about by sufficient amplification at higher frequencies.

      5. Conclusion

      The present study found significant improvement in the unaided speech discrimination score in patients with age-related hearing loss in the flat-type audiogram group following hearing aid use. Using hearing aids for a relatively long period was found to induce improvement in older individuals’ speech discrimination ability.
      Speech perception performance in older adults declines over time. Nonetheless, older adults can maintain or even improve their performance by using hearing aids. Our study indicates that the hearing aids are not merely sound amplification tools but may also serve as auditory rehabilitation tools.

      Disclosure statements

      The authors report no conflict of interest.

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