Starkey Research & Clinical Blog

On the Prevalence of Cochlear Dead Regions

Pepler, A., Munro, K., Lewis, K. & Kluk, K. (2014). Prevalence of Cochlear Dead Regions in New Referrals and Existing Adult Hearing Aid Users. Ear and Hearing 20(10), 1-11.

This editorial discusses the clinical implications of an independent research study and does not represent the opinions of the original authors.

Cochlear dead regions are areas in which, due to inner hair cell and/or nerve damage, responses to acoustic stimuli occur not at the area of peak basilar membrane stimulation but instead occur at adjacent regions in the cochlea. Professor Brian Moore defined dead regions as a total loss of inner hair cell function across a limited region of the basilar membrane (Moore, et al., 1999b). This hair cell loss does not result in an inability to perceive sound at a given frequency range, rather the sound is perceived via off-place or off-frequency listening, a spread of excitation to adjacent regions in the cochlea where inner hair cells are still functioning (Moore, 2004).  Because the response is spread across a broad tonotopic area, individuals with cochlear dead regions may perceive pure tones as “clicks”, “buzzes” or “whooshes”.

Cochlear dead regions are identified and measured by a variety of masking techniques. The most accurate method is the calculation of psychophysical tuning curves (PTCs), originally developed to measure frequency selectivity (Moore & Alcantara 2001). A PTC plots the level required to mask a stimulus frequency as a function of the masker frequency. For a normally hearing ear, the PTC peak will align with the point at which the stimulus can be masked by the lowest level masker.  In ears with dead regions, the tip of the PTC is shifted off of the signal frequency to indicate that the signal is being detected in an adjacent region. Though PTCs are an effective method of identifying and delineating the edges of cochlear dead regions, they are time consuming and ill-suited to clinical use.

The test used most frequently for clinical identification of cochlear dead regions is the Threshold Equalizing Test (TEN; Moore et al., 2000; 2004). The TEN test was developed with the idea that tones detected by off-frequency listening, in ears with dead regions, should be easier to mask with broadband noise than they would in ears without dead regions. With the TEN (HL) test, masked thresholds are measured across the range of 500Hz to 4000Hz, allowing the approximate identification of a cochlear dead region.

There are currently no standards for clinical management of cochlear dead regions. Some reports suggest that affect speech, pitch, loudness perception, and general sound quality (Vickers et al., 2001; Baer et al., 2002; Mackersie et al., 2004; Huss et al., 2005a; 2005b). Some researchers have specified amplification characteristics to be used with patients with diagnosed dead regions, but there is no consensus and different studies have arrived at conflicting recommendations. While some recommend limiting amplification to a range up to 1.7 times the edge frequency of the dead region (Vickers et al., 2001; Baer et al., 2002), others advise the use of prescribed settings and recommend against limiting high frequency amplification (Cox et al., 2012; see link for a review).  Because of these conflicting recommendations, it remains unclear how clinicians should modify their treatment plans, if at all, for hearing aid patients with dead regions.

Previous research on the prevalence of dead regions has reported widely varying results, possibly due to differences in test methodology or subject characteristics. In a study of hearing aid candidates, Cox et al. (2011) reported a dead region prevalence of 31%, but their strict inclusion criteria likely missed individuals with milder hearing losses, so their prevalence estimate may be different from that of hearing aid candidates at large. Vinay and Moore (2007) reported higher prevalence of 57% in a study that did include individuals with thresholds down to 15dB HL at some frequencies, but the median hearing loss of their subjects was higher than that of the Cox et al. study, which likely impacted the higher prevalence estimate in their subject group.

In the study being reviewed, Pepler and her colleagues aimed to determine how prevalent cochlear dead regions are among a population of individuals who have or are being assessed for hearing aids. Because dead regions become more likely as hearing loss increases, and established hearing aid patients are more likely to have greater degrees of hearing loss, they also investigated whether established hearing aid patients would be more likely to have dead regions than newly referred individuals.  Finally, they studied whether age, gender, hearing thresholds or slope of hearing loss could predict the presence of cochlear dead regions.

The researchers gathered data from a group of 376 patients selected from the database of a hospital audiology clinic in Manchester, UK. Of the original group, 343 individuals met inclusion criteria; 193 were new referrals and 150 were established patients and experienced hearing aid users.  Of the new referrals, 161 individuals were offered and accepted hearing aids, 16 were offered and declined hearing aids and 16 were not offered hearing aids because their losses were of mild degree.  The 161 individuals who were fitted with new hearing aids were referred to as “new” hearing aid users for the purposes of the study. All subjects had normal middle ear function and otoscopic examinations and on average had moderate sensorineural hearing losses.

When reported as a proportion of the total subjects in the study, Pepler and her colleagues found dead region prevalence of 36%.  When reported as the proportion of ears with dead regions, the prevalence was 26% indicating that some subjects had dead regions in one ear only. Follow-up analysis on 64 patients with unilateral dead regions revealed that the ears with dead regions had significantly greater audiometric thresholds than the ears without dead regions. Only 3% of study participants had dead regions extending across at three or more consecutive test frequencies. Ears with contiguous dead regions had greater hearing loss than those without.  Among new hearing aid users, 33% had dead regions while the prevalence was 43% among experienced hearing aid users. On average, the experienced hearing aid users had poorer audiometric thresholds on average than new users.

Pepler and colleagues excluded hearing losses above 85dB HL because effective TEN masking could not be achieved. Therefore, dead regions were most common in hearing losses from 50 to 85dB HL, though a few were measured below that range. There were no measurable dead regions for hearing thresholds below 40dB HL. Ears with greater audiometric slopes were more likely to have dead regions, but further analysis revealed that only 4 kHz thresholds had a significant predictive contribution and the slopes of high-frequency hearing loss only predicted dead regions because of the increased degree of hearing loss at 4 kHz.

Demographically, more men than women had dead regions in at least one ear, but their audiometric configurations were different: women had poorer low frequency thresholds whereas men had poorer high frequency thresholds. It appears that the gender effect actually due to the difference in audiometric configuration, specifically the men’s poorer high frequency thresholds. A similar result was reported for the analysis of age effects. Older subjects had a higher prevalence of dead regions but also had significantly poorer hearing thresholds.  Though poorer hearing thresholds at 4kHz did slightly increase the likelihood of dead regions, regression analysis of the variables of age, gender and hearing thresholds found that none of these factors were significant predictors.

Pepler et al’s prevalence data agree with the 31% reported by Cox et al (2012), but are lower than that reported by Vinay and Moore (2007), possibly because the subjects in the latter study had greater average hearing loss than those in the other studies.  But when Pepler and her colleagues used similar inclusion criteria to the Cox study, they found a prevalence of 59%, much higher than the report by Cox and her colleagues and likely due to the exclusion of subjects with normal low frequency hearing in the Cox study. The authors proposed that Cox’s exclusion of subjects with normal low frequency thresholds could have reduced the overall prevalence by increasing the proportion of subjects with metabolic presbyacusis and eliminating some subjects with sensory presbyacusis—sensory presbyacusis is often associated with steeply sloping hearing loss and involves atrophy of cochlear structures (Shuknecht, 1964).

 In summary:

The study reported here shows that roughly a third of established and newly referred hearing aid patients are likely to have at least one cochlear dead region, in at least one ear. A very low proportion (3% reported here) of individuals are likely to have dead regions spanning multiple octaves. The only factor that predicted the presence of dead regions was hearing threshold at 4 kHz.

On the lack of clinical guidance:

As more information is gained about prevalence and risk factors, what remains missing are clinical guidelines for management of hearing aid users with diagnosed high-frequency dead regions. Conflicting recommendations have been proposed for either limiting high frequency amplification or preserving high frequency amplification and working within prescribed targets. The data available today suggest that prevalence of contiguous multi-octave dead regions is very low and a further subset of hearing aid users with contiguous dead regions experience any negative effects of high-frequency amplification. With consideration to these observations, it seems prudent that the prescription of high-frequency gain should adhere to the prescribed targets for all patients at the initial fitting. Any reduction to high-frequency gains should be managed as a result of subjective feedback from the patient after they have completed a trial period with their hearing aids.

On frequency lowering and dead regions:

Some clarity is required regarding the role of frequency lowering and the treatment of cochlear dead regions. Because acoustic information in speech extends out to 10 kHz and because most hearing aid frequency responses roll off significantly after 4-5 kHz, the mild prescription of frequency lowering can be beneficial to many hearing aid users. It must be noted that the benefits of this technology arise largely from the acoustic limitations of the device and not the presence or absence of a cochlear dead region. There are presently no recommendations for the selection of frequency lowering parameters in cases of cochlear dead regions. In the absence of these recommendations, the best practice for the prescription of frequency lowering would follow the same guidelines as any other patient with hearing loss; validation and verification should be performed to document benefit with the algorithm and identify appropriate selection of algorithm parameters.

On the low-frequency dead region: 

The effects of low-frequency dead regions are not well studied and may have more significant impact on hearing aid performance.  Hornsby (2011) reported potential negative effects of low frequency amplification if it extends into the range of low-frequency dead regions (Vinay et al., 2007; 2008). In some cases performance decrements reached 30%, so the authors recommended using low-frequency gain limits of 0.57 times the low-frequency edge of the dead region in order to preserve speech recognition ability. Though dead regions are less common in the low frequencies than in the high frequencies, more study on this topic is needed to determine clinical testing and treatment implications.

References

Baer, T., Moore, B. C. and Kluk, K. (2002). Effects of low pass filtering on the intelligibility of speech in noise for people with and without dead regions at high frequencies. Journal of the Acoustical Society of America 112(3 Pt 1), 1133-44.

Cox, R., Alexander, G., Johnson, J., Rivera, I. (2011). Cochlear dead regions in typical hearing aid candidates: Prevalence and implications for use of high-frequency speech cues. Ear and Hearing 32(3), 339 – 348.

Cox, R.M., Johnson, J.A. & Alexander, G.C. (2012).  Implications of high-frequency cochlear dead regions for fitting hearing aids to adults with mild to moderately severe hearing loss. Ear and Hearing 33(5), 573-87.

Hornsby, B. (2011) Dead regions and hearing aid fitting. Ask the Experts, Audiology Online October 3, 2011.

Huss, M. & Moore, B. (2005a). Dead regions and pitch perception. Journal of the Acoustical Society of America 117, 3841-3852.

Huss, M. & Moore, B. (2005b). Dead regions and noisiness of pure tones. International Journal of Audiology 44, 599-611.

Mackersie, C. L., Crocker, T. L. and Davis, R. A. (2004). Limiting high-frequency hearing aid gain in listeners with and without suspected cochlear dead regions. Journal of the American Academy of Audiology 15(7), 498-507.

Moore, B., Huss, M. & Vickers, D. (2000). A test for the diagnosis of dead regions in the cochlea. British Journal of Audiology 34, 205-224.

Moore, B. (2004). Dead regions in the cochlea: Conceptual foundations, diagnosis and clinical applications. Ear and Hearing 25, 98-116.

Moore, B. & Alcantara, J. (2001). The use of psychophysical tuning curves to explore dead regions in the cochlea. Ear and Hearing 22, 268-278.

Moore, B.C., Glasberg, B. & Vickers, D.A. (1999b). Further evaluation of a model of loudness perception applied to cochlear hearing loss. Journal of the Acoustical Society of America 106, 898-907.

Pepler, A., Munro, K., Lewis, K. & Kluk, K. (2014). Prevalence of Cochlear Dead Regions in New Referrals and Existing Adult Hearing Aid Users. Ear and Hearing 20(10), 1-11.

Schuknecht HF. Further observations on the pathology of presbycusis. Archives of Otolaryngology 1964;80:369—382

Vickers, D., Moore, B. & Baer, , T. (2001). Effects of low-pass filtering on the intelligibility of speech in quiet for people with and without dead regions at high frequencies. Journal of the Acoustical Society of America 110, 1164-1175.

Vinay and Moore, B. C. (2007). Speech recognition as a function of high-pass filter cutoff frequency for people with and without low-frequency cochlear dead regions. Journal of the Acoustical Society of America 122(1), 542-53.

Vinay, Baer, T. and Moore, B. C. (2008). Speech recognition in noise as a function of high pass-filter cutoff frequency for people with and without low-frequency cochlear dead regions. Journal of the Acoustical Society of America 123(2), 606-9.

Cochlear Dead Regions and High Frequency Gain: How to Fit the Hearing Aid

Cox, R.M., Johnson, J.A. & Alexander, G.C. (2012). Implications of high-frequency cochlear dead regions for fitting hearing aids to adults with mild to moderately severe hearing loss. Ear and Hearing, May 2012, e-published ahead of print.

This editorial discusses the clinical implications of an independent research study. This editorial does not represent the opinions of the original authors.

Cochlear dead regions (DRs) are defined as a total loss of inner hair cell function across a limited region of the basilar membrane (Moore, et al., 1999b). This does not result in an inability to perceive sound at a given frequency range, rather the sound is perceived via a spread of excitation to adjacent regions in the cochlea where the inner hair cells are still functioning. Because the response is spread out over a broader tonotopic region, patients with cochlear dead regions may perceive some high frequency pure tones as “clicks”, “buzzes” or “whooshes” rather than tones.

Dead regions can be present at moderate hearing thresholds (e.g. 60dBHL) and are more likely to be present as the degree of loss increases. Psychophysical tuning curves are the preferred method for identifying cochlear dead regions in the laboratory, complicated and time consuming. Moore and his colleagues developed the Threshold Equalizing Noise (TEN) Test as a clinical means of identifying dead regions (Moore et al., 2000; Moore et al., 2004). The TEN procedure looks for shifts in masked thresholds beyond what would typically be expected for a given hearing loss.  A threshold obtained with TEN masking noise that shifts at least 10dB indicates the likely presence of a cochlear dead region.

Historically, there has been a lack of consensus among clinical investigators as to whether or not high frequency gain is beneficial for hearing aid users who have cochlear dead regions. Some studies suggest that high frequency gain could have deleterious effects on speech perception and should be limited for individuals with cochlear dead regions (Moore, 2001b; Turner, 1999; Padilha et al., 2007). For example, Vickers et al. (2001) and Baer et al. (2002) studied the benefit of high frequency amplification in quiet and noise for individuals with and without DRs. Both studies reported that individuals with DRs were unable to benefit from high frequency amplification. While Gordo & Iorio (2007) found that hearing aid users with DRs performed worse with high-frequency amplification than they did without it.

In contrast, Cox and her colleagues (2011) found beneficial effects of high frequency audibility whether or not the participants had dead regions. Others have reported equivalent performance for participants with and without dead regions for quiet and low noise conditions; however, in high noise conditions the individuals without dead regions demonstrated further improvement when additional high frequency amplification was provided, whereas participants with dead regions did not (Mackersie et al., 2004). This article is summarized in more detail here: http://blog.starkeypro.com/bid/66368/Recommendations-for-fitting-patients-with-cochlear-dead-regions

The current study was undertaken to examine the subjective and objective effect of high frequency amplification on matched pairs of participants (with and without DRs) in a variety of conditions. Participants were fitted with hearing aids that had two programs: the first (NAL) was based on the NAL-NL1 formula and the second (LP) was identical to the NAL-NL1 program below 1000Hz, with amplification rolled off above 1000Hz.  The goals of the study were to compare performance with these two programs, for individuals with and without dead regions. The following measures were conducted:

1) Speech discrimination in quiet laboratory conditions

2) Speech discrimination in noisy laboratory conditions

3) Subjective performance in everyday situations

4) Subjective preference for everyday situations

Participants were recruited from a pool of individuals who had previously been identified as typical hearing aid patients (Cox et. al., 2011). Participants had bilateral flat or sloping sensorineural hearing loss with thresholds above 25dB below 1kHz and thresholds of 60 to 90dB HL for at least part of the frequency range of 1-3kHz.

The TEN test (Moore et al., 2004) was administered to determine the presence of DRs. To be eligible for the study, participants needed to have one or more DRs in the better ear at or above 1kHz and no DRs below 1kHz. Participants were then divided into to two groups: the experimental group with DRs and the control group without DRs.  Individuals in the experimental group showed a diverse range of DR distribution across frequency. Almost half of the participants had DRs between 1-2kHz, whereas the remainder had DRs only at or above 3kHz. A little more than half of the participants had one DR only, whereas the others had more than one DR.

Individuals in the experimental group were matched in pairs with individuals from the control group. In total, there were 18 participant pairs; each matched for age, degree of hearing loss and slope of hearing loss. There were 24 men and 12 women. No attempt was made to match pairs based on gender.

Participants were fitted monaurally with behind-the-ear hearing aids coupled to vented skeleton earmolds. The monaural fitting was chosen to avoid complications when participants switched between the NAL and LP programs. Data collection was completed before the widespread availability of wireless hearing aids, so the participants would have had to reliably switch both hearing aids individually to the proper program every time to avoid making occasional subjective judgments based on mismatched programs.

The hearing aids had two programs: a program based on the NAL-NL1 prescription (NAL) and a program with high-frequency roll-off (LP). Participants were able to switch the programs themselves but could not identify the programs as NAL or LP. Half of the participants had NAL in P1 and LP in P2, whereas the other half had LP in P1 and NAL in P2.  Verification measures were conducted to ensure that the two programs matched below 1kHz and to make sure the participants judged the programs to be equally loud.

After a two week acclimatization period, participants returned for speech recognition testing and field trial training. Speech and noise stimuli were presented in a sound field with the unaided ear was plugged during testing. Speech recognition in quiet was evaluated using the Computer Assisted Speech Perception Assessment (CASPA; Mackersie, et al., 2001).  The CASPA test includes lists of 10 consonant-vowel-consonant words spoken by a female. Five lists were presented for each of the NAL and LP programs. Stimuli were presented at 65dB SPL.

Speech recognition in noise was evaluated with the Bamford-Kowell-Bench Speech in Noise (BKB-SIN test,  Etymotic Research, 2005), which contains sentences spoken by a male talker, masked by 4-talker babble. The test contains lists of 10 sentences with 31 scoring words. In each list, the signal-to-noise ratio (SNR) decreases by 3dB with each sentence, so that within any list the SNR ranges from +21dB to -6dB. Sentences were presented at 73dB, a “loud but OK” level, as recommended for this test.

Following the speech recognition testing, participants were trained in the field trial procedures for subjective ratings. They were asked to evaluate their ability to understand speech in everyday situations with the NAL and LP programs and identify occasions during which they felt they could understand some but not all of the words they were hearing. Participants were given booklets with daily rating sheets and listening checklists to record daily hours of hearing aid use and track the variety of their daily listening experiences.

After a two week field trial, participants returned to the laboratory for a second session of CASPA and BKB-SIN testing. They submitted ratings sheets and listening checklists and were interviewed about their preferred hearing aid program for everyday listening. The interview consisted of questions that covered program preferences related to:  understanding speech in quiet, understanding speech in noise, hearing over long distances, the sound of their own voice, sound quality, loudness, localization, the least tiring program and the one that provided the most comfortable sound. Participants were asked to indicate their preferred program for each of these criteria, as well as their preferred program for overall everyday use. They were asked to provide three reasons for overall preference.

Speech recognition testing in quiet revealed no difference in overall performance between the two groups, but there was a significant difference based on the hearing aid program that was used. Listeners from both the experimental group and the control group performed better with the broadband NAL program, though the difference between the NAL and LP programs was larger for the control group than the experimental group. This indicates that the individuals without DRs were able to derive more benefit from the additional high frequency information in the NAL program than the individuals with DRs did.

Speech recognition testing in noise revealed a similar finding but in this case the improvement with the NAL program was only significant for the control group. Although the experimental group’s mean scores with the NAL program were higher than those with the LP program, the difference did not reach statistical significance.  Because the BKB-SIN test used variable SNR levels, performance-intensity functions were constructed with scores obtained using the NAL and LP programs. These functions revealed that at any given SNR, speech was more intelligible with the NAL program. However, there was more of a difference between the NAL and LP functions for the control group than the experimental group, consistent with a program by group statistical interaction.

Subjective ratings of speech understanding revealed no significant difference between the experimental and control groups, but there was a significant difference based on program.  Participants from the control and experimental groups rated their performance better with the NAL program.

Interviews concerning program preference revealed that 23 participants preferred the NAL program and 11 preferred the LP program. There was no association with the presence of DRs. When the reasons supporting the participants’ preferences were analyzed, the most frequently mentioned reason for NAL preference was greater speech clarity. The most common reason for LP preference was that the other program (NAL) was too loud.

This investigation by Dr. Cox and her colleagues indicates that high-frequency amplification was beneficial to participants with single or multiple DRs, especially for speech recognition in quiet. In noise, participants with DRs still performed better with the NAL program, though the improvement was not as marked as it was for those without DRs. In field trials, participants with DRs reported more improvement with the NAL program than the control group did, indicating that perceived benefits in everyday situations exceeded any predictions of the laboratory results. At no point in the study did high-frequency amplification reduce performance for individuals with or without DRs.

This finding is in contrast with previous reports (Vinay & Moore, 2007a; Gordo & Iorio, 2007). Cox and her colleagues note that most of the participants in their study had only one or two DRs as opposed to several contiguous DRs. They allow that their findings might not relate to the performance of participants with several contiguous DRs, but point out that among typical hearing aid candidates, it is unlikely for individuals to have more than one or two DRs. With this consideration, the authors suggest that, high frequency amplification should not be reduced, even in cases with identified dead regions.

This study from the University of Memphis provides a recommendation for use of prescribed settings and against reduction of high frequency gain for hearing aid users with one or two DRs.  They found beneficial effects of high frequency amplification in laboratory and everyday environments and noted no circumstances in which listeners demonstrated deleterious effects of high frequency amplification. These results may not pertain to individuals with several contiguous DRs but they are pertinent to the majority of typical hearing aid wearers. Their findings also support the use of subjective performance measures, as these provided additional information that was sometimes in contrast to the laboratory results. They point out that laboratory results do not always predict performance in everyday life and it can be extrapolated that clinical measures of efficacy should always be supported with subjective reports of effectiveness, like self-assessment of comfort and acceptance.

References

Baer,  T., Moore, B.C. & Kluk, K. (2002). Effects of low pass filtering on the intelligibility of speecdh in noise for people with and without dead regions at high frequencies. Journal of the Acoustical Society of America 112(3 pt. 1), 1133-1144.

Ching, T.Y., Dillon, H. & Byrne, D. (1998). Speech recognition of hearing-impaired listeners: predictions from audibility and the limited role of high-frequency amplification. Journal of the Acoustical Society of America 103, 1128-1140.

Cox, R. M., Alexander, G.C., & Johnson, J.A. (2011). Cochlear dead regions in typical hearing aid candidates: prevalence and implications for use of high-frequency speech cues. Ear and Hearing 32, 339-348.

Cox, R.M., Johnson, J.A. & Alexander, G.C. (2012). Implications of high-frequency cochlear dead regions for fitting hearing aids to adults with mild to moderately severe hearing loss. Ear and Hearing, May 2012, e-published ahead of print.

Etymotic Research (2005). BKB-SIN Speech in Noise Test, Version 1.03. Elk Grove Village, IL: Etymotic Research.

Moore, B.C., Glasberg, B. & Vickers, D.A. (1999b). Further evaluation of a model of loudness perception applied to cochlear hearing loss. Journal of the Acoustical Society of America 106, 898-907.

Moore, B.C., Huss, M. & Vickers, D.A. (2000). A test for the diagnosis of dead regions in the cochlea. British Journal of Audiology 34, 205-224.

Moore, B.C., Glasberg, B. R. & Stone, M.A. (2004). New version of the TEN test with calibrations in dB HL. Ear and Hearing 25, 478-487.

Gordo, A. & Iorio, M.C. (2007). Dead regions in the cochlea at high frequencies: Implications for the adaptation to hearing aids. Revista Brasileira de Otorrinolaringologia 73, 299-307.

Hogan, C.A. & Turner, C.W. (1998). High frequency audibility: benefits for hearing-impaired listeners. Journal of the Acoustical Society of America 104, 432-441.

Mackersie, C.L., Boothroyd, A. & Minniear, D. (2001). Evaluation of the Computer-Assisted-Speech Perception Assessment Test (CASPA).  Journal of the American Academy of Audiology 12, 390-396.

Mackersie, C.L., Crocker, T.L. & Davis, R.A. (2004). Limiting high-frequency hearing aid gain in listeners with and without suspected cochlear dead regions. Journal of the American Academy of Audiology 15, 498-507.

Moore, B.C. (2001a). Dead regions in the cochlear: Diagnosis, perceptual consequences and implications for the fitting of hearing aids. Trends in Amplification 5, 1-34.

Moore, B.C., (2001b). Dead regions in the cochlear: Implications for the choice of high-frequency amplification. In R.C. Seewald & J.S. Gravel (Eds). A Sound Foundation Through Early Amplification, p 153-166. Stafa, Switzerland: Phonak AG.

Padilha, C., Garcia, M.V., & Costa, M.J. (2007).  Diagnosing cochlear dead regions and its importance in the auditory rehabilitation process. Brazilian Journal of Otolaryngology 73, 556-561.

Turner, C.W. (1999). The limits of high-frequency amplification. Hearing Journal 52, 10-14.

Turner, C.W. & Cummings, K.J. (1999). Speech audibility for listeners with high-frequency hearing loss. American Journal of Audiology 8, 47-56.

Vickers, D.A., Moore, B.C. & Baer, T. (2001). Effects of low-pass filtering on the intelligibility of speech in quiet for people with and without dead regions at high frequencies. Journal of the Acoustical Society of America 110, 1164-1175.

Vinay, B.T. & Moore, B.C. (2007a). Prevalence of dead regions in subjects with sensorineural hearing loss. Ear and Hearing 28, 231-241.

Recommendations for fitting patients with cochlear dead regions

Cochlear Dead Regions in Typical Hearing Aid Candidates:

Prevalence and Implications for Use of High-Frequency Speech Cues

Cox, R.M., Alexander, G.C., Johnson, J. & Rivera, I. (2011).  Cochlear dead regions in typical hearing aid candidates: Prevalence and implications for use of high-frequency speech cues. Ear & Hearing 32 (3), 339-348.

This editorial discusses the clinical implications of an independent research study. The original work was not associated with Starkey Laboratories and does not reflect the opinions of the authors.

Audibility is a well-known predictor of speech recognition ability (Humes, 2007) and audibility of high-frequency information is of particular importance for consonant identification.  Therefore, audibility of high-frequency speech cues is appropriately regarded as an important element of successful hearing aid fittings (Killion & Tillman, 1982; Skinner & Miller, 1983). In contrast to this expectation, some studies have reported that high-frequency gain might have limited or even negative impact on speech recognition abilities of some individuals (Murray & Byrne, 1986; Ching et al., 1998; Hogan & Turner, 1998). These researchers observed that when high-frequency hearing loss exceeded 55-60dB, some listeners were unable to benefit from increased high-frequency audibility.  A potential explanation for this variability was provided by Brian Moore (2001), who suggested that an inability to benefit from amplification in a particular frequency region could be due to cochlear “dead regions” or regions where there is a loss of inner hair cell functioning.

Moore suggested that hearing aid fittings could potentially be improved if clinicians were able to identify patients with cochlear dead regions (DRs). Working under the assumption that diagnosis DRs may contraindicate high-frequency amplification. He and his colleagues developed the TEN test as a method of determining the presence of cochlear dead regions (Moore et al., 2000, 2004). The advent of the TEN test provided a standardized measurement protocol for DRs, but there is still wide variability in the reported prevalence of DRs. Estimates range from as 29% (Preminger et a., 2005) to as high as 84% (Hornsby & Dundas, 2009), with other studies reporting DR prevalence somewhere in the middle of that range. Several potential factors are likely to contribute to this variability, including degree of hearing loss, audiometric configuration and test technique.

In addition to the variability in reported prevalence of DRs, there is also variability in the reports of how DRs affect the ability to benefit from high-frequency speech cues (Vickers et al., 2001; Baer et al., 2002; Mackersie et al., 2004). It remains unclear as to whether high-frequency amplification recommendations should be modified to reflect the presence of DRs.  Most research is in agreement that as hearing thresholds increase, the likelihood of DRs also increases.  Hearing aid users with severe to profound hearing losses are likely to have at least one DR. Because a large proportion of hearing aid users have moderate to severe hearing losses, Dr. Cox and her colleagues wanted to determine the prevalence of DRs in this population. In addition, they examined the effect of DRs on the use of high-frequency speech cues by individuals with moderate to severe loss.

Their study addressed two primary questions:

1) What is the prevalence of dead regions (DRs) among listeners with hearing thresholds in the 60-90dB range?

2) For individuals with hearing loss in the 60-90dB range, do those with DRs differ from those without DRs in their ability to use high-frequency speech cues?

One hundred and seventy adults with bilateral, flat or sloping sensorineural hearing loss were tested. All subjects had thresholds of 60 to 90dB in the better ear for at least part of the range from 1-3kHz and thresholds no better than 25dB for frequencies below 1kHz. Subjects ranged in age from 38 to 96 years, and 59% of the subjects had experience with hearing aids.

First, subjects were evaluated for the presence of DRs with the TEN test. Then, speech recognition was measured using high-frequency emphasis (HFE) and high-frequency emphasis, low-pass filtered (HFE-LP) stimuli from the QSIN test (Killion et al. 2004). HFE items on this test are amplified up to 32dB above 2.5kHz, whereas the HFE-LP items have much less gain in this range. Comparison of subjects’ responses to these two types of stimuli allowed the investigators to assess changes in speech intelligibility with additional high frequency cues. Presentation levels for the QSIN were chosen by using a loudness scale and bracketing procedure to arrive at a level that the subject considered “loud but okay”. Finally, audibility differences for the two QSIN conditions were estimated using the Speech Intelligibility Index based on ANSI 3.5-1997 (ANSI, 1997).

The TEN test results revealed that 31% of the participants had DRs at one or more test frequencies. Of the 307 ears tested, 23% were found to have a DR for one or more frequencies. Among those who tested positive for DRs, about 1/3 had DRs in both ears and 2/3 had DRs in one ear or the other in equal proportion. Mean audiometric thresholds were essentially identical for the two groups below 1kHz, but above 1kHz thresholds were significantly poorer for the group with DRs than for the group without DRs.  DRs were most prevalent at frequencies above 1.5kHz. There were no age or gender differences.

On the QSIN test, the mean HFE-LP scores were significantly poorer than the mean HFE scores for both groups.  There was also a significant difference in performance based on whether or not the participants had DRs. Perhaps more interestingly, there was a significant interaction between the DR group and test stimuli conditions, in that the additional high-frequency information in the HFE stimuli resulted in slightly greater performance gains for the group without DRs than it did for the group with DRs.  Furthermore, subjects with one or more isolated DRs were more able to benefit from the high frequency cues in the HFE lists than were those subjects with multiple, contiguous DRs. Although there were a few isolated individuals who demonstrated lower scores for the HFE stimuli, the differences were not significant and could have been explained by measurement error. Therefore, the authors conclude that the additional high frequency information in the HFE stimuli was not likely to have had a detrimental effect on performance for these individuals.

As had also been reported in previous studies, subject groups with DRs had poorer mean audiometric thresholds than the groups without DRs, so it was possible that audibility played a role in QSIN performance. Analysis of the audibility of QSIN stimuli for the two groups revealed that high frequency cues in the HFE lists were indeed more audible for the group without DRs. In accounting for this audibility effect, the presence of DRs still had a small but significant effect on performance.

The results of this study suggest that listeners with cochlear DRs still benefit from high frequency speech cues, albeit slightly less than those without dead regions.  The performance improvements were small and the authors caution that it is premature to draw firm conclusions about the clinical implications of this study.  Despite the need for further examination, the results of the current study certainly do not support any reduction in prescribed gain for hearing aid candidates with moderate to severe hearing losses.  The authors acknowledge, however, that because the findings of this and other studies are based on group data, it is possible that specific individuals may be negatively affected by amplification within dead regions. Based on the research to date, this seems more likely to occur in individuals with profound hearing loss who may have multiple, contiguous DRs.

More study is needed to determine the most effective clinical approach to managing cochlear dead regions in hearing aid candidates. Future research should be done with hearing aid users, including for example, the effects of noise on everyday hearing aid performance for individuals with DRs. A study by Mackersie et. al. (2004) showed that subjects with DRs suffered more negatives effects of noise than did the subjects without DRs. If there is a convergence of evidence to this effect, then recommendations about the use of high frequency gain, directionality and noise reduction could be determined as they relate to DRs. For now, Dr. Cox and her colleagues recommend that until there are clear criteria to identify individuals for whom high frequency gain could have deleterious effects, clinicians should continue using best-practice protocols and provide high frequency gain according to current prescriptive methods.

References

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Ching,T., Dillon, H. & Byrne, D. (1998). Speech recognition of hearing-impaired listeners: Predictions from audibility and the limited role of high-frequency amplification. Journal of the Acoustical Society of America 103, 1128-1140.

Cox, R.M., Alexander, G.C., Johnson, J. & Rivera, I. (2011).  Cochlear dead regions in typical hearing aid candidates: Prevalence and implications for use of high-frequency speech cues. Ear & Hearing 32 (3), 339-348.

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Moore, B.C.J., Huss, M., Vickers, D.A.,  et al. (2000). A test for the diagnosis of dead regions in the cochlea. British Journal of Audiology 34, 2-5-224.

Moore, B.C.J., Glasberg, B.R., Stone, M.A. (2004). New version of the TEN test with calibrations in dB HL. Ear and Hearing 25, 478-487.

Murray, N. & Byrne, D. (1986). Performance of hearing-impaired and normal hearing listeners with various high-frequency cut-offs in hearing aids. Australian Journal of Audiology 8, 21-28.

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