Starkey Research & Clinical Blog

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

ANSI ( 1997). American National Standard Methods for Calculation of the Speech Intelligibility Index (Vol. ANSI S3.5-1997). New York: American National Standards Institute.

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.

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.

Humes, L.E. (2007). The contributions of audibility and cognitive factors to the benefit provided by amplified speech to older adults. Journal of the American Academy of Audiology 18, 590-603.

Killion, M. C. & Tillman, T.W. (1982). Evaluation of high-fidelity hearing aids. Journal of Speech and Hearing Research 25, 15-25.

Moore, B.C.J. (2001). 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.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.

Skinner, M.W. & Miller, J.D. (1983). Amplification bandwidth and intelligibility of speech in quiet and noise for listeners with sensorineural hearing loss.  Audiology 22, 253-279.