COCHLEAR DEAD REGIONS IN HEARING-IMPAIRED ADULTS

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Abstract

ABSTRACT THE UNIVERSITY OF MANCHESTERAnna Caroline PeplerDoctor of PhilosophyCOCHLEAR DEAD REGIONS IN HEARING-IMPAIRED ADULTSDecember 2013Cochlear dead regions (DRs) are areas in the cochlea where inner hair cells and/or neurones are functioning so poorly that a sound that causes peak basilar membrane motion in that region is more efficiently detected via off-frequency listening. The Threshold Equalising Noise (TEN) test is a clinical test procedure for detecting DRs. Psychophysical Tuning Curves (PTCs) can be used to identify the boundary frequency of the DR although the clinical importance of doing this has yet to be determined. Some studies have suggested that the reduction of amplification well inside the DR may be beneficial; however, other studies have been unable to replicate these findings in a more typical clinical population. Three studies were completed in order to:1. determine the prevalence of DRs in a clinical sample of the UK adult population,2. investigate repeatability, agreement and clinical feasibility of the TEN-test and fast PTCs in a clinical setting, and 3. determine the benefit of high-frequency amplification in ears with and without DRs, when listening to nonsense syllable speech material in quiet and babble. In the first study, 343 hearing-impaired adults were tested for DRs using the TEN-test. In total, 36% (95% confidence interval 31-41) of these adults had a DR in at least one ear, but frequently at 4 kHz only. Only 3% (1-5) of participants had a DR spanning more than three consecutive frequencies. These findings suggest that DRs usually only span 1 or 2 clinically-relevant frequencies. In the second study, the TEN-test was completed on 70 ears at frequencies between 0.5 and 4 kHz. Fast PTCs were measured on 20 ears at ≥ 2 frequencies. The TEN-test and fast PTCs were highly repeatable on retest (97% and 100%, respectively). There was 87% agreement between the two procedures in terms of the presence of off-frequency listening, with the TEN-test less likely to detect a DR than fast PTCs. Compared to the TEN-test, fast PTCs had a 10% lower ‘conclusive finding’ rate and the test duration was typically 40 minutes longer. Therefore, the TEN-test is more clinically acceptable, but it may underestimate the extent of a DR because of its inability to precisely identify the boundary frequency. In the third study, 18 ears with a high-frequency DR and 18 matched ears without a DR were tested. Vowel-Consonant-Vowel (VCV) stimuli were presented in quiet and babble when listening with an unfiltered and three low-pass filtered hearing aid settings. Best performance was obtained in the unfiltered condition; however the DR group performed significantly poorer than the controls in babble. There was no evidence to support reducing amplification in ears with a DR. However, participants with DRs may benefit from counseling about the limitations of listening in noise. In summary, DRs are relatively prevalent in hearing-impaired adults and can be diagnosed most efficiently in a clinical setting using the TEN-test. However, DRs are often restricted to a narrow frequency range and, in the typical adult clinical population, there is no evidence to support deviating from prescription targets.

Keyword(s)

Dead Regions; High-frequency amplification; PTC test; Prevalence; TEN test; Test Agreement; Test Feasibility; Test Repeatability

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