Implications of High-Frequency Cochlear Dead Regions for Fitting Hearing Aids to Adults With Mild to Moderately Severe Hearing Loss

Abstract

A cochlear dead region (DR) occurs at a given frequency when there is a loss of normal functioning of inner hair cells tuned to that frequency. It has been suggested that existence of high-frequency DRs has implications for hearing aid fitting, and that the optimal amount of high-frequency gain is reduced for these patients. However, the data supporting this suggestion has been obtained using listeners with severe or profound hearing loss. It is uncertain whether these results would apply for listeners with the mild to moderately severe hearing loss that is more typical of hearing aid users. This investigation used laboratory and field measurements to examine the effects of reduced high-frequency gain in typical hearing aid users with high-frequency DRs compared with matched users without DRs. The study was a double-blinded, nonrandomized intervention design with 18 matched pairs of adult subjects. In each pair, one subject had high-frequency DRs (usually at one or two test frequencies) and the other subject had no DR. Each subject was fitted unilaterally with a hearing aid having two active programs. One program used a National Acoustics Laboratories (NAL) prescription target for average speech (NAL condition). The other program was identical to NAL except for reduced gain in the high frequencies (low-pass [LP] condition). Outcomes included aided speech recognition in quiet and noise measured in the laboratory, ratings of speech understanding in daily life, and final preference for the NAL or LP program. In laboratory testing, speech recognition in quiet was significantly better when using the NAL program. This result was seen for subjects with DRs and without DRs. When listening in noise, speech recognition was significantly better when using the NAL program for subjects without DRs. For subjects with DRs, results were equivalent for the NAL and LP programs. In daily life, the NAL program received significantly higher ratings for speech understanding, and this result was seen for subjects with and without DRs. When asked about their preference for using the NAL or LP program in daily life, subjects did not always choose the program they had rated as giving better speech understanding, but their preference was not associated with having DRs. About one-third of subjects preferred the LP program. The reason most frequently given for preferring the LP program was that the NAL was too loud. Overall, adult hearing aid wearers with mild to moderately severe hearing loss benefitted from high-frequency gain whether or not they had DRs. In laboratory testing, but not in daily life, subjects with DRs tended to obtain less benefit than those without DRs. However, provision of NAL high-frequency gain never resulted in poorer performance in either group. These results suggest that identification of high-frequency DRs at one or two frequencies does not call for any a priori modification of the target hearing aid prescription for listeners with mild to moderately severe hearing loss.