Impaired Tone-in-Noise Detection in Individuals Who Report Temporary Hearing Symptoms After Noise Exposure.

OBJECTIVES

Current hearing conservation standards assume that changes in the pure-tone audiogram fully capture the effects of noise-induced hearing damage. However, recent data suggest that individuals who frequently experience acute hearing symptoms after noise exposure may suffer long-term increases in hearing difficulty, even when their pure-tone thresholds remain unchanged. It is unclear whether these increases in subjective hearing difficulties correspond to objective measures of hearing ability. This study explores (1) whether tone-in-noise detection is negatively impacted by a history of noise-induced temporary threshold shifts, (2) whether noise exposure history or the number of temporary threshold shifts after exposure is the better predictor of tone-in-noise detection, and (3) whether tone-in-noise detection provides insights into subjective hearing complaints independent of the audiogram.

DESIGN

Two experiments were conducted to measure 500-Hz tone-in-noise detection thresholds among service members (SMs). Audiometric data were collected from SMs in addition to questionnaires about perceived hearing difficulties, noise exposure history, and history of hearing symptoms following noise exposure. In Experiment 1, 4253 SMs completed a 33-trial Masking Level Difference test that measured detection thresholds for a 500-Hz homophasic tone presented in homophasic noise (the N0S0 condition) and a 500 Hz antiphasic tone presented in homophasic noise (the N0Sπ condition). In Experiment 2, 17,658 SMs completed an 18-trial test measuring N0Sπ detection. Results from Experiment 2 were converted into N0Sπ detection thresholds equivalent to those in Experiment 1 using data from 3206 SMs who completed both tests. Linear models were then fit to the data to determine the effects of noise exposure history, symptoms experienced after exposure, and pure-tone audiometric thresholds on N0S0 and N0Sπ detection. Linear models were also used to determine the degree to which post-exposure symptoms, pure-tone audiometric thresholds, and N0Sπ detection could predict self-reported hearing complaints.

RESULTS

Individuals who reported frequent hearing symptoms after noise exposure had substantially worse N0S0 and N0Sπ detection thresholds than those without these symptoms, even after controlling for pure-tone audiometric thresholds. Group differences were smallest for individuals who had good hearing thresholds and increased systematically as hearing thresholds increased within the normal range. Noise exposure history also influenced N0Sπ thresholds, but had a smaller effect than that of post-exposure symptoms. Finally, combining N0Sπ and hearing thresholds explained roughly 50% more of the variance in hearing complaint scores than either factor alone.

CONCLUSIONS

These findings are consistent with results from animal models showing that temporary changes in hearing thresholds are associated with hearing damage. The results suggest that some listeners experience chronic hearing damage from the same noise exposures that have no effect on other listeners. Such auditory damage seems to impact subjective hearing difficulty, as reported previously, and objective tone-in-noise detection, as observed here. Evidence from this and other studies justifies the addition of questions regarding temporary threshold shift history and hearing complaints into all annual hearing tests conducted in hearing conservation programs. Adding a measure of binaural tone-in-noise detection to the clinical battery administered to noise-exposed patients who report hearing difficulties despite normal audiograms is also recommended.