Stimulus Rate Effect on Electrocochleogram Components in Adults with High Risk for Noise Exposure.

Background:  Hidden hearing loss (HHL) is a newly described condition that has been associated with hearing thresholds within normal limits and poorer speech-in-noise (SIN) performance. Furthermore, HHL has been associated with an elevated or absent middle ear muscle reflex (MEMR). One of the leading factors causing HHL is exposure to noise, specifically in patients who experience temporary threshold shift. This study investigated the SIN performance and MEMR in individuals at risk for noise exposure. Additionally, this study investigated the auditory nerve response measured to different stimulation rates in order to evaluate the auditory nerve adaptation and potentially identify neural loss in individuals at risk for noise exposure.

Research design:  Twenty-one young-adult participants (11 low-risk and 10 high-risk) were recruited to participate in the study. Participants were divided into a low-risk and high-risk group based on their noise exposure history and if they have experienced any signs of temporary threshold shift following noise exposure. SIN testing, distortion product otoacoustic emissions (DPOAEs), MEMR, hearing thresholds, and electrocochleography (ECochG) were measured. Experimental protocol included measuring MEMR, word recognition scores in quiet and in noise (0 dB signal-to-noise ratio), and ECochG recordings at three rates (9.1, 37.1, and 71.1 clicks/seconds).

Data analysis:  Descriptive statistics are provided for the different variables. Contingency tables were used to investigate the frequency of measuring normal MEMR between both groups. We also investigated the audiometric threshold, DPOAEs amplitude, and SIN results between groups using independent samples t-tests. Lastly, we investigated the effect of stimulation rate on the summating potential (SP) and action potential (AP) between groups using repeated measures analysis of variance with mixed effects.

Results:  No significant differences in hearing thresholds or DPOAEs amplitude at any tested frequency between both groups. Significantly larger number of participants in the high-risk group have abnormal MEMR compared with the low-risk group. Significantly poorer performance on SIN, smaller AP amplitude, and greater SP/AP amplitude ratio in the high-risk group. Furthermore, the AP amplitude adapted differently for the high-risk group with increasing stimulation rate compared with the low-risk group.

Conclusions:  The results of this study show that HHL is associated with poorer SIN performance and abnormal MEMR. Additionally, the significant reduction in AP amplitude and the degree of AP amplitude adaptation with change in stimulus rate might indicate loss of neural output in individuals with a history of noise exposure.