Envelope following response measurements in young veterans are consistent with noise-induced cochlear synaptopathy

Abstract

Animal studies have demonstrated that noise exposure can lead to the loss of the synapses between the inner hair cells and their afferent auditory nerve fiber targets without impacting auditory thresholds. Although several non-invasive physiological measures appear to be sensitive to cochlear synaptopathy in animal models, including auditory brainstem response (ABR) wave I amplitude, the envelope following response (EFR), and the middle ear muscle reflex (MEMR), human studies of these measures in samples that are expected to vary in terms of the degree of noise-induced synaptopathy have resulted in mixed findings. One possible explanation for the differing results is that synaptopathy risk is lower for recreational noise exposure than for occupational or military noise exposure. The goal of this analysis was to determine if EFR magnitude and ABR wave I amplitude are reduced among young Veterans with a history of military noise exposure compared with non-Veteran controls with minimal noise exposure. EFRs and ABRs were obtained in a sample of young (19-35 years) Veterans and non-Veterans with normal audiograms and robust distortion product otoacoustic emissions (DPOAEs). The statistical analysis is consistent with a reduction in mean EFR magnitude and ABR wave I amplitude (at 90 dB peSPL) for Veterans with a significant history of noise exposure compared with non-Veteran controls. These findings are in agreement with previous ABR wave I amplitude findings in young Veterans and are consistent with animal models of noise-induced cochlear synaptopathy.

Keywords: Auditory brainstem response; Cochlear synaptopathy; Envelope following response; Frequency following response; Hidden hearing loss; Noise-induced hearing loss.

Figure 1.. Audiograms are similar across noise exposure groups.

Mean audiometric thresholds from 0.25-16 kHz are plotted for each noise exposure group (thick lines) and for individual participants (thin lines). Color-coding indicates the noise exposure group.

Figure 2.. DP-grams are similar across noise exposure groups.

Mean DPOAE levels from 1-8 kHz obtained from the DP-gram are plotted for each noise exposure group (thick lines). DP-grams for individual participants are shown with the thin lines and are color-coded by group.

Figure 3.. Mean EFR magnitude is reduced for the Veteran High Noise group compared with controls.

Mean EFR magnitudes are plotted by noise exposure group for each modulation depth with thick color-coded lines. Thin lines show EFR magnitudes for individual participants.

Figure 4.. Modeled mean EFR magnitude by noise exposure category and sex.

Model fitted mean EFR magnitudes are plotted by noise exposure category and sex for each modulation depth. Thick lines show the interquartile range and thin lines show the 90% Bayesian confidence interval.

Figure 5.. Histogram illustrating the calculation of posterior probabilities for group contrasts.

The histogram shows the posterior probability distribution for Non-Veteran Control mean EFR magnitude minus High Noise Veteran mean EFR magnitude (for males at a modulation depth of 100%). The portion of the distribution that is greater than zero, indicating higher mean EFR magnitudes for Non-Veteran Controls than for High Noise Veterans, is shaded dark gray and represents 95% of the distribution. The portion of the distribution that is less than zero, indicating lower mean EFR magnitudes for Non-Veteran Controls, is shaded light gray and represents 5% of the distribution. Therefore, the posterior probability that male Non-Veteran Controls have a mean higher EFR magnitude than High Noise Veterans at 100% modulation depth is 95%, given the data and the fitted model.

Figure 6.. Mean ABR wave I amplitude is reduced for the Veteran High Noise group compared with controls.

Mean ABR wave I amplitudes are plotted by noise exposure group for each ABR stimulus level with thick color-coded lines. Thin lines show wave I amplitudes for individual participants.

Figure 7.. Modeled mean ABR wave I amplitude by noise exposure category and sex.

Model fitted mean ABR wave I amplitudes are plotted by noise exposure category and sex for each stimulus level. Thick lines show the interquartile range and thin lines show the 90% Bayesian confidence interval.