RP208 - Evaluation of Hidden Hearing Loss in Normal-Hearing Firearm Users

Abstract:
Some noise exposures that result in a temporary threshold shift (TTS) also result in permanent, cochlear synaptopathy. The purpose of this retrospective study was to evaluate a human population — firearm users — that might be at risk for noise-induced cochlear synaptopathy (i.e. “hidden hearing loss”). We tested the hypothesis that Firearm-User participants would exhibit a reduced cochlear nerve response compared to Non-Firearm User participants, despite normal-hearing sensitivity in both groups. No statistically significant group differences were detected in standard audiometry, extended high-frequency audiometry, Words-in-Noise performance, distortion product otoacoustic emission, middle ear muscle reflex, or auditory brainstem response.

Summary:
Rationale/Purpose
Across a variety of animal models, significant noise-induced temporary threshold shift (TTS) in the absence of permanent threshold shift (PTS) has been accompanied by permanent damage to the synaptic connections between the inner hair cells and auditory nerve. Notably, not every noise-induced TTS results in permanent, cochlear synapse loss, and more recent studies suggest that non-human primates are significantly less vulnerable to such a selective, neural injury. The purpose of this retrospective study was to evaluate the cochlear nerve response in a young, human population that was likely to have experienced prior, significant threshold shift following firearm use, yet presently exhibit normal-hearing audiometric thresholds.

Methods
70 total participants were enrolled in this study. Participants were 18-25 yr and exhibited audiometric thresholds < 20 dB HL from .25 - 8 kHz. Participants who reported “0 occasions of firearm use” were assigned to the Non-Firearm User (CNTRL) group (n=37; 25 female, 12 male). Participants who reported “5+ occasions of firearm use” were assigned to the Firearm User (EXP) group (n=33; 12 female, 21 male). EXP participants completed a survey estimating frequency of firearm use occasions, firearm type(s), and consistency/type of hearing protection use during those occasions. Participants also completed a survey to estimate the quantity of their average recreational and occupational noise exposure. Finally, participants underwent otoscopy, tympanometry, middle ear muscle reflex, distortion product otoacoustic emission, standard and extended high-frequency audiometry, auditory brainstem response, and Words-in-Noise analysis, in addition to real-ear attenuation at threshold measurements of hearing protection devices.

Results & Conclusions
No statistically significant group differences in auditory performance were detected between the CNTRL and EXP participants on standard audiometry, extended high-frequency audiometry, Words-in-Noise performance, distortion product otoacoustic emission, middle ear muscle reflex, or auditory brainstem response. Importantly, 91% of EXP participants reported that they wore hearing protection either “all the time” or “almost all the time” while using firearms. The data suggest that consistent use of hearing protection during firearm use can effectively protect cochlear and neural measures of auditory function, including suprathreshold responses. The current results do not exclude the possibility that neural pathology may be evident in firearm users with less consistent hearing protection use.