Background: Over the past decade, DOD and VA audiologists have reported large numbers of relatively young adult active-duty patients who have normal to near-normal audiometric thresholds (i.e., < 25 dB HL) but who report difficulty understanding speech in noisy environments. Many of these service members (SMs) also reported having experienced exposure to one or more explosive blasts as part of their military service. A two-phase, multi-site study was conducted to understand the functional hearing and communication deficits (FHCD) in blast-exposed SMs.

Methods: In the first phase roughly 3400 active-duty SMs were screened for FHCD on four auditory tests (two speech-in-noise and two tone-in-noise tests) and a brief 6-question survey about perceived speech understanding and sound quality. To investigate the potential sources of FHCD (Phase II), two groups of blast-exposed SMs — those with or without FHCD — were tested large of number of tasks with behavioral, cognitive, and electrophysiological measures. Performances of these two groups were compared with that of a control group consisting of SMs with normal hearing thresholds (≤ 20 dB HL) and no blast-exposure, who did not exhibit FHCD.

Results: The first phase of the study revealed blast exposure and mildly elevated hearing thresholds each increased the probability of FHCD by 2 to 3 times. SMs having both blast-exposure and mildly elevated hearing thresholds exhibited up to 4 times higher risk for FHCD. In the second phase, the 3-way comparison (i.e., among non-blast SMs, blast-exposed SMs with FHCD, and blast-exposed SMs without FHCD) indicated that blast-exposed SMs with FHCD performed significantly worse than the control group on metrics that measured peripheral auditory system performance such as pure-tone thresholds, high-frequency distortion product otoacoustic emissions, frequency-following response, and signal-to-noise ratio and stability of envelope-following responses. Performance of blast-exposed SMs without FHCD was not different that the control group SMs on most of these metrics. However, both blast- exposed SM groups (with or without FHCD) had significantly worse scores than the control group on measures of peripheral auditory processing (click ABR wave V amplitude) and measures of central auditory processing (Warrington Recognition Memory Test and Speed and Capacity of Language Processing Test).

Conclusion: Roughly 33.6% of active-duty SMs with normal to near-normal hearing thresholds (or approximately 423,000 by the current military size) are at some risk for FHCD, and about 5.7% (i.e. blast exposed and with elevated hearing thresholds, approximately 72,000) are at high risk for FHCD. Phase II results indicated that blast-exposed SMs with FHCD tended to have increased neural noise resulting in a poorer internal signal-to-noise ratio when compared to the control group as well as to blast-exposed SMs without FHCD. These data are consistent with studies showing benefit of low-gain hearing aids for this population in that small amounts of amplification would at least partially compensate, for degradation in peripheral auditory processing.