Traumatic brain injury (TBI) is a common injury among military service members and veterans (SMVs) (1). There has been increasing awareness that physical, sensory, cognitive, and/or affective symptoms are often reported many months or years following injury [e.g., (2)] or develop following subconcussive blast exposure (3). Critical for improving patient care is our ability to comprehensively assess the range of problems that individuals with a TBI or blast exposure experience. This goal is complicated by variability in the causes and symptoms associated with these deficits.

Of particular challenge has been assessing TBI- and blast-associated deficits in communication. Hearing loss and tinnitus are among the most prevalent service-connected disabilities for veterans (4) and the incidence of hearing difficulties in service members, particularly those deployed (5), exceeds that of the U.S. working population (6). The monumental increase of blast exposure in deployments has led to an increased incidence of TBI diagnoses and associated auditory impairment (7). Permanent sensorineural hearing loss is reportedly the most prevalent type of auditory impairment in blast trauma, accounting for 35–54% of auditory injury (8). Chandler (9) estimated that 64% of blast-injured service members being treated at a large, U.S. military treatment facility had ongoing hearing loss. Bothersome tinnitus is also often experienced in people with trauma-related injuries, exposure to high levels of occupational noise, and hearing loss; all of which are common in the military population. Approximately 20% of people with chronic tinnitus have bothersome tinnitus that can promote cognitive difficulties, mental health disorders, insomnia, and decreased psychosocial functioning (10–12).

SMVs with a history of TBI and/or blast exposure often report even greater difficulties understanding speech in noisy environments than would be predicted from clinical audiometric assessments, such as pure tone thresholds (13). Clinical tests are often not sensitive enough to quantify speech recognition difficulties (14). In general, individuals with a TBI may appear normal in clinical exams, but suffer in more complex, real-world environments (15).

These findings suggest multiple sources of impairment may occur with TBI or blast exposure that exacerbate speech understanding difficulties in challenging conditions. Though the mechanisms are still under investigation, damage along the peripheral to central auditory pathway may place greater demands on top-down, cognitive systems to compensate, especially in adverse conditions. Individuals with damage to these domain-general systems, as can occur with TBI or blast exposure, may thus be particularly unable to compensate. Indeed, a tight link between auditory and cognitive impairments has been noted in the epidemiological literature (16). Additionally, small-scale studies of civilians have observed associations between auditory and cognitive function in assessing auditory processing abilities (13, 17, 18), though these assessments did not consistently distinguish individuals with and without a history of TBI.

Accurate assessment of communication challenges is critical for mitigating the potential negative social and cognitive consequences of auditory dysfunction including poorer quality of life (19) and job performance and promotion (20). The presence of bothersome tinnitus can have a detrimental impact on a person's emotional, social, mental, and professional life. Tinnitus secondary to blast injury may even be more detrimental due to its sudden emergence instead of gradual onset with progressive sensorineural hearing loss (21).

Complicating the assessment of the impact of TBI and blast exposure on communication is the variability in the causes and symptoms associated with these injuries. For example, mild TBI (mTBI) resulting from blast exposure has been associated with more self-reported hearing difficulty than mTBI resulting from a non-blast mechanism (22). Comorbidities may also cloud our understanding of symptoms of TBI reported years following a TBI. Over 40% of soldiers with mTBI have a comorbid post-traumatic stress disorder (PTSD), and a number of health problems reported by service members with mTBI are strongly influenced by PTSD or depression (23, 24).

Due to increased concern of mental health disorders in the military population independent of auditory status (25, 26), it is imperative that those with bothersome tinnitus and auditory dysfunction are given timely and appropriate treatment options. However, tests of speech recognition in noise and other complex environments as well as tinnitus evaluations are often not part of a standard audiological evaluation.

Given the diversity of factors contributing to challenging speech understanding and hearing and tinnitus problems, we present an initial analysis of a large-scale study of SMVs. This study aims to highlight domains that may be important for comprehensive assessments of the subjective and objective hearing and tinnitus problems of SMVs with or without a history of TBI and/or blast exposure.

SMVs with a history of TBI or blast exposure often report difficulties understanding speech in adverse conditions that are difficult to capture with standard audiometric tests. This suggests multiple factors contribute to their speech recognition problems. This study's results are consistent with this: individuals with a self-reported history of blast exposure also reported more severe auditory symptoms than those without, but only those with a history of TBI exhibited significantly worse performance on an objective measure of speech recognition. Furthermore, having a history of a more severe TBI was associated with greater tinnitus complaints (though this model explained little variance overall).

As expected, impairments in pure tone audiometric thresholds contributed to objective and subjective hearing performance. Three of the four thresholds contributed to predicted objective performance. At high frequencies, speech-in-noise performance was dominated by the thresholds in the “better” ear, which reflects that listeners can extract speech information from one ear for binaurally-presented speech. At low frequencies, speech-in-noise performance was primarily determined by thresholds in the “worse” ear. This reflects the important role that low-frequency binaural processing plays in the perception of speech stimuli in complex auditory environments. The performance benefit that listeners get when a noisy signal is spatially separated from a target (i.e., binaural release from masking) depends on the auditory system's ability to compare the amplitude and phases of low-frequency sounds arriving at the two ears. Thus, one would expect performance to be limited by the fidelity of the neural representation of the sound in the worse ear. Previous studies have shown that binaural tasks like auditory localization tend to degrade when the hearing threshold at 500 Hz exceeds 40 dB in the worse ear (51).

MRT RT and executive function also predicted objective speech-in-noise recognition. These results align with previous findings that cognitive factors influence performance on speech-in-noise tests [e.g., (43, 44)]. They also extend the results of small-scale studies of civilians with and without TBI (each total n < 35) that observed associations between auditory and cognitive function in assessing speech-in-noise abilities (13, 18).

Surprisingly, an increase, rather than a decrease, in speech-in-noise performance was predicted with aging. This was one of the weaker effects in the model and likely reflects an offset related to another predictor that naturally increases with age, like hearing thresholds. This interpretation is supported by a weak, positive correlation between age and speech composite score [r₍₂₁₀₎ = 0.14, p = 0.04] in the overall data when other factors are not partialed out.

In contrast to objective recognition, subjective hearing difficulties were predominantly predicted by thresholds in the better ear at high and low frequencies. Notably, the actual performance level on the objective speech-in-noise tests was not a significant predictor. Responses to THS Hearing questions may have been more likely to be influenced by the audibility of soft sounds than by speech-in-noise understanding. Objective speech-in-noise tests in this study were generally presented at a high enough level to ensure audibility, which could explain why low frequency thresholds in the worse ear contributed more to the objective than the subjective speech measure. Additionally, symptoms of PTSD were related to subjective hearing problems. However, even when including these factors, self-reported blast exposure was still related to hearing complaints. This follows from the finding that mTBIs caused by blast exposure are associated with more self-reported hearing difficulty than those not caused by blast (22). Because blast exposure often accompanies acoustic noise exposure, more detailed blast and noise exposure history information will be needed to better understand this link.

In addition to greater TBI severity, subjective tinnitus complaints were also associated with measures of processing speed and attention/working memory. This finding aligns with research suggesting that bothersome tinnitus coincides with deficits in attention and executive functions (52). However, few factors predicted tinnitus problems in this sample, and the model explained a small amount of variance.

The results of this study highlight that standard audiometric measures may be insufficient to characterize the hearing-related problems of SMVs with a history of TBI or blast exposure. Although the mechanisms are unclear, these data suggest that SMVs with blast exposure or TBI suffer from hearing deficits that go beyond what would be expected from increased hearing thresholds, elevated PTSD and depressive symptoms, and degraded cognitive function that might result from their injuries. Indeed, there was an observable relationship between hearing difficulties and TBI or blast history even when these other factors were included in the models.

One takeaway from this data is that objective speech-in-noise performance, subjective hearing and tinnitus were associated with different injury mechanisms. Objective speech-in-noise performance was best predicted by the presence of any TBI. Subjective hearing was best predicted by a self-reported history of blast exposure and tinnitus was best predicted by the presence of severe TBI. Military audiologists have anecdotally noted that blast-exposed patients tend to report hearing problems that are difficult to validate with clinical tests of speech-in-noise recognition. These data are consistent with those anecdotal observations. However, it is not clear whether increased patient complaints reflect a true performance deficit that was not detected by our speech-in-noise tests or whether it reflects a tendency for blast-exposed listeners to experience greater listening effort even when they achieve the same level of objective performance.

This preliminary, cross-sectional analysis has focused on identifying broad factors that might differentiate the objective hearing performance and subjective hearing complaints of SMVs with TBI from those of SMVs who have not suffered head injuries. As data collection progresses, we hope to be able to identify more specific tests or combinations of tests that might be sensitive to the unique hearing pathologies that exist in this population. The longitudinal nature of the study will also make it possible to track how these hearing problems progress over time and the extent to which they might contribute to the overall quality of life experienced by SMVs with TBI. Perhaps most importantly, we hope to be able to conduct more nuanced analyses to help identify the specific exposures or injury mechanisms that might be responsible for the excess hearing difficulties attributed to blast exposure or TBI in this sample. In the short term, however, these results may serve to highlight the importance of including audiological measures beyond the pure-tone audiogram in studies evaluating chronic effects of TBI. Including cognitive and symptomological assessments may help to better characterize and ultimately better remediate these deficits. However, more work is needed to fully account for the challenges that SMVs with a history of TBI or blast exposure face.

The datasets presented in this article are not readily available because Department of Defense policy prohibits sharing of sensitive data for this DoD-funded research. Requests to access the datasets should be directed to Stefanie E. Kuchinsky, stefanie.e.kuchinsky.civ@mail.mil.

The studies involving human participants were reviewed and approved by the Institutional Review Board at Walter Reed National Military Medical Center. The patients/participants provided their written informed consent to participate in this study.

RL, LF, and TB designed and directed the larger longitudinal project. RL, LF, TB, SL, DB, and ME contributed to the overarching design of this study. DB, SK, and ME conceptualized the specific research question. SK conceptualized and implemented the analysis plan. SK, DB, and ME interpreted the results and wrote the initial draft of the paper. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.