Implications of blast exposure for central auditory function: a review

Additive effects of mild head trauma, blast exposure, and aging within white matter tracts: A novel Diffusion Tensor Imaging analysis approach

Existing diffusion tensor imaging (DTI) studies of neurological injury following high-level blast exposure (hlBE) in military personnel have produced widely variable results. This is potentially due to prior studies often not considering the quantity and/or recency of hlBE, as well as co-morbidity with non-blast head trauma (nbHT). Herein, we compare commonly used DTI metrics: fractional anisotropy and mean, axial, and radial diffusivity, in Veterans with and without history of hlBE and/or nbHT. We use both the traditional method of dividing participants into 2 equally weighted groups and an alternative method wherein each participant is weighted by quantity and recency of hlBE and/or nbHT. While no differences were detected using the traditional method, the alternative method revealed diffuse and extensive changes in all DTI metrics. These effects were quantified within 43 anatomically defined white matter tracts, which identified the forceps minor, middle corpus callosum, acoustic and optic radiations, fornix, uncinate, inferior fronto-occipital and inferior longitudinal fasciculi, and cingulum, as the pathways most affected by hlBE and nbHT. Moreover, additive effects of aging were present in many of the same tracts suggesting that these neuroanatomical effects may compound with age.

Therapeutic Function of Liraglutide for Mitigation of Blast-Induced Hearing Damage: An Initial Investigation in Animal Model of Chinchilla

INTRODUCTION

Auditory injuries induced by repeated exposures to blasts reduce the operational performance capability and the life quality of military personnel. The treatment for blast-induced progressive hearing damage is lacking. We have recently investigated the therapeutic function of liraglutide, a glucagon-like peptide-1 receptor agonist, to mitigate blast-induced hearing damage in the animal model of chinchilla, under different blast intensities, wearing earplugs (EPs) or not during blasts, and drug-treatment plan. The goal of this study was to investigate the therapeutical function of liraglutide by comparing the results obtained under different conditions.

MATERIALS AND METHODS

Previous studies on chinchillas from two under-blast ear conditions (EP/open), two blast plans (G1: 6 blasts at 3-5 psi or G2:3 blasts at 15-25 psi), and three treatment plans (blast control, pre-blast drug treatment, and post-blast drug treatment) were summarized. The auditory brainstem response (ABR), distortion product otoacoustic emission (DPOAE), and middle latency response (MLR) recorded within 14 days after the blasts were used. Statistical analysis was performed to evaluate the effect of liraglutide under different conditions.

RESULTS

ABR threshold shifts indicated that the conditions of the EP and open ears were substantially different. Results from EP chinchillas indicated that the pre-blast treatment reduced the acute ABR threshold elevation on the day of blasts, and the significance of such an effect increased with the blast level. Liraglutide-treated open chinchillas showed lower ABR threshold shifts at the later stage of the experiment regardless of the blast levels. The DPOAE was less damaged after G2 blasts compared to G1 when pre-blast liraglutide was administrated. Lower post-blast MLR amplitudes were observed in the pre-blast treatment groups.

CONCLUSIONS

This study indicated that the liraglutide mitigated the blast-induced auditory injuries. In EP ears, the pre-blast administration of liraglutide reduced the severity of blast-induced acute damage in ears with EP protection, especially under G2. In animals with open ears, the effect of liraglutide on the restoration of hearing increased with time. The liraglutide potentially benefits post-blast hearing through multiple approaches with different mechanics.

Mild Traumatic Brain Injury and the Auditory System: An Overview of the Mechanisms, Clinical Presentations, and Current Diagnostic Modalities

The acute and long-term consequences of mild traumatic brain injury (mTBI) are far reaching. Though it may often be overlooked due to the now expansive field of research dedicated to understanding the consequences of mTBI on the brain, recent work has revealed that substantial changes in the vestibulo-auditory system can also occur due to mTBI. These changes, termed "labyrinthine" or "cochlear concussion," include hearing loss, vertigo, and tinnitus that develop after mTBI in the setting of an intact bony labyrinthine capsule (as detected on imaging). In the review that follows, we focus our discussion on the effects of mTBI on the peripheral structures and pathways of the auditory and vestibular systems. Although the effects of indirect trauma (e.g., noise and blast trauma) have been well-investigated, there exists a profound need to improve our understanding of the effects of direct head injury (such as mTBI) on the auditory and vestibular systems. Our aim is to summarize the current evidentiary foundation upon which labyrinthine and/or cochlear concussion are based to shed light on the ways in which clinicians can refine the existing modalities used to diagnose and treat patients experiencing mTBI as it relates to hearing and balance.

Hearing rehabilitation of adults with auditory processing disorder: a systematic review and meta-analysis of current evidence-based interventions

Background

For adults with auditory processing disorder (APD), listening and communicating can be difficult, potentially leading to social isolation, depression, employment difficulties and certainly reducing the quality of life. Despite existing practice guidelines suggesting treatments, the efficacy of these interventions remains uncertain due to a lack of comprehensive reviews. This systematic review and meta-analysis aim to establish current evidence on the effectiveness of interventions for APD in adults, addressing the urgent need for clarity in the field.

Methods

Following the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines, we conducted a systematic search across MEDLINE (Ovid), Embase (Ovid), Web of Science and Scopus, focusing on intervention studies involving adults with APD. Studies that met the inclusion criteria were grouped according to intervention with a meta-analysis only conducted where intervention, study design and outcome measure were comparable.

Results

Out of 1,618 screened records, 13 studies were included, covering auditory training (AT), low-gain hearing aids (LGHA), and personal remote microphone systems (PRMS). Our analysis revealed: AT, Mixed results with some improvements in speech intelligibility and listening ability, indicating potential benefits but highlighting the need for standardized protocols; LGHA, The included studies demonstrated significant improvements in monaural low redundancy speech testing (p < 0.05), suggesting LGHA could enhance speech perception in noisy environments. However, limitations include small sample sizes and potential biases in study design. PRMS, Demonstrated the most consistent evidence of benefit, significantly improving speech testing results, with no additional benefit from combining PRMS with other interventions.

Discussion

PRMS presents the most evidence-supported intervention for adults with APD, although further high-quality research is crucial for all intervention types. The establishment and implementation of standardized intervention protocols alongside rigorously validated outcome measures will enable a more evidence-based approach to managing APD in adults.

Blast Exposure Alters Synaptic Connectivity in the Mouse Auditory Cortex

Blast exposure can cause auditory deficits that have a lasting, significant impact on patients. Although the effects of blast on auditory functions localized to the ear have been well documented, the impact of blast on central auditory processing is largely undefined. Understanding the structural and functional alterations in the central nervous system (CNS) associated with blast injuries is crucial for unraveling blast-induced pathophysiological pathways and advancing development of therapeutic interventions. In this study, we used electrophysiology in combination with optogenetics assay, proteomic analysis, and morphological evaluation to investigate the impairment of synaptic connectivity in the auditory cortex (AC) of mice following blast exposure. Our results show that the long-range functional connectivity between the medial geniculate nucleus (MGN) and AC was impaired in the acute phase of blast injury. We also identified impaired synaptic transmission and dendritic spine alterations within 7 days of blast exposure, which recovered at 28 days post-blast. Additionally, proteomic analysis identified a few differentially expressed proteins in the cortex that are involved in synaptic signaling and plasticity. These findings collectively suggest that blast-induced alterations in the sound signaling network in the auditory cortex may underlie hearing deficits in the acute and sub-acute phases after exposure to shockwaves. This study may shed light on the perturbations underlying blast-induced auditory dysfunction and provide insights into the potential therapeutic windows for improving auditory outcomes in blast-exposed individuals.

Imaging the Ear Anatomy and Function Using Optical Coherence Tomography Vibrometry

Optical coherence tomography (OCT) is a novel technology for performing real-time high-speed and high-resolution cross-sectional imaging on the micro-scale in situ. It is analogous to ultrasound imaging, except that it uses light instead of sound. OCT has recently been introduced in auditory research to visualize the various structures of the ear with a minimally invasive operation. In addition, OCT can be used as a vibrometry system that is capable to detect sound-induced sub-nanometer vibrations of the middle and inner ear. OCT-vibrometry measures depth-resolved vibrations into the specimen, which overcomes several limitations of classical vibrometry techniques (e.g., single surface point measurements using laser interferometry). In this article, we illustrate how to visualize the anatomy and function of the middle and inner ear (the cochlea) in a gerbil model using recently developed spectral-domain OCT. Our results demonstrate that the largest clinical impact of OCT for otology is to visualize various pathologies and quantify sound conduction and processing in the individual peripheral human ear.

Blast-Induced Central Auditory Neurodegeneration Affects Tinnitus Development Regardless of Peripheral Cochlear Damage

Blast exposure causes serious complications, the most common of which are ear-related symptoms such as hearing loss and tinnitus. The blast shock waves can cause neurodegeneration of the auditory pathway in the brainstem, as well as the cochlea, which is the primary receptor for hearing, leading to blast-induced tinnitus. However, it is still unclear which lesion is more dominant in triggering tinnitus, the peripheral cochlea or the brainstem lesion owing to the complex pathophysiology and the difficulty in objectively measuring tinnitus. Recently, gap detection tests have been developed and are potentially well-suited for determining the presence of tinnitus. In this study, we investigated whether the peripheral cochlea or the central nervous system has a dominant effect on the generation of tinnitus using a blast-exposed mouse model with or without earplugs, which prevent cochlear damage from a blast transmitted via the external auditory canal. The results showed that the earplug (+) group, in which the cochlea was neither physiologically nor histologically damaged, showed a similar extent of tinnitus behavior in a gap prepulse inhibition of acoustic startle reflex test as the earplug (-) group, in which the explosion caused a cochlear synaptic loss in the inner hair cells and demyelination of auditory neurons. In contrast, both excitatory synapses labeled with VGLUT-1 and inhibitory synapses labeled with GAD65 were reduced in the ventral cochlear nucleus, and demyelination in the medial nucleus of the trapezoid body was observed in both groups. These disruptions significantly correlated with the presence of tinnitus behavior regardless of cochlear damage. These results indicate that the lesion in the brainstem could be dominant to the cochlear lesion in the development of tinnitus following blast exposure.

Relationships Between Auditory Processing and Cognitive Abilities in Adults: A Systematic Review

PURPOSE

The contributions from the central auditory and cognitive systems play a major role in communication. Understanding the relationship between auditory and cognitive abilities has implications for auditory rehabilitation for clinical patients. The purpose of this systematic review is to address the question, "In adults, what is the relationship between central auditory processing abilities and cognitive abilities?"

METHOD

Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines were followed to identify, screen, and determine eligibility for articles that addressed the research question of interest. Medical librarians and subject matter experts assisted in search strategy, keyword review, and structuring the systematic review process. To be included, articles needed to have an auditory measure (either behavioral or electrophysiologic), a cognitive measure that assessed individual ability, and the measures needed to be compared to one another.

RESULTS

Following two rounds of identification and screening, 126 articles were included for full analysis. Central auditory processing (CAP) measures were grouped into categories (behavioral: speech in noise, altered speech, temporal processing, binaural processing; electrophysiologic: mismatch negativity, P50, N200, P200, and P300). The most common CAP measures were sentence recognition in speech-shaped noise and the P300. Cognitive abilities were grouped into constructs, and the most common construct was working memory. The findings were mixed, encompassing both significant and nonsignificant relationships; therefore, the results do not conclusively establish a direct link between CAP and cognitive abilities. Nonetheless, several consistent relationships emerged across different domains. Distorted or noisy speech was related to working memory or processing speed. Auditory temporal order tasks showed significant relationships with working memory, fluid intelligence, or multidomain cognitive measures. For electrophysiology, relationships were observed between some cortical evoked potentials and working memory or executive/inhibitory processes. Significant results were consistent with the hypothesis that assessments of CAP and cognitive processing would be positively correlated.

CONCLUSIONS

Results from this systematic review summarize relationships between CAP and cognitive processing, but also underscore the complexity of these constructs, the importance of study design, and the need to select an appropriate measure. The relationship between auditory and cognitive abilities is complex but can provide informative context when creating clinical management plans. This review supports a need to develop guidelines and training for audiologists who wish to consider individual central auditory and cognitive abilities in patient care.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.24855174.

Functional Hearing Difficulties in Blast-Exposed Service Members With Normal to Near-Normal Hearing Thresholds

OBJECTIVES

Estimated prevalence of functional hearing and communication deficits (FHCDs), characterized by abnormally low speech recognition and binaural tone detection in noise or an abnormally high degree of self-perceived hearing difficulties, dramatically increases in active-duty service members (SMs) who have hearing thresholds slightly above the normal range and self-report to have been close to an explosive blast. Knowing the exact nature of the underlying auditory-processing deficits that contribute to FHCD would not only provide a better characterization of the effects of blast exposure on the human auditory system, but also allow clinicians to prescribe appropriate therapies to treat or manage patient complaints.

DESIGN

Two groups of SMs were initially recruited: (1) a control group (N = 78) with auditory thresholds ≤20 dB HL between 250 and 8000 Hz, no history of blast exposure, and who passed a short FHCD screener, and (2) a group of blast-exposed SMs (N = 26) with normal to near-normal auditory thresholds between 250 and 4000 Hz, and who failed the FHCD screener (cutoffs based on the study by Grant et al.). The two groups were then compared on a variety of audiometric, behavioral, cognitive, and electrophysiological measures. These tests were selected to characterize various aspects of auditory system processing from the cochlear to the cortex. A third, smaller group of blast-exposed SMs who performed within normal limits on the FHCD screener were also recruited (N = 11). This third subject group was unplanned at the onset of the study and was added to evaluate the effects of blast exposure on hearing and communication regardless of performance on the FHCD screener.

RESULTS

SMs in the blast-exposed group with FHCD performed significantly worse than control participants on several metrics that measured peripheral and mostly subcortical auditory processing. Cognitive processing was mostly unaffected by blast exposure with the exception of cognitive tests of language-processing speed and working memory. Blast-exposed SMs without FHCD performed similarly to the control group on tests of peripheral and brainstem processing, but performed similarly to blast-exposed SMs with FHCD on measures of cognitive processing. Measures derived from EEG recordings of the frequency-following response revealed that blast-exposed SMs who exhibited FHCD demonstrated increased spontaneous neural activity, reduced amplitude of the envelope-following response, poor internal signal to noise ratio, reduced response stability, and an absent or delayed onset response, compared with the other two participant groups.

CONCLUSIONS

Degradation in the neural encoding of acoustic stimuli is likely a major contributing factor leading to FHCD in blast-exposed SMs with normal to near-normal audiometric thresholds. Blast-exposed SMs, regardless of their performance on the FHCD screener, exhibited a deficit in language-processing speed and working memory, which could lead to difficulties in decoding rapid speech and in understanding speech in challenging speech communication settings. Further tests are needed to align these findings with clinical treatment protocols being used for patients with suspected auditory-processing disorders.

Mitigation of Hearing Damage With Liraglutide Treatment in Chinchillas After Repeated Blast Exposures at Mild-TBI

INTRODUCTION

Although hearing protection devices (HPDs) have been widely used during training and combat, over one million veterans experience service-connected hearing loss. Hearing damage has been reported to be associated with blast-induced mild traumatic brain injury (mTBI) and there is a lack of understanding and treatment. Liraglutide is a glucagon-like peptide-1 receptor agonist and a potential treatment for TBI-induced memory deficits. This study aims to investigate the function of the liraglutide to prevent damage and facilitate hearing restoration in chinchillas exposed to multiple high-intensity, mTBI-level blasts.

MATERIALS AND METHODS

Chinchillas were divided into three treatment groups: blast control, pre-blast drug treatment, and post-blast drug treatment. On day 1, the chinchilla ears were protected by HPDs and exposed to three blasts with peak pressure levels of 15-25 psi. The auditory brainstem response (ABR), distortion product otoacoustic emission (DPOAE), and middle latency response (MLR) were recorded pre- and post-blast on day 1 and on days 4, 7, 14, and 28.

RESULTS

Substantial acute damage was observed and progressively recovered in chinchillas after the blast exposures. The pre-blast treatment group exhibited the lowest elevation of the ABR threshold and reduction of the wave I amplitude on day 1 after blasts. The liraglutide treatment insignificantly facilitated the recovery of the DPOAE levels and ABR thresholds on days 14 and 28. The pre-blast treatment chinchillas showed reduced MLR amplitudes on days 4 and 7.

CONCLUSIONS

This study indicated that the pre-blast liraglutide administration provided damage protection against blasts in addition to the HPDs. Current evidence suggests that the effect of liraglutide is more prominent in the early phase of the experiment.

Recommendations for a Military Health System Auditory Blast Injury Prevention Standard

INTRODUCTION

Although existing auditory injury prevention standards benefit warfighters, the Department of Defense could do more to understand and address auditory injuries (e.g., hearing loss, tinnitus, and central processing deficits) among service members. The Blast Injury Prevention Standards Recommendation (BIPSR) Process is designed to address the needs of all the Military Services for biomedically valid Military Health System (MHS) Blast Injury Prevention Standards.

MATERIALS AND METHODS

Through the BIPSR Process, stakeholders provided their intended uses and requested functionalities for an MHS Blast Injury Prevention Standard. The BIPSR Process established a broad-based, non-advocacy panel of auditory injury Subject Matter Expert (SME) Panel with members drawn from industry, academia, and government. The SME Panel selected evaluation factors, weighted priorities, and then evaluated the resulting candidate MHS Auditory Blast Injury Prevention Standards against the evaluation criteria. The SME Panel members provided rationales for their decisions, documented discussions, and used iterative rounds of feedback to promote consensus building among members. The BIPSR Process used multi-attribute utility theory to combine members' evaluations and compare the candidate standards.

RESULTS

The SME Panel identified and collated information about existing auditory injury datasets to identify gaps and promote data sharing and comprehensive evaluations of standards for preventing auditory blast injury. The panel evaluated the candidate standards and developed recommendations for an MHS Blast Injury Prevention Standard.

CONCLUSIONS

The BIPSR Process illuminated important characteristics, capabilities, and limitations of candidate standards and existing datasets (e.g., limited human exposure data to evaluate the validity of injury prediction) for auditory blast injury prevention. The evaluation resulted in the recommendation to use the 8-hour Equivalent Level (LAeq8hr) as the interim MHS Auditory Blast Injury Prevention Standard while the community performs additional research to fill critical knowledge gaps.

Central Auditory Processing Dysfunction in Service Members and Veterans: Treatment Considerations and Strategies

PURPOSE

Military risk factors such as blast exposure, noise exposure, head trauma, and neurotoxin exposure place Service members and Veterans at risk for deficits associated with auditory processing dysfunction. However, there is no clinical guidance specific to the treatment of auditory processing deficits in this unique population. We provide an overview of available treatments and their limited supporting evidence for use in adults, emphasizing the need for multidisciplinary case management and interdisciplinary research to support evidence-based solutions.

METHOD

We explored relevant literature to inform the treatment of auditory processing dysfunction in adults, with emphasis on findings involving active or former military personnel. We were able to identify a limited number of studies, pertaining primarily to the treatment of auditory processing deficits through the use of assistive technologies and training strategies. We assessed the current state of the science for knowledge gaps that warrant additional study.

CONCLUSIONS

Auditory processing deficits often co-occur with other military injuries and may pose significant risk in military operational and occupational settings. Research is needed to advance clinical diagnostic and rehabilitative capabilities, guide treatment planning, support effective multidisciplinary management, and inform fitness-for-duty standards. We emphasize the need for an inclusive approach to the assessment and treatment of auditory processing concerns in Service members and Veterans and for evidence-based solutions to address complex military risk factors and injuries.

Immune responses in mice after blast-mediated traumatic brain injury TBI autonomously contribute to retinal ganglion cell dysfunction and death

PURPOSE

The purpose of this study was to examine the role of the immune system and its influence on chronic retinal ganglion cell (RGC) dysfunction following blast-mediated traumatic brain injury (bTBI).

METHODS

C57BL/6J and B6.129S7-Rag1^tm1Mom/J (Rag^-/-) mice were exposed to one blast injury of 140 kPa. A separate cohort of C57BL/6J mice was exposed to sham-blast. Four weeks following bTBI mice were euthanized, and splenocytes were collected. Adoptive transfer (AT) of splenocytes into naïve C57BL/6J recipient mice was accomplished via tail vein injection. Three groups of mice were analyzed: those receiving AT of splenocytes from C57BL/6J mice exposed to blast (AT-TBI), those receiving AT of splenocytes from C57BL/6J mice exposed to sham (AT-Sham), and those receiving AT of splenocytes from Rag^-/- mice exposed to blast (AT-Rag^-/-). The visual function of recipient mice was analyzed with the pattern electroretinogram (PERG), and the optomotor response (OMR). The structure of the retina was evaluated using optical coherence tomography (OCT), and histologically using BRN3A-antibody staining.

RESULTS

Analysis of the PERG showed a decreased amplitude two months post-AT that persisted for the duration of the study in AT-TBI mice. We also observed a significant decrease in the retinal thickness of AT-TBI mice two months post-AT compared to sham, but not at four or six months post-AT. The OMR response was significantly decreased in AT-TBI mice 5- and 6-months post-AT. BRN3A staining showed a loss of RGCs in AT-TBI and AT-Rag^-/- mice.

CONCLUSION

These results suggest that the immune system contributes to chronic RGC dysfunction following bTBI.

Mitigation of Hearing Damage After Repeated Blast Exposures in Animal Model of Chinchilla

High-intensity sound or blast-induced hearing impairment is a common injury for Service members. Epidemiology studies revealed that the blast-induced hearing loss is associated with the traumatic brain injury (TBI), but the mechanisms of the formation and prevention of auditory injuries require further investigation. Liraglutide, a glucagon-like peptide-1 receptor (GLP-1R) agonist, has been reported as a potential treatment strategy for TBI-caused memory deficits; however, there is no study on therapeutics of GLP-1R for blast-induced hearing damage. This paper reports our current study on progressive hearing damage after repeated exposures to low-level blasts in the animal model of chinchilla and the mitigation of hearing damage using liraglutide. Chinchillas were divided into three groups (N = 7 each): blast control, pre-blast treatment, and post-blast treatment. All animals were exposed to six consecutive blasts at the level of 3-5 psi (21-35 kPa) on Day 1. The auditory brainstem response (ABR) was measured on Day 1 (pre- and post-blast) and Days 4, 7, and 14 after blast exposure. Upon the completion of the experiment on Day 14, the brain tissues of animals were harvested for immunofluorescence studies. Significant damage was revealed in blast-exposed chinchillas by increased ABR thresholds, decreased ABR wave I amplitudes, and cell apoptosis in the inferior colliculus in the blast control chinchillas. Treatment with liraglutide appeared to reduce the severity of blast-induced hearing injuries as observed from the drug-treated chinchillas comparing to the blast controls. This study bridges the gap between TBI and hearing impairment and suggests a possible intervention for blast-induced hearing loss for Service members.

Association Between TBI-Related Hearing Impairment and Cognition: A TRACK-TBI Study

OBJECTIVE

To examine the association between hearing impairment and cognitive function after traumatic brain injury (TBI).

SETTING

A total of 18 level I trauma centers throughout the United States in the T ransforming R esearch a nd C linical K nowledge in TBI (TRACK-TBI) study.

PARTICIPANTS

From February 2014 to June 2018, a total of 2697 participants with TBI were enrolled in TRACK-TBI. Key eligibility criteria included external force trauma to the head, presentation to a participating level I trauma center, and receipt of a clinically indicated head computed tomographic (CT) scan within 24 hours of injury. A total of 1267 participants were evaluated in the study, with 216 participants with hearing impairment and 1051 participants without hearing impairment. Those with missing or unknown hearing status or cognitive assessment were excluded from analysis.

DESIGN

Prospective, observational cohort study.

MAIN MEASURES

Hearing impairment at 2 weeks post-TBI was based on self-report. Participants who indicated worse hearing in one or both ears were defined as having hearing impairment, whereas those who denied worse hearing in either ear were defined as not having hearing impairment and served as the reference group. Cognitive outcomes at 6 months post-TBI included executive functioning and processing speed, as measured by the Trail Making Test (TMT) B/A and the Wechsler Adult Intelligence Scale, Fourth Edition, Processing Speed Index subscale (WAIS-IV PSI), respectively.

RESULTS

TBI-related hearing impairment had a small but significantly greater TMT B/A ratio than without TBI-related hearing impairment: mean difference ( B ) = 0.25; 95% CI, 0.07 to 0.43; P = .005. No significant mean differences on WAIS-IV PSI scores were found between participants with and without TBI-related hearing impairment: B = 0.36; 95% CI, -2.07 to 2.60; P = .825.

CONCLUSION

We conclude that TBI-related hearing impairment at 6 months postinjury was significantly associated with worse executive functioning but not cognitive processing speed.

A 3D Printed Human Ear Model for Standardized Testing of Hearing Protection Devices to Blast Exposure

Hypothesis

A 3D printed human temporal bone (TB) that is anatomically accurate would cost-effectively reproduce the responses observed in blast testing of human cadaveric TBs with and without passive hearing protection devices (HPDs).

Background

HPDs have become critical personal protection equipment against auditory damage for service members. Acoustic test fixtures and human TBs have been used to test and develop HPDs; however, the lack of a cost-effective, standardized model impedes the improvement of HPDs.

Methods

In this study, the 3D printed TB model was printed with flexible and rigid polymers and consisted of the ear canal, tympanic membrane (TM), ossicular chain, middle ear suspensory ligaments/muscle tendons, and middle ear cavity. The TM movement under acoustic stimulation was measured with laser Doppler vibrometry. The TB model was then exposed to blasts with or without HPDs and pressures at the ear canal entrance (P0) and near the TM in the ear canal (P1) were recorded. All results were compared with that measured in human TBs.

Results

Results indicated that in the 3D printed TB, the attenuated peak pressures at P1 induced by HPDs ranged from 0.92 to 1.06 psi (170-171 dB) with blast peak pressures of 5.62-6.54 psi (186-187 dB) at P0, and measured results were within the mean and SD of published data. Vibrometry measurements also followed a similar trend as the published results.

Conclusions

The 3D printed TB model accurately evaluated passive HPDs' protective function during blast and the potential for use as a model for acoustic transmission was investigated.

Cognitive-Communication Complaints and Referrals for Speech-Language Pathology Services Following Concussion

PURPOSE

Speech-language pathologists are increasingly being recognized as key members of concussion management teams. This study investigates whether self-report of communication problems postconcussion may be useful in identifying clients who could benefit from speech-language pathology services.

METHOD

Participants included 41 adolescents and adults from an outpatient specialty concussion clinic. All completed the La Trobe Communication Questionnaire (LCQ) at admission, and 23 repeated this measure at discharge. Participants were prospectively enrolled, with chart reviews providing demographic, injury, and medical factors. The analysis considered (a) communication complaints and resolution over time, including comparison to two previously published LCQ studies of typical adults and adults with and without traumatic brain injury (TBI); (b) the relationship between communication complaints, participant factors, and common concussion assessments; and (c) factors related to speech-language pathology service referral for rehabilitation.

RESULTS

At first visit, 12 of 41 participants (29%) reported communication problems, although 19 (46%) reported difficulty with greater than half of LCQ items. At a group level, compared to published reference data of both people with chronic mixed severity TBI and controls, participants in this study reported more problems at first visit with communication overall, as well as greater difficulty with the LCQ Initiation/Conversation Flow subscale. Partner Sensitivity subscale scores at first visit were also greater than published control data. LCQ subscale scores of Initiation/Conversation Flow and Partner Sensitivity decreased from first visit to last visit, demonstrating resolution over time. Only concussion symptom scales and not demographic, injury, or cognitive screenings were related to LCQ scores. The same two LCQ subscales, Initiation/Conversation Flow and Partner Sensitivity, predicted referral for speech-language pathology services, along with symptom scales and being injured due to motor vehicle crash.

DISCUSSION

A subset of people recovering from concussion report experiencing communication problems. Reporting of particular communication problems was related to referral for speech-language pathology rehabilitation services and may be useful in directing care after concussion.

Correlation of blast-induced tympanic membrane perforation with peripheral cochlear synaptopathy

The auditory organs, including the tympanic membrane, cochlea, and central auditory pathway, are the most fragile components of the human body when exposed to blast overpressure. Tympanic membrane perforation (TMP) is the most frequent symptom in blast-exposed patients. However, the impact of TMP on the inner ear and central auditory system is not fully understood. We aimed to analyze the effect of blast-induced TMP on the auditory pathophysiological changes in mice after blast exposure. Mice aged 7 weeks were exposed to blast overpressure to induce TMP and allowed to survive for 2 months. All TMP cases had spontaneously healed by week 3 following the blast exposure. Compared to controls, blast-exposed mice exhibited a significant elevation in hearing thresholds and an apparent disruption of stereocilia in the outer hair cells, regardless of the occurrence or absence of TMP. The reduction in synapses in the inner hair cells, which is known as the most frequent pathology in blast-exposed cochleae, was significantly more severe in mice without TMP. However, a decrease in the number of excitatory central synapses labeled by VGLUT-1 in the cochlear nucleus was observed regardless of the absence or presence of TMP. Our findings suggest that blast-induced TMP mitigates peripheral cochlear synaptic disruption but leaves the central auditory synapses unaffected, indicating that central synaptic disruption is independent of TMP and peripheral cochlear synaptic disruption. Synaptic deterioration in the peripheral and central auditory systems can contribute to the promotion of blast-induced hearing impairment, including abnormal auditory perception.

Concussion Management Guidelines Neglect Auditory Symptoms

Tinnitus, noise sensitivity, and hearing difficulties are commonly reported secondary to head injury. These auditory deficits have been shown to negatively impact daily functioning, and yet, often go unnoticed by health care professionals. The purpose of this editorial is to explain why it is essential for clinical practice guidelines that address the management of patients who have experienced a head injury to incorporate assessment and rehabilitation of auditory symptoms.

Predicting speech discrimination scores from pure-tone thresholds-A machine learning-based approach using data from 12,697 subjects

Diagnostic tests for hearing impairment not only determines the presence (or absence) of hearing loss, but also evaluates its degree and type, and provides physicians with essential data for future treatment and rehabilitation. Therefore, accurately measuring hearing loss conditions is very important for proper patient understanding and treatment. In current-day practice, to quantify the level of hearing loss, physicians exploit specialized test scores such as the pure-tone audiometry (PTA) thresholds and speech discrimination scores (SDS) as quantitative metrics in examining a patient’s auditory function. However, given that these metrics can be easily affected by various human factors, which includes intentional (or accidental) patient intervention, there are needs to cross validate the accuracy of each metric. By understanding a “normal” relationship between the SDS and PTA, physicians can reveal the need for re-testing, additional testing in different dimensions, and also potential malingering cases. For this purpose, in this work, we propose a prediction model for estimating the SDS of a patient by using PTA thresholds via a Random Forest-based machine learning approach to overcome the limitations of the conventional statistical (or even manual) methods. For designing and evaluating the Random Forest-based prediction model, we collected a large-scale dataset from 12,697 subjects, and report a SDS level prediction accuracy of 95.05% and 96.64% for the left and right ears, respectively. We also present comparisons with other widely-used machine learning algorithms (e.g., Support Vector Machine, Multi-layer Perceptron) to show the effectiveness of our proposed Random Forest-based approach. Results obtained from this study provides implications and potential feasibility in providing a practically-applicable screening tool for identifying patient-intended malingering in hearing loss-related tests.

Blast Exposure and Self-Reported Hearing Difficulty in Service Members and Veterans Who Have Normal Pure-Tone Hearing Sensitivity: The Mediating Role of Posttraumatic Stress Disorder

Purpose Evidence suggests that military blast exposure may lead to self-reported hearing difficulties despite audiometrically normal hearing. Research identifying potential mechanisms of this association remains limited. The purpose of this article is to evaluate the associations between blast, posttraumatic stress disorder (PTSD), and self-reported hearing difficulty, and to examine PTSD as a possible mediator of the association between blast exposure and hearing difficulty. Method We used baseline data from the Noise Outcomes in Service members Epidemiology (NOISE) study (n = 477). Participants in this study undergo a comprehensive hearing, and tinnitus if applicable, evaluation and complete a large number of surveys. Pertinent data extracted from these surveys included information on participant's demographics, military service history, including exposure to blast, and health conditions such as symptoms of PTSD. Using regression models and following a formal causal mediation framework, we estimated total associations, natural direct and indirect associations, and percent mediated. Results We found that individuals with blast exposure had higher prevalence of both probable PTSD and self-reported hearing difficulty than individuals who were not blast exposed. Compared with participants without blast exposure, those with blast exposure had twice the prevalence of self-reported hearing difficulty, with 41% of the association mediated through probable PTSD. Conclusion As PTSD is a possible mediator of the association between blast exposure and hearing difficulty, Service members and Veterans with normal pure-tone hearing sensitivity who report hearing difficulties and a history of blast exposure may benefit from evaluation for PTSD symptoms. Supplemental Material https://doi.org/10.23641/asha.16674247.

Estimated Prevalence of Functional Hearing Difficulties in Blast-Exposed Service Members With Normal to Near-Normal-Hearing Thresholds

OBJECTIVES

Over the past decade, U.S. Department of Defense and Veterans Affairs audiologists have reported large numbers of relatively young adult patients who have normal to near-normal audiometric thresholds but who report difficulty understanding speech in noisy environments. Many of these service members also reported having experienced exposure to explosive blasts as part of their military service. Recent studies suggest that some blast-exposed patients with normal to near-normal-hearing thresholds not only have an awareness of increased hearing difficulties, but also poor performance on various auditory tasks (sound source localization, speech recognition in noise, binaural integration, gap detection in noise, etc.). The purpose of this study was to determine the prevalence of functional hearing and communication deficits (FHCD) among healthy Active-Duty service men and women with normal to near-normal audiometric thresholds.

DESIGN

To estimate the prevalence of such FHCD in the overall military population, performance of roughly 3400 Active-Duty service members with hearing thresholds mostly within the normal range were measured on 4 hearing tests and a brief 6-question survey to assess FHCD. Subjects were subdivided into 6 groups depending on the severity of the blast exposure (3 levels: none, far away, or close enough to feel heat or pressure) and hearing thresholds (2 levels: audiometric thresholds of 20 dB HL or better, slight elevation in 1 or more thresholds between 500 and 4000 Hz in either ear).

RESULTS

While the probability of having hearing difficulty was low (≈4.2%) for the overall population tested, that probability increased by 2 to 3 times if the service member was blast-exposed from a close distance or had slightly elevated hearing thresholds (>20 dB HL). Service members having both blast exposure and mildly elevated hearing thresholds exhibited up to 4 times higher risk for performing abnormally on auditory tasks and more than 5 times higher risk for reporting abnormally low ratings on the subjective questionnaire, compared with service members with no history of blast exposure and audiometric thresholds ≤20 dB HL. Blast-exposed listeners were roughly 2.5 times more likely to experience subjective or objective hearing deficits than those with no-blast history.

CONCLUSIONS

These elevated rates of abnormal performance suggest that roughly 33.6% of Active-Duty service members (or approximately 423,000) with normal to near-normal-hearing thresholds (i.e., H1 profile) are at some risk for FHCD, and about 5.7% (approximately 72,000) are at high risk, but are currently untested and undetected within the current fitness-for-duty standards. Service members identified as "at risk" for FHCD according to the metrics used in the present study, in spite of their excellent hearing thresholds, require further testing to determine whether they have sustained damage to peripheral and early-stage auditory processing (bottom-up processing), damage to cognitive processes for speech (top-down processing), or both. Understanding the extent of damage due to noise and blast exposures and the balance between bottom-up processing deficits and top-down deficits will likely lead to better therapeutic strategies.

Longitudinal auditory pathophysiology following mild blast-induced trauma

Blast-induced hearing difficulties affect thousands of veterans and civilians. The long-term impact of even a mild blast exposure on the central auditory system is hypothesized to contribute to lasting behavioral complaints associated with mild blast traumatic brain injury (bTBI). Although recovery from mild blast has been studied separately over brief or long time windows, few, if any, studies have investigated recovery longitudinally over short-term and longer-term (months) time windows. Specifically, many peripheral measures of auditory function either recover or exhibit subclinical deficits, masking deficits in processing complex, real-world stimuli that may recover differently. Thus, examining the acute time course and pattern of neurophysiological impairment using appropriate stimuli is critical to better understanding and intervening in bTBI-induced auditory system impairments. Here, we compared auditory brainstem response, middle-latency auditory-evoked potentials, and envelope following responses. Stimuli were clicks, tone pips, amplitude-modulated tones in quiet and in noise, and speech-like stimuli (iterated rippled noise pitch contours) in adult male rats subjected to mild blast and sham exposure over the course of 2 mo. We found that blast animals demonstrated drastic threshold increases and auditory transmission deficits immediately after blast exposure, followed by substantial recovery during the window of 7-14 days postblast, although with some deficits remaining even after 2 mo. Challenging conditions and speech-like stimuli can better elucidate mild bTBI-induced auditory deficit during this period. Our results suggest multiphasic recovery and therefore potentially different time windows for treatment, and deficits can be best observed using a small battery of sound stimuli.NEW & NOTEWORTHY Few studies on blast-induced hearing deficits go beyond simple sounds and sparsely track postexposure. Therefore, the recovery arc for potential therapies and real-world listening is poorly understood. Evidence suggested multiple recovery phases over 2 mo postexposure. Hearing thresholds largely recovered within 14 days and partially explained recovery. However, midlatency responses, responses to amplitude modulation in noise, and speech-like pitch sweeps exhibited extended changes, implying persistent central auditory deficits and the importance of subclinical threshold shifts.

Effect of shock wave power spectrum on the inner ear pathophysiology in blast-induced hearing loss

Blast exposure can induce various types of hearing impairment, including permanent hearing loss, tinnitus, and hyperacusis. Herein, we conducted a detailed investigation of the cochlear pathophysiology in blast-induced hearing loss in mice using two blasts with different characteristics: a low-frequency dominant blast generated by a shock tube and a high-frequency dominant shock wave generated by laser irradiation (laser-induced shock wave). The pattern of sensorineural hearing loss (SNHL) was low-frequency- and high-frequency-dominant in response to the low- and high-frequency blasts, respectively. Pathological examination revealed that cochlear synaptopathy was the most frequent cochlear pathology after blast exposure, which involved synapse loss in the inner hair cells without hair cell loss, depending on the power spectrum of the blast. This pathological change completely reflected the physiological analysis of wave I amplitude using auditory brainstem responses. Stereociliary bundle disruption in the outer hair cells was also dependent on the blast’s power spectrum. Therefore, we demonstrated that the dominant frequency of the blast power spectrum was the principal factor determining the region of cochlear damage. We believe that the presenting models would be valuable both in blast research and the investigation of various types of hearing loss whose pathogenesis involves cochlear synaptopathy.

Approaches for Monitoring Warfighter Blast-related Exposures in Training to Develop Effective Safety Standards

INTRODUCTION

Traumatic brain injuries are of concern to the sports and military communities because of the age of the participants and costly burden to society. To markedly reduce the impact of traumatic brain injury and its sequela (TBI-S), it is necessary to determine the initial vulnerability of individuals as well as identify new technologies that indicate early signs of TBI-S.

MATERIALS AND METHODS

Currently, diverse methods have been used by the authors and others in laboratory settings to reveal early signs of persistent TBI-S including simulation modeling of the effect of rapid deceleration on the deviatoric strain (shear force) imposed on specific brain regions, auditory evoked potential (AEP) measurements to determine injury to the auditory cortex optokinetic nystagmus (OKN) measures sensitive to vestibular trauma, and optical coherence tomography (OCT) measures that reveal changes in central visual function obtained noninvasively by examination of the retina.

RESULTS

Simulation studies provided technical information on maximal deviatoric strain at the base of the sulci and interface of gray and white matter consistent with results from neuropathology and from magnetic resonance imaging. The AEP and OKN reveal measurable injury to similar regions below the Sylvian fissure including auditory cortex and midbrain, and the OCT reveals changes to the retina consistent with forceful deceleration effects.

CONCLUSIONS

The studies and results are consistent with prior work demonstrating that noninvasive tests may be sensitive to the presence of TBI-S, potentially in the training field as advances in the portability of test instruments are underway. When combined with baseline data gathered from individuals in quantitative form, key variances can emerge. Therefore, it is hypothesized that AEP, OKN, and OCT, taken together, may yield faster objective and quantitative neurophysiological measures serving as a "signature" of neural injury and more indicative of potentially persistent TBI-S-recommending larger scale longitudinal studies.

Effects of Signal Type and Noise Background on Auditory Evoked Potential N1, P2, and P3 Measurements in Blast-Exposed Veterans

OBJECTIVES

Veterans who have been exposed to high-intensity blast waves frequently report persistent auditory difficulties such as problems with speech-in-noise (SIN) understanding, even when hearing sensitivity remains normal. However, these subjective reports have proven challenging to corroborate objectively. Here, we sought to determine whether use of complex stimuli and challenging signal contrasts in auditory evoked potential (AEP) paradigms rather than traditional use of simple stimuli and easy signal contrasts improved the ability of these measures to (1) distinguish between blast-exposed Veterans with auditory complaints and neurologically normal control participants, and (2) predict behavioral measures of SIN perception.

DESIGN

A total of 33 adults (aged 19-56 years) took part in this study, including 17 Veterans exposed to high-intensity blast waves within the past 10 years and 16 neurologically normal control participants matched for age and hearing status with the Veteran participants. All participants completed the following test measures: (1) a questionnaire probing perceived hearing abilities; (2) behavioral measures of SIN understanding including the BKB-SIN, the AzBio presented in 0 and +5 dB signal to noise ratios (SNRs), and a word-level consonant-vowel-consonant test presented at +5 dB SNR; and (3) electrophysiological tasks involving oddball paradigms in response to simple tones (500 Hz standard, 1000 Hz deviant) and complex speech syllables (/ba/ standard, /da/ deviant) presented in quiet and in four-talker speech babble at a SNR of +5 dB.

RESULTS

Blast-exposed Veterans reported significantly greater auditory difficulties compared to control participants. Behavioral performance on tests of SIN perception was generally, but not significantly, poorer among the groups. Latencies of P3 responses to tone signals were significantly longer among blast-exposed participants compared to control participants regardless of background condition, though responses to speech signals were similar across groups. For cortical AEPs, no significant interactions were found between group membership and either stimulus type or background. P3 amplitudes measured in response to signals in background babble accounted for 30.9% of the variance in subjective auditory reports. Behavioral SIN performance was best predicted by a combination of N1 and P2 responses to signals in quiet which accounted for 69.6% and 57.4% of the variance on the AzBio at 0 dB SNR and the BKB-SIN, respectively.

CONCLUSIONS

Although blast-exposed participants reported far more auditory difficulties compared to controls, use of complex stimuli and challenging signal contrasts in cortical and cognitive AEP measures failed to reveal larger group differences than responses to simple stimuli and easy signal contrasts. Despite this, only P3 responses to signals presented in background babble were predictive of subjective auditory complaints. In contrast, cortical N1 and P2 responses were predictive of behavioral SIN performance but not subjective auditory complaints, and use of challenging background babble generally did not improve performance predictions. These results suggest that challenging stimulus protocols are more likely to tap into perceived auditory deficits, but may not be beneficial for predicting performance on clinical measures of SIN understanding. Finally, these results should be interpreted with caution since blast-exposed participants did not perform significantly poorer on tests of SIN perception.

Acquired Central Auditory Processing Disorder in Service Members and Veterans

Purpose A growing body of evidence suggests that military service members and military veterans are at risk for deficits in central auditory processing. Risk factors include exposure to blast, neurotrauma, hazardous noise, and ototoxicants. We overview these risk factors and comorbidities, address implications for clinical assessment and care of central auditory processing deficits in service members and veterans, and specify knowledge gaps that warrant research. Method We reviewed the literature to identify studies of risk factors, assessment, and care of central auditory processing deficits in service members and veterans. We also assessed the current state of the science for knowledge gaps that warrant additional study. This literature review describes key findings relating to military risk factors and clinical considerations for the assessment and care of those exposed. Conclusions Central auditory processing deficits are associated with exposure to known military risk factors. Research is needed to characterize mechanisms, sources of variance, and differential diagnosis in this population. Existing best practices do not explicitly consider confounds faced by military personnel. Assessment and rehabilitation strategies that account for these challenges are needed. Finally, investment is critical to ensure that Veterans Affairs and Department of Defense clinical staff are informed, trained, and equipped to implement effective patient care.

Update on treatment options for blast-induced hearing loss

PURPOSE OF REVIEW

The incidence of blast injuries has increased, and the ear is the highest risk organ. Ear injury induced by blast exposure is important in both military and civilian conditions. The permanent hearing loss caused by blast exposure is associated with a decline in the quality of life. In this review, I describe recent therapeutic strategies for each of the ear pathologies caused by blast exposure.

RECENT FINDINGS

For tympanic membrane perforation after blast exposure, basic fibroblast growth factor (bFGF) has been used as a less invasive treatment to repair the tympanic membrane. The closure rates of tympanic membrane perforations treated with bFGF were reported to be comparable to those following conventional tympanoplasty.For sensorineural hearing loss after blast exposure, treatment with neurotrophic factors, such as nerve growth factor (NGF) or neurotrophin-3, antioxidants, and Atoh1 induction have recently been applied, and some of them were considered for clinical application.

SUMMARY

Recent advances of therapeutics for blast-induced hearing loss, based on their pathologies, have been outlined. There are several promising therapeutic approaches for both middle and inner ear disorders after blast exposure; however, further research is needed to establish new treatments for blast-induced hearing dysfunction.

Identification of chronic brain protein changes and protein targets of serum auto-antibodies after blast-mediated traumatic brain injury

In addition to needing acute emergency management, blast-mediated traumatic brain injury (TBI) is also a chronic disorder with delayed-onset symptoms that manifest and progress over time. While the immediate consequences of acute blast injuries are readily apparent, chronic sequelae are harder to recognize. Indeed, the identification of individuals with mild-TBI or TBI-induced symptoms is greatly impaired in large part due to the lack of objective and robust biomarkers. The purpose of this study was to address these need by identifying candidates for serum-based biomarkers of blast TBI, and also to identify unique or differentially regulated protein expression in the thalamus in C57BL/6J mice exposed to blast using high throughput qualitative screens of protein expression. To identify thalamic proteins differentially or uniquely associated with blast exposure, we utilized an antibody-based affinity-capture strategy (referred to as "proteomics-based analysis of depletomes"; PAD) to deplete thalamic lysates from blast-treated mice of endogenous thalamic proteins also found in control mice. Analysis of this "depletome" detected 75 unique proteins, many with associations to the myelin sheath. To identify blast-associated proteins eliciting production of circulating autoantibodies, serum antibodies of blast-treated mice were immobilized, and their immunogens subsequently identified by proteomic analysis of proteins specifically captured following incubation with thalamic lysates (a variant of a strategy referred to as "proteomics-based expression library screening"; PELS). This analysis identified 46 blast-associated immunogenic proteins, including 6 shared in common with the PAD analysis (ALDOA, PHKB, HBA-A1, DPYSL2, SYN1, and CKB). These proteins and their autoantibodies are appropriate for further consideration as biomarkers of blast-mediated TBI.

Characterization of acute hearing changes in United States military populations

Until recently, most hearing conservation programs, including those in the military, have used permanent shifts in the pure-tone audiometric threshold as the gold standard for measuring hearing impairment in noise-exposed populations. However, recent results from animal studies suggest that high-level noise exposures can cause the permanent destruction of synapses between the inner hair cells and auditory nerve fibers, even in cases where pure-tone audiometric thresholds eventually return to their normal pre-exposure baselines. This has created a dilemma for researchers, who are now increasingly interested in studying the long-term effects that temporary hearing shifts might have on hearing function, but are also concerned about the ethical considerations of exposing human listeners to high levels of noise for research purposes. One method that remains viable to study the effects of high noise exposures on human listeners, or to evaluate the efficacy of interventions designed to prevent noise-related inner ear damage, is to identify individuals in occupations with unavoidable noise exposures and measure hearing before and as soon as possible after exposure. This paper discusses some of the important factors to be considered in studies that attempt to measure acute hearing changes in noise-exposed military populations.

Blast Preconditioning Protects Retinal Ganglion Cells and Reveals Targets for Prevention of Neurodegeneration Following Blast-Mediated Traumatic Brian Injury

Purpose

The purpose of this study was to examine the effect of multiple blast exposures and blast preconditioning on the structure and function of retinal ganglion cells (RGCs), to identify molecular pathways that contribute to RGC loss, and to evaluate the role of kynurenine-3-monooxygenase (KMO) inhibition on RGC structure and function.

Methods

Mice were subjected to sham blast injury, one single blast injury, or three blast injuries separated by either 1 hour or 1 week, using a blast intensity of 20 PSI. To examine the effect of blast preconditioning, mice were subjected to sham blast injury, one single 20-PSI injury, or three blast injuries separated by 1 week (5 PSI, 5 PSI, 20 PSI and 5 PSI, 5 PSI, 5 PSI). RGC structure was analyzed by optical coherence tomography (OCT) and function was analyzed by the pattern electroretinogram (PERG). BRN3A-positive cells were quantified to determine RGC density. RNA-seq analysis was used to identify transcriptional changes between groups.

Results

Analysis of mice with multiple blast exposures of 20 PSI revealed no significant differences compared to one 20-pounds per square inch (PSI) exposure using OCT, PERG, or BRN3A cell counts. Analysis of mice exposed to two preconditioning 5-PSI blasts prior to one 20-PSI blast showed preservation of RGC structure and function. RNA-seq analysis of the retina identified multiple transcriptomic changes between conditions. Pharmacologic inhibition of KMO preserved RGC responses compared to vehicle-treated mice.

Conclusions

Preconditioning protects RGC from blast injury. Protective effects appear to involve changes in KMO activity, whose inhibition is also protective.

Auditory and Cognitive Behavioral Performance Deficits and Symptom Reporting in Postconcussion Syndrome Following Mild Traumatic Brain Injury

Purpose This study examined auditory deficits and symptom reporting in individuals with long-term postconcussion symptoms following a single mild traumatic brain injury (mTBI) compared to age- and gender-matched controls without a history of mTBI. Method Case history interviews, symptom questionnaires, and a battery of central auditory and neuropsychological tests were administered to 2 groups. The mTBI group was a civilian population recruited from a local concussion management program who were seeking rehabilitation for postconcussion-related problems in a postacute period between 3 and 18 months following injury. Symptom validity testing was included to assess the rate of possible insufficient test effort and its influence on scores for all outcome measures. Analyses of group differences in test scores were performed both with and without the participants who showed insufficient test effort. Rates of symptom reporting, correlations among symptoms and behavioral test outcomes, and the relationships between auditory and cognitive test performance were analyzed. Results The mTBI group reported a high rate of auditory symptoms and general postconcussion symptoms. Performance on neuropsychological tests of cognitive function showed some differences in raw scores between groups, but when effort was considered, there were no significant differences in the rate of abnormal performance between groups. In contrast, there were significant differences in both raw scores and the rate of abnormal performance between groups for some auditory tests when only considering participants with sufficient effort. Auditory symptoms were strongly correlated with other general postconcussion symptoms. Conclusions Significant auditory symptoms and evidence of long-term central auditory dysfunction were found in a subset of individuals who had chronic postconcussion symptoms after a single mTBI unrelated to blast trauma. The rate of abnormal performance on auditory behavioral tests exceeded the rate of abnormal performance on tests of cognitive function. Supplemental Material https://doi.org/10.23641/asha.8329955.

Auditory Localization and Spatial Release From Masking in Children With Suspected Auditory Processing Disorder

OBJECTIVES

We sought to investigate whether children referred to our audiology clinic with a complaint of listening difficulty, that is, suspected of auditory processing disorder (APD), have difficulties localizing sounds in noise and whether they have reduced benefit from spatial release from masking.

DESIGN

Forty-seven typically hearing children in the age range of 7 to 17 years took part in the study. Twenty-one typically developing (TD) children served as controls, and the other 26 children, referred to our audiology clinic with listening problems, were the study group: suspected APD (sAPD). The ability to localize a speech target (the word "baseball") was measured in quiet, broadband noise, and speech-babble in a hemi-anechoic chamber. Participants stood at the center of a loudspeaker array that delivered the target in a diffused noise-field created by presenting independent noise from four loudspeakers spaced 90° apart starting at 45°. In the noise conditions, the signal-to-noise ratio was varied between -12 and 0 dB in 6-dB steps by keeping the noise level constant at 66 dB SPL and varying the target level. Localization ability was indexed by two metrics, one assessing variability in lateral plane [lateral scatter (Lscat)] and the other accuracy in the front/back dimension [front/back percent correct (FBpc)]. Spatial release from masking (SRM) was measured using a modified version of the Hearing in Noise Test (HINT). In this HINT paradigm, speech targets were always presented from the loudspeaker at 0°, and a single noise source was presented either at 0°, 90°, or 270° at 65 dB A. The SRM was calculated as the difference between the 50% correct HINT speech reception threshold obtained when both speech and noise were collocated at 0° and when the noise was presented at either 90° or 270°.

RESULTS

As expected, in both groups, localization in noise improved as a function of signal-to-noise ratio. Broadband noise caused significantly larger disruption in FBpc than in Lscat when compared with speech babble. There were, however, no group effects or group interactions, suggesting that the children in the sAPD group did not differ significantly from TD children in either localization metric (Lscat and FBpc). While a significant SRM was observed in both groups, there were no group effects or group interactions. Collectively, the data suggest that children in the sAPD group did not differ significantly from the TD group for either binaural measure investigated in the study.

CONCLUSIONS

As is evident from a few poor performers, some children with listening difficulties may have difficulty in localizing sounds and may not benefit from spatial separation of speech and noise. However, the heterogeneity in APD and the variability in our data do not support the notion that localization is a global APD problem. Future studies that employ a case study design might provide more insights.

Temporal dynamics and uncertainty in binaural hearing revealed by anticipatory eye movements

Accurate perception of binaural cues is essential for left-right sound localization. Much literature focuses on threshold measures of perceptual acuity and accuracy. This study focused on supra-threshold perception using an anticipatory eye movement (AEM) paradigm designed to capture subtle aspects of perception that might not emerge in behavioral-motor responses, such as the accumulation of certainty, and rapid revisions in decision-making. Participants heard interaural timing differences (ITDs) or interaural level differences in correlated or uncorrelated narrowband noises, respectively. A cartoon ball moved behind an occluder and then emerged from the left or right side, consistent with the binaural cue. Participants anticipated the correct answer (before it appeared) by looking where the ball would emerge. Results showed quicker and more steadfast gaze fixations for stimuli with larger cue magnitudes. More difficult stimuli elicited a wider distribution of saccade times and greater number of corrective saccades before final judgment, implying perceptual uncertainty or competition. Cue levels above threshold elicited some wrong-way saccades that were quickly corrected. Saccades to ITDs were earlier and more reliable for low-frequency noises. The AEM paradigm reveals the time course of uncertainty and changes in perceptual decision-making for supra-threshold binaural stimuli even when behavioral responses are consistently correct.

Neurosensory Deficits Vary as a Function of Point of Care in Pediatric Mild Traumatic Brain Injury

Neurosensory abnormalities are frequently observed following pediatric mild traumatic brain injury (pmTBI) and may underlie the expression of several common concussion symptoms and delay recovery. Importantly, active evaluation of neurosensory functioning more closely approximates real-world (e.g., physical and academic) environments that provoke symptom worsening. The current study determined whether symptom provocation (i.e., during neurosensory examination) improved classification accuracy relative to pre-examination symptom levels and whether symptoms varied as a function of point of care. Eighty-one pmTBI were recruited from the pediatric emergency department (PED; n = 40) or outpatient concussion clinic (n = 41), along with matched (age, sex, and education) healthy controls (HC; n = 40). All participants completed a brief (∼ 12 min) standardized neurosensory examination and clinical questionnaires. The magnitude of symptom provocation upon neurosensory examination was significantly higher for concussion clinic than for PED patients. Symptom provocation significantly improved diagnostic classification accuracy relative to pre-examination symptom levels, although the magnitude of improvement was modest, and was greater in the concussion clinic. In contrast, PED patients exhibited worse performance on measures of balance, vision, and oculomotor functioning than the concussion clinic patients, with no differences observed between both samples and HC. Despite modest sample sizes, current findings suggest that point of care represents a critical but highly under-studied variable that may influence outcomes following pmTBI. Studies that rely on recruitment from a single point of care may not generalize to the entire pmTBI population in terms of how neurosensory deficits affect recovery.

Impact of Blast Injury on Hearing in a Screened Male Military Population

Exposure to hazardous intensity levels of combat noise, such as blast, may compromise a person's ability to detect and recognize sounds and communicate effectively. There is little previous examination of the onset of hearing health outcomes following exposure to blast in representative samples of deployed US military personnel. Data from the prospective Blast-Related Auditory Injury Database were analyzed. We included only those participants with qualified hearing tests within a period of 12 months prior to, and following, injury (n = 1,574). After adjustment for relevant covariates and potential confounders, those who sustained a blast injury had significantly higher odds of postinjury hearing loss (odds ratio = 2.21; 95% confidence interval: 1.42, 3.44), low-frequency hearing loss (odds ratio = 1.95; 95% confidence interval: 1.01, 3.78), high-frequency hearing loss (odds ratio = 2.45; 95% confidence interval: 1.43, 4.20), and significant threshold shift compared with a group with non-blast-related injury. An estimated 49% of risk for hearing loss in these blast-injured, deployed military members could be attributed to the blast-related injury event. This study reinforced that it is imperative to identify at-risk populations for early intervention and prevention, as well as to consistently monitor the effects of blast injury on hearing outcomes.

Feasibility of using normobaric hypoxic stress in mTBI research

Studies of mild traumatic brain injury (mTBI) recovery generally assess patients in unstressed conditions that permit compensation for impairments through increased effort expenditure. This possibility may explain why a subgroup of individuals report persistent mTBI symptoms yet perform normally on objective assessment. Accordingly, the development and utilization of stress paradigms may be effective for enhancing the sensitivity of mTBI assessment. Previous studies, discussed here, indirectly but plausibly support the use of normobaric hypoxia as a stressor in uncovering latent mTBI symptoms due to the overlapping symptomatology induced by both normobaric hypoxia and mTBI. Limited studies by our group and others further support this plausibility through proof-of-concept demonstrations that hypoxia reversibly induces disproportionately severe impairments of oculomotor, pupillometric, cognitive and autonomic function in mTBI individuals.

Effects of Recreational Noise on Threshold and Suprathreshold Measures of Auditory Function

Noise exposure that causes a temporary threshold shift but no permanent threshold shift can cause degeneration of synaptic ribbons and afferent nerve fibers, with a corresponding reduction in wave I amplitude of the auditory brainstem response (ABR) in animals. This form of underlying damage, hypothesized to also occur in humans, has been termed synaptopathy , and it has been hypothesized that there will be a hidden hearing loss consisting of functional deficits at suprathreshold stimulus levels. This study assessed whether recreational noise exposure history was associated with smaller ABR wave I amplitude and poorer performance on suprathreshold auditory test measures. Noise exposure histories were collected from 26 men and 34 women with hearing thresholds ≤ 25 dB hearing loss (HL; 250 Hz to 8 kHz), and a variety of functional suprathreshold hearing tests were performed. Wave I amplitudes of click-evoked ABR were obtained at 70, 80, 90, and 99 dB (nHL) and tone-burst evoked ABR were obtained at 90 dB nHL. Speech recognition performance was measured in quiet and in competing noise, using the Words in Noise test, and the NU-6 word list in broadband noise (BBN). In addition, temporal summation to tonal stimuli was assessed in quiet and in competing BBN. To control for the effects of subclinical conventional hearing loss, distortion product otoacoustic emission amplitude, an indirect measure of outer hair cell integrity, was measured. There was no statistically significant relationship between noise exposure history scores and ABR wave I amplitude in either men or women for any of the ABR conditions. ABR wave I amplitude and noise exposure history were not reliably correlated with suprathreshold functional hearing tests. Taken together, this study found no evidence of noise-induced decreases in ABR wave I amplitude or signal processing in noise in a cohort of subjects with a history of recreational noise exposure.

Blast-induced hearing loss

The incidence of blast injury has increased recently. As the ear is the organ most sensitive to blast overpressure, the most frequent injuries seen after blast exposure are those affecting the ear. Blast overpressure affecting the ear results in sensorineural hearing loss, which is untreatable and often associated with a decline in the quality of life. Here, we review recent cases of blast-induced hearing dysfunction. The tympanic membrane is particularly sensitive to blast pressure waves, since such waves exert forces mainly at air-tissue interfaces within the body. However, treatment of tympanic membrane perforation caused by blast exposure is more difficult than that caused by other etiologies. Sensorineural hearing dysfunction after blast exposure is caused mainly by stereociliary bundle disruption on the outer hair cells. Also, a reduction in the numbers of synaptic ribbons in the inner hair cells and spiral ganglion cells is associated with hidden hearing loss, which is strongly associated with tinnitus or hyperacusis.

Cortical and Sensory Causes of Individual Differences in Selective Attention Ability Among Listeners With Normal Hearing Thresholds

PURPOSE

This review provides clinicians with an overview of recent findings relevant to understanding why listeners with normal hearing thresholds (NHTs) sometimes suffer from communication difficulties in noisy settings.

METHOD

The results from neuroscience and psychoacoustics are reviewed.

RESULTS

In noisy settings, listeners focus their attention by engaging cortical brain networks to suppress unimportant sounds; they then can analyze and understand an important sound, such as speech, amidst competing sounds. Differences in the efficacy of top-down control of attention can affect communication abilities. In addition, subclinical deficits in sensory fidelity can disrupt the ability to perceptually segregate sound sources, interfering with selective attention, even in listeners with NHTs. Studies of variability in control of attention and in sensory coding fidelity may help to isolate and identify some of the causes of communication disorders in individuals presenting at the clinic with "normal hearing."

CONCLUSIONS

How well an individual with NHTs can understand speech amidst competing sounds depends not only on the sound being audible but also on the integrity of cortical control networks and the fidelity of the representation of suprathreshold sound. Understanding the root cause of difficulties experienced by listeners with NHTs ultimately can lead to new, targeted interventions that address specific deficits affecting communication in noise.

PRESENTATION VIDEO

http://cred.pubs.asha.org/article.aspx?articleid=2601617.

Alterations in neurovascular coupling following acute traumatic brain injury

Following acute traumatic brain injury (TBI), timely transport to a hospital can significantly improve the prognosis for recovery. There is, however, a dearth of quantitative biomarkers for brain injury that can be rapidly acquired and interpreted in active, field environments in which TBIs are frequently incurred. We explored potential functional indicators for TBI that can be noninvasively obtained through portable detection modalities, namely optical and electrophysiological approaches. By combining diffuse correlation spectroscopy with colocalized electrophysiological measurements in a mouse model of TBI, we observed concomitant alterations in sensory-evoked cerebral blood flow (CBF) and electrical potentials following controlled cortical impact. Injury acutely reduced the peak amplitude of both electrophysiological and CBF responses, which mostly recovered to baseline values within 30 min, and intertrial variability for these parameters was also acutely altered. Notably, the postinjury dynamics of the CBF overshoot and undershoot amplitudes differed significantly; whereas the amplitude of the initial peak of stimulus-evoked CBF recovered relatively rapidly, the ensuing undershoot did not appear to recover within 30 min of injury. Additionally, acute injury induced apparent low-frequency oscillatory behavior in CBF ([Formula: see text]). Histological assessment indicated that these physiological alterations were not associated with any major, persisting anatomical changes. Several time-domain features of the blood flow and electrophysiological responses showed strong correlations in recovery kinetics. Overall, our results reveal an array of stereotyped, injury-induced alterations in electrophysiological and hemodynamic responses that can be rapidly obtained using a combination of portable detection techniques.

Hidden Hearing Loss? No Effect of Common Recreational Noise Exposure on Cochlear Nerve Response Amplitude in Humans

This study tested hypothesized relationships between noise exposure and auditory deficits. Both retrospective assessment of potential associations between noise exposure history and performance on an audiologic test battery and prospective assessment of potential changes in performance after new recreational noise exposure were completed.

Methods: 32 participants (13M, 19F) with normal hearing (25-dB HL or better, 0.25–8 kHz) were asked to participate in 3 pre- and post-exposure sessions including: otoscopy, tympanometry, distortion product otoacoustic emissions (DPOAEs) (f2 frequencies 1–8 kHz), pure-tone audiometry (0.25–8 kHz), Words-in-Noise (WIN) test, and electrocochleography (eCochG) measurements at 70, 80, and 90-dB nHL (click and 2–4 kHz tone-bursts). The first session was used to collect baseline data, the second session was collected the day after a loud recreational event, and the third session was collected 1-week later. Of the 32 participants, 26 completed all 3 sessions.

Results: The retrospective analysis did not reveal statistically significant relationships between noise exposure history and any auditory deficits. The day after new exposure, there was a statistically significant correlation between noise “dose” and WIN performance overall, and within the 4-dB signal-to-babble ratio. In contrast, there were no statistically significant correlations between noise dose and changes in threshold, DPOAE amplitude, or AP amplitude the day after new noise exposure. Additional analyses revealed a statistically significant relationship between TTS and DPOAE amplitude at 6 kHz, with temporarily decreased DPOAE amplitude observed with increasing TTS.

Conclusions: There was no evidence of auditory deficits as a function of previous noise exposure history, and no permanent changes in audiometric, electrophysiologic, or functional measures after new recreational noise exposure. There were very few participants with TTS the day after exposure - a test time selected to be consistent with previous animal studies. The largest observed TTS was approximately 20-dB. The observed pattern of small TTS suggests little risk of synaptopathy from common recreational noise exposure, and that we should not expect to observe changes in evoked potentials for this reason. No such changes were observed in this study. These data do not support suggestions that common, recreational noise exposure is likely to result in “hidden hearing loss”.

Auditory and Cognitive Factors Associated with Speech-in-Noise Complaints following Mild Traumatic Brain Injury

BACKGROUND

Auditory complaints following mild traumatic brain injury (MTBI) are common, but few studies have addressed the role of auditory temporal processing in speech recognition complaints.

PURPOSE

In this study, deficits understanding speech in a background of speech noise following MTBI were evaluated with the goal of comparing the relative contributions of auditory and nonauditory factors.

RESEARCH DESIGN

A matched-groups design was used in which a group of listeners with a history of MTBI were compared to a group matched in age and pure-tone thresholds, as well as a control group of young listeners with normal hearing (YNH).

STUDY SAMPLE

Of the 33 listeners who participated in the study, 13 were included in the MTBI group (mean age = 46.7 yr), 11 in the Matched group (mean age = 49 yr), and 9 in the YNH group (mean age = 20.8 yr).

DATA COLLECTION AND ANALYSIS

Speech-in-noise deficits were evaluated using subjective measures as well as monaural word (Words-in-Noise test) and sentence (Quick Speech-in-Noise test) tasks, and a binaural spatial release task. Performance on these measures was compared to psychophysical tasks that evaluate monaural and binaural temporal fine-structure tasks and spectral resolution. Cognitive measures of attention, processing speed, and working memory were evaluated as possible causes of differences between MTBI and Matched groups that might contribute to speech-in-noise perception deficits.

RESULTS

A high proportion of listeners in the MTBI group reported difficulty understanding speech in noise (84%) compared to the Matched group (9.1%), and listeners who reported difficulty were more likely to have abnormal results on objective measures of speech in noise. No significant group differences were found between the MTBI and Matched listeners on any of the measures reported, but the number of abnormal tests differed across groups. Regression analysis revealed that a combination of auditory and auditory processing factors contributed to monaural speech-in-noise scores, but the benefit of spatial separation was related to a combination of working memory and peripheral auditory factors across all listeners in the study.

CONCLUSIONS

The results of this study are consistent with previous findings that a subset of listeners with MTBI has objective auditory deficits. Speech-in-noise performance was related to a combination of auditory and nonauditory factors, confirming the important role of audiology in MTBI rehabilitation. Further research is needed to evaluate the prevalence and causal relationship of auditory deficits following MTBI.

Practical guidelines to minimise language and cognitive confounds in the diagnosis of CAPD: a brief tutorial

OBJECTIVE

To provide audiologists with strategies to minimise confounding cognitive and language processing variables and accurately diagnose central auditory processing disorder (CAPD).

DESIGN

Tutorial.

STUDY SAMPLE

None.

RESULTS

Strategies are reviewed to minimise confounding cognitive and language processing variables and accurately diagnose CAPD.

CONCLUSIONS

Differential diagnosis is exceedingly important and can be quite challenging. Distinguishing between two or more conditions presenting with similar symptoms or attributes requires multidisciplinary, comprehensive assessment. To ensure appropriate interventions, the audiologist is a member of the multidisciplinary team responsible for determining whether there is an auditory component to other presenting deficits or whether one condition is responsible for the symptoms seen in another. Choice of tests should be guided both by the symptoms of the affected individual, as established in an in-depth interview and case history, the individual's age and primary language, and by the specific deficits reported to be associated with specific clinical presentations. Knowing which tests are available, their strengths and limitations, the processes assessed, task and response requirements, and the areas of the central auditory nervous system (CANS) to which each test is most sensitive provides the audiologist with critical information to assist in the differential diagnostic process.

Auditory Processing After Traumatic Brain Injury

Auditory processing involves many diverse aspects of the peripheral and central nervous system. Where “simple” transformation of information ends and “signal processing” begins is difficult to say with any certainty, and the distinction between “automatic” processing and “controlled” processing is an even more philosophical question. For these reasons, the damage that occurs in one portion of the nervous system can have serious implications for many other parts of the system. For this reason, it is essential that clinicians involved in working with patients for whom any portion of the auditory system is at risk need to be in close communication with those who specialize in understanding the other parts of this delicate and interwoven system.

Effects of noise on speech recognition: Challenges for communication by service members

Speech communication often takes place in noisy environments; this is an urgent issue for military personnel who must communicate in high-noise environments. The effects of noise on speech recognition vary significantly according to the sources of noise, the number and types of talkers, and the listener's hearing ability. In this review, speech communication is first described as it relates to current standards of hearing assessment for military and civilian populations. The next section categorizes types of noise (also called maskers) according to their temporal characteristics (steady or fluctuating) and perceptive effects (energetic or informational masking). Next, speech recognition difficulties experienced by listeners with hearing loss and by older listeners are summarized, and questions on the possible causes of speech-in-noise difficulty are discussed, including recent suggestions of "hidden hearing loss". The final section describes tests used by military and civilian researchers, audiologists, and hearing technicians to assess performance of an individual in recognizing speech in background noise, as well as metrics that predict performance based on a listener and background noise profile. This article provides readers with an overview of the challenges associated with speech communication in noisy backgrounds, as well as its assessment and potential impact on functional performance, and provides guidance for important new research directions relevant not only to military personnel, but also to employees who work in high noise environments.

Pathophysiology of the inner ear after blast injury caused by laser-induced shock wave

The ear is the organ that is most sensitive to blast overpressure, and ear damage is most frequently seen after blast exposure. Blast overpressure to the ear results in sensorineural hearing loss, which is untreatable and is often associated with a decline in the quality of life. In this study, we used a rat model to demonstrate the pathophysiological and structural changes in the inner ear that replicate pure sensorineural hearing loss associated with blast injury using laser-induced shock wave (LISW) without any conductive hearing loss. Our results indicate that threshold elevation of the auditory brainstem response (ABR) after blast exposure was primarily caused by outer hair cell dysfunction induced by stereociliary bundle disruption. The bundle disruption pattern was unique; disturbed stereocilia were mostly observed in the outermost row, whereas those in the inner and middle rows stereocilia remained intact. In addition, the ABR examination showed a reduction in wave I amplitude without elevation of the threshold in the lower energy exposure group. This phenomenon was caused by loss of the synaptic ribbon. This type of hearing dysfunction has recently been described as hidden hearing loss caused by cochlear neuropathy, which is associated with tinnitus or hyperacusis.