Auditory problems are the most common service-connected disability (U.S. Department of Veterans Affairs, 2019) and the fourth leading cause of medical referral for Veterans of the United States military (McIlwain et al, 2008).
Although it is established that noise exposure is one of the major causes of hearing loss and auditory difficulties in Veterans, the recent conflicts (Operation Iraqi Freedom, Operation Enduring Freedom, and Operation New Dawn) have drawn the attention of clinicians and researchers in the Departments of Veterans Affairs (VA) and Defense (DOD) to another significant risk factor for listening difficulties: exposure to high-intensity blasts (Gallun et al, 2017).
Improvised explosive devices, rocket-propelled grenades, and mortars used in modern warfare produce incredibly powerful explosions with associated high-intensity noise. They also are responsible for some of the most serious injuries observed in the contemporary Veteran population.
In addition to limb amputations and traumatic brain injury (TBI), multiple parts of the auditory system also are susceptible to damage from these blasts. Subsequently, auditory injuries are the most common single type of injury consequent to blast exposure, resulting in tympanic membrane perforation, middle-ear, and/or cochlear damage (Gondusky and Reiter, 2005).
Gallun et al (2012) reported that U.S. Service members hospitalized after blast injury had a three-fold higher chance of abnormal performance on one or more tests of auditory processing relative to a control group matched in age and relatively minor differences in audiometric thresholds. Tests of competing speech and temporal processing were the most likely to be abnormal in this population.
Cognitive and Neuropsychological Factors
Listening requires cooperation between cognitive functions, such as attention and working memory, and neuropsychological factors, such as emotion regulation (Belanger et al, 2005). Dysfunction in one or more of these areas can manifest in auditory symptoms. For example, in some individuals, listening difficulties could be due to problems modulating attention between two competing sounds (Gallun and Jakien, 2019).
Some individuals may have global cognitive deficits that are most noticeable within the auditory domain, in part because humans rely on hearing to support other complex processes, such as speech perception and auditory language comprehension.
Research suggests that traumatic brain injury is reflected in difficulty comprehending complex language (Angeleri et al, 2008; Coelho et al, 2003). Sentence-level processing with attentional demands appears to be particularly vulnerable and sensitive to TBI (Salvatore et al, 2017).
Damage to the brain consequent to blast tends to be diffuse. Furthermore, not only do sensory processing difficulties negatively influence quality of life, but similarly, negative emotional reactions to life situations and function can influence listening abilities (Callahan and Lim, 2018).
Although there have been no studies involving animals who have been exposed to blasts where the auditory processing ability or specific auditory-pathways were studied, two recent animal studies have been completed.
Perez-Garcia et al (2019) found a relationship between post-traumatic stress disorder (PTSD) and mild TBI (mTBI) and Ratliff et al (2019) found damage to the amygdala.
Auditory cortical and subcortical processing areas are connected to the amygdala, and this connection is responsible for emotional regulation. These connections are implicated in the development and modulation of auditory fear responses (Cambiaghi et al, 2017). This may explain why, for some patients with cognitive difficulties, hypervigilance and anxiety are causing, increasing, and/or co-occurring with auditory processing difficulties.
Innovative Approaches to Helping Patients
Across multiple studies, it has been revealed that the deficits in those with brain injury are heterogeneous and complex. For example, in Gallun et al (2012; 2016), the participants with a history of blast exposure were much more likely to perform abnormally on one or more test of auditory processing than were the control participants, but there was very little consistency in the tests most likely to reveal abnormalities.
Though the rates of PTSD were high in the participants in the 2016 study, very few performed abnormally on more than three of the five auditory processing tests used, suggesting that overall deficits in attention, vigilance, or motivation were unlikely to be solely responsible for the results. This variability does complicate our understanding of how blasts damage the auditory system, but it also makes it quite clear that many Veterans with positive histories of blast exposure will require support from multiple disciplines.
Appropriate diagnostic and rehabilitative care for these patients requires a team of clinicians with the collective expertise needed to address all factors that are likely to influence performance and perceived handicap.
Although interprofessional approaches are regularly used to diagnose and treat individuals with head injuries, audiology is not typically a core member of the interprofessional team. Clinical audiologists can make a substantive impact in the diagnosis, monitoring, and management of anyone with communication difficulties.
Audiologists can also help by educating other professionals about (1) the high rates of listening difficulties in this population, (2) the gains that can be made in addressing these difficulties, and (3) the potential for listening deficits to provide deeper insight into the overall picture of damage caused by blast exposure and other sources of brain injury.
Although there are barriers to identifying auditory dysfunction in people with mTBI, there are many approaches that show promise for the future. Peripheral dysfunction can be revealed by otoacoustic emissions even when pure-tone thresholds are not elevated (Job et al, 2009).
In terms of the central auditory system, auditory brainstem measures can identify decreases in neural synchrony (Bramhall et al, 2019) and behavioral tests of temporal, spectral, and binaural sensitivity can reveal specific functional deficits (Mueller and Beck, 1987; Musiek et al, 2005).
In addition, there is evidence that speech-based tests can reveal damage to auditory brain areas (Mueller et al, 1987). In terms of visualizing the damage in an individual brain, advanced scanning technologies, such as diffusion tensor imaging, have been used to reveal abnormalities in white-matter connectivity (Asken et al, 2018).
The time, specialized equipment, and knowledge needed to administer the tests and interpret the results are all factors that keep these tests from being used in clinical settings. To address this, our group, working on a multisite VA rehabilitation R&D-funded grant, has implemented computerized versions of some of the most promising behavioral measures using simple graphical interfaces that make testing easier for both the clinician and the patient.
One example of this is the Portable Automated Rapid Testing (PART) system that was developed with a grant from the National Institutes of Health (NIH DC 015051) and is available for free on the Apple App store (Gallun et al, 2018). This app provides a calibration interface, as well as more than 12 basic tests that can be configured by varying any of several dozen parameters. This application has been used to collect research data on more than 600 participants in 12 different laboratories studying the effects of aging and hearing loss, as well as the development of children’s auditory abilities.
PART has also been used in the classroom by more than 100 students. We are also using automated tests of auditory verbal working memory, auditory language processing, and executive function in our research with blast-exposed Veterans (McNeil et al, 2015; Zhen et al, 2019). Over the next few years, our teams intend to provide outreach and education to ensure that these research and educational tools can be transitioned smoothly into clinical testing devices.
Recent and ongoing research from our labs and those of our colleagues suggests that the range of tests and specialists involved in the treatment of brain injury should be expanded. Blast exposure often manifests as a series of complex symptoms that are not always apparent upon a cursory review. Identifying individual deficit patterns and generating a responsive and effective treatment plan often requires multiple visits to a variety of specialists. Our labs are actively involved in both conducting the research studies to provide clinical evidence and developing the clinical tools needed to provide appropriate care for those who have “borne the battle.”