Chronic Effects of Exposure to High-Intensity Blasts: Results of Tests of Central Auditory Processing

Abstract

Clinical Trial Registration: ClinicalTrials.gov: "Central auditory processing deficits associated with blast exposure"; NCT01567020; https://clinicaltrials.gov/ct2/show/NCT01567020 INTRODUCTION For well over a decade, the U.S. military has been engaged in conflicts that have changed the nature of injury and survival among Veterans because of changes in weaponry, battle armor, and medical technology that have taken place over the course of the past 20 yr [1]. According to the Institute of Medicine (IOM), these changes have resulted in a ratio of wounded to dead soldiers of 9.7 to 1 for the Global War on Terror, in contrast to rates of 4.3 to 1 or lower for all wars preceding it in the history of the United States [1]. Both the increased reliance upon explosives and the higher survival rate have created a landscape of new questions that must be answered in order to provide the best medical care for Veterans. This is true for hearing healthcare as much as for any other area of medicine. In 2014, the IOM issued a report on the chronic effects of exposure to high-intensity blasts [1]. In a systematic review of the literature, the IOM report found that symptoms of hearing loss are often present following blast exposure [2-3]. The report went on to state that while peripheral auditory dysfunction is likely to persist even after other effects have diminished, there was "inadequate" evidence in the literature to answer the question of whether or not central auditory effects persist beyond a period of 6 mo after blast exposure. The current report was motivated by ongoing research in our laboratory that has revealed evidence of an association between remote high-intensity blast exposure and the ability to process auditory information. Previous reports have described the ways in which blast exposure could [4] and does [3] cause changes in the ability of listeners to perform on tests of central auditory processing measured within a year of blast exposure. Tasks upon which blast-exposed participants were most likely to perform poorly included those relying upon temporal pattern perception, auditory temporal resolution, binaural processing, and dichotic listening. In addition, it has been demonstrated [3] that these problems occur even when traditional audiometric test results and electrophysiological measures primarily assessing the auditory brainstem remain largely in the normal range. Furthermore, electrophysiological measures evaluating cortical function indicate that some of these same listeners show delayed and reduced peak amplitudes relative to the responses expected from young listeners with normal peripheral auditory function. This pattern is consistent with the hypothesis that blast exposure can cause damage to cortical areas responsible for auditory processing, as well as potentially damaging the connections among central auditory processing areas. Clinically, these patterns of dysfunction are unusual in younger listeners but have similarities with patterns of abnormal performance sometimes observed in older listeners. Data collection and recruitment are currently ongoing, but this report has been compiled in order to provide timely information to the healthcare community regarding the question posed by the IOM concerning the possible persistence of the effects of blast exposure on the ability to process auditory information. To address this question, the behavioral data collected so far will be presented in a manner allowing direct comparison to the behavioral data from the previous study [3] of more recently blast-exposed patients. Additional measures not reported here, including a range of electrophysiological tests and a number of behavioral tests with speech and nonspeech stimuli, will be reported separately once the full cohort of participants has been recruited and tested. METHODS Participants The goal of the study was to test blast-exposed Veterans, but no database of such Veterans exists and the military records that might help to identify these Veterans are not available through the Department of Veterans Affairs (VA) electronic medical record system. …