Traumatic brain injuries from blast exposure have been called the “signature injury” of military conflicts in Iraq and Afghanistan. This is largely due to our enemies’ unprecedented reliance on explosive weaponry such as improvised explosive devices (IED) (Figure 1). Estimates indicate that approximately 19.5% of deployed military personnel have suffered traumatic brain injuries since 2001 (Rand Report, Invisible Wounds of War, 2008). With more than 2 million service members deployed to Iraq and Afghanistan, this means that over 400,000 American Veterans are currently living with the chronic effects of blast exposure.

Fig1_IED2007

Figure 1 caption: An armored military vehicle lies on its side after surviving a buried IED blast on April 15, 2007. The vehicle was hit by a deeply buried improvised explosive device while conducting operations just south of the Shiek Hamed village in Iraq. Photograph courtesy of the U.S. Army: http://www.army.mil/article/9708/general-lee-rides-again/

When a blast wave from a high-intensity explosive impacts the head, a wave of intense heat and pressure moves through the skull and brain. Delicate neural tissues are stretched and compressed, potentially leading to cell damage, cell death, hemorrhaging, and inflammation. All regions of the brain are at risk of damage, and the auditory system is no exception. In recent years, increasing numbers of young Veterans with blast exposure have sought help from VA audiologists for hearing-related problems such as poor speech understanding. However, standard tests of hearing sensitivity often show no signs of hearing loss. This this combination of factors often suggests damage to areas in the brain dealing with auditory signals.   The efforts of hearing health professionals to help these Veterans are hampered by a lack of information regarding the effects of blast exposure on auditory function. The purpose of this presentation is to present some early results of a study currently underway at the National Center for Rehabilitative Auditory Research (NCRAR) investigating the long term consequences of blast exposure on hearing. Discovering the types of auditory problems caused by blast exposure is a crucial step toward developing effective rehabilitation options for this population.

Study participants include Veterans who experienced high-intensity blast waves within the past twelve years. The majority of participants have experienced multiple blast episodes, with the most severe events occurring approximately eight years prior to enrolling in the study. Another group of participants of similar age and gender but with no blast exposure are also included to serve as comparisons to the blast-exposed group (controls). On questionnaires assessing hearing ability in different contexts, blast-exposed Veterans described having more difficulties in many listening situations compared control participants. Common challenging situations reported by blast-exposed Veterans involve understanding speech in background noise, understanding when multiple people are talking simultaneously, and recalling multiple spoken instructions. Further, blast-exposed Veterans are more likely to rate the overall quality of sounds such as music and voices more poorly than control participants, and often report that listening requires greater effort. Different tests of listening abilities found many areas of difficulty which probably help explain self-reports. First, blast-exposed Veterans often have poorer ability to distinguish timing cues than control participants. Hence, sounds may seem blurry or smeared over time. Second, the ability to process sounds presented to both ears is often poorer in blast-exposed Veterans. Normally, listeners are able to utilize small differences in the timing and level of sound arriving at the two ears to improve listening performance, especially in noisy listening environments. This ability is often degraded in blast-exposed Veterans. Third, blast-exposed Veterans are poorer at distinguishing changes in the pitch of sounds compared to control participants, even when the pitch change is large. Lastly, blast-exposed Veterans often have greater difficulty ignoring distracting information in order to focus on listening.   This leads to problems such as trouble conversing with others when the television is on or when conversing at restaurants or parties. Our study results show that these listening difficulties are often great enough to impact daily life, causing blast-exposed Veterans to avoid social situations that they once enjoyed.

Fig2_P300

Figure 2 caption: Average EEG responses from blast-exposed and control participant groups in response to a large change in tone pitch. The horizontal axis shows time since the pitch changed (which occurred at time 0 on this axis). The vertical axis shows the magnitude of the neural response of the brain. Notice that the peak of activity in the control group (blue star labeled ‘P300’)is considerably larger and occurs earlier in time compared to the blast-exposed group (red star labeled ‘P300’).

Self-assessment and behavioral performance measures are supported by numerous direct measures of auditory processing. Using a type of electroencephalography (EEG), we non-invasively measure the response of the brain to sound by assessing the timing and size of neural activity associated with sound perception and processing. These tests reveal that the brains of blast-exposed Veterans require more time to analyze sound and respond less actively to changes in sounds. For example, the average EEG responses of blast-exposed and control participants are shown in Figure 2. These waveforms reflect neural detection of a large change in the pitch of tones presented to participants. Notice that the peak marked ‘P300’ is larger and occurs earlier in time in control participants compared to blast-exposed Veterans. Similar effects are seen in response to more complex sounds, such as when participants are asked to identify target words among non-target filler words. Overall, these EEG results suggest degraded sound processing in the brains of blast-exposed Veterans compared to control participants.

In summary, our results strongly suggest that blast exposure can cause chronic problems in multiple areas of the brain where sound is processed. Blast exposure has the potential to damage auditory areas of the brain as well as cognitive regions, both of which likely contribute to hearing difficulties. Thus, though Veterans may have normal hearing sensitivity, blast exposure may cause problems processing complex sounds. These difficulties may persist for many years after blast exposure.

 

Melissa Papesh – Melissa.Papesh@va.gov

Frederick Gallun – Frederick.Gallun@va.gov

Robert Folmer – Robert.Folmer@va.gov

Michele Hutter – Michele.Hutter@va.gov

Heather Belding – Heather.Belding@va.gov

  1. Samantha Lewis – Michele.Lewis3@va.gov

National Center for Rehabilitative Auditory Research
Portland VA Medical Center
3710 SW US Veterans Hospital Road
Portland, OR 97239

Marjorie Leek – Marjorie.Leek@va.gov

Loma Linda VA Medical Center
11201 Benton St
Loma Linda, CA 92354

Popular version of paper “4pPP3. Effects of blast exposure on central auditory processing

Presented Thursday afternoon, October 30, 2014

168th ASA Meeting, Indianapolis

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