Central Auditory Processing Deficits Associated With Blast Exposure

The current conflicts in Afghanistan (Operation Enduring Freedom; OEF) and Iraq (Operation Iraqi Freedom; OIF) have resulted in unprecedented rates of exposure to high-intensity blasts and resulting brain injury. Dennis (2009) reports that during 2005-2007, 68% of U.S. military personnel injured in the OEF/OIF conflicts had blast-related injuries and 28%-31% of those evacuated to Walter Reed Army Medical Center (WRAMC), Washington, DC had brain injuries. While the common focus of auditory evaluation is on damage to the peripheral auditory system, the prevalence of brain injury among those exposed to high-intensity blasts suggests that damage to the central auditory system is an equally important concern for the blast-exposed Veteran. Discussions with clinical audiologists and OEF/OIF Veterans Service Office personnel suggest that a common complaint voiced by blast-exposed Veterans is an inability to understand speech in noisy environments, even when peripheral hearing is within normal or near-normal limits (see attached letters of support). Such complaints are consistent with damage to neural networks responsible for higher-order auditory processing. This proposal is the second phase of a research project focused on examining the degree to which central auditory processing (CAP) dysfunction is a result of blast exposure. Over the initial period of funding, data collection at WRAMC and the VA RR&D National Center for Rehabilitative Auditory Research (NCRAR) established that CAP dysfunction is present in Warfighters exposed to high-intensity blasts while serving in combat. Recently blast-exposed patients with and without diagnoses of mild traumatic brain injury (mTBI) tested at WRAMC showed differences from controls tested at NCRAR on one or more behavioral and neurophysiological tests used to evaluate central auditory function. This project will 1) develop a more accurate estimate of the prevalence of central auditory dysfunction among Veterans exposed to blasts over the past ten years, 2) identify the functional outcomes associated with abnormal performance on tests of central processing, and 3) improve understanding of the ways in which blast-exposure resembles and differs from both the normal aging process and non-blast-related TBI in terms of performance on tests of central auditory processing.

Key Question 1: To what extent is CAP dysfunction observable among OEF/OIF Veterans who have been exposed to high intensity blasts? Based on preliminary data, the investigators hypothesize that the rate of abnormal performance on behavioral and neurophysiological tests of CAP dysfunction will be higher in a group of Veterans exposed to blasts than it will be in a control group of similar ages and hearing thresholds who have not been exposed to blasts.

Key Question 2: How well can behavioral and neurophysiological tests of CAP predict functional auditory deficits measured behaviorally and through self report? It is hypothesized that tests of CAP ability will predict performance in a testing situation involving multiple talkers delivering competing messages. CAP tests will also correlate with responses blast-exposed Veterans make on the Speech and Spatial Qualities of Hearing (SSQ) questionnaire, designed to examine functional hearing ability in various acoustically complex environments.

Key Question 3: To what extent do blast-exposed Veterans resemble older listeners and participants with mild TBI who have not been exposed to blasts in their performance on CAP tests and functional tests of hearing? It is hypothesized that comparisons of the blast-exposed group with an older group with matched pure-tone sensitivity and an age- and hearing-matched group with non-blast-related TBI will be consistent with premature aging in the blast-exposed group but demonstrate substantive differences with the non-blast group.

Inclusion criteria

• Aged 18-90
• Pure-tone sensitivity of 40 dB HL or better at all audiometric frequencies below 8 kHz
• English as first language.

Group membership inclusion criteria:

Group 1: Blast exposed Veterans

• Report having been exposed to high-intensity blast during the ten years prior to enrollment
• Cognitive and physical ability to take part in these auditory evaluations

Group 2. Non-blast TBI group

• Diagnosed with mild-to-moderate TBI

Group 3. Age matched control group -18-59 years.

Group 4. Older control group

• 60-90 years.
• Older group will be aged 60 and older

Audiometric status of these groups will be required to meet the same exclusion criteria as that of the other two groups described above

Exclusion criteria

• Evidence of conductive or retrocochlear dysfunction
• Hearing loss exceeding pure-tone averages for frequencies of .5, 1, 2, and 4 kHz of 35 dB HL

--Hearing loss of greater than 40 dB HL at any one of these frequencies in either ear

• Asymmetrical hearing thresholds exceeding 10 dB at any audiometric frequency below 4 kHz
• Abnormal cognitive function as indicated by scores of 23 or below on the Mini Mental State Exam
• Indications of clinical depression as evidenced by a score of 17 or greater on the Beck Depression Inventory