Abstract
The long-term effects of blast exposure are a major health concern for combat veterans returning from the recent conflicts in Iraq and Afghanistan. We used an optimized diffusion tensor imaging tractography algorithm to assess white matter (WM) fractional anisotropy (FA) in blast-exposed Iraq and Afghanistan veterans (n = 40) scanned on average 3.7 years after deployment/trauma exposure. Veterans diagnosed with a blast-related mild traumatic brain injury (mTBI) were compared to combat veterans with blast exposure but no TBI diagnosis. Blast exposure was associated with decreased FA in several WM tracts. However, total blast exposure did not correlate well with neuropsychological testing performance and there were no differences in FA based on mTBI diagnosis. Yet, veterans with mTBI performed worse on every neurocognitive test administered. Multiple linear regression across all blast-exposed veterans using a six-factor prediction model indicated that the amount of blast exposure accounted for 11–15% of the variability in composite FA scores such that as blast exposure increased, FA decreased. Education accounted for 10% of the variability in composite FA scores and 25–32% of FA variability in the right cingulum, such that as level of education increased, FA increased. Total blast exposure, age, and education were significant predictors of FA in the left cingulum. We did not find any effect of post-traumatic stress disorder on cognition or composite FA. In summary, our findings suggest that greater total blast exposure is a contributing factor to poor WM integrity. While FA was not associated with neurocognitive performance, we hypothesize that FA changes in the cingulum in veterans with multiple combat exposures and no head trauma prior to deployment may represent a marker of vulnerability for future deficits. Future work needs to examine this longitudinally.
Highlights
The long-term effects of blast exposure have become a growing health concern within the military due to the large number of Iraq and Afghanistan combat veterans who have experienced single or multiple blasts from improvised explosive devices, rocket propelled grenades, and mortar rounds
The authors stipulate that the magnitude of mild traumatic brain injury (mTBI)-related effects within each cognitive domain remains unclear as the respective effect sizes for “cognitive deficits” appear especially heterogeneous across meta-analyses (i.e., d = −0.11 to 0.72) and that these higher order functions appear most susceptible to multiple mTBI [d = 0.24 [11]]
Consistent with the hypotheses, the results suggest that white matter (WM) integrity is compromised in veterans with blast exposure in relation to the dose of exposure, which corroborates the findings of a recent study by Trotter et al [53]
Summary
The long-term effects of blast exposure have become a growing health concern within the military due to the large number of Iraq and Afghanistan combat veterans who have experienced single or multiple blasts from improvised explosive devices, rocket propelled grenades, and mortar rounds. One study found that Marines returning from combat with self-reported concussion and symptoms consistent with mTBI showed decline of cognitive performance on a computerized cognitive battery at a 3-month follow-up; these deficits recovered by month 6 [8] By contrast, another group reported that veterans with blast-related mTBI and combat controls with no history of blast exposure showed no performance differences on cognitive measures including the Controlled Oral Word Association Test, Trail Making Test, Color–Word Interference Test, and Verbal Selective Reminding Test [9]. Another group reported that veterans with blast-related mTBI and combat controls with no history of blast exposure showed no performance differences on cognitive measures including the Controlled Oral Word Association Test, Trail Making Test, Color–Word Interference Test, and Verbal Selective Reminding Test [9] The interpretation of such discrepancies is further complicated by the use of different methodologies to assess cognition (i.e., raw scores from paper and pencil tests vs reaction time on computerized tests). The nature and extent of cognitive impairment following blast exposure are varied in the literature, symptomatic complaints are common, and they do not seem to associate exclusively with any single profile
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