Abstract
Objective: Blast exposure (BE) and mild traumatic brain injury (mTBI) have been independently linked to pathological brain changes. However, the combined effects of BE and mTBI on brain structure have yet to be characterized. Therefore, we investigated whether regional differences in cortical thickness exist between mTBI Veterans with and without BE while on deployment. We also examined whether cortical thickness (CT) and cognitive performance differed among mTBI Veterans with low vs. high levels of cumulative BE.Methods: 80 Veterans with mTBI underwent neuroimaging and completed neuropsychological testing and self-report symptom rating scales. Analyses of covariance (ANCOVA) were used to compare blast-exposed Veterans (mTBI+BE, n = 51) to those without BE (mTBI-BE, n = 29) on CT of frontal and temporal a priori regions of interest (ROIs). Next, multiple regression analyses were used to examine whether CT and performance on an executive functions composite differed among mTBI Veterans with low (mTBI+BE Low, n = 22) vs. high (mTBI+BE High, n = 26) levels of cumulative BE.Results: Adjusting for age, numer of TBIs, and PTSD symptoms, the mTBI+BE group showed significant cortical thinning in frontal regions (i.e., left orbitofrontal cortex [p = 0.045], left middle frontal gyrus [p = 0.023], and right inferior frontal gyrus [p = 0.034]) compared to the mTBI-BE group. No significant group differences in CT were observed for temporal regions (p's > 0.05). Multiple regression analyses revealed a significant cumulative BE × CT interaction for the left orbitofrontal cortex (p = 0.001) and left middle frontal gyrus (p = 0.020); reduced CT was associated with worse cognitive performance in the mTBI+BE High group but not the mTBI+BE Low group.Conclusions: Findings show that Veterans with mTBI and BE may be at risk for cortical thinning post-deployment. Moreover, our results demonstrate that reductions in CT are associated with worse executive functioning among Veterans with high levels of cumulative BE. Future longitudinal studies are needed to determine whether BE exacerbates mTBI-related cortical thinning or independently negatively influences gray matter structure.
Highlights
The use of improvised and other explosive devices—such as rocket propelled grenades and mortar rounds—during the conflicts in Afghanistan and Iraq has led to a stark increase in the prevalence of combat-related blast exposure (BE) in the military population
Given that the combination of BE and mild traumatic brain injury (mTBI) may be especially deleterious to brain structure and cognition, we examined such effects by: [1] investigating regional gray matter morphological differences in vulnerable frontal and temporal regions in mTBI Veterans who were and were not exposed to blast; and [2] determining the relationship between cortical thickness and cognitive outcomes across different levels of BE
The mTBI Veterans who were exposed to blast (mTBI+BE) group did not differ from the mTBI Veterans were not exposed to blast (mTBI-BE) group on age, ethnicity, education, or psychiatric symptomatology
Summary
The use of improvised and other explosive devices—such as rocket propelled grenades and mortar rounds—during the conflicts in Afghanistan and Iraq has led to a stark increase in the prevalence of combat-related blast exposure (BE) in the military population. Among a convenience sample of Operation Enduring Freedom/Operation Iraqi Freedom/Operation New Dawn (OEF/OIF/OND) Veterans, nearly 80% reported at least one close range BE (within 100 m) while overseas [2]. Such BE, combined with the improvement of combat protective gear and medical response methods, has led to unprecedented rates of certain non-lethal blast-related injuries within returning service members. Explosive detonation results in an over-pressurized shockwave—or primary blast wave—that transmits through the skull and directly interfaces with, displaces, or damages neural tissue [5, 6] This primary blast wave may cause rapid physical displacement of blood from the abdominal area to the cranial vault, damaging the cerebrovasculature and blood brain barrier [7,8,9].
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