Exposure to explosive forces (primary blast) emanating from bombs and other devices is common in recent military personnel from the conflicts in Iraq and Afghanistan. Clinicians and experts have established minimum symptom criteria for diagnosis of mild TBI including altered sensorium (e.g. feeling dazed and confused), loss of consciousness (LOC), and amnesia following exposure to blast or impact. Diffusion Tensor Imaging (DTI) has demonstrated that these symptoms are associated with white matter damage. However, little attention has been given to investigating white matter following asymptomatic exposure to repetitive blast. Relatedly, several recent studies report white matter damage in elite athletes following repetitive impacts despite the absence of concussive symptoms. Our goal is to assess damage to white matter and impairments in cognitive performance in recent military personnel with primary blast exposure without clinical symptoms of TBI (subconcussive blast exposure). Animal models of blast support systemic mechanisms of white matter damage such as neuroinflammatory and neurotoxic processes that are likely to result in diffuse and widespread tissue injury that is spatially heterogeneous among affected individuals. We will use methods for quantifying spatially heterogeneous damage to white matter damage that are more sensitive than conventional voxelwise or region-of-interest approaches. We anticipate comparable white matter damage in a subconcussive blast exposed group and a mild TBI group that will be significantly greater than in blast- unexposed subjects. We will use a machine learning approach to generalize knowledge about the magnitude of white matter in clinically established cases of mild TBI. We will then test this knowledge (validation) to make diagnostic predictions for new cases, particularly blast-exposed individuals, who lack a clinical diagnosis of TBI. We will assess the association between spatially distributed injury to white matter and other measures including, gray matter volume, resting-state functional connectivity, cognitive performance, and symptoms of PTSD and depression. If we confirm our key predictions that tissue damage, cognitive impairment, and functional quality of life changes result from subconcussive blast exposure, it would argue for an augmentation of the established approach for making clinical symptom-based diagnoses of mild TBI with neuroimaging-based diagnostic criteria. The results would further imply that many blast-exposed individuals with chronic symptoms (e.g. mood symptoms, fatigue, inattention) are being incorrectly diagnosed with other conditions (e.g. depression, PTSD) in the absence of acute clinical diagnosis of TBI. Validation of our approach on a large scale including translation to civilian subconcussive exposure (e.g. sports), would argue for implementing advanced diffusion imaging and analytic techniques in the clinical setting.
The majority of mild traumatic brain injury (TBI) cases from recent military conflicts involve exposure to explosive forces (primary blast) emanating from bombs and other devices. Whereas, Diffusion Tensor Imaging (DTI) has demonstrated disruption of brain tissue in cases that meet the minimum symptom criteria for clinical diagnosis of mild TBI, the consequences of blast exposure that is insufficient to meet these criteria is unknown. We will investigate damage resulting to brain tissue from exposure to subconcussive blast.