Repeated Exposure to Military Low-Level Blast Is Associated With Regionally Specific Reductions in Gray Matter Volume in Breachers and Snipers

Objective: To examine the association between duration of repeated exposure to low-level blast and alteration in brain structure. Setting: Data were collected at 2 neuroimaging facilities. Participants: Canadian Armed Forces breachers ( n = 30) and snipers ( n = 31) with a history of repeated exposure to low-level blast (average years = 9.70, SD = 4.57) and sex- and age-matched active military controls ( n = 57). Design: MRI and neurocognitive data from breachers and snipers were collected prospectively at baseline (time 1) and after a training exercise involving a concentrated regimen of blast exposure (time 2) (time gap = 80.56 days, SD = 36.90). Time 1 and time 2 data were used to assess the chronic and training-related effects of blast exposure, respectively. MRI and neurocognitive data from controls were collected twice with a comparable time gap. Main measures: High-resolution magnetic resonance imaging (MRI) scans, plus neurocognitive measures. Results: Voxel-based morphometry conducted on MRI data collected at time 1 demonstrated that compared with controls, there was lower regional gray matter volume in left inferior frontal gyrus in snipers, and bilaterally in parahippocampal gyrus (bordering on amygdala) in breachers. Regional gray matter volume in left inferior frontal gyrus in snipers predicted neurocognitive function. We did not observe any difference in regional white matter volume between groups at time 1, nor any changes in regional gray or white matter volume between time 1 and time 2. Conclusion: Chronic but not training-related low-level blast exposure was associated with dissociable patterns of regional gray matter volume reduction in snipers and breachers, and regional gray matter volume predicted neurocognitive function in snipers. The differential pattern of structural alteration between breachers and snipers and its relationship with neurocognitive function in snipers can inform health monitoring programs for diagnosis and treatment of blast-exposed service members and Veterans.