One is not like the other: Examining the neural response to repetitive low-level blast exposure in experienced military personnel

Military members often report concussion-like symptoms from repetitive low-level blast (LLB) exposure, defined as overpressure from outgoing munitions like rifles and explosive breaching. Typically, the early stages of concussion and other neurological conditions (e.g., Alzheimer's Disease) lead to hyperconnectivity which is a transient and adaptive brain response to strengthen and establish neural connections to restore brain function. Over time, however, chronic hyperconnectivity can contribute to neurodegeneration. To determine whether LLB exposure also exhibits this connectivity trajectory, this study investigated the neural signature of LLB at two time points: At the chronic stage extrapolated from the duration of an individual's occupational career, and following a recent and concentrated blast regimen. Forty-six military breachers and snipers underwent a resting state functional magnetic resonance imaging brain scan before and after a training course. Graph theory was used to study the whole-brain network, cross-validated by a principal components analysis (PCA) conducted post-hoc . The pre-course scan was analyzed separately to examine the neural effects of chronic LLB exposure. The pre- and post-course scans were compared to examine the neural effects of recent blast exposure. Military controls without occupational breaching and/or sniping experience underwent the same protocol. At pre-course, breachers and snipers exhibited hyperconnectivity compared to controls. However, after undergoing a recent LLB regimen, only breachers showed hypoconnectivity post-course relative to pre-course compared to controls. The mechanism of repeated LLB overpressure and its associated neural response to this exposure appear to be specific to this condition. Characterizing LLB exposure can help refine assessment and treatment. • Repetitive low-level blast exposure is harmful for brain health in military members. • Low-level blast exposure elicits a neural response distinct from concussion. • Chronic low-level blast exposure leads to enhanced brain network connectivity. • Recent low-level blast exposure leads to reduced brain network connectivity. • Breachers and snipers show different neural connectivity profiles from each other.