Single and repeated high-level blast, low-level blast, and new-onset self-reported health conditions in the U.S. Millennium Cohort Study: An exploratory investigation

Impact of Low-Level Blast Exposure From Military Training and Career Cumulation on Hearing Outcomes

INTRODUCTION

Occupational trades expose military service members to unique sound exposure profiles, distinct in frequency and amplitude to those commonly studied in industrial settings. Characteristics of the energy exposure incurred through blast exposure exercises a distinct mechanism of injury in the auditory system that yield acute and cumulative health impacts. The following paper considers physiological and self-reported data attributed to repeated low-level blast exposure. We focused our analysis to examine objective audiological metrics of blast overpressure damage to quantify the impact of discrete and accumulated blast exposure for military service members.

MATERIALS AND METHODS

This study was reviewed and approved by the Human Research Ethics Committee of Defense Research and Development Canada. Data were collected in a cross-sectional, repeated measures observational design. Participants comprised service members who took part in a training course using controlled explosives (n = 46) and an age- and sex-matched control group (n = 42) with noise exposure but no history of blast exposure. Extensive demographic and subjective health data were captured additional to a battery of quantitative health data. Nonlinear mixed effect models with bivariate generalized additive architecture, Friedman's test, and repeated measures ANOVA models were applied.

RESULTS

Subjective tinnitus and hearing scores were sensitive to life-time blast overpressure exposure (P = .034) and a delta of low- (2-4 kHz) and high-frequency (4-8 kHz) audiometric threshold averages significantly differentiated by both cumulative (P = .0016) and training course exposure (P = .0142). Furthermore, audiometric values showed progressive increasing variance in high frequencies (>4  kHz) in blast-exposed participants with worsening audiometric outcomes. Variation in high-frequency hearing thresholds may be attributed to differences in energy exposure because of structural differences between individuals or differences in exposure profiles (produced either by diversity in PPE compliance or exposure sources).

CONCLUSION

Military members exposed to blasts in a training environment and accumulated through a career experience adverse hearing and audiometric impacts. However, better measures of hearing injury should be pursued to improve understanding of injury mechanism and injury progression which may inform risk and medical interventions. Blast-induced hearing injuries are dynamic and change over time between acute and chronic phases which are further exacerbated with multiple blast exposures presented consecutively.

Exploring Calcium Channels as Potential Therapeutic Targets in Blast Traumatic Brain Injury

Background/Objectives: Repeat low-level blast exposure has emerged as a significant concern for military populations exposed to explosive events. Blast-Related Traumatic Brain Injury (bTBI) is a unique form of brain trauma with poorly understood molecular mechanisms. Loss of calcium homeostasis has emerged as a mediator of early neuronal dysfunction after blast injury. This review aims to examine the role of calcium signaling in bTBI, focusing on the dual function of calcium channels as mediators and modulators of injury, and to explore therapeutic strategies targeting calcium homeostasis. Methods: We conducted a review of peer-reviewed articles published between 2000 and 2024, using the databases PubMed, Scopus, and EBSCO. Search terms included "blast traumatic brain injury", "calcium channels", and "calcium". Studies investigating intracellular calcium dynamics after bTBI were included. Exclusion criteria included studies lacking evaluation of calcium signaling, biomarker studies, and studies on extracellular calcium. Results: We identified 13 relevant studies, primarily using preclinical models. Dysregulated calcium signaling was consistently linked to cellular dysfunction, including plasma membrane abnormalities, cytoskeletal destabilization, mitochondrial dysfunction, and proteolytic enzyme activation. Studies highlighted spatially compartmentalized vulnerabilities across neurons and astrocytes, suggesting that targeting specific cellular regions, such as the neuronal soma or axons, could enhance the therapeutic outcome. Therapeutic strategies included pharmacological inhibitors, plasma membrane stabilizers, and modulators of secondary injury. Conclusions: Calcium signaling is implicated in the pathophysiology of bTBI. Standardized experimental approaches would reduce variability in findings and improve the understanding of the relationship between calcium channel dynamics and bTBI and help guide the development of neuroprotective interventions that mitigate injury and promote recovery.

A review of long-term outcomes of repetitive concussive and subconcussive blast exposures in the military and limitations of the literature

Objective: The purpose of this review is to summarize the long-term cognitive, psychological, fluid biomarker, and neuroimaging outcomes following repetitive concussive and subconcussive blast exposures sustained through a military career. Method/Results: A review of the literature was conducted, with 450 manuscripts originally identified and 44 manuscripts ultimately included in the review. The most robust studies investigating how repetitive concussive and subconcussive exposures related to cognitive performance suggest there is no meaningful impact. Although there are minimal studies that suggest some small impacts on neuroimaging and fluid biomarkers, most findings have been in very small samples and fail to replicate. Both repetitive blast mTBI and subconcussive blasts appeared to be associated with increased self-reported symptoms. Many of the studies suffered from small sample size, failure to correct for multiple comparisons, and inappropriate control groups. Conclusions: Overall, there is little evidence to support that repetitive blast mTBIs or subconcussive blast exposures have a lasting impact on cognition, neuroimaging, or fluid biomarkers. In contrast, there does appear to be a relationship between these exposures and self-reported psychological functioning, though it is unclear what mechanism drives this relationship. Small sample size, lack of correction for multiple comparisons, limited control groups, lack of consideration of important covariates, limited diversity of samples, and lack of reliable and valid measures for assessment of blast exposure are major limitations restricting this research. Patients should be encouraged that while research is ongoing, there is little to currently suggest long-term cognitive or neurological damage following repetitive blast exposure.

Circulating Brain-Reactive Autoantibody Profiles in Military Breachers Exposed to Repetitive Occupational Blast

Military breachers are routinely exposed to repetitive low-level blast overpressure, placing them at elevated risk for long-term neurological sequelae. Mounting evidence suggests that circulating brain-reactive autoantibodies, generated following CNS injury, may serve as both biomarkers of cumulative damage and drivers of secondary neuroinflammation. In this study, we compared circulating autoantibody profiles in military breachers (n = 18) with extensive blast exposure against unexposed military controls (n = 19). Using high-sensitivity immunoassays, we quantified IgG and IgM autoantibodies targeting glial fibrillary acidic protein (GFAP), myelin basic protein (MBP), and pituitary (PIT) antigens. Breachers exhibited significantly elevated levels of anti-GFAP IgG (p < 0.001) and anti-PIT IgG (p < 0.001) compared to controls, while anti-MBP autoantibody levels remained unchanged. No significant differences were observed for any IgM autoantibody measurements. These patterns suggest that repetitive blast exposure induces a chronic, adaptive immune response rather than a short-lived acute phase. The elevated IgG autoantibodies highlight the vulnerability of astrocytes, myelin, and the hypothalamic-pituitary axis to ongoing immune-mediated injury following repeated blast insults, likely reflecting sustained blood-brain barrier disruption and neuroinflammatory processes. Our findings underscore the potential of CNS-targeted IgG autoantibodies as biomarkers of cumulative brain injury and immune dysregulation in blast-exposed populations. Further research is warranted to validate these markers in larger, more diverse cohorts, and to explore their utility in guiding interventions aimed at mitigating neuroinflammation, neuroendocrine dysfunction, and long-term neurodegenerative risks in military personnel and similarly exposed groups.

Modeling Highly Repetitive Low-level Blast Exposure in Mice

Exposure to explosive blasts is a significant risk factor for brain trauma among exposed persons. Although the effects of large blasts on the brain are well understood, the effects of smaller blasts such as those that occur during military training are less understood. This small, low-level blast exposure also varies highly according to military occupation and training tempo, with some units experiencing few exposures over the course of several years whereas others experience hundreds within a few weeks. Animal models are an important tool in identifying both the injury mechanisms and long-term clinical health risks following low-level blast exposure. Models capable of recapitulating this wide range of exposures are necessary to inform acute and chronic injury outcomes across these disparate risk profiles. Although outcomes following a few low-level blast exposures are easily modeled for mechanistic study, chronic exposures that occur over a career may be better modeled by blast injury paradigms with repeated exposures that occur frequently over weeks and months. Shown here are methods for modeling highly repetitive low-level blast exposure in mice. The procedures are based on established and widely used pneumatic shocktube models of open-field blast exposure that can be scaled to adjust the overpressure parameters and the number or interval of the exposures. These methods can then be used to either enable mechanistic investigations or recapitulate the routine blast exposures of clinical groups under study.

Impact of repeated blast exposure on active-duty United States Special Operations Forces

United States (US) Special Operations Forces (SOF) are frequently exposed to explosive blasts in training and combat, but the effects of repeated blast exposure (RBE) on SOF brain health are incompletely understood. Furthermore, there is no diagnostic test to detect brain injury from RBE. As a result, SOF personnel may experience cognitive, physical, and psychological symptoms for which the cause is never identified, and they may return to training or combat during a period of brain vulnerability. In 30 active-duty US SOF, we assessed the relationship between cumulative blast exposure and cognitive performance, psychological health, physical symptoms, blood proteomics, and neuroimaging measures (Connectome structural and diffusion MRI, 7 Tesla functional MRI, [11C]PBR28 translocator protein [TSPO] positron emission tomography [PET]-MRI, and [18F]MK6240 tau PET-MRI), adjusting for age, combat exposure, and blunt head trauma. Higher blast exposure was associated with increased cortical thickness in the left rostral anterior cingulate cortex (rACC), a finding that remained significant after multiple comparison correction. In uncorrected analyses, higher blast exposure was associated with worse health-related quality of life, decreased functional connectivity in the executive control network, decreased TSPO signal in the right rACC, and increased cortical thickness in the right rACC, right insula, and right medial orbitofrontal cortex-nodes of the executive control, salience, and default mode networks. These observations suggest that the rACC may be susceptible to blast overpressure and that a multimodal, network-based diagnostic approach has the potential to detect brain injury associated with RBE in active-duty SOF.

Study protocol: Identifying transcriptional regulatory alterations of chronic effects of blast and disturbed sleep in United States Veterans

Injury related to blast exposure dramatically rose during post-911 era military conflicts in Iraq and Afghanistan. Mild traumatic brain injury (mTBI) is among the most common injuries following blast, an exposure that may not result in a definitive physiologic marker (e.g., loss of consciousness). Recent research suggests that exposure to low level blasts and, more specifically repetitive blast exposure (RBE), which may be subconcussive in nature, may also impact long term physiologic and psychological outcomes, though findings have been mixed. For military personnel, blast-related injuries often occur in chaotic settings (e.g., combat), which create challenges in the immediate assessment of related-injuries, as well as acute and post-acute sequelae. As such, alternate means of identifying blast-related injuries are needed. Results from previous work suggest that epigenetic markers, such as DNA methylation, may provide a potential stable biomarker of cumulative blast exposure that can persist over time. However, more research regarding blast exposure and associations with short- and long-term sequelae is needed. Here we present the protocol for an observational study that will be completed in two phases: Phase 1 will address blast exposure among Active Duty Personnel and Phase 2 will focus on long term sequelae and biological signatures among Veterans who served in the recent conflicts and were exposed to repeated blast events as part of their military occupation. Phase 2 will be the focus of this paper. We hypothesize that Veterans will exhibit similar differentially methylated regions (DMRs) associated with changes in sleep and other psychological and physical metrics, as observed with Active Duty Personnel. Additional analyses will be conducted to compare DMRs between Phase 1 and 2 cohorts, as well as self-reported psychological and physical symptoms. This comparison between Service Members and Veterans will allow for exploration regarding the natural history of blast exposure in a quasi-longitudinal manner. Findings from this study are expected to provide additional evidence for repetitive blast-related physiologic changes associated with long-term neurobehavioral symptoms. It is expected that findings will provide foundational data for the development of effective interventions following RBE that could lead to improved long-term physical and psychological health.

Twenty Years of Blast-Induced Neurotrauma: Current State of Knowledge

Blast-induced neurotrauma (BINT) is an important injury paradigm of neurotrauma research. This short communication summarizes the current knowledge of BINT. We divide the BINT research into several broad categories-blast wave generation in laboratory, biomechanics, pathology, behavioral outcomes, repetitive blast in animal models, and clinical and neuroimaging investigations in humans. Publications from 2000 to 2023 in each subdomain were considered. The analysis of the literature has brought out salient aspects. Primary blast waves can be simulated reasonably in a laboratory using carefully designed shock tubes. Various biomechanics-based theories of BINT have been proposed; each of these theories may contribute to BINT by generating a unique biomechanical signature. The injury thresholds for BINT are in the nascent stages. Thresholds for rodents are reasonably established, but such thresholds (guided by primary blast data) are unavailable in humans. Single blast exposure animal studies suggest dose-dependent neuronal pathologies predominantly initiated by blood-brain barrier permeability and oxidative stress. The pathologies were typically reversible, with dose-dependent recovery times. Behavioral changes in animals include anxiety, auditory and recognition memory deficits, and fear conditioning. The repetitive blast exposure manifests similar pathologies in animals, however, at lower blast overpressures. White matter irregularities and cortical volume and thickness alterations have been observed in neuroimaging investigations of military personnel exposed to blast. Behavioral changes in human cohorts include sleep disorders, poor motor skills, cognitive dysfunction, depression, and anxiety. Overall, this article provides a concise synopsis of current understanding, consensus, controversies, and potential future directions.

Lifetime Blast Exposure Is Not Related to White Matter Integrity in Service Members and Veterans With and Without Uncomplicated Mild Traumatic Brain Injury

This study examines the impact of lifetime blast exposure on white matter integrity in service members and veterans (SMVs). Participants were 227 SMVs, including those with a history of mild traumatic brain injury (mTBI; n = 124), orthopedic injury controls (n = 58), and non-injured controls (n = 45), prospectively enrolled in a Defense and Veterans Brain Injury Center (DVBIC)/Traumatic Brain Injury Center of Excellence (TBICoE) study. Participants were divided into three groups based on number of self-reported lifetime blast exposures: none (n = 53); low (i.e., 1-9 blasts; n = 81); and high (i.e., ≥10 blasts; n = 93). All participants underwent diffusion tensor imaging (DTI) at least 11 months post-injury. Tract-of-interest (TOI) analysis was applied to investigate fractional anisotropy and mean, radial, and axial diffusivity (AD) in left and right total cerebral white matter as well as 24 tracts. Benjamini-Hochberg false discovery rate (FDR) correction was used. Regressions investigating blast exposure and mTBI on white matter integrity, controlling for age, revealed that the presence of mTBI history was associated with lower AD in the bilateral superior longitudinal fasciculus and arcuate fasciculus and left cingulum (βs = -0.255 to -0.174; ps < 0.01); however, when non-injured controls were removed from the sample (but orthopedic injury controls remained), these relationships were attenuated and did not survive FDR correction. Regression models were rerun with modified post-traumatic stress disorder (PTSD) diagnosis added as a predictor. After FDR correction, PTSD was not significantly associated with white matter integrity in any of the models. Overall, there was no relationship between white matter integrity and self-reported lifetime blast exposure or PTSD.