Acute Cognitive Effects of Low Level Blast Exposure

Introduction: There is currently great interest in understanding and mitigating the impact of low level blast (LLB) exposures for military personnel. LLB is typically experienced during heavy weapons training (HWT). Although there are several studies demonstrating an increase in symptoms following LLB, published peer reviewed cognitive findings are limited, particularly in relation to the degree of media speculation and attention to military blast sequela. The objective of this study was to evaluate the acute cognitive effects of LLB exposures during HWT.

Methods: 75 active duty service members including Marine Corps School of Infantry (n=11) and Naval Special Warfare (n=64) completed a cognitive battery one week before HWT (Baseline/BL), within 30 minutes (Range/RG), and 72-96 hours after exposure (Follow/FW). The sample consisted of 60 trainees and 15 range safety officers (RSOs). Participants wore blast gauges and only those with at least one sensor reading ≥4 psi threshold were included in the analysis. Participants’ ages ranged from 21-37 years (M= 26.52, SD= 3.14), 92% were White/ Caucasian, 51% had a college degree, 86% were enlisted and all were male. 36% of the participants had a history of at least one combat deployment (range 1-7) and 4.45 years of active duty service (SD=2.46, range 2-17). 43% reported a history of at least one TBI (range 1-6) on a TBI history structured interview, self-reported close proximity blast exposure history varied ranging none to 240 (median = 5).

All participants received a cognitive battery consisting of measures for verbal learning and memory (Hopkins Verbal Learning Test-Revised, HVLT-R), processing speed (Trail Making Test A TMT-A), and executive functioning (Trail Making Test B, TMT-B). Twenty-three participants also completed a visual memory measure (Brief Visuospatial Memory Test – Revised, BVMT-R) and an additional processing speed measure (Symbol Digit Modality Test, SDMT). Self-report measures included the Neurobehavioral Symptom Inventory (NSI), Patient Health Questionnaire Depression Scale (PHQ-8), Combat Exposure Scale (CES), and the PTSD Checklist Military Version (PCL-M). Change in cognitive functioning was examined using reliable change interval (RCI) and corrected for practice effects to identify those who declined immediately following blast exposure. Z-statistics analysis was used to identify if the rate of decline was statistically significant based on the anticipated 5% rate in a normal distribution. Raw scores were used in all the analyses.

Results: The rate of decline from baseline to following assessments: immediate verbal memory (HVLT-R Total, 10.67%), delayed verbal memory (HVLT-R Delayed Recall= 17.33%, immediate visual memory (BVMT-R =21.74%), and delayed visual memory (BVMT-R Delayed Recall= 45.46%). The other cognitive measures including assessment of processing speed and executive functioning did not demonstrate significant decline. When comparing those who had at least one area of significant cognitive decline after exposure (n=27) to those with no areas of decline (n=48), those who demonstrated decline had significantly more years of active duty service (p=0.019), and were twice as likely to have experienced combat (p=0.009). Those who demonstrated cognitive decline also reported significantly more symptoms of depression (PHQ-8 p=0.038) and PTSD symptoms (PCL-M p=0.008) at Baseline. Of the PCL-M subscales, the significant change was noted only for Hyperarousal (p=0.002). The group that declined also self- reported more kinetic combat experiences (CES p=0.0078) and greater number of close proximity blast exposures (p=0.0119). Other variables including age, education, number of combat deployments, neurobehavioral symptoms (NSI), and TBI history were not significantly different between the groups at baseline.

Discussion: Future studies looking at the long term effects of LLB as well as better understanding of what is contributing to the changes noted is suggested as the next steps for this area of research. Possible low grade inflammation, protein aggregation, vascular permeability, oxidative stress, neurotransmitter imbalance or other changes may result in a lower threshold for additional injury and clinical symptoms. Additionally, occupational specialties who are at risk for blast exposures are also individuals who are exposed to higher rate of combat stress and trauma given their roles. The combination of both factors are likely contributory to the outcome and both need to be considered as points of intervention- physical protection of neurological functions when possible and mental health support with appropriate level of care to counter the psychological impact of war.

Conclusion: This study demonstrates an acute negative impact of LLB on verbal and visual learning and memory. Those who showed cognitive decline had greater years of active-duty service, more likely to have been deployed to combat, greater number of self-reported blast exposures, greater self-report symptoms of depression and PTSD, as well more kinetic combat experiences compared with those with no significant cognitive changes. Our analysis highlights the need to correct for practice effects because of the influence of learning on serial assessments given in close succession, as well as RCI analysis that allow for individualized baseline comparisons. Doing so allowed for capture of subtle changes and identification of the subset of individuals who were negatively impacted.