Research Scientist Walter Reed Army Institute of Research Silver Spring, Maryland
Abstract Text: Characterization of occupational exposure associated with explosive breaching attracted considerable attention since a seminal study conducted in 2007. ‘Breacher’s brain’ was an anecdotal term used by breachers to describe adverse effects that they experience from chronic exposure to repeated blasts. Until recently, there was no agency-wide orchestrated effort to capture the nature of the impact of repeated low-level blast (LLB) exposure on the performance and health of active duty Service Members. Not surprisingly, LLB characterization of weapon systems other than breaching and associated symptomatology has received less attention. One example of a knowledge gap is weapons with a high cycling rate of fire, such as machine guns, e.g., the M2A1 Browning machine gun. Exposure conditions in this type of environment are highly complex and consist of hundreds to thousands of events per session. Typically, a wide range of peak overpressures is registered when multiple guns are firing simultaneously.
In this study, 18 heavy weapons instructors were enrolled, and it was designed as a 6-days longitudinal evaluation. Baseline measures were collected in the morning of each day, and a follow-up post-exposure test battery was administered in the afternoons. On days 2, 5, and 6, the training with heavy weapons was conducted on two separate ranges. Range 1 was used for the M2A1 machine gun and Mk 19 grenade launcher, while on Range 2, AT4 bazooka training was conducted (using predominantly subcaliber ammunition). There were 9 instructors on each range (n =18), and these individuals received a standard battery of assessments: neurocognitive (DANA Rapid), self-reported symptoms, and blood draws. Levels of serum biomarkers of neurotrauma were measured using ELISA: glial fibrillary acidic protein (GFAP), ubiquitin C-terminal hydrolase-L1 (UCH-L1), neurofilament light (Nf-L), tau, and amyloid beta peptides (Aβ-40 and Aβ-42). Overpressure exposures were measured using wearable BlackBox Biometrics (B3) Blast Gauges and Larson-Davis LxT dosimeters.
The rate of fire and relatively low BOPs are two challenges the current generation of wearable sensors cannot meet. The net result is that the exposure data recorded by these sensors are only a fraction of the total daily dosage. For the calculation of the complete daily dosage (i.e., the cumulative impulse), an alternative method based on the peak overpressure recorded by Larson-Davis devices was developed. The personnel on Range 1 experienced relatively high cumulative impulse dosage that ranged between 155 and 1560 psi*ms. The cumulative impulse for Range 2 instructors ranged from 18 to 54 psi*ms. Self-reported symptoms were relatively scarce but did increase in severity on “range hot” days. Slowed reaction time on DANA Rapid was noted for 5 individuals supervising Range-1 and 2 individuals supervising Range-2. Even more significant changes were noticeable for the Go-or-No Go (GNG) test, where 12 and 5 instructors’ scores were indicative of decrements (Range-1 vs. -2, respectively, p < 0.05).
The temporal changes in serum biomarker concentrations indicate there is a considerable level of variability between subjects; e.g., min-max for Aβ-40 was 98-301 pg/mL. No single biomarker among the tested panel showed superior sensitivity. Instead, there were single-day upticks in certain serum peptides, e.g., Aβ-42 on Day-2 and -6 (range hot on both days), Aβ-40 on Day-2, and UCH-L1 on Day-2 among Range 1 instructors (p < 0.05, WITHIN group comparison). On the other hand, the BETWEEN group comparison (i.e., Range-1 vs. Range-2) has shown elevation in GFAP (Day-2), Nf-L (Day-2, -5, and -6), and UCH-L1 (Day -2). The exposure days biomarker data correlate with cumulative impulse only for Aβ-40 (.r = -0.44, p = 0.04) and Aβ-42 (.r = -0.46, p = 0.04). There was also a positive correlation between the cumulative symptoms score and serum biomarker for Aβ-40, Aβ-42, and Nf-L, but didn’t reach statistical criterion (p > 0.05).
The results of this study demonstrate that a single day exposure to a high number of relatively low overpressure can lead to an elevation in serum biomarkers and concomitant performance decrements. The limited sample size, typical for this kind of study design, is a limiting factor of this study. Thus, additional effort is required to validate the findings from this work.
Keywords: blast overpressure, low-level blast, heavy weapons, serum biomarkers, amyloid beta, DANA Rapid