A Distinct Metabolite Signature in Military Personnel Exposed to Repetitive Low-Level Blasts

Abstract

Military Breachers and Range Staff (MBRS) are subjected to repeated sub-concussive blasts, and they often report symptoms that are consistent with a mild traumatic brain injury (mTBI). Biomarkers of blast injury would potentially aid blast injury diagnosis, surveillance and avoidance. Our objective was to identify plasma metabolite biomarkers in military personnel that were exposed to repeated low-level or sub-concussive blast overpressure. A total of 37 military members were enrolled (18 MBRS and 19 controls), with MBRS having participated in 8-20 breaching courses per year, with a maximum exposure of 6 blasts per day. The two cohorts were similar except that the number of blast exposures were significantly higher in the MBRS, and the MBRS cohort suffered significantly more post-concussive symptoms and poorer health on assessment. Metabolomics profiling demonstrated significant differences between groups with 74% MBRS classification accuracy (CA). Feature reduction identified 6 metabolites that resulted in a MBRS CA of 98%, and included acetic acid (23.7%), formate (22.6%), creatine (14.8%), acetone (14.2%), methanol (12,7%), and glutamic acid (12.0%). All 6 metabolites were examined with individual receiver operating characteristic (ROC) curve analyses and demonstrated areas-under-the-curve (AUCs) of 0.82-0.91 (P ≤ 0.001) for MBRS status. Several parsimonious combinations of three metabolites increased accuracy of ROC curve analyses to AUCs of 1.00 (P < 0.001), while a combination of volatile organic compounds (VOCs; acetic acid, acetone and methanol) yielded an AUC of 0.98 (P < 0.001). Candidate biomarkers for chronic blast exposure were identified, and if validated in a larger cohort, may aid surveillance and care of military personnel. Future point-of-care screening could be developed that measures VOCs from breath, with definitive diagnoses confirmed with plasma metabolomics profiling.

Keywords: biomarkers; blast; metabolites; mild traumatic brain injury; military.

Figure 1

Metabolites identified with feature selection that determine military breacher/range staff (MBRS) status and their relationship with Rivermead post-concussion variables. (A) A rank order of six leading plasma metabolites that classify MBRS vs. non-MBRS with 98% classification accuracy. All six metabolites are significantly decreased in plasma from MBRS when compared to non-MBRS. Their relative % importance is shown. (B) A tSNE plot demonstrating that MBRS and non-MBRS can be easily separated and identified based on plasma levels of the leading six metabolites. The axes are dimension-less. (C) A heat map demonstrating the negative correlations between Rivermead post-concussion variables and plasma levels of the six leading metabolites. Brighter blue represents a stronger negative correlation. Statistically significant negative correlations are indicated with white asterisks (^*P < 0.05). (D) ROC curves illustrating that the Rivermead post-concussion variables are predictive of MBRS status, as well as with the metabolite parsimonious combinations listed in Table 4 [RPQ13 (late symptoms) AUC = 0.79 [0.65–0.94], RPQ3 (early symptoms) AUC = 0.77 [95%CI 0.62–0.93], Somatic AUC = 0.75 [95%CI 0.58–0.91], Cognitive AUC = 0.71 [95%CI 0.53–0.88], and Emotional AUC = 0.69 [95%CI 0.52–0.87].