Abstract
Concern is growing that subconcussive blast exposure during military exercises such as heavy weapons training (HWT) may adversely affect brain function. HWT involves firing shoulder-mounted weapons that can expose the gunner, assistant gunner, and instructors (range safety officers [RSOs]) to blast overpressure >4.0 psi. Every HWT participant sustains repeated blast exposure: gunner and assistant alternate firing within an event, and instructors supervise multiple firings at approximately 12 HWTs over two years. This five-year study was designed to quantify blast exposure and serially assess a broad array of brain function measures in HWT participants and corresponding controls. Two hundred and twenty-four participants were prospectively assessed: 104 exposed special operators (SOs) firing live rounds in HWT; 47 SOs completing land warfare training without firing live rounds during HWT (training controls); 36 conventional active duty controls (ADCs); and 37 instructors. Assessments were conducted at baseline and at 30 min, 6, 24, and 72 h, 2 weeks, and 3 months post-HWT. Instructors were also assessed 9 and 18 months after their first HWT, with repeat of all acute assessments (30 min through 3 months) after their 18-month HWT. Blast exposure was measured with the Black Box Biometrics Generation 7 Blast Gauge System. Assessments included blood biomarkers, symptom scales, memory, cerebrovascular reactivity, electroencephalography (EEG), oculomotor function, gait, tactile discrimination, and hearing acuity. SOs exhibited significant alterations in short-term memory, complexity of electroencephalographic signals, and the blood biomarkers tau and ubiquitin C-terminal hydrolase L1 within minutes after HWT, while gait and oculomotor function were measurably altered within hours. More than 20% of the exposed had at least one significant change in a hearing threshold on audiologic assessment, though this, as well as changes in levels of the blood biomarker neurofilament-light, were observed two weeks after HWT. Instructors who experienced repeated HWT exposure over two-year duty tours demonstrated greater magnitude and duration alterations in memory and gait toward the end of those tours. The blood biomarker glial fibrillary acidic protein, vasomotor reactivity of the middle cerebral arteries, and seven cognitive tests comprising the Automated Neuropsychological Assessment Metrics Version 4 showed no significant changes in association with subconcussive blast exposure in this setting. Blast exposure during HWT is associated with acute changes in some blood biomarker levels and measures of brain function such as short term recall memory and gait. The effects appear to be short in duration, but recovery may be delayed after repeated exposure.