Page 96 - JSOM Spring 2020

they used audiometric rather than blast pressure measures. attempt was made to correlate objective blast-exposure data
Gill et al. were the first to report elevated peripheral cytokine to performance data.
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levels (tumor necrosis factor-α and interleukin-6) in breachers
exposed to moderate primary blast (>5 psi), which correlated Neurologic
to peak overpressure. Gill et al. identified 1,803 differentially In the only study to specifically report on cranial nerve find-
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expressed genes after moderate blast exposure among breach- ings, Baker et al. reported vestibulocochlear nerve abnormal-
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ers, with the amyloid precursor protein (APP) network being ities in a minority of breaching instructors at baseline that did
suppressed and most affected. Changes in APP were detected not change with acute exposure during a 10-day training.
the day after moderate blast exposure and 2 days later but
returned to baseline 3 days after. This acute decrease may have Sleep
clinical consequences, given the reported onset of associated In the sole study to specifically address sleep, Kelley et al.
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headaches and concentration problems. used actigraphy watches to assess eight different sleep indices
over the course of a 12-day breacher training period for 18
Cognitive Outcomes Marines, who demonstrated no significant changes on these
Results of studies on cognitive performance are also mixed. indices. However, on one of the three subjective measures of
Some 12,19 found possible evidence of long-term cumulative sleep, the Marines self-reported increased sleepiness at the end
effect in instructors, whereas Kamimori et al. found no of the course as compared with the beginning. There was no at-
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significant changes over 5 years and Thiel et al. found no be- tempt to correlate these changes with degree of blast exposure.
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tween-group differences between control subjects and breach-
ers over 2 years. Two studies 16,22 reported an acute or postacute Symptoms
effect on a few tests within the context of many comparisons. In a model that included age, history of head injury, and his-
tory of blast exposure, including separate predictors for his-
Hearing tory of IED and shoulder-fired weapons, Carr et al. found
1
Only two studies examined potential effects of low-level blast only a history of shoulder-fired weapons was a reliable predic-
on hearing and results were varied. St Onge et al. reported tor of symptom severity in those with long-term exposure. In
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statistically and clinically significant changes in hearing, but a subsequent model, history of breaching blast exposure was
not vestibular functioning, 2 weeks after training in their also significant. However, most other studies found no effect
study with an unknown number of breachers. Kubli et al. , of long-term exposure on symptom reporting. 20,21,24,28,29
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in the most comprehensive study of hearing, did not find any
evidence of significant changes over time in several measures In the Baker et al. study, a minority of breachers complained
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within or between breachers and nonexposed engineers. of dizziness and headache at baseline, and this did not change
with acute exposure during a 10-day training. Carr et al.
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Brain Imaging found that headaches significantly increased after a 2-week
In their untreated (i.e., no jugular collar) group, Bonnette et breacher course. Instructors had a greater symptom increase
al. found altered electroencephalogram activity from before overall than students. No differences were noted on any of
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to after 1 day of blast training, which they hypothesized may the other measures included in the study, including measures
represent neural degeneration. Carr et al. found increased ac- of posttraumatic stress disorder, depression, and anxiety.
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tivation, using an n-back functional magnetic resonance imag- Blast-gauge data were collected but not used to correlate with
ing (fMRI) protocol, in instructors relative to students after a symptom reporting. On responses to four mood questions
2-week breacher course. In their untreated (i.e., no jugular col- from baseline to day 7 of training, Gill et al. did not find any
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lar) group, Yuan et al. also found increased activation from significant difference between breachers exposed to moderate
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before to after training (1 day of breacher training) on fMRI blast and those not so exposed. In contrast, in another study,
associated with a 2-back versus 0-back task. In addition, a Gill et al. found new onset of headaches and concentration
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significant correlation was found between the changes in brain problems after moderate blast exposure among breachers, but
activation and the average peak pressure recorded from blast only on the same day of moderate blast exposure.
gauges in the noncollar group.
The preponderance of evidence suggests no lasting effect of
Kamimori et al. found no significant changes using several blast exposure on symptom complaints. Generally, if there
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neuroimaging modalities in breachers measured over 5 years, were findings, increased headaches were a common theme,
including fMRI, diffusion tensor imaging, T1, T2, fluid-atten- which may increase in a subset of individuals and only be ex-
uated inversion recovery, and susceptibility weighted imaging. perienced acutely.
In summary, there may be some acute to postacute increases
in activation during a working-memory task associated with Vestibular
peak overpressure, though there is no evidence that these In the Baker et al. study, most participants scored poorly
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changes persist over a longer time. at baseline on a balance measure, and this finding did not
change with acute exposure during a 10-day training. St Onge
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Motor et al. similarly found no changes in vestibular functioning
Only one study included specific assessment of motor func- after a 2-week training. Littlefield et al. did not find any sub-
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tioning, although it resulted in no firm conclusions. Rhea et jective or objective vestibular changes over 17 months in a
al. found evidence of slower and greater variability in step- group of breachers. However, they did find that nystagmus
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ping in place in those with immediate cognitive decline af- was the most common abnormal finding associated with acute
ter military heavy-weapons training as compared with those exposure, particularly upbeat positional nystagmus, which
without cognitive decline. These effects dissipated by 72 to correlated with history of prior mild TBI. The prevalence of
96 hours after training. Though they used blast gauges, no nystagmus did not increase during the study. Breachers had

90 | JSOM Volume 20, Edition 1 / Spring 2020