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be detectable by advanced magnetic resonance imaging (MRI) skull entry (e.g., the orbits and ear canals) could interfere with
techniques. 26,30 Astroglial scarring is rarely seen in cases of vision and communication, the options for blast gauge place-
pure chronic traumatic encephalopathy (CTE), which has ment are inherently limited. Gauges may also malfunction in
been described primarily in athletes with repetitive blunt head the harsh environments in which SOF personnel operate, or
trauma. 31,32 Whereas CTE has been described as a tauopathy they may fail to detect rounds from a weapon with a rapid
(i.e., a neurodegenerative disorder characterized by accumula- firing speed. Blast gauges thus may not provide a measurement
tion of phosphorylated tau protein within neurons), rBBI may of the ground truth (i.e., the true magnitude and number of
be more aptly described as a polyproteinopathy (i.e., a process blasts an Operator experienced).
characterized by accumulation of multiple abnormal proteins
in the brain). 32-35 Using subjective self-report questionnaires, in which Opera-
tors are asked to provide a cumulative count of blast exposure
While the precise contributions of focal and diffuse patho- from various weapons systems, is a complementary approach
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physiologic mechanisms to rBBI are unknown, it is likely that to objective data gathered through blast gauges. This ap-
heterogeneous forces are exerted on the brain during thou- proach is currently the only method of eliciting exposures that
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sands of blast exposures. A single exposure to overpressure were not otherwise measured, witnessed, or treated. However,
may not be sufficient to alter brain structure or function or important limitations include recall bias and lack of validation
cause long-term symptoms. However, years of cumulative ex- against blast gauge or alternative measurements. Furthermore,
posure may contribute to a broad spectrum of cognitive symp- self-report assessments are unlikely to account for variations
toms such as memory loss and inattention, physical symptoms in Operator positioning with respect to the blast, physical bar-
such as headache and dizziness, and psychological symptoms riers between the Operator and the blast, and reverberations
related or similar to post-traumatic stress disorder (PTSD) and of blast waves off nearby objects and surfaces—all of which
depression. 3,8,37 Each of these sequelae—alone or in concert— influence the amount of overpressure that reaches the brain. 24
has been reported by SOF personnel with high levels of blast
exposure. 38-41 A recently published measure, the Generalized Blast Expo-
sure Value, asks respondents to self-report average lifetime
It is also unknown whether the pathophysiology of rBBI differs exposure to five categories of blast ranging from small- and
from that of a single blast-induced mTBI (Table 1). In other medium-sized arms to large explosives or targeted explosives
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words, do multiple subconcussive exposures to blast overpres- in close range. Other measures like the Blast Ordnance and
sure cause the same type of brain injury as a single mTBI from Occupational Exposure Measure incorporate information
blast overpressure? We can approach this question from the about recent exposures as well as history of breacher train-
perspective of civilian head trauma, in which the pathology of ing courses attended and taught, during which exposures are
multiple subconcussive blunt traumas (i.e., repetitive head im- especially frequent. These measures assess cumulative blast
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pacts) appears to differ from that of a single blunt mTBI. 43,44,45 exposure (i.e., over months to years), rather than incremental
By extension, we might expect rBBI to cause brain pathology changes in exposure, which would be required to measure the
that is distinct from that of a single blast-induced mTBI, but relationship between increasing blast exposure and changes
this reasoning awaits further evidence. in neuroimaging or blood-based biomarkers. In summary,
though a variety of sensors and self-report questionnaires have
In summary, the mechanisms underlying rBBI are complex, been designed to measure blast exposure, precise, reliable, and
heterogenous, and incompletely understood. Similarly, the longitudinal measurements remain elusive.
pathological distinctions between rBBI and a single blast-in-
duced mTBI have not been fully elucidated, in part because it Scientific Barrier – Accounting for Resilience
is difficult to isolate “pure” blast exposure from concurrent Additional barriers to development of a diagnostic test for rBBI
blunt head trauma exposure in training and combat. Given include the unique characteristics of Operators themselves.
current gaps in knowledge, translating findings from the labo- Within the military, the SOF community may be both the most
ratory to the war theater is an exceedingly complex and mul- exposed and the most resilient to the effects of blasts. 48,49 With
tidimensional challenge. A diagnostic test for rBBI must not respect to resilience – the ability to withstand or quickly re-
only detect the multitude of focal and diffuse effects of blasts cover from difficult situations – SOF personnel may possess
on the human brain but also distinguish rBBI from brain in- characteristics that affect brain structure and function in
jury caused by other exposures. unique ways. For example, the cognitive and physical capa-
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bilities that enable them to withstand the grueling selection
process and that are further refined during training and com-
Barriers to the Development of a Diagnostic Test
bat missions (which require continuous high performance)
Scientific Barrier – Measuring the may promote faster recovery from brain injury. 49,51,52 Resil-
Magnitude and Frequency of Blast Exposure ience has also been associated with more robust physiologic
There are no validated tools that accurately measure the health, as measured by cerebral blood flow velocity, and with
strength and number of blasts experienced by an Operator. cognitive reserve, which is the brain’s resistance to damage. 53-55
Studies using blast gauges to measure pounds per square inch Cognitive reserve can develop over a lifetime of pursuing men-
suggest that four pounds per square inch is a threshold at tally challenging tasks such as educational and occupational
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which a blast adversely affects the human brain. However, specialization and training. 56
these studies are limited by gauge placement (typically on the
back of the helmet, chest, and one shoulder), which precludes Scientific Barrier – Accounting for Additional Exposures
measurement of the exact amount of overpressure that reaches The broad spectrum of additional harmful exposures that SOF
the brain. Given that placement of an intracranial blast gauge personnel may experience poses a related challenge in isolat-
is not ethical and that placement of blast gauges at sites of ing blast effects on the brain. Many SOF personnel experience

Brain Health in U.S. Special Operations Forces | 49