Combat casualty care simulator — detecting and triaging traumatic brain injury from explosive blast overpressure
When an explosive charge detonates, it releases energy as a supersonic shock front of compressed air — the overpressure wave — followed by a longer-duration blast wind. This wave can traverse the body and disrupt tissue at air-fluid interfaces, including the brain, without ever breaking the skin. This is the defining and most historically underappreciated feature of blast injury.
Detonation converts a solid or liquid explosive into gas almost instantaneously, releasing enormous energy in microseconds. This creates a spherical shock front of highly compressed air that expands outward faster than the speed of sound, followed by a sub-atmospheric "negative phase" as air rushes back toward the seat of the blast.
Peak overpressure and impulse (the pressure-time integral) both fall off steeply with distance — roughly with the cube of range for a free-field blast — which is why standing a few extra meters from a charge, or having any intervening cover, dramatically changes injury severity. Charge size (yield), confinement (enclosed rooms/vehicles amplify overpressure through reflection), and ambient conditions all modulate the wave a casualty actually experiences.
Primary blast injury (PBI) is tissue damage caused directly by the pressure differential of the shock wave, independent of any object striking the body. It preferentially damages air-fluid interfaces and air-filled organs, because the wave causes rapid compression, spalling, and shearing at density boundaries inside the body:
• Lungs — "blast lung": alveolar hemorrhage, pulmonary contusion, potential air embolism • Ears — tympanic membrane rupture is the most common and most sensitive primary blast injury; often used as a rough proxy for exposure severity • GI tract — bowel wall hemorrhage, delayed perforation • Brain — increasingly recognized as a primary blast target; the wave transmits through skull and vasculature, causing diffuse axonal-type shear injury, microvascular disruption, and neuroinflammation — with no external wound at all.
For most of the 20th century, medicine treated blast injury as essentially synonymous with fragment wounds and burns. The idea that the pressure wave alone could injure the brain — with a normal-looking head and no loss of consciousness at the scene — was not systematically recognized until the IED-dominated conflicts in Iraq and Afghanistan, where blast TBI became known as the "signature injury" of those wars.
A casualty struck only by primary blast effects can walk, talk, and appear entirely uninjured in the immediate aftermath, while sustaining real brain injury that will only manifest as confusion, headache, or cognitive slowing minutes to hours later. This is fundamentally different from penetrating or blunt trauma, where the mechanism and the wound are visually linked.
The practical consequence for combat medicine: proximity to any blast event — even one that produced no fragments or visible casualties — must itself be treated as a potential TBI exposure and screened for, rather than waiting for a casualty to "look hurt."
Military medicine classifies blast injury into four distinct mechanisms. A single casualty near a detonation is frequently struck by all four at once, and each must be assessed independently — a casualty with an obvious shrapnel wound (secondary) can simultaneously have an occult brain injury (primary) that is easy to overlook once the visible wound draws attention.
Secondary blast injury results from objects propelled by the explosion striking the body: munitions casing fragments, nails or ball bearings packed into an improvised device, and environmental debris such as glass, gravel, or vehicle parts. These behave like high-velocity penetrating trauma and can strike any body region, including the head — a penetrating head wound is a distinct and separately assessed injury from primary blast TBI, even though both can occur in the same casualty.
Tertiary blast injury occurs when the blast wind itself displaces the casualty's body — throwing them against a wall, vehicle, or the ground — or when a structure collapses onto them. This produces blunt trauma patterns: fractures, closed head injury, traumatic amputation at a limb's point of impact, and soft-tissue crush. A casualty thrown several meters can sustain a closed traumatic brain injury from the impact alone, layered on top of any primary blast effect from the wave itself.
Quaternary blast injury is a catch-all for blast-related harm that is neither the pressure wave, a fragment, nor displacement: thermal burns from the fireball, inhalation of toxic combustion gases and fine dust, crush injury from collapsed structures, and acute exacerbation of pre-existing conditions (asthma, cardiac disease) triggered by the physical and psychological stress of the event. In enclosed-space or vehicle blasts, toxic inhalation and burns are frequently the dominant threat to survival even when fragments are absent.
| Product | Indication | Trial Design | Key Result |
|---|---|---|---|
| Primary | Air-filled organs: lungs, ears, GI tract, brain | Direct tissue effect of the overpressure shock wave itself | Tympanic rupture, blast lung, primary blast TBI |
| Secondary | Any body region, including head/torso | Projectiles — casing fragments, shrapnel, environmental debris | Penetrating wounds, fragment injuries |
| Tertiary | Skeleton, closed head, soft tissue | Body thrown by blast wind or struck by collapsing structure | Fractures, closed TBI, traumatic amputation |
| Quaternary | Skin, airway/lungs, whole-body systems | Burns, toxic gas/dust inhalation, crush, illness exacerbation | Thermal burns, inhalation injury, crush syndrome |
The Glasgow Coma Scale (GCS) is the standard rapid neurological exam used by combat medics to quantify level of consciousness in the field. It requires no equipment beyond observation and simple verbal/physical prompts, which makes it fast and reliable under fire — and it is the single most important tool for catching primary blast TBI in a casualty who has no visible wound at all.
GCS sums three independently scored responses:
• Eye opening (1–4): 4 = spontaneous, 3 = to voice, 2 = to pain, 1 = none • Verbal response (1–5): 5 = oriented, 4 = confused conversation, 3 = inappropriate words, 2 = incomprehensible sounds, 1 = none • Motor response (1–6): 6 = obeys commands, 5 = localizes pain, 4 = withdraws from pain, 3 = abnormal flexion, 2 = extension, 1 = none
The three sub-scores are always reported alongside the total (e.g. "GCS 13, E3 V4 M6") because the pattern of deficit — not just the number — guides clinical decisions. A dropping GCS on repeat exam is itself a critical warning sign of expanding intracranial injury, which is why GCS is reassessed serially, not scored once and forgotten.
A GCS of 8 or below indicates a level of impaired consciousness at which a casualty can no longer reliably protect their own airway — they lose the gag/cough reflexes needed to prevent aspiration and their tongue can obstruct the airway. This is the origin of the trauma teaching "GCS ≤8, intubate" — in the field this translates to airway management (positioning, adjuncts, or advanced airway if trained/equipped) becoming the immediate first priority, ahead of essentially everything except uncontrolled hemorrhage, followed by urgent evacuation.
A casualty can have a completely intact, unmarked skull and skin, and still present with a GCS of 6 from primary blast overpressure alone. GCS scoring — not the presence or absence of a visible wound — is what actually identifies severe TBI in the field.
GCS is excellent at flagging severe impairment but is a blunt instrument for mild TBI: a casualty can score a perfect 15 — fully oriented, obeying commands, eyes open — while still having a real concussion with impaired memory, concentration, and reaction time that GCS was never designed to detect. This is exactly why a second, more sensitive screening tool is used for casualties who pass GCS but were within blast range: the Military Acute Concussion Evaluation, covered next.
The Military Acute Concussion Evaluation (MACE) is a real, fielded cognitive screening tool used by military medics and corpsmen to evaluate any service member with a suspected concussion — most commonly after blast exposure. It is administered to casualties who are conscious, ambulatory, and would otherwise be assumed fine, precisely because primary blast TBI at the mild end of the spectrum produces no external signs.
MACE combines a history of the event (was there loss of consciousness, alteration of consciousness, amnesia?) with a structured cognitive exam covering four domains:
• Orientation — date, day, month, year, place, time • Immediate memory — recall of a five-word list across three learning trials • Concentration — reciting digits backward and months of the year in reverse order • Delayed recall — recall of the same five-word list after a several-minute delay filled with other tasks
A neurological screen (pupils, speech, coordination, balance) and a symptom checklist (headache, dizziness, memory problems, balance problems, nausea) are administered alongside the cognitive score. The tool takes only a few minutes, which is why it is practical to administer to every casualty within range of a blast, not just those who "seem" concussed.
During the Iraq and Afghanistan conflicts, the sheer volume of IED exposure meant huge numbers of service members experienced blast events without penetrating wounds. Department of Defense data from that era found that mild TBI/concussion from blast exposure affected an estimated 20% or more of deployed personnel in some studied cohorts — far higher than had been assumed when blast casualties were tracked mainly by visible wounds.
That realization drove a major doctrinal shift: any service member within a specified distance of a blast, or involved in a vehicle rollover/collision, is now required to undergo a MACE-style screen and mandatory rest period — regardless of how they say they feel — rather than relying on self-report alone.
A lower MACE cognitive score, an abnormal neuro exam, or a positive history of loss/alteration of consciousness or amnesia around the event all point toward a concussion diagnosis, which triggers the mandatory rest-and-reassess protocol described in the final stage. Because MACE performance can also be affected by pain, fatigue, or acute stress reaction, it is interpreted alongside the symptom checklist and clinical judgment — it is a screening and documentation tool, not a stand-alone diagnostic test in isolation.
Blast TBI triage resolves into two very different care pathways. Severe TBI (GCS ≤8) is a life-threatening emergency demanding airway protection and urgent evacuation. Mild TBI/concussion is not benign either — it demands a mandatory stand-down period, because returning a concussed service member to duty too soon carries real, well-documented risk.
A casualty with GCS ≤8 is triaged for urgent evacuation, with airway management as the first intervention priority given their inability to reliably protect their own airway, followed closely by control of any hemorrhage and rapid transport to a higher level of care where imaging and neurosurgical capability exist. Moderate TBI (GCS 9–12) is managed with close serial reassessment — a falling score at any point escalates the casualty to the severe pathway and urgent evacuation, because intracranial injury can evolve over the minutes and hours following a blast.
For casualties who pass GCS and are diagnosed with concussion on MACE, the protocol is a mandatory rest period away from combat duty, hydration, symptom monitoring, and a graduated, staged return to full duty only after symptoms resolve — not an immediate return to the line because the casualty "looks fine" or insists they are okay. This represents a major and relatively recent shift in military medical doctrine, moving away from treating mild concussion as something to push through.
Returning a concussed service member to duty before the brain has recovered carries two distinct risks that drove the current mandatory stand-down policy:
• Second-impact risk — a second concussive event before the first has resolved can produce disproportionately severe, sometimes catastrophic, neurological injury compared to either event alone • Cumulative effects — repeated concussions, even individually mild, are associated with prolonged recovery times and increased risk of long-term cognitive, mood, and neurodegenerative problems
Because a concussed service member with intact GCS can appear entirely combat-capable while still having impaired judgment, reaction time, and memory, self-report is not considered sufficient — the graduated return-to-duty process requires symptom resolution and passing follow-up cognitive screening before full duty resumes.
The recognition that a huge share of blast-exposed service members in Iraq and Afghanistan had occult mild TBI — invisible on the outside, real on the inside — is what forced mandatory rest-and-reassess policy into military medicine. Treating "no visible wound" as equivalent to "no injury" was, for decades, the single biggest gap in combat trauma care.