An explosion produces separate categories of injury, and only some of them involve being hit by anything.
**Primary blast injury** comes from the supersonic pressure wave that leaves the explosive itself — the direct effect of over- and under-pressurization on the body.
**Secondary blast injury** comes from shrapnel — fragments propelled outward, producing lacerations, contusions, and fractures.
**Tertiary blast injury** comes from structural collapse and from the blast wind throwing a person against a wall, a vehicle, or the ground. The injuries come from striking those objects.
**Quaternary blast injury** covers everything else the event produces: burns, asphyxia, and exposure to toxic inhalants. That taxonomy is the standard clinical framework (DePalma et al., 2005).
The tertiary category is the one usually pictured when a blast injury is described. Being thrown. That happens, and it matters a great deal. But the more insidious injury arrives first, and it doesn't require contact with anything at all.
How Does a Pressure Wave Reach the Brain Through the Chest?
Here is the part that reframes how blast injury should be evaluated.
The supersonic pressure wave strikes the torso and compresses it very rapidly. Your chest contains air-filled and fluid-filled cavities — the lungs, the heart, the great vessels — and the wave compresses all of them in a fraction of a second.
That compression drives a pressurized surge through the body's fluid. As one review of blast pathobiology describes it, the blast front couples with the elastic body wall, compresses the abdomen and chest, and transfers its kinetic energy into the body's fluid phase, generating oscillating waves in the blood that travel to organs remote from the point of contact — including the brain (Cernak, 2010).
The brain, in other words, can be injured from the inside by a hydraulic surge, without the head ever being struck by anything.
It is worth being straight about the state of this evidence. The thoracic route was first argued as a formal hypothesis (Courtney & Courtney, 2009), and while it is now widely discussed and supported by animal work, it is not settled the way the impact mechanism is. What has moved from speculation to accepted is the broader point: blast reaches the brain systemically, not only through the skull.
What Does the Surge Do When It Arrives?
That pressurized wave of blood reaches the blood-brain barrier — the tightly regulated boundary that controls what is permitted to enter brain tissue from the bloodstream.
The barrier can be breached by it. And unlike a localized injury, this happens broadly rather than in one spot, producing a widespread leak. Substances that the brain is specifically designed to exclude gain entry.
In animal models of blast exposure, that disruption has been measured directly. Blast produces barrier permeability that persists for hours to days and follows an unusual open-closed-open pattern, most pronounced in frontal cortex and hippocampus (Logsdon et al., 2018). Repetitive primary blast has been shown to disrupt the barrier structurally, with loss of astrocyte end-foot coverage and accompanying neuroinflammation (Uzunalli et al., 2021).
Those are animal studies. Human confirmation of blast-induced barrier breakdown is not yet available at that level of detail, and I want to be clear that applying it to a person in front of me is an extrapolation.
What the extrapolation predicts is a fundamentally different kind of injury than a bruise on the surface of the brain. Not a focal area of damaged tissue, but a system-wide failure of the environment the brain depends on to function, occurring all at once. That pattern is close to unique to the blast mechanism — it does not typically happen in an impact injury at anything approaching the same prevalence.
What Happens to the Neck at the Same Moment?
The pressure wave is only part of the picture, because the tertiary component is arriving too.
The knockdown force of a blast can produce rapid angular acceleration and deceleration of the head and neck at extreme levels. The figure I use in teaching is on the order of 160 Gs. I should be clear that this is a working figure from the blast-injury material I present rather than a number I can point you to a single paper for.
Two comparisons put it in context, and [I've written about those thresholds in detail elsewhere](post-how-much-force-injures-your-neck-versus-your-brain.html). Reconstructed concussions in football cluster near 100 Gs of head acceleration, with 50%-risk estimates across methods spanning roughly 65 to 192 Gs. The threshold for beginning to tear the ligaments of the craniocervical junction — where your skull meets the top of your spine — is dramatically lower. The figure I work from clinically is roughly 4.5 Gs, and I flag it as my working number rather than a verified constant.
Even taking the most conservative version of those comparisons, a tertiary blast force lands well above the brain's tolerance and enormously above the neck's.
Which means a blast-exposed person can sustain, in a single instant: a breached blood-brain barrier from the pressure wave, potential direct brain injury from the impact, and severe structural disruption at the craniocervical junction. Three distinct injuries, three distinct mechanisms, one event.
Why Don't Standard Concussion Protocols Fit?
Concussion management was built around impact. Rest, time, graded return to activity. For an impact injury with an intact blood-brain barrier and a body given the conditions to recover, that framework is reasonable.
A pressure-wave injury that breached the blood-brain barrier is not going to follow the impact timeline, because it is not primarily an impact. The mechanism is different, so the trajectory is different, and a protocol calibrated to one will not describe the other.
This is why blast-exposed veterans so often appear to fail standard concussion recovery. It is almost never a failure of effort, motivation, or resilience — and telling someone otherwise is both inaccurate and damaging. It is a mismatch between the care and the injury actually sustained.
Why Do Blast Injuries Collapse the Timeline?
There's a second difference that matters for how these cases present.
After a typical head or neck injury, problems tend to develop in stages over time. Structural disruption comes first. Compromised brain health follows. Chemical and neurological decline follow that. The recognizable end-stage conditions arrive months or years later. [That delay is one of the most useful clues in the whole picture](post-why-the-timing-of-treatment-after-a-head-injury-changes-the-outcome.html), because it means there's a chain to interrupt.
Blast injuries don't move down that sequence. They jump it.
The pressure wave produces breakdowns in brain chemistry and structure simultaneously rather than sequentially. Which is why a blast-exposed patient can present at end-stage severity almost immediately, with a symptom picture that would normally have taken years to develop. It's the same cascade, collapsed into a single moment.
Why Is Recovery Slower After Repeated Exposure?
There's one more factor specific to repeated blast exposure — which describes breachers and others exposed many times over a career.
That population has been studied directly. In career breachers compared with matched controls, researchers found significant group differences spanning brain structure, blood biomarkers, and functional connectivity, alongside frequently reported performance deficits and symptoms (Stone et al., 2020). The study is cross-sectional and small, so it establishes that something measurable is different rather than proving what caused it.
Alongside the barrier disruption and severe autonomic dysregulation, repeated exposure appears to produce what I'd describe as neuroplastic entrenchment. The brain becomes locked into a cyclical pattern that is difficult to break out of. It behaves like a wagon wheel settled into a deep rut: getting out requires considerably more effort than staying in. That description is mine, drawn from clinical work rather than from a study.
That entrenchment is part of why blast-exposed patients are frequently resistant to mental health interventions that would help someone else. The intervention isn't wrong and the person isn't failing it. The pattern is being held in place by something the intervention doesn't reach.
What Does This Argue For?
None of this suggests abandoning existing care. It argues for matching the evaluation to the mechanism that actually occurred.
That means asking what kind of energy was involved rather than assuming impact by default, and then looking everywhere that energy went. For a blast, that includes the chest and the vascular route into the skull, and it includes the [craniocervical junction](condition-concussion-and-mtbi.html) the wave and the knockdown force both passed through — assessed with imaging built to show motion and stability, not only fractures and bleeds.
Veterans deserve care matched to the injury they actually sustained rather than to the injury a model assumes. That begins with recognizing that blast exposure reaches the brain by a route standard concussion management was never designed to address.
Did Blast Exposure Leave Symptoms Standard Care Never Resolved?
If you were exposed to an explosion and your recovery never followed the timeline you were given, the injury may have reached your brain by a different route — one that deserves an evaluation built around it, including the [upper neck](condition-whiplash.html).
Investigating the actual mechanism is the work we do at Cerebral. If you or a veteran you know is still struggling, we're here.
References
- DePalma RG, Burris DG, Champion HR, et al. Blast injuries. *New England Journal of Medicine*. 2005;352(13):1335–1342. https://pubmed.ncbi.nlm.nih.gov/15800229/
- Cernak I. The importance of systemic response in the pathobiology of blast-induced neurotrauma. *Frontiers in Neurology*. 2010;1:151. https://pmc.ncbi.nlm.nih.gov/articles/PMC3009449/
- Courtney AC, Courtney MW. A thoracic mechanism of mild traumatic brain injury due to blast pressure waves. *Medical Hypotheses*. 2009;72(1):76–83. https://pubmed.ncbi.nlm.nih.gov/18829180/
- Cernak I, Noble-Haeusslein LJ. Traumatic brain injury: an overview of pathobiology with emphasis on military populations. *Journal of Cerebral Blood Flow & Metabolism*. 2010;30(2):255–266. https://pubmed.ncbi.nlm.nih.gov/19809467/
- Logsdon AF, Meabon JS, Cline MM, et al. Blast exposure elicits blood-brain barrier disruption and repair mediated by tight junction integrity and nitric oxide dependent processes. *Scientific Reports*. 2018;8(1):11344. https://pubmed.ncbi.nlm.nih.gov/30054495/
- Uzunalli G, Herr S, Dieterly AM, et al. Structural disruption of the blood-brain barrier in repetitive primary blast injury. *Fluids and Barriers of the CNS*. 2021;18(1):2. https://pubmed.ncbi.nlm.nih.gov/33413513/
- Stone JR, Avants BB, Tustison NJ, et al. Functional and structural neuroimaging correlates of repetitive low-level blast exposure in career breachers. *Journal of Neurotrauma*. 2020;37(23):2468–2481. https://doi.org/10.1089/neu.2020.7141