An explosion is, at its essence, a rapid release of heat and pressure. The physics are as complex as they are dispassionate; joules and kilopascals blend to impose grave injuries then categorized by mathematical calculations and arcane Latin. What you must truly understand about explosions though, is that none of that matters in the moment. It barely matters in the aftermath. What matters in the moment is the sudden overwhelming eruption of light and sound and pressure and heat amidst clouds of smoke and dirt and whirling steel. For the survivors, what matters next are the effects of heat and blast and metal fragments upon burned and tattered flesh. Orienting weapons for follow-on ambushes. Protecting against secondary blasts designed to catch those moving to aid the wounded and recover the dead. Maybe this is a single-device attack. Maybe this is just the start of a long day or night of fighting. You clarify the situation; identify the status of your force; ascertain the condition of your weapons and equipment. You prepare for whatever is coming next.

Nine of my Marines experienced it firsthand in December 2004. A Chinese anti-tank mine filled with 5.4 kilograms of explosive material designed to cripple an armored vehicle exploded under the front left tire of their metal and fiberglass Humvee. The blast pushed the floorboard to the steering wheel and launched the driver through the thankfully open roof of the vehicle, breaking his legs in multiple places, but sparing him a snapped neck. The truck and its remaining passengers slid 50 feet down an Iraqi street before they came to rest, the vehicle nose down in a huge roadside hole. 

Explosions are a common wartime experience, the nature of which has changed very little, even if the technology for delivering them has. British Pvt. Alfred Day experienced the same trauma firsthand in February 1916. The explosion of a trench mortar shell a mere five yards from his head left him bleeding in the mud near the river Somme, shrapnel wounds in his left thigh, his upper lip blooming like scarlet flowers. 

British gunners watching German prisoners, wounded and visibly distressed, passing after the taking of Guillemont. Chimpanzee Valley, near Montauban. (Photo: Wikimedia Commons)

Though 100 additional years of physics study more fully explain how explosions work, the experience of being hit by one hasn’t changed. You’re suddenly blind and deaf and struck stupid. The air rushes from your lungs as the force of the blast strikes your body like a surprise punch to the stomach from a trained fighter. You never saw it coming and you’re scared and mad and confused as the “fight or flight” half of your nervous system drives muscle memory to accomplish tasks for which your still-reeling brain is not yet prepared. Your heart rate and breathing skyrocket while you try to claw through what feels like being very suddenly drunk to get to a radio. A weapon. A wounded friend ejected from the vehicle with broken legs. Duty demands it. Duty drives everything in that moment. 

But once the immediate threat is rendered just one more in a long line of wartime explosions, the impact of injuries largely invisible at the moment — like the damage to Pvt. Day’s brain — begins metastasizing to stretch years beyond the actual event. The invisible supersonic ripple of air caused by the detonation of a high explosive, whether flung outward in all directions or diabolically focused to maximize the effect on flesh and bone, maims and kills without being seen by the naked eye. When a shock wave passes through a soldier’s skull, the physical damage in its wake is called traumatic brain injury, or TBI.

Though diminished from the height of operations between 2004 and 2012, such is life for American service members deployed to combat on the post-9/11 battlefields. Because the improvised explosive device (IED) has become ubiquitous in the wars in Iraq and Afghanistan, blast traumatic brain injury (bTBI) has been the impetus for much research. But it is critical to note that the rugged nature of military life means that the more general diagnosis of mild traumatic brain injury (mTBI), whether from single or accumulated TBIs, from blunt force blows and explosions, is shockingly prevalent. According to the Defense and Veterans Brain Injury Center, nearly 414,000 TBIs have been reported among U.S. service members worldwide since 2000. Most Americans can move on as the war dwindles, but biomechanists like my writing partner, Duke University’s Dr. Rachel Lance, will spend the rest of their careers studying TBI. And with 17 discrete TBIs during 27 years as a United States Marine, with incidents ranging from hard helicopter landings to difficult parachute jumps to blast exposure in training and combat, I may spend my life living it. 

Marine Raiders with 1st Marine Raider Battalion conduct free-fall training on Camp Pendleton, Calif., Dec. 7, 2017.

The wars that consumed the first twenty years of the 21st century are now shriveling in the American consciousness, ending from a seeming lack of interest rather than a treaty or a flag planted on a hill. With a presidential pledge to remove the last lingering troops from Afghanistan by Sept. 11, 2021, Americans will soon allocate my war to the history books alongside World War I. But as always, when the bands stop playing, there remains a population of current and former service members who must live with the aftermath. 

TBI is a condition as misunderstood by its victims as anyone. When related to the brain, the word trauma may give rise to concerns of mental illness. Especially in military culture, which values stoicism and stability, such concerns make service members loathe to report anything that may affect their duty status. Thus, those suffering symptoms that accompany TBI are often reluctant to acknowledge the common symptomology caused by the very service they seek to continue: headache, confusion, amnesia surrounding the time of the event, short-term memory problems, difficulty concentrating, mood alteration, difficulty sleeping, and anxiety. This unfounded stigma is so strong that articles, like this one, may even choose to use case reports from WWI such as Pvt. Day’s rather than contend with living soldiers’ reluctance to talk due to fears of being labeled as “crazy.”

By the time the trench mortar blew up next to him in February 1916, Alfred Day had already been hit in the head twice. The blunt force impacts to his skull during his previous 15 months of combat resulted in lacerations crisscrossing his scalp, but the stalwart volunteer of the West Yorkshire Regiment headed back into battle both times after the bleeding stopped. However, the trench mortar proved his undoing. He began to report a hand tremor. He stopped sleeping and began struggling with memory. Even his pupils became sluggish in their reflexive response to light, betraying that his “shell shock” could not be purely psychiatric. Given his distance from the blast, we now know that there was a calculable 40 percent chance that the trauma was severe enough to spread mild cobwebs of blood across small sections of Day’s brain, in addition to the more obvious shrapnel wounds to his leg and face. 

A shrapnel-damaged sign from World War I, now on display at the Imperial War Museum in London.

The possibility of TBI in my own brain was identified in a far less dramatic fashion. My brain scans appeared healthy most of the time during testing at my command, as displayed in green. But a portion of my brain was colored red and orange. Reviewing those results, a cognitive psychologist asked if it was possible I’d suffered exposure to blast injuries in the area behind my left ear. I laughingly asked: “Do about thirty-five different incidents of plastic explosive detonating within thirty feet of my head count?”

For a Marine officer and Marine Raider, even one in his third decade of service, there is an expectation of peak physical and mental performance. By September of 2019, it wasn’t happening anymore, at least not at a level that was acceptable for me. I had none of Pvt. Day’s more dramatic symptoms, no hand tremors or semi-catatonia. But I found myself having difficulty managing all the complexities I’d once handled with ease, an ability long a point of pride for me and a requirement central to virtually every assignment. Had Day come forward 103 years, we could have traded knowing looks about insomnia and memory issues. Attempting to maintain multiple schedules, coordinate complex and varied administrative requirements, and perform higher-order thinking, I felt as if my mind were shrouded in mist. I began tasks, inevitably got called away to deal with issues associated with helping to lead a large unit of worldwide deployable Marines, then later found myself confused about what I had been doing or why. 

Recovering from the physical training that is part and parcel of Marine life was harder too. Of course, I joked with my fellow older Marines about the chronic pain of years of accumulated injuries. Neck and back pain. Knees and ankles. Achilles tendonitis. Torn rotator cuffs. Where two years before I had been running ultramarathons up to 100 miles, now I could not walk down stairs without turning and moving sideways, like a toddler learning to master them. I had no motivation to run beyond the minimum expectation because I had to run through the pain, then deal with increased pain caused by running. I trained in the morning, as expected, then limped through the rest of the day with concerned Marines asking me what was wrong with my leg. I have never been thin, but without endurance running, I gained weight, a cardinal sin in the Corps. More critically, lengthy runs had long been my mental therapy. Without those long runs, I had a generally darker mood shift that settled in and became the normal backdrop to my days.

U.S. Marine Corps Raiders with the 3d Marine Raider Battalion fire an M224 mortar at Eglin Range, Fla., May 22, 2018. (U.S. Air Force photo by Senior Airman Joseph Pick) (Portions of this image were obscured for security reasons)

Worse yet, my symptoms affected my family. I was chronically tired, in pain from my injuries, and subconsciously angry and depressed about it all. After the war, Pvt. Day referred to his own sleep as “disturbed,” “delayed,” and sometimes “terrifying.” Those words applied to my own as I averaged about five hours per night. By 2019, it had been 15 years since I had slept normally. Three to four-hour nights of broken sleep were not uncommon. Correspondingly, my patience was thin enough after I got through a day at work that I was too grouchy and impatient to truly enjoy the fleeting joys that come with the blessing of being the father of a precocious, lively, creative 9-year-old daughter and husband to a brilliant, engaged, and hardworking wife. That felt like failure, anathema in the success-driven military culture to which I’ve belonged since my late teens. I sought help for insomnia during an assignment at the Pentagon. After seeing no improvement, my doctor told me it would probably get better when I retired. Retirement was at least three years away. The thought of slogging through that seemingly interminable distance made me feel figuratively mired in the same mud in which Pvt. Day lived at the Somme.

In 1916, Day’s physicians could not give him any solutions. But now there is hope that affected servicemembers can overcome TBI symptomology as well as regain specific functions, despite any scars buried in their neurons. Although the neurons of the brain do not heal, the good news is that the brain is remarkably flexible, a phenomenon called neuroplasticity. Amputees automatically repurpose the chunks of the brain that used to pilot now-missing limbs. Brain surgery patients can task new bits to re-learn languages or skills lost to accidental damage after removal of tumors. It is possible to recover functions after a blast TBI, too, even if the original neurons themselves do not revitalize. Military researchers are working on better chemical interventions to prevent TBI before it happens and learning new ways to regenerate damaged neurons for the warfighters of the future. In the meantime, there are skilled clinicians who can help retrain injured brains like mine.

After a standard neurological assessment for special operations personnel resulted in a possible TBI diagnosis, my doctor referred me to the five-week Return to Forces Program at Intrepid Spirit at Camp Lejeune, North Carolina. Largely funded by private donations, Intrepid Spirit is an interdisciplinary program maintained aboard eight U.S. military bases with two more to come in the future. Intrepid Spirit offers “a comprehensive means of delivering treatments [such as] neurology, nutrition, physical therapy, speech and hearing therapy, psychiatry, occupational therapy, speech pathology, neuropsychology, ophthalmology, and nursing [as well as] integrated health treatments, including acupuncture, yoga, mindfulness, art and music therapy.” 

Mr. Arnold Fisher, Honorary Chairman of the Intrepid Fallen Heroes Fund (center), cuts the ceremonial ribbon during the opening of the National Intrepid Center of Excellence (NICoE) Satellite Center aboard Camp Lejeune, N.C., Oct. 02, 2013. The NICoE helps veterans deal with battle-related injuries such as traumatic brain injury and post-traumatic stress. (U.S. Marine Corps photo by Lance Cpl. Andre Dakis, Combat Camera, MCI-East, Camp Lejeune/Released)

Part of Intrepid Spirit requires reprogramming how service members respond to questions about their health. Conditioned to accept pain and discomfort, to put mission and peers ahead of self, and often combat-experienced in Iraq and Afghanistan, we reflexively minimize our injuries in light of whatever the person next to us is dealing with. And it is a lot. As the war in Afghanistan pushes towards 20 years old, I’ve lost count of the service members I know with sleep and memory issues. Owing largely to TBI and post-traumatic stress (PTS), I know men and women who have not slept more than 60 minutes at a stretch in a decade or more; lying awake all night or checking doors and windows and sleeping family members before fighting to stay awake at work the next day. These are stable, reliable people who have seen friends killed in horrific ways and have themselves killed with guns and rockets and mortars and bombs. Yet they carry on normally and ensure no one knows what burdens they actually bear. Much is made of PTS, and rightly so. But Intrepid Spirit focuses on TBI and in that I found answers to questions I’ve been asking for years. Like me, to understand how TBI patients get better, you must first understand how we get hurt. 

The invention of the high explosive TNT and its major combat debut in World War I meant that the first time medics and physicians saw the symptoms of bTBI was inside mud-filled trenches. In those early years, bTBI was described with a nebulous shoulder shrug by overwhelmed doctors, the symptoms lumped under vague, unhelpful terms like “shell shock,” “neurasthenia,” or “commotion.” However, the human brain has not changed in the last 100 years, so while the injuries and experiences of today’s soldiers may be similar to those of Pvt. Day, a century later we are finally starting to explain what these traumas do to the tissues inside our skulls. 

Understanding TBI requires understanding what does — and does not — occur inside the skull during an impact or an explosion, which means the first theory to address is “brain slosh.” Before a shock wave can hit the brain it has to pass through the skull and a layer of liquid called cerebrospinal fluid, or CSF. Although the space between the brain and skull looks empty on an MRI, it’s filled with fluid, a mechanical cushion for the brain keeping it cocooned securely inside the skull. Since the brain and the CSF are nearly the same density, it’s not like a water balloon rattling inside a jug; it’s more like a water balloon in a jug filled with other water balloons. The brain is more likely to spin, albeit ever so slightly, than it is to move linearly, and rotation, not linear motion, is shown to be the driver behind TBIs from blunt force trauma.

The theory that the brain ping-pongs wildly back and forth inside the skull, so-called “brain slosh,” has never been supported by a single piece of experimental data. The theory’s lead modern proponents, who popularized the use of the term within the last eight years, are also financially invested in selling the invention they claim will prevent it.

(Left to right) Colonel Andrew R. Milburn, the outgoing Marine Raider Regiment commander, Col. Peter Huntley, the oncoming Marine Raider Regiment commander, and Maj. Gen. Joseph L. Osterman, Marine Corps Forces Special Operations Command commander, salute the colors during the pass and review of the MRR change of command ceremony at the MARSOC headquarters at Stone Bay, aboard Marine Corps Base Camp Lejeune, N.C., June 26, 2015.

Most people think of the brain as a squishy, pink, sponge, but trying to compress the brain into a smaller volume is like trying to compress a ball of gelatin. Real brains deform, not by getting smaller in volume like foam, but more like a stack of paper: smack your hand down on a stack of paper, and it doesn’t dramatically squish, change volume, or bounce back. But place the hand on top and pull to the side, and the stack slides apart in that direction. This type of deformation is officially called “shear,” but when it occurs it looks like gelatin wobbling. 

The human brain is over 737,000 times more willing to shear than it is to compress, which means that when the skull is impacted by any force, the brain behaves more like a stiff, macabre Jell-O mold than a bouncing rubber ball. The shear theory says that when a shock wave enters the skull, or when the skull is hit hard enough, the brain eagerly responds with the slightest of wobbles while still in place; no ricochet motion required. Thus, the damage is done. 

Shear wreaks such havoc because of the shape of the brain’s cells. Most cells in the body are blobs, shaped somewhere between a sphere and a brick. In contrast, neurons look more like leafless sunflowers. The neuron’s large main “flower” body, called a soma (somata for the plural), has narrow, jagged “petals,” referred to as dendrites, connecting neighboring neurons to one another so they can work in groups. But also like a sunflower, each main body has one elegant, fragile, disproportionately long stalk called an axon. The surface of the brain is covered in these nerve “flowers,” which when clumped together look greyish — this is grey matter. Those main bodies covering the surface shoot their long axons inward toward the spinal column. The axons, when clumped together like the stalks of a bouquet, look white — they are the white matter. 

Protected deep inside the core of the brain are the primeval bits that control the functions we need to survive, like breathing, which is why so many TBI victims walk away afterward. But in the brain’s outer shell live the neuronal bodies whose functions operate our fine motor control, our speech, our memories, our personalities. 

If shear is the cause of TBI, the brain’s wobble tears or at least over-stretches the fragile axon-stalks that run from that outer shell all the way through the brain. It’s the leading theory; however, science is still searching for the evidence that draws the final line between the laboratory experiments and the symptoms of the live soldiers in the field.

In severe cases of TBI, blood vessels on the surface of the brain can rupture and cause bleeding, or shear motion can disrupt sufficiently massive swaths of axon stalks in trauma labeled a Diffuse Axonal Injury (DAI). The long axons can tear like fragile threads attached to every sheet of paper in the sliding stack. The whole brain immediately rebounds back into shape, but once the axons are torn or even stretched beyond their functional limit during the wobble, the damage is done. The body can regenerate peripheral nerve cells, such as those in our limbs, but these central neurons do not heal. Even if the somata stay intact, they are now screaming messages into a void, with no pathway to carry those messages to the rest of the body. Eventually, they give up and go silent. Yet for most mild to moderate exposures, there is no visual trace of the lightning-fast insult and the damage it does. Since the brain has 86 billion neurons, it’s actually possible for a couple to die off harmlessly without being missed. But after a larger blast, or if damage accumulates through multiple injuries, the number of neuronal casualties can lead affected service members and their physicians to see the signs and symptoms of these injuries. 

After 15 months of trench combat, two blows to the head, and one explosion, Pvt. Alfred Day began to experience his TBI symptoms. After 27 years, three combat deployments, and 17 identified TBIs, I experienced mine. Unfortunately for Day, there was no Intrepid Spirit. But for me, Intrepid Spirit was not only there, it changed my life. Over five weeks, I was introduced to or had re-emphasized a host of therapies. The first week was entirely diagnostic. I met with a general practitioner, a neurologist, a psychologist, a neuropsychologist, a psychometrist, physical therapists, a chaplain, a music therapist, an art therapist, a nutritionist, and other supporting practitioners. Then, as individuals or in various group sessions, we met with practitioners and got to work for four weeks. 

A U.S. Marine with 3rd Marine Raider Battalion, Marine Corps Forces Special Operations Command (MARSOC), posts security during an urban operations raid on Camp Lejeune, N.C., Nov. 17, 2016. (U.S. Marine Corps photo by Cpl. Christopher A. Mendoza, 2d MARDIV Combat Camera)

My motto at Intrepid Spirit was, “I came here to say yes.” With a correspondingly open mind, I met for Cognitive Behavioral Therapy to specifically address insomnia. I had acupuncture. I sat through audiovisual entrainment to calm my brain and alpha wave stimulation to focus it. Physical therapists evaluated my balance and pain levels and used cupping techniques that left my back covered in round bruises but reduced stiffness and increased my mobility. To her great fascination, I told my daughter I’d gotten the marks fighting a giant squid then told her the truth when I ordered my own cupping set and asked her to apply it to my back and neck. After a week, my neurologist asked if I’d try a non-narcotic prescription medicine for pain. A week later, with my pain reduced, I began sleeping six to seven solid hours of restful sleep for the first time in 15 years. And I ran again. I ran every day for seven straight days, just to see if I could. My wife saw my mood improve and become more consistent, a predictability key to the happy, stable home in which I want my daughter to grow up. I noticed that on my hour long commute, I no longer had road rage. I tried to. I screamed curse words at someone who pulled out in front of me, forcing me to skid to a stop. Then I realized there was no heat behind my words and I resolved to just not do it anymore. Realizing how much anger and pain I no longer had illuminated how much I had been carrying. I was overcome with guilt and remorse when I realized my daughter, born while I was in Afghanistan, had never known me without insomnia or pain or anger. But the Intrepid Spirit staff helped me acknowledge that while I cannot change what is done, I can be better moving forward. I can feel better. I can act better. I can perform better. By taking advantage of my brain’s neuroplastic ability to re-learn the functions I need, I can emphasize positive functions like joy and enthusiasm and patience.

After his blast, Pvt. Day’s long list of symptoms left him 30 percent disabled by the estimate of the British Army. Relegated to alternate duty, he served his country honorably for the remainder of the war. But over time, even without any of our modern treatment methods at his disposal, Day’s brain did what brains do best: it adapted. By the time of his death, the severity of his disability had been downgraded to 6 percent. He lived a fairly normal life. He got married. It’s impossible to tell from his medical records alone, but hopefully, he was happy. I liken my own experience to working in a room with dirty windows and poor lighting. My room has not changed, but about 60% of the way through Intrepid Spirit, someone washed the windows and changed the bulbs and now I can see clearly enough to do all the things I want and need to do. I can once again enjoy, rather than endure, the prosaic moments that are the subtle joys of life. 


Russell Worth Parker is a United States Marine Corps Special Operations Officer retiring to write and teach. His more than 27 years of service included infantry and special operations assignments in Iraq and Afghanistan. He is grateful for the work of scientists like Rachel Lance and the expert medical and administrative staff at the Intrepid Spirit TBI Clinic at Camp Lejeune, North Carolina.

Rachel Lance, Ph.D., is a research professor at Duke University. She specializes in the biomechanics of trauma and survival in extreme environments, including blasts. Worth Parker found her online following her publication of an article about blast lung, and they decided to write this piece together to provide a similar explanation of TBI.

A note on sourcing from the authors:

In general, please make sure that any online articles providing medical insight cite peer-reviewed academic articles to support their claims, as the internet is chock full of both information and misinformation. The theories of shear waves and cavitation are buttressed every day with new data that they are likely the truth. More peer-reviewed academic publications to support this article, in addition to those linked in the text, can be found at these links.

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