For most of history, battlefield medicine was about treating only visible injuries. Cuts, fractures, and shrapnel wounds defined military trauma and critically impacted generations of soldiers. But the wars in Iraq and Afghanistan, characterized by increased use of improvised explosive devices (IEDs) introduced a new and invisible wound known as blast-induced traumatic brain injury (TBI). While more than 75% of these cases are classified as mild TBI, they typically lack visible or physical damage on patients through conventional scans and can lead to debilitating long-term symptoms such as headaches, memory loss, and post-traumatic stress disorder.
If there is no physical wound to analyze, what is happening inside the skull? To answer this, scientists have forgone traditional methods and looked towards new means of biomechanical research, using computer simulations, physical models, and even brain tissue taken directly from humans. This innovative, trifold, chronological journey is finally providing clear insights into the blurry history of the invisible wound.
The first major breakthrough came from advanced computer simulations, which are a powerful tool for studying invisible workings of the human body. In 2012, a study by Matthew Panzer used Finite Element Analysis (FEA) to simulate a blast wave interacting with the human head. His findings were revolutionary in that he showed that blast neurotrauma is less about the brain moving violently inside the skull and more so about a rapid change in its shape. The key factor was an incredibly fast speed of deformation, typically referred to in mechanics as a high strain rate, coupled with a change in the form of the brain, known as deviatoric strain. These forces are unique to blasts external from the body and help explain why the injury results in different side effects from a concussion.
With a theory established, the next step was to find physical proof of the symptoms. One hypothesis was that a blast wave could create bubbles in the cerebrospinal fluid, often referred to as cavitation, in which the collapse of these bubbles could damage brain tissue. A 2022 study by Xiancheng Yu and colleagues provided the first direct experimental evidence for this notion. In their study, a physical head model was exposed to blast waves in a controlled environment, which showed that a blast, unlike a blunt impact to the head, could produce cavitation.
The final step came from a study using human stem cells, essentially forming miniature versions of the brain grown in a lab. This allowed for the observation of mechanical and biological effects of blast waves on human brain tissue in a controlled environment. The 2022 study by David Brody subjected three different miniature brains to blast waves and found that a less severe blast temporarily stunned or froze the neural network, even though no cells died as a result. This finding provides a likely biological explanation for the symptoms, such as feeling dazed and confused, that patients suffering from blast induced trauma often report, but which have no detectable physical causes when scanned for in an MRI.
The journey to understanding traumatic brain injuries from blasts in war has moved from theoretical computer models to physical proof, and now towards explanations at the cellular level. This work is a critical step in developing more effective combat gear such as helmets for soldiers to use in battle, and will also aid in creating better diagnostic tools to help detect the invisible injury, thus ultimately providing better care for the veterans who have sacrificed so much for the sake of their country.
Featured Image from National Geographic and Peter Van Agtmael, Magnum Photos.