Military Post-Traumatic Headache: The state of the science and what is being done to find more answers.
Andrew H. Ahn, MD, PhD
Key Points
- Very little is known about how a knock to the head from a military explosion, called a concussion, leads to post-traumatic headache (PTH).
- Experts base what is known about concussion and headache on indirect evidence.
- How brain trauma affects structures and chemical signals in the brain is the focus of current research
- Trying to understand blast-injury is a big challenge.
- Researchers will need more information from the affected soldiers to guide future animal and human studies
The problem of post-traumatic headache
A large number of soldiers experience problems with sleep, concentration, and constant headaches who have served in Iraq and Afghanistan. These problems are due to the large number of hard knocks to the head suffered by the soldiers. Experts call these hard knocks concussions when they cause a brief sense of being dazed or a loss of consciousness. Current military duty related concussions result most often from the blast of improvised explosive devices (IED’s). Experts do not completely know how blast-related concussion causes constant headache and these other problems.
If concussions are so common, why do we know so little about them? Currently, no accepted way to test for concussion is the single most important reason. Like civilian sports and motor vehicle accidents, blast-related concussion shows no visible brain injury on standard brain images. That is, an MRI of the brain that your doctor can order is usually normal. Also, because concussions are not fatal, there are no autopsy studies of concussion or of PTH. Thus no detailed brain facts exist to compare with the clinical problems. A major research priority is to measure the amount of injury from a concussion. When this research is successful a “biomarker” will exist.
Researchers have proposed various brain changes in concussion. These ideas result from advanced brain imaging methods used in small groups of subjects. Experts hope one day that these facts will lead to a biomarker for concussion. These imaging changes in the brain relate to chemical signals, blood flow, and the relative size of certain brain areas. These tests are in their earliest stages. We need to regard the results as tentative. Many more subjects need testing.
Clues from related conditions
Let’s consider two related conditions. Traumatic brain injury (TBI) is the first. With this injury the trauma is severe enough to cause visible brain injury. Brain contusion is one type of such injury. This bruise is a small blood vessel leak. However, unlike a simple bruise under the skin, a brain contusion can lead to long-term changes in the normal function of that part of the brain. Shear injury is another kind of TBI. Diffuse-axonal injury is another name. Shear injury in this case refers to the sudden jarring of the brain. This causes layers of the brain to slide past each other. Shearing in the brain causes a break in axons, the parts of nerve cells that form connections with each other. The detection of even a small amount of this type of brain injury predicts the presence of severe problems like sleep, mood changes, attention difficulties and headaches.
The experience with brain imaging of TBI has identified a common pattern of injuries. Though the pattern is not a perfect predictor, it does provide clues as to the parts of the brain most likely affected in the mildest cases. This pattern includes brain contusions at the surface of the brain directly under the area hit. In addition, there is sometimes an area of injury at the opposite side of the brain, called a contra coup injury. Bounce-back of the brain against the skull after the initial blow causes the contra coup. In addition, there is a vulnerable region of the brain, along the elongated lobes that sit alongside each side of the brain, called the temporal lobes. Their vulnerability is significant due to the importance of the temporal lobes in memory, emotions, and communication.
Blast-related concussion closely relates to sport-related concussion. Experts in the field of sports medicine accept that repeated concussions are bad. Sports that involve repetitive blows to the head, such as professional boxing, are proof of that. These blows have long been known to greatly increase the risk of severe and progressive problems later in life. These problems include memory, attention span, problem solving, speech and physical movement abnormalities. All of these problems occur in the elderly with dementia. Abnormal findings in the brain at autopsy occur so often in boxers that experts call this dementia pugilistica. More recent studies confirm that other athletes receive repetitive head trauma. The sports of football, hockey, X games, also place the athlete at high risk for brain injury. This injury goes by the term chronic traumatic encephalopathy, or CTE. The media has discussed CTE recently related to professional football and boxing. Among those with the clinical picture of CTE, the brain has strong similarities to the problems seen in Alzheimer’s and Parkinson’s disease. These changes include the abnormal build-up of proteins called beta-amyloid and tau, as well as another protein called TDP-43.
Animal models of concussion
In the laboratory, researchers have well-established experiments meant to copy the conditions of TBI in animals. However, because these tests produce a well-defined brain injury, they are for the most part not good tests for concussion.
The so-called cortical impact model involves a controlled blow of a weight onto an exposed brain surface. Because the researcher removes a portion of the skull this model is a very good way to produce a local region of brain injury. However, this test is a model of “open head injury” in which the force of the injury exposes the brain. By contrast, a concussion is by definition a “closed head injury.” The skull is still intact. The mechanism of how the energy of the injury is sent throughout the brain is very likely to be different. Another method, called fluid percussion, involves the spread of a fixed blow through a column of fluid. Researchers usually use an exposed brain; thus this is a form of “open head injury.”
There are a few animal models of “closed head injury.” One such model involves the controlled blow of a weight on an anesthetized animal held in a secured position. We all should regard these studies as preliminary. In any case, they remain only a gross likeness of the real-life conditions of a concussion.
Towards new test models of concussion
The ability to copy the conditions of an explosive blast in the laboratory is a technical challenge. One unique aspect of an explosion is the “primary blast wave.” This refers to the energy of the blast explosion first carried through the air. This blast wave is the rapid sequence of high pressure followed by a wave of very low pressure. The subsequent events, such as collisions with hard surfaces, the sudden deceleration with impact, etc, have similar injuries in the civilian setting. The understanding of the physics of blast injury on living tissues is poor at this time, but is the subject of intensive study.
There are still several basic questions that would go a long way towards understanding military PTH. For the time being, the most direct way to address these questions appears to be through further studies of the soldiers returning from combat (see article by Ann Scher). We all hope that these key insights will provide clearer direction on how to design laboratory models and how they lead to ongoing sleep, attention and headache and other symptoms.
Andrew H. Ahn, MD PhD, Assistant Professor of Neurology and Neuroscience, University of Florida College of Medicine, Gainesville, Florida