U.S. Army Rangers fire AT4s at a range at Camp Roberts, California. Stocktrek Images/Getty Images/Stocktrek Images hide title
When military personnel fire certain powerful weapons, they are affected by shock waves that rush blood from the body to the brain.
This "tsunami within the body" is one way the blast damages blood vessels that supply oxygen and nutrients to structures deep in the brain. Dr. Iberia CernakBlast injury specialist at Belmont University in Nashville.
"We're talking about the brainstem, we're talking about the cerebellum," Chernak said, "all well-cushioned brain structures."
In animals, there is now strong evidence that these structures may be damaged by disruption of blood vessels by blast exposure. For veterans, the impact is harder to measure.
Imaging studies conducted by the military found that "blood flow is affected in people with a history of occupational exposure to explosions," said Stephen Ahlers of the research center. naval medical research command.
These studies also show reduced brain volume, increased brain inflammation, and disruption of connections between neurons.
"There are these long-lasting cumulative effects," Allers said.
When it comes to brain injuries, an explosion involves more than just a blow to the head.
Head impacts tend to damage structures near the surface. The shockwaves continued.
The result can be impaired blood flow in areas such as the brainstem that control breathing, heart rate and blood pressure.
"Veterans may be coming to the doctor because of uncontrolled blood pressure," Chernak said. What doctors may miss, she said, is that "the problem is in the brain stem."
In addition to the surge of blood passing through the brain, shock waves tend to cause damage as they travel from one tissue to another.
"So at the boundary between the blood vessels and the brain, we're going to have shearing and potential structural damage," Senac said.
When the shock wave comes from a large explosion, such as a bomb, it can cause catastrophic hemorrhage. But repeated exposure to what the military calls low-intensity blasts, including those from rocket and missile launchers, appears to cause more subtle changes in blood vessels.
The blood-brain barrier is a particularly fragile structure, a protective layer of cells on the inner surface of blood vessels. These cells form a membrane that normally keeps toxins out of the brain while allowing oxygen and nutrients in.
but animal research Showed that exposure to repeated shock waves can damage the system.
"The blood-brain barrier starts to leak," Chernak said.
This leaky barrier makes the brain more susceptible to infection and more difficult to remove waste products. It also causes inflammation, which disrupts connections in the brain.
A damaged barrier reduces the flow of oxygen and nutrients to neurons (the cells that allow movement and thinking).
"Neurons are starving right now," Chernak said. "So we end up causing nerve cells to die."
The military became interested in low-intensity explosions about a decade ago.
Thousands of roadside bombs in Iraq and Afghanistan show that large explosions can cause concussions or worse without leaving any visible damage. So Ahlers and other researchers started thinking about smaller explosions.
They studied military saboteurs, who are experts at using explosives to clear obstacles or gain access to secure buildings. The study involved participants in a two-week saboteur training program.
Allers said a series of tests found the program had no effect on the students' brains.
"But we decided to commit five instructors to this study," he said. "They showed some psychological effects," including memory problems.
The results sparked a series of studies on low-intensity blasts targeting animals and military personnel.
Initially, researchers focused on damage to neurons. Recent studies have added other targets, including blood vessels.
Research shows that repeated exposure to low-intensity blasts causes many of the same brain changes as high-intensity blasts. However, it's unclear how much blast exposure or overpressure was a problem.
"We tend to focus on the largest weapons systems, which are shoulder-fired weapons that are very close to the head, and the weapons systems that have the most stress," Ahlers said. "But we're not quite sure what the threshold is."
At some point, however, blast exposure begins to affect what's called the neurovascular unit, a complex system that ensures brain cells get enough blood.
One of Ahlers' own Research The researchers looked at the protective layer of protein that lines the inner walls of blood vessels.
"After exposure to blast overpressure, it was essentially stripped away," he said. "It takes about a month to grow back."
Other changes may be permanent. For example, repeated exposure to blasts can damage the smooth muscle cells in blood vessels that regulate blood pressure and blood flow.
"Under normal conditions, you might not be able to tell a difference in blood flow," Ahlers said, "but when you're stimulated, you see the smooth muscles change the way they contract."
Muscle cells contract with less force.
Researchers are just beginning to understand the many ways low-intensity shock waves affect blood vessels. However, there are already signs that these changes may be profound and long-lasting.
2023 study For example, it has been found in mice that blast exposure can cause arteriovenous malformations, an abnormality in which veins and arteries become tangled and can lead to fatal bleeding.