U.S. Army Rangers fire an AT4 at a range at Camp Roberts, California.
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When soldiers fire certain powerful weapons, they are exposed to a shock wave that sends blood from the body to the brain.
This “tsunami in the body” is one of the ways an explosion can damage the blood vessels that supply oxygen and nutrients to deep structures in the brain, according to Dr Ibolja Cernakan expert on blast injuries at Belmont University in Nashville.
“We’re talking about the brainstem, we’re talking about the cerebellum,” Cernak says, “all very well-padded brain structures.”
In animals, there is now strong evidence that these structures can be damaged by exposures to blasts that disrupt blood vessels. Among veterans, the effects are more difficult to measure.
Imaging studies by the military have shown that “blood circulation is affected in people who have been exposed to blasts during their careers,” says Stephen Ahlers, of the Naval Medical Research Command.
These studies also suggest a decrease in brain volume, an increase in brain inflammation, and a disruption of connections between neurons.
“There are long-term cumulative effects,” says Ahlers.
From explosion to blood to brain
An explosion is not just a blow to the head when it comes to a brain injury.
An impact to the head tends to injure structures near the surface. A shock wave continues.
The result can be impaired blood flow to an area like the brainstem, which controls breathing, heart rate and blood pressure.
“It could be that the veteran is coming (to the doctor) because of uncontrolled blood pressure,” Cernak says. What a doctor may not know, she says, is that “the problem lies in the brainstem.”
In addition to the rush of blood to the brain, a shock wave tends to cause injury when it passes from one type of tissue to another.
“So at the boundaries of the blood vessels and the brain, we will experience shearing and potentially structural damage,” says Cernak.
When the blast wave comes from a large explosion, such as a bomb, it can cause catastrophic bleeding. But repeated exposure to what the military calls low-level blasts, including those from rocket launchers and missiles, appears to cause more subtle changes in blood vessels.
One particularly vulnerable structure is the blood-brain barrier, a protective layer of cells lining the inner surface of blood vessels. These cells form a membrane that generally prevents toxins from entering the brain while allowing oxygen and nutrients to enter.
But animal studies show that exposure to repeated blast waves alters this system.
“The blood-brain barrier starts to leak,” says Cernak.
This leaky barrier makes the brain more vulnerable to infection and less able to eliminate waste. It also leads to inflammation, which can disrupt connections in the brain.
A compromised barrier reduces the flow of oxygen and nutrients to neurons, the cells that enable movement and thought.
“The neurons are now starving,” explains Cernak. “So we end up with neural cell death.”
Blast: a brief history
The military became interested in exposure to low-intensity explosions about ten years ago.
Thousands of roadside bombs in Iraq and Afghanistan showed that a large explosion could cause a concussion or worse without leaving visible injuries. So Ahlers and other researchers began to wonder about the small explosions.
They conducted a study on military land clearers, specialists who use explosives to remove obstacles or gain access to a secure building. The study involved participants in a two-week hacker training program.
A battery of tests revealed no impact on the brains of students in the program, Ahlers says.
“But we had five instructors that we kind of decided to include in the study,” he says. “They are the ones who showed psychological effects”, notably memory problems.
The result led to a series of studies on low-intensity explosions in animals and military personnel.
Initially, researchers focused on damage to neurons. More recent studies have added other targets, including blood vessels.
Research has shown that repeated exposure to low-intensity blasts can cause many of the same brain changes seen with high-intensity blasts. However, it is still unclear to what extent exposure to blast, or overpressure, constitutes a problem.
“We’ve tended to focus on the larger weapon systems, the long guns where they’re very close to your head and the ones that have the most overpressure,” Ahlers says. “But we don’t really know what the thresholds are.”
However, at some point, exposure to blasts begins to affect what is known as the neurovascular unit, a complex system that ensures that brain cells receive enough blood.
One of the Ahler studies examined a protective layer of proteins in the lining of blood vessels.
“After exposure to excess blast pressure, it is virtually removed,” he says. “It takes about a month for it to grow back.”
Other changes may be permanent. For example, repeated exposure to blasts impairs the smooth muscle cells that allow blood vessels to regulate blood pressure and flow.
“You may not be able to distinguish differences in blood flow under normal conditions,” says Ahlers, “but when you stimulate, you see that the smooth muscles have changed the way they contract.”
Muscle cells contract with less force.
Researchers are just beginning to understand the many ways that low-intensity blast waves affect blood vessels. However, there are already indications that the changes could be profound and permanent.
A 2023 study in rats, for example, it was found that exposure to an explosion could cause arteriovenous malformation, an abnormal tangle of veins and arteries that could cause fatal bleeding.
