Explosions from military weapons can injure the brain through its blood vessels: gunshots

When military personnel fire certain powerful weapons, they are exposed to a pressure wave that causes blood to flow from the body to the brain.

This “tsunami in the body” is one way an explosion can damage blood vessels that supply oxygen and nutrients to structures deep in the brain, he says Dr. Ibolja Cernaka blast injury expert at Belmont University in Nashville.

“We’re talking about the brainstem, we’re talking about the cerebellum,” Cernak says, “all the very nicely padded brain structures.”

In animals, there is now strong evidence that these structures can be damaged by blasts that destroy blood vessels. For veterans, the impact is more difficult to measure.

Imaging studies conducted by the military have shown that “blood flow is impaired in people who have been exposed to blasts in their occupational past,” says the military’s Stephen Ahlers Naval Medical Research Command.

These studies also suggest a reduction in brain volume, an increase in brain inflammation, and disrupted connections between neurons.

“There are these long-lasting cumulative effects,” says Ahlers.

From explosion to blood to brain

An explosion isn’t just a blow to the head when it comes to a brain injury.

A head impact tends to injure structures near the surface. A pressure wave continues.

The result can be impaired blood flow to an area such as 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,” says Cernak. What a doctor might miss, she says, is that “the problem is in the brain stem.”

In addition to the rush of blood through the brain, a pressure wave can cause injury as it passes from one type of tissue to the next.

“So there will be shearing and possibly structural damage at the boundaries of the blood vessel and the brain,” says Cernak.

If 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-pressure explosions, including those from rockets and rocket launchers, appears to cause more subtle changes in blood vessels.

One structure particularly at risk is the blood-brain barrier, a protective layer of cells that lines the inner surface of blood vessels. These cells form a membrane that normally keeps toxins out of the brain while allowing oxygen and nutrients to enter.

But Animal studies show that exposure to repeated pressure waves affects this system.

“The blood-brain barrier starts to leak,” says Cernak.

This leaky barrier makes the brain more susceptible to infection and less able to remove waste products. 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 thinking.

“The neurons are now starving,” says Cernak. “So we are dealing with the death of neuronal cells.”

Explosion: A Short History

The military interest aroused with low explosion impact about a decade ago.

Thousands of roadside bombs in Iraq and Afghanistan had shown that a large explosion could cause a concussion or worse without leaving visible injuries. So Ahlers and other researchers started thinking about smaller explosions.

They conducted a study of military breachers, specialists who use explosives to clear obstacles or gain entry to a secured building. The study involved participants in a two-week security breach training program.

A series of tests found no effects on the brains of the program’s students, Ahlers says.

“But we had five lecturers that we just wanted to include in the study,” he says. “They were the ones who showed some psychological effects,” including memory problems.

The result led to a series of studies on small explosions in both animals and military personnel.

Initially, the researchers focused on damage to neurons. More recent studies have added additional targets, including blood vessels.

Research has shown that repeated exposure to low-level explosions can produce many of the same brain changes seen with high-level explosions. However, it is still not clear how much blast loading or overpressure is a problem.

“We tended to focus on the largest weapon systems, the shoulder weapons, where they are very close to the head, and the ones that apply the most excess pressure,” says Ahlers. “But we’re not entirely sure what the thresholds are.”

At some point, however, the explosion begins to affect the so-called neurovascular unit, a complex system that ensures that brain cells receive enough blood.

One of Ahlers Studies examined a protective layer of proteins in the lining of blood vessels.

“After it’s exposed to the blast pressure, it’s essentially stripped away,” he says. “It takes about a month for it to grow back.”

Other changes may be permanent. For example, repeated blast exposure affects the smooth muscle cells that allow blood vessels to regulate blood pressure and blood flow.

“Under normal conditions, you may not be able to see the differences in blood flow,” says Ahlers, “but when you stimulate, you can see that the contraction of the smooth muscle has changed.”

The muscle cells contract with less force.

Researchers are just beginning to understand the diverse effects of low pressure waves on blood vessels. However, there is already evidence that the changes may be profound and lasting.

A 2023 study In rats, for example, it has been found that blast exposure can cause arteriovenous malformation, an abnormal intertwining of veins and arteries that can cause fatal bleeding.