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We've been hearing a lot about concussions, recently, most of it related to football players. Football teams have become more cautious about putting players back in action after theyve suffered a concussion. Now, brain injury experts are trying to take that approach from the football field to the battlefield.

NPR's Jon Hamilton reports that those experts also have made some surprising discoveries, about how energy from an explosion reaches the brain and causes damage.

JON HAMILTON: An enemy truck packed with artillery shells and gasoline caused Jake Mathers first concussion in Iraq. Mathers was in a firefight near Ramadi when the truck blew up.

Mr. JAKE MATHERS (U.S. Marine Corps): All I saw was sparks and then I went out like a light. And I woke up and my nose was bleeding, my ears was bleeding, you know, like my tear ducts were bleeding.

HAMILTON: Mathers' body was intact, though. So, he stayed with his Marine unit and suffered another concussion.

Mr. MATHERS: Oh, there were several. An RPG hit my truck one time. I got knocked out. And another time, we were going down this street called Ice Cream, probably the worst street in Ramadi, one of them. And one of the guys shot an RPG and it hit the hood of our vehicle and blew up.

HAMILTON: Mathers says he can't remember how many concussions he had during his seven-month tour. He has trouble remembering a lot of things, now that he's back home in Monroe, Louisiana.

Mr. MATHERS: I'll lose my pack of smokes like three or four times a day and I'll buy different packs of smokes 'cause I forget, you know, that I bought 'em. Or sometimes I'll be driving down the road and I'll forget where I'm going or lose my car keys, cell phone or...

HAMILTON: Then there are the headaches, the nightmares, the problems sleeping. David Hovda directs the Brain Injury Research Center at UCLA. He says Iraq and Afghanistan have produced a lot of people like Mathers - people who had multiple concussions in a short period of time. He says each brain injury is usually the result of a relatively small explosion.

Dr. DAVID HOVDA Brain Injury Research Center, UCLA): Which, when it occurs close to an individual, can produce a concussive wave, or a blast wave, that actually moves the head and body very violently and that can in fact cause a concussion.

HAMILTON: And Hovda says brain scans show that for days or even weeks after an injury like this, the brain's metabolism slows down, which leaves some cells starved for energy.

Dr. HOVDA: During the time when this metabolism is altered, the brain not only is dysfunctional, but it's also extremely vulnerable. So that if it's exposed to another mild injury, which normally you would be able to tolerate really well, now there can be long-term devastating consequences.

HAMILTON: It's the second hit or the third that often does the damage. So Hovda and others have been urging the military to act more like the NFL and order troops off the field when they've had a head injury.

That message seems to be getting through. A few months ago, Admiral Mike Mullen, chairmen of the Joint Chiefs of Staff, announced a policy saying troops near a bomb blast MUST be removed from combat for 24 hours and must be checked for traumatic brain injury. Experts on brain injury say implementing this policy is important for several reasons. Hovda says one is that concussions often cause fuzzy thinking.

Dr. HOVDA: If you're going to call in a mortar strike, you have to do some fancy math and some really sophisticated calculations in your head, and you could create a problem if this isn't done correctly.

HAMILTON: Another reason to aggressively look for brain injury is that it's very hard to tell who might have sustained one. Ibolja Cernak studied brain injuries on the battlefields of Kosovo. Now she works at the Johns Hopkins Applied Physics Laboratory in Maryland. She says research in animals shows that shock waves from a blast can cause brain damage even if the head is completely protected. The shock waves compress the abdomen or chest wall violently, she says, transferring the bomb's kinetic energy to blood and other fluids.

Ms. IBOLJA CERNAK (Johns Hopkins Applied Physic Laboratory): So this kinetic energy now inside the body generates oscillating pressure waves which travel everywhere, to the organs, but also to the brain.

HAMILTON: Where they may kill or damage brain cells. Cernak says shock waves also can reach structures deep in the brain that control things like blood pressure, balance, and speech. And she says the blast can cause a breach in the so-called blood-brain barrier, which usually protects the central nervous system from substances that could cause dangerous inflammation.

Ms. CERNAK: These inflammatory molecules and mechanisms start to give irreversible damages and changes in the brain, very comparable to Alzheimer.

HAMILTON: Repeated injuries in a short period of time greatly increase the risk that will happen. Jake Mathers, the Marine, says people probably ignored concussions until recently, because they don't leave any visible scar.

Mr. MATHERS: It makes me a little mad. You know people are like, oh, well you seem fine. Well, you know, I'm not. And yeah, you know, it's not like I'm missing a leg. But I'm missing something that I can never get back.

HAMILTON: Mathers says he hopes his story will help protect the next generation of fighters.

Jon Hamilton, NPR News.

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