WASHINGTON -- After more than a decade of war, the U.S. military cannot precisely diagnose concussion-related brain injuries resulting from the bomb blasts that are the most common cause of combat casualties.

The mechanism of the injury -- how the shock of blast can cause lingering headaches, dizziness, memory loss and insomnia -- is not understood either, according to the Pentagon's top expert on Traumatic Brain Injury (TBI). Without a precise diagnosis or understanding of the injury, the treatment offered by the Pentagon consists mainly of rest and reassurance.

As a result, tens of thousands of Americans who have served in Iraq or Afghanistan may have been injured but were never diagnosed properly -- or at all. They may be unnecessarily worried for their health. Or, they could be lacking the kind of therapy and other treatment that might help prevent more severe problems later in life.

Most of those who experience a blast-related brain injury recover relatively quickly. Defense Department experts say soldiers will suffer no consequences of the injury. But while some researchers suggest that the lifetime consequences of a blast-related brain injury could be significant, there are no long-term studies on the effects of concussion on combat veterans.

Moderate and severe brain injuries can be readily diagnosed with brain imaging scanners, which can detect lesions in the brain or other physical damage. But brain injury caused by concussion, which the military labels mild Traumatic Brain Injury, or mTBI, is far more common, often leaving the injured soldier dazed and unable to function properly. But while mTBI is serious, it leaves behind no obvious physical damage, making it impossible to diagnose directly.

Only by asking the injured service member "What happened?" can medics and doctors begin to piece together a guess that the trooper has suffered a concussion and could have mTBI. But concussions can result in short-term memory loss, or even brief unconsciousness.

So the current testing is "Not good enough," said Katherine M. Helmick, the Pentagon's senior TBI expert. "With moderate and severe TBI we have very clear markers. With mTBI what we have is a story. We are asking [the wounded] to remember what they can't remember! It's very faulty."

Quick and accurate diagnosis of mTBI is critical, Helmick said, so that battlefield medics can pull injured soldiers off duty to prevent a second injury. Current military policy is for medics to quickly determine if a soldier has had a concussion, and if so to order him to a safe place for rest and observation for a day or two.

Helmick is deputy director for TBI, Defense Centers of Excellence for Psychological Health and Traumatic Brain Injury, which oversees clinical prctice and research related to Traumatic Brain Injury.

Without a clear idea of how concussion affects memory, concentration, balance, sleep and other brain functions, doctors and therapists have found that the best treatment is rest -- and reassuring the patient that he or she will get better.

"We don't have pills that are FDA-approved for TBI, we don't have a bunch of therapies that are FDA-approved or supported for TBI," Helmick told The Huffington Post. "What has been shown to be an effective treatment is letting them know they have had a concussion, that this is what you can expect and you will get better."

The most effective treatment for mTBI, she said, is "if you set that expectation, that they will improve."

Recognizing the drawbacks of current diagnosis and treatment of mTBI, the Pentagon and the Department of Veterans Affairs (VA) this week announced a new $100 million research effort to find better ways to diagnose and treat mTBI and Post Traumatic Stress Disorder, a form of combat trauma with many of the same symptoms as mTBI.

"PTSD and mTBI are two of the most prevalent injuries suffered by our warfighters in Iraq and Afghanistan, and identifying better treatments for those impacted is critical," Dr. Jonathan Woodson, assistant defense secretary for health affairs, said in a statement .

For some, the research will come too late.

Jaime M. Beavers was a 26-year-old private first class when he was deployed into combat in Iraq. He spent 27 months at war over two tours; he was exposed to IED blasts six times, including one explosion that knocked him unconscious. He was given medication to help him cope with anxiety, pain and sleeplessness, but he was never diagnosed with TBI until he was dismissed from active duty for drug abuse and eventually made his way to a VA hospital for help.

Beavers still struggles with dizziness and has difficulty with speech, memory and focus, making it hard for him to find a job or finish school.

Almost a quarter-million military men and women have been diagnosed with some form of TBI. Most of them were not injured in a battlefield blast, but from training accidents or motor vehicle -- and especially motorcycle -- wrecks.

But the battlefield toll of brain injury is significant. Since 2003, the military has identified 43,299 service men and women who deployed to Iraq or Afghanistan and experienced some form of TBI, according to the Armed Forces Health Surveillance Center . The vast majority of them suffered mTBI. In the summer of 2010, for instance, the military's Theater Medical Data Store listed 1,331 cases of war-zone mTBI, 45 cases of "moderate" TBI and two cases of "severe" TBI, according to data assembled for The Huffington Post.

As early as 2003, combat medics in Iraq and Afghanistan were reporting patients they believed had suffered a concussion. But not until four years later, in 2007, did the Defense Department institute widespread battlefield testing of injured troops suspected of having suffered a concussion.

"There were some indications early on that that the previous administration really didn't want to address this problem," said Dr. James Kelly, director of the National Intrepid Center for TBI at Walter Reed National Military Medical Center in Bethesda, Md.

Within the Defense Department, he told The Huffington Post, "There was the top-down message that said, 'Suck it up, we don't need to hear about this' -- especially the mTBI, even though there were lots of people saying that this was a serious problem.

"That was the [former Defense Secretary Donald] Rumsfeld era within the Defense Department," Kelly said.

But as evidence of widespread combat brain injuries accumulated in 2006, the Defense Department began a concerted effort to screen troops wounded in bomb blasts or vehicle collisions in which they might have suffered a concussion, or worse. The test that began to be widely used in 2007, called the Military Acute Concussion Evaluation (MACE), included a list of five words that the injured person is supposed to repeat back to the medic.

The test met with resistance among combat troops, who didn't want to be pulled away from their units or tagged with a "mental" condition. Medics soon were reporting that some soldiers and Marines tested were turning in perfect scores -- they had memorized the word lists beforehand.

The MACE was quickly revised to include a variety of different word lists. And the testing confirmed the suspicion of many researchers that huge numbers of troops were suffering from concussion-related brain injuries. Between 2006 and 2007, when the test was widely used, the number of diagnosed TBI injuries doubled, from 2,974 to 6,208, according to Pentagon data, even while the pace of overall battlefield casualties was declining.

That data suggests that thouands of combat troops may have received a mild Traumatic Brain Injury and, like Jaime Beavers, were not diagnosed.

Hundreds of research projects are already underway to find out more about mTBI. Together the VA and the Pentagon have already invested some $600 million in brain injury research, Helmick said. But answers are elusive.

"I wish I knew exactly what in the brain, from a structural and physiological standpoint, is causing terrible headaches, loss of memory -- is it something with blood vessels? It's very mystifying," said Helmick. And for diagnosis, "I would love to have a blood test so I could say, 'Yup -- you've had a concussion.' Science is moving in that direction," but right now, she said, "We don't have anything like that."

Above all, Helmick is frustrated that the military cannot tell someone exposed to a blast if or how severely they've been injured.

"We don't have all the answers," she said. "But I don't want to tell an 18-year-old kid with two tours in Iraq and exposure to blast that I'm worried about his functionality, unless I had a good strong reason."