What is it? And how does it differ from a concussion?

Traumatic brain injury, or TBI, is a sudden trauma to the brain caused by force. A severe TBI can leave a person almost incapable of functioning. But even a mild TBI -- a concussion -- can lead to a range of debilitating symptoms: headaches, balance problems, hearing problems, lack of self-control, mood changes, ringing in the ears, problems sleeping and memory loss. While most people recover from a mild TBI, it can take months, even years.

According to the Centers for Disease Control and Prevention, "approximately 1.7 million people sustain a traumatic brain injury annually" in the United States, and "the majority of TBIs each year are concussions or other forms of mild TBI." For some basics on the science behind TBIs, see NOVA Science Now's "Brain Trauma" video and website.

The brain injuries sustained by soldiers serving in Iraq and Afghanistan are most often caused by explosives. See ProPublica's graphic showing what happens to the brain during and after a blast.

How many U.S. soldiers have TBI?

The exact number is hard to pinpoint. The Pentagon says about 115,000 soldiers have mild TBI, while the RAND Corporation study Invisible Wounds of War suggests the much higher number of 400,000 total TBIs, the majority of which are mild.

How is TBI diagnosed?

Diagnosing TBI can be hard. Symptoms of moderate to severe TBI can be obvious -- extended loss of consciousness and severe neurological disorders -- but diagnosing a mild TBI is trickier, especially during combat. Often soldiers don't even realize they have a mild brain injury after a blast. In some cases, combat medical records are lost or destroyed in theater.

And NPR and ProPublica found that "the military’s doctors and screening systems routinely miss brain trauma in soldiers."

The military uses two basic tests to diagnose the injury: The first, the Military Acute Concussion Evaluation, or MACE, is a survey taken immediately after an injury. NPR and ProPublica found that often soldiers learn to cheat on this test because they want to return to their platoon. The second, Automated Neuropsychological Assessment Metrics, or ANAM, "failed to catch nearly half of all soldiers who had suffered a concussion, according to a recent unpublished study obtained by NPR and ProPublica. Lt. Gen. Eric Scoomaker, the Army's top medical official, recently testified in Congress that results from the test are no better than a 'coin flip.'"

Another consideration in diagnosing TBI is its comorbidity with PTSD. In a response to NPR and ProPublica, Gen. Peter W. Chiarelli, the Army vice chief of staff, said "it was a mistake to focus solely on TBIs, since many soldiers are also suffering from post-traumatic stress, or PTS[D], a debilitating psychological wound that can be caused by the intense terror of being involved in a roadside blast. ...He said the military was diagnosing and treating soldiers suffering from both wounds."

(See a timeline tracking the effort to diagnose and treat TBI.)

How is TBI treated?

Treatment varies widely. There's no standard treatment. Although NPR and ProPublica found that regular and consistent cognitive rehabilitation therapy -- techniques to compensate for decreases in mental function -- benefited several veterans, this type of treatment is rarely available through military medical care. Many soldiers have sought rehabilitative treatment at private facilities. For each of the three soldiers profiled in FRONTLINE's The Wounded Platoon the most common treatment given for their diagnosed TBI was pharmacological.

There is some hope that treatment in hyperbaric oxygen chambers might aid in recovery. A study on this therapy gets underway in 2011 and will be conducted at five U.S. bases by the Defense Centers of Excellence for Psychological Health and Traumatic Brain Injury.

The most common way to prevent chronic TBI is to rest after receiving an injury to the head. The Pentagon recently released a new policy for the treatment of TBI, which includes a mandatory 24-hour rest period after a blast, and a complete neurological assessment for soldiers who have had three concussions. If a soldier with TBI is not taken out of theater to properly recover, any additional brain injuries can exacerbate the damage. Yet, as noted above, some soldiers may not realize they have TBI, or they brush off symptoms in order to rejoin their platoon.