NASHVILLE, Tenn. (Sept. 9) - The war in Iraq  is not over, but one legacy is already here in this city and others across America: a "silent epidemic" of brain-damaged soldiers.

Thousands of troops have been diagnosed with traumatic brain injury, or TBI. These blast-caused head injuries are so different from the ones doctors are used to seeing from falls and car crashes that treating them is as much faith as it is science.

"I've been in the field for 20-plus years dealing with TBI. I have a very experienced staff. And they're saying to me, 'We're seeing things we've never seen before,"' said Sandy Schneider, director of Vanderbilt University's brain injury rehabilitation program.

Doctors also are realizing that symptoms overlap with post-traumatic stress disorder, and that both must be treated. Odd as it may seem, brain injury can protect against PTSD by blurring awareness of what happened.

But as memory improves, emotional problems can emerge: One of the first "graduates" of Vanderbilt's program committed suicide three weeks later.

"Of all the ones here, he would not have been the one we would have thought," Schneider said. "They called him the Michelangelo of Fort Campbell" - a guy who planned to go to art school.

As more troops return from the war, brain injuries are a growing burden - for them, for the few programs to treat them, and for taxpayers who pay for their care and disability if they cannot hold jobs.

Most TBIs are mild, and most of these patients recover within a year. But one-fifth of the troops with these mild injuries will have prolonged or lifelong symptoms and need continuing care, the military estimates. Nearly all of the moderate and severe ones will, too.

Though the full number of those suffering from TBI is still unknown, the problem is straining the U.S. Department of Veterans Affairs. Until now, "they were dealing with a cohort of aging veterans with diabetes, heart disease, lung disease," said Dr. Jeffrey Drazen, editor-in-chief of the New England Journal of Medicine and a VA adviser.

Now, these young, brain-injured troops need highly specialized care, and how much it will help long-term is unknown, he said.

People with TBI have frequent headaches, dizziness, and trouble concentrating and sleeping. They may be depressed, irritable and confused, and easily provoked or distracted. Speech or vision also can be impaired.

Some sufferers have been misdiagnosed with personality disorders. Others have lost jobs because of unrecognized and untreated symptoms.

"It's the so-called invisible injury. It's where a troop takes 10 times the normal time to pack his rucksack ... a complicated injury to the most complicated part of the body," said Dr. Alisa Gean, a neurosurgeon at the University of California, San Francisco.

Diagnosing it is imprecise - damage rarely shows up on CAT scans or other tests.

Treating it is even more difficult. Lacking a cure, doctors focus on symptoms - headaches, anxiety, vision problems, etc. But they lack good treatments for some of these, too, and are considering some experimental approaches being pushed by private companies with little proof they work.

Many troops get no care at all. Some are sent back to fight with their brain injuries undetected, especially if they had no obvious wounds.

What happened to Eric O'Brien and Bryan Malone shows the scope of this problem.

O'Brien, a 32-year-old Army staff sergeant from Iowa's Quad Cities, was teasing Malone, 22, a specialist from Haughton, La., in a Baghdad gym last summer.

"I told him and his workout partner: 'Put some more weight on it,"' prompting the men to get up. Seconds later, a rocket hit where they had sat. They survived, but a pressure wave from the blast coursed through their brains.

"I patted myself down head to toe, making sure I wasn't missing a limb," and felt odd, like "I must be missing a chunk of my head,"' O'Brien said. He remembers little else except walking through debris to pick up his iPod and sunglasses.

As for Malone, an air conditioning vent had fallen on his head and he had shrapnel wounds. He had multiple surgeries, spent several months in Walter Reed Army Medical Center and now has titanium mesh reinforcing his skull.

O'Brien, however, had shrapnel removed from his scalp and then was sent back to his unit - "no antibiotics, no pain medication or anything. They just sent me on my way."

When he later complained of pain, doctors gave him Motrin. When he discovered a trickle of blood from his hip, they said he would be fine. Six weeks later, when he could barely walk, tests revealed shrapnel in his hip. By then, he was having headaches and trouble sleeping.

O'Brien had been through multiple previous explosions - troops average one a month, a study found - and each raises the risk that the next one will do harm. Soldiers and Marines are proud and reluctant to go "off mission" just because "they get their bell rung," said Dr. Michael Kilpatrick, a top Defense Department physician.

"Most of the treatment is explaining the situation and giving the tincture of time - giving it time to heal," he said. If no big symptoms appear in eight to 12 hours, "they're probably ready to go back."

Officers also face pressure to return troops to duty, said Jordan Grafman, a neuroscientist who studies TBI at the National Institutes of Health.

"People don't want to lose these guys from their command - they can't replace them fast enough," he said.

During a surprise visit to Iraq with President Bush  on Labor Day, Gen. Peter Pace, chairman of the Joint Chiefs of Staff, said the military was "much smarter about this now," and urged troops to watch for signs of TBI and post-traumatic stress.

"They are every bit as much battle injuries as is a bullet or shrapnel. It is OK, it is OK to seek help for those kinds of war wounds, and I ask you all to help your buddies understand what you see in them," he said.

But that was long after O'Brien was hurt. His TBI was not diagnosed for months, until his hip injury landed him back at Fort Campbell in Kentucky. By then, the Army needed help treating TBI and was contracting with private rehab centers like Schneider's at Vanderbilt.

Malone and O'Brien had become friends, helping each other cope with wounds.

"They were sent to us together," Schneider said.

"I'll need to get milk and bread and eggs. Milk and bread and eggs. Next thing you know, I drive right by Wal-Mart," O'Brien said.

"I can vaguely tell you what we talked about at the beginning of this conversation," Malone said.

Memory trouble is a common sign of TBI. It isn't like Alzheimer's disease, where people are so disconnected from reality that they forget things like how a key works or where they live. It isn't like amnesia, where a chunk of the past is missing.

"I don't have any problem remembering the past. I have trouble with now," O'Brien said.

Multiple or complex tasks confound and irritate people with TBI. Therapists challenge them through exercises, like a computer game where they run a hot dog stand and must manage inventory, set prices, do banking and anticipate demand according to the weather.

Other therapy focuses on life skills like following directions while paying attention to something else.

"I counted three trash cans," O'Brien announced after a scouting mission to find landmarks using a map and tally cans along the way.

"I counted five," said therapist Jenny Owens.

Improving these skills is key to living a normal life, especially driving.

"Most of them don't drive. A van brings them down. They were hitting mailboxes, they'd get lost. We draw them maps and they forget when they're supposed to be here," Schneider said.

The Army gives some injured soldiers Palm Pilots - handheld computers to help manage their lives.

"It costs them more for us to miss two appointments than to give us one of these," O'Brien explained.

But devices and mental exercises do only so much. Troops must be able to use information and reason, but TBI keeps many from being aware of their gaps.

"They don't realize their judgment is impaired," said Vanderbilt neuropsychologist Elizabeth Fenimore.

The training that helped them in combat situations is hurting them now.

"These guys are taught to be alert all the time," so they sleep poorly, Schneider said.

"Their nervous system becomes acclimated to being constantly on alert - fight or flight," Fenimore said.

Malone knows it well.

"I worry about every little thing - people breaking into my house, loud booms ... I'm jumpy," he said.

"I'm going to Afghanistan  next year," said O'Brien, determined to stay in the Army and support his two daughters, who live with his ex-wife in Texas.

"I'm trying," added Malone. "They're telling me they don't think my brain can take it. I think, 'Why don't you let me decide?"'

Doctors don't know whether either will return. But after all they've been through, if one does and the other does not, "it's going to be tough," Malone said. "It's going to be tough for whichever one stays back."

Associated Press writer Christine Simmons in Washington contributed to this report.