WASHINGTON — Many of the thousands of troops who suffered traumatic brain injuries in Iraq and Afghanistan are at risk of long-term health problems including depression and Alzheimer's-like dementia, but it's impossible to predict how high those risks are, researchers say.

About 22 percent of wounded troops have a brain injury, concluded the prestigious Institute of Medicine — and it urged precise steps for studying how these patients fare years later so chances to help aren't missed.

The Veterans Affairs Department, which requested the report, and the Pentagon already are taking some of the recommended steps. But a report out Thursday highlights the urgency.

"I don't think we really knew how big a hole in scientific knowledge there is about blast-induced brain injuries," said Dr. George Rutherford of the University of California, San Francisco, the report's lead researcher.

Traumatic brain injury, or TBI, is a signature injury of the Iraq war. Most do not involve penetrating head wounds but damage hidden inside the skull caused by an explosion's pressure wave. It can range from a mild concussion to severe injury. And because symptoms may not be immediately apparent, troops may not seek care.

"If you have a gunshot wound to some specific part of your brain, I can tell you the consequences," Rutherford said. But with blast concussions, it's not even possible to say "if you have six of these, are you six times more likely to have something bad happen to you than if you've had one?"

Returning soldiers have reported headaches, dizziness, memory loss, confusion, irritability, insomnia and depression. The military has said most of the TBI-injured troops recover with treatment.

"There's clearly a whole bunch of people who have mild TBI who have no negative outcomes," Rutherford agreed.

But his committee examined decades of studies into mostly civilian injuries and found:

—Moderate-to-severe TBI is linked with later-in-life risks including Alzheimer's-like dementia, Parkinson's-like symptoms, seizures, problems with social functioning and unemployment.

—TBI in general is linked to depression, aggressive behavior and post-concussion symptoms such as dizziness and amnesia.

—If mild TBI caused loss of consciousness, a risk of later memory, movement and seizure problems cannot be ruled out.

The report recommends that every soldier exposed to a blast, even a low-intensity one, be screened for TBI — and that everyone get a pre- and post-deployment brain-function test. The military has begun those steps.

It also urges the Defense Department to conduct rigorous studies and a VA-run registry of TBI patients to identify long-term risks and factors that improve or worsen outcomes, in comparison to deployed troops with non-brain injuries.

The VA said it would consider the recommendations, and it has 60 days to decide if those long-term disorders will be presumed linked to brain-injured veterans' military service.