By Gene Emery

BOSTON, Jan 30 (Reuters) - Tests of 2,525 U.S. combat veterans after returning from Iraq have found that depression and post-traumatic stress disorder play key roles in determining who will suffer from health problems following a mild brain injury.

"We thought the symptoms would be related to concussion, but they turned out to be most strongly related to PTSD," said Dr. Charles Hoge, a psychiatrist at the Walter Reed Army Institute of Research.

The research, published on Wednesday in the New England Journal of Medicine, also suggests that the rate of such injuries is high.

"In this study, nearly 15 percent of soldiers reported an injury during deployment that involved loss of consciousness or altered mental status," Hoge and his colleagues reported.

More than 1.5 million Americans have gone to Iraq or Afghanistan since 2001, and some estimates of the rate of head and neck injuries as high as one in four.

Hoge and his colleagues surveyed veterans within four months of their return from a year-long deployment. In addition to being asked if they had been knocked out, left dazed by a head injury or had a head injury they did not remember, they were asked to rate their health.

Nearly 5 percent reported losing consciousness for up to three minutes and 10 percent said they had been left dazed, confused or seeing stars.

By comparing them to the 17 percent who had received some other type of injury, the Hoge team found the highest PTSD rates among soldiers with head injuries.

"Overall, 43.9 percent of soldiers who reported loss of consciousness met the criteria for PTSD, as compared with 27.3 percent of those with altered mental status, 16.2 percent of those with other injuries, and 9.1 percent of those with no injuries," they said.

"The concussion with loss of consciousness conferred an elevated risk of PTSD compared with other injuries, and then it's probably the PTSD, in turn, generating a lot of in the health symptoms," he said in a telephone interview.

Losing consciousness tripled the risk of PTSD and nearly quadrupled the chance of major depression. Not surprisingly, soldiers faced with the heaviest combat were nearly 12 times more likely to be affected by PTSD than those who saw the least amount of action.

In an editorial in the journal, Richard Bryant of Australia's University of New South Wales said although soldiers who have mild traumatic brain injury face a greater risk of health problems, servicemen and women should not be led to believe that a concussion or similar problem will produce permanent damage.

"This could be damaging to morale and to the person's future mental health, because it could lead to the expectation of poor recovery," Bryant said. (Editing by Will Dunham)

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