That would be good news because there are successful treatments for those conditions, said several nonmilitary doctors who praised the research.
Thousands of soldiers returning from Iraq have struggled with memory loss, irritability, trouble sleeping and other problems. Many have suffered mild blast-related concussions, but there is no easy way to separate which symptoms are due to physical damage and which are from mental problems caused by the traumatic stress of war. Imaging of the brain is being tested, but hasn't yet proven to be helpful.
The new study, based on a survey of 2,500 soldiers, found that brain injury made traumatic stress more likely. The study tied only one symptom - headaches - specifically to brain injury.
"We found that the symptoms and health concerns that we expected to be due to the concussion actually proved to be more strongly related to PTSD," or post-traumatic stress disorder, and depression, said Dr. Charles Hoge, a colonel and psychiatry chief at Walter Reed Army Institute of Research who led the study. "There isn't a clear delineation between a psychological and a physical problem."
Other doctors were optimistic about treatment efforts.
"It gives us hope, because we've got good treatments for PTSD," said Barbara Rothbaum, a psychologist who heads a trauma recovery program at Emory University in Atlanta. "If we can relieve the PTSD and depression, I'm hoping we'll see alleviation of a lot of these physical symptoms."
Hoge was to report on the survey Wednesday at a military health conference in Washington. Results also are being published in Thursday's New England Journal of Medicine.
The journal's editor-in-chief, Dr. Jeffrey Drazen, said editors initially were skeptical of the findings, which depart from the gloom-and-doom picture some have painted for soldiers with brain injuries.
However, the solid research methods and the "strong and robust" data linking stress and concussion symptoms persuaded them, said Drazen, who is a scientific adviser to the Veterans Administration.
The case of Eric O'Brien, a 33-year-old Army staff sergeant from Iowa's Quad Cities, suggests the researchers may be right.
After an explosion in Baghdad in 2006, O'Brien was treated at Vanderbilt University's brain injury rehabilitation program and at Fort Campbell, Ky., for post-traumatic stress. Now he is preparing to redeploy, this time to Afghanistan.
"I retested on a lot of the tests and they showed a pretty decent increase," he said of his mental function tests. As for stress, "I don't know if it's something you just learn to deal with or if it just gets a little bit better over time," he said. "It's not as bad as it was."
The vast majority of brain injuries, or concussions, are mild, but the military previously estimated that one-fifth cause symptoms lasting a year or more.
The new study tried to pin down the potential long-term effects of mild brain injury, through an anonymous survey of two Army combat brigades - one active and one Reserve - in 2006, several months after they returned home from Iraq.
Fifteen percent of soldiers reported a mild brain injury - having been knocked unconscious or left confused or "seeing stars" after a blast. They were more likely than other soldiers to report health problems, missing work, and symptoms such as trouble concentrating.
The worst symptoms were in soldiers who lost consciousness. About 44 percent of them met the criteria for post-traumatic stress, compared with 16 percent of soldiers with non-head injuries, and only 9 percent of those with no injuries.
"The same incident might have triggered both processes," Rothbaum said, noting that after World War I, "they thought that shell shock was a neurological disorder and it turned out to have a lot of overlap with the psychological disorder."
Concussions may compound stress by damaging brain areas that tamp down responses to fear, Richard Bryant, a psychologist at the University of New South Wales in Sydney, Australia, writes in an editorial in the journal.
"PTSD and depression may be the primary problem," he writes. "Soldiers should not be led to believe that they have a brain injury that will result in permanent change."
The military recently started screening all returning troops for concussions. Any soldiers who saw intense combat should be similarly checked for stress disorder, said Anthony Stringer, director of Emory University's neuropsychology rehabilitation program.
The new study can be viewed as positive "if the results are used to make sure that soldiers have the care they need when they return," he said.