Now, a study conducted by military doctors and praised by outside experts suggests that post-traumatic stress disorder (PTSD), and not the physical injuries themselves, are most likely to blame for many of the long-term problems.
In the study, published Wednesday in the New England Journal of Medicine, Walter Reed Army Institute of Research psychiatry chief Charles Hoge and his colleagues surveyed 2,525 soldiers from two brigades, three to four months after the soldiers' return from a year-long deployment to Iraq.
The researchers found that nearly 5 percent of the soldiers reported losing consciousness for up to three minutes after a head injury in Iraq, and 10 percent said that they head been left dazed.
Soldiers who had suffered a concussion -- and especially those who had lost consciousness -- were more likely than others to experience ongoing problems such as headaches, trouble sleeping and missed workdays.
But that difference disappeared when PTSD was taken into account. Forty-four percent of soldiers who reported loss of consciousness after a head injury met the criteria for PTSD, as compared to 27 percent of those who had been left dazed, 16 percent of those with other injuries, and nine percent of uninjured soldiers.
That makes sense, Hoge told Nature News in an interview: "Concussion during a deployment is a very close call on one's life," he said. "It's obviously a very life-threatening situation. We certainly don't see post-traumatic stress disorder in football players who get a concussion."
Richard Bryant, a psychologist at the University of New South Wales in Australia who wrote an editorial accompanying that study, suggests that the physical injury might also contribute to PTSD. The force of knocking the front of the brain against the skull could damage the amygdala, the part of the brain that regulates fear response.
"These neural networks that are required to regulate fear are impaired or diminished in people with post-traumatic stress disorder," Bryant told Nature News.
The study could have implications for treatment, doctors say, because treatments for brain-injured patients and treatments for those with PTSD are very different.
"It gives us hope, because we've got good treatments for PTSD," Emory University psychologist and trauma researcher Barbara Rothbaum told AP. "If we can relieve the PTSD and depression, I'm hoping we'll see alleviation of a lot of these physical symptoms."