A 2007 study led by Dr. Charles Hoge, a colonel and psychiatry chief at Walter Reed Army Institute of Research, was based on a survey of nearly 2,500 soldiers. The study found that one in six soldiers returning from Iraq had suffered concussions and that brain injury made traumatic stress more likely. The study tied only one symptom—headaches—specifically to brain injury.
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
Kevin Guskiewicz, professor of exercise and sport science and director of the Sports Medicine Research Laboratory at the University of North Carolina, Chapel Hill, led research into brain injuries resulting from head impacts in sports. The research showed that even low-impact head hits can cause brain injuries. Players who look like they have been hit really hard are not necessarily the ones who will sustain the most brain damage; there is no relationship between the magnitude of the impact and the clinical outcome.