A study of active duty Marines provides some of the best evidence to date that post traumatic stress disorder (PTSD) can be caused by physical injury to the brain, not just psychological trauma. The study found that if a Marine suffered a traumatic brain injury while on duty in Afghanistan or Iraq the likelihood of a developing PTSD doubled.

Blast injuries from improvised explosive devices (the homemade bombs called IEDs) are all too common among military personnel fighting in Iraq and Afghanistan, with more than half (52%) of all reported cases of traumatic brain injury (TBI) traced to them.

Suffering a traumatic brain injury while on duty in Afghanistan or Iraq nearly doubled the likelihood of a Marine developing PTSD.

Many factors can affect a soldier's risk of developing PTSD, making it hard to tease out the role of one factor from another. After all, war is psychologically traumatizing. Even though service members with mild TBIs report PTSD symptoms at approximately double the rate compared to those with no TBI, it's still difficult to prove that the brain injury caused the PTSD.

In the current study, researchers tried to determine the exact numerical effect that several factors had on Marines developing PTSD during active duty. They interviewed and tested Marines one month before a seven-month combat deployment in Iraq or Afghanistan and repeated the interviews and tests three months after the tour of duty.

The results were used to analyze how well each of three factors predicted a serviceman would develop PTSD after their tour of duty: combat severity, having PTSD symptoms prior to their tour of duty, and receiving a traumatic brain injury during the tour of duty.

All three increased the probability of testing positive for PTSD after the combat tour. But brain injury was the strongest predictor. It nearly doubled the likelihood of developing PTSD in servicemen who had no or minimal PTSD symptoms before their tour of duty began.

Brain injury and PTSD also seem to go together in non-military situations. Motor vehicle accidents are common causes of both TBI and PTSD. And while you can't really apply the results of a military study to civilian conditions, the study certainly leaves open the possibility that physical injury to the brain is the reason victims of car accidents and other brain trauma develop PTSD afterwards.

The study appears in JAMA Psychiatry.