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It seems obvious that living through the terrors of military combat would increase someone's risk of developing post-traumatic stress disorder (PTSD). But a new report published in this week's New England Journal of Medicine suggests that PTSD isn't a simple psychological reaction to a scary situation. When researchers at Walter Reed Army Medical Center and the Uniformed Services University of Health Sciences examined the cases of 2,525 soldiers three to four months after serving in Iraq, they found that a remarkable one-sixth had suffered at least one concussion during their yearlong deployment — typically during combat or from a blast. These same soldiers turned out to be at higher risk for PTSD than those who hadn't had a concussion — about three times higher for soldiers who had been knocked unconscious by concussion, and nearly twice as high for those who had a concussion without blackout.

The study found that soldiers who had suffered concussions were also more likely than soldiers with other injuries to report post-concussive symptoms such as irritability and problems with memory, balance and concentration. But when researchers factored in the soldiers' higher risk for PTSD, the psychiatric disorder seemed to account for the presence of most of those post-concussive symptoms. That would suggest, according to an accompanying editorial by psychologist Richard Bryant of the University of New South Wales in Sydney, that "psychological factors play a significant role in post-concussive symptoms" and that the physical and psychological consequences of suffering a concussion may be tightly bound.

That doesn't mean that every soldier who gets a concussion will develop PTSD, nor, conversely, that concussion is a necessary prerequisite for the disorder. It does mean that doctors should be aware of the potential. People who have suffered even mild concussions in situations where there's psychological stress are probably at elevated risk for PTSD, and should probably be monitored.

The new finding may also have implications for another, ongoing controversy about head trauma in war. According to a feature story in Science magazine last week, a neurologist named Ibolja Cernak, currently working at Johns Hopkins' Applied Physics Laboratory, has been arguing for years that soldiers can suffer brain damage even when they haven't had so much as a concussion. Cernak first observed these effects in a Belgrade hospital during the Balkan wars of the 1990s, according to the writer, Science staffer Yudhijit Bhattacharjee. Soldiers who had survived blasts, but had no head injury whatever, came to the hospital complaining of typical post-concussive symptoms, including dizziness, difficulty in decision-making, memory problems and sleep deficits. MRIs revealed physical damage in many cases — and after further research, Cernak and several colleagues came up with a radical new idea: Shock waves from an explosion, they suggested, might ripple through the body and up into the head. A helmet, or even motorcycle-style protective headgear, wouldn't protect a soldier from brain trauma, according to this theory. Nor would it protect a soldier from the psychological trauma associated with surviving a blast.

While other researchers acknowledge that Cernak might have something, they remain unconvinced about her explanation of exactly how shock waves could be transmitted from the body to the head. Cernak thinks it happens through the vascular system — rapid oscillations in pressure in the blood vessels connecting body to head, induced by blast waves from a nearby explosion. She's got more research under way to try and bolster that case.

But given her research, and the study in this week's New England Journal, it's clear that brain injuries don't have to be massive to cause significant emotional and mental problems, and that "shell shock," as it used to be called, may be caused by physical injury or, in turn, cause physical symptoms — it's not just a reaction to the horrors of war. And if that's the case, better and earlier medical and psychological intervention, along with better protective armor that shields the body as well as the head, could make life after combat a lot easier to endure.

• Michael D. Lemonick
• New research suggests that post-traumatic stress disorder may be linked to concussions suffered in battle