Many soldiers who return home from war are psychologically devastated by post-traumatic stress disorder (PTSD), in which the trauma is relived through nightmares and flashbacks, but others who have fought the same battles remain healthy. Work published in the 15 October online edition of Nature Neuroscience suggests that inherited differences in brain structure make some people far more sensitive to the trauma of war than others.
Part of the brain called the hippocampus, which is important for learning and memory, is clearly smaller than normal in PTSD patients. But why? Trauma might shrink the hippocampus, because extreme stress can flood the bloodstream with hormones that might damage hippocampal neurons. Another possibility is that the hippocampi of PTSD patients were small to begin with. Researchers couldn't test this idea because soldiers with PTSD hadn't had their brains scanned before combat.
To get around this problem, a team led by Mark Gilbertson, a neurobiologist at Harvard Medical School in Boston, studied 40 veterans of the Vietnam War who had left a twin brother at home. Because the sizes of brain structures are very similar between identical twins, the nontraumatized brothers provide a "before" snapshot of the PTSD brain. If the hippocampi of the veterans with PTSD tend to be smaller than those of their twin brothers who stayed home, then this would support the idea that the trauma of war can shrink the hippocampus. But magnetic resonance imaging revealed no difference between the hippocampi of healthy and PTSD twins, suggesting that a small hippocampus might be a cause of PTSD, not the other way around. They also found that hippocampus volume correlates with PTSD severity: The smaller the hippocampus, the worse the symptoms of PTSD. These results suggest that the military might be able to predict which soldiers are more vulnerable to the trauma of combat by scanning brains during peacetime.
"The paper adds an interesting new dimension to the PTSD story," says Bruce McEwen of Rockefeller University in New York City. But McEwen points out that the shared childhood environment of twins could be a bias in the data. "Clearly this deserves much more investigation."