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Brain Size May Affect Reactions to Trauma - 2002-10-16

The size of a brain region involved with memory and stress might affect how a person reacts to trauma. A new study suggests that people who have suffered persistent post-traumatic stress disorder were born with a smaller brain area called the hippocampus.

Previous studies have shown that patients with post-traumatic stress disorder (PTSD) have smaller than normal hippocampal regions. The hippocampus is believed to be the location where memory is first processed before being transferred to another region for long term storage. It is crucial to learning.

Because stress can damage the hippocampus, most researchers concluded that PTSD causes the shrinkage found in people suffering the disorder. But until now, no one had looked at brain size before and after PTSD developed to see if this is always true.

The new research led by U.S. government psychologist Mark Gilbertson does this and challenges the conventional view that shrinkage follows trauma. "There does seem to be some evidence that having a smaller hippocampus prior to the trauma may make individuals more vulnerable to develop stronger, more persistent fear responses," says Mr. Gilbertson.

Mr. Gilbertson and colleagues used a brain scanning technique called magnetic resonance imaging to examine 40 pairs of identical male twins. In each set one brother was a Vietnam War combat veteran while the other had stayed home. As expected, the hippocampus was smaller than normal in the combat veterans who suffered PTSD, a condition with symptoms that include flashbacks, nightmares, and emotional problems.

But to the researchers' surprise, this brain region was also smaller in the twins who had not gone to war. "What this suggested to us was that this is a pre-existing condition," says Mr. Gilbertson. "This suggested there wasn't any exposure effect. In other words, it wasn't a neurotoxic effect of being in combat that produced the smaller hippocampal volume."

The study also shows that the worst cases of post-traumatic stress disorder were in the combat veterans with the smallest hippocampuses. The results indicate that a smaller hippocampus may predispose a person to PTSD and conversely a larger hippocampus could protect against it.

An editorial in the journal Nature Neuroscience, the magazine where the study appears, warns that much more research is necessary before a definite link between PTSD and hippocampus size can be made. If it can be, could hippocampal volume help predict which individuals might be vulnerable to stressful situations -- say, soldiers or police officers?

Mr. Gilbertson says such measures might not be effective for this purpose yet because the stress response is complex and involves more than the hippocampus. "At this point, the utility is mostly in terms of getting a better understanding of what structure or function in the brain may underlie vulnerability to stress disorders so that we can develop more effective prevention or treatment strategies in the future."