Psychiatrists know post-traumatic stress disorder, or PTSD, develops in people exposed to chronic stress. Researchers have studied the brains of adults with the psychological disorder and found a region of their brains called the hippocampus to be smaller than in people without PTSD. The hippocampus is a part of the brain associated with the storage and retrieval of memories.

Dr. Victor Carrion, a psychiatrist at California's Stanford University wondered if the same phenomenon occurred in children, and if exposure to severe stress caused their hippocampi to shrink.

"We're talking about interpersonal trauma," he says, "the experience of physical abuse, sexual abuse, witnessing violence, and these experiences tend to be severe and they tend to be chronic. But anything really ... man-made things like shootings, for example, or natural catastrophes like earthquakes."

Carrion and his colleagues evaluated 15 pre-adolescent children with symptoms of PTSD, including nightmares and uncontrollable flashbacks, extreme agitation and emotional numbness. They assessed the children's behavior at the beginning of the study and again after 12 to 18 months. They also measured blood levels of the stress hormone cortisol and took images of their brains.

They found that kids with more severe PTSD symptoms had more cortisol in their blood and their hippocampi decreased in volume. "What that means is that the higher your symptoms of PTSD, or the higher your level of cortisol, the higher your chances of having a decrease in the size of this structure," Carrion notes, adding that this was the first time researchers have really seen that connection, indicating how cortisol might be related to the hippocampus.

Carrion says he was surprised at how quickly the changes occurred, even though the children were not experiencing trauma during the time of the study.

"This is a trauma that they experienced in the past," he explains, "so we're seeing the consequences of the trauma having an effect on the individual, not only the trauma."

Carrion says knowing about the physiologic changes in the hippocampus might help researchers understand why PTSD becomes a chronic condition and develop better treatments to help children. His study appears in the journal Pediatrics.