Years ago, it was noted that the region of the brain known as the hippocampus, which is involved in memory and stress responses, was smaller in men suffering from post-traumatic stress disorder (PTSD) than in those who didn't suffer from the disorder.
But one question has persisted: Which came first, the smaller hippocampus or the PTSD?
Now researchers say the smaller hippocampal size came first. While it's still unclear if this smaller size actually contributes to the development of PTSD, it is clear it is not a result of the disorder.
"The study is extremely interesting, and it sort of culminates a debate that has been going on for several years," says Dr. Randall Marshall, director of trauma studies at the New York State Psychiatric Institute and an associate professor of clinical psychiatry at Columbia University College of Physicians & Surgeons, both in New York City.
"It's fairly dramatic," he says.
PTSD is a debilitating disorder that can affect people who have been exposed to severe psychological trauma, such as combat or kidnapping. The National Vietnam Veterans Readjustment Study estimates that almost 31 percent of male Vietnam veterans and almost 27 percent of female Vietnam veterans suffer from the disorder. Among other things, individuals with PTSD often have flashbacks of the initial trauma, are irritable, have difficulty concentrating and sleeping, and have trouble coping with daily life.
In this study, researchers used magnetic resonance imaging (MRI) to examine the brains of two different sets of identical twins. The first set was comprised of 17 pairs of twins, one of whom had gone to Vietnam and developed PTSD and the other who had not been exposed to combat and who did not have the disorder. In the second set of twins (23 pairs), one brother had gone to Vietnam but had not developed PTSD, while his twin had stayed at home and also had not developed PTSD.
The twins were recruited primarily through the Vietnam Era Twin (VET) registry, which was developed by the U.S. Veterans Affairs Administration.
The authors of the study found a 10 percent difference in total hippocampal volume between Vietnam vets with PTSD as compared to veterans without PTSD. This was not surprising.
Much more surprising was the fact that the twins of men with PTSD also had smaller hippocampal volumes and that those volumes were also significantly less than the set of twins without PTSD.
"It's one of the first demonstrations that brain structure can serve as a preexisting vulnerability factor for a stress disorder, so it helps us begin to understand additional factors in who develops PTSD," says Mark Gilbertson, lead author of the study. A psychologist at Manchester Veterans Affairs Medical Center in New Hampshire, Gilbertson is also an instructor in psychology at Harvard University School of Medicine's department of psychiatry.
The research effectively settles a debate that had, at times, become heated and dramatic.
"There have been data for a number of years that have shown that individuals with PTSD tend to have smaller hippocampi, but it was a chicken-and-egg thing," Gilbertson says.
A number of scientists had vigorously promoted the notion that hippocampus size was a result of PTSD. "People were too enthusiastic to latch onto an explanatory theory without thinking it through. So that's why this is really exciting because it pretty much answers this question," Marshall says. "It's the perfect design, the perfect study to answer exactly this question."
But it does not answer the question of whether the smaller hippocampal size is actually a risk factor for PTSD.
"You can't really assume that," Marshall says. "We know it's a preexisting condition. It may or may not be an abnormality. Especially in genetics, it could be that there is some third variable or process that is influencing both hippocampal volume and PTSD."
It's also not clear that heredity is responsible for the smaller hippocampal size. It could be shared environment, as most twins grow up in the same household.
Much work still needs to be done but, Gilbertson says, "it's certainly a starting point indicating that certain aspects of brain structure, and presumably function as well, may either create vulnerability or be a protective factor."
What To DoFor more information on post-traumatic stress disorder, visit the National Center for PTSD or the National Institute of Mental Health.