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CHICAGO (Reuters) - A study of identical twins -- one with combat experience and one without -- suggests both genetic and environmental factors contribute to post-traumatic stress disorder (PTSD), U.S. researchers said Tuesday.

The 10-year study aimed to determine whether the often-debilitating anxiety disorder arises from an inherent vulnerability or is strictly caused by a traumatic event.

"The way we have been getting at this question is to study a group of more than 100 Vietnam vets who each have an identical twin who didn't serve in combat," said Dr. Roger Pitman of Harvard Medical School and Massachusetts General Hospital, who presented his findings at a meeting of the American College of Neuropsychopharmacology in Scottsdale, Arizona.

"Because twins have the same genes and family environment, the unexposed twin represents what the combat-exposed twins would be like except for the combat exposure," Pitman said in a telephone briefing.
Prior research had found that diminished brain volume in the hippocampus, a part of the brain involved in memory and stress response, was associated with PTSD.

Pitman and colleagues found this characteristic was shared in twins in which one had developed PTSD.
They also found shared abnormalities in the dorsal anterior cingulate cortex, which plays a role in the fear response.

"We've identified several abnormalities that combat vets share with their unexposed twins. These must represent pre-existing risk factors," he said.

However, they also found a number of abnormalities not shared by the twins, such as a highly attuned startle response resulting in increased heart rates in the combat-exposed twin.
"This tends to refute the suggestion that a person with PTSD would have psychiatric problems even if they had not been exposed to a traumatic event," he said.
He said the findings may help guide treatment for people with PTSD.

PTSD often begins three months after an event such as wartime combat or injury, rape, assault, a car or plane crash or other traumas. But sometimes PTSD appears years later.
A study by the RAND Corp. earlier this year estimated about 300,000 troops, or 18.5 percent, of the more than 1.5 million troops sent to Iraq and Afghanistan exhibit symptoms of either PTSD or depression.