Twin study may quiet doubts over PTSD-trauma link

Trauma really is the trigger of post-traumatic stress disorder, or PTSD, suggests a new study that could help settle an ongoing debate.

"It's been argued by some that PTSD is not a bona fide disorder, that these people are just maladjusted and the trauma doesn't have anything to with it," senior researcher Dr. Roger Pitman of Boston's Harvard University told Reuters Health.

This new study provides "a formidable piece of evidence against that notion. It's a nail in the coffin," Pitman said.

But not all experts are as convinced that the link is sealed between PTSD and traumatic events.

PTSD, which by definition results from exposure to trauma, affects nearly 8 million Americans, according to the National Institute of Mental Health. Still, just a fraction of individuals exposed to such disturbing events subsequently develop the disorder. An estimated 19 percent of Vietnam veterans experienced its symptoms, including flashbacks and emotional detachment, at some point after the war.

Pitman and colleagues from Harvard and the U.S. Department of Veterans Affairs looked to 103 pairs of identical male twins from the Vietnam Era Twin Registry to test whether trauma truly causes PTSD, or if its sufferers would have developed symptoms of the disorder regardless. One brother from each pair had been exposed to combat in the Vietnam War; the other had not. Fifty of the combat-exposed men had PTSD.

The researchers found a substantial difference in mental disorders between the twins: Men exposed to combat and diagnosed with PTSD had three-fold more symptoms than their brothers, as well as compared to the combat veterans without PTSD and their co-twins.

Similar patterns appeared when considering only PTSD-related psychiatric symptoms, report the researchers in the Journal of Clinical Psychiatry.

According to Pitman, "If you assume that the identical twin is a representation of what the veteran would have been like had he not been exposed to combat, with the same genes and same family upbringing, then the conclusion is that psychiatric trauma causes substantial psychiatric symptoms in a portion of the population."

Given the similarities in symptoms between co-twins of combat-exposed brothers that did and did not develop PTSD, the findings also refute the presence of any predisposing genetic or environmental traits that increase vulnerability to trauma-induced PTSD, the authors say.

In an email to Reuters Health, Richard McNally of Harvard University, who was not involved in the study, said this "ingenious twin method" is a powerful tool for distinguishing such preexisting vulnerabilities from those resulting from the trauma exposure itself.

"They showed that nonspecific symptoms of PTSD that do not refer to any specific event, such as irritability, numbness, sleep disturbance, social withdrawal and loss of interest, are just as attributable to trauma-induced PTSD as are symptoms that by definition presuppose exposure to a stressful event, such as intrusive thoughts, nightmares about a trauma and avoidance of reminders of a trauma," McNally said.