MONTREAL. -- Predisposition is an important factor, but a traumatic event remains the necessary trigger in the development of posttraumatic stress disorder, a new study of identical twins indicates.
"Embedded within the diagnostic criteria of PTSD is a presumed causal event, but this assumption has come under scrutiny, as a recent study suggested that the symptoms of PTSD may merely represent general psychiatric symptoms that would have developed even in the absence of a trauma (J. Anxiety Disord. 2007;21:176-82), explained Dr. Roger Pitman, director of the PTSD and psychophysiology laboratory at Massachusetts General Hospital and professor of psychiatry at Harvard Medical School, both in Boston.
Speaking at the meeting, Dr. Pitman launched new evidence to support the widely held theory that trauma is central to the development of PTSD.
The study comprised 104 Vietnam combat veterans and their nonveteran identical twins. Of the veterans, 50 had PTSD and 54 did not, whereas none of the nonveteran identical twins had the disorder (J. Clin. Psychiatry 2010;71:1324-30).
"If the PTSD-affected veterans had predisposing vulnerability to psychopathology on a genetic or environmental basis, then that ought to be shared by their twins," he explained.
Psychometric measures - including the Symptom Checklist-90-Revised, the Clinician-Administered PTSD Scale (CAPS), and the Mississippi Scale for Combat-Related PTSD - were used to assess symptoms for all veterans and their twins. For the nonveterans, questions about combat trauma were replaced with questions about their most traumatic experience.
As expected, the evaluations revealed higher scores on all measures for the PTSD-affected veterans, compared with their identical twins. All nonveteran twins had scores similar to those of the veterans without PTSD.
"These results do not support the idea that the people with PTSD would have been symptomatic even without the traumatic event," Dr. Pitman said. "They do support the conclusion that the mental disorders found in PTSD result from a trauma."
About one-third of individuals who were exposed to a traumatic event will go on to develop PTSD.
This suggests that certain people might have an underlying predisposition to developing the disorder, Dr. Pitman said.
"We called the twins of the PTSD-affected veterans 'high risk' because they had a shared familial environment and shared genes," he noted. …