PTSD Risk Rooted in Stress and

"This suggests this 'fragile egg' hypothesis -- that some people are probably genetically a little bit less resilient [to PTSD] than other people," said Dr. Julie Fudge, an associate professor of psychiatry and neurobiology and anatomy at the University of Rochester Medical Center, in New York.

However, the ventromedial prefrontal cortex area of the brain (involved in decision-making) is smaller in the combat-exposed brothers, compared to their unexposed twins.

"We discovered a number of abnormalities in PTSD combat veterans not shared with their twins and . . . we infer that the abnormality was caused by combat," Pitman said. "One of these is increased heart rate response when the combat veteran is startled. Another example is loss of gray matter in the rostral anterior cingulated cortex, which plays a role in inhibiting the fear response."

"The assumption for the last 10 years has been that the hippocampus has shrunken due to stress basically because this is what happens in animals. But the Pitman study shows that this is probably not the case for the hippocampus," said Dr. Keith A. Young, vice chair for research at Texas A&M Health Science Center College of Medicine and Neuroimaging and Genetics Core Leader for the VA Center of Excellence for Research on Returning War Veterans. "In other areas of the brain they may very well be being damaged by the excess stress," he said.

A thus-far unpublished finding, Pittman added, is that combat-exposed men had a broad array of psychiatric symptoms that were not seen in their brothers.

"This tends to refute that persons with PTSD would have had psychiatric problems if they hadn't been exposed to a traumatic event," he said.

More information

For more on this condition, visit Root Cause of PTSD.

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