Researchers at Duke University and the Durham VA Medical Center have linked post-traumatic stress disorder (PTSD) in recently diagnosed combat veterans to a significantly smaller size for the brain section critical for regulating fear and anxiety responses.

It is not clear if the smaller size of the brain’s amygdala was caused by PTSD or another traumatic event, or whether the syndrome develops more readily in people who naturally have smaller amygdalas.

Whatever the case, the result provided clear evidence that smaller amygdala volume is associated with PTSD, regardless of the severity of the trauma—but not with the severity, frequency, or duration of trauma, indicating that such exposures do not cause the amygdala to shrink. That and other findings were published Nov. 5 in the journal Archives of General Psychiatry.

“Our results may trigger a renewed impetus for investigating structural differences in the amygdala, its genetic determinants, its environmental modulators, and the possibility that it reflects an intrinsic vulnerability to PTSD,” researchers concluded in their study.

Rajendra A. Morey, M.D., an assistant professor at Duke and the study’s lead author, noted in a statement that while studies in animals have established the amygdala playing a role in regulating fear, anxiety, and stress responses, its effect on human behavior is less well known.

“Researchers found 20 years ago that there were changes in volume of the hippocampus associated with PTSD, but the amygdala is more relevant to the disorder,” Dr. Morey said.

Dr. Morley said it appears more likely that people with measurably smaller amygdala to begin with are susceptible to PTSD based on evidence from their study. Researchers have begun to explore that question, but need more evidence before making that determination.

In the most recent study, 200 combat veterans who served in Iraq and Afghanistan after Sept. 11, 2001, were enrolled. Almost half (99) had PTSD, while the other 101 patients had been exposed to trauma, but not PTSD. Amygdala and hippocampus volumes were computed from MRI scans of the veterans.

The researchers found significant evidence that PTSD was associated with smaller volume in both the left and right amygdala, and confirmed previous studies linking the disorder to a smaller left hippocampus.

The study ruled out a link between differences in brain volumes between the two patient groups and the extent of depression, substance abuse, trauma load, or PTSD severity—factors the researchers took into account in their statistical model.

PTSD has affected nearly 14% of combat veterans serving in Iraq and Afghanistan, according to the Department of Veterans Affairs. PTSD is also estimated to afflict 6.8% of adults in the general population who have suffered abuse, crimes and other traumas.

Funding for the study came from NIH and the U.S. Department of Veterans Affairs.

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