Recent combat veterans diagnosed with post traumatic stress disorder (PTSD) have significantly smaller volume in the amygdala, a small structure in the brain critical for regulating fear and anxiety responses, according to new research.

Researchers at Duke University and the Durham VA Medical Center say their study provides evidence that smaller amygdala volume is associated with PTSD, regardless of the severity of trauma. But, they add, it’s not clear whether the physiological difference was caused by a traumatic event, or whether PTSD develops more readily in people who naturally have smaller amygdalas.

“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,” said Rajendra A. Morey, M.D., M.S., assistant professor at Duke and lead author of the study.

Morey noted that studies in animals have established the amygdala’s role in regulating fear, anxiety and stress responses, but its effect on human behavior is less well known. “It’s associated with how fear is processed, especially abnormal fear processing,” he said. “So it makes sense to look at the structure of the amygdala.”

The researchers recruited 200 combat veterans who served in Iraq and Afghanistan after Sept. 11, 2001; half had PTSD and the other half had been exposed to trauma, but did not developed PTSD. Amygdala and hippocampus volumes were computed from MRI scans of all 200.

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 researchers emphasize that the differences in brain volumes were not due to the extent of depression, substance abuse, trauma load or PTSD severity, factors they took into account in their statistical model.

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

“The next step is to try to figure out whether a smaller amygdala is the consequence of a trauma, or a vulnerability that makes people get PTSD,” Morey said.

He said the study showed that amygdala volume does not appear to be affected by the severity, frequency or duration of trauma, indicating that these factors do not cause the amygdala to shrink. It appears more likely, according to the researchers, that people with measurably smaller amygdala to begin with are susceptible to PTSD, but more studies are needed to make that determination.

Morey said he and colleagues are exploring that question, and are intrigued by evidence from their study that suggests people may have a propensity for developing PTSD based on inherently smaller amygdala volume.

“This is one piece in a bigger puzzle to understanding why some people develop PTSD and others do not,” Morey said. “We are getting closer to that answer.”

Funding for the study, which was published in the journal Archives of General Psychiatry, came from the U.S. Department of Veterans Affairs and the National Institutes of Health.

Source: Duke University Medical Center

PTSD Linked to Smaller Brain Area That Helps Regulate Fear