LOS ANGELES — A study of 80 men — 40 who saw combat in Vietnam and their twins who did not — suggests the size of a region of the brain involved in storing memories can predict one’s vulnerability to post-traumatic stress disorder.
Previous studies have found the region, called the hippocampus, is smaller than normal in the brains of veterans who suffer from the disorder, marked by flashbacks and sometimes overwhelming memories of traumatic experiences.
The assumption has been that stress caused the region to shrink in volume.
Now, a study that involved 40 sets of identical twins found the smaller volume is likely inherited and not a consequence of the trauma of combat.It suggests the hippocampus can increase one’s vulnerability to the syndrome’s effects.
“That would probably be the most likely explanation of the results,” said psychologist Mark Gilbertson of the Veterans Administration Medical Center in Manchester, N.H., and co-author of a study appearing Tuesday in the electronic edition of the journal Nature Neuroscience.
The study was sponsored by the Veterans Administration.
Post-traumatic stress disorder has afflicted nearly 31 percent of all Vietnam combat veterans at some time, according to government estimates.
In the new study, about half the 40 combat-veteran patients suffered from chronic, unremitting post-traumatic stress disorder.
The other half had never been affected, nor had any of the 40 stay-at-home twins.
In veterans who were affected, hippocampal volume was 10 percent smaller on average than in those who had never suffered from the syndrome, but who had seen combat.
Twins of the combat veterans who reported problems also had smaller hippocampi, even though they had seen no combat.
Most had served in the armed forces, however, Gilbertson said.
Since identical twins have similar brain structures, the finding suggests those veterans suffering from the disorder had smaller hippocampi before they entered combat.
The volume differences remained significant even when patients who reported childhood sexual or physical abuse were subtracted, further suggesting that a smaller hippocampus size increased vulnerability to the syndrome and was not caused by earlier trauma, Gilbertson said.
Gilbertson cautioned that smaller hippocampus volume did not guarantee a combat veteran would suffer from the syndrome; the severity of the combat experience is still a better predictor of that outcome.
Dr. J. Douglas Bremner of Emory University said it is still possible that environmental stresses caused the smaller volumes, ruling out a purely hereditary effect.
Early environmental stresses were presumably shared by the twins, explaining the similarity in their hippocampus volumes.
“It’s too early to say it’s totally genetic,” Bremner said.
“That stress has no effect on that part of the brain,” Bremner said.