PTSD: An Interaction of Combat Stress and Hippocampal Volume

Animal studies demonstrated that repeated and prolonged stress for an animal can produce damage to the hippocampus.  The hippocampus is an area of the brain that has been implicated in declarative memory for humans.  Declarative memory include memories that were created from  our school learning as an example,  and those personal memories such as what our 12th birthday was like.

Given the intriguing findings about the effects of extreme stress on  the hippocampus in animals, MRI researchers looked into the structural aspects of the hippocampus in humans who suffered chronic unremitting forms of posttraumatic stress disorder (PTSD).  They found that those persons who had been diagnosed with PTSD had smaller hippocampal volume.  Given these findings, 2 questions  remained:  was smaller hippocampal volume a neurotoxic effect of combat stress, or 2) was smaller hypocampal volume a pre-existing condition that rendered the person vulnerable to PTSD?

To address this question, Mark Gilbertson, of the Dept of Psychiatry at Harvard University, and colleagues obtained a sample of monozygotic (or identical) twins who were discordant for combat duty  (one twin had been in combat, the other had not).

Gilbertson, et al. found that there was a positive correlation between the volume of the hippocampus in the twin pairs irrespective of the presence of PTSD in one of the twins.  Further correlations showed significant negative relationship between hippocampal volume and PTSD symptom severity; also there was a significant negative correlation between hippocampal volume in the unexposed (to combat) twin and PTSD severity in the exposed brother (a smaller volume in the unexposed brother = greater PTSD symptom severity in the exposed co-twin).

To further test for the neurotoxic vs. pre-existing condition question, Gilbertson, et al. set up a series of four contrasts.  1) The first contrast examined if there was  a significant volume difference in those veterans with PTSD vs. those without PTSD? (a replication of earlier findings).  2) a comparison of hippocampal volume between the exposed twin vs. the unexposed twin (neurotoxic effect test). 3) contrast of hippocampal volumes in the two groups of combat-unexposed co-twins whose combat exposed brothers did versus did not develop PTSD (this interaction effect was a test for the pre-existing vulnerability hypothesis).

The contrast results confirmed a vulnerability model and not a neurotoxic model.  Given the diathesis of reduced hippocampal volume, and given the sufficient condition of combat exposure, the person is at risk for developing a severe form of PTSD.  This outcome remained even when rival hypotheses including – prior traumatic experience, severity of alcoholism history, history and severity of depression history, and the severity of combat experience  were controlled.

Heredity is the most likely explanation for these results.  However, this design did not allow for an assessment of  shared enviornment effects.  An  addition of a sample of dizygotic (fraternal) twins discordant for combat experience would allow for a separation of heredity and shared environment effects.

Reference

Gilbertson, M.W., Shenton, M.E., Ciszewski, A., Kasai, K., Lask, N.B., Orr, S.P., & Pitman, RK. (2002).  Smaller hippocampal volume predicts pathologic vulnerability to psychological trauma. Nature Neuroscience, 5, 1242 – 1247.

An abstract of this study can be obtained here.  The email address of the lead author is available at the preceding link.

I sincerely thank Dr. Gilbertson for promptly emailing me a copy of the full study.

4 Comments

Filed under Healthcare, Psychological Disorders, Psychology Ramblings..., Research, WAR

4 responses to “PTSD: An Interaction of Combat Stress and Hippocampal Volume

  1. Interesting study, iggy. It suggests that in a situation where an individual will be exposed to the stress of combat, examination of the hippocamal volume should be undertaken; or, alternatively, in the making of decisions concerning redeployment, the hippocamal volume of the potentially redeployed member be considered prior to another tour.

    Being vaguely familiar with the procedures involved in determining service connected disability, I am concerned that there will be some attempts made to use this study as a basis for denial of benefits to those suffering from PTSD. Was this traditional tort law, one takes the injured party as one found him; this is not the general rule in disability cases.

  2. iggydonnelly

    6176,

    You bring up an implication of these findings I had not thought of. Denying people with the more severe forms of this disorder disability benefits would save a great deal of money.

    The memory problem for people with PTSD is not that they have repressed trauma, but rather they can’t forget trauma. There was a very long controversy about repressed sexual abuse trauma that would suddenly appear during a course of psychotherapy. I think there is general agreement now that sexual abuse trauma does not get repressed, and instead the person has more PTSD type features:
    1) reliving the trauma when exposed to stimuli similar to the trauma,
    2) having nightmares about the trauma
    3) flashbacks
    4) emotional numbing
    5) severe anxiety;

    A few criteria I can recall off the top of my head.

    BTW, odors are often potent triggers for flashbacks. Smells are processed directly through the amygdala – no other sense does that. The amygdala is a brain center that evaluates the emotional meaning of incoming stimuli. The fact that smells are processed directly through this center makes me think that this rapid evaluation function had an importance in our evolutionary history. But I believe the world and man evolved for longer than 10K years – so be advised of that bias.

    http://biology.about.com/library/organs/brain/blamygdala.htm

  3. Iggy, be advised that you and I share the same bias.

    On the possible use of the study to deny benefits; I must admit to my legal training coming to the fore as soon as I read your post. Again, my familiarity with the rabbit warren of regulations, etc., concerning disability ratings is extremely limited, but based upon that limited knowledge combined with the results of the study, it was a natural conclusion to draw (well, natural for me).

  4. lilacluvr

    On the MSNBC Chris Matthews show last night, it was reported that a Salon (?) reporter got hold of a taped recording of some military doctor telling people that military doctors are under alot of pressure to not diagnose PTSD. The top brass would rather see, severe anxiety – than PTSD.

    Does anyone know how Obama feels about this issue? Has it even been brought up before or is this first rumbling of this particular storm?