Can Pre-Existing Neurological "Soft Signs" Predict PTSD Vulnerability?

Neurological "soft signs" are

neurological abnormalities that are not readily localizable to a specific brain region, while hard signs provide some indication of the underlying brain systems or regions that are affected (Ismail et al., 1998).

Pre-existing, mild neurological deficits may have increased the risk of PTSD in a sample of Vietnam combat veterans, says a study published in the May 2006 issue of Archives of General Psychiatry. How did they surmise the pre-existing neurological state of the vets?

Gurvits et al. examined 45 neurological soft signs in Vietnam combat veterans and their combat-unexposed, identical co-twins. The unexposed co-twins of the combat veterans with posttraumatic stress disorder (PTSD) had significantly higher neurological soft sign scores than the unexposed co-twins of the veterans without PTSD. This result supports the conclusion that subtle neurological dysfunction represents an antecedent familial vulnerability factor for developing chronic PTSD on exposure to a traumatic event.

The authors argue that the stress--> cortisol--> hippocampal shrinkage--> PTSD pathway [Sapolsky RM. Why stress is bad for your brain. Science. 1996;273:749-750] isn't as directly causal as the arrows above might suggest. This is an expansion of their earlier work published in Nature Neuroscience:

Gilbertson MW, Shenton ME, Ciszewski A, Kasai K, Lasko NB, Orr SP, Pitman RK. Smaller hippocampal volume predicts pathologic vulnerability to psychological trauma. Nat Neurosci. 2002;5:1242-1247.

It's incredibly impressive that they drew from a sample of 103 male identical twin pairs discordant for combat in Vietnam. 49 twin pairs participated in the current study: 25 pairs in which the combat vet had PTSD, and 24 pairs in which the combat vet never had PTSD.

Tamara V. Gurvits, MD, PhD; Linda J. Metzger, PhD; Natasha B. Lasko, PhD; Paul A. Cannistraro, MD; Alexandra S. Tarhan, MD, PhD; Mark W. Gilbertson, PhD; Scott P. Orr, PhD; Anna M. Charbonneau, BA; Michelle M. Wedig, BS; Roger K. Pitman, MD

Subtle Neurologic Compromise as a Vulnerability Factor for Combat-Related Posttraumatic Stress Disorder: Results of a Twin Study

Arch Gen Psychiatry. 2006;63:571-576.

Conclusions These results replicate previous findings of increased NSSs in Vietnam combat veterans with PTSD. Furthermore, results from their combat-unexposed identical co-twins support the conclusion that subtle neurologic dysfunction in PTSD is not acquired along with the trauma or PTSD but rather represents an antecedent familial vulnerability factor for developing chronic PTSD on exposure to a traumatic event.