Subtle neurologic compromise as a vulnerability factor for combat-related posttraumatic stress disorder: results of a twin study
Gurvits TV, Metzger LJ, Lasko NB, Cannistraro PA, Tarhan AS, Gilbertson MW et al.;
Commented by , 23 May 2006
Background
Recent studies have found neurologic soft signs in patients with chronic posttraumatic stress disorder (PTSD).
Aims of the study
To clarify the relationship between traumatic events, PTSD and neurologic soft signs.
Method
45 neurologic soft signs were assessed in 49 twin pairs. One twin served in combat in Vietnam and his co-twin did not. In 25 pairs, the combat veteran had current combat-related PTSD. In 24 pairs, the combat veteran had never had combat related PTSD. Characteristics of combat-exposed Vietnam Veterans with and without PTSD were compared with those of their combat-unexposed identical co-twins.
Results
Combat veterans with PTSD had significantly higher neurologic soft sign scores than combat veterans without PTSD. The "high-risk", unexposed co-twins of the former also had significantly higher NSS scores than the "low-risk", unexposed co-twins of the latter
Professor Pull's comments
In a previous study by the same authors (ref. 1), neurologic soft sign scores of women with PTSD owing to sexual abuse in childhood and veteran men with combat-related PTSD were significantly higher than those of women sexually abused as children and combat veteran men without PTSD.
As stated by the authors of the present study, it is important to keep in mind that although PTSD is (indisputably) the result of a psychologically traumatic event, this does not mean that all abnormalities associated with PTSD are as well.
Associated abnormalities may be related to the traumatic event in two ways:
- they may have been present prior to the occurrence of the traumatic event and have increased the individual's likelihood of developing PTSD on traumatic exposure
- they may be acquired PTSD signs that developed after the traumatic exposure, along with the PTSD.
Studying identical twins discordant for traumatic exposure offers an excellent opportunity to clarify the nature of the association between such abnormalities, PTSD and the traumatic event.
Some abnormalities are acquired signs of PTSD rather than familial vulnerability factors. In a study (ref. 2) involving identical twins discordant for traumatic event, Orr et al investigated physiologic responses (i.e. heart rate, skin conductance and orbicularis oculi electromyogram) to sudden, loud tones in monozygotic twins discordant for combat exposure.
Averaged heart rate responses to loud tones were larger in Vietnam combat veterans with PTSD. These larger responses were not shared by their non-combat-exposed co-twins, whose responses were similar to those of the non-PTSD combat veterans and their non-combat-exposed co-twins.
Other abnormalities do not reflect brain damage acquired along with the PTSD but instead represent familial vulnerability factors, which likely antedate the traumatic exposure. The finding in the present study that the (high-risk) unexposed co-twins of the combat veterans with PTSD had significantly higher average NSS scores than the (low-risk) unexposed co-twins of the combat veterans without PTSD clearly supports the interpretation that neurologic soft signs are premorbid vulnerability factors for PTSD.
On the whole, the present investigation is an important contribution to the ongoing debate about the nature of the relationship between traumatic events, PTSD and the presence of associated abnormalities, including neurologic soft signs.
References
1. Gurvits TV, Gilbertson MW, Lasko NB, Tarhan AS, Simeon D, Macklin ML, et al. Neurologic soft signs in chronic posttraumatic stress disorder. Archives of General Psychiatry; 57 (2); 181-6
2. Orr SP, Metzger LJ, Lasko NB, Macklin ML, Hu FB, Shalev AY, et al. Physiologic responses to sudden, loud tones in monozygotic twins discordant for combat exposure: association with posttraumatic stress disorder. Archives of General Psychiatry; 60 (3); 283-8