Twin studies of posttraumatic stress disorder: differentiating vulnerability factors from sequelae.

Posttraumatic stress disorder (PTSD) is defined by one's response to an environmental event. However, genetic factors are important in determining people's response to that event, and even their likelihood of being exposed to particular traumatic events in the first place. Classical twin designs can decompose genetic and environmental sources of variance. Such studies are reviewed extensively elsewhere, and we cover them only briefly in this review. Instead, we focus primarily on the identical co-twin control design. This design makes it possible to resolve the "chicken-egg" dilemma inherent in standard case-control designs, namely, distinguishing risk from sequelae. Abnormalities that are present in both the twin with PTSD and the unaffected co-twin suggest pre-existing vulnerability indicators. These include smaller hippocampal volume, large cavum septum pellucidum, more neurological soft signs, lower general intellectual ability, and poorer performance in the specific cognitive abilities of executive function, attention, declarative memory, and processing of contextual cues. In contrast, abnormalities in a twin with PTSD that are not present in the identical co-twin suggest consequences of PTSD or trauma exposure. These include psychophysiological responding, higher resting anterior cingulate metabolism, event-related potential abnormalities associated with attentional processes, recall intrusions, and possibly some types of chronic pain. Most co-twin control studies of PTSD have been small and come from the same twin registry of middle-aged male veterans. Consequently, there is a great need for replication and extension of the findings, particularly in women and younger individuals. The creation of new twin registries would do much toward accomplishing this goal. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.