Cavum septum pellucidum in monozygotic twins discordant for combat exposure: relationship to posttraumatic stress disorder.

BACKGROUND Abnormally large cavum septum pellucidum has been reported in posttraumatic stress disorder; however, the origin of this association is uncertain. METHODS We utilized magnetic resonance imaging to measure cavum septum pellucidum in pairs of identical twins discordant for combat exposure in Vietnam. RESULTS Presence of abnormal cavum septum pellucidum was significantly correlated between exposed and unexposed twins, indicating that it is partially determined by heredity and/or shared environment. There was a greater proportion of cavum septum pellucidum in combat-exposed twins with posttraumatic stress disorder and their noncombat-exposed co-twins. CONCLUSIONS The presence of abnormally large cavum septum pellucidum is a familial vulnerability factor for posttraumatic stress disorder.