Peritraumatic tonic immobility in a large representative sample of the general population: association with posttraumatic stress disorder and female gender

Background: Tonic immobility is an involuntary response to inescapable life-threatening events. Peritraumatic tonic immobility has been reported in convenience samples of female victims of sexual assault and in mixed-gender victims of different types of trauma. This study evaluated peritraumatic tonic immobility in a representative general population sample and its association with posttraumatic stress disorder (PTSD) and gender. Methods: 3231 victims of traumatic events aged 15–75 years responded to the Tonic Immobility Scale. PTSD and traumatic events were assessed using the Composite International Diagnostic Interview (CIDI 2.1). We calculated the means and the standard deviations of Tonic Immobility Scale scores stratified by PTSD and gender. The association between tonic immobility scores and gender was explored controlling for potential confounders through a multiple linear regression model. Results: Tonic immobility scores were more than double in those who met criteria for PTSD and were almost four points higher in women. Gender differences remained statistically significant even after adjustment for confounding variables. Limitations: The cross-sectional and retrospective design may have given rise to recall bias. Results presented here may not apply to small and medium rural areas and the CIDI 2.1 can lead to a certain degree of misclassification. Conclusions: We have expanded the scope of previous investigations on peritraumatic tonic immobility which were based on convenience samples only, showing its occurrence in victims of traumatic events using a large representative sample of the general population. Furthermore, we confirmed in an unbiased sample the association between peritraumatic tonic immobility and PTSD and female gender.