Tonic immobility during sexual assault - a common reaction predicting post-traumatic stress disorder and severe depression.

INTRODUCTION: Active resistance is considered to be the 'normal' reaction during rape. However, studies have indicated that similar to animals, humans exposed to extreme threat may react with a state of involuntary, temporary motor inhibition known as tonic immobility. The aim of the present study was to assess the occurrence of tonic immobility during rape and subsequent post-traumatic stress disorder and severe depression.

MATERIAL AND METHODS: Tonic immobility at the time of the assault was assessed using the Tonic Immobility Scale in 298 women who had visited the Emergency clinic for raped women within 1 month of a sexual assault. Information about the assault and the victim characteristics were taken from the structured clinical data files. After 6 months, 189 women were assessed regarding the development of post-traumatic stress disorder and depression.

RESULTS: Of the 298 women, 70% reported significant tonic immobility and 48% reported extreme tonic immobility during the assault. Tonic immobility was associated with the development of post-traumatic stress disorder (OR 2.75; 95% CI 1.50-5.03, p = 0.001) and severe depression (OR 3.42; 95% CI 1.51-7.72, p = 0.003) at 6 months. Further, previous trauma history (OR 2.36; 95% CI 1.48-3.77, p < 0.001) and psychiatric treatment history (OR 2.00; 95% CI 1.26-3.19, p = 0.003) were associated with the tonic immobility response.

CONCLUSIONS: Tonic immobility during rape is a common reaction associated with subsequent post-traumatic stress disorder and severe depression. Knowledge of this reaction in sexual assault victims is important in legal matters and for healthcare follow up.