Tonic immobility does not uniquely predict posttraumatic stress symptom severity.

Abstract

Tonic immobility (TI) is an involuntary state of temporary motor inhibition believed to occur in response to events that provoke extreme fear and the perception of inescapability. Human TI has been documented in a range of traumatic events and several researchers have reported associations between TI and posttraumatic stress symptoms (PTSS); however, it remains unclear if TI is a unique predictor of PTSS. This study was designed to determine whether TI severity would account for variance in PTSS severity over and above the influence of peritraumatic dissociation and trait anxiety. Participants were community members (n 75; 88% women; ages 18–65, Mage 31.49, SD 12.21) who reported TI during a traumatic event. TI, peritraumatic dissociation, and trait anxiety were assessed as part of a larger investigation. Results of hierarchical regression analyses indicated trait anxiety and peritraumatic dissociation, but not TI, were significant and substantive predictors of PTSS scores. In all analyses TI scores failed to account for significant variance in PTSS scores (all ps .05). Results suggest the TI construct may add little to understanding PTSS beyond what can be ascertained by assessing peritraumatic dissociation and trait anxiety. Given mixed findings to date, further investigation is required to disentangle what is shared and what is distinct among these constructs.