How does exposure therapy work? A comparison between generic and gastrointestinal anxiety-specific mediators in a dismantling study of exposure therapy for irritable bowel syndrome.

Objective: Systematic exposure is potentially an effective treatment procedure for treating irritable bowel syndrome (IBS), but little is known about the processes by which it achieves its effect on outcome. The aim of this study was to identify mediators in a previously published randomized dismantling trial in which participants with IBS were randomized to Internet-delivered cognitive-behavioral treatment (ICBT) that incorporated systematic exposure or to the same treatment protocol without exposure (ICBT-WE). Method: Weekly measurements of gastrointestinal anxiety-specific process variables (behavioral avoidance, gastrointestinal-specific anxiety) based on the gastrointestinal symptom-specific anxiety model, generic process variables (self-efficacy and mindful nonreactivity), and treatment outcome (IBS symptoms) were obtained from 309 participants with IBS. Growth models and cross-lagged panel models, estimated within structural equation modeling, were employed to evaluate mediators of outcome. Results: Parallel process growth modeling showed that behavioral avoidance, gastrointestinal-specific anxiety, self-efficacy mediated the incremental effect of ICBT compared to ICBT-WE. The mediated effect of avoidance was stronger for individuals scoring high on the avoidance variable at 1st measurement point. Cross-lagged regression analyses with random effects revealed that behavioral avoidance and gastrointestinal-specific anxiety had a stronger effect on subsequent symptom change rather than vice versa, whereas mindful nonreactivity and self-efficacy displayed the opposite pattern. Conclusions: The evidence collectively provided support for the hypothesis that exposure for IBS achieves its positive results by virtue of changing gastrointestinal anxiety-specific processes rather than generic processes. IBS-specific behavioral avoidance emerged as the most clear-cut mediator of the specific effect of exposure on outcome.