Objectives: Exposure-based cognitive behavioral therapy via internet (Internet-CBT) has been shown to reduce symptoms and increase quality of life for children with functional abdominal pain disorders (FAPDs), but the mechanisms of change are unknown. The objective was to examine whether a change in symptom-specific fear and avoidance, i.e., gastrointestinal-specific anxiety (GI-anxiety) and gastrointestinal-specific avoidance (GI-avoidance), mediated changes in parent-reported abdominal symptoms for children receiving Internet-CBT compared with children receiving treatment as usual. A further aim was to assess if baseline levels of the proposed mediators moderated the mediation.

Methods: Weekly assessments of child-reported mediators and parent-reported outcome from 90 children aged 8-12 who were included in a randomized controlled trial were used in univariate and multivariate growth models to test the direct effect of treatment on outcome and the indirect effects via mediators and moderated mediation.

Results: Treatment condition significantly predicted the slope of the mediators (a-path), in favor of Internet-CBT, and mediators were correlated with the outcome (b-path). The indirect effects of the mediators on the outcome (cross-product of the a and b-paths) were significantly different from zero for both GI-avoidance, ab = 1.43, 95%CI [0.42, 3.23]; and GI-anxiety ab = 1.58, 95%CI [0.43, 3.62]. Baseline levels of the proposed mediators moderated the size of the mediation.

Conclusions: GI-anxiety and GI-avoidance were mediators of change in Internet-CBT and high levels of the mediators at baseline were associated with larger mediated effects. Healthcare professionals should be aware of, and inform families about, the potential benefits of reducing symptom-specific fear and avoidance.