Mediators of symptom improvement in transdiagnostic internet cognitive behavioural therapy for mixed anxiety and depression

Objectives Transdiagnostic internet cognitive behavioural therapy (iCBT) improves depression and anxiety symptoms, but the factors mediating these improvements are unknown. Design In a secondary analysis of a randomised controlled trial, we compared the impact of a transdiagnostic iCBT program (n = 39) on emotion regulation, repetitive negative thinking, and experiential avoidance relative to treatment-as-usual (TAU) control (n = 35). The extent to which changes in these processes mediated treatment gains in iCBT relative to TAU was also explored. Intention-to-treat mixed models were used to assess outcomes, and unmoderated simple mediation models were used to explore mediators of change of each outcome (depression/anxiety). Results Results showed iCBT outperformed TAU in reducing all three processes, with largebetween-group effect sizes for repetitive negative thinking (g = 1.04), and medium effect sizes for emotion regulation (g = 0.78), and experiential avoidance (g = 0.54). Changes in symptoms of depression and anxiety were partly mediated through pre-to-post changes in all three variables, but not pre-to-mid-treatment changes. Conclusion These findings provide evidence that transdiagnostic iCBT for depression and anxiety improves emotion regulation, reduces repetitive thinking and experiential avoidance, and provides preliminary evidence that symptom improvements are associated with improvements in these variables. KEY POINTS What is already known about this topic: (1) Transdiagnostic interventions aim to target the common symptoms and the functional emotional, cognitive, and behavioural processes shared across emotional disorders. (2) Transdiagnostic internet-delivered cognitive behavioural therapy (iCBT) improves depression and anxiety symptoms. (3) However, the factors mediating these symptom improvements are largely unknown. What this topic adds: (1) Transdiagnostic iCBT for depression and anxiety improves emotion regulation and reduces repetitive thinking and experiential avoidance in participants with an anxiety and/or depressive disorder. (2) Symptom improvements are associated with improvements in these transdiagnostic variables; emotion regulation, repetitive negative thinking, and experiential avoidance appear to be significant mediators of post-treatment improvements in symptoms of depression and anxiety. (3) Targeting these shared underlying processes is likely important to improve symptoms of depression and anxiety.