Predictors of Treatment Success of Psychotherapy in Functional Disorders: A Systematic Review of the Literature

Objective: Functional disorders (FDs) benefit from psychotherapy. However, the determinants predicting their efficacy remain largely unexplored. Methods: A systematic literature review was conducted. PubMed, Web of Science, Embase, Cochrane collaboration and grey literature were screened from inception to November 2024. Randomized controlled trials on predictors of success of psychotherapy for FDs (e.g., somatoform disorders, irritable bowel syndrome [IBS], chronic fatigue syndrome/myalgic encephalomyelitis [CFS/ME], fibromyalgia [FM]) in adults (i.e., ≥ 18 years of age) were included. The review yielded 24 eligible studies and included 3382 participants. A standardized quality assessment via ROB-2 Tool was performed. PRISMA guidelines were followed. Results: Most studies applied CBT-based interventions (n = 19), mainly face-to-face, with some internet-based (n = 5), while fewer used emotional-based (n = 4), mindfulness-based (n = 3), psychodynamic (n = 1) or operant behavioural therapy (n = 1). The primary factors identified as predictive of treatment success in FM and somatization were the intensity of experienced pain. Moreover, the presence of mental disorders, i.e., depression and anxiety disorders, emerged as predictors for a range of disorders including FM, IBS, somatization disorder, hypochondriasis, medically unexplained symptoms and dissociative seizures. Symptom severity was recognized as a predictor across various FDs with findings indicated that severe severity could predict treatment outcomes. Conclusion: The body of research concerning predictors of treatment success in the context of FDs can help clinicians identifying appropriate psychotherapy trajectories. Trial registration: Not applicable. PROSPERO no. CRD42022379791; OSF (https://osf.io/8q7z9).