The role of illness perception and behaviour in the treatment of adolescents with multisystem functional somatic disorders: a post hoc mediation analysis of the AHEAD trial.

Psychological interventions can reduce symptom load and disability in adolescents with various functional somatic disorders (FSD). However, mechanisms of change are less investigated, especially in those with multisystem symptomatology. In a post-hoc analysis, illness perception, illness behaviour, and psychological inflexibility (experiential avoidance and fusion) were examined as mediators of changes in self-perceived physical health in adolescents 3 months after receiving group-based Acceptance and Commitment Therapy versus Enhanced Usual Care. Baseline levels of proposed mediators were also assessed for moderation effects. Data from 91 adolescents (15-19 years) with multisystem FSD (duration ≥1 year) from the RCT "ACT for Health in Adolescents" (AHEAD) were used in a classic mediation analysis. Mediators were measured at baseline (before assessment), 5.5 months (2 weeks after end-of-treatment), and 8 months (3 months after end-of-treatment). Improvements in negative illness perceptions and psychological inflexibility (experiential avoidance) mediated the effect of AHEAD on physical health at 8 months. The mediation effect increased with higher baseline levels, but the moderation effects were non-significant. AHEAD reduced negative illness perception and psychological inflexibility (experiential avoidance) and, consequently, improved physical health in adolescents with multisystem FSD. Reducing these factors may be important treatment targets in future treatment programmes for adolescents with FSD.