Abstract
OBJECTIVE: This study conducted a preliminary naturalistic effectiveness evaluation of Individualised Metacognitive Therapy (MCT+) delivered via videoconferencing for individuals with bipolar I disorder (BD-I) in a real-world clinical setting in Chile. It also explored how therapist characteristics-specifically mentalisation capacity-influence the therapeutic alliance in digital psychotherapy. METHODS: A longitudinal quantitative case series design was implemented across 14 therapist-patient dyads. Patients received 12 weekly sessions of MCT+ online. Standardised measures assessed anxiety (GAD-7), depression (PHQ-9), metacognitive beliefs (MCQ-30), psychological distress (CORE-10) and quality of life (WHOQOL-BREF). Therapeutic alliance was tracked session-by-session (WAI-S). Therapist mentalisation and attachment were evaluated at baseline (MASC-SP, RFQ-8, ECR-12). Changes in outcome measures were analysed using paired t-tests, effect sizes (Cohen's d), correlations, reliable change indices (RCI) and hierarchical linear modelling (HLM). RESULTS: Across the intervention, there were significant decreases in anxiety (d = 0.64) and improvements in metacognitive beliefs (d = 0.37). Depression showed a modest improvement (d = 0.34), while changes in quality of life were negligible (d = -0.21). Hierarchical modelling indicated a significant interaction between automatic and controlled mentalisation (b = -0.45, p = 0.008), suggesting that flexible adjustment supported therapeutic alliance development. Attachment style showed no significant associations with the alliance (largest unadjusted effect: ρ = -0.54, p = 0.073; all adjusted ps > 0.99). CONCLUSIONS: Digital MCT+ showed preliminary effectiveness in reducing anxiety and maladaptive metacognitive beliefs among individuals with BD-I, with more limited effects on depression and quality of life. Importantly, therapist mentalisation flexibility-the capacity to shift between automatic and controlled modes, or the mentalisation switch-emerged as a central mechanism for alliance building and engagement in digital contexts, highlighting a key target for clinical training and future research.