BACKGROUND: Internet-delivered therapist-guided therapy (e-therapy) represents a promising approach for enhancing accessibility, treatment fidelity, and scalability within child and adolescent mental health services (CAMHS).

OBJECTIVE: This systematic review aimed to (1) identify and synthesize determinants of implementation, specifically barriers to and facilitators of e-therapy in CAMHS structured according to the Consolidated Framework of Implementation Research (CFIR); and (2) provide pooled benchmark estimates of key implementation outcomes for fidelity, cost-effectiveness, and acceptability.

METHODS: A PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses)-compliant systematic review was performed across PsycINFO, MEDLINE, Web of Science, CINAHL, Embase, Cochrane, and ProQuest Dissertations & Thesis on June 6, 2025-to identify peer-reviewed studies assessing implementation outcomes or determinants of e-therapy in the context of outpatient CAMHS (ages 8-18 years). Barriers and facilitators were synthesized qualitatively with thematic analysis applying CFIR. A parallel quantitative synthesis of Proctor et al's taxonomy of implementation outcomes was performed using Bayesian multilevel random-effects meta-analyses to estimate pooled effect sizes and 95% credible intervals (CIs). By combining quantitative benchmarks of implementation success with qualitative insights into contextual determinants, the review provides an integrated understanding of what drives effective e-therapy implementation in CAMHS. Study quality was assessed using the CASP (Critical Appraisal Skills Programme) checklist, Cochrane Risk of Bias tool, and Risk Of Bias In Non-randomized Studies-of Interventions tool. Small study effects were evaluated using funnel plots, sensitivity analyses, and the Egger test.

RESULTS: From 50,026 screened reports, 50 studies published between 2007 and 2025 were included: 18 randomized controlled trials, 17 cohort, and 15 qualitative or mixed methods studies. Most studies originated from Western Europe (n=34), Northern America (n=11), and Oceania (n=5), targeting anxiety (n=24) and depression (n=9), through cognitive behavioral therapy-based programs (n=47), with parallel parent content (n=31). Therapist guidance was primarily asynchronous (n=43). Among the 39 studies reporting determinants, common barriers and facilitators were identified across intervention, organization, therapist, and patient domains, structured via CFIR. Pooled implementation outcomes showed modest dropout rates (~20%, CI 14%-27%), high module completion (~68%, CI 60%-75%), low therapist time (24 min per wk per patient, 95% CI 19-28), and high patient satisfaction (24/32 on Client Satisfaction Questionnaire-8, 95% CI 22-27; and 76% satisfaction rate, 95% CI 62%-87%), suggesting e-therapy is resource efficient and acceptable if implemented successfully.

CONCLUSIONS: This review provided the first integrated synthesis of pooled benchmarks for implementation outcomes of e-therapy in CAMHS and modifiable determinants to inform future service planning and scale-up. These findings highlighted service-level enablers, such as leadership anchoring, targeted use, technical stability, structured patient flow, and therapist training, that organizations could prioritize to strengthen sustainable e-therapy implementation in CAMHS.