The purpose of this trial is to evaluate the clinical efficacy, the cost-effectiveness and the 6-month durability of a therapist-guided, Internet-delivered cognitive-behavior therapy programme for children and adolescents with body dysmorphic disorder.

Primary objective: To determine the clinical efficacy of a therapist-guided, Internet-delivered cognitive-behaviour therapy (ICBT) programme for body dysmorphic disorder (BDD) in reducing BDD symptom severity in children and adolescents with BDD, compared to a control intervention consisting of a therapist-guided, Internet-delivered relaxation treatment (IRT) for BDD.

Secondary objective: To establish the 6-month durability of the treatment effects and to assess the cost-effectiveness of ICBT, compared with IRT, from multiple perspectives and to conduct a health-economic evaluation of ICBT for BDD at the primary endpoint from a health organisation payer, healthcare resource use, and societal perspective.

Type of trial: A multisite parallel-group randomised controlled superiority trial.

Rationale: BDD is a prevalent and impairing disorder that tends to have a chronic course if left untreated. Adolescent-onset BDD is associated with more severe symptoms, greater lifetime comorbidity, and higher rates of attempted suicide compared to adult-onset BDD. Therefore, early intervention is crucial. BDD can be effectively treated with cognitive-behaviour therapy (CBT), although the current evidence is rather weak and more evidence is needed. Furthermore, CBT for BDD is a highly specialised treatment and many young people do not have access to it. ICBT can be a way to increase the availability of an effective, evidence-based treatment for children and adolescents with BDD.

Trial design and methods: Participants will be recruited nationally across Sweden and will be offered 12 modules of therapist-guided ICBT or 12 modules of therapist-guided IRT delivered over 12 weeks. Under certain circumstances, such as illness or holidays, the design allows participants to pause their therapist-support for a maximum of two weeks, which may extend the treatment length to a maximum of 14 weeks. All potential participants will be initially screened via the telephone or at one of the three participating sites. This will be followed by an inclusion/baseline assessment conducted either at one of the three clinics (BUP OCD och relaterade tillstånd, BUP Specialmottagning or BUP Skåne) or, if face-to-face assessments are not feasible, via a secure video application. Participants who are eligible and have consented will be randomised to one of two trial arms. Participants in the comparator group (IRT) will be offered to cross-over to the ICBT intervention after the primary endpoint.

The primary outcome variable: The primary outcome variable is BDD symptom severity measured by the Yale-Brown Obsessive Compulsive Scale modified for BDD, Adolescent version (BDD-YBOCS-A) at the primary endpoint (1-month follow-up post-treatment). Based on BDD-YBOCS-A scores, responder and remission rates at all follow-up points will be calculated. Response will be defined as ≥30% reduction from baseline; full or partial remission will be defined as a score ≤16.

Planned trial sites: The study will be coordinated from the Department of Clinical Neuroscience at Karolinska Institutet (the Sponsor). There will be 3 collaborating study sites: BUP OCD och relaterade tillstånd (Region Stockholm), BUP Specialmottagning (Västra Götalandsregionen), and BUP Skåne (Region Skåne). Each of the three sites will assess and treat participants from their own region, and occasionally from other regions.

Sample: A total of 154 children and adolescents diagnosed with BDD and their primary caregivers.

Statistical methodology and analysis: Data will be analysed using a pre-specified intention-to-treat statistical analysis plan.

Inclusion Criteria:

1. A primary diagnosis of BDD, based on the diagnostic criteria of the 5th edition of the Diagnostic and Statistical Manual of Mental Disorders. Confirmed by the assessor at the inclusion assessment using a semi-structured diagnostic interview. In cases where it is challenging to rule out the presence of actual physical defects that are clearly noticeable (e.g., if the area of concern is the genitalia), we will make a referral to an appropriate service for an expert opinion.
2. A total BDD symptom severity score on the BDD-YBOCS-A ≥24. Confirmed by the assessor at the inclusion assessment.
3. Age between 12 and 17 years. Confirmed by the caregiver/parent and subsequently by the medical record.
4. A minimum of one available caregiver/parent being able to co-participate and support the child/adolescent throughout the treatment. Confirmed by the caregiver/parent at the inclusion assessment.
5. Regular access to a computer connected to the Internet and a mobile phone to receive SMS messages (one of each per family is enough). Confirmed by the caregiver/parent at the inclusion assessment.

Exclusion Criteria:

1. Previous CBT for BDD for a minimum of five sessions with a qualified therapist within the 12 months prior to the inclusion assessment. Confirmed by the caregiver/parent at the telephone screening and/or the inclusion assessment and by the medical record.
2. Simultaneous psychological treatment for BDD or for any psychiatric comorbidity. Confirmed by the caregiver/parent at the telephone screening and/or the inclusion assessment and by the medical record.
3. Initiation, change of dosage or cessation of any selective serotonin reuptake inhibitors (SSRI) or antipsychotic drugs within the six weeks prior to the inclusion assessment. Confirmed by the caregiver/parent at the telephone screening and/or the inclusion assessment and by the medical record.
4. A diagnosis of organic brain disorder, intellectual disability, psychosis, bipolar disorder, eating disorder, severe depression or alcohol/substance dependence. Confirmed by the caregiver/parent at the telephone screening and/or the inclusion assessment, with supplemental information from the semi-structured diagnostic interview and by the medical record.
5. Immediate risk to self or others requiring urgent medical attention or inpatient care, such as suicidality, or repeated self-injurious behaviours. Confirmed by the caregiver/parent at the telephone screening and/or the inclusion assessment.
6. A documented suicide attempt in the last 12 months. Confirmed by the caregiver/parent at the telephone screening and/or the inclusion assessment or by documentation in the medical record.
7. Child/adolescent and caregiver/parent not able to read and communicate in Swedish. Confirmed by the caregiver/parent at the telephone screening and/or the inclusion assessment.
8. Having a close relationship to an already included participant (e.g., sibling, cousin), to avoid being randomised into two different arms, with the risk of information "leaking" between the groups. Confirmed by the caregiver/parent or assessor at the telephone screening and/or at the face-to-face or video conference inclusion assessment.

All study personnel who can be blind to study aims/hypotheses and group allocation, will be blind. Assessors conducting post-treatment and follow-up assessments will be external to the research team and blind to study aims/hypotheses and treatment allocation for the full duration of the trial (up to the 6-month follow-up). The assessors will not receive any information about the study design, objectives or treatment conditions in order to maximise the blinding integrity. At each follow-up assessment, participants will be reminded by their assessor to not reveal details about their treatment.