The goal of this feasibility study is to test whether a new blended digital intervention for adolescents with anxiety disorders is feasible and acceptable for use, and to help plan future larger scale studies. The main questions it aims to answer are: * A: Can the investigators recruit appropriate participants? * B: How appropriate are the data collection procedures and outcome measures? * C: Are the study procedures and interventions suitable for and acceptable to the participants? * D: Does the research team have the resources and ability to manage the study and intervention? * E: Does the intervention show promise of being successful for adolescents with anxiety disorders? Participants will: * Receive outpatient anxiety treatment, typically every other week or weekly. * Use the Sidekick app as a homework supplement to plan, carry out, and evaluate exposure exercises between treatment sessions. * Complete questionnaires before, during, after, and three months after using the app. * Some participants and therapists will be invited to take part in qualitative interviews following the intervention.
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Sidekick app engagement and completion of tasks
Timeframe: From pre-treatment/baseline to post-treatment (variable and participant-dependent, estimated range: 4-12 weeks)
Credibility and Expectancy Questionnaire (CEQ-1)
Timeframe: At post-treatment (variable and participant-dependent, estimated range: 4-12 weeks post-baseline)
Client Satisfaction Questionnaire (CSQ-3)
Timeframe: At post-treatment (variable and participant-dependent, estimated range: 4-12 weeks post-baseline)
EuroQoL 5D-5L (EQ-5D-5L)
Timeframe: At pre-treatment/baseline and post-treatment (variable and participant-dependent, estimated range: 4-12 weeks post-baseline)
Level of anxiety before, during and after exposure tasks
Timeframe: From pre-treatment/baseline to post-treatment (variable and participant-dependent, estimated range: 4-12 weeks) after every completed exposure task (variable and participant-dependent, estimated range: 1-20 exposure tasks per month)
Recruitment
Timeframe: At pre-treatment/baseline.
Adherence
Timeframe: From pre-treatment/baseline to post-treatment (variable and participant-dependent, estimated range: 4-12 weeks post-baseline), and from post-treatment to 3-month follow-up.
Qualitative interviews
Timeframe: At post-treatment (variable and participant-dependent, estimated range: 4-12 weeks post-baseline).
Revised Child Anxiety and Depression Scale (RCADS 47)
Timeframe: At pre-treatment/baseline, post-treatment (variable and participant-dependent, estimated range: 4-12 weeks post-baseline), and 3-month follow-up.
General self-efficacy scale (GSES)
Timeframe: At pre-treatment/baseline, post-treatment (variable and participant-dependent, estimated range: 4-12 weeks post-baseline), and 3-month follow-up.
KIDSCREEN-10
Timeframe: At pre-treatment/baseline, post-treatment (variable and participant-dependent, estimated range: 4-12 weeks post-baseline), and 3-month follow-up.
Patient Exposure and Response Prevention Adherence Scale (PEAS)
Timeframe: From pre-treatment/baseline to post-treatment (variable and participant-dependent, estimated range: 4-12 weeks) after every completed exposure task (variable and participant-dependent, estimated range: 1-20 exposure tasks per month)