Effect of transdiagnostic therapy on psychological variables and symptoms of adolescents with sub-clinical body dysmorphic disorder: a randomized clinical trial

Body dysmorphic disorder (BDD) is a debilitating condition often accompanied by self-focused attention biases, depressive symptoms, and social adjustment difficulties. While disorder-specific treatments reduce core symptoms, they frequently fall short of addressing comorbid psychopathology. This study evaluated the efficacy of the Unified Protocol for Adolescents (UP-A) as a transdiagnostic cognitive-behavioral therapy (CBT) in adolescents with subclinical BDD. This randomized clinical trial employed a parallel-group superiority design with a 1:1 allocation ratio and included 62 adolescents aged 12–18 years in Zanjan, Iran, who exhibited subclinical body dysmorphic disorder (BDD), operationalized as an Anxiety Disorder Interview Schedule (ADIS-IV-C) Clinical Severity Rating (CSR) of 2 or 3 (i.e., ≥ 2 and < 4). Participants were randomly assigned to either the UP-A intervention group ( n = 31, delivered online over 12 sessions) or a waitlist control group ( n = 31), which received educational pamphlets after assessment. Assessments were conducted at baseline, post-intervention (2 months), and at 6-month follow-up using the Focus of Attention Questionnaire (FAQ), Dysmorphic Concerns Questionnaire (DCQ), Hamilton Depression Rating Scale (HDRS), and the Social Adjustment subscale of the Adjustment Inventory for School Students (AISS). Marginal longitudinal model analysis indicated that the intervention group achieved significantly greater reductions in self-focused attention (FAQ) and BDD symptoms (DCQ) compared to the control group at both post-test and 6-month follow-up ( P < 0.05). For social adjustment (AISS), a significant group-by-time interaction was observed at post-test (β = −1.83, P < 0.001), indicating an acute improvement in the intervention group; however, between-group differences were not statistically significant at post-test ( P = 0.29) or at follow-up ( P = 0.40). Regarding depressive symptoms (HDRS), although the intervention group showed significant within-group improvements over time ( P < 0.05), between-group comparisons revealed no statistically significant differences at post-test ( P = 0.34) or at 6-month follow-up ( P = 0.11). The UP-A is an effective transdiagnostic intervention for adolescents with subclinical BDD, yielding significant reductions in dysmorphic concerns and self-focused attention, alongside gradual improvements in social adjustment. However, its specific clinical efficacy in reducing comorbid depressive symptoms beyond waitlist and temporal effects was not supported, highlighting the need for targeted clinical monitoring. Iranian Registry of Clinical Trials (IRCT ID: IRCT20210428051113N2; Registration Date: 16/03/2023).