This study aims to investigate the characteristics of the microbiota-gut-brain axis in pediatric patients with obsessive-compulsive disorder (OCD) through performing an integrated analysis of gut microbiota, serum metabolomics, neuroimaging and electroencephalography (EEG), conducted before and after treatment with selective serotonin reuptake inhibitors (SSRIs). The results will be compared with those of pediatric OCD paitents comorbid with tic disorder (TD) and healthy controls. The findings are expected to further elucidate the potential pathogenesis of OCD, providing a theoretical basis for identifying novel clinical intervention targets and optimizing treatment strategies.

We recruited outpatient children and adolescents with OCD confirmed by the Kiddie Schedule for Affective Disorders and Schizophrenia for School-Age Children-Present and Lifetime Version (4th edition). The following inclusion and exclusion criteria were also applied:

Inclusion Criteria:

(1) 7-17 years old, (2) Y-BOCS score ≥16, (3) with or without a history of TD, (4) Drug-naïve or drug-free for 8 weeks, (5) Wechsler IQ score ≥ 80.

Exclusion Criteria:

• History of serious medical, neurological illness or other psychotic disorders other than OCD (tic disorder excepted).
• Previous exposure to cognitive behavioral therapy (CBT) for OCD or TD.
• serious suicide risk.
• Has taken antibiotics, probiotics, prebiotics, corticosteroids, nonsteroidal anti-inflammatory drugs (NSAIDs), or immunosuppressants within the past 8 weeks.
• Has a history of gastrointestinal diseases such as inflammatory bowel disease, irritable bowel syndrome, constipation, hemorrhoids, or a history of gastrointestinal surgery, or has a history of autoimmune diseases such as rheumatoid arthritis or systemic lupus erythematosus.
• Participants with claustrophobic, heart pacemaker, mechanical heart valve, mechanical implant such as an aneurysm clip, hip replacement, or any other pieces of metal that have accidentally entered their body.

Drug-naïve or drug-free pediatric patients with OCD, drug-naïve or drug-free pediatric OCD patients comorbid with TD, and healthy control.