P323 Defining Normal Bowel Wall Thickness on Intestinal Ultrasound in an International Multicenter Cohort of Children with Inflammatory Bowel Disease in Sustained Deep Remission

Abstract Background Intestinal ultrasound (IUS) is a non-invasive, patient-centric tool for detecting and monitoring activity in patients with inflammatory bowel disease (IBD). Bowel wall thickness (BWT) is the most important parameter for assessing IBD activity, however, normal values are extrapolated from adults. We aimed to define normal BWT in children with IBD who achieved sustained deep remission. Methods International, multicentre, retrospective, pediatric IUS database including IBD patients ( 18 years) without a prior history of surgery or complications who underwent IUS after achieving sustained deep remission: absence of ulcerations on ileocolonoscopy and/or transmural healing on magnetic resonance enterography or computed tomography enterography and steroid-free clinical remission from 2020-2023. Descriptive statistics summarized data as percentages for categorical variables and median IQR for continuous variables. Primary outcome was median BWT by segment: sigmoid colon (SC), descending colon (DC), transverse colon (TC), ascending colon (AC), and terminal ileum (TI), assessed using Kruskall Wallis. Secondary outcomes included univariate association of BWT and age, sex, disease duration, diagnosis (Crohn’s disease (CD) vs. ulcerative colitis (UC)/IBD-unspecified (IBD-U) and medication exposure. Results 80 patients (33 (41%) female; 51 (64%) CD:26 (33%)UC:3 (4%)IBD-U, median age 15.2 14-16.6 years) with 400 bowel segments were included. Forty (50%) received anti-tumor necrosis factor therapy (anti-TNF), 18 (23%) ustekinumab (UST), 10 (12%) mesalamine(5-ASA), 6 (8%) vedolizumab (VDZ), 5 (6%) immunomodulator monotherapy (IM), and 1 (1%) upadacitinib (UPA) (Table 1). Median BWT was 1.3 1.0-1.6mm for SC, 1.3 1.1-1.7mm for DC, 1.3 1.0-1.6 mm for TC, 1.3 1.0-1.6mm for AC, and 1.3 1.1-1.5mm for TI (Figure 1). There was no difference in BWT between segments (p=0.7952), age (p=0.1952) or sex (p=0.781). BWT was negatively associated with disease duration (p0.05). UC patients had a 0.16 mm thinner BWT than CD (p0.05). Mean BWT in patients on VDZ was thinner than those on anti-TNF monotherapy (-0.19mm, p0.05). BWT in patients on anti-TNF combination therapy was thicker than patients on anti-TNF monotherapy (0.21mm, p0.05). There was no BWT difference between patients on IM, 5-ASA and UST compared to anti-TNF (p0.05). Conclusion Normal BWT for children with IBD in deep remission is less than adults, and unaffected by age, sex and bowel segment. BWT may be affected by disease duration, phenotype and medication exposure. These findings are crucial to the standardized assessment of transmural healing in children with IBD.