Abstract Background Intestinal ultrasound (IUS) total bowel wall thickness (BWT) correlates well with endoscopic inflammatory bowel disease (IBD) activity. The intestine’s layers are also of interest. In adults, increased total BWT (3 mm) and submucosal thickness (SMT) to BWT ratio (0.50) has been reported in active ulcerative colitis (UC).1-3 In children, a thinner total BWT cut-off (2 mm) has been found for active IBD, but the SMT and ratio have not yet been reported, which was the aim of our study. Methods Pediatric patients (18 years) with suspected IBD were prospectively enrolled (between 2019-2024) and assessed (clinical, biochemical, endoscopic, IUS). IUS included two total BWT measurements for each intestine segment. For this study, IUS images were retrospectively reviewed and the SMT was measured twice within 0.5 cm of the original total BWT measurements. Statistical analyses included descriptive, Spearman rho, Mann-Whitney U, and ROC curve analyses. Results 132 patients (n = 624 intestine segments) were included (n = 86/132, 65% male), with a median age at diagnosis of 14 years IQR 11-16, range 3-17, majority with CD (n = 71/132, 54%). There were significant moderate correlations between endoscopic disease severity and thickness of both the total BWT and SMT for CD (rho=0.55 BWT, rho=0.48 SMT) and ulcerative colitis (rho=0.53 BWT, rho=0.48 SMT) (p 0.001). SM/BWT ratio weakly correlated with endoscopy in UC (rho=0.24, p 0.001), and didn’t significantly correlate in CD (rho=0.11, p = 0.06). The median SM was significantly thicker in endoscopically inflamed intestine in CD (SES-CD 0; 1.1 mm 0.7-2 vs. 0.8 mm 0.6-1, p 0.001) and UC (Mayo sub-score 0; 1.3 mm 0.8-2 vs. 0.7 mm 0.6-0.6, p 0.001). The optimal cut-off for detecting any endoscopic inflammation was a SM ≥ 1.1 mm (for CD) (AUC 0.67, 95th CI 0.61-0.73, p 0.001, Youden’s index 0.32) and ≥ 0.8 mm (for UC) (AUC 0.81, 95th CI 0.74-0.87, p 0.001, Youden’s index 0.55) (Figure 1). The optimal SM/BWT ratio cut-off was ≥0.5 for UC (AUC 0.66, 95th CI 0.58-0.75, p 0.001, Youden’s index 0.28) (Figure 1). Conclusion Thickening of the SM bowel wall layer (≥0.8 mm UC, ≥1.1 mm CD) was found to be a new helpful indicator of active pediatric IBD, correlating well to endoscopic disease activity. The SM/BWT ratio ≥0.5 identified endoscopic UC inflammation, the same cut-off as reported in adult UC. The ratio was not helpful in pediatric CD. This may provide insight into the progression of inflammation in UC starting with submucosal expansion, compared to more uniform thickening of all bowel layers with inflammation in CD. References: 1. Miyoshi J, Ozaki R, Yonezawa H, et al. Ratio of submucosal thickness to total bowel wall thickness as a new sonographic parameter to estimate endoscopic remission of ulcerative colitis. J Gastroenterol. 2022;57(2):82-89. doi:10.1007/s00535-021-01847-3. 2. Komatsu H, Morikubo H, Kimura Y, et al. A combination of bowel wall thickness and submucosa index is useful for estimating endoscopic improvement in ulcerative colitis: external validation of the Kyorin Ultrasound Criterion. J Gastroenterol. 2024;59(3):209-215. doi:10.1007/s00535-024-02077-z. 3. de Voogd FA, Bots SJ, van Wassenaer EA, et al. Early intestinal ultrasound predicts clinical and endoscopic treatment response and demonstrates drug-specific kinetics in moderate-to-severe ulcerative colitis. Inflamm Bowel Dis. 2024;30(11):1992-2003. doi:10.1093/ibd/izad274. Conflict of interest: Hong, Jacqueline M.: No conflict of interest Chamseddine, Ehab: No conflict of interest Carroll, Matthew: No conflict of interest Migliarese Isaac, Daniela: No conflict of interest Huynh, Hien: Grant: AbbVie and Janssen, Takada Personal Fees: Abbvie adboard Other: Abbvie and Janssen education support Dr. Hudson, Alexandra: No conflict of interest
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