P191 Inflammatory bowel disease type unclassified in paediatric-onset IBD: A nationwide cohort study in Scotland with up to 20 years follow-up shows reclassification in the majority and mild course in those remaining as IBDU

Abstract

Abstract Background Inflammatory bowel disease type unclassified (IBDU) is the least common subtype of IBD accounting for around 10% of incident cases in children. There are a lack of long-term longitudinal data to fully elucidate both disease course and reclassification rate in IBDU. Our aim is to clarify disease course and reclassification rate by presenting nationwide data of an IBDU cohort with long-term (9-20 years) follow-up. Methods In a nationwide study, we analysed our prospectively identified 11 year cohort of IBDU patients diagnosed 01/01/03-31/12/13 via all Scottish paediatric IBD centres (Aberdeen, Edinburgh, Glasgow) and followed-up into adult services until 01/01/23. All had a diagnosis of IBDU based on the Porto diagnostic criteria and Paris PIBD phenotypic classification using a combination of clinical, endoscopic, histological and radiological findings. Follow-up data were obtained retrospectively from electronic medical records (demographics, diagnostic plus all repeat endoscopic and radiological assessments, longitudinal disease severity based on defined global clinician assessment, medical treatment and surgical outcomes) at fixed time points (5 and 10 years post-diagnosis) and at the last follow-up. Results A total of 116 patients were initially identified as IBDU, with 14 patients excluded as they emigrated out of the area before the first time point. 102 patients were included in the analysis (57/102 (56%) male, median (IQR) age at diagnosis 11.5 (9.1-13.2) years) with a median (IQR) length of follow-up of 10.5 (8.6-14.0) years. A change of diagnosis was made in 61/102 (60%) patients with 30/102 (29%) reclassified as Crohn’s disease (CD) and 31/102 (30%) as ulcerative colitis (UC) after a median (IQR) disease duration of 5.0 (2.0-8.2) and 4.7 (1.8 – 7.7) years respectively. Patients who remained as IBDU had a milder disease course compared to those reclassified as CD/UC with higher 1-5 year remission rates (30/39 (77%) in IBDU vs 16/57 (28%) in CD/UC-reclassified, p<0.05), lower rates of moderate-to-severe disease (3/39 (8%) in IBDU vs 31/57 (54%) in CD/UC-reclassified, p<0.05), less need for biologics across all time points (all p<0.05) and a higher proportion managed on aminosalicylates or no medication (all p<0.05). Higher rates of surgical resections were observed by last documented FU in CD/UC-reclassified patients (IBDU 1/41 (2%) vs CD/UC-reclassified 11/61 (18%), p=0.02). Conclusion In our nationwide paediatric IBDU cohort the majority (60%) of patients were reclassified as either UC or CD over the longest published median follow-up (10.5 years); those who remained with an IBDU classification had a milder disease course and decreased need of biologic escalation.