When Bowel Preparation Worsens With Time: The Case of an Unknown Crohn's Fistula

Colonoscopy is essential for diagnosing and managing lower gastrointestinal diseases, but adequate bowel preparation is critical for success. We report the case of a 66-year-old man with quiescent Crohn's disease who experienced repeated bowel preparation failure despite adherence to intensified regimens. Imaging later revealed an ileo-sigmoid fistula diverting laxative flow, preventing proximal colonic cleansing and causing stool reaccumulation in distal segments. Internal fistulas, common in Crohn's disease, are often asymptomatic but can significantly impair preparation quality. This case highlights the need to consider anatomical abnormalities such as internal fistulas in patients with recurrent bowel preparation failure and poor visualization during colonoscopy.