Differential diagnosis of ulcerative colitis with increased diarrhea; collagenous colitis or irritable bowel syndrome? A case report.

A 50-year-old man with a 20-year history of left-sided ulcerative colitis (UC) presented to our hospital with sudden onset of watery diarrhea. To this point, he had been treated with mesalazine 2.0 g/day for UC and had maintained remission. We considered that the UC had worsened. We immediately performed surveillance colonoscopy, which revealed a normal mucous membrane. The results of blood laboratory examinations were normal. Histopathology of colonic biopsies revealed new-onset collagenous colitis (CC), with a thickened subepithelial collagen band (SECB) and inactive UC. We herein report the importance of random colonic biopsies to diagnose CC even when the endoscopic appearance of the colon is normal in patients with inflammatory bowel disease with worsened diarrhea.