Synchronous collagenous and ulcerative colitis: A case report and review of the literature.

Collagenous colitis (CC) and ulcerative colitis (UC) are chronic inflammatory bowel diseases with distinct pathophysiological mechanisms and histological features. Their coexistence is exceptional and poses significant diagnostic and therapeutic challenges. We report the case of a 51-year-old asymptomatic woman who underwent colonoscopy after a positive faecal occult blood test, which revealed mild endoscopic changes. Serial biopsies from three colonic segments (ascending, transverse, and descending and sigmoid colon) demonstrated histological findings characteristic of active ulcerative colitis (architectural distortion, basal plasmacytosis, and crypt abscesses) coexisting with diagnostic criteria for collagenous colitis (a subepithelial collagen band measuring 15-23μm with capillary entrapment). Initial faecal calprotectin was 77.4mg/kg, rising to 1779mg/kg at three months, with subsequent normalisation (77mg/kg) following optimisation of mesalazine therapy. We review the most relevant literature on the synchronous or metachronous coexistence of these two conditions. This case underscores the importance of considering coexisting disease entities when faced with complex histological findings, even in the absence of significant symptoms.