Sigmoid colonic tuberculosis presenting as a colovesical fistula mimicking colorectal malignancy: a case report

Intestinal tuberculosis (ITB) most commonly involves the ileocecal region. Isolated sigmoid colonic tuberculosis complicated by a colovesical fistula is extremely rare and may closely mimic colorectal malignancy or Crohn's disease (CD).

A 73-year-old man presented with subacute diarrhea, fever, and lower urinary tract symptoms. Laboratory tests showed markedly elevated inflammatory markers and anemia. Cross-sectional imaging demonstrated segmental thickening of the sigmoid colon, pericolic lymphadenopathy, multiple serous effusions, and findings consistent with a colovesical fistula, including bladder wall disruption and intravesical gas. Colonoscopy revealed a circumferential stenosing lesion with irregular ulceration, raising strong suspicion for colorectal malignancy or CD.

Initial histopathology showed only mixed inflammatory cell infiltration without granulomas or malignant cells, and empirical antimicrobial therapy failed to control the fever. Given the positive immunological testing for tuberculosis and persistent clinical suspicion, acid-fast bacilli staining and metagenomic next-generation sequencing (mNGS) were performed on colonic biopsy tissue. Acid-fast bacilli were detected, and mNGS identified Mycobacterium tuberculosis complex, confirming ITB. Standard anti-tuberculosis therapy was initiated, leading to rapid clinical improvement, complete endoscopic mucosal healing, and radiological resolution of the colovesical fistula.

This case highlights that ITB can present as an isolated tumor-like sigmoid lesion complicated by fistula formation. When routine histology is nondiagnostic, especially in the absence of granulomas, integration of imaging, immunological testing, special staining, and molecular diagnostics may be crucial for early diagnosis, avoidance of misdiagnosis, and timely targeted treatment.