Retroperitoneal urinoma presenting as a posterior knee sinus following emphysematous pyelitis

A woman in her late 40s presented with abdominal sepsis secondary to emphysematous pyelitis complicated by a retroperitoneal urinoma. Despite initial antimicrobial therapy, percutaneous drainage and ureteric stenting, she developed persistent thigh pain and a posterior knee sinus. Repeat cross-sectional imaging demonstrated caudal spread of infected retroperitoneal fluid and gas along the iliopsoas and fascial planes into the thigh. Subsequent imaging identified a fistulous communication involving the colon. The exact sequence of fistula formation could not be determined, and causation between the urinary pathology and colonic communication remains uncertain. Management required multidisciplinary input, broad-spectrum antimicrobial therapy, urinary decompression, radiological drainage, bowel diversion and repeated surgical debridement. The patient recovered with resolution of sepsis and restoration of mobility. This case highlights the importance of recognising atypical limb symptoms in retroperitoneal infection and repeating imaging when clinical progress is not as expected.