Polymicrobial Solitary Retroperitoneal Abscess Due to Sigmoid Colon Perforation.

We treated an 85-year-old man with an abscess perforating into the retroperitoneal space from the sigmoid colon, with retroperitoneal drainage combined with antibiotics. CT showed no abscess formation in the intraperitoneal space. The patient consulted a doctor with a chief complaint of left-side low back pain and fever. He was first diagnosed with bacteremia due to Escherichia coli and close examination by CT revealed a retroperitoneal abscess. On referral to our hospital, we determined by CT that the cause of abscess formation was perforation of the intestine into the retroperitoneal space and spreading into the psoas muscle compartment. We then performed colostomy and abscess drainage through the retroperitoneal space to prevent the abscess disseminating into the intraperitoneal space. The abscess and necrotic tissue cultures were polymicrobial, including Enterobacteriaceae and Bacteroides spp. The abscess almost disappeared after drainage, and the patient's general condition gradually improved. The retroperitoneal abscess did not relapse by follow-up CT. In conclusion, this rare case presented with perforation of the intestine (Sigmoid colon) disseminated only to the retroperitoneal space without no intraperitoneal space abscess formation. We performed drainage only by a retroperitoneal approach without entering the intraperitoneal space.