A Case of a Huge Iliopsoas Abscess that Perforated and Communicated with the Ureter.

We experienced a rare case in which iliopsoas abscess (IPA), caused by an Extended Spectrum β-Lactamase (ESBL)-producing Proteus mirabilis, perforated and communicated with the ureter and caused sepsis. An 84-year-old woman, bedridden due to sequelae of a cerebral hemorrhage, was brought to our hospital with a chief complaint of fever lasting for 3 weeks. Computed tomography (CT) revealed a huge 180 × 110 × 100 mm IPA in the right iliopsoas muscle. The ureter was also found to communicate with the iliopsoas muscle abscess, ureteral stenosis was detected at the same site, and dilatation of the renal pelvis occurred above the area of the ureteral stenosis, indicating hydronephrosis. Considering the mechanism of this case, if the ureter first ruptures and urine leaks, followed by the formation of an IPA, urine will flow along the surrounding fatty tissue and cause an abscess around the ureter and kidney. However, because almost no abscess was detected around the ureter, the abscess was thought to have originated from the iliopsoas muscle located near the center of the ureter. In summary, in this case, an abscess first formed within the iliopsoas muscle, which gradually expanded and compressed the right ureter, resulting in hydronephrosis. The upper ureter, which had become dilated and thinned due to ureteral obstruction, became even more fragile because of the spread of inflammation from the IPA, and the IPA perforated and communicated with the ureter. In patients who have difficulty communicating, the diagnosis of IPA may be delayed because the only symptom is fever. As in this case, if the diagnosis is delayed, the abscess may become large and perforate the ureter; thus, IPA should always be considered as a cause of fever of unknown origin.