Renal Abscess

 Focal renal parenchymal inflammation may progress to liquefaction and abscess formation. If inadequately treated, acute pyelonephritis may also progress to tissue necrosis resulting in renal abscess. It may also be a sequel to hematogenous spread of infection. The former is usually due to Gram negative or anaerobic bacilli, the latter to Staphylococcal aureus and is associated with a septic site elsewhere (skin or nasal cavity focus, intravenous drug abuse, infected central line) or impaired immunity, including diabetes. Hematogenous spread may lead to multiple and/or bilateral renal abscesses as well as abscesses elsewhere in the body (particularly vertebral, pulmonary and cerebral metastatic spread). There may be marked loin pain and tenderness and severe pyrexia. Negative urine analysis or culture is seen in upto 20 % of cases.

IVU usually shows only a non-specific focal mass displacing collecting system. Ultrasound shows a heterogenous mass (slightly hypoechoic earlier, later hypo to anechoic) containing areas of cystic necrosis. There is often considerable echogenic debris, increased through transmission, septations, microbubbles of gas within the cystic areas on ultrasound.
On contrast CT scans, renal abscesses usually have a attenuation values of about 30 HU; this feature distinguishes them from fluid density renal cysts. Their contents do not enhance. They may have distinct, rounded margins and thick, enhancing walls. Alternatively abscesses may be ill-defined and surrounded by zones of decreased parenchymal enhancement, representing inflammation that has not yet progressed to necrosis. Gas in occasionally found in renal abscesses. Focal thickening of the adjacent renal fascia and stranding in the adjacent perirenal fat are common CT findings. A perinephric abscess may ultimately result if the infection extends through the renal capsule. Renal abscesses should be differentiated from cystic renal cell carcinoma.
Although small renal abscess may respond to antibiotic therapy alone, percutaneous drainage is usually treatment of choice, which can be done with use of CT or ultrasonographic guidance. Perinephric absess always requires drainage.