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Urogenital Imaging

Pyonephrosis

collection of pus in a dilated, obstructed collecting system of the kidney associated with a variable degree of renal impairment. If only one calyx is affected, the term pyocalyx is used. Patients have shaking chills, fever, and flank pain, but afebrile courses are not uncommon. In patients with terminal renal insufficiency, pyonephrosis is fairly common and is often overlooked. The obstruction is most frequently caused by a stone, and rarely by a tumour, postoperative changes, retroperitoneal fibrosis or a neurogenic bladder. Gram-negative bacteria predominate.

The calyces and pelvis become irregularly dilated and filled with pus. Obstructive hydronephrosis is a common precursor of pyonephrosis, which occurs when it becomes infected. Usually the involved kidney fails to excrete contrast medium on urography. The kidney is usually enlarged, although it may infrequently be normal in size or smaller. If the pelvis is opacified, the calyces are seen to be irregular in outline, and dilated. Multiple filling defects representing pus or necrotic tissue may be present. Spread into the perinephric spaces may occur with formation of perinephric abscess. Fistula formation is also known to occur. Ultrasound (US) is particularly valuable in establishing the diagnosis of pyonephrosis. The dilated fluid-filled collecting system contains echogenic material representing pus and cellular debris. This echo pattern may be limited to the dependent part of the pelvis and form a fluid debris level that shifts with changes in the position of the patient. Other US patterns are dense peripheral echoes caused by bubbles of gas and diminished sound transmission.

CT shows a dilated collecting system, depending on the level of obstruction, with ill-defined borders to the parenchyma (Fig.1). The density of the infected fluid may be well above that of normal urine. Since the pus is heavier than urine, a urine pus level may result. The cause of the obstruction can usually be discerned by CT imaging.

Therapy consists of drainage, usually by percutaneous methods under antibiotic coverage. Immediate and early intervention can usually salvage renal function.

HH

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Fig.1

Axial contrast enhanced CT section demonstrating an obstructing stone in the left renal pelvis, with associated leftsided pyonephrosis with gas-forming organisms.
Pyonephrosis, Fig.1