Last revised by Naim Qaqish on 10 May 2021

Pyonephrosis is a term given to an infection of the kidney with pus in the upper collecting system which can progress to obstruction.

Pyonephrosis may be suspected when the clinical symptoms of fever and flank pain are combined with the radiologic evidence of urinary tract obstruction 1. Debris in the upper collecting system on imaging raises even more suspicion.

Ultrasound usually shows dilatation of the pelvicalyceal system with the following additional features:

  • echogenic debris in the collecting system: considered the most reliable sign 4
  • fluid-fluid levels within the collecting system
  • incomplete ("dirty") shadows of gas in the collecting system may occasionally be seen

The presence of clinical signs of infection with hydronephrosis on CT is considered a more sensitive indicator of pyonephrosis than many of the CT findings alone 6. In addition to features of obstruction, CT may demonstrate:

  • thickening of the renal pelvic wall (>2 mm)
  • parenchymal or perinephric inflammatory changes
  • dilatation and obstruction of the collecting system
  • higher than usual attenuation values of the fluid within the renal collecting system
  • gas-fluid levels in the intrarenal collecting system
  • layering of contrast material above / anterior to the purulent fluid on excretory studies

A caveat to CT evaluation is that it is often difficult to distinguish simple hydronephrosis from pyonephrosis by fluid attenuation measurements.

MRI may demonstrate features similar to those seen on CT. Some promising preliminary work has been done with both diffusion-weighted imaging and apparent apparent diffusion coefficient maps in an attempt to distinguish hydronephrosis from pyonephrosis, but these techniques require further investigation 4.

Emergent insertion of a percutaneous nephrostomy to drain the infected collecting system is usually performed after review and close collaboration with a urologist and/or and interventional radiologist.

Complications of pyonephrosis may include 7:

  • sepsis
  • xanthogranulomatous pyelonephritis
  • renal abscess
  • perinephric abscess
  • fistula to pleura, colon and duodenum

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Cases and figures

  • Figure 1: pyonephrosis (right) normal urine (left)
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  • Case 1: echogenic material in the collecting system
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  • Figure 2: pyonephrosis
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  • Case 2 : fluid-fluid levels
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  • Case 3: emphysematous pyelonephritis and pyonephrosis
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  • Case 4 : with ureteric calculus 
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  • Case 5
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  • Case 6
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  •  Case 7
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