Pyonephrosis: infected obstructed kidney
Pyonephrosis is pus within an obstructed (blocked) renal collecting system: the cup-shaped calyces and funnel-shaped renal pelvis that gather urine and pass it on to the ureter.
Remember
When a blocked kidney shows signs of sepsis, think pyonephrosis: it needs urgent drainage alongside antibiotics, since antibiotics alone leave the infected urine trapped behind the blockage.
Memory aid
A blocked drain full of infected water
Medicine cannot clear it until the blockage is relieved, so the kidney is drained first.
Pyonephrosis starts with a blockage, often a stone in the ureter, that stops urine draining from the kidney. The urine builds up behind it, bacteria multiply, and the trapped urine becomes pus. Pressure inside the kidney rises and bacteria can spill into the blood, causing sepsis (a dangerous, whole-body response to infection).
Pus is what separates pyonephrosis from simple (uncomplicated) hydronephrosis, which is swelling of the collecting system from obstruction without infection. Acute pyelonephritis is infection of the kidney tissue and the renal pelvis, and in an unobstructed kidney urine can still drain. A renal abscess is a pocket of pus in the kidney tissue itself (the parenchyma).
An obstructed kidney that develops signs of sepsis, such as fever and rigors (violent shaking with chills), should be suspected of pyonephrosis.
Pyonephrosis requires urgent drainage, because infection behind a blockage cannot flush out. The choices are a ureteric stent (a tube passed up from the bladder to bypass the stone) or a percutaneous nephrostomy (a tube that goes through the skin and into the kidney). Immediate antibiotics and sepsis management go alongside. Antibiotics do not relieve an infected obstruction on their own, because the infected urine has nowhere to go. Delay in drainage can be fatal, and the stone (if there is one) is dealt with separately afterwards.
Key facts
- Definition
- pus within an obstructed renal collecting system.
- Suspect it when
- an obstructed kidney that produces fever, rigors or other signs of sepsis.
- Treatment
- urgent drainage of the obstructed collecting system, using a ureteric stent or a percutaneous nephrostomy, with immediate antibiotics and sepsis management.
- Why antibiotics alone fail
- the infected urine is trapped behind the blockage and cannot drain.
- Risk if drainage is delayed
- delay can be fatal, and the kidney can be left permanently damaged.
- Not the same as
- pyelonephritis in an unobstructed kidney, where urine can still drain, or simple hydronephrosis, which has no infection.
Exam traps
- Pus in the fat around the kidney (a perinephric abscess) is not pyonephrosis, because the pus lies around the kidney rather than within the collecting system, even though it can cause sepsis.
- Acute pyelonephritis in a kidney that is not obstructed can look similar, but without a blocked outflow there is no trapped pus under pressure, so drainage is not the central treatment.
- An infected, obstructed kidney does not settle with oral antibiotics alone, because the trapped infected urine has nowhere to go and urgent drainage is needed.
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