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Renal abscess

A renal abscess, like any other abscess, is a collection of infective fluid at abuts the kidney. It is usually a sequelae of acute pyelonephritis, where severe vasospasm and inflammation may occasionally result in liquefactive necrosis and abscess formation.

Epidemiology

It can affect all ages and has no recognised gender predilection.

Clinical presentation

The most common signs or symptoms are : 

  • fever
  • flank / abdominal pain
  • chills and 
  • dysuria

Radiographic features

Ultrasound

Typically a renal abscess appears as a well defined hypoechoic area within the cortex or in corticomedullary parenchyma. It shows internal echoes within and associated diffusely hypoechoic kidney due to acute pyelonephritis may be seen. Perinephric collection may also be seen.

CT

CT is currently the most accurate modality for diagnosis and follow-up of renal abscesses1.  An abscess appears as a well defined mass of low attenuation with a thick, irregular wall or pseudocapsule, which can be better seen on contrast enhanced scans. Gas within a low attenuation/cystic mass strongly suggests abscess formation. Renal parenchyma around the abscess cavity may appear hypoenhancing in nephrogram phase, and may appear hyperattenuating in delayed images. Associated fascial and septal thickening is seen with obliteration of perinephric fat.

In some cases formation of a discrete abscess is preceded by acute focal lobar nephronia which is a focal inflammation of the kidney without liquefaction. It appears as a solid mass. 

Complications

The main complication is abscess rupture. When it does into calyceal system then it's called pyonephrosis, to the perinephric space (perinephric abscess), beyond Gerota fascia (paranephric abscess) and into peritoneum (subdiaphragmatic / pelivc abscess). These complications may lead to renal atrophy secondary to compression or obstruction.

Treatment and prognosis

Treatment consists of intravenous antibiotics and drainage, which may be performed under either ultrasound or CT guidance.

Differential diagnosis

General imaging differential considerations include

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