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Paediatric Imaging

Abscess, kidney

Abscess formation in the kidney is unusual, but may occur with haematogenous spread of pyogenic organisms, and is a form of suppurative pyelonephritis. Ascending infection from the bladder also occurs in some patients. In neonates fungal infections are the most frequent.

The abscess formation may occur in four forms: a renal abscess; sometimes called a renal carbuncle (Fig.1), a perinephric collection; renal xanthogranulomatous pyelonephritis and finally infection within the collecting system (pyoureteronephrosis). In all situations the child presents as being unwell with fever and loin pain and palpable loin mass may be present. There may be pyuria and haematuria.

Imaging

Plain radiographs show an enlarged and distorted renal outline, often with displaced bowel gas and a localized ileus. Calcification may be present in xanthogranulomatous pyelonephritic lesions and in obstructed systems with hydroureteronephrosis.

Ultrasound quickly establishes the diagnosis by identifying the abscess mass as a lesion of mixed echodensity (Fig.2) often with fluid debris which alters with position. The initial sonographic change is alteration of the echogenicity focally within the kidney. As the abscess develops the area of altered echogenicity becomes relatively transonic with liquefaction of the central part of the infected kidney. Xanthogranulomatous lesions may have mass appearances that simulate neuroblastoma or Wilms tumour or may contain calcification. In pyoureteronephrosis (Fig.3) the normally echo free urine is replaced by pus, but normal surrounding renal architecture is maintained. A perinephric abscess occurs in a traumatized or ruptured kidney or if there has been rupture of a hydronephrosis. A mixed echodensity, though mainly fluid collection, will be seen around the kidney but some normal architecture is preserved.

Cross-sectional imaging is often carried out to better identify the lesions and to plan whether drainage under CT or ultrasound control is preferable. On cross-sectional imaging with xanthogranulomatous pyelonephritis, in addition to the mass and calcification, areas of fat are identifiable within the mass indicating the true nature of the lesion. Functional imaging with radionuclides is required in all cases to establish base line function in the damaged kidney and to show normal contralateral renal function. Also, see abscess perinephric.

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Fig.1

a. Intravenous urography in a girl with left-sided renal abscess. Note the displacement of the collecting system laterally. b, c. CT scan and DMSA scan in another child (same child as Fig. 2). The CT scan demonstrates the non-enhancing abscess centrally in the kidney. The DMSA scan confirms the photopaenia secondary to the abscess.
Abscess, kidney, Fig.1 (a)
Abscess, kidney, Fig.1 (b)
Abscess, kidney, Fig.1 (c)
Abscess, kidney, Fig.2
Abscess, kidney, Fig.3 (a)
Abscess, kidney, Fig.3 (b)