A 13-year-old previously healthy girl presented to an outside hospital with 2 weeks of right upper quadrant pain and fever. A sonogram revealed a renal abscess. The patient was transferred to our hospital for further evaluation. The abscess was percutaneously drained and cultures grew Escherichia coli. Following a 4-week course of antibiotics, a repeat sonogram showed a residual abscess. Computed tomography (CT) was performed to look for complications, including perinephric abscess or rupture, needing further treatment. Following the CT scan, the patient was continued on antibiotic treatment with resolution of the abscess shown on a 3-month follow-up sonogram.
The CT examination was performed on a Somatom Definition 64 AS scanner (Siemens, Erlangen, Germany). This scanner has CAREDOSE 4D and CARE kV, which optimize milliamperage (mAs) and kilovoltage (kVp), respectively, so that optimal image quality and lowest dose are achieved. In Figure 1, axial (A), multiplanar coronal (B) and sagittal (C), and color-enhanced (D) CT images show a fluid-filled cavity with a thick, enhancing rim and a small low-attenuation perinephric fluid collection. The CT features are diagnostic of renal abscess.
Infection is the most common disorder of the urinary tract in children. It usually occurs in the setting of ascending infection with pyelonephritis. The common inciting microorganism is Escherichia coli, which elaborates a protein termed p fimbriae that facilitates bacterial adhesion to the cells of the uroepithelium. A renal abscess is a necrotic cavity filled with purulent material. Most are the result of inadequately treated interstitial infection that ultimately liquefies. However, it also occurs in the setting of bacteremia with hematogenous seeding, most commonly due to Staphylococcus aureus or contiguous spread from other organs.
Clinical manifestations of renal abscess are similar to acute pyelonephritis and include fever, flank pain, abdominal pain, and dysuria and/or frequency. Urinalysis usually shows pyuria and proteinuria when the abscess is associated with acute pyelonephritis. However, bacteria and pyuria may be absent if the abscess is secondary to hematogenous spread and does not communicate with the collecting system.
In patients with renal infection, CT scanning is extremely useful to characterize renal infections as diffuse or focal and to detect the presence of perinephric extension and abscess formation. On contrast-enhanced CT, the characteristic appearance of renal abscess consists of a low-attenuation (0-20 Hounsfield units) unilocular or multilocular mass with thick enhancing walls.1-4 Septations may be present, particularly when multiple small abscesses coalesce. Gas bubbles or gas-fluid levels are uncommonly seen. The center of the abscess fails to enhance after contrast administration, reflecting the presence of purulent material and necrosis. Other features may include perinephric fluid and inflammatory stranding and thickening of Gerota fascia.
Perinephric abscess is the most common complication of intrarenal abscess, occurring in the setting of extension through the renal capsule and spread into the perinephric space. Perinephric abscesses are usually confined to the perinephric space by Gerota fascia. However, because the kidneys are retroperitoneal structures, extrarenal extension of infection can affect structures in the anterior perirenal space (pancreas, bowel, and colon), and perinephric space (Gerota fascia and adrenal glands). Perinephric abscesses may also extend into the flank muscles or into the psoas muscle or less commonly into the peritoneal cavity.
Small abscesses are usually treated conservatively with antibiotic therapy directed against culture-specific bacteria. Percutaneous drainage plus parenteral antibiotics is commonly indicated as the initial treatment for larger abscesses (>3 to 5 cm in size). Renal abscesses may be drained percutaneously under CT or sonographic guidance.
Disadvantages of CT scanning include radiation exposure. Therefore, low dose techniques should be used in performing CT in children. The major factors that need to be adjusted are mAs, kVp, slice thickness, and pitch. In this patient, CT scan allowed diagnosis of a renal abscess mass with excellent characterization of anatomy and extent, with exceptionally low radiation levels.
CT findings of renal abscess include a fluid-filled mass with thick enhancing walls in the renal parenchyma. Most children recover with conservative management with antibiotics and show no long-term sequelae of disease. Low-dose CT is the study of choice to document the presence of abscess formation and extension outside of the renal capsule.