A 16-year-old female patient presented with a two-week history of increasing fever, back pain, and dysuria. The clinical evaluation revealed tenderness in the left flank, an elevated C-reactive protein, and leukocytosis. An ultrasound (US) of the kidneys demonstrated a focal lesion of uncertain etiology in the left kidney. A CT scan was obtained for further evaluation and showed two focal wedge-shaped hypodense lesions in the left kidney (Fig. 60.1) suggestive of acute focal pyelonephritis. The patient was placed on antibiotic treatment, which led to resolution of the lesions.
In the clinical setting of fever and laboratory signs of an infection, focal lesions in the kidneys as described above should be considered to be acute bacterial pyelonephritis, previously called “lobar nephronia.”
Renal US examination in a child with acute pyelonephritis is often completely normal and is less sensitive than other imaging modalities such as CT, MR, or nuclear scintigraphy. There may be focal hypo- or hyperechoic areas, depending on the degree of associated edema and/or hemorrhage. In some cases, the focal area of inflammation may produce mass effect on adjacent structures. Color Doppler US, especially power Doppler, tends to show decreased flow in these areas. Diffuse renal involvement may produce diffuse enlargement with hypo- or hyperechogenicity, loss of corticomedullary differentiation, and uro-epithelial thickening. There may be additional features of an underlying renal obstructive anomaly predisposing to infection, including an obstructed duplicated upper pole collecting system, ureteropelvic or ureterovesical obstruction.