emDocs - August 10, 2020 - By Pillai S and Desai S
Reviewed by: Chavez S; Koyfman A and Long B
"Summary
- Acute pyelonephritis (APN) is an infection of the upper urinary tract with a clinical presentation that may be challenging.
- Uncomplicated pyelonephritis occurs in a non-pregnant, immunocompetent female of reproductive age with previously normal renal function while complicated pyelonephritis occurs in those that do not fit this criterion.
- The lack of consensus regarding a diagnostic criterion (classic triad of fever, flank pain, and nausea and/or vomiting has a varying prevalence of 35 to 80%) as well as the significant mortality rates (up to 20% in APN) have made CT imaging an integral part of diagnosis and management of complicated APN.
- There is no role for imaging in management of uncomplicated APN unless there is uncertainty in diagnosis (few leukocytes in urine, vague symptoms, etc.). Imaging may be considered if fever or leukocytosis persist beyond 72 hours.
- Kidneys in APN may either appear normal or focally edematous in non-contrast CT. Renal calculi (obstructive APN), gas (emphysematous APN), hemorrhage, renal enlargement, inflammatory masses, and non-calculi obstructions can be visualized with a non-contrast CT.
- CT imaging of the abdomen and pelvis with IV contrast is the modality of choice in APN. It characteristically shows one or more focal or ill-defined wedge-like regions with reduced enhancement and poor corticomedullary differentiation. Other common findings better appreciated compared to non-contrast CT include delayed calyceal opacification, perinephric stranding, inflammatory parenchyma or gas formation. A “striated nephrogram” (alternating cortical radial bands of hyper- and hypo-attenuation) is common but nonspecific.
- Contrast CT findings in complications of APN include: abscess (sharply marginated area of low attenuation with peripheral enhancing), obstructing APN with pyonephrosis (calculi with dilated calyces, pelvic wall thickening, and gas in the collecting system), and papillary necrosis (poorly marginated hypo-attenuated lesions in papilla).
- Emphysematous pyelonephritis is an often-fatal necrotizing complication of APN occurring commonly in diabetics and necessitates emergent nephrectomy or percutaneous drainage. Non-contrast CT may demonstrate pararenal or parenchymal gas. In addition to these findings, CT imaging with contrast will also show parenchymal destruction, fluid collections or focal tissue necrosis."