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Right flank pain.
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The right kidney is enlarged, however, it retains its kidney-shaped morphology. It identifies an obstructive stone at the level of the renal pelvis that reaches 2 cm in diameter. The major calyces are dilated, with the presence of hypodense content inside (16 HU). These generate the sign "paw sign". The renal cortex is thinned, the perirenal fat is striated, and Gerota's fascia is thickened. However, perirenal collections are not observed.
The left kidney also presented an obstructive stone in the ureteropelvic junction, in addition to other non-obstructive punctate stones in its major calyces.
Multiple cholelithiases and calcified granuloma in the right lower lobe as incidental findings.
Xanthogranulomatous pyelonephritis (XGP) is a rare chronic infection involving the kidney and adjacent tissues, characterized by the destruction of the renal parenchyma and its replacement with lipid-laden histiocytes (xanthoma cells) and granulomatous infiltrates 1. This is generally associated with the prolonged presence of an obstructive stone (75%) and chronic urinary infections 1,2.
Computed tomography (CT) is the most accurate diagnostic tool for its diagnosis, with a sensitivity close to 84% and an excellent ability to assess the extent of the disease; which is essential data for planning therapy 1,2. Its main imaging characteristics are the presence of an obstructive central stone (in most cases with a staghorn appearance) inside a contracted renal pelvis, increased kidney size with preservation of its kidney-shaped morphology and circumscribed contours, expansion of the renal calyces with low-attenuation material inside (paw sign) 3 and, in some cases, perinephric extension 2.
The pathogens that are generally involved are E. Coli and Proteus mirabilis 1,2. Although S. Aureus, Klebsiella and Pseudomonas can also be isolated. However, in 40% of cases, the urine culture is negative 1.
Its main differential diagnoses are pyonephrosis, renal abscesses, renal cell carcinoma and urothelial carcinoma 1.
Depending on the location of the stone, XGP can be diffuse or focal, with the diffuse form being the most frequent (80-90%) 1,2.
Possible complications include psoas abscess, nephrocutaneous fistula, nephrocolonic fistula, and perinephric abscess 1.
Its incidence is higher in female patients who are between the fifth and sixth decades of life 1. The most frequent symptoms are abdominal or flank pain, fever, hematuria, palpable mass and weight loss 1. Diabetes mellitus is a risk factor 2. Treatment is generally surgical (nephrectomy) and antibiotic therapy 1.
- Li L & Parwani A. Xanthogranulomatous Pyelonephritis. Archives of Pathology & Laboratory Medicine. 2011;135(5):671-4. doi:10.5858/2009-0769-rsr.1 - Pubmed
- Craig W, Wagner B, Travis M. Pyelonephritis: Radiologic-Pathologic Review. Radiographics. 2008;28(1):255-76. doi:10.1148/rg.281075171 - Pubmed
- Garrido Abad P, Rodríguez-Cabello M, Vera-Berón R, Platas-Sancho A. Bear Paw Sign: Xanthogranulomatous Pyelonephritis. Radiology Case. 2018;12(11):18-24. doi:10.3941/jrcr.v12i11.3415 - Pubmed