Xanthogranulomatous pyelonephritis: classic CT signs of an uncommon disease

A 48-year-old woman presented to the emergency department with anorexia, weight loss and left flank pain with a few months of evolution. Physical examination revealed left flank tenderness. Blood results showed leukocytosis and elevated C-reactive protein. Relevant medical history included urolithiasis and recurrent urinary tract infections.

An abdominal radiogram obtained outside our institution revealed radiopaque branching densities overlying the left renal outline with the shape of the renal pelvis and calyces, compatible with a left staghorn calculus. Suspecting complicated pyelonephritis, an abdominopelvic computed tomography (CT) was requested. Topogram (figure 1) and unenhanced CT images (figure 2) confirmed the findings described for the abdominal X-ray. Contrast-enhanced CT images showed an enlarged left kidney with multiple hypodense rounded spaces, some with air-fluid levels, consistent with dilated calyces, surrounded by a thin rim of renal parenchyma – bear paw sign (figure 3a). There was also perinephric fat stranding and thickening of the Gerota, Zuckerkandl's, and lateroconal fasciae (figure 3a, b, and c). Some perinephric fluid collections were present, one of them being in contact with the descending colon, which was thickened (figure 3b and c). Excretory phase images were obtained and there was no visible left renal contrast excretion.

Xanthogranulomatous pyelonephritis (XGP) is a severe and uncommon chronic granulomatous infection that destroys renal parenchyma and results in loss of renal function [1]. It is believed to result from an atypical, incomplete immune response to a subacute/chronic bacterial infection, usually in the setting of obstructive renal stones, that causes the parenchyma to be replaced by lipid-laden macrophages [1,2]. The most commonly involved organisms are Proteus mirabilis and Escherichia coli [3,4]. The disease can be confined to the kidney or may involve adjacent structures. 

It is predominantly a unilateral disease, with no predilection for either kidney. XGP can be diffuse or focal; the diffuse form represents about 85-90% of cases [1,5]. Middle-aged female patients with a history of recurrent urinary tract infections (UTI) are most frequently affected, but XGP can occur at any age and affect both genders [4,6]. Almost all patients are symptomatic but clinical presentation is highly variable and nonspecific. The most frequently reported symptoms are fever, flank pain, dysuria, malaise, anorexia, and weight loss [1,2,4,6]. Laboratory results may show leukocytosis, anaemia, and elevated erythrocyte sedimentation rate [1,4,6]. Urinalysis is usually compatible with a UTI [6]. 

Abdominal radiogram usually reveals renal calculi [4]. Although ultrasound findings may be characteristic, the preferred imaging method is an enhanced CT of the abdomen and pelvis. CT typically reveals a non or poorly functioning kidney, diffuse enlargement of the kidney, a contracted renal pelvis with a staghorn calculus, dilated calyces with thinning of the cortex (bear paw sign), and perinephric inflammatory changes [1-6]. Extrarenal involvement may present as perirenal abscesses, which can involve adjacent organs (psoas muscle, pancreas, colon) [1,3,4,6]. 

With diffuse XGP, CT allows a confident diagnosis in most cases because of the presence of a highly specific set of features [2]. When typical findings are not present, as is the case of focal XGP, the diagnosis is more challenging, and other entities, such as renal abscess or carcinoma, need to be considered [3,4]. 

XGP is associated with high mortality if untreated [1]. Preoperative management with antibiotics and early surgical intervention (partial or total nephrectomy) are key factors for successful treatment [3], with CT playing a key role in adequate preoperative planning due to accurate assessment of the extrarenal involvement if present [2]. 

This patient was submitted to nephrectomy and the pathological examination confirmed the suggested imaging diagnosis. 

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