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Xanthogranulomatous pyelonephritis

Xanthogranulomatous pyelonephritis
Author:
Alain Meyrier, MD
Section Editor:
Stephen B Calderwood, MD
Deputy Editor:
Allyson Bloom, MD
Literature review current through: Jan 2024.
This topic last updated: Jan 28, 2021.

INTRODUCTION — Xanthogranulomatous pyelonephritis (XPN) is an unusual variant of chronic pyelonephritis. Most cases occur in the setting of obstruction due to infected renal stones [1-6]. Affected patients usually have massive destruction of the kidney due to granulomatous tissue containing lipid-laden macrophages; the appearance may be confused with renal malignancy.

Studies performed in the 1970s found XPN in 8.2 percent of kidneys surgically removed or biopsied for chronic pyelonephritis and in as many as 25 percent in patients with pyonephrosis [1,7].

CLINICAL PRESENTATION — The clinical picture of XPN is somewhat different in adults and children.

Adults — XPN most often occurs in middle-aged women with a history of recurrent urinary tract infections [2]. The typical presenting symptoms include flank pain, fever, malaise, anorexia and weight loss. A unilateral renal mass can usually be palpated on physical examination. Blood tests reveal nonspecific abnormalities including anemia, an increased erythrocyte sedimentation rate, and liver function abnormalities reflecting mild biliary retention.

Examination of the urine confirms the presence of urinary tract infection. The urinalysis reveals pyuria and bacteriuria. Urine culture typically demonstrates Enterobacteriaceae. The most common organisms associated with XPN are Escherichia coli, Proteus mirabilis, Pseudomonas, Enterococcus faecalis, and Klebsiella [6]. The urine cultures are sterile in 25 percent of cases [4].

Children — There are two different presentations of XPN in children [4,8-10]. The most common form affects boys and girls equally and involves the entire kidney. The other form, which is more frequent in girls, is localized and may mimic a tumor.

The presenting symptoms include flank and abdominal pain, fever, and growth and weight retardation. Approximately one-half of children have a palpable abdominal mass. Bacteriuria and pyuria are found in 50 to 70 percent of cases with Proteus (60 percent) and E. coli being the most common causative organisms.

Secondary amyloidosis — Some patients have a relatively prolonged course of chronic inflammation before the diagnosis of XPN is established. In this setting, secondary amyloidosis (amyloid AA) can occur, leading in some cases to the nephrotic syndrome [11]. (See "Causes and diagnosis of AA amyloidosis and relation to rheumatic diseases".)

Renal transplant recipients — Xanthogranulomatous pyelonephritis rarely may complicate the course of renal transplantation from the early weeks to up to 14 years after transplantation [12,13]. Immunosuppression, rejection and lymphatic blockade may be etiologic factors, and it may be difficult to distinguish XPN from rejection.

RADIOLOGIC DIAGNOSIS — Although the urinalysis and urine culture typically indicate the presence of urinary tract infection, the diagnosis of XPN is confirmed by radiologic testing and pathology. Intravenous pyelography is abnormal but generally not diagnostic. It usually reveals the shadow of a nonfunctioning, enlarged kidney containing several renal stones or a staghorn calculus. In cases in which part of the kidney is still functioning, the IVP shows a space-occupying lesion resembling renal cancer.

CT scan — Computed tomography (CT) scan has replaced renal angiography as the preferred diagnostic tool in the evaluation of XPN (image 1). The CT scan has several advantages [14]:

It shows the characteristic replacement of the renal tissue by several rounded, low-density (15 to 18 Hounsfield units) areas that are surrounded by an enhanced rim of contrast medium, resulting in a multiloculated appearance (image 2). These findings are similar in appearance to the paw print of a bear and are known as the "bear's paw sign" [15,16]. The imaging abnormalities correspond to dilated calyces lined with necrotic xanthomatous tissue extending into the renal parenchyma.

It can determine the extent of the lesion and any extension into the perirenal area. The extent of the lesion can be classified into three stages:

Stage 1 – Localized disease confined to renal parenchyma.

Stage 2 – Extent of inflammation involves the perinephric fat.

Stage 3 – Extent of inflammation involves the perinephric spaces and/or abdominal wall. In the most advanced forms, the mass of xanthogranulomatous tissue can involve the adjacent gastrointestinal tract, and fistulas may be created into the colon or duodenum [1].

Renal stones which, as noted above are a frequent cause of XPN, are clearly visible on CT scan films.

CT scan may disclose the infrequent association of XPN and renal cancer, with or without renal stones [17]. In those patients without stones, the malignancy may be a causative factor in XPN, probably through urinary tract obstruction.

PATHOLOGY — XPN is almost always unilateral. On macroscopic examination, the kidney is enlarged and usually destroyed by the inflammatory process. Necrotic yellow material surrounded by layers of orange-colored tissue is typically seen (picture 1). Renal stones, including staghorn calculi, are usually present within the mass. The inflammatory tumor is adherent to the surrounding structures if there has been perirenal extension. Extension to the duodenum, the spleen, the lung and the skin is uncommon, but may lead to fistula formation and require extensive surgical intervention [13].

On microscopic examination, the lesion comprises three layers centered by a calyx [18]. The inner zone consists of necrosis, leukocytes, lymphocytes, plasma cells and macrophages. The middle zone contains vascularized granulation tissue interspersed with hemorrhage. The inflammatory cells are largely lipid-laden macrophages, which account for the yellow color (picture 2). The outermost part of the lesion is characterized by giant cells and cholesterol clefts [14].

PATHOPHYSIOLOGY — XPN is due to inflammation which may occur as a result of a defect in macrophage processing of bacteria. In the case of XPN complicating renal transplantation, renal ischemia, lymphatic blockade and immunosuppression may also be important factors [12]. The factors responsible for the accumulation of lipids and cholesterol in the lesion are not well understood.

DIFFERENTIAL DIAGNOSIS — XPN is most frequently confused with renal carcinoma in its clinical presentation and radiographic appearance. There is also report of pulmonary septic emboli masquerading as metastatic malignancy [19]. Evidence of chronic urinary tract infection and CT scan findings usually facilitates differentiation of these disorders, although rarely the two can occur together. XPN must also be distinguished histologically from two other inflammatory conditions, renal parenchymal malakoplakia and megalocytic interstitial nephritis [20]. The clinical differential diagnosis is similar to that of renal abscess, although the imaging findings are distinct. (See "Renal and perinephric abscess", section on 'Differential diagnosis'.)

TREATMENT — XPN is almost always unilateral and is associated with virtually complete destruction of the kidney. Treatment is therefore surgical (after an initial course of antimicrobials to control the local infection) and consists of en-bloc nephrectomy, in which all the involved tissue is removed and any fistulas closed. Patients with a localized form (usually children) or with bilateral disease can be treated with partial nephrectomy [21]. Laparoscopic nephrectomy is a potential approach, depending on the extent of the lesions and the urologist's expertise [22].

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Urinary tract infections in adults".)

SUMMARY

Xanthogranulomatous pyelonephritis (XPN) is an unusual variant of chronic pyelonephritis in which there is massive destruction of the kidney by granulomatous tissue containing lipid-laden macrophages. It is thought to be due to aberrant inflammation as a result of a macrophage defect in microbial processing. (See 'Introduction' above.)

In adults, xanthogranulomatous pyelonephritis most commonly occurs in middle-aged women with a history of recurrent urinary tract infections, particularly with Escherichia coli, Proteus mirabilis, Pseudomonas aeruginosa, Enterococcus faecalis, and Klebsiella species. In children, xanthogranulomatous pyelonephritis can involve the entire kidney or may be localized, mimicking a tumor, and is most commonly associated with Proteus and E. coli. Xanthogranulomatous pyelonephritis is a rare complication of renal transplantation. (See 'Clinical presentation' above.)

Presenting symptoms include flank or abdominal pain, fever, malaise, and weight loss or growth retardation. A unilateral renal mass can usually be palpated, and blood tests can reveal evidence of nonspecific systemic inflammation. In some cases, chronic inflammation can lead to secondary amyloidosis and the nephrotic syndrome. (See 'Clinical presentation' above.)

The diagnosis is confirmed by characteristic findings on computed tomography (CT) scan and pathology. On CT, renal tissue appears replaced by several rounded low-density areas that are surrounded by an enhanced rim of contrast medium and correspond to dilated calyces lined with necrotic xanthomatous tissue (image 2). (See 'Radiologic diagnosis' above.)

On gross examination, the kidney is enlarged with necrotic yellow material surrounded by layers of orange-colored tissue. Microscopically, the lesion consists of three layers of inflammatory cells surrounding the calyx. (See 'Pathology' above.)

Xanthogranulomatous pyelonephritis is almost always unilateral and is most frequently confused clinically and radiographically with renal carcinoma. (See 'Differential diagnosis' above.).

Xanthogranulomatous pyelonephritis is treated with partial or full nephrectomy following an initial course of antibiotics to control the local infection. (See 'Treatment' above.)

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