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Xanthogranulomatous pyelonephritis

Dr Yuranga Weerakkody and Dr Frank Gaillard et al.

Xanthogranulomatous pyelonephritis (XGP) is a rare form of chronic pyelonephritis and represents a chronic granulomatous disease resulting in a non-functioning kidney. Radiographic features are usually specific.

Epidemiology

Xanthogranulomatous pyelonephritis is seen essentially in all age groups, but most frequently presents in middle-aged to elderly patients ^1,5. There is a female predilection (F:M 2:1) presumably relating to an increased incidence of urinary tract infections and thus struvite (staghorn) calculi.

Clinical presentation

Clinical presentation is typically vague, consisting of constitutional symptoms such as malaise, weight loss and low grade fever. Haematuria and flank pain are sometimes encountered ⁴.

Despite often absent urinary tract symptoms, pyuria and positive urinary cultures are present in the majority of cases (95 and 60% respectively) ².

Pathology

Xanthogranulomatous pyelonephritis is, as the name suggests, a chronic granulomatous process believed to be the result of subacute chronic infection inciting a chronic but incomplete immune reaction ^1,4.

The kidney is eventually replaced by a mass of reactive tissue, surrounding the usually present (90%) inciting staghorn calculus with associated hydronephrosis of a greater or lesser degree. Foamy (lipid laden) macrophages predominate ^1,4.

Inflammatory process eventually extends into perinephric tissues and even adjacent organs ⁵.

Various bacteria are isolated, however the most commonly isolated species are E coli and P mirabilis ^1,4.

Radiographic features

Two forms of the disease are recognised both macroscopically and on imaging ^1,5:

diffuse : 90% of cases
focal : 10% of cases
- sometimes truly a focal process in a normal kidney
- in other instances this represents diffuse XGP of one moiety of a duplex system

Plain film

Plain film findings are difficult to distinguish from a routine staghorn calculus, although fragmentation and enlargement of the the renal outline may be seen. A calculus is not always present; in such cases it is not possible to make a plain film diagnosis.

Ultrasound

Ultrasound examination demonstrate an enlarged and distorted renal outline, with loss of the normal renal architecture and (usually) a centrally located shadowing calculus.

CT

CT findings are most helpful in reaching the correct diagnosis. The normal renal outline is lost and enlarged with paradoxical contracted renal pelvis. The calyces in contrast, are dilated giving a multloculated appearance that has been likened to the paw print of a bear (bear's paw sign) ³.

CT or conventional urography

In most cases there is little if any renal function in the affected kidney ¹.

MRI

MRI appearances mirror the heterogeneous nature of the mass with solid and cystic components surrounding a central staghorn calculus. As such signal is heterogeneous on all sequences.

Treatment and prognosis

By the time xanthogranulomatous pyelonephritis has become established, no conservative or medical therapies exist. Surgical nephrectomy is usually curative ^4-5. The presence of inflammatory reaction in adjacent tissues often requires a large operative field and an anterolateral transperitoneal approach ⁵.

Differential diagnosis

The differential is narrow when the entire kidney is affected and cross-sectional imaging has been obtained, and is largely limited to renal tuberculosis, which however usually results in a shrunken calcified putty kidney.

In cases where typical features are not present (e.g. no staghorn calculus, focal disease only) then other entities to be considered include: