Bear paw sign | Radiology Signs

Answer:

The bear paw sign is a reniform kidney containing multiple rounded, low-attenuation cavities that radiate outwards towards the cortex, separated and outlined by thin rims of enhancing residual parenchyma, with a paradoxically contracted renal pelvis. The cavities are dilated calyces filled with pus and lipid-laden debris. It is the classic CT appearance of xanthogranulomatous pyelonephritis, a chronic destructive granulomatous infection in which lipid-laden macrophages progressively replace the renal parenchyma. Around 80% of cases are associated with an obstructing staghorn calculus, and the usual organisms are Proteus mirabilis and Escherichia coli. The diffuse form accounts for about 90% of cases and the focal (tumefactive) form for the remainder. Differentials include renal cell carcinoma, renal tuberculosis, malakoplakia and pyonephrosis; the distinction from pyonephrosis matters, since that is an obstructed infected collecting system without the parenchymal destruction. The sign is highly suggestive but histopathology is often still required, particularly for the focal form.

Why is it called so?

The rounded cavities radiating from the contracted pelvis resemble the pads and toes of a bear’s paw.

Pathophysiology

Chronic obstruction, usually by a staghorn calculus, combined with persistent infection provokes a granulomatous response in which lipid-laden foamy macrophages progressively destroy and replace the renal parenchyma. The calyces dilate behind the obstruction and fill with pus and necrotic debris, so each becomes a rounded low-attenuation cavity, while the intervening rims of surviving parenchyma and inflammatory tissue enhance. The renal pelvis itself is contracted rather than dilated because the inflammatory process encases and fibroses it, which is the feature that separates the appearance from ordinary hydronephrosis. Inflammation frequently extends beyond the capsule into perinephric fat, psoas and the abdominal wall, producing fistulae.

Alternative names: Bear’s paw sign

Other associated named signs: Staghorn calculus, faceless kidney, and the rim sign of chronic obstruction

References

1. Xiang H, Han J, Ridley WE, Ridley LJ. Bear paw sign: Xanthogranulomatous pyelonephritis. J Med Imaging Radiat Oncol. 2018;62 Suppl 1:56. PMID: 30309087.