What causes the bear claw appearance on renal ultrasound?
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Answer:
The bear claw or bear paw appearance of the kidney describes dilated, rounded calyces radiating from a dilated renal pelvis, so that the anechoic spaces resemble the pads and claws of a bear’s paw. In its usual usage it describes marked hydronephrosis, in which the dilated calyces communicate with each other and with the pelvis, and that communication is the feature that distinguishes hydronephrosis from multiple parapelvic cysts, which do not communicate and do not connect to the renal pelvis on careful scanning or on delayed excretory imaging. The report should grade the dilatation, comment on cortical thickness as an indicator of chronicity and functional reserve, look for a cause including a calculus at the pelviureteric junction, in the ureter or at the vesicoureteric junction, a bladder outlet abnormality, and a pelvic mass, and note the ureteric jets and bladder appearance. A dilated collecting system is not synonymous with obstruction, since reflux, a full bladder, pregnancy, congenital megacalyces and a previously obstructed system all dilate without current obstruction, and the resistive index, diuretic renography or CT urography resolve the question. In children the Society for Fetal Urology and the urinary tract dilation classification systems standardise reporting. The same term is used for the bear paw sign of xanthogranulomatous pyelonephritis on CT, an important and different entity that should not be confused with it.
Why is it called so?
The dilated calyces splaying out from the pelvis resemble the toes and claws of a bear’s paw.
Pathophysiology
Obstruction of urinary outflow raises pressure within the collecting system, which distends the calyces and pelvis. The normally cupped calyces become blunted and then rounded and finally balloon into the parenchyma, and if the obstruction persists, sustained pressure and the resulting reduction in renal blood flow cause progressive parenchymal atrophy so that the cortex thins. The communication between the dilated spaces is anatomical, since they are simply distended parts of a single continuous system, whereas parapelvic cysts are lymphatic in origin and lie in the renal sinus without any connection to the urothelium, which is the anatomical basis for distinguishing them.
Alternative names: Bear paw appearance; hydronephrotic kidney
Other associated named signs: Bear paw sign of xanthogranulomatous pyelonephritis on CT and the soap-bubble nephrogram of severe obstruction
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