Also known as benign renal tubular ectasia, medullary sponge kidney is a cystic dilatation of the collecting tubules in 1 or more renal pyramid. Urine stasis in the collecting tubules may lead to stone formation within the ectatic tubules.

The etiology of medullary sponge kidney is unknown. It is seen in 0.5% of patients examined with excretory urography and is usually incidental (asymptomatic) unless complicated by infection, stones, or hematuria. Most cases are sporadic. It occurs in males more than females, at any age, and is more frequently bilateral than unilateral.

Plain radiograph shows nephrocalcinosis, clustered pyramidal medullary calcifications, and calculi in renal pelvis, ureter, bladder may be seen. Kidney size is usually normal. Calyces are broad, shallow, and distorted, with groups of calculi arrayed medially emanating from the renal papilla.

Contrast urography shows normal collecting system and renal pelvis with striated and saccular collections of contrast in the renal papilla. When contrast fills the cystically dilated tubules and obscures medullary nephrocalcinosis seen on the scout film, it is referred to as a “growing calculus” sign, and is diagnostic of MSK.

Unenhanced CT scan of abdomen in a patient with hematuria shows bilateral punctate calcifications in the region of the medullary rays consistent with medullary nephrocalcinosis in a patient with dilated distal collecting tubules (medullary sponge kidney).

Plain film of abdomen in patient with MSK and resultant nephrocalcinosis.  Note that the calcifications seem to be grouped in the region of the experted renal pyramids where the tubular ectasia is located.  Statis in these dilated tubules results in stone formation and predisposed to stone migration and subsequent ureteral obstruction.

Delayed phase of contrast enhanced CT scan of the abomen shows dilated renal tubules in the left medullary pyramid (yellow arrow).  On the left, a renal stone has migrated and obstructed the collecting system.  On the left, we see dilated collectin system including calcyes (c) and renal pelvis (p) with contrated and urine layering in the dilated renal pelvis - note contrast urine level (arrow).