The term nephrocalcinosis refers to radiologically demonstrable diffuse renal parenchymal calcifications. Although medullary nephrocalcinosis is characteristic of medullary sponge kidney [MSK], a diverse group of diseases can produce this appearance including those that cause systemic metabolic alterations of hypercalcemia and hypercalciuria. Some of the common conditions associated with medullary nephrocalcinosis are medullary sponge kidney, hyperparathyroidism, renal tubular acidosis, sarcoidosis and metastatic disease.

Medullary sponge kidney is a developmental anomaly characterized by cystic dilatation of the collecting tubules in one or more renal medullary pyramids. It is of uncertain etiology and almost always diagnosed radiologically. This disease may be asymptomatic or may be characterized by repeated renal colics, calculous disease, hematuria or repeated urinary tract infections.

Ultrasound features in patients with MSK, which are quite non-specific, include-
•Predominantly bilateral involvement, but may be unilateral or segmental [focal].
•Normal sized kidneys if no associated complications.
•Echogenic medullary pyramids, irrespective of medullary nephrocalcinosis. Some of these may cast posterior acoustic shadowing as illustrated in this case, or may not cast any posterior shadowing [click here to view].

Ultrasound may also detect associated complications such as-
•Calculi- seen as dense, echogenic foci casting a shadow.
•Obstructive nephropathy- enlarged kidneys, hydronephrosis.
•If repeated UTI, patients may develop pyelonephritis [not detected well on ultrasound] and renal abscess. An abscess is seen on ultrasound as a focal hypoechoic mass that may be solid or liquefying. These findings are better delineated by CT or MR.

Role of ultrasound in MSK-
•Establish a diagnosis.
•Monitor adult patients to detect complications.
•In children with MSK, an association is noted with Wilm’s tumors and other abdominal tumors; hence, periodic surveillance is performed with ultrasound.

Excretory urograms in MSK have a classic radiological appearance. Pools of contrast media collection in the ectatic ducts give it the appearance of ‘bouquet of flowers’ or ‘bunch of grapes’ and this is the diagnostic sign. CT scan is very sensitive in the detection of papillary calcifications and complications occurring in MSK, however no characteristic finding of MSK has been described.