Medullary Sponge Kidney Workup

The diagnosis of medullary sponge kidney is usually suspected in patients presenting with renal calculi, hematuria, or multiple urinary tract infections (UTIs). Prior clinical suspicion of medullary sponge kidney is important because it helps in ordering the most relevant radiologic tests.

Metabolic acidosis may occur secondary to renal tubular acidosis (RTA) (type 1 RTA or distal RTA) and is usually diagnosed during an acidification test as an inability to reduce the pH of urine to less than 5.3. Incomplete distal RTA is more common than frank type 1 RTA and, thus, may be missed on electrolyte testing because incomplete RTA does not show resting metabolic acidosis. However, incomplete distal RTA has been known to contribute to stone formation. ^[8]

A defective urinary concentrating ability is the inability to achieve maximal urine osmolality (ie, 900 mOsm/kg or greater) with water restriction. However, the defect in urinary concentrating ability is usually mild, and patients are asymptomatic.

Hypercalciuria may occur in 30-50% of cases. Patients with medullary sponge kidney nephrolithiasis who are hypercalciuric have a higher incidence of renal leak–type hypercalciuria than do patients with hypercalciuric calcium stones without medullary sponge kidney. A higher incidence of hypocitraturia occurs in medullary sponge kidney.

Hyperparathyroidism has been reported in a few patients with medullary sponge kidney. Both diseases can manifest as kidney stones and hypercalciuria, and the relationship between them remains uncertain, with some authors proposing that hyperparathyroidism is a cause of medullary sponge kidney, while others suggest that parathyroid adenoma may be secondary to long-term negative calcium balance caused by high urinary calcium in patients with medullary sponge kidney. ^[9]

Pediatric patients

Patients with medullary sponge kidney identified in childhood must be evaluated for other urologic abnormalities and must undergo surveillance for future urologic and abdominal tumors. See Pediatric Medullary Sponge Kidney.

Patients with medullary sponge kidney and microscopic hematuria

In rare instances, this condition may be associated with nephrocalcinosis. However, other causes of microhematuria, such as bladder cancer, renal tumors, and benign prostatic hyperplasia, must be excluded and often require a urology referral.

Patients with medullary sponge kidney and gross hematuria

Bladder tumors and pelvic obstruction must be ruled out aggressively. Children with medullary sponge kidney and gross hematuria must be evaluated for Wilms tumor.

Stone risk factors

Fabris and colleagues indicated that patients with medullary sponge kidney should be screened using 24-hour urine collection to evaluate for stone risk factors (ie, hypocitraturia, hypercalciuria, hyperuricosuria, hyperoxaluria). ^[10]

These patients must be evaluated routinely using renal function tests, radiologic surveillance, and ultrasonography. Urinary pH of more than 5.3 in adults and more than 5.6 in children in the setting of non–anion gap metabolic acidosis can indicate the presence of distal RTA (type 1). However, urinary pH can also be alkaline in the presence of UTI because of urea-splitting organisms.

The diagnosis of distal RTA can be established simply by raising the plasma bicarbonate concentration toward the reference range (ie, 18-20 mEq/L) with an intravenous infusion of sodium bicarbonate at a rate of 0.5-1.0 mEq/kg/h. The fractional excretion of bicarbonate is less than 3%, and the urine pH remains relatively stable in type 1 disease. Raising the plasma bicarbonate concentration to 18-20 mEq/L has little effect on bicarbonate excretion in type 1 RTA because no defect in proximal reabsorptive capacity exists.

The presence of urinary infection with urea-splitting organisms can lead to struvite stones and subsequent kidney insufficiency. This must be identified and promptly treated. Moreover, patients with medullary sponge kidney and other renal or congenital abnormalities must be evaluated for oncologic tumors.

Imaging study findings in medullary sponge kidney are summarized in Table 2, below. See also Imaging in Medullary Sponge Kidney.

Table 2. Radiologic Appearance in Medullary Sponge Kidney (Open Table in a new window)

Urography

The diagnosis of medullary sponge kidney is usually confirmed by findings on excretory urography, which reveals radial, linear striations in the papillae. These striations, which are often referred to as "brushlike" patterns, result from the collection of contrast in dilated and cystic collecting tubules. ^[7] Cystic collections of contrast media in the ectatic collecting duct are referred to as "bunches of grapes" or "bouquets of flowers."

High-quality excretory urography with renal tomograms obtained before and after injection of contrast medium and then every 4 minutes during the next 20 minutes has been described as the most accurate method of identifying medullary sponge kidney. The diagnosis of medullary sponge kidney can be missed if the preparation is suboptimal.

Computed tomography

That noncontrast computed tomography (CT) has higher sensitivty than excretory urography for the detection of kidney stones and hematuria is well established. CT is also more sensitive in detecting papillary calcifications; however, noncontrast CT is not sensitive for detecting ectasia of precalyceal tubules, which is required for the diagnosis of medullary sponge kidney. ^[11]

CT urography performed with multidetector CT (MDCT) has been shown to create images of the renal collecting system with similar detail as excretory urography. ^[11] CT scanning can also be used to help identify other renal abnormalities, such as associated cysts, horseshoe kidney, renal abscess, and papillary necrosis. Helical CT scanning has been demonstrated to improve the definition of the abnormalities in medullary sponge kidney. ^[12] (See the images below.)

IVU-like, volume-rendered (VR) computed tomography (CT) scan of both kidneys demonstrates brushlike densities throughout multiple papillae of both kidneys consistent with renal tubular ectasia. Correlation of the stone disease with the ectatic tubules is diagnostic of medullary sponge kidney. Image courtesy of Dr. Terri J. Vrtiska, Consultant, Department of Radiology, Mayo Clinic College of Medicine, Rochester, Minnesota, USA.

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Unenhanced coronal, volume-rendered (VR) computed tomography (CT) scan of the kidneys demonstrates 2 small calculi in the midportion of the right kidney and 2 small calculi in the lower pole of the left kidney (arrowheads). A large, low-density lesion in the lower pole of the right kidney and a small, low-density lesion in the upper pole of the left kidney (short arrows) were shown to represent benign simple renal cysts on the contrast-enhanced CT scans. Image courtesy of Dr. Terri J. Vrtiska, Consultant, Department of Radiology, Mayo Clinic College of Medicine, Rochester, Minnesota, USA.

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Radiography and ultrasonography

Abdominal plain film may reveal nephrocalcinosis. Ultrasonography of the kidney along with clinical and laboratory finding can be used in the diagnosis of medullary sponge kidney. Typical findings on ultrasound include hypoechoic medullary areas, hyperechoic spots, microcystic dilatations of papillary zone, and multiple calcifications in each papilla. ^[13]  In early-stage medullary sponge kidney without calcification, the papillae may appear bright on an ultrasonogram.