Medullary Sponge Kidney

Pathophysiology

The most important abnormality in medullary sponge kidney is the spherical, oval, or irregular dilatation of the medullary and papillary portions of the collecting ducts. The underlying abnormality responsible for this developmental anomaly is unknown.

The disease is bilateral in 70% of cases, and unilateral involvement of only one pyramid is uncommon. The dilated duct often communicates proximally with the collecting duct of normal size and shows a constriction of normal diameter at the point of communication with the calyx.[2]

Cysts in medullary sponge kidney usually measure 1-7 mm and contain clear, jellylike material and, frequently, small calculi. The kidney may appear to be slightly enlarged when several papillae are involved.

Microscopically, communicating cysts are lined with columnar or cuboidal epithelium and rarely by transitional epithelium, which is caused by the effects of calculi. Closed cysts are lined with atrophic epithelium. The rest of the kidney usually is normal, unless pyelonephritis or renal obstruction complicates the course of the disease.

Etiology

Most cases of medullary sponge kidney are sporadic. Theories suggest that dilatation of a collecting duct may occur, caused by occlusion by uric acid during fetal life or resulting from tubular obstruction due to calcium oxalate calculi secondary to infantile hypercalciuria.

Most patients with medullary sponge kidney have no family history of the condition, although there is a rare, familial, autosomal dominant form of the disease. A rare, autosomal recessive form is associated with Caroli disease.

Occurrence in the United States

The exact prevalence of medullary sponge kidney is unknown. The frequency of medullary sponge kidney in the general population has been estimated to be 1 case per 5000 population, and the prevalence may be as much as 1 case per 1000 population in urology clinics. In addition, medullary sponge kidney has been identified in 12-20% of patients who form calcium stones.[4]

Approximately 0.5% of patients undergoing intravenous urography are estimated to have medullary sponge kidney, while another 1% have papillary blush. No autopsy series have examined the prevalence of medullary sponge kidney specifically. In patients with nephrolithiasis, up to 20% may have mild degrees of medullary sponge kidney.

Sex- and age-related demographics

Women are affected by medullary sponge kidney more frequently than are men. In addition, while the overall incidence of calcium stones is 15-20% in medullary sponge kidney, the incidence is even higher in women, reaching 20-30%. In patients with nephrolithiasis, a higher relative prevalence of medullary sponge kidney occurs in females than in males.

Medullary sponge kidney is commonly diagnosed during the second or third decade of life. The mean age at diagnosis is approximately 27 years.

Medullary sponge kidney is usually a benign disorder without any serious morbidity or mortality. However, renal insufficiency may occur in as many as 10% of patients with the disorder.

Occasionally, patients can have recurrent nephrolithiasis, which can lead to significant morbidity. Very rarely, patients may develop renal failure as a result of repeated pyelonephritis or urinary tract obstruction.

In approximately 10% of patients with medullary sponge kidney, major morbidity is caused by repeatedly passing renal stones and by recurrent UTI. Complete obstruction of the kidney by renal stones is rare. Surgery is rarely required to remove the stones because they are usually very small and pass spontaneously.

A patient with medullary sponge kidney is estimated to pass 1.23 stones per year, compared with 0.66 stones per year in other people who form calcium stones.

Some physicians may encounter patients with medullary sponge kidney who describe severe, chronic renal pain but have no manifestation of infection, stones, or obstruction. The source of this pain is unclear. These patients may be treated best by physicians comfortable with chronic pain management.

A grading system has been devised that could identify patients with medullary sponge kidney who are at an increased risk for complications based on intravenous urographic findings.[5] Higher grades correlated with more frequent symptomatic stone episodes, number of hospital admissions, and number of procedures required per year. Classifications were as follows:

• Grade 1 -  One calyx, unilateral
• Grade 2 - One calyx, bilateral
• Grade 3 - More than one calyx, unilateral
• Grade 4 - More than one calyx, bilateral

Patients should receive educational materials, including handouts, informing them of the importance of maintaining volume expansion in order to maintain a daily urinary output in excess of 2 L. Handouts should indicate that medullary sponge kidney is a benign disorder and requires no specific therapy. A medullary sponge kidney fact sheet is available from the National Kidney Foundation.

Patients should also be informed of the possibility of a genetic role in their condition if other members of their families have a history of renal stones. For patient education information, see Blood in the Urine and Kidney Stones.

Patients with medullary sponge kidney often are asymptomatic. Not infrequently, the diagnosis is made during radiologic investigations, including abdominal radiography and excretory pyelography, performed for other clinical situations.[6, 7]

Hematuria is frequent, and gross hematuria may be present in 10-20% of cases. Gross hematuria usually results from pelvic obstruction due to stones. Microscopic hematuria may be present with or without urinary tract infection (UTI). (See Table 1, below.)

Renal stones also result in UTI, urinary obstruction, and nephrocalcinosis. The stones in medullary sponge kidney are usually composed of calcium phosphate (apatite) and calcium oxalate. (See Table 1, below.)

UTI is common in medullary sponge kidney, with and without nephrolithiasis. Sterile pyuria also is common. Patients with medullary sponge kidney have more UTIs than other patients with nephrolithiasis, and the incidence of urinary infection is higher in women than in men. (See Table 1, below.)

Recurrent nephrolithiasis is a major complication of medullary sponge kidney. The most common presenting symptom of this complication is renal colic, often accompanied by hematuria. (See Table 1, below.)

Table 1. Clinical Features of Medullary Sponge Kidney and Etiologic Correlation (Open Table in a new window)

Medullary sponge kidney can occur in conjunction with other congenital abnormalities, as follows:

• Anodontia
• Autosomal dominant polycystic kidney
• Beckwith-Wiedemann syndrome
• Caroli syndrome
• Congenital hemihypertrophy - As many as 10% of patients with hemihypertrophy have medullary sponge kidney, and as many as 25% of patients with medullary sponge kidney have hemihypertrophy
• Congenital pyloric stenosis
• Distal renal tubular acidosis
• Ehlers-Danlos syndrome
• Horseshoe kidney
• Marfan syndrome
• Parathyroid adenomas
• Renal artery stenosis
• Ureteral duplication

When associated with medullary sponge kidney, Beckwith-Wiedemann syndrome (high birth weight, macroglossia, omphalocele, visceromegaly, mental retardation, cysts in the adrenal cortex, enlarged kidneys, medullary sponge kidney, and hemihypertrophy) has a high tumor rate, especially Wilms tumor, adrenal gland cancer, and hepatoblastoma.

If any suggestion of a history of medullary sponge kidney in other family members exists, further investigations may be indicated to unravel the genetic pattern of transmission.

The following may be encountered on physical examination:

• No physical findings are usually present except for hematuria
• Renal colic may occur
• Costovertebral angle tenderness occurs in cases of pyelonephritis or ureteral obstruction by a calculus
• Hemihypertrophy is present in 25% of cases
• Other signs of associated congenital disorders may be present (as listed in the History section)

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.

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.

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. 

Asymptomatic adults with medullary sponge kidney

Advise patients about the benign nature of medullary sponge kidney, and inform them about the possibility of renal calculi and urinary tract infections (UTIs) due to the condition. Obtaining periodic urinalysis and abdominal radiographs is recommended, although guidelines for the frequency of radiologic surveillance in asymptomatic adults are unclear.

Asymptomatic children with medullary sponge kidney

In asymptomatic children with medullary sponge kidney, conduct regular surveillance for Wilms tumor and other abdominal tumors.

Patients with medullary sponge kidney and recurrent nephrolithiasis

Patients are advised to drink plenty of fluids in order to have a daily urinary output in excess of 2L. A 24-hour urine collection for potential kidney stone risk factors (eg, calcium, citrate, uric acid, magnesium, sodium, oxalate, phosphate) can be very helpful in treating the metabolic factors contributing to nephrolithiasis.[2, 14]

Patients with medullary sponge kidney and distal RTA

Adjust the dosage and timing of potassium citrate supplementation to increase the urinary pH to a maximum of 7.0-7.2. Overalkalinization can lead to calcium phosphate precipitation and stone formation.

Patients with medullary sponge kidney and UTI

Medullary sponge kidney in these patients must be treated aggressively until the urine is clear. Proteus infection can lead to the formation of struvite stones and requires aggressive antibacterial therapy.

Prevention of stone formation

As previously mentioned, Fabris et al recommended that patients with medullary sponge kidney be screened using 24-hour urine collection to evaluate for stone risk factors (ie, hypocitraturia, hypercalciuria, hyperuricosuria, hyperoxaluria).[10] Patients who demonstrate abnormality in any of these test results should be considered for treatment with potassium citrate to prevent stone formation.

The starting dose of potassium citrate is 20 mEq/day. The dosage should be adjusted to maintain a urinary citrate level of 450 mg/day and a urinary pH of less than 7.5.

Patients with medullary sponge kidney and symptomatic nephrolithiasis can be treated with extracorporeal shock wave lithotripsy (ESWL), percutaneous surgery, or ureteroscopy. Partial nephrectomy must be performed in severe cases with segmental renal involvement.

In rare cases in which patients with medullary sponge kidney are suffering from severe urosepsis, unilateral nephrectomy is performed.

Urologic consultation may be essential in patients with medullary sponge kidney presenting with recurrent nephrolithiasis or acute urinary tract obstruction. Patients with congenital abnormalities associated with medullary sponge kidney may also need urologic surveillance.

Patients with medullary sponge kidney and hypercalciuria should avoid a high-protein diet. Patients are advised to drink plenty of fluids in order to generate a daily urinary output in excess of 2 L.

Thiazide therapy, in addition to restriction of animal protein intake, and aggressive diuresis may be helpful in patients with medullary sponge kidney and hypercalciuria.

Patients with medullary sponge kidney who demonstrate risk factors for stone formation (ie, hypocitraturia, hypercalciuria, hyperuricosuria, hyperoxaluria) should be considered for treatment with potassium citrate to prevent stone formation.[10]

Class Summary

The primary role of thiazide diuretics is to decrease hypercalciuria.

Hydrochlorothiazide (Esidrix, Microzide, HydroDIURIL)

This agent inhibits the reabsorption of sodium in distal tubules, causing increased excretion of sodium and water, as well as of potassium and hydrogen ions. Its use is indicated in patients with medullary sponge kidney with hypercalciuria.

Class Summary

Patients with medullary sponge kidney who demonstrate risk factors for stone formation (ie, hypocitraturia, hypercalciuria, hyperuricosuria, hyperoxaluria) should be considered for treatment with potassium citrate to prevent stone formation.

Potassium citrate (Urocit K)

Potassium citrate is a potassium-based alkalinizer. The starting dose of potassium citrate is 20 mEq/day and should be adjusted to keep a urinary citrate level of 450 mg/day and a urinary pH level of less than 7.5.