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Diagnosis and Treatment of Medullary Sponge Kidney

Definition of the Medullary Sponge Kidney

A medullary sponge kidney is a congenital kidney malformation with cystic enlargement and calcification of the collecting ducts (Foster et al., 2007).

Urography of a medullary sponge kidney: the contrast medium fills the ectatic collecting ducts with delayed excretion: paintbrush appearance. With kind permission, Dr. G. Antes, Kempten.

Epidemiology:

The prevalence is approximately 1 in 5,000. It is more common in patients with recurrent kidney stones.

Etiology:

In most sporadic cases, the exact cause of medullary sponge kidneys is unknown. In some patients, mutations can be detected in the RET (rearranged during transfection) and GDNF (glial cell line-derived neurotrophic factor) genes; the patients may also show extrarenal malformations (Ria et al., 2017).

Stone formation (calcium phosphate and calcium oxalate stones) is promoted not only by ectatic collecting ducts and urinary stasis, but also by accompanying tubular dysfunction. Frequently described abnormalities include hypocitraturia, hypercalciuria, impaired urinary concentrating ability, and incomplete distal renal tubular acidosis.

Pathology of the Medullary Sponge Kidney

The cut surface of the renal medulla looks like a sponge. The distal collecting ducts are dilated and have cysts with a diameter of 1–8 mm. Calcifications (70% calcium phosphate and 30% calcium oxalate) develop within the cysts. In 75%, medullary sponge kidney is bilateral.

Signs and Symptoms of a Medullary Sponge Kidney

• A medullary sponge kidney is often asymptomatic (incidental finding in urography or computed tomography)
• Recurrent nephrolithiasis
• Recurrent urinary tract infections
• Symptoms of chronic kidney disease (rare)

Diagnostic Workup of a Medullary Sponge Kidney

Ultrasound of the kidneys:

Hyperechoic papillae and nephrolithiasis are unspecific findings.

Computed tomography

Non-contrast CT is highly sensitive for nephrocalcinosis and urolithiasis. CT urography can demonstrate the characteristic contrast retention within ectatic collecting ducts and thereby confirm the diagnosis: linear or streaky contrast deposits adjacent to the papillae (papillary blush) on maximum intensity projection images.

Urography:

The contrast medium fills the ectatic collecting ducts, and the excretion is delayed: "paintbrush appearance" [fig. medullary sponge kidney in urography]. Progressive disease leads to calcifications in the distal collecting ducts: appearance suggestive of a "bouquet of flowers".

Laboratory tests:

In stone-forming patients with medullary sponge kidney: urine sediment, urine culture, serum creatinine, electrolytes, calcium, and parathyroid hormone should be assessed. In the presence of abnormalities or recurrent nephrolithiasis, a metabolic evaluation is advisable; common findings include hypercalciuria, hypocitraturia, and evidence of distal renal tubular acidosis.

Differential Diagnosis of Bilateral Renal Calcification:

• Medullary sponge kidney
• Hyperparathyroidism
• Distal renal tubular acidosis
• Sarcoidosis
• Vitamin D intoxication
• Multiple myeloma
• Urogenital tuberculosis
• Milk-alkali syndrome

Treatment of the Medullary Sponge Kidney

Treatment in medullary sponge kidney is only necessary in complications like kidney stones, urinary tract infection or renal insufficiency.

Treatment of Nephrolithiasis:

The mainstays of management are high fluid intake, dietary stone prevention, and treatment according to the same principles used for other calcium stone formers. Potassium citrate is often appropriate in patients with hypocitraturia or distal renal tubular acidosis. Thiazide diuretics may be considered in cases of hypercalciuria. Interventional stone treatment does not differ in any relevant way.

Treatment of Urinary Tract Infections:

Urinary tract infections are treated according to the clinical presentation, urine culture, and antimicrobial susceptibility pattern.

Prognosis of Medullary Sponge Kidney

In previous studies, the risk of renal failure in medullary sponge kidneys for symptomatic patients was 10%. Due to improved minimally invasive stone therapy options, this figure should be lower nowadays.

References

Forster, J. A.; Taylor, J.; Browning, A. J. & Biyani, C. S. A review of the natural progression of medullary sponge kidney and a novel grading system based on intravenous urography findings.
Urol Int, 2007, 78, 264-269

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