Medullary sponge kidney is a benign congenital abnormality that was first described in 1939. Anatomically it is characterized by cystic dilatation of the renal medullary collecting ducts. These numerous small cysts range in diameter from 1 to 8 millimeters and give the kidney, when cut, the appearance of a sponge, thus the name.
Medullary sponge kidney is usually bilateral but can affect one kidney. The condition is bilateral in 70% of cases, and it is a relatively rare disorder with a prevalence of about 1/5,000 population. It is usually asymptomatic but can present with hematuria, urinary tract infections (UTIs), or renal stone formation. The age of presentation is usually 20 to 30 years old.
Distinguishing medullary sponge kidney from medullary nephrocalcinosis is important. Medullary sponge kidney is one of several common causes of medullary nephrocalcinosis. Medullary nephrocalcinosis is defined as the deposition of calcium salts in the medulla of the kidney. Other causes of medullary nephrocalcinosis include hyperparathyroidism, renal tubular acidosis type I, hypervitaminosis D, milk-alkali syndrome, and sarcoidosis.
Medullary sponge kidney has no known cause. Most cases are sporadic. Some cases are thought to run in families, but there is no known specific genetic cause. Some studies suggest a possible relationship between hyperparathyroidism and medullary sponge kidney. There is an association between medullary sponge kidney and Beckwith-Wiedemann syndrome.
The incidence is similar among racial and ethnic groups.
There is an association between medullary sponge kidney and hemihyperplasia, previously known as hemihypertrophy, which is a disorder in which one side of the body grows significantly more than the other side.
Some cases of medullary sponge kidney have mutations in the gene for glial cell-derived neurotrophic factor (GDNF).
Five percent of cases are hereditary and autosomal dominant.
The prevalence of medullary sponge kidney is one out of every 5000 persons, but among patients with calcific renal stones, 12% to 20% will have the disorder.
Women are affected by medullary sponge kidney more frequently than men.
The mean age of patients diagnosed is approximately 27 years.
The incidence of medullary sponge kidney worldwide is similar to the prevalence in the United States.
In medullary sponge kidney, the primary abnormality is the dilatation of the medullary and papillary portions of the collecting ducts. The dilated duct often communicates proximally with a collecting duct that is of normal size.
The cysts themselves typically measure 1 to 8 millimeters in diameter and contain clear, jelly-like material. Small calculi are often present. The kidney may appear enlarged with the involvement of multiple papillae. The exact pathogenesis of medullary sponge kidney is unclear. During embryogenesis, a disruption in the ureteric bud–metanephros interface has been postulated as a possible cause.
The collecting ducts of the inner medullary and papillary portions of the kidney are directly involved in this disorder. On gross pathology, the cysts generally measure less than 1 centimeter and are within the renal pyramids.
On histology, there are dilated papillary collecting ducts. These dilated papillary collecting ducts are lined with flattened or cuboidal epithelium. Within the cyst, there may be inflammation and/or calculi.
The cortex of the kidney is normal and unaffected.
Patients with medullary sponge kidney usually are asymptomatic. In symptomatic patients, hematuria, renal colic, fever, and dysuria are the most common presenting symptoms. Gross hematuria has been reported in about 10% to 20% of patients. Complications such as nephrolithiasis, renal calculi, and urinary tract infections may be seen. Patients are prone to renal calculi because of urinary stasis, hypercalciuria, increased risk of UTIs and distal renal tubular acidosis.
Some patients will describe chronic renal pain without any obvious infection, obstruction, hydronephrosis or stones. The etiology of this pain is unclear.
The diagnosis often is made by radiologic studies such as renal ultrasound and CT urogram and, occasionally, plain abdominal films involving the kidneys, ureters, and bladder (KUB).
It has been estimated that 12% to 20% of all recurrent calcium stone formers may have medullary sponge kidney. In women and younger patients (younger than 20 years), the estimated incidence is even higher at 20% to 30%.
Hemihypertrophy is noted in about 25% of cases of medullary sponge kidney. Conversely, 10% of patients with hemihypertrophy have medullary sponge kidney.
A detailed medical and family history can help diagnose medullary sponge kidney as it should be suspected when a patient has repeated urinary tract infections or kidney stones.
A patient with medullary sponge kidney does not usually have physical signs, except for occasional hematuria.
Several imaging modalities can diagnose medullary sponge kidney. A plain film KUB may show calcifications, but this is the least sensitive and least specific imaging test.
Renal ultrasound will show characteristic echogenic medullary pyramids, but ultrasound is a very technologist dependent modality which can easily miss the diagnosis if performed by inexperienced personnel.
At intravenous urography (IVU), a classic paintbrush-like appearance within the dilated medullary collecting ducts is characteristic, but this test is no longer used in most clinical practices.
Multidetector contrast CT urography will show a distinctive papillary blush. On delayed imaging from a CT urogram, the characteristic finding of medullary sponge kidney is parallel striations in contrast which extend from the papilla to the medulla and persist on delayed imaging.
CT can also detect related complications such as hydronephrosis, calculi, and pyelonephritis.
Treatment consists of managing the complications of medullary sponge kidney.
For UTIs, antibiotics and meticulous personal hygiene practices are recommended.
For calcium stones, initial recommendations include a high fluid intake sufficient to generate 2000 cc of urine per day. In general, a diet that is low in sodium, normal in calcium, high in potassium, and low to normal in protein may be helpful.
A 24-hour urine test is recommended to help optimize the urinary chemistry in motivated patients with medullary sponge kidney who develop stones. These patients will tend to have a higher incidence of renal leak type hypercalciuria and hypocitraturia than most calcium stone formers. If this is confirmed by the 24-hour urine test, these disorders can be treated with thiazide diuretics for the hypercalciuria and potassium citrate supplements for the hypocitraturia.
The potassium citrate supplementation also seems to help minimize the long-term bone loss that is sometimes associated with medullary sponge kidney. This bone loss is thought to be due primarily to the persistent renal leak type hypercalciuria although impaired urinary acidification has also been suggested. There is also a possible association with hyperparathyroidism.
Most of the stones in patients with medullary sponge kidney tend to be small and will usually pass spontaneously, but occasionally surgery, ureteroscopy, or lithotripsy may be needed. Overall, medullary sponge kidney patients who produce calcium stones tend to make about twice as many stones as other calcium stone formers.
Some patients will also have distal type renal tubular acidosis (RTA) which will demonstrate hypocitraturia and can then be treated with supplemental potassium citrate. The dosage of the potassium citrate should be titrated to approach an optimal 24-hour urinary citrate level (usually greater than 500 mg/24 hours) with a urinary pH around 6.5 if possible. A urinary pH over 7.2 to 7.5 should generally be avoided to minimize the production of calcium phosphate calculi. Serum potassium should also be monitored periodically to avoid hyperkalemia.
The other causes of medullary nephrocalcinosis comprise the differential diagnosis of medullary sponge kidney. The differential diagnosis of medullary nephrocalcinosis is as follows: hyperparathyroidism, hypervitaminosis D, milk-alkali syndrome, and other pathological hypercalcemic or hypercalciuric states. Papillary necrosis can also cause medullary calcifications. A history of analgesic abuse will favor papillary necrosis.
Although medullary sponge kidney is a benign condition, 10% of patients with medullary sponge kidney will ultimately develop renal failure. The majority of patients will have normal renal function throughout their lives. The renal failure is thought to arise from recurrent severe infections and extensive calculi formation.
Associations with the following disease entities have been reported:
Although medullary sponge kidney is generally described as a benign condition, as many as 10% of patients will develop renal insufficiency or failure over the course of their lives.
Since medullary sponge kidney is associated with hypercalciuria, especially the renal calcium leak type, those with the disease are at risk of developing osteopenia and secondary hyperparathyroidism.