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.