Medullary Sponge Kidney
Medullary sponge kidney is formation of diffuse, bilateral medullary cysts caused by abnormalities in pericalyceal terminal collecting ducts.
(See also Overview of Cystic Kidney Disease.)
The cause of medullary sponge kidney is unknown, but genetic transmission occurs in < 5% of cases.
Most patients are asymptomatic, and the disorder usually remains undiagnosed. It predisposes to calculus formation (often with increased urinary calcium excretion) and UTI, so the most common presenting symptoms are the following:
Medullary sponge kidney is benign, and long-term prognosis is excellent. Obstruction by renal calculi may transiently reduce GFR and increase serum creatinine.
The diagnosis is suspected in patients with recurrent calculi or UTIs or on the basis of incidental radiographic findings such as medullary nephrocalcinosis and dilated contrast-filled collecting ducts. Urinalysis typically shows evidence of incomplete distal renal tubular acidosis (overt metabolic acidosis is rare) and decreased urine-concentrating ability in patients without symptomatic polyuria.
Diagnosis is generally confirmed by CT, but IVU can be used. Ultrasonography is not helpful because cysts are small and located deep in the medulla.
Treatment is indicated only for UTIs and for recurrent calculus formation. Thiazide diuretics (eg, hydrochlorothiazide 25 mg po once/day) and high fluid intake may inhibit calculus formation by reducing urinary calcium excretion and preventing urinary stasis. These effects may reduce incidence of obstructive complications in patients with recurrent calculi.