MEDULLARY SPONGE KIDNEY
Medullary sponge kidney (MSK) is a congenital disor- der in which there is cystic dilation of the medullary collecting ducts. The dilated ducts generally have a diameter of 1 to 3 mm, although in some cases they may be as large as 5 to 7 mm. In most cases, both kidneys are involved, but in some cases the disorder is limited to one kidney or even to one or two pyramids.
MSK can occur either as an isolated phenomenon or in association with various genetic syndromes, such as congenital hemihypertrophy, Beckwith-Wiedemann syndrome, Marfan syndrome, and Ehlers-Danlos syndrome. It is generally considered to be sporadic, although there have been some reports of families with autosomal dominant inheritance. The pathogenesis of MSK is not known but is thought to involve abnormalities in the ureteric buds, which give rise to the ureters, renal pelves, calices, and collecting ducts during development of the metanephros (see Plate 2-2).
PRESENTATION AND DIAGNOSIS
MSK is generally a benign disorder that does not cause any symptoms. Therefore, for many patients, the abnormality is either never discovered or is only incidentally seen during radiographic evaluation of the abdomen for some other indication. In some cases, however, patients have nephrolithiasis, urinary tract infection, and/or hematuria, generally during adulthood.
The increased formation of nephrolithiasis in this disorder is thought to reﬂect numerous factors, including urine stasis in the dilated collecting ducts, as well as an increase in urine pH secondary to poor acidiﬁcation in the dilated collecting ducts. Moreover, for uncertain reasons, patients are at increased risk of hypocitraturia and hypercalciuria, both of which promote calcium stone formation. The increased risk of urinary tract infection likely results from the presence of urinary stasis and stones. Finally, hematuria can occur with or without stones or infection, and it can be either gross or microscopic.
In many patients, the dilated collecting ducts also become dysfunctional. Impaired urine concentration may occur, for example, although it generally does not lead to polyuria. Likewise, incomplete distal renal tubular acidosis (see Plate 3-25) is common, but it does not generally lead to systemic acidosis.
The diagnosis of MSK is typically established with intravenous pyelography or contrast-enhanced CT. During the excretory phase, the dilated, contrast-ﬁlled medullary collecting ducts form striations that may be seen radiating from the calices in a brushlike pattern. Using CT, small calculi may also be seen in the dilated collecting ducts.
There is no directed treatment for MSK. Patients who experience nephrolithiasis should increase their water intake and may beneﬁt from treatment with thiazides and potassium citrate. When stones occur, they are generally small and undergo spontaneous passage; however, an intervention such as ureteroscopy (Plate 10-33) or extracorporeal lithotripsy (Plate 10-12) may sometimes be required.
In general, the prognosis for patients with MSK is excellent. In rare cases, however, repeated episodes of nephrolithiasis or infection may lead to some degree of renal insufﬁciency.