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.
PATHOGENESIS
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.
TREATMENT
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.
