MEDULLARY SPONGE KIDNEY

medullary sponge kidney (MSK) - Definition
A cystic condition of the kidneys characterized by dilatation of the distal collecting ducts associated with the formation of multiple cysts and diverticula within the medulla of the kidney.

Prevalence
Difficult to know as it may be asymptomatic (diagnosed on an IVU done for other reasons or at postmortem). Estimated to affect between 1 in 5000 to 1 in 20,000 people in the general population; 1 in 200 in those undergoing IVU (a select population). In 75% of cases both kidneys are affected.

Pathology
The renal medulla resembles a sponge in cross-section due to dilated collecting ducts in the renal papillae and the development of numerous small cysts. This is associated with urinary stasis and the formation of small calculi within the cysts. It has a reported familial inheritance and is associated with other malformations (hemihypertrophy).

Presentation
The majority of patients are asymptomatic. When symptoms do occur, they include ureteric colic, renal stone disease (calcium oxalate calcium phosphate), UTI, and haematuria (microscopic or macroscopic). Up to 50% have hypercalciuria due to renal calcium leak or increased gastrointestinal calcium absorption. Renal function is normal, unless obstruction occurs (secondary to renal pelvis or ureteric stones).

Differential diagnosis
Other causes of nephrocalcinosis (deposition of calcium in the renal medulla) (e.g. TB, healed papillary necrosis).

Investigation
Intravenous urogram (IVU)
The characteristic radiological features of MSK, as seen on IVU, are dilatation of the distal portion of the collecting ducts with numerous associated cysts and diverticula (the dilated ducts are said to give the appearance of  bristles on a brush ). The collecting ducts may become filled with calcifications, giving an appearance described as a  bouquet of flowers  or  bunches of grapes .

Biochemistry
24-h urinary calcium may be elevated (hypercalciuria). Detection of hypercalciuria requires further investigation to exclude other causes (i.e. raised serum parathyroid hormone levels (PTH) indicate hyperparathyroidism).

Treatment
Asymptomatic MSK disease requires no treatment. General measures to reduce urine calcium levels help reduce the chance of calcium stone formation (high fluid intake, vegetarian diet, low salt intake, consumption of fruit and citrus fruit juices). Thiazide diuretics may be required for hypercalciuria resistant to dietary measures designed to lower urine calcium concentration. Intra-renal calculi are often small and, as such, may not require treatment, but if indicated this can take the form of ESWL or flexible ureteroscopy and laser treatment. Ureteric stones are again usually small and will therefore pass spontaneously in many cases, with a period of observation. Renal function tends to remain stable in the long term.