336 CHAPTER 7 Miscellaneous urological diseases of kidney
Cystic renal disease: medullary sponge kidney (MSK)
Definition
MSK is 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
Prevalence is difficult to know, as MSK may be asymptomatic (diagnosed on an IVP done for other reasons or at postmortem). It is estimated to affect between 1 in 5000 and 1 in 20,000 people in the general population, and 1 in 200 in those undergoing IVP (a select population). In 75% of cases both kidneys are affected.
Pathology
The renal medulla resembles a sponge in cross-section because of 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 hematuria (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
This includes other causes of nephrocalcinosis (deposition of calcium in the renal medulla) (e.g., TB, healed papillary necrosis).
Investigation
Intravenous pyelogram (IVP)
The characteristic radiological features of MSK, as seen on IVP, 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
Levels of 24-hour 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).
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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.
Intrarenal 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.
338 CHAPTER 7 Miscellaneous urological diseases of kidney
Acquired renal cystic disease (ARCD)
Definition and epidemiology
ARCD is cystic degenerative disease of the kidney with greater than 5 cysts visualized on CT scan. By definition, this is an acquired condition, in contrast to adult polycystic kidney disease (ADPKD), which is inherited (in an autosomal dominant fashion). It is predominantly associated with chronic and end-stage renal failure (originally, it was thought to specifically affect patients on hemodialysis).
It is clinically important because it may cause pain or hematuria and is associated with the development of benign and malignant renal tumors.
Approximately one-third of patients develop ARCD after 3 years of dialysis. The male-to-female ratio is 2:1.
Pathology
Usually multiple, bilateral cysts are found mainly within the cortex of small, contracted kidneys. Cysts vary in size (average 0.5–1 cm) and are filled with a clear fluid, which may contain oxalate crystals. They usually have cuboidal and columnar epithelial linings and are in continuity with renal tubules (and thus cannot be defined as simple cysts).
Atypical cysts have a hyperplastic lining of epithelial cells, which may represent a precursor for tumor formation. Renal transplantation can cause regression of cysts in the native kidneys.
Etiology
The exact pathogenesis is unknown, but several theories have been proposed. Obstruction or ischemia of renal tubules may induce cyst formation. Renal failure may predispose to the accumulation of toxic endogenous substances or metabolites that alter the release of growth factors and result in changes in sex steroid production or cause cell proliferation (secondary to immunosuppressive effects) that results in cyst formation.
Associated disorders
There is an increased risk of benign and malignant renal tumors. The chance of developing renal cell carcinoma is 3–6 times greater than that in the general population (males > females).
ARCD may also be associated with tubulointerstitial nephritis and membranoproliferative glomerulonephritis.
Presentation
Flank pain, UTI, macroscopic hematuria, renal colic (stone disease), and hypertension can occur.
Investigation
This depends on the presenting symptoms.
-For suspected UTI—culture urine
-For hematuria—urine cytology, flexible cystoscopy, and renal ultrasound. On ultrasound the kidneys are small and hyperechoic, with multiple cysts of varying size, many of which show calcification. If the nature of the cysts cannot be determined with certainty on ultrasound, arrange for a renal CT.
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Treatment
Persistent macroscopic hematuria can become problematic, exacerbated by heparinization (required for hemodialysis). Options include transfer to peritoneal dialysis, renal embolization, or nephrectomy.
Infected cysts that develop into abscesses require percutaneous or surgical drainage. Radical nephrectomy is indicated for renal masses with features suspicious of malignancy.
Smaller asymptomatic masses require surveillance.