Urolithiasis is calculus formation at any level in the urinary collecting system, but most often calculi arise in the kidney. Symptomatic urolithiasis is more common in men than in women. The stones occur frequently; it is estimated that by 70 years of age, 11% of men and 5.6% of women in the United States will have developed a symptomatic kidney stone. A familial tendency toward stone formation has long been recognized.
Enteric(4.5%)
Primary(0.5%)Hyperuricosuria(20%)Noknownmetabolicabnormality(15%to
Struvite(Mg,NH3,PO4)
There are three major types of renal stones:
About 80% are composed of either calcium oxalate or calcium oxalate mixed with calcium phosphate.
About 10% are composed of magnesium ammonium phosphate.
Approximately 6% to 9% are either uric acid or cystine stones.
In all cases, an organic matrix of mucoprotein is present that makes up about 2.5% of the stone by weight.
The cause of stone formation is often obscure, particularly in the case of calcium-containing stones. Probably involved is a confluence of predisposing conditions, including the concentration of the solute, changes in urine pH, and bacterial infections. The most important cause is increased urinary concentration of the stone’s constituents, so that it exceeds their solubility in urine (supersaturation). As shown in
Table 14.5 , 50% of patients who develop calcium stones have hypercalciuria that is not associated with hypercalcemia. Most patients in this group absorb calcium from the gut in excessive amounts (absorptive hypercalciuria) and promptly excrete it in the urine, and some have a primary renal defect of calcium reabsorption (renal hypercalciuria).
The causes of the other types of renal stones are better understood. Magnesium ammonium phosphate (struvite) stones almost always occur in individuals with a persistently alkaline urine resulting from UTIs. In particular, infections with urea-splitting bacteria, such as Proteus vulgaris and staphylococci, predispose individuals to urolithiasis. Moreover, bacteria may serve as particulate nidi for the formation of any kind of stone. In avitaminosis A, desquamated cells from the metaplastic epithelium of the collecting system act as nidi.
Gout and diseases involving rapid cell turnover, such as leukemias, lead to high uric acid levels in the urine and the possibility of uric acid stones. About one half of individuals with uric acid stones, however, have neither hyperuricemia nor increased urine urate but demonstrate an unexplained tendency to excrete a persistently acidic urine (with a pH < 5.5). This low pH favors uric acid stone formation, in contrast with the high pH that favors formation of stones containing calcium phosphate. Cystine stones are almost invariably associated with a genetically determined defect in the renal transport of certain amino acids, including cystine. Like uric acid stones, cystine stones are more likely to form when the urine is relatively acidic.
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MORPHOLOGYStonesareunilateralinabout80%ofpatients.Commonsitesofformationaretherenalpelvisandcalycesandthebladder.Often,manystonesarefoundinonekidney.Theytendtobesmall(averagediameter,2–3mm)andmaybesmoothorjagged.Occasionally,progressiveaccretionofsaltsleadstothedevelopmentofbranchingstructuresknownasstaghorn calculi, whichcreateacastoftherenalpelvisandcalycealsystem.Thesemassivestonesusuallyarecomposedofmagnesiumammoniumphosphate.