Renal and Urinary Tract Stone

Overview
  • Because development of stones (calculi) in both the kidney and the urinary tract occur by the same pathophysiologic mechanisms, they are considered together.
Stone Types and Pathogeneses
  • Overview
    • Calculi can be composed of different materials and the pathogenesis for each is slightly different. In general though, stones form when the urinary concentration of the offending material increases to the point that it begins to precipitate and form aggregates. Consequently, stone-formation generally occurs when the urinary excretion of the offending material is increased and is exacerbated when patients are dehydrated.
  • Calcium Calculi
    • Composed of calcium in complex with either oxalate or phosphate. These are by far the most common type of calculi. Calcium calculi can occur in completely healthy individuals although it is thought that in such cases there is a mild, idiopathic increase in urinary calcium excretion due to increased GI calcium absorption. Conditions which raise blood calcium (thus causing mild hypercalcemia) and thus raise urinary calcium excretion, significantly increase the risk of forming calcium calculi. Consequently, patients with primary hyperparathyroidism, Vitamin D toxicity, milk-alkali syndrome, and sarcoidosis are at risk for developing calcium calculi. Certain malignancies can also increase the risk of developing calcium calculi due to hormonal activity by tumor cells. For example, non-small cell lung cancers elaborates PTH, thus leading to hyperparathyroidism and thus increased calcium levels.
  • Uric Acid Calculi
    • Uric Acid stones occur in individuals with increased rates uric acid urinary excretion. Thus, individuals with gout are prone to developing such stones. Those with leukemia are also at increased risk as the high levels of neoplastic cell death and proliferation generate uric acid which is leaked into the blood and thus must be excreted.
  • Struvite Calculi
    • Struvite Calculi are composed of magnesium ammonium phosphate. Struvite precipitates only in cases where urine pH is extremely high. This occurs in the context of urinary tract infections with bacteria, most notably Proteus mirabilis, that can split urea into ammonium, thus increasing urine pH.
  • Cysteine Stones
    • Cysteine Stones are composed of the amino acid cysteine and develop in those with cystinuria (see page).
Morphology
  • Location
    • Calculi can develop at any level of the urinary tract or within the renal pelvis and calyces. The most common location for stone development is the renal pelvis and calyces.
  • Size and Shape
    • Most calculi tend to be small, between 1-3mm in size, and can have a smooth or rugged contour. However, in some cases stones can grow to be enormous, filling the entire renal pelvis, and thus looking like a Stag's Horns. Such "Staghorn" calculi are generally composed of struvite, and can slowly damage the kidneys.
Clinical Consequences
  • Larger Stones
    • Large stones within the renal pelvis may be completely asymptomatic and be found incidentally. Sadly, staghorn calculi can silently destroy renal function without early clinical consequences.
  • Smaller Stones
    • Very small stones can be passed through the urinary tract completely painlessly. However, in many cases, passage of stones will be highly painful, resulting in intense episodic flank pain accompanied by hematuria. Pain likely results during peristaltic attempts by the ureters to push urine past the obstructing stone.
  • Complications
  • Radiography
    • Calcium calculi are radio-opaque on radiography while struvite and uric acid stones are radio-lucent. However, CT scans can generally detect all stone subtypes.