The majority (65 to 75%) of stones are composed of either pure or mostly of calcium salts, including those of calcium oxalate, mixed calcium oxalate with uric acid, and calcium phosphate (brushite). Uric acid, cystine, and magnesium ammonium phosphate (struvite) compose the remainder of the stones. In the United States the lifetime risk for stone formation is 12% in men and 5% in women. Recurrence rates of new stone formation are high. If untreated, stones will recur at the rate of 50% in 5 to 10 years. An episode of renal colic has a sudden onset, with fluctuation and intensification over 15 to 45 minutes. Stones may obstruct the urinary tract and impair renal function. There is increased risk of infection with chronic obstruction. Bleeding may be chronic and accompany obstruction. The size, number, and metabolic composition of new stones strongly influence the natural history and complication rates. Composition of the stone reflects metabolic abnormalities in the urine. High urine calcium leads to calcium oxalate or calcium phosphate stones, high urine uric acid or low urine uric acid solubility (low pH) lead to uric acid stones; hyperoxaluria results in calcium oxalate stones; cystinuria is complicated by cysteine stones; and urinary tract infection with gram negative bacteria that produce urease and promote the infection and magnesium ammonium phosphate stones (struvite). The acute passage of a kidney stone is the 9th most common reason for visits to an emergency room. In the differential diagnosis of acute abdominal/flank pain, renal ultrasound is almost as good as abdominal CT for diagnosis. Diagnosis includes inspection of a first morning urine for crystals, chemical analysis of a stone if available, and fasting serum chemistries and 24 hour urine collection for chemical analyses of calcium, creatinine, phosphorus, oxalate, citrate, pH, volume, sodium and potassium, bicarbonate, ammonium. Supersaturation calculations of calcium oxalate, calcium phosphate, uric acid and other ions with the use of software is helpful. Fasting serum calcium, phosphorus, magnesium, creatinine and uric acid are also indicated. Treatment depends upon the biochemical blood and urine tests which will identify the composition of the stone. Hypercalciuria with normal serum calcium is most likely due to idiopathic hypercalciuria (IH). Lowering high urine calcium is best accomplished by daily thiazide therapy. Calcium oxalate or calcium phosphate kidney stone recurrence is reduced by 80 to 90%. Hyperoxaluria depends on whether oxalate overproduction or over-absorption is present. Low urine volume and low urine citrate should be corrected. Uric acid stones can be reduced by decreasing purine precursor intake, or correcting a very acidic urine which reduces uric acid solubility. Infection stones, often staghorn, requires careful surgical removal of all of the stone material. Cystine stone of infancy and childhood requires aggressive control of the high urine cysteine by specific agents. For complete coverage of all related areas of Endocrinology, please see our FREE on line web-book, www.endotext.org.
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