Types of Stones
Uric Acid

The incidence of uric acid calculi is approximately 5% - 10% of all renal stones. Men are affected four times more often than women. Approximately 25% of patients with gout will form uric acid stones. However, the majority of patients who form uric acid calculi have no detectable abnormalities in uric acid metabolism.

Factors that may contribute or predispose to uric acid stone formation include acidic or strongly concentrated urine, excess urinary excretion of uric acid, distal small bowel disease or resection (regional enteritis), ileostomies, myeloproliferative disorders being treated with chemotherapy and inadequate caloric or fluid intake. Treatment is tailored toward increasing urine volume and urinary pH. If hyperuricosuria is present, it can be corrected with appropriate dietary management and/or administration of allopurinol, a xanthine oxidase inhibitor. Unlike most other renal calculi, existing uric acid stones can often be dissolved with either systemic or topical alkalinizing agents.

Facts
Occurrence: 3-15%
Produced in acidic urine (pH <5.5)
Radiolucent stones
Amenable to medical therapy

Medical History
Pathophysiological factors: anemia, neoplastic disorders, intoxication, cardiac infarction, irradiation and treatment with cytotoxic agents.
Metabolic abnormalities: primary gout, Lesch-Nyhan syndrome.
Pharmacologic influence on the excretion of uric acid: uricosuric agents (probenecid, sulfinpyrazone, salycilates), diuretics (thiazide, furosemide) and analgesics, vitamin C.

Treatment
In contrast to other types of stones, medical therapy is the mainstay of treatment and prophylaxis. Treatment includes hydration, and limitation of dietary sodium (<150 meq/day) and protein intake. Potassium citrate at a dose titrated to alkalinize the urine to a pH of 6-7 will dissolve uric acid stones.

Suggested readings
Low RK, Stoller ML. Uric acid-related nephrolithiasis. Urol Clin North Am 1997, 24:1:135-148.

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