Uric acid nephrolithiasis

Available estimates indicate that about 10 per cent of all renal calculi encountered in the U.S.A. as a whole are composed of uric acid and that the prevalence of uric acid nephrolithiasis in the population at large is of the order of 0.01 per cent. In pathogenesis the critical factors are those that lead to supersaturation of the urine with respect to undissociated (free) uric acid, which is more sparingly soluble than urates, and the factors that lead to separation of solid phase uric acid from its saturated solution in the urine, with organized crystal overgrowth. In regard to the first, the importance of persistent undue acidity of the urine, hyperuricosuria and contraction of the urine volume is stressed; the controversial role of deficient urinary ammonium excretion in undue acidity of the urine is discussed. In regard to the second, it is concluded that initiating uric acid crystals probably form the nidus in most cases, seeds of calcium oxalates in some. An etiologic classification of uric acid nephrolithiasis is proposed and the various categories discussed: idiopathic uric acid nephrolithiasis; uric acid stones associated with inborn errors of metabolism, neoplastic disorders and hyperuricemia of undetermined cause; with dehydration; and with hyperuricosuria without hyperuricemia. Conventional medical management is reviewed. The beneficial effects of supplementary treatment with allopurinol in 108 difficult cases of uric acid nephrolithiasis are summarized, with illustrative case reports.