To elucidate the pathophysiology of mixed stone formation in cystinuria, 27 patients with documented cystine nephrolithiasis underwent an inpatient evaluation under a constant dietary regimen. All patients had homozygous cystinuria, since the daily urinary cystine excretion exceeded 250 mg. per gm. creatinine. Hypercalciuria was noted in 5 patients (18.5 per cent), 4 of whom had fasting hypercalciuria. Hyperuricosuria was found in 6 patients (22.2 per cent) and it was not caused by a consumption of a diet rich in animal proteins, since urinary pH was higher and urinary sulfate lower than in control subjects. Serum uric acid was slightly lower and uric acid clearance was higher in hyperuricosuric patients than in control subjects. Hypocitraturia was found in 12 patients (44.4 per cent) and it was associated with defective renal acidification in 4 of 5 patients in whom it was tested. Thus, hypercalciuria, hyperuricosuria and hypocitraturia frequently accompany cystinuria in patients with cystine nephrolithiasis. These conditions might be renal in origin, rather than a result of dietary or environmental aberrations. They may contribute to the formation of calcium and uric acid stones, which sometimes complicate cystine nephrolithiasis.
ASJC Scopus subject areas