Stone clustering of patients with cystine urinary stone formation.

OBJECTIVES To explore the history of symptomatic cystine stone formation. Cystinuria is a genetic defect that may result in the formation of recurrent cystine calculi. METHODS Thirty-four patients with cystinuria were retrospectively evaluated for treatment patterns, medical prophylaxis, and renal outcome. Patients were offered a conservative surgical regimen and routine radiographic and laboratory follow-up and were encouraged to use medical prophylaxis. A Poisson regression model was used to analyze the patterns of stone formation. RESULTS The mean age at presentation and at last follow-up was 18 and 38 years, respectively. Patients underwent a total of 249 procedures, with an average of 7.3 procedures per patient, including 37% percutaneous nephrolithotomies; 25.7% shock wave lithotripsy procedures; 22.1% ureteroscopies; 12.9% open lithotomies; and 1.6% nephrectomies. In 29% of patients, unilateral surgery only was required. With a conservative treatment regimen, no patient developed renal insufficiency or failure. The overdispersion estimated from the Poisson model was 5.03 (P <0.001) if patients were evaluated from birth to last follow-up and 2.06 (P <0.001) if followed from first presentation to last-follow-up, suggesting a clustering of stone events. Overdispersion was moderately related to increased age (older than 34 years). CONCLUSIONS The results of our study showed that patients with cystinuria develop symptomatic calculi in clusters, with a slight predominance of stone formation after the age of 34. The cause of the stone clustering is unclear. Renal function can be preserved with a conservative surgical treatment protocol. Contemporary medical prophylaxis may be ineffective.