Conversion from Cystine to Noncystine Stones: Incidence and Associated Factors

Purpose: Patients with cystinuria are often treated with medical alkalization and shock wave lithotripsy, although each treatment is hypothesized to increase the risk of calcium phosphate stones. We performed a multicenter retrospective review to evaluate whether stones of another composition develop in patients with cystinuria and with what frequency. Materials and Methods: We retrospectively reviewed the records of a multi-institutional cohort of patients with cystinuria. We assessed medications, stone analyses, 24-hour urinalyses and types of procedures. We compared patients who formed only cystine stones vs those with noncystine stones. Results: We identified 125 patients from a total of 5 institutions who were followed a mean of 5.2 years (range 0 to 26). Stones with noncystine components were submitted by 37 patients (29.6%). Potassium citrate medication was not associated with a noncystine composition (p = 0.1877). Regarding surgical management 18 patients (13%) underwent at least 1 shock wave lithotripsy session (range 0 to 9) and 79 (63%) underwent percutaneous nephrolithotomy at least once (range 0 to 10). When stratified based on pure cystine vs converted stones, the average total number of shock wave lithotripsy and percutaneous nephrolithotomy procedures was higher in the group with cystine and subsequent noncystine stone compositions (0.94 vs 0.10, p <0.0001, and 1.7 vs 1.5, p = 0.0053, respectively). On logistic regression male gender (OR 3.1, p = 0.0280) and the number of shock wave lithotripsy sessions (OR 3.0, p = 0.0170) were associated with an increased likelihood of the development of stones with a noncystine composition. Conclusions: Stones with noncystine components develop in more than 25% of patients with cystinuria, underscoring the importance of continued stone analysis. In this study prior shock wave lithotripsy was associated with conversion to a noncystine stone composition while urinary alkalization therapy was not associated.