New England Section of the American Urological Association

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Cystinuric Stone Conversion and Associations with Medical and Surgical Interventions
Lael Reinstatler, MD, MPH1; Cody M. Rissman, MD1; Karen Stern, MD2; Hunt Batter, BS3, Kymora Scotland, MD; PhD4, Gholamreza S. Ardekani, MD; PhD4; Manuel Rivera, MD5; Ben Chew, MD4; Brian Eisner, MD6; Amy Krambeck, MD5; Manoj Monga, MD2; Vernon M. Pais, Jr., MD1
1Dartmouth Hitchcock Medical Center, Lebanon, NH; 2Cleveland Clinic, Cleveland, OH; 3Harvard School of Medicine, Boston, MA; 4University of British Columbia, British Columbia, BC; 5Indiana University, Indianapolis, IN; 6Massachussetts General Hospital, Boston, MA

Background: Although cystinurics are assumed to exclusively make cystine stones, conversion to non-cystine stones has been observed. Prior research suggested SWL may alter future stone composition. Additionally, urinary alkalization, a cornerstone of cystinuria medical management, may contribute to altered stone composition.
The incidence of and predictors for stone conversion among cystinurics are unknown. If such conversion occurs, it may alter management and thus would underscore the need for continued stone analyses.
Materials & Methods: We constructed a multi-institutional database of patients with cystinuria. We analyzed medications, stone analyses, 24-hour urinalyses, and prior interventions. We divided the patients into those who only formed cystine stones and those with other stone components and compared these two groups to identify factors associated with conversion.
Results: A total of 104 patients from 4 institutions were studied. There were 279 stone analyses and 417 stone procedures. Thirty-six (35%) patients converted to non-cystine components of which 77% were calcium phosphate. 15 (43%) converted more than once. K citrate medication was used in 38 (60%) of pure cystine stone patients, and in 24 (75%) of converters (p = 0.1303). Converters took K citrate for an average duration of 4.5 years. Surgically, 13 (12.4%) had at least one SWL (range 0 10) and 71 (67.6%) had at least one PCNL (range 0-9). When stratified based on pure cystine vs converted stone, the average number of procedures was higher in the conversion group (5.9 vs 3.5, p=0.0130). There was a higher average SWL amount in the conversion group (0.97 vs 0.07, p=0.0012) but no difference in PCNL (1.8 vs 1.8, p=0.9597). On logistic regression, male gender (OR 3.2, p-value 0.0331) and number of SWL (OR 3.05, p-value 0.0229) were associated with increased likelihood of stone conversion.
Conclusion: Conversion to non-cystine stones may occur in over one third of cystinurics. This appears to be more common in men and is associated with prior SWL. Future research with larger cohorts is indicated to further characterize cystine stone conversion and guide best management options.


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