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Calcium phosphate nephrolithiasis in patients with cystinuria
Cody M. Rissman, BS1, Vernon M. Pais, Jr., MD2.
1Geisel School of Medicine at Dartmouth, Hanover, NH, USA, 2Dartmouth-Hitchcock Medical Center, Lebanon, NH, USA.

BACKGROUND: Cystinuria is a rare autosomal recessive disorder characterized by decreased renal reabsorption of filtered cystine and resultant recurrent cystine nephrolithiasis. Standard treatment for cystinuria includes dietary modification, cystine-binding agents, and urinary alkalinization in order to increase cystine solubility. However, urinary alkalinization may theoretically predispose to calcium phosphate stone formation, as the solubility of calcium phosphate markedly decreases with increasing pH. Although many of the implications of cystinuria have been well studied, it is unknown whether treatment side effects may alter the actual type of stone formed. We sought to investigate the incidence of conversion to calcium phosphate stones in treated cystinurics.
METHODS: We identified 16 patients with cystinuria that were followed for a mean 9.4 years (range 1-24 years). Data collected as part of routine clinical practice including periodic 24hr urinalysis, stone analysis, and procedural data were reviewed retrospectively with institutional IRB approval.
RESULTS: Of the patients with cystinuria followed at our institution, 3 of 16 (18.8%) developed some component of calcium phosphate in their stones. One of the three developed a primary calcium phosphate stone (80% by composition) with no cystine component. Data from 24hr urinalyses were available for 14 of 16 patients. The mean pH assessed by 24hr urine collection occurring within 1 year of stone event in calcium phosphate stone formers was 7.117 vs. 6.817 in those without any calcium phosphate component (p=0.42). There was no significant difference in stone events per year in the two populations (P=0.901). Additionally, there was no significant difference in the cystine capacity measured via 24hr urine collection in the two populations (P=.648).
CONCLUSIONS: We have identified an incidence of de novo calcium phosphate stone formation in treated cystinurics of 18.8%. Complete conversion to calcium phosphate, however, occurred in only 6.2%. Events precipitating the formation of calcium phosphate nephrolithiasis in patients with cystinuria are unclear. Although elevated pH was identified in this cohort, our small sample size did not allow us to identify statistical differences between these groups. Nevertheless, these findings confirm the utility of routine stone analysis, even in those with otherwise “predictable” cystine stones, as established treatment regimens may inadvertently induce stone formation. Early identification of these conversions may allow for treatment adjustments given that therapeutic algorithms for calcium phosphate stones in cystinurics are not as well defined as those for their primary cystinuria.


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