Objective To characterize the endoscopic and histologic renal papillary lesions in a cohort of uric acid (UA) stone formers (SF). Methods Data were prospectively obtained during percutaneous nephrolithotomy between 2009 and 2013. Renal papillae were endoscopically analyzed to quantitate surface area occupied by plaque or plug, and biopsies were obtained. UA SF were compared with non-SF controls and patients with >50% calcium oxalate (CaOx) in the absence of UA. Results There were 23 UA SF; of which 19 stones (83%) were admixed with CaOx and 4 (17%) were pure. Compared with CaOx SF and controls, UA SF had a higher prevalence of diabetes and obesity, greater serum creatinine and UA levels, lower estimated glomerular filtration rate and urine pH, and elevated UA supersaturation. Characteristics of UA SF were compared with 95 CaOx SF and 19 controls. Overall, 23 (100%) UA SF had endoscopic plaque and 13 (57%) plugs. Endoscopically, UA SF displayed a greater incidence of plugging (57% vs 45% vs 11%; P =.006) relative to CaOx SF and controls. Likewise, UA SF had a greater percentage surface area of plugging (0.1 vs 0.0; P =.002) and plaque (2.0 vs 0.9; P =.006) than controls but similar amounts to CaOx SF. Histologic plugs were similar in UA and CaOx SF, although CaOx SF demonstrated greater interstitial inflammation on endoscopic biopsy. Conclusion UA and CaOx SF have similar amounts of plaque, whereas UA SF have more endoscopic but not histologic collecting duct plugs. These data suggest an overlap between the pathogenesis of UA and CaOx stones. The anchoring site for UA stones remains uncertain.
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