Characterization of Inner Medullary Collecting Duct Plug Formation Among Idiopathic Calcium Oxalate Stone Formers

Objective To study the prevalence of, risk factors for, and renal functional consequences of ductal plug formation in idiopathic calcium oxalate (iCaOx) stone formers (SF). Patients and Methods Accessible renal papillae were videotaped to determine the percent surface area (SA) occupied by plaque and ductal plug in a consecutive cohort of iCaOx SF undergoing percutaneous nephrolithotomy for stone removal. Results Between 2009 and 2014, iCaOx SF comprised 96 of 240 enrolled patients. Of these, 41 (43%) had ductal plugs. Mean plaque SA did not differ between the low and high % plug groups (2.1% vs 3.4%, respectively). The amounts of mean % SA plaque and ductal plug were not strongly correlated (Spearman's ρ = 0.12, P = .3). Patients with >1% mean SA plug had a higher urinary pH (median 6.5 vs 6.0, P = .02) and elevated urinary hydroxyapatite supersaturation (median 5.4 vs 3.7 delta G; P = .04). Those with >1% plugging had more extensive ductal dilation (P = .002) compared to those with ≤1%. However, estimated glomerular filtration rate was the same (median 75.4 mL/min/1.73 m² vs 74.7 mL/min/1.73 m²). Number of prior stone events was associated with mean and maximum papillary SA occupied by plug (P < .05 for both), but not plaque (P = .3 and p = .5, respectively). Conclusion Within a cohort of iCaOx SF, macroscopic plaque and ductal plugs often coexist. Intraluminal features known to favor calcium phosphate crystallization appear to play a role in plug formation. The pathogenic significance of these plugs remains to be established, although their extent appears to correlate with stone burden.