Medullary sponge kidney: what kind of stones?

Medullary sponge kidney (MSK) is an anomaly of the kidney with cystic enlargement of distal tubules generating stasis zones where calcium salt deposits may form and grow locally. From an epidemiological point of view, MSK is reported in 2% to more than 20% of calcium stone formers. The association of stasis and various metabolic disorders explains a high recurrence rate of stones in affected patients. The composition of stones has been poorly investigated. The aim of this study is to compare stone composition and morphology in MSK and non-MSK patients. Material and methods: 1036 stones from MSK patients and 31,494 stones from non-MSK patients were submitted to a morpho-constitutional analysis based on morphological typing under stereo microscope and precise characterization of chemical and crystalline phases by Fourier transform infrared spectroscopy. Results and discussion: Regarding patients, the male to female ratio was significantly lower in MSK vs non-MSK subjects (1.48 vs 2.09, p < 0 . 00001 ). The recurrence rate was significantly higher in MSK patients (85.0% vs 39.5% in non-MSK, p < 0 . 000001 ). Stones were more often spontaneously passed in MSK vs non-MSK subjects (45.7% vs 34.6%, p < 0 . 000001 ). Stones were mainly composed of calcium oxalate in both groups (66.3% vs 72.5%). However, the slight decrease in the occurrence of calcium oxalate stones was mainly supported by weddellite calculi, less frequent in MSK (17.0% vs 21.6% in non-MSK, p < 0 . 001 ). The main point was the high proportion of stones mainly composed of calcium phosphates: 28.9% in MSK vs 13.9% in non-MSK patients) with a high proportion of stones exhibiting a IVa2 morphology which is a marker for distal tubular acidification defect (12.1% in MSK vs 0.4% in non-MSK patients, p < 0 . 000001 ). Finally, while stones nucleated from a papillary Randall’s plaque were found with a similar frequency in both groups, calcium phosphate nucleus was significantly more frequent in MSK than in non-MSK stones (68.8% vs 48.9%, p < 0 . 000001 ), suggesting a frequent initiation of stones from calcium phosphate plugs in distal tubules ectasias.