# Medullary sponge kidney: in-depth phenotyping for a better understanding of functional and structural abnormalities

## TL;DR

This study uses MRI to investigate kidney function and structure in patients with medullary sponge kidney, revealing impaired oxygenation but no fibrosis.

## Contribution

The study introduces a detailed phenotyping approach using fMRI to assess functional and structural abnormalities in medullary sponge kidney.

## Key findings

- Patients with medullary sponge kidney showed higher R2* cortex-to-medulla ratios, indicating impaired renal oxygenation.
- MRI revealed medullary cysts in 60% of medullary sponge kidney patients.
- Estimated GFR was inaccurate in medullary sponge kidney patients, while measured GFR was significantly lower.

## Abstract

Medullary sponge kidney is an entity characterized by pre-calyceal dilatation of the renal tubules, whose pathophysiology is unknown. Tubular anomalies have been described, suggesting impaired medullary function. To better characterize these patients, tools for assessing medullary function and structure are needed. The latter can be evaluated with functional magnetic resonance imaging (fMRI), using blood-oxygen-level-dependent imaging, which quantifies tissue oxygenation, and diffusion-weighted-imaging and T1-mapping sequences which allow fibrosis assessment. The aim of this study was to deeply phenotype medullary sponge kidney patients.

We carried out fMRI, measured glomerular filtration rate (mGFR) by iohexol clearance, and metabolic assessment of urolithiasis in patients with medullary sponge kidney and in healthy controls. The primary endpoint was the comparison of R2*, inversely proportional to oxygen content, measured by blood-oxygen-level-dependent MRI. Secondary endpoints included comparison of T1 and apparent diffusion coefficient, comparison of GFR between medullary sponge kidney patients and controls, and the correlations between fMRI, GFR and biological abnormalities in medullary sponge kidney.

Twenty patients with medullary sponge kidney were included, as well as 13 controls. We observed a higher R2* cortex-to-medulla ratio in medullary sponge kidney patients compared to controls (0.60 vs. 0.55; p = 0.04). No difference was observed for T1 and apparent diffusion coefficient cortex-to-medulla ratio. mGFR was significantly lower in medullary sponge kidney patients (90 ml/min/1.73m2 vs 78 ml/min/1.73m2; p = 0.008) although estimated GFR did not differ. Medullary cysts were visible on MRI in 60% of medullary sponge kidney patients.

We identified impaired renal oxygenation in patients with medullary sponge kidney. We did not find evidence of kidney fibrosis in medullary sponge kidney. GFR estimation was not accurate in medullary sponge kidney patients. MRI can visualize medullary cystic appearance of medullary sponge kidney.

The online version contains supplementary material available at 10.1007/s40620-025-02413-3.