High Resolution Ultrasound Appearance of Medullary Sponge Kidney in Primary Distal Renal Tubular Acidosis

A 12-year-old boy presented to the pediatric outpatient department with a history of growth faltering, polyuria, polydipsia, bowing of legs, and recurrent vomiting since 2 years of age. He had high 24-h urine output (2.01 L/m²), hypokalemia (2.69 mmol/L), normal anion-gap metabolic acidosis, and high urine pH (7.15), increased urinary Beta-2-microglobulin (32,242 ng/mL), and hypercalciuria (urine calcium-to-creatinine ratio of 0.6 mg/mg). Bicarbonate loading test revealed a low urine-to-blood carbon dioxide gradient (1.2 mmHg) and normal fractional excretion of bicarbonate (2%), suggesting distal renal tubular acidosis (dRTA) along with proximal tubular dysfunction. Ultrasound [Figure 1] demonstrated medullary nephrocalcinosis, macroscopic medullary renal cysts, and numerous tubular medullary cysts leading to a diagnosis of medullary sponge kidney (MSK).¹ On genetic evaluation, two variants (pathogenic in exon 19 and likely pathogenic in exon 17) were identified in the gene encoding solute carrier family 4 member 1 (SLC4A1). Therapy with oral potassium citrate led to consistent improvement in serum potassium and bicarbonate levels.

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Figure 1: Medullary sponge kidney. Screening with a low-resolution curvilinear probe reveals simple cysts (arrows in a) in the medulla and echogenic calyces with punctate echogenic foci in the medulla (arrowheads in a), suggesting medullary nephrocalcinosis. A high-resolution ultrasound with a linear ultrasound probe reveals numerous tubular cysts (arrows in b & c) within the medulla, representing the ectatic collecting ducts, and larger macrocysts (asterisks in b-d) that are confluent with the tubular cysts (arrows in b-d), and medullary nephrocalcinosis (arrowheads in b & c). No cortical cysts are seen. The tubular microcysts are visualized only on high-resolution ultrasound.

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While the classical ‘flower bouquet’ appearance on intravenous urography is diagnostic of MSK, high-resolution ultrasound can identify the microcystic dilatation of the papillary zone and nephrocalcinosis.² MSK is occasionally associated with inherited forms of dRTA caused by genetic variants of the adenosine triphosphatase proton transporter isoforms (ATP6V1B1 or ATP6V0A4) and SLC4A1, possibly as a consequence of chronic metabolic acidosis.^3,4 The dRTA is likely primary, while MSK is a coexisting structural abnormality identified on imaging.

This case highlights the utility of high-resolution ultrasonography in diagnosing MSK in patients with genetically proven dRTA. Whole-exome sequencing can ascertain the inherited basis of MSK, thereby aiding sibling screening and prenatal genetic counselling.