Medullary Calcinosis (Nephrocalcinosis)
Medullary calcinosis, also known as medullary nephrocalcinosis, is the deposition of calcium salts specifically within the renal medullary tissue, particularly in the renal pyramids, which appears as echogenic structures on ultrasound and calcifications on CT imaging. 1
Pathophysiology and Mechanism
The condition results from impaired calcium handling in the renal tubules, leading to calcium salt precipitation in the medullary interstitium. 1 In conditions like Bartter syndrome, impaired salt reabsorption in the thick ascending limb causes reduced calcium reabsorption, resulting in hypercalciuria and subsequent nephrocalcinosis. 1
Clinical Context and Associated Conditions
Medullary nephrocalcinosis occurs as a non-specific renal manifestation in various metabolic and genetic disorders: 2
- Bartter syndrome types 1 and 2 (hypercalciuria with subsequent nephrocalcinosis developing after 1-2 months of life is typical) 1
- Primary hyperparathyroidism (associated with hypercalcemia and hypercalciuria) 2
- Distal renal tubular acidosis type 1 (incomplete forms commonly associated with medullary sponge kidney) 3, 4, 2
- Medullary sponge kidney (MSK) - a renal malformation typically associated with nephrocalcinosis and recurrent calcium stones 3, 4
- Hypophosphatemic rickets 2
- Chronic renal insufficiency requiring dialysis 2
Diagnostic Approach
Imaging Characteristics
Ultrasound is the primary diagnostic modality, showing echo-enhanced structures in the region of the renal pyramids. 2 However, a critical caveat exists: in the current era of non-contrast CT imaging, distinguishing true nephrocalcinosis (tissue calcification) from nephrolithiasis (urinary calculi) requires direct visual inspection via endoscopy, making nephrocalcinosis diagnosis essentially endoscopic rather than purely radiologic. 5
CT imaging may show calcifications in close association with renal papillae, but these may actually represent stones rather than tissue calcifications in conditions like primary hyperparathyroidism, distal renal tubular acidosis, and medullary sponge kidney. 5
Biochemical Evaluation
When medullary nephrocalcinosis is suspected, assess: 1
- Serum electrolytes (sodium, chloride, potassium, calcium, magnesium) 1
- Acid-base status 1
- Renin and aldosterone levels 1
- 24-hour urinary excretion of calcium, phosphate, oxalate, uric acid, and citrate 3, 4
- Morning urine pH 3, 4
- Fractional excretion of chloride and urinary calcium-creatinine ratio 1
Important Clinical Distinctions
Bartter Syndrome Subtypes
Bartter syndrome type 3 patients typically have a milder defect without hypercalciuria and partial capacity to concentrate urine, distinguishing them from types 1 and 2 where hypercalciuria and nephrocalcinosis are characteristic. 1
Medullary Sponge Kidney Phenotypes
Two clinical phenotypes exist among patients with medullary sponge kidney: 3
- Those with stone risk factors (hypercalciuria, hypocitraturia, hyperuricosuria, hyperoxaluria) who have high stone recurrence rates
- Those without stone risk factors who have very low stone rates 3
Clinical Significance and Complications
Beyond the medullary calcifications themselves, patients commonly develop: 1
- Recurrent calcium nephrolithiasis 3, 5
- Isosthenuria (impaired ability to dilute or concentrate urine due to blunted osmotic gradient in renal medulla) 1
- Bone disease (very frequent in medullary sponge kidney patients with primary stone risk factors) 4
- Progressive chronic kidney disease (particularly with prolonged hypercalcemia or chronic hypokalemia) 1
In extreme hypercalcemia, glomerular calcification may exceed cortical and outer medullary tubular calcification, potentially causing segmental glomerulosclerosis and nephrotic-range proteinuria. 6