What is the clinical significance of milk of calcium calcifications in adults, particularly those with a history of kidney disease or urinary tract infections?

Clinical Significance of Milk of Calcium Calcifications

Milk of calcium calcifications are benign, gravity-dependent colloidal suspensions of calcium salts within renal cysts or dilated calyces that require recognition to avoid unnecessary surgical intervention, as they are not true kidney stones and should not be treated with lithotripsy. 1, 2

What Milk of Calcium Actually Is

• Milk of calcium is a viscous colloidal suspension of calcium carbonate, calcium phosphate, or calcium oxalate that settles in dependent portions of cystic cavities or dilated calyces due to gravity 2, 3
• The condition develops in association with stasis, obstruction, and infection as predisposing factors 2
• This entity was first described in 1940 and remains a rare finding that can be mistaken for nephrolithiasis 2, 3

Critical Diagnostic Features to Prevent Misdiagnosis

Radiographic Characteristics

• On upright plain films, milk of calcium demonstrates a characteristic crescent or half-moon contour with a sharp horizontal upper border at the calcium-fluid interface 1, 3
• On supine radiographs, these calcifications appear as poorly defined, faint radio-opacities that can be misinterpreted as renal stones 1, 3
• CT scan is the definitive diagnostic modality, showing calcific debris with horizontal layering in dependent positions within dilated calyces 2, 3

CT Imaging Specifics

• Hounsfield units range from 114 to 612, which is lower than typical renal calculi 2
• Stones are located in the posterior aspect of dilated calyces due to gravitational effect when scanned supine 2
• Fluid levels or semilunar patterns on the anterior surface are pathognomonic features 2
• CT in both supine and prone positions demonstrates gravity-dependent layering, confirming the diagnosis 3

Clinical Significance and Management Implications

Why Recognition Matters

• Milk of calcium can masquerade as nephrolithiasis, leading to unnecessary surgical interventions including shock wave lithotripsy, which is ineffective for this condition 1, 3
• The material is soft and does not require disintegration, unlike true kidney stones 2
• Shock wave lithotripsy will fail because the material is not a solid calculus but rather a colloidal suspension 3

Appropriate Management

• When intervention is needed, percutaneous nephrolithotomy (PCNL) with suction is the effective treatment modality, allowing retrieval of the soft material without need for stone fragmentation 2
• Conservative management may be appropriate in asymptomatic patients, as the condition itself is benign 1
• Ureteric stenting may facilitate drainage of milk of calcium from the ureter when present 4

High-Risk Populations

Spinal Cord Injury Patients

• Bilateral renal milk of calcium has been reported in patients with spinal cord injury who remain in prolonged supine positions 3, 4
• Tetraplegia and paraplegia patients with recurrent urinary tract infections should be evaluated with CT rather than plain films when calcifications are detected 3, 4
• The combination of immobility, urinary stasis, and recurrent infections creates ideal conditions for milk of calcium formation 3, 4

Patients with Chronic Kidney Disease

• While milk of calcium is a distinct entity from the vascular calcifications associated with elevated calcium-phosphate product in CKD, both involve calcium deposition 5, 6
• CKD patients with calcium-phosphate product above 55 mg²/dL² face increased risk of vascular and soft tissue calcification, which represents a different pathophysiologic process than milk of calcium 5, 6
• Total calcium intake in CKD patients should not exceed 2,000 mg/day to minimize vascular calcification risk, though this does not directly relate to milk of calcium formation 5, 6

Common Pitfalls to Avoid

• Do not perform shock wave lithotripsy based solely on ultrasound findings without CT confirmation, as milk of calcium will not respond to this treatment 1, 3
• Do not assume all radio-opaque shadows in the kidney represent true calculi—faint opacities with indistinct margins should prompt CT evaluation 3
• Do not rely on supine imaging alone—upright films or CT in multiple positions reveals the characteristic layering 1, 3
• Do not confuse intracystic calcification (eggshell pattern) with milk of calcium, as these represent different entities 7

Differential Diagnosis Considerations

• Sloughed papillae may produce triangular or ring-shaped calcifications that differ from the horizontal layering of milk of calcium 7
• True renal calculi demonstrate uniform radiopacity rather than the gravity-dependent layering seen with milk of calcium 7
• Intracystic calcification appears as thin peripheral "eggshell" calcification rather than dependent layering 7