Milk of Calcium in the Kidney

Milk of calcium is a suspension of fine calcium salts, mainly carbonate and phosphate, in a stagnant cavity of the kidney. The cavity is usually a calyceal diverticulum, and less often a hydronephrotic calyx or an obstructed upper pole.

It is an uncommon finding on imaging, usually incidental and asymptomatic. The key point is recognition, so that a harmless finding is not treated as a stone or tumour.

Pathophysiology. Urinary stasis in the cavity allows calcium salts to crystallise and layer out. The collection moves with gravity, giving a dependent layer that changes shape with position.

Calyceal diverticula are narrow necked cavities lined by urothelium that connect to the collecting system. Stasis there also predisposes to true stones, infection and pain.

Most patients are asymptomatic. Symptoms are loin pain, haematuria, and recurrent infection when the cavity is infected or contains a true stone. A fever with loin pain needs urine culture and imaging for abscess.

• Upright plain film: a crescent or layered meniscus of calcification with a horizontal upper margin. On a supine film it spreads into a round or ill defined opacity.
• Cross-table lateral film or CT with change of position confirms the shift.
• CT: a dependent layer of high attenuation (often several hundred HU) in a cavity with a fluid level. Delayed images after intravenous contrast may fill the cavity, showing connection with the collecting system.
• Ultrasound: a dependent echogenic layer with shadowing that moves with position.
• Urinalysis, culture, creatinine. Stone analysis if retrieved.

Differential diagnosis.

• Calcified renal cyst (wall calcification, no gravity dependent layer)
• Stone within a diverticulum
• Nephrocalcinosis
• Tuberculous calcification
• Calcified tumour
• Hydatid cyst
• Renal artery aneurysm
• Calcified haematoma

• Asymptomatic: no treatment and no routine follow up.
• Symptomatic or infected: culture directed antibiotics for 7 to 14 days, and analgesia (paracetamol 1 g every 6 hours, maximum 4 g daily).
• Persisting symptoms: treat the diverticulum.
  • Ureteroscopy with laser incision of the neck and fulguration of the lining, for upper and mid-pole diverticula with accessible necks.
  • Percutaneous access with neck dilation, stone clearance and fulguration, for lower pole or posterior diverticula with stones.
  • Laparoscopic marsupialisation or diverticulectomy for anterior or refractory diverticula.
• Shock wave lithotripsy often fails to clear fragments because the neck is narrow and drainage is poor.
• Metabolic assessment (24-hour urine, serum calcium and parathyroid hormone) when true stones are present.

Monitoring. Imaging at 3 months after intervention for residual cavity and stones, and review for recurrent infection.

Referral and escalation. Refer to urology for symptoms, infection, a true stone or doubt about the diagnosis. Urgent referral for infected obstruction or abscess.