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Milk of calcium stones: radiological signs and management outcome

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Introduction:Milk of calcium (MOC) is a rare type of stone that was first described in 1940 by Ludin and Howald who reported MOC in renal cysts. Milk of calcium is a viscous colloidal suspension of calcium salts.  Stasis, obstruction and infection are important predisposing factors. Due to a layering effect, characteristic radiological signs especially in CT can help in diagnosis to avoid unsuccessful shock wave lithotripsy.

Milk of calcium stones are well reported in association with calyceal diverticulae and renal cysts and can be also found in extra-urinary sites as the gastrointestinal tract (gall-bladder) or bronchogenic cysts.

 Only few cases of renal milk of calcium in hydronephrotic kidneys have been reported. The etiology of MOC is unclear, however, its relation to urine stagnation (obstruction) and infection is well described. As a colloid suspension, the calcium salts gravitate to the most dependent portion of the cavity. This layering (gravitational) effect leads to the radiological finding of crescent-shaped dense lesion with a fluid level at the upper (anterior) border of the stone.

Patients and methods: All cases of renal milk of calcium stones in our department over 7 years (2008-2015) were included in this case series. Preoperative urine culture was done in all cases.  Preoperative imaging included X-ray KUB, abdominal US and non-contrast CT.  All cases were treated with PCNL.

Stones retrieved were sent for stone culture and chemical composition using infrared method. All patients underwent non contrast CT two days postoperatively prior to removal of nephrostomy tube to detect any residual stones that may require second look PCNL.

Results:

Nine cases of milk of calcium renal stones were included in this study. These stones were faint radio-opaque in 4 cases and radiolucent in 5 cases. All cases were diagnosed preoperatively with non contrast CT. Their Hounsfield units (HU) ranged from 112 to 622 (Median value 322). All stones were located in a dependent position (gravitational effect) in the posterior aspect of dilated calyces. Seven cases exhibited the typical fluid level  and two cases demonstrated semilunar (half moon) pattern in the anterior surface of the stones. All cases underwent PCNL with suction and retrieval of soft stones without the need for disintegration.

Discussion:

This is the largest reported case series of milk of calcium renal stones, in which radiological signs by CT scan to predict renal MOC stones, clinical picture and management outcome are described in details.

Conclusions:

Diagnosis of milk of calcium stones preoperatively requires a high degree of suspicion.  When stones demonstrate a low Hounsfield unit, are arranged in dependent positions within dilated calyces and exhibit a fluid level or semilunar pattern on non contrast CT,  milk of calcium stones should be considered. PCNL is an effective modality for management of renal milk of calcium stones.

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