Struvite and Staghorn Calculi Workup

Updated: Jan 02, 2019
  • Author: Maxwell Meng, MD; Chief Editor: Bradley Fields Schwartz, DO, FACS  more...
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Workup

Laboratory Studies

Prior to treatment of struvite stones, complete serum laboratory studies are required, including the following:

  • Complete blood cell count
  • Prothrombin and activated partial thromboplastin times
  • Serum electrolyte evaluations
  • Creatinine measurements

Chronic anemia may necessitate preoperative blood typing and screening for potential blood transfusion, especially if open or percutaneous surgery is planned.

More importantly, urinalysis and urine culture should be performed several days before surgery, and specific antibiotic therapy should be initiated at least 24 hours prior to treatment. Findings from cultures of voided urine may not accurately reflect renal microbiology, with a negative culture finding or discrepant organisms.

Additional aspects of the metabolic evaluation of urinary stones should be pursued because up to 50% of patients with infection-related stones have concomitant metabolic abnormalities. Thus, a 24-hour urinary collection (for calcium, oxalate, uric acid, citrate, phosphate, uric acid, magnesium, sodium, total volume, and pH) and simultaneous serum tests for calcium, uric acid, electrolytes, and phosphate are indicated. If the serum calcium level is elevated, it should be rechecked along with serum parathyroid hormone levels.

If the patient has undergone prior stone removal surgery, information regarding the chemical composition of any previous stones is extremely important.

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Imaging Studies

Plain abdominal radiography usually documents the extent of struvite staghorn calculi; however, additional imaging tests that reveal the anatomy of the renal collecting system can be helpful. In addition, guidelines from the American Urological Association/Endourological Society advise that in some patients with staghorn calculi and compromised renal function, adequate assessment of function requires a nuclear renal scan or a contrast-enhanced imaging study (eg, computed tomography [CT], magnetic resonance [MR], or intravenous [IV] urography. [10, 11]

Intravenous urography can clearly delineate the pelvic calyceal anatomy. Currently, however, noncontrast CT scanning followed by intravenous contrast CT scanning is obtained most often in the evaluation of urinary stones. CT scans also display the adjacent structures and may aid in selecting the safest percutaneous tract to access the renal collecting system.

Narrow, scarred infundibula indicate the need for percutaneous nephrostomy (PCN), while wide, large renal infundibula suggest that extracorporeal shockwave lithotripsy (SWL) might be adequate. If the passageway between the calyces and renal pelvis is open and unrestricted, stone fragments produced during SWL are much more likely to pass.

Traditionally, staghorn calculi were defined as partial if the renal pelvic stone extended into at least 2 calyceal groups or complete if at least 80% of the collecting system was filled. Some experts argue that, to compare published stone-free rates, especially in the era of minimally invasive modalities, an improved classification system based on stone size should be implemented.

CT scanning with 3-dimensional reconstruction offers accurate stone volumes, but the added radiographic analysis is costly, time consuming, and neither practical nor readily available. Lam and associates reported a simple 2-dimensional electronic computerized tracing technique that calculated stone surface area, which correlated well with stone volume. [12]

Performing nuclear renography is not necessary, but the findings are helpful for determining the relative function of the affected kidney. If the kidney has minimal function, nephrectomy may be needed. However, overall and relative renal function must be considered prior to removal of the kidney.

Dual energy CT with advanced postacquisition processing has shown some promise in preoperative determination of stone composition. In a small sample of patients, Zilberman et al demonstrated that all struvite stones have a measured attenuation of 900 Hounsfield Units (HU) or less. [13] This type of work was reproduced in both human models and another small population of test subjects. [14] Accurate preoperative prediction of stone composition may assist with planning for perioperative antibiotic and surgical management.

Ultrasonography alone is insufficient, but images show coexisting hydronephrosis. MRI does not help visualize urinary calculi; therefore, this modality has no role in preoperative and postoperative imaging of struvite stones.

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