Clinical Pearls: Shock-wave Lithotripsy
What conditions contribute to calcium nephrolithiasis?
Calcium-based stones (calcium oxalate, calcium phosphate, and brushite) comprise approximately 80% of upper urinary tract stones. The pathogenesis of stone formation is complex and involves not only urinary supersaturation with stone-forming salts but also processes localized to microenvironments within the renal papilla. Calcium stone formation is multifactorial and a variety of pathophysiologic abnormalities may contribute to the risk of stone formation, including hypercalciuria, hyperoxaluria, hyperuricosuria, hypocitraturia and low urine pH. Dietary factors have also been implicated in stone formation.
When is an interventional treatment warranted in the setting of renal calculi?
Patients with renal calculi that are not symptomatic, obstructing, or associated with infection may be observed, although initiation of a medical prophylactic program is advisable to prevent stone progression. Renal calculi associated with pain, obstruction, infection, or continued growth should be treated in order to prevent sepsis or loss of renal function. The primary therapeutic options for surgical management of renal calculi are shock-wave lithotripsy, ureteroscopy, and percutaneous nephrolithotomy.
For which type of kidney stones is shock-wave lithotripsy the most effective treatment?
Shock-wave lithotripsy is generally most effective for stones less than 1.5 to 2.0 cm in diameter. It is generally not recommended for branched, or staghorn, calculi. The outcome for patients with lower-pole renal calculi is generally poorer than for stones in other locations in the kidney, likely due to impaired clearance of fragments from the dependent lower-pole calyces. As such, shock-wave lithotripsy treatment of lower pole stones should be limited to those less than 10 mm in diameter. Less dense stones, those with an attenuation coefficient <900 Hounsfield units on computerized tomography scans, are more likely to be associated with successful treatment with shock-wave lithotripsy than denser stones. Skin-to-stone distance also correlates with shock-wave lithotripsy success; shorter distances, less than 10 cm, are associated with a greater likelihood of success.
What are the contraindications to shock-wave lithotripsy?
Contraindications to shock-wave lithotripsy include active urinary tract infection, uncorrected bleeding diathesis or coagulopathy, distal obstruction, and pregnancy. Obesity and orthopedic or spinal deformities may preclude shock-wave lithotripsy due to inability to properly position or image the patient.