Maximizing SWL Outcomes for Calculi

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Besides stone size and location, urologists need to consider stone density, skin-to-stone distance.

 

Urothiliasis afflicts 7%-13% of the population and has a recurrence rate of up to 50% at five years. In the United States, the economic burden of treatment and lost productivity was estimated at $2.1 billion in 2001. Stone size and location determined from non-contrast CT and plain radiographs have long been used to triage patients for either shock wave lithotripsy (SWL) or  endoscopic management. In one of the few randomized trials in stone disease, the Lower Pole Study Group demonstrated stone-free rates after SWL for lower pole stones of 63%, 23% and 14% for stones 1-10, 11-20, and 21-30 mm2, respectively. This compares to stone free rates of 100%, 93%, and 86% following percutaneous nephrolithotomy for the same stone size ranges. It is clear from these data that as calculus size increases, success rates with SWL plummet.

 

Stone-related variables on CT are recognized as being increasingly important in predicting stone-free-status following SWL. The skin-to-stone distance (SSD) as measured on CT was demonstrated to predict the success of SWL by Stephen Nakada, MD. He and his colleagues found that 85% of patients with residual fragments had an SSD greater than 10 cm in diameter. Additionally, 80% of patients who were rendered stone free had an SSD less than 10 cm. Stone density as measured on CT also has been shown to predict SWL success. Several groups have demonstrated a significantly better stone- free rate (80%-100%) in patients with calculi that had densities below 1000 Hounsfield Units (HU) compared with 30% for patients with stones greater than 1000 HU.

 

Randomized trial data from our center have demonstrated the value of slower treatment rates and altering post treatment management of residual fragments. Slowing the rate of shock delivery from 120 to 60 per minute was associated with a significant improvement of treatment success rate from 32% to 71% at three months for stones larger than 100 mm2. In addition, for patients with lower pole fragments after SWL, mechanical percussion inversion and diuresis (MPID) resulted in a higher stone free rate when compared against standard observation. Those treated with MPID had a stone-free rate of 40% compared with only 4% in the observation arm. Patients in the observation arm achieved the same success rate after crossover into the MPID arm.

Available data would suggest the need to incorporate stone size and location, as well as stone density and SSD into the counseling and decision-making process for patients with urolithiasis to ensure optimization of treatment results for each patient.

 

Dr. Schuler is a clinical fellow at St. Michael's Hospital in Toronto, Ontario, Canada. Dr. Honey is the division head of urology at St. Michael's Hospital and is on the editorial advisory board of Renal & Urology News.

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