Stone Size, Density Matter

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Characteristics predict likelihood of shock wave lithotripsy success.

 

QUEBEC CITY—Studies presented at recent North American medical conferences have identified factors that can predict the success or failure of shock wave lithotripsy (SWL) for clearing renal and ureteral stones. These factors could help doctors decide on the most appropriate treatment modality for a given stone patient.

 

At the recent annual meeting of the Canadian Urological Association here, investigators at the University of Toronto reported that renal stone density greater than 900 Hounsfield Units (HU) and a skin-to-stone distance (SSD) greater than 9 cm—as determined from non-contrast CT scans—independently predict SWL failure. Trevor Schuler, MD, and his colleagues noted that identifying factors that can predict SWL failure would streamline the care of stone patients. 

 

“Obviously one has to be careful extrapolating the results of a retrospective review to all patient populations,” Dr. Schuler told Renal & Urology News. “However, we are in the process of evaluating the predictors of SWL failure in a prospective trial at our institution.”

 

The study enrolled 111 patients (mean age 52 years) with renal stones imaged by CT. The study only enrolled patients with stones 5-20 mm in diameter. The stones had a mean area of 90.5 mm2. The investigators considered treatment successful if patients were rendered stone free or had asymptomatic fragments less than 5 mm in diameter (complete fragmentation). Of 111 patients, 71 had successful treatment (44 were stone free and 27 had complete fragmentation).

 

Dr. Schuler's group divided patients into four risk groups: those with a stone density less than 900 HU and SSD less than 9 cm; density less than 900 HU and SSD greater than 9 cm; density greater than 900 HU and SSD less than 9 cm; and density greater than 900 HU and SSD greater than 9 cm. Success rates were 90.5%, 78.8%, 58.3%, and 40.9%, respectively. Patients with a stone density greater than 900 HU or a SSD greater than 9 cm, or both, had a sixfold greater risk of SWL failure. 

 

“In this series, it is pretty clear that those with SSD more than 9 cm and stone attenuation greater than 900 HU are destined for failure with SWL and should be counseled towards other treatment modalities for their stone,” Dr. Schuler said. “It should be kept in mind that our outcomes are likely conservative since follow-up in this series was based on two-week post-treatment imaging.”

 

Meanwhile, at the American Urological Association annual meeting in Anaheim, Calif., Eric A. Singer, MD, MA, and his colleagues at the University of Rochester Medical Center in New York reported that proximal ureteral stones smaller than 29 mm2 or with a density less than 644 HU independently predicted successful stone clearance.

 

The study focused on 50 patients (mean age 46.6 years; range 15-93 years) who underwent SWL for proximal ureteral stones diagnosed with non-contrast CT (NCCT). The investigators calculated stone cross-sectional area as the product of stone length and width.

 

Three months after SWL treatment, 24 (48%) patients were stone free and 26 had residual stones, Dr. Singer reported. The mean stone size was 29 mm2 for the stone-free group compared with 43 mm2 for the patients with residual stones. The mean stone density was 644 HU for the stone-free group versus 833 HU for the residual-stone group. Patient age, BMI, and SSD did not predict success.

 

The study findings are “making us rethink which patients we are recommending for shock wave lithotripsy versus ureteroscopy and laser lithotripsy,” said Dr. Singer, who is a urology resident. He noted that many patients seen at his institution have NCCT scans as part of their initial evaluation, which enable calculation of stone size and density.

 

In another study presented in Anaheim, a team from Mansoura University in Egypt showed that obesity and increased stone density were significant predictors of SWL failure. The study, led by Ahmed R. El-Nahas, MD, involved 120 patients with renal stones (71 men and 49 women; mean age 42.6 years) evaluated by NCCT and treated with SWL. The study only included patients whose stones were 0.5 to 2.5 cm in the longest dimension. A total of 3,000 shocks (at a frequency of 100 per minute) were delivered per session.

 

The investigators considered treatment to be successful if the patients had complete clearance of stone fragments or they had residual fragments less than 4 mm in diameter three months after treatment. They defined treatment failure as no breakage of stones after three SWL sessions.

 

A total of 105 patients had successful treatment and 15 failed. Compared with patients whose stones had a density of 1000 HU or less, those with larger stones were 8.1 times more likely to fail SWL treatment. In addition, the researchers found that each 1 kg/m2 increase in BMI was associated with a 12% increased risk of treatment failure. Dr. El-Nahas' group concluded that alternative treatments should be used for obese patients with stone density greater than 1000 HU.

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