The following represent unfavourable characteristics for successful extracorporeal shockwave lithotripsy to a lower pole stone except:
A. Infundibulopelvic angle <90 degreesAnswer B
The success of ESWL depends on factors that relate to the stone characteristics, the renal anatomy, the patient anatomy and the type of lithotriptor. Stone factors include the stone size, hardness, and location within the kidney. Success rates for lower pole stones are less than for stones located in the renal pelvis or other calyces. For example, in the Lower Pole I study only 21% of patients with stones larger than 10 mm located in a lower pole calyx were stone free after lithotripsy [5]. Adverse anatomic features include an infundibulopelvic angle <90 degrees and a narrow (<5 mm) or long (>30 mm) calyceal infundibulum. The role for adjunctive measures to improve the outcome of ESWL for lower pole stones such as PDI (percussion, diuresis and inversion) is yet to be established.
The effectiveness of lithotripsy is also dependent on the hardness of the stone. For example, calcium oxalate monohydrate, dicalcium phosphate dihydrate (brushite) and cystine stones are relatively resistant to shockwaves although they are not contraindications to ESWL. Uric acid stones are radiolucent and must therefore be localised using ultrasound rather than fluoroscopy but are soft and may fragment well with lithotripsy.
Obesity reduces the effectiveness of ESWL and skin to stone difference has been shown to be an independent predictor of success. Modern lithotriptors are less effective than the Dornier HM3 lithotriptor but are safer, better tolerated and do not require general anaesthesia. A recent meta-analysis has shown that reducing the shockwave frequency from 120 to 60–90 per minute improves stone clearance.