EXTRACORPOREAL SHOCK WAVE LITHOTRIPSY
Extracorporeal shock wave lithotripsy (ESWL) is a noninvasive procedure for the treatment of nephrolithiasis. In this procedure, acoustic shock waves are generated external to the patient and focused on the renal stones, which are fragmented into small pieces that can be spontaneously passed in the urine. The skin and surrounding renal parenchyma receive a much smaller dose of energy and therefore remain largely unharmed.
Shockwaves fragment renal stones through multiple mechanisms. A direct effect occurs because of the difference in density between the stones and surrounding ﬂuid. As the waves enter and then exit the stones, they create compressive and then tensile forces that contribute to stone fragmentation. An indirect effect occurs because of the cavitation bubbles that form behind the advancing shock waves, which exert forces on the stones as they collapse.
In current practice, lithotripters create shock waves using electrohydraulic, electromagnetic, or piezoelectric generators. Electrohydraulic lithotripters consist of two underwater electrodes that spark, producing a vaporization bubble that rapidly collapses and generates shock waves. An ellipsoid reﬂector refocuses these waves so they converge onto the stones. Electromagnetic lithotripters, meanwhile, consist of an electromagnetic coil beneath a metal plate in a water bath. As a charge is passed through a coil, a repulsive magnetic force pushes the plate against the water, generating a shock wave that is focused using an acoustic lens. Finally, piezoelectric lithotripters contain thousands of small piezoelectric crystals arranged under water on a hemispherical surface. These crystals rapidly expand in response to an electrical pulse, generating a shock wave. Because of the conﬁguration of the crystals, no additional focusing is necessary.
INDICATIONS AND PREOPERATIVE EVALUATION
ESWL is indicated for most uncomplicated upper urinary tract calculi where renal anatomy is normal and the combined diameters of the stones is less than 2 cm. ESWL is also considered an appropriate option for the management of stones anywhere in the ureter, with the exception of middle and lower ureteral stones in women of childbearing age. Of note, ESWL is less effective for “hard” stones composed of calcium oxalate monohydrate, calcium phosphate, or cystine. In addition, certain factors may lower the likelihood of fragment passage, such as lower pole calculi, long and narrow renal infundibula, narrow infundibulopelvic angles, and severe hydronephrosis.
Absolute contraindications to ESWL include pregnancy, severe skeletal malformations (because of altered anatomic relationships), signiﬁcant coagulopathy, urinary tract infection, and large abdominal aortic aneurysm (because of possible rupture). Relative contraindications include obesity, which diminishes efﬁcacy; cardiac pacemakers, due to concerns over inducing arrhythmias; renal artery aneurysms and chronic pancreatitis, which may be worsened by the procedure; and uncontrolled hypertension, due to an increased bleeding risk.
Most patients are positioned supine on the lithotripter bed; however, those with stones in anteriorly located kidneys, medial portions of a horseshoe kidney, or transplanted kidneys should be positioned prone to reduce the skin-to-stone distance and remove skeletal structures from the shock wave path. Once the patient is positioned, the stones are localized using ﬂuoroscopy and, in some cases, ultrasound.
During lithotripsy, the body must be coupled with the shock wave source. This process eliminates the transition between ambient air and the patient’s skin, which would otherwise attenuate the shock wave and cause complications such as ecchymoses and skin breakdown. Because soft tissue has an acoustical impedance similar to that of water, coupling can be achieved either by submerging the patient in a water bath, as in earlier systems, or by applying a water cushion with a silicone membrane directly to the patient’s skin.
The number of shock waves applied to the stones will impact the degree of fragmentation. Each manufacturer determines a machine-speciﬁc dosage that should not be exceeded.
POSTOPERATIVE CARE AND COMPLICATIONS
After lithotripsy, patients are encouraged to ambulate and to increase ﬂuid intake to promote stone passage. Postprocedural gross hematuria is common and transient. A mild to moderate degree of abdominal or ﬂank pain is also common; however, severe and persistent pain suggests complications such as hematoma or impaction of stone fragments, which should prompt evaluation with CT scan.
In the absence of a more acute indication, follow-up is typically performed at 2 weeks and includes repeat imaging to assess the success of stone fragmentation and passage. If necessary, residual stones may be treated with a repeat ESWL procedure or other techniques.