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.
SHOCKWAVE
PHYSICS
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.
PROCEDURE
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.
