Extracorporeal Shockwave Lithotripsy 

Updated: Jun 18, 2018
Author: Michael Grasso, III, MD; Chief Editor: Bradley Fields Schwartz, DO, FACS 

Overview

Practice Essentials

Prior to the introduction of extracorporeal shockwave lithotripsy (ESWL) in 1980, the only treatment available for calculi that could not pass through the urinary tract was open surgery. Since then, ESWL has become the preferred tool in the urologist’s armamentarium for the treatment of renal stones, proximal stones, and midureteral stones. Compared with open and endoscopic procedures, ESWL is minimally invasive, exposes patients to less anesthesia, and yields equivalent stone-free rates in appropriately selected patients.

The efficacy of ESWL lies in its ability to pulverize calculi in vivo into smaller fragments, which the body can then expulse spontaneously. Shockwaves are generated and then focused onto a point within the body. The shockwaves propagate through the body with negligible dissipation of energy (and therefore damage) owing to the minimal difference in density of the soft tissues. At the stone-fluid interface, the relatively large difference in density, coupled with the concentration of multiple shockwaves in a small area, produces a large dissipation of energy. Via various mechanisms, this energy is then able to overcome the tensile strength of the calculi, leading to fragmentation. Repetition of this process eventually leads to pulverization of the calculi into small fragments (ideally < 1 mm) that the body can pass spontaneously and painlessly.

Technical Aspects

All lithotripsy machines share 4 basic components: (1) a shockwave generator, (2) a focusing system, (3) a coupling mechanism, and (4) an imaging/localization unit.

Shockwave generator

Shockwaves can be generated in 1 of 3 ways, as follows:

  • Electrohydraulic: The original method of shockwave generation (used in the Dornier HM3) was electrohydraulic, meaning that the shockwave is produced via spark-gap technology. In an electrohydraulic generator, a high-voltage electrical current passes across a spark-gap electrode located within a water-filled container. The discharge of energy produces a vaporization bubble, which expands and immediately collapses, thus generating a high-energy pressure wave.

  • Piezoelectric: The piezoelectric effect produces electricity via application of mechanical stress. The Curie brothers first demonstrated this in 1880. The following year, Gabriel Lippman theorized the reversibility of this effect, which was later confirmed by the Curie brothers. The piezoelectric generator takes advantage of this effect. Piezoelectric ceramics or crystals, set in a water-filled container, are stimulated via high-frequency electrical pulses. The alternating stress/strain changes in the material create ultrasonic vibrations, resulting in the production of a shockwave.

  • Electromagnetic: In an electromagnetic generator (as seen below), a high voltage is applied to an electromagnetic coil, similar to the effect in a stereo loudspeaker. This coil, either directly or via a secondary coil, induces high-frequency vibration in an adjacent metallic membrane. This vibration is then transferred to a wave-propagating medium (ie, water) to produce shockwaves.

    Electromagnetic generator system. Electromagnetic generator system.

Focusing systems

The focusing system is used to direct the generator-produced shockwaves at a focal volume in a synchronous fashion. The basic geometric principle used in most lithotriptors is that of an ellipse. Shockwaves are created at one focal point (F1) and converge at the second focal point (F2). The target zone, or blast path, is the 3-dimensional area at F2, where the shockwaves are concentrated and fragmentation occurs.

Focusing systems differ, depending on the shockwave generator used. Electrohydraulic systems used the principle of the ellipse; a metal ellipsoid directs the energy created from the spark-gap electrode. In piezoelectric systems, ceramic crystals arranged within a hemispherical dish direct the produced energy toward a focal point. In electromagnetic systems, the shockwaves are focused with either an acoustic lens (Siemens system) or a cylindrical reflector (Storz system).

Coupling mechanisms

In the propagation and transmission of a wave, energy is lost at interfaces with differing densities. As such, a coupling system is needed to minimize the dissipation of energy of a shockwave as it traverses the skin surface. The usual medium used is water, as this has a density similar to that of soft tissue and is readily available. In first-generation lithotriptors (Dornier HM3), the patient was placed in a water bath. However, with second- and third-generation lithotriptors, small water-filled drums or cushions with a silicone membrane are used instead of large water baths to provide air-free contact with the patient's skin. This innovation facilitates the treatment of calculi in the kidney or the ureter, often with less anesthesia than that required with the first-generation devices.

Localization systems

Imaging systems are used to localize the stone and to direct the shockwaves onto the calculus, as well as to track the progress of treatment and to make alterations as the stone fragments. The 2 methods commonly used to localize stones include fluoroscopy and ultrasonography.

Fluoroscopy, which is familiar to most urologists, involves ionizing radiation to visualize calculi. As such, fluoroscopy is excellent for detecting and tracking calcified and otherwise radio-opaque stones, both in the kidney and the ureter. Conversely, it is usually poor for localizing radiolucent stones (eg, uric acid stones). To compensate for this shortcoming, intravenous contrast can be introduced or (more commonly) cannulation of the ureter with a catheter and retrograde instillation of contrast (ie retrograde pyelography) can be performed.

Ultrasonographic localization allows for visualization of both radiopaque and radiolucent renal stones and the real-time monitoring of lithotripsy. Most second-generation lithotriptors can use this imaging modality, which is much less expensive to use than radiographic systems. Although ultrasonography has the advantage of preventing exposure to ionizing radiation, it is technically limited by its ability to visualize ureteral calculi, typically due to interposed air-filled intestinal loops. In particular, smaller stones may be difficult to localize accurately.

History of the Procedure

Evolution of shockwave lithotriptors

The Dornier HM3, originally designed to test supersonic aircraft parts, was the first shockwave lithotriptor introduced in the United States. Despite being somewhat dated, it is still one of the most effective lithotriptors and has become the standard to which other devices are compared. The design of the HM3 is based on an electrohydraulic shockwave generator; the shockwaves are focused via an ellipsoid metal water-filled tub in which both the patient and the generator are submerged. Biplanar fluoroscopy is used for localization, allowing placement of the calculi to be fragmented in the target zone.

Second-generation lithotriptors typically use piezoelectric or electromagnetic generators as the energy source. When coupled with the appropriate focusing device, these shockwave generators commonly have a smaller focal zone. Although a smaller focal zone may minimize damage to the surrounding tissue, this comes at a price. During respiratory excursion, the stone may move in and out of the focal zone; this may compromise fragmentation rates. The coupling device in a second-generation lithotriptor is a silicone-encased water cushion that coapts to the patient, a design that greatly simplifies the positioning of patients.

The newest-generation lithotriptors have been designed to offer greater portability and adaptability. These systems often provide imaging with both fluoroscopy and ultrasonography. The ability to alternate between imaging modalities allows the urologist to compensate for the deficiencies of either system.

Most current lithotriptors are powered by an electromagnetic generator. Electromagnetic generators and their focusing units are capable of delivering shockwaves that are similar in intensity to those of the HM3, but usually to a smaller focal zone. As mentioned above, this has the theoretical advantage of minimizing damage to surrounding soft tissue. However, because of the smaller focal zone, respiration may cause the stone to move out of the target zone for portions of the treatment. Although improved localization techniques and anesthetic manipulation can be used to account for this, the shockwaves applied while the stones are out of the target zone do not cause fragmentation. Thus, certain second- and third-generation machines are associated with higher failure rates, incomplete treatment, and the need for retreatment.

Pathophysiology

A stone is fragmented when the force of the shockwaves overcomes the tensile strength of the stone. Although incompletely understood, fragmentation is thought to occur through a combination of methods, including compressive and tensile forces, erosion, shearing, spalling, and cavitation. Of these various forces, the generation of compressive and tensile forces and cavitation are thought to be the most important.

When a shockwave is propagated through a medium (water), it loses very little energy until it crosses into a medium with a different density. If the medium is denser, compressive forces are produced on the new medium. Similarly, if the new medium is less dense, tensile stress is produced on the first medium. Upon hitting the anterior surface of a stone, the change in density creates compressive forces, causing fragmentation. As the wave proceeds through the stone to the posterior surface, the change from high to low density reflects part of the shockwave’s energy, producing tensile forces, which again disrupt and fragment the stone.

In cavitation, shockwave energy applied at a focal point leads to failure of the liquid with generation of water-vapor bubbles. These gaseous bubbles collapse explosively, creating microjets that fracture and erode the calculus. This process can be monitored with real-time ultrasonography during the treatment and appears as swirling fragments and liquid in the focal zone.

Indications

The current options available for the treatment of renal and ureteral calculi include conservative management (watchful waiting for spontaneous passage), extracorporeal shockwave lithotripsy (ESWL), endoscopic techniques (rigid or flexible ureteroscopic lithotripsy), and percutaneous treatments.

The American Urological Association Stone Guidelines Panel has classified ESWL as a potential first-line treatment for ureteral and renal stones smaller than 2 cm.

In the pediatric population, those with uncomplicated, non-infectious calculi can undergo ESWL with an age-dependent response.[1]

Indications for ESWL include the following:

  • Individuals who work in professions in which unexpected symptoms of stone passage may prompt dangerous situations (eg, pilots, military personnel, physicians) (In such individuals, definitive management is preferred to prevent adverse outcomes.)

  • Individuals with solitary kidneys in whom attempted conservative management and spontaneous passage of the stone may lead to an anuric state

  • Patients with hypertension, diabetes, or other medical conditions that predispose to renal insufficiency

Relevant Anatomy

See Preoperative details.

Contraindications

Absolute contraindications to extracorporeal shockwave lithotripsy (ESWL) include the following:

  • Acute urinary tract infection or urosepsis

  • Uncorrected bleeding disorders or coagulopathies

  • Pregnancy

  • Uncorrected obstruction distal to the stone

Relative contraindications include the following:

  • Body habitus: Morbid obesity and orthopedic or spinal deformities may complicate or prevent proper positioning. In these situations, attempting to position the patient prior to anesthetic induction is useful to ensure the practicality of the approach.

  • Renal ectopy or malformations (eg, horseshoe kidneys and pelvic kidneys)

  • Complex intrarenal drainage (eg, infundibular stenosis)

  • Poorly controlled hypertension (due to increased bleeding risk)

  • Gastrointestinal disorders: In rare cases, these may be exacerbated after ESWL treatment.

  • Renal insufficiency: Stone-free rates in patients with renal insufficiency (57%) (serum creatinine level of 2–2.9 mg/dL) were significantly lower than in patients with better renal function (66%) (serum creatinine level < 2 mg/dL).

  • History of previous Open Renal Stone Surgery: Overall stone-free rates after ESWL treatment found to be significantly lower in patients with a history of open stone surgery, especially for those with stones in the lower calyx (48.4% vs. 64%) [2]

Preexisting pulmonary and cardiac problems are not contraindications, provided they are appropriately addressed both preoperatively and intraoperatively. In patients with a history of cardiac arrhythmias, the shockwave can be linked to electrocardiography (ECG), thus firing only on the R wave in the cardiac cycle, coinciding with the refractory period of the cardiac cycle (ie, gated lithotripsy).

Ganem and Carson retrospectively reviewed patients treated with gated and ungated lithotripsy. The study population included those with preexisting hypertension and cardiac disease and those taking cardiac medications. Of the patients in the ungated group, 20% developed arrhythmias, although they were universally benign, resolving with conversion to a gated procedure. Conversely, only 1 of 357 patients in the gated lithotripsy group developed any arrhythmia.[3]

Eaton and Erturk studied 51 patients who underwent ungated lithotripsy, including several patients with preexisting cardiac arrhythmias. The 21 patients who had more than 6 premature ventricular contractions (PVC) intraoperatively had troponin measured 24 hours postoperatively. A selected sample of patients who did not develop arrhythmias also had troponin measured as a control group and the troponin levels did not vary significantly between the 2 groups.[4]

Investigators concluded that ESWL-induced ventricular ectopy was probably reflective of mechanical stimulation of the myocardium rather than myocardial injury. However, the authors caution that as rare reports exist of myocardial injury after ESWL, one should exercise caution when treating patients with renal stones who may be at increased risk for cardiac damage.

Based on these studies, patients with preexisting cardiac disease, not including documented preoperative arrhythmia, can probably undergo ungated lithotripsy safely. Close monitoring is imperative as those who develop arrhythmias can be safely converted to gated lithotripsy.

Cardiac pacemakers are also not contraindicated, although seeking assistance from a cardiologist for possible changes to pacemaker settings would be prudent.

Oral anticoagulants (eg, clopidogrel [Plavix] and warfarin [Coumadin]) should be discontinued to allow normalization of clotting parameters. Platelet function is normalized by discontinuing aspirin-containing products and nonsteroidal anti-inflammatory drugs (NSAIDs) 7 days before treatment.

 

Workup

Laboratory Studies

See the list below:

  • CBC count

  • Anticoagulation profile (PT/aPTT)

  • Urinalysis, with or without urine culture

Imaging Studies

See the list below:

  • Renal ultrasonography

  • Noncontrast CT scanning

  • Intravenous pyelography

Other Tests

See the list below:

  • Electrocardiography in patients older than 50 years and in patients with a history of cardiac disease

 

Treatment

Preoperative Details

Several factors related to the stone, including stone burden (size and number), composition, and location, affect the outcome of extracorporeal shockwave lithotripsy (ESWL).

Stone size

As stone size approaches 2 cm, the likelihood of success with ESWL decreases, and the need for retreatment and adjunctive therapy increases. ESWL has also been found to be most efficacious in treating nonobstructing renal calculi. In patients with a large stone burden, pre-ESWL stenting may secure drainage and prevent obstructive urosepsis. A study where stone volume was calculated based on a 3D rendered image corroborated that smaller stones are more likely to fragment than larger stones, with 500 microL as the cutoff.[5]

Stone composition

The density and ability of a stone to resist ESWL is based in part on the composition of the stone. Stones composed of calcium oxalate dihydrate, magnesium ammonium phosphate, or uric acid tend to be softer and to fragment more easily with ESWL. Stones composed of calcium oxalate monohydrate or cystine, on the other hand, are less susceptible to ESWL. To a degree, this can be predicted with CT scanning by measuring the radio-opacity of stones. A recent retrospective study showed that ESWL monotherapy is more likely to be effective against stones with a Hounsfield units [HU] < 815 Hounsfield units [HU]) than those with a higher radio-opacity.[6]

In addition, certain radiolucent stones (uric acid, indinavir [Crixivan]) are difficult to visualize on fluoroscopy and therefore require either ultrasonography-guided localization or the addition of retrograde or intravenous contrast to localize a calculus.

Stone location

See the list below:

  • Lower-pole calculi: Although ESWL can fragment stones in the lower pole of the kidney, the resulting stone-free rate is decreased because of the difficulty in passing stones from this location. Recent studies have delineated renal morphology associated with improved stone-free rates (eg, lower infundibular length–to–diameter ratio of < 7, lower-pole infundibular diameter of >4 mm, single minor calyx), as well as factors associated with decreased stone-free rates (infundibulopelvic angle of < 70°, an infundibular length of >3 cm, an infundibular width of < 5 mm). Regardless of anatomy, ESWL tends to yield better results in patients with smaller stone burdens.

  • Calyceal diverticula with infundibular stenosis: In patients with diverticula caused by or related to infundibular stenosis, fragmented stones cannot easily bypass the obstruction, with resultant retained stone fragments. These patients are best served by more invasive techniques that allow the surgeon to address the obstruction and the stones simultaneously, either with retrograde ureteroscopy or in an antegrade percutaneous fashion.

  • Ureteral calculi: Fragmentation of proximal stones is more effective than mid or distal stones. In addition, when associated with hydronephrosis, ureteroscopy yields better stone-free rates for stones larger than 15 mm.

Skin to stone distance

Skin to stone distance, which can be easily measured on CT scan, appears to predict the success of ESWL. Distances reaching greater than 10cm appears to have a negative effect on successful stone treatment.[7, 8]

Preoperative and intraoperative stenting

In the modern setting, where access to ESWL and ureteroscopy is readily available, the indications for stenting prior to definitive treatment are much fewer. These indications include (1) obstructed pyelonephritis or pyelitis and (2) newly onset renal insufficiency or renal failure. In these situations, the stent helps to ensure internal drainage and allows passive dilatation of the ureter, facilitating future endoscopic evaluation and treatment. With the advent of newer and smaller ureteroscopic equipment, the rates of endoscopic complications (ie, strictures) have subsequently declined. When preoperative stenting is required, the authors believe that ureteroscopy, especially for ureteral stones, may yield higher stone-free rates without a significant increase in morbidity, time, or cost.

The need for intraoperative manipulation of stones for ESWL (eg, stone pushback) or placement of a ureteral catheter to assist with stone visualization has decreased, as newer machines are capable of treating proximal ureteral stones or visualizing radiolucent stones with ultrasonography. That said, intraoperative ureteral stents should be considered in patients with larger stones, as the rate of steinstrasse (German for “stone street”) increases with stone burden (1-4% in general vs 10% for stones >2 cm).

Intraoperative Details

The optimal shockwave lithotripsy treatment is thought to be about 80-90 shocks per minute. Faster rates have been shown to be associated with decreased stone-free rates, especially for larger stones (11-20 mm). The difference in stone-free rates is less significant for smaller stones. Conversely, slower rates obviously increase the total operative time.

Ramping has been shown to improve stone fragmentation as well as have a protective effect on the kidney. This technique entails delivering ≤100 shocks to the kidney at a lower energy setting. After a short time of allowing the kidney to rest, the remainder of the high energy shocks are given. The protective effect of the kidney is thought to be due to vasoconstriction. The lower power shocks condition the stone which leads to improved comminution.[9, 10, 11]

During shockwave lithotripsy, tracking the stone burden becomes an important issue, in part because of the natural movement of the kidney during respiration, with subsequent movement of the stone burden in and out of the focal zone. The smaller focal zone of the newer devices allows for minimal anesthesia, but the patient’s increased ability and susceptibility to cough, shift, or otherwise move requires vigilance to ensure the appropriate targeting accuracy in the application of energy to the stone. This means that the targeting of the machine needs to be adjusted more often.

It should be noted that the type of gel and method in which it is applied to the water filled cushion can affect the efficacy of ESWL. Specifically, standard ultrasound gel results in the most air bubbles when compared to therasonic gel and silicon oil. This leads to decreased fragmentation of in vivo and ex vivo stone models. Removing the air bubbles from the cushion-patient interface leads to improved success.[12]

The decreased anesthetic need during lithotripsy when using the later generation devices provides obvious advantages, such as rapid recovery time and rapid turnover between patients. However, a balance must be struck between the minimal invasiveness of the procedure and the stone disintegration efficacy. The optimal anesthetic regimen to facilitate this remains a subject of debate.

Patient-controlled analgesia has been suggested to enable urologists to achieve better patient compliance through more accurate pain control and, hence, more effective treatment.[13] To this end, Parkin et al studied preoperative diclofenac alone versus diclofenac and alfentanil PCA. Pain scores based on a visual analog score (VAS) were similar between the 2 groups but PCA patients showed a statistically significant increased level of satisfaction with the experience.[14]

Kumar et al compared 3 forms of adjunctive analgesia for ESWL patients: preoperative diclofenac, topical eutectic mixture of lidocaine/prilocaine (EMLA), and the 2 in combination. Stone free rates at 3 months were the highest in the combination group at 88.75%, and the retreatment rate was the lowest, both reaching statistical significance.[15]

Thus, excellent stone free rates can be achieved with minimal to no general anesthesia using modern lithotripters. Analgesic adjuncts certainly have a role in facilitating these outcomes.

Postoperative Details

Common adverse effects associated with ESWL include flank petechiae, hematuria, and passage of stone fragments with associated renal colic. Many patients are issued a urine strainer to help collect stone fragments, which can later be chemically analyzed to assist with prevention of future stones. Hydration and analgesia alleviate most flank discomfort and symptoms caused by the passage of fragments.

Some groups have initiated trials of pharmacologic aids similar to those involved in medical stone-passage protocols to facilitate stone passage. In the treatment arm, pharmacologic aids (stone-free rate of 86% with nifedipine and 82% with tamsulosin) were superior to placebo (stone-free rate of 52-57%). Quantification of residual stone burden and resolution of hydronephrosis was defined with postoperative radiography or ultrasonography.

It is the practice of the authors to obtain postoperative imaging, typically a KUB or ultrasound, within 6 weeks following the procedure or sooner if the patient is symptomatic.

Follow-up

Stone-prevention strategies

All patients who undergo surgery for stones should be given information about kidney-stone prevention. General measures include increased fluid intake and restriction of dietary sodium and purine. In patients with calcium oxalate stones, intake of foods high in oxalate (eg, spinach, nuts, beer, chocolate, rhubarb, green leafy vegetables) should be discouraged. Calcium intake should be moderated; extremely high or low levels of calcium can increase stone production.

Blood work and 24-hour urine collections measuring for pH, urinary volume, citrate, calcium, oxalate, uric acid, sodium, magnesium, phosphates, and electrolytes can assist in identifying and alleviating risk factors for future stone production. Following treatment of the initial stone event, testing should be performed in all children and in patients with solitary kidneys, chronic diarrhea, a history of bariatric surgery, renal failure, and nephrocalcinosis, as well as in any patient with kidney stones who has sufficient motivation to follow long-term treatment recommendations to prevent future stones. Twenty-four–hour urine-testing protocols are available from a number of sources, including Mission, Dianon, UroCor, Quest, LabCorp, and Litholink.

The National Kidney and Urologic Diseases Information Clearinghouse (NKUDIC), which is part of the National Institutes of Health (NIH), is a good general patient information Web site.

The NIH has also recommended The Kidney Stones Handbook (Savitz and Leslie, 2000). This award-winning patient guide to kidney stones can be ordered directly from the publisher (Grant Gibbs) by email (gsavitz@earthlink.net) or by telephone (530-889-1727).

Complications

Renal complications

Bacteriuria develops in 7.7-23.5% of patients undergoing extracorporeal shockwave lithotripsy (ESWL) and is more likely to develop in patients with infection-related stones. Bacteremia is less common, developing in up to 14% of patients, with fewer than 1% developing clinical sepsis (although this number increases to 2.7% in patients with staghorn calculi). Although preoperative antibiotic coverage remains controversial, antibiotics may be recommended in patients with infection-related stones, positive urine cultures results, or recurrent urinary tract infections.

Post-ESWL hematuria is usually mild and transient. In the event of significant hematuria with clots or frank clot retention, imaging the kidneys should be considered to identify a perinephric hematoma. Perinephric, subcapsular, or intranephric hematomas can present with severe pain, ileus, and, infrequently, shock or hypotension, with preoperative risk factors including hypertension, elevated BMI, and vascular comorbidities[16, 17] . Unexplained or unusually severe pain or any unusual drop in blood pressure may suggest a hematoma. Subcapsular hematoma following ESWL usually responds to bedrest, transfusions, and supportive care. If the patient requires multiple transfusions, arteriography and selective embolization should be considered.

Stone fragments may pass with a minimal amount of discomfort. In some patients, the comminuted stone fragments pile up in the ureter, creating a virtual column of stone called steinstrasse. The overall rate of steinstrasse is 1-4%, with the rate progressively increasing for greater stone burdens (10% for stone burdens >2 cm2) and approximately 40% for complete staghorn calculi. Patients with asymptomatic nonobstructing steinstrasse can be monitored closely with serial imaging.

Asymptomatic or mildly symptomatic steinstrasse with mild dilatation of the upper urinary tract can be managed conservatively. If fragments fail to progress within 3-4 weeks or if patients develop significant symptoms or obstruction, endoscopic lithotripsy or percutaneous drainage should be performed. Patients with high-grade obstruction and concomitant pyelonephritis require prompt percutaneous nephrostomy drainage with appropriate antibiotic coverage, followed by staged endoscopic removal of stone fragments.

Failure to completely fragment a stone with ESWL is not predictive of an unfavorable outcome with subsequent ureteroscopy and laser lithotripsy. There is, however, an increased need for ureteral stenting following the salvage procedure.[18]

Renal atrophy, although uncommon, can result from renal vascular or severe atherosclerotic disease. Patients with underlying renal parenchymal disease are at a higher risk of renal atrophy. However, studies of ESWL in patients with a solitary kidney have shown no statistical evidence of renal function deterioration secondary to shockwave lithotripsy.

Medical complications

Hypertension is an unusual complication of ESWL but may occur as a sequela of a large perinephric hematoma (ie, page kidney). Older patients with abnormal renal perfusion may develop hypertension within 26 months after the ESWL session.

Patients who undergo ESWL may have a slightly higher likelihood of eventually developing hypertension and diabetes than patients who undergo other therapies for stone removal, based on a 2006 study by Krambeck et al. This study retrospectively identified 630 patients who were treated with ESWL 19 years prior, and queried them regarding development of hypertension and diabetes mellitus. Compared with a control group treated conservatively for nephrolithiasis at the same time, diabetes and hypertension were more common, a finding that persisted in multivariate analysis when BMI was controlled for. The authors suggested injury to pancreatic islet cells by the shock waves as an explanation for this observation.[19]

In response to the above study, Wendt-Nordahl et al measured pancreatic enzymes post-ESWL in patients treated for proximal ureteral and renal stones to evaluate for acute pancreatic injury. The control group consisted of patients treated with ESWL for lower ureteral stones and no difference was demonstrated between the 2 groups. The authors concluded that the hypothesis that ESWL causes acute trauma to pancreatic islet cells, leading to an endocrine insufficiency resulting in DM, therefore seems unlikely.[20]

Makhlouf et al examined a cohort of almost 2000 patients who underwent ESWL between 1999 and 2002. The control group consisted of matched individuals from a national database. At 6 years of follow-up, the number of patients in the study group and the control group who had developed DM were the same.[21]

To assess the risk of DM post-ESWL, Rashed et al reviewed data over a 15-year period, evaluating patients who were treated between 1991-1994. Their findings demonstrated the highest prevalence of new onset DM with bilateral ESWL. They also noted a correlation with intensity of shock waves, particularly those greater than 15.5 KV[22] .

Based on these studies, the relationship between ESWL and the development of diabetes remains unclear, and further studies are necessary to elaborate this relationship. Potential future areas of investigation include the impact of device generation, incidence of disease over time and effects of treatment laterality in disease incidence.

There are well documented acute effects to the kidney in the pediatric ESWL population. However, a meta-analysis in 2014 showed no long term decline in GFR or renal scarring on DMSA scan. ESWL is a safe method for treating urolithiasis in children.[23]

Other possible complications

Less-common complications may include (1) pulmonary contusion, (2) pancreatitis, (3) splenic hematoma, (4) elevated liver functions (transient), and (5) biliary colic with inadvertent fragmentation of adjacent biliary stones.

By a similar mechanism, ESWL has also been reported to disrupt calcified vessel walls, resulting in plaque fragmentation and rupture.[24]

There was some controversy as to the effect of ESWL on patients' auditory function. Several studies support the notion that ESWL does not appear to affect hearing function of treated patients.[25]

Outcome and Prognosis

In appropriately selected patients, the overall success rate of extracorporeal shockwave lithotripsy (ESWL) is higher than 90% for stone clearance, with patients remaining stone-free for up to 2 years. Compared with ureteroscopic removal of stones, ESWL leads to less complications and shorter hospital stays. However, ureteroscopy was shown in one review to achieve a greater stone-free rate than ESWL.[26]

ESWL is safe and effective. Small series have shown successful treatment of stones in young children, with an acceptable short-term safety profile. For example, a 2012 study by Fayad et al demonstrated equivalent renal growth at one year in children who underwent ESWL compared to a healthy control group who did not have the intervention.[27] However, long-term follow-up of the potential complications, including hypertension and decreased renal function, are not yet mature.

As the degree of stone burden increases and exceeds 2 cm, the stone-free rate drops significantly. In patients with stones sized 2-3 cm, the stone-free rate with ESWL monotherapy is typically 50%. Stone-free rates in patients with larger stones (complete and incomplete staghorn calculi) are correspondingly lower.

The location of the stone also affects the efficacy of ESWL. In a meta-analysis of 2927 patients from 14 centers, Lingeman et al (1996) found that the overall stone-free rate for all lower-pole stones treated with ESWL (59.2%) was lower than the stone-free rate associated with percutaneous nephrolithotomy (90%).[28] Some studies have suggested that select patients with appropriate renal collecting system anatomy may see good results with ESWL despite lower-pole stone location. In these studies, the overall stone-free rate was approximately 50%, with a stone-free rate of 85% in patients with favorable anatomy versus 7% in those with unfavorable anatomy.

In a prospective, randomized, multicenter clinical trial performed by the Lower Pole Study Group, patients with lower-pole stones treated with ESWL or percutaneous nephrolithotomy had overall stone-free rates of 37% and 95%, respectively. In contrast, this prospective study did not show any difference in stone-free rates based on renal anatomy, but an inverse relationship was found between stone size and stone-free rate. In patients with stones or stone aggregates measuring larger than 1 cm, percutaneous nephrolithotomy was the most efficacious modality to render patients stone-free.

Sheir et al (2003) evaluated the safety and efficacy of ESWL in patients with an anomalous kidney, including 49 patients with a horseshoe kidney, 120 patients with a malrotated kidney, and 29 patients with a duplex kidney.[29] Two second-generation lithotriptors were used. Although the type of renal anomaly and the type of lithotriptor did not affect the stone-free rate, stone length and number (stone burden) significantly influenced the stone-free rate. The prone position facilitated treatment in 38% of the patients with a horseshoe kidney and in 31% of patients with a duplex kidney. The overall retreatment success rate was 64.1%. However, with an overall stone-free rate of 72.2%, Sheir et al deemed ESWL to be safe and reliable in patients with an anomalous kidney and to be considered the primary treatment option for stones smaller than 20 mm.

Early-generation lithotriptors required pushback of stones into the renal pelvis for treatment. With advancements, specifically higher-amplitude waveforms with smaller focal zones, newer lithotriptors are able to treat ureteral stones in situ. Results tend to be better for proximal stones, with stone-free rates of 65-81%, versus 58-67% for distal ureteral stones.

Future and Controversies

Technical improvements, such as bidirectional synchronous twin-pulse technique with variable angles between the shockwave reflectors, have been attempted to increase the quality and rate of stone disintegration. With this technique, shock waves are produced simultaneously from separate reflectors through nonopposing directions to the same F2 and appear to be particularly effective with right angle orientation between the two. This effect was demonstrated in vitro and then demonstrated by Sheir in a study of 50 patients with renal or ureteral stones (mean size, 12.3 mm; range, 9-18 mm). Using this technique, 17 patients (34%) were rendered stone-free, 20 patients (40%) had less than 5 mm of residual stone, and 13 patients (26%) had 6-9 mm of residual stone at 14 day follow-up. Thirteen patients (26%) with more than 5 mm of residual stone underwent repeat ESWL.[30]

The same authors followed this initial clinical study with one that randomized patients to the twin-pulse technique versus standard ESWL. The study group included 240 patients with single radio-opaque stones less than 2.5 cm. Stone disintegration rates were significantly greater in the twin-pulse group while stone free rates were high in both groups but not significantly different for stones less than 10 mm (67% and 74%; standard and twin-pulse, respectively).[31]

An additional purpose of this study was to compare collateral parenchymal damage caused by each technique. N-acetyl-B-glucosaminidase (NAG), a high-molecular weight enzyme not typically filtered by the glomerulus, was measured post-ESWL. The degree to which this enzyme is lost in the urine immediately after ESWL is thought to be reflective of the severity of tubular damage. Both groups in this study had elevated post-ESWL NAG levels, but the levels normalized in the twin-pulse group after 2 days and remained elevated up to 7 days in the control group. Patients also underwent dynamic MRI post-ESWL to assess changes in renal perfusion. Decreased renal perfusion was demonstrated in the standard ESWL group (control), but not in the twin-pulse group.

The authors of these studies concluded that bidirectional synchronous twin-pulse lithotripsy has superior efficacy to standard lithotripsy while possibly decreasing damage to surrounding parenchyma.

Other groups have attempted to improve the fragmenting capability of the cavitation bubbles created during lithotripsy by forcing their collapse with a second weaker pulse timed immediately after the initial pulse. Using a porcine model with BegoStone phantoms, Young et al (2003) used a 22-kV shock from an HM3 followed with a 4-kV shockwave 500-600 ms later from a separate piezoelectric source. Their initial results showed increased stone comminution rates with reduced renal injury.

The smaller focal zone and newer lithotripter tabletop designs have increased the indications for treatment and lowered the anesthetic requirements, but some have demonstrated decreased overall efficacy of the treatment. Many newer generators require precise localization, with little margin for error in light of the greatly reduced focal zones. Future studies are necessary to define the preferable anesthetic regimen, localization technique, and shock-wave delivery sequence to optimize outcomes.

A prospective study comparing the clinical effectiveness of the HM3 to the newer MODULITH SLX-F2 lithotripter found that the HM3 showed higher stone-free rates for solitary ureteral stones and multiple stones at 3-month follow-up. The HM3 also required fewer shock waves and led to fewer kidney hematomas.[32]

Certainly ESWL has a role in the armamentarium against urolithiasis. Recent trends however, show that its popularity is yielding to endoscopic management of stones. Perhaps this decline can be attributed not to any shortcomings of ESWL, but rather to the rapid improvement of endoscopic instruments used in minimally invasive approaches.[33, 34]