Renal stones pose a significant burden on the health care system. The prevalence of renal stones has increased from 3.8% in 1970 to 8.8% in 2010 in the United States, with annual healthcare costs of USD 3.8 billion. It is estimated that more than half a million people a year visit the emergency room for kidney stone problems. The most common presenting symptoms include hematuria, pain in the flank, groin, or abdomen.
There are many treatment modalities available for managing renal stones. They range from completely non-invasive outpatient procedures to invasive procedures requiring hospital admission and increased risks of complications. Extracorporeal shockwave lithotripsy (ESWL) is a truly non-invasive procedure as opposed to other surgical treatments used, such as retrograde intrarenal surgery (RIRS) and percutaneous nephrolithotomy (PCNL). The efficacy of ESWL lies in its ability to pulverize calculi in vivo into smaller segments, which are them expelled spontaneously by the boy. Shockwaves are generated and are then focused on a point within the body.
The shockwaves propagate through the body with minimal dissipation of energy (and therefore damage) due to minimal difference in density of the soft tissues. At the stone fluid interface, a relatively significant difference in density, combined with a large concentration of multiple shockwaves in a small area, produces a significant dissipation of energy. Through the different mechanisms, this energy can overcome the tensile strength of the stone resulting in fragmentation. Repetition of this process leads to the pulverization of the stone into small fragments that the body then can pass painlessly and spontaneously.
Stones can form in the urinary tract anywhere from kidneys to the urethra. They can form in the calyceal system (major or minor calyx), ureteropelvic junction, ureter, or bladder. The physiology of stone formation is complex, with many factors that come into play.
Most renal stones start as Randall plaque in the kidney. It is a calcium plaque that is deposited in the interstitial tissue of the renal papilla. In calcium oxalate stone formers, the calcification is invariably located in the basement membrane of the loops of Henle. This is where urinary stones are believed to be formed. These plaques initially start in membranes below the kidney urothelium and then progressively increase in size into the urinary system until regions of plaques are exposed to urine. Once they are in continuous contact with urine, crystal layers typically begin to organize on the nidus through aggregation and epitaxy. The supersaturation of urine with crystals is a common phenomenon occurring in all cases of renal calculi.
Calcium oxalate stones are the most common urolithiasis and tend to form when the urinary pH is under 7.2, as well as uric acid stones. Acidic urine (not hyperuricosuria) is the leading cause of uric acid stone formation. Calcium phosphate predominant stones tend to form in more alkaline urine. The most common renal calculi are calcium-containing, followed by urate crystals.
The choice between shockwave lithotripsy (SWL) and other treatment modalities depends on several factors, including stone size, stone burden, stone composition, etc. Another compounding factor in choosing the treatment modality is patient preference and expectation.
For non-staghorn calculi, renal and ureteral stones less than 2 cm size SWL is considered a viable first-line treatment. Success rates of SWL are considerably lower for stones that are larger than 2 cm. If the patient is not a candidate for general anesthesia, SWL can be considered in larger stones since it can be performed under local anesthesia or intravenous sedation. Ureteric stent placement should be considered when treating larger stones (size greater than 15 mm × 15 mm in its largest dimensions). It minimizes the risk of post-SWL colic and obstruction in large stones.
Stone location is an essential predictor of SWL success rates. Stones in lower calyces (LC) can be fragmented by SWL just as well as those present in other locations, but because of the dependent nature of the lower pole, the clearance of stone fragments after SWL is reduced. Lower pole stones of more than 10 mm size are treated with SWL, have lower stone-free-rates (SFR) outcomes than percutaneous nephrolithotripsy (PCNL) in that same region.
The stone composition, as well as visibility, also has a significant impact on the outcomes of SWL. SWL easily fragments uric acid stones; however, it sometimes cannot be visualized on fluoroscopy since they are radiolucent. Calcium oxalate monohydrate, cysteine, and calcium phosphate stones are considered relatively SWL-resistant; however, they often have variable compositions, making them amenable to fragmentation. Stone density, measured by Hounsfield units (HU) on CT scan, is another factor to consider before undergoing SWL. Several studies have demonstrated that as the HU increases, the SFR decreases.
In addition to stone factors, several patient factors determine the treatment of choice for the nephrolithiasis in the given patient. Obesity provides challenges with on-table positioning and radiographic quality. Skin-to-stone distance of less than 10 to 11 cm is an independent predictor of a higher SFR after SWL. Similarly, a higher body mass index (BMI, more than 30 kg/m2) is associated with decreased success. Anatomic abnormalities such as calyceal diverticuli, kidney rotation, and kidney fusion warrant careful assessment and judicious use of SWL.
Retrograde intrarenal surgery (RIRS) is emerging as an alternative to SWL. RIRS has higher success rates as well as lower retreatment rates compared to SWL, especially when treating radiolucent stones.
Extracorporeal shock wave lithotripsy (ESWL) in pregnancy has been associated with many complications, including low birth weight, miscarriage, and displacement of the placenta. Therefore SWL is contraindicated in pregnancy.
Patients with an aortic aneurysm are at increased risk of bleeding and rupture if they undergo SWL.
Patients who have a bleeding diathesis, or are on antiplatelet, antithrombotic, or anticoagulant drugs are at increased risk of bleeding. It is essential to stop these medications well in advance of the procedure. In high-risk patients, if it is not safe to hold these medications, SWL should be delayed, or alternative treatment plans (such as ureteroscopy) should be discussed with the patient as it can be performed in anticoagulated patients.
Severe or untreated hypertension is a significant risk factor for bleeding and perinephric hematoma after SWL and is an absolute contraindication.
Patients with infected stones, untreated urinary tract infections, or even bacteriuria are at an increased risk of pyelonephritis, bacteremia, and sepsis if they undergo ESWL.
In extracorporeal shock wave lithotripsy (ESWL), shockwaves are generated by a lithotripter and are focused on the stones to disintegrate them. Lithotripters are divided based on the underlying mechanism of shockwave generation. There are currently three types of shock wave generators: electrohydraulic, electromagnetic, and piezoelectric.
Electrohydraulic generators produce a vaporization bubble. The bubble produces a high-energy pressure wave by expanding and then immediately collapsing. The shock wave is then focused after hitting a reflector.
Electromagnetic generators produce a magnetic field. A shock wave is generated when the magnetic field causes membrane repulsion. It is focused on stone with a reflector or acoustic lens. Electrohydraulic technology requires replacement of electrodes every several thousand shockwaves, while electromagnetic generators can last for millions of shock waves.
Piezoelectric generators utilize the piezoelectric effect to generate shockwaves. Current is passed through crystals that make the crystals vibrate. This steady speed vibration is used to generate shockwaves.
Other pieces of equipment required in ESWL include ultrasound or fluoroscopy to localize the stone, and a coupling agent to maximize the delivery of shockwaves to stone without acoustic impedance.
Medical professionals involved in extracorporeal shock wave lithotripsy (ESWL) include the performing provider, an assistant, and an anesthetist. The role of the anesthetist is essential in ensuring adequate anesthesia/analgesia to reduce the patient's stress and prevent movement of the patient due to pain during the procedure that may result in decoupling.
The provider should be present to localize the stone at the beginning of the procedure and ensure proper targeting throughout the procedure. It is common for the stone to disintegrate, move, or even start with multiple stones. Therefore it is paramount that the provider is present to primarily plan the treatment session as well as to adapt the targeting to any variation during the procedure itself.
A detailed history, along with complete physical examination, should be conducted to rule out any contraindication of extracorporeal shock wave lithotripsy (ESWL). The investigations are based around locating the stones, identifying any complication due to stones, and finding any hidden risk factor. X-ray KUB is used as an initial screening test for significant nephrolithiasis. Ultrasound KUB can detect non-calcified stones if they are bigger than 5 mm. It can also detect obstruction and hydronephrosis.
CT scan is the most sensitive test in localizing the calculi and identifying any complication. Other investigations include urinalysis, urine culture, and white blood cell count with differential to rule out infections. Coagulation profiling should be done to detect any bleeding diathesis. The patient should be educated about different treatment options available to him and the benefits and potential risks of each of them. Informed consent should be obtained before initiating the procedure. With ESWL, the key factors that require consideration and discussion with the patient include the possibility of retreatment and post-ESWL complications and their management.
Stone localization is done with ultrasound or fluoroscopy.
To maximize the efficiency of extracorporeal shock wave lithotripsy (ESWL), proper positioning of the patient is necessary. Skeletal structures should not interfere with the delivery of shockwaves. Usually, the patient is placed in the supine or modified supine position, but if it is not possible or recommended, then prone positioning must be done.
For the effective delivery of shockwaves, there should be no impedance. It is done by coupling the shock head and the skin using ultrasound gel, silicon oil, or other media, abolishing the air barrier to prevent scattering. The introduction of air bubbles in the coupling zone may attenuate the delivery of shockwave and reduce the efficacy of ESWL.
Proper analgesia or anesthesia is necessary to minimize the movement of the patient, to avoid the introduction of air bubbles in the coupling medium, and also to allow maximum delivery of shockwaves to the targeted location.
The optimal shock wave rate is not clear. Studies suggest that when compared to 120 shocks/min, SWL at 60 to 90 shocks/min has better fragmentation and decreased risk of renal injury.
The total number of shocks delivered per session depends on patient habitus and stone characteristics, including stone location, stone density, stone composition, etc., and ranges between 2000 and 4500 shocks per session.
The energy voltage is gradually “ramped up” to minimize the injury to renal tissue, and to better manage the pain during the procedure.
Stone passage after the procedure is facilitated by using nonsteroidal anti-inflammatory drugs, alpha-blockers, calcium channel blockers, and corticosteroids.
Like all other procedures, this procedure is also not without complications. Complications of extracorporeal shock wave lithotripsy (ESWL) include:
The primary treatment modalities available for nephrolithiasis are extracorporeal shockwave lithotripsy (ESWL), ureteroscopy (URS), and percutaneous nephrolithotomy (PCNL). ESWL is the only method that is truly non-invasive and can achieve the stone-free-rates that approach 75%. ESWL is considered as first-line treatment for smaller stones (less than 2 cm size).
For stones that are larger than 2 cm, a more invasive approach (such as PCNL) is better suited. However, more patients are willing to accept multiple treatments with ESWL instead of an invasive procedure. While the single-treatment success rates of ESWL are not equal to those of URS or PCNL, success rates can be maximized with the use of adjunctive treatment before or after ESWL, proper patient selection, and optimization of ESWL technique.
When a patient presents to a provider with renal calculi, a complete understanding of medical and surgical treatment options is mandatory for the provider. The patient should be educated about the available options and risks and benefits of them. With extracorporeal shock wave lithotripsy (ESWL), the chances of retreatment and adjunctive treatments should be explained.
Preoperative evaluation and preparation will allow the provider to maximize the success rate of ESWL with minimal chances of unforeseen complications. The role of the anesthetist is necessary to ensure adequate analgesia. The provider must be present during the procedure to ensure an optimum SFR. The nurse or assistant should be present for patient monitoring and administrating medications if necessary. After ESWL, the patient should be educated about diet alterations to reduce the risk of recurrence of renal calculi.
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