Shockwave lithotripsy (SWL) remains the least invasive treatment modality for renal and ureteral stones. Other management options for renal stones include observation (with intervention if symptoms develop), ureteroscopic stone extraction and percutaneous stone extraction. Management alternatives for ureteral stones include medical expulsive therapy, ureteroscopy and, rarely, percutaneous extraction. In brief, SWL involves the production in a generator of shockwaves which are focused on a stone within the kidney or ureter. Numerous adaptations to the equipment itself, the methods of patient selection, treatment and follow-up have been developed since the introduction of SWL into clinical practice in the early 1980s. This article reviews the current status of SWL therapy.
by Dr T. Schuler
Basics of shockwave lithotripsy
Shockwave lithotripters serve to fragment renal and ureteral stones sufficiently such that the patient will pass the fragments spontaneously, without the need for more invasive therapy, rendering them stone and symptom-free. Shockwaves are generated by electrohydraulic, electromagnetic or piezo-electric systems, depending on the design of the lithotripter and are propagated through the patient where they are focused on the stone. The stones are targeted by means of fluoroscopy or ultrasonic imaging.
The origin of clinical SWL was with the Dornier HM3 lithotripter. Although very powerful, this instrument was cumbersome in that patients were required to be submerged in a water bath in order to couple the shockwaves to the soft tissue of the patient and, although resulting in an excellent clinical outcome, its sheer power necessitated anaesthesia. More modern lithotripters have been simplified by advances in the coupling mechanisms that have alleviated the need for water submersion. In addition, they have been in essence “de-tuned” in order to improve patient tolerance and eliminate the need for anaesthesia. Typically patients can be treated with intravenous sedation. This has led to a reduction in the success rates of modern SWL series, but has increased patient acceptance and satisfaction so that SWL remains as a reasonable non-invasive treatment option.
Methods of increasing successful outcomes with SWL
The success SWL can be optimised by careful patient selection, consideration of stone characteristics, alteration of treatment parameters and augmenting fragment passage.
A discussion regarding patient selection should begin with a review of absolute contraindications to SWL, which include pregnancy, the presence of distal urinary tract obstruction and patients who are anticoagulated or having bleeding diatheses. Relative contraindications include proximity of the stone to be treated to abdominal or renal aneurysms, morbid obesity, uncontrolled hypertension and patients whose body habitus precludes adequate positioning to target the stone.
Patient factors
Patient factors which may contribute to poor outcomes with SWL include obesity and the distance of the skin to the stone. Obesity has been reported as an independent predictor of SWL failure on multivariate analysis of series. More recently, several authors have reported skin to stone distance (SSD), measured by pretreatment non-contrast computed tomography, as being predictive of SWL failure. Published data have reported a decrease in success rates when SSD was greater than 10cm and 9cm respectively.These data may not apply to all lithotriptors however, given the differences in focal zones between machines. Patients with congenital renal anomalies such as a horseshoe kidney may also have problems passing
fragments created by SWL.
Stone factors
It is well known that SWL success correlates with stone burden. Patients with multiple stones should expect to undergo more than one treatment to be rendered stone-free, and therefore may be considered for more invasive procedures such as ureteroscopy or percutaneous nephrolithotripsy should they desire to be stone free with a single treatment. Stone size correlates with success rates with 79.9%, 64.1%, and 53.7% of patients with renal stones respectively less than 10mm, 11-20mm and greater than 20mm being rendered stone free. It is generally thought that for stones less than 20mm in diameter, SWL is the first line treatment option, whilst recognising that more than one SWL session may be required, and that it may take several weeks to pass the fragments created. Stones that are known to be of hard composition such as cystine, brushite and calcium oxalate monohydrate may be best treated by another modality. It may be difficult to know, however, what type of stone the patient has unless a history of cystine stones, or known hard compositions from previous stone events, can be elucidated.
Stone location deserves special mention, particularly stones within the lower pole of the kidney. The lower pole study group compared SWL and percutaneous stone removal for lower pole stones and found stone free rates of 37% and 95% respectively for stones measuring 11-20mm in diameter. In a follow-up study, the group found no difference between SWL and ureteroscopy with respect to stone-free rates for lower pole stones of less than 10mm. SWL was however favoured from the point of view of patient acceptance and convalescence compared to the more invasive ureteroscopic approach. Methods to improve fragment clearance from the lower pole will be discussed in further detail later. With respect to ureteral stones, recent jointly developed guidelines of the EUA and AUA recommend SWL as being superior to ureteroscopy for stones less than 10mm in size within the upper ureter, with ureteroscopy being superior for both stones in the distal ureter and stones greater than 10mm in size in the upper ureter.
More recently, several authors have reported on the predictive value of Hounsfield Unit (HU) attenuation of the stone on computed tomography in predicting the outcome of SWL therapy. A 100% clearance of stones less than 500 HU was noted compared with 55% for stones >1000 HU. Further to this, several authors have reported a significant reduction in successful treatments when stone attenuation was >900 HU.
Treatment parameters
Once the urologist and patient have elected to proceed with SWL as a stone management modality, it is important to optimise the treatment itself in order to achieve the best possible outcome. As decribed earlier, the initial method of coupling the shockwaves to the patient was with emersion in a water bath. This has largely been abandoned by the development of dry treatment heads, which have enclosed water cushions that are positioned against the patient’s skin. The patient must however be coupled to the water filled cushion. Numerous coupling agents have been described, with the end goal being elimination of air bubbles within the coupling medium. It has been suggested that optimal results are achieved by applying 250cc of gel directly to the treatment head and subsequently using the inflation of the water cushion to collapse any air pockets. This minimises handling of the coupling medium and the potential for introduction of air, which scatters the shockwaves. In order to overcome difficulties with coupling, some modern lithotripters utilise a shallow water bath. Once coupling is established, it is important to try to minimise patient movement, which may introduce air and reduce the efficiency of coupling.
Little clinical data exist on the ideal way in which to increase the shockwave energy during treatment. A small clinical study showed that step-wise escalation of shockwave energy resulted in a significant improvement in overall success rate eight weeks post treatment. Additionally, the benefit of priming the kidney with low energy shockwaves may reduce renal injury induced by shockwaves.
The rate at which the shockwaves are delivered during treatment has also been demonstrated to impact on success rates. A recent meta-analysis revealed a 10.2% weighted risk difference in success rates in favour of patients treated at slower rates. Traditionally, patients have been treated with 120 shocks per minute, with recent data suggesting the additional benefits of treatment rates of 60-90 shocks per minute. This does however increase the time required for treatment, and in areas with long waiting lists for SWL it may negatively impact the access to care. Notably, the increase in success rate for stones treated at slower rates was most significant for stones >100mm2 in a recent study in which larger renal stones treated at 60 shocks per minute were successfully treated in 75.7% of cases compared with 40% treated at 120 shocks per minute. The same group from the University of Toronto have recently published their data demonstrating an improvement in outcomes in a series of exclusively ureteral stones randomised to treatment at 60 versus 120 shocks per minute. Again the slow treatment groupachieved a significantly higher success rate of 64.9% compared with 48.8%. More studies are required to determine if the effect of slowing shockwave delivery rates will be effective on all types of lithotripters and to identify stone and patient characteristics that would most benefit from reduction in treatment rate.
Augmentation of fragment passage
Fragment passage may be augmented by physical treatment to aid in stone passage (mechanical percussion, inversion and diuresis [MPID]) as well as treatments with medical agents to expel the fragments (medical expulsive therapy [MET]) or medications such as postassium citrate to dissolve fragments.
The first study to report on MPID was a crossover randomisation of 69 patients who had fragments in the lower pole of the kidney post-SWL. Patients were inverted on a treatment table, treated with an intravenous diuretic and had their backs percussed with a physiotherapy mechanical chest percussor. This resulted in 40% of the treatment arm become stone free as compared to 3% in the observation arm.
The use of medical expulsive therapy to aid in the passage of fragments was recently summarised in a meta-analysis. This identified four studies in which MET was used to aid in fragment passage after SWL. It included trials with both renal and ureteral stones and a variety of medical expulsive agents including alpha blockers, calcium channel blockers and an herbal agent Phyllanthus niruri alone or in combination with an oral corticosteroid. The analysis revealed a weighted risk difference of 17% in favour of MET in addition to SWL with respect to treatment success compared to SWL. MET was well tolerated and was most beneficial for stones greater than 10mm. Subsequent this study, further trials have suggested the benefit of tamsulosin, an alpha adrenergic blocker, with repect to aiding in fragment passage.
The utility of potassium citrate (60mEq/day in divided doses) to aid in reduction of stone burden, post SWL has been demonstrated. In patients treated with potassium citrate, 44% were stone free while 56% of patients had a stable residual stone burden as opposed to the control group, of whom only 12.5% became stone free, 25% had a stone burden that was unchanged and 62.5% had an increase in their stone burden during follow-up.
Conclusion
Shockwave lithotripsy represents the least invasive and most tolerable option for most urinary calculi. The decision to undergo SWL versus other stone treatment modalities must be discussed with the patient and should be based on stone factors, patient factors and the patient’s expectation of treatment outcome. Once the decision to use SWL has been made, consideration should be given to methods which optimise fragmentation of stones and their subsequent passage in order to prevent the need for additional SWL or more invasive therapies.
Further reading
Schuler TD, Shahani R, Honey RJ, Pace KT. Medical expulsive therapy as an adjunct to improve shockwave lithotripsy outcomes: a systematic review and meta-analysis. J Endourol 2009; 23: 387-393.
A complete bibiography is available from the author
The author
Dr Trevor Schuler,
Assistant Professor , Surgery
University of Alberta
Off Campus
Edmonton, AB,
Canada.
e-mail: ts9@ualberta.ca
