Is Lithotripsy Injurious to Your Health

  By John A. Connolly, M.D.

A study from the Mayo Clinic by Krambeck et al published in the May edition of the Journal of Urology suggests that shock wave lithotripsy (SWL) of renal stones may not be as safe a procedure over the long term as we urologists have assumed. The authors found an increased incidence of hypertension and diabetes mellitus in 19-year follow-up of a cohort of patients treated for nephrolithiasis with SWL. 

The treatment of renal and upper ureteral stones was revolutionized in the 1980s with the advent of the Dornier HM3 lithotripter. This machine and subsequent generations of lithotripters are more than 90 percent successful in fragmenting renal and upper ureteral stones and facilitating the passage of stone fragments. Because of its non-invasive nature and perceived safety, SWL quickly gained widespread acceptance and has largely eliminated open stone surgery. 

That SWL can cause acute deleterious effects through cavitation effects and shearing forces within tissues is well recognized. Hematuria with clot colic or retention, renal hematoma, acute pancreatitis and colon or other adjacent organ injury has been described in the immediate post-procedure setting. Clinically evident problems, however, are rare and the great majority of patients undergo SWL suffering nothing more than mild hematuria and transient flank discomfort. The potential for the acute renal injury associated with SWL to be translated into later hypertension has been looked at before with conflicting results. Early reports suggested an association with acute onset hypertension, but subsequent publications (including a randomized control trial) failed to demonstrate any association between SWL and elevated blood pressure. The patient follow-up in these studies was less than five years.

The Mayo Clinic study is the first to look at very long-term problems related to SWL. The authors describe a significantly increased incidence of hypertension in SWL patients compared with a matched control group at 19 years follow-up (36.4 percent vs. 27.9 percent, p=0.034). Patients who underwent bilateral SWL had the highest rate of late hypertension. Large population-based studies have demonstrated a correlation between hypertension and stone formation, and the rate of treated hypertension in both the study group and the control group in the Mayo Clinic study was high. However, after excluding patients with preexisting hypertension, the authors found that the development of new hypertension was significantly higher in the SWL group during the 19 years of follow-up. 

In addition they noted a significantly increased incidence of diabetes mellitus (16.8 percent of the SWL treated group vs. 6.7 percent of the control group, p=0.001). This is the only study to have ever suggested an association between diabetes and SWL. The authors found this association with diabetes to be still present after controlling for obesity and changes in body mass index during the follow-up period. 

The Mayo Clinic study suggests that these observed increases in the incidence of hypertension and diabetes are secondary to renal and pancreatic injury during SWL. This injury presumably has an effect on renin secretion in the kidney and insulin secretion by pancreatic islet cells. Although others have shown that acute injury after SWL is related to the number of shocks and the power (kV) used, the current study did not demonstrate any correlation between these parameters and the late complications described. The type of lithotripter used in the study may, however, be very important in the interpretation of the results because there are considerable technical differences between the older generation of lithotripters and the newer machines. 

The patients in the Mayo Clinic study all were treated with the Dornier HM3 lithotripter and matched to a group of stone patients treated nonsurgically. The HM3 lithotripter, introduced in 1984, is still used today (although not extensively) and is considered by some to be the gold standard because of its high stone-free rates (greater than 90 percent) and low re-treatment rates (less than 10 percent). It is a machine with a large focal area (F2) measuring 90x15mm. More modern mobile electrohydrolic lithotripters have smaller F2 zones (the Medispec device has an F2 measuring 60x13mm) and electromagnetic lithotripters can have the smallest F2 zones (as small as 6mm). The latter have higher peak pressures over the smaller F2 zone. Despite the higher peak pressures, these electromagnetic lithotripters have lower early stone-free rates and higher re-treatment rates than the electro-hydrolic machines. It is felt that the smaller F2 combined with normal respiratory-related movement of the stone during treatment means that fewer shocks actually hit the stone. Using the larger F2 devices, such as the Dornier HM3, means that the stone usually is in the targeted area irrespective of respiratory movement. The smaller focal zone of newer lithotripters also requires more and better fluoroscopy during the procedure to ensure that the stone is very precisely targeted. 

When these issues are taken into account, it may be that the observed increase in the incidence of hypertension and diabetes in the Mayo Clinic patients is related to renal and pancreatic damage from the use of a lithotripter with a relatively large F2. Newer generation machines might be expected to do less overall damage to soft tissue because of their smaller focal zones even though the peak pressure of the shock wave can be higher with these lithotripters. However, there is the issue of higher re-treatment rates in patients having treatment with the small F2 machines and the risk of additional tissue injury secondary to multiple treatments.

Beyond this question, of course, is the validity of the Mayo Clinic study in the first place. It is a retrospective case control study with only an approximately 60 percent response rate to a questionnaire sent to patients by the study authors. Also patients who died before 2004 were not included in the study and therefore adverse long-term outcomes in these patients were not studied. There may be data collection bias since long-term outcome information on patients treated with SWL was obtained by questionnaire, and outcome data on controls was obtained by retrospective chart review of patients followed at the clinic. 

Until further retrospective and, in the longer term, prospective data is acquired, patients considering lithotripsy should be informed of the questions raised by this study. Discussion with patients should emphasize that lithotripsy is still a highly successful and safe treatment for kidney stones.

Until further retrospective and, in the longer term, prospective data is acquired, patients

considering lithotripsy should be informed of the questions raised by this study

Alternate treatments for renal and upper ureteral stones should be discussed including ureteroscopic manipulation with lasers, lithoclast devices, and baskets and also percutaneous nephrolithotomy (PNL) for larger stones in the kidney. These treatments have a small but real risk of injury to the ureter (and kidney in the case of PNL), which can require extensive reparative surgery or even loss of the renal unit. Deeper anesthesia is also required for these procedures compared with modern lithotripters. Institutions using the Dornier HM3 lithotripter should point out to their patients that this machine was used on the study patients. We hope newer generation lithotripters will not show the same long-term trends for hypertension and diabetes because of their smaller F2. However, there cannot at this time be excessive complacency on the part of those urologists who use newer lithotripters because of the factors mentioned already, i.e. higher peak pressures at the focal zone and higher re-treatment rates. Careful targeting of the stone, with deeper anesthesia if necessary to reduce respiratory movement of the stone during treatment (and hence "wasted" shocks), using the least number of shocks to effect stone destruction and deferring treatment of certain incidentally discovered asymptomatic stones (e.g. small calyceal stones without obstruction) should be considered in patients undergoing lithotripsy until the issue of long-term effects is resolved.