"Patients undergoing shock wave lithotripsy should be counseled that there may be a small increase in the risk of hypertension following the procedure," said senior author Manoj Monga, MD, who was professor of urologic surgery at the University of Minnesota, Minneapolis at the time of the study and is now director of the Stevan B. Streem Center for Endourology & Stone Disease at Cleveland Clinic.
Dr. Monga based his recommendation on results of a survey and chart review showing that there is a modest but significant risk of developing hypertension among patients undergoing SWL. His findings appear to confirm similar results of a Mayo Clinic, Rochester, MN, study that was published in the Journal of Urology (2006; 175:1742-7). That study concluded that SWL was associated with an increased risk both of hypertension and diabetes. Several studies that followed the Mayo Clinic study found either no relation between SWL and hypertension or a modest, equivocal risk.
Dr. Monga elaborated on the data by noting that among patients who did not report having hypertension at the time of treatment, 15% reported the condition at the time of the survey. The expected rate, using age-matched controls, would be 6.25%.
In the 2006 Mayo Clinic study, the investigators found that at 19 years of follow-up, patients treated with SWL were more likely to develop hypertension (OR=1.47) and more likely to develop diabetes (OR=3.75).
Dr. Monga and colleagues looked at questionnaires returned by 1,892 patients whose names and charts were drawn from the National Health and Nutrition Examination Survey (NHANES). Data on patient demographics, body mass index, stone location, number and size of stones, treatment parameters such as the number and size of stones, and outcomes such as complications, re-treatment rates, and stone-free rates were recorded prospectively. The investigators did not look at the incidence of diabetes. All the NHANES patients were treated with a Medstone STS electrohydraulic lithotripter, a third-generation lithotripter. (The Mayo Clinic patients were treated in 1985 with a Dornier HM3 lithotripter.) Treated patients and controls were matched by age and BMI at the time of treatment and at follow-up.
"There is an increased risk of hypertension in men, the elderly, and patients with higher BMI, but we found no correlation that would suggest dose dependency. There was no correlation with the number of shocks, shock intensity, bilateral versus unilateral treatment, or with people who had undergone multiple treatments," Dr. Monga said.
He also noted that patients with an abnormal blood pressure reading on the day of SWL but without a diagnosis of hypertension were at greater risk of developing hypertension later on.
Causative mechanism unknown
These observations create a challenge for those trying to hypothesize a causative mechanism.
"Normally, if you are going to draw a positive relationship, you would expect to see a dose-dependent effect. You would expect that more shocks would net a higher risk of hypertension, or that treating both kidneys as opposed to one kidney would create a higher risk, but we did not find that. This makes it very difficult to define a cause and effect," Dr. Monga told Urology Times.
Dr. Monga issued his findings with caveats. The NHANES data did not indicate tobacco use, family history of stones, and other potentially confounding factors.
One urologist attending the presentation asked Dr. Monga if there was a way of determining whether stones alone or stones with treatment increased hypertension risk.
"Unfortunately, in a retrospective design, there is no good way to define a control group," Dr. Monga answered. "I believe the only real way to answer those questions is to conduct a prospective study in which patients are randomized or preselect themselves to observation, SWL, or ureteroscopy."