Defining the Rate of Primary Ureteroscopic Failure in Unstented Patients: A Multi-Institutional Study

Introduction

Ureteroscopy (URS) has undergone great progression over the past several decades with miniaturization of scopes, advanced fragmentation technology, and improved extraction devices. As the field of endourology evolves, URS is rapidly becoming the most common method to treat urinary tract stones in the United States. ^1,2 While overall success rate is high, urologists performing primary ureteroscopic intervention for kidney or ureteral stones occasionally encounter difficulty with passage of the ureteroscope in the initial procedure. These patients will require a second procedure after prestenting.

Ureteral stents interfere with quality of life in the vast majority of patients. Nearly 80% of patients note bothersome urinary symptoms or pain and 58% of patients report stent symptoms having a negative economic impact due to work interruption. ^3,4 Increasingly urologists, especially young urologists, attempt to avoid this morbidity and do not routinely prestent. ⁵

This study aims to define the contemporary failure rate of primary URS and identify predictive factors that necessitate prestenting. These results will improve preoperative patient counseling, facilitating the informed consent process by clearly defining the probability of needing subsequent procedures and the factors that may increase that likelihood. In addition, it will assist clinicians with decision-making to identify patients who may benefit the most from selective prestenting. In this study, we report the overall rate of failure to access the patient's ureter during initial treatment in a large multi-institutional cohort and identify patient characteristics that are associated with failure.

Materials and Methods

We conducted a multi-institutional retrospective review of consecutive retrograde primary ureteroscopic procedures for kidney and/or ureteral stones over a 2-year period from August 2011 to August 2013 at three academic medical centers. Institutional Review Board approval was obtained at two sites and the third site followed the Declaration of Helsinki protocol for a Quality Improvement project.

Deidentified data, including patient age, gender, primary stone size and location, prior procedures for nephrolithiasis, and/or prior spontaneous stone passage were collected. The location and size of the most distal stone were used for those variables. Stone location was recorded as kidney, proximal ureter (above the level of bony pelvis), mid-ureter (overlying the region of the bony pelvis), and distal ureter (below region of bony pelvis).

Flexible and semirigid ureteroscopic procedures were included. All participating institutions used the Karl Storz Flex-X2^™ flexible ureteroscopes and Karl Storz or Gyrus-ACMI semirigid ureteroscopes. All procedures were initially attempted with a safety wire in place. A variety of strategies were employed at the discretion of the surgeon to gain access to the difficult distal ureter. Of the participating institutions, after an initial attempt to traverse the distal ureter with the ureteroscope failed, one center utilized ureteral balloon dilation (4 cm × 12F), another center utilized lubriglide serial ureteral dilators, and the third did not perform ureteral dilation to manage a difficult distal ureter during the primary procedure. Rate of ureteroscopic failure, defined as the inability to access the ureter with a ureteroscope to the level of the patient's primary stone, was determined. Access failure rate was compared among difficult distal ureter management techniques.

The significance of stone location was evaluated as renal vs all ureteral stone locations and renal vs specific ureteral location (proximal, mid, distal). In addition, the ureteroscopic failure rate of the proximal ureter was compared to the mid and distal ureter. Patient age, stone size, prior procedures for nephrolithiasis, and prior spontaneous stone passage were compared between ureteroscopic failure and success groups.

Statistical analysis, including Fisher's exact test, rank-sum test, and multivariable logistic regression of patient demographics and ureteroscopic failure, was performed using the STATA version 14 software package. Univariable analysis was performed for all variables to assess the impact on primary ureteroscopic failure. Multivariable logistic regression using only variables significant on univariable analysis was then performed. At model development, we aim for, at least, internally valid predictions, that is, predictions that are valid for subjects from the underlying population. Preferably, the predictions are also generalizable to “plausibly related” populations. These concepts are reflected in the calibration and discrimination of the final model. Goodness-of-fit, or calibration, is established through the Hosmer and Lemeshow test. Discrimination was assessed using receiver operating characteristic curve analysis. An alpha level of 0.05 was considered to indicate statistical significance. Ultimately, the prediction model based on age, stone size, and location had good discrimination with a c-statistic of 0.7 and high calibration (p ≥ 0.47). Rigorous model validation is reported using likelihood ratios and Wald test with p-values of 0.0010 and 0.0031, respectively, indicating that the logistic model with age, size, and location predicts access failure better than random chance alone.

Results

A total of 535 patients undergoing primary URS were reviewed. Variables, including patient demographics, stone location, stone size, and prior interventions for nephrolithiasis, were collected (Table 1). Ureteroscopic failure rate among these unstented patients was 7.7% with 41 failures. All failures were stented and subsequently underwent URS at a later date. The failure rate by participating institution was 22/96 (22.9%), 10/224 (4.5%), and 9/215 (4.2%). The higher failure rate was a surgeon in their first years of practice and is likely, partially, a result of conservative practice and the preference not to perform dilation of the intramural tunnel.

As indicated in the “Materials and Methods” section, two of the three institutions performed dilation of the narrow intramural distal ureter with either 4 cm × 12F balloon dilators or lubriglide serial ureteral dilators. Out of the 535 unstented patients, 88 underwent sequential dilation of the ureteral orifice and eight distal ureters were balloon dilated. There were five access failures after serial dilation (5.7%) and there were no access failures after balloon dilation. Complication data specific to ureteral dilation technique were not collected.

In the unstented ureter, the median age of patients with failed access was significantly lower at 51 (95%; confidence interval [CI] 37–59) vs 56 (95%; CI 54–58) years in the successfully accessed group (p = 0.0228). This appears to be due to a difference in the female population mostly, as the median age of the females with primary ureteroscopic failure was significantly lower than in females who had successful ureteroscopic access, 34 (CI 23–53) vs 52 (CI 50–55) years, respectively; (p = 0.0041). The trend remained the same, although not statistically significant if the center with the highest rate of failure that did not dilate the distal ureter was removed from the analysis. The median age of female access failures was 53 and 41.5 for female access success and failures, respectively (p = 0.084). There was no difference in the median age of males with successful access vs failure, 58 (CI 56–60) vs 57 (CI 47–60) p = 0.35, and overall gender itself was not independently associated with failure (Table 2).

For patients where their prior surgical stone history was known, there was no statistically significant relationship between prior intervention for nephrolithiasis and ureteroscopic failure in unstented patients (Table 3). For the 498 patients with known stone size, the failure rate for patients with stones >10 mm was significantly lower than with stones <10 mm in size, 5.1% (12/236) vs 10.3% (27/262), p = 0.0438.

Failure of primary ureteroscopic access for a renal stone was less likely compared to all ureteral stone locations combined, with a 4.68% (13/278) failure rate vs 10.89% (28/257), respectively, (p = 0.009). Proximal ureteral stones had the highest failure rate for ureteral access at 15.45% (17/110), which was significantly higher than the failure rate for renal stones (4.68%, 13/278) and distal ureteral stones (6.73%, 7/104), p = 0.001 and p = 0.049, respectively (Table 4).

On univariable analysis, age, stone size, and location were significant predictors of ureteroscopic failure (p = 0.0228, p = 0.0020, and p = 0.006). Limiting this to only ureteral stones, there was no difference in stone size and access failure, p = 0.22. When comparing renal stones to ureteral stones on univariable logistic regression, ureteral stones were more likely to have access failure with odds ratio (OR) 2.49 (1.26, 4.93) p = 0.009. This became stronger when only comparing proximal ureteral stones to renal stones as well, OR 3.72 (1.74, 7.97) p = 0.001.

On multivariable logistic regression, proximal ureteral stone location remained a significant predictor of failure when compared to renal stones, OR 3.14 (1.4, 7.0) p = 0.006. Overall, smaller stone size was a significant predictor of failure, OR 0.90 (0.82, 0.99) p = 0.039. Interestingly, age was no longer a significant predictor (p = 0.117). In addition, when including only ureteral stones in the multivariable analysis, stone size was no longer a significant predictor, p = 0.206. Location in the proximal ureter compared to the distal ureter was not significant, OR 0.38 (0.15, 1.00) p = 0.050 (Table 5).

Discussion

There are multiple variables that must be considered and discussed with the patient before the ureteroscopic treatment of nephrolithiasis or ureterolithiasis. These include the likelihood of successful ureteral access, the probability of needing subsequent procedures, and how these specific factors are affected by unique patient characteristics. While there is some literature reporting failure of primary URS, much of the data are several decades old, describing rates with older technology and larger ureteroscopes. In addition, the reports are small, single-center series with failure rate generally recorded as a secondary outcome. There are several relatively small series studies reporting failure rate of primary URS. ^6,7 To our knowledge, this is the largest contemporary series that reports this information with modern technology. It is the only multicenter study where we are aware of where failure rate of primary URS is reported as the primary outcome and where predictive factors are determined, with a goal of enhancing patient counseling and the informed consent process.

In 1990, Jones et al. ⁸ first published that stenting after a failed attempted URS was associated with increased secondary success of stone removal and decreased need for ureterolithotomy. They also found a failure rate of primary URS with a 9.5F or 11F semirigid ureteroscope of 11%. ⁸ Cetti and colleagues updated this experience in a more modern cohort in 119 patients using 7.5F or 6F and 8.4F or 7.5F semirigid and flexible ureteroscopes, respectively, with active dilation of the distal ureter (12F dilator or 11–13 access sheath) when necessary, and showed an overall failure rate of 8.4%. ⁶ This was a single-surgeon report and did not identify risk factors for failure. We update this figure in a larger, multi-institutional cohort and show failed access of the ureter in 7.7% (41/535) of unstented patients. Patients who had narrow intramural/distal ureters and had active dilation (either balloon or sequential dilation) performed had a failure rate of 5.2% (5/96).

In addition, we identified risk factors for failed access during primary URS. A prior study suggested that prior ipsilateral ureteral stent or stone surgery increased access rates. ⁷ This study found no relationship between prior ureteroscopic intervention, shock wave lithotripsy, percutaneous nephrolithotomy, or spontaneous stone passage on future access failure. Our data reveal that proximal ureteral stones are less likely to be accessed for treatment primarily than those in the kidney, mid-ureter, or distal ureter. It may be that stones in the proximal ureter that have failed to advance or spontaneously pass are a marker for a smaller ureter, explaining our findings. In addition, this may be a subgroup that could benefit from prestenting at the discretion of the surgeon in certain circumstances, particularly when there is a large proximal ureteral stone suspicious for impaction. Chu et al. ⁹ has previously shown that this is a reasonable and conservative approach reporting reduced reoperative rates when prestenting for large stone burdens (>1 cm), especially those in the kidney and proximal ureter (p = 0.001). Netsch and colleagues ¹² further found improved stone-free rates (SFR) for stones ≥5 mm with prestenting. The recently published study of the Clinical Research Office of Endourological Society report supports this result. They found prestenting increased the SFR for renal, but not ureteral stones. ^{9 –11}

When all locations were included, stone size >10 mm favored successful primary access. However, this was likely an artifact from the larger stones within the kidney, as we ultimately found no association between stone size and ureteral access for only ureteral stones. In addition, younger patients, specifically young females (median 34 years old vs 52), were less likely to have successful primary ureteral access. The reason for this is unknown; however, may be secondary to inherent ureteral tissue properties in this subpopulation and further study is needed. Furthermore, we did not separate out parous vs nonparous females and this could potentially explain the difference in tissue properties. Stone size, laterality, previous procedures for upper urinary tract stones, gender, and prior stone passage were not statistically significant predictors of primary ureteroscopic access failure and should have minimal impact on preoperative counseling.

There are limitations of this study, including its retrospective nature. Moreover, all participating urologists are fellowship-trained endourologists with experience treating difficult stones and are referral centers for their management. Failure rates may not be translatable to urologists with lower case volumes. Between participating institutions, the management of the poorly accommodating distal ureter varied between stenting with passive dilation vs active dilation with balloons or serial dilators, which recent literature supports as safe and efficacious. ¹¹ The overall failure rate in this report therefore is likely slightly overestimated for those who are more aggressive and slightly underestimated for those who may be more conservative.

We do not report SFR in our study. Multiple studies have addressed the safety, efficacy, and SFRs of prestenting vs not prestenting. ^{12 –15} The purpose of our study was to define failure rate of URS when prestenting is not done with the goal of enhancing patient counseling and the informed consent process. SFR was chosen not to be recorded due to variability in postoperative imaging practices across institutions and its lack of relevance to our primary goal.

This study is not applicable to all patients as there are many reasons why patients may not have attempted primary URS, for example in the setting of active urinary tract infection.

Conclusions

There is a low failure rate of 7.7% to access the ureter and initiate stone treatment with primary URS. Stones within the proximal ureter and young female patients are least likely to be accessed primarily and most at risk of needing a secondary procedure. These findings will improve preoperative counseling, enhancing the overall informed consent process for the patient. In addition, these data may aid the surgeon in decision -making by identifying patients who may benefit the most from selective prestenting.