Ureteral access sheath (UAS) use has become of great interest due to the multiple benefits associated with their deployment during ureteroscopic (URS) lithotripsy. Of note, the UAS has facilitated rapid, safe, and repeated passage of the flexible ureteroscope, lowered intrarenal pressures, decreased postoperative sepsis, and allowed for more efficient stone fragmentation/removal.^1-7

The downside of UAS passage is the potential for ureteral injury during insertion. Indeed, Traxer and Thomas noted injury in up to 47% of patients with actual splitting of the urothelium in 13% of the patients overall.⁸ These results have been further corroborated in a more recent report by Fulla et al, in which splitting of the ureter was noted in 26% of cases.⁹ In both situations, these injuries occurred during passage of the intermediate size 14F UAS. These concerns may explain the hesitancy to use UAS during URS lithotripsy, as reflected in the state of Michigan study where urologists were using UAS in only 38% of cases.¹⁰

To further investigate ways to mitigate tissue damage, the UCI (University of California, Irvine) Department of Urology in collaboration with UCI’s California Institute of Telecommunications and Information Technology developed a novel force sensor (FS) to be used at the time of UAS insertion (parts A and B of Figure). The device was initially used in a porcine study where, of the 5 pigs in which the insertion force exceeded 8 newtons (N), 3 pigs sustained a high-grade ureteral injury. In contrast, insertion forces ≤6 N incurred no high-grade ureteral injuries.¹¹ These findings were further corroborated in a subsequent clinical study in which 200 patients undergoing URS did not sustain any ureteral injuries when insertion forces were limited to ≤6 N. Indeed, the largest UAS (16F) was safely passed in 61% of patients.¹² Of note, no grade 4 or 5 post-ureteroscopy lesion scale (PULS) injuries occurred in any of the 200 patients.¹² Armed with this information, we next sought to define the natural distensibility of the human ureter by the sequential passage of 37 cm long urethral dilators (ranging from 10F to 24F; Cook Medical) into the ureter at the outset of URS using the UAS-FS.

Figure. A, The University of California, Irvine ureteral access sheath force sensor which measures applied forces in 1/100th of a Newton. B, Sample force output reading from the force sensor. C, Urethral dilator set circumferences. D, urethral dilator set tips. Fr indicates French.

As previously noted in our original force study, the UAS-FS contains an internal load cell that allows for continuous force readings in hundredths of a Newton up to 12 N. Force measurements are recorded in real time and then exported for analysis via a Bluetooth interface between the device and a computer tablet (parts A and B of Figure). At the outset of the procedure, the urethral dilators were sequentially passed in 2F increments (ie 10F to 24F) over a guidewire until the UAS-FS registered an insertion force of 6 N. Based on the size dilator passed at 6 N, the appropriate-size UAS was then placed.

Seventy-five patients (46 female and 29 male) undergoing URS stone removal underwent sequential sizing of the ureter (parts C and D of Figure). At the end of the procedure, ureteroscopy was performed and a PULS grade was determined.¹³

Urethral dilators were successfully passed using the 6 N threshold in 37.3% of patients with dilators ≤12F (ie 10F and 12F), 24% of patients with a 14F dilator, 24% of patients with a 16F dilator, and 14.6% of patients with dilators ≥18F (ie 18F-24F). The median maximum dilator diameter was 14F. The mean peak force most frequently occurred in the distal ureter (52%). High-grade ureteral injuries (PULS 3 and above) were noted in 3 separate cases at insertion forces of 5.9 N, 5.1 N, and 5.9 N during 14F, 16F, and 24F dilator insertion, respectively. Following ureteral sequential dilation, UAS placement of 16F, 14F, and 12F was achieved in 28 (38%), 25 (33%), and 22 (29%) patients, respectively.

Successful passage of dilator sizes ≥16F was more commonly observed in patients presenting with a preoperative indwelling stent. Fifteen of the 18 pre-stented patients (83%) were able to accept these larger dilators (≥16F) compared to only 14 of the 57 unstented patients (24.6%). A multivariate logistic regression analysis was performed to assess for the impact of age, gender, tamsulosin, preoperative indwelling ureteral stent, and the combination of tamsulosin and a preoperative indwelling ureteral stent on the ability to pass dilators ≥16F. Among these factors, only a preoperative indwelling ureteral stent favored passage of ≥16F dilators (see Table). Preoperative administration of tamsulosin, alone or in the presence of an indwelling stent, was not found to be of benefit.

In conclusion, the unstented human ureter could be safely distended to an average circumference of 14F; among preoperatively stented patients, the ureter could be safely distended to an average circumference of 16F. Preoperative administration of tamsulosin was not found to be beneficial.

Acknowledgments

We thank Cook Medical Inc for in-kind support in supplying the urethral dilators used in this study.