Editorial Comment for Sener et al.

In the preceding article by Sener and colleagues ¹ , the authors examine important relationships between the size of ureteral access sheaths (UASs), ureteroscopes, and the pressure and flow rate measured inside a bench top ureteroscopy model. The use of the UAS has become more popular in the past decade, but it has a dual nature that articles like this important submission are helping to clarify. On the positive side, the UAS has been shown to reduce pressures in the kidney during ureteroscopy and may thereby decrease pyelovenous backflow and risk for sepsis. ^2,3 It may also provide improved irrigation flow and allow faster more efficient basket extraction of stones and repeat access to the upper tract. ^{4 –6} On the negative side, larger UASs have been shown to produce ureteral ischemia with a theoretical increased risk for stricture, ⁷ to require routine stenting following their use, and to result in injury to the ureter in up to 46.5% of patients ⁸ and may also increase costs. ⁹

There are some important points that can be gleaned from this article. First, it is not the outer diameter of the UAS that determines the flow rate, rather it is the internal diameter. Second, it is important to pair the size of the ureteroscope with the appropriate internal diameter of the UAS to provide an optimal flow rate. It makes intuitive sense that a smaller UAS would result in less risk of ureteral injury, although this parameter was not specifically tested in this study. It is also important to remember that in an in vivo system, there are many other characteristics of a UAS that may influence safety other than the size and flow rate. These factors cannot be entirely controlled for in a bench top study and include lubricity, stiffness, the amount of taper of the tip, and the interaction between the UAS and the ureter itself. This article suggests that the use of a UAS that is too small may result in higher irrigation pressures and a higher risk for sepsis, while the use of a UAS that is too large may increase the risk of injury to the ureter. Hence, the authors conclude that it is important that the flexible ureteroscope in use be matched with an appropriate-sized UAS. We have recently been studying pressures during percutaneous nephrolithotomy and have identified that pressures that are too high may result in pyelovenous backflow and increased infectious risk, while pressures that are too low may result in less venous tamponade, more bleeding, poorer visualization, greater difficulty in mapping the calyces due to collapse of the system, and could lead to a lower stone-free rate. ¹⁰ Perhaps, we are facing the Goldilocks Paradox where we need a UAS that is not too large, or too small, but is just right. This is certainly an important area of research and studies like this one are helping to clarify these issues.