Peristalsis prevents ureteral dilation

The etiology of ureteral dilation in primary nonrefluxing, nonobstructing megaureters is still not well understood. Impaired ureteral peristalsis has been theorized as one of the contributing factors. However, ureteral peristalsis and its "normal" function is not well defined. In this study, using mathematical modeling techniques, we aim to better understand how ureteral peristalsis works. This is the first model to consider clinically observed, back-and-forth, cyclic wall longitudinal motion during peristalsis. We hypothesize that dysfunctional ureteral peristalsis, caused by insufficient peristaltic amplitudes (e.g., circular muscle dysfunction) and/or lack of ureteral wall longitudinal motion (e.g., longitudinal muscle dysfunction), promotes peristaltic reflux (i.e., retrograde flow of urine during an episode of peristalsis) and may result in urinary stasis, urine accumulation, and consequent dilation.

Based on lubrication theory in fluid mechanics, we developed a two-dimensional (planar) model of ureteral peristalsis. In doing so, we treated ureteral peristalsis as an infinite train of sinusoidal waves. We then analyzed antegrade and retrograde flows in the ureter under different bladder-kidney differential pressure and peristalsis conditions.

There is a minimum peristaltic amplitude required to prevent peristaltic reflux. Ureteral wall longitudinal motion decreases this minimum required amplitude, increasing the nonrefluxing range of peristaltic amplitudes. As an example, for a normal bladder-kidney differential pressure of 5 cmH₂ O, ureteral wall longitudinal motion increases nonrefluxing range of peristaltic amplitude by 65%. Additionally, ureteral wall longitudinal motion decreases refluxing volumetric flow rates. For a similar normal bladder pressure example of 5 cmH₂ O, refluxing volumetric flow rate decreases by a factor of 18. Finally, elevated bladder pressure, not only increases the required peristaltic amplitude for reflux prevention but it increases maximum refluxing volumetric flow rates. For the case without wall longitudinal motion, as bladder-kidney differential pressure increases from 5 to 40 cmH₂ O, minimum required peristaltic amplitude to prevent reflux increases by 40% while the maximum refluxing volumetric flow rate increases by approximately 100%.

The results presented in this study show how abnormal ureteral peristalsis, caused by the absence of wall longitudinal motion and/or lack of sufficient peristaltic amplitudes, facilitates peristaltic reflux and retrograde flow. We theorize that this retrograde flow can lead to urinary stasis and urine accumulation in the ureters, resulting in ureteral dilation seen on imaging studies and elevated infection risk. Our results also show how chronically elevated bladder pressures are more susceptible to such refluxing conditions that could lead to ureteral dilation.