Applied trigonometry in elucidating the mechanism of post-HoLEP urinary incontinence

Stress urinary incontinence (SUI) after holmium laser enucleation of the prostate (HoLEP) is traditionally explained by longitudinal overstretching of the external urethral sphincter (EUS) during enucleation. We propose extreme radial forces exerted on the EUS as a novel mechanism for post-HoLEP SUI.

We assessed the resectoscope angles during enucleation as seen on external intraoperative pictures followed by fluoroscopically calibrated measurements and trigonometric calculations of EUS dilation as a function of the resectoscope angle and diameter and the membranous urethra length and width. Preoperative magnetic resonance imaging studies (n = 55) validated the trigonometric formula relative to SUI at one month post-HoLEP.

The maximal median angles for the non-SUI (n = 28) and SUI (n = 27) groups were 54° vs. 57° (p = 0.002), and the calculated sphincter dilations were 39mm vs. 49mm (p < 0.001), respectively. The sphincter dilation and resectoscope angle were the only independent predictors of SUI post-HoLEP in the univariate and multivariate regression models. A receiver-operating characteristic analysis found an area under the curve (AUC) of 0.849 for our formula, 0.764 for sphincter dilation, and 0.745 for the resectoscope angle. These AUCs were significantly better than those for prostate volume and patient age (p < 0.05).

The EUS dilation caused by radial forces exercised during resectoscope angulation is a plausible cause for post-HoLEP UI. It may also explain its temporality.