In this study, we explored the meaning of segmental urethral mobility of the urethra by perineal US. We found that the mobility in all urethral segments, especially the proximal urethra, along with the RVAδ played a significant role in the pathogenesis of SUI.
Sendag14 reported that a descending distance of the bladder neck > 15 mm and a posterior RVA > 120° were related to poor support of the bladder neck in patients with SUI with 96% and 53% sensitivity and 85% and 100% specificity, respectively. During straining condition, SUI women demonstrated > 1 cm of caudal motion of their urethrovesical junction by transrectal US, while control had < 1 cm of that15. Another study speculated that a bladder neck distance (BND) ≥ 25 mm may be abnormal and associated with SUI.16 Our results showed that the sensitivity and specificity of cut-off point A (bladder neck > 10.8 mm) as a diagnostic criterion were 71% and 68%, respectively. The inter-study differences in the results related to the descending distance of the bladder neck may be due to different measurement methods, as well as race. We combined the vertical and horizontal motion of the bladder neck, which may provide a better representation of the spatial movement in the bladder neck. However, a recent study confirmed that BND alone cannot comprehensively evaluate SUI.17 Similar to our results, one Chinese report suggested that the specificity of BND in SUI diagnosis was only 68.9%.18 Thus, we need to explore the abnormal urethral support structure to understand the etiology and pathogenesis of SUI. Most studies have focused on bladder neck mobility and provided little information about the rest of the urethra.19
The phenomenon where there is a rotational movement around the tip of the symphysis pubis during stress was observed on US in all patients. This rotational movement occurs in a cranial-to-caudal and dorsal-to-ventral direction, which is why we decided to use a mathematical description of urethral mobility. Our results showed that this change in mobility was similar for all urethral segments and was not restricted to the mid-urethra, suggesting that mobility at all stages of the urethra should be considered in UI studies. One previous study divided the urethra into five equal segments with six points from the bladder neck to the external urethral meatus, and results showed that the proximal urethra was consistently more mobile than the distal urethra (P < 0.001).20 However, they did not compare horizontal movement. In our study, the mean mobility and vertical distance of Points A (YA) and B (YB) in SUI were much greater than those in NSUI, and horizontal movement was one of the most influential factors. The horizontal distance of Point A in SUI was significantly greater than that in NSUI. However, there was no significant difference between SUI and NSUI at Point B. We speculated that there is more horizontal and vertical movement of the urethra closer to the neck of the bladder. Points C and D were the least mobile, suggesting that loss of proximal urethral stability leads to abnormalities in urethral mobility, resulting in SUI.
SUI is related to both urethral support and morphological changes in the junction of the urethra and bladder. In this study, there was no obvious change in urethral inclination, but the mean RVAδ at rest and during the Valsalva maneuver was clearly different in SUI and NSUI patients. A similar study showed that SUI was significantly associated with RVAδ. The mean RVA during maximum Valsalva was 152°, which was similar to our results (most effective Valsalva maneuver 143.4 ± 15.7°). We believe that the difference in RVA may be related to bladder volume and Valsalva maneuvers during detection. We further tested the mean RVAδ at rest and during the Valsalva maneuver, and speculated that this indicator may better explain the differences in bladder volumes. However, a previous study confirmed that there are no differences in BND, URA, or posterior urethro-vesical angle (PUVA) for different bladder capacities (100–500 ml).21 One study22 revealed that BND, URA, and RVA were not significantly different in mild to moderate and severe patients. Thus, perineal US is a useful tool for identifying SUI, but not its severity. Real time assessment of bladder neck and proximal urethra behavior using transperineal US indicated that the median RVA (166°) in the standing position was significantly greater than RVA (133°) in the supine position, and the median URA of 35° was significantly smaller than that of 64° in the supine position.23 In our study, the measurements were performed with patients in the supine position, and the mean URA of 25.3 ± 18.4° and mean RVA of 143.4 ± 15.7° were smaller than reported by the previously mentioned study. This may be due to different patient variables and the standard of Valsalva maneuvers. We found no obvious difference in RVA at rest between SUI and NSUI, but there was a significant difference in RVA during the Valsalva maneuver. Another report showed similar results whereby the difference in PUVA at rest and during Valsalva maneuver in SUI was significantly higher than in NSUI.24.
In terms of BND results (optimal cut-off, 24 mm), the area under the ROC curve (AUC) value was 0.804, with 66.4% sensitivity and 84.5% specificity.25 In our results, the ROC area of Point A was 0.73 with the optimal cut-off 10.8 mm, and this cut-off point provided 71% sensitivity and 68%, specificity. Moreover, Point B (cut-off 9.4 mm) provided 67% sensitivity and 75% specificity. We believe that bladder neck activity and the first one-third of the length from the urethra to the bladder neck has value in predicting UI. The AUC value of RVAδ was 0.62 (cut-off 10.7°), with 72% sensitivity and 54% specificity. This represents a weak predictive index for SUI.