Understanding the urethral closure mechanism and dysfunction in female stress urinary incontinence (SUI)  has been debated by many investigators for decades. There are numerous hypotheses attempting to accurately explain the function of the urethral structures. The severity of SUI depends upon the ability of the urethra to maintain a robust urethral closure pressure during fluctuations in intra-abdominal pressure. Pelvic floor ultrasound is a useful tool to visualize urethral shape and motion during Valsalva. This study aimed to establish whether the closure mechanism of the urethra can be quantified by studying changes in the urethra’s shape and position during strain across varying degrees of SUI.  Our hypothesis was that excessive urethral motion and urethral shape would differ between continent, mild SUI, and severe SUI groups.

This was a prospective cohort study. Women who presented to our tertiary urogynecology center for urodynamic testing for urinary incontinence evaluation or pre-operative assessment for occult urinary incontinence were recruited to the study. Women with urinary retention, history of significant central nervous system disease, floor reconstructive surgery, anti-incontinence surgery, or third line treatment for overactive bladder were excluded. Additionally, patients were excluded after the urodynamic test if they had newly diagnosed urinary retention, low bladder capacity, or high-pressure detrusor instability. 

This urodynamic study was performed according to the International Continence Society criteria, using a 7-Fr transurethral double-lumen catheter and an 8-Fr rectal/vaginal pressure sensor with a bladder filling rate of 70 mL/min. Serial Valsalva maneuvers were performed during the filling phase to replicate stress urinary leakage. Urethral pressure profiles and urethral closure pressures were obtained and then permission to void was given.

Pelvic floor ultrasound imaging was obtained at the time of the study visit using the BK Medical BK5000 X14L4 12 MHz transducer. All ultrasound studies were performed with the patient in the dorsal lithotomy position with hips flexed and abducted. Patients were instructed to arrive with a partially full bladder. The probe was inserted into the vagina in a neutral position and dynamic ultrasound videos were saved.

Dynamic anterior compartment ultrasound took 5 second videos of the midsagittal plane. The view included the bladder and urethra, with the urethral meatus, pubic symphysis, and bladder neck serving as landmarks. Patients were asked to perform a squeeze and then a Valsalva maneuver. Total urethral length and retropubic and infrapubic urethral length, urethral thickness at three levels, bladder neck retropubic angle, and the bladder neck and distal urethra’s swing angle relative to the pubic bone were measured during Valsalva.

Statistical shape modeling was carried out by calculating corresponding points using Deformetrica and then using the Procrustes method and performing a principal component analysis (PCA) in Mathematica. This output modes of variation describing shape variance. PC scores were calculated for the significant modes for subsequent statistical analyses.

For statistics, only patients with a maximum urethral closure pressure of 40 cm H20 were included and categorized into “No SUI”, “Mild SUI”, or “Severe SUI” groups based on the symptoms present on a daily basis. Using SAS 9.4, patients’ demographics, symptoms, POP-Q, urodynamic, and changes in dynamic ultrasound measurements were compared between these groups using Fisher’s exact or chi-squared test for categorical variables and ANOVA (parametric) or Kruskal-Wallis test (nonparametric) for continuous variables. Changes in dynamic ultrasound were quantified by subtracting values at rest from those at Valsalva. Position and shape variables were evaluated together using a multinomial logistic regression. Using IBM SPSS Statistics v26, PC scores were analyzed with a Two-Way Mixed MANOVA with univariate ANOVAs and Benjamini-Hochberg corrections (with a false discovery rate of 10%) post hoc performed to evaluate the influence of the between-subjects variable, SUI severity, and the within-subjects variable, maneuver (Rest vs Valsalva).

76 women met the inclusion criteria for the final analysis (23 with no SUI, 31 with mild SUI, and 22 with severe SUI). There were no statistically significant differences in age, parity or BMI among groups. 12 modes of variation explained significant shape variance and maneuver significantly influenced the overall urethra shape (p<0.001). The difference between rest and Valsalva was significant for modes 1 and 2 specifically (p<0.001).  Qualitatively, mode 1 described variation in “c” shape concavity and mode 2 variation in “s” shape concavity that exists in all SUI severity groups. During Valsalva, the urethra became more “s” shaped with distal urethral wall thickening (Figure 1).

Changes in bladder neck retropubic angle, infrapubic urethral length, and the distal urethra’s swing angle relative to the pubic bone from rest to Valsalva were significantly different between groups (p=0.0157, 0.0154, and 0.0098, respectively) (Figure 2). SUI severity influenced the overall urethral shape significantly (p<0.001) and was significant for modes 5 (p=0.001), 7 (p=0.001), and 11 (p=0.009) specifically. For these modes, the continent and severe SUI groups differed significantly (p=0.001, 0.002, and 0.007, respectively) and the continent and mild SUI groups differed for mode 7 (p=0.007). Modes 5, 7, and 11 describe variation in the proportional wall thickness of specific regions of the urethra, indicative of more or less “pinching” or “squeezing” across SUI severity, regardless of maneuver. Together, these modes describe increased “s” shape concavity, distal urethral wall thickening, and proximal urethral pinching in mild and severe SUI urethras compared those in the no SUI group. 

The multiple logistic regression comparing mild and severe SUI patients to those with no SUI demonstrated that increased “s” shape concavity and distal urethral wall thickening (described by modes 2, 5, 7, and 11) and urethral swing angle were significant predictors of severe SUI as indicated by their respective odds ratios (OR=1.728, 0.228, 0.309, 9.357, and 1.005) and p-values (p=0.0321, 0.0015, 0.0030, 0.0014, and 0.0012).

These results suggest that dynamic endovaginal anterior compartment ultrasound can visualize and allow for quantification of passive closure of the urethra during Valsalva due to shape changes and motion caused by increased intra-abdominal pressure. 

The instability and excessive swinging motion of the bladder neck and distal urethra relative to the pubic bone, increasing “s” shape concavity and urethral wall thickness (mode 2), and increasing distal urethral thickening (mode 11) were associated with a higher likelihood of having severe SUI. Meanwhile, increased distal urethral pinching (mode 5) and more exaggerated bending of the ends of the urethra towards the pubic symphysis to make a more pronounced “c” shape (mode 7) were associated with an increased likelihood of being continent.

Endovaginal dynamic anterior compartment ultrasound can visualize and allow for quantification of the mechanisms involved in urinary continence. This imaging modality and subsequent analyses could provide improved understanding of female SUI and guide future treatment planning.