Urethral mobility and urinary incontinence

Authors


Abstract

Objective

Urethral mobility is considered an important factor in female urinary incontinence. We therefore undertook a study to correlate segmental urethral mobility, as described by the urethral motion profile (UMP), with symptoms and urodynamic findings. Our null hypothesis was that there would be no statistically significant relationship between female urinary incontinence and segmental urethral mobility.

Methods

We performed a retrospective study in 198 women who had undergone multichannel urodynamic testing and four-dimensional translabial ultrasound for symptoms of lower urinary tract dysfunction or prolapse. Segmental urethral mobility was described by vectors of movement from rest to maximum Valsalva, relative to the posteroinferior pubosymphyseal margin. We described the mobility of six equidistant points located along the length of the urethra from the bladder neck to the external urethral meatus. The results were tested against symptoms and urodynamic findings.

Results

Stress urinary incontinence (SUI) and urodynamic stress incontinence (USI), but not urge incontinence, detrusor overactivity or voiding dysfunction, were strongly associated with mobility of the mid-urethra.

Conclusion

Impairment of mid-urethral fixation, rather than bladder neck fixation, seems important in the pathophysiology of SUI and USI. Copyright © 2010 ISUOG. Published by John Wiley & Sons, Ltd.

Introduction

Stress urinary incontinence (SUI) is an embarrassing condition that has a negative impact on women's health, both physically and mentally1, 2. It is the most common type of urinary incontinence in community-dwelling women3. To date, the pathophysiology of SUI is not fully understood. It seems likely that the etiology is multifactorial, involving a combination of pathologies affecting one or more of the bladder neck, urethra and pelvic floor muscles. While the relative importance of these mechanisms remains to be elucidated, it is generally believed that SUI occurs if urethral support is defective, impairing pressure transmission during increased abdominal pressure. This altered pressure transmission may result in a failure of the bladder neck or urethral sphincter to close4, 5. Current data in the literature, on urethral mobility as a measure of urethral support, focus mainly on the bladder neck6–8. There is minimal information available on the mobility of the remainder of the urethra, which we believe is important in understanding urethral support and the pathophysiology of SUI and urodynamic stress incontinence (USI).

The urethral motion profile (UMP) is a methodology that was recently developed to study urethral mobility9. The aims of this study were to determine the correlation between segmental urethral mobility and both lower urinary tract symptoms and urodynamic diagnosis. Our null hypothesis was that there is no statistically significant relationship between symptoms and urodynamic diagnosis of female urinary incontinence on the one hand and segmental urethral mobility on the other hand.

Methods

This was a retrospective, cross-sectional study spanning a 20-month period from December 2006 to July 2008. In total, 305 women complaining of symptoms of prolapse or lower urinary tract dysfunction underwent a routine assessment that included a standardized interview, clinical examination, multichannel urodynamic testing (Neomedix Acquidata; Neomedix, Sydney, Australia) and four-dimensional (4D) transperineal ultrasound examination. Symptoms of incontinence were rated positive if urinary leakage occurred at least once a month. Symptoms of voiding dysfunction included hesitancy, poor stream, stop–start voiding and straining to void. Prolapse grading was performed using the Pelvic Organ Prolapse Quantification System of the International Continence Society (ICS POP-Q)10. Urodynamic testing was evaluated according to the standardization document of the International Continence Society11. Urodynamic voiding dysfunction was defined as a residual of over 50 mL on free flowmetry and/or a maximum flow rate below the 10th centile on the Liverpool nomogram12.

Translabial ultrasound examination was performed with patients in the supine position, after bladder emptying, using a GE Voluson 730 Expert system (GE Medical Systems, Zipf, Austria) equipped with an 8–4-MHz curved-array volume transducer that had an acquisition angle of 85°, as previously described13. Volume acquisition was performed at rest and on maximum Valsalva maneuver. At least three Valsalva maneuvers were performed for each patient. Volume data of the maximal Valsalva maneuver (i.e. the Valsalva resulting in the greatest degree of pelvic organ descent) was used in the subsequent analysis of urethral mobility.

Of the original 305 women, 79 were excluded as they had undergone previous incontinence or prolapse procedures that were expected to impact on urethral mobility, leaving 226 women. In a further 28 cases, ultrasound volume data were unavailable either because of clerical error or data corruption, or because it was technically inadequate for UMP determination. The remaining 198 patients with complete UMP datasets were included in the subsequent analysis. The data were obtained in the context of studies approved by the local Human Research Ethics Committee.

UMP determination was performed on a desktop computer using the proprietary software 4D Sonoview, Version 5.0 (GE Medical Systems). The measurements were performed, at least 5 months after the patient's assessment, by operators blinded to all clinical and other ultrasound data. Our method of determining the UMP was shown to be highly reproducible in a previous study, with the intraclass correlation coefficients (ICCs) ranging between 0.73 and 0.97 for the coordinate measurements and mobility vectors9. To determine the UMP, the urethral length of each patient was manually traced on ultrasound images in the mid-sagittal plane, both at rest and on maximum Valsalva. This traced length was then divided into five segments with six equidistant points, termed Point 1 (bladder neck) to Point 6 (external urethral meatus), as shown in Figure 1. The horizontal distance (x coordinate) and the vertical distance (y coordinate) of the six points relative to the inferoposterior margin of the pubic symphysis were then determined, as illustrated by distances 7 and 8 in Figure 1, both at rest and on Valsalva. As the manual measurement of these coordinates is laborious, a semi-automatic method was developed using a macro written for use in Microsoft Excel that allowed automatic determination of the x and y coordinates, as well as the automatic division of the traced urethra into five segments on a bitmap imported from the 4D view. Urethral mobility was described by the vectors of movement from rest to maximum Valsalva of these six equidistant points using the formula SQRT ((xvalsalva− xrest)2 + (yvalsalva− yrest)2)9. A test–retest series conducted in another study to test the agreement between the manual method and the semi-automated program has shown excellent repeatability (ICC = 0.93; 95% CI, 0.90–0.96)14. The first author (A.P.) undertook all UMP measurements after training by the second author (K.L.S). A test–retest series of 10 UMPs (60 mobility vectors) between the two authors revealed excellent repeatability, with ICCs for x and y coordinates and for segmental urethral mobility of 0.95 (95% CI, 0.92–0.97), 0.98 (95% CI, 0.96–0.99) and 0.93 (95% CI, 0.90–0.96), respectively. Mobility vectors of the six points were tested against symptoms of SUI and the diagnosis of USI. The potential confounders of age, vaginal parity and prolapse were controlled for in a multivariable analysis.

Figure 1.

Translabial ultrasound image of a urethra at rest and on Valsalva. The manual trace shows the six equidistant points from the bladder neck (Point 1) to the external urethral meatus (Point 6). Points 7 and 8 demonstrate the method used to determine the x and y coordinates of Point 1. A, anal canal; BN, bladder neck; S, pubic symphysis; U, urethra; V, vagina.

Maximum urethral closure pressure (MUCP) is a measure of urethral sphincter function, which is believed to be important for the maintenance of SUI. Women with a low MUCP are more likely to have SUI15. In this study, the MUCP had been obtained in a subgroup of 74 women using fluid-filled catheters and manual pull-through. MUCP and the mobility vectors of each urethral segment were analyzed against both SUI and USI in a multivariable analysis, in order to account for any confounding effect of MUCP. All definitions are in agreement with the standardization document of the International Continence Society11, unless otherwise indicated.

Statistical evaluation was undertaken using SPSS 12 for Windows (SPSS Inc., Chicago, IL, USA), Minitab version 13 for PC (Minitab Inc., State College, PA, USA) and the Excel plug-in ‘Analyse it’ (Analyse-it Software, Leeds, UK). Data were tested for normal distribution using Kolmogorov–Smirnov analysis. All data were normally distributed unless indicated otherwise. Two-sample t-tests were used for statistical analyses.

Results

The mean age of the 198 women included in this study was 53.7 (range, 19–85) years. Ninety-two per cent of the women (n = 182) were vaginally parous, 81% (n = 160) presented with symptoms of SUI, 73% (n = 144) presented with urge incontinence and 23% (n = 45) presented with symptoms of voiding dysfunction. Thirty-nine per cent (n = 77) also complained of symptoms of female organ prolapse. On clinical assessment, significant female pelvic organ prolapse (ICS POP-Q Grade 2 or higher) was found in 59% (n = 118). There was a cystocele in 46% (n = 91), uterine prolapse in 5% (n = 10), a rectocele in 34% (n = 67) and an enterocele in 5.5% (n = 11).

Urodynamic testing was incomplete in two of the 198 cases; this was because of urethral stenosis in one case and compliance problems in the other. Therefore, 196 complete urodynamic datasets were available for analysis. Of the 196 women, 68% (n = 133), were diagnosed with USI, 24% (n = 47) with detrusor overactivity and 26% (n = 51) with voiding dysfunction. Free flowmetry was obtained for 163 women. The mean volume voided was 264 mL and the mean maximum flow rate centile was 33.5 (range, 0–98). The median residual was 0 (interquartile range, 0–20) mL. The mean MUCP in the subgroup analysis of 74 women was 36.3 mmHg (SD = 14).

All ultrasound data used in this analysis were normally distributed. The mean mobility vectors from the bladder neck to the external urethral meatus in women with and without SUI/USI are shown in Tables 1 and 2. There was a generalized increase in segmental urethral mobility in women with SUI. However, it reached significance only at Points 2, 3 and 4. The differences in the mean mobility vectors at these points were 0.377 (P = 0.014), 0.351 (P = 0.006) and 0.254 (P = 0.021) cm, respectively. On controlling for the potential confounders of age, parity and prolapse in a multivariable analysis, the relationships between SUI and segmental urethral mobility became nonsignificant for all vectors (P = 0.39 to P = 0.07). There was no significant relationship between segmental urethral mobility and urge incontinence (P = 0.30 to P = 0.59) or symptoms of voiding dysfunction (P = 0.27 to P = 0.45). USI was also found to be associated with a generalized increase in urethral mobility (all P < 0.02) most notably at the mid-urethra (P≤0.001). The differences in the mean mobility vectors measured at the six points along the urethra from Point 1 to Point 6 were 0.575 (P = 0.001), 0.516 (P≤0.001), 0.428 (P≤0.001), 0.341 (P≤0.001), 0.286 (P = 0.002) and 0.245 (P = 0.016) cm, respectively. On multivariable analysis (controlling for age, vaginal parity and prolapse) the relationship between USI and segmental urethral mobility weakened slightly but remained significant for all vectors (P = 0.039 to P = 0.001). The strongest relationships were again observed for the mid-urethra (Points 3 and 4). There was no significant relationship between urethral mobility and detrusor overactivity (all P > 0.10) or urodynamically diagnosed voiding dysfunction (all P > 0.25).

Table 1. Mean mobility vectors of Point 1 (bladder neck) to Point 6 (external urethral meatus) for women with and without stress urinary incontinence (SUI)
 Mobility vector (cm) 
PointSUI (mean ± SD)Non-SUI (mean ± SD)Difference between means (95% CI)P
13.01 ± 1.052.69 ± 1.04− 0.32 (−0.70 to 0.05)0.091
22.63 ± 0.882.25 ± 0.82− 0.38 (−0.68 to − 0.08)0.014
32.29 ± 0.731.94 ± 0.67− 0.35 (−0.60 to − 0.10)0.006
42.01 ± 0.631.75 ± 0.59− 0.26 (−0.47 to − 0.04)0.021
51.86 ± 0.611.75 ± 0.59− 0.16 (−0.38 to 0.05)0.124
61.89 ± 0.661.76 ± 0.65− 0.13 (−0.37 to 0.10)0.263
Table 2. Mean mobility vectors of Point 1 (bladder neck) to Point 6 (external urethral meatus) for women with and without urodynamic stress incontinence (USI)
 Mobility vector (cm) 
PointUSI (mean ± SD)Non-USI (mean ± SD)Difference between means (95% CI)P
13.14 ± 0.982.56 ± 1.09− 0.58 (−0.89 to − 0.25)0.001
22.72 ± 0.812.20 ± 0.88− 0.52 (−0.78 to − 0.26)≤ 0.001
32.36 ± 0.681.93 ± 0.73− 0.43 (−0.64 to − 0.21)≤ 0.001
42.07 ± 0.601.73 ± 0.62− 0.34 (−0.53 to − 0.16)≤ 0.001
51.93 ± 0.591.64 ± 0.60− 0.29 (−0.47 to − 0.11)0.002
61.95 ± 0.641.70 ± 0.67− 0.25 (−0.44 to − 0.05)0.016

A subgroup analysis was performed of 74 cases in whom MUCP had been obtained. MUCP was not shown to impact statistically on the relationship between segmental urethral mobility and symptoms of SUI or a urodynamic diagnosis of USI (P = 0.53).

Discussion

Urethral hypermobility, secondary to impairment of urethral support, is generally considered to be important in the pathophysiology of SUI and USI. While increased bladder neck mobility has been found to be associated with SUI15–17, little is known about the mobility of the rest of the urethra. In this study, combining multichannel urodynamics and ultrasound assessment of segmental urethral mobility, both symptoms of SUI and USI, were found to be associated with a generalized increase in urethral mobility, which is consistent with current assumptions regarding the pathophysiology of SUI and USI. However, symptoms of SUI were significantly associated with mid-urethral rather than bladder-neck mobility. Furthermore, while there was a significant association between bladder neck mobility and urodynamically diagnosed stress incontinence, the association was again strongest for the mid-urethra. It is hypothesized that this association may reflect an impairment of mid-urethral fixation and therefore pressure transmission.

To date, the structures responsible for urethral support remain controversial18. The posterior pubourethral ligament, the vagina and the endopelvic fascia, as well as the levator ani muscle, have been proposed as important urethral-supporting structures and therefore to contribute to the pathophysiology of SUI/USI4, 19–21. Lovegrove et al. have recently used two-dimensional (2D) ultrasound and motion tracking to describe urethral mobility during a cough, suggesting that reflex levator ani contraction contributes to urethral support22. However, another study on levator avulsion and UMP has challenged the importance of the levator ani muscle in supporting the urethra23. Future effort is clearly needed to resolve this issue, which appears to be more complex than previously assumed24. Our method of determining the UMP may be useful in future studies in this area.

Furthermore, the recent work on pelvic floor trajectory, pioneered by C. Constantinou's group at Stanford University, suggests that the timing of movement and deformation of pelvic floor structures in response to fast increases in intra-abdominal pressure may be important in understanding SUI25. Future studies on urethral trajectory using our semi-automated methodology may enable us to gain a better understanding of the importance of neuromuscular control in the pathophysiology of SUI.

Several limitations of this study must be acknowledged. First, this was a retrospective cross-sectional study in mostly Caucasian women attending for urodynamic testing. Thus, our results may not be applicable to the general population, or to other ethnic groups. Second, urethral sphincter function, believed to be important in the urinary continence mechanism, may confound the correlation between segmental urethral mobility and SUI/USI. Owing to the retrospective nature of this study, data on urethral closure pressure were incomplete. In a subgroup analysis of 74 women with available MUCP data, we were unable to demonstrate a confounding effect of MUCP on the relationship with segmental urethral mobility and SUI/USI. The lack of effect is probably due to the small sample size. Another potential confounder is voluntary or involuntary reflex activation of the levator ani during the Valsalva maneuver. This is common, especially in nulliparous women, and such levator co-activation is probably not an unusual cause of false-negative urodynamics test results26. We are unable to assess whether increased urethral mobility in parous women may be explained partly by altered pelvic floor muscle function rather than by changes to the ligamentous fixation of the urethra. Finally, the fact that we did not standardize Valsalva maneuvers may be another potential confounder. However, previous attempts to standardize Valsalva pressures have been largely unsuccessful27.

In conclusion, in this retrospective study combining multichannel urodynamic testing and translabial ultrasound, symptoms of SUI and the urodynamic diagnosis of USI were found to be associated with an increase in urethral mobility as defined by the UMP, particularly of the mid-urethra. An impairment of mid-urethral fixation, and therefore pressure transmission, may be important in the pathophysiology of this common form of urinary incontinence.

Acknowledgements

We would like to thank Miss W. Y. Kay for assisting us in the development of the semi-automated program.

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