Ultrasound Obstet Gynecol 2004; 23: 272–276 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/uog.987
Correlating structure and function: three-dimensional ultrasound of the urethral sphincter D. ROBINSON*, P. TOOZS-HOBSON†, L. CARDOZO* and A. DIGESU* *Department of Urogynaecology, Kings College Hospital, London and †Department of Obstetrics and Gynaecology, Birmingham Women’s Hospital, Birmingham, UK
K E Y W O R D S: pregnancy; three-dimensional ultrasound; urethral pressure profile; urethral sphincter
ABSTRACT Objective Three-dimensional (3D) ultrasound facilitates assessment of the anatomy of the urethral sphincter, a decrease in sphincter volume being associated with stress incontinence. Urethral pressure profilometry (UPP) is used to assess urethral sphincter function although it is not diagnostic of stress incontinence. The aim of this pilot study was to determine whether 3D sphincter volume can be correlated with the area under the UPP curve. Methods Primigravid women were recruited antenatally between 32 and 42 weeks’ gestation. 3D transvaginal ultrasound scans of the urethra were performed in modified lithotomy position with a comfortably full bladder. Length and cross-sectional area were measured allowing the sphincter volume to be calculated. UPP measurements were performed with a Gaeltec solid-state pressure transducer and the area under the curve mapped out and calculated. All scans and measurements were repeated between 3 and 6 months postpartum. Results Nineteen women participated in the study. Antenatal and postnatal data were available in all cases. There was a statistically significant correlation between antenatal sphincter volumes and area under the UPP curve although this relationship was lost following delivery. Conclusions There appears to be a correlation between structural anatomy and functional anatomy in the antenatal period although following delivery this is lost. Copyright 2004 ISUOG. Published by John Wiley & Sons, Ltd.
INTRODUCTION Urinary incontinence, the ‘complaint of any involuntary leakage of urine’1 is a common and distressing condition that affects up to 30% of women following delivery2 , the
commonest cause being urodynamic stress incontinence3 . Whilst the pathogenesis of stress incontinence is multifactorial, bladder neck hypermobility and intrinsic sphincter deficiency have been postulated to be the most important contributory mechanisms4 . Three-dimensional (3D) ultrasound has previously been described in the assessment of ovarian pathology5 allowing improved accuracy in the estimation of cyst volume over that of conventional two-dimensional (2D) ultrasound. More recently 3D ultrasound has been described as an accurate and reproducible method of assessing urethral sphincter volume in women6 , a reduction in volume being associated with urodynamic stress incontinence7 . Although not accurate in the diagnosis of urodynamic stress incontinence, urethral pressure profilometry (UPP) remains useful in the assessment of the physiology of the urethral sphincter8 . Whilst this enables the measurement of intrinsic and extrinsic factors contributing to urethral closure pressure, it provides no information on the relative contributions of each of these components. Women with a ‘low pressure urethra’ have been found to have an increased risk of failure following continence surgery suggesting that the intrinsic urethral closure mechanism is important in maintaining continence9 . Whilst the precise etiology of urodynamic stress incontinence remains to be determined, we hypothesized that a reduction in urethral sphincter volume may be associated with a fall in intra-urethral pressure and this may contribute to the development of stress incontinence postpartum. Consequently the two null hypotheses for this study were as follows. (1) There is no correlation between 3D ultrasound measures of urethral anatomy and UPP measures of urethral function. (2) There is no difference between 3D ultrasound and UPP measurements performed in the third trimester and those obtained 3–6 months postpartum.
Correspondence to: Prof. L. Cardozo, 8 Devonshire Place, London W1G 6HP, UK (e-mail:
[email protected]) Accepted: 8 December 2003
Copyright 2004 ISUOG. Published by John Wiley & Sons, Ltd.
ORIGINAL PAPER
3D ultrasound of the urethral sphincter
273
METHODS This was a prospective observational pilot study. Primigravid women were recruited prospectively from the antenatal clinic during the third trimester (between 32 and 42 weeks’ gestation). All women were given written information concerning the study and consent was obtained. This study group represents a subgroup of those included in a previously published study6 . A 3D transvaginal ultrasound scan of the urethra was performed in all the women using a Kretz Combison 530 ultrasound machine (Kretztechnik, Zipf, Austria) with a 7.5-MHz mechanical sector probe. The scan image allows simultaneous visualization of sagittal, transverse and coronal sections and enables the image to be manipulated through 360◦ . The active plane can be altered and the image repositioned allowing analysis of a cross-sectional area. In addition the images can be rotated 360◦ in all three planes. If the urethral sphincter was visualized inadequately at first the scan was repeated. Satisfactory images were then stored using a SyQuest 88-Mb interchangeable hard disc drive storage system. Women were placed in a modified lithotomy position with a comfortably full bladder. In the sagittal plane the urethra was imaged along its entire length and care was taken not to distort the anatomy during the procedure. A 3D scan was then performed. The mechanical sector probe automatically swept through 140◦ recording 250 2D sections. These images were then computer-regenerated to form a 3D image (Figure 1). The sections allowed a cross-sectional image to be shown of the urethral endothelium and submucosal vascular plexus as a hyperechogenic central core10 . In the transverse plane the rhabdosphincter appeared as a hypoechogenic Sagittal plane
incomplete band around the central core. The sagittal image showed the bladder neck and the vagina with the rhabdosphincter image superiorly. The coronal section showed the rhabdosphincter laterally. The limits of the urethral sphincter were identified proximally and distally and the sphincter length recorded. The cross-sectional area of the total sphincter was serially traced recording each area from one end to the other using the roller ball on the ultrasound machine to calculate the area. When the entire sphincter had been traced a volume was computed using the supplied software. This process was repeated for the inner core of the sphincter. To calculate the volume of the rhabdosphincter the inner sphincter volume was subtracted from the total sphincter volume. UPP was performed in the supine position with the bladder comfortably full. A Gaeltec (Gaeltec Ltd, Dunvegan, Isle of Skye, UK) solid state 7 French microtransducer with two pressure transducers 6 cm apart was withdrawn at a rate of 2 mm/s and the subtracted urethral pressure profile recorded. Measurements were repeated, with the pressure transducers orientated at the 3 o’clock or 9 o’clock position, until the trace was reproducible and the maximum urethral closure pressure (MUCP) and functional urethral length (FUL) were recorded using a chart recorder (Figure 2). The area under the UPP curve was then mapped out and calculated. All investigations were initially performed in the third trimester of the antenatal period and then again at 3–6 months following delivery. Correlation of total sphincter volume and area under the UPP curve was then performed using the Pearson method (SPSS v10, SPSS, Chicago, IL, USA). This study was approved by the local ethics committee.
Transverse plane Pubic bone Urethral mucosa and internal sphincter
Striated muscle of rhabdosphincter
Coronal plane
Pyramid represents area scanned
Figure 1 Three-dimensional ultrasound scan of the urethral sphincter.
Copyright 2004 ISUOG. Published by John Wiley & Sons, Ltd.
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Robinson et al.
274 1.8
p Ves
1.6
p Ura MUCP
Area under the curve
ANAREA (cm2)
1.4 1.2 1.0 0.8 0.6
p Ves − p Ura
FUL
0.4
Figure 2 Urethral pressure profile. FUL, functional urethral length; MUCP, maximum urethral closure pressure; p Ves, bladder pressure; p Ura, urethral pressure; p Ves – p Ura, subtracted urethral pressure.
Parameter Mean total sphincter volume (cm3 ) Mean rhabdosphincter volume (cm3 ) Mean MUCP (cm H2 O) Mean FUL (cm)
Antenatal
Postnatal
2.22 1.15 65.16 1.85
2.03 1.01 59.16 1.78
FUL, functional urethral length; MUCP, maximum urethral closure pressure.
RESULTS In total 19 women were recruited to the study. All women were primigravid, the mean age being 30.9 years. Fifteen (79%) women had a vaginal delivery whilst four (21%) had a Cesarean section. Antenatal and postnatal assessments were performed in all cases, providing 19 sets of paired data (Table 1). Following delivery there was a reduction in mean sphincter volume and mean rhabdosphincter volume although this reduction was not significant. In addition there was also a reduction in mean MUCP and FUL. Correlation revealed a statistically significant correlation between antenatal total sphincter volumes and area under the UPP curve in the antenatal period (Figure 3) although this relationship was lost following delivery (Figure 4). These findings are summarized in Table 2.
DISCUSSION To our knowledge this is the first study to compare urethral anatomy, assessed using 3D ultrasound, with urethral function using UPP. 3D ultrasound has already been reported as an accurate and reproducible method of assessing the urethral sphincter6 by our group, and may be performed by the transvaginal or transrectal route11 . In this study all women were scanned transvaginally. Since 3D ultrasound allows measurements to be made in three planes rather than two using conventional ultrasound it
Copyright 2004 ISUOG. Published by John Wiley & Sons, Ltd.
1.4
1.6
1.8
2.0 2.2 2.4 ANTVOL (cm3)
2.6
2.8
3.0
Figure 3 Scatter plot showing the correlation of antenatal total sphincter volume (ANTVOL) and area under the urethral pressure profilometry curve (ANAREA).
1.8 1.6 1.4 PNAREA (cm2)
Table 1 Antenatal and postnatal urethral sphincter volumes and urethral pressure profilometry measurements
0.2 1.2
1.2 1.0 0.8 0.6 0.4 0.2 1.4
1.6
1.8
2.0 2.2 2.4 PNTVOL (cm3)
2.6
2.8
3.0
Figure 4 Scatter plot showing the correlation of postnatal total sphincter volume (PNTVOL) and area under the urethral pressure profilometry curve (PNAREA).
allows a more accurate measurement of sphincter volume and has also been shown to correlate with histology in cadaver specimens10 by our group. UPP remains the standard technique for measuring urethral sphincter function although it has limited clinical application. The urethral pressure profile is a direct measurement of pressure changes along the length of the urethra and as such is the sum of extrinsic and intrinsic urethral compression forces. To remain continent the MUCP must remain higher than intravesical pressure. At the time of voiding this relationship is reversed and micturition is initiated. In this study we postulated that, in continent nulliparous women, there may be a correlation between urethral sphincter volume and urethral pressure and that this relationship may change following delivery. In order to assess the pressure along the entire length of the urethra we used the area under the UPP curve rather than simply
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Table 2 Correlation of three-dimensional total urethral sphincter volume and area under the urethral pressure profilometry curve
Parameter
n
Mean
SD
Pearson coefficient
Antenatal UPP area (cm2 ) Antenatal volume (cm3 ) Postnatal UPP area (cm2 ) Postnatal volume (cm3 )
19 19 19 19
1.01 2.22 0.85 2.03
0.44 0.48 0.33 0.37
r = 0.508 P = 0.026 r = 0.062 P = 0.802
95% CI 0.07–0.78 − 0.40–0.50
UPP, urethral pressure profilometry.
measuring the pressure at one point, such as MUCP. The results show that there is a reduction in both mean urethral sphincter volume and MUCP following delivery. In the antenatal period there was a positive correlation between urethral pressure and urethral sphincter volume although following delivery this correlation was lost. Pregnancy and childbirth are known to affect the pelvic floor musculature as well as the continence mechanism. Changes in the anatomy of the pelvic floor have been demonstrated during pregnancy showing the bladder to lie lower with increased bladder neck opening and mobility12 . Connective tissue changes also occur during pregnancy and delivery leading to changes in the fascial support mechanisms of the bladder neck and urethra13 . In addition, hormonal factors may also play an important role in changes in the lower urinary tract during pregnancy and delivery. Estrogen and progesterone receptors have been demonstrated in the bladder, urethra and pelvic floor muscles14 , whilst progesterone has been shown to inhibit alpha-adrenergic activity in the bladder base and urethral sphincter15 . This may explain the findings of a reduced urethral closure pressure during pregnancy16 . MUCP and FUL have also been shown to be reduced following vaginal delivery in the early postpartum period although there was no correlation with duration of second stage, infant birth weight or episiotomy17 . However, a reduction in MUCP and FUL has been associated with the development of stress incontinence in the both antenatal and postpartum period18 . As well as changes occurring in late pregnancy, events at delivery also have a major impact on the pelvic floor musculature and lower urinary tract. Anatomical changes following delivery have been demonstrated using magnetic resonance scanning, showing a reduced levator muscle thickness, increased levator angle and increased levator hiatus in primiparous women19 . In addition to anatomical changes, muscle damage has also been reported following vaginal delivery with changes in muscle fiber diameter and fibrosis being reported in muscle biopsies from postmortem specimens20 . Whilst myogenic injury has been documented, neurogenic injury at delivery may also be important; 40–80% of women following vaginal delivery have prolongation of the pudendal nerve terminal motor latency21 . The implications of these findings are uncertain. Prolongation of terminal motor latency is not equivalent to denervation although it may be a sign of early neuromuscular damage. Further evidence is provided by electromyographic studies demonstrating
Copyright 2004 ISUOG. Published by John Wiley & Sons, Ltd.
re-innervation in the pelvic floor consistent with partial denervation in 80% of nulliparous women22 . Since this was only a small-scale pilot study we were unable to examine the effect of delivery by Cesarean section and whether this had any effect on the relationship between urethral sphincter volume and urethral pressure. The long-term outcome regarding continence following childbirth also could not be assessed in this relatively short-term follow-up study and further work may clarify the clinical relevance of urethral sphincter changes following delivery. In conclusion, our findings are supportive of previously reported studies examining the effect of pregnancy and childbirth on the lower urinary tract and pelvic floor. Following delivery the relationship between urethral sphincter volume and urethral pressure transmission is lost. Changes occurring at the time of delivery are most likely responsible for this effect; a decrease in urethral sphincter volume being in keeping with the concept of intrinsic sphincter deficiency and the development of urodynamic stress incontinence. Whilst our work does not offer an explanation for these changes it is probable that they are due to anatomical, myogenic, neurogenic and hormonal changes occurring at this time. Further studies investigating the effect of pregnancy and delivery on urethral sphincter volume and bladder neck mobility may help clarify the precise mechanism of postpartum urodynamic stress incontinence in addition to quantifying the relative contributions of intrinsic sphincter deficiency and urethral hypermobility.
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