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ORIGINAL RESEARCH |
From the Urogynaecology Unit, Department of Obstetrics and Gynaecology, King’s College Hospital, London, United Kingdom.
Address reprint requests to: Linda Cardozo, FRCOG Urogynaecology Unit, Department of Obstetrics and Gynaecology King’s College Hospital, Ruskin Wing Denmark Hill, 6th Floor SE5 9RS, London United Kingdom
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Abstract |
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Methods: Women with no previous continence or prolapse surgery had transvaginal sonography using a 7.5-MHz mechanical sector endoprobe with real-time and three-dimensional facilities. Three perpendicular planes were displayed simultaneously on the screen. Manipulation of the stored images allowed detailed morphologic assessment of the urethra at different levels using several parallel cross-sectional planes along its length. The length, thickness, and volume of the rhabdosphincter were measured.
Results: Forty-six women with genuine stress incontinence (27–73 years, mean 48) and 48 continent controls (23–76 years, mean 49) were studied. In the transverse plane, the inner part of the urethra, which consists of urothelium and smooth muscle, appeared hyperechogenic compared with the outer hypoechogenic ring of striated muscle. The two groups studied were not different in age and parity. Women with genuine stress incontinence, compared with continent controls, had a significantly shorter (mean ± standard deviation 16.9 ± 1.9 mm compared with 19.2 ± 3.6 mm; P = .001), thinner (2.1 ± 0.5 mm compared with 2.5 ± 0.4 mm; P < .001), and smaller volume (0.8 ± 0.2 mL compared with 1.2 ± 0.2 mm; P < .001) of the striated urethral sphincter. There was a close correlation between the urethral sphincter volume and the degree of incontinence assessed on videocystourethrography (r = -.65, P < .001).
Conclusion: Three-dimensional ultrasound allowed examination of the female urethra in planes that could not be visualized by conventional sonography. The rhabdosphincter had a smaller volume in women with genuine stress incontinence than in continent women.
Urinary incontinence in women is a common problem that causes social morbidity and reduced quality of life.1,2 Urodynamic diagnosis of lower urinary tract dysfunction helps improve the results of treatment and reduce postoperative failure of continence surgery. For the last 2 decades, the investigation of women with urinary incontinence has been based on pressure measurements and radiologic imaging.3 Although these are useful in diagnosing the type of urinary incontinence, the anatomic and pathologic etiologies for lower urinary tract and pelvic floor dysfunction remain poorly understood.
Recent enhancements in imaging have improved our ability to see anatomic structures that contribute to pelvic organ support and maintain urinary continence. In particular, ultrasound has been used to evaluate anatomy of the urethrovesical junction and mobility of the bladder neck.4–8 Various techniques with different positioning of patients, types of transducers, and modes of scanning have been advocated, but all have limitations. Transvaginal,6 transrectal,4 and transperineal5,7 approaches have been used to image the bladder base and urethra; however, pelvic anatomy and the retropubic position of the urethra significantly reduce the available ultrasound scanning planes, making evaluation of urethral anatomy difficult without substantial distortion of anatomic relations.
Recently introduced into clinical practice, three-dimensional ultrasound9,10 overcomes some of the limitations of conventional B-mode sonography and offers an unlimited number of ultrasound sections, even at planes perpendicular to the axis of the ultrasound beam.
This study was done to assess the feasibility and usefulness of three-dimensional, transvaginal ultrasound imaging of the urethra and periurethral tissue in women to compare measurements of the striated urethral sphincter in stress-incontinent and continent women.
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Materials and Methods |
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Women were placed in the supine position for trans-vaginal sonography of the urethra and periurethral tissues. The operator was masked to the results of the videocystourethrography. A 7.5-MHz mechanical sector endoprobe (Combison 530; Kretz Technik, Zipf, Austria) with real-time, B-mode, and three-dimensional facilities was inserted into the distal vagina. The scanning angle was 360° perpendicular to the axis of the probe and 100° in the axial plane of the transducer.
Two-dimensional ultrasonography was done initially to identify the area of interest and to estimate the size of the volume to be imaged. The urethra was viewed in the sagittal plane to ensure that its whole length from the bladder neck to the external urethral meatus was visible. The striated urethral sphincter (rhabdosphincter) was seen and identified easily. The ultrasound probe was held steady and each woman was asked to lie still on the examination couch. The volume mode was then switched on. Three-dimensional ultrasound volume was generated by the automatic rotation of the mechanical transducer through 140° (Figure 1A
). Up to 250 ultrasound slices were recorded, digitized, and stored on a removable hard disk. Any structure within the stored volume could be viewed and measured immediately or later.
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). The other two planes were used to ensure that the whole of the striated urethral sphincter was included in the measurements. Up to 15 parallel sections (about 1–2 mm apart) were taken in the transverse plane. The striated sphincter volume was calculated by subtracting the volumes of the smooth muscle and urothelium from the total urethral volume. Reliability was assessed by examining intraobserver variance in 14 women and interobserver agreement in 14 other subjects.12 The women were chosen from both groups. Comparisons between groups were done using Student t test (SPSS Inc., Chicago, IL). Correlation was tested using the Spearman rank test. Data were expressed as mean, standard deviation (SD), minimum, and maximum.
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Results |
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Using three-dimensional ultrasonography, we were able to visualize the urethra, the striated urethral sphincter (rhabdosphincter), and the periurethral tissues in all women examined. The examinations were well tolerated by all subjects. In the transverse plane, the middle portion of the urethra looked like a target (Figure 2). The inner part, which consists of the urothelium and smooth muscle, appeared hyperechogenic compared with the outer hypoechogenic ring that represents the striated muscle. The rhabdosphincter was thicker on the ventral and lateral aspects and thin or absent dorsally. The echogenicity of the rhabdosphincter was increased in older women compared with younger ones. Hyperechogenic areas within the hypoechoic muscle were visible in six incontinent women and one control subject. The striated urethral sphincter was seen clearly in the coronal and sagittal planes as a hypoechoic area around the urethra running along its midportion (Figure 3).
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), the hyperechogenic urothelium was surrounded by detrusor muscle fibers that could be seen as an irregular hypoechogenic ring. Moving more distally at the midurethra (Figure 5), the rhabdosphincter was clearly shown surrounding the circular and longitudinal smooth muscle and urothelium. More distally (Figure 6), the urethral circumference progressively diminished, mainly because of a reduction of the smooth-muscle component, not thinning of the striated muscle. Therefore, smooth rather than striated muscle contributes to the oval shape of the urethra, an interesting finding that was consistent and contradicted the concept of an oval rhabdosphincter. In some cases, the striated urethral muscle was thinner in the middle and thicker at its distal end. The retropubic space, which contains periurethral fascia and blood vessels, was imaged as a hyperechogenic nonhomogeneous structure anterior to the urethra (Figure 2).
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and 2). The t test for paired samples was used for statistical analysis. Mean differences between the assessments were almost zero, with narrow limits of agreement for all measurements (urethral sphincter length, thickness, and volume).
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). There was no overlap between the groups for the urethral sphincter volume (95% CIs). There was a good correlation between the urethral sphincter volume and the severity of incontinence assessed by videocystourethrography (r =-.65, P , .001).
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Discussion |
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The ultrasonographic pictures we took correlate well with known anatomy. Histomorphologic studies have shown the female urethra to be a multilayered tube.13–15 The submucosa and the urothelium lie in the middle surrounded by an inner, longitudinally arranged layer of smooth muscle, which in turn is surrounded by a thin outer layer of circular smooth muscle. The outermost layer, the striated urethral muscle (rhabdosphincter), consists of striated muscle fibers and is believed to have an important effect on maintaining urinary continence. Using ultrasound in the transverse plane, the midurethra looks like a target. The hypoechoic outer ring corresponds to the striated urethral sphincter and is thicker on the ventral and lateral aspects and thinner dorsally. The smooth muscle and the submucosa appear hyperechogenic.
The echogenicity of different parts of the urethra can be explained by the orientation of collagen and muscle fibers relative to the direction of the ultrasound beam. When the collagen fibers run parallel to the ultrasound beam, the tissue appears hypoechoic, whereas the fibers that are perpendicular to the ultrasound beam appear echogenic. Using the transvaginal approach, fibers of the rhabdosphincter are parallel to the ultrasound waves and lateral to the urethra, so the rhabdosphincter appears hypoechoic in those places. The smooth-muscle fibers run along the urethra, perpendicular to the incident ultrasound waves, so they appear more echogenic. The echogenicity of different parts of the urethra and urethral sphincter has been investigated by scanning cadavers with the transperineal and transvaginal approaches. Tissues have been measured histologically to correlate the tissue measurements and the images. The transperineal approach imaged the longitudinal fibers and core of the urethra as hypoechoic and the surrounding rhabdosphincter as hyperechoic. Using the transvaginal approach, the longitudinal fibers appear hyperechoic and the rhabdosphincter laterally appears hypoechoic, the exact opposite image of the transperineal approach.
Our findings indicated that the rhabdosphincter is thicker in its distal third, which concurs with histologic findings in cadaveric urethras. However, we have been unable to view the morphology of the distal portions of the urethral sphincter (ie, the compressor urethrae muscle and the urethrovaginal sphincter muscle), which is probably because those two structures are composed of muscle fibers that run perpendicular to the ultrasound beam and appear hyperechogenic like the other periurethral tissues. The bulkiness of the urethra in its middle third seems to be related to the width of the smooth muscle and the submucosa layers, rather than the striated urethral sphincter.
The echogenicity of the rhabdosphincter varied among women. The rhabdosphincter was usually more echogenic in older women than in younger women. Although this observation is subjective because echogenicity is difficult to quantify objectively, one could hypothesize that this finding was secondary to increases in collagen and decreases in the proportion of striated muscle fibers within the rhabdosphincter with increasing age.13,14,16
Hyperechogenic spots within the hypoechoic rhabdosphincter were also visible in some women. It is impossible to know whether these areas represent scarring from previous trauma or variations of normal anatomy. Concentric-needle electromyography of the urethral sphincter under ultrasound control has enabled us to detect the characteristic striated-muscle-fiber electric activity within the hypoechoic area of the rhabdosphincter. We are currently using the same technique to assess whether those hyperechogenic areas visible in the urethras of some women are electromyo-graphically silent.
Urethral anatomy is difficult to evaluate by ultrasonography regardless of the technique. The ultrasound beam of the transvaginal and transperineal approaches is almost parallel to the urethral axis, and transverse planes cannot be obtained without substantial distortion of anatomy. Most studies have shown that ultrasound can be useful in the evaluation of bladder-neck mobility in urinary incontinence.4,6–8 Using transabdominal sonography, Leonor de Gonzalez et al17 saw a hypoechoic rounded or ovoid structure below the bladder base in women and suggested that it resembled the male prostate and corresponded to the urethral sphincter. Recently, intraurethral ultrasound using high-frequency probes has been used for assessment of the urethral sphincter.18,19 The maximal cross-sectional area of the striated urethral sphincter was significantly smaller in incontinent women than in continent controls, but clear visualization of that structure was possible in only 60–75% of cases. The disadvantage of this approach is that the imaging plane cannot be determined and the penetration of ultrasound is limited to 1–2 cm because of the higher frequency. No information can be obtained about the anatomy of the periurethral tissues using this technique.
The advantage of the three-dimensional technique compared with B-mode imaging is that sonographic sections can be obtained easily by viewing the saved volume using the controls of the keyboard, without changing the position of the probe during examination and thus avoiding distortion of anatomy. Three-dimensional ultrasonography also allows volume measurements of irregularly shaped structures. One can be certain of the level at which each section was taken by displaying the three perpendicular planes of the urethra simultaneously on the screen, ensuring accurate and reproducible measurements.
Planar sections of the female urethra can also be obtained with magnetic resonance imaging.20–22 The comparative advantages of three-dimensional ultrasonography are its low cost, convenience of ultrasound, and ease with which unlimited planar reformatted sections can be obtained even after the woman has left the clinic. Dynamic imaging can be done with conventional ultrasonography before scanning tissue volumes, providing full assessment of the static and functional anatomy of the lower urinary tract.
Ultrasound measurements of the rhabdosphincter are reproducible, with satisfactory intra- and interobserver variance. Our results showed that women with genuine stress incontinence had smaller, thinner, and shorter rhabdosphincters, and these measurements correlated with the degree of incontinence. Although the exact mechanism by which the urethral sphincter contributes to urinary continence is not fully understood, our data suggest that the size of this structure may have an important effect on maintaining urinary continence, and urethral sphincter volumes may be of value for predicting the outcome of incontinence surgery.
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Footnotes |
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Received July 9, 1998. Received in revised form January 20, 1999. Accepted January 28, 1999.
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References |
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2. Kelleher CJ, Cardozo LD, Khullar V, Salvatore S. A new questionnaire to assess the quality of life of urinary incontinent women. Br J Obstet Gynaecol 1997;104:1374–9.[Medline]
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6. Quinn MJ, Beynon J, Mortensen NJ, Smith PJ. Transvaginal endosonography: A new method to study the anatomy of the lower urinary tract in urinary stress incontinence. Br J Urol 1988;62: 414–8.[Medline]
7. Kolbl H, Bernaschek G, Wolf G. A comparative study of perineal ultrasound scanning and urethrocystography in patients with genuine stress incontinence. Arch Gynecol Obstet 1988;244:39–45.[Medline]
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18. Klein HM, Kirschner-Hermanns R, Lagunilla J, Gunther RW. Assessment of incontinence with intraurethral US: Preliminary results. Radiology 1993;187:141–3.
19. Kirschner-Hermanns R, Klein HM, Muller U, Schafer W, Jakse G. Intra-urethral ultrasound in women with stress incontinence. Br J Urol 1994;74:315–8.[Medline]
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22. Kirschner-Hermanns R, Wein B, Niehaus S, Schaefer W, Jakse G. The contribution of magnetic resonance imaging of the pelvic floor to the understanding of urinary incontinence. Br J Urol 1993;72: 715–8.[Medline]
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