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ORIGINAL RESEARCH |
From the Division of Gynecology, Department of Obstetrics and Gynecology, University of Vienna Medical School, Vienna, Austria.
Address reprint requests to: Wolfgang H. Umek, MD, University of Vienna Medical School, Department of Obstetrics and Gynecology, Division of Gynecology, Währinger Gürtel 18-20, A-1090 Vienna, Austria; E-mail: wolfgang.umek{at}univie.ac.at.
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ABSTRACT |
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METHODS: Twenty female patients with benign gynecologic disorders (mean age: 29 years; range: 19–40) had transrectal sonography using a 7.5-MHz mechanical sector endoprobe with three-dimensional features during both pelvic floor muscle relaxation and pelvic floor muscle contraction. The multiplanar display of the scanned volumes allowed detailed morphologic assessment of the urethra and the measurement of distances and volumes of the urethral components. Statistical end points were maximum sagittal and transverse urethral diameter, maximum sphincter length and thickness, maximum smooth muscle thickness, and the volumes of the sphincter, the smooth muscle, and the entire urethra.
RESULTS: All 20 rectal scans were feasible. Two patients had to be excluded from analysis because of poor image quality, leaving 18 patients for evaluation. When compared with pelvic floor relaxation, the following measures were smaller during pelvic floor contraction: sagittal urethral diameter (10.4 versus 11.5 mm; P = .004), transverse urethral diameter (14.1 versus 15.0 mm; P = .009), urethral sphincter thickness (2.4 versus 2.7 mm; P = .012), urethral sphincter volume (0.5 versus 0.6 mL; P = .003), and total urethral volumes (1.4 versus 1.5 mL; P = .007). Sphincter length and smooth muscle thickness, as well as smooth muscle volume, did not change significantly during pelvic floor contraction.
CONCLUSION: On three-dimensional ultrasound, the morphologic changes of the female urethra during pelvic floor contraction suggest external compression of the urethra rather than contraction of the sphincter muscle.
The urinary continence mechanism in women is a controversial issue.1,2 In a study examining the extrinsic continence mechanism, DeLancey described a sling of fibrous connective tissue underneath the urethra, which is attached to the levator ani and the arcus tendineus fasciae pelvis on both sides. This observation is paramount to the concept that during a pelvic floor muscle contraction the sling is pulled anteriorly toward the pubic symphysis, thereby compressing the urethra.3
In contrast to this concept, Bo et al described an intrinsic continence mechanism during pelvic floor muscle contraction in a combined electromyographic and pressure study. The authors in this study concluded that as part of the intrinsic mechanism the striated sphincter urethrae muscle contracts simultaneously with the pelvic floor and thereby increases the urethral closure pressure.4
Three-dimensional ultrasound has been used for the depiction and investigation of the lower urinary tract, in particular the female urethra.5–8 Three-dimensional ultrasound offers advantages over two-dimensional ultrasound because every plane including sagittal, transverse, and coronal views can be depicted within the scanned volume, and volumes can be measured accurately.9,10
We performed this three-dimensional ultrasound study to investigate how pelvic floor contractions influence the sonographic appearance of the female urethra.
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MATERIALS AND METHODS |
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. Each patient gave written informed consent to participate in the study.
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The patients were placed on an examination bed in the supine position with an empty bladder. A 7.5-MHz mechanical sector endoprobe with three-dimensional features (Combison 530D, Kretztechnik, Austria; probe diameter, 21 mm) was inserted into the rectum. For three-dimensional scans, the rotational scanning angle was a maximum of 120° perpendicular to the axis of the probe and 100° in the axial plane of the transducer (Figure 1).
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The hand-held ultrasound-transducer was kept steady, and patients were asked to lie still during the scan. Attention was paid to holding the transducer head loosely. The acquisition time for one volume data set was 2–5 seconds. Volumes could only be evaluated if women were able to maintain a pelvic floor contraction during the entire scan. Otherwise the scan was repeated. No contrast medium was used. Volumes were immediately composed and depicted in the multiplanar mode, which shows sagittal, transverse, and coronal slices simultaneously (Figure 2). The examination of one patient took approximately 15 minutes, and all scans were done by the same person (WU). Data were stored on 540-MB magnetic-optical discs for later evaluation.
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Based on studies that compared ultrasound appearance and histology of the urethra, we assumed the hyperechoic center to be urothelium, submucous vascular plexus, and smooth muscle (here called "smooth muscle complex") and the hypoechoic ring around it to be the striated sphincter urethrae muscle.6,11 We did not measure the length of the entire urethra because the external urethral meatus has no clearly defined appearance on ultrasound. However, the length of the urethral sphincter muscle could be measured accurately. All values of volumes refer to the part of the urethra that contains urethral sphincter. Only this part of the urethra was subject to the measurements described below. Variables included maximum sagittal and transverse urethral diameter, maximum sphincter length and thickness, maximum thickness of the smooth muscle complex, and the volumes of sphincter, smooth muscle complex, and the entire urethra. Figure 2 explains how these variables were measured.
Total urethral volumes were measured by outlining the cross-sectional area of the urethra in the transverse plane using a rollerball cursor. Up to ten of these cross-sections, 2 mm apart, were measured along the urethra’s axis over the length of the urethral sphincter muscle. The same technique was used to measure the volume of the smooth muscle complex. The sphincter volume was calculated by subtracting the volume of the smooth muscle complex from the total urethral volume.
All two-dimensional measures were exact to 1 mm; volumes were measured in milliliters.
Assessment of normal distribution was based on visual evaluation of box-plot-diagrams and Kolmogorov-Smirnov test. As a result, normal distribution was not refuted for any variable. However, it could not be distinguished whether this was owing to the sample size or to actual normal distribution. As a consequence, Wilcoxon signed rank test was used for all calculations. Statistical significance was accepted at P <. 05.
All analyses were performed using SPSS 8.0 Statistical Software (SPSS Inc., Chicago, IL).
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RESULTS |
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The urethra could be visualized in all women examined. On transverse planes, the urethra appeared as a round or oval-shaped structure. A central hyperechogenic core was surrounded by a hypoechogenic ring, the striated sphincter urethrae muscle (transverse view, Figure 2). Both the hyperechogenic core and the hypoechogenic ring appeared homogenous in all patients, although they differed in echogenicity individually. Anteriorly and posteriorly, the sphincter was thinner than laterally, and it was more difficult to differentiate it from the smooth muscle complex. On coronal planes (coronal view, Figure 2), the sphincter was seen as a parenthesis-like structure on both sides of the urethra, which extended from its distal part almost to the bladder neck.
All variables are presented in Table 2 as median, minimum, and maximum.
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DISCUSSION |
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This finding indicates a compression of the urethra by the paraurethral tissue rather than a contraction of the urethral sphincter itself. It is consistent with the studies of DeLancey, who demonstrated a paraurethral sling of connective tissue, which compresses the urethra against the symphysis pubis during pelvic floor contractions.3
Comparing the volumes of the urethra’s components between pelvic floor relaxation and contraction shows corresponding results. During pelvic floor contractions, the volume of the entire urethra decreases. This is owing to a decrease in sphincter muscle volume, whereas the smooth muscle complex does not change in volume. If the sphincter played an active part during pelvic floor contraction, we would expect thickening of the muscle layer rather than a decrease in size. Moreover, a visible effect of a contracting sphincter on the center of the urethra, as described by Frauscher et al, would cause decreases in size of the smooth muscle complex, which contains mucosa, submucosal vascular plexus, and smooth muscle.12 We did not observe such an effect.
Various two-dimensional techniques, including transvaginal,13 transrectal,14 transperineal,15 and intraurethral ultrasound11,12,16 have been used previously to investigate the urethral anatomy. Three-dimensional ultrasound has also been used previously and could distinguish between smooth muscle and striated sphincter muscle.6–8 Two of these studies used a transvaginal approach. In our own earlier study,8 we compared the transvaginal and the transrectal approach. We could demonstrate compression of the urethra under the symphysis by a vaginally applied ultrasound probe, which was consistent with the data for two-dimensional ultrasound studies.17 This is the reason we chose the transrectal approach in this study. Nevertheless, we cannot rule out an alteration of the urethra’s anatomy, even by a transrectally applied probe. In such a case we would, however, expect the probe to compress the sagittal more than the transverse urethral diameters. We could not demonstrate such an effect.
Patients’ ability to contract their pelvic floor muscles voluntarily was checked by visual inspection of the perineum during pelvic floor muscle contraction and by palpating the levator ani muscle. A recent study by Peschers et al18 showed that neither electromyography, pressure perineometry, nor perineal ultrasound selectively records pelvic floor muscle activity in general or pelvic floor muscle strength in particular better than digital palpation. To eliminate possible confounders, we excluded patients with an ongoing pregnancy, cancer, and previous pelvic floor surgery, as well as patients after menopause or with a history of urinary incontinence.
We did not verify the histologic nature of the components measured. Like other authors before us,6,7 we assumed the hyperechoic center to be mucosa, adventitia, submucosal vascular plexus, and smooth muscle. Accordingly, the well-distinguishable hypoechoic ring around the hyperechoic center represents striated sphincter urethrae.
Based on these assumptions, the morphologic changes of the female urethra during voluntary pelvic floor contractions as seen on three-dimensional ultrasound suggest compression of the urethra rather than contraction of the urethral sphincter muscle.
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Footnotes |
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Received January 10, 2002. Received in revised form April 17, 2002. Accepted May 13, 2002.
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REFERENCES |
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2. Thind P. The significance of smooth and striated muscles in the sphincter function of the urethra in healthy women. Neurourol Urodyn 1995;14:585–618.[Medline]
3. DeLancey JOL. Structural aspects of the extrinsic continence mechanism. Obstet Gynecol 1988;72:296–301.
4. Bo K, Stien R. Needle EMG registration of striated urethral wall and pelvic floor muscle activity patterns during cough, valsalva, abdominal, hip adductor, and gluteal muscle contractions in nulliparous healthy females. Neurourol Urodyn 1994;13:35–41.[Medline]
5. Khullar V, Salvatore S, Cardozo L, Abbott D, Hill S. Three-dimensional ultrasound of the urethra and urethral pressure profiles. Int Urogynecol J 1994;5:319.
6. Athanasiou S, Khullar V, Boos K, Salvatore S, Cardozo L. Imaging the urethral sphincter with three-dimensional ultrasound. Obstet Gynecol 1999;94:295–301.
7. Toozs-Hobson P, Khullar V, Cardozo L. Three-dimensional ultrasound: A novel technique for investigating the urethral sphincter in the third trimester of pregnancy. Ultrasound Obstet Gynecol 2001;17:421–4.[Medline]
8. Umek WH, Obermair A, Stutterecker D, Häusler G, Leodolter S, Hanzal E. Three-dimensional ultrasound of the female urethra: Comparing transvaginal and transrectal scanning. Ultrasound Obstet Gynecol 2001;17:425–30.[Medline]
9. Elliot TL, Downey DB, Tong S, McLean CA, Fenster A. Accuracy of prostate volume measurements in vitro using three-dimensional ultrasound. Acad Radiol 1996;3:401–6.[Medline]
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12. Frauscher F, Helweg G, Strasser H, Enna B, Klauser A, Knapp R, et al. Intraurethral ultrasound: Diagnostic evaluation of the striated urethral sphincter in incontinent females. Eur Radiol 1998;8:50–3.[Medline]
13. 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]
14. Kuo HC. Transrectal sonographic investigation of urethral and paraurethral structures in women with stress urinary incontinence. J Ultrasound Med 1998;17:311–20.[Abstract]
15. Kölbl 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]
16. Kirschner-Hermanns R, Klein HM, Müller U, Schäfer W, Jakse G. Intra-urethral ultrasound in women with stress incontinence. Br J Urol 1994;74:315–8.[Medline]
17. Wise BG, Burton G, Cutner A, Cardozo LD. Effect of vaginal ultrasound probe on lower urinary tract function. Br J Urol 1992;70:12–6.[Medline]
18. Peschers UM, Gingelmaier A, Jundt K, Leib B, Dimpfl T. Evaluation of pelvic floor muscle strength using four different techniques. Int Urogynecol J 2001;12:27–30.
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