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ORIGINAL RESEARCH |
From the Urogynecology Section, Department of Obstetrics and Gynecology, Methodist Hospital, Indiana School of Medicine, Indianapolis, Indiana; and the University of Louisville Health Science Center, Louisville, Kentucky.
Address reprint requests to: John R. Fischer, MD, 304 Volusia Avenue, Oakwood, OH 45409, E-mail: jfischer317{at}msn.com
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Methods: Twenty-three nulliparas, each less than 35 years old and without pelvic floor disorders, were recruited at Methodist Hospital in Indianapolis, Indiana, and the University of Louisville in Louisville, Kentucky. Each had concentric needle electromyography of their urethra to localize their striated urethral sphincter. Intraurethral ultrasound was used to identify the needle tip and layer in which it was found, examine the sonographic appearance of periurethral anatomy, and measure the thickness of hypoechoic and outer hyperechoic layers.
Results: Three layers were seen on intraurethral ultrasound: a mildly hyperechoic inner layer, a hypoechoic middle layer, and a hyperechoic outer layer. The concentric needle tip was seen in all subjects and showed motor unit action potentials when located in the outer hyperechoic layer. The mean thickness of the hypoechoic layer was 2.5 mm, and the mean thickness of the outer hyperechoic layer was 2.6 mm.
Conclusion: Motor unit action potentials showed that striated muscle was present in the outer hyperechoic layer on intraurethral ultrasound, implying that it contains the striated urethral sphincter.
Endoanal ultrasound enables clinicians to evaluate the structural anatomy of anal sphincters directly in the work-up of fecal incontinence. Characteristic findings in women are of an internal hypoechoic layer that corresponds to the smooth muscle of the internal anal sphincter and a hyperechoic layer that corresponds to the skeletal muscle of the external anal sphincter.1 The urethral sphincter has a similar histologic structure with an inner smooth-muscle layer and an outer skeletal muscle layer.2 There has been recent interest in using intraurethral ultrasound to evaluate urinary incontinence in women to correlate urethral sphincter morphology with function.3–5 The histologic similarities between urethral sphincter and anal sphincter imply that sonographic appearance should be comparable.5
Interpretations of the images obtained with intraurethral ultrasound have varied, making it difficult to know what is being measured. An oval hypoechoic structure has been reported as either the internal3 or external4 urethral sphincter. Others reported that the sonographic findings corresponded with those seen with endoanal ultrasound, an inner hypoechoic longitudinal smooth-muscle layer and an outer hyperechoic band made up of the circular smooth-muscle and striated muscle layers.5 Proper interpretation of images from intraurethral ultrasound requires understanding normal sonographic anatomy of the urethral sphincter.
Concentric needle electromyography uses a needle electrode to record motor unit action potentials of striated muscle. Those motor unit action potentials represent the summation of electrical activity of single muscle fiber action potentials from muscle fibers of a given motor unit that is innervated by a single anterior horn cell. For a motor unit action potential to be suitable for evaluation, it must have a rise time, measured as the time lag from the initial positive peak to the subsequent negative peak, of approximately 300–500 microseconds. A rise time of that magnitude confirms the needle tip is within 200–300 µm of the muscle fiber. Motor unit action potentials are not seen when electromyography needles are inserted into smooth muscle.
A previous study that evaluated intraurethral ultrasound in animals used a needle inserted near the urethra to aid identification of periurethral anatomy.6 The needle tip was seen as a strong reflection with distal acoustic shadowing. The purpose of the current study was to use concentric needle electromyography to localize the striated urethral sphincter by presence of motor unit action potentials, then to localize the needle tip using intraurethral ultrasound. That would allow definitive identification of the various layers seen in the periurethral anatomy with intraurethral ultrasound.
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Each subject had topical anesthetic (lidocaine 2.5%, and prilocaine 2.5%, Emla Cream; Astra, Westborough, MA) applied to the urethral meatus 20 minutes before the procedure. A Nicolet Viking IIe (Nicolet, Madison, WI) with an amplifier gain of 50 µV per division and a sweep speed of 10 milliseconds per division was used. A 1-in. 30-gauge concentric needle (Vickers Medical, Antwerp, Belgium) was inserted approximately 5 mm anterior to the urethral meatus to a depth of at least 20 mm to ensure placement beyond the urethrovaginal sphincter and compressor urethrae muscles. Audio output of the Nicolet was used to guide needle placement into the bulk of the urethral rhabdosphincter. Only motor unit action potentials with rise times under 500 microseconds were accepted. After localizing the urethral rhabdosphincter, the needle was left in place.
Intraurethral ultrasound was done using a 6.2-F, 12.5-MHz Sonicath intravascular ultrasound catheter (Microvasive; Boston Scientific Inc., Boston, MA) at both study sites. The commercially available polyethylene devices contain a miniaturized ultrasound transducer enclosed in acoustic housing at the distal end of the round-tipped catheter. A one-way valve at the end of the acoustic housing is punctured with a 27-gauge needle to fill it with 0.3 mL of sterile saline, which provides a good acoustic medium for ultrasound imaging. The catheter assembly houses a flexible drive that rotates the transducer when connected to the motored drive of the Sonicath interface, by which sound waves are transmitted radially, rendering a 360° real-time cross sectional image of the urethra that is offset 10° from perpendicular. The resolution of the image is approximately 0.1 mm with a depth of penetration of 1 to 2 cm. Catheters were connected to a Diasonics Model IVUS ultrasound scanning system (Diasonics, Milpitas, CA) in Indianapolis, and to a B&K Model 3535 ultrasound scanning system (U.S. Medical Co., Cincinnati, OH) with a Model 1880 Sonicath interface (U.S. Medical Co.) in Louisville.
With a subject in the lithotomy position, the ultrasound catheter was passed transurethrally into the bladder. The catheter was manually withdrawn, and the total length of the urethral sphincter was imaged to locate the needle and select the point of maximal sphincter thickness. To ensure that the ultrasound catheter did not move the needle from its place in the striated urethra muscle, motor unit action potentials were measured with the needle and ultrasound catheter in place. Ventral thickness of the hypoechoic and hyperechoic layers of the urethra were measured at the point of maximal urethral thickness.
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). After confirming the location of the needle in the outer hyperechoic layer, it was repositioned into the hypoechoic layer. No motor unit potentials were noted in that layer, implying that it is made of smooth muscle (Figure 2). The mean thickness of the hypoechoic layer was 2.5 mm, and the mean thickness of the outer hyperechoic layer was 2.6 mm. Histologic comparisons to sonographic findings showed that the inner hyperechoic layer corresponded to the mucosa and adventia of the urethra5; therefore, that layer was not measured.
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Schaer et al found that the longitudinal smooth-muscle layer corresponded to the internal hypoechoic layer, whereas the more irregular hyperechoic outer layer corresponded to the circular smooth and striated sphincter muscle layers.5 They were unable sonographically to show a well-defined border between circular smooth muscle and striated muscle. Our study had similar findings, with a well-defined internal hypo-echoic layer that did not show motor unit action potentials and was believed to be consistent with smooth muscle, and an outer hyperechoic layer that did show motor unit action potentials, indicating striated muscle fibers. We could not show separate longitudinal and circular smooth-muscle layers because our study did not include histologic analysis. Schaer et al found the mean thickness of the longitudinal smooth-muscle layer, including the mucosal layer and adventia, to be 2.64 mm. That was similar to the mean thickness of 2.5 mm of the hypoechoic layer in our study. We did not include the inner hyperechoic layer because it represents the mucosa of the urethra.3–5 Our measurement of the thickness of outer hyperechoic layers probably included striated and circular muscle layers.
Denervation has been implicated as a possible cause of urinary incontinence and other pelvic floor disorders.10 Denervation of a skeletal muscle can result in its atrophy, with subsequent decrease in size.11 In the striated urethral sphincter, that might manifest as a thinner hyperechoic layer on intraurethral ultrasound. It is unknown how smooth muscle of the urethral sphincter responds to similar denervation injury. Interruption of the nerve supply to smooth muscles can lead to supersensitivity of the muscle to agonist stimulation.12 How that would manifest in smooth muscle of the urethra is unknown.
Changes in thickness of the urethral sphincter measured by ultrasound might not be the only criterion to be considered when evaluating incontinent women. Histologic examination of striated urethral sphincter has shown considerable variation in density of striated muscle. In some specimens, striated muscle was replaced largely by fibrous tissue. Sonographic images from those areas remained hyperechoic despite lacking striated muscle.5 Women with intrinsic sphincter deficiencies have had increased fibrosis on urethral biopsy and fewer motor unit action potentials on concentric needle electromyography of the urethra,13 which implies that function might not correlate well with the area occupied by this echogenic layer,5 and simple sonographic evaluation of the urethral sphincter might not sufficiently determine the status of urethral function.
All the subjects in this study were young nulliparas without urinary incontinence or other pelvic floor disorders; therefore, it is doubtful that there was a significant amount of fibrosis in the urethrae of those imaged. Sonographic appearance of hyperechoic layers was the same and motor unit action potentials were easily located in all subjects. There was no evidence of increased fibrosis on concentric needle electromyography in any subject.
A limitation of this study was that intra- and inter-observer reliability of measuring thicknesses of hypo-echoic and hyperechoic layers of urethra was not determined. The original goals of the study were to describe the sonographic appearance of the periurethral anatomy in young, nulliparous, continent women and to correlate the sonographic appearance with results of concentric needle electromyography of the urethra.
Intraurethral ultrasound is potentially useful for evaluating urinary incontinence. It is easily done and well tolerated by women. Although data suggest there was a decrease in size of urethral sphincter in women with urinary incontinence,3,4 it is too early to advocate intraurethral ultrasound except as a research tool. Studies are needed to compare sonographic findings between continent and incontinent age-matched subjects and correlate sonographic findings with urodynamic testing. Optimal ultrasound frequencies and points of urethra measurement need to be determined. Image quality also needs improvement. Those variables can affect measurements and should be considered when comparing data between studies. Future studies also need to determine reliability of intraurethral ultrasound, as the cost of the catheters is prohibitive, and they are approved only for single use.
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Footnotes |
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The opinions and conclusions in this article are those of the author and do not represent the official position of the Department of Defense, United States Air Force, or any other government agency.
Received March 18, 1999. Received in revised form June 15, 1999. Accepted July 8, 1999.
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References |
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2. DeLancey JOL. Anatomy of the female bladder and urethra. In: Ostergard DR, Bent AE, ed. Urogynecology and urodynamics: Theory and practice. Baltimore, Maryland: Williams & Wilkins, 1996:3–18.
3. Klein HM, Kirschner-Hermanns R, Lagunilla J, Gunther RW. Assessment of incontinence with intraurethral US: Preliminary results. Radiology 1993;187:141–3.
4. Kirshner-Hermanns R, Klein HM, Muller U, Schafer W, Jakse G. Intraurethral ultrasound in women with stress incontinence. Br J Urol 1994;74:315–8.[Medline]
5. Schaer GN, Schmid T, Peschers U, Delancey JOL. Intraurethral ultrasound correlated with urethral histology. Obstet Gynecol 1998;91:60–4.[Abstract]
6. Goldberg BB, Liu JB, Merton DA, Kurtz AB. Endoluminal US: Experiments with nonvascular uses in animals. Radiology 1990; 175:39–43.
7. Colleselli K, Stenzl A, Eder R, Strasser H, Poisel S, Bartsch G. The female urethral sphincter: A morphological and topographical study. J Urol 1998;160:49–54.[Medline]
8. Borirakchanyavat S, Aboseif SR, Carroll PR, Tanagho EA, Lue TF. Continence mechanism of the isolated female urethra: An anatomical study of the intrapelvic somatic nerves. J Urol 1997;158:822–6.[Medline]
9. Strohbehn K, Quint LE, Prince MR, Wojno KJ, DeLancey JOL. Magnetic resonance imaging anatomy of the female urethra: A direct histologic comparison. Obstet Gynecol 1996;88:750–6.[Abstract]
10. Allen RE, Hosker GL, Smith ARB, Warrell DW. Pelvic floor damage and childbirth: A neurophysiological study. Br J Obstet Gyaecol 1990;97:770–9.
11. Dumitru D. Myopathies. In: Dumitru D, ed. Electrodiagnostic medicine. St. Louis, MO: CV Mosby, 1995:1031–129.
12. Brading AF. A myogenic basis for the overactive bladder. Urology 1997;50:57–67.[Medline]
13. Hale DS, Benson JT, Brubaker L, Heidkamp MC, Russell B. Histologic analysis of needle biopsy of urethral sphincter from normal and stress incontinent women with comparison of electromyographic findings. Am J Obstet Gynecol 1999;180:342–8.[Medline]
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  H. Major, P. Culligan, and M. Heit Urethral Sphincter Morphology in Women With Detrusor Instability Obstet. Gynecol., January 1, 2002; 99(1): 63 - 68. [Abstract] [Full Text] [PDF]
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