Does vaginal closure force differ in the supine and standing positions?

American Journal of Obstetrics and Gynecology (2005) 192, 1722 8 www.ajog.org Does vaginal closure force differ in the supine and standing positions? Daniel M. Morgan, MD, a, * Gurpreet Kaur, BS, a Yvonne Hsu, MD, a Dee E. Fenner, MD, a Kenneth Guire, MS, a Janis Miller, PhD, b James A. Ashton-Miller, PhD, c John O. L. Delancey, MD a Department of Obstetrics and Gynecology, Pelvic Floor Research Group, a School of Nursing, b Departments of Mechanical Engineering and Biomechanical Engineering, and Institute of Gerontology, c University of Michigan Medical School, Ann Arbor, Mich KEY WORDS Pelvic floor Posture Levator ani Vaginal closure force Objective: This study was undertaken to quantify resting vaginal closure force (VCF REST ), maximum vaginal closure force (VCF MAX ), and augmentation of vaginal closure force augmentation (VCF AUG ) when supine and standing and to determine whether the change in intra-abdominal pressure associated with change in posture accounts for differences in VCF. Study design: Thirty-nine asymptomatic, continent women were recruited to determine, when supine and standing, the vaginal closure force (eg, the force closing the vagina in the mid-sagittal plane) and bladder pressures at rest and at maximal voluntary contraction. VCF was measured with an instrumented vaginal speculum and bladder pressure was determined with a microtip catheter. VCF REST was the resting pelvic floor tone, and VCF MAX was the peak pelvic floor force during a maximal voluntary contraction. VCF AUG was the difference between VCF MAX and VCF REST. T tests and Pearson correlation coefficients were used for analysis. Results: VCF REST when supine was 3.6 G 0.8 N and when standing was 6.9 G 1.5 Nda 92% difference (P!.001). The VCF MAX when supine was 7.5 G 2.9 N and when standing was 10.1 G 2.4 Nda 35% difference (P!.001). Bladder pressure when supine (10.5 G 4.7 cm H 2 O) was significantly less (P!.001) than when standing (31.0 G 6.4 cm H 2 O). The differences in bladder pressure when either supine or standing did not correlate with the corresponding differences in VCF at rest or at maximal voluntary contraction. The supine VCF AUG of 3.9 G 2.7 N, was significantly greater than the standing VCF AUG of 3.3G1.9 N. Conclusion: With change in posture, vaginal closure force increases because of higher intraabdominal pressure and greater resistance in the pelvic floor muscles. Ó 2005 Elsevier Inc. All rights reserved. Support by the National Institutes of Health grant NICHD R01 HD 38665, P50 HD44406, R01HD38665-05. Presented at the Joint Scientific Meeting of the Society of Gynecologic Surgeons and the American Urogynecologic Society, San Diego, Calif, July 29-31, 2004. * Reprint requests: Daniel M Morgan, MD, University of Michigan, Department of Obstetrics and Gynecology, Women s Hospital L4000, 1500 East Medical Center Dr, Ann Arbor, MI 48109. E-mail: morgand@umich.edu Stress urinary incontinence and pelvic organ prolapse can be highly morbid conditions with a significant impact on quality of life. The pathophysiology of symptomatic disease is not well understood, but the upright posture of humans is generally considered a contributing factor. The pelvic floor soft tissues close the genital hiatus (GH), support the pelvic viscera, and play an important role in bladder and bowel continence. 0002-9378/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.ajog.2004.11.050

Morgan et al 1723 The levator ani muscles, which are an important component of this soft tissue, exhibit constant activity 1,2 and have evolved to meet the increased support needed with upright posture. To visualize the perineum and vagina optimally, examinations are usually performed with patients in lithotomy position. Although this approach is understandable, it may not adequately reflect the physiologic function of the pelvic floor soft tissues. Vaginal closure force (VCF), which is that force exerted by the pelvic floor soft tissues to close the vaginal opening, can be quantified using an instrumented speculum. 3-5 In previous work, our research group has shown that the VCF is correlated with levator ani muscle damage seen on magnetic resonance imaging scans. 6 The relationship between posture and vaginal closure pressures has been studied by other investigators who used a balloon device, 7,8 but with this methodology, it has not been possible to differentiate intraabdominal from vaginal pressure. The objectives of this study were to determine posture-related differences in resting vaginal closure force (VCF REST ), maximum vaginal closure force (VCF MAX ), and augmentation of vaginal closure force (VCF AUG) and to explore whether the change in intra-abdominal pressure is related to differences in VCF. Methods Recruitment and protocol A convenience sample of 39 asymptomatic, continent women was recruited through 3 Institutional Review Board approved projects. Information on age, body mass index (BMI), and obstetric history were collected. A pelvic examination, a cystometrogram, and a full bladder stress test were performed. GH was measured from the mid-urethra to perineal body. Women were excluded if the vaginal wall descended below the hymen with valsalva or if they experienced incontinence caused by a detrusor contraction, a cough, or a valsalva effort. An 8F micro-tip dual sensor (Gaeltec Ltd, Isle of Skye, Scotland) was placed in the bladder to monitor pressure during filling with 300 ml of normal saline solution and was used throughout testing as a surrogate for intra-abdominal pressure. The subject was instructed in pelvic floor muscle contraction until satisfactory performance of a contraction was demonstrable. The upward motion of the perineum and vaginal palpation of the levator ani muscle were used by the examiner to confirm proper technique. If pelvic floor muscle contraction caused the bladder pressure to rise above its baseline, patients were reinstructed on technique and advised to avoid use of the abdominal and gluteal muscles. Figure 1 Photograph of speculums. The instrumented vaginal speculum (top) and a standard vaginal speculum are shown. The depth of insertion was 7 cm, similar to a standard speculum, and the cross-sectional area of the anterior bill was 18.8 cm 2. An instrumented vaginal speculum (Figure 1) was used to measure VCF in newtons. Similar to a standard speculum with respect to size and shape, the 2- billed instrumented vaginal speculum is equipped at the handle with a strain gauge that sums the forces on the anterior and posterior bills. VCF was measured at rest (VCF REST ) and during maximum voluntary contraction (VCF MAX ). Augmentation of VCF (VCF AUG ) was the difference of VCF MAX and VCF REST (Figure 2). The subject voluntarily contracted the pelvic floor muscles 3 times when supine and when standing. A trial was repeated if intravesical pressure increased and if it seemed that an improvement in technique was possible. Values are expressed as the mean and SD. Statistics Two-sided paired t tests were performed to compare the means of VCF and bladder pressure when supine and standing. Pearson correlation coefficients were used to evaluate the relationship of age, BMI, parity, most dependent point of vaginal wall support, and GH with VCF REST, VCF MAX, and VCF AUG when supine and standing. A level of P!.05 was considered significant. Results Subjects Mean age (GSD) was 45.8 G 9.5 years, BMI 28.7 G 4.9 kg/m 2, and parity 2.2 G 1.3. Mean GH was 2.7 G 0.8 cm. Thirty-two women had delivered by vaginal delivery only, 3 women had delivered by cesarean section only, and 3 women had delivered by both vaginal and cesarean delivery. One woman was nulliparous. There

1724 Morgan et al Figure 2 Example of a vaginal closure force tracing. A trial in the supine and standing position is shown to illustrate how a tracing is analyzed to determine VCF REST, VCF MAX, and VCF AUG. Bolded vertical lines before pelvic muscle contraction indicate when the computer is available for data capture and do not indicate the latency between instruction to contract and time of contraction. Table Mean VCF when supine and standing VCF REST VCF MAX VCF AUG Supine 3.6 G 0.8 7.5 G 2.9 3.9 G 2.7 Standing 6.9 G 1.5 10.1 G 2.4 3.3 G 1.9 Arithmetic 3.3 2.6 0.6 difference (N) Percent 92 35 18 difference (%) Statistical significance P!.0001 P!.0001 P =.009 was a trend, which was not statistically significant, between increasing parity and GH. VCFs and bladder pressures Standing VCF REST was 92% greater than supine VCF REST and standing VCF MAX was 35% greater than supine VCF MAX (Table and Figure 3). Bladder pressure when standing (31.0 G 6.4 cm H 2 O) was significantly greater (P!.001) than when supine (10.5 G 4.7 cm H 2 O). The difference in resting bladder pressure when standing and supine did not correlate with the corresponding difference in VCF (r = 0.1, P =.58, Figure 4). The difference in bladder pressure at maximal voluntary contraction when supine and standing also did not correlate with the corresponding difference in VCF (r = 0.01, P = 0.93). The mean difference in bladder pressure during a voluntary pelvic floor muscle contraction was 1.7 G 2.5 cm H 2 O when supine and 7.5 G 5.1 cm H 2 O when standing. Figure 5 illustrates that almost all subjects when standing had more difficulty contracting the pelvic floor muscles without a concomitant increase in intra-abdominal Figure 3 Mean VCF REST and mean VCF MAX in the supine and standing positions. There was a 92% difference (P!.001) between supine and standing VCF REST and a 35% difference (P!.001) between supine and standing VCF MAX. pressure. VCF AUG when supine was 18% greater (P =.009) than when standing (Table). Factors possibly influencing VCF Factors that could influence VCF were evaluated. Supine VCF REST was inversely correlated with genital hiatus (r = 0.36, P =.02) but not with age, BMI, parity, or most dependent point of vaginal wall support. Standing VCF REST was positively correlated with BMI (r = 0.36, P =.02) but not with age, parity, most dependent point of vaginal wall support, or GH. There were no statistically significant correlations between supine VCF MAX, standing VCF MAX, supine VCF AUG, or standing VCF AUG and age, BMI, parity, most dependent point of vaginal wall support, or GH.

Morgan et al 1725 Figure 4 Difference in bladder pressure and VCF associated with change in the supine and standing position. The slope of the regression line demonstrates that there is a weak correlation between these 2 factors. Comment This study revealed a 92% increase in VCF REST when a woman moves from the supine to the standing position. What might have caused this? An increase in intra-abdominal pressure or greater resistance from the pelvic floor muscles are 2 potential factors. The simultaneous measurement of these 2 measures allows the relative contribution of increased intra-abdominal pressure on VCF to be explored. The lack of a significant correlation between the differences of them when supine and standing suggests that an increase in intra-abdominal pressure does not entirely account for the increase in VCF. The design of the instrumented vaginal specula and the definition of the term VCF have implications regarding what is measured. VCF is an assessment of the resultant force acting normal to the vaginal axis in the midsagittal plane. VCF REST consists of 3 components: 1. The passive force on both the anterior and posterior bills from the circumferential stretch of the fibromuscular tissue of the vagina and pelvic floor muscles on insertion of the speculum; 2. The superiorly directed force on the posterior bill of the levator ani muscle resisting intra-abdominal pressure; and 3. The inferiorly directed force on the anterior bill from the intra-abdominal pressure acting perpendicular to the speculum s longitudinal axis on the cross-sectional area and from the presence of the pubic symphysis. The force related to intra-abdominal pressure on the anterior bill results from the hydrostatic pressure caused

1726 Morgan et al Figure 5 Change in bladder pressure with maximal voluntary contraction when (A) supine and (B) standing. When supine, subjects were able to isolate the pelvic floor muscles and avoid increasing intra-abdominal pressure. When standing, they had difficulty contracting the pelvic floor muscles without increasing intra-abdominal pressure. by the weight of the superincumbent intra-abdominal contents and intra-abdominal pressurization caused by abdominal muscle contraction. Unlike the balloon pressure devices, the speculum is insensitive to pressures acting in the mediolateral direction or craniocaudally along the axis of the vagina. Our findings of increased intra-abdominal pressure with change in posture are consistent with previous studies. The mean resting intra-abdominal pressure we reported was 31 cm H 2 O, which is similar to the findings of Henriksson et al 9 and Iosif et al. 10 When standing, higher intravesical pressures occur for 2 reasons: (1) the height of the superincumbent intraabdominal contents (eg, the height of the column of water) above the bladder is increased and (2) in comparison to supine positioning, abdominal muscle tone increases, 11 leading to additional intra-abdominal pressurization. It is instructive to estimate the contributions to VCF when supine and standing. The force that is due to supine intra-abdominal pressure can be calculated by converting the measured intravesical pressure to units of N/cm 2 and then multiplying the result by the crosssectional area of the anterior bill of the speculum: 10 cm H 2 O = (0.1 N/cm 2 )(18.8 cm 2 ) = 1.9 N This suggests that 1.9 N of the 3.6 N of supine VCF REST is an inferiorly directed force on the anterior bill caused by intra-abdominal pressure and the remaining 1.7 N is due to the passive tissue stretch and the presence of the pubic symphysis. Because the speculum is in equilibrium, the sum of the forces on the 2 bills must be equal. Therefore, the pelvic floor muscles provide a superiorly directed force on the posterior bill of 3.6 N as they hold the speculum against the pubic symphysis and resist the action of intra-abdominal pressure. Now, what happens when standing? We can again estimate the

Morgan et al 1727 contribution of intra-abdominal pressure to the standing VCF REST if we multiply the intra-abdominal pressure by the area of the anterior blade: 31 cm H 2 O = (0.31 N/cm 2 )(18.8 cm 2 ) = 5.6 N This means that 5.6 N of the 6.9 N of average standing VCF REST is the inferiorly directed force of intra-abdominal pressure on the anterior blade. The remaining 1.3 N is due to the passive forces of tissue stretch and the presence of the pubic symphysis. The superiorly directly force from the pelvic floor muscles is now 6.9 N. The weak correlation between the difference in supine and standing VCF REST and bladder pressure can be explained by interindividual variability in the recruitment and tone of the abdominal and pelvic floor muscles. However, the fact that VCF AUG was 3 to 4 N in the supine and standing positions suggests that the maximal volitional increase in the pelvic muscle force elevating the speculum is independent of body orientation and that the instrumented speculum captures the increase of VCF in the sagittal plane. The closure of the vagina with change in posture has been evaluated by previous investigators. By using vaginal pressure transducers, both Bo and Finkenhagen 7 and Miller 8 found differences between resting supine and standing measures. However, their findings at maximum voluntary contraction with respect to vaginal closure pressure were not consistent. With change in posture, Bo and Finkenhagen 7 found a difference in intravaginal pressure at maximum voluntary contraction but Miller 8 did not. It is not clear if this discrepancy could be due to the methodology used or differences in the population of women studied (eg, Bo and Finkenhagen 7 only studied women who were stress incontinent and Miller 8 studied both asymptomatic and stress incontinent women). The associations of clinical factors with standing VCF REST and supine VCF REST deserve further investigation. It was not possible to determine how parity and route of delivery affected VCF with this sample size and the distribution of obstetric events. The importance of the correlation between higher BMI and standing VCF REST is interesting given the known influence of obesity on symptoms of urinary incontinence. The clinical significance of each of these findings will need further examination. This study has certain limitations. The number of neuromuscular units recruited, the presence/absence of neurologic damage, and the role of muscle fatigue cannot be quantified with this technique. Supplemental studies with electromyography may help clarify these issues but this technique too has limitations. Electromyography can only be used to study intact muscle and it has been shown that pelvic floor muscle can be missing. 12,13 Multiple techniques of examination may be needed to better understand pelvic floor structure and function. Second, the most effective technique of pelvic floor contraction may involve coactivation of the abdominal muscles. 14 When standing, women tended to tighten their abdominal muscles during contraction, even if they had demonstrated excellent technique when supine. Third, the sample of subjects should be considered. Although this cohort was asymptomatic and continent, they were also parous and may have some underlying, subclinical neuromuscular damage. 15 The BMI is also high and may not be generalized to all populations. The levator ani muscles close the urogenital hiatus and supports the abdominopelvic organs. Injury to them can result in increased risk for pelvic organ prolapse and stress urinary incontinence. 16,17 Physiologic differences in levator ani function may have broad implications for patients. They may explain differences in symptom control and account for many of the successes and failures of both conservative and surgical treatments. Although it will always be more convenient to study pelvic floor muscle action when supine, data taken in the upright posture captures the natural action of the muscle in the position in which it must function daily. Investigations of pelvic floor muscle in upright postures may bring new insights to pelvic floor function. References 1. Shafik A, Doss S, Asaad S. Etiology of the resting myoelectric activity of the levator ani muscle: physioanatomic study with a new theory. World J Surg 2003;27:309-14. 2. Taverner D. An electromyographic study of the normal function of the external anal sphincter and pelvic diaphragm. Dis Col Rect 1959;2:153-60. 3. Sampselle CM, Miller JM, Mims BL, DeLancey JOL, Ashton- Miller JA, Antonakos CL. Effect of pelvic muscle exercise on transient incontinence during pregnancy and after birth. Obstet Gynecol 1998;91:406-11. 4. Ashton-Miller JA, DeLancey JOL, Warwick DN. Method and apparatus for measuring properties of the pelvic floor muscles. US patent 6,468,232 B1. October 2002. 5. Dumoulin C, Bourbonnais D, Lemieux MC. Development of a dynamometer for measuring the isometric force of the pelvic floor musculature. Neurourol Urodyn 2003;22:648-53. 6. DeLancey J, Kearney R, Umek W, Ashton-Miller JA. Levator ani muscle structure and function in women with prolapse compared to women with normal support. Neurourol Urodynam 2003;22:542-3. 7. Bo K, Finckenhagen HB. Is there any difference in measurement of pelvic floor muscle strength in supine and standing position? Acta Obstet Gynecol Scand 2003;82:1120-4. 8. Miller JM. Effect of standing and lying postures on the distal intravaginal pressure measured in activities that relate to stress urinary incontinence. Doctoral Dissertation, Chapter V, School of Nursing, University of Michigan, 1996. 9. Henriksson L, Ulmsten U, Andersson K. The effect of changes of posture on the urethral closure pressure in stress-incontinent women. Scand J Urol Nephrol 1977;11:207-10. 10. Iosif S, Ulmsten U, Andersson K. The effect of changes of posture on the urethral closure pressure in healthy women. Scand J Urol Nephrol 1977;11:201-6. 11. De Troyer A. Mechanical role of the abdominal muscles in relation to posture. Resp Physiol 1983;53:341-538.

1728 Morgan et al 12. Hoyte L, Schierlitz L, Zou K, Flesh G, Fielding JR. Two- and 3- dimensional MRI comparison of levator ani structure, volume, and integrity in women with stress incontinence and prolapse. Am J Obstet Gynecol 2001;185:11-9. 13. Singh K, Jakab M, Reid WM, Berger LA, Hoyte L. Threedimensional magnetic resonance imaging assessment of levator ani morphologic features in different grades of prolapse. Am J Obstet Gynecol 2003;188:910-5. 14. Sapsford RR, Hodges PW, Richardson CA, Cooper DH, Markwell SJ, Jull GA. Co-activation of the abdominal and pelvic floor muscles during voluntary exercises. Neurourol Urodyn 2001;20:31-42. 15. Snooks SJ, Swash M, Setchell M, Henry MM. Injury to innervation of pelvic floor sphincter musculature in childbirth. Lancet 1984;2:546-50. 16. DeLancey JOL, Kearney R, Chou Q, Speights S, Binno S. The appearance of levator ani muscle abnormalities in magnetic resonance images after vaginal delivery. Obstet Gynecol 2003;101:46-53. 17. Tunn R, Paris S, Fischer W, Hamm B, Kuchinke J. Static magnetic resonance imaging of the pelvic floor muscle morphology in women with stress urinary incontinence and pelvic prolapse. Neurourol Urodyn 1998;17:579-89.