Objective
We used a novel technique, high-definition manometry (HDM) that utilizes 256 tactile sensitive microtransducers to define the characteristics of vaginal high-pressure zone.
Study Design
Sixteen nullipara asymptomatic women were studied using HDM, transperineal 2-dimensional dynamic ultrasound and dynamic magnetic resonance (MR) imaging.
Results
Vaginal high-pressure zone revealed higher contact pressures in anterior and posterior directions compared with lateral directions, both at rest and squeeze. At rest, anterior pressure cluster is located 10 mm cephalad to posterior pressure cluster; with squeeze the latter moves in the cranial direction by 7 mm. Ultrasound and MR images revealed that the anorectal angle moves cephalad and ventrally during squeeze. Cephalad movement of posterior pressure cluster during squeeze is similar to the cranial movement of anorectal angle.
Conclusion
We propose that the vaginal high-pressure zone represents the constrictor function and cranial movement of the posterior pressure cluster represents the elevator function of pelvic floor. HDM may be used to measure the constrictor and elevator functions of pelvic floor muscles.
Vaginal pressure is a key measure of the strength of the pelvic floor muscles. Kegel was the first to use a pneumatic resistance chamber to measure vaginal pressure and perform biofeedback therapy using his device to improve the strength of pelvic floor muscle. Since then, several investigators have used various types of devices to measure vaginal pressure/force as a measure of the pelvic floor strength.
Pressures in general are directionless and should be symmetrical on all sides. However, such is not the case in various sphincters or high-pressure zones of the body because these are contact and not the cavity or fluid pressures. Using a side-hole infusion manometery technique that can measure contact pressure at a point location, we found that there is a high-pressure zone (HPZ) in the distal part of the vagina that shows axial and circumferential asymmetry of the contact pressures both at rest and during contraction. Furthermore, the contact pressures increase significantly during pelvic floor contraction.
Using a water-filled bag placed in the vagina and 3-dimensional ultrasound (US) imaging of the pelvic floor, we visualized the vaginal HPZ to be located just above the level of hymen. Shape of the deformed water bag located in the vagina suggests that the forces responsible for genesis of vaginal HPZ are directed in the anterior-posterior direction. Based on the aforementioned characteristics, we postulated that the vaginal HPZ is most likely related to the ventral movement of the puborectalis component of pelvic floor muscles.
The goal of our current study was to define the static and dynamic characteristics of vaginal HPZ using a novel, tactile pressure–sensing technology (ie, high-definition manometry [HDM]). HDM can measure contact pressures at closely spaced intervals, and therefore, it has the potential to provide information on the direction of forces and specific muscles responsible for the genesis of vaginal HPZ. To further understand the characteristics of vaginal HPZ revealed by HDM, we recorded dynamic, 2-dimensional (2D) US images of the pelvic floor muscles. Additionally, dynamic magnetic resonance (MR) imaging of the pelvis and pelvic floor muscles was performed to study the anatomical relationship between vaginal HPZ and the adjacent pelvic floor structures.
Materials and Methods
The institutional review board of the University of California, San Diego, approved the study protocol, and each subject signed an informed consent prior to participation in the study protocol. These subjects responded to an advertisement and were reimbursed a nominal amount of money for participation in the study.
The study was conducted in 16 nulliparous women with a mean age of 37.4 years (range, 21–61 years). 2D US and HDM were performed in 11 subjects and MR imaging was obtained in 5 subjects. Each subject completed medical history and a previously validated urinary incontinence and anal incontinence scoring questionnaires to confirm the absence of urinary and anal incontinence symptoms. Prior to starting the study, each subject was instructed to contract the pelvic floor by a prompt “squeeze as if you were trying to stop your stream of urine.” A simultaneous digital vaginal examination by the investigator ensured the contraction of the pelvic floor muscle.
High-definition manometry
Vaginal pressures were recorded using a newly developed HDM probe (ManoScan 360 HD; Sierra Scientific Instruments Inc, Los Angeles, CA) that has the following features: (1) the probe is 10 mm in diameter and the pressure sensitive part of the probe is 64 mm in length, (2) there are 256 transducers on the surface of the HDM probe that form a continuous grid in both the axial and circumferential directions, and (3) each transducer is 4 mm long (axially) and 2 mm wide (circumferentially).
The HDM probe has the following functional characteristics: (1) pressures from all transducers are recorded digitally and displayed on a personal computer as color plots (ManoView HD beta; Sierra Scientific Instruments), (2) in vitro testing revealed that an externally applied pressure on each transducer does not influence the output of the adjacent transducer; and (3) pressure recordings have an accuracy of 5 mm Hg.
The HDM probe was placed in the vagina in such a fashion that the entire vaginal HPZ was captured; the most cranial part of the probe recorded abdominal pressure and the most caudal part measured the atmospheric pressures. The circular orientation of the probe in relation to the anterior midline, posterior midline, and left lateral and right lateral orientation of the vagina was noted. Measurements were obtained while the subject was at rest and then during 3 sustained maximal pelvic floor contractions (squeeze) and relaxations.
Two-dimensional dynamic ultrasonography
A 3-dimensional US system (Phillips HD11; Phillips Medical Systems, Bothell, WA) was used to acquire images of pelvic organs and pelvic floor muscles. With the subject in the lithotomy position, 2D US dynamic cinematic loops were obtained by placing a 3-9 MHz, transvaginal US transducer on the perineum.
To increase the field of view of structures close to the skin, a custom-built standoff, made of agar, was placed on the US transducer. The US probe with the standoff in place was directed cranially to image the pelvic floor hiatus and surrounding muscles. The 2D US cinematic loops were recorded during pelvic floor contraction; they were archived on compact disk and viewed off-line and analyzed with the Q lab 4.2 software program (Phillips Medical Systems).
Magnetic resonance imaging
Dynamic MR imaging of the pelvis was recorded in the midsagittal plane with and without a water-filled bag placed in the vagina. The bag, built from a noncompliant polyethylene material was 10 cm long and had a maximal diameter of 3.5 cm when fully distended. MR imaging was performed using 1.5 T super conducting magnet (Symphony; Siemens Medical Systems, Erlangen, Germany) machine.
The following MR parameters were used for MR imaging: echo time of 2.52 milliseconds, repetition time of 5.03 milliseconds, slice thickness of 10 mm, and slice gap of 20%, yielding an image matrix of 256 × 100. Images were recorded at rest and during contraction of pelvic floor muscles. The MR sequence allowed the capture of 2D MR images in the midsagittal plane at a temporal resolution of 1 Hz. The bag was filled with 50 mL of water before the acquisition of dynamic MR images. Images were recorded at rest and continuously during a sustained and maximal pelvic floor contraction. MR images were analyzed using Syngo Fast View software (AG 2004; Siemens, Munich, Germany).
Data analysis
Vaginal HDM pressures were displayed as color plots and revealed 3 distinct pressure zones. The upper zone (distal part of the probe) shows uniform pressures that reflect transmitted abdominal pressure into the vagina, the middle zone represents the axial and circumferential asymmetric contact pressures of the vaginal HPZ, and the lower zone (proximal part of the probe) represents pressures from the part of the probe located below the hymen that represents atmospheric pressure ( Figure 1 ).
From the first 2 (proximal) and the last 2 rows (distal) of pressure sensors, the mean values and SD were computed to determine the mean atmospheric and mean abdominal pressures, respectively, which were used to define the caudal and cranial edges of the vaginal HPZ, respectively. To define the upper and lower borders of the vaginal HPZ, the raw data from 256 sensors were exported to an Excel sheet and viewed as a 16 × 16 table.
At rest and during squeeze, contact pressures of the vaginal HPZ were averaged over 2 second periods. The cranial edge of the vaginal HPZ was defined at a location at which the pressure was noted to be 2 SD higher than the abdominal pressure while the caudal edge was defined at which the pressure was noted to be 2 SD higher than the atmospheric pressures.
Measurements of the length, peak contact pressures, and location of peak contact pressures in the vaginal HPZ were determined from the reconstructed vaginal HPZ. Lastly, contact pressures from the vaginal HPZ of the 11 subjects were averaged to obtain a composite vaginal HDM profile at rest and pelvic floor contraction.
The mean pressure profile was calculated by optimally aligning the pressure plots of all subjects. The optimal alignment was achieved by calculating the correlation coefficients between the pressure values of the 2 subjects at a time and then sliding the transducer position axially (±3 transducer position) and circumferentially (±1 transducer position). The optimal pressure transducer alignment was the one that yielded a maximal correlation coefficient. This method of alignment allowed correction for variations in the depth of probe insertion and the possible slight misalignment in the circumferential direction.
Two-dimensional dynamic ultrasonography
Two-dimensional US images of the pelvic floor, at rest and at peak pelvic floor contraction, were extracted from the cinematic loops and analyzed with the help of sigma scan. The X-axis ( horizontal line ) was drawn parallel to the transducer surface where it touched the skin and was used as a reference line against which the craniocaudal distance moved by the anorectal angle was measured. The Y-axis ( vertical line ) was drawn tangential to the inferior and posterior point of pubic symphysis and perpendicular to the first line and was used as a reference line against which the dorsoventral distance moved by the anorectal angle was measured ( Figure 2 ).
These reference lines were part of the Cartesian coordinate system as described previously. Our measurement system is slightly different from the one suggested by others and was designed to measure both the horizontal and vertical movements of anorectal angle on US images.
MR imaging
The anorectal angle moves in the cranial and ventral direction during pelvic floor contraction, and these movements were analyzed using Syngo Fast View (AG 2004; Siemens). A horizontal line, passing tangentially through the lower edge of the pubic symphysis, was used to measure the craniocaudal movement of the anorectal angle (ARA). A vertical line, passing tangentially through the lower edge of the pubic symphysis, was used to measure the dorsovental movement of anorectal angle ( Figure 3 ). Similar to US image analysis, all measurements were obtained using a Cartesian coordinate system.
