The most convenient transducer for translabial use is an abdominal/ obstetric curved array of a frequency of 3.5-7 MHz. The transducer surface is covered with gel and a nonpowdered glove, condom or clingwrap before placement on the perineum (see Figure 1). It may be necessary to part the labia for optimal image quality. The urethra anteriorly and the anal canal posteriorly define the midline, and the inferior margin of the symphysis pubis serves as a convenient bony point of reference. Usually scanning is performed in the supine (modified lithotomy) position and after bladder emptying, but sometimes it is necessary to examine the patient while standing in order to optimise prolapse development.
Figure 1: Preparation of an abdominal curved array transducer for translabial use, placement in the midsagittal plane. Images courtesy of Nelinda Pangilinan, Manila.
Bladder Neck Mobility
Bladder neck position and mobility can be assessed with a high degree of reliability. Points of reference is the inferoposterior margin of the symphysis pubis, see Figure 2. Imaging can be undertaken supine or erect, with the bladder full or empty. The full bladder is less mobile and may prevent complete development of pelvic organ prolapse. In the standing position, the bladder is situated lower at rest but descends about as far as in the supine patient on Valsalva. Either way, it is essential not to exert undue pressure on the perineum so as to allow full development of pelvic organ descent, although this may be difficult in women with severe prolapse such as vaginal eversion or procidentia.
Measurements of bladder neck position relative to the symphysis pubis are generally performed at rest and on maximal Valsalva manoeuvre. The difference yields a numerical value for bladder neck descent (BND). Intraclass correlations for BND are between 0.75 and 0.98, indicating excellent agreement.
There is no definition of ‘normal’ for bladder neck descent although cut-offs of 15, 20 and 25 mm have been proposed to define hypermobility. Mean values in stress incontinent women are consistently around 30 mm (own unpublished data). Bladder filling (Dietz and Wilson, 1999), patient position (Dietz and Clarke, 2001) and catheterization all have been shown to influence measurements, and it can occasionally be quite difficult to obtain an effective Valsalva manoeuvre, especially in nulliparous women, due to levator co-activation (Oerno et al., 2007), see Figure 3. The author has obtained BND measurements of 1.2- 40.2 mm (mean 17.3 mm) in a group of 106 stress continent nulligravid young women of 18- 23 years of age (Dietz 2004).
Bladder neck descent is probably a predictor of success after suburethral slings. The less mobility, the more difficult it may be to achieve just the right degree of tension to avoid either excessive obstruction, resulting in voiding dysfunction, or insufficient compression, resulting in recurrent stress leakage. However, urethral quality (measured by MUCP) seems to be an even stronger predictor of success. The aetiology of increased bladder neck descent is likely to be multifactorial. There likely is a congenital component (Dietz et al, 2005), and vaginal childbirth seems the most significant environmental factor. Trauma to the levator ani muscle is associated with markedly increased bladder neck mobility (Dietz and Lanzarone, 2005) although damage to structures tethering the urethra to the pubic rami probably also occurs. Having said that, it seems that mid- urethral mobility matters more than bladder neck mobility (Pirpiris et al., 2012), and it seems to be pregnancy rather than childbirth that has the most effect on the supports of the mid- urethra (Shek et al., 2012). Figure 4 shows differential movement of urethral segments on Valsalva, implying mid- urethral tethering due to the pubourethral ligaments.
Figure 2: Measuring bladder neck descent on Valsalva, relative to the inferoposterior symphyseal margin.
Figure 3: Co- activation of the levator ani during Valsalva manoeuver. The middle images (‘first valsalva’) show a contraction of the levator at first Valsalva, resulting in a false-negative assessment for prolapse (from Oerno et al., Ultrasound Obstet Gynecol 2007; 30: 346-350)
Figure 4: Differential movement of urethral segments, showing that the mid- urethra is the least mobile part of the organ.
In patients with stress incontinence, but also in asymptomatic women, funneling of the internal urethral meatus may be observed on Valsalva (see Videos 1 and 2) and sometimes even at rest. Funneling is often associated with leakage. Other indirect signs of urine leakage on B- mode realtime imaging are weak grayscale echoes (‘streaming’) and the appearance of two linear (‘specular’) echoes defining the lumen of a fluid- filled urethra. However, funneling may also be observed in urge incontinence. Its anatomical basis is unclear. Marked funneling has been shown to be associated with poor urethral closure pressures.
Leakage of urine on Valsalva can also be documented using Colour Doppler ultrasound (Dietz et al., 1999), see Video 1. As a rule, it makes sense to set Doppler gain and scale to values that just permit the pickup of venous flow signals, e.g. from vessels posterior to the symphyseal margin, avoiding marked flash artefact as tissues move with a Valsalva manoeuvre.
Video 3: Contraction of the levator ani results in a cranioventral displacement of the bladder neck.
Video 1: Valsalva, demonstrating the typical anatomical equivalent of uncomplicated stress urinary incontinence
Video 2: Colour Doppler (CDV) showing urethral loss of urine during Valsalva. A reflex contraction of the external perineal muscles precedes the leak.
Translabial ultrasound can quantify pelvic floor muscle function via observation of a cranioventral displacement of the internal urethral meatus or a reduction of the levator hiatus in the midsagittal plane (see Video 2). These measurements are associated with contraction strength as quantified by perineometer and vaginal palpation (Dietz, Jarvis and Vancaillie, 2002). Narrowing of the hiatus without cranioventral displacement of the bladder neck implies that the patient has increased intraabdominal pressure while contracting the levator ani. This is a common problem, especially in nulliparous women (Oerno et al., 2007) and should be corrected by teaching proper technique, e.g. by visual ultrasound biofeedback (Figure 3). Occasionally, levator co-activation can not be overcome, and the patient has to be examined standing.
Of course, we often want to assess the effect of a pelvic floor muscle contraction (PFMC). The usual instruction would be to ask the patient to contract the muscles around vagina and back passage, to pull up and in. Sometimes a reference to intercourse helps as well. Often enough, women will simultaneously activate abdominal muscles, and visual biofeedback can be very useful, with the patient watching the effect of her PFMC on the monitor.
There are a number of different measures of pelvic floor muscle function that can be obtained from 2D pelvic floor ultrasound. Figure 5 shows the most commonly used parameters: a change in levator plate angle, measured relative to the central axis of the symohysis pubis, a reduction in midsagittal hiatal diameter, and bladder neck displacement relative to the symphysis pubis. All seem to be similarly valid and correlate well with Oxford grading, the most commonly used clinical measure of pelvic floor function.
Figure 5: Three different measures of pelvic floor function: Change in levator plate angle (A,B), reduction in hiatal diameter (C,D) and bladder neck displacement (E,F).
The Time Factor
There are several other confounders of a proper Valsalva maneuver. The most important is time- that is, the length of a Valsalva (Orejuela et al., 2011). This is generally ignored, but a Valsalva maneuver should last at least 5 seconds in order to achieve near- maximal organ descent, as shown in Fig. 6.
The Pressure Factor
Pressure, on the other hand (and contrary to what one might assume) is much less of an issue (Mulder et al., 2012). The great majority of women, even the elderly, can generate pressures in excess of 80 cm H2O, and this means that a maximal Valsalva does not need to be standardised further (see Figure 7).
Figure 7: Percentage of maximum bladder neck descent reached at different pressures, showing that 80% of maximum is reached at a mean pressure of less than 60 cm H2O (from Mulder et al., ANZJOG 2012, DOI: 10.1111/j.1479-828X.2012.01446.x).
Figure 6: Percentage of maximum organ descent reached with duration of Valsalva (approx. one volume registered per second) (from Orejuela et al., Int Urogynecol J 2011; DOI 10.1007/s00192-011-1533-x
2D imaging of the levator muscle
The levator ani muscle can in fact be imaged by 2D ultrasound, using an oblique parasagittal approach (Dietz and Shek, 2008). This is less repeatable than axial plane imaging due to the absence of an easy point of reference, but may be useful if 3D imaging is unavailable. Video 3 shows the appearances in the parasagittal plane and demonstrates the effect of a pelvic floor contraction.
Video 4: The levator ani as seen in an oblique parasagittal orientation. This is a normal muscle with excellent contractile function.