The original indication for pelvic floor ultrasound was (and still is) the assessment of bladder neck mobility and funneling of the bladder neck, both of which are considered important in women with urinary incontinence. Figure 40-3 shows the standard orientation used to describe bladder neck mobility. The position of the bladder neck is determined relative to the inferoposterior margin of the symphysis pubis, and comparative studies have shown good correlations with radiological methods previously used for this purpose (for an overview see19). The one remaining advantage of x-ray fluoroscopy may be the ease with which the voiding phase can be observed, although some investigators have used specially constructed equipment to document voiding with ultrasound.20
Translabial ultrasound can be used to determine residual urine (Figure 40-6), using a formula originally developed for transvaginal ultrasound.21 Although it is reasonable using this formula: [(X × Y × 5.9) − 14.9 = residual in mL], as it should matter little as to whether a midsagittal image is obtained with a vaginal or a perineal probe, the method has not been validated for translabial ultrasound.
Determination of residual urine according to Haylen.20 The product of the 2 maximal diameters in centimeters is multiplied by 5.9, and subtraction of 14.9 mm results in the residual volume in milliliters: 4.31 × 1.69 × 5.9 − 14.9 − 28.9 mL.
Bladder neck position and mobility can be assessed with a high degree of reliability. Points of reference are the central axis of the symphysis pubis10 or its inferoposterior margin22; see Figure 40-3). The full bladder is less mobile23 and may prevent complete development of pelvic organ prolapse. It is essential to not exert undue pressure on the perineum so as to allow full development of pelvic organ descent. Measurements of bladder neck position are performed at rest and on maximal Valsalva, and the difference yields a numerical value for bladder neck descent. On Valsalva, the proximal urethra may be seen to rotate in a posteroinferior direction. The extent of rotation can be measured by comparing the angle of inclination between the proximal urethra and any other fixed axis. Some investigators measure the retrovesical angle (RVA, or posterior urethrovesical angle [PUV]) between proximal urethra and trigone; others determine the angle between the central axis of the symphysis pubis and a line from the inferior symphyseal margin to the bladder neck.24 The reproducibility of bladder neck descent seems good, with intraclass correlations between 0.75 and 0.98, indicating "excellent" agreement.25
There is no definition of "normal" for bladder neck descent, although cutoffs of 20 and 25 mm have been proposed to define hypermobility. Bladder filling, patient position, and catheterization all have been shown to influence measurements (see Ref.  for an overview), and it can occasionally be quite difficult to obtain an effective Valsalva maneuver, especially in nulliparous women who routinely coactivate the levator muscle.9 Perhaps not surprisingly, publications to date have presented widely differing reference measurements in nulliparous women. Although 2 series documented mean or median bladder neck descent of only 5.1 mm26 and 5.3 mm27 in continent nulliparous women, another study on 39 continent nulliparous volunteers measured an average of 15 mm of bladder neck descent.28 The author has obtained measurements of 1.2 to 40.2 mm (mean 17.3 mm) in a group of 106 stress continent, nulligravid young women of 18 to 23 years of age. It is likely that methodological differences account for the above discrepancies, with all known confounders tending to reduce descent.
The etiology of increased bladder neck descent is likely to be multifactorial. The wide range of values obtained in young nulliparous women suggests a major congenital component, confirmed in a twin study.29 Vaginal childbirth is probably the most significant environmental factor,30,31 with a long second stage of labor and vaginal operative delivery associated with increased postpartum descent. Figure 40-7 shows markedly increased bladder neck mobility after a vacuum delivery at term. This association between increased bladder descent and vaginal parity is also evident in older women with symptoms of pelvic floor dysfunction.32 It is not clear what pathophysiological changes are responsible for increased pelvic organ descent. Both fascial disruption and damage to the levator ani may play a role.
Immobile bladder neck (BND 6 mm) prior to first delivery (left pair of images), and a marked increase in bladder neck mobility (BND 38.1 mm) after childbirth (right pair of images). (From Dietz, HP, Bennett, MJ. The effect of childbirth on pelvic organ mobility Obstet Gynecol. 2003; 102: 223-228, with permission.)
Funneling and Stress Incontinence
In patients with stress incontinence, but also in asymptomatic women, funneling of the internal urethral meatus may be observed on Valsalva (see Figure 40-4C) and sometimes even at rest.33 Funneling is often (but not necessarily) associated with leakage. Other indirect signs of urine leakage on B-mode real-time imaging are weak gray-scale echoes ("streaming") and the appearance of 2 linear ("specular") echoes defining the lumen of a fluid-filled urethra. However, funneling may also be observed in urge incontinence and cannot be used to prove urodynamic stress incontinence (USI). Its anatomical basis is unclear. Marked funneling has been shown to be associated with poor urethral closure pressures.34,35
Until recently, clinicians and researchers have focussed on bladder neck (rather than urethral) mobility. A recently developed method now allows assessment of the entire urethra.36 The distal and central urethra are consistently less mobile than the bladder neck, suggesting mid-urethral tethering of the organ to the pelvic sidewall. This agrees with the finding that the midurethra seems to matter much more for continence than the bladder neck.37 Childbirth seems to affect the bladder neck more than the central urethra.38 It is likely that this new methodology will be useful in the assessment of anti- incontinence procedures and in research into the aetiology and pathogenesis of stress urinary incontinence.39
Color Doppler Imaging of Stress Incontinence
Color Doppler ultrasound can demonstrate urine leakage on Valsalva maneuver or coughing (Figures 40-8 and 40-9).40 Agreement between color Doppler and fluoroscopy was high in a controlled group with indwelling catheters and identical bladder volumes.41 Both velocity and energy mapping were able to document leakage. Color Doppler velocity (CDV) was slightly more likely to show a positive result, probably due to its better motion discrimination. Color Doppler imaging may also facilitate the documentation of leak point pressures.42 Whether this is, in fact, desired will depend on the clinician and his or her preferences, and one may argue that urine leakage and leak point pressures can be determined much more easily.
Color Doppler ultrasound (CDV) demonstrating urine leakage (arrowhead) through the urethra on Valsalva maneuver. (From Dietz HP. Ultrasound Imaging of the Pelvic Floor: Part 1: 2D aspects. Ultrasound Obstet Gynecol 2004; 23: 80-92, with permission.)
Color Doppler energy (CDE) imaging in Urodynamic Stress Incontinence (USI). The Doppler signal outlines most of the proximal urethra. (From Dietz HP. Ultrasound Imaging of the Pelvic Floor: Part 1: 2D aspects. Ultrasound Obstet Gynecol 2004; 23: 80-92, with permission.)
Clinical examination is limited to grading anterior compartment prolapse, which we call "cystocele." In fact, imaging will identify a number of anatomical situations that are difficult, if not impossible, to distinguish clinically. There are at least 2 types of cystoceles with very different functional implications (Figure 40-10). A cystourethrocele is associated with above average flow rates and urodynamic stress incontinence (left images), whereas a cystocele with intact retrovesical angle (right images) is generally associated with voiding dysfunction and a low likelihood of stress incontinence.43 In addition, occasionally a cystocele will turn out to be due to a urethral diverticulum (Figure 40-11 shows a 3D representation of an unusual anterior urethral diverticulum), a Gartner duct cyst (Figure 40-12), or an anterior enterocele, all likely to be missed on clinical examination, or even a leiomyoma.44
The 2 most common presentations of cystocele. The left 2 images show typical findings in a patient with mild stress urinary incontinence and anterior vaginal wall descent (clinically a cystourethrocele grade II). The right 2 images demonstrate appearances in a patient with a cystocele with intact retrovesical angle. Bladder neck and proximal urethra are virtually inverted compared to their position at rest, and there is marked urethral kinking. Usually such women present with prolapse and are often continent. (From Dietz HP. Pelvic Floor Ultrasound: a review. Am J Obstet Gynecol 2010; 202: 321-334, with permission.)
Urethral diverticulum as seen on 3D translabial ultrasound. The extent of the diverticulum is clearly apparent, both in sectional planes (A-C) and in the rendered volume (D). This diverticulum is unusual in that it mainly develops into the space of Retzius (ie, toward the bottom left corner of the image in A). The diverticular tract was identified cystoscopically at 12 o'clock. (From Dietz HP. Pelvic Floor Ultrasound: a review. Am J Obstet Gynecol 2010; 202: 321-334, with permission.)
Gartner duct cyst close to bladder neck (arrow). (From Dietz HP. Ultrasound Imaging of the Pelvic Floor: Part 1: 2D aspects. Ultrasound Obstet Gynecol 2004; 23: 80-92, with permission.)
Urethral and Paraurethral Pathology
Modern B-mode systems allow a detailed assessment of the urethra. The true dimensions of the urethral rhabdosphincter have only recently become clear as imaging in the midsagittal plane does not show the ventral aspect of the muscle well, and as transvaginal ultrasound results in artifacts due to the varying angle between incident beam and muscle fibers, leading to underestimation of sphincter area and volume. Figure 40-13 shows a normal urethra and illustrates how much easier it is to identify the donut-shaped rhabdosphincter in the axial compared to the sagittal plane. Although textbooks usually describe the rhabdosphincter as omega-shaped, this is clearly not the case on axial plane imaging. Figure 40-14 demonstrates typical findings in a patient with urodynamic stress incontinence and a completely normal pelvic floor muscle.
The urethral rhabdosphincter is much more easily seen in the axial plane due to poor tissue discrimination between fibrofatty tissue anterior to the urethra and the rhabdosphincter. In the axial plane, the ring structure of the sphincter is surprisingly obvious on this VCI (volume contrast imaging) slice of 3-mm thickness.
Typical findings in a patient with urodynamic stress incontinence. The left image shows marked funneling, with the bladder neck reaching the inferior margin of the symphysis pubis. The right image demonstrates an open bladder neck and a completely normal pelvic floor in an axial plane rendered volume.
Urethral diverticulae are often overlooked for years in women with recurrent bladder infections and symptoms of frequency, urgency, and pain or burning on voiding, until imaging is undertaken. Urethral structure and spatial relationships are much better appreciated in the axial plane (see Figure 40-11), which is particularly useful in the differential diagnosis of Gartner cyst and urethral diverticulum. Often a Valsalva maneuver will help in improving visibility, allowing insonation of the structure from varying angles. Standard textbooks suggest that MR is the investigation of choice in women suspected of having a urethral diverticulum, but it is difficult to see what advantages MR should have over ultrasound for this indication. Unfortunately, the condition is too uncommon to allow studies of diagnostic efficacy.
The thickness of the bladder wall (bladder wall thickness [BWT] or detrusor wall thickness [DWT]) can easily be determined on translabial ultrasound (Figure 40-15). As increasing bladder filling reduces bladder wall thickness due to distension, measurements should only be undertaken at bladder volumes of 50 mL or less.45 Although DWT has probably been overrated as a diagnostic tool in the context of detrusor overactivity,46,47 increased DWT is associated with symptoms of the overactive bladder,47,48 and may be a predictor of postoperative de novo urge incontinence and/or detrusor overactivity after anti-incontinence procedures.49 As opposed to the situation in the male, DWT in women is not predictive of voiding dysfunction.50
Measurement of bladder wall thickness at the dome in 4 women with non-neuropathic bladder dysfunction. In all cases residual urine is well below 5 0 mL. (From Lekskulchai O, Dietz HP. Detrusor wall thickness as a test for detrusor overactivity in women. Ultrasound Obstet Gynecol 2008; 32: 535-539, with permission.)
Occasionally a foreign body, eroded mesh, or even a bladder tumor (Figure 40-16) may be picked up on translabial ultrasound,51 and a careful examination using parasagittal planes may show bladder diverticulae. A cystic structure that varies markedly over seconds or minutes and is located 1 to 3 cm lateral and posterior to the bladder neck is likely to be a ureterocele, a generally harmless sacculation of the distal ureter due to stenosis of the ureterovesical junction. If the respective upper tract appears normal on renal ultrasound then no further action is required.
Transitional cell carcinoma of the bladder as seen on para-sagittal translabial ultrasound (arrow). (From Dietz HP. Ultrasound Imaging of the Pelvic Floor: Part 1: 2D aspects. Ultrasound Obstet Gynecol 2004; 23: 80-92, with permission.)