Chapter 40: PELVIC FLOOR ULTRASOUND

It has taken over 20 years for imaging to develop as a mainstream diagnostic tool in the investigation of female pelvic organ prolapse, urinary and fecal incontinence, and defecation disorders. Physicians have been slow in realizing that clinical assessment alone is a very inadequate tool to assess pelvic floor function and anatomy. Our examination skills are poor, focusing on surface anatomy rather than true structural abnormalities, and recurrence after pelvic reconstructive surgery is common.¹ The uptake of pelvic floor ultrasound by clinicians has varied substantially from one specialty to another, with gynecologists having a major advantage over urologists and colorectal surgeons due to the fact that an entire generation of OB/GYN specialists has now grown up with ultrasound imaging. Sonography is an accepted component of any clinical assessment in both obstetrics and gynecology, so why should it be any different in urogynecology and female urology?

In theory, clinical assessment skills could be improved to such a degree as to make imaging unnecessary in many cases. However, this clearly is not the case at present, and it is unlikely to happen unless we allow imaging techniques to demonstrate what (and where) the actual problems are. To give just one example: the missing link between vaginal childbirth and prolapse—major levator trauma in the form of avulsion of the anteromedial aspects of the puborectalis muscle off the pelvic sidewall^2,3—is palpable, but palpation of levator trauma requires considerable skill and teaching,^{4,5, and 6} preferably with imaging confirmation. Certainly, diagnosis by imaging is more reproducible than diagnosis by palpation,⁶ and easier to teach. And suspected levator trauma or abnormal distensibility ("ballooning") are by no means the only reason to perform pelvic floor imaging (Table 40-1).

Two-Dimensional Imaging

Basic requirements for translabial pelvic floor ultrasound include a B-mode capable two-dimensional (2D) ultrasound (US) system with cine loop function, a 3.5- to 6-MHz curved array transducer, and a video printer. A midsagittal view is obtained by placing a transducer (usually a curved array with frequencies between 3.5 and 8 MHz) on the perineum (Figure 40-1), after covering the transducer with a glove, condom, or thin plastic wrap. Sterilization as for intracavitary transducers is usually considered unnecessary. We use alcoholic wipes to clean the transducer between patients, but regulations may vary between jurisdictions.

Powdered or coated gloves can impair imaging quality due to reverberations and should be avoided. It is worthwhile testing several types of probe covers for their effect on image quality and ease of application. Imaging is usually performed in dorsal lithotomy, with the hips flexed and slightly abducted, or in the standing position. Requiring the patient to place heels close to the buttocks and then move hips towards the buttocks will result in an improved pelvic tilt. Bladder filling should be specified; usually prior voiding is preferable. The presence of a full rectum can impair diagnostic accuracy and sometimes necessitates a repeat assessment after bowel emptying. Parting of the labia can improve image quality, which generally is best in pregnancy and poorest in menopausal women with marked atrophy, most likely due to varying hydration of tissues. Vaginal scar tissue can also impair visibility, but obesity virtually never seems to be a problem.

The transducer can usually be placed firmly on the perineum and the symphysis pubis without causing discomfort, unless there is marked atrophy. The resulting image includes the symphysis pubis anteriorly, the urethra and bladder neck, the vagina, cervix, rectum and anal canal (see Figure 40-1). Posterior to the anorectal junction a hyperechogenic area indicates the central portion of the levator plate, ie, the puborectalis muscle. The cul-de-sac may also be seen, filled with a small amount of fluid, echogenic intraperitoneal fat, or peristalsing small bowel. Parasagittal or transverse views may yield additional information, eg, enabling assessment of the levator ani muscle and its insertion on the arcus tendineus of the levator ani, and for imaging of implants.

Although there has been disagreement regarding image orientation in the midsagittal plane, the author prefers an orientation as on conventional transvaginal ultrasound (cranioventral aspects to the left, dorsocaudal to the right). The latter also seems more convenient when using three-dimensional (3D)/four-dimensional (4D) systems.

Three-Dimensional/Four-Dimensional Imaging

The introduction of 4D ultrasound has had a major impact on pelvic floor imaging. One could argue that 4D enhances our diagnostic capabilities to a greater degree than in any other area of OB/GYN. This is mainly due to the fact that 4D ultrasound gives access to the axial plane to a degree and with an ease that far surpasses what was possible using intracavitary transducers in the past. A single volume obtained at rest with aperture and acquisition angles of 70 degrees or higher will include the entire levator hiatus with symphysis pubis, urethra, paravaginal tissues, the vagina, anorectum, and levator ani muscle from the pelvic sidewall to the posterior aspect of the anorectal junction.

Basic requirements for 3D/4D pelvic floor ultrasound would include a system that allows acquisition, reconstruction, and analysis of volume datasets, including the capability to measure distances and areas in this volume. This implies motorized acquisition or free-hand acquisition with an external position sensor, although the latter is now regarded as obsolete. Currently, the most common 3D probes are those that combine an electronic curved array of 3 to 8 MHz with mechanical sector technology, allowing fast motorized sweeps through a field of vision, but in the future it is quite likely that such transducers will be replaced by electronic 3D arrays or "matrix transducers", allowing much higher acquisition speeds.

In essence, any system that allows satisfactory 3D imaging using an abdominal obstetric probe will be suitable provided the acquisition angle is sufficient to include the entire levator hiatus (ie, 70 degrees or higher. Optimally, one should be able to obtain volumes at an acquisition angle of 80 to 85 degrees and store at least 5 seconds' worth of sequential volumes on the system's hard disk for later evaluation.

Display Modes

Figure 40-2 demonstrates the 2 basic display modes currently in use on 3D ultrasound systems. The multiplanar or orthogonal display mode shows cross-sectional planes through the volume in question. For pelvic floor imaging, this most conveniently means the midsagittal (top left), the coronal (top right), and the axial (bottom left) planes. Imaging planes on 3D ultrasound can be varied in a completely arbitrary fashion in order to enhance the visibility of a given anatomical structure, either at the time of acquisition or offline at a later time. The levator ani, for example, usually requires an axial plane that is slightly tilted in a ventrocaudal to dorsocranial direction.

The 3 orthogonal images are complemented by a "rendered image," that is a semitransparent representation of all voxels in an arbitrarily definable "box," the "region of interest." Figure 40-2D shows a standard rendered image of the levator hiatus, with the rendering direction set from caudally to cranially, which is the most appropriate for imaging the hiatus. The possibilities for postprocessing are restricted only by the software used for this purpose.

Four-Dimensional Imaging

Four-dimensional imaging implies the real-time acquisition of volume ultrasound data, which can then be represented in orthogonal planes or rendered volumes. Many systems are now capable of storing cine loops of dozens of volumes, which is of major importance in pelvic floor imaging as it allows enhanced documentation of functional anatomy. Even on 2D single plane imaging, a static assessment at rest gives little information compared with the evaluation of maneuvers such as a levator contraction and Valsalva. Their observation will allow assessment of levator function and delineate levator or fascial trauma more clearly.

The ability to perform a real-time 3D (or 4D) assessment of pelvic floor structures makes the technology superior to magnetic resonance imaging (MRI). Prolapse assessment by MR requires ultra-fast acquisition,⁷ which is of limited availability and will not allow optimal resolutions. Alternatively, some systems allow imaging of the sitting or erect patient, but again accessibility will be limited for the foreseeable future. The sheer physical characteristics of MRI systems make it much harder for the operator to ensure efficient maneuvers as over 50% of all women will not perform a proper pelvic floor contraction when asked,⁸ and a Valsalva is often confounded by concomitant levator activation.⁹ Without real-time imaging, these confounders are impossible to control for. Therefore, ultrasound has major potential advantages when it comes to describing prolapse, especially when associated with fascial or muscular defects, and in terms of defining functional anatomy. Offline analysis packages allow distance, area, and volume measurements in any user-defined plane (oblique or orthogonal), which is much superior to what is currently possible with Digital Imaging and Communications in Medicine (DICOM) viewer software on a standard set of single-plane MRI images.

Valsalva

The Valsalva maneuver, that is a forced expiration against a closed glottis and contracted diaphragm and abdominal wall, is routinely used to effect downward displacement of pelvic organs, reveal the symptoms and signs of female pelvic organ prolapse, and demonstrate distensibility of the levator hiatus. The result is a dorsocaudal displacement of urethra and bladder neck that can be quantified using a system of coordinates based on the inferoposterior symphyseal margin (Figure 40-3) or the central axis of the symphysis pubis.¹⁰ There also is downward movement of the uterine cervix and the rectal ampulla, and frequently the development of a rectocele, ie, a sacculation of the anterior wall of the rectal ampulla, toward the vaginal introitus or beyond (see below). In the axial plane the hiatus is distended, and the posterior aspect of the levator plate is displaced caudally, resulting in a varying degree of perineal descent. All this can be observed on pelvic floor ultrasound, but it is important to let the transducer move with the tissues, avoiding undue pressure on the perineum, which would prevent full development of a prolapse. In order to achieve maximal or near-maximal organ descent it is necessary to obtain Valsalva pressures of at least 60 cm H2O for at least 5-6 seconds¹¹ and on clinical examination this is frequently not achieved.

In addition, especially in young, nulliparous women, a Valsalva is frequently confounded by levator activation,⁹ as any obstetrician is able to observe on a daily basis. Levator coactivation during Valsalva is highly inconvenient—not just in the labor ward during the expulsive phase of a woman's first vaginal delivery, but also when we assess women for pelvic floor dysfunction and prolapse. Levator coactivation is visible as a reduction in the anteroposterior diameter of the levator hiatus on Valsalva (Figure 40-4). It has to be avoided to obtain an accurate assessment of pelvic organ descent. Any imaging assessment of organ descent requires real-time observation of the effect of a Valsalva maneuver to correct suboptimal efforts, especially if leakage from bladder or bowel is likely. At times, levator coactivation can prevent adequate assessment in the supine position, in particular in women with a strong, intact levator shelf. Sometimes it is necessary to repeat imaging in the standing position, which seems to increase the likelihood of an adequate bearing-down effort.

Pelvic Floor Muscle Contraction

Ultrasound is a highly useful tool in the assessment of the pelvic floor musculature, both in purely anatomical terms (see below) and for function. A levator contraction will reduce the size of the levator hiatus in the sagittal plane and elevate the anorectum, changing the angle between the levator plate and symphysis pubis. As an indirect effect, other pelvic organs such as the uterus, bladder, and urethra are displaced cranially (Figure 40-5), and there is compression of the urethra, vagina, and anorectal junction, explaining the importance of the levator ani for urinary and fecal continence as well as for sexual function.

In its most basic form, transabdominal B-mode imaging can demonstrate elevation of the bladder base on pelvic floor muscle contraction (PFMC), but quantification is difficult and repeatability is lower than for translabial ultrasound.¹² The latter has been employed for the quantification of pelvic floor muscle function, both in women with stress incontinence and continent controls.¹³ as well as before and after childbirth.^14,15 A cranioventral shift of pelvic organs imaged in a sagittal midline orientation is taken as evidence of a levator contraction. The resulting displacement of the internal urethral meatus is measured relative to the inferoposterior symphyseal margin (see Figure 40-5). Care has to be taken to avoid concomitant activation of the abdominal muscles, especially the rectus abdominis or the diaphragm, as this would tend to cause caudal displacement of the bladder neck.

Another means of quantifying levator activity is to measure reduction of the levator hiatus in the midsagittal plane, or to determine the changing angle of the hiatal plane relative to the symphyseal axis. The method can also be utilized for pelvic floor muscle exercise teaching by providing visual biofeedback¹⁶ and has helped validate the concept of "the knack," ie, of a reflex levator contraction immediately prior to increases in intraabdominal pressure such as those resulting from coughing.¹⁷ Correlations between cranioventral shift of the bladder neck on the one hand and palpation/perineometry on the other hand have been shown to be good.¹⁸

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

Residual Urine

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.

Bladder Neck Mobility

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. [19] 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.

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

Urethral Mobility

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.

Cystocele

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

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.

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.

Detrusor Wall Thickness

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

Other Bladder Pathology

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.

Uterine Descent

Tranbslabial ultrasound is somewhat less useful in the assessment of central compartment prolapse. Generally, uterine prolapse is obvious clinically, as is vault descent. An unusually low cervix is iso- to hypoechoic, often with distal shadowing. Its distal margin is evident as a specular line, and it may well contain the cystic structures of nabothian follicles (Figure 40-17). A Valsalva maneuver will result in relative movement, distinguishing a nabothian follicle from Gartner duct cysts or urethral diverticulae. Translabial ultrasound can graphically show the effect of an anteriorized cervix in women with an enlarged, retroverted uterus, explaining symptoms of voiding dysfunction, and supporting surgical intervention in order to improve voiding. On the other hand, mild descent of an anteverted uterus may result in compression of the anorectum, explaining symptoms of obstructed defecation—a situation that is termed a "colpocele" on defecation proctogram.

Vault Prolapse

It is frequently possible to image vault descent in women after hysterectomy, but just as often the thin iso- to hypoechoic structure of the vaginal wall is obscured by a descending rectocele or enterocele, which limits the usefulness of assessing the position of the vault. If imaging of the vault is required, depositing 20 to 50 mL of bubble-free ultrasound gel in the vagina will help with delineation of the upper vagina.

Pelvic floor ultrasound is particularly useful in the posterior compartment. Clinically, we diagnose "rectocele," quite unaware that several different conditions can lead to prolapse of the posterior vaginal wall. A second-degree rectocele could be due to a true rectocele, ie, a defect of the rectovaginal septum (which is most common and is associated with symptoms of prolapse, incomplete bowel emptying, and straining at stool)⁵²; an abnormally distensible, intact rectovaginal septum (which is common and is associated only with prolapse symptoms); a combined recto-enterocele (which is less common); an isolated enterocele (which is uncommon); or just a deficient perineum giving the impression of a "bulge."⁵³ Occasionally, a "rectocele" turns out to be due to rectal intussusception, an early stage of rectal prolapse, where the wall of the rectum is inverted and enters the anal canal on Valsalva.

Rectocele

An anterior rectocele is evident as a diverticulum of the anterior wall of the rectal ampulla into the vagina, generally much more evident on Valsalva than at rest. Posterior rectoceles are uncommon in adult women and may rather be a form of intussusception than an actual rectocele. A rectocele usually contains iso- to hyperechoic feces, and often there is bowel gas as well, resulting in specular echoes and reverberations. Occasionally, there is no stool in the ampulla that could be propelled into the rectocele, and as a result it remains smaller and filled only with rectal mucosa. Since distension of a rectocele will depend on the presence and quality of stool, appearances may vary considerably from one day to the next. Stool quality may also affect the symptoms arising from a rectocele of a given size: In women with normal stool consistency even large rectoceles are sometimes completely asymptomatic. The severity of a rectocele can be quantified by measuring maximal descent relative to the inferior symphyseal margin, and by determining the maximal depth of the sacculation as seen in Figure 40-18, which compares ultrasound and radiological findings in a patient with rectocele.

An enterocele is visualized as downward displacement of abdominal contents into the vagina, ventral to the anal canal. Small bowel may be identifiable due to its peristalsis, and sometimes a small amount of intraperitoneal fluid outlines the apex of the enterocele. Distal shadowing is much less common that with rectocele, and contents often have an irregular isoechoic or ground-grass-like appearance (Figure 40-19).

An intussusception is seen as splaying of the normally tubular anal canal, with the anterior wall of the rectal ampulla (and sometimes the posterior wall as well) being inverted and propelled into the opening anal canal. Figure 40-20 shows a comparison of radiological and ultrasound findings in a patient with rectal intussusception. Usually, a rectal intussusception is due to an enterocele of small bowel that progresses not into the vagina, but down the anal canal—but other abdominal contents such as sigmoid (as in Figure 40-20), omentum, and even the uterus can act in a similar way. Rectal intussusception is associated with abnormal distensibility of the levator ani and avulsion of the puborectalis muscle.⁵⁴

When a rectocele or rectal intussusception is identified on translabial imaging, one may want to provide the patient with visual biofeedback. Demonstrating that straining at stool is obviously counterproductive (propelling feces not into the anal canal but into the vagina, or pushing down not feces but small bowel) may help in modifying behavior. Several studies have recently shown that ultrasound is much better tolerated than defecation proctography, and of course it is much cheaper. As a result it is likely that ultrasound will replace the radiological technique in the initial investigation of women with defecation disorders. If there is a rectocele or a rectal intussusception/prolapse on ultrasound, this condition is very likely to be confirmed on x-ray imaging.^{55,56, and 57} In addition, anismus may be evident as an inability to relax the levator ani, which can be diagnosed qualitatively or by measuring the anorectal angle,⁵⁸ although it is often unclear whether observations on diagnostic imaging bear any relation to what happens during spontaneous defecation.

Assessment of the anal sphincter is covered elsewhere and will not be discussed in any detail here. The anal sphincter is generally imaged by endo-anal ultrasound. This method is firmly established as one of the cornerstones of a colorectal diagnostic workup for anal incontinence and beyond the scope of this review. Because of the limited availability of such probes in gynecology, obstetricians and gynecologists have taken to using high-frequency curved array or endovaginal probes placed exo-anally, ie, transperineally, in the coronal rather than the midsagittal plane.^{59,60,61, and 62} Lately, 3D pelvic floor imaging has also been used to demonstrate the anal canal.^61,63

There are advantages to this approach—not just from the point of view of the patient. Exo-anal imaging (Figure 40-21) reduces distortion of the anal canal and allows dynamic evaluation of the anal sphincter and mucosa at rest and on sphincter contraction, which seems to enhance the definition of muscular defects. However, resolutions are likely to be inferior⁶⁴ to those obtained by endo-anal ultrasound, and good comparative studies are still lacking.

Anterior Colporrhaphy/Vaginal Paravaginal Repair

Anatomical results after traditional anterior repair and vaginal paravaginal repair are highly variable. There are no distinct ultrasonographic features. In the short term, fluid collections are frequently seen and usually do not require any action. Suture material, detritus, and patient discomfort will impair imaging conditions for the first few postoperative weeks. In the medium term, ultrasound imaging sometimes demonstrates no significant change after such surgery. This is true even for patients who consider themselves cured of their prolapse symptoms, raising the possibility that symptom relief may at times be due to vaginal wall denervation rather than altered anatomy. Levator avulsion seems to be a risk factor for recurrence.⁶⁵

Burch/Marshall-Marchetti-Krantz (MMK) Colposuspension

Contrary to the situation with anterior repair, ultrasound findings after colposuspension are quite distinct (Figure 40-22). Bladder neck mobility is often greatly reduced and the internal meatus of the urethra is anteriorized to a varying degree, depending on surgical technique. Recurrent hypermobility is uncommon—anatomical changes after open Burch colposuspension are usually permanent.

After laparoscopic colposuspension similar findings are observed, although over-elevation seems very difficult to achieve by the laparoscopic route, and recurrent bladder neck hypermobility may be more common. The typical appearance is most obvious on Valsalva in the form of a "colposuspension ridge" underneath the trigone.^{24,66,67, and 68} Occasionally the bladder neck is pulled up and anteriorized so far that it is difficult to image behind the symphysis pubis. Such appearances are associated with postoperative voiding dysfunction and possibly with symptoms of bladder overactivity,^24,69 although the latter may only be true for markedly over-elevated colposuspensions.⁷⁰

After Marshall-Marchetti urethropexies and laparoscopic urethropexy there usually is no distinct "colposuspension ridge." Although the bladder neck is immobilized, there is no support underneath the trigone, resulting in a sharply sloping trigone on Valsalva and a low retrovesical angle (≥80 degrees) on Valsalva (Figure 40-23).⁷¹

Fascial/Synthetic Traditional Slings

Synthetic or fascial traditional bladder neck slings performed according to the Aldridge technique or modifications of the same often result in complete fixation of the bladder neck. There is little mobility or change in the retrovesical angle on Valsalva (Figure 40-24), and appearances are similar to those after Burch colposuspension. Fascial implants are of varying visibility, but synthetic implants are generally hyperechoic and show distal shadowing. Because of the highly abnormal anatomy, voiding function is often poor, resulting in a chronic residual.

Injectables

The echogenicity of injectables varies considerably. Because of the small market share of those procedures after the introduction of modern suburethral slings, the literature on sonographic appearances is very scant. Collagen injections do not seem to cause longer-term changes visible on ultrasound, but silicone macroparticles (Macroplastique™) remains visible permanently as a hyperechogenic donut shape surrounding the urethra (Figure 40-25). A special case are aseptic abscesses that occur after certain injectables, resulting in unusual iso- to hypoechoic collections surrounding the urethra.

Suburethral Slings

Recently, synthetic suburethral slings such as the TVT™, IVS™, Sparc™, TVT-O™, Monarc™, etc, have become very popular, replacing other surgical methods for the treatment of urodynamic stress incontinence. Ultrasound can confirm the presence of such a sling; distinguish between transobturator and transretzius implants,⁷² especially when examining the axial plane (Figure 40-26); and allow an educated guess regarding the type and material of the tape.⁷³ As these implants are highly echogenic, ultrasound is superior to MR in identifying such meshes⁷⁴ and has helped elucidate their mode of action.⁷⁵ A successful sling reduces mid-urethral mobility on Valsalva and funnelling,⁷⁶ although the latter is often still visible postoperatively and certainly does not imply failure. The location of the tape relative to the urethra does not seem to matter much,^{77,78, and 79} and dislodgment or migration of suburethral wide-weave polypropylene tapes is highly unlikely.⁸⁰ The effect of such slings largely seems to be limited to the mid-urethra.⁸¹

Ultrasound is particularly useful in women who present with postoperative complications such as voiding dysfunction, symptoms of overactive bladder, recurrent urinary tract infections, and pain.^82,83 Urethral kinking is sometimes very marked, and on its own does not necessarily imply that there is a problem. However, tape that is curled into a tight C shape at rest, shows minimal mobility, and leaves a gap of less than 0.8 cm between tape and symphysis pubis is likely to be obstructive. Consequently, one would expect tape division to improve the situation. Finally, ultrasound may help in locating the implant if tape division is contemplated.

Modern Mesh Implants

There is a worldwide trend toward implantation of permanent vaginal wall meshes, especially for recurrent prolapse, and complications such as failure and mesh erosion are not uncommon. In October 2008, the US Food and Drug Administration (FDA) issued a caution toward the use of vaginal meshes due to erosion or mesh exposure in the vagina or pain. Polypropylene meshes such as the Perigee™, Prolift™ Avaulta™, and Apogee™ are highly echogenic, and their visibility is limited only by persistent prolapse and transducer distance (Figure 40-27). Translabial and introital ultrasound have been used to show that the implanted mesh often "shrinks," or more likely, does not remain as flat as it was prior to implantation.^84,85 For Perigee™, the author's group have been unable to demonstrate any reduction in mesh dimensions after implantation over an observation period of 60 woman-years.⁸⁶ Surgical technique seems to play a major role in determining mesh appearance as fixation of mesh to underlying tissues results in a flatter, more even appearance. Surgical technique may also have an effect on the likelihood of postoperative stress urinary incontinence. The closer the mesh is to the symphysis pubis and the higher it remains on Valsalva, the lower the prevalence of postoperative stress incontinence.⁸⁷ The position, extent, and mobility of vaginal wall mesh can be determined, helping with the assessment of individual technique, and ultrasound may uncover complications such as dislodgment of anchoring arms (Figure 40-28).⁸⁴ Another (completely inapparent) form of "failure" is shown in Figure 40-29. The patient is cured of her symptoms of vaginal prolapse after a clinically successful posterior compartment mesh. However, she now complains of symptoms of obstructed defecation, and the reason is an enterocele that now descends into the anal canal rather than the vagina, causing a rectal intussusception. Clearly, translabial 4D ultrasound will be useful in determining functional outcome and location of implants, and will help in optimizing both implant design and surgical technique.

The main current use of 3D translabial ultrasound and axial plane imaging is the assessment of the levator ani muscle, occasionally extending to paraurethral tissues in patients with diverticulae or strictures. Translabial ultrasound has confirmed 60-year-old clinical data⁸⁸ and MRI studies⁸⁹ showing that major morphological abnormalities of levator structure and function are common in vaginally parous women.⁹⁰ Recently it has been conclusively proven that such morphological abnormalities are due to vaginal delivery²; see Figure 40-30 for a comparison of clinical findings, ultrasound and MR in a patient with unilateral levator avulsion, and Figure 40-31 for a comparison of antepartum and postpartum findings in patients with unilateral and bilateral avulsion injuries. Such trauma can be documented on 2D ultrasound—either with a side-firing endocavitary probe⁹¹ or with a para-sagittal probe orientation.⁹² When using a standard 2D abdominal probe for para-sagittal imaging the probe has to be tilted obliquely in order to follow the V shape of the puborectalis muscle between the inferior pubic ramus anteriorly and the anal canal posteriorly. Figure 40-32 shows 2D para-sagittal findings in a patient with unilateral trauma as verified by tomographic ultrasound (TUI). Although levator trauma can be seen on 2D imaging, the most convenient and reproducible approach is by using an abdominal 3D probe. Just as when imaging a fetal face, a rendered volume, with the rendering direction set from distally to proximally, results in optimal images. Tomographic imaging seems to be the most repeatable technique and can be used to bracket the entire puborectalis muscle.^93,94

Major delivery-related levator trauma, affecting the inferomedial aspects of the puborectalis muscle ("avulsion injury") clearly is part of the missing link between vaginal childbirth and prolapse. While there are other factors, probably including microtrauma or altered biomechanics of otherwise intact muscle, levator trauma seems to enlarge the hiatus⁹³ and results in anterior and central compartment prolapse.⁹⁰ Patients with avulsion defects are 2.3 times more likely to have a significant cystocele, and 4 times as likely to have uterine prolapse.⁹⁵ Avulsion reduces pelvic floor muscle function by about one-third^96,97 and has a marked effect on hiatal biometry and distensibility⁹⁸ (Figure 40-33).

The larger the defect, the higher the likelihood of prolapse,⁹³ as quantified on multislice or tomographic ultrasound (Figure 40-34). Levator defects seem to be associated with cystocele recurrence after anterior repair^65,98,99 and may well be a risk factor for prolapse recurrence after pelvic reconstructive surgery in general. These defects are palpable (Figure 40-35), but palpation requires significant teaching^4,5 and is clearly less repeatable (κ = 0.41)¹⁰² than identification by ultrasound (κ = 0.83 on analysis of whole volumes and κ = 0.61 for single slices in author's data).^93,101 Identification of an avulsion injury is aided by measurement of the "levator-urethra gap" (Figure 40-36), the distance from the center of the urethral lumen to the most medial aspect of the puborectalis muscle,¹⁰² analogous to the measurement of the levator-symphysis gap (LSG) on magnetic resonance imaging.¹⁰³

Levator trauma seems to be more likely in women who are older at the time of their first delivery, after delivery of large babies, after Forceps, and after a long second stage of labor.^{2,105,106,107, and 108} There seems to be a high prevalence of levator defects in women with anal sphincter defects, which is not really surprising given the overlap in risk factors.^104,105 Bilateral defects (see Figure 40-37 for a comparison of ultrasound and MR in a patient with bilateral trauma) are more difficult to detect on palpation, since there is no normal side to compare with, but they have a particularly severe impact on pelvic floor function and organ support.

Another factor only apparent on axial plane imaging is the degree of hiatal distension on Valsalva. Figure 40-38 illustrates the identification of the plane of minimal hiatal dimensions. After locating the minimal distance between the dorsal aspect of the symphysis pubis and the ventral aspect of the puborectalis loop, this line is then identified in the axial plane, defining the "plane of minimal dimensions." This is a useful approximation and helps to make both qualitative and quantitative assessment more reproducible, even though it is understood that the true plane of minimal dimensions is "non-euclidean," ie, warped.¹¹¹ Figure 40-39 gives an impression of the range of hiatal area measurements in patients attending a pelvic floor clinic. Measures of hiatal dimensions seem highly repeatable^{110,111,112,113,114, and 115} and correlate well with findings on magnetic resonance imaging.¹¹⁶ Hiatal enlargement to over 25 cm² on Valsalva is defined as "ballooning" on the basis of receiver operator characteristics statistics¹¹⁷ and normative data in young nulliparous women.^110,112 Hiatal dimensions are associated with the length of the second stage of labor and may be a surrogate measure of soft tissue compliance of the birth canal.^118,119 The degree of distension is strongly associated with prolapse¹²⁰ and symptoms of prolapse.¹¹⁷ It seems that ballooning is associated with rectal intussusception⁵⁴ and prolapse recurrence after rectocele repair,¹²¹ and the same probably holds for other forms of prolapse surgery.

If delivery-related trauma and excessive distensibility of the levator are indeed risk factors for female pelvic organ prolapse and recurrence after reconstructive surgery, then of course we should know about it preoperatively and adjust our surgical approach accordingly. Some forms of prolapse are probably impossible to cure surgically unless one uses mesh implants such as the transobturator mesh shown in Figure 40-40, reducing the effective size of the hiatus by bridging the defects. In the future, we should aim to develop surgical methods that reduce the size and distensibility of the hiatus or reconnect the detached muscle in an attempt to prevent recurrence—and in 2010 this was no longer a hypothetical goal. Over the next few years we are likely to see a rapid translation of imaging research into everyday surgical practice.

Even prior to the widespread introduction of 3D/4D imaging, pelvic floor ultrasound was a highly useful diagnostic tool for physicians dealing with pelvic floor disorders. While the uptake of this diagnostic method has varied enormously from one country to another, and from one specialty to another, there now is a substantial body of literature dealing with translabial/perineal/introital ultrasound and its use in women with pelvic floor dysfunction. As of 2010, clinicians using the method include not just gynecologists, urologists, urogynecologists, and radiologists, but also colorectal surgeons and gastroenterologists.

The near universal introduction of 4D ultrasound, new software options, and increasing availability of training will lead to more general acceptance of ultrasound as a standard diagnostic option in pelvic floor medicine. The issue of levator trauma, one of the most significant developments in clinical obstetrics since the introduction of fetal monitoring, will take pelvic floor ultrasound from a niche application into the mainstream and speed the convergence of clinical specialties dealing with pelvic floor disorders. Very likely, ultrasound imaging will help us to resolve some of the many open questions in pelvic floor medicine and lead to improvements in clinical skills and surgical techniques, benefiting our patients. The crucial issue, as always, is teaching and the provision of up-to-date resources.