Pelvic Floor Ultrasound
Female pelvic floor dysfunction encompasses a number of prevalent clinical conditions, including female pelvic organ prolapse, urinary and fecal incontinence, obstructed defecation, and sexual dysfunction. In most cases, neither etiology nor pathophysiology are well understood. Imaging has great potential to enhance both our research and clinical management capabilities in this field, and to date this potential is underutilised. Of the available techniques such as X-ray, computed tomography, magnetic resonance imaging, and ultrasound, the last is generally superior for pelvic floor imaging, especially in the form of perineal or translabial imaging. The technique is safe, simple, cheap, and easily accessible, and provides high spatial and temporal resolutions. Translabial or perineal ultrasound is useful in determining residual urinary volume, detrusor wall thickness, and bladder neck mobility, and in assessing pelvic organ prolapse as well as levator function and anatomy. It is also useful for the evaluation of obstructed defecation and fecal incontinence. It is at least equivalent to other imaging techniques in diagnosing conditions as diverse as urethral diverticulum, rectal intussusception, and avulsion of the puborectalis muscle. Ultrasound is currently the only imaging method capable of visualizing modern slings and mesh implants, and it may help in selecting patients for prolapse mesh surgery by identifying those most at risk of recurrence. Birth trauma to the levator ani muscle seems to be the central etiological factor for pelvic organ prolapse and recurrence after prolapse surgery, and it is easily diagnosed by pelvic floor ultrasound. Similarly, delivery-related trauma to the external and internal anal sphincter is central to the aetiology of anal incontinence in women and is equally easily diagnosed using either endo- or exoanal (translabial or perineal) ultrasound. This review will focus on translabial/perineal ultrasound as the least invasive, cheapest, simplest, and most commonly available method for pelvic floor imaging. I will discuss the main current uses of the method in clinical practice and research, and will try and provide a perspective for likely future developments in this field.
KeywordsFemale pelvic organ prolapse Levator ani Pelvic floor Ultrasound Incontinence Mesh
The constantly growing interest in pelvic floor imaging is attributable to a number of distinct factors. Female pelvic floor dysfunction encompasses several highly prevalent conditions. The estimated lifetime risk of surgery for POP or urinary incontinence in women is quoted as 10–20 % [6, 7]. Despite the high prevalence, however, our understanding of the aetiology and pathophysiology of female pelvic floor dysfunction is limited, resulting in largely empirical treatment without solid scientific basis. This is reflected in a high re-operation rate of about 1 in 3 .
The recent re-discovery of the link between childbirth-related pelvic floor muscle trauma and female POP is a good example of how imaging may help us understand the pathophysiology of pelvic floor dysfunction [8, 9, 10, 11, 12•], allowing primary [13•] and secondary prevention trials and hopefully resulting in effective treatment based on translational research [14, 15]. It is more and more obvious that clinical assessment alone is insufficient to assess pelvic floor function and anatomy. Our clinical examination generally focuses on the description of surface anatomy which is often unable to reveal true underlying structural abnormalities. To give one example related to the diagnosis of prolapse of the posterior vaginal compartment, which is common in women with symptoms of prolapse and obstructed defecation: gynecologists call posterior vaginal wall descent a ‘rectocele’, but this appearance may be caused by at least five distinct anatomical conditions which are difficult to distinguish without imaging. These include true radiological rectocele, perineal hypermobility, enterocele, rectoenterocele, and rectal intussusception [16, 17]. Colorectal surgeons commonly avoid this particular pitfall by employing defecation proctography, but this method involves radiation exposure, is very unpleasant for the patient, and is unable to demonstrate problems in other compartments and abnormalities of the levator ani, all of which are common in women with obstructed defecation.
Of more importance in the realm of the gynecologist is the fact that a clinical assessment for female POP is to a large extent determined by confounders that are commonly overlooked, such as bladder  and rectal filling, levator co-activation , and the duration of a Valsalva maneuver . It is not surprising that findings in the operating theatre often vary from those obtained in the outpatient setting, since all potential confounders act to increase the likelihood of false-negative results. Hence, one should ensure bladder (and, if possible, rectal) emptying, a sufficiently long maximal Valsalva of at least 5 s, and relaxation of the levator ani. Real-time imaging makes it much easier for the operator to avoid these confounders by using visual biofeedback.
This review is limited to translabial/perineal ultrasound imaging. Some investigators prefer to use ‘introital ultrasound’, a technique involving the use of front-firing vaginal endo-probes placed in the vaginal introitus. Such probes allow higher resolutions, but there are limitations in assessing the effect of maneuvers and in imaging of the levator ani, which is why I am not going to discuss this technique further. However, most of the issues described here also apply to introital ultrasound.
Instrumentation and Basic Methodology
The basic requirements for perineal or translabial imaging include a B-mode capable 2-dimensional (2D) ultrasound system with cine-loop function, a 3.5- to 6-MHz curved array transducer and a video-printer. Such systems can be obtained at low cost and are commonly available, as they are widely used for imaging of the fetus or the paediatric and adult abdomen. The examination is performed in dorsal lithotomy, with hips flexed and slightly abducted and the heels placed close to the buttocks, or in the standing position.
Indications for pelvic floor ultrasound imaging
Recurrent urinary tract infections
Urgency, frequency, nocturia, and/or urge urinary incontinence
Stress urinary incontinence
Insensible urine loss
Symptoms of voiding dysfunction
Symptoms of prolapse, i.e., sensation of lump or dragging sensation
Symptoms of obstructed defecation, e.g., straining at stool, chronic constipation, vaginal or perineal digitation, and sensation of incomplete bowel emptying
Pelvic or vaginal pain after anti-incontinence or prolapse surgery
Vaginal discharge or bleeding after anti-incontinence or prolapse surgery
Vaginal laxity, psychosexual dysfunction
Translabial ultrasound has long been used in the evaluation of stress urinary incontinence. Due to the assumption that urethral hypermobility is important in the etiology of female stress urinary incontinence, bladder neck mobility was one of the first proposed indications for ultrasound imaging in urogynaecology. On Valsalva, the proximal urethra can be seen to rotate postero-inferiorly around the fulcrum of the symphysis pubis. To measure bladder neck position and mobility, points of reference are either the central axis of the symphysis pubis  or its infero-posterior margin , with the latter more practicable due to transducer dimensions and calcification of the interpubic disc in older women.
Female Pelvic Organ Prolapse
Female pelvic organ prolapse (FPOP) is a common indication for surgical treatment, with a lifetime risk of between 10 and 20 % [6, 7]. Most prolapse procedures are performed by gynecologists and urogynecologists, but urologists and colorectal surgeons also undertake such surgery. The aetiology of FPOP is likely to be multifactorial and is not completely understood as yet . Congenital factors clearly play a role , and lifestyle factors such as obesity and asthma are said to contribute. However, pregnancy and childbirth, especially vaginal delivery, are the commonest and most substantial modifiable risk factors. The relative contribution of factors appears to vary for different forms of prolapse: childbirth-related pelvic floor trauma seems to play only a minor role in the aetiology of rectocele [43, 44], but it is of paramount importance for uterine and bladder prolapse [45, 46], likely at least partly mediated through the increasingly well-defined factor of levator trauma [9, 11] (see below).
As regards the central compartment, the uterus can at times be difficult to identify because of its iso-echoic nature, similar in echotexture to the vaginal wall, especially in postmenopausal women with atrophic uteri (see Fig. 6). Occasionally ultrasound can show the effect of an anteriorised cervix in women with an enlarged, retroverted uterus, explaining symptoms of voiding difficulty. On the other hand, a mobile acutely anteverted uterus can result in compression or ‘plugging’ of the anorectum, explaining symptoms of obstructed defecation—what radiologists call a ‘colpocele’ on defecation proctography.
While many women with such anatomical abnormalities of the posterior compartment are asymptomatic, defecatory symptoms are common in patients with pelvic floor dysfunction. Defecation proctography is the gold standard in the investigation of patients with obstructed defecation by imaging specialists, but the technique involves the use of ionising radiation, is invasive, and costly. It is therefore not surprising that colorectal surgeons and gastroenterologists have started using ultrasound [54, 55, 56], which is much better tolerated  and cheaper, in the initial investigation of women with defecatory symptoms. If there is a rectocele or a rectal intussusception on ultrasound, this condition is very likely to be found on defecation proctography [17, 57•]. It is highly probable that ultrasound will replace radiological techniques in the initial investigation of women with defecatory symptoms [50••, 56].
Of course, diagnostic information only contributes to improved outcomes if it is interpreted correctly and utilized by the clinician. Unfortunately, it is not always clear what kind of therapeutic consequences one should draw from imaging findings such as that of a rectal intussusception. However, one would certainly not expect a rectocele repair to alleviate symptoms due to intussusception. If there is a true rectocele then clearly a defect specific rectocele repair  is the surgical treatment of choice. On the other hand, if posterior compartment descent is due to a hyperdistensible fascia as in the case of perineal hypermobility then one is unlikely to find an RVS defect in theatre. Fascial plication or even a levatorplasty may be a better surgical treatment option in such patients. However, this approach would not be expected to resolve symptoms of obstructed defecation, which may be functional in origin. Finally, it seems to make little sense to remove portions of rectal wall, as in the stapled transanal rectal resection (STARR) procedure, and expect cure of symptoms of obstructed defecation in someone who has a herniation of the rectal wall due to a defect of the RVS. Clearly, there is much we still have to learn in the management of patients with posterior compartment prolapse and obstructed defecation.
Endoanal ultrasound is routinely used to evaluate the anal sphincters, especially in women with fecal incontinence. This involves placement of an ultrasound probe inside the anal canal which distorts anatomy and precludes assessment on sphincter contraction which can enhance tissue definition. Exoanal ultrasound imaging on the other hand, first described by an obstetrician in 1997 , does not have these disadvantages. It is now widely used to image the anal sphincter using either endovaginal or transabdominal probes and is especially useful in its 3D/4D incarnation [60, 61, 62, 63]. The method has been shown to correlate well with 2D endoanal imaging . It will however be necessary to validate both methods against symptoms in a comparative manner to determine their relative value.
Provided the current controversy surrounding mesh use does not preclude any chance of further development, translabial 4D ultrasound will be useful in determining functional outcome and location of implants, and will help in optimizing implant design and surgical technique. We have identified three distinct anatomical forms of failure after anterior compartment mesh use: (1) anterior failure: cystocele ventral and caudal to a well supported mesh; (2) apical failure: cystocele/anterior enterocele/uterine prolapse dorsal and caudal to a mesh with high mobility of the cranial mesh aspect; and finally (3) global failure: cystocele with high mobility of cranial and caudal mesh aspects on Valsalva. Most recurrent cystocele after mesh placement is due global or apical failures, suggesting dislodgment of lateral and/or apical anchors or fixation . A small minority are anterior failures, implying dislodgment of the mesh from the bladder base due to faulty surgical technique.
Hopefully, the above illustrates the potential impact of functional imaging on postoperative audit and technological innovation and development in pelvic reconstructive surgery. Over the last decade, there has been a trend towards patient-centered outcomes in surgery, generally assessed using questionnaires. However, questionnaires are poor outcome measures for surgical trials as they are not very sensitive to change, and of course subjective outcomes allow no insight into pathophysiological mechanisms. Ultrasound may be a more powerful tool in assessing surgical outcomes than subjective symptoms and questionnaires . Imaging measures of prolapse seem to be more strongly associated with symptoms rather than the results of a clinical assessment . Hence, it seems reasonable to assume that sonographic assessment of surgical outcomes can enhance research and technological development in pelvic reconstructive surgery.
Axial Plane Imaging: The Levator Muscle and Hiatus
Direct imaging of the levator ani, which requires access to the axial plane, has experienced a boost with the development of 3D ultrasound. Side-firing vaginal transducers can image the axial plane on 2D, but they were never widely used. The translabial use of standard, commonly available abdominal 4D probes has advantages over endosonography, even if spatial resolutions are often inferior. With an acquisition angle of 70° or higher a single volume dataset includes the entire levator hiatus with symphysis pubis, urethra, paravaginal tissues, the vagina, anorectum, and puborectalis muscle. A Valsalva maneuver can result in displacement of parts of the puborectalis muscle outside the field of vision, especially in women with significant hiatal ballooning , which is why acquisition angles of 80° or even 85° are preferable.
Figure 3 demonstrates the main display modes on 3D/4D pelvic floor ultrasound imaging. The orthogonal display mode (A or midsagittal, B or coronal, C or axial plane) shows cross-sectional planes through the volume in question, with each plane situated at right angles to the other two. Distinct from MR, ultrasonic imaging planes can be varied arbitrarily to enhance the visibility of any structure. Imaging of the levator ani usually requires an axial plane that is slightly tilted, and this tilt varies substantially between patients and with maneuvers. Hence, it is very difficult to obtain correct axial planes on dynamic MR imaging, with a consequent reduction in accuracy .
The three orthogonal planes are often complemented by a ‘rendered image’, that is, a semitransparent representation of all pixels in a region of interest or ‘ROI’ defined by the investigator. Section d in Fig. 2 shows a ‘rendered image’, with the rendering direction set from caudally to cranially. The result is a representation that corresponds to seeing the patient’s pelvis from below. Rendering can substantially enhance the visibility of certain structures (e.g., synthetic implants) which helps patients and caregivers understand anatomical relationships. Figure 9 shows orthogonal and rendered volume representations of a typical true rectocele. 4D imaging involves the real-time acquisition of volume ultrasound data, that is, a succession of volumes over time, not just a single volume. This is particularly useful for the evaluation of functional anatomy, that is, for observing morphological changes during maneuvers (Valsalva or pelvic floor muscle contraction), which is virtually impossible on MR. And even if one was able to obtain true dynamic MR imaging, the lack of real-time control would preclude any opportunity for assessing the quality of such maneuvers and to avoid levator coactivation . Hence, ultrasound has substantial advantages for prolapse assessment, especially when associated with fascial or muscular defects, and for defining functional anatomy. Finally, offline analysis software allow distance, area, angle, and volume measurements in any user-defined plane in a manner that is superior to what is currently possible with DICOM viewer software on MRI images.
Axial plane imaging is highly useful for the assessment of the levator ani muscle and hiatus, and for the examination of para-urethral tissues in patients with diverticula or strictures. Translabial ultrasound has confirmed forgotten 70-year-old clinical data  and recent MRI studies , showing that major structural abnormalities of the levator ani muscle are common in vaginally parous women  and that they are clearly due to vaginal childbirth . Such defects are common and have now been confirmed by multiple research groups [12•, 79, 80, 81, 82].
Major delivery-related levator injury clearly plays a substantial role in the aetiology of FPOP, although there are likely to be other birth-related factors, including ‘microtrauma’ or altered biomechanics of otherwise intact muscle [12•] and fascial trauma. Avulsion results in enlargement of the levator hiatus [87, 88] and is associated with reduced contractile strength . It increases the likelihood of prolapse, especially in the anterior and central compartments [9, 10]. The larger the defect, the higher is the likelihood of prolapse , as quantified on multi-slice or tomographic ultrasound (see Fig. 15). Levator defects are also associated with cystocele recurrence after anterior repair with or without mesh [90••, 91•, 92]. These defects can be diagnosed clinically with palpation, which, however, does require significant teaching and is probably less repeatable than imaging .
Currently, there is a highly controversial debate on mesh use. On the one hand, mesh implants have been shown to be effective in reducing prolapse recurrence [102••], especially in patients with levator avulsion . On the other hand, mesh implants are associated with a higher rate of potentially serious complications such as chronic pain. Clearly, the balance between risk and benefit of mesh use will vary from one individual to the other; the real question is probably not whether to use mesh but in whom. Together with other published risk factors, such as young age, advanced prolapse stage before operation, family history of prolapse, previous failed surgery, enlarged urogenital hiatus, and Oxford grading [90••, 91•, 104, 105], the state of the patient’s pelvic floor may help identify patients at high risk of recurrence, justifying the consideration of mesh use.
Imaging may also allow the development of innovative surgical approaches that could obviate the need for intravaginal mesh use. Surgical attempts to reconnect the detached muscle both as a primary procedure in delivery suite  and as a secondary procedure during prolapse surgery  have been reported; however, results to date have been disappointing. In another development, surgical reduction of hiatal ballooning appears to be feasible  through accessing the ischiorectal fossa. The effect of hiatal reduction surgery on prolapse recurrence remains doubtful and will require a randomized controlled trial.
Sonographic imaging of pelvic floor structures, in particular in the form of translabial 3D/4D ultrasound, is becoming increasingly useful in the hands of physicians, surgeons, and researchers investigating pelvic floor disorders. The near-universal distribution of 4D ultrasound systems, new software options, and the increasing availability of training will likely lead to the acceptance of this method as an integral part of pelvic floor medicine. The issue of levator trauma, one of the most significant developments in clinical obstetrics in the last decade or two, will enhance this trend and enable an entirely new degree of cooperation and understanding between the different clinical specialties dealing with patients affected by pelvic floor disorders. As always, the provision of up-to-date resources for teaching and training of the next generation of healthcare providers is of paramount importance.
Hans Peter Dietz has served as a consultant for Materna and has received grant support and had travel/accommodation expenses covered/reimbursed by GE Medical.
Compliance with Ethics Guidelines
This article does not contain any studies with human or animal subjects performed by any of the authors.