(Radiology. 2001;218:621-641.)
© RSNA, 2001
Pelvic Floor Imaging1
Jaap Stoker, MD, PhD,
Steve Halligan, MD, MRCP, FRCR and
Clive I. Bartram, FRCR, FRCP, FRCS
1 From the Department of Radiology, Academic Medical Center, University of Amsterdam, PO Box 22700, 1100 Amsterdam, the Netherlands (J.S.), and the Intestinal Imaging Centre, St Mark’s Hospital, London, England (S.H., C.I.B.). Received June 18, 1999; revision requested August 9; revision received November 9; accepted November 16; updated September 28, 2000. Address correspondence to J.S. (e-mail: j.stoker@amc.uva.nl).
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ABSTRACT
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A greater awareness of the therapies now available for pelvic floor dysfunction has increased demand for specialized imaging of this region. Some of the techniques required are available at relatively few centers, and the purpose of this review is to introduce the emerging subspecialty of pelvic floor imaging to a more general readership. Pelvic floor anatomy is complex and is being unraveled by means of magnetic resonance (MR) imaging. This is discussed in detail by using a global, rather than a compartmentalized, anatomic approach. The physiology of normal urinary and anal function and the routine clinical tests applied to them are outlined. The imaging techniques involved include MR imaging, endosonography, and fluoroscopy. The main investigations include video urodynamic imaging, evacuation proctography, dynamic cystoproctography, dynamic MR imaging of the pelvic floor, and endoluminal imaging of the anal sphincters with MR imaging and ultrasonography. These are described in detail, and their role with regard to the main pathologic conditions of the pelvic floor—urinary and anal incontinence, constipation, and prolapse—are discussed.
Index terms: Bladder, abnormalities, 83.832, 83.835 • Colon, abnormalities, 75.133, 75.15, 75.27, 75.73, 75.79, 75.791 • Colon, MR, 75.121411, 75.121412 • Colon, US, 75.12981, 75.12989 • Pelvic organs, 80.11 • Pelvic organs, MR, 80.121411, 80.121412 • Pelvic organs, US, 80.12981, 80.12989 • State of the Art • Urine, incontinence, 82.835
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INTRODUCTION
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The pelvic floor is a complex system, with passive and active components that provide pelvic support, maintain continence, and coordinate relaxation during urination and defecation. Dysfunction may manifest as prolapse, incontinence, pelvic pain, or constipation. Imaging is becoming increasingly central to the management of pelvic floor dysfunction. The success of medical and surgical treatments coupled with a growing awareness among primary physicians and the general public of investigative and therapeutic possibilities are leading to an increasing number of referrals to radiologists with a special interest in this field (1). However, detailed knowledge of pelvic floor imaging is not widespread within the radiologic community. The purpose of this review is to describe the role of imaging in the diagnosis of adult male and female patients with pelvic floor dysfunction. More attention will be given to pelvic floor diseases in women, in whom these conditions are more prevalent.
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ANATOMY
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A basic knowledge of pelvic floor anatomy is fundamental to the imaging interpretation and understanding of dysfunction. The pelvic floor is classically divided into three compartments: anterior, middle, and posterior, to which a fourth compartment, the peritoneal cavity and fascia, is sometimes added. This segregation reflects historical boundaries between the various professional groups involved and is, to a large extent, artificial, because the pelvic floor structures are closely interrelated and patients with abnormalities in one compartment often have disorders in others (1–3). The anatomy of the pelvic floor will be described in an integrated fashion, with greater emphasis on female anatomy.
Pelvic Floor
The pelvic floor is a complex, integrated, multilayer system that provides active and passive support. Fascia and ligaments provide passive support, while the muscles of the pelvic floor, mainly the levator ani, provide active support. The fascia is attached to the bone ring of the pelvis, with the ligaments formed from fascial condensations. The pelvic floor has three layers from superior to inferior: the pelvic fascia, pelvic diaphragm, and urogenital diaphragm, with their associated supportive structures, which are intimately related to the urogenital region, urethra, anal sphincter, and vagina in women.
Pelvic Fascia
Pelvic fasciae are delicate structures, most of which are below imaging resolution. The most cephalad layer covers the levator ani muscle and viscera in a continuous sheet. At the uterine level, this layer is called the parametrium; at the vaginal level, it is called the paracolpium (4). Two dense aggregations of obturator and levator ani fasciae, the arcus tendineus fasciae pelvis and arcus tendineus levator ani, provide important passive lateral support (Figs 1, 2). The arcus tendineus fasciae pelvis provides lateral anchoring for the anterior vaginal wall where it underlies and supports the urethra, while the arcus tendineus levator ani provides anchoring for the levator ani muscles. The arcus tendineus levator ani can be identified on magnetic resonance (MR) images as the origin of a part the levator ani muscle (iliococcygeus muscle) at the internal obturator fascia (Figs 2–4) (4). The arcus tendineus fasciae pelvis (Figs 1, 2), although visualized at high-spatial-resolution MR imaging in a cadaver (5), is difficult to identify with certainty in vivo. This is due to the difficulty in differentiating this fascial aggregation from other closely aligned fascial structures and vessels.
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Figure 2. Drawing shows the retropubic space (space of Retzius) viewed from above, with the bladder and vagina transected at a level just below the bladder, revealing the attachment of the vagina to the levator ani muscles. ATFP = arcus tendineus fascia pelvis, ATLA = arcus tendineus levator ani, LA = levator ani muscles, OIF = obturatorius internus fascia, PS = pubic symphysis, PVM = pubovesical muscle, PVP = periurethral vascular plexus, R = rectum, U = urethra, VLA = vaginolevator attachment, VW = vaginal wall. (Reprinted, with permission, from reference 6.)
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Figure 3a. (a) Midvaginal coronal T2-weighted fast spin-echo (repetition time msec/echo time msec, 2,826/120) MR image obtained with an endovaginal coil and (b) corresponding drawing in a 23-year-old asymptomatic female volunteer demonstrate the vaginal wall (VW), urogenital diaphragm (UG), puborectal muscle (PR), bulbocavernous muscle (BC), levator ani muscle (LA), internal obturator muscle (IOM), arcus tendineus levator ani (ATLA), bladder (B), and uterus (UT).
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Figure 3b. (a) Midvaginal coronal T2-weighted fast spin-echo (repetition time msec/echo time msec, 2,826/120) MR image obtained with an endovaginal coil and (b) corresponding drawing in a 23-year-old asymptomatic female volunteer demonstrate the vaginal wall (VW), urogenital diaphragm (UG), puborectal muscle (PR), bulbocavernous muscle (BC), levator ani muscle (LA), internal obturator muscle (IOM), arcus tendineus levator ani (ATLA), bladder (B), and uterus (UT).
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Figure 4a. (a) Coronal T2-weighted fast spin-echo (2,500/100) MR image obtained through the anterior anal sphincter complex with an endoanal coil and (b) corresponding drawing in a 30-year-old asymptomatic female volunteer demonstrate the internal sphincter (IS), external sphincter (ES), puborectal muscle (PR), levator ani muscle (LA), arcus tendineus levator ani (ATLA), internal obturator muscle (IOM), and urogenital diaphragm (UG).
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Figure 4b. (a) Coronal T2-weighted fast spin-echo (2,500/100) MR image obtained through the anterior anal sphincter complex with an endoanal coil and (b) corresponding drawing in a 30-year-old asymptomatic female volunteer demonstrate the internal sphincter (IS), external sphincter (ES), puborectal muscle (PR), levator ani muscle (LA), arcus tendineus levator ani (ATLA), internal obturator muscle (IOM), and urogenital diaphragm (UG).
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The structure of these fasciae differs greatly from connective tissue that forms the tendons and ligaments in other parts of the body. For example, the endopelvic fascia comprises a meshwork of collagen, smooth muscle cells, fibroblasts, elastin, and neurovascular and fibrovascular bundles (4,6).
Pelvic Diaphragm
The levator ani is the major muscle of the pelvic diaphragm; it is attached to the pubis and supportive fascia by the arcus tendineus levator ani (Figs 3–5). The levator ani is readily visible at MR imaging. Several segments of the levator ani have been described in terms of their visceral insertions, although these are inseparable parts of a single unit (5). The ventromedial part of the levator ani, variously called the pubococcygeus muscle, pubovisceralis muscle, or puborectal muscle, is a thick, slinglike bundle of fibers arising from the inner aspect of the pubis, passing beside the urethra, vagina (Figs 3, 6), and anorectum (Figs 5, 7), and attaching to the vagina and anorectum (5). The slinglike configuration of the ventromedial part of the puborectal muscle may give the false impression that it is a separate muscle. Tonic contraction of the two parts of this sling closes the urogenital and anorectal hiatus, which provides a supportive platform during normal activity and standing. Constant tone is maintained by means of slow-twitch (type I) muscle fibers (7). The iliococcygeus and coccygeus muscles also are part of the levator ani. The origin of the iliococcygeus muscle is the arcus tendineus levator ani (Figs 2–4). The levator ani is innervated by sacral nerve roots S2 through S4 via the pudendal nerve.
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Figure 5a. (a) Coronal midanal T2-weighted fast spin-echo (2,500/100) MR image obtained with an endoanal coil and (b) corresponding drawing in a 21-year-old asymptomatic male volunteer demonstrate the internal sphincter (IS), intersphincteric space (ISS), longitudinal muscle (LM), external sphincter (ES), puborectal muscle (PR), and levator ani muscle (LA).
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Figure 5b. (a) Coronal midanal T2-weighted fast spin-echo (2,500/100) MR image obtained with an endoanal coil and (b) corresponding drawing in a 21-year-old asymptomatic male volunteer demonstrate the internal sphincter (IS), intersphincteric space (ISS), longitudinal muscle (LM), external sphincter (ES), puborectal muscle (PR), and levator ani muscle (LA).
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Figure 6a. (a) Transverse T2-weighted fast spin-echo (2,826/120) MR image obtained with an endovaginal coil and (b) corresponding drawing in an asymptomatic 31-year-old female volunteer show the vaginal wall (VW), puborectal muscle (PR), urethral smooth muscle (SM) and striated muscle (ST), urethral mucosa and submucosa (MS), urethropelvic (or parapelvic) ligament (UP) and parapelvic ligament (PU), anorectum (A), and internal anal sphincter (IS).
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Figure 6b. (a) Transverse T2-weighted fast spin-echo (2,826/120) MR image obtained with an endovaginal coil and (b) corresponding drawing in an asymptomatic 31-year-old female volunteer show the vaginal wall (VW), puborectal muscle (PR), urethral smooth muscle (SM) and striated muscle (ST), urethral mucosa and submucosa (MS), urethropelvic (or parapelvic) ligament (UP) and parapelvic ligament (PU), anorectum (A), and internal anal sphincter (IS).
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Figure 7a. (a) Transverse T2-weighted fast spin-echo (2,500/100) MR image obtained through the upper part of the anal sphincter with an endoanal coil and (b) corresponding drawing in an asymptomatic 30-year-old female volunteer show the internal sphincter (IS), longitudinal muscle (LM), puborectal muscle (PR), vagina (V), urethral mucosa and submucosa (MS), urethral smooth muscle (SM) and striated muscle (ST), urethropelvic (or parapelvic) ligament (UP), and parapelvic ligament (PU).
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Figure 7b. (a) Transverse T2-weighted fast spin-echo (2,500/100) MR image obtained through the upper part of the anal sphincter with an endoanal coil and (b) corresponding drawing in an asymptomatic 30-year-old female volunteer show the internal sphincter (IS), longitudinal muscle (LM), puborectal muscle (PR), vagina (V), urethral mucosa and submucosa (MS), urethral smooth muscle (SM) and striated muscle (ST), urethropelvic (or parapelvic) ligament (UP), and parapelvic ligament (PU).
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Urogenital Diaphragm
The urogenital diaphragm, also called the deep perineal space or perineal membrane, is situated caudal to the pelvic diaphragm and anterior to the anorectum. In men, it lies directly superior to the root of the penis and inferior to the prostate gland. The urogenital diaphragm is penetrated by the urethra and additionally by the vagina in women. The primary muscle of the urogenital diaphragm is the deep transverse muscle of the perineum, which originates at the inner surface of the ischial ramus and is readily visible at MR imaging (Figs 3, 4). The urogenital diaphragm has attachments to surrounding structures, including the vagina, perineal body, external anal sphincter, and bulbocavernous muscle (8). The puborectal (pubococcygeus) muscle courses directly superior to the urogenital diaphragm toward the pubic bone (Fig 3).
Urethra
The female urethra is approximately 4.5 cm long, with two-thirds of the urethra above the levator ani (pelvic diaphragm). The proximal one-third of the female urethra and the proximal (prostatic) male urethra are lined with transitional cell epithelium. The more distal portion of the urethra is lined with (pseudo-) stratified columnar epithelium. Infoldings of urothelial tissue with rich submucosal vascular plexuses, mucosal secretions, and urethral smooth muscle all contribute to passive urethral closure (urethral coaptation, mucosal seal) (9,10).
The urethral sphincter is composed of involuntary inner smooth muscle that is continuous with the bladder, as well as the voluntary external sphincter (rhabdosphincter), which is composed of striated muscle. Mucosa and submucosa, inner smooth muscle, and external urethral striated sphincter are readily visible at high-resolution MR imaging in all individuals (Figs 6, 7). The inner smooth muscle sphincter extends throughout the proximal two-thirds of the urethra, and its tension is distributed relatively uniformly and contributes to about one-third of intraurethral pressure (10). The smooth muscle of the urethra has high signal intensity on T2-weighted MR images and demonstrates enhancement after intravenous administration of contrast medium (8). This manifestation is common to smooth muscle sphincters, including the smooth muscle internal anal sphincter. Although this manifestation probably is related to the specific histologic characteristics of smooth muscles, no definite explanation is available, to our knowledge.
The external sphincter contributes mainly to resting pressure by means of slow-twitch muscle fibers. The urogenital diaphragm muscle (deep transverse muscle of the perineum) predominantly contributes to voluntary and reflex muscle contraction. There is debate with regard to the neural pathways to the external urethral sphincter (11).
Urethral and Bladder Neck Supporting Structures
Fascial and ligamentous support of the urethra and the bladder neck is vital to preserve urinary continence. Recent research with cadaver dissection in combination with MR imaging has provided insights into the anatomy of urethral and bladder neck support. However, the exact anatomy remains to be established, and the terminology used is confusing. A description of the most important structures will be provided in this article, but for a detailed description the reader is referred to other sources (4,8,12–15).
Vesicopelvic, urethropelvic, and pubourethral ligaments and fascia give anterior and lateral support to the bladder neck and urethra by means of attachment to the pubic bone and arcus tendineus fasciae pelvis (Fig 2) (12,13). Current research with high-resolution MR imaging in cadavers and in vivo is directed to revealing which ligaments and muscles can be identified. The urethropelvic ligaments can be easily identified with high-resolution MR imaging in almost all individuals, but there is no consensus about whether these structures insert directly into the levator ani muscle or through another (periurethral) ligament (Figs 6, 7) (8,13). Other ligaments between the urethra and pubic arch (pubourethral ligament) and between the bladder and pubic arch (vesicopelvic ligament) can be identified in some individuals. The pubovesical ligament or muscle (Fig 2), an extension of detrusor smooth muscle coursing through the retropubic space to the arcus tendineus fasciae pelvis, has been identified at high-resolution MR imaging in a cadaver (5) and may assist in opening the bladder neck during voiding. Bladder neck position is influenced by connections between the puborectal muscle, vagina, and proximal urethra (16).
Urogenital Region
The urogenital region forms the superficial part of the anterior pelvic floor. The external genital muscles—the superficial transverse muscle of the perineum, the bulbocavernous (bulbospongiosus) muscle, and the ischiocavernous muscle—lie within this region, as does the urethrovaginal sphincter in women (Fig 8). The urethrogenital sphincter encircles the urethra and vagina. This sphincter has been considered part of the deep transverse muscle of the perineum (urogenital diaphragm), but the authors of a recent endovaginal MR imaging study (8) suggested that it might be part of the puborectal (pubococcygeus) muscle. In women, the bulbocavernous muscles inserts into the pubic arch and the root and dorsum of the clitoris and has attachments to the vagina and urogenital diaphragm (8). The external genital muscles are visible with high-resolution MR imaging in all individuals (Fig 8).
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Figure 8a. (a) Transverse T2-weighted fast spin-echo (2,500/100) MR image obtained through the lower edge of the anal sphincter with an endoanal coil and (b) corresponding drawing in a 25-year-old asymptomatic female volunteer show the external sphincter (ES), anococcygeal ligament (AC), bulbocavernous muscle (BC), ischiocavernous muscle (IC), superficial transverse perineal muscle (STP), and vaginal introitus (VI).
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Figure 8b. (a) Transverse T2-weighted fast spin-echo (2,500/100) MR image obtained through the lower edge of the anal sphincter with an endoanal coil and (b) corresponding drawing in a 25-year-old asymptomatic female volunteer show the external sphincter (ES), anococcygeal ligament (AC), bulbocavernous muscle (BC), ischiocavernous muscle (IC), superficial transverse perineal muscle (STP), and vaginal introitus (VI).
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Vagina
The vaginal wall is composed of smooth muscle and connective tissue (collagen and elastin) (4). The anterior wall is bridged bilaterally between the arcus tendineus fasciae pelvis (Fig 1) (4,17). This "hammock" of vaginal tissue lies beneath the urethra and may be a vital component of urinary continence (12,18). The urethra is compressed against this tissue when abdominal pressure is increased, as during coughing (12). The vagina is attached to the levator ani at the level of the urogenital diaphragm (16). Its wall is surrounded by adipose tissue containing an extensive venous plexus. Posteriorly, this forms the rectovaginal septum.
Perineal Body
Directly anterior to the anal sphincter is the perineal body (central tendon of the perineum). In men, it is posterior to the spongious and cavernous bodies and their related muscles, whereas in women it lies within the anovaginal septum (Fig 9). Many structures insert fibers into the perineal body, including the external anal sphincter, the deep and superficial transverse muscles of the perineum (urogenital diaphragm), and the bulbocavernous and puborectalis (pubococcygeus) muscles. The superficial transverse muscle of the perineum spans the dorsal edge of the urogenital diaphragm and elevates the perineal body.
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Figure 9a. (a) Midanal sagittal T2-weighted fast spin-echo (2,826/120) MR image obtained with endoanal coil and (b) corresponding drawing in a 30-year-old asymptomatic female volunteer demonstrates the internal sphincter (IS), longitudinal muscle (LM), external sphincter (ES), puborectal muscle (PR), levator ani muscle (LA), perineal body (P), vagina (V), urethra (U), bladder (B), and rectum (R) (window settings optimized for demonstration of the external sphincter).
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Figure 9b. (a) Midanal sagittal T2-weighted fast spin-echo (2,826/120) MR image obtained with endoanal coil and (b) corresponding drawing in a 30-year-old asymptomatic female volunteer demonstrates the internal sphincter (IS), longitudinal muscle (LM), external sphincter (ES), puborectal muscle (PR), levator ani muscle (LA), perineal body (P), vagina (V), urethra (U), bladder (B), and rectum (R) (window settings optimized for demonstration of the external sphincter).
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Anal Sphincter
The anal canal sphincter contains muscular and neurovascular components and is surrounded by the fat-containing ischioanal space. The epithelial lining of the distal part of the anal canal is stratified squamous epithelium, richly supplied by sensory receptors that are concentrated mostly at the dentate line. This line demarcates the junction with the proximal columnar epithelium, which is devoid of sensory receptors. The mechanism of anal continence is similar to that of urinary control, with the anal lining and its underlying vascular spaces, the anal cushions, playing a major role in sealing the anal canal (19). The anal sphincters form two cylindric layers between which lies the longitudinal muscle (Fig 5). The intersphincteric space is a plane, composed of fat, usually located between the longitudinal muscle and external sphincter.
The internal sphincter forms the innermost muscular layer and is the terminal condensation of the circular rectal smooth muscle (Fig 5). The internal sphincter extends from the anorectal junction to approximately 1–1
cm below the dentate line. It is composed of smooth muscle fibers with autonomous innervation from sympathetic presacral nerves. The longitudinal muscle (Fig 5) is the continuation of the longitudinal muscle of the rectal wall. There is a large fibroelastic element derived from the pelvic fascia that invests both sphincters. The longitudinal muscle is closely related to the subcutaneous external sphincter. Some authors have stated that the longitudinal muscle ends in the subcutaneous external sphincter, while others have stated that this muscle passes through the subcutaneous external sphincter to terminate in the perianal skin (20).
The external sphincter is the outermost muscle of the distal anal canal (Figs 5, 8, 10) and is composed of several parallel bundles (20). It is a circular structure and is shorter anteriorly in women, approximately 1 cm. The external sphincter extends approximately 1 cm beyond the internal sphincter (Fig 5). The deep part of the external sphincter is fused with or intimately related to the puborectalis muscle. Anteriorly, it is closely related to the superficial transverse muscle of the perineum and the perineal body (Figs 8, 9). Posteriorly, the muscle is continuous with the anococcygeal ligament (Figs 8–10). All sphincter muscles are readily seen at endoanal MR imaging. The muscle is under voluntary control and is innervated by the pudendal nerves (S2 through S4). The puborectalis muscle has separate innervation from S3 and S4.
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Figure 10a. (a) Transverse T2-weighted fast spin-echo (2,500/100) MR image obtained through the lower part of the anal sphincter with an endoanal coil and (b) corresponding drawing in a 40-year-old asymptomatic male volunteer show the internal sphincter (IS), longitudinal muscle (LM), intersphincteric space (ISS), external sphincter (ES), anococcygeal ligament (AC), and bulbocavernous muscle (BC).
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Figure 10b. (a) Transverse T2-weighted fast spin-echo (2,500/100) MR image obtained through the lower part of the anal sphincter with an endoanal coil and (b) corresponding drawing in a 40-year-old asymptomatic male volunteer show the internal sphincter (IS), longitudinal muscle (LM), intersphincteric space (ISS), external sphincter (ES), anococcygeal ligament (AC), and bulbocavernous muscle (BC).
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Normal variants of anal sphincter anatomy have been identified, such as differing relationships between the superficial transverse perineal muscle and the external sphincter (21). The inferior edge of the external sphincter may have a closed circular configuration or may be open anteriorly and posteriorly. After trauma (eg, obstetric), the anal sphincter anatomy may be disturbed without clinical symptoms (22,23). These findings may possibly be related to late-onset incontinence.
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PHYSIOLOGY
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Urinary Continence
The mechanism of urinary control has been intensively studied, but its elucidation remains incomplete. Urinary continence requires integration of the central and peripheral nervous systems, the bladder wall and detrusor muscle, the bladder neck, the urethra, and the pelvic support structures (10). Central nervous system perception of bladder filling results in the generation of efferent impulses that inhibit bladder contraction. Increased intraabdominal pressure is countered by urethral closure, which requires adequate pelvic support (9). Passive urethral closure is reliant on good urethral coaptation from a mucosal seal formed by the submucosal vascular plexuses and urethral smooth muscle (9). The pelvic floor reflexly contracts in response to sudden increases in abdominal pressure, resulting in active urethral closure and passive compression. It is widely believed that increased intraabdominal pressure simultaneously increases intravesicular and intraurethral pressure because the bladder neck and the proximal female urethra normally lie above the pelvic floor (24). Alternatively, it has been suggested that urethral compression against the endopelvic fascia and vagina are responsible for closure—the "hammock hypothesis" (12).
Micturition
The bladder can accommodate large volumes without substantially increased intravesicular pressure because of the passive viscoelastic properties of the smooth muscle and connective tissue of its wall. Filling leads to reflex stimulation of 
-adrenergic receptors within the bladder neck and proximal urethral smooth muscle, which increases outlet resistance. The efferent somatic nerves also stimulate the striated muscular external urethral sphincter. During bladder contraction, there is coordinated opening first of the proximal sphincter, with funneling of the vesical neck and proximal urethra and then of the distal sphincter, at which time voiding will ensue. If the sphincter contracts during bladder contraction, continence will be maintained or voiding will cease if the pressure is greater than that within the urethra or bladder.
Anal Continence
Anal continence is maintained by means of a complex interrelationship between anal and pelvic floor musculature integrated by means of somatic and autonomic nervous control. The smooth muscle of the internal sphincter maintains a tonic contraction, keeping the anal canal closed at rest. The puborectalis muscle and external anal sphincter also display some resting tone but contract rapidly to prevent incontinence in response to any sudden increase in intraabdominal pressure.
Defecation
It is likely that defecation is initiated by colonic smooth muscle contractions, which are provoked by waking and after eating. These contractions propel stool from the sigmoid colon into the normally empty rectum and stimulate rectal sensory nerves that produce an urge to defecate. These nerves are also used to determine the nature of rectal content. The sensation of a full rectum and the ability to discriminate gaseous, liquid, and solid content are important components of continence. Interestingly, sensation is retained after rectal excision, suggesting that some sensory receptors reside in the pelvic floor (25). Rectal filling causes reflexive internal sphincter relaxation (the rectoanal inhibitory reflex), rectal contraction, and contraction of the puborectalis muscle and external sphincter, both of which are heavily modulated with conscious control. Stool in the anal canal contacts sensory receptors concentrated at the level of the dentate line and greatly intensifies the urge to defecate, which is resisted by vigorous striated muscle contraction until circumstances for defecation are appropriate. When such circumstances are present, pelvic floor relaxation and increased intraabdominal pressure create a positive pressure gradient from the rectum to the anus to allow evacuation.
Internal sphincter disorders result in passive incontinence (ie, fecal leakage without awareness) (26), whereas external sphincter damage results in urge incontinence (ie, the patient is unable to delay defecation).
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FUNCTIONAL DIAGNOSTIC TESTS
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Urinary Functional Tests
Urodynamic tests are the most important functional tests in urinary dysfunction. Electromyography (EMG), performed with surface electrodes, or needle EMG may be used to evaluate neurologic dysfunction. Many urodynamic tests have been developed (27,28), and an extensive description is beyond the scope of this article. The simple description provided here is limited to lower urinary tract studies. Unfortunately, the sensitivity and specificity of many of these techniques has not been fully evaluated (29).
Flow rate measurement is an index of the volume voided in a unit time, expressed as millimeters per second. Detrusor underactivity, instability, and outlet obstruction produce abnormal flow patterns. Cystometrography (CMG) is used to measure the relationship between bladder pressure and volume for evaluation of the detrusor functions of compliance and contractility. Urethral pressure profiles are records of intraurethral pressure and can demonstrate pressure equalization between the bladder and urethra at the high-pressure area, which, in the absence of detrusor activity, is considered to be indicative of genuine stress incontinence (ie, urinary incontinence caused by hypermobility of the bladder neck) (29). Detrusor leak-point pressure and abdominal leak-point pressure represent those bladder pressures at which leakage occurs in the absence of increased intraabdominal pressure and at straining, respectively (9). An abnormality suggests an intrinsic sphincter deficiency, and the examination should be performed in conjunction with fluoroscopy to identify potential pitfalls such as the presence of a cystocele (9).
Video urodynamics (VUDO), also called video cystourethrography, is considered to be the best method for investigation of lower urinary function but remains relatively underevaluated (29,30). It is a combination of CMG and voiding cystourethrography that integrates pressure and imaging studies (9). Common indications include failed incontinence surgery and equivocal results after urodynamics. VUDO is a routine technique in many tertiary referral centers. An intravesicular catheter is used to measure bladder pressure during filling with contrast medium, as in CMG, to provide information on bladder capacity and compliance. Simultaneous measurement of intraabdominal pressure is achieved by means of a rectal or vaginal catheter, and subtraction of intraabdominal pressure from intravesicular pressure determines the true detrusor pressure. Urethral closure pressure can be calculated from the difference between urethral and bladder pressures. Voiding flow rate and voided volume are measured by using a transducer placed under a collecting receptacle. Interruption of voiding allows observation of bladder neck closure and urethral "milk-back." Urodynamic measurements are integrated with fluoroscopic findings.
Anorectal Physiology Testing
A variety of techniques are available to test anorectal nerve integrity, conduction, and muscular performance. Few are absolutely diagnostic, and most must be considered together with symptoms, clinical findings, and imaging results. However, these techniques provide valuable complementary information that radiologists working in this field should be aware of. Normal values vary between laboratories.
Manometry
Manometry is used to determine rectal and anal pressures. The systems in use vary in complexity from simple balloons connected to a pressure transducer to perfused multichannel catheters capable of simultaneous measurement of pressures at several sites to ambulatory systems that can record for 24 hours or longer. The pressure recorded will increase when a rectal catheter is withdrawn into the anus and decrease again when it reaches the anal margin, indicating the functional anal canal length. This is usually longer than its anatomic length. A static anal catheter will measure resting anal canal pressure, which is predominantly an internal sphincter function (31).
Resting pressure is reduced in incontinence, owing to abnormality of the internal sphincter. In contrast, squeeze pressure, the incremental increase over resting pressure elicited when the patient is asked to voluntarily contract his or her anus, reflects external sphincter function. This may be reduced when incontinence is due to external sphincter tears, such as those that occur with obstetric injury. A dual sphincter abnormality is suggested when both resting and squeeze pressures are abnormal.
Pudendal Nerve Latency
Pudendal nerve terminal motor latency can be determined on the basis of the time needed for a digitally delivered pudendal nerve stimulus to elicit anal contraction. This is measured by using a disposable glove with a stimulating electrode at the fingertip coupled with a pressure sensor at the base of the glove. The nerve is stimulated near the ischial spine and has both sensory and motor components. Slow conduction is thought to be due predominantly to a stretch-induced injury. This may follow childbirth (32) or chronic straining and is a transient phenomenon even in healthy subjects asked to strain excessively. The clinical relevance of pudendal neuropathy remains unclear, not least because it is a normal feature of aging. The degree of neuropathy and pelvic floor descent should be directly related, although this has not been demonstrated (33). Incontinence is usually attributed to neuropathic sphincter degeneration if the latencies are abnormal and the sphincters intact. Sphincter repair is less successful if there is an underlying neuropathy (34).
EMG Studies
A needle electrode inserted into the external sphincter can determine both its electric activity and quality. Denervation is followed by reinnervation via neighboring healthy axons and is identified at EMG when the recorded action potentials become polyphasic. Prior to endoanal ultrasonography (US), EMG was the only reliable method to preoperatively identify external sphincter tears. No muscle potential would be seen in an area of scarring. Circumferential needle passes around the anus would "map" the extent of the muscle tear or "defect." EMG is painful, and a local anesthetic cannot be used because it interferes with recording. Fortunately, endoanal US is superior for detecting sphincter defects (35).
EMG may be combined with defecography to record external sphincter activity during evacuation. Normally, striated muscle should switch off completely during evacuation. Increased activity suggests anismus. Anismus can be defined as functional evacuatory failure, frequently associated with involuntary contraction, instead of relaxation, of striated pelvic floor musculature during attempted evacuation. Anismus may also be diagnosed manometrically if anal canal pressure increases paradoxically during straining.
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IMAGING TECHNIQUES
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US of the Bladder, Bladder Base, Urethrovesical Junction, and Urethra
Bladder US to determine the residual volume is frequently combined with flow rate estimation. Bladder volume can be determined and calculated by means of various methods that use correction factors for the nonspherical shape of the bladder (36). The residual volume may be increased in outlet obstruction, overflow incontinence (incontinence caused by overdistention of the bladder), and decreased bladder contractility. An increased wall thickness and increased flow at the fundus of the bladder during color Doppler US may indicate detrusor instability (37). US evaluation of urethrovesical junction mobility and bladder base morphology was initially performed by using transabdominal techniques and later by using endorectal, endovaginal, and perineal approaches. These techniques can be used for accurate evaluation of urethrovesical junction mobility and may be comparable to voiding cystourethrography (38–40).
A low resting position for the bladder neck and basal descent of 1 cm or more are used to identify hypermobility of the bladder neck in stress incontinence, although there is some overlap with findings in healthy subjects (38,39,41). Rotational descent of the proximal urethra is common in stress urinary incontinence (42). The volume of the striated muscular sphincter (rhabdosphincter), determined with three-dimensional transperineal US, is decreased in genuine stress incontinence (43).
Voiding Cystourethrography
Voiding cystourethrography (VCUG) is performed primarily to detect a cystocele and evaluate urethrovesical junction mobility. When combined with evacuation proctography, VCUG is termed cystoproctography (44–47). Lateral fluoroscopy at rest, during coughing, and during voiding helps differentiate between bladder neck descent or urethrovesical junction hypermobility, defined by bladder base descent below the inferior margin of the pubic symphysis (45), and bladder base descent (cystocele) (Fig 11) (46). A cystocele is frequently overlooked during physical examination, and VCUG or cystoproctography are more accurate. Urethrovesical junction mobility can be related to bone landmarks (pubococcygeal line) or a small radiopaque intraurethral tube (45), with travel of more than 1 cm indicative of hypermobility (46). An open bladder neck and proximal urethra at rest in the absence of detrusor contraction (funneling) may indicate an intrinsic sphincter deficiency (urethral sphincter defect). However, there is a weak relationship between bladder neck funneling at rest and urodynamic parameters of intrinsic sphincter deficiency (46,47).
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Figure 11. Lateral dynamic cystoproctogram in a 62-year-old woman with urinary and fecal incontinence and feelings of incomplete evacuation reveals a cystocele (C) and enterocele (E). Part of the perspex seat is visible as a horizontal bar. A = anterior.
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The results of VCUG in patients with stress urinary incontinence are influenced by concomitant detrusor instability, which can be identified only with simultaneous urodynamics (47). Considerable intra- and interobserver variation during VCUG has been reported (48). Currently, the main role of VCUG or cystoproctography in cases of stress incontinence is for help in the detection of a concomitant cystocele (Table 1) (45,49,50). Urethrovesical junction mobility and intrinsic sphincter deficiency are better evaluated with US and urodynamics, whereas MR imaging may prove to be valuable in the near future. VCUG can help detect other pathologic conditions such as urethral diverticula, vesicoureteric reflux, and fistulas (51).
MR Imaging of Bladder Neck, Urethra, and Supportive Structures
MR imaging has been applied to anterior pelvic floor imaging because of the intrinsic soft-tissue contrast capability of MR imaging, which permits detailed visualization of the pelvic floor (13,14, 52,53). The high spatial resolution possible with endoluminal coils has further improved anatomic evaluation, and the potential value of this technique in urinary incontinence is currently being assessed (8). T2-weighted fast spin-echo MR sequences can be used to delineate the multiple layers of the urethra and its supportive structures and for the detection of urethral diverticula. Faster MR techniques such as fast gradient-echo and echo-planar imaging allow dynamic imaging (2,53–55). In an experimental study (56), urine volume and flow rate were measured by using these techniques during voiding.
Endoanal US
Endoanal US of the anal sphincters is achieved by the simple expedient of replacing the balloon system used for rectal scanning with a hard cone (57). The cylindric nature of the anal structures favors the 360° axial view at right angles to the lumen obtained with a mechanically rotated endoprobe. The cone is filled with degassed water for acoustic coupling and is covered with a lubricated condom. It is introduced into the rectum, aligned in standard orientation with the anterior end uppermost, and then slowly withdrawn down the anal canal. Images are obtained at proximal, middle, and distal levels. Some image asymmetry may be induced if the patient is in a left-lateral position, and it is preferable to examine the patient prone.
The puborectalis muscle and transverse perinea demarcate the proximal portion of the canal. The former blends into the external sphincter in the middle part of the canal, forming a complete ring anteriorly. The internal sphincter is thickest in the middle part of the anal canal and is of uniformly low reflectivity, contrasting sharply against the more heterogeneous subepithelial tissues medially and the longitudinal muscle laterally (Fig 12). The subcutaneous external sphincter, lying below the termination of the internal sphincter, defines the distal portion of the canal. Although the external sphincter, intersphincteric plane, and longitudinal muscle are each relatively heterogeneous, there are characteristic features visible when the latest generation of high-frequency (10-MHz) transducers, which allow these to be distinguished, are used.
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Figure 12. Transverse endoanal US image obtained with a 10-MHz transducer shows normal sphincter anatomy in a 37-year-old asymptomatic male volunteer. Subepithelial tissues (SE), the internal sphincter (IS), the intersphincteric space and longitudinal muscle (IL), and the external sphincter (ES) are visible.
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The intact sphincters form the cornerstone of a normal examination because observations of morphology are more variable. External sphincter measurements are subject to considerable interobserver variation, owing to the lack of any precise outer US demarcation and variable acoustic shadowing from the intersphincteric-fat plane interface (58). In contrast, the internal sphincter has clearly defined borders and may be measured with some degree of reproducibility. The internal sphincter thickens with age, measuring approximately 2–3 mm in adults (59).
MR Imaging of the Anal Sphincter
The anal sphincter can be visualized with the body coil alone or with a phased-array or endoluminal coil. Examination with an endoluminal coil results higher spatial resolution but a limited field of view (Fig 5). Examination with an endoanal coil is especially suitable for demonstrating subtle changes within the sphincters, since these lie close to the coil surface. The spatial resolution provided by either a phased-array or a body coil is probably insufficient to aid in the diagnosis of sphincter abnormalities (60). Rigid endoanal coils are preferred for optimal image quality; the diameter of such coils ranges from 7 to 19 mm (21,61). The diameter used is a compromise between imaging of an effective volume and overcompression of adjacent structures. Muscle relaxants may be used to reduce artifacts due to peristalsis (21).
The optimal sequence for evaluating anal sphincter anatomy and abnormalities has not been established. We routinely use a T2-weighted sequence (eg, fast spin-echo) as our basic sequence. The use of T1-weighted sequences, either standard or dynamic with contrast medium, increase cost, and their superiority over other sequences has not been established. The transverse plane, oriented to be at a right angle to the anal canal, is the most relevant surgically and is supplemented by use of longitudinal planes to provide additional information on the extent of disease (62). Similar to the urethra, the smooth muscle of the internal sphincter is relatively hyperintense, and the striated muscle of the external sphincter, puborectal muscle, and levator ani muscle are relatively hypointense on T2-weighted images.
Endoanal MR imaging is well tolerated by nearly all patients and is easily performed (21). Discomfort is limited and comparable to that at endoanal US, although the procedure is more time-consuming (approximately 30 minutes vs 5 minutes) (21).
Colonic Transit Studies
The colon and rectum operate as a single functional unit, so investigation of constipation usually incorporates assessment of colonic transit. A variety of techniques of varying complexity are available, and the level of information needed largely determines which is chosen. Accurate assessment of segmental transit requires scintigraphy to quantify the passage of radioisotope through the colon (63). This is time-consuming and expensive, however, and a simple assessment of whole gut transit by using radiopaque markers is usually all that is necessary, particularly because the relevance of segmental transit disorders remains controversial. A simple method involves acquisition of a single abdominal image 6 days after radiopaque markers have been ingested. This method provides an overall estimation of whole gut transit (64) and has been validated by comparing such images with radionuclide studies (63).
Evacuation Proctography
Evacuation proctography is a simple radiologic technique that involves imaging of rectal voiding of a barium paste enema. Radiologic studies of rectal evacuation have been performed for nearly 50 years but became more widely utilized after description of a simplified technique in 1984 (65). Although evacuation proctography is now well established, its use is predominantly confined to specialist centers. Furthermore, the role of this technique remains controversial despite several years of clinical use. The examination is also termed defecography, but the evacuation during the test is not associated with any of the colonic reflexes that accompany normal defecation, so this term may be misleading; rather, it is a test of voluntary rectal evacuation. Constipation—namely, difficult or infrequent rectal evacuation—is the main indication for performance of evacuation proctography. Some authors have stressed a role in incontinence, since an obtuse anorectal angle may help identify patients likely to benefit from postanal repair if the anal sphincters appear normal at US. It has been argued, however, that involuntary loss of barium alone may be sufficient for diagnosis, and many investigators believe that evacuation proctography has a limited role in this scenario.
Technique.—Evacuation proctography is a rapid and simple technique to perform. The rectum may be emptied prior to evacuation proctography, either by administering glycerin suppositories or an enema. These maneuvers have been criticized as unphysiologic, but they do standardize the examination if a fixed volume of contrast medium is given. This allows comparison with normal of evacuation time and completeness, as well as with any change on follow-up studies. The passage of stool during the examination may interfere with the visualization of an intussusception.
In general, a prepared examination is more acceptable to both patients and staff. It is generally accepted that the consistency of the contrast medium should approximate that of feces: a barium suspension mixed with either potato starch or methylcellulose. Commercial preparations are also available. The total volume used is variable; some investigators instill contrast medium until a strong urge to evacuate is provoked, whereas others use an identical volume in all cases, for the reasons given earlier (we use 120 mL).
The paste is injected with a syringe into the rectum, with the patient in the left-lateral position on the fluoroscopy table. The injection is continued during withdrawal of the syringe, to mark the anal canal and verge. The table is moved into the upright position, and a commode is placed on the footrest. The commode must incorporate some filtration to prevent screen flare. Four millimeters of copper provides sufficient attenuation (66). Although many authors stress the importance of the more physiologic and sensitive seated position, the examination can be performed in the left-lateral position if a commode is unavailable or if the patient is incontinent, but static values for pelvic floor position are higher (67). It is essential to perform continuous or rapid recording of rectal evacuation by means either of spot imaging or videofluoroscopy. Although spot imaging provides the best spatial resolution, the ability with videofluoroscopy to replay the entire examination at any speed is an invaluable feature if evacuation is rapid or findings are subtle; this is also possible with some digital systems. Patient dose is also lower than that for standard imaging, although newer digital systems allow substantial reduction in dose. A suitable compromise is to videotape the examination with spot images obtained at key events (68). To reduce dose, imaging should be intermittent if evacuation is prolonged.
Normal study.—Based on the findings in 56 asymptomatic patients, Mahieu et al (65) defined five criteria for a normal evacuation proctographic study: increase in anorectal angulation, obliteration of the puborectal impression, wide anal canal opening, total evacuation of contrast medium, and normal pelvic floor resistance. Several subsequent studies with asymptomatic volunteers have revealed a wide range of normal values, and some overlap with pathologic states (66,69). Nevertheless, there is a consensus regarding normal findings, with broad agreement with the description by Mahieu et al.
The proctographic component of the examination can be considered in three stages: the preevacuation, evacuation, and postevacuation stages. The initial lateral view records the preevacuation anorectal configuration and pelvic floor position. The anorectal angle is thought to be important in maintaining continence and is usually measured between the axis of the anal canal and the posterior rectal wall. Although considerable attention has been devoted to this measurement, there is little evidence that it is important. The junction between the rectal ampulla and the anal canal, the anorectal junction, is easy to appreciate at rest. The pubococcygeal line (Fig 13) is traditionally used to define the level of the pelvic floor but may be difficult to identify with a limited field of view, in which case the inferior surface of the ischial tuberosities are used instead. At rest the anorectal junction should be at or just above this plane, and the anal canal should be closed without leakage. The anorectal angle should be approximately 90°. Some investigators advocate the acquisition of views during a "squeeze" maneuver to evaluate the strength of voluntary pelvic floor musculature, during coughing to stress the continence mechanism, and during straining to assess pelvic floor descent. However, the anorectal angle may increase during these maneuvers in up to 30% of normal subjects, which reflects paradoxical pelvic floor contraction to maintain continence (70).
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Figure 13a. Dynamic sagittal T1-weighted gradient-echo (79/240; flip angle, 55°) MR images obtained in a 45-year-old woman with pelvic pain. B = bladder, line = pubococcygeal line. (a) Image obtained at rest. (b) Image obtained during maximal straining shows a large cystocele (c).
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Figure 13b. Dynamic sagittal T1-weighted gradient-echo (79/240; flip angle, 55°) MR images obtained in a 45-year-old woman with pelvic pain. B = bladder, line = pubococcygeal line. (a) Image obtained at rest. (b) Image obtained during maximal straining shows a large cystocele (c).
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Initiation of evacuation should be rapid and involves pelvic floor descent and loss of the puborectal impression, with the anorectal angle becoming more obtuse by approximately 20°, and the anal canal shortening as it opens widely. After evacuation, the canal closes and the anorectal angle and junction return to their original positions. Patients who digitate to aid emptying should be instructed to do so, so that the effect of such maneuvers can be evaluated. It may be relevant to examine the patient in the frontal position, to reveal the coronal configuration of the rectum, because this helps diagnose intussusception (71).
Modifications to the basic technique.—The technique described in the preceding paragraphs will demonstrate the rectal configuration during evacuation and help determine the rate and completeness of evacuation. It is now well recognized that weakness of the middle and anterior pelvic floor often accompanies posterior weakness, so there may be any combination of cystocele, rectocele, enterocele, sigmoidocele, and gynecologic prolapse (1).
An enterocele is defined as prolapse of the small bowel into the rectogenital space; and a sigmoidocele, as prolapse of the sigmoid colon into this space. These are notoriously difficult to detect clinically. In one study (72) with 300 women, enteroceles were revealed with dynamic cystoproctography in 111, of which 93 (84%) were missed at clinical examination. With standard proctographic techniques, these entities will also be missed, and the authors of that study stressed the benefits of an integrated approach to pelvic floor imaging. To achieve this, the most common proctographic modification is oral administration of a barium suspension approximately 1–2 hours before the procedure to opacify the small bowel (68). Alternatively, a vaginal marker in women can help diagnose prolapse by demonstrating rectovaginal separation during or after evacuation. The best choice is probably a barium paste, because a tampon may inhibit prolapse by splinting the vagina (73). However, enteroceles may develop without substantial rectovaginal separation. In addition, liquid barium can be injected before administration of the paste, to opacify the sigmoid colon and identify a sigmoidocele.
Dynamic cystoproctography is essentially evacuation proctography preceded by cystography (70,72,74). Catheterization of the bladder is performed with a 5-F feeding tube (after manual reduction of a prolapse, if necessary). The bladder is then drained and filled with water-soluble contrast medium (45). Lateral views of the bladder are obtained at rest and during maximal straining (45). The bladder should then be emptied completely, because the space in the pelvic cavity is limited; a cystocoele will prevent formation of an enterocoele and vice versa. To overcome the "crowded pelvis," the cystographic phase should also be performed before rectal filling, because rectal distention elevates the bladder base and may mask a cystocele (1).
Some investigators (75,76) have also introduced water-soluble contrast medium directly into the peritoneal cavity to image the pelvic peritoneal recesses during voiding, a practice that is unnecessarily invasive since the advent of dynamic MR imaging. Evacuation proctography has also been combined with simultaneous cystography, EMG, and intrarectal pressure measurement, but the benefits of these combinations remain uncertain.
Dynamic MR Imaging
Although evacuation proctography is rapid and easy to perform, the modifications needed to image other organs are time-consuming and invasive. Also, the musculature of the pelvic floor is not visualized directly, and irradiation of younger patients is an important factor. To overcome these limitations, MR imaging has been applied to pelvic floor dynamics with promising results, although the experimental nature of this procedure must be emphasized. Initial study results were necessarily compromised by slow acquisition times (77), but advances in MR technology now allow multisection imaging during a single straining effort (2,78,79).
The basic examination requires no patient preparation. Vaginal and rectal lumina may be intubated with a soft catheter to facilitate identification. The patient is positioned supine in the magnet, on protective pads, and the pelvis is imaged at rest by using a rapid sequence (eg, fast spoiled gradient-echo acquisition in steady state). Images may be obtained in sagittal, coronal, and transverse planes. The sagittal plane best demonstrates the relationship of the pelvic viscera to each other and the pelvic floor (Fig 13). Imaging is then repeated while the patient performs a maximal straining effort, which has been practiced with the patient beforehand. The resulting images are best viewed as a cine loop to appreciate organ dynamics.
This is an evolving field, and an optimal technique has yet to emerge. Like cystoproctography, some authors catheterize and fill both the bladder and the rectum and encourage the patient to evacuate within the magnet, an examination that has been termed colpocystorectography and undoubtedly provides the most informative study, at the expense of some inconvenience (54). Direct comparison has been made with proctography, and, not surprisingly, there is considerable discrepancy between results (80). This is probably a consequence of the necessary supine position, which can be overcome by those fortunate enough to have access to a vertical open MR system (81). Normal values will have to be determined for this evolving technique, but a study (82) with 50 asymptomatic individuals found some crossover with values assumed to indicate a pathologic condition. The technique is especially valuable for help in the diagnosis of perineal hernia, because both the herniated bowel (usually the rectum) and the defect in the levator plate can be seen.
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PATHOLOGIC CONDITIONS
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Urinary Incontinence
Urinary incontinence has been defined as "a condition in which involuntary loss of urine is a social or a hygienic problem and is objectively demonstrable" (83). Urinary incontinence afflicts 13 million Americans—85% of whom are women—at a cost to the economy of $16 billion annually (30,84). The incidence increases with age (85,86) and leads to social withdrawal in approximately 20% of patients (30). Common types of incontinence are urge incontinence, stress incontinence, or a combination of the two (30). Less common causes include urethral diverticula and ov
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ABSTRACT
INTRODUCTION
