(Radiographics. 1999;19:S171-S177.)
© RSNA, 1999
Endoanal MR Imaging of the Anal Sphincter in Fecal Incontinence1
Elena Rociu, MD,
Jaap Stoker, MD,
Andries W. Zwamborn and
Johan S. Laméris, MD
1 From the Department of Radiology, Erasmus Medical Center, Rotterdam, the Netherlands (E.R., J.S., A.W.Z.); and the Department of Radiology, G 1-211, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ Amsterdam, the Netherlands (E.R., J.S., J.S.L.). Presented as a scientific exhibit at the 1998 RSNA scientific assembly. Received January 27, 1999; revision requested March 16 and received April 12; accepted April 12. Address reprint requests to E.R.
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Abstract
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Fecal incontinence is a major medical and social problem. The most frequent cause is a pathologic condition of the anal sphincter. Endoanal magnetic resonance (MR) imaging allows detailed visualization of the normal anatomy and pathologic conditions of the anal sphincter. The hyperintense internal sphincter appears as a continuation of the smooth muscle of the rectum; the hypointense external sphincter surrounds the lower part of the internal sphincter. A sphincteric defect is seen as a discontinuity of the muscle ring. Scarring appears as a hypointense deformation of the normal pattern of the muscle layer. Two external sphincteric patterns may be misdiagnosed as defects: a posterior discontinuity (often seen in young male patients) and an anterior discontinuity (often seen in female patients). Atrophy of the external sphincter is easily detected on coronal MR images by comparing the thicknesses of all anal muscles. Endoanal MR imaging is superior to endoanal ultrasonography because of the multiplanar capability and higher inherent contrast resolution of the former. Use of endoanal MR imaging may lead to better selection of candidates for surgery and therefore better surgical results. Endoanal MR imaging is the most accurate technique for detection and characterization of sphincteric lesions and planning of optimal therapy.
Index Terms: Anus, abnormalities, 757.40 • Anus, MR, 757.12141 • Magnetic resonance (MR), intracavitary coils, 757.12141
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INTRODUCTION
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Fecal incontinence is the inability to voluntarily control defecation. The prevalence of fecal incontinence ranges from three to 10 per 1,000 but may actually be much higher (1). Most affected persons do not seek help, chiefly because of embarrassment or lack of awareness that help is available. This chronic disability has a serious emotional impact that may result in an increased risk for behavioral problems and social isolation. Women are affected more often than men; the condition is often a consequence of childbirth (2).
At first, fecal incontinence was thought to be caused by neurogenic dysfunction of the anal complex. Therefore, diagnostic techniques were focused mostly on functional information. The development of imaging techniques that
involve the use of endoluminal devices—endoluminal ultrasonography (US) and endoluminal magnetic resonance (MR) imaging—resulted in good visualization of the anal sphincter. This development made possible a better understanding of the pathologic condition by showing that sphincteric lesions are the main cause of fecal incontinence (3,4). Consequently, detailed imaging of the sphincter became important in the diagnosis and treatment of the disease.
The main lesion is usually confined to the sphincter, but sometimes the cause of fecal incontinence occurs higher in the digestive tract (eg, chronic diarrhea or rectal prolapse) (5). Fecal incontinence caused by rectal prolapse is imaged with defecography (6), which allows visualization of morphologic characteristics of the pelvic floor during defecation; defecography will not be discussed in this article.
Because the most frequent cause of fecal incontinence is a pathologic condition of the anal sphincter, the purpose of this article is to demonstrate the value of endoanal MR imaging of the anal sphincter in patients with fecal incontinence. Specific topics discussed are as follows: technique, normal anatomy and pathologic conditions, relative values of other modalities, and effect on treatment of fecal incontinence.\.
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TECHNIQUE
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MR imaging with a dedicated endoanal coil was developed during the past 5 years (7,8). Endoanal MR imaging has multiplanar capability and higher inherent contrast resolution than other techniques. We perform endoanal MR imaging with a 1.5-T unit (Gyroscan ACS-NT; Philips, Best, the Netherlands) and a dedicated endoanal coil (diameter, 19 mm). Axial proton-density–weighted three-dimensional gradient-echo (GRE) imaging (repetition time msec/echo time msec = 30/13, 60° flip angle, 140 x 112-mm field of view, 205 x 256 matrix, 2-mm section thickness) and sagittal and coronal T2-weighted fast spin-echo imaging (2,800/120, echo train length of 10, 120 x 90-mm field of view, 228 x 256 matrix, 4-mm section thickness) are performed.
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NORMAL ANATOMY AND PATHO-LOGIC CONDITIONS
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Endoanal MR imaging allows detailed visualization of the normal anatomy and pathologic conditions of the anal sphincter. The hyperintense internal sphincter is visible as a continuation of the circular smooth muscle of the rectum (Fig 1). The hypointense external sphincter surrounds the lower part of the internal sphincter (Fig 1). The puborectal muscle swings around the upper part of the external sphincter and is continued cranially by the levator ani muscle (Fig 2).
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Figures 1, 2. Normal anatomy. (1) Axial proton-density-weighted GRE endoanal MR image (30/13, 60° flip angle) shows a normal internal sphincter (IS) and external sphincter (ES). IAS = ischioanal space. (2) Axial proton-density-weighted GRE endoanal MR image (30/13, 60° flip angle) shows a normal puborectal muscle (PR). IAS = ischioanal space, U = urethra.
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Figures 1, 2. Normal anatomy. (1) Axial proton-density-weighted GRE endoanal MR image (30/13, 60° flip angle) shows a normal internal sphincter (IS) and external sphincter (ES). IAS = ischioanal space. (2) Axial proton-density-weighted GRE endoanal MR image (30/13, 60° flip angle) shows a normal puborectal muscle (PR). IAS = ischioanal space, U = urethra.
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A sphincteric defect is defined as a discontinuity of the muscle ring (Figs 3–6). Scarring is defined as a hypointense deformation of the normal pattern of the muscle layer due to replacement of muscle cells by fibrous tissue (Figs 6, 7). Two external sphincteric patterns may be misdiagnosed as defects. The first, a posterior discontinuity of the muscle ring (Fig 8), is often seen in young male patients. This finding may be interpreted as a defect but is in fact a normal variant produced by the anococcygeal ligament. On images obtained in higher planes, the external sphincteric fibers merge posteriorly, reestablishing the common shape. The second pattern, an anterior discontinuity of the external sphincter (Fig 9), is often seen in female patients. This discontinuity is created by the direction of the external sphincteric fibers. Most fibers of the superficial and deep parts of the external sphincter insert into the central tendon of the perineum, but some fibers continue forward and insert anteriorly in the perineal raphe. Especially in women, this insertion may be large and the imbrication of the muscle fibers may be pronounced. The resulting open elliptic shape is in fact a normal variant but may be misdiagnosed as an anterior defect.
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Figure 3. Sphincteric defect in a patient with fecal incontinence after hemorrhoidectomy. Axial proton-density-weighted GRE endoanal MR image (30/13, 60° flip angle) shows a defect of the internal sphincter (IS) (arrows). ES = external sphincter.
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Figures 4, 5. (4) Sphincteric defect in a woman with fecal incontinence after giving birth. Axial proton-density-weighted GRE endoanal MR image (30/13, 60° flip angle) shows a defect of the external sphincter (ES) (arrows). A small coil artifact is evident (arrowhead). IS = internal sphincter. (5) Sphincteric fragmentation in a patient with fecal incontinence after rape. Axial proton-density-weighted GRE endoanal MR image (30/13, 60° flip angle) shows fragmentation of the external sphincter (ES) (arrows). IS = internal sphincter.
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Figures 4, 5. (4) Sphincteric defect in a woman with fecal incontinence after giving birth. Axial proton-density-weighted GRE endoanal MR image (30/13, 60° flip angle) shows a defect of the external sphincter (ES) (arrows). A small coil artifact is evident (arrowhead). IS = internal sphincter. (5) Sphincteric fragmentation in a patient with fecal incontinence after rape. Axial proton-density-weighted GRE endoanal MR image (30/13, 60° flip angle) shows fragmentation of the external sphincter (ES) (arrows). IS = internal sphincter.
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Figure 6. Complex lesion. Axial proton-density-weighted GRE endoanal MR image (30/13, 60° flip angle) shows defects of the external sphincter (ES) (white arrows) and internal sphincter (IS) (black arrows), scarring (S), and asymmetry of the anal complex.
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Figure 7. Sphincteric scarring. Axial proton-density-weighted GRE endoanal MR image (30/13, 60° flip angle) shows scar tissue (S) of the internal sphincter (IS) in the intersphincteric space. ES = external sphincter.
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Figures 8, 9. Normal variant. (8) Axial proton-density-weighted GRE endoanal MR image (30/13, 60° flip angle) shows a posterior discontinuity of the external sphincter (ES) (arrow). IS = internal sphincter. (9) Axial proton-density-weighted GRE endoanal MR image (30/13, 60° flip angle) shows an anterior discontinuity of the external sphincter (ES) (curved arrows). This finding may be interpreted as a defect but is in fact a normal variant produced by the proximity of the transverse perineal muscle (TPM), which makes visualization of the external sphincteric fibers difficult. A small coil artifact is evident (straight arrow). IS = internal sphincter.
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Figures 8, 9. Normal variant. (8) Axial proton-density-weighted GRE endoanal MR image (30/13, 60° flip angle) shows a posterior discontinuity of the external sphincter (ES) (arrow). IS = internal sphincter. (9) Axial proton-density-weighted GRE endoanal MR image (30/13, 60° flip angle) shows an anterior discontinuity of the external sphincter (ES) (curved arrows). This finding may be interpreted as a defect but is in fact a normal variant produced by the proximity of the transverse perineal muscle (TPM), which makes visualization of the external sphincteric fibers difficult. A small coil artifact is evident (straight arrow). IS = internal sphincter.
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Because of the fine demonstration of muscle layers, endoanal MR imaging may also allow detection of local thinning and atrophy of the sphincter. Atrophy of the external sphincter, defined as an extreme generalized thinning (Fig 10), has been proved to be a predictive factor for a negative outcome of sphincteric surgery (9). On coronal images, the thickness of all anal muscles may be compared; such comparison makes external sphincteric atrophy easy to detect (Fig 11). Several studies have demonstrated a good correlation between atrophy as seen at endoanal MR imaging and the findings at surgery and histopathologic investigation of the biopsy specimens (8,9). Therefore, endoanal MR imaging is of major importance in the preoperative assessment of fecal incontinence.
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Figure 11a. Normal anatomy versus external sphincteric atrophy at coronal MR imaging. ES = external sphincter, IS = internal sphincter, LAM = levator ani muscle, PR = puborectal muscle. (a) Coronal T2-weighted fast spin-echo endoanal MR image (2,800/120) shows a normal sphincteric complex. (b) Coronal T2-weighted fast spin-echo endoanal MR image (2,800/120) shows atrophy of the external sphincter.
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Figure 11b. Normal anatomy versus external sphincteric atrophy at coronal MR imaging. ES = external sphincter, IS = internal sphincter, LAM = levator ani muscle, PR = puborectal muscle. (a) Coronal T2-weighted fast spin-echo endoanal MR image (2,800/120) shows a normal sphincteric complex. (b) Coronal T2-weighted fast spin-echo endoanal MR image (2,800/120) shows atrophy of the external sphincter.
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RELATIVE VALUE OF OTHER MODALITIES
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In patients with fecal incontinence, pathologic conditions of the anal sphincter include neurogenic dysfunction and traumatic sphincteric lesions. Several diagnostic techniques provide information about the structure or function of the anal sphincter. The detection of structural lesions has the most impact because the treatment of neuropathic sphincteric conditions often produces disappointing results. The patients with sphincteric lesions may benefit from surgical repair. Sphincteric lesions may be caused by vaginal delivery, pelvic surgery (for prolapse, hemorrhoids, or hysterectomy), or rape.
Digital examination may reveal perianal hypoesthesia, decreased sphincteric resting and squeezing tone, or sometimes sphincteric defects (3). Anorectal manometry is important for measurement of the resting pressure (mainly due to the internal sphincter) and the squeezing pressure (mainly due to the external sphincter) (10). Both resting and squeezing pressures are reduced in patients with fecal incontinence. Manometry is also important to establish the presence of the rectoanal inhibitory reflex, which is often reduced or absent in patients with fecal incontinence. Pudendal nerve terminal motor latencies are valuable in the study of the innervation of the external sphincter and are often prolonged in neurogenic fecal incontinence (10). Electromyography is useful in mapping defects of the external and internal sphincters and studying the density of muscle fibers, which is increased in patients with neurogenic fecal incontinence. This blind, painful, and invasive method is being replaced by endoluminal imaging (11).
Endoanal US is useful in the detection of internal sphincteric defects. However, it is limited by inherent poor contrast, which leads to cumbersome identification of the external sphincter. Surgery or vaginal delivery often damages this muscle, which is of major importance in continence (2). Endoanal US does not accurately demonstrate the external sphincter and therefore may not allow detection of external sphincteric thinning and atrophy. This technique is also operator dependent.
US is cheaper, more widely available, and quicker than endoluminal MR imaging. Earlier studies demonstrated that US (Fig 12) or MR imaging performed with a body coil (Fig 13) does not result in adequate contrast and spatial resolution (12). The role of MR imaging performed with a phased-array coil has not been evaluated, to our knowledge, but the local spatial resolution of this technique is inferior to that of endoluminal techniques (Fig 14). Endoanal MR imaging is superior to endoanal US because of the multiplanar capability and higher inherent contrast resolution of the former. Endoanal MR imaging has been proved to be more accurate than endoanal US in the detection of sphincteric lesions (13). Although endoanal MR imaging is not operator dependent, it is more expensive than endoanal US. A recent study indicated that if only one technique were to be used, MR imaging would provide the optimal decision in more cases than would US (13). The better performance of endoanal MR imaging could prevent unnecessary operations and may make MR imaging cost-effective.
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Figures 13, 14. (13) Appearance at body coil MR imaging. Axial T2-weighted fast spin-echo MR image (5,200/ 132) obtained with a body coil shows that the external sphincter (ES) and internal sphincter (IS) are not well demonstrated. Accurate evaluation of sphincteric pathologic conditions is therefore hardly possible. (14) Appearance at phased-array coil MR imaging. Axial T2-weighted fast spin-echo MR image (5,200/132) obtained with a phased-array coil shows more detail of the anal sphincters than does body coil MR imaging (cf Fig 13) but less detail than endoanal MR imaging (cf Fig 1). ES = external sphincter, IS = internal sphincter.
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Figures 13, 14. (13) Appearance at body coil MR imaging. Axial T2-weighted fast spin-echo MR image (5,200/ 132) obtained with a body coil shows that the external sphincter (ES) and internal sphincter (IS) are not well demonstrated. Accurate evaluation of sphincteric pathologic conditions is therefore hardly possible. (14) Appearance at phased-array coil MR imaging. Axial T2-weighted fast spin-echo MR image (5,200/132) obtained with a phased-array coil shows more detail of the anal sphincters than does body coil MR imaging (cf Fig 13) but less detail than endoanal MR imaging (cf Fig 1). ES = external sphincter, IS = internal sphincter.
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EFFECT ON TREATMENT OF FECAL INCONTINENCE
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To treat fecal incontinence, the physician may choose among several modalities. The conservative approach consists of constipation agents, electrical biofeedback therapy, and exercises to strengthen the pelvic floor. The most used surgical therapy is anterior anal repair. Unfortunately, it has only a 65% success rate. A recent study indicated that external sphincteric atrophy detectable only with endoanal MR imaging could be predictive of a poor outcome of anterior anal repair (9). In this respect, the use of endoanal MR imaging may lead to better patient selection and therefore better results. The alternative for patients with persistent fecal incontinence after sphincteroplasty commonly was creation of a stoma. New and promising surgical treatments include transposition of striated skeletal muscles combined with implantation of neurostimulators (eg, dynamic plastic surgery of the gracilis muscle), artificial anal sphincters based on the same principle as artificial urinary sphincters, and direct stimulation of the sacral nerve (14).
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CONCLUSIONS
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Careful assessment of the medical history and physical examination will orient the physician to the cause of fecal incontinence. Fecal incontinence caused by dynamic conditions such as prolapse, enterocele, cystocele, and intussusception may be assessed with defecography.
After assessment of sphincteric function with digital examination, manometry, and pudendal nerve terminal motor latencies, visualization of the sphincter and detection of sphincteric lesions will play a crucial role in diagnosis of fecal incontinence due to sphincteric pathologic conditions. Selection of patients with fecal incontinence who may benefit from surgical therapy is based on the detection and evaluation of sphincteric damage. To perform optimal surgery, an accurate description of the position, extent, and type of lesion is necessary. If available, endoanal MR imaging should be performed instead of endoanal US. In our experience, endoanal MR imaging is the most accurate technique for detection and characterization of sphincteric lesions and planning of optimal therapy.
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Acknowledgments
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We thank Teun Rijsdijk for the high-quality photographs and Lambertus van Heerebeek and Ruud Smit for preparing the images.
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Footnotes
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Abbreviation: GRE = gradient echo
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References
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Abstract
INTRODUCTION
