(Radiology. 2000;217:395-401.)
© RSNA, 2000
Normal Anal Sphincter Anatomy and Age- and Sex-related Variations at High-Spatial-Resolution Endoanal MR Imaging1
Elena Rociu, MD,
Jaap Stoker, MD,
Marinus J. C. Eijkemans, MSc and
Johan S. Laméris, MD
1 From the Departments of Radiology (E.R., J.S.) and Public Health (M.J.C.E.), Erasmus University Medical Center, Rotterdam, the Netherlands, 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.). From the 1999 RSNA scientific assembly. Received August 24, 1999; revision requested October 14; revision received March 2, 2000; accepted March 30. Address correspondence to E.R. (e-mail: e.rociu@amc.uva.nl).
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ABSTRACT
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PURPOSE: To describe the various patterns of normal sphincter anatomy as seen at endoanal magnetic resonance (MR) imaging and to assess sex- and age-related variations in the dimensions of the anal sphincter to refine the diagnosis of sphincter disorders.
MATERIALS AND METHODS: Endoanal MR imaging (1.5 T) was performed in 100 healthy volunteers (50 women, 50 men) evenly distributed between ages 20 and 85 years. The essential anatomic structures were evaluated, and various patterns in men and women were recorded. The thickness of the anal sphincter muscles and the length of the anal canal were measured, and age- and sex-related correlations were studied.
RESULTS: Sex-related differences included a significantly shorter external sphincter in women than in men both laterally (mean, 27.1 mm ± 5.4 vs 28.6 mm ± 4.3; P < .05) and anteriorly (mean, 14.0 mm ± 3.0 vs 27.0 mm ± 53.0; P < .051). The superficial transverse perineal muscle is located more superiorly in women than in men. The central perineal tendon in men is a central muscular insertion point; in women, it represents an area where muscle fibers imbricate. Age-related variations included a significant decrease in the thickness of the external sphincter in men (P < .01). Significant decrease in the thickness of the longitudinal muscle and increase in the thickness of the internal sphincter were noted in both sexes (P < .01).
CONCLUSION: High-spatial-resolution endoanal MR imaging provides excellent visualization of pelvic floor structures. Severe atrophy as it occurs in incontinent patients should be differentiated from physiologic, age-related thinning of the external sphincter and longitudinal muscle.
Index terms: Aging • Anus, 757.92 • Anus, MR, 757.121411, 757.121412 • Magnetic resonance (MR), coils, 757.121419 • Magnetic resonance (MR), three-dimensional
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INTRODUCTION
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Knowledge of the normal anatomic features of the anal canal is essential for the diagnosis and treatment of anal diseases. Unlike the morphology of the rectum, however, the anatomy of the anal canal is often subject to contradictory theories (1,2). Initial descriptions were based on dissection studies and observations during surgery. Introduction of endoanal imaging techniques opened new possibilities for in vivo evaluation of the pelvic structures. Endoanal ultrasonography (US) facilitates demonstration of anal structures but is still limited by the often inaccurate delineation of the external sphincter (3). Endoanal magnetic resonance (MR) imaging provides multiplanar, high-spatial-resolution demonstration of anal anatomy and a fine delineation of all sphincter muscles. This led to recent revision of the knowledge of anal sphincter anatomy (4-6). Especially when dealing with sphincter atrophy as a cause of fecal incontinence in the elderly, a refined knowledge of anal anatomy and of the effect of aging on sphincter structures is extremely useful.
The purpose of this study was to analyze possible variations of normal sphincter anatomy as seen with high-spatial-resolution endoanal MR imaging. Furthermore, sex- and age-related variations in the thickness and length of the anal sphincter were evaluated to refine the diagnosis of sphincter atrophy. In addition, potential pitfalls of endoanal MR imaging were assessed.
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MATERIALS AND METHODS
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Following approval by the institutional review board and after obtaining written informed consent, 100 consecutive healthy volunteers were included in the protocol. Exclusion criteria were a history of any pelvic floor disease (eg, prolapse, urinary incontinence, fecal incontinence), previous anorectal surgery, accidental anal trauma, laxative or antidiarrheal drug use, and any anal or rectal symptoms (eg, constipation, diarrhea, pain, blood loss). The study group consisted of 50 women and 50 men evenly distributed between ages 20 and 85 years (median age, 52 years) as follows: 10 women and 10 men in each of the age categories 20–35, 36–45, 46–55, 56–65, and 66–85 years. The number of deliveries was recorded for every female subject.
High-spatial-resolution endoanal MR imaging was performed at 1.5 T (Gyroscan ACS-NT; Philips Medical Systems, Best, the Netherlands) without bowel preparation. Subjects were asked to empty their bladder before the study. Before imaging, 1 mL of butylscopolamine bromide (Buscopan, 20 mg/mL; Boehringer, Ingelheim, Germany) was injected intramuscularly to reduce bowel motion. The endoanal coil (Philips Medical Systems) with a diameter of 19 mm was covered with a condom, and lubricant was applied to the surface.
A transverse, intermediate-weighted, three-dimensional gradient-echo (GRE) sequence (acquisition time, 6.5 minutes; 30/13 [repetition time msec/echo time msec]; flip angle, 60°; field of view, 140 x 112 mm; imaging matrix, 205 x 256; section thickness, 2 mm; contiguous sections; two signals acquired) was performed perpendicular to the long axis of the endoanal coil.
For sagittal and coronal images, T2-weighted turbo spin-echo (SE) sequences were performed (acquisition time, 5.0 minutes; 2,800/120 [effective]; echo train length, 10; echo spacing, 21.8 msec; field of view, 120 x 90 mm; imaging matrix, 186 x 256; section thickness, 4.0 mm with an intersection gap of 0.4 mm; eight signals acquired). The coronal and sagittal sections were parallel to the long axis of the endoanal coil.
Until now, the optimal imaging sequences for endoluminal MR imaging of the rectum and anus have not been definitely established. The use of T1-weighted imaging with intravenously administered contrast medium, short tau inversion recovery , or STIR, and T2-weighted turbo SE has been advocated (7–9). In our experience, intermediate-weighted GRE and T2-weighted turbo SE sequences provide good results without the need of intravenously administered contrast medium. The intermediate-weighted GRE sequence allows thin sections, important for the scrupulous examination of the anus; the T2-weighted turbo SE sequence, less susceptible to motion artifacts than is the GRE sequence, provides fine overviews. These sequences are, therefore, the mainstay of our imaging protocols for endoluminal MR imaging of the anus.
Image Analysis
Two experienced radiologists (E.R., J.S.) evaluated the images separately by using an Easy Vision workstation (Philips Medical Systems). The quality of the images was scored for the presence of artifacts: good, no artifacts; moderate, some artifacts; or poor, artifacts interfering with image analysis. The lengths of the anal canal, external sphincter, and puborectal muscle were measured. The thicknesses of the anal sphincter muscles (external and internal sphincter, longitudinal and puborectal muscles) and levator ani muscle were measured as well. All measurements were performed with electronic calipers on midcoronal images, except for the anterior part of the external sphincter for which midsagittal images were used. From each muscle, we used the measurement of its largest portion to avoid misdiagnosing the muscle as thinner than it actually was. Measurements were performed on the left and right sides of the sphincter complex; each side was separately evaluated.
Statistical Analyses
Sex-related differences and interobserver variation in measurements were assessed by using the Mann-Whitney test. To determine agreement between measurements of different investigators, the interclass correlation coefficient was used. Age-related variation was evaluated by using the Pearson product moment correlation. A P value less than .05 was considered to indicate statistical significance.
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RESULTS
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The endoluminal device was well tolerated by all volunteers. The quality of the MR images was good in 91 (91%) of 100 patients, moderate in seven (7%), and poor in two (2%). In the last two cases, motion artifacts occurred.
MR Anatomy
Figures 1 and 2 depict the anatomic appearance on T2-weighted turbo SE images in the coronal plane. The anal sphincter surrounds the most distal part of the digestive tract, the anal canal. The mucosa has relatively high signal intensity, and the submucosa has relatively low signal intensity. At the anorectal junction, the muscularis propria of the rectum changes: The inner, circular layer thickens and becomes the internal sphincter with high signal intensity. The outer, longitudinal layer of the muscularis propria becomes the relatively hypointense longitudinal muscle. First, the external anal sphincter, then the puborectal muscle, and, at the anorectal junction, the most inferior part of the levator ani muscle surround the longitudinal muscle from caudal to cranial. These muscles have low signal intensity. The external sphincter consists of a subcutaneous, a superficial, and a deep part. In the coronal plane, the subcutaneous bundle of the external sphincter manifests with a characteristic J shape that is curved medially (Fig 1). The sphincter complex is embedded in the fat-containing ischioanal space, which is relatively hyperintense.
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Figure 1a. (a, c) Midcoronal T2-weighted turbo SE MR images (2,800/120 [effective]; echo train length, 10; field of view, 120 x 90 mm) and (b, d) diagrams depict the (a, b) female and (c, d) male anatomy: external (Es) and internal (Is) sphincters, levator ani muscle (Lam), longitudinal muscle (Lm), puborectal muscle (Pr), and submucosa (Sm).
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Figure 1b. (a, c) Midcoronal T2-weighted turbo SE MR images (2,800/120 [effective]; echo train length, 10; field of view, 120 x 90 mm) and (b, d) diagrams depict the (a, b) female and (c, d) male anatomy: external (Es) and internal (Is) sphincters, levator ani muscle (Lam), longitudinal muscle (Lm), puborectal muscle (Pr), and submucosa (Sm).
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Figure 1c. (a, c) Midcoronal T2-weighted turbo SE MR images (2,800/120 [effective]; echo train length, 10; field of view, 120 x 90 mm) and (b, d) diagrams depict the (a, b) female and (c, d) male anatomy: external (Es) and internal (Is) sphincters, levator ani muscle (Lam), longitudinal muscle (Lm), puborectal muscle (Pr), and submucosa (Sm).
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Figure 1d. (a, c) Midcoronal T2-weighted turbo SE MR images (2,800/120 [effective]; echo train length, 10; field of view, 120 x 90 mm) and (b, d) diagrams depict the (a, b) female and (c, d) male anatomy: external (Es) and internal (Is) sphincters, levator ani muscle (Lam), longitudinal muscle (Lm), puborectal muscle (Pr), and submucosa (Sm).
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Figure 2a. Anterior coronal T2-weighted turbo SE MR images (2,800/120 [effective]; echo train length, 10; field of view, 120 x 90 mm) depict the (a) female and (b) male anatomy. Image in a shows the bulbospongiosus (bulbocavernosus) muscle (Bsm), external sphincter (Es), levator ani muscle (Lam), puborectal muscle (Pr), and superficial transverse perineal muscle (Tpm). Image in b shows the bulbospongiosus (bulbocavernosus) muscle (Bsm), corpus cavernosum (Cc), corpus spongiosum (Cs), prostate (P), puborectal muscle (Pr), and urogenital diaphragm (Ud).
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Figure 2b. Anterior coronal T2-weighted turbo SE MR images (2,800/120 [effective]; echo train length, 10; field of view, 120 x 90 mm) depict the (a) female and (b) male anatomy. Image in a shows the bulbospongiosus (bulbocavernosus) muscle (Bsm), external sphincter (Es), levator ani muscle (Lam), puborectal muscle (Pr), and superficial transverse perineal muscle (Tpm). Image in b shows the bulbospongiosus (bulbocavernosus) muscle (Bsm), corpus cavernosum (Cc), corpus spongiosum (Cs), prostate (P), puborectal muscle (Pr), and urogenital diaphragm (Ud).
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Figures 3 and 4 depict the anatomic appearance on T2-weighted turbo SE images in the sagittal plane. The external sphincter is shorter anteriorly in women than in men (Table 1) and has a more horizontal orientation in women. In women, the longitudinal muscle terminates just cranial to the external sphincter, whereas in men it extends to the caudal part of the external sphincter. The superficial transverse perineal muscle is close to the external sphincter, and their relation in the craniocaudal direction is different between the sexes. In women, the superficial transverse perineal muscle is directly superior to the external sphincter, often with some overlap (Fig 3). At first sight, this close relationship can appears as one muscle on the midsagittal image. In men, the superficial perineal muscle is directly anterior to the external sphincter. The central perineal tendon is an insertion common to all the striated muscles, which anchors the anal sphincter to the surrounding structures (superficial transverse perineal muscle, bulbospongiosus muscle, urogenital diaphragm). In men, this structure is more like a central point, whereas in women this insertion is larger, and the imbrication of the muscle fibers is more pronounced; therefore, it is often described as the perineal body (Fig 3). Anterior to the perineal body is the urogenital diaphragm (deep transverse perineal muscle and fascia).
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Figure 3a. (a, c) Midsagittal T2-weighted turbo SE MR images (2,800/120 [effective]; echo train length, 10; field of view, 120 x 90 mm) and (b, d) diagrams depict the (a, b) female and (c, d) male anatomy. Images in a and b show the central perineal tendon (curved arrow in a), external (Es) and internal (Is) sphincters, levator ani muscle (Lam), longitudinal muscle (Lm), puborectal muscle (Pr), submucosa (Sm), superficial transverse perineal muscle (Tpm), and vagina (V). Images in c and d show the bulbospongiosus (bulbocavernosus) muscle (Bsm), central perineal tendon (curved arrow in c), corpus spongiosum (Cs), external (Es) and internal (Is) sphincters, levator ani muscle (Lam), longitudinal muscle (Lm), puborectal muscle (Pr), submucosa (Sm), superficial transverse perineal muscle (Tpm), and urogenital diaphragm (Ud).
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Figure 3b. (a, c) Midsagittal T2-weighted turbo SE MR images (2,800/120 [effective]; echo train length, 10; field of view, 120 x 90 mm) and (b, d) diagrams depict the (a, b) female and (c, d) male anatomy. Images in a and b show the central perineal tendon (curved arrow in a), external (Es) and internal (Is) sphincters, levator ani muscle (Lam), longitudinal muscle (Lm), puborectal muscle (Pr), submucosa (Sm), superficial transverse perineal muscle (Tpm), and vagina (V). Images in c and d show the bulbospongiosus (bulbocavernosus) muscle (Bsm), central perineal tendon (curved arrow in c), corpus spongiosum (Cs), external (Es) and internal (Is) sphincters, levator ani muscle (Lam), longitudinal muscle (Lm), puborectal muscle (Pr), submucosa (Sm), superficial transverse perineal muscle (Tpm), and urogenital diaphragm (Ud).
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Figure 3c. (a, c) Midsagittal T2-weighted turbo SE MR images (2,800/120 [effective]; echo train length, 10; field of view, 120 x 90 mm) and (b, d) diagrams depict the (a, b) female and (c, d) male anatomy. Images in a and b show the central perineal tendon (curved arrow in a), external (Es) and internal (Is) sphincters, levator ani muscle (Lam), longitudinal muscle (Lm), puborectal muscle (Pr), submucosa (Sm), superficial transverse perineal muscle (Tpm), and vagina (V). Images in c and d show the bulbospongiosus (bulbocavernosus) muscle (Bsm), central perineal tendon (curved arrow in c), corpus spongiosum (Cs), external (Es) and internal (Is) sphincters, levator ani muscle (Lam), longitudinal muscle (Lm), puborectal muscle (Pr), submucosa (Sm), superficial transverse perineal muscle (Tpm), and urogenital diaphragm (Ud).
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Figure 3d. (a, c) Midsagittal T2-weighted turbo SE MR images (2,800/120 [effective]; echo train length, 10; field of view, 120 x 90 mm) and (b, d) diagrams depict the (a, b) female and (c, d) male anatomy. Images in a and b show the central perineal tendon (curved arrow in a), external (Es) and internal (Is) sphincters, levator ani muscle (Lam), longitudinal muscle (Lm), puborectal muscle (Pr), submucosa (Sm), superficial transverse perineal muscle (Tpm), and vagina (V). Images in c and d show the bulbospongiosus (bulbocavernosus) muscle (Bsm), central perineal tendon (curved arrow in c), corpus spongiosum (Cs), external (Es) and internal (Is) sphincters, levator ani muscle (Lam), longitudinal muscle (Lm), puborectal muscle (Pr), submucosa (Sm), superficial transverse perineal muscle (Tpm), and urogenital diaphragm (Ud).
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Figure 4a. Lateral sagittal T2-weighted turbo SE MR images (2,800/120 [effective]; echo train length, 10; field of view, 120 x 90 mm) depict the (a) female and (b) male anatomy: central perineal tendon (curved arrow), corpus spongiosum (Cs), external (Es) and internal (Is) sphincters, levator ani muscle (Lam), longitudinal muscle (Lm), puborectal muscle (Pr), and superficial transverse perineal muscle (Tpm).
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Figure 4b. Lateral sagittal T2-weighted turbo SE MR images (2,800/120 [effective]; echo train length, 10; field of view, 120 x 90 mm) depict the (a) female and (b) male anatomy: central perineal tendon (curved arrow), corpus spongiosum (Cs), external (Es) and internal (Is) sphincters, levator ani muscle (Lam), longitudinal muscle (Lm), puborectal muscle (Pr), and superficial transverse perineal muscle (Tpm).
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Figures 5–7 depict the anatomic appearance on intermediate-weighted GRE images in the transverse plane. In the transverse plane, most fibers of the superficial and deep parts of the external sphincter are convergent to the central tendon of the perineum, but especially in men, some fibers may continue forward and insert in the perineal raphe. The first transverse sections do not contain the internal sphincter, and longitudinal muscle because those structures begin cranial to the level of the subcutaneous external sphincter. These anterior and posterior insertions create an open elliptical shape of this part of the external sphincter, which can be misdiagnosed as anterior or posterior defects (Fig 7). Posteriorly, the puborectal muscle forms a sling around the upper part of the sphincter (Fig 6).
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Figure 5a. (a, c) Transverse intermediate-weighted GRE MR images (30/13, 60° flip angle; field of view, 140 x 112 mm) obtained in the mid anal canal and (b, d) diagrams depict the (a, b) female and (c, d) male anatomy. Images in a and b show the anococcygeal ligament (Acl), external (Es) and internal (Is) sphincters, submucosa (Sm), superficial transverse perineal muscle (Tpm), urethra (U), and vagina (V). Images in c and d show the corpus cavernosum (Cc), corpus spongiosum (Cs), external (Es) and internal (Is) sphincters, ischioanal space (IAS), ischiocavernosus muscle (Icm), longitudinal muscle (Lm), submucosa (Sm), and superficial transverse perineal muscle (Tpm).
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Figure 5b. (a, c) Transverse intermediate-weighted GRE MR images (30/13, 60° flip angle; field of view, 140 x 112 mm) obtained in the mid anal canal and (b, d) diagrams depict the (a, b) female and (c, d) male anatomy. Images in a and b show the anococcygeal ligament (Acl), external (Es) and internal (Is) sphincters, submucosa (Sm), superficial transverse perineal muscle (Tpm), urethra (U), and vagina (V). Images in c and d show the corpus cavernosum (Cc), corpus spongiosum (Cs), external (Es) and internal (Is) sphincters, ischioanal space (IAS), ischiocavernosus muscle (Icm), longitudinal muscle (Lm), submucosa (Sm), and superficial transverse perineal muscle (Tpm).
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Figure 5c. (a, c) Transverse intermediate-weighted GRE MR images (30/13, 60° flip angle; field of view, 140 x 112 mm) obtained in the mid anal canal and (b, d) diagrams depict the (a, b) female and (c, d) male anatomy. Images in a and b show the anococcygeal ligament (Acl), external (Es) and internal (Is) sphincters, submucosa (Sm), superficial transverse perineal muscle (Tpm), urethra (U), and vagina (V). Images in c and d show the corpus cavernosum (Cc), corpus spongiosum (Cs), external (Es) and internal (Is) sphincters, ischioanal space (IAS), ischiocavernosus muscle (Icm), longitudinal muscle (Lm), submucosa (Sm), and superficial transverse perineal muscle (Tpm).
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Figure 5d. (a, c) Transverse intermediate-weighted GRE MR images (30/13, 60° flip angle; field of view, 140 x 112 mm) obtained in the mid anal canal and (b, d) diagrams depict the (a, b) female and (c, d) male anatomy. Images in a and b show the anococcygeal ligament (Acl), external (Es) and internal (Is) sphincters, submucosa (Sm), superficial transverse perineal muscle (Tpm), urethra (U), and vagina (V). Images in c and d show the corpus cavernosum (Cc), corpus spongiosum (Cs), external (Es) and internal (Is) sphincters, ischioanal space (IAS), ischiocavernosus muscle (Icm), longitudinal muscle (Lm), submucosa (Sm), and superficial transverse perineal muscle (Tpm).
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Figure 6a. Transverse intermediate-weighted GRE MR images (30/13, 60° flip angle; field of view, 140 x 112 mm) obtained high in the anal canal depict the (a) female (b) male anatomy: internal sphincter (Is), longitudinal muscle (Lm), prostate (P), puborectal muscle (Pr), urethra (U), and vagina (V).
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Figure 6b. Transverse intermediate-weighted GRE MR images (30/13, 60° flip angle; field of view, 140 x 112 mm) obtained high in the anal canal depict the (a) female (b) male anatomy: internal sphincter (Is), longitudinal muscle (Lm), prostate (P), puborectal muscle (Pr), urethra (U), and vagina (V).
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Figure 7a. Images demonstrate the pitfalls of transverse intermediate-weighted GRE MR images (30/13, 60° flip angle; field of view, 140 x 112 mm), which depict (a) the female anatomy in the mid anal canal and (b) the male anatomy in the low anal canal. Image in a shows the external (Es) and internal (Is) sphincters, the longitudinal muscle (Lm), and superficial transverse perineal muscle (Tpm). Image in b shows the anococcygeal ligament (Acl), and external (Es) and internal (Is) sphincters. Note the open elliptical shape of the external sphincter.
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Figure 7b. Images demonstrate the pitfalls of transverse intermediate-weighted GRE MR images (30/13, 60° flip angle; field of view, 140 x 112 mm), which depict (a) the female anatomy in the mid anal canal and (b) the male anatomy in the low anal canal. Image in a shows the external (Es) and internal (Is) sphincters, the longitudinal muscle (Lm), and superficial transverse perineal muscle (Tpm). Image in b shows the anococcygeal ligament (Acl), and external (Es) and internal (Is) sphincters. Note the open elliptical shape of the external sphincter.
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Sphincter Measurements
The differences between the measurements by the two radiologists were not significant (P > .05, Mann-Whitney test). The interobserver agreement was good (interclass correlation coefficient, 0.91). The number of deliveries was recorded for every female subject but did not correlate with any of the other parameters.
Sex-related variation.—Table 1 shows the mean sex-related variations in measurements of length and thickness. The external sphincter measurements were significantly shorter in women than in men in both the lateral (P < .05) and anterior (P < .01) parts. No statistically significant sex-related differences in lengths of the anal canal and the puborectal muscle were found.
The measurements of the thickness of the sphincter muscles were performed left and right on midcoronal images. There was a high correlation (Pearson correlation, P < .001) between all left and right measurements at the same level. Therefore, for each subject, a mean muscle thickness ([left + right] ÷ 2) was calculated; this is the value referred to herein as "muscle thickness" of a subject. The mean muscle thickness of all subjects was also calculated; this is the value referred to herein as "mean muscle thickness" of all subjects. There was no significant difference between the women and men in the thickness of the sphincter muscles (P > .05).
Age-related variation.—Table 2 shows the mean age-related variations in measurements of length and thickness. No statistically significant age-related differences in the lengths of the anal canal, external sphincter, and puborectal muscle were found. In women, the thickness of the longitudinal muscle significantly decreased with age (P < .01), the thickness of the internal sphincter significantly increased with age (P < .01), and the thicknesses of the external sphincter, levator ani muscle, and puborectal muscle decreased with age but not significantly (P > .05).
In men, the thicknesses of the external sphincter and longitudinal muscle significantly decreased with age (P < .01 for each). The thickness of the internal sphincter significantly increased with age (P < .01). The thicknesses of levator ani and puborectal muscles did not correlate with age (P > .05 for each).
The difference between women and men was that the thickness of the external sphincter in men decreased significantly with age, but in women the decrease was not statistically significant.
The variation of thickness with age was evaluated as presented in Figure 8. The thickness of the external sphincter in young (
35 years) women was significantly less than in young men (4.32 mm ± 0.7 vs 5.21 mm ± 0.8; Mann-Whitney test, P < .05). With increasing age, there was a decrease in sphincter thickness for both women and men, but the decrease was more pronounced in men. This resulted in a smaller difference between the thickness of the external sphincter in the older group (>65 years); however, this was not statistically significant (mean, 3.9 mm ± 1.4 in women vs 3.45 mm ± 1.1 in men; Mann-Whitney test, P > .05). This was probably due to the large spread of values in the female group.
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DISCUSSION
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The anal sphincter is part of the complex anatomy of the pelvic floor. There have been many contradictory and often confusing theories of the anatomy of this region. Debates on this subject persisted because there was little scientific information to define the morphology of the anal muscles. Anal US is the imaging technique currently used for visualization of this region and has proved superior to conventional-body-coil MR imaging without an endoluminal device (3,10). A study in which endoanal US was used (11) revealed similar thickness, and its findings suggested variation with age of the internal and external sphincter. However, the authors state that thickness of the external sphincter was "difficult to define with endoanal sonography"; the results were based on a small number of subjects (15 women and 11 men), and no age distribution of the studied subjects was mentioned.
The results of other studies (12,13) indicate that endoanal MR imaging is more accurate than endoanal US in characterizing the external sphincter and can also depict external sphincter atrophy. Studies with use of endoanal MR imaging (4,5) revealed similar thickness of the sphincter muscles but were based on no more than 10 subjects and did not assess age- and sex-related variations. It is possible that the measured thickness of the sphincter can be influenced by the distention grade of the sphincter determined by the thickness of the coil. Nevertheless, the measured sex- and age-related variations of the muscle thickness is a value statistically independent from the coil thickness.
To our knowledge, the role of phased-array-coil MR imaging has not been evaluated yet, but the local spatial resolution of this technique is inferior to that of endoluminal techniques and, in our opinion, does not allow enough delineation of pelvic muscles to accurately detect sphincter defects and atrophy. Endoanal MR imaging facilitates the understanding of the anal anatomy by providing multiplanar high-spatial-resolution demonstration in vivo and was therefore the chosen technique to perform this study (4,6).
As a result, we found that the major variation of the anatomy was visible on the anterior part of the anal sphincter in men and women. The central perineal tendon in men is a central insertion point. In women, it is not a point but more an insertion area with woven muscular fibers, consequently allowing more elasticity and being called the "perineal body." The superficial transverse perineal muscle in women is superior to the external sphincter. In men, this muscle is directly anterior to the external sphincter, reinforcing this part of the anal sphincter.
Quantitative differences were shorter external sphincter in women (almost half that in men) and thinner external sphincter in young women. All of those features make it less resistant to pressure. This is important during vaginal delivery when the anterior part of the external sphincter closely related to the vagina should allow wide extension. In women, this region is the most common location of external sphincter lacerations (14,15). Lesions of the anal sphincter are the most common cause of fecal incontinence and are commonly treated by means of surgical anterior anal repair. Exact visualization of the sphincter anatomy with endoanal MR imaging before planning surgery is therefore mandatory (12,16).
External sphincter atrophy has been observed in patients with late-onset fecal incontinence several years after childbirth and explained as having both neurologic and ischemic causes (5). It can only be visualized with endoanal MR imaging and represents a negative predictive factor for the success of anterior anal repair (13). Patients with anterior external sphincter lacerations and external sphincter atrophy are eligible for other types of surgical treatments like dynamic repair of the gracilis muscle (17). This preoperative selection can be performed only by using endoanal MR imaging. Although atrophy of the external sphincter is visible in all planes, the coronal plane allows the easiest evaluation because the volume of all sphincter muscles can be compared.
In the young population, "complete continence" is a common state, but in the elderly, the prevalence of fecal incontinence is reported as 18% and estimated to be much higher. Because we required healthy, "completely continent" elderly subjects in our study, a careful selection took place (18). On the basis of our experience of visualizing fecal incontinence with endoanal MR imaging, we believe the general, unselected, elderly population, in whom a certain degree of incontinence is quite common, has even thinner sphincter muscles. Further research is required to assess the scientific basis of this difference and to detect below which level the thinning of anal muscles manifests itself clinically. Nevertheless, the radiologist should be aware that, with aging, thinning of the external sphincter and longitudinal muscle and compensatory hypertrophy of the internal sphincter occur in healthy, continent subjects. The thinning of the external sphincter and longitudinal muscle in the elderly should be seen as physiologic and differentiated from severe atrophy as it occurs in incontinent patients.
In conclusion, detailed knowledge of anatomic variations among sexes, age groups, and individuals is crucial for a correct evaluation of pelvic floor morphology. High-spatial-resolution endoanal MR imaging provides a fine demonstration of deformity and integrity of the pelvic floor structures and earlier studies proved it as being an excellent tool in the preoperative diagnosis and for surgical planning (9,11). As the general population lives longer, the number of incontinent patients increases, and there is a growing demand for imaging in patients with fecal incontinence. Accurate assessment is, therefore, extremely important to support successful treatment and thereby increase the quality of life in the elderly.
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ACKNOWLEDGMENTS
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The authors thank Teun Rijsdijk for the high- quality photographs and Andries W. Zwamborn and Bert van Heerebeek for preparing the images.
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FOOTNOTES
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Abbreviations: GRE = gradient echo,
SE = spin echo
Author contributions: Guarantor of integrity of entire study, E.R.; study concepts and design, E.R., J.S., J.S.L.; definition of intellectual content, all authors; literature research, E.R.; clinical studies, E.R., J.S., J.S.L.; data acquisition, E.R., J.S.; data analysis, M.J.C.E., E.R., J.S.; statistical analysis, M.J.C.E.; manuscript preparation, E.R., J.S.; manuscript editing, E.R.; manuscript review, all authors.
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ABSTRACT
INTRODUCTION
and solid regression line) and women (
and dashed regression line).