British Journal of Radiology (2005) 78, 308-311
© 2005 British Institute of Radiology
doi: 10.1259/bjr/72038963
Three-dimensional anal endosonography may improve diagnostic confidence of detecting damage to the anal sphincter complex
A F Christensen, MD
B Nyhuus, MD
M B Nielsen, MD, DMSc
and
H Christensen, MD, PhD
Department of Radiology, Section of Ultrasound, Rigshospitalet and Department of Neurology, Bispebjerg Hospital, University of Copenhagen, Denmark
Correspondence: Anders Fogh Christensen, Department of Radiology, Section of Ultrasound x4123, Rigshospitalet, Blegdamsvej 9, 2100 Copenhagen, Denmark
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Abstract
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The aim of study was to investigate the differences between three-dimensional (3D) endosonography and two-dimensional (2D) endosonography in visualizing damage to the anal sphincter complex. 33 patients with a history of damage to the anal sphincters were examined with a 10 MHz rotating endoprobe. Cross-sectional images of the anal sphincters were stored on a 3D system during retraction of the endoprobe through the anal canal. Cross-sectional images (2D) were compared with reconstructed projections (3D) according to five parameters relating to damage of different parts of the sphincter muscles as well as scar-tissue formation in the rectovaginal septum. Depending on whether the investigator felt confident in diagnosis of scar tissue being present or not a numerical value of 1 or 0 was assigned. In this way a scale from 0 to 5 points was achieved, which ideally should be identical in 3D and 2D. Overall both observers felt diagnostic confidence in a median of five parameters (range 4–5) using 3D, compared with a median of four parameters (range 3–5) using 2D (p=0.001). When only assessing the four parameters relating to damage of different parts of the sphincter-muscles the observers felt diagnostic confidence in eight more cases using 3D than 2D. This difference did not reach statistical significance. The overall agreement between the two observers comparing all five parameters was 98.2% using 3D and 87.9% using 2D. 3D anal endosonography improves diagnostic confidence in detecting damage to the anal sphincter complex. The agreement between the two observers was acceptable using 2D but better when using 3D. The 3D method may improve the selection of patients for surgical repair of the anal sphincter complex.
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Introduction
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Anal incontinence following damage to the anal sphincter muscles during vaginal delivery occurs in up to 7% of cases [1, 2]. The treatment in severe cases is surgery. This is preceded by anal endosonography [3], which has been shown to provide highly accurate imaging of anal sphincter morphology with a high spatial resolution, and has become the gold standard for demonstration of sphincteric defects [4, 5]. However, the technique is limited to a two-dimensional (2D) visualization of the structures. With a new technique, three-dimensional (3D) endosonography, any projection (e.g. coronal and sagittal) can be reconstructed [6, 7]. The aim of this study was to investigate the differences between the two endosonographic techniques in visualization of damage to the anal sphincter complex.
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Materials and methods
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33 consecutive female patients, median age 33 years (range 24–58 years), with suspected damage to the anal sphincter muscles were examined with anal endosonography in the period from July 2001 to September 2002. Endosonography was performed using a B-K Medical (Herlev, Denmark) type 2102 ultrasound apparatus with a rotating endoprobe type 1850 and a 10 MHz transducer type 6004 (focus 1.5–4 cm), which produces 360° axial images of the anal sphincter muscles. The transducer was covered with a water-filled hard sonolucent plastic cone (diameter 1.7 cm) [5]. The ultrasound apparatus had a built-in 3D module (B-K Medical, Herlev, Denmark), with which the axial images of the anal sphincters were stored on a hard disc during retraction of the endoprobe through the anal canal using a mechanical retractor with a constant retraction rate. Afterwards, by means of the 3D module, any projection (e.g. coronal or sagittal) could be reconstructed.
All examinations were retrospectively reviewed by two observers and the axial (2D) projections (Figure 1a)
, i.e. the true scanning plane, reviewed from the 3D dataset, were compared with reconstructed projections (3D: coronal, sagittal as well as the original axial projections) (Figure 1b) for the following five parts of the anal sphincter apparatus: (1) internal anal sphincter; (2) puborectalis muscle; (3) external anal sphincter in the middle third of the anal canal; (4) external anal sphincter in the distal third of the anal canal; (5) distal part of the rectovaginal septum. Depending on whether the investigator felt diagnostic confidence about damage/scar tissue being present or not (Figure 2) a numerical value of 1 or 0 was assigned for each of the five locations. In this way a scale from 0 to 5 points were achieved, that ideally should be identical in 3D and 2D.
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Figure 1. (a) Transverse anal endosonogram (2D), corresponding to the original axial image, shows large hypoechoic defect in the external anal sphincter (arrows). (b) 3D projection of the same patient (right) shows the sphincteric defect (arrows) extending cranially (upper right) towards the rectovaginal septum.
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Figure 2. 3D coronal projection of the anal sphincter muscles. To the right the hypoechoic internal sphincter is clearly seen (yellow arrows), to the left hypoechoic scar tissue in the external sphincter (arrows) involves the internal anal sphincter as well (*).
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Data analysis was performed by SPSS for Windows version 9.0 (SPSS Inc., Chicago, IL). Non-continuous related variables were compared by the Wilcoxon signed rank-test. Interobserver agreement was calculated as (N–agreement x 100)/N–total, where N is number of patients.
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Results
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Data are presented in Table 1. Both observers felt diagnostic confidence in median five parameters (range 4–5) using 3D, compared with median four parameters (range 3–5) using 2D (p=0.001). The lowest number of cases in which both observers shared diagnostic confidence was 31 in 3D and 1 in 2D, both concerning presence of scar tissue in the rectovaginal septum.
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Table 1. Number of cases in which the two observers A and B were in agreement of whether 2D or 3D anal endosonography were able or unable to determine possible damage to the anal sphincter complex
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When isolating the four parameters relating to damage to different parts of the sphincter muscles the observers felt diagnostic confidence in eight more cases using 3D than 2D. This difference did not reach statistical significance when the parameters were compared separately, but only when they were compared together.
The overall agreement between the two observers comparing all five parameters were 98.2% using 3D and 87.9% using 2D. The agreement ranged from 63.6% to 100% with the parameters on 2D and from 93.9% to 100% with the parameters in 3D.
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Discussion
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Anal endosonography using a single scanning plane (2D) has been validated histologically [8], intraoperatively [9] and physiologically [10] as the primary tool in the assessment of sphincteric defects and has replaced the previously used needle electromyography mapping of the external anal sphincter [3, 11, 12]. In detail endosonography has been reported to be reproducibly better than MRI with endocoil at visualizing the internal anal sphincter [13] and possibly for differentiating scar tissue from the external anal sphincter whereas MRI is considered the most accurate in evaluation of the external anal sphincter itself [14]. 2D endosonography has not previously been compared with the new technique of 3D endosonography. In this retrospective study, based on the diagnostic confidence concerning possible damage to the anal sphincter complex in five well demarcated areas, the use of 3D endosonography increased the diagnostic confidence and offered an improved agreement between two observers.
A detailed assessment of the possible damage to the anal sphincter complex is important for selection of patients for surgical repair of the anal sphincters [3, 12, 15, 16]. The results of the present study indicate that 3D endosonography is better at providing these data than 2D endosonography for the anal sphincter muscles and especially in detecting scar tissue in the distal rectovaginal septum. The rectovaginal septum, named by some the rectovaginal fascia, has been defined as a sheet of strong connective-type tissue between the rectum and the vagina [17]. The septum fuses with the vaginal wall anteriorly, the fascia over the levator muscles laterally and the anterior rectal wall posteriorly. Obstetric trauma is associated with damage/defects primarily to the distal part of the rectovaginal septum [18] as well as elongation of the length of the septum [19] and defects in the septum have been associated with rectocele formation [17]. Surgical repair involving repair of the rectovaginal septum has been shown to be an effective treatment of rectocele and symptoms of obstructed defecation/incontinence caused by the rectocele [20, 21]. However surgical repair in combination with sphincteric repair does not seem to give any clear benefits [21, 22]. Perhaps the lack of a way to visualize the extent of the scar tissue can explain these results. A recent study [23] has reported a sensitivity and positive predictive value for standard pre-operative evaluation of rectovaginal septum support that was less than 40%, and the authors emphasize the need for new methods to evaluate the area.
With 2D endosonography and transverse images the anal sphincter muscles are usually described at three levels only (the upper, middle or lower third of the anal canal) and the radial extent of scar tissue in hours of the clock. An additional advantage of 3D anal endosonography is the possibility to measure the size of a sphincter tear in any projection as well as the length of the remaining intact sphincter muscle. We did not include measurements of maximum size of tear in this study because 2D and 3D are not comparable in this respect.
The interobserver agreement for diagnosing sphincter disruption using 2D endosonography has previously been shown to be high with values exceeding 80% and complete agreement between observers in cases of large tears involving both the internal and the external anal sphincter [24]. This correlates well with an overall agreement between observers in this study of 88% using 2D. In comparison values below 50% have been described for endoanal MRI [13]. To our knowledge no comparison of interobserver agreement for diagnosing sphincteric damage using 3D endosonography has been performed. In this study the values for interobserver agreement using 3D reached 98% clearly exceeding the values found using 2D.
The ability of 3D endosonography to review images in all projections resembles MRI, but the resolution appears to be better. For endosonography, slice thickness for reconstruction is 0.2 mm, which corresponds to the obtained resolution in a voxel size of 0.2 mm3 in longitudinal projections. In comparison the best resolution by MRI will have a voxel size of at least 0.6 mm3 [25, 26]. The high resolution in the reconstructed projections facilitates differentiation of damage to the thin structures of the anal sphincter complex.
In conclusion, 3D may improve the diagnostic confidence in detecting damage to the anal sphincter complex and especially the presence of scar tissue in the distal rectovaginal septum. The interobserver agreement is improved using 3D endosonography. This may result in a better selection of patients for surgical sphincter repair.
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Footnotes
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This study was supported by grants from: Mogens Andreasen fonden and Ragnhild Ibsens Legat for Medicinsk forskning. 
Received for publication March 22, 2004.
Revision received August 23, 2004.
Accepted for publication November 15, 2004.
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Abstract
Introduction
