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Prospective Assessment of Accuracy of Endoanal MR Imaging and Endosonography in Patients with Fecal Incontinence

¹ Intestinal Imaging Centre, St. Mark's Hospital, Northwick Park, Watford Road, Harrow, Middlesex, HA1 3UJ, United Kingdom.
² Sir Alan Parks Physiology Unit, St. Mark's Hospital, Northwick Park, London, HA1 3UJ, United Kingdom.

Received October 4, 1999; accepted after revision February 21, 2000.

 
Partly supported by a Kodak bursary via the Royal College of Radiologists.

Address correspondence to S. Halligan.

Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
OBJECTIVE. Endoanal MR imaging was prospectively compared with anal endosonography to determine any superiority in the characterization of sphincter morphology in fecal incontinence.

SUBJECTS AND METHODS. Fifty-two consecutive patients with fecal incontinence were examined with anal endosonography and endoanal MR imaging after a detailed bowel history, clinical examination, and complete anorectal physiologic testing. External and internal anal sphincter integrity was noted on both endosonograms and MR images by two radiologists in consensus, who read individual scans in a random order to avoid recall bias. Imaging findings were subsequently compared, and arbitration of any disagreement between endosonography and MR imaging was made in consensus by a surgeon and a gastroenterologist who also had access to the patient's history, clinical examination, and anorectal physiologic testing results.

RESULTS. Complete agreement was found between anal endosonographic and MR imaging interpretations in 32 patients (62%): 10 with combined external and internal sphincter injuries, two with isolated internal sphincter injury, and 20 with intact sphincters. Of 20 patients in whom results of the scans were disparate, incorrect interpretation was found on endosonography in six patients, on MR imaging in 15. Overall, one error relating to the internal sphincter was made on endosonography versus 12 on MR imaging (p = 0.002), and five errors relating to the external sphincter were made on endosonography versus six on MR imaging (p = 1.0).

CONCLUSION. This study suggests that endoanal sonography and endoanal MR imaging are equivalent in diagnosing external anal sphincter injury, but MR imaging is inferior in diagnosing internal anal sphincter injury.

Introduction

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Anal endosonography is the accepted gold standard for the in vivo assessment of anal sphincter anatomy. It has been validated both intraoperatively [1] and histologically [2] and has a high degree of accuracy in characterizing sphincter abnormalities in incontinence [3, 4]. The internal sphincter is well visualized on endosonography because hypoechogenic smooth muscle is strongly differentiated from echogenic subepithelial tissues medially and the longitudinal muscle laterally. In contrast, the external sphincter is of mixed and variable echogenicity so that its boundaries are more difficult to define. Endoanal MR imaging has recently been described, and initial work suggests that it may be superior to endosonography for the diagnosis of anal sphincter disruption, primarily because the lateral external sphincter boundaries are better delineated [5, 6]. This study compared prospectively the diagnostic accuracy of anal endosonography and endoanal MR imaging in patients presenting with fecal incontinence.

Subjects and Methods

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Fifty-two consecutive patients (mean age, 53 years; range, 15-78 years; 47 women, five men) were referred to the anorectal physiology unit for investigation of fecal incontinence. All patients underwent a detailed bowel history, clinical examination, complete anorectal physiologic testing, anal endosonography, and endoanal MR imaging on the same day.

History assessed in detail the type, degree, and frequency of incontinence; current bowel habits; previous anorectal surgery; obstetric details in women; and the temporal relationship between any surgery and childbirth and incontinence. Sphincter function and integrity were assessed digitally by a gastroenterologist and colorectal surgeon before full anorectal physiologic testing.

Anorectal physiologic testing included anal manometry performed with an eight-channel water-perfused (0.5 mL/min) radial manometry catheter (Coller type A; Mui Scientific, Mississauga, Ont., Canada) using a pull-through technique with 0.5-cm stations [7]. Maximum anal resting pressure (normal range, 60-160 cm H₂O) and squeeze pressure (normal range, 50-220 cm H₂O) were recorded as the mean of all eight-channel recordings. The latter was calculated as the incremental rise above resting pressure. The functional anal canal length was measured in centimeters. Pudendal nerve terminal motor latencies were measured bilaterally with a stimulating electrode (St. Mark's pudendal electrode; Dantec Medical, Skovlunde, Denmark) [8]. Anal and rectal mucosal electrosensitivity thresholds were determined with a urethral ring electrode mounted on a catheter (Dantec Keypoint EMG/EP; Dantec Medical) [9], and rectal sensitivity to distention was determined with incremental inflation of an intrarectal balloon. Threshold, urge, and maximum tolerated volumes were recorded.

Anal endosonography was performed with the patient in the prone position with a 3535 scanner (B&K Medical, Gentofte, Denmark) with an 1850 endoscopic probe and type 6004 10-MHz transducer [10]. This transducer images in the axial plane only and has a focal range of 5-45 mm, an axial resolution of less than 0.05 mm, and a lateral resolution of 0.5-1.0 mm within the focal range. A plastic cone of 19-mm external diameter covered the transducer head and was specially constructed to match exactly the diameter of the endoanal MR receiver coil. The cone was filled with degassed water and covered with a lubricated condom. Images of the proximal, middle, and distal anal canal were recorded by a laser imager at two different magnifications.

Endoanal MR imaging was performed with a 1.0-T superconducting static magnet (Gyroscan T10-NT; Philips Medical Systems, Hammersmith, U.K.) and an endoanal receiver coil that has been described previously [11]. This coil consisted of a fixed tuned rectangular rigid coil, 60 mm long and 16 mm wide, that was contained within a cylindric coil holder 80 mm long and 19 mm wide. The coil holder was covered with a condom, lubricated, and inserted with the patient in the left lateral position so that the distal extent of the coil, indicated by a marker on the coil holder, lay at the anal verge. When the patient turned supine, the coil was supported by soft rubber pads and connected to a preamplifier box after its position was checked. The patient was moved into the magnet and fast spin-echo T2-weighted images were obtained in the axial, coronal, and sagittal planes with respect to the longitudinal axis of the coil with the following parameters: TR/TE, 3768/120; field of view, 120 mm; matrix size, 512 x 512; slice thickness, 3 mm; interslice gap, 0.3 mm; and excitations, six. Each examination was recorded by a laser imager.

The sonograms and MR images were analyzed on separate occasions and reported in consensus by two consultant gastrointestinal radiologists with a specialty interest in anorectal imaging, who assessed the integrity of both internal and external sphincters. To avoid recall bias, film analysis occurred in a random order at least 3 months after the patient had been examined. Endosonographic and MR imaging criteria for a defect were any breech in sphincter integrity. The radiologists were unaware of clinical history, examination, and anorectal physiologic findings. Finally, a consensus diagnosis was made for each patient by a colorectal surgeon and a gastroenterologist, both with a special interest in anorectal disorders and experience in radiologic, physiologic, and clinical assessment of fecal incontinence, who had access to all historic, clinical, physiologic, and imaging data. Corresponding endosonographic and MR imaging findings for each patient were then compared with this diagnosis and with each other. When a discrepancy occurred on imaging, arbitration was based on the final clinical consensus diagnosis that considered the clinical history and temporal relationship to symptoms (obstetric factors, anal canal surgery), the type of incontinence experienced (urge or passive or both) [12], clinical examination, and the results of anorectal physiologic testing. Internal sphincter dysfunction was diagnosed if the manometric resting pressure was below the normal range [12]. External sphincter dysfunction was diagnosed if the manometric voluntary contraction (squeeze) pressure was below the normal range [12].

Error frequencies for anal endosonography and MR imaging were compared using Fisher's exact test, and calculations were performed with Arcus Quickstat Biomedical 1.2 software (Research Solutions, Cambridge, U.K.). Statistical significance was assigned at a probability level of 0.05.

Results

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Of the 52 patients studied, nine (17%) complained solely of urge incontinence, 15 (29%) complained solely of passive incontinence, and 28 (54%) patients experienced symptoms of both urge and passive incontinence.

Complete agreement was found between anal endosonographic and MR imaging findings and the final diagnosis in 32 (62%) of the 52 subjects studied. These included 10 patients with combined external and internal sphincter defects caused by obstetric injury, two patients with isolated internal sphincter defects after anal canal surgery, and 20 patients with intact sphincters with either a functional disorder or degenerative sphincter disease [4] (Fig. 1A,1B).

View larger version (202K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —47-year-old woman with anal incontinence caused by functional disorder. Anal endosonogram shows intact external (curved arrows) and internal (straight arrow) anal sphincters. Anterior is uppermost.

View larger version (159K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —47-year-old woman with anal incontinence caused by functional disorder. T2-weighted endoanal MR image also shows intact external (black arrow) and internal (white arrow) anal sphincters.

Of the 20 patients in whom the scan interpreters disagreed, anal endosonographic findings were considered correct after arbitration in 14 and MR imaging findings correct in five (p = 0.049). MR imaging findings were incorrect in 15 patients. In five patients with intact sphincters, reviewers diagnosed internal sphincter defects in four and an external sphincter defect in the fifth using MR imaging findings.

With MR images, reviewers also missed isolated internal sphincter defects in four patients with a history of previous anal surgery (Fig. 2A,2B). In two patients with isolated external sphincter injuries, reviewers diagnosed an internal sphincter injury in one and an intact sphincter in the other. In four patients with combined sphincter injury caused by obstetric trauma, reviewers using MR images missed both injuries in two (because of movement artifacts), the internal sphincter injury in the third, and the external sphincter defect in the fourth (in whom the internal sphincter injury was also missed on anal endosonograms).

View larger version (195K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —57-year-old woman with incontinence after anal surgery for hemorrhoids. Anal endosonogram shows internal sphincter defect between 10-and 3-o'clock positions (arrows).

View larger version (164K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —57-year-old woman with incontinence after anal surgery for hemorrhoids. T2-weighted endoanal MR image shows internal sphincter to be intact (arrow).

Interpretation of anal endosonograms was incorrect in six patients. Reviewers incorrectly diagnosed external sphincter defects in two patients with isolated internal sphincter injury (Fig. 3A,3B) and missed three external sphincter defects and one internal sphincter defect in four patients with injuries of both sphincters. The patient with the internal sphincter defect was the same patient in whom the external sphincter injury had been missed with endoanal MR imaging.

View larger version (188K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —60-year-old man with incontinence after surgery for fistula in ano. Anal endosonogram shows internal sphincter defect, with general fragmentation of sphincter. Straight arrows indicate sphincter remnants. External sphincter defect in right posterior quadrant was also visualized (curved arrow).

View larger version (158K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —60-year-old man with incontinence after surgery for fistula in ano. T2-weighted endoanal MR image at level corresponding to A also shows internal sphincter fragments (straight arrows). External sphincter was shown to be intact (curved arrow).

Overall, one error related to the internal sphincter and five errors related to the external sphincter were made using endosonograms, whereas 12 errors related to the internal sphincter and six related to the external sphincter were made with MR imaging findings. The frequency of internal sphincter errors was significantly higher on MR imaging when compared with anal endosonography (12 versus one, respectively; p = 0.002), but no significant difference was seen for external sphincter errors on MR imaging compared with this technique (six versus five, respectively; p = 1.0).

Of the 20 patients in whom reviewers of endosonographic and MR imaging findings disagreed, three have undergone surgery at the time of writing. Two of these patients underwent sphincter repair, and the final consensus diagnosis was proved to be correct in each patient: one woman with combined sphincter defects correctly identified on endosonography and a second woman with an internal sphincter defect in whom MR imaging had correctly predicted an intact external sphincter. No surgical information on the sphincter was available in the third woman in whom the consensus diagnosis suggested intact sphincters and who subsequently underwent a Delorme's procedure [13] for associated rectal prolapse that was thought to be contributing to her symptoms of incontinence. At the time of writing, all remaining 17 patients were being treated with either constipating drugs, biofeed-back, or a combination of both.

Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Before the advent of anal endosonography, an internal anal sphincter injury was inferred from a low anal canal resting pressure, and an external sphincter injury was inferred by a low squeeze pressure or the absence of increased electric activity during voluntary contraction of the external sphincter on electromyographic recording from a needle electrode placed within it. Anal endosonography was the first technique to directly visualize the separate components of the sphincter [14], and endosonography has the ability to characterize sphincter abnormalities in a variety of clinical subgroups of incontinent patients. Women who become incontinent after childbirth often have disruption of the anterior external sphincter and often have internal sphincter injury as well. The mechanism of the injury may involve extension of perineal tears toward the anal canal [3]. In contrast, patients incontinent after anal surgery frequently have injuries confined to the internal sphincter because of direct involvement in sphincterotomy [15] or secondary involvement after hemorrhoidectomy [16] or anal dilation [17]. Furthermore, incontinence caused by primary internal sphincter degeneration can be diagnosed when internal sphincter thinning occurs without evidence of muscle disruption [4].

Although anal endosonography has been adequately validated histologically [2], intraoperatively [1], and physiologically [3], characterization of external sphincter injury can remain problematic because its echogenicity is frequently similar to that of ischioanal fat. A study of intraobserver agreement for anal endosonography found complete agreement for internal sphincter integrity in 51 consecutive patients but disagreement related to isolated external sphincter injury in nine (18%) [18]. Furthermore, complete agreement occurred between reviewers when external sphincter injury was accompanied by internal sphincter injury, suggesting that the latter facilitates diagnosis, presumably because either the internal sphincter is easier to visualize on sonography or the tears are larger.

Endoanal MR imaging can also visualize the anal sphincters with high spatial resolution. In contrast to endosonography, MR imaging sharply defines the lateral border of the external sphincter because fat and striated muscle return different signal intensities on T2-weighted images. It has been suggested that this may improve specificity for external sphincter injury; a retrospective study of 22 women undergoing sphincter repair revealed that endoanal MR imaging was superior to anal endosonography for the diagnosis of external sphincter injury [5]. We found that neither technique showed any advantage for diagnosing injuries of the external sphincter. External anal sphincter injuries may be easier to diagnose on MR imaging if adjacent scarring in ischioanal fat is present because the low-signal-intensity scar tissue is highly conspicuous against the brighter fat. Diagnosis is more difficult when scar tissue is localized to the external sphincter because the signals from scar tissue and striated muscle are similar on fast spin-echo sequences.

Although the internal sphincter shows relatively high signal intensity on T2-weighted MR images, in our experience the internal sphincter was more difficult to identify on MR images than on anal endosonography. Reviewers of MR images made significantly more errors regarding internal sphincter integrity when compared with endosonography (12 compared with one). This series comprised a relatively high proportion of patients with isolated iatrogenic internal sphincter injuries, reflecting the tertiary referral pattern to our unit. The internal sphincter enhances after IV gadolinium administration [6], and contrast-enhanced scans could have been used to improve the sensitivity of MR imaging. However, T2-weighted sequences without contrast enhancement are a well-established protocol [5, 11], and using gadolinium would have been more invasive and considerably more expensive. Two MR examination reviewers who each missed combined sphincter defects probably did so because of movement artifacts and coil migration. We attempted to immobilize the coil with foam pads but an external clamp may be preferable [6].

Incontinent patients undergoing surgery have a disproportionate prevalence of external sphincter tears because anterior external sphincter repair is the most commonly practiced operation [19]. However, most incontinent patients will not have surgery, either because their sphincters are intact or because they have isolated internal sphincter damage, for which generally no surgical option is available. Therefore, operative validation of isolated internal sphincter damage is rarely possible.

To avoid this spectrum bias, we chose to image a consecutive unselected cohort of patients. This approach will inevitably introduce the problem of a "true" gold standard because most of these patients will not be operated on for the reasons described. In an attempt to overcome this limitation, we additionally used anorectal physiologic testing to independently characterize sphincter abnormality. A low resting pressure indicates internal sphincter dysfunction, whereas a low voluntary contraction (squeeze) pressure indicates external sphincter dysfunction. Furthermore, imaging arbitration was made in consensus by the gastroenterologist and surgeon to whom the patient had been referred, both of whom had full access to the clinical history, examination, and anorectal physiologic results. It is well established that the clinical type of incontinence can indicate the abnormal muscle; patients with internal sphincter dysfunction experience passive incontinence, whereas those with external sphincter dysfunction suffer urge incontinence [12]. Additionally, the temporal relationship of symptoms to events such as vaginal delivery or anorectal surgery can help indicate the likely muscle involved. Because of this approach, our series included a higher proportion of patients with isolated internal sphincter dysfunction, either because of primary or secondary iatrogenic damage or primary degeneration [4], than in series in which findings were validated through surgery [5]. When anal endosonography is validated surgically, it has a high sensitivity for external and internal sphincter defects [2].

It is possible that a bias toward endosonography occurred because both reviewers have performed many thousands of examinations over several years and are likely to be especially capable performing the technique. However, both have also performed endoanal MR imaging since its introduction. Experience in endoanal MR imaging is generally limited because endoanal probes (as opposed to transrectal probes) remain unavailable outside a few specialist centers.

In summary, this study suggests that anal endosonography and endoanal MR imaging are comparable for the diagnosis of external anal sphincter defects but that internal anal sphincter defects are less well assessed on MR imaging.

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This Article Abstract Figures Only Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Malouf, A. J. Articles by Kamm, M. A. Search for Related Content PubMed PubMed Citation Articles by Malouf, A. J. Articles by Kamm, M. A. Social Bookmarking                      What's this?