Endoanal Ultrasound-Guided Surgery for Anal Fistula

• Introduction
• Patients and Methods
• Endoanal Ultrasound
• Hydrogen Peroxide Injection
• 3-D EAUS imaging
• Use of EAUS Findings for Decisions Regarding Surgery
• Statistical Evaluation
• Results
• Agreement Between EAUS and Surgery Findings
• Contribution of Hydrogen Peroxide Enhancement
• Contribution of 3-D Imaging
• EAUS and Surgical Decision Making
• Discussion
• References

Background and Study Aims: Outcomes following surgical treatment of patients with anal fistula are related to eradication of tracts and the internal opening. In this study, the results of surgery based on endoanal ultrasound (EAUS) findings were evaluated.
Patients and Methods: A total of 102 patients with primary cryptogenetic anal fistula were prospectively examined with EAUS, using a 360-degree rotating 10-MHz probe, equipped with a three-dimensional (3-D) imaging system. Injection of hydrogen peroxide through the external opening was also used. Patients underwent operation on the basis of the EAUS findings. The agreement between findings from EAUS and from surgery was calculated. Clinical results were reported as treatment success, fistula recurrence, and fecal incontinence.
Results: Amongst 102 patients, the overall concordance between EAUS and surgical findings was 94.1 % for primary tracts, 91.2 % for internal openings, 96.1 % for secondary tracts, 100 % for abscesses, and 96.1 % for horseshoe tracts. Diagnostic accuracy was improved when hydrogen peroxide injection or 3-D imaging were used. Fistulotomy was performed in 46 patients (45.1 %), fistulectomy in 17 (16.7 %), fistulotomy plus seton placement in 19 (18.6 %), fistulectomy plus seton in 18 (17.6 %), and mucosal flap advancement in 2 (2.0 %). The operation was curative in 100 patients (98.0 %), and unsuccessful in 2 (2.0 %) due to recurrence of the fistula. Fecal continence was preserved in all patients.
Conclusions: These data highlight the diagnostic accuracy of EAUS, particularly when hydrogen peroxide injection or 3-D imaging are used. Basing our surgical decision making on EAUS findings allowed us to carry out curative operations in a significantly large number of patients; the recurrence rate was very low. The accurate EAUS assessment of the relationship between fistulas and sphincters has been the main factor in choosing a sphincter-saving surgical procedure, avoiding fecal incontinence.

Introduction

In the treatment of anal fistula, an appropriate surgical approach is of primary importance since two main problems may be encountered in this field: the strong possibility of recurrence of the fistula, and iatrogenic damage to the anal sphincter that could lead to impairment of fecal continence. These complications are frustrating both for the patient and surgeon and it is mandatory to pay maximum attention to both possibilities during surgery. It has been suggested that detailed knowledge of fistula characteristics may allow a reduction in the occurrence of both these adverse events. Thus the preoperative assessment of the fistula plays a key role in successful treatment. However, clinical evaluation alone, including fistula probing, has severe limitations, on account of poor reliability in detecting the location of the internal opening and the possible existence of secondary tracts [1] [2] [3] [4] [5]. On the other hand, fistulography, traditionally adopted in the diagnostic work-up, has been shown to be of poor accuracy in delineating fistula anatomy [1] [6] [7] [8] [9]. Moreover, computerized tomography (CT) is of limited usefulness since it is extremely uncomfortable for the patient due to the need to give a dose of radiation and, moreover, offers very little clinical information [10] [11] [12].

Over the last two decades, endoanal ultrasound (EAUS) has been demonstrated to be a very helpful diagnostic tool, showing interesting features in accurately assessing all fistula characteristics [2] [3] [4] [9] [13] [14] [15] [16] [17]. The ease of use, intra- and interobserver reproducibility of results, and low costs are just some of the reasons for the increasing interest in this method of examination for patients with fistulas. The addition of hydrogen peroxide injection through the external opening of the fistula appears to improve the diagnostic accuracy of standard EAUS [5] [6] [18] [19] [20] [21] [22] [23] [24] [25]; where it is injected, hydrogen peroxide produces a significant increase in the echogenicity of the fistulous tract which then appears as hyperechoic instead of hypoechoic. This method could thus be helpful in identifying tracts which had not been observed at the standard EAUS examination or whose presence had not been definitively established. Moreover, the EAUS views obtained using the three-dimensional (3-D) system of acquisition and revision of the ultrasound images might be able to provide a significant contribution to accurate diagnosis [23].

Magnetic resonance imaging (MRI) would provide comparable accuracy in uncomplicated fistulas, which would be better in cases of complex fistulas [18] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]. However, the limited availability of MRI instruments and of specific expertise with fistulas, and the high costs, are some of the major problems preventing its widespread use in the clinical setting.

In the present study, the accuracy of EAUS in identifying primary and secondary fistula tracts and the internal opening has been evaluated. The primary aim has been to assess the therapeutic significance, in the management of anal fistula, of surgical decision making based on EAUS findings.

Patients and Methods

Between 1998 and 2002, 102 patients (76 men, 26 female; mean age 45 years, range 18-75) with primary cryptogenetic anal fistula were entered into the study. Patients with recurrent anal fistula, Crohn anal fistula, or previous anorectal surgery were excluded from the study. After clinical examination, which included probing of the fistula, the patients underwent EAUS.

Endoanal Ultrasound

The examination was performed using a multifrequency (5 - 10 Mhz range) 360-degree rotating endoprobe (model 1850; B-K Medical, Herlev, Copenhagen, Denmark), connected to the ultrasound machine (Leopard; B-K Medical). During EAUS, the patient is placed on their left side. The endoprobe is covered with a condom; some gel is placed on the smooth tip of the endoprobe. After introduction of the endoprobe, the appropriate crystal frequency is selected: the examination is generally started using 10Mhz, changing to 7 or 5Mhz to optimize visualization of the deeper structures external to the anal sphincters.

A regular pull-through allows visualization of all the anal canal structures. The puborectal muscle, external, longitudinal, and internal sphincters are all identified and used as referents for the spatial orientation of the fistula or abscess. An anal fistula appears as a hypoechoic tract which is followed along its crossing of the subepithelium, internal or external sphincters, and through the perianal spaces. With regard to the anal sphincters, the fistulous primary tract was classified as intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric, according to Parks’ classification [35]. Secondary tracts, when present, were classified as intersphincteric, extrasphincteric, or suprasphincteric. Similarly, horseshoe fistula tracts, when identified, were categorized as intersphincteric, extrasphincteric, or suprasphincteric. The internal opening was identified as hypoechoic (when acute inflammation is present) or hyperechoic (when chronically inflamed). The site of the internal opening was defined according to the criteria of Cho et al. [36], and categorized as being above, at, or below the dentate line (in relation to the presumed location of the dentate line at the middle third of the anal canal). In addition, the site of the internal opening was also characterized by the clock position, being classified from 1 to 12 o’clock.

An anal abscess appears as a hypoechoic dyshomogeneous area, sometimes with hyperechoic spots within it, possibly in connection with a fistulous tract directed through the anal canal lumen. Abscesses were classified as intersphincteric, extrasphincteric, or suprasphincteric.

During the examination, the operator classified all of the fistula characteristics, recording them on the patient’s file. In addition, all images of significant clinical interest were printed out (in a 2-D format).

Hydrogen Peroxide Injection

In 89 patients (87.3 %) in whom the external fistula opening was “patent”, 3 % hydrogen peroxide was injected very slowly using a smooth needle via this opening. Information about fistula characteristics obtained from hydrogen peroxide-enhanced EAUS (HPUS) was compared with that obtained during standard EAUS.

3-D EAUS imaging

Since 2001, a 3-D system (B-K Medical) for EAUS imaging has also been used. With this system, connected to the ultrasound component, all images were acquired, either as parallel transverse 2-D or as sagittal views, by the endoprobe (which can be mounted on a specifically designed recto-anal mover) during regular pull-through along the anal canal. In this system, the images are elaborated into a 3-D geometric figure. This reconstruction instantly converts a set of 2-D images into a 3-D image, correcting geometric errors arising from the transducer. Within a few seconds, the final 3-D image is visualized. Real time manipulation is possible, viewing (and reviewing, at any time after the end of the examination) any desired region following the coronal, longitudinal, and also oblique projections; it is possible to produce the volume rendering of the 3-D image.

A total of 66 patients (64.7 %) were examined with the 3-D imaging system. In these patients, findings obtained using the 3-D system were compared with those provided by 2-D imaging.

Use of EAUS Findings for Decisions Regarding Surgery

When an intersphincteric or low transsphincteric fistula was diagnosed by EAUS, patients were given locoregional anesthesia, which was administered using a posterior perineal block (40 ml of 0.475 % ropivacaine), in a day-surgery setting. The other types of fistula were operated on with patients under general or spinal anesthesia.

When EAUS showed intersphincteric or low transsphincteric fistulas, patients were scheduled for fistulotomy. With middle transsphincteric fistulas, fistulotomy was planned only when the fistula tract involved less than 50 % of the external sphincter. However, when a middle transsphincteric fistula was found by EAUS to be located on the anterior side of the anal canal in female patients, a cutting seton placement or a mucosal flap procedure was planned. In the case of high transsphincteric, suprasphincteric, or extrasphincteric fistulas, a partial fistulectomy plus cutting seton or a mucosal flap procedure were scheduled. EAUS imaging of an abscess indicated the need for surgical drainage and, if possible, a loose seton placement.

Statistical Evaluation

The mean and standard error of the mean (SEM) were calculated for each recorded parameter. Student’s t-test for paired data was used to compare the means. P values of less than 0.05 were considered to be statistically significant.

Results

Agreement Between EAUS and Surgery Findings

The data showing the extent of agreement between EAUS and surgical findings are shown in Table [1]. For the primary tract, very good agreement was observed in the categorization of intersphincteric tracts (15/17, 88.2 %) and transsphincteric tracts (76/76, 100 %), while for suprasphincteric tracts (even though they were detected in only five instances) the concordance was low (1/5, 20 %); all four extrasphincteric fistulas (100 %) were correctly diagnosed.

An intersphincteric secondary tract was correctly diagnosed by EAUS in 83.3 % of cases (5/6), while only a single secondary extrasphincteric tract was detected (by both the procedures). EAUS findings excluded a secondary tract in 92 out of the 95 patients eventually found at surgery not to have a secondary tract (96.8 %).

A 91.7 % concordance between EAUS and surgical findings was obtained for the diagnosis of ischiorectal horseshoe tract (11/12 patients). EAUS correctly identified the intersphincteric horseshoe tract in one patient, but not the single supralevator horseshoe tract. EAUS correctly excluded a horseshoe tract in 86 out of 88 cases (97.7 %).

Anterior internal openings were correctly identified by EAUS in 91.8 % of cases (45/49), and posterior internal openings in 96.2 % (51/53). In the characterization of the site of the internal opening in relation to the dentate line, agreement between EAUS and surgery was found in 96.6 % of patients in whom the opening was at the dentate line (84/87); in 100 % of those with the opening above the dentate line (2/2); in 44.4 % when the opening was below the dentate line (4/9), and in 75 % with the opening located in the lower rectum (3/4). When the agreement between EAUS and surgical findings was evaluated for the location of the internal opening according to clock position, the concordance was 94.2 % (96/102) (Figure [1]).

Anal abscesses were correctly identified in all those patients presenting this lesion (located in the ischioanal space in all 36 instances). EAUS also excluded the presence of an abscess in 66 patients (concordance 100 %).

Figure [2] shows a few examples of EAUS images used in the diagnosis of fistula and abscesses.

Contribution of Hydrogen Peroxide Enhancement

A comparison between standard EAUS and HPUS findings in the 89 patients examined using both modalities is outlined in Table [2]. HPUS was superior to standard EAUS in the accurate classification of all fistula characteristics. In particular, a marked improvement in the percent accuracy was obtained in the diagnosis of primary intersphincteric (+ 15.4 %), transsphincteric (+ 10.1 %), suprasphincteric (+ 33.3 %), and extrasphincteric (+ 50 %) tracts; in the diagnosis of secondary tracts (+ 60 % to + 100 %); of horseshoe tracts (+ 33.3 % to + 100 %); of internal openings, both anterior (+ 9.5 %) and posterior (+ 8.5 %), or located at the dentate line (+ 9.9 %), below the dentate line (+ 33.4 %), above the dentate line (+ 100 %), or rectum (+ 66.7 %).

Contribution of 3-D Imaging

Comparative data for the 2-D and 3-D imaging systems, obtained in 66 patients, are reported in Table [3]. The use of 3-D imaging improved the accuracy in categorizing primary tracts (intersphincteric, + 12.5 %; transsphincteric, + 3.8 %; suprasphincteric, + 33.3 %; extrasphincteric, + 33.3 %); secondary tracts (intersphincteric, + 33.3 %; extrasphincteric, + 100 %); horseshoe tracts (intersphincteric, + 100 %; extrasphincteric, + 14.3 %); and internal openings (anterior, + 6.5 %; posterior, + 11.4 %; at the dentate line, + 6.6 %; below the dentate line, + 66.7 %; above the dentate line, + 100 %; rectum, + 50 %).

EAUS and Surgical Decision Making

Details of the surgical approach eventually used in patients compared with the operation scheduled on the basis of the EAUS findings are shown in Table [4]. A fistulotomy was planned in 67 patients on the basis of the results of EAUS imaging, but this operation was actually carried out in 65 of them, fistulectomy being preferred in the other two patients (concordance 97.0). In another 33 patients, fistulectomy, as suggested by the EAUS findings, was performed (concordance 100 %). Finally, in two patients, EAUS correctly guided the choice of treating the fistula by a mucosal flap. Use of a seton was suggested by the results of EAUS in 40 patients, and was effected in 35 of these (87.5 %) during surgery. The operation was performed under locoregional anesthesia in 25 patients and general anesthesia in 77 patients, in each case in agreement with the approach planned on the basis of the EAUS data.

The long-term results were evaluated after a median follow up of 20 months (range 10 - 33). Complete healing of the anal fistula was observed in 99 patients (97.1 %). In three patients (2.9 %), fistula recurrence was detected; all were re-evaluated by EAUS in order to confirm the relapse, and two patients underwent re-operation with success, while the third, who had a very thin recurrent anal fistula, refused further surgery. Severe fecal incontinence was not reported by any of the patients; only one patient presented transient soiling which disappeared within 6 months.

Discussion

The present study included only patients with primary cryptogenetic anal fistula, those with recurrent or Crohn fistulas, or previous anorectal surgery having been excluded. Compared with our previous report [6], data from this study show a significant improvement in diagnostic accuracy. Various factors could have contributed to the improvement in results, namely, greater experience of the operator in using EAUS; the larger number of patients studied; the rational use of hydrogen peroxide; and the addition of 3-D ultrasound imaging.

With injection of hydrogen peroxide into the external fistula opening it is possible not only to confirm the tract(s) already seen by standard EAUS, but also to identify other tracts, in most cases secondary, that were not observed during the standard procedure. Indeed, this step could be particularly useful when an active fistulous tract needs to be distinguished from post-surgical or post-trauma scar tissue [37], which is of primary importance for successful surgical treatment [35]. The reported diagnostic accuracy of HPUS ranges from 71 % to 95 % for primary tracts [5] [6] [18] [19] [20] [21] [22] [23], and from 63 % to 96.1 % for secondary tracts ([5] [6] [20] [23] present series), while that of standard EAUS ranges from 50 % to 91.7 % for the primary tract [5] [6] [14] [15] [19] [21] [23], and from 60 % to 68 % for secondary tracts [5] [6] [19] [23]. The highest concordance is usually reported for primary transsphincteric fistulas, for both standard EAUS and HPUS [3,6,18,21, present series], while the major diagnostic difficulty is still the adequate identification of primary supra- and extrasphincteric fistulas [3, 6, 18, 21, present series]. In the present series, the most powerful contribution of HPUS was in accurately diagnosing secondary and horseshoe tracts, which were identified in every instance.

Moreover, HPUS could contribute to a more accurate identification of the internal opening, which also plays a key role in surgical treatment to prevent a recurrence of the fistula [4]. In comparison to results reported using standard EAUS, with an accuracy ranging from 5.3 % to 93.5 % [3 - 6, 14, 15, 19, 21, 23, 36, present series], the reported accuracy of HPUS is higher, ranging from 48 % to 96.6 % [5, 6, 20 - 23, present series]. In our patients, although poor results were obtained in the attempts to identify internal openings located below the dentate line (though accuracy improved when HPUS or 3-D imaging was used), very high accuracies were obtained for all the other types. In this regard, Moscowitz et al. [38] reported a false-negative result in seven out of 57 patients (12.3 %) using hydrogen peroxide; four out of these seven patients had supra- and extrasphincteric fistulas. Lengyel et al. [21] reported a 93 % accuracy in the detection of the internal opening by EAUS. Gustafsson et al. [19] examined 23 patients by EAUS and found concordance between EAUS and surgery in 74 % of cases, and between MRI and surgery in 43 %. Moreover, for both the anterior and, particularly posterior internal openings, the accuracy was very high in our patients. Preoperative location of the internal opening, according to clock position, could be of interest to surgeons since it might help in identifying this during surgery; in our series, as far as classification of this fistula characteristic is concerned, EAUS showed very good agreement with surgical findings. Regardless of the examination modality preferred, the availability of accurate criteria for locating the internal opening (in the present series, the criteria of Cho et al. [36] were used) seems to play a fundamental role in achieving the best results ([3] [36] [39], present series).

More recently, the availability of the 3-D imaging system has provided another very useful tool for an accurate assessment of endoanal and endorectal ultrasound examination. With regard to anal fistulas, with this method the operator can review the entire series of ultrasound images, reconstructed, without distortions, along all the planes desired (coronal, sagittal, oblique). Buchanan et al. [23], in 19 patients with recurrent or complex fistulas, reported accurate results in detecting primary tracts (81 %), secondary tracts (68 %), and internal openings (90 %); the addition of hydrogen peroxide did not improve these features (accuracies of 71 %, 63 %, and 86 %, respectively). Using the 3-D imaging system in our patients, the accuracies were 98.5 % for primary tracts, 98.5 % for secondary tracts, and 96.4 % for internal openings (compared with 89.4 %, 83.3 %, 87.9 %, respectively, when the 2-D system was used).

In the present series, all anal abscesses were correctly diagnosed using EAUS; this finding should have clinical implications in the management of anal abscesses which, far too frequently, are submitted for surgery on the basis of an external clinical evaluation. Although it might be argued that an endoanal examination could be painful for the patient with an anal abscess, in our experience, the delicate introduction of the EAUS probe was possible in all patients with only slight discomfort, thus allowing very useful clinical information to be collected. Performing EAUS before surgery should significantly reduce the incidence of unidentified abscess cavities which, not being completely drained surgically, are one of the main factors responsible for treatment failures [40].

Even if the improvement in diagnostic accuracy is a notable feature in the present study, the most interesting aspect concerns the impact of these findings on surgical strategy. Theoretically, the best way to establish the influence of EAUS on fistula surgery would be a comparison between groups of patients in whom EAUS is or is not performed before surgery. However, our previous data [6] and those of others [3] [4] [5] [14] [15] [16] [17] [18] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40], showing the high diagnostic value of EAUS, suggest that it would be unethical to operate on patients with anal fistula without performing a diagnostic instrumental examination. All our patients, therefore, underwent operation with the EAUS findings being borne in mind, and these were improved with the use of HPUS and the 3-D imaging system. Thus, the anesthesiologic approach was also chosen in relation to the fistula course (locoregional for low intersphincteric or transsphincteric tracts, general or spinal for all others). This anesthesiologic approach remained unchanged, thus allowing completion of the scheduled operation in all patients. EAUS guided the choice of surgical procedure to be performed in almost all patients: indeed, in almost all instances, the planned surgical option was actually adopted. Furthermore, use of a seton was also correctly indicated by EAUS in a large percentage of patients. The long-term outcome was the most gratifying proof of correct choice of surgery, with a high rate of successful operations, a low rate of fistula recurrence, and only one case of transient soiling, but no incidence of major fecal incontinence in any of our patients.

Thus, in our opinion, anal continence was preserved, in our patients, when sphincterotomy was guided by the knowledge (from EAUS findings) about the amount of sphincter above the anal fistula and which, indeed, remained intact during surgery. EAUS data offered a reliable delineation of the anatomy of the fistula and thus the surgeon could effectively plan the surgical procedure; the treatment programme was discussed with the patients prior to surgery in order to also establish the anesthesiologic approach and predict the time for healing.

In conclusion, integration between EAUS (including also HPUS and the 3-D imaging) and surgery appears to improve the logistic and clinical management of anal fistulas, with a significant improvement in the surgical outcome.

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C. Ratto, M. D.

Department of Surgical Sciences, Catholic University

Largo A. Gemelli 8 · 00168 Rome · Italy

Fax: +39-06-6693413

Email: carloratto@tiscali.it

References

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C. Ratto, M. D.

Department of Surgical Sciences, Catholic University

Largo A. Gemelli 8 · 00168 Rome · Italy

Fax: +39-06-6693413

Email: carloratto@tiscali.it