Digestive and Liver Disease
Volume 38, Issue 8 , Pages 537-543, August 2006

Endoanal ultrasound in perianal fistulas and abscesses

Department of Gastroenterology and Hepatology, VU University Medical Centre, P.O. Box 7057, 1007 MB Amsterdam, The Netherlands

Received 26 September 2005; accepted 24 February 2006.

Article Outline

Abstract 

Anal ultrasound has demonstrated to be accurate in staging perianal cryptoglandular fistulae and fistulae in Crohn's disease. When there is an external fistula opening, H2O2 can be introduced with a plastic infusion catheter. The tract becomes then visible as a hyperechoic lesion (‘white’). It has been shown that this corresponds excellent with surgical findings. It is equally sensitive as endoanal MRI. Because recurrent cryptoglandular fistulae are complex in about 50% and Crohn's fistula in 75%, it is mandatory to perform anal ultrasound preoperative in these patients to avoid missed tracks during surgery and subsequent recurrences. Anal ultrasound can also be used to monitor (medical) therapy in patients with Crohn's disease. Especially considering the easiness of performing and lesser costs then MRI, endoanal ultrasound merits more attention.

Keywords: Anal endosonography, Anal ultrasound, Perianal fistula, Perianal sepsis

 

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1. Introduction 

Endoanal ultrasound (EUS) was introduced 20 years ago by the urologists to evaluate the prostate. Later, EUS was used by other specialists to stage rectal tumours and to investigate benign disorders of the anal sphincters and pelvic floor.

The technique is easy to perform, has a short learning curve and causes no more discomfort than a routine digital examination. A rotating probe with a 360° radius and a frequency between 5 and 16MHz is introduced into the rectum. The probe is then slowly withdrawn so that the pelvic floor and the subsequent sphincter complex is seen. Recently, it is also possible to reconstruct 3D images.

EUS has been used for almost every possible disorder in the anal region and delineating the anatomy has increased insight in anal pathology.

Clinical indications for EUS are the faecal incontinence for the detection of defects and atrophy, perianal fistulae and abscesses to demonstrate the fistula tract and rectal and anal carcinomas for staging and follow-up.

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2. Normal anatomy and morphology 

Endosonographic findings in healthy volunteers have been thoroughly investigated [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Moreover, a number of studies have established basic endosonographic anatomy by making comparisons with anatomical preparations [1], [4], [7], [10].

The puborectalis muscle is, in almost all cases, easily visualised and can serve as a point of orientation: it appears as a V-shaped echogenic band, which slings dorsally around the rectum (Fig. 1a). When withdrawing the probe, the echogenic band closes anteriorly, thus forming the external anal sphincter (Fig. 1b). The thickness of the external anal sphincter is approximately 4–10mm [3], [5], [8], [9], [11], [12]. Fig. 1c and d represents the lateral and coronal views, respectively.

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  • Fig. 1. 

    Anal endosonography. Normal anatomy of the anal sphincter and puborectal muscle in 3D imaging: (a) frontal view of puborectal muscle (PR); (b) frontal view of the anal sphincters; (c) lateral view; and (d) coronal view. SM, submucosa; IAS, internal anal sphincter; and EAS, external anal sphincter.

In women the external sphincter is anteriorly thinner and shorter [2], [8], [11], which makes it more vulnerable to obstetric damage. Besides being related to gender, the external sphincter thickness is also correlated to body weight [8]. There is no clear relationship between the external anal sphincter thickness and age [11], [12], [13], [14].

Inside the external sphincter lies the internal anal sphincter, which presents as a thin echo lucent band of approximately 1–3mm thickness [3], [5], [6], [8], [9], [11], [12], [13], [14], [15]. The internal anal sphincter increases in thickness and echogenicity with age, both in patients [14], [16] and healthy volunteers [6], [8], [11], [12], [13], [15]. These findings are suggestive for sclerosis of the internal sphincter in the elderly, which has been demonstrated histologically [17]. The internal sphincter thickness is not related to sex, body weight or length [6].

The submucosal layer has a mixed echogenic aspect and is partly collapsed by pressure of the endoprobe [10]. The submucosal thickness increases slightly with age [11]. This has also been found to a larger extent in internal haemorrhoids [18] and might be caused by physiological distal displacement or enlargement of the anal cushions [19]. The mucosa cannot be identified separately with the frequencies used.

Other pelvic floor structures around the sphincter complex can also be visualised. There are some reports on visualising the longitudinal muscle of the external anal sphincter, but the importance is controversial [2], [5], [7], [10], [20]. The anococcygeal ligament appears as an echo poor triangle and causes tapering of the external sphincter or puborectalis muscle [2], [11]. Furthermore, the transverse perineal muscles, the ischiocavernous muscles, the urethra and the pubic bones may be visualised [4], [8], [11].

Vaginal endosonography to visualise the perianal area and especially the perineum, is an alternative when rectal endosonography is not possible; for instance, when the anus is asymmetrical causing air artefacts, extreme anal stenosis or pain [21] (Fig. 2).

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3. Perianal fistulae 

3.1. Imaging technique 

Parks has classified perianal fistulae as inter, trans, extra or suprasphincteric [22]. To reduce the risk of postoperative faecal incontinence and recurrences, it is important to identify the anatomic course of the fistula in relation to the anal sphincters and to be informed about the existence of anal sphincter defects before surgery. Furthermore, secondary extensions must be detected to reduce the risk of recurrence.

With proctological examination and probing of the fistula, accurate assessment is not often possible. Fistulography is an obsolete technique.

The use of contrast agents like hydrogen peroxide (Fig. 3) has significantly improved the results of endosonography in the assessment of perianal fistulae [23], [24], [25], [26], [27], [28], [29], [30]. The infusion of hydrogen peroxide generates the formation of small air bubbles and this changes the fistula track from hypoechoic to bright hyperechoic (‘white’). The original study [24] described in two patients with a recurrent complex perianal fistula that EUS accurately demonstrated the fistulous tract anatomy, which was confirmed during surgery. Two larger studies confirmed these good results with accuracy rates of 95% [23] and 92% [25], regarding classification of the fistula. Hydrogen peroxide-enhanced endosonography must be considered a safe, economic and reliable procedure for the assessment of perianal fistulas. It assists in delineating the anatomic course of perianal fistulas and is therefore of value in planning surgical strategy. Levovist has also been used to visualise perianal fistula and was better at assessing the anal fistula than the physical examination and conventional ultrasound [31]. It seems strange that with these results, studies about EUS still emerge without the use of contrast [32], possibly because of doubt about the improved results [30].

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  • Fig. 3. 

    (a) Normal image of a patient with a perianal fistula; (b) same patient, after infusion of hydrogen peroxide through the external fistulous opening reveals a white transsphincteric fistula (arrows); (c) a transsfincteric fistula. Internal opening is indicated with an arrow; and (d) complex horseshoe fistula with suprasphincteric extension in a patient with Crohn's disease.

3.2. EUS compared with MRI 

MRI has also been proven to be highly accurate in diagnosing fistulae. Several studies have compared EUS to MRI (Fig. 4), with different conclusions.

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  • Fig. 4. 

    Transsphincteric fistula and internal opening (arrow): transversal image with anal EUS (a) and endoanal MRI (b) and lateral image with EUS (c) and endoanal MRI (d).

One study determined the agreement between hydrogen peroxide-enhanced 3D endoanal ultrasonography (3D HPUS) and endoanal MRI in preoperative assessment of perianal fistulas and compared these results to surgical findings [33].

Twenty-one patients (aged 26–71) with clinical symptoms of a cryptoglandular perianal fistula and a visible external opening underwent preoperative 3D HPUS, endoanal MRI with subsequent surgical exploration. Experienced observers, blinded for each other's findings, assessed the results separately. A description of each fistula was made and the following characteristics were recorded: classification of the primary fistula tract according to Parks (intersphincteric, transsphincteric, extrasphincteric, suprasphincteric, horseshoe or not classified), presence of secondary tracts (circular or linear) and location of an internal opening.

The median time between 3D HPUS and endoanal MRI was 66 days and the median time between the last study (3D HPUS or endoanal MRI) and surgery was 154 days. Agreement for the primary fistula tract was 81% for 3D HPUS and surgery, 90% for endoanal MRI and surgery as well as for 3D HPUS and endoanal MRI. For secondary tracts, agreement for circular tracts was 67% for 3D HPUS and surgery, 57% for endoanal MRI and surgery, and 71% for 3D HPUS and endoanal MRI and for linear tracts, 76% for 3D HPUS and surgery, 81% for endoanal MRI and surgery, and 71% for 3D HPUS and endoanal MRI. Agreement for the location of an internal opening was 86% for 3D HPUS and surgery as well as for endoanal MRI and surgery and 90% for 3D HPUS and endoanal MRI. Therefore, evaluating perianal fistulas with 3D HPUS and endoanal MRI have a good agreement especially for the classification of the primary fistula tract and in the location of an internal opening. These results also show good agreement when compared to surgical findings. It was concluded that both 3D HPUS and endoanal MRI could be used as a reliable method for preoperative evaluation of perianal fistulas.

In another study, 40 patients who were not yet operated were compared concerning their 3D HPUS and endoanal MRI findings [34]. The interval between 3D HPUS and endoanal MRI was shorter with 14 days. Comparison of both the techniques and the patient preference regarding these techniques was assessed. The methods agreed in 88% for the primary fistula tract, in 90% for the location of the internal opening, in 78% for secondary tracts and in 88% for fluid collections. There was no significant difference between the amount of discomfort experienced during 3D HPUS and endoanal MRI or in patient preference for one procedure over the other.

Several other studies have compared the results of anal ultrasonography and MRI for assessing perianal fistulas [32], [35], [36], [37], [38], [39], [40]. Three studies [32], [37], [38], [39] found MRI to be more accurate than EUS, not using HPUS. A study in patients with Crohn's disease [35] found EUS more sensitive modality for imaging perianal Crohn's disease than the pelvic MRI, another study found comparable results for body coil MRI [36]. Others [39], [40] concluded that EUS, MRI and examination under general anaesthesia were all accurate tests for determining fistula anatomy in patients with perianal Crohn's disease.

In most of the studies mentioned, surgery was also used as a gold standard although EUS and MRI are both well-established techniques for assessing fistulas. The use of surgery as a gold standard has been questioned. An EUS study [23] demonstrated two secondary tracts with HPUS that were not found during surgery, these patients developed a recurrent fistula, suggesting that these branches were actually present at the time of HPUS. In a follow-up study with 37 patients, the body coil MRI was shown to make better predictions regarding patient outcome than surgical findings [41]. Another study [36] with 23 patients found that in two patients who did not heal after surgery, EUS showed an extension and/or abscess, which were not identified at the time of operation. For body coil MRI the corresponding figure was three. In the same study no internal opening was found during surgery in three patients.

The results in the studies mentioned above [32], [33], [34], [35], [36], [37], [38], [39], [40] differ and different techniques for MRI and EUS were used. One of the reasons for the different results found could be the use of different probes for EUS. Better results were found when a linear probe or biplane probe [35], [39] was used; 3D EUS imaging may also improve imaging [33], [34]. Most of the studies mentioned earlier used a body coil [35], [36], [37]. Body coil MRI may give additional information on structures further away from the anal canal but it can be difficult to make a distinction between fistula tracts and vessels. With an endocoil, more precise information can be obtained from the anal sphincter [33].

An important issue is that hydrogen peroxide was not used in many of the studies mentioned above. This could be a reason for some of the disappointing results found while using EUS. Excellent results have been reported for the assessment of perianal fistulas when hydrogen peroxide is used as a contrast medium [23], [24], [25], [26], [27], [28], [29], [30], [42]. The accuracy for determining the Parks’ classification can rise by 30% with HPUS [23]. A problem encountered by conventional EUS is that it is difficult to distinguish between scar tissue and active fistulas. When hydrogen peroxide is introduced into the external opening, a fistula tract appears as hyperechoic. This way it is easier to identify a fistula tract as well as the internal opening or any secondary tracts. The use of hydrogen peroxide has proved to be a safe method. In our centre, neither complications have occurred due to the introduction of hydrogen peroxide nor are any documented in the studies [23], [24], [25], [26], [27], [28], [29], [30], [42] mentioned. Although this method provides good results and is safe an external opening must be visible to introduce hydrogen peroxide.

3.3. Perianal fistula patterns in cryptoglandular disease 

Parks’ classification (inter, trans, supra and extrasphincteric) is used to describe the anatomical track of fistulas-in-ano in cryptoglandular disease. The frequency in which these types occur and whether visualisation is necessary was examined by retrospective reviewing of all the HPUS studies to investigate the fistula track in never operated (n=48) and recurrent (n=33) cryptoglandular fistulas [27]. All never operated cryptoglandular fistulas-in-ano were inter or transsphincteric. An extra track was found in 5%. Recurrent fistulas-in-ano were supra or extrasphincteric in 15% and ramified in 27%, thus complicated in a total of 44% of cases. Therefore, never operated fistulas-in-ano do not require any special investigation before surgical treatment in contrast to recurrent fistulas, where visualisation by HPUS is recommended to detect supra or extrasphincteric fistulas or ramification.

3.4. Perianal fistulae in Crohn's disease 

A similar study in patients with Crohn's disease [28] showed that only 22% of the patients appeared to have single inter or transsphincteric fistula, 12% single supra or extrasphincteric fistula (high fistula), 34% ramified fistula (more than one fistula track) from which the main track follows the Parks’ classification but ramifications had a bizarre pattern, which is not in agreement with this classification. Anovaginal fistulas were found in 32%. Thus, 78% of patients had a complicated fistula. Therefore, optimal documentation by means of hydrogen peroxide-enhanced transanal ultrasound is mandatory before surgery or before immune-modulating treatments.

The effect of three gifts infliximab (5mg/kg) was studied in an open label trial in eight patients with Crohn's disease-associated fistula-in-ano using hydrogen peroxide-enhanced transanal ultrasound [43]. Patients with vaginal or perineal fistulas did not respond clinically to therapy, whereas patients with perianal fistulas improved considerably. However, in all patients, remaining of the fistulous tracts were demonstrated by endosonographic techniques.

In a double-blind placebo-controlled study in 24 patients, they were treated with three gifts infliximab (5mg/kg) and randomly assigned to receive 500mg ciprofloxacin twice daily or a placebo for 12 weeks [44]. At week 18, patients treated with ciprofloxacin tended to respond better. 3D HPUS improved only in three patients with a clinical response.

Therefore, short-term treatment of Crohn's disease-associated fistulas with infliximab and/or antibiotics does not induce disappearance of fistulous tracts, irrespective of therapeutic response. Further conservative and/or surgical therapy needs to be evaluated to reach the closure of these fistulae.

Besides being of value in the assessment of fistulas and abscesses in IBD [43], [44], [45], [46], [47], [48], endosonography has shown that anal wall thickness is increased in active perianal Crohn's disease [49]. Fibrosis of the anus may be visible as heterogeneity of the anal sphincter complex [48].

Conflict of interest statement

None declared.

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PII: S1590-8658(06)00070-3

doi:10.1016/j.dld.2006.02.016

Digestive and Liver Disease
Volume 38, Issue 8 , Pages 537-543, August 2006

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