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Imaging of Fistula in Ano¹

¹ From the Department of Specialist Radiology, University College Hospital, Level 2, Podium, 235 Euston Rd, London NW1 2BU, England (S.H.), and Department of Radiology, Academic Medical Center, Amsterdam, the Netherlands (J.S.). Received June 11, 2004; revision requested August 24; revision received October 15; accepted January 10, 2005; final version accepted March 2; updated September 29. Address correspondence to S.H.

	ABSTRACT

TOP ABSTRACT INTRODUCTION ANATOMY AND ETIOLOGY FISTULA CLASSIFICATION ASSESSMENT AND TREATMENT IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FOR DIFFERENTIAL... WHICH PATIENTS SHOULD BE... CONCLUSION ESSENTIALS References

 
Fistula in ano is a common condition that often recurs despite seemingly adequate surgery, usually because of infection that was missed at surgery. It is now increasingly recognized that preoperative imaging can help identify infection that would have otherwise gone unidentified. In particular, magnetic resonance (MR) imaging findings have been shown to influence surgery and markedly diminish the chance of recurrence; thus, preoperative imaging will become increasingly routine in the future. In this article, the authors describe the pathogenesis, classification, and imaging of fistula in ano, with an emphasis on MR imaging. Most important, the authors describe how the radiologist is well placed to answer the surgical riddles that must be solved for treatment to be effective.

© RSNA, 2006

	INTRODUCTION

TOP ABSTRACT INTRODUCTION ANATOMY AND ETIOLOGY FISTULA CLASSIFICATION ASSESSMENT AND TREATMENT IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FOR DIFFERENTIAL... WHICH PATIENTS SHOULD BE... CONCLUSION ESSENTIALS References

 
Fistula in ano (anal fistula) is a common condition that has a tendency to recur despite seemingly adequate surgery. Recurrence is usually due to infection that has escaped surgical detection and has thus gone untreated. It is now increasingly recognized that preoperative imaging, notably magnetic resonance (MR) imaging, can help identify infected tracts and abscesses that would otherwise have been missed. Preoperative MR imaging findings have been shown to influence subsequent surgery and markedly diminish the chance of recurrent disease as a result. Because of this, preoperative imaging is likely to become increasingly routine in the future, especially in patients with recurrent disease.

This review will detail the pathogenesis of fistula in ano, explain how pathogenesis causes the different types of fistula encountered, and describe how these types can be imaged, with the emphasis on MR. Most important, we will describe how the radiologist is well placed to answer the surgical questions that must be solved for treatment to be effective.

	ANATOMY AND ETIOLOGY

TOP ABSTRACT INTRODUCTION ANATOMY AND ETIOLOGY FISTULA CLASSIFICATION ASSESSMENT AND TREATMENT IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FOR DIFFERENTIAL... WHICH PATIENTS SHOULD BE... CONCLUSION ESSENTIALS References

 
To fully understand the role of imaging with regard to fistula in ano, an appreciation of its etiology and how the various fistula types are defined by anatomic boundaries is mandatory.

The anal canal is essentially a cylinder surrounded by two muscular sphincters, the internal and external anal sphincters, which are composed of smooth and striated muscle, respectively (Fig 1). The external sphincter has posterior attachments to the anococcygeal ligament and anterior attachments to the perineal body and urogenital diaphragm (and bulbocavernous muscle in boys and men) and merges proximally with the slinglike puborectalis muscle (which defines the anorectal junction), which itself merges with the levator plate of the pelvic floor.

View larger version (67K): [in this window] [in a new window] [Download PPT slide]   Figure 1: Illustration of anal canal anatomy in the coronal plane. EAS = external anal sphincter, IAS = internal anal sphincter, IS = intersphincteric space, LA = levator ani, LM = longitudinal muscle, PR = puborectalis muscle.

The intersphincteric space is the surgical plane of dissection between the internal and external sphincters and is most frequently found between the longitudinal muscle and external sphincter, where it exists as a sheet of fat containing loose areolar tissue. The fat-filled ischioanal fossa lies lateral to the sphincter complex and is traversed by a network of fibroelastic connective tissue fibers. We prefer the term ischioanal fossa to ischiorectal fossa because this space predominantly surrounds the anus rather than the rectum. However, the two terms are interchangeable.

With regard to the lining of the anal canal, the proximal half is characterized by longitudinal mucosal folds, the anal columns of Morgagni (2). The distal aspect of each column is linked to its neighbor by a small semilunar fold (the anal valves), which in turn forms small pockets (the anal sinuses, or crypts of Morgagni). The distal undulating limit of these valves is the dentate (pectinate) line, which also marks the most distal aspect of the anal transitional zone, a histologic junction between anal squamous epithelium and rectal columnar epithelium.

The dentate line lies approximately 2 cm proximal to the anal verge and is a crucial landmark in fistula in ano because the anal glands empty into the crypts that lie proximal to the valves. These glands were first linked to the genesis of fistula in ano by Chiari (3), who suggested that they were the source of infection. Their purpose is unclear, although they may help lubricate the anus by secreting mucus into the anal crypts. The origin of the anal glands within the surrounding tissues is variable. For example, they are present in the subepithelium and may be present in the internal sphincter, and approximately one- to two-thirds of these glands are deeply sited within the intersphincteric space (4) (Fig 1). Most authorities believe that it is infection of these intersphincteric glands that is the initiating event in fistula in ano, in a process known as the "cryptoglandular hypothesis" (5). Furthermore, lymphoid aggregates surround the anal glands, which may partly explain the increased incidence of anal fistula in Crohn disease (6,7).

It is believed that gland infection results in an intersphincteric abscess if the draining duct becomes blocked by infected debris. This abscess may resolve by means of spontaneous drainage into the anal canal or may progress to an acute anorectal abscess, which is a common acute surgical emergency and is familiar to all general and coloproctologic surgeons (8). Treatment generally consists of incision and drainage of the most fluctuant part of the abscess; however, this procedure does not pay due attention to the source of infection in the intersphincteric space, with the result that as many as 87% of patients with an acute abscess may subsequently develop a fistula (9). Acute anorectal abscess and fistula in ano are, therefore, generally believed to be acute and chronic manifestations, respectively, of the same disease. Because of this, the search for intersphincteric infection and an anal canal internal opening followed by treatment of these at the time of acute manifestation has been advocated in an attempt to reduce the incidence of subsequent fistula (10).

Fistula in ano develops when an intersphincteric infection is allowed to continue unabated. It has a prevalence of approximately 0.01% and predominantly affects young adults (11). Fistula in ano is commoner in men, who dominate in all published series, with a male-to-female ratio of approximately 2:1 (11). Patients most commonly present with discharge (65%), but local pain due to inflammation is also frequent (12). However, some fistulas may be entirely asymptomatic.

	FISTULA CLASSIFICATION

TOP ABSTRACT INTRODUCTION ANATOMY AND ETIOLOGY FISTULA CLASSIFICATION ASSESSMENT AND TREATMENT IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FOR DIFFERENTIAL... WHICH PATIENTS SHOULD BE... CONCLUSION ESSENTIALS References

 
By definition, a fistula is an abnormal tract that connects two epithelial surfaces. The anatomic course of an anal fistula will be dictated by the location of the infected anal gland and the anatomic planes and boundaries that surround it. There will usually be an internal enteric opening in the anal canal at the level of the dentate line—that is, at the original site of the duct draining the infected gland. In most cases this is at the 6-o'clock position, because anal glands are more abundant posteriorly (radial positions around the anus are referenced with respect to a clock face, with 12 o'clock being directly anterior) (13). The fistula can reach the perianal skin by a variety of routes, some more tortuous than others, and by penetrating and involving the muscles of the anal sphincter and surrounding tissues to a variable degree. Fistulas may thus be classified according to the route taken by this "primary tract" that links the internal and external openings.

Furthermore, classification largely determines treatment. In 1934, Milligan and Morgan (14) stressed the importance of the "anorectal ring" (anatomically, the puborectalis muscle) and categorized fistulas as those that entered the anal canal above or below this structure, warning that postoperative incontinence was highly likely if high fistulas were surgically divided without due attention. This classification was subsequently modified and refined by other authors, but the most comprehensive and practical classification in use today is that of Parks et al (6), who carefully analyzed a consecutive series of 400 patients referred to the surgeons of St Mark's Hospital, London, England. Parks et al found that they were able to assign all fistulas into one of four groups; intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric (6). Of importance, most of these groupings could be explained in terms of the cryptoglandular hypothesis.

The path of least resistance for festering intersphincteric infection is straight down the intersphincteric space, which creates an intersphincteric fistula; this type of fistula composed 45% of cases in the series of Parks et al (6) (Fig 2). Of importance, this fistula does not penetrate the adjacent external sphincter, which forms a relative barrier to spread. However, some truculent fistulas can cross the external sphincter and reach the ischioanal fossa by doing so (Fig 2). This results in a transsphincteric fistula, which composed 30% of cases in the series of Parks et al. Other fistulas may spread upward in the intersphincteric space and arch over the puborectalis muscle, where they must cross the levator plate to reach the perianal skin. This type, the suprasphincteric fistula (Fig 2), composed 20% of cases in the series of Parks et al.

View larger version (53K): [in this window] [in a new window] [Download PPT slide]   Figure 2: Illustrations in coronal plane show classification of fistula in ano according to Parks et al (6): intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric.

While most fistulas probably start as a simple single primary tract, unabated infection may result in ramifications that branch away from this. These secondary tracts are generally known as known as "extensions" (Fig 3). Extensions may be intersphincteric, ischioanal, or supralevator (pararectal), and their morphology may suggest tracts or abscesses. Exactly when a tract becomes an abscess is not precisely defined, but both terms describe regions of infection. The ischioanal fossa is the commonest site for an extension (6), especially one that arises from the apex of a transsphincteric fistula (A in Fig 3). Extensions also occur in the horizontal plane and are known as "horseshoes" if there is ramification each side of the internal opening (6) (Fig 3). The anatomic description of the path taken by the primary fistula tract and the location of any associated extension forms its "classification."

View larger version (107K): [in this window] [in a new window] [Download PPT slide]   Figure 3: Illustration in coronal plane shows fistula extensions (secondary tracts): A = extension into roof of ischioanal fossa, arising from apex of a transsphincteric fistula; B = supralevator pararectal extension arising from apex of a transsphincteric fistula; C = supralevator extension originating from intersphincteric plane, D = intersphincteric horseshoe.

	ASSESSMENT AND TREATMENT

TOP ABSTRACT INTRODUCTION ANATOMY AND ETIOLOGY FISTULA CLASSIFICATION ASSESSMENT AND TREATMENT IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FOR DIFFERENTIAL... WHICH PATIENTS SHOULD BE... CONCLUSION ESSENTIALS References

It is the right balance between eradication of infection and preservation of function that is the art of fistula surgery. To achieve this, two surgical questions need to be answered preoperatively: (a) What is the relationship between the fistula and the anal sphincter (ie, can the tract be safely laid open with only a low risk of postoperative incontinence), and (b) are there any extensions from the primary tract that need to be treated to prevent recurrence, and, if so, where are they?

Surgeons have traditionally relied on examination of patients who have been administered a general anesthetic, a procedure referred to as examination under anesthesia (EUA), to answer these questions. At EUA, the surgeon attempts to classify the fistula by palpating it to determine its relationship to the sphincter. However, anesthesia and consequent loss of tone impair precise identification of underlying muscles. A metal probe is inserted into the external opening and is directed toward the dentate line to find the internal opening. This is also not as straightforward as it sounds. For example, the internal opening is frequently not obvious, and the surgeon may need to inject hydrogen peroxide into the external opening while inspecting the anal canal.

The height of the internal opening relative to the anal canal and sphincters is also crucial; the higher the opening, the more sphincter will be divided. Although Milligan and Morgan (14) realized the importance of the puborectalis muscle for continence as long ago as 1934—"Whereas, if this ring be cut, loss of control surely results"—they suggested that most of the remaining sphincter could be sacrificed (14). However, it is now generally accepted that functional disability can follow even minimal division of the sphincter. Perhaps most important, injudicious probing during EUA can create new secondary tracts very easily. For example, forceful probing in the roof of the ischioanal fossa can rupture through the levator plate, causing a supralevator extension or even a rupture into the rectum, which would cause an extrasphincteric fistula (6,12).

Identification of extensions at EUA is central to curing the fistula. It is well recognized that missed extensions are the commonest cause of recurrence, which has reached 25% in some series (16). Extensions require specific treatment and inevitably necessitate more extensive surgery. For example, supralevator extensions are particularly difficult both to diagnose (because they are high above the anal canal) and to treat (because the levator plate forms a barrier to drainage).

The net result is that it can be very difficult at EUA to classify the primary tract with confidence, and there is ample opportunity to make matters worse. Patients with recurrent disease are a particular case in point. Such patients are more likely to harbor missed disease but are also difficult to assess. Multiple failed operations are the rule rather than the exception, with the result that digital palpation frequently cannot help distinguish between scarring due to repeated surgery and induration due to an underlying extension. Furthermore, patients in this group are also more likely to have extensions that travel several centimeters away from the primary tract, frustratingly in almost any direction, which further hampers detection of the extensions. The more chronic the fistula, the more complicated the associated extensions tend to be. The inevitable result is that these patients become progressively more difficult to treat, with both patient and surgeon becoming ever more exasperated. The key to breaking this loop is accurate preoperative assessment.

	IMAGING FISTULA IN ANO: FISTULOGRAPHY

TOP ABSTRACT INTRODUCTION ANATOMY AND ETIOLOGY FISTULA CLASSIFICATION ASSESSMENT AND TREATMENT IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FOR DIFFERENTIAL... WHICH PATIENTS SHOULD BE... CONCLUSION ESSENTIALS References

 
For many years radiologists have attempted to help answer the surgical questions posed above, with varying degrees of success. Contrast material–enhanced fistulography was the first modality used. In fistulography, the external opening is catheterized with a fine cannula, and a water-soluble contrast agent is injected gently to define the fistula tract (Fig 4).

View larger version (157K): [in this window] [in a new window] [Download PPT slide]   Figure 4: Fistulography in a male patient. Coronal image shows that it is obvious that there are several high extensions (arrows) surrounding the anorectal junction; however, the exact anatomic location of these is unclear because the pelvic floor (ie, levator ani in this case) cannot be directly visualized. Definition of extension location (supra- or infralevator) is central to surgical management.

Very little has been written on fistulography for fistula in ano, probably because the modality is so fraught with errors. Kuijpers and Schulpen (17) attempted to determine its value by retrospectively reviewing fistulographic images in 25 patients. They found that the internal opening and associated extensions were demonstrated and correctly interpreted in only four (16%) subjects. Moreover, false-positive diagnoses of rectal openings and supralevator extensions were made in three (12%) patients, which would have resulted in serious surgical errors if acted on. The authors concluded that fistulography was "inaccurate and unreliable," although they admitted prior bias against the technique (17). In contrast, Weisman and co-workers (18) found fistulography to be more useful in that it provided helpful information in nearly half of the 27 subjects in their study.

It has been suggested that the prime reason why fistulography is generally unhelpful is that radiologists are not familiar with the concepts of fistula pathogenesis and anatomy and the relevant surgical questions (17). One of the most hazardous misinterpretations is to diagnose a direct rectal opening merely because there is contrast material in the rectal lumen; usually, the contrast agent has merely refluxed up from the internal anal opening. Such radiology reports only encourage the surgeon to look for nonexistent openings and extensions, which can result in iatrogenic secondary tracts.

	IMAGING FISTULA IN ANO: CT

TOP ABSTRACT INTRODUCTION ANATOMY AND ETIOLOGY FISTULA CLASSIFICATION ASSESSMENT AND TREATMENT IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FOR DIFFERENTIAL... WHICH PATIENTS SHOULD BE... CONCLUSION ESSENTIALS References

 
Computed tomography (CT) may depict fistula in ano, especially if rectal and intravenous contrast material are used, and initial reports were encouraging (19–21). However, fistula depiction is not enough; fistulas must be classified correctly, and more recent and better data suggest that CT cannot be used for this purpose with sufficient accuracy. This is because the CT attenuation of the anal sphincter and pelvic floor is similar to that of the fistula itself, unless the latter contains air or contrast material. This is compounded by the inability to image in the surgically relevant coronal plane. Comparative studies are sparse: A study of 25 patients with 17 fistulas found that CT could be used to correctly classify only four fistulas, in contrast to the 14 correct classifications achieved with endosonography (22). Potentially, the disadvantages of CT might be overcome by using multi–detector row CT fistulography, which offers the possibility of isotropic voxels and multiplanar imaging, but results are awaited and the motivation to perform these studies is likely limited by the ready availability of MR imaging. It should be borne in mind that CT is frequently used to search for abscesses in the context of Crohn disease, and fistula in ano may be encountered at the time of examination. At present, however, accurate classification of these fistulas is best left to MR imaging or anal endosonography, with CT probably limited to the diagnosis of fistula-associated pelvic abscesses where other imaging is unavailable or cannot be tolerated.

	IMAGING FISTULA IN ANO: ANAL ENDOSONOGRAPHY

TOP ABSTRACT INTRODUCTION ANATOMY AND ETIOLOGY FISTULA CLASSIFICATION ASSESSMENT AND TREATMENT IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FOR DIFFERENTIAL... WHICH PATIENTS SHOULD BE... CONCLUSION ESSENTIALS References

 
Anal endosonography, developed by Clive Bartram, FRCP, FRCS, FRCR, was the first technique to directly depict the anal sphincter complex in detail (23). Simple modification of a rotating rectal endoprobe by covering it with a nondeformable plastic cone allowed the transducer to be withdrawn through the anal canal itself, situating it very close to the target structures and thus providing images of high spatial resolution. The technique has attracted considerable attention because of its ability to demonstrate the presence and extent of anal sphincter disruption, notably after vaginal delivery (24). Anal endosonography has also been extensively used for the preoperative classification of fistula in ano.

The examination is simple, rapid, and well tolerated by patients. The patient lies in the left lateral position or in the prone position if female (25). The probe is gently inserted into the distal rectum and then withdrawn through the anal canal. The internal sphincter is visualized as a hypoechoic ring encircling the anal canal, whereas the external sphincter is of mixed echogenicity (Fig 5). The intersphincteric space and longitudinal muscle lie between these and are of mixed echogenicity and are easily identified by using modern 10-MHz transducers (26).

View larger version (200K): [in this window] [in a new window] [Download PPT slide]   Figure 5: Intersphincteric fistula. Anal endosonogram in transverse plane at mid–anal canal level in a male patient shows fistula with hypoechoic tract located in intersphincteric plane between external (EAS) and internal (IAS) anal sphincters. Internal sphincter is markedly hypoechoic. At surgery, the internal opening was located at 6 o'clock posteriorly and was correctly predicted from anal endosonographic visualization because of the radial position of the fistula within the intersphincteric plane.

View larger version (219K): [in this window] [in a new window] [Download PPT slide]   Figure 6: Transsphincteric fistula shown on anal endosonogram in the transverse plane at the mid–anal canal level in a female patient. In contrast to Figure 5, the fistula (*) has penetrated the external anal sphincter (EAS). The internal opening was correctly predicted at 7-o'clock position. Note that the internal sphincter is relatively thinned here, which is a clue to the site of the internal opening, but there is no tract extending to the anal mucosa.

View larger version (209K): [in this window] [in a new window] [Download PPT slide]   Figure 7: Anal endosonogram at upper anal canal level in a male patient shows extensive hypoechoic horseshoe extension (*). Because endosonography is limited to the transverse plane, it is difficult to determine whether this extension is infra- or supralevator.

However, there are undoubtedly several areas where anal endosonography has specific disadvantages. For example, insufficient penetration of the ultrasound beam beyond the external sphincter, especially with high-frequency transducers, limits the ability to resolve ischioanal and supralevator infections, with the result that extensions from the primary tract may be missed at endosonography. Also, anal endosonography cannot be used to reliably distinguish infection from fibrosis, because both have a hypoechoic appearance (30). This causes particular difficulties in patients with recurrent disease, since infected tracts and fibrotic scars are frequently combined. Attempts have been made to clarify the course of patent tracts by injecting hydrogen peroxide or sonography contrast agents into the external opening during examination (34,35). However, gas formed within the tract as a result may cause acoustic shadowing that mimics an extension. Indeed, this phenomenon can occur with any tract that contains air, leading, for example, to intersphincteric fistulas being inadvertently classified as transsphincteric (Fig 8).

View larger version (211K): [in this window] [in a new window] [Download PPT slide]   Figure 8: Transverse anal endosonogram at upper anal canal level in a female patient shows intersphincteric horseshoe extenstion (arrows). Gas in the fistula causes acoustic shadowing (*), which could be mistaken for transsphincteric tracts.

View larger version (158K): [in this window] [in a new window] [Download PPT slide]   Figure 9: Three-dimensional anal endosonogram (coronal view) in a female patient shows transsphincteric tract (arrows) that has been outlined by injecting hydrogen peroxide into the external opening.

	IMAGING FISTULA IN ANO: MR IMAGING

TOP ABSTRACT INTRODUCTION ANATOMY AND ETIOLOGY FISTULA CLASSIFICATION ASSESSMENT AND TREATMENT IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FOR DIFFERENTIAL... WHICH PATIENTS SHOULD BE... CONCLUSION ESSENTIALS References

 
In recent years, MR imaging has emerged as the leading contender for preoperative classification of fistula in ano. The ability of MR imaging to help not only accurately classify tracts but also identify disease that otherwise would have been missed has had a palpable effect on surgical treatment and, ultimately, patient outcome.

Technique
Magnetic field strength does not appear to be a critical factor for good results (38). Although higher field strength might be relevant for more subtle differentiation of sphincter anatomy and tracts, no definite diagnostic benefit has been demonstrated, to our knowledge.

Authors of initial reports (39–43) on MR imaging necessarily used the body coil, with good results. However, authors of some subsequent studies (44–45) reported lower accuracy when a body coil was used; this was thought to be possibly due to lower spatial resolution. The introduction of external phased-array surface coils increased the signal-to-noise ratio and spatial resolution, to good effect (46,47), and these coils quickly became generally available.

The best spatial resolution is achieved by using a dedicated endoluminal anal coil (48), which may be combined with a surface coil to increase the field of view (49). It should be noted that these endoluminal coils are not the same as a rectal coil but are of smaller diameter and are designed to be placed in the anus. The receiver coil is generally housed in a plastic cover that allows placement across the anal canal (Fig 10). The external diameter generally ranges between 12 and 19 mm, although smaller coils have been used for pediatric examinations (50,51). Endoluminal coils are susceptible to motion artifact, but this can be reduced by means of careful patient preparation. For example, patients should be asked to try and relax the sphincter and pelvic floor as much as possible, and due attention should be paid to comfort, including support for the coil and patient with pads (50). Spasmolytics such as 20 mg of hyoscine butylbromide (Buscopan; Boehringer Ingelheim, Ingelheim, Germany) or 1 mg of glucagon administered intramuscularly may help to reduce motion-induced artifacts. (Hyoscine butylbromide is not licensed for use in the United States.)

View larger version (114K): [in this window] [in a new window] [Download PPT slide]   Figure 10: Endoanal receiver coil for MR imaging.

These results clearly suggest that a large field of view is necessary whenever extensions are suspected—for example, in patients with recurrent fistula or Crohn disease. The high spatial resolution of endoluminal coils makes them ideal for precise demonstration of the location and height of the internal opening, and they may have a special role for demonstrating ano- or rectovaginal fistulas, which are notoriously difficult to image (54). Endoluminal coils are also valuable when simultaneous information on the degree of sphincter disruption is needed, which may be the case in patients who have undergone previous surgery. Endoluminal coils are sometimes difficult to place owing to anal stenosis or local pain as a result of extensive infection. Halligan and Bartram (52) found that an endoluminal coil could not be placed in 17% of their patients, whereas Stoker and colleagues (50) failed to place the coil in only 3%, which may reflect differences in patient population since the coil used in both studies was identical.

When circumstances allow, it is likely that an optimal examination will be achieved by using a combination of both external and endoluminal coils. However, it should be borne in mind that accuracy with a body or external coil alone remains high (41–43,47,49,52), and lack of an endoluminal coil alone is insufficient reason to avoid preoperative MR imaging of fistula in ano. Indeed, examination with a body or phased-array coil has become standard practice, not least because endoluminal coils specifically designed for anal imaging remain relatively unavailable.

MR Sequences
Various investigators have adopted different strategies with respect to the MR sequences used to image fistula in ano. All agree that anatomic precision is needed so that the course of the fistula with respect to adjacent structures can be judged accurately, and all use some method with which infection (usually pus) can be highlighted. These aims can be achieved by a variety of means. Many investigators use the rapid and convenient fast spin-echo T2-weighted sequence, which provides good contrast between hyperintense fluid in the tract and the hypointense fibrous wall of the fistula while simultaneously enabling good discrimination between the several layers of the anal sphincter (50,55). Others have used T1-weighted sequences, which must be combined with intravenous contrast material for the fistula to be highlighted (43). Fat-suppression techniques are widely used with both T2-weighted (56) and gadolinium-enhanced T1-weighted sequences (the latter may be especially valuable in patients with Crohn disease, to differentiate between fluid and inflammatory tissue that are both hyperintense on fat-suppressed T2-weighted and/or short tau inversion-recovery [STIR] images). Normal anorectal structures do not enhance substantially, except for the internal anal sphincter and blood vessels, including hemorrhoids. Investigators have also successfully employed STIR, a sequence that combines fat suppression with high conspicuity of active tracts (41,47,52). Other approaches have included saline instillation into the external opening in an attempt to increase tract conspicuity (57) or rectal contrast medium (58,59). However, such measures increase the complexity of the examination in the face of the already excellent results achieved with more standard procedures. Imaging protocols are detailed in the Table.

View larger version (193K): [in this window] [in a new window] [Download PPT slide]   Figure 11: Correct orientation for MR imaging of anal canal. Sagittal T2-weighted scout image through patient's midline is used to plan images that are truly transverse with respect to anal canal, as shown by white lines. Coronal imaging is then performed at 90° to the transverse plane.

View larger version (145K): [in this window] [in a new window] [Download PPT slide]   Figure 12a: Coronal (a) T2-weighted fast spin-echo (2500/70; echo train length, 16; field of view, 300 mm; matrix, 256 x 512; section thickness, 4 mm; gap, 0.4 mm) and (b) coronal STIR (4000/42, inversion time of 150 msec; echo train length, 16; matrix, 224 x 256; section thickness, 4 mm; gap 0.4 mm; two signals acquired) MR images acquired with external phased-array coil show complex transsphincteric fistula with tract (short straight arrows) in left ischioanal fossa that extends below ischial bone (I) toward the upper leg (not shown). At the ischial tuberosity, bone marrow edema (long straight arrow) is visible on b. Arrowhead = external opening, curved arrow = small abscess, AS = anal sphincter.

View larger version (130K): [in this window] [in a new window] [Download PPT slide]   Figure 12b: Coronal (a) T2-weighted fast spin-echo (2500/70; echo train length, 16; field of view, 300 mm; matrix, 256 x 512; section thickness, 4 mm; gap, 0.4 mm) and (b) coronal STIR (4000/42, inversion time of 150 msec; echo train length, 16; matrix, 224 x 256; section thickness, 4 mm; gap 0.4 mm; two signals acquired) MR images acquired with external phased-array coil show complex transsphincteric fistula with tract (short straight arrows) in left ischioanal fossa that extends below ischial bone (I) toward the upper leg (not shown). At the ischial tuberosity, bone marrow edema (long straight arrow) is visible on b. Arrowhead = external opening, curved arrow = small abscess, AS = anal sphincter.

Interpretation
The success of MR imaging for preoperative classification of fistula in ano is a direct result of the sensitivity of MR for tracts and abscesses combined with high anatomic precision and the ability to image in surgically relevant planes. Accurate preoperative classification is achieved by correctly relating the imaged fistula to the anal sphincter.

Primary Tract
Active tracts are filled with pus and granulation tissue and, thus, appear as hyperintense longitudinal structures on T2-weighted or STIR images (Fig 12). On contrast-enhanced T1-weighted images, active granulation tissue will enhance while fluid in the tract itself remains hypointense (Fig 13). Active tracts are often surrounded by hypointense fibrous walls (Fig 13), which can be relatively thick, especially in patients with recurrent disease and previous surgery. Occasionally, some hyperintensity in this fibrous area may be seen, probably reflecting edema. Hyperintensity may also extend beyond the tract and its fibrous sleeve, where it represents adjacent inflammation (Fig 13).

View larger version (153K): [in this window] [in a new window] [Download PPT slide]   Figure 13a: Transsphincteric fistula in a man with Crohn disease. (a, b) Transverse T2-weighted fast spin-echo MR images (echo train length, 16; field of view, 300 mm; matrix 256 x 512; section thickness, 4 mm; gap, 0.4 mm; two signals acquired) obtained (a) without (2500/70) and (b) with (4000/85) fat saturation and (c) transverse fat-saturated contrast-enhanced T1-weighted fast spin-echo image (see Table for parameters) show two separate fistula tracts (straight and curved arrows) in left posterior ischioanal space, close to the anal sphincter (A). Both tracts show confluent high signal intensity centrally, which represents pus in the tract lumen. On a and b, the surrounding inflammatory tissue (arrowheads) is of low signal intensity (a), which increases with fat-saturation (b) and especially with contrast enhancement (c). Anterior tract (curved arrow) demonstrates more adjacent inflammation (arrowheads) than does posterior tract (straight arrow).

View larger version (136K): [in this window] [in a new window] [Download PPT slide]   Figure 13b: Transsphincteric fistula in a man with Crohn disease. (a, b) Transverse T2-weighted fast spin-echo MR images (echo train length, 16; field of view, 300 mm; matrix 256 x 512; section thickness, 4 mm; gap, 0.4 mm; two signals acquired) obtained (a) without (2500/70) and (b) with (4000/85) fat saturation and (c) transverse fat-saturated contrast-enhanced T1-weighted fast spin-echo image (see Table for parameters) show two separate fistula tracts (straight and curved arrows) in left posterior ischioanal space, close to the anal sphincter (A). Both tracts show confluent high signal intensity centrally, which represents pus in the tract lumen. On a and b, the surrounding inflammatory tissue (arrowheads) is of low signal intensity (a), which increases with fat-saturation (b) and especially with contrast enhancement (c). Anterior tract (curved arrow) demonstrates more adjacent inflammation (arrowheads) than does posterior tract (straight arrow).

View larger version (116K): [in this window] [in a new window] [Download PPT slide]   Figure 13c: Transsphincteric fistula in a man with Crohn disease. (a, b) Transverse T2-weighted fast spin-echo MR images (echo train length, 16; field of view, 300 mm; matrix 256 x 512; section thickness, 4 mm; gap, 0.4 mm; two signals acquired) obtained (a) without (2500/70) and (b) with (4000/85) fat saturation and (c) transverse fat-saturated contrast-enhanced T1-weighted fast spin-echo image (see Table for parameters) show two separate fistula tracts (straight and curved arrows) in left posterior ischioanal space, close to the anal sphincter (A). Both tracts show confluent high signal intensity centrally, which represents pus in the tract lumen. On a and b, the surrounding inflammatory tissue (arrowheads) is of low signal intensity (a), which increases with fat-saturation (b) and especially with contrast enhancement (c). Anterior tract (curved arrow) demonstrates more adjacent inflammation (arrowheads) than does posterior tract (straight arrow).

View larger version (113K): [in this window] [in a new window] [Download PPT slide]   Figure 14: Intersphincteric fistula in a male patient. Transverse STIR MR image (1500/15; field of view, 375 mm; matrix, 256 x 256; section thickness, 4 mm; gap, 1 mm; four signals acquired) shows that lateral margin of external sphincter (long arrow) contrasts against fat in the ischioanal fossa (*). Fistula (short arrow) is in the intersphincteric space posteriorly at 6 o'clock and is contained by the external sphincter. There is no tract in the ischioanal fossa.

It should be noted that in some patients the puborectalis muscle is rather gracile, unlike the bulky muscle suggested in many anatomy texts, and it frequently and imperceptibly segues into the external sphincter, all of which hampers precise identification of the mid–anal canal level. Nevertheless, with experience it is possible to estimate the exact height of the internal opening with reasonable precision (47).

Any tract that penetrates the pelvic floor above the level of the puborectalis muscle is potentially a suprasphincteric or extrasphincteric fistula. The level of the internal opening distinguishes between these types of fistula; specifically, the internal opening is anal in suprasphincteric fistulas (Fig 15) and rectal in extrasphincteric fistulas (Fig 16).

View larger version (113K): [in this window] [in a new window] [Download PPT slide]   Figure 15a: Fistula classified as suprasphincteric on coronal STIR MR images (same parameters as for Fig 14) in a female patient. (a) Primary tracts in right (long arrow) and left (short arrow) ischioanal fossae are shown. (b) Image obtained just posterior to a shows that right-sided primary tract (white arrows) arches over puborectalis muscle (*) to reach a lower internal opening at the dentate line level (black arrow).

View larger version (104K): [in this window] [in a new window] [Download PPT slide]   Figure 15b: Fistula classified as suprasphincteric on coronal STIR MR images (same parameters as for Fig 14) in a female patient. (a) Primary tracts in right (long arrow) and left (short arrow) ischioanal fossae are shown. (b) Image obtained just posterior to a shows that right-sided primary tract (white arrows) arches over puborectalis muscle (*) to reach a lower internal opening at the dentate line level (black arrow).

View larger version (143K): [in this window] [in a new window] [Download PPT slide]   Figure 16: Fistula classified as extrasphincteric in a female patient on coronal T2-weighted MR image (4563/150; field of view, 350 mm; matrix, 256 x 256; section thickness, 6 mm; gap, 0.6 mm; four signals acquired). Fistula tract (horizontal white arrows) is seen in left ischioanal fossa. Levator plates (vertical white arrows) are well depicted bilaterally. Tract penetrates the left levator plate, and the internal opening (top horizontal white arrow) is into the rectum, above the level of the puborectalis muscle (*) and well above the dentate line (black arrow).

View larger version (146K): [in this window] [in a new window] [Download PPT slide]   Figure 17: Transsphincteric fistula in a male patient. Transverse STIR MR image (same parameters as for Fig 14) shows primary tract (vertical arrow) in right ischioanal fossa, where it can be clearly seen to penetrate external sphincter (*) to reach the intersphincteric space. Internal opening is posterior at 6 o'clock (horizontal arrow), at dentate line level.

View larger version (170K): [in this window] [in a new window] [Download PPT slide]   Figure 18: Transsphincteric fistula in a male patient. Transverse STIR MR image (same parameters as for Fig 14) at level of the internal opening shows primary tract (vertical arrow) at 4–5 o'clock. Unlike Figure 17, the tract cannot be traced right to the anal mucosa, and the adjacent internal sphincter (horizontal arrow) appears intact. However, an internal opening at 4–5 o'clock was reported because this position indicated site of maximal infection in the intersphincteric plane. The internal opening was confirmed at this site during subsequent EUA. Intersphincteric plane is well seen in this patient between hyperintense internal sphincter and the external sphincter.

The commonest type of extension is one that arises from the apex of a transsphincteric tract and extends into the roof of the ischioanal fossa (Figs 3, 19). The major benefit of MR imaging findings is that they can alert the surgeon to extensions that would otherwise be missed. For example, extensions may be several centimeters from the primary tract (Fig 20), which makes them difficult to detect during clinical examination or EUA. This is especially the case when extensions are contralateral to the primary tract (Fig 21). It is also important to search for supralevator extensions (Fig 22), since these are not only difficult to detect but pose specific problems with regard to treatment. Horseshoe extensions spread across both sides of the internal opening and are recognized on MR images by their unique configuration (Fig 23); horseshoe extensions may be intersphincteric, ischioanal, or supralevator. Complex extensions are especially common in patients with recurrent fistula in ano (Fig 21) or in those who have Crohn disease (Fig 13).

View larger version (165K): [in this window] [in a new window] [Download PPT slide]   Figure 19: Left-sided transsphincteric tract (short arrow) in a female patient. Coronal STIR MR image (same parameters as for Fig 14) shows large extension (long arrow) from apex of tract into roof of ipsilateral ischioanal fossa.

View larger version (139K): [in this window] [in a new window] [Download PPT slide]   Figure 20: Left-sided transsphincteric fistula (short straight arrows) with internal opening at 6 o'clock (long straight arrow) in a female patient. Transverse STIR MR image (same parameters as for Fig 14) shows remote extension (curved arrow) into ipsilateral buttock that was unsuspected at clinical examination but is well demonstrated at MR imaging. The extension was found at surgery guided by MR findings.

View larger version (163K): [in this window] [in a new window] [Download PPT slide]   Figure 21: Transsphincteric primary tract (short arrow) in the right posterior quadrant of a female patient. Transverse STIR MR image (same parameters as for Fig 14) shows two left-sided contralateral extensions (long arrows) that were undetected at EUA until results of patient's MR examination were revealed to the surgeon in the operating theater.

View larger version (153K): [in this window] [in a new window] [Download PPT slide]   Figure 22: Bilateral supralevator extensions (long arrows) in a female patient. Coronal STIR MR image (same parameters as for Fig 14) clearly show levator plates (short arrows) bilaterally, so that it is easy for the radiologist to be confident that infection extends above them.

View larger version (167K): [in this window] [in a new window] [Download PPT slide]   Figure 23: Horseshoe extension (arrows) arising from intersphincteric fistula in a male patient. Transverse STIR MR image (same parameters as for Fig 14) shows that, in this case, the horseshoe practically encircles the anal canal.

Spencer and colleagues (63) independently classified 37 patients into those with simple or those with complex fistulas on the basis of MR imaging and EUA and found that MR results were the better predictor of outcome, with positive and negative predictive values, respectively, of 73% and 87% for MR and 57% and 64% for EUA. These results clearly implied that MR imaging and outcome were closely related and again raised the possibility that preoperative MR could help identify features that cause postoperative recurrence.

Beets-Tan and colleagues (46) extended this hypothesis by investigating the therapeutic effect of preoperative MR imaging; the MR imaging findings in 56 patients were revealed to the surgeon after he or she had completed an initial EUA. MR imaging provided important additional information that precipitated further surgery in 12 (21%) of 56 patients, predominantly in those with recurrent fistula or Crohn disease (46).

Buchanan and co-workers (47) hypothesized that the therapeutic influence and, thus, beneficial effect of preoperative MR imaging would be greatest in patients with recurrent fistula, since these patients had the greatest chance of harboring occult infection, while such fistulas were also the most difficult to evaluate clinically. After an initial EUA, Buchanan et al revealed the findings of preoperative MR imaging to the surgeons for 71 patients with recurrent fistulas and left any further surgery to the discretion of the operating surgeon. They found that postoperative recurrence was only 16% for surgeons who always acted if MR findings suggested that areas of infection had been missed, whereas recurrence was 57% for those surgeons who instead always chose to ignore imaging results (47). Furthermore, in the 16 patients who needed further unplanned surgery, MR initially correctly predicted the site of this disease in all cases (47). Using a similar approach, Buchanan and colleagues (64) also investigated the effect of preoperative MR imaging on clinical outcome in patients with fistula in ano at initial presentation and found that the scheduled surgical approach changed in 10% of this group.

Ever since the results of Lunniss et al (41) suggested that EUA might be an imperfect reference standard with which to judge MR imaging, comparative studies have been plagued by the lack of a genuine reference standard. It is now well recognized that surgical findings at EUA are often incorrect. In particular, there are frequent false-negatives. In a recent comparative study of endosonography, MR imaging, and EUA in 34 patients with fistula due to Crohn disease, Schwartz and co-workers (32) found that a combination of the results of at least two modalities was necessary to arrive at a correct classification. Indeed, it is well established that many false-negative surgical results will only reveal themselves during long-term clinical follow-up, and, at this point in time, comparative studies that ignore clinical outcome are likely to be seriously flawed.

	IMAGING FOR DIFFERENTIAL DIAGNOSIS

TOP ABSTRACT INTRODUCTION ANATOMY AND ETIOLOGY FISTULA CLASSIFICATION ASSESSMENT AND TREATMENT IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FOR DIFFERENTIAL... WHICH PATIENTS SHOULD BE... CONCLUSION ESSENTIALS References

 
Not all cases of perianal sepsis are due to fistula in ano. For example, acne conglobata, hidradenitis suppurativa, pilonidal sinus, actinomycosis, tuberculosis, proctitis, human immunodeficiency virus, lymphoma, and anal and rectal carcinoma may all cause perianal infection. While clinical examination results are often conclusive, this is not always the case, and imaging may help with the differential diagnosis. The cardinal feature of fistula in ano is intersphincteric infection, which is not generally found in other conditions. Whenever imaging suggests that infection is superficial rather than deep seated and that there is no sphincteric involvement, other conditions such as hidradenitis suppurativa should be considered (Fig 24). For example, authors of a recent study (65) of patients with pilonidal sinus and fistula in ano found that MR imaging could be used to reliably distinguish between the two on the basis of intersphincteric infection and an enteric opening.

View larger version (175K): [in this window] [in a new window] [Download PPT slide]   Figure 24: Hidradenitis suppurativa in a male patient. Transverse STIR MR image (same parameters as for Fig 14) shows extensive superficial infection (arrows). Absence of any infection related to anal canal and intersphincteric space meant that diagnosis could be confidently made preoperatively by using imaging.

	WHICH PATIENTS SHOULD BE IMAGED?

TOP ABSTRACT INTRODUCTION ANATOMY AND ETIOLOGY FISTULA CLASSIFICATION ASSESSMENT AND TREATMENT IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FOR DIFFERENTIAL... WHICH PATIENTS SHOULD BE... CONCLUSION ESSENTIALS References

 
While fistula in ano is simple to diagnose and simple to treat in most patients, many other patients will benefit from detailed and accurate preoperative investigation. Where there is easy access to MR imaging, it could be argued that all patients should undergo preoperative imaging. For example, it has been estimated that the therapeutic effect of MR imaging is 10% in patients presenting for the first time with a seemingly simple fistula (64). Where access to imaging is more restricted, however, the clinician and radiologist will need to select those patients who are most likely to benefit. Since there is now overwhelming evidence that MR imaging alters surgical therapy and improves clinical outcome in patients with recurrent disease, MR should be routine in such cases. Patients presenting for the first time with a fistula that appears complex at clinical examination should also be referred, as should patients with known Crohn disease, since the preponderance of complex fistulas is increased.

There are also surgical situations where imaging is likely to be particularly beneficial, even when the fistula itself is simple. For example, the anterior external sphincter is very short in women, and division of this sphincter during fistula incision is particularly associated with postoperative incontinence, even when the fistula itself is simple and has no extensions. Faced with such a dilemma, the surgeon may choose to pass a thread (seton) through the tract rather than incise the fistula to provide drainage. The patient can undergo postoperative imaging so that the potential extent of sphincter division can be assessed by visualizing the relationship of the seton to the external sphincter. A decision can then be made whether to progress with fistulotomy or to keep the seton in place for a few months, after which time the internal opening can be closed with a rectal mucosal advancement flap. A seton can also be placed at EUA when the surgeon is uncertain about the relationship between the tract and the sphincter; postoperative imaging can then be used to help answer this question. In patients who undergo novel therapies, such as use of fibrin glue, imaging may be necessary during instillation to be confident that the whole tract has been filled (71).

MR imaging is not restricted to surgical assessment. Infliximab, a chimeric monoclonal antibody to human tumor necrosis factor–, currently has a prominent role for the medical treatment of Crohn disease, especially in patients with a chronic fistula (72). However, infliximab therapy is contraindicated if an abscess is present, and MR imaging may be used to search for this (70,73). Indeed, MR imaging may be used to monitor infliximab therapy, since it seems that fistulas may persist in the face of clinical findings that suggest remission (74). For example, investigations (70,73) of MR in patients whose external opening has closed have revealed that underlying infection can still be present, indicating a need for continued therapy. Further studies are needed to determine whether MR monitoring improves outcome.

Where MR imaging is unavailable or where competent interpretation of the images is not possible, then anal endosonography is a viable and useful alternative. While recent comparisons have shown that MR imaging outperforms anal endosonography in all respects, the latter is far superior to simple clinical examination, and its performance in some areas is very close to that of MR (37). Notably, anal endosonography is very adept at depicting the internal opening.

	CONCLUSION

TOP ABSTRACT INTRODUCTION ANATOMY AND ETIOLOGY FISTULA CLASSIFICATION ASSESSMENT AND TREATMENT IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FOR DIFFERENTIAL... WHICH PATIENTS SHOULD BE... CONCLUSION ESSENTIALS References

 
In those patients with fistula in ano who have a high likelihood of complex disease, the evidence that preoperative MR imaging influences the surgical approach and the extent of exploration and improves the ultimate outcome is now overwhelming. We hope that this article will stimulate radiologists to provide this service to their surgeons in the expectation that this will reduce the incidence of recurrent fistula in ano and the misery that this causes.

	ESSENTIALS

TOP ABSTRACT INTRODUCTION ANATOMY AND ETIOLOGY FISTULA CLASSIFICATION ASSESSMENT AND TREATMENT IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FOR DIFFERENTIAL... WHICH PATIENTS SHOULD BE... CONCLUSION ESSENTIALS References

 

• Recurrent fistula in ano is usually caused by infection that was missed during surgical exploration.
  
• MR imaging depicts remote foci of infection better than any other modality, including surgical exploration.
  
• MR image–guided surgery helps reduce postoperative recurrence by 75% in patients with complex disease.
  
• Anal endosonography is a viable alternative to MR imaging when the latter is not available.
  

	FOOTNOTES

 

Abbreviations: EUA = examination under anesthesia • STIR = short tau inversion recovery

	References

TOP ABSTRACT INTRODUCTION ANATOMY AND ETIOLOGY FISTULA CLASSIFICATION ASSESSMENT AND TREATMENT IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FISTULA IN ANO:... IMAGING FOR DIFFERENTIAL... WHICH PATIENTS SHOULD BE... CONCLUSION ESSENTIALS References

 

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