imaging of fistula in ano - afrh · 2010. 10. 10. · fistula in ano develops when an...

Imaging of Fistula in Ano1

Steve Halligan, MD, FRCP, FRCRJaap Stoker, MD, PhD Fistula in ano is a common condition that often recurs

despite seemingly adequate surgery, usually because ofinfection that was missed at surgery. It is now increasinglyrecognized that preoperative imaging can help identifyinfection that would have otherwise gone unidentified. Inparticular, magnetic resonance (MR) imaging findingshave been shown to influence surgery and markedly dimin-ish the chance of recurrence; thus, preoperative imagingwill become increasingly routine in the future. In this arti-cle, the authors describe the pathogenesis, classification,and imaging of fistula in ano, with an emphasis on MRimaging. Most important, the authors describe how theradiologist is well placed to answer the surgical riddles thatmust be solved for treatment to be effective.

� RSNA, 2006

1 From the Department of Specialist Radiology, UniversityCollege Hospital, Level 2, Podium, 235 Euston Rd, Lon-don NW1 2BU, England (S.H.), and Department of Radiol-ogy, Academic Medical Center, Amsterdam, the Nether-lands (J.S.). Received June 11, 2004; revision requestedAugust 24; revision received October 15; accepted Janu-ary 10, 2005; final version accepted March 2; updatedSeptember 29. Address correspondence to S.H.

� RSNA, 2006

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18 Radiology: Volume 239: Number 1—April 2006

F istula in ano (anal fistula) is a com-mon condition that has a tendencyto recur despite seemingly ade-quate surgery. Recurrence is usuallydue to infection that has escaped surgi-cal detection and has thus gone un-treated. It is now increasingly recog-nized that preoperative imaging, nota-bly magnetic resonance (MR) imaging,can help identify infected tracts and ab-scesses that would otherwise have beenmissed. Preoperative MR imaging find-ings have been shown to influence sub-sequent surgery and markedly diminishthe chance of recurrent disease as aresult. Because of this, preoperative im-aging is likely to become increasinglyroutine in the future, especially in pa-tients with recurrent disease.

This review will detail the pathogen-esis of fistula in ano, explain how patho-genesis causes the different types of fis-tula encountered, and describe howthese types can be imaged, with the em-phasis on MR. Most important, we willdescribe how the radiologist is wellplaced to answer the surgical questionsthat must be solved for treatment to beeffective.

Anatomy and Etiology

To fully understand the role of imagingwith regard to fistula in ano, an appreci-ation of its etiology and how the variousfistula types are defined by anatomicboundaries is mandatory.

The anal canal is essentially a cylin-

der surrounded by two muscularsphincters, the internal and externalanal sphincters, which are composed ofsmooth and striated muscle, respec-tively (Fig 1). The external sphincterhas posterior attachments to the ano-coccygeal ligament and anterior attach-ments to the perineal body and urogen-ital diaphragm (and bulbocavernousmuscle in boys and men) and mergesproximally with the slinglike puborec-talis muscle (which defines the anorec-tal junction), which itself merges withthe levator plate of the pelvic floor.

The internal sphincter is the distaltermination of the circular muscle of thegut tube. The rectal longitudinal smoothmuscle interdigitates between the inter-nal and external sphincters and isthought to have no obvious sphinctericeffect; rather, its role is probably tobind the anus together (1).

The intersphincteric space is thesurgical plane of dissection between theinternal and external sphincters and ismost frequently found between the lon-gitudinal muscle and external sphincter,where it exists as a sheet of fat contain-ing loose areolar tissue. The fat-filledischioanal fossa lies lateral to thesphincter complex and is traversed by anetwork of fibroelastic connective tissuefibers. We prefer the term ischioanalfossa to ischiorectal fossa because thisspace predominantly surrounds theanus rather than the rectum. However,the two terms are interchangeable.

With regard to the lining of the analcanal, the proximal half is characterizedby longitudinal mucosal folds, the analcolumns of Morgagni (2). The distal as-pect of each column is linked to itsneighbor by a small semilunar fold (theanal valves), which in turn forms smallpockets (the anal sinuses, or crypts ofMorgagni). The distal undulating limit ofthese valves is the dentate (pectinate)line, which also marks the most distalaspect of the anal transitional zone, ahistologic junction between anal squa-mous epithelium and rectal columnarepithelium.

The dentate line lies approximately2 cm proximal to the anal verge and is acrucial landmark in fistula in ano be-cause the anal glands empty into the

crypts that lie proximal to the valves.These glands were first linked to thegenesis of fistula in ano by Chiari (3),who suggested that they were thesource of infection. Their purpose is un-clear, although they may help lubricatethe anus by secreting mucus into theanal crypts. The origin of the analglands within the surrounding tissues isvariable. For example, they are presentin the subepithelium and may bepresent in the internal sphincter, andapproximately one- to two-thirds ofthese glands are deeply sited within theintersphincteric space (4) (Fig 1). Mostauthorities believe that it is infection ofthese intersphincteric glands that is theinitiating event in fistula in ano, in aprocess known as the “cryptoglandularhypothesis” (5). Furthermore, lym-phoid aggregates surround the analglands, which may partly explain the in-creased incidence of anal fistula inCrohn disease (6,7).

It is believed that gland infection re-sults in an intersphincteric abscess if thedraining duct becomes blocked by in-fected debris. This abscess may resolveby means of spontaneous drainage intothe anal canal or may progress to anacute anorectal abscess, which is a com-mon acute surgical emergency and isfamiliar to all general and coloprocto-logic surgeons (8). Treatment generallyconsists of incision and drainage of themost fluctuant part of the abscess; how-ever, this procedure does not pay dueattention to the source of infection inthe intersphincteric space, with the re-sult that as many as 87% of patientswith an acute abscess may subsequentlydevelop a fistula (9). Acute anorectalabscess and fistula in ano are, there-fore, generally believed to be acute andchronic manifestations, respectively, ofthe same disease. Because of this, thesearch for intersphincteric infection andan anal canal internal opening followed

Published online10.1148/radiol.2391041043

Radiology 2006; 239:18–33

Abbreviations:EUA � examination under anesthesiaSTIR � short tau inversion recovery

Essentials

� Recurrent fistula in ano is usuallycaused by infection that wasmissed during surgical explora-tion.

� MR imaging depicts remote foci ofinfection better than any othermodality, including surgical explo-ration.

� MR image–guided surgery helpsreduce postoperative recurrenceby 75% in patients with complexdisease.

� Anal endosonography is a viablealternative to MR imaging whenthe latter is not available.

STATE OF THE ART: IMAGING OF FISTULA IN ANO Halligan and Stoker

Radiology: Volume 239: Number 1—April 2006 19

by treatment of these at the time ofacute manifestation has been advocatedin an attempt to reduce the incidence ofsubsequent fistula (10).

Fistula in ano develops when an in-tersphincteric infection is allowed tocontinue unabated. It has a prevalenceof approximately 0.01% and predomi-nantly affects young adults (11). Fistulain ano is commoner in men, who domi-nate in all published series, with a male-to-female ratio of approximately 2:1(11). Patients most commonly presentwith discharge (65%), but local paindue to inflammation is also frequent(12). However, some fistulas may beentirely asymptomatic.

Fistula Classification

By definition, a fistula is an abnormal tractthat connects two epithelial surfaces. Theanatomic course of an anal fistula will bedictated by the location of the infectedanal gland and the anatomic planes andboundaries that surround it. There willusually be an internal enteric opening inthe anal canal at the level of the dentateline—that is, at the original site of theduct draining the infected gland. In mostcases this is at the 6-o’clock position, be-cause anal glands are more abundant pos-teriorly (radial positions around the anusare referenced with respect to a clockface, with 12 o’clock being directly ante-rior) (13). The fistula can reach the peri-anal skin by a variety of routes, some

more tortuous than others, and by pene-trating and involving the muscles of theanal sphincter and surrounding tissues toa variable degree. Fistulas may thus beclassified according to the route taken bythis “primary tract” that links the internaland external openings.

Furthermore, classification largelydetermines treatment. In 1934, Milliganand Morgan (14) stressed the importanceof the “anorectal ring” (anatomically, thepuborectalis muscle) and categorized fis-tulas as those that entered the anal canalabove or below this structure, warningthat postoperative incontinence washighly likely if high fistulas were surgicallydivided without due attention. This classi-fication was subsequently modified andrefined by other authors, but the mostcomprehensive and practical classifica-tion in use today is that of Parks et al (6),who carefully analyzed a consecutive se-ries of 400 patients referred to the sur-geons of St Mark’s Hospital, London, En-gland. Parks et al found that they wereable to assign all fistulas into one of fourgroups; intersphincteric, transsphinc-teric, suprasphincteric, and extrasphinc-teric (6). Of importance, most of thesegroupings could be explained in terms ofthe cryptoglandular hypothesis.

The path of least resistance for fester-ing intersphincteric infection is straightdown the intersphincteric space, whichcreates an intersphincteric fistula; thistype of fistula composed 45% of cases inthe series of Parks et al (6) (Fig 2). Of

importance, this fistula does not pene-trate the adjacent external sphincter,which forms a relative barrier to spread.However, some truculent fistulas cancross the external sphincter and reach theischioanal fossa by doing so (Fig 2). Thisresults in a transsphincteric fistula, whichcomposed 30% of cases in the series ofParks et al. Other fistulas may spread up-ward in the intersphincteric space andarch over the puborectalis muscle, wherethey must cross the levator plate to reachthe perianal skin. This type, the supras-phincteric fistula (Fig 2), composed 20%of cases in the series of Parks et al.

Parks et al (6) also noted a fourthtype of fistula in 5% of cases. This typewas characterized by the surprising ab-sence of intersphincteric infection. In-stead, the fistula entered the rectum oranorectal junction directly (Fig 2).Parks et al termed these extrasphinc-teric fistulas. Clearly, infection of theanal glands cannot explain this type offistula, and Parks et al stressed that pri-mary rectal or pelvic disease (eg, diver-ticular disease, rectal Crohn disease,carcinoma) should be sought when thistype was encountered. It should benoted that the Parks et al series inevita-bly suffered spectrum bias owing to thespecialized nature of St Mark’s Hospital,a bias acknowledged by Parks himself,with the result that complex fistulaswere almost certainly overrepresented.For example, Parks et al did not de-scribe submucosal fistulas, which are

Figure 1

Figure 1: Illustration of anal canal anatomy inthe coronal plane. EAS � external anal sphincter,IAS � internal anal sphincter, IS � intersphinc-teric space, LA � levator ani, LM � longitudinalmuscle, PR � puborectalis muscle.

Figure 2

Figure 2: Illustrations in coronal plane show classification of fistula in ano according to Parks et al (6):intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric.



very superficial and do not involve thesphincter at all.

While most fistulas probably start asa simple single primary tract, unabatedinfection may result in ramificationsthat branch away from this. These sec-ondary tracts are generally known asknown as “extensions” (Fig 3). Exten-sions may be intersphincteric, ischio-anal, or supralevator (pararectal), andtheir morphology may suggest tracts orabscesses. Exactly when a tract be-comes an abscess is not precisely de-fined, but both terms describe regionsof infection. The ischioanal fossa is thecommonest site for an extension (6),especially one that arises from the apexof a transsphincteric fistula (A in Fig 3).Extensions also occur in the horizontalplane and are known as “horseshoes” ifthere is ramification each side of theinternal opening (6) (Fig 3). The ana-tomic description of the path taken bythe primary fistula tract and the locationof any associated extension forms its“classification.”

Assessment and Treatment

Treatment is usually straightforwardand involves laying open the fistula by

means of surgical incision, usually bycutting down onto a metal probe thathas been inserted into the primarytract. Indeed, James Syme (in 1838)thought laying open a fistula to bestraightforward and wondered why theprocedure was performed in a theaterwith “all the pomp and circumstance ofa great operation” (15). Unfortunately,he was mistaken; there are many trapswaiting for the unwary. Inappropriateincision and injudicious exploration canall too easily convert a simple fistula intoa surgical nightmare, with disastrousconsequences for the patient. The pri-mary objectives are to eradicate thetract and drain all associated sites ofinfection while simultaneously preserv-ing anal continence.

It is the right balance between eradi-cation of infection and preservation offunction that is the art of fistula surgery.To achieve this, two surgical questionsneed to be answered preoperatively:(a) What is the relationship between thefistula and the anal sphincter (ie, can thetract be safely laid open with only a lowrisk of postoperative incontinence), and(b) are there any extensions from the pri-mary tract that need to be treated to pre-vent recurrence, and, if so,where are they?

Surgeons have traditionally reliedon examination of patients who havebeen administered a general anesthetic,a procedure referred to as examinationunder anesthesia (EUA), to answerthese questions. At EUA, the surgeonattempts to classify the fistula by palpat-ing it to determine its relationship to thesphincter. However, anesthesia andconsequent loss of tone impair preciseidentification of underlying muscles. Ametal probe is inserted into the externalopening and is directed toward the den-tate line to find the internal opening.This is also not as straightforward as itsounds. For example, the internal open-ing is frequently not obvious, and thesurgeon may need to inject hydrogenperoxide into the external openingwhile inspecting the anal canal.

The height of the internal openingrelative to the anal canal and sphinctersis also crucial; the higher the opening,the more sphincter will be divided. Al-though Milligan and Morgan (14) real-

ized the importance of the puborectalismuscle for continence as long ago as1934—“Whereas, if this ring be cut,loss of control surely results”—they sug-gested that most of the remainingsphincter could be sacrificed (14). How-ever, it is now generally accepted thatfunctional disability can follow evenminimal division of the sphincter. Per-haps most important, injudicious prob-ing during EUA can create new second-ary tracts very easily. For example,forceful probing in the roof of the ischio-anal fossa can rupture through the leva-tor plate, causing a supralevator exten-sion or even a rupture into the rectum,which would cause an extrasphinctericfistula (6,12).

Identification of extensions at EUAis central to curing the fistula. It is wellrecognized that missed extensions arethe commonest cause of recurrence,which has reached 25% in some series(16). Extensions require specific treat-ment and inevitably necessitate moreextensive surgery. For example, supral-evator extensions are particularly diffi-cult both to diagnose (because they arehigh above the anal canal) and to treat(because the levator plate forms a bar-rier to drainage).

The net result is that it can be verydifficult at EUA to classify the primarytract with confidence, and there is am-ple opportunity to make matters worse.Patients with recurrent disease are aparticular case in point. Such patientsare more likely to harbor missed dis-ease but are also difficult to assess. Mul-tiple failed operations are the rulerather than the exception, with the re-sult that digital palpation frequentlycannot help distinguish between scar-ring due to repeated surgery and indu-ration due to an underlying extension.Furthermore, patients in this group arealso more likely to have extensions thattravel several centimeters away fromthe primary tract, frustratingly in al-most any direction, which further ham-pers detection of the extensions. Themore chronic the fistula, the more com-plicated the associated extensions tendto be. The inevitable result is that thesepatients become progressively more dif-ficult to treat, with both patient and sur-

Figure 3

Figure 3: Illustration in coronal plane showsfistula extensions (secondary tracts): A � exten-sion into roof of ischioanal fossa, arising fromapex of a transsphincteric fistula; B � supraleva-tor pararectal extension arising from apex of atranssphincteric fistula; C � supralevator exten-sion originating from intersphincteric plane, D �intersphincteric horseshoe.



geon becoming ever more exasperated.The key to breaking this loop is accuratepreoperative assessment.

Imaging Fistula in Ano: Fistulography

For many years radiologists have at-tempted to help answer the surgicalquestions posed above, with varying de-grees of success. Contrast material–en-hanced fistulography was the first mo-dality used. In fistulography, the exter-nal opening is catheterized with a finecannula, and a water-soluble contrastagent is injected gently to define the fis-tula tract (Fig 4).

Unfortunately, fistulography has twomajor drawbacks. First, extensions fromthe primary tract may fail to fill with con-trast material if they are plugged with de-bris, are very remote, or there is exces-sive contrast material reflux from eitherthe internal or external opening. Second,the sphincter muscles themselves are notdirectly imaged, which means that the re-lationship between any tract and thesphincter must be guessed. Furthermore,an inability to visualize the levator platemeans that it can be difficult to decidewhether an extension has a supra- or aninfralevator location. Similarly, the exactlevel of the internal opening in the analcanal is often impossible to determinewith sufficient accuracy to help the sur-geon. The net result is that fistulographicfindings are both difficult to interpret andunreliable.

Very little has been written on fistu-lography for fistula in ano, probably be-cause the modality is so fraught witherrors. Kuijpers and Schulpen (17) at-tempted to determine its value by retro-spectively reviewing fistulographic im-ages in 25 patients. They found that theinternal opening and associated exten-sions were demonstrated and correctlyinterpreted in only four (16%) subjects.Moreover, false-positive diagnoses ofrectal openings and supralevator exten-sions were made in three (12%) pa-tients, which would have resulted in se-rious surgical errors if acted on. Theauthors concluded that fistulographywas “inaccurate and unreliable,” al-though they admitted prior bias againstthe technique (17). In contrast, Weis-

man and co-workers (18) found fistulog-raphy to be more useful in that it pro-vided helpful information in nearly halfof the 27 subjects in their study.

It has been suggested that the primereason why fistulography is generally un-helpful is that radiologists are not familiarwith the concepts of fistula pathogenesisand anatomy and the relevant surgicalquestions (17). One of the most hazard-ous misinterpretations is to diagnose adirect rectal opening merely becausethere is contrast material in the rectallumen; usually, the contrast agent hasmerely refluxed up from the internal analopening. Such radiology reports only en-courage the surgeon to look for nonexist-ent openings and extensions, which canresult in iatrogenic secondary tracts.

Imaging Fistula in Ano: CT

Computed tomography (CT) may depictfistula in ano, especially if rectal and intra-venous contrast material are used, andinitial reports were encouraging (19–21).However, fistula depiction is not enough;fistulas must be classified correctly, andmore recent and better data suggest thatCT cannot be used for this purpose withsufficient accuracy. This is because theCT attenuation of the anal sphincter andpelvic floor is similar to that of the fistulaitself, unless the latter contains air or con-trast material. This is compounded by theinability to image in the surgically relevantcoronal plane. Comparative studies aresparse: A study of 25 patients with 17fistulas found that CT could be used tocorrectly classify only four fistulas, in con-trast to the 14 correct classificationsachieved with endosonography (22). Po-tentially, the disadvantages of CT mightbe overcome by using multi–detector rowCT fistulography, which offers the possi-bility of isotropic voxels and multiplanarimaging, but results are awaited and themotivation to perform these studies islikely limited by the ready availability ofMR imaging. It should be borne in mindthat CT is frequently used to search forabscesses in the context of Crohn disease,and fistula in ano may be encountered atthe time of examination. At present, how-ever, accurate classification of these fistu-las is best left to MR imaging or anal en-

dosonography, with CT probably limitedto the diagnosis of fistula-associated pel-vic abscesses where other imaging is un-available or cannot be tolerated.

Imaging Fistula in Ano: AnalEndosonography

Anal endosonography, developed by CliveBartram, FRCP, FRCS, FRCR, was thefirst technique to directly depict the analsphincter complex in detail (23). Simplemodification of a rotating rectal endo-probe by covering it with a nondeform-able plastic cone allowed the transducerto be withdrawn through the anal canalitself, situating it very close to the targetstructures and thus providing images ofhigh spatial resolution. The technique hasattracted considerable attention becauseof its ability to demonstrate the presenceand extent of anal sphincter disruption,notably after vaginal delivery (24). Analendosonography has also been exten-sively used for the preoperative classifica-tion of fistula in ano.

Figure 4

Figure 4: Fistulography in a male patient.Coronal image shows that it is obvious that thereare several high extensions (arrows) surroundingthe anorectal junction; however, the exact ana-tomic location of these is unclear because thepelvic floor (ie, levator ani in this case) cannot bedirectly visualized. Definition of extension location(supra- or infralevator) is central to surgicalmanagement.



The examination is simple, rapid, andwell tolerated by patients. The patient liesin the left lateral position or in the proneposition if female (25). The probe isgently inserted into the distal rectum andthen withdrawn through the anal canal.The internal sphincter is visualized as ahypoechoic ring encircling the anal canal,whereas the external sphincter is ofmixed echogenicity (Fig 5). The inter-sphincteric space and longitudinal musclelie between these and are of mixed echo-genicity and are easily identified by usingmodern 10-MHz transducers (26).

Endosonography is particularly wellsuited to identification of the internalopening, because this opening is usuallypositioned right at the probe surface. It isimportant to realize, however, that a tractextending up to the anal mucosal surfaceis rarely seen. Although a breach in thesubepithelial layer of the anal canal is oc-casionally present, it is more common forthe position of the internal opening to berevealed as a hypoechoic focus in the in-tersphincteric space that abuts the inter-nal sphincter, often with a small corre-sponding defect in the internal sphincter(Fig 6). Because intersphincteric fistulasdo not stray beyond the intersphinctericspace, they are usually very well visual-ized at anal endosonography. Trans-sphincteric fistulas are revealed by tracts

that cross the external sphincter to reachthe ischioanal fossa (Fig 6). As would beexpected, extensions are revealed as hy-poechoic fluid collections (Fig 7).

It was initially hoped that anal en-dosonography would revolutionize preop-erative fistula classification, a view sup-ported by the results of early studies (27).However, subsequent work has been in-conclusive. For example, some investiga-tors (28,29) have found the technique tobe useful, while others (30) have found itto be no better than digital rectal exami-nation. Furthermore, in direct compari-sons with MR imaging, anal endosonogra-phy has been variously found to be supe-rior (31), equivalent (32,33), or inferior(34,35). Much of this discrepancy is prob-ably related to operator expertise, sinceanal endosonography, similar to fistulog-raphy, is highly operator dependent.

However, there are undoubtedly sev-eral areas where anal endosonographyhas specific disadvantages. For example,insufficient penetration of the ultrasoundbeam beyond the external sphincter, es-pecially with high-frequency transducers,limits the ability to resolve ischioanal andsupralevator infections, with the resultthat extensions from the primary tractmay be missed at endosonography. Also,anal endosonography cannot be used toreliably distinguish infection from fibro-sis, because both have a hypoechoic ap-pearance (30). This causes particular dif-ficulties in patients with recurrent dis-ease, since infected tracts and fibroticscars are frequently combined. Attemptshave been made to clarify the course ofpatent tracts by injecting hydrogen perox-ide or sonography contrast agents intothe external opening during examination(34,35). However, gas formed within thetract as a result may cause acoustic shad-owing that mimics an extension. Indeed,this phenomenon can occur with any tractthat contains air, leading, for example, tointersphincteric fistulas being inadvertentlyclassified as transsphincteric (Fig 8).

Anal endosonography is also disad-vantaged by the inability to image in thesurgically important coronal plane, sothat it may be very difficult to distin-guish supra- from infralevator exten-sions (Fig 7). Some investigators (36)have attempted to overcome this disad-

Figure 5

Figure 5: Intersphincteric fistula. Anal en-dosonogram in transverse plane at mid–anal canallevel in a male patient shows fistula with hypo-echoic tract located in intersphincteric plane be-tween external (EAS) and internal (IAS) analsphincters. Internal sphincter is markedly hypo-echoic. At surgery, the internal opening was lo-cated at 6 o’clock posteriorly and was correctlypredicted from anal endosonographic visualiza-tion because of the radial position of the fistulawithin the intersphincteric plane.

Figure 6

Figure 6: Transsphincteric fistula shown onanal endosonogram in the transverse plane at themid–anal canal level in a female patient. In con-trast to Figure 5, the fistula (�) has penetrated theexternal anal sphincter (EAS). The internal open-ing was correctly predicted at 7-o’clock position.Note that the internal sphincter is relatively thinnedhere, which is a clue to the site of the internalopening, but there is no tract extending to the analmucosa.

Figure 7

Figure 7: Anal endosonogram at upper analcanal level in a male patient shows extensive hy-poechoic horseshoe extension (�). Because en-dosonography is limited to the transverse plane, itis difficult to determine whether this extension isinfra- or supralevator.



vantage by employing a three-dimen-sional acquisition (Fig 9), but this tech-nique remains relatively experimental.

There is no doubt that anal en-dosonography is a valuable technique inthe right hands. A recent study (37) inwhich anal endosonography was com-pared with digital rectal evaluation andMR imaging in 108 primary tracts foundthat digital evaluation resulted in cor-rect classification of 61% of fistulas; analendosonography, 81%; and MR imag-ing, 90%. Anal endosonography wasparticularly adept with regard to thesite of the internal opening, with correctprediction in 91% of cases comparedwith 97% for MR imaging (37). How-ever, there is little doubt that MR imag-ing is a superior technique overall and isnow also generally available. Given this,the major role of anal endosonographyin fistula disease is probably in the as-sessment of the degree of sphincter dis-ruption in patients who become anallyincontinent after surgery for a fistula.Endosonography has high spatial reso-lution, so it also has a particular role inpatients who potentially have a smallintersphincteric abscess that might bedifficult to resolve by using standardbody- or surface-coil MR imaging.

Imaging Fistula in Ano: MR Imaging

In recent years, MR imaging has emergedas the leading contender for preoperativeclassification of fistula in ano. The abilityof MR imaging to help not only accuratelyclassify tracts but also identify diseasethat otherwise would have been missedhas had a palpable effect on surgical treat-ment and, ultimately, patient outcome.

TechniqueMagnetic field strength does not appearto be a critical factor for good results(38). Although higher field strengthmight be relevant for more subtle differ-entiation of sphincter anatomy andtracts, no definite diagnostic benefit hasbeen demonstrated, to our knowledge.

Authors of initial reports (39–43) onMR imaging necessarily used the bodycoil, with good results. However, au-thors of some subsequent studies (44–45) reported lower accuracy when a

body coil was used; this was thought tobe possibly due to lower spatial resolu-tion. The introduction of externalphased-array surface coils increased thesignal-to-noise ratio and spatial resolu-tion, to good effect (46,47), and thesecoils quickly became generally available.

The best spatial resolution is achievedby using a dedicated endoluminal anal coil(48), which may be combined with a sur-face coil to increase the field of view (49).It should be noted that these endoluminalcoils are not the same as a rectal coil butare of smaller diameter and are designedto be placed in the anus. The receiver coilis generally housed in a plastic cover thatallows placement across the anal canal(Fig 10). The external diameter generallyranges between 12 and 19 mm, althoughsmaller coils have been used for pediatricexaminations (50,51). Endoluminal coilsare susceptible to motion artifact, but thiscan be reduced by means of careful pa-tient preparation. For example, patientsshould be asked to try and relax thesphincter and pelvic floor as much as pos-sible, and due attention should be paid tocomfort, including support for the coiland patient with pads (50). Spasmolyticssuch as 20 mg of hyoscine butylbromide(Buscopan; Boehringer Ingelheim, In-gelheim, Germany) or 1 mg of glucagonadministered intramuscularly may help toreduce motion-induced artifacts. (Hyo-

scine butylbromide is not licensed for usein the United States.)

The choice of coil depends on per-sonal preference, availability, the pa-tient group studied, and the clinicalquestion in each patient. In a study of 10patients with cryptoglandular fistula(45), an endoluminal coil was found tobe superior to a surface coil; in a subse-quent study of 30 patients (52), how-ever, a body coil was found to be supe-rior overall because the limited field ofview inevitable with endoluminal imag-ing meant that distant extensions weremissed, a phenomenon that is especially

Figure 8

Figure 8: Transverse anal endosonogram atupper anal canal level in a female patient showsintersphincteric horseshoe extenstion (arrows).Gas in the fistula causes acoustic shadowing (�),which could be mistaken for transsphincterictracts.

Figure 9

Figure 9: Three-dimensional anal endosono-gram (coronal view) in a female patient showstranssphincteric tract (arrows) that has been out-lined by injecting hydrogen peroxide into the ex-ternal opening.

Figure 10

Figure 10: Endoanal receiver coil for MRimaging.



frequent in patients with Crohn disease(53). In a third study (49), endoluminaland phased-array coils were comparedin 20 patients, and the investigatorsfound that while the endoluminal coilwas superior for classification of the pri-mary tract, extensions were better im-aged by using the superior field of viewof the external coil.

These results clearly suggest that alarge field of view is necessary when-ever extensions are suspected—for ex-ample, in patients with recurrent fistulaor Crohn disease. The high spatial reso-lution of endoluminal coils makes themideal for precise demonstration of thelocation and height of the internal open-ing, and they may have a special role fordemonstrating ano- or rectovaginal fis-tulas, which are notoriously difficult toimage (54). Endoluminal coils are alsovaluable when simultaneous informa-tion on the degree of sphincter disrup-tion is needed, which may be the case inpatients who have undergone previoussurgery. Endoluminal coils are some-times difficult to place owing to anal ste-nosis or local pain as a result of exten-sive infection. Halligan and Bartram(52) found that an endoluminal coilcould not be placed in 17% of their pa-tients, whereas Stoker and colleagues(50) failed to place the coil in only 3%,

which may reflect differences in patientpopulation since the coil used in bothstudies was identical.

When circumstances allow, it islikely that an optimal examination willbe achieved by using a combination ofboth external and endoluminal coils.However, it should be borne in mindthat accuracy with a body or externalcoil alone remains high (41–43,47,49,52), and lack of an endoluminalcoil alone is insufficient reason to avoidpreoperative MR imaging of fistula inano. Indeed, examination with a bodyor phased-array coil has become stan-dard practice, not least because endolu-minal coils specifically designed for anal im-aging remain relatively unavailable.

MR SequencesVarious investigators have adopteddifferent strategies with respect to theMR sequences used to image fistula inano. All agree that anatomic precisionis needed so that the course of thefistula with respect to adjacent struc-tures can be judged accurately, and alluse some method with which infection(usually pus) can be highlighted.These aims can be achieved by a vari-ety of means. Many investigators usethe rapid and convenient fast spin-echo T2-weighted sequence, which

provides good contrast between hy-perintense fluid in the tract and thehypointense fibrous wall of the fistulawhile simultaneously enabling gooddiscrimination between the severallayers of the anal sphincter (50,55).Others have used T1-weighted se-quences, which must be combinedwith intravenous contrast material forthe fistula to be highlighted (43). Fat-suppression techniques are widelyused with both T2-weighted (56) andgadolinium-enhanced T1-weighted se-quences (the latter may be especiallyvaluable in patients with Crohn dis-ease, to differentiate between fluidand inflammatory tissue that are bothhyperintense on fat-suppressed T2-weighted and/or short tau inversion-recovery [STIR] images). Normal ano-rectal structures do not enhance sub-stantially, except for the internal analsphincter and blood vessels, includinghemorrhoids. Investigators have alsosuccessfully employed STIR, a se-quence that combines fat suppressionwith high conspicuity of active tracts(41,47,52). Other approaches have in-cluded saline instillation into the ex-ternal opening in an attempt to in-crease tract conspicuity (57) or rectalcontrast medium (58,59). However,such measures increase the complex-

Figure 11

Figure 11: Correct orientation for MR imagingof anal canal. Sagittal T2-weighted scout imagethrough patient’s midline is used to plan imagesthat are truly transverse with respect to anal canal,as shown by white lines. Coronal imaging is thenperformed at 90° to the transverse plane.

Figure 12

Figure 12: Coronal (a) T2-weighted fast spin-echo (2500/70; echo train length, 16; field of view, 300 mm;matrix, 256 � 512; section thickness, 4 mm; gap, 0.4 mm) and (b) coronal STIR (4000/42, inversion time of150 msec; echo train length, 16; matrix, 224 � 256; section thickness, 4 mm; gap 0.4 mm; two signals ac-quired) 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 (notshown). At the ischial tuberosity, bone marrow edema (long straight arrow) is visible on b. Arrowhead � ex-ternal opening, curved arrow � small abscess, AS � anal sphincter.



ity of the examination in the face of thealready excellent results achieved withmore standard procedures. Imagingprotocols are detailed in the Table.

Imaging PlanesIt is central to success that imagingplanes are correctly aligned with re-spect to the organ of interest, namelythe anal canal. Because the anal canal istilted forward from the vertical by ap-

proximately 45°, straight transverseand coronal images will fail to achievethis alignment because of marked par-tial volume effect. Oblique transverseand coronal planes oriented orthogonaland parallel, respectively, to the analsphincter are therefore necessary andare most easily planned by using a mid-line sagittal image (Fig 11). It may benecessary to align supplementary exam-inations with the rectal axis in complex

cases of an internal opening high in therectum, but this is seldom needed.

It is important that the imaged volumeextend several centimeters above the leva-tors and include the whole presacral space,both of which are common sites for exten-sions. The entire perineum should also beincluded. On occasion, tracts may extendfor several centimeters, even leaving thepelvis or reaching the legs, and any tractvisible must be followed to its termination if

Figure 13

Figure 13: 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, 300mm; matrix 256 � 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) trans-verse fat-saturated contrast-enhanced T1-weighted fast spin-echo image (see Table for parameters) show two separate fistula tracts (straight and curved arrows) in leftposterior 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).

External Phased-Array 1.5-T MR Protocols for Imaging Perianal Fistula

Sequence and Plane* TR/TE†Echo TrainLength

Field ofView (mm) Matrix

Section Thickness/Intersection Gap(mm)‡

T2-weighted fast spin echo without fat suppressionSagittal, coronal, transverse 2500/70 10 300§ 256 � 512 3/0.3

T2-weighted fast spin echo with fat suppressionTransverse 2500/70 10 300§ 256 � 512 3/0.3

T1-weighted fast spin echo with fat suppressionTransverse# 600/minimal 3 450 256 � 256 4/0.4

Note.—Parameters are typical and were established with EchoSpeed imager (GE Healthcare, Milwaukee, Wis). Field of view should include rectum and perineum; at endoanal MR, volume shouldencompass entire sensitive region of coil. For all sequences, two signals are acquired and bandwidth is 20.83 kHz.

* Transverse and coronal planes are off axis, orthogonal and parallel, respectively, to anal canal.† TR/TE � repetition time (msec)/echo time (msec).‡ For endoanal MR, transverse section thickness and gap are 2–3 mm and 0.2–0.3 mm, respectively.§ For endoanal MR, field of view is 160 mm for coronal and sagittal planes and 100 mm for transverse plane.# Additional value of routine use of this contrast-enhanced sequence is not yet determined, but it may help differentiate inflammation from abscess.



this has not been included on the standardimage volume (Fig 12). The imaged volumeshould encompass the whole sensitive re-gion of the coil when an endoanal receiveris used. The precise location of the primarytract (eg, ischioanal or intersphincteric) isusually most easily appreciated by usingtransverse images; the radial site of the in-ternal opening is also well seen on images inthis plane. Coronal images best depict thelevator plate,which helps distinguish supra-from infralevator infection. The height ofthe internal opening may also be best ap-preciated on coronal images, with the ca-veat that the anal canal must be imagedalong its entire craniocaudal extent.

The radial plane (as used for imagingmenisci of the knee) is seldom used butseems attractive because it has the poten-tial to depict fistulas along their full cranio-caudal extent, something that is onlyachieved with standard coronal imaging

when the fistula is at the 3- or 9-o’clockposition. Instead of being obtained parallelto the anal canal, the image planes radiatefrom it like the spokes of awheel (60). Littleresearch has specifically addressed the ben-efits of various imaging planes, but in astudyof 20patients (60) investigators foundthat the combination of a transverse seriesand a longitudinal series (coronal, sagittal,radial, or a combination) provided all nec-essary information for successful interpre-tation.

InterpretationThe success of MR imaging for preoper-ative classification of fistula in ano is adirect result of the sensitivity of MR fortracts and abscesses combined withhigh anatomic precision and the abilityto image in surgically relevant planes.Accurate preoperative classification isachieved by correctly relating the im-aged fistula to the anal sphincter.

Primary TractActive tracts are filled with pus andgranulation tissue and, thus, appear as

hyperintense longitudinal structures onT2-weighted or STIR images (Fig 12).On contrast-enhanced T1-weighted im-ages, active granulation tissue will en-hance while fluid in the tract itself re-mains hypointense (Fig 13). Activetracts are often surrounded by hypoin-tense fibrous walls (Fig 13), which canbe relatively thick, especially in patientswith recurrent disease and previoussurgery. Occasionally, some hyperin-tensity in this fibrous area may be seen,probably reflecting edema. Hyperinten-sity may also extend beyond the tractand its fibrous sleeve, where it repre-sents adjacent inflammation (Fig 13).

The external anal sphincter is clearlyvisualized by using MR imaging. It is rela-tively hypointense, and its lateral bordercontrasts against the fat in the ischioanalfossa, both on STIR (Fig 14) and especiallyon fast T2-weighted MR studies (Fig 13a).Thus, it is relatively easy to determinewhether a fistula is contained by the exter-nal sphincter or has extended beyond it. If afistula remains contained by the externalsphincter throughout its course, then it is

Figure 14

Figure 14: Intersphincteric fistula in a malepatient. Transverse STIR MR image (1500/15; fieldof view, 375 mm; matrix, 256 � 256; sectionthickness, 4 mm; gap, 1 mm; four signals ac-quired) shows that lateral margin of externalsphincter (long arrow) contrasts against fat in theischioanal fossa (�). Fistula (short arrow) is in theintersphincteric space posteriorly at 6 o’clock andis contained by the external sphincter. There is notract in the ischioanal fossa.

Figure 15

Figure 15: Fistula classified as suprasphincteric on coronal STIR MR images (same parameters as for Fig14) in a female patient. (a) Primary tracts in right (long arrow) and left (short arrow) ischioanal fossae areshown. (b) Image obtained just posterior to a shows that right-sided primary tract (white arrows) arches overpuborectalis muscle (�) to reach a lower internal opening at the dentate line level (black arrow).



highly likely to be intersphincteric (Fig 14).In contrast, any evidence of a tract in theischioanal fossa effectively excludes an in-tersphincteric fistula. However, trans-sphincteric, suprasphincteric, and extra-sphincteric fistulas all share the commonfeature of a tract that lies beyond the con-fines of the external sphincter. While atranssphincteric fistula will be the common-est cause of a tract in the ischioanal fossa(Fig 13), differentiation between thesethree fistulas is only possible by locating theinternal opening and determining thecourse between this and the primary tract.

Internal OpeningThe exact location of the internal openingcan be difficult to define, whatever theimaging modality used. Two questionsneed to be answered. What is the radialsite of the internal opening, and what isits level? The vast majority of anal fistulasopen into the anal canal at the level of thedentate line, commensurate with thecryptoglandular hypothesis of fistulapathogenesis. Furthermore, most fistulasalso enter posteriorly, at the 6-o’clock po-sition. Unfortunately, the dentate linecannot be identified as a discrete ana-tomic entity, even when endoanal re-ceiver coils are used, but its general posi-tion can be estimated with sufficient pre-cision for the imaging assessment to beworthwhile. The dentate line lies at ap-proximately the mid–anal canal level.This is generally midway between the su-perior border of the puborectalis muscleand the most caudal extent of the subcu-taneous external sphincter. These land-marks define the “surgical” anal canal (asdistinct from the “anatomic” anal canal,which is shorter and is defined as thecanal caudal to the anal valves). The den-tate level is probably best appreciated oncoronal views, which allow the craniocau-dal extent of the puborectalis muscle andexternal sphincter to be appreciated; withexperience, however, its location can beestimated with reasonable precision byusing transverse views.

It should be noted that in some pa-tients the puborectalis muscle is rathergracile, unlike the bulky muscle sug-gested in many anatomy texts, and itfrequently and imperceptibly seguesinto the external sphincter, all of which

hampers precise identification of themid–anal canal level. Nevertheless, withexperience it is possible to estimate theexact height of the internal opening withreasonable precision (47).

Any tract that penetrates the pelvicfloor above the level of the puborectalismuscle is potentially a suprasphinctericor extrasphincteric fistula. The level ofthe internal opening distinguishes be-tween these types of fistula; specifically,the internal opening is anal in supra-sphincteric fistulas (Fig 15) and rectal inextrasphincteric fistulas (Fig 16).

Transsphincteric fistulas penetratethe external sphincter, a feature that canbe easily appreciated on transverse (Fig17) or coronal views. However, recentstudies (61) in which MR imaging wasused have revealed that a transsphinc-teric tract may cross the sphincter at avariety of angles. For example, it may

Figure 16

Figure 16: Fistula classified as extrasphinc-teric in a female patient on coronal T2-weightedMR image (4563/150; field of view, 350 mm; ma-trix, 256 � 256; section thickness, 6 mm; gap, 0.6mm; four signals acquired). Fistula tract (horizon-tal white arrows) is seen in left ischioanal fossa.Levator plates (vertical white arrows) are well de-picted bilaterally. Tract penetrates the left levatorplate, and the internal opening (top horizontalwhite arrow) is into the rectum, above the level ofthe puborectalis muscle (�) and well above thedentate line (black arrow).

Figure 17

Figure 17: Transsphincteric fistula in a malepatient. Transverse STIR MR image (same param-eters as for Fig 14) shows primary tract (verticalarrow) in right ischioanal fossa, where it can beclearly seen to penetrate external sphincter (�) toreach the intersphincteric space. Internal openingis posterior at 6 o’clock (horizontal arrow), at den-tate line level.

Figure 18

Figure 18: Transsphincteric fistula in a malepatient. Transverse STIR MR image (same param-eters as for Fig 14) at level of the internal openingshows primary tract (vertical arrow) at 4 –5o’clock. Unlike Figure 17, the tract cannot betraced right to the anal mucosa, and the adjacentinternal sphincter (horizontal arrow) appears in-tact. However, an internal opening at 4 –5 o’clockwas reported because this position indicated siteof maximal infection in the intersphincteric plane.The internal opening was confirmed at this siteduring subsequent EUA. Intersphincteric plane iswell seen in this patient between hyperintenseinternal sphincter and the external sphincter.



arch upward as it passes through the ex-ternal sphincter and thus cross the mus-cle at a higher level than would be de-duced merely by inspecting the level ofthe internal opening. This is important,because such tracts will require a greaterdegree of sphincter incision during fistu-lotomy, with a corresponding increase inthe risk of postoperative incontinence.MR imaging in the coronal plane is bestfor estimates of the precise angulation ofthe tract with respect to the surroundingmusculature (61).

The radial site of the internal openingis easy to identify if the fistula tract can be

traced right to the anal mucosa (Fig 17).The radial site is reported with respect toa clock face, with 12 o’clock being directlyanterior. However, like endosonography,it is frequently impossible to trace a tractright up to the anal mucosa, especially ifan endoanal coil has not been used. Insuch cases, an intelligent deduction mustbe made as to where the internal openingis likely to be. This is best accomplishedby looking for the area of maximal inter-sphincteric sepsis, since the internalopening is likely to lie very close to this(Fig 18). The intersphincteric space andlongitudinal layer are often seen as a hy-pointense ring between the internal andexternal sphincters (Fig 18). The internalsphincter is hyperintense on both T2-weighted fast spin-echo and STIR images,especially if contrast material has beenused (62).

ExtensionsThe major advantage of MR imaging isthe facility with which it can demonstrateextensions associated with a primarytract. Morphologically, extensions fre-quently take the form of complex tractsystems, regions of which have often be-come dilated to create an abscess (al-though a precise radiologic distinction be-tween abscess and a large tract remainselusive). Extensions appear as hyperin-tense regions on T2-weighted and STIRimages and enhance if intravenous con-trast material is used. Again, collateralinflammation can be present to a variableextent.

The commonest type of extension is

one that arises from the apex of a trans-sphincteric tract and extends into theroof of the ischioanal fossa (Figs 3, 19).The major benefit of MR imaging find-ings is that they can alert the surgeon toextensions that would otherwise bemissed. For example, extensions maybe several centimeters from the pri-mary tract (Fig 20), which makes themdifficult to detect during clinical exami-nation or EUA. This is especially thecase when extensions are contralateralto the primary tract (Fig 21). It is alsoimportant to search for supralevator ex-tensions (Fig 22), since these are notonly difficult to detect but pose specificproblems with regard to treatment.Horseshoe extensions spread acrossboth sides of the internal opening andare recognized on MR images by theirunique configuration (Fig 23); horse-shoe extensions may be intersphinc-teric, ischioanal, or supralevator. Com-plex extensions are especially commonin patients with recurrent fistula in ano(Fig 21) or in those who have Crohndisease (Fig 13).

Effect of Preoperative MR on Surgeryand OutcomeOver the past few years, imaging, nota-bly MR, has revolutionized the treat-ment of patients with fistula in ano. Thisis because MR can be used to classifyfistulas preoperatively with high accu-racy while also alerting the surgeon todisease that would otherwise have beenmissed. While there are reports of thetechnique dating from 1989 (40), it wasnot until the description by Lunniss andco-workers (41) that the true potentialof MR imaging was fully appreciated.Lunniss et al imaged 16 patients withcryptoglandular fistula in ano and com-pared the MR classifications they ob-tained with those from subsequentEUA. MR imaging proved correct in 14(88%) cases, which immediately sug-gested that it was the most accuratepreoperative assessment yet available.However, the remaining two patients,in whom MR suggested disease but EUAyielded normal findings, re-presentedsome months later with disease at thesite initially indicated on MR images.This led the authors to conclude that

Figure 19

Figure 19: Left-sided transsphincteric tract(short arrow) in a female patient. Coronal STIR MRimage (same parameters as for Fig 14) showslarge extension (long arrow) from apex of tract intoroof of ipsilateral ischioanal fossa.

Figure 20

Figure 20: Left-sided trans-sphincteric fistula (short straightarrows) with internal opening at 6o’clock (long straight arrow) in afemale patient. Transverse STIRMR image (same parameters as forFig 14) shows remote extension(curved arrow) into ipsilateralbuttock that was unsuspected atclinical examination but is welldemonstrated at MR imaging. Theextension was found at surgeryguided by MR findings.



MR imaging “is the most accuratemethod for determining the presenceand course of anal fistulae” (41). Thiswork was rapidly confirmed by othersworking in the field and was subse-quently elaborated on.

Spencer and colleagues (63) inde-pendently classified 37 patients intothose with simple or those with complexfistulas on the basis of MR imaging andEUA and found that MR results werethe better predictor of outcome, withpositive and negative predictive values,respectively, of 73% and 87% for MRand 57% and 64% for EUA. These re-sults clearly implied that MR imagingand outcome were closely related andagain raised the possibility that preop-erative MR could help identify featuresthat cause postoperative recurrence.

Beets-Tan and colleagues (46) ex-tended this hypothesis by investigatingthe therapeutic effect of preoperative MRimaging; the MR imaging findings in 56patients were revealed to the surgeon af-ter he or she had completed an initialEUA. MR imaging provided important ad-ditional information that precipitated fur-ther surgery in 12 (21%) of 56 patients,predominantly in those with recurrent fis-tula or Crohn disease (46).

Buchanan and co-workers (47) hypoth-esized that the therapeutic influence and,thus, beneficial effect of preoperative MRimaging would be greatest in patients withrecurrent fistula, since these patients hadthe greatest chance of harboring occult in-fection, while such fistulas were also themost difficult to evaluate clinically. After aninitial EUA, Buchanan et al revealed thefindings of preoperative MR imaging to thesurgeons for 71 patients with recurrent fis-tulas and left any further surgery to thediscretion of the operating surgeon. Theyfound that postoperative recurrence wasonly 16% for surgeons who always acted ifMR findings suggested that areas of infec-tion had been missed, whereas recurrencewas 57% for those surgeons who insteadalways chose to ignore imaging results (47).Furthermore, in the 16 patients whoneeded further unplanned surgery, MR ini-tially correctly predicted the site of this dis-ease in all cases (47). Using a similar ap-proach, Buchanan and colleagues (64) alsoinvestigated the effect of preoperative MR

imaging on clinical outcome in patients withfistula in ano at initial presentation andfound that the scheduled surgical approachchanged in 10% of this group.

Ever since the results of Lunniss et al(41) suggested that EUA might be an im-perfect reference standard with which tojudge MR imaging, comparative studieshave been plagued by the lack of a genu-ine reference standard. It is now well rec-ognized that surgical findings at EUA areoften incorrect. In particular, there arefrequent false-negatives. In a recent com-parative study of endosonography, MRimaging, and EUA in 34 patients with fis-tula due to Crohn disease, Schwartz andco-workers (32) found that a combinationof the results of at least two modalitieswas necessary to arrive at a correct clas-sification. Indeed, it is well establishedthat many false-negative surgical resultswill only reveal themselves during long-term clinical follow-up, and, at this pointin time, comparative studies that ignoreclinical outcome are likely to be seriouslyflawed.

Imaging for Differential Diagnosis

Not all cases of perianal sepsis are due tofistula in ano. For example, acne conglo-bata, hidradenitis suppurativa, pilonidal

sinus, actinomycosis, tuberculosis, proc-titis, human immunodeficiency virus, lym-phoma, and anal and rectal carcinomamay all cause perianal infection. While

Figure 21

Figure 21: Transsphincteric primary tract(short arrow) in the right posterior quadrant of afemale patient. Transverse STIR MR image (sameparameters as for Fig 14) shows two left-sidedcontralateral extensions (long arrows) that wereundetected at EUA until results of patient’s MRexamination were revealed to the surgeon in theoperating theater.

Figure 22

Figure 22: Bilateral supralevator extensions(long arrows) in a female patient. Coronal STIRMR image (same parameters as for Fig 14) clearlyshow levator plates (short arrows) bilaterally, sothat it is easy for the radiologist to be confident thatinfection extends above them.

Figure 23

Figure 23: Horseshoe extension (arrows) aris-ing from intersphincteric fistula in a male patient.Transverse STIR MR image (same parameters asfor Fig 14) shows that, in this case, the horseshoepractically encircles the anal canal.



clinical examination results are often con-clusive, this is not always the case, andimaging may help with the differential di-agnosis. The cardinal feature of fistula inano is intersphincteric infection, which isnot generally found in other conditions.Whenever imaging suggests that infectionis superficial rather than deep seated andthat there is no sphincteric involvement,other conditions such as hidradenitis sup-purativa should be considered (Fig 24).For example, authors of a recent study(65) of patients with pilonidal sinus andfistula in ano found that MR imaging couldbe used to reliably distinguish betweenthe two on the basis of intersphinctericinfection and an enteric opening.

The possibility of underlying Crohndisease should always be considered inpatients who have a particularly com-plex fistula, especially if the history isrelatively short. Indeed, a perianal fis-tula is the presenting condition in 5% ofpatients (34), and 30%–40% of patientswith Crohn disease will experience analdisease at some time (66–68). Specificscoring systems for perianal Crohn dis-ease have been proposed, such as thePerianal Crohn’s Disease Activity Index(69) and a recently described systembased solely on MR imaging findings

(70). Small-bowel imaging may be usedto search for Crohn disease when it issuspected, and the possibility of under-lying pelvic disease should be consid-ered in any patient with an extrasphinc-teric fistula, whether thought due toCrohn disease or otherwise.

Which Patients Should Be Imaged?

While fistula in ano is simple to diag-nose and simple to treat in most pa-tients, many other patients will benefitfrom detailed and accurate preopera-tive investigation. Where there is easyaccess to MR imaging, it could be ar-gued that all patients should undergopreoperative imaging. For example, ithas been estimated that the therapeuticeffect of MR imaging is 10% in patientspresenting for the first time with aseemingly simple fistula (64). Where ac-cess to imaging is more restricted, how-ever, the clinician and radiologist willneed to select those patients who aremost likely to benefit. Since there is nowoverwhelming evidence that MR imag-ing alters surgical therapy and improvesclinical outcome in patients with recur-rent disease, MR should be routine insuch cases. Patients presenting for thefirst time with a fistula that appearscomplex at clinical examination shouldalso be referred, as should patients withknown Crohn disease, since the prepon-derance of complex fistulas is increased.

There are also surgical situationswhere imaging is likely to be particularlybeneficial, even when the fistula itself issimple. For example, the anterior exter-nal sphincter is very short in women,and division of this sphincter during fis-tula incision is particularly associatedwith postoperative incontinence, evenwhen the fistula itself is simple and hasno extensions. Faced with such a di-lemma, the surgeon may choose to passa thread (seton) through the tractrather than incise the fistula to providedrainage. The patient can undergo post-operative imaging so that the potentialextent of sphincter division can be as-sessed by visualizing the relationship ofthe seton to the external sphincter. Adecision can then be made whether toprogress with fistulotomy or to keep the

seton in place for a few months, afterwhich time the internal opening can beclosed with a rectal mucosal advance-ment flap. A seton can also be placed atEUA when the surgeon is uncertainabout the relationship between the tractand the sphincter; postoperative imag-ing can then be used to help answer thisquestion. In patients who undergo noveltherapies, such as use of fibrin glue, im-aging may be necessary during instilla-tion to be confident that the whole tracthas been filled (71).

MR imaging is not restricted to sur-gical assessment. Infliximab, a chimericmonoclonal antibody to human tumornecrosis factor–�, currently has aprominent role for the medical treat-ment of Crohn disease, especially in pa-tients with a chronic fistula (72). How-ever, infliximab therapy is contraindi-cated if an abscess is present, and MRimaging may be used to search for this(70,73). Indeed, MR imaging may beused to monitor infliximab therapy,since it seems that fistulas may persistin the face of clinical findings that sug-gest remission (74). For example, inves-tigations (70,73) of MR in patientswhose external opening has closed haverevealed that underlying infection canstill be present, indicating a need forcontinued therapy. Further studies areneeded to determine whether MR mon-itoring improves outcome.

Where MR imaging is unavailable orwhere competent interpretation of theimages is not possible, then anal en-dosonography is a viable and useful al-ternative. While recent comparisonshave shown that MR imaging outper-forms anal endosonography in all re-spects, the latter is far superior to sim-ple clinical examination, and its perfor-mance in some areas is very close tothat of MR (37). Notably, anal en-dosonography is very adept at depictingthe internal opening.

Conclusion

In those patients with fistula in ano whohave a high likelihood of complex dis-ease, the evidence that preoperativeMR imaging influences the surgical ap-proach and the extent of exploration

Figure 24

Figure 24: Hidradenitis suppurativa in a malepatient. Transverse STIR MR image (same param-eters as for Fig 14) shows extensive superficialinfection (arrows). Absence of any infection relatedto anal canal and intersphincteric space meant thatdiagnosis could be confidently made preopera-tively by using imaging.



and improves the ultimate outcome isnow overwhelming. We hope that thisarticle will stimulate radiologists to pro-vide this service to their surgeons in theexpectation that this will reduce the in-cidence of recurrent fistula in ano andthe misery that this causes.

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