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Imaging of Fistula in Ano1 STATE of the ART Ⅲ

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Imaging of Fistula in Ano1 STATE of the ART Ⅲ Imaging of Fistula in Ano1 STATE OF THE ART Ⅲ Steve Halligan, MD, FRCP, FRCR Fistula in ano is a common condition that often recurs Jaap Stoker, MD, PhD despite seemingly adequate surgery, usually because of infection that was missed at surgery. It is now increasingly recognized that preoperative imaging can help identify REVIEWS AND COMMENTARY infection that would have otherwise gone unidentified. In particular, magnetic resonance (MR) imaging findings have been shown to influence surgery and markedly dimin- ish the chance of recurrence; thus, preoperative imaging will 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 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 1 From the Department of Specialist Radiology, University College 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 requested August 24; revision received October 15; accepted Janu- ary 10, 2005; final version accepted March 2; updated September 29. Address correspondence to S.H. ஽ RSNA, 2006 18 Radiology: Volume 239: Number 1—April 2006 STATE OF THE ART: IMAGING OF FISTULA IN ANO Halligan and Stoker istula in ano (anal fistula) is a com- der surrounded by two muscular crypts that lie proximal to the valves. mon condition that has a tendency sphincters, the internal and external These glands were first linked to the Fto recur despite seemingly ade- anal sphincters, which are composed of genesis of fistula in ano by Chiari (3), quate surgery. Recurrence is usually smooth and striated muscle, respec- who suggested that they were the due to infection that has escaped surgi- tively (Fig 1). The external sphincter source of infection. Their purpose is un- cal detection and has thus gone un- has posterior attachments to the ano- clear, although they may help lubricate treated. It is now increasingly recog- coccygeal ligament and anterior attach- the anus by secreting mucus into the nized that preoperative imaging, nota- ments to the perineal body and urogen- anal crypts. The origin of the anal bly magnetic resonance (MR) imaging, ital diaphragm (and bulbocavernous glands within the surrounding tissues is can help identify infected tracts and ab- muscle in boys and men) and merges variable. For example, they are present scesses that would otherwise have been proximally with the slinglike puborec- in the subepithelium and may be missed. Preoperative MR imaging find- talis muscle (which defines the anorec- present in the internal sphincter, and ings have been shown to influence sub- tal junction), which itself merges with approximately one- to two-thirds of sequent surgery and markedly diminish the levator plate of the pelvic floor. these glands are deeply sited within the the chance of recurrent disease as a The internal sphincter is the distal intersphincteric space (4) (Fig 1). Most result. Because of this, preoperative im- termination of the circular muscle of the authorities believe that it is infection of aging is likely to become increasingly gut tube. The rectal longitudinal smooth these intersphincteric glands that is the routine in the future, especially in pa- muscle interdigitates between the inter- initiating event in fistula in ano, in a tients with recurrent disease. nal and external sphincters and is process known as the “cryptoglandular This review will detail the pathogen- thought to have no obvious sphincteric hypothesis” (5). Furthermore, lym- esis of fistula in ano, explain how patho- effect; rather, its role is probably to phoid aggregates surround the anal genesis causes the different types of fis- bind the anus together (1). glands, which may partly explain the in- tula encountered, and describe how The intersphincteric space is the creased incidence of anal fistula in these types can be imaged, with the em- surgical plane of dissection between the Crohn disease (6,7). phasis on MR. Most important, we will internal and external sphincters and is It is believed that gland infection re- describe how the radiologist is well most frequently found between the lon- sults in an intersphincteric abscess if the placed to answer the surgical questions gitudinal muscle and external sphincter, draining duct becomes blocked by in- that must be solved for treatment to be where it exists as a sheet of fat contain- fected debris. This abscess may resolve effective. ing loose areolar tissue. The fat-filled by means of spontaneous drainage into ischioanal fossa lies lateral to the the anal canal or may progress to an sphincter complex and is traversed by a acute anorectal abscess, which is a com- Anatomy and Etiology network of fibroelastic connective tissue mon acute surgical emergency and is To fully understand the role of imaging fibers. We prefer the term ischioanal familiar to all general and coloprocto- with regard to fistula in ano, an appreci- fossa to ischiorectal fossa because this logic surgeons (8). Treatment generally ation of its etiology and how the various space predominantly surrounds the consists of incision and drainage of the fistula types are defined by anatomic anus rather than the rectum. However, most fluctuant part of the abscess; how- boundaries is mandatory. the two terms are interchangeable. ever, this procedure does not pay due The anal canal is essentially a cylin- With regard to the lining of the anal attention to the source of infection in canal, the proximal half is characterized the intersphincteric space, with the re- by longitudinal mucosal folds, the anal sult that as many as 87% of patients Essentials columns of Morgagni (2). The distal as- with an acute abscess may subsequently Ⅲ Recurrent fistula in ano is usually pect of each column is linked to its develop a fistula (9). Acute anorectal caused by infection that was neighbor by a small semilunar fold (the abscess and fistula in ano are, there- missed during surgical explora- anal valves), which in turn forms small fore, generally believed to be acute and tion. pockets (the anal sinuses, or crypts of chronic manifestations, respectively, of Ⅲ MR imaging depicts remote foci of Morgagni). The distal undulating limit of the same disease. Because of this, the infection better than any other these valves is the dentate (pectinate) search for intersphincteric infection and modality, including surgical explo- line, which also marks the most distal an anal canal internal opening followed ration. aspect of the anal transitional zone, a Ⅲ MR image–guided surgery helps histologic junction between anal squa- Published online reduce postoperative recurrence mous epithelium and rectal columnar 10.1148/radiol.2391041043 by 75% in patients with complex epithelium. disease. The dentate line lies approximately Radiology 2006; 239:18–33 Ⅲ Anal endosonography is a viable 2 cm proximal to the anal verge and is a Abbreviations: alternative to MR imaging when crucial landmark in fistula in ano be- EUA ϭ examination under anesthesia the latter is not available. cause the anal glands empty into the STIR ϭ short tau inversion recovery Radiology: Volume 239: Number 1—April 2006 19 STATE OF THE ART: IMAGING OF FISTULA IN ANO Halligan and Stoker by treatment of these at the time of more tortuous than others, and by pene- importance, this fistula does not pene- acute manifestation has been advocated trating and involving the muscles of the trate the adjacent external sphincter, in an attempt to reduce the incidence of anal sphincter and surrounding tissues to which forms a relative barrier to spread. subsequent fistula (10). a variable degree. Fistulas may thus be However, some truculent fistulas can Fistula in ano develops when an in- classified according to the route taken by cross the external sphincter and reach the tersphincteric infection is allowed to this “primary tract” that links the internal ischioanal fossa by doing so (Fig 2). This continue unabated. It has a prevalence and external openings. results in a transsphincteric fistula, which of approximately 0.01% and predomi- Furthermore, classification largely composed 30% of cases in the series of nantly affects young adults (11). Fistula determines treatment. In 1934, Milligan Parks et al. Other fistulas may spread up- in ano is commoner in men, who domi- and Morgan (14) stressed the importance ward in the intersphincteric space and nate in all published series, with a male- of the “anorectal ring” (anatomically, the arch over the puborectalis muscle, where to-female ratio of approximately 2:1 puborectalis muscle) and categorized fis- they must cross the levator plate to reach (11). Patients most commonly present tulas as those that entered the anal canal the perianal skin. This type, the supras- with discharge (65%), but local pain above or below this structure, warning phincteric fistula (Fig 2), composed 20% due to inflammation is also frequent that postoperative incontinence was of cases in the series of Parks et al. (12). However, some fistulas may be highly likely if high fistulas were surgically Parks et al (6) also noted a fourth entirely asymptomatic. divided without due attention. This classi- type of fistula in 5% of cases. This type fication was subsequently modified and was characterized by the surprising ab- refined by other authors, but the most sence of intersphincteric infection. In- Fistula Classification comprehensive and practical classifica- stead, the fistula entered the rectum or By definition, a fistula is an abnormal tract tion in use today is that of Parks et al (6), anorectal junction directly (Fig 2). that connects two epithelial surfaces. The who carefully analyzed a consecutive se- Parks et al termed these extrasphinc- anatomic course of an anal fistula will be ries of 400 patients referred to the sur- teric fistulas.
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