Perianal Fistulas: Magnetic Resonance Characterization

Pictorial essay

Rev. Argent. Radiol. 2017;81(2): 129-134 129

Abstract Perianal fistulas are a prevalent problem in current medical practice, with a high impact on morbidity. Treat-ment is not exempt from complications, such as local recurrence and fecal incontinence. Magnetic resonance imaging (MRI) plays an essential role in surgical planning in these patients, since the high spatial resolution of the latest equip-ment provides more precise information on the pelvis anatomy, indicating the number and relationship of the fistulous tracks with the anal sphincter. The aim of this work is to identify the different types of fistulas based on MRI features, with particular emphasis on the local anatomy and study techniques.© 2016 Sociedad Argentina de Radiología. Published by Elsevier Spain, S.L.U. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Key words: Anal canal; Fistula; Morbidity; Magnetic resonance imaging.

Introduction

A fistula is defined as an abnormal communication between two epithelial lined surfaces1. In the case of perianal fistulas, such communication is established between the lining of the anal canal or the inferior rectus muscle and the perianal skin.Surgery is the treatment of choice. However, despite the suc-cess achieved in most cases, this treatment is not exempt from complications and a high rate of recurrence, mainly in complex cases2.Magnetic resonance imaging (MRI) plays an essential role, since it allows identification of millimeter tracts that are undetectable by other methods and provides details of the relationship between the fistula and the sphincter complex, contributing to surgical planning.This article reviews the regional anatomy, study techniques and MRI classification of perianal fistulas.

Etiology and pathogenesis

Even if the etiology of perianal fistulas may be due to various conditions, the most widely accepted theory is the crypto-glandular hypothesis. According to this theory, intersphinc-teric gland infection represents the initial event, starting with gland obstruction at the pectinate line1. Infection gener-ally spreads longitudinally towards the anal verge, although sometimes it may spread upwards to the rectal wall thickness

or through the external sphincter into the ischiorectal fossa. The final consequence of acute infection is the formation of an abscess which, when drained (either spontaneously or sur-gically), becomes a chronic fistula.However, the cryptoglandular hypothesis cannot explain for-mation of fistulas in inflammatory processes such as Crohn disease and diverticulitis, which result in development of ex-trasphincteric fistulas, with a direct communication between the perineum and rectum or other visceral structures such as the vagina, with no involvement of the anal canal3.In this respect, other predisposing factors are pelvic infection (actinomycosis, venereal lymphogranuloma or tuberculosis), trauma during childbirth, pelvic malignancy (anorectal carci-noma), radiation therapy and previous surgeries (hemorrhoid-ectomy or episiotomy)1,3. Hematologic diseases (leukemia, granulocytopenia or lymphoma) predispose the development of anal fistulas and abscesses, which are often serious1. How-ever, most are idiopathic and are generally thought to repre-sent the chronic phase of anal gland infection1,3.

Epidemiology

Perianal fistulas are a rare condition, with a prevalence of ap-proximately 0.01%3, predominantly affecting young adults, with a male-to-female ratio of 2:11,3. The most common symptom is local pain (65% of cases), although they may also be completely asymptomatic3.

Perianal Fistulas: Magnetic Resonance CharacterizationA. Domínguez*, A. Pitrella and M. Noceti

Fundación Escuela de Medicina Nuclear (FUESMEN), City of Mendoza, Mendoza, Argentina.

Rev. Argent. Radiol. 2017;81(2): 129-134

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Anatomy

Knowledge of the anatomy of the anal sphincter complex and surrounding spaces is essential for imaging interpretation (fig. 1). The anal canal is the terminal portion of the large in-testine, extending from the insertion of the levator ani muscle to the anal verge in cephalocaudal direction3,4. It is a cylindri-cal structure 2.5 to 3.5 cm long, surrounded by two muscular layers: the internal and external sphincters.The internal sphincter surrounds the upper two thirds of the anal canal and is composed of smooth muscle (the fibers are continuous with the circular smooth muscle of the rec-tum). This sphincter contracts involuntarily and is responsible for 85% of the resting tone of the anal canal. The external sphincter, in turn, forms a broad band on each side of the inferior third of the anal canal. It is composed of striated mus-cle, which merges with the puborectalis muscle to form the levator ani muscle, and contributes only 15% of the resting anal tone, despite its strong voluntary contractions that pre-vent defecation4. The two sphincters are separated by the in-tersphincteric space, which contains fat and the longitudinal muscle. This space is the place in which most fistulas spread.In the middle third of the anal canal, there is a transition zone where the squamous epithelium is continuous with columnar epithelium. This zone is characterized by the presence of lon-gitudinal mucosal folds, known as the columns of Morgagni. In the distal part of each column, there are anal valves, which form small pockets, the crypts of Morgagni. This distal part of the valves is known as the pectinate line and it marks the most distal region of the transition zone, approximately 2 cm from the anal verge4. Outside and on both sides of the sphinc-ter complex lie the ischiorectal and ischioanal fossae, fat-filled spaces where transsphincteric fistulas or abscesses may occur.

The role of magnetic resonance imaging

MRI is the technique of choice for preoperative evaluation of perianal fistulas3,5. Its high degree of tissue differentiation al-lows demonstration of hidden areas of infection and second-ary tracks (which contribute to post-surgery recurrence) and definition of anatomic relationships of the fistula to predict the likelihood of postoperative fecal incontinence.Inadequate assessment may result in the development of a complex fistula, and failure to recognize secondary exten-sions may result in recurrent sepsis and an unnecessarily pro-longed clinical course.In addition, it should be noted that treatment differs between different types of fistulas. In cases of simple, intersphincteric fistulas with or without abscesses, surgery is generally simple, while for transsphincteric or suprasphincteric fistulas, surgery is more complex, sometimes requiring temporary colostomy3,6,7.

Study technique and protocol

MRI examinations are currently performed with phased-array coils, which require no special patient preparation and pro-vide excellent anatomic detail of the anal sphincters and the anatomic boundaries of the pelvis3. An important advantage of MRI is the ability to assess the sphincter complex in any plane. For this reason, it is essential that images are acquired following the axis of the anal canal.To achieve the correct orientation, a sagittal fast spin-echo (FSE) T2-weighted sequence should be initially performed, which will provide an overview of the pelvis, showing the extent and axis of the canal.

Figure 1 (a and b) Drawing showing anatomy of the anal sphincter and perirectal spaces.

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At our institution, the protocol used consists of the follow-ing sequences: sagittal T2-weighted sequences (useful for planning subsequent slices), followed by axial T2-weighted images and 3-mm thick short inversion time inversion-recov-ery (STIR) (without gap and with a resolution of 0.6 x 0.6 mm); coronal T2-weigthed images; axial diffusion weighted-sequences (DWI); and finally axial fat-suppressed T1-weigted (FS T1) images with and without gadolinium enhancement, all oriented to the anal canal.

Appearance on magnetic resonance imaging

Fistulous tracks, inflammation and abscesses appear as areas of low signal intensity on fat-saturated T1-weighted images, and it is difficult to distinguish them from normal structures, such as sphincters and levator ani muscles. Therefore, admin-istration of intravenous contrast is helpful.Contrast enhancement determines the active status of inflam-matory disease. On gadolinium-enhanced FS T1-weighted images, fistulous tracks and active granulation tissue demon-strate intense enhancement, while fluid in the track remains hypointense. Unenhanced T1-weighted images may be help-ful in detection of hemorrhagic material or with high-protein content, which has an intermediate or high signal intensity3,6.T2-weighted images provide good contrast between the hyperintense fluid in the fistula and the hypointense fibrous wall, and allow anatomic differentiation from internal and external sphincters. Active fistulas and secondary extensions

have high signal intensity on T2-weighted images, while the sphincters and muscles have low signal intensity. Chronic fis-tulous tracks or scars appear as areas of low signal intensity, both on T1- and T2-wighted images, unenhanced or with de-layed post-contrast enhancement. Abscesses also have high signal intensity on T2-weighted images due to the presence of pus in the central cavity.Normal anorectal structures do not enhance significantly af-ter the administration of intravenous contrast, except for anal sphincters and internal blood vessels, which should not be confused with fistulous tracks of fluid collections.

Figure 2 Scheme showing how to locate the fistulous track using a clock position: 12 o’clock refers to the anterior perine-um, 6 o’clock to the natal cleft, 9 o’clock to the right anal verge and 3 o’clock to the left verge.

Figure 3 Grade 1 fistula: (a) Axial T2-weighted image shows an intersphincteric track at the 1 o’clock position (arrow), while (b) the coronal T2-weighted image in the same patient identifies the track descending through the intersphincteric space to reach the natal cleft on the left side (arrowheads).

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Location of fistulas using clock position: “the anal clock”

It is essential to adequately describe the point of origin in the anal canal and the path of the fistulous track. The “anal clock” is an analogy of axial MR images, observed by sur-geons with patients in the lithotomy position: the anterior

perineum is located at 12 o’clock and the natal cleft is at 6 o’clock, while the left lateral aspect of the anal canal is at 3 o’clock and the right lateral aspect at 9 o’clock (fig. 2). In most cases, the point of origin is at the 6 o’clock position, since anal glands are more abundant in this site3,4.

Classification by magnetic resonance imaging

Fistulas are classified according to the course of the main fistulous track from the anal canal to the skin, taking into account its relationship to anatomic structures, mainly the anal sphincter muscles, as their preservation is essential to maintain rectal continence, which especially applies to the external sphincter and puborectalis muscle.The most widely used classification system is the adaptation of Park’s surgical classification, performed by St James’s Uni-versity Hospital1,3,5,7,8. This system has been proposed by radi-ologists on the basis of imaging findings in axial and coronal planes, and it consists of five grades (table 1):Grade 1. Simple linear intersphincteric fistula: the track ex-tends from the anal canal through the intersphincteric space to reach the skin of the perineum or natal cleft, with no sec-ondary extensions or associated abscesses (fig. 3).Grade 2. Intersphincteric fistula with an abscess or secondary track: the main track and a secondary track or abscess occur in the intersphincteric space. They are always confined by the external sphincter, which is not crossed. Secondary exten-

Table 1: Saint James classification.

Grade ISimple intersphincteric fistula. The most common type, which corresponds to a straight track between the anal canal and the perineal skin, with no other associated tracks or abscesses.Grade IIIntersphincteric fistula, but with an abscess in the un-derlying fatty tissue or secondary tracksGrade IIIIntersphincteric fistula. A track that crosses the external sphincter to reach the ischiorectal fossa.Grade IVTranssphincteric fistula with an abscess or secondary track in the ischiorectal or ischioanal fossa.Grade VSupralevator and translevator disease. Complex fistula that passes through the levator ani to the supralevator space.

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Figure 4 Grade 2 fistula: (a) Axial T2-weighted image shows 2 simple fistulous tracks at the 10 o’clock and 1 o’clock positions (arrows), while (b) the coronal T2-weighted image shows a small abscess in the right ischioanal fossa, secondary to an inter-sphincteric fistula (arrowheads).


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sions and abscesses may be of the horseshoe type, cross the midline or they may ramify in the ipsilateral intersphincteric plane (fig. 4).Grade 3. Transsphincteric fistula: it pierces both layers of the sphincter complex and takes a downward course through the

ischiorectal and ischioanal fossae before reaching the perine-al skin. It is not associated with secondary tracks or abscesses (fig. 5).Grade 4. Transsphincteric fistula with an abscess or secondary track in the ischiorectal or ischioanal fossa: the track crosses

a b

Figure 5. Grade 3 fistula: (a and b) Axial STIR and T2-weighted images show a fistulous track crossing the external sphincter (transsphincteric) at the 1 o’clock position, with no secondary tracks or associated abscesses (arrows).

a b

Figure 6 Grade 4 fistula: (a) Gadolinium-enhanced axial FS T1-weighted image identifies a transsphincteric track at the 1 o’clock position (arrow) with an abscess in the ischioanal fossa (arrowheads) and (b) Axial T2-weighted image in a different patient shows transsphincteric tracks at the 11 o’clock and 6 o’clock positions, and an intersphincteric track at the 1 o’clock position (arrows).



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the external sphincter to reach the ischiorectal and ischioanal fossae, where it is complicated by an abscess or a secondary extension (fig. 6).Grade 5. Supralevator and translevator disease: fistulous dis-ease extends above the insertion of the levator ani muscle. Supralevator fistulas extend upwards through the inter-sphincteric plane, pass over the top of the levator ani and pu-borectalis muscles, and descend through the ischiorectal and ischioanal fossae to reach the skin. In translevator disease, the fistulous track extends directly from its origin in the pelvis through the perianal skin and the ischiorectal and ischioanal fossae, with no involvement of the anal canal. These fistulas indicate the existence of primary pelvic disease with exten-sion through the levator plate (fig. 7).

Conclusion

Magnetic resonance is the imaging method of choice for the evaluation of perianal fistulas. Detailed knowledge of the anatomy and an adequate study technique are required for their proper characterization. The role of radiologists is to provide surgeons with an accurate map of the type of track and its relations to regional anatomy, to allow adequate sur-gical planning.

Confidentiality of dataThe authors declare that they have followed the protocols of their work center on the publication of patient data and that all the patients included in the study have received sufficient information and have given their informed consent in writing.

Conflicts of interestThe authors declare no conflicts of interest.

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Figure 7 Grade 5 fistula: (a) Axial T2-weighted and (b) Coronal STIR images detect a track with its origin at the 6 o’clock posi-tion, passing through the levator ani muscle on the right side involving the supralevator space and descending to the natal cleft at the right parasagittal level (arrowheads).

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Perianal Fistulas: Magnetic Resonance Characterization