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MRI of perianal fistulae: a pictorial kaleidoscope
Clinical Radiology xxx (2015) e1ee11
Contents lists available at ScienceDirect
Clinical Radiology journal homepage: www.clinicalradiologyonline.net
Pictorial Review
MRI of perianal fistulae: a pictorial kaleidoscope N. Kumar a, *, Y. Agarwal a, A. Singh Chawla b, R. Jain c, B. Bhushan Thukral a a
Department of Radiodiagnosis, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi 110029, India b Department of Surgery, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi 110029, India c Department of Radiology, Primus Super Speciality Hospital, Chandragupt Marg, Chanakyapuri, New Delhi 110021, India
art icl e i nformat ion Article history: Received 1 November 2014 Received in revised form 9 August 2015 Accepted 3 September 2015
Perianal fistulae are an abnormal communication between the anorectum and the perianal skin. A seemingly benign condition, it can be a cause of considerable distress to the patient if it is not mapped out adequately before embarking upon surgical correction. The persistence of residual disease complicates and up-stages the grade of the remnant fistula with increased risk of anal incontinence following surgery secondary to damage to the anal sphincter complex. Magnetic resonance imaging (MRI) can play a critical role in mapping the fistulae tract in relation to the anal sphincter complex and hence, act as a reliable guide for the surgeon to chart the optimised management of perianal fistulae. This review illustrates the role of MRI in the imaging evaluation of perianal fistulae, to facilitate a well-planned surgical course. Ó 2015 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction
The majority of perianal fistulous disease may be divided into two groups with distinct pathophysiology, prognosis, and strategies for imaging and treatment: perianal fistulas related to crypto-glandular disease and those related to secondary pelvic pathology. Most of the of perianal fistulae (almost 90%) are non-specific and related to the infection of the anal glands.5 In this review, we illustrate the role of MRI in depicting the perianal pelvic anatomy, correctly classifying the disease process and triage the complexity of the fistula tract by demonstrating the secondary ramifications, abscesses, and horseshoe fistulae.
Perianal fistula is an abnormal communication between the anal canal and the perianal skin. A common disease of middle-aged males with perianal discharge being the usual manifestation, it can be a cause of significant morbidity if not mapped out adequately. The treatment in most of these fistulas is surgery, which is complicated by the risk of residual/recurrent disease and of faecal incontinence secondary to the damage to anal sphincter.1e3 Magnetic resonance imaging (MRI) by highlighting the morphological features of the fistula tract in relation to the anal sphincter complex provides the operating surgeon with an adequate roadmap.4
Anatomy at MRI: perianal pelvic region
* Guarantor and correspondent: N. Kumar, Department of Radiodiagnosis, VMM College and Safdarjung Hospital, New Delhi 110029, India. Tel.: þ91 9999814147; fax: þ91 11 26198075. E-mail address: [email protected] (N. Kumar).
Knowledge of the anatomy of the anal canal and its relationship to the surrounding structures is important in the understanding of the disease process and the imaging interpretation of perianal fistulae. The anal canal is a cylindrical structure surrounded by two muscular sphincters,
http://dx.doi.org/10.1016/j.crad.2015.09.001 0009-9260/Ó 2015 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Kumar N, et al., MRI of perianal fistulae: a pictorial kaleidoscope, Clinical Radiology (2015), http://dx.doi.org/ 10.1016/j.crad.2015.09.001
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N. Kumar et al. / Clinical Radiology xxx (2015) e1ee11
Figure 1 Schematic diagram showing the anatomy of anal canal in axial/transverse (a) and coronal planes (b).
the internal and external anal sphincters, which are composed of smooth and striated muscle, respectively, with an intervening intersphincteric space (Fig 1a). Surgical importance of the sphincter complex lies in the fact that the division of internal sphincter would not cause incontinence whereas disproportionate division of external sphincter can lead to incontinence, a major challenge in fistula surgery. The two sphincters are separated by intersphincteric space, which is a potential fat-containing space with additional
extension from the rectal smooth muscle in the form of longitudinal muscle (Fig 1b). The role of longitudinal muscle in continence is limited.6,7 The epithelial lining of anal canal is columnar in the proximal half and the distal half is lined by the advancing squamous epithelium of the perineal skin. There are longitudinal folds in the proximal anal canal, which are joined side to side by anal folds. The dentate line, which demarcates the site of squamo-columnar junction and also
Figure 2 MRI anatomy of perianal region. The anal sphincter complex appears as muscular rings bounded laterally by ischio-anal fat on either side. The outer/external sphincter muscle appears hypointense on T1W (a) and T2W images (b). The inner/internal sphincter muscle appears hyperintense on T2W and (b) and fat-saturated images (c). The levator ani muscle forms the superior border of the ischio-anal fossa and merges inferiorly with puborectalis, which continues as the external sphincter muscle (d). Please cite this article in press as: Kumar N, et al., MRI of perianal fistulae: a pictorial kaleidoscope, Clinical Radiology (2015), http://dx.doi.org/ 10.1016/j.crad.2015.09.001
N. Kumar et al. / Clinical Radiology xxx (2015) e1ee11
Figure 3 Schematic anal clock represented on axial T2W image of the perineum. The anal clock system is used to describe the fistula tract in relation to the anal canal with anterior midline perineum at the 12 o’clock position and the natal cleft (NC) corresponding to the 6 o’clock position. The right and left lateral anal canal relates to the 9 o’clock and 3 o’clock positions, respectively.
delineates the inferior margin of semilunar folds in the mucosa of the mid- to upper anus, is an important landmark in the pathophysiology of perianal fistulae as it is the site of opening of the deep-seated anal glands, which initiates fistulae formation (Fig 1b). Although the dentate line is not visualised directly at MRI, it is located approximately 2 cm above the anal verge.6 The outer layer of the sphincter complex, which is formed by the external sphincter, appears hypointense on T1-weighted (W), T2W, and on T2W fat-suppressed images (Fig 2aeb). The internal sphincter appears relatively hyperintense on T2W images (Fig 2c) and enhances following contrast medium administration. The external sphincter muscle merges proximally with the puborectalis, which continues as the levator ani muscle. Coronal MRI depicts the levator plate well, and thus, is able to differentiate between the infra levator tracts from the supralevator one (Fig 2d).
Localisation and classification of fistulae: clinicoradiological perspective
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with respect to the anatomical point of origin and the path of the fistula trajectory within the perineum. Thus, the external and the enteric opening of the fistula tract is described in accordance with the “anal clock” scheme where the perineal anatomy of the lithotomy patient correlates with the axial pelvic MRI anatomy of the supine decubitus patient (Fig 3). The anterior midline perineum is at the 12 o’clock position, whereas the natal cleft corresponds to the 6 o’clock position, and the right and left lateral anal canal relates to the 9 o’clock and 3 o’clock positions, respectively. The anatomical course for most of the primary fistula is related to the location of the anal glands and thus the fistula tracts are classified according to the route taken by the tract, which links the internal and external opening. There are two main fistula classification systems: the Parks classification and the St James’s University Hospital classification.8,9 Parks et al.8 classified the perianal fistulae into four types depending on the location and course of the primary tract. More recently, Morris et al.9 presented a classification system for perianal fistulae based on MRI and classified them into five grades, relating the Parks clinical classification to the MRI anatomy.9 This system is easy to use because it utilises familiar axial anatomical landmarks and more elaborative as it considers the primary as well as the secondary tracts and abscesses in classifying the condition (Table 1). Thus, it has been shown to correlate better with surgical assessment and with favourable long-term outcome.10
Grade1: simple linear intersphincteric fistula A Grade 1 fistula tract arises from the anal canal, penetrates through the internal anal sphincter, and traverses caudally within the intersphincteric space to open in the perianal skin. The external sphincter is intact with no evidence of the disease in the ischio-anal/ischiorectal fossa and absence of secondary tracts/abscess or cranial extension above the levator plate (Fig 4).
Grade 2: complex intersphincteric fistula By definition, a fistula is an abnormal tract that connects two epithelial surfaces; therefore, it has an internal enteric opening, fistulous tract, and an external opening in the perianal region. The location of the fistula tract is detailed
A grade 2 intersphincteric fistula is one associated with abscess/secondary tract within the intersphincteric plane, which is limited by the external sphincter (Fig 5).
Table 1 Differences between the clinical and radiological classification systems for perianal fistulae. Discriminating features
Parks classification8
St. James University Hospital Fistula Classification9
1. Basis of categorisation 2.Number of classes/Grades
Clinical Four: Intersphincteric, Trans-sphincteric, Suprasphincteric and Extrasphincteric fistula External sphincter with cranio-caudal detailing of the tract Not included
Relates Parks surgical classification to MRI findings Five grades of increasing complexity
3.Anatomical reference 4.Secondary tracts and abscesses
Both internal as well as external sphincter with description of the cranio-caudal and circumferential extension of the disease process Grading is inclusive of the complex features in form of secondary tracts and abscesses
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Figure 4 Grade 1: simple intersphincteric perianal fistula. Schematic diagram (a) and STIR image (b) of the perianal region in the coronal plane shows a simple intersphincteric fistula to the right of anal canal.
Figure 5 Grade 2: complex intersphincteric perianal fistula. Schematic diagram (a) and STIR image (b) of the perianal region in the coronal plane shows a left-sided intersphincteric fistula with widening of the tract and peripheral rim enhancement (c) and intermediate signal intensity necrotic centre (asterisk in c) consistent with abscess formation. Please cite this article in press as: Kumar N, et al., MRI of perianal fistulae: a pictorial kaleidoscope, Clinical Radiology (2015), http://dx.doi.org/ 10.1016/j.crad.2015.09.001
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Figure 6 Grade 3: simple trans-sphincteric perianal fistula. Schematic diagram (a), T1W image (b), and T1W fat-saturation image (c) of the perianal region in the coronal plane shows a fistula tract traversing the internal and external sphincter to the right side of the anal canal.
Figure 7 Grade 4: complex trans-sphincteric perianal fistula. Schematic (a) and STIR (b) image of the perianal region shows a left-sided transsphincteric fistula with associated widening of the tract cranially (curved arrow). Please cite this article in press as: Kumar N, et al., MRI of perianal fistulae: a pictorial kaleidoscope, Clinical Radiology (2015), http://dx.doi.org/ 10.1016/j.crad.2015.09.001
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Figure 8 Grade 5: supra levator perianal fistula. Schematic (a), T1W (b), and STIR (c) image of the perianal region shows a right-sided supralevator fistula with abscess formation.
Grade 3: simple trans-sphincteric fistula The fistula tract originates from the anal canal, penetrates the internal and external sphincter, and traverses through the ischio-anal fossa to reach the perianal skin; however, there is no evidence of abscess/secondary tract or cranial extension above levator plate (Fig 6).
and have a cranial extension in the intersphincteric space, and then traverses caudally across the levator ani muscle descending through the ischiorectal and ischio-anal fossae before opening in the perianal skin (Fig 8). Extrasphincteric fistulas are a separate entity wherein the fistula tract arises from the pelvis secondary to the primary pelvic disease and pierces the levator plate to open into the perianal skin outside the sphincter complex (Fig 9).
Grade 4: complex trans-sphincteric fistula
MRI technique Grade 4 refers to a trans-sphincteric fistula tract with associated secondary tracts or abscess in the ischio-anal or ischiorectal fossa (Fig 7).
Grade 5: supralevator and translevator fistula Grade 5 encompasses complex fistula tracts, which traverse through the levator plate. Supralevator fistulas originate from the anal canal, penetrate the internal sphincter,
Different MRI techniques have been utilised in the imaging of perianal fistula. All provide multiplanar highresolution images of the perianal region in surgically relevant planes. Thus, the most important aspect of image acquisition is related to obtaining axial and coronal oblique images, which are orthogonal and parallel to the anal canal as the anal canal is oriented at an angle of about 45o anteriorly in the sagittal plane.6 To achieve such orthogonal
Please cite this article in press as: Kumar N, et al., MRI of perianal fistulae: a pictorial kaleidoscope, Clinical Radiology (2015), http://dx.doi.org/ 10.1016/j.crad.2015.09.001
Figure 9 Grade 5: extra sphincteric perianal fistula secondary to tubercular spondylo-discitis. Schematic (a), T1W pre- (b) and post-CE (c) image of the female pelvis shows a left-sided intermediate signal intensity tract in the ischio-anal and ischiorectal fossa outside the sphincter complex (brown arrow) with abscess formation (arrow). Sagittal CE images (d) reveal infective spondylo-discitis with presacral abscess and fistula formation (arrowhead).
Figure 10 Sagittal FSE T2W image of the pelvis showing the planning for acquisition of oblique axial (a) and oblique coronal (b) scans at an angle of 45o to the midline such that the images are orthogonal and parallel to the anal canal. Please cite this article in press as: Kumar N, et al., MRI of perianal fistulae: a pictorial kaleidoscope, Clinical Radiology (2015), http://dx.doi.org/ 10.1016/j.crad.2015.09.001
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Figure 11 Axial images of the pelvis show bilateral active trans-sphincteric fistulae. T1W image (a) shows an intermediate signal intensity tract with hyperintensity on the STIR image (b). The CE image at a slightly higher level shows horseshoeing and abscess formation (curved arrow in c) with small air foci (vertical arrow in c). The MRI findings correlated well with the post-fistulectomy surgical findings, which showed a bilateral tract with a walled-off abscess (d).
orientation of the anal canal, sagittal fast spin-echo (FSE) T2W sequences should be performed as a localiser scan, providing an overview of the pelvis and displaying the correct axis of imaging. The localiser sequence is used to derive oblique axial (Fig 10a) and coronal (Fig 10b) images
along the long axis of the anal canal and enabling appropriate assessment of perianal fistulas in the surgically relevant planes. The levator plate and the entire perineum should be included to identify areas of sepsis and infected tracks that may lead to recurrence.
Figure 12 Pelvic MRI image in a chronic healed fistula. T1W (a) axial image and T2W (b) coronal image shows a right-sided healed transsphincteric fistula with T1 and T2 hypointense signal intensity. Please cite this article in press as: Kumar N, et al., MRI of perianal fistulae: a pictorial kaleidoscope, Clinical Radiology (2015), http://dx.doi.org/ 10.1016/j.crad.2015.09.001
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Figure 13 Axial CE image of the perianal region shows characteristic horseshoeing within the posterior supralevator compartment with abscess formation (arrow).
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Imaging at our institution (Department of Radiology, VMMC and Safdarjung Hospital) was performed utilising the phased-array pelvic coil at 1.5 T (Philips Inter Achieva Systems). The most suitable protocol used at our institution (Department of Radiology, VMMC and Safdarjung Hospital) for the evaluation of perianal fistulas consists of the following sequences: T1W FSE and T2W FSE sequence in axial and coronal oblique planes followed by fat-suppressed T2W sequences, such as short tau inversion-recovery (STIR) or frequency-selective fat-saturated T2W FSE sequence. The use of contrast-enhanced (CE)-MRI (i.e., fat-suppressed T1W FSE post-gadolinium-based contrast medium) is reserved for high-grade complex fistulae/abscesses or fistulae associated with primary pelvic disease. The predominant role of CE-MRI includes (1) differentiation of active and chronic perianal fistula tracts: active tracts appear as high intensity on fat-suppressed T2W images with intense enhancement on post-contrast images,
Figure 14 Axial STIR (a) and CE image (b) of the perianal region shows a horseshoe perianal abscess (white arrow) with airefluid level (brown arrow).
Figure 15 Axial T1W fat-saturated image of the perianal region delineates hyperintense secondary tracts in the ischio-anal fossa in two different patients. Please cite this article in press as: Kumar N, et al., MRI of perianal fistulae: a pictorial kaleidoscope, Clinical Radiology (2015), http://dx.doi.org/ 10.1016/j.crad.2015.09.001
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Table 2 Reporting schema for imaging of perianal fistulae. A. THE PRIMARY TRACT I. Location and course External opening
Tick in reference to the anal clock
Course
Start at the external opening and follow course in towards the anal canal d “out-to-in”. Tick in reference to the anal clock
Internal opening II. Morphometrics Cranio-caudal length
.........cm
Total length
..........cm
Measure the vertical length of the tract from anal verge Measure the length of the tract from anal verge to the mucosal defect
III. Relationship with anal sphincter complex Type Intersphincteric: Simple Complex Trans-sphincteric Simple Complex Extrasphincteric Horseshoeing Yes No B. SECONDARY TRACT/S Location Single Multiple Number Interconnection Yes No C. ABSCESSES Location Number Single Multiple D. SUPRALEVATOR EXTENSION Primary tract Yes/No Secondary tract Yes/No E. ANCILLARY FINDINGS Enumerate findings that may indicate the root cause of the pathology. Look for imaging signs of tuberculosis, inflammatory bowel disease, diverticulitis, pelvic malignancy, and pelvic irradiation Conclusion/Impression:
whereas chronic healed tracts appear hypointense on T1/ T2W images and do not enhance following contrast medium administration; (2) differentiating abscess from inflammatory changes: abscess shows widening of the tract with central non-enhancing pus and peripheral wall enhancement, whereas inflammatory granulation tissue shows heterogeneous enhancement on CE sequences; (3) in patients with perianal Crohn’s disease: dynamic CE-MRI parameters shows significant correlation with perianal Crohn’s disease activity. Such dynamic imaging is done by acquisition of multiple pre- and post-contrast T1W fast spoiled gradient-echo sequences for a total of 6 minutes
duration with a five-section volume that is imaged 20 consecutive times with a temporal resolution of 5 seconds. Image analysis includes evaluation of the region of interest around the fistula for an area of maximum enhancement, slope of enhancement, and timeeintensity curves.
Perianal fistulae: morphological imaging characteristics T1Wimages provide the anatomical details of the sphincter complex and the levator plate whereas T2W and fat-saturation images highlight the tract against the relevant pelvic anatomical structures. Fistula tracts show low to intermediate signal intensity on T1W sequences and appear hyperintense on T2W and STIR sequences with contrast enhancement in active disease and early phases of healing (Fig 11). Chronic fibrosed fistula tracts show hypointense signal intensity on T1W and T2W images and do not enhance following contrast medium injection (Fig 12).5e7 Thus, apart from characterising the details of fistula tract, MRI is also useful in determining the response to treatment. Loss of hyperintense signal intensity on T2W imaging followed by absence of contrast enhancement is a predictable sequence of morphological changes occurring with healing of the fistula tract.7 In patients evaluated after fistula surgery, it is important to identify the postoperative findings in form of fat packing (hyperintense on T1W imaging with suppression of the signal on fat-saturation sequences), seton threads (hypointense on T1W and T2W images), and air foci (focal low signal on T1W and T2W imaging; Fig 11c). Horseshoe extensions are identified by their unique configuration when the extension occurs in the horizontal plane on either side of the midline (Fig 13). Horseshoeing may occur in the intersphincteric plane or outside the sphincter complex and may be associated with abscess and or secondary tracts. Perianal abscesses are defined as widening of the tract by >10 mm and by peripheral rim/marginal enhancement (Fig 14). Secondary ramifications are defined as fistula tracts arising from the primary tract (Fig 15). The radiological report of perianal fistulae should include the position of the tract, its internal opening with reference to the anal clock position, additional secondary tracts/ ramifications, and horseshoeing (Table 2). Providing an accurate grade to the fistula, aside from the above-mentioned morphological characteristics, which has a definitive impact on the management of the fistula tracts, should also be included in the radiological report.
Role/usefulness of MRI in imaging of perianal fistulae The role of MRI in the evaluation of perianal fistula is to relate the fistula tract with the sphincter complexelevator plate anatomically, grade the fistulae appropriately, and depict the secondary tracts and deep-seated hidden foci of sepsis in a multiplanar dimension, and thus, reduce the risk
Please cite this article in press as: Kumar N, et al., MRI of perianal fistulae: a pictorial kaleidoscope, Clinical Radiology (2015), http://dx.doi.org/ 10.1016/j.crad.2015.09.001
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of recurrent disease and likelihood of postoperative faecal incontinence. MRI has had a major impact on the preoperative assessment of perianal fistulas as treatment strategies for perianal fistula has to be individualised depending on the type of fistula and the extent of involvement of pelvic structures. Depending upon the involvement of the ischioanal and ischiorectal fossae, a decision between simple surgical procedures with favourable outcome or a complex surgery with the risk of incontinence or diversion colostomy may be taken. When the fistula tract traverses the levator plate, a translevator fistula (Grade 5) is present, and a likely source of pelvic sepsis should be sought; hence, classification of the fistulae helps to triage the surgical management and prognostication of perianal fistula disease. MRI is also useful in evaluating the results of surgical treatment in perianal fistula.11e13 Currently, there is an ongoing randomised controlled trial to compare SurgisisÒ, lyophilised porcine submucosal anal plug with the surgeon’s preferred alternative treatment (advancement flap/fistulotomy/cutting seton) for trans-sphincteric perianal fistulae. By assessing the imaging signs of healing of fistulae, MRI helps in accomplishing the objective evaluation of the trial.14
Conclusion MRI sets the trajectory of surgical management in perianal fistulae and must therefore be an essential component of the preoperative work-up. Each of the morphological characteristics that it delineates carries extremely significant important information relevant to the eventual surgical outcome. Failure to do so may predispose the patient to complicated recurrence. In addition, providing a systematic reporting schema for perianal fistulae during MRI reporting provides an accurate and effective clinicoradiological communication of the primary presentation and facilitates proper follow-up imaging, thus resulting in improved patient care.
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Acknowledgements We extend our heartiest thanks to Abhimanyu Sinha, illustrator at Hindustan times and HT media, New Delhi for all medical illustrations used in this article.
References 1. Roig JV, Jord an J, García-Armengol J, et al. Changes in anorectal morphologic and functional parameters after fistula in ano surgery. Dis Colon Rectum 2009;52:1462e9. 2. Van Koperen PJ, Wind J, Bemelman WA, et al. Long-term functional outcome and risk factors for recurrence after surgical treatment for low and high perianal fistulas of cryptoglandular origin. Dis Colon Rectum 2008;51(10):1475e81. 3. Garcia-Aguilar J, Belmonde C, Wong WD, et al. Anal fistula surgery. Dis Colon Rectum 1996;39(7):723e9. 4. Finlay IG, Lunniss PJ, Philips RKS. Objectives in management of anal fistula. Chapman and Hall; 1996. p. 78e80. 5. George U, Sahota A, Rathore S. MRI in evaluation of perianal fistula. J Med Imaging Radiat Oncol 2011;55(4):391e400. 6. de Miguel Criado J1, del Salto LG, Rivas PF, et al. MR imaging evaluation of perianal fistulas: spectrum of imaging features. RadioGraphics 2012;32(1):175e94. 7. Alaat El Essawy MT. Magnetic resonance imaging in assessment of anorectal fistulae and its role in management. J Gastroint Dig Syst 2013;139(3). http://dx.doi.org/10.4172/2161-069X.1000139. 8. Parks AG, Gordon PH, Hardcastle JD. A classification of fistula in ano. Br J Surg 1976;63:1e12. 9. Morris J, Spencer JA, Ambrose NS. MR imaging classification of perianal fistulas and its implications for patient management. RadioGraphics 2000;20(3):623e35. discussion 635e637. 10. Spencer JA, Ward J, Ambrose NS. Dynamic contrast-enhanced MR imaging of perianal fistulae. Clin Radiol 1998;53:96e104. 11. Buchanan GN, Halligan S, Bartram CI, et al. Clinical examination, endosonography and MR imaging in preoperative assessment of fistula in ano. Comparison with outcome-based reference standard. Radiology 2004;233(3):674e81. 12. Sahni VA, Ahmad R, Burling D. Which method is best for imaging perianal fistula. Abdom Imaging 2008;33(1):26e30. 13. Scholefeild JH, Berry DP, Armitage NC, et al. Magnetic resonance imaging in the management of fistula in ano. Int J Colorectal Disease 1997;12:276e9. 14. Tabry H, Farrands PA. Update on anal fistulae: surgical perspectives for the gastroenterologist. Can J Gastroenterol 2011;25(12):675e80.
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Contents lists available at ScienceDirect
Clinical Radiology journal homepage: www.clinicalradiologyonline.net
Pictorial Review
MRI of perianal fistulae: a pictorial kaleidoscope N. Kumar a, *, Y. Agarwal a, A. Singh Chawla b, R. Jain c, B. Bhushan Thukral a a
Department of Radiodiagnosis, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi 110029, India b Department of Surgery, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi 110029, India c Department of Radiology, Primus Super Speciality Hospital, Chandragupt Marg, Chanakyapuri, New Delhi 110021, India
art icl e i nformat ion Article history: Received 1 November 2014 Received in revised form 9 August 2015 Accepted 3 September 2015
Perianal fistulae are an abnormal communication between the anorectum and the perianal skin. A seemingly benign condition, it can be a cause of considerable distress to the patient if it is not mapped out adequately before embarking upon surgical correction. The persistence of residual disease complicates and up-stages the grade of the remnant fistula with increased risk of anal incontinence following surgery secondary to damage to the anal sphincter complex. Magnetic resonance imaging (MRI) can play a critical role in mapping the fistulae tract in relation to the anal sphincter complex and hence, act as a reliable guide for the surgeon to chart the optimised management of perianal fistulae. This review illustrates the role of MRI in the imaging evaluation of perianal fistulae, to facilitate a well-planned surgical course. Ó 2015 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction
The majority of perianal fistulous disease may be divided into two groups with distinct pathophysiology, prognosis, and strategies for imaging and treatment: perianal fistulas related to crypto-glandular disease and those related to secondary pelvic pathology. Most of the of perianal fistulae (almost 90%) are non-specific and related to the infection of the anal glands.5 In this review, we illustrate the role of MRI in depicting the perianal pelvic anatomy, correctly classifying the disease process and triage the complexity of the fistula tract by demonstrating the secondary ramifications, abscesses, and horseshoe fistulae.
Perianal fistula is an abnormal communication between the anal canal and the perianal skin. A common disease of middle-aged males with perianal discharge being the usual manifestation, it can be a cause of significant morbidity if not mapped out adequately. The treatment in most of these fistulas is surgery, which is complicated by the risk of residual/recurrent disease and of faecal incontinence secondary to the damage to anal sphincter.1e3 Magnetic resonance imaging (MRI) by highlighting the morphological features of the fistula tract in relation to the anal sphincter complex provides the operating surgeon with an adequate roadmap.4
Anatomy at MRI: perianal pelvic region
* Guarantor and correspondent: N. Kumar, Department of Radiodiagnosis, VMM College and Safdarjung Hospital, New Delhi 110029, India. Tel.: þ91 9999814147; fax: þ91 11 26198075. E-mail address: [email protected] (N. Kumar).
Knowledge of the anatomy of the anal canal and its relationship to the surrounding structures is important in the understanding of the disease process and the imaging interpretation of perianal fistulae. The anal canal is a cylindrical structure surrounded by two muscular sphincters,
http://dx.doi.org/10.1016/j.crad.2015.09.001 0009-9260/Ó 2015 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Kumar N, et al., MRI of perianal fistulae: a pictorial kaleidoscope, Clinical Radiology (2015), http://dx.doi.org/ 10.1016/j.crad.2015.09.001
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Figure 1 Schematic diagram showing the anatomy of anal canal in axial/transverse (a) and coronal planes (b).
the internal and external anal sphincters, which are composed of smooth and striated muscle, respectively, with an intervening intersphincteric space (Fig 1a). Surgical importance of the sphincter complex lies in the fact that the division of internal sphincter would not cause incontinence whereas disproportionate division of external sphincter can lead to incontinence, a major challenge in fistula surgery. The two sphincters are separated by intersphincteric space, which is a potential fat-containing space with additional
extension from the rectal smooth muscle in the form of longitudinal muscle (Fig 1b). The role of longitudinal muscle in continence is limited.6,7 The epithelial lining of anal canal is columnar in the proximal half and the distal half is lined by the advancing squamous epithelium of the perineal skin. There are longitudinal folds in the proximal anal canal, which are joined side to side by anal folds. The dentate line, which demarcates the site of squamo-columnar junction and also
Figure 2 MRI anatomy of perianal region. The anal sphincter complex appears as muscular rings bounded laterally by ischio-anal fat on either side. The outer/external sphincter muscle appears hypointense on T1W (a) and T2W images (b). The inner/internal sphincter muscle appears hyperintense on T2W and (b) and fat-saturated images (c). The levator ani muscle forms the superior border of the ischio-anal fossa and merges inferiorly with puborectalis, which continues as the external sphincter muscle (d). Please cite this article in press as: Kumar N, et al., MRI of perianal fistulae: a pictorial kaleidoscope, Clinical Radiology (2015), http://dx.doi.org/ 10.1016/j.crad.2015.09.001
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Figure 3 Schematic anal clock represented on axial T2W image of the perineum. The anal clock system is used to describe the fistula tract in relation to the anal canal with anterior midline perineum at the 12 o’clock position and the natal cleft (NC) corresponding to the 6 o’clock position. The right and left lateral anal canal relates to the 9 o’clock and 3 o’clock positions, respectively.
delineates the inferior margin of semilunar folds in the mucosa of the mid- to upper anus, is an important landmark in the pathophysiology of perianal fistulae as it is the site of opening of the deep-seated anal glands, which initiates fistulae formation (Fig 1b). Although the dentate line is not visualised directly at MRI, it is located approximately 2 cm above the anal verge.6 The outer layer of the sphincter complex, which is formed by the external sphincter, appears hypointense on T1-weighted (W), T2W, and on T2W fat-suppressed images (Fig 2aeb). The internal sphincter appears relatively hyperintense on T2W images (Fig 2c) and enhances following contrast medium administration. The external sphincter muscle merges proximally with the puborectalis, which continues as the levator ani muscle. Coronal MRI depicts the levator plate well, and thus, is able to differentiate between the infra levator tracts from the supralevator one (Fig 2d).
Localisation and classification of fistulae: clinicoradiological perspective
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with respect to the anatomical point of origin and the path of the fistula trajectory within the perineum. Thus, the external and the enteric opening of the fistula tract is described in accordance with the “anal clock” scheme where the perineal anatomy of the lithotomy patient correlates with the axial pelvic MRI anatomy of the supine decubitus patient (Fig 3). The anterior midline perineum is at the 12 o’clock position, whereas the natal cleft corresponds to the 6 o’clock position, and the right and left lateral anal canal relates to the 9 o’clock and 3 o’clock positions, respectively. The anatomical course for most of the primary fistula is related to the location of the anal glands and thus the fistula tracts are classified according to the route taken by the tract, which links the internal and external opening. There are two main fistula classification systems: the Parks classification and the St James’s University Hospital classification.8,9 Parks et al.8 classified the perianal fistulae into four types depending on the location and course of the primary tract. More recently, Morris et al.9 presented a classification system for perianal fistulae based on MRI and classified them into five grades, relating the Parks clinical classification to the MRI anatomy.9 This system is easy to use because it utilises familiar axial anatomical landmarks and more elaborative as it considers the primary as well as the secondary tracts and abscesses in classifying the condition (Table 1). Thus, it has been shown to correlate better with surgical assessment and with favourable long-term outcome.10
Grade1: simple linear intersphincteric fistula A Grade 1 fistula tract arises from the anal canal, penetrates through the internal anal sphincter, and traverses caudally within the intersphincteric space to open in the perianal skin. The external sphincter is intact with no evidence of the disease in the ischio-anal/ischiorectal fossa and absence of secondary tracts/abscess or cranial extension above the levator plate (Fig 4).
Grade 2: complex intersphincteric fistula By definition, a fistula is an abnormal tract that connects two epithelial surfaces; therefore, it has an internal enteric opening, fistulous tract, and an external opening in the perianal region. The location of the fistula tract is detailed
A grade 2 intersphincteric fistula is one associated with abscess/secondary tract within the intersphincteric plane, which is limited by the external sphincter (Fig 5).
Table 1 Differences between the clinical and radiological classification systems for perianal fistulae. Discriminating features
Parks classification8
St. James University Hospital Fistula Classification9
1. Basis of categorisation 2.Number of classes/Grades
Clinical Four: Intersphincteric, Trans-sphincteric, Suprasphincteric and Extrasphincteric fistula External sphincter with cranio-caudal detailing of the tract Not included
Relates Parks surgical classification to MRI findings Five grades of increasing complexity
3.Anatomical reference 4.Secondary tracts and abscesses
Both internal as well as external sphincter with description of the cranio-caudal and circumferential extension of the disease process Grading is inclusive of the complex features in form of secondary tracts and abscesses
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Figure 4 Grade 1: simple intersphincteric perianal fistula. Schematic diagram (a) and STIR image (b) of the perianal region in the coronal plane shows a simple intersphincteric fistula to the right of anal canal.
Figure 5 Grade 2: complex intersphincteric perianal fistula. Schematic diagram (a) and STIR image (b) of the perianal region in the coronal plane shows a left-sided intersphincteric fistula with widening of the tract and peripheral rim enhancement (c) and intermediate signal intensity necrotic centre (asterisk in c) consistent with abscess formation. Please cite this article in press as: Kumar N, et al., MRI of perianal fistulae: a pictorial kaleidoscope, Clinical Radiology (2015), http://dx.doi.org/ 10.1016/j.crad.2015.09.001
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Figure 6 Grade 3: simple trans-sphincteric perianal fistula. Schematic diagram (a), T1W image (b), and T1W fat-saturation image (c) of the perianal region in the coronal plane shows a fistula tract traversing the internal and external sphincter to the right side of the anal canal.
Figure 7 Grade 4: complex trans-sphincteric perianal fistula. Schematic (a) and STIR (b) image of the perianal region shows a left-sided transsphincteric fistula with associated widening of the tract cranially (curved arrow). Please cite this article in press as: Kumar N, et al., MRI of perianal fistulae: a pictorial kaleidoscope, Clinical Radiology (2015), http://dx.doi.org/ 10.1016/j.crad.2015.09.001
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Figure 8 Grade 5: supra levator perianal fistula. Schematic (a), T1W (b), and STIR (c) image of the perianal region shows a right-sided supralevator fistula with abscess formation.
Grade 3: simple trans-sphincteric fistula The fistula tract originates from the anal canal, penetrates the internal and external sphincter, and traverses through the ischio-anal fossa to reach the perianal skin; however, there is no evidence of abscess/secondary tract or cranial extension above levator plate (Fig 6).
and have a cranial extension in the intersphincteric space, and then traverses caudally across the levator ani muscle descending through the ischiorectal and ischio-anal fossae before opening in the perianal skin (Fig 8). Extrasphincteric fistulas are a separate entity wherein the fistula tract arises from the pelvis secondary to the primary pelvic disease and pierces the levator plate to open into the perianal skin outside the sphincter complex (Fig 9).
Grade 4: complex trans-sphincteric fistula
MRI technique Grade 4 refers to a trans-sphincteric fistula tract with associated secondary tracts or abscess in the ischio-anal or ischiorectal fossa (Fig 7).
Grade 5: supralevator and translevator fistula Grade 5 encompasses complex fistula tracts, which traverse through the levator plate. Supralevator fistulas originate from the anal canal, penetrate the internal sphincter,
Different MRI techniques have been utilised in the imaging of perianal fistula. All provide multiplanar highresolution images of the perianal region in surgically relevant planes. Thus, the most important aspect of image acquisition is related to obtaining axial and coronal oblique images, which are orthogonal and parallel to the anal canal as the anal canal is oriented at an angle of about 45o anteriorly in the sagittal plane.6 To achieve such orthogonal
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Figure 9 Grade 5: extra sphincteric perianal fistula secondary to tubercular spondylo-discitis. Schematic (a), T1W pre- (b) and post-CE (c) image of the female pelvis shows a left-sided intermediate signal intensity tract in the ischio-anal and ischiorectal fossa outside the sphincter complex (brown arrow) with abscess formation (arrow). Sagittal CE images (d) reveal infective spondylo-discitis with presacral abscess and fistula formation (arrowhead).
Figure 10 Sagittal FSE T2W image of the pelvis showing the planning for acquisition of oblique axial (a) and oblique coronal (b) scans at an angle of 45o to the midline such that the images are orthogonal and parallel to the anal canal. Please cite this article in press as: Kumar N, et al., MRI of perianal fistulae: a pictorial kaleidoscope, Clinical Radiology (2015), http://dx.doi.org/ 10.1016/j.crad.2015.09.001
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Figure 11 Axial images of the pelvis show bilateral active trans-sphincteric fistulae. T1W image (a) shows an intermediate signal intensity tract with hyperintensity on the STIR image (b). The CE image at a slightly higher level shows horseshoeing and abscess formation (curved arrow in c) with small air foci (vertical arrow in c). The MRI findings correlated well with the post-fistulectomy surgical findings, which showed a bilateral tract with a walled-off abscess (d).
orientation of the anal canal, sagittal fast spin-echo (FSE) T2W sequences should be performed as a localiser scan, providing an overview of the pelvis and displaying the correct axis of imaging. The localiser sequence is used to derive oblique axial (Fig 10a) and coronal (Fig 10b) images
along the long axis of the anal canal and enabling appropriate assessment of perianal fistulas in the surgically relevant planes. The levator plate and the entire perineum should be included to identify areas of sepsis and infected tracks that may lead to recurrence.
Figure 12 Pelvic MRI image in a chronic healed fistula. T1W (a) axial image and T2W (b) coronal image shows a right-sided healed transsphincteric fistula with T1 and T2 hypointense signal intensity. Please cite this article in press as: Kumar N, et al., MRI of perianal fistulae: a pictorial kaleidoscope, Clinical Radiology (2015), http://dx.doi.org/ 10.1016/j.crad.2015.09.001
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Figure 13 Axial CE image of the perianal region shows characteristic horseshoeing within the posterior supralevator compartment with abscess formation (arrow).
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Imaging at our institution (Department of Radiology, VMMC and Safdarjung Hospital) was performed utilising the phased-array pelvic coil at 1.5 T (Philips Inter Achieva Systems). The most suitable protocol used at our institution (Department of Radiology, VMMC and Safdarjung Hospital) for the evaluation of perianal fistulas consists of the following sequences: T1W FSE and T2W FSE sequence in axial and coronal oblique planes followed by fat-suppressed T2W sequences, such as short tau inversion-recovery (STIR) or frequency-selective fat-saturated T2W FSE sequence. The use of contrast-enhanced (CE)-MRI (i.e., fat-suppressed T1W FSE post-gadolinium-based contrast medium) is reserved for high-grade complex fistulae/abscesses or fistulae associated with primary pelvic disease. The predominant role of CE-MRI includes (1) differentiation of active and chronic perianal fistula tracts: active tracts appear as high intensity on fat-suppressed T2W images with intense enhancement on post-contrast images,
Figure 14 Axial STIR (a) and CE image (b) of the perianal region shows a horseshoe perianal abscess (white arrow) with airefluid level (brown arrow).
Figure 15 Axial T1W fat-saturated image of the perianal region delineates hyperintense secondary tracts in the ischio-anal fossa in two different patients. Please cite this article in press as: Kumar N, et al., MRI of perianal fistulae: a pictorial kaleidoscope, Clinical Radiology (2015), http://dx.doi.org/ 10.1016/j.crad.2015.09.001
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Table 2 Reporting schema for imaging of perianal fistulae. A. THE PRIMARY TRACT I. Location and course External opening
Tick in reference to the anal clock
Course
Start at the external opening and follow course in towards the anal canal d “out-to-in”. Tick in reference to the anal clock
Internal opening II. Morphometrics Cranio-caudal length
.........cm
Total length
..........cm
Measure the vertical length of the tract from anal verge Measure the length of the tract from anal verge to the mucosal defect
III. Relationship with anal sphincter complex Type Intersphincteric: Simple Complex Trans-sphincteric Simple Complex Extrasphincteric Horseshoeing Yes No B. SECONDARY TRACT/S Location Single Multiple Number Interconnection Yes No C. ABSCESSES Location Number Single Multiple D. SUPRALEVATOR EXTENSION Primary tract Yes/No Secondary tract Yes/No E. ANCILLARY FINDINGS Enumerate findings that may indicate the root cause of the pathology. Look for imaging signs of tuberculosis, inflammatory bowel disease, diverticulitis, pelvic malignancy, and pelvic irradiation Conclusion/Impression:
whereas chronic healed tracts appear hypointense on T1/ T2W images and do not enhance following contrast medium administration; (2) differentiating abscess from inflammatory changes: abscess shows widening of the tract with central non-enhancing pus and peripheral wall enhancement, whereas inflammatory granulation tissue shows heterogeneous enhancement on CE sequences; (3) in patients with perianal Crohn’s disease: dynamic CE-MRI parameters shows significant correlation with perianal Crohn’s disease activity. Such dynamic imaging is done by acquisition of multiple pre- and post-contrast T1W fast spoiled gradient-echo sequences for a total of 6 minutes
duration with a five-section volume that is imaged 20 consecutive times with a temporal resolution of 5 seconds. Image analysis includes evaluation of the region of interest around the fistula for an area of maximum enhancement, slope of enhancement, and timeeintensity curves.
Perianal fistulae: morphological imaging characteristics T1Wimages provide the anatomical details of the sphincter complex and the levator plate whereas T2W and fat-saturation images highlight the tract against the relevant pelvic anatomical structures. Fistula tracts show low to intermediate signal intensity on T1W sequences and appear hyperintense on T2W and STIR sequences with contrast enhancement in active disease and early phases of healing (Fig 11). Chronic fibrosed fistula tracts show hypointense signal intensity on T1W and T2W images and do not enhance following contrast medium injection (Fig 12).5e7 Thus, apart from characterising the details of fistula tract, MRI is also useful in determining the response to treatment. Loss of hyperintense signal intensity on T2W imaging followed by absence of contrast enhancement is a predictable sequence of morphological changes occurring with healing of the fistula tract.7 In patients evaluated after fistula surgery, it is important to identify the postoperative findings in form of fat packing (hyperintense on T1W imaging with suppression of the signal on fat-saturation sequences), seton threads (hypointense on T1W and T2W images), and air foci (focal low signal on T1W and T2W imaging; Fig 11c). Horseshoe extensions are identified by their unique configuration when the extension occurs in the horizontal plane on either side of the midline (Fig 13). Horseshoeing may occur in the intersphincteric plane or outside the sphincter complex and may be associated with abscess and or secondary tracts. Perianal abscesses are defined as widening of the tract by >10 mm and by peripheral rim/marginal enhancement (Fig 14). Secondary ramifications are defined as fistula tracts arising from the primary tract (Fig 15). The radiological report of perianal fistulae should include the position of the tract, its internal opening with reference to the anal clock position, additional secondary tracts/ ramifications, and horseshoeing (Table 2). Providing an accurate grade to the fistula, aside from the above-mentioned morphological characteristics, which has a definitive impact on the management of the fistula tracts, should also be included in the radiological report.
Role/usefulness of MRI in imaging of perianal fistulae The role of MRI in the evaluation of perianal fistula is to relate the fistula tract with the sphincter complexelevator plate anatomically, grade the fistulae appropriately, and depict the secondary tracts and deep-seated hidden foci of sepsis in a multiplanar dimension, and thus, reduce the risk
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of recurrent disease and likelihood of postoperative faecal incontinence. MRI has had a major impact on the preoperative assessment of perianal fistulas as treatment strategies for perianal fistula has to be individualised depending on the type of fistula and the extent of involvement of pelvic structures. Depending upon the involvement of the ischioanal and ischiorectal fossae, a decision between simple surgical procedures with favourable outcome or a complex surgery with the risk of incontinence or diversion colostomy may be taken. When the fistula tract traverses the levator plate, a translevator fistula (Grade 5) is present, and a likely source of pelvic sepsis should be sought; hence, classification of the fistulae helps to triage the surgical management and prognostication of perianal fistula disease. MRI is also useful in evaluating the results of surgical treatment in perianal fistula.11e13 Currently, there is an ongoing randomised controlled trial to compare SurgisisÒ, lyophilised porcine submucosal anal plug with the surgeon’s preferred alternative treatment (advancement flap/fistulotomy/cutting seton) for trans-sphincteric perianal fistulae. By assessing the imaging signs of healing of fistulae, MRI helps in accomplishing the objective evaluation of the trial.14
Conclusion MRI sets the trajectory of surgical management in perianal fistulae and must therefore be an essential component of the preoperative work-up. Each of the morphological characteristics that it delineates carries extremely significant important information relevant to the eventual surgical outcome. Failure to do so may predispose the patient to complicated recurrence. In addition, providing a systematic reporting schema for perianal fistulae during MRI reporting provides an accurate and effective clinicoradiological communication of the primary presentation and facilitates proper follow-up imaging, thus resulting in improved patient care.
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Acknowledgements We extend our heartiest thanks to Abhimanyu Sinha, illustrator at Hindustan times and HT media, New Delhi for all medical illustrations used in this article.
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