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Year : 2018  |  Volume : 31  |  Issue : 2  |  Page : 494-501

Role of magnetic resonance imaging in the diagnosis of perianal fistulae

Department of Radiodiagnosis; Department of Surgery, Menoufia University Hospitals, Shibin El Kom, Egypt

Date of Submission16-Jul-2016
Date of Acceptance30-Dec-2016
Date of Web Publication27-Aug-2018

Correspondence Address:
Mohamed E.M. Bakr
8 Almadares Street, Al-Santa, Gharbia 31696
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/mmj.mmj_386_16

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The aim of this study was to clarify the role of MRI in the diagnosis and classification of perianal fistula and to evaluate the additional clinical value of preoperative MRI and its benefit to the surgeon.
An anorectal abscess originates from an infection arising in the cryptoglandular epithelium lining of the anal canal that can penetrate through the internal sphincter. Extension of the infection can involve the intersphincteric space, ischiorectal space, or even the supralevator space.
Patient and methods
This prospective study included 20 (15 men and five women; age range: 19–59 years; mean age: 36.85 years) patients selected from 34 patients referred to the Radiodiagnosis Department with clinically suspected perianal fistula. The study was conducted between March 2013 and November 2015 after preoperative clinical examination. Pulse sequences used were STIR, T2WI, T1WI, and T1WI after contrast with fat suppression sequences in axial, coronal, and, sometimes, sagittal planes.
MRI revealed six (30%) patients with grade 1 simple linear intersphincteric fistula, five (25%) with grade 2 intersphincteric fistula with abscess or secondary track, three (15%) with grade 3 trans-sphincteric fistula, four (20%) with grade 4 trans-sphincteric fistula with abscess or secondary track, and two (10%) with grade 5 supralevator and translevator disease. One of the two supralevator fistulae showed horseshoe extension. MRI findings were concordant with the surgical findings in 18 (90%) patients (P = 0.003). Fallacies of MRI were observed in two (10%) patients, which proved to be sinus at surgery.
Our results support that MRI is the imaging technique of choice for preoperative evaluation of perianal fistulae, providing a highly accurate, rapid, and noninvasive means of performing presurgical assessment as it displays the anatomy of the sphincter muscles orthogonally with a background of good contrast resolution.

Keywords: intersphincteric, magnetic resonance imaging, perianal fistulae, trans-sphincteric

How to cite this article:
Elzawawi MS, Abdullah MS, Bakr ME. Role of magnetic resonance imaging in the diagnosis of perianal fistulae. Menoufia Med J 2018;31:494-501

How to cite this URL:
Elzawawi MS, Abdullah MS, Bakr ME. Role of magnetic resonance imaging in the diagnosis of perianal fistulae. Menoufia Med J [serial online] 2018 [cited 2020 Sep 23];31:494-501. Available from: http://www.mmj.eg.net/text.asp?2018/31/2/494/239729

  Introduction Top

Fistula-in-ano is a benign condition but may cause considerable distress to the patient and difficulty for the surgeon. Fistulae are intimately related to the anal–sphincter complex, so that the incision and drainage may damage those muscles to a variable degree with the risk for anal incontinence. The correct balance between the eradication of infection and maintenance of continence depends upon accurate preoperative assessment of fistula geographically, namely, the site and level of any internal opening, the anatomy of primary track, and the presence of any secondary ramifications [1].

Although imaging techniques played a limited role in evaluation of perianal fistulae in the past, it is now increasingly recognized that imaging techniques, especially MRI, may play a crucial role. MRI allows identification of infected tracks and abscesses that would otherwise remain undetected. Furthermore, radiologists can provide detailed anatomic descriptions of the relationship between the fistula and the anal–sphincter complex, thereby allowing surgeons to choose the best surgical treatment significantly reducing the recurrence of the disease or possible secondary effects of surgery, such as fecal incontinence [2],[3].

Imaging of perianal fistulae relies on the inherent high soft tissue. Despite the fact that anal fistulae are very common and have been studied extensively, some complex forms still continue to pose a difficult surgical problem [4],[5].

Conventional radiography and proctosigmoidoscopy are often insufficient to demonstrate the extraintestinal extent of the disease caused by the transmuraly penetrating nature of the inflammatory process. These fistulae have been traditionally imaged by conventional fistulograms; the procedure involves cannulation of the external opening and injection of a water-soluble contrast into the fistula. This method has two main disadvantages: first, the primary tract and its extensions do not fill with contrast if they are plugged by pus or debris, and, second, the sphincter muscle anatomy is not imaged and hence the relation between the tract, the internal/external sphincter, and the levator ani muscle is not revealed [6].

The classification described the course and relationship of perianal fistulae to the sphincter mechanism with reference to the coronal plane as intersphincteric, trans-sphincteric, suprasphincteric, and extrasphincteric types. In the more detail anatomical classification, the position of fistulae is used ('clockwise') in respect to the clock hands to avoid any misinterpretation [6].

Imaging of perianal fistulae relies on the inherent high soft tissue. Despite the fact that anal fistulae are very common and have been studied extensively, some complex forms still continue to pose a difficult surgical problem [4],[5].

MRI has been shown to demonstrate accurately the anatomy of the perianal region. In addition to showing the anal–sphincter mechanism, MRI clearly shows the relationship of fistulae to the pelvic diaphragm (levator plate) and the ischiorectal fossae. This relationship has important implications for surgical management and outcome [7].

The limitation of this study is the difficulty of alignment of MRI with the anal canal axis, especially in severe sepsis and in severe anal stenosis in Crohn's disease, and another limitation of MRI is that it is unable to identify the dentate line. The use of MRI also adds a considerable cost component to the patient's care. In addition, some patients have relative or absolute contraindications to MRI, including those with specific implanted devices such as pacemakers and certain cerebral aneurysm clips. Imaging will also be of limited utility for patients who cannot receive intravenous contrast due to renal insufficiency [8].

The aim of treatment for an anal fistula is to permanently eliminate abscess formation and achieve healing while preserving anal function and continence. Over-aggressive fistulotomy can lead to postoperative fecal incontinence, whereas inappropriate conservative treatment could lead to fistula recurrence. Therefore, accurate preoperative assessment of a fistula is necessary for optimal surgical results [9].

  Patients and Methods Top

The present study was conducted on 20 (15 men and five women; age range: 19–59 years; mean age: 36.85 years) patients with perianal sepsis in the period between March 2013 and November 2015. The patients gave an informed consent and the study was approved by the ethical committee of faculty of Medicine Menoufia University. The patients were selected from 36 patients referred to the Radiodiagnosis and Imaging Department. They had clinical manifestation of suspected perianal sepsis and a visible external opening. Sixteen patients were ruled out as they had mild skin infection or small abscesses.

MRI protocol

The examination was done after explanation of the procedure to the patients and ensuring that there were no contraindications to the study like cardiac pacemakers and metal implants. Examination was done for all patients in the MR Unit, Radiodiagnosis and Imaging Department, Menoufia University Hospitals, using 1.5T system (Excelart Vantage; Toshiba, Irvine, California, USA), using a body coil.

All patients were examined in the supine position. Saline was injected through the external opening of the fistulous tract in 13 patients, in four patients the external opening could not be cannulated because of fibrosis, and three patients refused the injection for fear of pain.

Initial three-plane images were obtained. On the sagittal images the axial cuts of the T1-weighted fast-spin echo, T2-weighted fast-spin echo, and STIR images were planed parallel to the pelvic diaphragm. Then on the same sagittal plane the coronal cuts were planed parallel to the anal canal tilted forward from the vertical by ∼45°. T1-weighted fast-spin echo with fat suppression (FOV 300 mm and matrix 256 × 512) and T2-weighted fast-spin echo with fat suppression (FOV 300 mm, matrix 256 × 512 and TR/TE 2500/150) were obtained in some complex and recurrent cases. After obtaining the axial and coronal images, contrast material (gadolinium-DTPA) (Magnevist, Bayer, Leverkusen, Germany) was injected intravenouslly in a dose of 0.1–0.2 mmol/kg in recurrent cases to get better details.

MRI findings from each sequence were reviewed for the detection of internal opening, extension of fistulous track, presence of scars, side branches, or complicated abscess. Anatomic overviews of the sphincter complex, levator plate, and ischiorectal fossa were obtained. On T2WI and STIR images, pathologic processes, including fistulous tracks and fluid collections, appeared as areas of high-signal intensity in contrast to sphincters, muscles, and fat. Location of the internal opening was determined in axial images with respect to the clock face with 12 o'clock being directed anterior.

Perianal fistulae were classified according to the St James's University Hospital Classification into the following:

Grade 1: Simple linear intersphincteric fistula.

Grade 2: Intersphincteric fistula with abscess or secondary track.

Grade 3: Trans-sphincteric fistula.

Grade 4: Trans-sphincteric fistula with abscess or secondary track within the ischiorectal fossa.

Grade 5: Supralevator and translevator disease.

Our results were confirmed and correlated with surgical findings.

Statistical analysis

Surgical findings served as the diagnostic standard of reference in all cases. We determined the sensitivity (how accurate the test is in positive cases), specificity (how accurate the test is in negative cases), overall accuracy of the test, and positive predictive value and negative predictive value (how accurate the test is when it gives a negative result).

The proportion of surgically confirmed perianal fistula was defined as true positive and the proportion of surgically confirmed absence of perianal fistula was defined as true negative [Table 1].
Table 1: Comparison of MRI findings with surgical findings taken as the gold standard

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All statistical analyses were carried out using the statistical package for the social sciences (SPSS), v. 22.0 (IBM Corp., Armonk, New York, USA).

  Results Top

This study included 20 cases with perianal fistula – five (25%) women and 15 (75%) men. The youngest patient included in our study was 19 years of age and the oldest was 59 years of age, with a mean age of 36.85 + 11.10 years. Primary fistulae were recorded in eight cases and 12 cases were recurrent. MRI findings were as follows: six (30%) patients with grade 1 simple linear intersphincteric fistula [Figure 1], five (25%) with grade 2 intersphincteric fistula with abscess or secondary track [Figure 2], three (15%) with grade 3 trans-sphincteric fistula [Figure 3], four (20%) with grade 4 trans-sphincteric fistula with abscess or secondary track within the ischiorectal fossa [Figure 4], and two (10%) with grade 5 supralevator and translevator disease [Figure 5]. One of the two supralevator fistulae showed a horseshoe extension [Figure 6].
Figure 1: Grade 1 short nonbranching fistulous tract at the left perianal region (curved arrow) extending for about 1 cm in the intersphincteric space (arrow) to end at the anterolateral wall of the anal canal (arrowhead). (a and b) Coronal STIR images. (c and d) Axial T2 FSE images.

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Figure 2: Grade 2 linear nonbranching left intersphincteric fistulous track starting at the perianal skin at 3 o'clock, passing upward, anteriorly and medially, in the intersphincteric space (arrow) to end at 2 o'clock location of the anal canal (arrowhead). Small intersphincteric abscess and signs of tissue inflammation and active sepsis could be seen (asterisk). (a and b) Coronal T2 fat saturation images. (c and d) Axial T1 postcontrast images.

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Figure 3: Grade 3 nonbranching thick fistulous tract (arrow) with its external opening at the medial aspect of the left gluteal region (curved arrow) passing superiorly to the ischiorectal fossa, then, curved medially, piercing the external and internal anal sphincters to end in the anal canal at its left posterolateral wall, just below the level of the left levator ani muscles (arrowhead). (a and b) Coronal STIR images. (c and d) Axial T2 FSE images.

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Figure 4: High trans-sphinctericgrade 4 fistulous tract. (arrow) extending from the left side of the perianal skin to the ischioanal fossa, passing anteriorly, superiorly, and, arching medially, penetrating both external and internal anal sphincters ending at posterolateral wall of the anal canal. (arrowhead). There is a small intersphincteric abscess measuring 1. cm in all diameters. (asterisk). (a and b) Coronal STIR images. (c and d) Axial STIR images.

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Figure 5: Left-sided grade 5 perianal fistula is seen starting at 5 o'clock extending cranially for about 5.5 cm, acquiring intersphincteric location (arrow), and then supralevator extension (arrowhead). It has a small left-sided intersphincteric and perirectal cystic collection, and abscess. (asterisk) are more evident opposite 4–6 o'clock measuring 2.5 × 1.2 cm. There is more evidence of previous anal surgery with surgical suture artifact and smudged ischiorectal fat; more on the left side. (a and b) Coronal STIR images. (c and d) Axial T2 FSE images.

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Figure 6: Grade 5 perianal fistulous tract on the right side of perianal skin running cranially and forming horseshoe tract (arrow) to the left side, positioned in intersphincteric space with more superior extension on the left side just above the levator ani level with small abscess collection measuring 1 cm in all diameters (asterisk). (a and b) Coronal STIR images. (c and d) Axial T2 FSE images.

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After surgery we found that 18 patients showed fistulous tract and two showed the tract partially replaced with the fibrous tissue and not connected to the anal canal (perianal sinus). Out of 18 patients, five (27.8%) patients showed grade 1 simple linear intersphincteric fistula, five (27.8%) showed grade 2 intersphincteric fistula, three (16.8%) showed grade 3 trans-sphincteric fistula (two mid and one high trans-sphincteric), four (22.2%) showed grade 4 trans-sphincteric fistula (two mid, one high, and one low trans-sphincteric; two with abscess and two with secondary track within the ischiorectal fossa), and one (5.6%) showed grade 5 supralevator (showed a horseshoe extension) [Table 2].
Table 2: MRI findings compared with surgical assessment

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Simple linear intersphincteric fistula and intersphincteric fistula with abscess or secondary track were the most frequent findings. The correlation between MRI findings and surgical findings demonstrated that MRI correctly depicted primary tracks in 18 out of 20 cases, six out of six abscess collections, all secondary branches, and horseshoe extension.

We had 18 true positives, 0 false negatives, and two false positives of the primary track. The two false negatives were one intersphincteric and one supralevator, which were misdiagnosed at MRI, the two of them proving to be sinus at the time of surgery.

MRI findings were concordant with the surgical findings in 18 (90%) patients, which is highly significant (P = 0.003). Fallacies of MRI were observed in two (10%) patients who had a history of recurrent fistulae. Correlation between MRI findings and surgical findings are shown in [Table 2].

Analytical statistics including sensitivity, specificity, and positive and negative predictive values are shown in [Table 3].
Table 3: The sensitivity, specificity, positive and negative predictive values, and overall accuracy of MRI in the detection of perianal fistula

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The comparison of sensitivity and specificity in our study with the international literature is shown in [Table 4].
Table 4: Comparison of sensitivity and specificity of our study with the international literature

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  Discussion Top

Not so long ago, surgeons performed operations on perianal fistulae without any previous radiological assessment. The surgical examination under anesthetics consisted of visual inspection and palpation with the probe of a fistulous passage under general anaesthesia. Numerous diagnostic modalities failed in the visualization and classification of perianal fistulae. The diagnostic assessment of patients with fistula-in-ano is challenging even for specialized colorectal surgeons and radiologists. Clinical examination can often be difficult, because of induration and inflammation in patients with anal sepsis. Previous fistula surgery, complexity, lack of identification of the internal fistulous opening, wrongly diagnosed primary tracks, and missed secondary tracks have all been identified as independent risk factors associated with a poor outcome after surgery [10].

Various imaging methods can be used to delineate the fistula tracks and evaluate the additional inflammatory changes. Fistulography, computed tomography, anal endosonography, and MRI can all be used but each has certain limitations [11]. MRI has become the reference standard for the anatomic evaluation of perianal and anovaginal or rectovaginal fistulas [12]. Body coils, pelvic phased array coils, and endoanal coils can be used to image and classify fistulae. Currently, MRI examinations are performed with a body coil. MRI may include various sequences and additional techniques. It is possible to visualize the whole sphincter complex and the track itself [13],[14].

In this study the peak incidence of perianal fistulae was in the third decade of life. Men were affected more frequently than women, with a men to women ratio of 3:1. These findings were consistent with the results of Halligan and Stocker [6], who found that the disease predominantly strikes young adults, and that men are more commonly affected.

We favored an MRI protocol without bowel preparation or special receiver coil to ensure noninvasiveness and comparability, although the patients were advised to keep their perianal regions clean for cannulation and fast for 6-h at least to avoid vomiting, which may be induced by contrast medium.

We injected saline in the external opening to facilitate the detection of the fistula tracks by distending the collapsed parts and expelling air from the fistula, which caused a signal void that can be easily confused with abscess. But this technique was troublesome and was refused by three patients. It also depended on the existence of an external opening, which we failed to detect it four patients. These findings are consistent with the results of Myhr et al. [15] who found that direct injection of the fistulae with saline has been shown to improve their delineation.

The commonly used image matrix is 256 × 512. The slice thickness of the sequences varied between 3 and 8 mm. Gadolinium-DTPA was used as an IV contrast agent, and was injected with a standard dose of 0.1 mmol/kg. MRI protocols for 'anal fistula' consist of spin-echo, spin-echo with fat saturation, STIR, and T1WI with application of a contrast agent [3].

In our study, we easily detected the exact location of the primary tract (ischioanal or intersphincteric) on axial images; the presence of disruption of the external anal sphincter and inflammation of the ischioanal fat differentiates a trans-sphincteric fistula from an intersphincteric one. The internal opening of the fistula is also best seen in this plane and thus it was easily indicated by reference to a clock face [1],[2],[3],[4],[5],[6],[7],[8],[9],[10],[11],[12] (anal clock). In addition, additional findings, such as the presence of a horseshoe extension or inflammation, were evaluated separately. Two fibrous tracts were misclassified, one as a simple intersphincteric fistula (grade 1) and the other one as a supralevator fistula (grade 5). When the images were evaluated, it was apparent that both tracts open into the anal canal, but the surgical evaluation was unable to demonstrate any internal opening in the anal canal. The main errors of MRI compared with surgery were in over-reporting of the internal opening, with a specificity of only 90.9%. We are not alone in observing a poor outcome for cases of MR-positive, surgery-negative fistulae; Spencer et al. [16] reported a poor specifity of only 67% of internal opening reporting.

We used St James's University Hospital Classification of perianal fistula, according to which fistula can be classified into five groups and fistulae can be differentiated from abscesses with excellent illustrations of the relationship between the fistula types and the anal sphincter muscles and surrounding soft tissues [3]. This was useful regarding the optimal surgical approach. When this classification was used, the MRI grading of fistulae was significantly associated with outcome (P < 0.003). St James's University Hospital Classification of perianal fistula employs simple anatomic discriminators identifiable on the axial and coronal MRI. By using this system, we were able to alert the referring clinician to the presence of complex diseases that may require expert surgical management. The essential teaching points can be summarized as follows:

  • If the ischioanal and ischiorectal fossae are unaffected at MRI, disease is likely confined to the sphincter complex (intersphincteric fistulization, grade 1 or 2). Outcome following a simple surgical management is favorable
  • If there is any track or abscess within the ischiorectal fossa, it is usually related to a complex perianal fistula (typically trans-sphincteric fistulization, grade 3 or 4). Correspondingly, more complex surgery may be required that may threaten continence or may require fecal diversion (colostomy) to allow healing. When the track traverses the levator plate, a translevator fistula (grade 5) is present, and a source of pelvic sepsis should be sought.

In our study, STIR sequence results were well-correlated with the surgical findings for fistulous track, abscess, internal opening, and horseshoe extension evaluation. In contrast, there was a significant correlation with inflammation. This finding is probably due to the fact that MRI is able to detect inflammation with greater sensitivity, whereas inflammation is not a straightforward assessment during surgery. T1W sequences with fat saturation after intravenous gadolinium administration also help in detecting the tracks that are less conspicuous on T2W sequences and differentiate a fluid-filled tract from an area of inflammation. The tract wall enhances, whereas the central portion is hypointense. Abscesses are also very well depicted on postgadolinium images. T2W images (TSE and fat-suppressed) provide good contrast between the hyperintense fluid in the tract and the hypointense fibrous wall of the fistula, while providing good delineation of the layers of the anal sphincter [13],[17]. Coronal images depict the levator plane, thereby allowing differentiation of supralevator from infralevator infection. A combination of an axial and a longitudinal series (coronal, sagittal, or radial) will provide all the necessary details [18].

In our study the overall accuracy of MRI in relation to surgical findings reached 93.9%. In one of the early studies on MRI fistulography, Beckingham et al. [2] reported a concordance rate of 86–88% between MRI and surgical findings. Moreover, in our study, MRI accuracy for the depiction of both primary tracts (sensitivity, 100%; specificity, 90.9%) and abscesses (sensitivity, 100%; specificity, 100%) was significantly high. That was consistent with Villa et al. [19] who reported that MRI accuracy for depiction of both the primary tracts (sensitivity, 100%; specificity, 86%) and abscesses (sensitivity, 96%; specificity, 97%) was high.

Surgical exploration may be difficult in the presence of inflammatory swelling or fibrosis. A particular advantage of MRI is its ability to demonstrate occult intersphincteric space sepsis (i.e. pus is trapped within the intersphincteric space with no cutaneous exit and thus cannot be found by probing). In cases of 'high' fistulae (trans-sphincteric and extrasphincteric, grades 3–5), probing or exploration may be abandoned when anatomic landmarks are uncertain and the operator is unsure whether he or she is above or below the levator plate [14].

Studies comparing new techniques for investigations of fistula-in-ano have used surgical exploration as the gold standard investigation with which to compare [20]. We did the same in our study. But there were some limitations. Even though surgical findings have been accepted as the reference standard, it is known that surgery may miss some fistulae and accompanying pathologies. Therefore, surgery may not offer a reliable 'reference standard' for perianal fistula disease. Long-term follow-up results also would be helpful for evaluating the MRI examination findings.

In this study 13 patients were managed by fistulotomy (five patients with simple linear intersphincteric fistula, two with intersphincteric fistula with secondary track, three with trans-sphincteric fistula, two with trans-sphincteric fistula with secondary track within the ischiorectal fossa, and one with intersphincteric sinus tract), four cases by fistulotomy and drainage of abscess cavity (three patients with intersphincteric fistula with abscess and one with trans-sphincteric fistula with abscess), and three patients by fistulotomy with cutting setons and drainage of abscess (two with supralevator with abscess and one with high trans-sphincteric with abscess). The surgical treatment for this disorder involves incision of the fistula track over a metal probe placed into the track (fistulotomy or fistulectomy), seton placement, and more complex techniques [21].

  Conclusion Top

MRI evaluation of perianal fistulae can be challenging, and knowledge of relevant pelvic anatomy and fistula classification remains crucial in the diagnosis. MRI is highly accurate for fistula depiction and, by providing an accurate assessment of disease status and extension, can help surgical planning to minimize recurrence and detect clinically unapparent disease. MRI has been shown to demonstrate accurately the anatomy of the perianal region. In addition to showing the anal–sphincter mechanism, MRI clearly shows the relationship of fistulae to the pelvic diaphragm (levator plate) and the ischiorectal fossae, especially for patients with complex and recurrent fistulae.

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Conflicts of interest

There are no conflicts of interest.

  References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]

  [Table 1], [Table 2], [Table 3], [Table 4]


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