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Significance of MR imaging in setting the ball path of surgical management in perianal fistulae
a p o l l o m e d i c i n e 1 1 ( 2 0 1 4 ) 1 7 9 e1 8 3
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/apme
Original Article
Significance of MR imaging in setting the ball path of surgical management in perianal fistulae Nishith Kumar a, Yatish Agarwal b,*, Avneet Singh Chawla c, Brij Bhushan Thukral d a
Senior Resident, Department of Radiodiagnosis, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi 110029, India b Professor and Consultant, Department of Radiodiagnosis, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi 110029, India c Professor and Consultant, Department of Surgery, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi 110029, India d Consultant and Head, Department of Radiodiagnosis, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi 110029, India
article info
abstract
Article history:
Objective: To study the role of magnetic resonance imaging (MRI) in surgical management of
Received 4 August 2014
perianal fistulae.
Accepted 11 August 2014
Materials and methods: This study comprises of 30 patients: 19 with complex and 11 with
Available online 4 September 2014
recurrent perianal fistulae. Each had a DRE and pelvic MRI examination, and the imaging features were correlated with intraoperative findings. Since the position of internal
Keywords:
opening, class of fistula and presence of secondary ramifications and/or abscess dictate the
Gastrointestinal imaging
surgical management and its success, special attention was paid to these characteristics
MRI
during MR imaging. Sensitivity, specificity, positive predictive value (PPV) and negative
Perianal fistulae
predictive value (NPV) both for DRE and MR imaging were calculated with respect to these
Intersphincteric
characteristics with intraoperative data as gold standard.
Transsphincteric
Results: The sensitivity of DRE in detection of internal opening was 33.33%, and of MRI 96.67%. DRE could classify the disease accurately in 33.33%, whereas MRI was able to do so in 86.67%. DRE could detect horseshoeing in 63.63% with a NPV of 82.60%, while MRI carried a sensitivity and specificity of 100%. Conclusion: The biggest Achilles heel in perianal fistulae surgery is the risk of recurrence. Since MR imaging identify the internal fistulous opening, classify the fistulae, and delineate the secondary tracts and extensions with a high degree of sensitivity and specificity, a preoperative MRI study can be extremely useful in charting the ball path of surgical management in complex and recurrent perianal fistulae. Forewarned of possible complicating factors, surgeon can plan the surgery well, and achieve a complete eradication of the disease. Copyright © 2014, Indraprastha Medical Corporation Ltd. All rights reserved.
Abbreviations: MRI, magnetic resonance imaging; DRE, digital rectal examination. * Corresponding author. Tel.: þ91 9811681790; fax: þ91 11 26198075. E-mail addresses: [email protected], [email protected] (Y. Agarwal). http://dx.doi.org/10.1016/j.apme.2014.08.005 0976-0016/Copyright © 2014, Indraprastha Medical Corporation Ltd. All rights reserved.
180
1.
a p o l l o m e d i c i n e 1 1 ( 2 0 1 4 ) 1 7 9 e1 8 3
Introduction
Perianal fistula is an abnormal communication between the anorectum and the perianal skin. Such a communication may be associated with one or more secondary ramifications and/or abscesses. A high internal opening and/or transsphincteric location can exaggerate the complexity of a fistula. The surgical challenge lies in finding success in eradication of fistula in toto, including all its branches. The persistence of residual disease complicates and upstages the complexity of remnant fistula. The situation may be compounded if the anal sphincter complex suffers any damage. This can lead to incontinence. A mild to moderate incontinence may occur in up to 50% cases.1e3 If the level and site of internal opening, anatomy of primary tract and presence of secondary ramifications and/or abscesses can be accurately identified before the surgeon embarks upon the surgery, such complicating factors can be nullified, and a complete eradication of the disease can be achieved. A preoperative pelvic MRI can help achieve these primary goals.4
2.
Materials and methods
This prospective study comprises of 30 consecutive patients with complex or recurrent perianal fistulae. Each was suitably counseled, a written informed consent was obtained, and the findings on digital rectal examination (DRE) were recorded. This was followed by a pelvic MRI examination. A high FOV (field of view) localiser sequence was used to plan out the T1 and T2 weighted sequences followed by pre and post contrast T1 weighted fat-saturation sequence in axial and coronal oblique plane. The characteristics of perianal fistulae were recorded with regard to the site of internal fistulous opening, class of fistula,5 presence of secondary ramifications and/or abscess and horseshoeing. During the course of surgical exploration, intraoperative findings were recorded. These observations were correlated with pelvic MRI data. With intraoperative findings as gold standard, sensitivity, specificity, positive predictive value and negative predictive value both for DRE and MR imaging were calculated.
3.
Results
This study includes 19 first-time patients with complex perianal fistulae and 11 with recurrent disease. DRE could identify the external opening in all 30 patients. The internal opening was felt in 10 (33.33%); induration of the tract with a possibility of supralevator disease in 13; while in 7 the disease was thought to be extrasphincteric with a high internal opening in the rectum. DRE could also detect secondary extensions and abscesses in 8, and horseshoeing in 7 patients. On MR examination, the internal opening was visualized in 29 (96.67%) patients; while 1 was classified as perianal sinus. Abscess were found in 9, horseshoeing in 11 and secondary tracts in 18 patients. The study cohorts were classified in accordance with St. James University Classification5 (Table 1). Nine (30.0%)
Table 1 e Classification of perianal fistulaea in the study cohort (n ¼ 30). Fistula classification Grade 1 (Intersphincteric with no extensions) Grade 2 (Intersphincteric with secondary extensions) Grade 3 (Transsphincteric with no extensions) Grade 4 (Transsphincteric with secondary extensions) Grade 5 (Extrasphincteric/ Suprasphincteric) Total a
Number
Percentage
4
13.33
4
13.33
7
23.34
9
30.00
6
20
30
100
St. James University MRI Classification.5
patients had grade 4 (transsphincteric fistula with secondary extensions/abscesses) perianal fistulae; 7 (23.34%) had grade 3 (transsphincteric with no extensions) perianal fistulae; 6 (20%) had grade 5 (extrasphincteric/suprasphincteric) perianal fistulae; and 4 (13.33%) patients each had grade 1 (intersphincteric with no extensions) and grade 2 (intersphincteric with secondary extensions) perianal fistulae. While DRE could correctly grade the disease in 10/30 patients, MRI succeeded in doing so in 26/30 patients. The comparative sensitivity, therefore, was 33.33% for DRE, and 86.67% for MRI (Table 2). DRE could identify 8/9 associated abscesses with a sensitivity of 88.89%, while MRI could pick all, with a sensitivity of 100%. DRE identified horseshoeing in 7/11 patients, with a sensitivity of 63.63% and NPV of 82.60%. MRI identified all 11, with a sensitivity, specificity, PPV and NPV of 100%.The secondary tracts were felt in 8/19 patients on DRE with a detection rate of 42.11%, while MRI detected secondary tracts in 18 cases with a sensitivity of 94.74% (Table 3).
4.
Discussion
This study probes into the clinical usefulness of MRI in operative management of complex and recurrent perianal fistulae. This usefulness hinges on accurate localization of site and level of internal opening, delineating the primary tract and identifying its secondary ramifications. In this study, MRI demonstrated a high degree of accuracy in identifying each of the three characteristics. The external opening was localized on DRE in all 30 subjects. Of them, 17 (56.67%) were situated in posterior and posterolateral position. The high precedence of this location is
Table 2 e Comparative accuracy of clinical vs. MRI findings in classification of perianal fistulae. Disease characteristic Fistulae correctly classified Fistulae falsely classified Total
Clinical classification
MR imaging classification
Surgical validation
10(33.33%)
26(86.67%)
30(100%)
20(66.67%)
4(13.33%)
0
30
30
30
a p o l l o m e d i c i n e 1 1 ( 2 0 1 4 ) 1 7 9 e1 8 3
181
Table 3 e Correlation of preoperative clinical evaluation, MRI and intraoperative findings. Disease characteristics
Clinical evaluation
Internal opening Abscesses Horse shoeing Secondary extensions
10 8 7 8
(33.33%) (88.89%) (63.64%) (42.11%)
MR imaging 29 9 11 18
(96.67%) (100%) (100%) (94.74%)
Surgical data 30 9 11 19
(100%) (100%) (100%) (100%)
related to anatomy of anal glands, which open posteriorly into the anal crypts most commonly. Similar results were found in 400 subjects, with the external opening in 44.7% subjects being posterior and lateral in location.6 This evaluation of external opening on DRE is critical from the perspective of triage making use of the Goodsall's rule in pre-empting the complexity of fistula. MR imaging was far superior to DRE in defining the internal opening of perianal fistulae. The sensitivity of DRE in detection of internal opening was 33.33% and on MRI 96.67% and both had a PPV of 100%. This failure of DRE in its inability to detect the internal opening may relate to several reasons: in some patients, the internal opening was flush with the rectal mucosa, and in others, the induration and inflammation from preceding surgery made the differentiation between the granulation tissue at the internal opening and the healed scar of previous surgery difficult. Even on MR imaging, the detection of internal opening of perianal fistulae is not always easy. The opening must be inferred by following the course of fistulous tract in the intersphincteric space and the area of maximum intersphincteric sepsis. In the solitary case, where MRI failed us, the failure occurred due to confusion between possible postoperative signal alteration and active intersphincteric sepsis. Since this was a patient with recurrent disease, we mistook the intersphincteric sepsis as a postoperative tissue change. On MR imaging, the largest number (16/30; 53.34%) were transsphincteric fistulas, i.e., St. James University Hospital Classification Grade 3 and 4. These results are divergent from other studies,5,7 which report intersphincteric fistulas to be the commonest. This difference in the type of fistulae possibly relates to inclusion of far more complex recurrent perianal fistulas in the present series. The MR imaging is able to delineate the pelvic anatomy well and with high resolution (Fig. 1a and b). Due to these virtues, it is capable of demonstrating the relationship of the perianal fistula with sphincter complex and helps in accurate categorization of perianal fistula (Fig. 2). In this series, we could classify the fistulae accurately with MR imaging in 26 (86.67%) patients. Of the 4 patients where we failed, 3 were transsphincteric fistulae. We mistook them as intersphincteric. Each of them had recurrent disease, with gross distortion of perianal anatomy which interfered with the visualization of outer interface of external sphincter muscle. When we retrospectively analyzed the MR images in these patients, we found the primary fistulous tract was crisscrossing the external sphincter muscle randomly, and this could have contributed to the blemish. The results of this study show a linear increasing trend between clinical and MR imaging for their accuracy in
Fig. 1 e (a and b): Normal MR Anatomy of sphincter complex. Axial T1 weighted image (a) of perianal region shows the intermediate signal intensity internal (short arrow) and external sphincter (long arrow) muscles. The high signal intensity ischioanal fat bounds them on either side. Coronal T1 weighted image (b) shows puborectalis muscle (short arrow) which continues as external sphincter (long arrow).
classifying the disease as the sensitivity for correctly classifying the disease was 33.33% on DRE as against 86.67% on MRI. This trend is similar to that recorded by Steve Halligan et al. who reported a significant linear trend (p ¼ 0.001) in the proportion of fistula tracks (n ¼ 108) correctly classified with each modality, as follows: clinical examination, 66 (61%) patients; endosonography, 87 (81%) patients; MR imaging, 97 (90%) patients.8 A study from the St Mark's Hospital Intestinal Imaging Centre has also recently concluded that MR imaging is an optimal technique for discriminating complex from simple perianal fistula. While the sensitivity of MRI in this study was found to be 95%, that of clinical assessment was restricted to 75%.9
182
a p o l l o m e d i c i n e 1 1 ( 2 0 1 4 ) 1 7 9 e1 8 3
Fig. 2 e Relationship of the fistula tract with sphincter complex. Coronal T1 weighted MR image of perianal region shows slightly hyperintense fistula tract (white long arrow) in the right perianal region traversing the external (short colored arrow) and internal anal sphincter muscle (long colored arrow) with uninvolved levator ani muscles bilaterally (vertical arrow) consistent with right sided trans-sphincteric fistula (Grade 3).
The identification of the secondary tracts also poses great difficulty in patients with complex recurrent fistulae. Such patients tend to have secondary extensions several centimeters away from the anal canal and, to make things worse, these tracts traverse virtually in any direction (Fig. 3). In the present study, 19 patients were found to have secondary extensions at the time of surgery. Of them, 18 (95%) could be picked on MR imaging. This failure could be due to spuriously high signal in scarred (healed) tract or faulting secondary tract for adjacent vessel. The sensitivity of DRE was 42%, while that of MRI was a robust 94.74%, with 100%specificity and 91.67% NPV. Similar results have been reported by others. Spencer et al., in a study of 42 patients with perianal fistulae, found DRE failed to pick abscesses in 8 of the 22 patients, and was unable to detect complex secondary tracts in 3/6 (50%) patients with complex perianal fistula.5 Horseshoe extensions can be identified by their unique configuration when the extension occurs in horizontal plane on either side of midline (Fig. 4). In the present study, DRE identified the associated abscesses and horseshoeing well, with a detection rate of 89% (8/9) and 64% (7/11) respectively. This finding however is in contrast to findings of Halligan et al.; they could identify only 23/68 (36%) horseshoe extensions.8 In the present study, MRI identified the abscess and
Fig. 3 e Axial T1 weighted MRI of perianal region. Multiple secondary tracts (arrows) are seen on either side of anal canal in a complex trans-sphincteric fistula (Grade 4).
horseshoeing in all cases and enjoyed 100% sensitivity, specificity, PPV and NPV. The information gleaned from MR imaging in the present study had a palpable effect on the patient's surgical management. Besides the 10 (33%) internal openings identified on clinical assessment, MR imaging could pick the internal
Fig. 4 e Axial post contrast T1 weighted MR image of perianal region. There is evidence of a complex transsphincteric fistula (Grade 4) with horseshoeing across the midline posteriorly and widening of fistula tract (vertical arrow) with low signal air foci within the abscess.
a p o l l o m e d i c i n e 1 1 ( 2 0 1 4 ) 1 7 9 e1 8 3
opening in another 19 (63%) patients. If clinical evaluation could correctly classify the disease in 10 (33.33%) patients, MR imaging could do so in 16 (53.33%) more. Likewise, besides the 8 secondary tracts detected on DRE, MRI could identify 10 (52.63%) more. MR imaging is therefore an optimal modality for the evaluation of complex and recurrent perianal fistulae. It can identify the internal opening, classify the disease, and delineate the secondary tracts and extensions well. This provides an excellent roadmap to the operating surgeon, who can achieve a complete eradication of disease by excising the fistula in toto.
5.
Conclusion
A precise preoperative anatomic detailing of the fistula is essential from the standpoint of its complete eradication. This can be best achieved with MR imaging of the perianal region, particularly in such cases, where a perianal fistula is thought to be complex or the disease is recurrent.
Conflicts of interest All authors have none to declare.
183
references
n J, Garcı´a-Armengol J, Esclapez P, Solana A. 1. Roig JV, Jorda Changes in anorectal morphologic and functional parameters after fistula in ano surgery. Dis Colon Rectum. 2009;52(8):1462e1469. 2. Van Koperen PJ, Wind J, Bemelman WP, et al. Dis Colon Rectum. 2008;51(10):1475e1481. 3. Garcia-Aguilar J, Belmonde C, Wong WD, Goldberg SM, Madoff RD. Anal fistula surgery. Dis Colon Rectum. 1996;39(7):723e729. 4. Finlay IG, Lunniss PJ, Philips RKS. Objectives in Management of Anal Fistula. Chapman and Hall; 1996:78e80. 5. Spencer JA, Ward J, Ambrose NS. Dynamic contrast enhanced MR imaging of perianal fistulae. Clin Radiol. 1998;53:96e104. 6. Abdul Kawy R. Classification of anal fistulas based on clinicopathological evidence. Bull Alexandria Fac Med. 2007;43(2):1e8. 7. Parks AG, Gordon PH, Hardcastle JD. A classification of fistula in ano. Br J Surg. 1976;63:1e12. 8. Buchanan GN, Halligan S, Bartram CI, Williams AB, Tarroni D, Cohen RG. Clinical examination, endosonography and MR imaging in preoperative assessment of fistula in ano. Comparison with outcome-based reference standard. Radiology. 2004;233(3):674e681. 9. Sahni VA, Ahmad R, Burling D. Which method is best for imaging perianal fistula? Abdom Imaging. 2008;33(1):26e30.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/apme
Original Article
Significance of MR imaging in setting the ball path of surgical management in perianal fistulae Nishith Kumar a, Yatish Agarwal b,*, Avneet Singh Chawla c, Brij Bhushan Thukral d a
Senior Resident, Department of Radiodiagnosis, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi 110029, India b Professor and Consultant, Department of Radiodiagnosis, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi 110029, India c Professor and Consultant, Department of Surgery, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi 110029, India d Consultant and Head, Department of Radiodiagnosis, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi 110029, India
article info
abstract
Article history:
Objective: To study the role of magnetic resonance imaging (MRI) in surgical management of
Received 4 August 2014
perianal fistulae.
Accepted 11 August 2014
Materials and methods: This study comprises of 30 patients: 19 with complex and 11 with
Available online 4 September 2014
recurrent perianal fistulae. Each had a DRE and pelvic MRI examination, and the imaging features were correlated with intraoperative findings. Since the position of internal
Keywords:
opening, class of fistula and presence of secondary ramifications and/or abscess dictate the
Gastrointestinal imaging
surgical management and its success, special attention was paid to these characteristics
MRI
during MR imaging. Sensitivity, specificity, positive predictive value (PPV) and negative
Perianal fistulae
predictive value (NPV) both for DRE and MR imaging were calculated with respect to these
Intersphincteric
characteristics with intraoperative data as gold standard.
Transsphincteric
Results: The sensitivity of DRE in detection of internal opening was 33.33%, and of MRI 96.67%. DRE could classify the disease accurately in 33.33%, whereas MRI was able to do so in 86.67%. DRE could detect horseshoeing in 63.63% with a NPV of 82.60%, while MRI carried a sensitivity and specificity of 100%. Conclusion: The biggest Achilles heel in perianal fistulae surgery is the risk of recurrence. Since MR imaging identify the internal fistulous opening, classify the fistulae, and delineate the secondary tracts and extensions with a high degree of sensitivity and specificity, a preoperative MRI study can be extremely useful in charting the ball path of surgical management in complex and recurrent perianal fistulae. Forewarned of possible complicating factors, surgeon can plan the surgery well, and achieve a complete eradication of the disease. Copyright © 2014, Indraprastha Medical Corporation Ltd. All rights reserved.
Abbreviations: MRI, magnetic resonance imaging; DRE, digital rectal examination. * Corresponding author. Tel.: þ91 9811681790; fax: þ91 11 26198075. E-mail addresses: [email protected], [email protected] (Y. Agarwal). http://dx.doi.org/10.1016/j.apme.2014.08.005 0976-0016/Copyright © 2014, Indraprastha Medical Corporation Ltd. All rights reserved.
180
1.
a p o l l o m e d i c i n e 1 1 ( 2 0 1 4 ) 1 7 9 e1 8 3
Introduction
Perianal fistula is an abnormal communication between the anorectum and the perianal skin. Such a communication may be associated with one or more secondary ramifications and/or abscesses. A high internal opening and/or transsphincteric location can exaggerate the complexity of a fistula. The surgical challenge lies in finding success in eradication of fistula in toto, including all its branches. The persistence of residual disease complicates and upstages the complexity of remnant fistula. The situation may be compounded if the anal sphincter complex suffers any damage. This can lead to incontinence. A mild to moderate incontinence may occur in up to 50% cases.1e3 If the level and site of internal opening, anatomy of primary tract and presence of secondary ramifications and/or abscesses can be accurately identified before the surgeon embarks upon the surgery, such complicating factors can be nullified, and a complete eradication of the disease can be achieved. A preoperative pelvic MRI can help achieve these primary goals.4
2.
Materials and methods
This prospective study comprises of 30 consecutive patients with complex or recurrent perianal fistulae. Each was suitably counseled, a written informed consent was obtained, and the findings on digital rectal examination (DRE) were recorded. This was followed by a pelvic MRI examination. A high FOV (field of view) localiser sequence was used to plan out the T1 and T2 weighted sequences followed by pre and post contrast T1 weighted fat-saturation sequence in axial and coronal oblique plane. The characteristics of perianal fistulae were recorded with regard to the site of internal fistulous opening, class of fistula,5 presence of secondary ramifications and/or abscess and horseshoeing. During the course of surgical exploration, intraoperative findings were recorded. These observations were correlated with pelvic MRI data. With intraoperative findings as gold standard, sensitivity, specificity, positive predictive value and negative predictive value both for DRE and MR imaging were calculated.
3.
Results
This study includes 19 first-time patients with complex perianal fistulae and 11 with recurrent disease. DRE could identify the external opening in all 30 patients. The internal opening was felt in 10 (33.33%); induration of the tract with a possibility of supralevator disease in 13; while in 7 the disease was thought to be extrasphincteric with a high internal opening in the rectum. DRE could also detect secondary extensions and abscesses in 8, and horseshoeing in 7 patients. On MR examination, the internal opening was visualized in 29 (96.67%) patients; while 1 was classified as perianal sinus. Abscess were found in 9, horseshoeing in 11 and secondary tracts in 18 patients. The study cohorts were classified in accordance with St. James University Classification5 (Table 1). Nine (30.0%)
Table 1 e Classification of perianal fistulaea in the study cohort (n ¼ 30). Fistula classification Grade 1 (Intersphincteric with no extensions) Grade 2 (Intersphincteric with secondary extensions) Grade 3 (Transsphincteric with no extensions) Grade 4 (Transsphincteric with secondary extensions) Grade 5 (Extrasphincteric/ Suprasphincteric) Total a
Number
Percentage
4
13.33
4
13.33
7
23.34
9
30.00
6
20
30
100
St. James University MRI Classification.5
patients had grade 4 (transsphincteric fistula with secondary extensions/abscesses) perianal fistulae; 7 (23.34%) had grade 3 (transsphincteric with no extensions) perianal fistulae; 6 (20%) had grade 5 (extrasphincteric/suprasphincteric) perianal fistulae; and 4 (13.33%) patients each had grade 1 (intersphincteric with no extensions) and grade 2 (intersphincteric with secondary extensions) perianal fistulae. While DRE could correctly grade the disease in 10/30 patients, MRI succeeded in doing so in 26/30 patients. The comparative sensitivity, therefore, was 33.33% for DRE, and 86.67% for MRI (Table 2). DRE could identify 8/9 associated abscesses with a sensitivity of 88.89%, while MRI could pick all, with a sensitivity of 100%. DRE identified horseshoeing in 7/11 patients, with a sensitivity of 63.63% and NPV of 82.60%. MRI identified all 11, with a sensitivity, specificity, PPV and NPV of 100%.The secondary tracts were felt in 8/19 patients on DRE with a detection rate of 42.11%, while MRI detected secondary tracts in 18 cases with a sensitivity of 94.74% (Table 3).
4.
Discussion
This study probes into the clinical usefulness of MRI in operative management of complex and recurrent perianal fistulae. This usefulness hinges on accurate localization of site and level of internal opening, delineating the primary tract and identifying its secondary ramifications. In this study, MRI demonstrated a high degree of accuracy in identifying each of the three characteristics. The external opening was localized on DRE in all 30 subjects. Of them, 17 (56.67%) were situated in posterior and posterolateral position. The high precedence of this location is
Table 2 e Comparative accuracy of clinical vs. MRI findings in classification of perianal fistulae. Disease characteristic Fistulae correctly classified Fistulae falsely classified Total
Clinical classification
MR imaging classification
Surgical validation
10(33.33%)
26(86.67%)
30(100%)
20(66.67%)
4(13.33%)
0
30
30
30
a p o l l o m e d i c i n e 1 1 ( 2 0 1 4 ) 1 7 9 e1 8 3
181
Table 3 e Correlation of preoperative clinical evaluation, MRI and intraoperative findings. Disease characteristics
Clinical evaluation
Internal opening Abscesses Horse shoeing Secondary extensions
10 8 7 8
(33.33%) (88.89%) (63.64%) (42.11%)
MR imaging 29 9 11 18
(96.67%) (100%) (100%) (94.74%)
Surgical data 30 9 11 19
(100%) (100%) (100%) (100%)
related to anatomy of anal glands, which open posteriorly into the anal crypts most commonly. Similar results were found in 400 subjects, with the external opening in 44.7% subjects being posterior and lateral in location.6 This evaluation of external opening on DRE is critical from the perspective of triage making use of the Goodsall's rule in pre-empting the complexity of fistula. MR imaging was far superior to DRE in defining the internal opening of perianal fistulae. The sensitivity of DRE in detection of internal opening was 33.33% and on MRI 96.67% and both had a PPV of 100%. This failure of DRE in its inability to detect the internal opening may relate to several reasons: in some patients, the internal opening was flush with the rectal mucosa, and in others, the induration and inflammation from preceding surgery made the differentiation between the granulation tissue at the internal opening and the healed scar of previous surgery difficult. Even on MR imaging, the detection of internal opening of perianal fistulae is not always easy. The opening must be inferred by following the course of fistulous tract in the intersphincteric space and the area of maximum intersphincteric sepsis. In the solitary case, where MRI failed us, the failure occurred due to confusion between possible postoperative signal alteration and active intersphincteric sepsis. Since this was a patient with recurrent disease, we mistook the intersphincteric sepsis as a postoperative tissue change. On MR imaging, the largest number (16/30; 53.34%) were transsphincteric fistulas, i.e., St. James University Hospital Classification Grade 3 and 4. These results are divergent from other studies,5,7 which report intersphincteric fistulas to be the commonest. This difference in the type of fistulae possibly relates to inclusion of far more complex recurrent perianal fistulas in the present series. The MR imaging is able to delineate the pelvic anatomy well and with high resolution (Fig. 1a and b). Due to these virtues, it is capable of demonstrating the relationship of the perianal fistula with sphincter complex and helps in accurate categorization of perianal fistula (Fig. 2). In this series, we could classify the fistulae accurately with MR imaging in 26 (86.67%) patients. Of the 4 patients where we failed, 3 were transsphincteric fistulae. We mistook them as intersphincteric. Each of them had recurrent disease, with gross distortion of perianal anatomy which interfered with the visualization of outer interface of external sphincter muscle. When we retrospectively analyzed the MR images in these patients, we found the primary fistulous tract was crisscrossing the external sphincter muscle randomly, and this could have contributed to the blemish. The results of this study show a linear increasing trend between clinical and MR imaging for their accuracy in
Fig. 1 e (a and b): Normal MR Anatomy of sphincter complex. Axial T1 weighted image (a) of perianal region shows the intermediate signal intensity internal (short arrow) and external sphincter (long arrow) muscles. The high signal intensity ischioanal fat bounds them on either side. Coronal T1 weighted image (b) shows puborectalis muscle (short arrow) which continues as external sphincter (long arrow).
classifying the disease as the sensitivity for correctly classifying the disease was 33.33% on DRE as against 86.67% on MRI. This trend is similar to that recorded by Steve Halligan et al. who reported a significant linear trend (p ¼ 0.001) in the proportion of fistula tracks (n ¼ 108) correctly classified with each modality, as follows: clinical examination, 66 (61%) patients; endosonography, 87 (81%) patients; MR imaging, 97 (90%) patients.8 A study from the St Mark's Hospital Intestinal Imaging Centre has also recently concluded that MR imaging is an optimal technique for discriminating complex from simple perianal fistula. While the sensitivity of MRI in this study was found to be 95%, that of clinical assessment was restricted to 75%.9
182
a p o l l o m e d i c i n e 1 1 ( 2 0 1 4 ) 1 7 9 e1 8 3
Fig. 2 e Relationship of the fistula tract with sphincter complex. Coronal T1 weighted MR image of perianal region shows slightly hyperintense fistula tract (white long arrow) in the right perianal region traversing the external (short colored arrow) and internal anal sphincter muscle (long colored arrow) with uninvolved levator ani muscles bilaterally (vertical arrow) consistent with right sided trans-sphincteric fistula (Grade 3).
The identification of the secondary tracts also poses great difficulty in patients with complex recurrent fistulae. Such patients tend to have secondary extensions several centimeters away from the anal canal and, to make things worse, these tracts traverse virtually in any direction (Fig. 3). In the present study, 19 patients were found to have secondary extensions at the time of surgery. Of them, 18 (95%) could be picked on MR imaging. This failure could be due to spuriously high signal in scarred (healed) tract or faulting secondary tract for adjacent vessel. The sensitivity of DRE was 42%, while that of MRI was a robust 94.74%, with 100%specificity and 91.67% NPV. Similar results have been reported by others. Spencer et al., in a study of 42 patients with perianal fistulae, found DRE failed to pick abscesses in 8 of the 22 patients, and was unable to detect complex secondary tracts in 3/6 (50%) patients with complex perianal fistula.5 Horseshoe extensions can be identified by their unique configuration when the extension occurs in horizontal plane on either side of midline (Fig. 4). In the present study, DRE identified the associated abscesses and horseshoeing well, with a detection rate of 89% (8/9) and 64% (7/11) respectively. This finding however is in contrast to findings of Halligan et al.; they could identify only 23/68 (36%) horseshoe extensions.8 In the present study, MRI identified the abscess and
Fig. 3 e Axial T1 weighted MRI of perianal region. Multiple secondary tracts (arrows) are seen on either side of anal canal in a complex trans-sphincteric fistula (Grade 4).
horseshoeing in all cases and enjoyed 100% sensitivity, specificity, PPV and NPV. The information gleaned from MR imaging in the present study had a palpable effect on the patient's surgical management. Besides the 10 (33%) internal openings identified on clinical assessment, MR imaging could pick the internal
Fig. 4 e Axial post contrast T1 weighted MR image of perianal region. There is evidence of a complex transsphincteric fistula (Grade 4) with horseshoeing across the midline posteriorly and widening of fistula tract (vertical arrow) with low signal air foci within the abscess.
a p o l l o m e d i c i n e 1 1 ( 2 0 1 4 ) 1 7 9 e1 8 3
opening in another 19 (63%) patients. If clinical evaluation could correctly classify the disease in 10 (33.33%) patients, MR imaging could do so in 16 (53.33%) more. Likewise, besides the 8 secondary tracts detected on DRE, MRI could identify 10 (52.63%) more. MR imaging is therefore an optimal modality for the evaluation of complex and recurrent perianal fistulae. It can identify the internal opening, classify the disease, and delineate the secondary tracts and extensions well. This provides an excellent roadmap to the operating surgeon, who can achieve a complete eradication of disease by excising the fistula in toto.
5.
Conclusion
A precise preoperative anatomic detailing of the fistula is essential from the standpoint of its complete eradication. This can be best achieved with MR imaging of the perianal region, particularly in such cases, where a perianal fistula is thought to be complex or the disease is recurrent.
Conflicts of interest All authors have none to declare.
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