MRI of perianal fistulas: bridging the radiological-surgical divide

Abstract

Perianal fistula is a clinical entity with multiple surgical treatment options. Recently, magnetic resonance imaging (MRI) has emerged as an important imaging modality in the management of perianal fistulas. It provides accurate description of the fistula within the anal canal in relation to the sphincter complex and other pelvic floor structures as well as the associated complications such as abscess. By understanding the surgical viewpoint, the appearance of perianal fistulas, associated complications, and post-treatment findings of commonly used surgical interventions can more accurately be interpreted to aid clinicians. The objective of the article is to review MRI indications and findings, radiological versus surgical classification schemes, and surgical treatment options for perianal fistulas.

Fig 1

The standard anal fistula MRI imaging protocol at our institution utilizes a tri-planar scout view and defines the extent of the scan along the sagittal images. The field of view is centered along the anal canal as shown.

Fig 2

Pelvic floor and anal canal anatomy. 2A – Coronal anatomic view of the pelvis. The anal glands are the starting location for cryptoglandular disease and emerge in the anal crypts along the dentate line. The interstitial tissue between the internal and external sphincters allows access to the pelvic compartment and pararectal space above the levator ani. The puborectalis muscle forms the superior aspect of the external sphincter and is functionally part of the levator ani muscle complex (17). 2B – Axial anatomic view of the female pelvis and uppermost anal canal mechanism. Cryptoglandular disease can spread circumferentially in the interstitial tissue between the internal and external sphincter. The fat filled ischioanal fossa can also become infected once both sphincters are breached.

Fig 2

Pelvic floor and anal canal anatomy. 2A – Coronal anatomic view of the pelvis. The anal glands are the starting location for cryptoglandular disease and emerge in the anal crypts along the dentate line. The interstitial tissue between the internal and external sphincters allows access to the pelvic compartment and pararectal space above the levator ani. The puborectalis muscle forms the superior aspect of the external sphincter and is functionally part of the levator ani muscle complex (17). 2B – Axial anatomic view of the female pelvis and uppermost anal canal mechanism. Cryptoglandular disease can spread circumferentially in the interstitial tissue between the internal and external sphincter. The fat filled ischioanal fossa can also become infected once both sphincters are breached.

Fig 3

Anatomic drawings of the various types of perianal fistulae using both the St. James and Parks classification systems. 3A – Coronal anatomic view showing different fistulae in green. The tract labeled A represents a Grade 1 simple intersphincteric fistula in the St. James University Hospital (SJUH) classification system. The tract labeled B represents a Grade 2 or complex intersphincteric fistula. Both would be considered intersphincteric fistula in the Parks classification system. 3B – Axial anatomic view of the female pelvis with an example of a horseshoe abscess interposed between the internal and external sphincter mechanisms. Intersphincteric abscesses can be associated with complex intersphincteric fistulae (grade 2). This can also be seen in association with complex transphincteric fistulae (grade 4), and fistulae with supralevator disease (grade 5).

Fig 3

Anatomic drawings of the various types of perianal fistulae using both the St. James and Parks classification systems. 3A – Coronal anatomic view showing different fistulae in green. The tract labeled A represents a Grade 1 simple intersphincteric fistula in the St. James University Hospital (SJUH) classification system. The tract labeled B represents a Grade 2 or complex intersphincteric fistula. Both would be considered intersphincteric fistula in the Parks classification system. 3B – Axial anatomic view of the female pelvis with an example of a horseshoe abscess interposed between the internal and external sphincter mechanisms. Intersphincteric abscesses can be associated with complex intersphincteric fistulae (grade 2). This can also be seen in association with complex transphincteric fistulae (grade 4), and fistulae with supralevator disease (grade 5).

Fig 4

49 year-old male with Grade 1 perianal fistula due to prior perirectal abscesses. 4A – Axial T2W fat suppressed view (TR/TE in milliseconds (ms); 5542/80 ms) of the anal canal at the level of the penis base demonstrating the origin of a Grade 1 (simple intersphincteric) fistula (arrowhead) exiting the internal sphincter. 4B – Coronal T2W fat suppressed view (3347/100 ms) of the anal canal demonstrating the tract of a Grade 1 (simple intersphincteric) fistula (arrowheads) interposed between the internal (single arrow) and external (two arrows) sphincter mechanism.

Fig 4

49 year-old male with Grade 1 perianal fistula due to prior perirectal abscesses. 4A – Axial T2W fat suppressed view (TR/TE in milliseconds (ms); 5542/80 ms) of the anal canal at the level of the penis base demonstrating the origin of a Grade 1 (simple intersphincteric) fistula (arrowhead) exiting the internal sphincter. 4B – Coronal T2W fat suppressed view (3347/100 ms) of the anal canal demonstrating the tract of a Grade 1 (simple intersphincteric) fistula (arrowheads) interposed between the internal (single arrow) and external (two arrows) sphincter mechanism.

Fig 5

26 year-old female with Crohn’s disease and horseshoe pattern Grade 2 fistula Axial T2W high resolution view (5200/80 ms) of the lower pelvis reveals a fistulous track between in the internal and external (arrow) sphincters in a “horseshoe” pattern extending from the 3:30 to 9:30 clock positions (arrowheads).

Fig 6

18 year-old male with ileocolonic Crohn’s disease and Grade 2 (complex intersphincteric) perianal fistula. Coronal fat suppressed T2WI (6750/102 ms) of the anal canal. The inferior extent of a Grade 2 (complex intersphincteric) fistula and abscess is seen (arrowhead) as is the perianal abscess (asterisk).

Fig 7

Coronal anatomic view showing different fistulae in green. The tract labeled C represents a Grade 3 (SJUH) simple transphincteric fistula. The tract labeled D represents a Grade 4 or complex transphincteric fistula. Both would be considered transphincteric fistula in the Parks classification system.

Fig 8

49 year-old male with prior perirectal abscess presenting with a Grade 3 (simple transphincteric) perianal fistula. The fistula would also be a transphincteric fistula in the Parks’ classification system. Axial T2W fat saturated image (4250/107) of the lower pelvis shows a simple transphincteric fistula arising from the right aspect of the superior anal canal (arrow) violating the external sphincter (arrowheads). Prostate (*).

Fig 9

25 year-old woman with Grade 4 (complex transphincteric) perianal fistula due to prior perirectal abscess. Post contrast T1W GRE MRI image (6.35/1.53 ms; flip angle 13°) through the lower pelvis and anal canal with rim enhancing abscess (arrow) with a transsphincteric grade 4 fistula penetrating the external sphincter on the left (arrowhead). The vagina (asterisk) is seen anterior to the rectum.

Fig 10

Coronal anatomic view showing different fistulae in green. The tract labeled E represents a suprasphincteric fistula in the Parks classification system. The tract labeled F represents an extrasphincteric fistula. Both would be considered Grade 5 fistula in the SJUH classification system due to the presence of supralevator disease. The tract labeled G is a superficial fistula which does not violate any of the sphincter muscles and is part of a modified Parks classification system.

Fig 11

45 year-old man with Grade 5 perianal fistula (suprasphincteric in Parks’ classification system) due to prior perirectal abscess. T2W fat suppressed coronal MRI (5870/84 ms) through the lower pelvis and anal canal with hyperintense fluid collection and edema above (arrow) and below (asterisk) the levator plate (arrowheads). The supralevator component classifies the fistula as a Grade 5.

Fig 12

26 year-old female with Crohn’s disease and multiple fistulae, including a Grade 4 (complex transphincteric) and Grade 5 (suprasphincteric) perianal fistula. 12A – Coronal 3D turbo spin echo view (1874/200 ms) of the lower pelvis with no fat saturation reveals a well-defined fistulous tract and a collection in the ischioanal fossa (double asterisk) outside of the external sphincter (arrows) on the right side and a small abscess (asterisk) beneath the levator plate (arrowheads) on the left qualifying as Grade 4 fistulae. 12B – Coronal fat suppressed T2WI of the lower pelvis (5457/100 ms; 40 mm slab MIP) demonstrating the extent of the complex perianal fistula and abscess. There is extensive disease with supralevator extension (asterisks) above the levator plate (dotted lines) making this Grade 5 disease. Transphincteric setons are seen on both sides (arrows). Also note the improved visualization of the fistulous tracks and the collections on the fat saturated images.

Fig 12

26 year-old female with Crohn’s disease and multiple fistulae, including a Grade 4 (complex transphincteric) and Grade 5 (suprasphincteric) perianal fistula. 12A – Coronal 3D turbo spin echo view (1874/200 ms) of the lower pelvis with no fat saturation reveals a well-defined fistulous tract and a collection in the ischioanal fossa (double asterisk) outside of the external sphincter (arrows) on the right side and a small abscess (asterisk) beneath the levator plate (arrowheads) on the left qualifying as Grade 4 fistulae. 12B – Coronal fat suppressed T2WI of the lower pelvis (5457/100 ms; 40 mm slab MIP) demonstrating the extent of the complex perianal fistula and abscess. There is extensive disease with supralevator extension (asterisks) above the levator plate (dotted lines) making this Grade 5 disease. Transphincteric setons are seen on both sides (arrows). Also note the improved visualization of the fistulous tracks and the collections on the fat saturated images.

Fig 13

Seton placement through a Grade 3 (SJUH) simple transphincteric fistula. 13A – Coronal anatomic drawing demonstrating seton placement through a Grade 3 (SJUH) simple transphincteric fistula 13B – Coronal post contrast T1W GRE MRI (5.76/2.83 ms; flip angle 12°) images demonstrate a seton (white arrow) through a Grade 3 (SJUH) simple transphincteric fistula. External sphincter (arrowhead).

Fig 13

Seton placement through a Grade 3 (SJUH) simple transphincteric fistula. 13A – Coronal anatomic drawing demonstrating seton placement through a Grade 3 (SJUH) simple transphincteric fistula 13B – Coronal post contrast T1W GRE MRI (5.76/2.83 ms; flip angle 12°) images demonstrate a seton (white arrow) through a Grade 3 (SJUH) simple transphincteric fistula. External sphincter (arrowhead).

Fig 14

Diagrammatic representation of endorectal advancement flap for repair of an anal fistula. 14A – Internal opening of the fistula is identified using a probe. 14B – The fistula track is cored out. A flap of tissue is then incised around the internal opening of the fistula. 14C – The flap of tissue is then sutured to cover the resected internal opening.

Fig 14

Diagrammatic representation of endorectal advancement flap for repair of an anal fistula. 14A – Internal opening of the fistula is identified using a probe. 14B – The fistula track is cored out. A flap of tissue is then incised around the internal opening of the fistula. 14C – The flap of tissue is then sutured to cover the resected internal opening.

Fig 14

Diagrammatic representation of endorectal advancement flap for repair of an anal fistula. 14A – Internal opening of the fistula is identified using a probe. 14B – The fistula track is cored out. A flap of tissue is then incised around the internal opening of the fistula. 14C – The flap of tissue is then sutured to cover the resected internal opening.