Perianal fistula in active setting usually T2 hyperintense, T1 hypointense and enhancing on T1 C+ MR
Old healed fibrotic fistulas usually hypointense on T1/T2WI and non-enhancing
Parks classification of perianal fistulas
– Intersphincteric fistula: Fistula traverses internal anal sphincter and extends downwards to skin surface
– Transsphincteric fistula: Fistula traverses both internal and external anal sphincters
– Extrasphincteric fistula: Fistula extends from supralevator space into ischioanal fossa without involving sphincter complex
– Suprasphincteric fistula: Fistula crosses internal sphincter, rises into supralevator space, and then crosses into ischioanal fossa
• Fistulogram probably best modality for definition of enterocutaneous fistulas
• Other fluoroscopic studies may have utility depending on location of fistulas (e.g., water soluble contrast enema for colovesical and colovaginal fistulas)
• CT: Detection of fistulas requires attention to both primary (e.g., direct fistulous tract filled with ectopic gas or contrast) and secondary (e.g., ectopic gas, abnormally tethered bowel loops) signs
CLINICAL ISSUES
• Symptoms dependent on type of fistula
Perianal fistulas most commonly present with purulent discharge or local pain and inflammation
Enterocutaneous fistulas often result in infected wound with purulent drainage
(Left) Graphic illustration of the perianal region demonstrates the 4 different types of perianal fistulas in the Parks classification and their relationship to the internal/external anal sphincters and the levator musculature.
(Right) Axial T2 FS MR demonstrates a linear T2 hyperintense fistulous track in the intersphincteric space. The track is outside the internal sphincter, but does not cross the external sphincter, compatible with an intersphincteric perianal fistula.
(Left) Axial T1 C+ FS MR demonstrates enhancement along the track of a transsphincteric perianal fistula arising from the anal canal at the 5-o’clock position. Enhancement along fistulous tracts suggests that the fistula is active, rather than chronic and healed.
(Right) Coronal T1 C+ FS MR demonstrates a transsphincteric fistula with an internal low signal seton catheter . Seton catheters, often utilized to keep fistulous tracts open and facilitate drainage, appear low signal on all pulse sequences.
TERMINOLOGY
Definitions
• Enteric fistula: Abnormal connection between bowel and another epithelial-lined surface (e.g., bladder, vagina, skin)
• Enteric sinus tract: Blind-ending tract originating from bowel
• Perianal fistula: Abnormal communication between anal canal and surrounding soft tissues or skin surface
IMAGING
General Features
• Best diagnostic clue
Presence of discrete enhancing tract connecting bowel and another epithelial-lined surface
Presence of unexpected gas, debris, or enteric contrast medium within bladder, vagina, etc.
• Morphology
Fistula tracts are usually linear, sometimes multiple
– Walls of tract are often hyperemic on CECT
Imaging Recommendations
• Best imaging tool
MR considered best modality for perianal fistulas and sinus tracts due to superior soft tissue resolution and ability to discriminate different anatomic components of perianal region
– CECT is probably better cross-sectional modality for many other locations due to superior spatial resolution
Fluoroscopic small bowel follow-through or contrast enema may be best choice for demonstrating internal (e.g., gut-to-gut) fistulas between bowel loops
Fistulogram for definition of enterocutaneous fistulas
• Protocol advice
CECT: Consider use of oral, rectal, or IV contrast media
– Not always advisable to use all at once, as this may confuse origin of opacified lumen
Radiographic Findings
• Radiography
May show ectopic gas with abscess, gas in bladder with colovesical fistula
MR Findings
• MR now considered imaging modality of choice for evaluation of perianal fistulas
Fistulous tracts in this region often not visible on CT due to limited soft tissue resolution
MR can determine relationship of fistulous tract with anal sphincters, identify exact site of opening of fistulous tract in anal canal, and differentiate fistulas which arise from distal rectum from those that arise from anal canal
MR proven to be very accurate, with sensitivity of 97% and specificity of 100% for detection of fistulas
Possible that MR at 3T may offer advantages in delineating fistulas due to superior signal-to-noise ratio and improved spatial resolution
– Pelvic surface coils often utilized, but endorectal coil typically not necessary
• MR protocol design (for perianal fistulas)
High resolution T2-weighted images through perianal region are critical to diagnosis
– Images must be appropriately oriented perpendicular and parallel to anal canal
Images oriented using localizer images through pelvis
– Typically fat saturation (either frequency-selective fat saturation or inversion recovery/STIR) included in at least 1 plane
STIR images have advantage of less susceptibility artifact from sutures
Fistulas and sinus tracts typically are T2 hyperintense and are more conspicuous with fat saturation
T2 weighted images without fat saturation helpful to better identify sphincter anatomy, although fistulous tracts may be more difficult to visualize
– High-resolution 3D T2-weighted images (i.e., T2 SPACE) are increasingly utilized to provide more precise delineation of fistulous tracts
3D images can be reconstructed into any plane due to isotropic voxel size, but at expense of longer acquisition time
– Post-contrast T1 weighted images also included, as active fistulous tracts typically show enhancement
– DWI may help in tract identification, but carries no other prognostic significance
• MR findings of perianal fistulas
Fistulous tracts are usually T2 hyperintense and T1 hypointense and originate from anal canal
– Site of origin of fistula described using clock face, which corresponds to surgeon’s view of perianal region in lithotomy position
6-o’clock is posterior, 12-o’clock is anterior, 3-o’clock is to patient’s left, and 9-o’clock is to patient’s right
Active fistulas typically show enhancement on T1 C+ images, while old, healed fistulous tracts do not
– Old, healed fibrotic fistulas usually hypointense on both T1 and T2WI
Abscesses appear as focal collections of T2 bright fluid which demonstrate peripheral enhancement on T1 C+ images
Secondary fistulous tracts or ramifications are common and can impact treatment
Seton catheters placed in fistula tracts typically appear as thin, curvilinear structures which are low signal on all pulse sequences
• Classification of perianal fistulas
Parks classification based primarily on surgical findings and secondarily applied to MR
– Intersphincteric fistula (45% of cases): Fistula traverses internal anal sphincter and extends downwards to skin surface (no involvement of external sphincter)
Fistula completely confined within intersphincteric space (between internal and external sphincters)
– Transsphincteric fistula (30% of cases): Fistula traverses both internal and external anal sphincters before extending through ischioanal fossa to skin surface
– Extrasphincteric fistula (5% of cases): Fistula extends from supralevator space across levator complex into ischioanal fossa without involving sphincter complex
Usually caused by primary pelvic inflammation (e.g., Crohn disease, pelvic abscess, rectal cancer, diverticulitis, etc.)
– Suprasphincteric fistula (20% of cases): Fistula crosses internal sphincter, rises upward into supralevator space, and then crosses over levator complex into ischioanal fossa
St. James University Hospital classification (based primarily on MR findings)
– Grade I: Simple intersphincteric fistula without secondary tracts or abscess
– Grade II: Intersphincteric fistula with abscess or secondary tract
– Grade III: Transsphincteric fistula without secondary tracts or abscess
– Grade IV: Transsphincteric fistula with secondary tracts or abscess
– Grade V: Fistula with supralevator or translevator involvement (suprasphincteric or extrasphincteric fistulas)
– Direct correlation between MR grade of fistula and clinical outcomes: Higher grades associated with worse outcomes
Sinus tracts (not described in either classification system): Superficial tracts which do not communicate with anal canal
Submucosal fistulas (not described in either classification system): Superficial tracts arising from inferior anal canal which extend to skin surface without involving internal or external sphincters
Fluoroscopic Findings
• Upper GI
Linear extravasation of water soluble medium along fistula
More useful for duodenal fistulas, but generally less useful for small bowel
May be helpful to demonstrate enteroenteric fistulas, but can be difficult to detect without high index of suspicion on part of radiologist
– Unlike fistulograms, provide global view of GI tract which may help surgical planning
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Parks classification of perianal fistulas
in the intersphincteric space. The track is outside the internal sphincter, but does not cross the external sphincter, compatible with an intersphincteric perianal fistula.
arising from the anal canal at the 5-o’clock position. Enhancement along fistulous tracts suggests that the fistula is active, rather than chronic and healed.
. Seton catheters, often utilized to keep fistulous tracts open and facilitate drainage, appear low signal on all pulse sequences.
MR considered best modality for perianal fistulas and sinus tracts due to superior soft tissue resolution and ability to discriminate different anatomic components of perianal region
MR can determine relationship of fistulous tract with anal sphincters, identify exact site of opening of fistulous tract in anal canal, and differentiate fistulas which arise from distal rectum from those that arise from anal canal
High resolution T2-weighted images through perianal region are critical to diagnosis
Active fistulas typically show enhancement on T1 C+ images, while old, healed fistulous tracts do not
Parks classification based primarily on surgical findings and secondarily applied to MR
St. James University Hospital classification (based primarily on MR findings)
May be helpful to demonstrate enteroenteric fistulas, but can be difficult to detect without high index of suspicion on part of radiologist
