Perianal fistula in active setting usually T2 hyperintense, T1 hypointense and enhancing on T1 C+ MR

– Intersphincteric fistula: Fistula traverses internal anal sphincter and extends downwards to skin surface
• 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







IMAGING
General Features
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

MR Findings
• MR now considered imaging modality of choice for evaluation of perianal fistulas
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 protocol design (for perianal fistulas)
High resolution T2-weighted images through perianal region are critical to diagnosis

– Typically fat saturation (either frequency-selective fat saturation or inversion recovery/STIR) included in at least 1 plane
– High-resolution 3D T2-weighted images (i.e., T2 SPACE) are increasingly utilized to provide more precise delineation of fistulous tracts
• MR findings of perianal fistulas
Active fistulas typically show enhancement on T1 C+ images, while old, healed fistulous tracts do not

• Classification of perianal fistulas
Parks classification based primarily on surgical findings and secondarily applied to MR
St. James University Hospital classification (based primarily on MR findings)

– Intersphincteric fistula (45% of cases): Fistula traverses internal anal sphincter and extends downwards to skin surface (no involvement of external sphincter)
– Transsphincteric fistula (30% of cases): Fistula traverses both internal and external anal sphincters before extending through ischioanal fossa to skin surface

Fluoroscopic Findings
• Upper GI
CT Findings
• Gut-to-gut fistula (e.g., enteroenteric, enterocolic, colocolic)
• Enterocutaneous fistula
• Colovesical fistula to thick-walled bladder
• Diverticulitis with fistula
• Pancreatitis with fistula
PATHOLOGY
General Features
• Etiology
Surgical causes

– Probably represents most common cause; usually result from bowel injury during surgery, including inadvertant enterotomy, anastomotic leak, or erosion of foreign body into bowel
CLINICAL ISSUES
Presentation
• Most common signs/symptoms
Enterocutaneous fistulas often present with fever, abdominal pain, distension, and drainage from wound (with signs of wound infection)
Other symptoms include sepsis, pneumaturia (colovesical fistula), feces per vagina (rectovaginal fistula), diarrhea (due to fistulization to distal colon), bowel obstruction (due to inflammation from fistula), or gastrointestinal bleeding


Treatment
• Fluid support, correction of electrolyte abnormalities, and antibiotics to treat underlying infection
• Enterocutaneous fistulas may close spontaneously with bowel rest, IV fluids, parenteral nutrition, ± somatostatin
• Internal gut-to-gut fistulas involving small or large bowel loops do not typically close spontaneously and often require surgical correction
• Perianal fistulas in patients with idiopathic fistulas usually surgically managed
Preoperative knowledge of fistula’s relationship with sphincter complex is critical for preservation of continence

































































