Enteric Fistulas and Sinus Tracts

Published on 19/07/2015 by admin

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

image Old healed fibrotic fistulas usually hypointense on T1/T2WI and non-enhancing
image 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

image Perianal fistulas most commonly present with purulent discharge or local pain and inflammation
image Enterocutaneous fistulas often result in infected wound with purulent drainage
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(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.

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(Right) Axial T2 FS MR demonstrates a linear T2 hyperintense fistulous track image in the intersphincteric space. The track is outside the internal sphincter, but does not cross the external sphincter, compatible with an intersphincteric perianal fistula.
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(Left) Axial T1 C+ FS MR demonstrates enhancement along the track of a transsphincteric perianal fistula image 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.

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(Right) Coronal T1 C+ FS MR demonstrates a transsphincteric fistula with an internal low signal seton catheter image. 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

image Presence of discrete enhancing tract connecting bowel and another epithelial-lined surface
image Presence of unexpected gas, debris, or enteric contrast medium within bladder, vagina, etc.
• Morphology

image Fistula tracts are usually linear, sometimes multiple

– Walls of tract are often hyperemic on CECT

Imaging Recommendations

• Best imaging tool

image 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
image Fluoroscopic small bowel follow-through or contrast enema may be best choice for demonstrating internal (e.g., gut-to-gut) fistulas between bowel loops
image Fistulogram for definition of enterocutaneous fistulas
• Protocol advice

image 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

image 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

image Fistulous tracts in this region often not visible on CT due to limited soft tissue resolution
image 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
image MR proven to be very accurate, with sensitivity of 97% and specificity of 100% for detection of fistulas
image 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)

image High resolution T2-weighted images through perianal region are critical to diagnosis

– Images must be appropriately oriented perpendicular and parallel to anal canal

image 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

image STIR images have advantage of less susceptibility artifact from sutures
image Fistulas and sinus tracts typically are T2 hyperintense and are more conspicuous with fat saturation
image 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

image 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

image 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

image 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
image 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
image Abscesses appear as focal collections of T2 bright fluid which demonstrate peripheral enhancement on T1 C+ images
image Secondary fistulous tracts or ramifications are common and can impact treatment
image Seton catheters placed in fistula tracts typically appear as thin, curvilinear structures which are low signal on all pulse sequences
• Classification of perianal fistulas

image 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)

image 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

image 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
image 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
image Sinus tracts (not described in either classification system): Superficial tracts which do not communicate with anal canal
image 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

image Linear extravasation of water soluble medium along fistula
image More useful for duodenal fistulas, but generally less useful for small bowel
image 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
– May not identify fistula if contrast moves too rapidly, patient cannot ingest sufficient contrast, or if fistula is located distally and previously opacification of loops of bowel makes it difficult to identify fistula
• Contrast enema

image Water soluble contrast enema may demonstrate colovesical and colovaginal fistulas
image Better than upper GI for demonstrating fistulas that involve colon due to increased intraluminal pressures
• Fistulagram

image Injection of contrast via catheter placed into cutaneous fistula opening

– Defines presence and location of communication to gut
– Important to depict specific site of connection, as proximal bowel involvement causes more fluid and electrolyte losses and is usually associated with greater morbidity
– Greater sensitivity for enterocutaneous fistulas compared to small bowel follow-through
– Provides only limited information about disease upstream or downstream from enterocutaneous fistula and may need to be supplemented by CT or small bowel follow-through

CT Findings

• Gut-to-gut fistula (e.g., enteroenteric, enterocolic, colocolic)

image Can be difficult to reliably identify on cross-sectional imaging, although discrete tract filled with enteric contrast or gas may sometimes be visualized
image Enteric contrast may not consistently fill fistula depending on amount of contrast ingested, speed of contrast transit through bowel, size of fistula, etc.
• Enterocutaneous fistula

image May be difficult to demonstrate with certainty on CT, particularly if discrete gas or contrast filled tract is not identified

– Administration of positive oral contrast may be helpful, but may not always fill fistulous track
image Involved bowel loops often thickened and tethered to anterior abdominal wall in close contiguity to ectopic gas within abdominal wall
• Colovesical fistula to thick-walled bladder

image Should be suspected in presence of gas, debris, or rectal contrast material within bladder even if direct track is not visible
image Bladder wall may appear focally thickened at site of contact between inflamed colon and bladder
image Most often occur at dome of bladder
• Colovaginal fistula should be suspected in presence of gas, debris, or contrast within vagina
• Diverticulitis with fistula

image Wall thickening, luminal narrowing and pericolonic inflammation (± ectopic gas) in site with multiple diverticula (usually sigmoid colon)
image Hyperemic granulation tissue along fistulous tract
image Can result in fistulas between colon and bladder, vagina, or other bowel loops
• Pancreatitis with fistula

image Most often due to necrotizing pancreatitis, with sinus tracts possible between pancreatic duct and skin, bowel, or left pleural space
image Tract connecting pancreatic or peripancreatic tissue to skin or bowel is sinus tract, not fistula
• Crohn disease with fistula

image Active Crohn disease: Mucosal hyperenhancement, submucosal edema, mesenteric fat infiltration
image Strong tendency to form fistulous tracts (e.g., to other small bowel loops, colon, skin, bladder, vagina) or sinus tracts into mesenteric fat
image Perianal fistulas and abscesses are common
image Even if direct fistulous tracts are not visible, presence of ectopic gas in mesentery with surrounding thickened, tethered loops of inflamed small bowel should raise concern for “complex fistulizing Crohn disease” (e.g., multiple complex interloop fistulas and sinus tracts)
• Duodenal fistula

image May result from peptic ulcer disease, sphincterotomy, gastric resection, or gallstone erosion

Ultrasonographic Findings

• Patients with Crohn disease and diverticulitis

image Mural thickening on grayscale sonography
image Hyperemia on color Doppler
• Enteric fistulas are often difficult to identify sonographically

DIFFERENTIAL DIAGNOSIS

 

•  

PATHOLOGY

General Features

• Etiology

image Spontaneous (nonsurgical) causes

– Diverticulitis: Most common cause of fistulization in industrialized countries
– Crohn disease: Accounts for 20-30% of all enterocutaneous fistulas
– Other causes include bowel perforation, malignancy, or infections
image 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

image Associated risk factors: Malnutrition, infection, immunosuppression, radiation therapy, or emergent surgical procedure
– Abdominoperineal or low anterior resection (LAR) for rectal cancer is especially prone to anastomotic leak and development of fistulas

image Commonly associated with fistulas to vagina (5-10% of women after LAR)
– Hysterectomy associated with increased risk of fistula when performed for endometrial or cervical carcinoma

image Radiation therapy or coexisting diverticulitis increases risk of colovaginal or rectovaginal fistula
image Radiation: Associated with rectovaginal fistula
image Perianal fistulas

– 90% are idiopathic and arise due to impaired drainage of anal glands leading to infection and subsequent development of perianal abscess or fistula

image Inadequately treated abscesses often result in development of fistula, explaining high incidence of both abscess and fistula in same patient (∼ 90% with perianal abscess go on to develop fistula)
– Other causes account for remaining 10% of cases

image Crohn disease (incidence of ∼ 25% after 20 years of disease)
image Diverticulitis
image Tuberculosis
image Trauma
image Anorectal cancer
image Radiation treatment
image Sequelae of other pelvic infections (e.g., actinomycosis, lymphogranuloma venereum, etc.)
image Childbirth
image HIV/AIDS

Gross Pathologic & Surgical Features

•  

CLINICAL ISSUES

Presentation

• Most common signs/symptoms

image Symptoms dependent on type of fistula and bowel segment involved
image Enterocutaneous fistulas often present with fever, abdominal pain, distension, and drainage from wound (with signs of wound infection)
image 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
image Perianal fistulas most commonly present with purulent discharge (2/3 of cases) or local pain and inflammation
image High output enterocutaneous fistulas (i.e., duodenal stump leak) may cause major electrolyte loss and more often originate in upper GI tract

– Lower output fistulas arise more distally
• Other signs/symptoms

Demographics

• Epidemiology

image Perianal fistulas most common in young males (M:F = 2:1)

– Perianal fistulas very uncommon, with prevalence of only 0.01%
image 33% of Crohn disease patients develop fistulas after 10 years; 50% after 20 years

Natural History & Prognosis

 

Treatment

• Fluid support, correction of electrolyte abnormalities, and antibiotics to treat underlying infection

image Presence of discrete abscess associated with fistula may require percutaneous drainage
• 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
• Colovesical/colovaginal fistulas usually require surgical repair

image Colostomy to divert fecal stream, followed by fistula excision (either open or percutaneous technique)
• Perianal fistulas in patients with idiopathic fistulas usually surgically managed

image Fistulotomy or fistulectomy of tracts and drainage of abscesses
image Preoperative knowledge of fistula’s relationship with sphincter complex is critical for preservation of continence

– Internal sphincter can generally be divided without loss of continence
– External sphincter division can lead to incontinence
image Combination of medical and surgical therapy utilized to treat fistulas due to Crohn disease
image Setons (thread placed in fistulous track during fistula incision) can keep fistula open and allow continuous drainage
image Fibrin glue and omentoplasty also represent treatment options

DIAGNOSTIC CHECKLIST

Consider

• MR is far superior to CT in identification of perianal fistulas and abscesses, as well as classifying fistula’s relationship with anal sphincters and levator complex

Image Interpretation Pearls

• Active perianal fistulas demonstrate high signal on T2WI and enhancement on T1 C+ MR, whereas old, healed fistulas are nonenhancing with low signal on T1 and T2WI

image
(Left) Axial T1 C+ FS MR in a patient with Crohn disease demonstrates avid enhancement and thickening of the rectum, compatible with Crohn colitis. Note the fistulous track image arising from the rectum above the levator muscle complex.
image
(Right) Coronal T1 C+ FS MR in the same patient nicely demonstrates the extrasphincteric fistula image in this patient arising in the supralevator space and directly crossing the levator muscle complex into the ischioanal fossa without involving the anal sphincters.
image
(Left) Axial T2 MR demonstrates a large transsphincteric perianal fistula image crossing the external sphincter and extending into the posterior soft tissues on the left. Notice that the fistula is contiguous with a large T2 hyperintense “horseshoe” type abscess image in the intersphincteric space.

image
(Right) Axial T1 C+ FS MR in the same patient demonstrates the prominent rim enhancement associated with both the abscess image and the fistula image.
image
(Left) Coronal T1 C+ FS MR demonstrates an enhancing fistulous track image rising upward from the intersphincteric space into the supralevator space, before crossing the levator ani muscle complex, compatible with a suprasphincteric fistula.

image
(Right) Axial T1 C+ FS MR in a patient with rectal cancer demonstrates a sinus track image extending posteriorly at the 6-o’clock position into the presacral space.

image
(Left) Axial CECT shows sigmoid colonic mucosal hyperenhancement, submucosal edema image, and pericolonic infiltration in a 54-year-old woman with a long history of Crohn disease who developed foul-smelling vaginal discharge.
image
(Right) More caudal axial CECT section in the same patient shows extensive rectal and perirectal inflammation image.
image
(Left) Axial CECT section in the same patient through the low rectum shows severe inflammation and the enhancing walls of a fistulous tract image extending toward the vagina.

image
(Right) Spot film from a water soluble contrast enema in the same case shows opacification of the rectosigmoid colon image and the vagina image through a fistulous tract image starting low in the rectum. Contrast spilling out of the anus and vagina stains the overlying sheets.
image
(Left) After removing the soiled sheets, a repeat film from the contrast enema in the same patient shows contrast medium within the rectum image, vagina image, and fistula image.

image
(Right) Lateral film from the contrast enema in the same patient shows the contrast-opacified lumens of the rectum image and vagina image. Crohn disease is a common cause of spontaneous enteric fistulas, as it is a chronic, transmural inflammatory disease.

image
(Left) Axial CECT shows extensive diverticulosis image in an elderly woman who presented with urinary sepsis and high fever. The colonic lumen is opacified by rectal contrast medium.
image
(Right) Axial NECT in the same patient shows a thick-walled urinary bladder image that is filled with gas, particulate (fecal) debris, and rectally administered contrast medium image, confirming a colovesical fistula. Diverticulitis was confirmed as the etiology.
image
(Left) Axial CECT following rectal contrast administration shows a thick-walled sigmoid colon image in a 60-year-old woman who had undergone a prior hysterectomy and developed foul-smelling vaginal discharge.

image
(Right) Axial CECT in the same patient shows adherence of the anterior rectal wall image to the back wall of the vagina or cervix image.
image
(Left) Axial CECT in the same patient shows rectally administered contrast medium within the vagina image, confirming a colovaginal fistula.

image
(Right) Sagittally reformatted CECT section in the same patient shows the fistula image from the distal colon (or rectum) image to the vagina (or cervix) image. Diverticulitis and hysterectomy are both associated with colovaginal fistula.

image
(Left) Axial CECT of a 29-year-old woman with chronic, intermittent abdominal pain and perianal inflammation shows part of a long segment of thick-walled ileum image, mucosal hyperenhancement, and extraluminal gas and fluid image.
image
(Right) Axial CECT in the same patient shows a fistula image extending from the low rectum or anus toward the skin. These are characteristic features of Crohn disease. Please note that CT is far inferior to MR in both the identification and classification of perianal fistulas.
image
(Left) Axial CECT of a morbidly obese young woman with pain and a foul-smelling discharge from a cutaneous site shows a walled-off abscess image adjacent to the sigmoid colon. There is a tract of gas and fluid image leading to the anterior abdominal wall defect.

image
(Right) Fistulagram (injection of a catheter image inserted into the abdominal wall defect), in the same patient, opacifies an abdominal abscess cavity image and the sigmoid colon lumen image. Diverticulitis was the etiology of this spontaneous colocutaneous fistula.
image
(Left) Sagittal CECT in a patient with a history of diverticulitis and purulent vaginal drainage demonstrates a fistulous communication between the sigmoid colon image and uterus image near the fundus.

image
(Right) Coronal CECT demonstrates several signs of a colovesical fistula in this patient with acute diverticulitis image, including direct contact between the inflamed colon and the bladder, focal wall thickening of the bladder at the site of contact, and the presence of ectopic gas image within the bladder lumen.

 

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