Anorectal Fistula

Anorectal Fistula 

An anorectal fistula is an inflammatory tract with a secondary (external) opening in the perianal skin and a primary (internal) opening in the anal canal at the dentate line.

Anorectal fistulae usually originate as a complication of a perianal abscess, which develops from an infected anal crypt gland. If that abscess ruptures and the tract epithelializes, then a fistula develops.

Anorectal fistulae can be classified as follows (based on their relationship to the anal sphincter complex):

• •Intersphincteric: Fistula tract passes within the intersphincteric plane to the perianal skin (most common).
• •Transsphincteric: Fistula tract passes from the internal opening, through the internal and external sphincter, and into the ischiorectal fossa to the perianal skin (frequent).
• •Suprasphincteric: After passing through the internal sphincter, fistula tract passes above the puborectalis and then tracts downward, lateral to the external sphincter, into the ischiorectal space to the perianal skin (uncommon); if abscess cavity extends cephalad, a supralevator abscess possibly palpable on rectal examination.
• •Extrasphincteric: Fistula tract passes from the rectum, above the levators, through the levator muscles to the ischiorectal space and perianal skin (rare).
• •Submucosal: Originate at the level of the dentate line in an infected crypt not involving the sphincter muscles.
• •With a horseshoe fistula, the tract passes from one ischiorectal fossa to the other behind the rectum.

Synonym

• Fistula-in-ano

Epidemiology & Demographics

• •Common in all ages (average age is 39 yr)
• •Occurs twice as often in men as compared to women (12.1/100,000 vs. 5.5/100,000)
• •Associated with inflammatory bowel disease and constipation
• •Pediatric age group: More common in infants; boys more than girls

Physical Findings & Clinical Presentation

• •Acute stage: Perianal swelling, pain with defecation and sitting, and fever
• •Chronic stage: Pain, malodorous perianal drainage or bleeding, anal pruritus
• •Tender, inflamed external fistulous opening, within 2 to 3 cm of the anal verge, with purulent or serosanguineous drainage on compression; the greater the distance from the anal margin, the greater the probability of a complicated upward extension
• •Goodsall rule:
  • 1.Location of the internal opening related to the location of the external opening
  • 2.With external opening anterior to an imaginary line drawn horizontally across the midpoint of the anus: Fistulous tract runs radially into the anal canal
  • 3.With opening posterior to the transanal line: Tract is usually curvilinear, entering the anal canal in the posterior midline
  • 4.Exception to this rule: An external anterior opening that is >3 cm from the anus. In this case the tract may curve posteriorly and end in the posterior midline
• •If perianal abscess recurs, presence of a fistula is suggested

Etiology

• •Most common: Nonspecific infection of anal crypt gland (skin or intestinal flora)
• •Fistulas more common when intestinal microorganisms are cultured from the anorectal abscess
• •Tuberculosis
• •Lymphogranuloma venereum
• •Actinomycosis
• •Inflammatory bowel disease (IBD): Crohn disease, ulcerative colitis
• •Trauma: Surgery (episiotomy, prostatectomy), foreign bodies, anal intercourse
• •Malignancy: Carcinoma, leukemia, lymphoma
• •Treatment of malignancy: Surgery, radiation

Differential Diagnosis

• •Anal abscess
• •Anal ulcer or fissure
• •Hidradenitis suppurativa
• •Pilonidal sinus
• •Bartholin gland abscess or sinus
• •Infected perianal sebaceous cysts

Workup

• •Digital rectal examination:
  • 1.Assess sphincter tone and voluntary squeeze pressure
  • 2.Determine the presence of an extraluminal mass
  • 3.Identify an indurated fistula tract
  • 4.Palpate an internal opening or pit
• •Gentle probing of external orifice to avoid creating a false tract; 50% do not have clinically detectable opening
• •Exam alone can be adequate for most simple anorectal fistulas
• •Anoscopy
• •Proctosigmoidoscopy to exclude inflammatory or neoplastic disease
• •All studies done under adequate anesthesia

Laboratory Tests

• •Complete blood count
• •Rectal biopsy if diagnosis of IBD or malignancy suspected; biopsy of external orifice is useless

Imaging Studies

• •Transanal endosonography (EUS) can be useful to predict the amount of sphincter that would be divided if primary fistulotomy is performed and to identify undrained sepsis, complex anatomy, and high blind tracts. MRI can be used to identify suprasphincteric or extrasphincteric tracts. MRI and EUS are more accurate in describing fistulas than clinical exam. CT lacks adequate resolution to identify tracts and their relationship to the sphincters and levators.
• •Fistulography, in which contrast is injected into the external opening and the fistula course is traced with fluoroscopy, is rarely necessary in view of recent technological advancements.
• •Colonoscopy or barium enema if:
  • 1.Diagnosis of IBD or malignancy is suspected.
  • 2.History of recurrent or multiple fistulas.
  • 3.Patient <25 yr.
• •Small bowel series: Occasionally obtained for reasons similar to previous advancements.

Treatment

Nonpharmacologic Therapy

• Sitz baths

Pharmacologic Therapy (Rarely Recommended)

• Immunomodulators (Infliximab)

Acute General Treatment

• •Treatment of choice: Surgery
• •Broad-spectrum antibiotic given if:
  • 1.Cellulitis present
  • 2.Patient is immunocompromised
  • 3.Valvular heart disease present
  • 4.Prosthetic devices present
• •Stool softener/laxative

Chronic Treatment

• •Surgery: The below table summarizes management strategy by fistula classification
• •Surgical goals are as follows:
  • 1.Remove the fistula
  • 2.Prevent recurrence
  • 3.Preserve sphincter function
  • 4.Minimize healing time
• •Methods for the management of anal fistulas: Fistulotomy, fistulectomy, setons (maintains fistula patent for drainage while spontaneous healing occurs), fibrin glue (clot formation within the fistulous tract), fistula plugs (promote closure of fistula), LIFT procedure, rectal advancement flaps, and colostomy

Management Strategy by Fistula Classification

From Cameron JL, Cameron AM: Current surgical therapy, ed 12, Philadelphia, 2017, Elsevier.

Type of Fistula	Fistulotomy	Seton	LIFT	Advancement Flap	Fibrin Glue	Collagen Plug	Address Intraabdominal or Pelvic Source
Superficial/intersphincteric	•
Low transsphincteric	•	•	•	•	•	•
High transsphincteric		•	•	•	•	•
Suprasphincteric		•		•		•	•
Extrasphincteric		•	•

LIFT, Ligation of intersphincteric fistula tract.

∗ Seton placement can be a first stage or definitive treatment option.

Disposition

• Outpatient surgery

Referral

• Refer to a surgeon with expertise in this area.

Pearls & Considerations

• •HIV-positive and diabetic patients with perirectal abscesses/fistulas are true surgical emergencies.
• •Risk of septicemia, Fournier gangrene, and other septic complications make immediate drainage imperative.

Suggested Reading

• Sneider E.B., Maykel J.A.: Anal abscess and fistula. Gastroenterol Clin N Am 2013; 42: pp. 773-784.