Fig. 4.4 Common sites of anal fistulae. Note subcutaneous fistulae do not traverse the sphincters, whereas low and high fistulae do.

Anorectal fistula

BASIC INFORMATION
Subcutaneous, low and high anal fistulae are the commonest types (Fig. 4.4). Anorectal fistulae are rarer forms. The fistulae usually present as abscesses and heal after the abscess is incised. In other cases a small discharging sinus may be noted by the patient. Endoanal ultrasonography, magnetic resonance and/or examination under anaesthetic is usually required to define the primary and any secondary tracks and detect any associated disease. Management is usually surgical with approximately 90% of fistulae being laid open or excised.
A 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. It originates in an abscess in the intersphincteric space of the anal canal. Fistulas can be classified as follows:
1. Intersphincteric: fistula track passes within the intersphincteric plane to the perianal skin; most common.
2. Transsphincteric: fistula track passes from the internal opening, through the internal and external sphincter, and into the ischiorectal fossa to the perianal skin; frequent.
3. Suprasphincteric: after passing through the internal sphincter, fistula tract passes above the pu-borectalis 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.
4. Extrasphincteric: fistula tract passes from the rectum, above the levators, through the levator muscles to the ischiorectal space and perianal skin; rare With a horseshoe fistula, the tract passes from one ischiorectal fossa to the other behind the rectum.
SYNONYMS
Fistula in ano
Anal fistula
EPIDEMIOLOGY & DEMOGRAPHICS
• Common in all ages
• Occurs equally in men and women
• Associated with constipation
• Pediatric age group: more common in infants; boys > girls

ETIOLOGY
• Most common: nonspecific crypto-glandular infection (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’s disease, ulcerative colitis
• Trauma: surgery (episiotomy, prostatectomy), foreign bodies, anal intercourse
• Malignancy: carcinoma, leukemia, lymphoma
• Treatment of malignancy: surgery, radiation
DIAGNOSIS
DIFFERENTIAL DIAGNOSIS
• Hidradenitis suppurativa
• Pilonidal sinus
• Bartholin’s 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 track
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
• Anoscopy
• Proctosigmoidoscopy to exclude inflammatory or neoplastic disease
• All studies done under adequate anesthesia
LABORATORY TESTS
• CBC
• Rectal biopsy if diagnosis of IBD or malignancy suspected; biopsy of external orifice is useless
IMAGING STUDIES
• Colonoscopy or barium enema if:
1. Diagnosis of IBD or malignancy is suspected
2. History of recurrent or multiple fistulas
3. Patient <25 yr old
• Small bowel series: occasionally obtained for reasons similar to above
• Fistulography: unreliable; but may be helpful in complicated fistulas
TREATMENT
NONPHARMACOLOGIC THERAPY
Sitz baths
ACUTE GENERAL Rx
• 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 Rx
• Surgery
• Surgical goals are as follows:
1. Cure the fistula
2. Prevent recurrence
3. Preserve sphincter function
4. Minimize healing time
• Methods for the management of anal fistulas: fistulotomy, setons, rectal advancement flaps, colostomy

PHYSICAL FINDINGS & CLINICAL PRESENTATION
• Acute stage: perianal swelling, pain, and fever
• Chronic stage: history of rectal drainage or bleeding; previous abscess with drainage
• Tender external fistulous opening, with 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’s 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.