eMedicine Specialties > General Surgery > Colorectal

Fistula-in-Ano

Author: Dennis F Zagrodnik II, MD, FACS, Consulting Staff, Premier Surgical of Wisconsin, SC
Contributor Information and Disclosures

Updated: Jul 16, 2009

Introduction

History of the Procedure

References to fistula-in-ano date to antiquity. Hippocrates made reference to surgical therapy for fistulous disease. The English surgeon John Arderne (1307-1390) wrote Treatises of Fistula in Ano; Haemmorhoids, and Clysters in 1376, which described fistulotomy and seton use. Historical references indicate that Louis XIV was treated for an anal fistula in the 18th century. In the late 19th and early 20th centuries, prominent physician/surgeons, such as Goodsall and Miles, Milligan and Morgan, Thompson, and Lockhart-Mummery, made substantial contributions to the treatment of anal fistula. These physicians offered theories on pathogenesis and classification systems for fistula-in-ano.^1,2 (See image below and Image 1.)

For excellent patient education resources, visit eMedicine's Esophagus, Stomach, and Intestine Center. Also, see eMedicine's patient education articles Anal Abscess, Rectal Pain, and Rectal Bleeding.

Problem

A fistula-in-ano is a hollow tract lined with granulation tissue connecting a primary opening inside the anal canal to a secondary opening in the perianal skin. Secondary tracts may be multiple and from the same primary opening.

Frequency

The prevalence rate is 8.6 cases per 100,000 population. The prevalence in men is 12.3 cases per 100,000 population. In women, it is 5.6 cases per 100,000 population. The male-to-female ratio is 1.8:1. The mean age of patients is 38.3 years.³

Etiology

Fistula-in-ano is nearly always caused by a previous anorectal abscess. Anal canal glands situated at the dentate line afford a path for infecting organisms to reach the intramuscular spaces.

Other fistulae develop secondary to trauma, Crohn disease, anal fissures, carcinoma, radiation therapy, actinomycoses, tuberculosis, and chlamydial infections.

Pathophysiology

The cryptoglandular hypothesis states that an infection begins in the anal gland and progresses into the muscular wall of the anal sphincters to cause an anorectal abscess. Following surgical or spontaneous drainage in the perianal skin, occasionally a granulation tissue–lined tract is left behind, causing recurrent symptoms. Multiple series have shown that the formation of a fistula tract following anorectal abscess occurs in 7-40% of cases.^4,5

Presentation

History

Patients often provide a reliable history of previous pain, swelling, and spontaneous or planned surgical drainage of an anorectal abscess.

Signs and symptoms (in order of prevalence)

• Perianal discharge
• Pain
• Swelling
• Bleeding
• Diarrhea
• Skin excoriation
• External opening

Past medical history

Important points in the history that may suggest a complex fistula include the following:

• Inflammatory bowel disease
• Diverticulitis
• Previous radiation therapy for prostate or rectal cancer
• Tuberculosis
• Steroid therapy
• HIV infection

Review of symptoms

• Abdominal pain
• Weight loss
• Change in bowel habits

Physical examination

Physical examination findings remain the mainstay of diagnosis. The examiner should observe the entire perineum, looking for an external opening that appears as an open sinus or elevation of granulation tissue. Spontaneous discharge via the external opening may be apparent or expressible upon digital rectal examination.

Digital rectal examination may reveal a fibrous tract or cord beneath the skin. It also helps delineate any further acute inflammation that is not yet drained. Lateral or posterior induration suggests deep postanal or ischiorectal extension.

The examiner should determine the relationship between the anorectal ring and the position of the tract before the patient is relaxed by anesthesia. The sphincter tone and voluntary squeeze pressures should be assessed before any surgical intervention to delineate whether preoperative manometry is indicated. Anoscopy is usually required to identify the internal opening.

Differential diagnoses

The following do not communicate with the anal canal:

• Hidradenitis suppurativa
• Infected inclusion cysts
• Pilonidal disease
• Bartholin gland abscess in females

Indications

Therapeutic intervention is indicated for symptomatic patients. Symptoms usually involve recurrent episodes of anorectal sepsis. An abscess develops easily if the external opening on the perianal skin seals itself.

If patients are without symptoms and a fistula is found during a routine examination, no therapy is required.

Relevant Anatomy

A thorough understanding of the pelvic floor and sphincter anatomy is a prerequisite for clearly understanding the classification system for fistulous disease (see image below and Image 1).

The internal sphincter muscle is a smooth muscle under autonomic control and is an extension of the circular muscle of the rectum.

In simple cases, the Goodsall rule can help to anticipate the anatomy of fistula-in-ano. The rule states that fistulae with an external opening anterior to a plane passing transversely through the center of the anus will follow a straight radial course to the dentate line. Fistulae with their openings posterior to this line will follow a curved course to the posterior midline (see image below and Image 2). Exceptions to this rule are external openings more than 3 cm from the anal verge. These almost always originate as a primary or secondary tract from the posterior midline, consistent with a previous horseshoe abscess.^6,7

The Parks classification system defines 4 types of fistula-in-ano that result from cryptoglandular infections.

• Intersphincteric
  • Common course - Via internal sphincter to the intersphincteric space and then to the perineum
  • Seventy percent of all anal fistulae
  • Other possible tracts - No perineal opening; high blind tract; high tract to lower rectum or pelvis
• Transsphincteric
  • Common course - Low via internal and external sphincters into the ischiorectal fossa and then to the perineum
  • Twenty-five percent of all anal fistulae
  • Other possible tracts - High tract with perineal opening; high blind tract
• Suprasphincteric
  • Common course - Via intersphincteric space superiorly to above puborectalis muscle into ischiorectal fossa and then to perineum
  • Five percent of all anal fistulae
  • Other possible tracts - High blind tract (ie, palpable through rectal wall above dentate line)
• Extrasphincteric
  • Common course - From perianal skin through levator ani muscles to the rectal wall completely outside sphincter mechanism
  • One percent of all anal fistulae

Current procedural terminology codes classification

• Subcutaneous
• Submuscular (intersphincteric, low transsphincteric)
• Complex, recurrent (high transsphincteric, suprasphincteric and extrasphincteric, multiple tracts, recurrent)
• Second stage

Contraindications

Surgery for fistula-in-ano should not be performed for definitive repair of the fistula in the setting of anorectal abscess (ie, unless the fistula is superficial and the tract is obvious). In the acute phase, simple incision and drainage of the abscess are sufficient. Only 7-40% of patients will develop a fistula. Recurrent anal sepsis and fistula formation are 2-fold higher after an abscess in patients younger than age 40 years and are almost 3-fold higher in nondiabetics.

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