- Author: Vassiliki L Tsikitis, MD; Chief Editor: Kurt E Roberts, MD more...
Anorectal abscess, which can be an incapacitating condition, originates from a cryptoglandular infection in the anal canal. Anal fistula, or fistula-in-ano, is a persistent, abnormal tract from the anal canal to the perianal skin, and is estimated to occur in 50% of patients with anorectal abscess.
Anal fistulae are hollow tracts lined with granulation tissue connecting a primary (internal) opening inside the anal canal to a secondary (external) opening in the perianal skin. Obstruction of anal crypt glands leads to suppuration, which then forms a tract into an anorectal space; the direction taken determines the anorectal abscess location and hence the type of fistula. To understand the different types of anal fistulae (see Technical Considerations), clinicians must be familiar with the different anorectal spaces from which abscesses arise (see the image below).
Whereas treating an abscess can be rather straightforward, treating an anorectal fistula can be difficult for the surgeon and frustrating for the patient. Treatment of anorectal fistulae also varies according to the location, severity, and chronicity of the fistula tract.
All anal fistulae should be treated surgically. The goal of treatment is to obliterate the internal fistulous opening, including associated epithelialized tracts, with minimal sphincter division (preservation of sphincter function) and prevention of recurrence.
The type of procedure performed depends on the type of fistula (see Technical Considerations). Thus, the first step in surgical treatment is to identify the anatomy, including the external and internal opening, and define the course of all tracts relative to the sphincter muscles. This maneuver almost always requires that the patient be anesthetized. The external opening is usually more apparent, and identifying the internal opening can be challenging.
Many principles and maneuvers have been devised to assist in this task, including the Goodsall rule, which is as follows (see the image below):
All fistula tracts with external openings within 3 cm of the anal verge and posterior to a line drawn through the ischial spines travel in a curvilinear fashion to the posterior midline; if the distance between the external opening and anal margin exceeds 3 cm, there is an increased chance of complicated extensions of the fistula tract
All tracts with external openings anterior to this line enter the anal canal in a radial fashion
According to the literature, Goodsall’s rule accurately predicts the location of the internal opening in 49-81% of patients. The external opening location can be a poor predictor of fistula location in patients with long fistula tracts, recurrent fistulae, or Crohn disease.[3, 4, 5]
All symptomatic anorectal fistulae require anal fistulotomy. The only exceptions are in patients with Crohn disease. The primary treatment for perianal Crohn fistulae is medical (ie, immunologic agents), and surgery is reserved for control of perianal sepsis, where less is more (eg, placement of draining setons).
Anatomy of anal canal and surrounding structures
A solid knowledge of the anatomy of the anal canal, the perirectal tissues and the sphincteric muscles is a prerequisite for any operative treatment of anal fistula.
The surgical anal canal, which is approximately 2-4 cm long, is located between the anorectal ring (a palpable convergence of the internal sphincter, the deep external sphincter, and the puborectalis) superiorly and extends inferiorly to the anal verge (the junction of the anal canal and the hair-bearing keratinized perineal skin). The lining of the anal canal is composed of columnar cells, transitional epithelium, and non-hair-bearing squamous epithelium.
The anal canal is surrounded by two layers of funnel-shaped musculature. The inner muscular structure is the internal anal sphincter, which is the lowermost continuation of the inner, circular muscle layer of the rectum and is under involuntary control. The outer musculature is formed by the puborectalis (innermost fibers of the levator ani) and the external anal sphincter. The external anal sphincter has three parts—subcutaneous, superficial, and deep—and is under voluntary control .
The dentate line, which is about 1-2 cm proximal to the anal verge or the midportion of the anal canal, is the embryologic fusion point between endoderm and ectoderm and marks a separation between innervation, arterial-venous blood supply, and lymphatic drainage. At the level of the dentate line, a cryptoglandular complex consisting of four to eight apocrine anal glands from the intersphincteric space exist that empty via anal ducts through the internal anal sphincter into the anal canal.
The intersphincteric groove is the space between the internal and external anal sphincters and can be palpated approximately 1 cm below the dentate line near the level of the anal verge. The anal margin is outside of the anal verge and is characterized by radial skin folds, thicker skin, pigmentation, and skin with adnexal tissues.
For more information about the relevant anatomy, see Anal Canal Anatomy.
Classification of fistula types
The Parks classification system, which describes the fistula tracts in relation to the anal sphincter complex, defines the following four types of fistula-in-ano resulting from cryptoglandular infections :
Intersphincteric fistulae (see the image below) account for about 70% of all anal fistulae and usually result from a perianal abscess. The tract is via the internal sphincter to the intersphincteric space, and then out to the perineum. No external sphincter is involved. Other possible tracts include a blind tract with no perineal opening and a high tract to the lower rectum or pelvis.
Transsphincteric fistulae (see the image below) account for about 25% of all anal fistulae and usually arise from an ischioanal abscess. The tract traverses both internal and external sphincters and passes into the ischiorectal fossa and then to the perineum. Other possible tracts include a high tract with perineal opening and a high blind tract.
Suprasphincteric fistulae (see the image below) account for about 5% of all anal fistulae and usually result from a supralevator abscess. The tract arises in the intersphincteric space and courses superiorly above the puborectalis into the ischiorectal fossa and then to the perineum.
Extrasphincteric fistulae (see the image below) account for only about 1% of all anal fistulae. Causes include iatrogenic injury from probing, penetrating injury to the perineum or rectum, Crohn disease, and carcinoma or its treatment. The tract passes from the rectum above the levators and through the levator ani muscles to the perianal skin completely outside the sphincter mechanism.
Additionally, anal fistulae can be classified as either simple or complex, as follows :
Simple fistulae include intersphincteric fistulae and low transsphincteric fistulae that cross less than 30% of the external sphincter
Complex fistulae include high transphincteric fistulae (with or without a high blind tract), suprasphincteric fistulae, and extrasphincteric fistulae, in addition to recurrent fistulae; rectovaginal fistulae, fistulae involving multiple tracts, anterior fistulae in women (higher risk for postfistulotomy incontinence); and fistulae associated with inflammatory bowel disease, radiation, malignancy or preexisting incontinence
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