Anorectal Abscess and Fistula in Ano

How to Cite This Chapter: Yoon HM, Eskicioglu C, Bartnik W. Anorectal Abscess and Fistula In Ano. McMaster Textbook of Internal Medicine. Kraków: Medycyna Praktyczna. Accessed January 18, 2021.
Last Updated: February 3, 2017
Last Reviewed: June 10, 2019
Chapter Information

Definition, Etiology, ClassificationTop

Anal canal anatomy and anorectal spaces: Figure 6.1-3.

Anorectal abscess and fistula in ano are entities along a spectrum of disease with the same underlying pathogenesis.

Anorectal abscesses are infections that usually develop as a result of obstruction of anal crypts causing stasis in ducts (also described as of cryptoglandular origin). Less common causes are inflammatory bowel disease, trauma, and malignancy. Anorectal abscesses occur in patients aged 20 to 40 years and are more frequent in men than in women.

A fistula in ano is an abnormal communication between the anal canal (usually at the level of the dentate line) and the skin. It develops due to chronic infection and epithelialization of an abscess drainage tract. Fistula in ano is found in 30% to 70% of patients presenting with an anorectal abscess. About a third of patients that do not have a concomitant fistula will develop a fistula after drainage of the abscess.

Abscesses are classified based on the anatomic location in which they develop as perianal, ischiorectal, intersphincteric, or supralevator (Figure 6.1-4). Horseshoe abscesses are those that track to the contralateral side via the deep postanal space. Perianal and ischiorectal abscesses compose about 80% of anorectal abscesses.

Fistulas may be classified based on their relationship to the external anal sphincter (under voluntary control; Figure 6.1-5). Intersphincteric fistulas penetrate only through the internal anal sphincter (under involuntary control). Transsphincteric fistulas penetrate through both internal and external sphincters. Suprasphincteric fistula tracts loop over the external sphincter and perforate the levator ani. Extrasphincteric fistulas are completely external to the sphincter complex. Intersphincteric and transsphincteric fistulas are most common.

Fistulas can also be classified as simple versus complex fistulas. Simple fistulas are intersphincteric or transsphincteric fistulas that do not involve a significant portion of the external sphincter (<30%). The following are classified as complex fistulas: transsphincteric fistulas involving a significant amount of the external sphincter; anterior fistulas in women; suprasphincteric fistulas; extrasphincteric fistulas; horseshoe fistulas; and fistulas associated with inflammatory bowel disease, radiation, or malignancy.

Clinical FeaturesTop

The most common symptoms of anorectal abscesses are acute perianal pain and swelling. Patients may report spontaneous drainage. Less commonly fever and malaise develop. Supralevator abscesses may present with referred pain in the lower back and buttocks.

The most common history suggestive of a fistula is intermittent perianal pain and swelling with constant or occasional drainage from the external opening of the fistula. Exacerbation of the inflammatory process is often observed following a spontaneous closure of the external opening (ie, increased pain and swelling after drainage stops).

It is important to elicit information about sphincter function, prior anorectal surgery, and any associated gastrointestinal, urologic, or gynecologic history.

Physical examination starts with inspection of the perineum. Erythema, signs of perianal Crohn disease, spontaneous drainage, or external openings of fistula in ano are noted. External perianal and digital rectal examination (DRE) are done to identify areas of induration, fluctuance, and tenderness. An intersphincteric abscess should be suspected if there is exquisite tenderness and fluctuance on DRE with a relatively unremarkable external examination. Careful examination of the contralateral side should also be performed to exclude a horseshoe abscess. Physical examination is not very helpful in supralevator abscesses.


Clinical examination is usually sufficient for diagnosis. However, if there is uncertainty or physical examination cannot be performed due to pain, the patient may require imaging or examination under anesthesia with anoscopy.

Imaging studies may be helpful in occult or complicated anorectal abscesses and Crohn disease. Computed tomography (CT) or magnetic resonance imaging (MRI) may identify and characterize abscesses and fistulas. MRI is more helpful in identifying and delineating fistulas.


1. Abscess: Prompt incision and drainage of the abscess is required. Incision should be large enough for adequate drainage and as close as possible to the anal verge to minimize the length of the potential fistula.

Antibiotic therapy is usually not necessary. It may be considered in patients with significant cellulitis, systemic signs, or underlying immunosuppression.Evidence 1Weak recommendation (benefits likely outweigh downsides, but the balance is close or uncertain; an alternative course of action may be better for some patients). Low Quality of Evidence (low confidence that we know true effects of the intervention). Quality of Evidence lowered due to the observational nature of studies and/or indirectness of populations and outcome measures. Seow-En I, Ngu J. Routine operative swab cultures and post-operative antibiotic use for uncomplicated perianal abscesses are unnecessary. ANZ J Surg. 2017 May;87(5):356-359. doi: 10.1111/ans.12936. Epub 2014 Nov 21. PubMed PMID: 25413131.Sözener U, Gedik E, Kessaf Aslar A, et al. Does adjuvant antibiotic treatment after drainage of anorectal abscess prevent development of anal fistulas? A randomized, placebo-controlled, double-blind, multicenter study. Dis Colon Rectum. 2011 Aug;54(8):923-9. doi: 10.1097/DCR.0b013e31821cc1f9. PubMed PMID: 21730779.

2. Fistula: The first step in treatment is an examination under anesthesia, which is performed to confirm diagnosis, identify the internal opening(s), and characterize the fistula tract(s). Goodsall rule is helpful in locating the internal opening of the fistula (Figure 6.1-6). To apply the rule, an imaginary line is drawn transversely across the buttocks, through the anus. If the external opening of a fistula is anterior to this line, the tract will likely be a straight radial tract with the internal opening at the end of this straight tract. If the external opening is posterior to this line, the tract will likely be curved, with the internal opening in the posterior midline location. An exception is a long anterior fistula with an external opening anterior to this line but further away from the anus (>3 cm) that has a curved tract with a posterior internal opening.

Surgical treatment may require multiple stages, depending on the characteristics of the fistula. Surgical treatment for a simple fistula with normal sphincter function is a fistulotomy.Evidence 2Strong recommendation (benefits clearly outweigh downsides; right action for all or almost all patients). Moderate Quality of Evidence (moderate confidence that we know true effects of the intervention). Quality of Evidence lowered due to the observational nature of the studies that demonstrate high success rates of fistulotomy (>90% healing rate, high patient satisfaction) and imprecision of the randomized controlled trials comparing surgical options. Abramowitz L, Soudan D, Souffran M, et al; Groupe de Recherche en Proctologie de la Société Nationale Française de Colo-Proctologie and the Club de Réflexion des Cabinets et Groupe d'Hépato-Gastroentérologie. The outcome of fistulotomy for anal fistula at 1 year: a prospective multicentre French study. Colorectal Dis. 2016 Mar;18(3):279-85. doi: 10.1111/codi.13121. PubMed PMID: 26382623. Jacob TJ, Perakath B, Keighley MR. Surgical intervention for anorectal fistula. Cochrane Database Syst Rev. 2010 May 12;(5):CD006319. doi: 10.1002/14651858.CD006319.pub2. Review. PubMed PMID: 20464741. Hall JF, Bordeianou L, Hyman N, et al. Outcomes after operations for anal fistula: results of a prospective, multicenter, regional study. Dis Colon Rectum. 2014 Nov;57(11):1304-8. doi: 10.1097/DCR.0000000000000216. PubMed PMID: 25285698. In patients with complex fistulas, when continence and/or wound healing may be compromised with surgical treatment, it is necessary to place a seton (drain) to ensure drainage. Other surgical treatments that are effective include ligation of the intersphincteric fistula tract (LIFT) and endoanal advancement flaps. Fistula plugs and fibrin glue are relatively ineffective.


Figure 6.1-3. Anal canal anatomy. Illustration courtesy of Dr Shannon Zhang.

Figure 6.1-4. Location of abscesses. Illustration courtesy of Dr Shannon Zhang.

Figure 6.1-5. Types of fistula in ano. Type 1, intersphincteric fistula. Type 2, transsphincteric fistula. Type 3, suprasphincteric fistula. Type 4, extrasphincteric fistula. Illustration courtesy of Dr Shannon Zhang.

Figure 6.1-6. Goodsall rule. Illustration courtesy of Dr Shannon Zhang.

We would love to hear from you

  • Do you have any comments?
  • Have you found a mistake?
  • Would you like to suggest a feature?

We use cookies to ensure you get the best browsing experience on our website. Refer to our Cookies Information and Privacy Policy for more details.