Anal fistula

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Anal fistula
Other names: Anal fistulae, fistula-in-ano, anorectal fistula
This is a modified version of the original by Armin Kübelbeck [CC BY 3.0 (], via Wikimedia Commons, Specifically, it was made fistula-specific by removing abscess labels.
Different types of anal fistula
SpecialtyGeneral surgery
SymptomsPain, drainage of pus.[1]
Usual onset20s to 40s[2]
DurationLong standing[3]
TypesSimple, complex[2]
CausesAnal abscess[4]
Risk factorsObesity, diabetes, smoking, high blood lipids, lack of exercise[2]
Diagnostic methodExamination under anesthesia, medical imaging[2]
Differential diagnosisAnal carcinoma, anal fissure, hemorrhoids, perianal abscess, diverticulitis, hidradenitis suppurativa, pilonidal cyst[2][5]
Frequency12 per 100,000 (males)[2]
6 per 100,000 (females)[2]

An anal fistula is a abnormal communication between the anal canal and usually the skin around the anus that is long standing in nature.[1][3] Symptoms may include rectal pain and drainage of pus.[1] Complications may include fecal incontinence, anal stenosis, and recurrence of the fistula.[2]

The most common cause is a prior anal abscess which does not heal properly.[4] Risk factors include Crohn disease, radiation therapy, obesity, diabetes, smoking, high blood lipids, and a lack of exercise.[2] They generally start from an anal gland, which are located between the internal and external anal sphincter and drain into the anal canal.[5] If there is a single tract, less than 30% involvement of the external sphincter, and no underlying cause it is classified as simple, otherwise it is classified as complex.[2] Diagnosis may require examination under anesthesia or medical imaging.[2]

Treatment is generally via surgery using one of a number of techniques.[2] Antibiotics are typically not needed.[2] Though, those related to Crohn disease may be, at least initially, treated with medications.[2] About 12 in 100,000 males and 6 per 100,000 females are affected.[2] Onset is usually between the ages of 20 and 40.[2] It occurs in 30% to 50% of individuals with anal abscess.[4]

Signs and symptoms

Marking of the complex anal fistula with a flexible metallic probe.
Japan: A man with an anal fistula. From the Yamai no Soshi, late 12th century.

Anal fistulae can present with the following symptoms:[6]

  • skin maceration
  • pus, serous fluid and/or (rarely) feces discharge — can be bloody or purulent
  • pruritus ani — itching
  • depending on presence and severity of infection:
  • pain
  • swelling
  • tenderness
  • fever
  • unpleasant odor
  • Thick discharge, which keeps the area wet


An anal fistula can be described as a narrow tunnel with its internal opening in the anal canal and its external opening in the skin near the anus.[7] If the outlet of these glands becomes blocked, an abscess can form which can eventually extend to the skin surface. The tract formed by this process is a fistula.[1] Abscesses can recur if the fistula seals over, allowing the accumulation of pus. It can then extend to the surface again – repeating the process.[1]


Diagnosis is by examination, either in an outpatient setting or under anaesthesia (referred to as EUA or Examination Under Anaesthesia). The fistula may be explored by using a fistula probe (a narrow instrument). In this way, it may be possible to find both openings. The examination can be an anoscopy. Diagnosis may be aided by performing a fistulogram, proctoscopy and/or sigmoidoscopy.

Possible findings:

  • The opening of the fistula onto the skin may be observed
  • The area may be painful on examination
  • There may be redness
  • An area of induration may be felt; thickening due to chronic infection
  • A discharge may be seen


  • Park's classification: This was done by Parks et al. from the UK[8] in 1976, before MRI or endoanal ultrasound was available. It classified the fistula in four grades:
  • St James University Hospital Classification: This was done by Morris et al. in the year 2000.[9] This classification was improvement over Parks classification as it was based on MRI studies. It classified the fistula in five grades.
  • Garg classification: This was done by Pankaj Garg in 2017.[10] This classification is improvement over both Parks and St James University Hospital Classification. This was based on MRI studies and operative findings in 440 patients. It classified the fistula in five grades. The grades of this classification correlate quite well with the severity of the disease. Grade I & II are simpler fistulas and can be managed by Fistulotomy whereas Grade III-V are complex fistulas in which fistulotomy should be not be done. They should be managed by Fistula experts. Unlike Park's and St James University Hospital Classification, this correlation is quite accurate with Garg's classification. Therefore this new classification is useful to both surgeons and radiologists.


Depending on their relationship with the internal and external sphincter muscles, fistulae are classified into five types:

  • Extrasphincteric fistulae begin at the rectum or sigmoid colon and proceed downward, through the levator ani muscle and open into the skin surrounding the anus. Note that this type does not arise from the dentate line (where the anal glands are located). Causes of this type could be from a rectal, pelvic or supralevator origin, usually secondary to Crohn's disease or an inflammatory process such as appendiceal or diverticular abscesses.[11][12][13]
  • Suprasphincteric fistulae begin between the internal and external sphincter muscles, extend above and cross the puborectalis muscle, proceed downward between the puborectalis and levator ani muscles, and open an inch or more away from the anus.[11][12][13]
  • Transphincteric fistulae begin between the internal and external sphincter muscles or behind the anus, cross the external sphincter muscle and open an inch or more away from the anus.[11][12][13] These may take a 'U' shape and form multiple external openings. This is sometimes termed a 'horseshoe fistula.'[7]
  • Intersphincteric fistulae begin between the internal and external sphincter muscles, pass through the internal sphincter muscle, and open very close to the anus.[11][12][13]
  • Submucosal fistulae pass superficially beneath the submucosa and do not cross either sphincter muscle.[13]

Differential diagnosis

Other conditions in which infected perianal "holes" or openings may appear include pilonidal cyst.


There are several stages to treating an anal fistula:

Definitive treatment of a fistula aims to stop it recurring. Treatment depends on where the fistula lies, and which parts of the internal and external anal sphincters it crosses.

  • Lay-open of fistula-in-ano – this option involves an operation to cut the fistula open. Once the fistula has been laid open it will be packed on a daily basis for a short period of time to ensure that the wound heals from the inside out. This option leaves behind a scar, and depending on the position of the fistula in relation to the sphincter muscle, can cause problems with incontinence. This option is not suitable for fistulae that cross the entire internal and external anal sphincter.
  • Cutting seton – if the fistula is in a high position and it passes through a significant portion of the sphincter muscle, a cutting seton (from the Latin seta, "bristle") may be used. This involves inserting a thin tube through the fistula tract and tying the ends together outside of the body. The seton is tightened over time, gradually cutting through the sphincter muscle and healing as it goes. This option minimizes scarring but can cause incontinence in a small number of cases, mainly of flatus. Once the fistula tract is in a low enough position it may be laid open to speed up the process, or the seton can remain in place until the fistula is completely cured. This was the traditional modality used by physicians in Ancient Egypt and formally codified by Hippocrates,[14] who used horsehair and linen.
  • Seton stitch – a length of suture material looped through the fistula which keeps it open and allows pus to drain out. In this situation, the seton is referred to as a draining seton. The stitch is placed close to the ano-rectal ring – which encourages healing and makes further surgery easy.
  • Fistulotomy – till anorectal ring
  • Colostomy – to allow healing
  • Fibrin glue injection is a method explored in recent years, with variable success. It involves injecting the fistula with a biodegradable glue which should, in theory, close the fistula from the inside out, and let it heal naturally. This method is perhaps best tried before all others since, if successful, it avoids the risk of incontinence, and creates minimal stress for the person.
Fistula plug
  • Fistula plug involves plugging the fistula with a device made from small intestinal submucosa. The fistula plug is positioned from the inside of the anus with suture. According to some sources, the success rate with this method is as high as 80%. As opposed to the staged operations, which may require multiple hospitalizations, the fistula plug procedure requires hospitalization for only about 24 hours. Currently, there are two different anal fistula plugs cleared by the FDA for treating ano-rectal fistulae in the United States. This treatment option does not carry any risk of bowel incontinence. In the systematic review published by Dr Pankaj Garg, the success rate of the fistula plug is 65–75%.[15]
  • Endorectal advancement flap is a procedure in which the internal opening of the fistula is identified and a flap of mucosal tissue is cut around the opening. The flap is lifted to expose the fistula, which is then cleaned and the internal opening is sewn shut. After cutting the end of the flap on which the internal opening was, the flap is pulled down over the sewn internal opening and sutured in place. The external opening is cleaned and sutured. Success rates are variable and high recurrence rates are directly related to previous attempts to correct the fistula.
  • LIFT technique is a novel modified approach through the intersphincteric plane for the treatment of fistula-in-ano, known as LIFT (ligation of intersphincteric fistula tract) procedure. LIFT procedure is based on secure closure of the internal opening and removal of infected cryptoglandular tissue through the intersphincteric approach. Essential steps of the procedure include, incision at the intersphincteric groove, identification of the intersphincteric tract, ligation of intersphincteric tract close to the internal opening and removal of intersphincteric tract, scraping out all granulation tissue in the rest of the fistulous tract, and suturing of the defect at the external sphincter muscle.[16] The procedure was developed by Thai colorectal surgeon, Arun Rojanasakul, The first reports of preliminary healing result from the procedure were 94% in 2007.[17] Additional ligation of the intersphincteric fistula tract did not improve the outcome after endorectal advancement flap.[18]
  • Fistula clip closure (OTSC Proctology) is the latest surgical development, which involves the closure of the internal fistula opening with a superelastic clip made of nitinol (OTSC). During surgery, the fistula tract is debrided with a special fistula brush and the clip is transanally applied with the aid of a preloaded clip applicator. The surgical principle of this technique relies on the dynamic compression and permanent closure of the internal fistula opening by the superelastic clip. Consequently, the fistula tract dries out and heals instead of being kept open by continuous feeding with stool and fecal organisms. This minimally-invasive sphincter-preserving technique has been developed and clinically implemented by the German surgeon Ruediger Prosst.[19][20] First clinical data of the clip closure technique demonstrate a success rate of 90% for previously untreated fistulae[21] and a success rate of 70% for recurrent fistulae.[22]
  • PERFACT Procedure is another latest addition to the armamentarium to treat complex and highly complex fistula-in-ano. It is a minimally cutting procedure as both the anal sphincters (internal and external sphincters) are not cut/damaged at all. Therefore, the risk of incontinence is minimal. PERFACT procedure (proximal superficial cauterization, emptying regularly fistula tracts and curettage of tracts) entails two steps: superficial cauterization of mucosa at and around the internal opening and keeping all the tracts clean. The principle is to permanently close the internal opening by granulation tissue. This is achieved by superficial electrocauterization at and around the internal opening and subsequently allowing the wound to heal by secondary intention. Early results of this procedure are quite encouraging for complex fistula-in-ano (86.4% in highly complex anal fistulae). The procedure is effective even in fistula associated with abscess, supralevator fistula-in-ano and fistula where the internal opening is non-localizable.[23]


Some people will have an active infection when they present with a fistula, and this requires clearing up before definitive treatment can be decided.

Antibiotics can be used as with other infections, but the best way of healing infection is to prevent the buildup of pus in the fistula, which leads to abscess formation. This can be done with a seton.


A literature review published in 2018 showed an incidence as high as 21 people per 100,000. "Anal fistulas are 2–6 times more prevalent in males than females, with the condition occurring most frequently in patients in their 30s and 40s." [24]


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  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 Carr, S; Velasco, AL (January 2020). "Fistula In Ano". PMID 32491449. {{cite journal}}: Cite journal requires |journal= (help)
  3. 3.0 3.1 Taylor, Robert B. (2002). Manual of Family Practice. Lippincott Williams & Wilkins. p. 337. ISBN 978-0-7817-2652-8. Archived from the original on 2021-08-27. Retrieved 2020-10-30.
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  9. Morris J, Spencer JA, Ambrose NS (May 2000). "MR imaging classification of perianal fistulas and its implications for patient management". Radiographics. 20 (3): 623–35. doi:10.1148/radiographics.20.3.g00mc15623. PMID 10835116.
  10. Garg P (13 April 2017). "Comparing existing classifications of fistula-in-ano in 440 operated patients: Is it time for a new classification?". Int J Surg. 42: 34–40. doi:10.1016/j.ijsu.2017.04.019. PMID 28414118.
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  13. 13.0 13.1 13.2 13.3 13.4 Shawki, Sherief; Wexner, Steven D (2011-07-28). "Idiopathic fistula-in-ano". World Journal of Gastroenterology. 17 (28): 3277–3285. doi:10.3748/wjg.v17.i28.3277. ISSN 1007-9327. PMC 3160530. PMID 21876614.
  14. "Hippocrates, "On Fistulae", translation by Francis Adams, Internet Classics Archive, Massachusetts Institute of Technology". Archived from the original on 2012-12-02. Retrieved 2012-12-14.
  15. Garg P, Song J, Bhatia A, Kalia H, Menon GR (October 2010). "The efficacy of anal fistula plug in fistula-in-ano: a systematic review". Colorectal Dis. 12 (10): 965–70. doi:10.1111/j.1463-1318.2009.01933.x. PMID 19438881.
  16. Rojanasakul A (September 2009). "LIFT procedure: a simplified technique for fistula-in-ano". Tech Coloproctol. 13 (3): 237–40. doi:10.1007/s10151-009-0522-2. PMID 19636496. S2CID 11643866.
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  21. Prosst RL, Ehni W, Joos AK (September 2013). "The OTSC Proctology clip system for anal fistula closure: first prospective clinical data". Minim Invasive Ther Allied Technol. 22 (5): 255–9. doi:10.3109/13645706.2013.826675. PMID 23971828. S2CID 25219225.
  22. Mennigen R, Laukötter M, Senninger N, Rijcken E (April 2015). "The OTSC(®) proctology clip system for the closure of refractory anal fistulas". Tech Coloproctol. 19 (4): 241–6. doi:10.1007/s10151-015-1284-7. PMID 25715788. S2CID 23284320.
  23. Garg P, Garg M (7 April 2015). "PERFACT procedure: a new concept to treat highly complex anal fistula". World Journal of Gastroenterology. 21 (13): 4020–9. doi:10.3748/wjg.v21.i13.4020. PMC 4385552. PMID 25852290.
  24. Yamana, Tetsuo (July 25, 2018). "PRACTICE GUIDELINES-Japanese Practice Guidelines for Anal Disorders II. Anal fistula". J Anus Rectum Colon. 2 (3): 103–109. doi:10.23922/jarc.2018-009. PMC 6752149. PMID 31559351.

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