A fistula-in-ano is an abnormal hollow tract or cavity that is lined with granulation tissue and that connects a primary opening inside the anal canal to a secondary opening in the perianal skin; secondary tracts may be multiple and can extend from the same primary opening. It should be differentiated from the following processes, which do not communicate with the anal canal:

Most fistulas are thought to arise as a result of cryptoglandular infection with resultant perirectal abscess. The abscess represents the acute inflammatory event, whereas the fistula is representative of the chronic process. Symptoms generally affect quality of life significantly, and they range from minor discomfort and drainage with resultant hygienic problems to sepsis.

References to fistula-in-ano date to antiquity. The fascination fistula-in-ano has exerted for more than 2000 years is manifested by the numerous papers and books on the subject. Hippocrates, in about 430 BCE, made reference to surgical therapy for fistulous disease, and he was the first person to advocate the use of a seton (from Latin seta "bristle").

In 1376, the English surgeon John Arderne (1307-1390) wrote Treatises of Fistula in Ano; Haemmorhoids, and Clysters, which described fistulotomy and seton use. Historical references indicate that Louis XIV was treated for an anal fistula in the 18th century. Salmon established a hospital in London (St. Mark's) devoted to the treatment of fistula-in-ano and other rectal conditions.[1]

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.[2, 3]

Since this early progress, little has changed in the understanding of the disease process. In 1976, Parks refined the classification system that is still in widespread use. Over the past few decades, many authors have presented new techniques and case series in an effort to minimize recurrence rates and incontinence complications, but despite more than two millennia of experience, fistula-in-ano remains a perplexing surgical disease.

Treatment of fistula-in-ano remains challenging.[4]  No definitive medical therapy is available for this condition, though long-term antibiotic prophylaxis and infliximab may have a role in recurrent fistulas in patients with Crohn disease. Surgery is the treatment of choice, with the goals of draining infection, eradicating the fistulous tract, and avoiding persistent or recurrent disease while preserving anal sphincter function.[5, 6]

For patient education information, see the Digestive Disorders Center, as well as Anal Abscess, Rectal Pain, and Rectal Bleeding.


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

View Image

Anatomy of the anal canal and perianal space.

The external sphincter muscle is a striated muscle under voluntary control by three components: submucosal, superficial, and deep muscle. Its deep segment is continuous with the puborectalis and forms the anorectal ring, which is palpable upon digital examination.

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 anticipate the anatomy of a fistula-in-ano. This rule states that fistulas 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. Fistulas with their openings posterior to this line will follow a curved course to the posterior midline (see the image below). Exceptions to this rule are external openings lying 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.[7, 8]

View Image

Fistula-in-ano. Goodsall rule.

Parks classification system

The classification system developed by Parks, Gordon, and Hardcastle (generally known as the Parks classification) is the one most commonly used for fistula-in-ano. This system (see the image below) defines four types of fistula-in-ano that result from cryptoglandular infections, as follows[9] :

View Image

Parks classification of fistula-in-ano.

An intersphincteric fistula-in-ano is characterized as follows:

A transsphincteric fistula-in-ano is characterized as follows:

A suprasphincteric fistula-in-ano is characterized as follows:

An extrasphincteric fistula-in-ano is characterized as follows:

Current procedural terminology codes classification

Current procedural terminology coding includes the following:

Unlike the current procedural terminology coding, the Parks and colleagues classification system developed by Parks et al does not include the subcutaneous fistula. These fistulas are not of cryptoglandular origin but are usually caused by unhealed anal fissures or anorectal procedures (eg, hemorrhoidectomy or sphincterotomy).


In the vast majority of cases, fistula-in-ano is caused by a previous anorectal abscess. Typically, there are eight to 10 anal crypt glands at the level of the dentate line in the anal canal, arranged circumferentially. These glands penetrate the internal sphincter and end in the intersphincteric plane. They provide a path by which infecting organisms can reach the intramuscular spaces. The cryptoglandular hypothesis states that an infection begins in the anal canal glands and progresses into the muscular wall of the anal sphincters to cause an anorectal abscess.

After surgical or spontaneous drainage in the perianal skin, a granulation tissue–lined tract is occasionally left behind, causing recurrent symptoms. Multiple series have shown that formation of a fistula tract after anorectal abscess occurs in 7-40% of cases.[10, 11]

Other fistulas develop secondary to trauma (eg, rectal foreign bodies), Crohn disease, anal fissures, carcinoma, radiation therapy, actinomycoses, tuberculosis, and lymphogranuloma venereum secondary to chlamydial infection.


The true prevalence of fistula-in-ano is unknown. The incidence of a fistula-in-ano developing from an anal abscess ranges from 26% to 38%.[5, 12] One study showed that the prevalence of fistula-in-ano is 8.6 cases per 100,000 population. In men, the prevalence is 12.3 cases per 100,000 population, and in women, it is 5.6 cases per 100,000 population. The male-to-female ratio is 1.8:1. The mean patient age is 38.3 years.[13]


Patients often provide a reliable history of previous pain, swelling, and spontaneous or planned surgical drainage of an anorectal abscess. Signs and symptoms of fistula-in-ano, in order of prevalence, include the following:

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

A review of symptoms may reveal the following in patients with a fistula-in-ano:

Physical Examination

Physical findings are 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 of pus or blood via the external opening may be apparent or expressible on digital rectal examination.

Digital rectal examination (DRE) may reveal a fibrous tract or cord beneath the skin. It also helps to 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 determine whether preoperative manometry is indicated. Anoscopy is usually required to identify the internal opening. Proctoscopy is also indicated in the presence of rectal disease (eg, Crohn disease or other associated conditions). Most patients cannot tolerate even gentle probing of the fistula tract in the office, and this should be avoided.

Laboratory Studies

No specific laboratory studies are required in the diagnosis of fistula-in-ano (though the normal preoperative studies are performed, based on age and comorbidities). Instead, physical examination findings remain the mainstay of diagnosis.

Imaging Studies

Radiologic studies are not performed for routine fistula evaluation, because in most cases, the anatomy of a fistula-in-ano can be determined in the operating room. However, such studies can be helpful when the primary opening is difficult to identify or when recurrent or persistent disease is present. In the case of recurrent or multiple fistulas, such studies can be used to identify secondary tracts or missed primary openings.[14]  Several imaging diagnostic modalities are available to evaluate fistula-in-ano. The efficacy of each modality is reviewed.


This technique involves injection of contrast via the internal opening, which is followed by anteroposterior, lateral, and oblique radiographic images to outline the course of the fistula tract.

Fistulography is relatively well tolerated but it can be painful when injecting the contrast material into the fistulous tract. It requires the ability to visualize the internal opening. Questions have been raised about its accuracy, which has been reported to range from 16% to 48%.[15] .

Because of these limitations, fistulography is generally reserved for cases in which there is a concern about a fistulous connection between the rectum and adjacent organs such as the bladder, where it may be slightly more useful than a careful examination under anesthesia.

Endoanal or endorectal ultrasonography

Endoanal or endorectal ultrasonography involves the passage of a 7- or 10-MHz ultrasound transducer into the anal canal to help define the muscular anatomy and thereby help differentiate intersphincteric from transsphincteric lesions. A standard water-filled balloon transducer can facilitate evaluation of the rectal wall for any suprasphincteric extension.

Investigations have shown that the addition of hydrogen peroxide via the external opening can aid in outlining the course of the fistula tract. This may be useful for helping to identify missed internal openings.

Endoanal/endorectal ultrasonography has been reported to be 50% better than physical examination alone in helping to detect an internal opening that is difficult to localize. This modality has not been used widely for routine clinical fistula evaluation.[16]

Magnetic resonance imaging

Findings on magnetic resonance imaging (MRI) show 80-90% concordance with operative findings when a primary tract course and secondary extensions are observed. MRI is becoming the study of choice for the evaluation of complex fistulas and recurrent fistulas. It has been shown to reduce recurrence rates by providing information on otherwise unknown extensions.[17, 18]

Computed tomography

Computed tomography (CT) is more helpful in the setting of perirectal inflammatory disease than in the setting of small fistulas because it is better for delineating fluid pockets that require drainage than for delineating small fistulas. CT requires administration of oral and rectal contrast. Muscular anatomy is not well delineated.

Barium enema/small bowel series

These studies may be useful for patients with multiple fistulas or recurrent disease to help rule out inflammatory bowel disease.

Anal Manometry

Anal manometry is rarely used in the evaluation of patients with fistula-in-ano. However, pressure evaluation of the sphincter mechanism is helpful in certain patients for operative planning, including the following:

If a decrease in pressure is found, surgical division of any portion of the sphincter mechanism should be avoided.


Examination under anesthesia

Examination of the perineum, digital rectal examination (DRE), and anoscopy are performed after the anesthesia of choice is administered. This must be done before surgical intervention is initiated, especially if outpatient evaluation causes discomfort or has not helped to delineate the course of the fistulous process.

Several techniques have been described to help locate the course of the fistula and, more important, identify the internal opening. They include the following:


Rigid sigmoidoscopy can be performed at the initial evaluation to help rule out any associated disease process in the rectum. Further colonic evaluation is performed only as indicated.

Approach Considerations

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.

Crohn disease of the perineum with multiple and often complex fistulas requires careful surgical treatment. Acute perianal abscess requires incision and drainage. Definitive repair of fistulas in these patients requires that the intra-abdominal disease be under control with medical therapy. If the disease is controlled, routine therapy is warranted. Recurrent fistulous disease to the rectum and perineum with persistent anorectal sepsis is an indication for panproctocolectomy.

Studies have identified a role in Crohn disease for fistula therapy with infliximab, with 50-60% response rates for perianal fistulas.[19, 20]  Adipose-derived stem-cell therapy is currently being studied for use in the treatment of Crohn fistula and other complex fistulas.[21, 22]

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

Surgery for fistula-in-ano should not be performed for definitive repair of the fistula in the setting of anorectal abscess (unless the fistula is superficial and the tract is obvious). In the acute phase, simple incision and drainage of the abscess are sufficient.[23]  Only 7-40% of patients will develop a fistula. Recurrent anal sepsis and fistula formation are twofold higher after an abscess in patients younger than 40 years and are almost threefold higher in nondiabetics.

Preoperative considerations include the following:

Intraoperative considerations include the following:


The laying-open technique (fistulotomy) is useful for 85-95% of primary fistulas (ie, submucosal, intersphincteric, and low transsphincteric; see the image below).[24, 25, 26, 27]

View Image

Schematic of intersphincteric and low transsphincteric fistulotomy.

A probe is passed into the tract through the external and internal openings. The overlying skin, subcutaneous tissue, and internal sphincter muscle are divided with a knife or electrocautery, and the entire fibrous tract is thereby opened.

At low levels in the anus, the internal sphincter and subcutaneous external sphincter can be divided at right angles to the underlying fibers without continence being affected. This is not the case if the fistulotomy is performed anteriorly in female patients. If the fistula tract courses higher into the sphincter mechanism, seton placement should be performed. Curettage is performed to remove granulation tissue in the tract base.

Opening the wound out on the perianal skin for 1-2 cm adjacent to the external opening with local excision of skin promotes internal healing before external closure. Some advocate marsupialization of the edges to improve healing times. Perform a biopsy on any firm, suggestive tissue.

Complete fistulectomy creates larger wounds that take longer to heal and offers no recurrence advantage over fistulotomy.

Seton Placement

A seton can be placed alone, combined with fistulotomy, or in a staged fashion. This technique is useful in patients with the following conditions[28, 29, 30] :

Beyond giving a visual identification of the amount of sphincter muscle involved, the purposes of setons are to drain, to promote fibrosis, and to cut through the fistula. Setons can be made from large silk suture, silastic vessel markers, or rubber bands that are threaded through the fistula tract.

Single-stage seton (cutting)

Pass the seton through the fistula tract around the deep external sphincter after opening the skin, subcutaneous tissue, internal sphincter muscle, and subcutaneous external sphincter muscle. The seton is tightened down and secured with a separate silk tie.

With time, fibrosis occurs above the seton as it gradually cuts through the sphincter muscles and essentially exteriorizes the tract. The seton is tightened on subsequent office visits until it is pulled through over 6-8 weeks. A cutting seton can also be used without associated fistulotomy. (See the image below).

View Image

Schematic of high transsphincteric fistulotomy with seton.

Recurrence and incontinence are important factors to consider when this technique is employed. The success rates for cutting setons range from 82-100%; however, long-term incontinence rates can exceed 30%.[31, 32, 33]

Two-stage seton (draining/fibrosing)

Pass the seton around the deep portion of the external sphincter after opening the skin, subcutaneous tissue, internal sphincter muscle, and subcutaneous external sphincter muscle.

Unlike the cutting seton, the seton is left loose to drain the intersphincteric space and to promote fibrosis in the deep sphincter muscle. Once the superficial wound is healed completely (2-3 months later), the seton-bound sphincter muscle is divided.

Two studies (74 patients combined) supported the two-stage approach with a 0-nylon seton. Once wound healing is complete, the seton is removed without division of the remaining encircled deep external sphincter muscle. The researchers reported eradication of the fistula tract in 60-78% of cases.

Mucosal Advancement Flap

A mucosal advancement flap is reserved for use in patients with chronic high fistula but is indicated for the same disease process as seton use.[19, 34, 35]  Advantages include a one-stage procedure with no additional sphincter damage. A disadvantage is poor success in patients with Crohn disease or acute infection.

This procedure involves total fistulectomy, with removal of the primary and secondary tracts and complete excision of the internal opening.

A rectal mucomuscular flap with a wide proximal base (two times the apex width) is raised. The internal muscle defect is closed with an absorbable suture, and the flap is sewn down over the internal opening so that its suture line does not overlap the muscular repair.

Plugs and Adhesives

Advances in biotechnology have led to the development of many new tissue adhesives and biomaterials formed as fistula plugs. By their less-invasive nature, these therapies lead to decreased postoperative morbidity and risk of incontinence, but long-term data are lacking for eradication of disease, especially in complex fistulas, which carry high recurrence rates.[36, 37, 35]

Reported series exist of fibrin glue treatment of fistula-in-ano, with 1-year follow-up showing recurrence rates approaching 40-80%.[38, 39, 40]  The Surgisis fistula plug has also had mixed long-term results in direct clinical trials.[41, 42, 43]

Early success rates have been reported for newer materials, such as acellular dermal matrix and the bioabsorbable Gore Bio-A fistula plug, in low fistulas and good animal model data.[44]  Assessment of long-term success rates with plug techniques for complex disease will be based on further data from randomized trials.

In a randomized, controlled study designed to evaluate the efficacy and safety of the anal fistula plug in patients with fistulizing anoperineal Crohn disease, Senéjoux et al did not find the plug to be superior to seton removal for achieving fistual closure, regardless of whether the fistula was simple or complex.[45]

A combined sphincter-sparing repair that includes both an anal fistula plug and a rectal advancement flap has been proposed for the treatment of transsphincteric fistula-in-ano.[46]

LIFT Procedure

Ligation of the intersphincteric fistula tract (LIFT) is a sphincter-sparing procedure for complex transsphincteric fistulas first described in 2007. It is performed by accessing the intersphincteric plane with the goal of performing a secure closure of the internal opening and by removing the infected cryptoglandular tissue.[47]

The intersphincteric tract is identified and isolated by performing meticulous dissection through the intersphincteric plane after making a small incision overlying the probe connecting the external and internal openings. Once isolated, the intersphincteric tract is hooked with a small right-angle clamp, and the tract is ligated close to the internal sphincter and then divided distal to the point of ligation. Hydrogen peroxide is injected through the external opening to confirm the division of the correct tract. The external opening and the remnant fistulous tract are curetted to the level of the proximity of the external sphincter complex.

Finally, the intersphincteric incision is loosely reapproximated with an absorbable suture. The curettaged wound is left opened for dressing.[47, 48, 49]

Because of its relative novelty, LIFT has not been extensively researched. In a randomized trial of 39 patients with complex fistula-in-ano who had failed previous procedures and were treated with LIFT technique, success rates were comparable to those seen with the anorectal advancement flap technique.[50]  The probability of recurrence at 19 months was 8% for the LIFT technique versus 7% for the anorectal advancement flap. Time to return to work was shorter in the LIFT group (1 vs 2 wk), but there was no difference in incontinence scores.[50]

Further randomized surgical trials are needed to determine whether this technique is a viable—or, possibly, a better—alternative to the other previously mentioned procedures for the treatment of fistula-in-ano.


In rare cases, the creation of a diverting stoma may be indicated to facilitate the treatment of a complex persistent fistula-in-ano. The most common indications include, but are not limited to, patients with perineal necrotizing fasciitis, severe anorectal Crohn disease, reoperative rectovaginal fistulas, and radiation-induced fistulas. Fecal diversion alone is effective in these select patients to control sepsis and symptoms; however, long-term success rates after reanastomosis are low because of recurrence from the underlying disease. Thus, this approach should be avoided unless the underlying fistula-in-ano disease process is repaired or has healed completely, which is unlikely.

Postoperative Care

After the operation, most patients can be treated in an ambulatory setting with discharge instructions and close follow-up care. Sitz baths, analgesics, and stool-bulking agents (eg, bran and psyllium products) are used in follow-up care.


Early postoperative complications may include the following:

Delayed postoperative complications may include the following:

Postoperative rates of recurrence and incontinence vary according to the procedure performed, as follows:

Long-Term Monitoring

Frequent office visits within the first few weeks help to ensure proper healing and wound care.

It is important to ensure that the internal wound does not close prematurely, causing a recurrent fistula. Digital examination findings can help distinguish early fibrosis. Wound healing usually occurs within 6 weeks.


Juan L Poggio, MD, MS, FACS, FASCRS, Associate Professor of Surgery, Chief, Division of Colon and Rectal Surgery, Department of Surgery, Drexel University College of Medicine

Disclosure: Nothing to disclose.

Chief Editor

John Geibel, MD, DSc, MSc, AGAF, Vice Chair and Professor, Department of Surgery, Section of Gastrointestinal Medicine, Professor, Department of Cellular and Molecular Physiology, Yale University School of Medicine; Director of Surgical Research, Department of Surgery, Yale-New Haven Hospital; American Gastroenterological Association Fellow

Disclosure: Nothing to disclose.


Oscar Joe Hines, MD Assistant Professor, Department of Surgery, University of California at Los Angeles School of Medicine

Oscar Joe Hines, MD is a member of the following medical societies: Alpha Omega Alpha, American Association of Endocrine Surgeons, American College of Surgeons, Association for Academic Surgery, Society for Surgery of the Alimentary Tract, and Society of American Gastrointestinal and Endoscopic Surgeons

Disclosure: Nothing to disclose.

David L Morris, MD, PhD, FRACS Professor, Department of Surgery, St George Hospital, University of New South Wales, Australia

David L Morris, MD, PhD, FRACS is a member of the following medical societies: British Society of Gastroenterology

Disclosure: RFA Medical None Director; MRC Biotec None Director

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Medscape Salary Employment

Dennis F Zagrodnik II, MD, FACS Consulting Staff, Premier Surgical of Wisconsin, SC

Dennis F Zagrodnik II, MD, FACS is a member of the following medical societies: American College of Chest Physicians, American College of Surgeons, American Medical Association, Phi Beta Kappa, Society of American Gastrointestinal and Endoscopic Surgeons, Southeastern Surgical Congress, and Wisconsin Medical Society

Disclosure: Nothing to disclose.


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Anatomy of the anal canal and perianal space.

Fistula-in-ano. Goodsall rule.

Parks classification of fistula-in-ano.

Schematic of intersphincteric and low transsphincteric fistulotomy.

Schematic of high transsphincteric fistulotomy with seton.

Anatomy of the anal canal and perianal space.

Fistula-in-ano. Goodsall rule.

Parks classification of fistula-in-ano.

Schematic of intersphincteric and low transsphincteric fistulotomy.

Schematic of high transsphincteric fistulotomy with seton.