INTRODUCTION — An anorectal fistula is the chronic manifestation of the acute perirectal process that forms an anal abscess [1]. When the abscess ruptures or is drained, an epithelialized track can form that connects the abscess in the anus or rectum with the perirectal skin [2]. Anal fistulas are sometimes also referred to as "fistula-in-ano."
EPIDEMIOLOGY — The true prevalence of anal fistulas is unknown, as anorectal discomfort is often attributed to symptomatic hemorrhoids. The incidence of an anal fistula developing from an anal abscess ranges from 15 to 38 percent [3-7]. The mean age for presentation of anal abscess and fistula disease is 40 years (range 20 to 60) [7-11]. Adult males are twice as likely to develop an abscess and/or fistula compared with women [1,7,11].
ETIOLOGY — The most common etiology of an anorectal fistula is an infected anal crypt gland. Other causes include:
●Crohn disease – Anoperineal and anovaginal fistulas are common manifestations of perianal Crohn disease. While the incidence of anovaginal fistula is relatively low (<5 percent after 10 years with disease), anoperineal fistulas are diagnosed in 15 percent after 10 years and in 20 to 30 percent at 20 years after Crohn diagnosis [12-14]. Anal fistula may be the initial manifestation of Crohn disease or occur after the diagnosis of proximal luminal disease [12]. The incidence of anal fistula increases as the luminal Crohn disease extends distally and is highest in the setting of Crohn proctitis [15]. (See "Perianal Crohn disease", section on 'Perianal fistula'.)
●Obstetric injury – Anovaginal and rectovaginal fistulas most frequently result from obstetric trauma, especially in undeveloped countries where prolonged obstructed labor can lead to pressure necrosis of the rectovaginal septum. (See "Rectovaginal and anovaginal fistulas".)
●Radiation proctitis – Patients undergoing pelvic radiation can develop bleeding, rectal pain, and fistulas. (See "Radiation proctitis: Clinical manifestations, diagnosis, and management".)
●Rectal foreign bodies – A retained rectal foreign body can be a rare cause of an anorectal fistula. An anorectal mucosal laceration is the most common complication from anal insertion of a foreign body and can result in an abscess and fistula formation [16-18]. (See "Rectal foreign bodies".)
●Infectious diseases – Lymphogranuloma venereum is chronic infection in the lymphatic system caused by Chlamydia trachomatis and can cause inflammatory perirectal masses and anal fistula. Anorectal tuberculosis may also cause anal fistula [19]. Primary perianal actinomycosis is a rare condition that can cause a simple fistula-in-ano or an inflamed perirectal mass in immunocompromised individuals [20,21]. Anal fistula may also be a perianal manifestation of AIDS [22,23]. (See "Abdominal actinomycosis" and "Lymphogranuloma venereum".)
●Malignancy – In rare instances, malignant transformation of a Crohn anal fistula may occur [24,25]. Uncommonly, anal squamous cell carcinoma may fistulize to the vagina [26].
PATHOGENESIS — Cryptoglandular fistulas originate from an infected anal crypt gland [2,27,28]. There are typically 8 to 10 anal crypt glands, arranged circumferentially within the anal canal at the level of the dentate line. The glands penetrate the internal sphincter and end in the intersphincteric plane. An anorectal fistula is the connection between two epithelial structures and connects the anal abscess from the infected anal crypt glands to the perirectal skin and occasionally to other pelvic organs.
In Crohn disease, anal fistulas are caused by penetrating inflammation rather than infection of a perianal gland, which warrants a more nuanced and multidisciplinary approach [15,29,30]. (See "Perianal Crohn disease", section on 'Perianal fistula'.)
The anatomy of the anal region is described elsewhere (figure 1 and figure 2 and figure 3). (See "Operative management of anorectal fistulas", section on 'Anatomy of the anal region'.)
CLINICAL FEATURES
Patient presentation — Patients with an anorectal fistula usually present with a "nonhealing" anorectal abscess following drainage or with chronic purulent drainage and a pustule-like lesion in the perianal or buttock area (picture 1). Patients may experience intermittent rectal pain, particularly during defecation, but also with sitting and activity. Patients may also experience intermittent and malodorous perianal drainage and pruritus [31].
Patients should be questioned about their baseline anal sphincter function; prior anorectal surgery; any associated gastrointestinal, genitourinary, or gynecologic symptoms; and, when appropriate, their risk factors for infectious diseases that have been linked to anal fistula formation. (See 'Etiology' above.)
Physical examination — The perianal skin may be excoriated and inflamed. The external opening may be visualized or palpated as induration just below the skin if the external opening is incomplete or blind-ended. The external opening may be inflamed, tender, and/or draining purulent fluid. A palpable cord leading from the external opening to the anal canal may be present.
A fistula can be explored with a fistula probe by an experienced clinician, using caution to avoid creating a false passage by penetrating the fistula wall. The probe is gently inserted into the external opening on the perianal skin and passed through the internal opening into the anus or rectum.
The internal opening in the anus can be viewed by an anoscopic examination, while a sigmoidoscope may be required to view an internal opening in the rectum. In some cases, the internal opening can be palpated on digital rectal examination.
Imaging studies — Whereas simple fistulas do not require imaging to guide treatment, complex fistulas, especially those associated with perianal Crohn disease, and recurrent fistulas can benefit from imaging studies. Simple and complex fistulas are defined below. (See 'Classification' below.)
Magnetic resonance imaging (MRI) of the pelvis without and with contrast and endosonography (EUS) are the preferred imaging studies to determine the anatomy of the fistula tract and the extent of anal sphincter involvement [32-34]. Computed tomography (CT) imaging of the pelvis with contrast is inferior to MRI in its ability to discern soft tissue involvement with the fistula. Anal fistulography is also limited by its inability to define the extent of anal sphincter involvement (image 1 and image 2 and image 3 and image 4) [31,35]. (See "The role of imaging tests in the evaluation of anal abscesses and fistulas".)
A meta-analysis showed that, for assessment of anal fistulas, MRI has a sensitivity of 87 percent and a specificity of 69 percent; EUS has a sensitivity of 87 percent and a specificity of 43 percent [36]. In a comparative study, both MRI and EUS were more accurate in classifying anal fistulas than clinical examination (awake, no probing) [37].
In another study, fistulography correctly identified the primary tract, internal opening, secondary tracts, and associated abscess in 100, 74, 92, and 88 percent of patients, respectively [38]. Finally, a study of perianal Crohn disease showed that combining any two techniques from EUS, MRI, and examination under anesthesia accurately diagnosed all 39 fistulas [39].
Classification — The classification of anal fistulas described by Parks, Gordon, and Hardcastle is the most common classification used [1,40]. This classification accurately describes the anatomic track of the fistula and is useful for predicting the complexity of the operative procedure to treat the fistula. (See "Operative management of anorectal fistulas".)
Anal fistulas are classified in terms of their relationship to the anal sphincter muscles (figure 4 and figure 5):
●Intersphincteric – The fistula begins at the dentate line and ends at the anal verge, tracking along the intersphincteric plane between the internal and external anal sphincters, and terminates in the perianal skin (Parks type 1).
●Transsphincteric – The fistula tracks through the external sphincter into the ischiorectal fossa, encompasses a portion of the internal and external sphincter, and terminates in the skin overlying the buttock (Parks type 2).
●Suprasphincteric – The fistula originates at the anal crypt and encircles the entire sphincter apparatus, and terminates in the ischiorectal fossa (Parks type 3).
●Extrasphincteric – The fistula is usually very high in the anal canal, located proximal to the dentate line. It encompasses the entire sphincter apparatus, including the levators, and terminates in the skin overlying the buttock (Parks type 4). Extrasphincteric fistulas are typically not cryptoglandular in origin but may result from trauma, rectal foreign bodies, Crohn disease, or an iatrogenic injury.
●Superficial – Superficial, or submucosal, fistula was not included in the original Parks classification. It does not involve any sphincter muscle.
Intersphincteric and transsphincteric fistulas are more common than suprasphincteric, extrasphincteric, and superficial fistulas [41]. Each of these types of fistulas may be associated with one or more extensions, and accessory or adjacent communicating blind tracks [1].
Anal fistulas may also be classified by complexity [42]:
●Complex anal fistulas include high transsphincteric fistulas that involve ≥30 percent of the external sphincter; suprasphincteric, extrasphincteric, or horseshoe fistulas; and fistulas associated with Crohn disease, radiation, malignancy, or existing fecal incontinence [43,44].
●Simple anal fistulas do not have any of the above attributes and generally include superficial, intersphincteric, and low transsphincteric fistulas that involve <30 percent of the external sphincter [45].
DIAGNOSIS — The diagnosis of fistula is primarily based upon characteristic findings on history and physical examination: pain, purulent drainage, and a perirectal skin lesion. (See 'Patient presentation' above and 'Physical examination' above.)
Imaging studies are not required for diagnosis of simple fistulas; however, they may be helpful for diagnostic evaluation of complex or recurrent fistulas. (See 'Imaging studies' above and "The role of imaging tests in the evaluation of anal abscesses and fistulas".)
DIFFERENTIAL DIAGNOSIS — In patients who present with anorectal pain and a perirectal skin lesion, the differential diagnosis includes:
●Anal abscess – An anal abscess is the acute manifestation of the infectious perirectal process. Most fistulas are a result of an abscess, and approximately one-half of anorectal abscesses result in a fistula. Abscesses may be associated with fever, but generally not fistulas. (See "Perianal and perirectal abscess".)
●Anal fissure – An acute anal fissure is a superficial linear tear in the anoderm lining of the anal canal distal to the dentate line and most commonly occurs in the posterior midline. The majority of acute anal fissures are caused by local trauma to the anal canal, such as after passage of hard stool. Pain is more severe with anal fissures than the fistula. Chronic anal fissure is associated with hypertrophy of the anal sphincter and pain with defecation for a period of several weeks or more. Anal fissures can also be seen in patients with Crohn disease, tuberculosis, and leukemia. (See "Anal fissure: Clinical manifestations, diagnosis, prevention".)
●Anal ulcer or sores – Anal ulcers can be caused by granulomatous diseases (eg, Crohn disease, tuberculosis [19,46]) or syphilis [47]. (See "Cutaneous manifestations of tuberculosis", section on 'Metastatic tuberculous abscesses'.)
●Hidradenitis – Hidradenitis suppurativa is a chronic follicular occlusive disease involving the intertriginous skin of the axillary, groin, perianal, perineal, and inframammary regions (picture 2). Hidradenitis suppurativa can occur in the perirectal area and present with purulent drainage, but it is usually and easily distinguished by its typical location in the perineal or inguinal area and also by gentle probing using an anal probe. (See "Hidradenitis suppurativa: Pathogenesis, clinical features, and diagnosis".)
●Pilonidal disease – Pilonidal disease is an infection of the skin and subcutaneous tissue at or near the upper part of the natal cleft of the buttocks (figure 6). It can typically be diagnosed by physical examination findings of one or more pores (pits) or sinuses in the natal cleft region, rather than the perianal area. There are often a series of pilonidal pits (pores appearing as small dimples) in the midline extending caudad in the natal cleft. (See "Pilonidal disease".)
MANAGEMENT PRINCIPLES — Surgical treatment is the mainstay of therapy and is required in patients with symptomatic anorectal fistulas, with the exception of some patients with Crohn disease. (See "Perianal Crohn disease", section on 'Management'.)
The goal of surgical therapy is to eradicate the fistula while preserving fecal continence. Alternatively, a draining seton may be used to keep the fistula tract open, which often prevents recurrent abscess. The surgical approach depends upon correct classification of the fistula. (See 'Classification' above.)
In order to identify the external and internal opening, the course of the track, and the amount of sphincter muscle it incorporates, it is often necessary to examine the patient in the operating room under anesthesia, which begins by gently probing the fistula to determine its anatomy. Identifying the internal opening of the fistula track is not always straightforward. Many principles and maneuvers have been devised to assist in this task. For example, instillation of dilute hydrogen peroxide into the external opening, while performing anoscopy, is often helpful to reveal the internal opening.
One of the most commonly cited principles to assist in the surgical management of an anal fistula is Goodsall's rule, which states (figure 7) [48]:
●All fistula tracks 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.
●All tracks with external openings anterior to this line enter the anal canal in a radial fashion.
Fistula tracks longer than 3 cm from the anal verge do not necessarily follow Goodsall's rule; they often have an internal opening in the posterior midline.
Although Goodsall's rule is often quoted, it may not always be accurate. In one series of 216 patients who underwent surgery for complete submuscular anal fistulas, Goodsall's rule was accurate only when applied to fistulas with posterior external anal openings. It was inaccurate for predicting the course of complete submuscular anal fistulas with an anterior external opening [49]. In other series, Goodsall's rule was more accurate for anterior than for posterior fistulas [50,51].
A detailed review of the preoperative evaluation of an anorectal fistula, procedure selection process, intraoperative preparation, operative procedures for simple and complex fistulas, and outcomes is discussed in another topic. (See "Operative management of anorectal fistulas".)
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Anal abscess and anal fistula".)
INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.
Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)
●Basics topics (see "Patient education: Anal abscess and fistula (The Basics)")
SUMMARY AND RECOMMENDATIONS
●The majority of anorectal fistulas originate from an infected anal crypt gland (cryptoglandular). In Crohn disease, anal fistulas are caused by penetrating inflammation rather than infection of a perianal gland. (See 'Pathogenesis' above.)
●Patients with an anorectal fistula usually present with a "nonhealing" anorectal abscess following drainage or with chronic purulent drainage and a pustule-like lesion in the perianal or buttock area. (See 'Clinical features' above.)
●Anorectal fistulas are classified in relationship to the anal sphincter and include superficial, intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric fistulas (figure 4). Fistulas can be simple or complex, and thorough knowledge of the pelvic anatomy is essential for surgical management. (See 'Classification' above.)
●Whereas simple fistulas do not require imaging to guide treatment, complex fistulas, especially those associated with perianal Crohn disease, and recurrent fistulas can benefit from imaging studies. Magnetic resonance imaging (MRI) and endosonography (EUS) are the preferred imaging studies to determine the anatomy of the fistula tract and the extent of anal sphincter involvement. (See 'Imaging studies' above.)
●Goodsall's rule states that all fistula tracks 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, and all tracks with external openings anterior to this line enter the anal canal in a radial fashion. While often quoted, it may not always be accurate. (See 'Management principles' above.)
●Surgical management is the mainstay of therapy. The goal of surgical therapy is to eradicate the fistula while preserving fecal continence. The surgical approach depends upon the type of fistula. (See 'Management principles' above and "Operative management of anorectal fistulas".)
ACKNOWLEDGMENTS — The editorial staff at UpToDate acknowledge Elizabeth Breen, MD, and Ronald Bleday, MD, who contributed to an earlier version of this topic review.
