Anal abscesses and fistulas
Authors
Elizabeth Breen, MD
Ronald Bleday, MD
Section Editors
Martin Weiser, MD
Lawrence S Friedman, MD
Deputy Editor
Rosemary B Duda, MD, MPH, FACS
Disclosures
All topics are updated as new evidence becomes available and our peer review process is complete.
Literature review current through: Jan 2012.
INTRODUCTION — Anal abscesses and fistulas are the acute and chronic manifestations of the same perirectal process, with the abscess being the acute phase and the fistula the chronic phase of suppuration [1]. Benign anorectal diseases, including fissures, abscesses, fistulas, and hemorrhoids, are common. The prevalence of specific disorders is difficult to estimate since almost any anorectal discomfort is often attributed to symptomatic hemorrhoids.
General issues related to anal abscesses and fistulas will be reviewed here. The anatomy of the anal canal and ischiorectal fossa, and the procedure selection and surgical outcomes for simple and complex fistulas are discussed elsewhere. (See "Operative management of anorectal fistulas".)
BACKGROUND — The prevalence of anal abscesses and fistulas in the general population is probably much higher than seen in clinical practice since the majority of patients with symptoms referable to the anorectum do not seek medical attention. The incidence of an anal fistula developing from an anal abscess ranges from 26 to 38 percent [2-4]. The incidence of anal abscesses was extrapolated from the incidence of anal fistulas and is estimated between 68,000 and 96,000 cases per year in the United States [1]. The mean age for presentation of anal abscess and fistula disease is 40 years (range 20 to 60) [5-8]. Adult males are twice as likely to develop an abscess and/or fistula compared with women [1,8].
PATHOGENESIS — The majority of anal abscesses and fistulas originate from an infected anal crypt gland [9]. 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 anal abscess develops when an anal crypt gland becomes obstructed with inspissated debris, which permits bacterial growth and abscess formation. The suppuration follows the path of least resistance and the infected fluid collects in the space where the gland terminates. An anal fistula is a connection between two epithelial structures and connects the anal abscess from the infected anal crypt glands to the skin of the buttocks. There is no model to predict when an abscess will develop a fistula [10].
In addition to the cryptoglandular etiology, anorectal fistulas can develop in patients with a variety of other disorders including [11]:
•Anorectal malignancy
•Actinomycosis
•Lymphogranuloma venereum
•Radiation proctitis
•Leukemia
The anatomy of the anal region is described elsewhere. (See "Operative management of anorectal fistulas", section on 'Anatomy of the anal region'.)
ANAL ABSCESS
Classification — Anal abscesses can involve different planes in the anorectum, which can have distinct presentations. Anal abscesses are classified based upon their location including [9]:
•Perianal (superficial)
•Ischiorectal (perirectal)
•Intersphincteric
•Supralevator
•Horseshoe (posterior anorectal space)
Clinical manifestations and diagnosis — Patients with abscesses often present with severe pain in the anal area. The pain is constant and not necessarily associated with a bowel movement. Constitutional symptoms such as fever and malaise are common. Purulent rectal drainage may be noted if the abscess has begun to drain spontaneously.
On physical examination, an area of fluctuance or a patch of erythematous, indurated skin overlying the perianal or ischiorectal space may be noted. However, in some patients and with some types of abscesses there are no findings on physical inspection, and the abscess can only be felt via digital rectal examination or seen on CT scan. The role of imaging tests in the evaluation of anal abscesses is presented separately. (See "The role of imaging tests in the evaluation of anal abscesses and fistulas".)
Differential diagnosis — Anorectal abscesses should be distinguished from other disorders that may be associated with cutaneous abscess formation and drainage including [11]:
•Infected presacral epidermal inclusion cysts
•Hidradenitis suppurativa (a chronic suppurative condition of apocrine glands) (see "Pathogenesis, clinical features, and diagnosis of hidradenitis suppurativa")
•Pilonidal disease is a common skin lesion of the gluteal cleft seen most frequently in young men, which involves hair follicles that become infected and lead to abscess formation. (See "Pilonidal disease".)
•Bartholin's abscess (see "Disorders of Bartholin's gland")
General principles of treatment — Recommendations for the treatment of anal abscesses and fistulas have been established by the American Society of Colon and Rectal Surgeons [12]. These guidelines are available online at: www.fascrs.org.
The following principles of treatment were proposed:
•Anal abscesses should be drained in a timely manner. Lack of fluctuance should not be a reason to delay treatment.
•Antibiotics may have a role in special circumstances including valvular heart disease, immunosuppression, extensive cellulitis, or diabetes.
Approximately half of anal abscesses will result in the development of a chronic fistula from the inciting anal gland to the skin overlying the drainage site. The risk of developing a fistula is not influenced by whether the fistula drained spontaneously or whether surgical drainage was performed.
Because of the high risk of developing a fistula, primary fistulotomy has been investigated in patients in whom the inciting anal gland can be identified at the time of drainage. However, prophylactic fistulotomy is not recommended because of the high frequency of anorectal dysfunction associated with this procedure. In one prospective randomized study, for example, 70 patients with an anorectal abscess were randomized to incision, drainage, and fistulotomy with primary partial internal sphincterotomy, or to incision and drainage alone [13]. After a median follow-up of 42.5 months, the combined recurrence or persistence rate was significantly higher in patients who did not undergo fistulotomy (40 versus 3 percent). However, patients who underwent fistulotomy were more likely to have anal dysfunction (40 versus 21 percent).
Thus, more than 50 percent of patients treated with incision and drainage alone will have a good outcome. Since primary fistulotomy may be associated with a high rate of anorectal dysfunction, it should be reserved for patients who have refractory symptoms. An exception to this general rule is in patients who have a horseshoe abscess (see 'Horseshoe abscess' below).
Role of abscess culture — Cultures are generally not required in the treatment of an anal abscess as antibiotic therapy is typically not needed following an incision and drainage in most patients. However, cultures may be useful in the following settings:
•To distinguish between a cryptoglandular abscess (typically due to colonic flora) and an abscess of the perianal skin (typically due to staphylococcal species)
•Patients who would typically be treated with antibiotics, including those with valvular heart disease, immunosuppression, extensive cellulitis, diabetes
•Patients who have been on multiple courses of antibiotics
•Patients with pain out of proportion to findings, who may have an unusual pathogen
•Patients with leukemia or lymphoma who may have unusual or resistant bacteria
Treatment according to site
Perianal — Perianal abscesses travel distally in the intersphincteric groove into the perianal skin, where they present as a tender, fluctuant mass (picture 1). Perianal abscesses can often be drained in the office. The overlying skin is anesthetized with local anesthesia and the cavity is drained with a cruciate incision as close to the anal verge as possible. The proximity of the drainage site to the anal verge is important in minimizing the potential fistula tract. After drainage, the area can be kept clean by frequent sitz baths or hand-held showers.
Ischiorectal — Ischiorectal abscesses penetrate through the external anal sphincter into the ischiorectal space and present as a diffuse, tender, indurated, fluctuant area within the buttocks. Small ischiorectal abscesses can sometimes be drained in the office, but larger abscesses are best drained in the operating room where adequate anesthesia, lighting, and exposure are helpful. The overlying skin is anesthetized with local anesthesia and the cavity is drained with a cruciate incision as close to the anal verge as possible. The proximity of the drainage site to the anal verge is important in minimizing the potential fistula tract. After drainage, the area can be kept clean by frequent sitz baths or hand-held showers.
Intersphincteric — Intersphincteric abscesses account for only 2 to 5 percent of all anorectal abscesses. They are located in the intersphincteric groove between the internal and external sphincters. As a result, they often do not cause perianal skin changes, but can be palpated as a fluctuant mass protruding into the lumen during digital rectal examination. Intersphincteric abscesses must be drained in the operating room where a retractor can be placed within the anal canal allowing visualization and access to the abscess cavity through the internal sphincter muscle.
Supralevator — Supralevator abscesses can originate from two different sources: the typical cryptoglandular infection that travels superiorly within the intersphincteric plane to the supralevator space and an inflammatory pelvic process such as perforated diverticular disease or Crohn's disease or a perforated neoplastic process. As a result, it is important to obtain a history of potential sources of pelvic infection.
Patients with a supralevator abscess present with severe anorectal pain, fever, and sometimes urinary retention. Physical examination usually reveals no obvious external findings. On digital examination, an area of induration or fluctuation can often be felt above the level of the anorectal ring. Because of the paucity of physical examination findings, an abdominopelvic CT scan may be required to establish the diagnosis. (See "The role of imaging tests in the evaluation of anal abscesses and fistulas".)
Supralevator abscesses can be drained in one of two ways depending upon the site of origin of the abscess. Although the origin is not always readily apparent, abscesses stemming from an extension of an ischiorectal abscess should be drained via the skin overlying the buttock. Abscesses originating from a pelvic process should be drained into the rectum to avoid creating an extrasphincteric fistula.
Horseshoe abscess — A horseshoe abscess is a complex abscess. Horseshoe abscesses form in the potential space posterior to the anal canal that is bounded by the pelvic floor superiorly, the anococcygeal ligament inferiorly, and the coccyx and anal canal. Because of these relatively rigid boundaries, abscesses in this space are forced into the ischiorectal space, where they can be unilateral or bilateral. Drainage of a horseshoe abscess is the one setting in which a primary fistulotomy is advised for anal abscess [14]. Fistulotomy should be performed through the posterior midline into the deep post-anal space. A counter incision should be made over the ischiorectal abscess. (See "Operative management of anorectal fistulas", section on 'Fistulotomy' and "Operative management of anorectal fistulas", section on 'Fistulotomy and setons'.)
ANAL FISTULA
Classification — The classification of anal fistulas described by Parks, Gordon, and Hardcastle is the most common classification used [1,15]. This classification accurately describes the anatomic tract of the fistula and is useful for predicting the complexity of the operative procedure to treat the fistula. (See "Operative management of anorectal fistulas".) Fistulas can have complicated anatomy, with one or more extensions and accessory tracts possible (figure 1). The four general types of anal fistulas described include:
•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.
•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.
•Suprasphincteric — The fistula originates at the anal crypt and encircles the entire sphincter apparatus, and terminates in the ischiorectal fossa.
•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.
Each of these types of fistulas may be associated with an adjacent communicating blind tract [1].
Clinical manifestations and diagnosis — Patients with anorectal fistulas usually present with a "non-healing" anorectal abscess following drainage, or with chronic drainage and a pustule-like lesion in the perianal or buttock area. The patient may have pain during defecation, which is usually much less severe than in patients with fissures. The perianal skin may be excoriated and itchy.
Fistulas complicating Crohn's disease or other intraabdominal inflammatory processes may be accompanied by associated bowel symptoms such as diarrhea and abdominal pain. Anorectal fistulas preceding other clinical manifestations of Crohn's disease are uncommon. As an example, in one surgical series of 136 patients with Crohn's disease, fistulas preceded the intestinal manifestations of the disease in only six patients (5 percent) [16]. (See "Perianal complications of Crohn's disease".)
Optimal treatment depends upon correctly classifying the fistula, which requires careful investigation. The external and internal opening, the course of the tract, and the amount of sphincter muscle it incorporates should be identified. To accomplish this, it is often necessary to examine the patient in the operating room under anesthesia. Patients with chronic recurrent fistulas are often evaluated by imaging modalities such as transanal ultrasound, MRI, or a fistulogram [17,18]. (See "The role of imaging tests in the evaluation of anal abscesses and fistulas".)
Treatment — Surgical treatment is required in patients with symptomatic anorectal fistulas, with the exception being patients with Crohn's disease. (See "Perianal complications of Crohn's disease".) The goal of surgical therapy is to eradicate the fistula while preserving fecal continence. The surgical approach depends upon the type of fistula. Thus, surgery begins by gently probing the fistula to determine its anatomy. This maneuver almost always requires that the patient be anesthetized. Identifying the internal opening of the fistula tract is not always straightforward. Many principles and maneuvers have been devised to assist in this task.
One of the most commonly cited principles to assist in the surgical management of a fistula is Goodsall's rule, which states [19]:
•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.
•All tracts with external openings anterior to this line enter the anal canal in a radial fashion.
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 [19].
A detailed review of the preoperative evaluation of an anorectal fistula, procedure selection process, intraoperative preparation, the operative procedures for simple and complex fistulas, and outcomes can be found on the following link. (See "Operative management of anorectal fistulas".)
SUMMARY AND RECOMMENDATIONS — Benign anorectal diseases, including fissures, abscesses, fistulas, and hemorrhoids, are common. However, the prevalence in the general population is probably much higher than seen in clinical practice since the majority of patients with symptoms referable to the anorectum do not seek medical attention.
•The majority of anal abscesses and fistulas originate from infected anal glands. (See 'Pathogenesis' above.)
•Patients with abscesses often present with severe pain in the anal area that may be constant but not necessarily associated with a bowel movement. Additional symptoms may include fever and malaise. (See 'Clinical manifestations and diagnosis' above.)
•The physical examination on most patients reveals an area of fluctuance or a patch of erythematous, indurated skin overlying the perianal or ischiorectal space. In some patients, there are no findings on physical inspection, and the abscess can only be felt via digital rectal examination. (See 'Clinical manifestations and diagnosis' above.)
•Abscesses should be incised and drained and lack of fluctuance is not sufficient to delay treatment. (See 'Anal abscess' above and 'General principles of treatment' above.)
•Anal fistulas are classified in relationship to the anal sphincter and include: intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric fistulas. (See 'Classification' above.) Fistulas can be complex and a thorough knowledge of the pelvic anatomy is essential for surgical management.
•Surgical treatment is usually required in patients with symptomatic anorectal fistulas, with the exception being patients with Crohn's disease. (See 'Anal fistula' above and 'Treatment' above.)
•Goodsall's rule states that 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, and all tracts 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 'Anal fistula' above and 'Treatment' above.)
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