Anorectal Abscess and Fistula- When to Call the Surgeon

Charles A. Ternent, M.D.

Anorectal abscess and fistula are common pathologic findings which have been described from the beginning of medical history. Anorectal abscess is the acute inflammatory process that often is the initial manifestation of an underlying anorectal fistula. An abscess in the anorectal area may also be associated with other pathology such as foreign body, trauma, malignancy, radiation, immunocompromised states (leukemia, AIDS), infectious dermatitides like hidradenitis suppurativa, tuberculosis, actinomycosis, Crohn’s diseases and an anal fissure. Anorectal abscess may also be seen after anorectal surgery such as hemorrhoidectomy and sphincterotomy. The cryptoglandular theory of anorectal abscesses proposes the etiology of this disease process to be secondary to plugging of one of the six to twelve anal ducts that are found circumferentially at the dentate line. Anal gland infection or cryptitis can lead to abscesses that are perianal, ischiorectal, intersphincteric and supralevator. Abscess and fistula occur more commonly in men than women with a ratio of 3 to 1, most individuals are in their third or fourth decade and the highest incidence is in the Spring and Summer.

Perianal abscess if the most common form of the acute anorectal infections (40-45%) and presents as a superficial tender mass outside the anal verge. The presentation is that of a short history of anorectal pain and swelling generally without fever or leukocytosis. Physical examination shows an area of erythema, induration or fluctuance. An abscess should be drained in a timely manner. A superficial abscess can be drained in the office under local anesthesia. A large-bored hypodermic needle inserted into an area of induration is a simple diagnostic test. If purulent material is present then a cruciate incision under local anesthesia can be made with drainage of the abscess. We usually recommend excision of the skin corners of the cruciate incision to allow adequate drainage over the ensuing days. We do not recommend packing of the abscess wound to allow drainage and instruct the patient to start sitz bath TID and after bowel movements. If the patient is too tender to permit an exam then and exam under anesthesia and abscess drainage can be carried out in the operating room. Antibiotics have a limited role in the treatment of anorectal abscesses and their use is limited to cases of valvular heart disease, immunosuppresion, extensive cellulites or diabetes. The patients are usually seen back in the office in 7 to 14 days for a re-evaluation of the area and to rule out an underlying fistula or recurrent abscess. Patients should be made aware of the almost 50% risk of developing an anal fistula following drainage of an anorectal abscess.

Ischiorectal abscess may present as a large, erythematous and indurated tender buttock mass or with pain alone and encompasses 20-25% of anorectal abscesses. Needle aspiration will usually resolve any questions regarding the presence of purulent material. This type of abscess is commonly associated with the development of anal fistulas and therefore should be drained as close to the anal opening as possible to minimize the length of any fistula tract. Following drainage, patients are instructed to return to the office in 7-14 days to re-evaluate. Contraindications for drainage in the office include the patient’s unwillingness or inability to undergo drainage under local anesthesia alone, lack of adequate instruments and exposure, sepsis, insulin dependent diabetes and immunocompromised hosts. In these cases parenteral antibiotic use along with drainage under anesthesia would be advisable. A deep postanal space abscess usually presents as the initial manifestation of a posterior transsphincteric fistula. Patients usually complain of severe rectal discomfort with radiation to the sacrum, coccyx or buttocks. The anorectal pain may be exacerbated by sitting or bowel movements, but differs from proctalgia fugax in that usually pyrexia is present and the pain is continuous in nature. Physical exam is remarkable for posterior rectal tenderness, but these “hidden” posterior abscesses are easily missed and can lead to sepsis, Fournier’s gangrene and to expansion to a half or fully circumferential horse-shoe abscess. These postanal and horse shoe abscesses require drainage in the operating room where the deep postanal space can be entered through a limited posterior internal sphincterotomy over the affected anal gland and crypt. The splayed superficial external sphincter leads way to the deep postanal space. A counter incision or incisions may need to be made to drain extensions of the posterior abscess to one or both ischiorectal fossas (horse-shoe components).

An intersphincteric abscess also starts at an anal crypt and tracts between the internal and external sphincters in cephalad or caudal form. They represent 2-5% of anorectal abscesses. Patients usually complain of anorectal discomfort, fullness and may have purulent discharge. Physical exam may show a submucosal rectal mass and in 25% these abscesses are associated with an anal fissure . Purulent discharge from an internal opening usually establishes the diagnosis. Drainage of this type of abscess is also not an office procedure and requires a limited internal sphincterotomy which is left open to granulate.

A supralevator abscess is rare and comprises less than 2.5% of anorectal abscesses. These have been associated with diabetes and obesity and present with fever, leukocytosis and buttock pain. There is usually a history of recent abdominal surgery, underlying pelvic inflammatory disease or Crohn’s disease. These abscesses may also be associated with a cephalad extension of an intersphincteric or transsphincteric abscess. Treatment mandates knowledge of the underlying pathology. Pelvic inflammation can usually be drained through the rectum or vagina whereas abscesses secondary to trans or intersphincteric pathology benefit from external perianal drainage. These are complex processes that require imaging studies such as CT scan to determine the nature of the pelvic pathology and a thorough exam in the operating room to identify potential internal openings at the dentate line.

Anal fistula usually presents with symptoms of anal pain, swelling and discharge. Most patients with an anal fistula have a history of a prior drained anorectal abscess. The classification of anal fistulas follows that for anorectal abscesses and includes submucosal, intersphincteric, transsphincteric, suprasphincteric and extrasphincteric. Fistulas are connecting tracts between two epithelialized surfaces. Intersphincteric fistulas account for 55 to 70% of anorectal fistulas and are the most common. They are usually treated with fistulotomy and unroofing of the tract from the internal to the external opening. A transsphincteric fistula passes through both the internal and external anal sphincters en route to the skin. Theses are always drained externally and may require placement of a seton to protect the involved external sphincter component as part of a staged fistulotomy. Other alternative therapies include the use of biomaterials such as fibrin glue (40-50% success rate) or the anal fistula plug (up to 85% effective in some studies) and the use of endoanal advancement flaps to close the internal opening with failure rates as high as 30-50% as well as the use of staged partial fistulotomies and setons. Transsphincteric fistulas account for 20 to 25% in most series.

Suprasphincteric fistulas account for 1 to 3% of anorectal fistulas and usually start in the intersphincteric plane, tract above the levators into the pelvis and then tract caudal toward the ischiorectal fossa external opening. Treatment of these fistulas usually depends on the underlying pelvic inflammatory process (such as diverticular disease, Crohn’s disease or a failed anastomosis from a recent pelvic bowel resection) and its successful treatment. Extrasphincteric fistulas have an internal opening above the pelvic floor and an external opening in the perianal or buttock skin outside the sphincter mechanism. These can also be treated with staged fistulotomy and seton placement or endoanal advancement flaps or biomaterials to attempt and close the internal opening. The fistulotomy route with seton placement in the more complex fistulas involving a significant amount of external sphincter carries a greater success rate than the glue or advancement flap techniques but is also associated with a greater degree of fecal incontinence issues. The fistula plug may be another alternative treatment in this scenario.

Identification of the fistula tract path can be performed with the application of Goodsall’s rule, physical examination looking for the thickened tract proceeding into the anal canal, probing of the tract and the use of injection and imaging techniques. Goodsall’s rule dates back to a chapter on anal fistulas written by Dr. Goodsall in 1900 and it helps to identify the likely place of an internal fistula opening at the dentate line relative to the perianal location of the external opening. This rule is a guide and states that an anterior external opening has a radially inward located internal opening. When the external opening is located posteriorly then the internal opening tends to be localized in the posteromidline. In most circumstances examination under anesthesia is required to identify and treat the fistula and seldom would this procedure be performed in the office setting.

Fistulography to evaluate the nature of an anorectal fistula has been replaced by endoluminal ultrasonography and MRI. Endoluminal ultrasound is an accurate and minimally invasive technique for delineating the relationship between fistula tracts and the sphincter mechanism and deeper areas of sepsis. The addition of hydrogen peroxide injected through the external opening to this technique, further enhances the ability to identify the tract and the internal opening. Similarly MRI and endoluminal MRI can also provide high-resolution images of collections and tracts. Although simple fistulas may not require these imaging techniques, complex or recurrent fistulas and perianal septic conditions may benefit from their selective use.

Ultimately, the success of fistula surgery is measured in terms of recurrence, delayed healing and fecal incontinence. The most likely cause of recurrence is the failure to identify and appropriately treat the internal opening. Factors associated with recurrence include complex type of fistula, horseshoe extension, lack of identification or lateral location of an internal fistula opening, prior fistula surgery and the surgeon variable. Recurrence rates are at least 4-10% for conventional fistulas. Studies have shown that following treatment for intersphincteric fistulas 17% of patients demonstrate incontinence to flatus and occasional soiling. One third of patients treated for transsphincteric fistulas had one or the other complaint of incontinence.

In summary, the treatment of anorectal abscesses and fistulas should be tapered to the extent of the acute and chronic suppurative process. Except for the simple perianal abscesses or a readily drained ischiorectal abscess, therapy must be carried in the operating room under adequate anesthesia and exposure. In view of the high rate of fistula formation after abscess drainage, patients should be informed of this 50% risk and the possibility of requiring further therapy at a later date with associated risk of varied levels of fecal incontinence.

Selected Reading

Corman ML. Anal fistula in Colon and Rectal Surgery, 5th Edition, Lippincot Williams and Wilkins, Philadelphia 2005:295

Corman ML. Anorectal abscess in Colon and Rectal Surgery, 5th Edition, Lippincot Williams and Wilkins, Philadelphia 2005:279

Parks AG. Pathogenesis and treatment of fistula in ano. Br Med J 1961;1:463.

Ramanujam PS, Prasad ML, Abcarian H, Tan AB. Perianal abscesses and fistulas: a study of 1023 patients. Dis Colon Rectum 1984;27:593

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Anorectal Abscess and Fistula- When to Call the Surgeon

Charles A. Ternent, M.D.