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Anal Fistula / Abscess

Christopher Mantyh, MD,
Assistant Professor of Surgery
Duke University Medical Center
Durham, NC


Treatment of perianal abscesses and fistulas has a long history in surgical lore. Special institutes such as St Mark’s Hospital in London (originally named St. Mark’s Hospital for Fistula and other Diseases of the Rectum) were originally established to treat this vexing disease. Curing the fistula while maintaining continence was and still remains of critical importance. This review will discuss the etiology, diagnosis, and treatment options of anorectal abscesses and fistulas.


The most commonly held theory on the etiology of fistulas suggests that an infection of an anal gland results in an abscess, which subsequently develops into a fistula-in-ano. This ‘cryptograndular’ theory contends that one of the approximately ten to fifteen anal glands becomes obstructed with fecal material, foreign bodies or trauma and results in stasis and infection in the gland (1). Additionally, abnormal bowel habits such as forceful diarrhea or hard bowel movements may force stool into the anal gland. A chronically infected gland will eventually lead to epithelialization of the tract if left untreated. If the abscess ruptures or is surgically drained, the epithelialized tract from the anal gland to the abscess then communicates with the perianal skin and results in a draining fistula-in-ano. The crytoglandular theory is supported by a classic manuscript by Parks (2) in which he demonstrated that 90% of specimens from fistula patients had either grossly or microscopically infected glands. Other etiologies of anal fistulas include Crohn’s disease, malignancy, leukemic or neutropenic patients, or tuberculosis.


Patients with an acute anal abscess most often present with pain and peri-anal swelling. The pain is constant and may be quite intense and aggravated with movement. Fever and malaise may also be present if the abscess has been present for several days. On examination, the patient commonly has the classic appearance of infection with redness, warmth, induration, and fluctuance. Pus may be seen emanating from the external abscess or at the internal opening in the anus. Importantly, intersphincteric abscesses may present with similar complaints of intense throbbing pain, but no overt peri-anal inflammation. Digital examination of the anal canal often helps determine the location of intersphincteric abscesses. Supralevator abscesses, although rare, may also present with similar constitutional symptoms but no physical findings. Anal or vaginal examination may again reveal a tender mass.

Chronic abscesses that have been epithelialized eventually become a fistula-in-ano. The external opening is often visualized as a small dimpled area of granulation tissue with drainage of pus or blood on manual compression. Additionally, a small cord-like structure may be palpated representing the fistula tracking into the anal canal. Anoscopy and proctoscopy are important adjuncts to any examination of peri-anal fistulas since it allows for documentation of the internal opening or the presence of other disease states such as inflammatory bowel disease. The internal openings of cryptoglandular fistulas-in-ano are often seen at the dentate line at the origin of the anal gland. Using a small probe, gentle manipulation may reveal the course of the fistula. This maneuver should be done with caution, as it is often quite painful, and forceful probing may result in the creation of false channels. To aid in detection of an occult internal opening, a small amount of dilute peroxide may be infused via an angiocathater into the external opening while examining the anal canal with an anoscope. Bubbling of the peroxide at the internal opening confirms the presence of a fistula. Goodsall’s rule, stating that posteriorly based external openings tract to the posterior midline while anterior openings tend to tract radially is often useful in predicting the course of the fistulous tract.

The differential diagnosis in perianal and perirectal abscesses and fistulas include other diseases that may cause suppuration of the perineum. Pilonidal disease, Bartholin’s glands, or hidradinitis are often confused with abscesses and fistulas, however a good history and examination will usually allow the correct diagnosis. Unusual infections such as tuberculosis or actinomycosis may also present with peri-anal infections. Finally, anal or even rectal cancers can result in pain and infection if left untreated.



The cornerstone of treatment of all anorectal abscesses is surgical drainage. The diagnosis is usually rapidly made and the procedure can be safely carried out either in the office, emergency room, or operating room. Antibiotics are not required unless the patient is immunocompromised or has a prosthetic device in place. The patient is placed in the prone jackknife or left lateral position and the area surrounding the abscess cavity is prepped with an antiseptic solution then anesthetized with an injection of 1% lidocaine. A large-bore needle may be used to locate the abscess cavity by carefully aspirating while probing the cavity. A scalpel is then used to open the skin over the abscess cavity. Larger cavities may require digital or hemostat-assisted exploration to break-up any undrained abscesses. If at any point during the procedure it becomes evident that the abscess cavity is too large to be drained in the office setting or the patient is too uncomfortable, the patient should be taken directly to the operating room for a complete exam under anesthesia. After the initial drainage, one may either chronically drain the abscess cavity either by the placement a small mushroom drain (#12 or 14 pezzer) or by making a wide cruciate incision with removal of the skin edges. Either of these techniques will allow any undrained pus to be expelled. The patient is provided pain medications and a follow-up appointment in one to two weeks at which point the drainage catheter is removed.

As mentioned previously, intersphincteric and supralevator abscesses may not present with any overt signs of sepsis. Both of these are best treated in the operating room with proper surgical equipment including lighted anal retractors. For intersphincteric abscesses, the internal sphincter is divided from its lower end to the dentate line and hemostasis is achieved. Supralevator abscesses are fortunately rare and require careful determination as to their cause. If it is a result of upward extension of an intersphincteric abscess, the abscess should be drained directly into the rectum, however if it is a result of an ischioanal abscess it should be drained via the ischioanal fossa. Supralevator abscesses that are caused from extra-pelvic disease such as Crohn’s disease, diverticulitis, or appendicitis should be approached with circumspection. Cross-sectional imaging with CT scans or contrast studies are useful in determining their origin. These abscesses may be initially drained via the rectum or the ischioanal fossa; however, the offending organ must be addressed, usually via laparatomy.

There is some controversy as to the appropriateness of performing a fistulotomy at the time of diagnosis of an anal or rectal abscess. Advantages include limiting the patient to one definitive operative and anesthetic procedure and a lower recurrence rate (3). Disadvantages include operating in inflamed tissue planes where false passages can be easily made. Additionally, it has been reported that only 16-48% of abscesses will lead to fistula formation making the addition of a fistulotomy often unnecessary and potentially dangerous.


All medically fit patients with a symptomatic fistula in ano should have some sort of surgical intervention. If left untreated, the external opening may temporarily close with a resultant recurrent abscess formation or formation of a more complex fistula. These complex fistulas often traverse deep to the external sphincter making operative repair more hazardous. Additionally, long-standing fistulas have a low but real risk of malignant transformation.

As it was true hundreds of years ago, the treatment of fistula surgery still involves the delicate balance of curing the patient of the fistula while maintaining continence. By definition, all anal and rectal fistulas traverse some portion of the sphincter complex. Pre-operative assessment of the location of the external and internal opening, determination of any contributing medical conditions such as Crohn’s disease, and evaluation of the sphincter function is critical prior to any fistula surgery. Patients should be advised on the potential risks of the surgery as well as the potential change in operative plan if unforeseen conditions are encountered.

Parks, Gordon and Hardcastle (4) have developed a classification of anal fistulas that is based on the their relationship to the external sphincter. This classification is not simply academic; instead the type of fistula found at the time of surgery dictates the proper surgical technique. Fistula surgery is best treated in the operating room under local, spinal or general anesthesia with all available equipment including lighted anal retractors.

Intersphincteric fistulas are the most common fistulas-in-ano representing 70% of all perianal fistulas. These fistulas traverse only the internal sphincter and may extend downward to the perianal skin, extend upwards as a blind tract, or may open into the rectum itself. Excising the fistulous tract over a small fistula probe and removing the epithelialized tract with a curette effectively treats these fistulas. All side branches are also excised. If the internal opening cannot be found with gentle probing, a dilute solution of peroxide, methylene blue, or milk may be injected into the external opening with an angiocathater while examining the anus with an anoscope.

Transsphincteric fistulas pass through the internal and external sphincter into the ischioanal fossa and to the perianal skin. These fistulas usually involve only a small portion of the external sphincter and may be effectively treated with a fistulotomy. If a greater portion (over half) of the external sphincter is involved, a cutting seton may be employed. Silk ties, vessel loops, or even penrose drains are brought around the fistulous tract and secured with a tie to form a cutting seton. The remaining tract is then excised. The seton can then be sequentially tightened in the clinic with additional ties. The seton will allow fibrosis and scaring proximal to the ligature allowing the muscle to hold together while the fistulous tract fibroses.

Suprasphincteric fistulas cross the internal sphincter and pass upward around the external sphincter above the puborectalis muscle. The fistula then tracts downward into the ischioanal fossa and out to the skin. Suprasphincteric fistulas are often treated with a partial fistulotomy of the lower half of the internal sphincter and placement of a seton around the external sphincter.

Extrasphincteric fistulas represent only 2% of all fistulas but some of the most difficult to treat. The fistulous tract extends from the perineal skin to through the ischioanal fossa and levators, eventually existing into the true rectum. The most common cause of these fistulas is iatrogenic from over-aggressive probing of a transsphincteric fistula. Other causes include inflammatory bowel disease, cancer, diverticular disease, or trauma from foreign bodies. Repair of extrasphincteric fistulas includes removal of the fistulous tract and oversewing of the rectal wall. The addition of fecal diversion may be needed depending on the extent and size of the fistulous tract. Extrasphincteric fistulas caused by other diseases such as diverticular abscesses are best treated by control of the septic origin and simple curettage of the tract.

Horseshoe fistulas are the result of circumferential spread of the septic process into the deep post-anal space with secondary extension laterally into the ischiorectal space. Gentle probing or injection with peroxide into the external openings usually confirms the diagnosis. These fistulas are often treated by unroofing of the lateral tracts and opening the deep post-anal space via an extrasphincteric incision posterior to the anus, division of the internal sphincteric component, and placement of a cutting seton. Alternatively, a horseshoe fistula may be managed as a modified Hanley technique where the lateral tracts are incised and curetted and the posterior aspect of the fistula is treated by division of the internal sphincter and placement of a seton around the external sphincter by entry into the deep post-anal space.

Other treatment options

Extensive fistulotomies may result in large unhealed wounds or worse, incontinence. For high-risk fistulas that cross the external sphincter at a high level or in women with anteriorly based fistulas, a rectal advancement flap has been used with success. This technique has been borrowed from the repair of rectovaginal fistulas and includes excision of the internal opening, curettage of the fistulous tract, suture ligation of the fistula, and advancement of a flap of healthy mucosa and submucosa over the previous opening via a tension-free closure. Multiple non-randomized trials attest to the success of this technique (5-8), with healing seen in 70-90% patients. However, factors such as location, type of fistula, and use of temporary fecal diversion were not separated or discussed in the analysis. However, this clearly represents a viable option for complex fistulas where incontinence may occur with a more aggressive approach.

Fibrin glue has been recently added to the armamentarium in the treatment of anorectal fistulas. In this technique, the external and internal opening of the fistula is found and the fistula is extensively cleaned of the epithelial lining using a curette or gauze strip. A double-armed catheter is then inserted into the external opening and the solutions of fibrinogen and thrombin are injected slowing into the fistula tract. Original studies use autologous cryoprecipitate and reconstituted bovine thrombin (9,10) while more recent reports use commercially available donor plasma (11). The collection of fibrinogen and thrombin results in a fibrin clot in the fistula tract that may enhance the migration and activation of fibrinoblasts and the formation of a collagen network. Initial studies demonstrated a high degree of success, however more recent studies have shown less favorable results (12,13). The infusion of fibrin glue does not disrupt the sphincter mechanism and appears to have little to no complications. This technique may lend itself best to high transsphincteric, suprasphincteric fistulas, or recurrent fistulas.


Treatment of anorectal abscesses and fistulas remains one of the benchmark procedures for the colorectal surgeon. Understanding the etiology and anatomy of perianal sepsis will enable the practitioner to effectively treat these diseases. Newer treatments to cure the disease while preserving continence are being actively investigated.


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