The large majority of anorectal abscesses result from infection of the anal glands and crypts, called crypto-glandular infection, and are thought to be part of the same disease process as anorectal fistula, which is discussed later. The abscesses are the acute manifestation of disease and fistula represent the chronic stage. Anorectal abscesses can affect patients at all ages; however, they most often present during the third decade of life. They are more common in men than women and typically affect healthy individuals; however, there are some conditions that predispose patients to abscess and these include diabetes mellitus, trauma (i.e., foreign body or surgery), Crohn’s disease, malignancy, radiation, human immunodeficiency virus (HIV), or other immunosuppressed states that may leave the patient susceptible to opportunistic infection.

The pathogenesis of an anorectal abscess is thought to start with infection of one of the 6–10 anal glands that lie in the intersphincteric space and normally function to secrete mucous and lubricate stools. These glands traverse the internal anal sphincter and empty into the 10–15 anal crypts, which lie circumferentially around the dentate line. Therefore, infection of the anal gland or crypt, usually by blockage, follows the path of least resistance and spreads along one of several planes in the anorectal region to form a perianal or perirectal abscess. Indeed, anorectal abscesses are classified according to location (Table 31.2), which aids in diagnosis as well as treatment and requires the surgeon to be familiar with the anatomy of anorectal spaces.

The presentation of an anorectal abscess may depend on its location; however, the initial presentation of most abscesses, regardless of location, is anal pain. This pain is usually described as dull or achy and is often independent of defecation. However, patients may note that the pain worsens with straining, coughing, or even walking. In rare circumstances, patients may present with fever, chills, urinary retention, and signs of sepsis suggestive of systemic illness, which should raise the suspicion of a necrotizing soft tissue infection.

Diagnosis starts with a complete history and physical eliciting pertinent past medical history including comorbidities and predisposing risk factors as listed previously, as well as a focus on prior abscesses or prior anorectal surgery. Physical exam should include a thorough abdominal and rectal exam, as well as a bimanual exam in women to rule out involvement of the vaginal wall. In the case of a perianal abscess, external inspection may reveal perianal swelling with associated erythema, cellulitis, and/or fluctuance. Intersphincteric abscess are usually without external signs, yet digital rectal exam will often elicit severe tenderness. Ischiorectal abscesses have the potential to be large and DRE may elicit lateral swelling and pain, however, with less obvious external findings. Supralevator abscess are the most difficult to diagnose since they may be the result of a cephalad progression of perianal infection or a manifestation of an intra-abdominal process such as diverticulitis. Therefore, computed tomography (CT) scan may be required to confirm the diagnosis. If there is ever a doubt as to the diagnosis or location of an anorectal abscess, an exam under anesthesia should be performed to allow for confirmation of the diagnosis as well as an opportunity for treatment.

The treatment for an anorectal abscess is incision and drainage. Perianal abscesses are often superficial and the easiest to drain, and can be performed in the emergency room under local anesthesia. An elliptical or cruciate incision should be made over the most prominent, fluctuant part of the abscess, taking careful measures to avoid injury to the sphincter muscles. The incision should also be made large enough so as to prevent premature closure of the skin before complete drainage of the abscess has occurred, and should also be made close to the anal verge in order to limit the extent of any potential fistulas that may develop in the future. The abscess cavity should then be thoroughly irrigated and loculations broken by either the surgeon’s finger or a blunt hemostat. No packing is necessary if the incision is adequate; however, a superficial dressing is ideal to prevent drainage onto the patient’s clothing. An ischiorectal abscess can be drained by a similar method, but may require a larger incision with a more thorough evacuation of the abscess. In the case of a horseshoe abscess that spreads posteriorly to both ischiorectal fossas, drainage often necessitates regional or general anesthesia in the operating room. At that time, an incision is made either posterior to the anus or over each ischiorectal fossa with a Penrose placed to allow for adequate drainage of the postanal space. The surgical drainage for intersphincteric abscesses is slightly more complicated in that it requires an incision in the anal mucosa overlying the abscess, followed by a partial division of the internal anal sphincter in order to access and fully clear the abscess. Again, packing is not necessary after drainage. Finally, in the case of supralevator abscesses, the treatment requires accurate diagnosis and identification of location prior to drainage. If the cause originates from cephalad spread of an ischiorectal or intersphincteric abscess then the drainage should be performed as discussed previously via the ischiorectal fossa or the rectum, respectively. For those supralevator abscesses caused by an intra-abdominal source, drainage is performed via the most direct route and often requires CT-guided drainage. Post-procedurally, patients should be given analgesia and instructed to take sitz baths three to four times a day to keep the area clean.

If a fistula is identified at the time of draining the abscess, then a seton may be placed through the fistula tract. This will keep the fistula open at both the internal and external openings, and promote drainage. Suture material or silastic vessel loops may be used as the seton. The fistula will not heal while the draining seton is in place, but it will protect the patient from recurrent abscess. Subsequently, the seton must be removed, or the fistula treated operatively in order to heal.

The success of incision and drainage for an anorectal abscess averages 50% with approximately 20–30% developing a recurrent abscess or fistula [16]. Recurrence is thought to be more common with ischiorectal fossa abscesses, potentially secondary to inadequate primary drainage; however, recurrence should also prompt the surgeon to look for a possible underlying disease such as Crohn’s disease or malignancy. Historically, surgeons prescribed post-procedure antibiotics to decrease this relatively high recurrence of abscess or fistula; however, data from a recent randomized control trial demonstrate that adjuvant therapy with antibiotics does not decrease the incidence of fistula formation at 1 year [17]. Certain patient populations should, however, be given adjuvant antibiotics, and likely require hospitalization. These groups include those patients who are immunocompromised, patients with diabetes mellitus, those with prosthetic devices or valvular heart disease as well as those patients with extensive cellulitis or a necrotizing soft tissue infection [18].

The main complication with abscesses, as previously discussed, is abscess recurrence or development of a fistula at the time of diagnosis. The risk of abscess recurrence increases significantly if there is a concurrent anal fistula. Fittingly, the role of primary fistulotomy at the time of anorectal abscess drainage has been debated as a method to reduce recurrence. While initial studies looking at the success of a combined procedure concluded no difference in the rate of abscess recurrence in those patients treated with incision and drainage alone versus the addition of a fistulotomy, a recent Cochrane review demonstrates that simultaneous fistula treatment and incision and drainage of the anorectal abscess does indeed decrease the incidence of persistent or recurrent abscess as well as the need for repeat surgical procedures [19]. A small number of patients may experience incontinence after this combined procedure; however, it is often transient. In our experience, when a fistula is discovered at the same time of abscess drainage, it is best to avoid extensive exploration of the tract due to the risk of creating false passages, and instead we place a seton to identify the tract for future definitive therapy once the inflammation surrounding the abscess has resolved. With proper drainage of the abscess, the fistula may heal on its own without the need for any further procedures.

After definitive incision and drainage of an anorectal abscess, complete healing takes approximately 4–8 weeks. Postoperatively, patients should be evaluated soon after surgery, within 1–2 weeks, and then again closer to 8 weeks to ensure proper healing. Afterward, patients only need to be seen on an as-needed basis based on recurrence of symptoms. Follow-up in patients with underlying Crohn’s disease or those who are immunosuppressed should, however, be more aggressive with almost weekly office visits to ensure healing without signs of perianal sepsis.