Anorectal abscess

An anorectal abscess is a collection of pus in the anal or rectal region. It may be caused by infection of an anal fissure, sexually transmitted infections or blocked anal glands.

The following anatomical types have been identified¹:

• Perianal abscess: the most common (60%) - caused by direct extension of sepsis in the intersphincteric plane caudal to the perianal skin.

• Ischiorectal abscess: (20%) - results from extension of sepsis through the external sphincter into the ischiorectal space.

• Intersphincteric abscess: (5%) - depending on the effort made to find them, sepsis confined to the intersphincteric space.

• Supralevator abscess: (4%) - produces horseshoe abscess track.

• Postanal abscess: posteriorly based below the level of the ano-coccygeal ligament.

Epidemiology

• High-risk groups include those with diabetes, immunocompromised patients, people who engage in receptive anal sex and patients with inflammatory bowel disease.

• Deep rectal abscesses may be caused by intestinal disorders such as Crohn's disease or diverticulitis².

• Studies suggest that most patients are between the ages of 20 to 60 with a mean age of 40. The male-to-female ratio is 2:1³.

Presentation

• Symptoms include painful, hardened tissue in the perianal area, discharge of pus from the rectum, a lump or nodule, tenderness at the edge of the anus, fever, constipation or pain associated with bowel movements.

• The perianal pain is usually constant, throbbing and worse when sitting down.

• A rectal examination may confirm the presence of an anorectal abscess.

• Superficial perianal abscesses may occur in infants and toddlers. The abscess often appears as a swollen, red, tender lump at the edge of the anus. The infant may have discomfort but no other symptoms.

Differential diagnosis

• Inflammatory bowel disease - eg, Crohn's disease, ulcerative colitis.

• Anal carcinoma.

• Colorectal cancer.

Investigations

• A digital rectal examination is usually sufficient for the diagnosis and the treatment planning of anal abscesses and fistulae.

• Initial investigation will depend on presentation but may include a screen for sexually transmitted infections and/or investigation for inflammatory bowel disease, diverticular disease or lower gastrointestinal tract malignancy.

• Proctosigmoidoscopy may be performed to exclude associated diseases.

• MRI scan: allows the assessment of³:
  

  • Location of any fistular tracts.

  • Location of the internal and external opening(s) of any fistula.

  • Location of deep abscesses.

  • The state of the anorectal wall and the perirectal spaces.

  • Any damage to the anal sphincter.

• Transperineal ultrasound may be a useful adjunct⁴.

Associated diseases

Fistula in ano³

• Fistulae occur in approximately 40% of patients with anorectal abscesses. Anorectal fistulae may also be associated with diverticular disease, inflammatory bowel disease, malignancy, tuberculosis and actinomycosis.

• One study found that 68% of recurrent abscesses were associated with a fistula.

• Goodsall's rule: an external opening situated behind the transverse anal line will open into the anal canal in the midline posteriorly. An anterior opening is usually associated with a radial tract. Goodsall's rule has been challenged as being inaccurate; cases have been reported where the external opening is off-midline⁵.

• Fistulae may be classified as intersphincteric (25% in one study), trans-sphincteric (29.16%), suprasphincteric (8.33%), and extrasphincteric (12.5%). Extrasphincteric fistulae are usually not associated with intersphincteric sepsis⁶.

Management

• Prompt surgical drainage.

• Medication for pain relief.

• Traditionally, antibiotics have not been considered necessary unless there was an underlying condition such as diabetes or immunosuppression. However, one study found that postoperative prophylactic antibiotic therapy including ciprofloxacin and metronidazole played an important role in preventing fistula-in-ano formation after incision and drainage of a simple perianal abscess⁷.

• A Cochrane review found that fistula surgery with abscess drainage significantly reduced recurrence or persistence of abscess/fistula, or the need for repeat surgery, compared to abscess drainage alone⁸.

• Low fistulae: lay open with either fistulotomy or fistulectomy⁹.

• High fistulae: may require a defunctioning proximal colostomy; there is also a risk of postoperative faecal incontinence¹⁰. Faecal diversion, however, is not always necessary. One study described the treatment of high fistula by fistulectomy and reconstruction (primary suture repair) of anal sphincter without stool diversion¹¹.

• An advancement flap (a section of mucosa removed from the anal lining) may be used to close the defect. This technique has been found particularly helpful in Crohn's disease¹². A biosynthetic anal fistula plug has been developed and the National Institute for Health and Care Excellence (NICE) recommends its use, providing standard consent and audit procedures are followed¹³.

• Clinical trials assessing adipose stem cells, collagen, dermal matrix and numerous other techniques are ongoing¹⁴¹⁵.

Complications

• Systemic infection.

• Fissure in ano occurs in up to 30% of patients (the risk is reduced by early surgical drainage).

• Recurrence.

• Scarring.

Prognosis

• The outcome is good if the abscess is treated promptly.

• Fistula in ano in children often resolves spontaneously and immediate surgical treatment should be withheld¹⁶.