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:

• Perianal abscess: the most common (around 50%) - caused by direct extension of sepsis in the intersphincteric plane caudal to the perianal skin.
• Ischiorectal abscess: (22-39%) - results from extension of sepsis through the external sphincter into the ischiorectal space.
• Intersphincteric abscess: (0-18%) - depending on the effort made to find them, sepsis confined to intersphincteric space.
• Supralevator abscess: (2-9%) - produce horseshoe abscess track.
• Postanal abscess: posteriorly based below the level of the anococcygeal ligament.

Epidemiology

• High-risk groups include diabetics, 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 disease¹ or diverticulitis.

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
• 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 diseases, and/or investigation for inflammatory bowel disease, diverticular disease or lower gastrointestinal tract malignancy.
• Proctosigmoidoscopy may be performed to exclude associated diseases.
• Ultrasound scan
• MRI scan: allows the assessment of:²
  • 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

Associated diseases

Fistula-in-ano

• Fistulae occur in 30-60% of patients with anorectal abscesses. Anorectal fistulae may also be associated with diverticular disease, inflammatory bowel disease, malignancy, tuberculosis and actinomycosis.
• 80% of recurrent abscesses are 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.
• Fistulae may be classified as intersphincteric (70%), transphincteric (25%), suprasphincteric (5%), extrasphincteric (less than 1%). Extrasphincteric fistulae are usually not associated with intersphincteric sepsis.

Management

• Prompt surgical drainage
• Medication for pain relief
• Antibiotics are usually not necessary unless there is associated diabetes or immunosuppression
• Low fistulae: lay open with either fistulotomy or fistulectomy³
• High fistulae: may require a defunctioning proximal colostomy; there is also a risk of postoperative faecal incontinence

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.
• However approximately two thirds of patients with rectal abscesses treated by incision and drainage or by spontaneous drainage will develop a chronic anal fistula.⁴
• The number of recurrences requiring surgery can be significantly reduced by initial fistulotomy.³
• Infants and toddlers usually recover very quickly.

Document references

1. Lewis RT, Maron DJ; Anorectal Crohn's disease. Surg Clin North Am. 2010 Feb;90(1):83-97, Table of Contents. [abstract]
2. Cuenod CA, de Parades V, Siauve N, et al; MR imaging of ano-perineal suppurations. J Radiol. 2003 Apr;84(4 Pt 2):516-28. [abstract]
3. Knoefel WT, Hosch SB, Hoyer B, et al; The initial approach to anorectal abscesses: fistulotomy is safe and reduces the chance of recurrences. Dig Surg. 2000;17(3):274-8. [abstract]
4. Hebra A; Perianal Abcess, eMedicine, Feb 2009.

Acknowledgements