Anorectal Abscess

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Last updated: January 23, 2022
Revisions: 21

Last updated: January 23, 2022
Revisions: 21

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Introduction

An anorectal abscess refers to a collection of pus in the anal or rectal region. They are more common in men than in women and have high rates of recurrence. One third of patients with an anorectal abscess have an associated perianal fistula at the time of presentation.

Pathophysiology

Anorectal abscess are though to be caused by plugging of the anal ducts, which drain the anal glands in the anal wall (normally helping to ease the passage of faecal matter using mucus secretions). Blockage of an anal duct results in fluid stasis, which will lead to infection. Common causative organisms include E. coli, Bacteriodes spp., and Enterococcus spp..

The anal glands are located in the intersphincteric space (between the internal and external anal sphincters), therefore infection from the glands here spreads to adjacent areas. Anorectal abscesses can be categorised by the area (Fig. 1) in which they occur: (1) Perianal* (2) Ischiorectal (3) Intersphincteric (4) Supralevator

*The perianal area is the most common site of abscess formation

Fig 1 - Classification of anorectal abscess by location.

Figure 1 – Classification of anorectal abscess by location

Clinical Features

Anorectal abscesses present with severe pain in the perianal region, worse with direct pressure (i.e. when sat down), alongside potential perianal discharge or bleeding. Severe abscesses may present with systemic features* such as fever or rigors, general malaise, or clinical features of sepsis.

On examination, there will be a erythematous, fluctuant, tender perianal mass (Fig. 1), which may be discharging pus or have surrounding erythema and induration. Deeper abscesses may not have any obvious external signs, however produce severe tenderness on PR examination.

Fig 2 - A perianal abscess, with overlying skin changes.

Figure 2 – A perianal abscess, with overlying skin changes

Investigations

The diagnosis is typically a clinical one. However, as the majority will require surgical intervention (see below), then routine bloods (FBC, U&Es, clotting, and Group & Screen) are typically performed.

Patients with an anorectal abscess without any known fistula-in-ano or other rectal pathology should have a HbA1c sent to check for any underlying diabetes mellitus.

For patients with atypical presentation or in those suspicious for a supralevator abscesses, complex perianal fistula, or perianal Crohn’s disease, then additional imaging is required where possible (ideally an MRI pelvis scan, however CT imaging if not available)

Management

Patients should be started on antibiotic therapy, as guided by local protocol, and provided with sufficient analgesia.

The main management* for anorectal abscesses is with an EUA Rectum and I&D (incision and drainage) of the abscess, which should always be performed under general anaesthetic wherever possible. These can be left to heal by secondary intention(with or without packing)

Intra-operative proctoscopy should be performed to check for the presence of any identifiable fistula-in-ano. If a fistula is identified, the insertion of a seton can be considered by experienced surgeons.

Limited data has suggested that use of post-operative antibiotics following drainage of anorectal abscess may lower the risk of fistula formation.

*In immunocompetent patients with a small perianal abscess and without systemic signs of sepsis, then rarely an antibiotic approach alone (i.e. without surgery) can be trialled

Key Points

  • Anorectal abscess are caused by blockage of the anal ducts, resulting in stasis and bacterial flora overgrowth
  • Diagnosis is typically clinical, however MRI imaging can be used in complicated or chronic disease
  • Management is via incision and drainage followed by proctoscopy, to heal via secondary intention