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.
Anorectal abscess are though to be caused by plugging of the anal ducts, the ducts that drain the anal glands in the anal wall, helping to ease the passage of faecal matter through mucus secretion.
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 are thus 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
Anorectal abscesses present with pain in the perianal region, which becomes exacerbated when sat down. Other symptoms include localised swelling, itching, or discharge. Severe abscesses may present with systemic features* such as fever, rigors, general malaise, or features of sepsis.
On examination, there will be a erythematous, fluctuant, tender perianal mass (Fig. 1), which may be discharging pus or have surrounding cellulitis.
Deeper abscesses may not have any obvious external signs, however produce severe tenderness on digital rectal exam, therefore require a further examination under anaesthesia for full assessment.
Complicated, unclear, or chronic disease may require additional imaging, either a CT or MRI scan.
*These features are more likely in patients who are immunocompromised or those with ischiorectal abscesses.
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 incision and drainage procedure, which should always be performed under general anaesthetic. These can be left to heal by secondary intention.
Once drained, 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, however this should only be performed if the tract is clearly identifiable with minimal probing.
Limited data has suggested that use of post-operative antibiotics following drainage of anorectal abscess may lower the risk of fistula formation.
- 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