Introduction

A perianal abscess is a collection of pus within the subcutaneous tissue of the anus that has tracked from the tissue surrounding the anal sphincter.

Epidemiology

  • Incidence: 25.00 cases per 100,000 person-years
  • Peak incidence: 30-40 years
  • Sex ratio: more common in males 2.5:1
<1 1-5 6+ 16+ 30+ 40+ 50+ 60+ 70+ 80+

Aetiology

Associated conditions
  • Any anorectal abscess can be caused by an underlying inflammatory bowel disorder, especially Crohn's;
  • Diabetes mellitus is a risk factor due to its ability to affect wound healing;
  • Underlying malignancy can cause these abscesses as well as other anorectal lesions due to the risk of bowel perforation.

Pathophysiology

Causes
  • They are generally colonised by gut flora such as E. coli;
  • Those caused by organisms such as Staph. aureus are more likely to be an infection of the skin rather than originating from the digestive tract.

Clinical features

Features
  • Patients may describe pain around the anus, which may be worse on sitting;
  • They may have also discovered some hardened tissue in the anal region;
  • There may be pus-like discharge from the anus;
  • If the abscess is longstanding, the patient may have features of systemic infection.

Investigations

Investigations
  • Most perianal abscesses can be detected through inspection of the anus and digital rectal examination;
  • When querying the underlying cause, colonoscopy and blood tests such as cultures and inflammatory markers may be of use;
  • Imaging such as MRI and transperineal ultrasound can be useful tools, with the former being the gold standard in imaging anorectal abscesses. They are however rarely used except for cases where the abscess has complications or is part of a more serious underlying process such as IBD.

Differential diagnosis

As stated above 'perianal abscess' refers to a simple abscess of the subcutaneous tissue. There are numerous other anorectal abscesses which can be classified by the layers and planes that they occupy.
  • Ischiorectal abscesses are found between the obturator internus muscles and the external anal sphincter;
  • Supralevator abscesses form when infection tracks superiorly from the peri-sphincteric area to above the levator ani;
  • Intersphincteric abscesses are rare (2-5% of cases) and as their name suggests are sited between the internal and external anal sphincters;
  • The pelvis is notorious for the presence of potential spaces, which can become sites of infection. One such incidence of this is the formation of a horseshoe abscess, a reference to their shape. These are found in a potential space between the coccyx and the anal canal, and can be the result of complication of another type of anorectal abscess, such as a supralevator abscess.

Management

Treatment
  • Treatment is usually surgical, with incision and drainage being first line, usually under local anaesthetic. The wound can then either be packed or left open, in which case it will heal in around 3-4 weeks;
  • Antibiotics can be of use, but are only usually employed if there is systemic upset secondary to the abscess, as they do not seem to help with healing of the wound after drainage.