An anorectal abscess refers to a collection of pus in the anal or rectal region. They are more common in men than in women.
In this article, we shall look at the pathophysiology, clinical features and management of an anorectal abscess
Anorectal abscesses are thought to be caused by the plugging of the anal ducts. These are structures found in the wall of the anal canal which drain the anal glands. They secrete mucus into the anal canal to ease the passage of faecal matter.
Blockage of the anal ducts causes stasis and allows the normal bacterial flora to overgrow, leading to infection. Common causative organisms include E. Coli, Bacteriodes, and Enterococcus.
The anal glands are located in the intersphincteric space (between the internal and external anal sphincters). Infection can then spread to adjacent areas. Hence anorectal abscesses can be categorised by the area (Fig. 1) in which they occur:
- The most common site of abscess formation
Clinical Features and Diagnosis
Anorectal abscesses are typically intermittent in nature. They present with pain in the perianal region, which is exacerbated when sitting down. Other symptoms include localised swelling, itching, or discharge. Severe abscesses may present with systemic features* such as fever, rigors, general malaise or sepsis.
On examination, an abscess will be red and tender and may be discharging purulent or haemorrhagic fluid. There will likely be some degree of surrounding cellulitis.
The diagnosis of a superficial anorectal abscess is made clinically. In patients where the diagnosis is less clear, a digital rectal examination will often reveal a fluctuant tender mass. However, many patients cannot tolerate this and require examination under anaesthesia. Complicated or chronic disease may require imaging, typically a MRI scan.
*These features are more likely in patients who are immunocompromised or those with ischiorectal abscesses.
Anorectal abscesses have high rates of recurrence and the development of fistulae. There is little room for conservative management; antibiotics may be utilised initially for acute infective states (especially in diabetics or immunocompromised patients) but should not be used routinely.
The main management for anorectal abscesses is surgical drainage, typically performed under general anaesthetic. Drainage should be in a timely manner, preventing tissue damage, including anal sphincter dysfunction. They are then allowed to heal by secondary intention*.
*Any attempt at early closure of a drained abscess is not advised. The insertion of a seton may be considered by experienced surgeons, however this is typically only performed if the tract is clearly identifiable with minimal probing.
- Anorectal fistula 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 surgical drainage followed by packed for healing via secondary intention