A clear understanding of anorectal anatomy and pathology are essential for the successful management of this common surgical problem.

The anal canal and rectum are surrounded by a number of potential tissue spaces, and anorectal abscesses are classified according to which of these they occupy (Fig. 1) 1169. Perianal and ischiorectal abscesses are encountered most frequently, representing 80 per cent of cases. Intermuscular and perirectal abscesses are much less common. It should be remembered that sepsis can spread with time and more than one space can therefore be affected simultaneously.

In about 20 per cent of patients with anorectal abscess there is a clear predisposing cause, such as inflammatory bowel disease (especially Crohn's disease), anorectal cancer, anal fissure, complicated haemorrhoids, or local trauma. Perirectal abscesses, lying between the levator ani and the pelvic peritoneum, can occur secondary to infection of another pelvic structure such as a fallopian tube or the prostate. In the majority of cases, however, no obvious cause can be demonstrated, and sepsis arising in an anal gland is frequently, but not invariably the culprit. There are 6 to 10 such glands distributed around the anal canal which drain into the base of the anal crypts. Glands commonly ramify into the internal anal sphincter and can extend as far as the intersphincteric plane. Perianal infection may develop when a gland fails to drain adequately. If this results in abscess formation, then the communication with the anal canal often leads to involvement of either the internal anal sphincter or the intersphincteric plane (or both). Other sources of infection are organisms that invade from the perianal skin, and organisms that enter from a distant site by haematogenous spread. The former is the more common of these.

Culture of pus from an anorectal abscess generally discloses one of two populations of pathogens whose presence correlates with the underlying aetiology: skin-derived organisms such as Staphylococcus aureus and members of the normal gut flora. The former are rarely associated with a communication between the abscess and the gastrointestinal tract, whilst isolation of the latter makes such a communication very likely. This difference has an important bearing on management: it is essential to send a specimen of pus for microbiological examination in all cases of anorectal abscess. Microbiological investigation may also uncover infection with less commonly encountered pathogens such as Mycobacterium tuberculosis or Actinomyces.

Anorectal abscesses affect both sexes but are 2 to 3 times more common in men than women. Abscesses can occur at any age, but are most common in the fourth to the sixth decades of life.

The position of an anorectal abscess largely determines its mode of presentation as well as any associated symptoms. Pain is a prominent initial feature of perianal and superficial ischiorectal abscesses, followed by local signs of inflammation. Such symptoms are less evident or may even be absent with deep infections, which tend to develop insidiously with pyrexia and systemic upset. This can lead to diagnostic confusion.

In all cases, it is important to establish whether there is a history of previous episodes of anorectal sepsis and how these were treated. The possibility of a predisposing cause should be explored.

Superficial lesions produce obvious signs of acute inflammation. In the case of a perianal abscess, there is a localized, fluctuant, red, hot, and tender swelling close to the anus. Such signs are more diffuse in patients with ischiorectal sepsis, where fluctuance is a late finding. Other features that might be noted are skin necrosis, if there is gross swelling, and crepitus if a gas-forming organism is present.

Deeper infections produce less obvious abnormalities, and these are only apparent on digital rectal examination. The diagnostic clue in such instances is the presence of a tender mass or an area of induration. Fluctuance may be detected. It is important to ascertain the position of such lesions with respect to the gut tube and the pelvic floor as this can have an important bearing on subsequent management.

In most instances, diagnosis is established from the history and examination. Where doubt remains, endoluminal ultrasound (anal and rectal) as well as computerized axial tomography (CAT) may be helpful. Examination under anaesthesia as a primary diagnostic procedure may be required.

If the clinical findings suggest a predisposing condition, it should be investigated on its own merits. Locally invasive procedures such as sigmoidoscopy should be performed under general anaesthetic to prevent undue discomfort to the patient.

This is achieved by incision and drainage after the patient has been examined under an appropriate anaesthetic. The site of drainage will generally be dictated by the clinical findings, but should be placed over the most dependent part of the abscess, as close to the anal canal as possible. Linear incisions are recommended as these cause minimum tissue damage. The abscess cavity should be examined for possible extensions and all loculi must be broken down. The cavity should be curetted and necrotic tissue excised. Additional treatment with antibiotics is not usually required.

It is clearly important to establish whether there is a communication between the abscess and the gastrointestinal tract. A specimen of pus should be sent for microbiological examination and the anal canal and abscess cavity should be examined for the presence of a fistula. Gentle pressure on the abscess may cause release of pus into the anal canal, demonstrating the site of an internal opening. If a fistula (see Section 20.5) 148 is thought to exist, its path can be explored with a Lockhart–Mummery probe, and it can either be laid open, if it is very low, or it can be marked with a seton for subsequent definitive treatment. If a fistula cannot be demonstrated then no further immediate measures are required.

Further intervention is not indicated if the pus specimen contains skin flora only. However, additional measures are recommended if gut-derived organisms are cultured, since the possibility of an underlying fistula remains. Ideally, the patient should be reviewed under anaesthetic 7 to 10 days after the initial procedure. This should be a sufficient delay to allow inflammation to subside and so increase the chance of demonstrating any fistula that may be present. A detailed account of fistula management is given in Section 20.5. 148

Treatment protocols for anorectal abscesses include simple drainage only and drainage followed by primary closure combined with systemic antibiotic therapy. Such measures can be effective but they reduce the chance of demonstrating an underlying communication with the gut and thus increase the possibility of future anorectal sepsis.

Goligher J. Anorectal abscess. In: Goligher J, ed. Surgery of the Anus, Rectum and Colon. London: Bailliere Tindall, 1984: 167–177

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Grace RH. The management of acute anorectal sepsis. Ann R Coll Surg 1990; 72: 160–2.