Anorectal abscesses

In 60 percent of cases the pus from the abscess yields a pure culture of Escherichia coli; in 23 per cent a pure culture of Staphylococcus aureus is obtained. In diminishing frequency, pure cultures of Bacteroides, a Streptococcus or Proteus strain are found. In many cases the infection is mixed. In a high per­centage of cases — some estimate it as high as 90 per cent —the abscess commences as an infection of an anal gland (Figs 61.34 and 61.35). Other causes are penetration of the rectal wall, e.g. by a fish bone, a blood-borne infection or an extension of a cutaneous boil. Underlying rectal disease, such as neoplasm and particularly Crohn’s disease, may be the cause. Similarly patients with generalised disorders, such as diabetes and more recently AIDS, may present with an anorectal abscess. The latter patients usually have abscesses which run an aggressive course.

A large percentage of anorectal abscesses coincides with a fistula in ano. For this reason, anorectal abscess becomes a highly important subject. Moreover, as antibiotics cannot reach the contents of an abscess in adequate concentration, no reliance can be placed on antibiotic therapy alone. A fis­tula is much more likely if bacterial culture of the pus discloses bowel (as opposed to skin) organisms (Grace).

Differential diagnosis

The only conditions with which an anorectal abscess is likely to be confused are an abscess connected with a pilonidal sinus, Bartholin’s gland or Cowper’s gland.

Classification

A clear understanding of suppuration in this area is dependent on a concise knowledge of the anatomy (Figs 61.34 and 61.35). There are four main varieties: perianal, ischiorectal, submucous and pelvirectal.

Perianal (60 per cent)

This usually occurs as the result of suppuration in an anal gland, which spreads superficially to lie in the region of the subcutaneous portion of the external sphincter (Fig. 61.35a).

It may also occur as a result of a thrombosed external pile. If the haematoma is not evacuated, it may become infected and a perianal abscess results. This is the most common abscess of the region. Persons of all ages are affected and the condition is not uncommon, even in infancy and childhood. The constitutional symptoms and the pain are less pronounced than in the ischiorectal abscess because the pus can expand the walls of this part of the intermuscular space comparatively easily. Early diagnosis is made by inspecting the anal margin, when an acutely tender, rounded, cystic lump about the size of a cherry is seen and felt at the anal verge below the dentate line.

Treatment. No time should be lost in evacuating the pus.

Operation. Thorough drainage is achieved by making a cruciate incision over the abscess and excising the skin edges  this completely removes the ‘roof’ of the abscess.

Ischiorectal abscess (30 per cent)

Commonly, this is due to an extension laterally through the external sphincter of a low intermuscular anal abscess (Fig. 61.35b). Rarely, the infection is either lymphatic or blood borne. The fat, which fills the ischiorectal fossa (Fig. 61.36), is particularly vulnerable because it is poorly vascularised; consequently it is not long before the whole space becomes involved. The ischiorectal fossa communicates with that of the opposite side via the postsphincteric space, and if an ischiorectal abscess is not evacuated early, involvement of the contralateral fossa is not uncommon. Should an internal opening into the anal canal ensue, a ‘horseshoe’ abscess develops enveloping the whole of the posterior part of the circumference of the anal canal (cf. horseshoe fistula).

An ischiorectal abscess gives rise to a tender, brawny induration palpable on the corresponding side of the anal canal and the floor of the fossa. Constitutional symptoms are severe, the temperature often rising to 38—390C. Men are affected more often than women.

Treatment. Operation should be undertaken early — as soon as it is certain that an abscess is present in this area — remembering that antibiotic therapy often masks the general signs.

Operation. Stage 1. A cruciate incision (Fig. 61.37 inset) is made into the abscess. A portion of skin is sometimes excised (Fig. 61.37) but deroofing is not necessary in every case.

Stage 2. As soon as the acute infection has subsided, the wound should be re-examined, preferably under general anaesthesia. A careful search is made for a fistulous opening communicating with the anal canal. If such is found, the treatment should be as for fistula. If no fistula is found, the cavity should be lightly packed with gauze wrung out in any weak antiseptic favoured by the operator. A T-bandage is applied. When the cavity has become covered with granulation tissue, skin grafting may help to expedite final epithelialisation.

Submucous abscess

Submucous abscess (5 per cent) occurs above the dentate line (Fig. 61.35c). When it occurs after the injection of haemorrhoids, it always resolves. Otherwise, it can be opened with sinus forceps when adequately displayed by a proctoscope.

Pelvirectal abscess

Pelvirectal abscess is situated between the upper surface of the levator ani and the pelvic penitoneum (Fig. 61.35d). It is nothing more or less than a pelvic abscess and, as such, is usually secondary to appendicitis, salpingitis, diverticulitis or parametritis. Abdominal Crohn’s disease is an important cause of pelvic disease that can present as perianal sepsis (cf. fistula in ano). A relevant point to remember is that, rarely, a supra­levator abscess/fistula may be due to overenthusiastic attempts to drain an ischiorectal abscess or to display a fistula, when a probe is forced through the levator ani/rectal wall from below.

Fissure abscess

This is the name given to a subcutaneous abscess lying in immediate association with an anal fissure. Drainage is achieved at the same time as the fissure is treated by sphincterotomy.