Abscesses in the anorectal region occur in all age groups and are a common, distressing problem. Predisposing factors include diabetes, Crohn disease, previous perianal surgery, and impairment of immunity, including human immunodeficiency virus (HIV); however, most abscesses occur spontaneously in healthy persons. Diagnosis is usually established by clinical examination, and adequate drainage is the treatment of choice. Drainage results in healing in more than half of cases. Persistence or recurrence is due to inadequate drainage, predisposing factors, or a fistula in ano.
Based on a study of patients treated for anorectal abscesses at Cook County Hospital and followed up for a 35-month period, Read and Abcarian reported in 1979 that the peak incidence was in the third decade, with males affected 1.76 times more frequently than females. The most common abscess in this series was perianal (42%), followed by ischiorectal in 20% and supralevator in 7%. An anal fistula could be demonstrated in 34% of patients. A subsequent report from Cook County Hospital in 1984 confirmed a similar relative frequency of anorectal abscesses (42.7% perianal, 22.7% ischiorectal, 21.4% intersphincteric, and 7.3% supralevator). Intersphincteric and supralevator abscesses had a higher incidence of concomitant fistula. Other study investigators have also found that the perianal variety is the most common and the intersphincteric variety is most commonly associated with a fistula in ano.
A brief review of the anatomy of the spaces surrounding the anorectum ( Fig. 4-1 ) helps in the understanding of the etiology, pathways of the spread of infection, and clinical presentation, all of which have a bearing on the management of anorectal abscesses.
The perianal space surrounds the anus and becomes continuous with the fat of the buttocks. The intersphincteric plane separates the external and internal sphincter muscles, is continuous with the perianal space, and extends superiorly into the rectal wall. Anal glands are found in the intersphincteric plane, traverse the internal sphincter, and empty into the anal crypts at the dentate line. Lateral to the anus is the ischiorectal space, which is bounded superiorly by the levators, medially by the external sphincter, laterally by the ischial tuberosity, and inferiorly by the transverse perineal septum. The two ischiorectal fossae are connected posteriorly through the deep postanal space between the levators and the anococcygeal ligament. The supralevator space lies superior to the levator ani on either side of the rectum.
The cryptoglandular theory, which states that sepsis originates in the anal glands because gland openings in the crypts are blocked, is the most widely accepted explanation for the development of anorectal abscesses. The glands extend into the surrounding sphincter muscles, and when infection is present, sepsis extends to a variable extent into and between these muscles and tracks along lines of least resistance. The findings of one study in the United Kingdom suggest that patients with fistula in ano in addition to an abscess are more likely to have gut aerobes (predominantly Escherichia coli ) or gut-specific anaerobes ( Bacteroides fragilis ) isolated from the pus than are persons without fistulae.
Natural History of the Disease and Spread Pathways
The intersphincteric plane is involved first, leading to an intersphincteric abscess. Spread of infection in a downward direction leads to presentation as a perianal abscess. When pus penetrates the external sphincter below the puborectalis and enters the ischiorectal fossa, it may point further laterally as an ischiorectal abscess. From here, pus may track into the postanal space and into the opposite ischiorectal space, leading to the formation of a horseshoe abscess. Upward extension of intersphincteric sepsis results in a supralevator abscess. Abscesses may enlarge and burst spontaneously in the perianal or ischiorectal area or into the rectum. Once drained, the infection usually settles, but occasionally a fistula develops, which may lead to a recurrent abscess.
The cardinal clinical signs of inflammation described by Celsus (i.e., rubor [redness], calor [warmth], dolor [pain], and tumor [swelling]), along with the additional sign of Virchow (i.e., functio laesa [difficulty in sitting down and painful defecation]), are usually present. Patients sometimes present with a partially burst abscess, and persistent residual sepsis and examination may reveal induration. Physical examination findings depend on the location of the abscess and associated disease.
Most abscesses in the anorectal region are perianal abscesses, which are seen in 40% to 45% of patients. Patients present with constant perianal pain and a localized swelling. Examination reveals an erythematous, tender swelling adjacent to the anus with varying amounts of induration, cellulitis, and fluctuance. Rectal examination usually does not demonstrate any fluctuance or tenderness above the dentate line. Some patients may not have a fluctuant swelling—just erythema.
An ischiorectal abscess presents further laterally than a perianal abscess, in the region of the ischiorectal fossa. Because there is more room for the abscess to expand in this area, it may present as a diffuse swelling in the gluteal region. The abscess may extend posteriorly to communicate with the opposite ischiorectal fossa, forming a horseshoe abscess. The deep anterior anal space also may be involved.
The relative incidence of this variety of anorectal abscess is 2% to 5%. Intersphincteric abscesses were first described by Eisenhammer and subsequently divided into high and low types. Symptoms and signs are similar to those of other anorectal abscesses, but findings are not as prominent. Patients report dull anal or rectal pain and occasionally may present with a high temperature. A sense of fullness in the rectum and painful defecation may be present. Mucus discharge from the anus also may occur. Usually no external findings are present, but upon rectal examination, exquisite tenderness and swelling may be present in the region of the abscess. Adequate examination may be precluded by pain. Intersphincteric abscesses are most commonly associated with fistulae and also are most likely to recur.
Supralevator abscesses are uncommon, with an incidence ranging from 2.5% to 9.1% in different studies. A supralevator abscess results from the upward spread of infection from an intersphincteric abscess or a downward spread of infection from diverticulitis or pelvic inflammatory disease. The presentation is similar to an intersphincteric abscess, and diagnosis is difficult because of the absence of significant local findings. Symptoms include a dull, aching, rectal pain accompanied by fever with chills. Urinary symptoms may be present as a result of local pressure effects. Digital rectal examination may suggest the presence of an abscess or diffuse anorectal fullness. Imaging usually plays an important role in the diagnosis of this variety of abscess.
Deep Postanal Abscess
Patients with a deep postanal abscess present with anal pain and tenderness but have no swelling. Digital examination reveals a fullness posteriorly, just above the sphincters.
Submucosal abscesses come from high intermuscular abscesses and may present after rupture into the rectum. Symptoms may include a dull ache with a sense of fullness in the rectum. The only finding may be a tender, smooth, submucosal swelling.