Perianal Abscess 

A perianal abscess represents an infection of the soft tissues surrounding the anal canal, with formation of a discrete abscess cavity. The severity and depth of the abscess are quite variable, and the abscess cavity is often associated with formation of a fistulous tract. For that reason, along with perianal abscess, perianal fistula also is discussed in this article.

For excellent patient education resources, visit eMedicine's Esophagus, Stomach, and Intestine Center. Also, see eMedicine's patient education articles Anal Abscess and Rectal Pain.

An anorectal abscess originates from an infection arising in the cryptoglandular epithelium lining the anal canal. The internal anal sphincter is believed to serve normally as a barrier to infection passing from the gut lumen to the deep perirectal tissues. This barrier can be breached through the crypts of Morgagni, which can penetrate through the internal sphincter into the intersphincteric space. Once infection gains access to the intersphincteric space, it has easy access to the adjacent perirectal spaces. Extension of the infection can involve the intersphincteric space, ischiorectal space, or even the supralevator space. In some instances, the abscess remains contained within the intersphincteric space. The variety of anatomic sequelae of the primary infection is translated into variable clinical presentations.

Frequency

The peak incidence of anorectal abscesses is in the third and fourth decades of life. Men are affected more frequently than are women, with a male-to-female predominance of 2:1 to 3:1. Approximately 30% of patients with anorectal abscesses report a previous history of similar abscesses that either resolved spontaneously or required surgical intervention.

A higher incidence of abscess formation appears to correspond with the spring and summer seasons. While demographics point to a clear disparity in the occurrence of anal abscesses with respect to age and sex, no obvious pattern exists among various countries or regions of the world. Although suggested, a direct relationship between the formation of anorectal abscesses and bowel habits, frequent diarrhea, and poor personal hygiene remains unproved.

The occurrence of perianal abscesses in infants also is quite common. The exact mechanism is poorly understood but does not appear to be related to constipation. Fortunately, this condition is quite benign in infants, rarely requiring any operative intervention in these patients other than simple drainage.^[1]

Perirectal abscesses and fistulas represent anorectal disorders arising predominately from the obstruction of anal crypts. Infection of the now static glandular secretions results in suppuration and abscess formation within the anal gland. Typically, the abscess forms initially in the intersphincteric space and then spreads along adjacent potential spaces.

As mentioned above, perirectal abscesses and fistulas represent anorectal disorders that arise predominately from the obstruction of anal crypts. Normal anatomy demonstrates anywhere from 4-10 anal glands drained by respective crypts at the level of the dentate line. Anal glands normally function to lubricate the anal canal. Obstruction of anal crypts results in stasis of glandular secretions and, when subsequently infected, suppuration and abscess formation within the anal gland results. The abscess typically forms in the intersphincteric space and can spread along various potential spaces.

Common organisms implicated in abscess formation include Escherichia coli, Enterococcus species, and Bacteroides species; however, no specific bacterium has been identified as a unique cause of abscesses.

Less common causes of anorectal abscess that must be considered in the differential diagnosis include tuberculosis, squamous cell carcinoma, adenocarcinoma, actinomycosis, lymphogranuloma venereum, Crohn's disease, trauma, leukemia, and lymphoma. These may result in the development of atypical fistula-in-ano or complicated fistulas that fail to respond to conventional surgical treatment.

The classic locations of anorectal abscesses listed in order of decreasing frequency are as follows: perianal 60%, ischiorectal 20%, intersphincteric 5%, supralevator 4%, and submucosal 1%. These major types are illustrated in the image below. Clinical presentation correlates with the anatomic location of the abscess.

Patients with a perianal abscess typically complain of dull perianal discomfort and pruritus. Their perianal pain often is exacerbated by movement and increased perineal pressure from sitting or defecation. Physical examination demonstrates a small, erythematous, well-defined, fluctuant, subcutaneous mass near the anal orifice.

Patients with an ischiorectal abscess often present with systemic fevers, chills, and severe perirectal pain and fullness consistent with the more advanced nature of this process. External signs are minimal and may include erythema, induration, or fluctuancy. On digital rectal examination (DRE), a fluctuant, indurated mass may be encountered. Optimal physical assessment of an ischiorectal abscess may require anesthesia to alleviate patient discomfort that would otherwise limit the extent of the examination.

Patients with an intersphincteric abscess present with rectal pain and exhibit localized tenderness on DRE. Physical examination may fail to identify an intersphincteric abscess. Although rare, supralevator abscesses present a similar diagnostic challenge. As a result, clinical suspicion of an intersphincteric or supralevator abscess may require confirmation through computed tomography (CT) scanning, magnetic resonance imaging (MRI), or anal ultrasonography. Use of the last modality is limited to confirming the presence of an intersphincteric abscess.

As a rule, the presence of an abscess is an indication for incision and drainage. Watchful waiting while administering antibiotics is inadequate.

Classification of anorectal abscess

Abscesses are classified based on their anatomic location. The most commonly described locations are perianal, ischiorectal, intersphincteric, and supralevator. The image below illustrates the different anatomic locations of anorectal abscesses.

Perianal abscesses represent the most common type of anorectal abscesses, accounting for approximately 60% of reported cases. These superficial collections of purulent material are located beneath the skin of the anal canal and do not transverse the external sphincter.

The next most common types of abscesses, in descending order of frequency, are ischiorectal, intersphincteric, and supralevator. An ischiorectal abscess forms when suppuration transverses the external sphincter into the ischiorectal space. Intersphincteric abscesses result from suppuration contained between the internal and external anal sphincters. A supralevator abscess results either from primary disease in the pelvis (eg, appendicitis, diverticular disease, gynecologic sepsis) or from suppuration extending cranially from an origin in the intersphincteric space, through the longitudinal muscle of the rectum and reaching above the levators.

Horseshoe abscesses, while rare, result from circumferential infiltration of pus within the intersphincteric planes.

The Goodsall rule for perianal fistulas

The Goodsall rule states that the external opening of a fistulous tract located anterior to a transverse line drawn across the anal verge is associated with a straight radial tract of the fistula into the anal canal/rectum. Conversely, an external opening posterior to the transverse line follows a curved, fistulous tract to the posterior midline of the rectal lumen. This rule is important for planning surgical treatment of the fistula and is illustrated in the images below.

Clinical suspicion of anorectal abscess warrants aggressive identification and surgical drainage. Delayed surgical intervention results in chronic tissue destruction, fibrosis, and stricture formation and may impair anal continence. Delayed incision and drainage of an anorectal abscess is contraindicated.