An anorectal abscess originates from an infection arising in the cryptoglandular epithelium lining the anal canal. Anorectal abscesses are classified according to their anatomical location; the following are the most common locations:
Perianal abscesses are the most common type of anorectal abscesses. These superficial collections of purulent material are located beneath the skin of the anal canal and do not transverse the external sphincter.
Ischiorectal abscesses are the next most common type. These abscesses form when suppuration transverses the external anal sphincter into the ischiorectal space. An ischiorectal abscess may traverse the deep postanal space into the contralateral side, forming a so-called horseshoe abscess.
Intersphincteric abscesses, the third most common type, result from suppuration contained between the internal and external anal sphincters. They may lie completely within the anal canal, leading to severe pain, and may only be found by digital rectal examination or anoscopy.
Supralevator abscesses, the least common of the four major types, may form from cephalad extension of the intersphincteric abscess above the levator ani or from caudal extension of a suppurative abdominal process (eg, appendicitis, diverticular disease, gynecologic sepsis) into the supralevator space. These abscesses may be diagnosed using CT, and they cause pelvic and rectal pain.
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Cite this: Richard H. Sinert. Fast Five Quiz: Anal Conditions - Medscape - Mar 26, 2021.