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 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, fistulas are also discussed in this article where relevant.The variety of anatomic sequelae of the primary infection is translated into variable clinical presentations. The relatively simple perianal abscess is to be distinguished from the more complex perirectal abscesses. Treatment also differs according to the type of abscess present.