Anatomic Description

Anorectal abscess are defined by their anatomic relationship to the internal and external sphincter and levator musculature (Fig. 27-1). Abscesses that remain localized to the body of the gland in the potential intersphincteric space, between internal and external sphincters, are termed intersphincteric abscesses. Abscesses that perforate laterally through the external sphincter into the lower extrarectal space are called ischiorectal abscesses. The ischiorectal space is a pyramidal area bordered by the rectum and anus medially and pelvic side wall laterally. The apex of the ischiorectal space is formed by the levator ani muscle, and posteriorly the sacrotuberous ligament and gluteus maximus muscle form its borders. Importantly, the pudendal and internal pudendal vessels run through the superolateral wall of the ischiorectal space.

Most often, the infection will track through the intersphincteric space into the base of the ischiorectal space and into perianal soft tissue. This is termed a simple perirectal (perianal) abscess (PRA). This space contains both the external hemorrhoidal plexus and the subcutaneous part of the external anal sphincter.

Rarely, the infection will track cephalad and is termed supralevator abscess. More frequently, infections in the supralevator space originate in the pelvis, usually as a result of a diverticular abscess eroding through the pelvic floor. This space is bordered inferiorly by the muscles of the levator ani, laterally by the obturator fascia, and medially by the rectum.

Surgical Management of Anorectal Abscess

The main issue in the management of PRA is control of sepsis by draining the abscess. Surgical management requires not only adequate drainage but also effective anesthesia, for perioperative management as well as early postoperative pain control. An appropriate perianal block must be administered at surgery and relies on blocking nociceptive impulses from the pudendal nerve bilaterally. This approach allows for maximal relaxation and also sphincter relaxation, which augments exposure.

A perianal block is administered by injection of local anesthetic at the root of the pudendal nerve as it exits from Alcock’s canal just medial to the pubic tubercle (Fig. 27-2, A). The tubercle is easily palpated through the skin, and the needle is introduced medial to this, as deeply as possible. Additional local anesthetic is fanned out in a diamond shape adjacent to the sphincters, to infiltrate the ramifying branches of the nerve. Another option is to perform a ring block, in which local anesthetic is introduced into the perianal skin and the underlying sphincter muscle.

Lastly, the skin immediately surrounding the abscess can be infiltrated. For all these methods, a small-bore needle (25 gauge) should be used because rapid infiltration through a large-bore needle can cause pain. Further, the acidic milieu that results from a purulent environment leads to less effective anesthesia if directly infiltrated; therefore the nonerythematous skin in the area should be targeted.