The ischioanal fossa (ischiorectal fossa) is the wedge-shaped fat-filled space lateral to the anal canal and below the levator ani. It is bounded medially by the external anal sphincter and levator ani, laterally by the obturator internus and its fascia (Alcock canal), and posteriorly by the sacrotuberous ligament. The pudendal nerve and internal pudendal vessels traverse Alcock canal in the lateral wall.
| Location | Each side of the anal canal, below the pelvic floor |
|---|---|
| Boundaries |
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| Contents |
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The fat pad lets the anal canal and rectum expand during defecation. The two fossae connect behind the anal canal, so an abscess can spread from one side to the other (horseshoe abscess).
The ischioanal fossa is the most common site of anorectal abscess (perirectal abscess), which may track across the midline posteriorly to form a horseshoe abscess. Surgical drainage requires knowledge of the fossa boundaries to decompress the abscess without damaging the sphincter complex. Anal fistula tracts course through the ischioanal fossa from the internal opening in the anal canal to the external opening. The fossa is accessed for pudendal nerve block at Alcock canal for perineal anaesthesia.
Perirectal abscess extending from one ischioanal fossa posteriorly through the retroanal space to the contralateral fossa forms a horseshoe configuration requiring drainage of both fossae through bilateral incisions with an internal opening division through the posterior midline sphincter to prevent recurrence as a fistula.
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