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Ischioanal Fossa

fossa ischioanalis

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.

Region: Pelvis & Hip

Clinical Relevance

Clinical Notes

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.

Pathology

Common Injuries & Conditions

Horseshoe Anorectal Abscess

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.