Describe the boundaries and contents of the ischiorectal fossa. Add a note on ischiorectal abscess.

Definition

Boundaries

• It is wedge-shaped in coronal section, with the base below and the apex above

Contents

• Ischiorectal fat — loose, in a fine areolar network; it allows the anal canal and rectum to distend during defaecation
• Inferior rectal vessels and nerve — crossing the fossa to the anal canal
• Pudendal canal (of ALCOCK) — on the lateral wall, in the obturator fascia, containing the internal pudendal artery and vein, the pudendal nerve, and the nerve to obturator internus
• Perineal branch of S4 and the perforating cutaneous nerve
• Anterior and posterior recesses — the anterior above the urogenital diaphragm, the posterior beneath the sacrotuberous ligament

Ischiorectal Abscess

• Presents with severe throbbing perianal pain, swelling, fever and difficulty sitting; there may be little to see externally at first because the fossa is deep
• Treatment is early incision and drainage; antibiotics alone are insufficient
• Commoner in diabetics and the immunocompromised, in whom it can progress to necrotising fasciitis of the perineum (Fournier gangrene)

Applied Aspects

• Fistula-in-ano follows about a third of drained abscesses; classified by its relation to the sphincters (intersphincteric, transsphincteric, suprasphincteric, extrasphincteric)
• Goodsall's rule predicts the internal opening — anterior openings track radially, posterior ones curve to the posterior midline
• The external anal sphincter must be preserved when laying open a fistula, or incontinence results; a seton is used for high tracks
• The fat is the reason the fossa is a good site for abscess but a poor one for healing — sparse blood supply favours infection and delays repair
• Pudendal nerve block is given at the ischial spine, which is at the apex of the fossa, approached transvaginally