Perineum and ischioanal fossa

Perineum Boundaries - The Diamond Down Under

The perineum is the diamond-shaped outlet of the pelvis. An imaginary line connecting the ischial tuberosities divides it into two triangles.

• Boundaries of the Diamond:

  • Anterior: Pubic symphysis
  • Posterior: Tip of the coccyx
  • Anterolateral: Ischiopubic rami
  • Posterolateral: Sacrotuberous ligaments
  • Roof: Pelvic diaphragm

• Triangles:

  • Urogenital (UG) Triangle (Anterior): Contains external genitalia & urethra.
  • Anal Triangle (Posterior): Contains the anal canal.

⭐ The perineal body is a central fibromuscular mass in the perineum, crucial for pelvic floor integrity. Tearing it during childbirth weakens pelvic support, risking organ prolapse.

Urogenital Triangle - The Front Door

• Boundaries: Defined by the pubic symphysis, ischiopubic rami, and the posterior border of the perineal membrane.
• Superficial Perineal Pouch:
  • Contents: Erectile tissues (crura & bulb of penis/clitoris), Ischiocavernosus, Bulbospongiosus, and Superficial Transverse Perineal muscles.
  • Innervation: Pudendal nerve.
• Deep Perineal Pouch:
  • Contents: Urogenital diaphragm (External urethral sphincter, Deep Transverse Perineal muscle), Bulbourethral glands (males).

⭐ The perineal body is a fibromuscular node central to the perineum; its disruption during childbirth can lead to pelvic organ prolapse.

Anal Triangle & Ischioanal Fossa - The Back Passage

• Anal Triangle: Posterior half of the perineum, containing the anal canal and sphincters.
  • Boundaries: Line between ischial tuberosities, sacrotuberous ligaments, coccyx.
• Ischioanal (Ischiorectal) Fossa: Fat-filled, wedge-shaped space lateral to the anal canal.
  • Contents: Adipose tissue, inferior rectal nerves & vessels.
  • Pudendal Canal (Alcock's): Runs in the lateral wall (obturator fascia); contains the pudendal nerve (S2-S4) & internal pudendal vessels.

⭐ High-Yield: The ischioanal fossa is a common site for abscesses. Infection can spread behind the anal canal to the opposite fossa, forming a horseshoe abscess.

Neurovasculature - The Supply Lines

• Artery: Internal Pudendal Artery (branch of Internal Iliac A.).
• Nerve: Pudendal Nerve (from sacral plexus roots S2-S4).
• Pathway: The neurovascular bundle exits the pelvis via the greater sciatic foramen, loops around the ischial spine, and enters the perineum through the lesser sciatic foramen.
• Canal: Travels within the pudendal (Alcock's) canal, a fascial sheath on the obturator internus muscle.
• 📌 S2, S3, S4 keeps the penis and pelvic floor off the floor.

⭐ For a pudendal nerve block, the needle is passed transvaginally toward the ischial spine to anesthetize the perineum, often used in the second stage of labor.

High‑Yield Points - ⚡ Biggest Takeaways

• The pudendal nerve (S2-S4) is the primary nerve of the perineum; a nerve block is performed at the ischial spine.
• The ischioanal fossa is a common site for abscesses, which can spread between sides posterior to the anal canal.
• Alcock's canal (pudendal canal) contains the pudendal nerve and internal pudendal vessels.
• The perineal body is a critical fibromuscular point; its rupture during childbirth compromises pelvic floor integrity.
• A mediolateral episiotomy is preferred to minimize risk to the anal sphincter.