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Applied Anatomy of Ischiorectal fossa and Pelvic Floor

         Dr.Barin Bose MS, FACRSI, FAIS 
         Consultant Surgeon
         Jabalpur Hospital and Research Centre.
         Jabalpur (MP)

Potential spaces around the anorectum are -
(a) The Ischiorectal fossa
(b) Perianal space -The Perianal fascia is in the form of septa which passes laterally from the lower end of the intersphincteric longitudinal fibers to the pudendal canal. It separates a shallow subcutaneous Perianal space from the deep Ischiorectal space. The Perianal space surrounds the anal canal below the white line of Hilton and contains subcutaneous part of external sphincter, external rectal venous plexus terminal branches of inferior rectal vessels and nerve. The fat in the Perianal space is tightly arranged in small loculi. The infection in this space is therefore very painful due to tension caused by swelling.
(c) Sub mucous space - Above the white line of Hilton between mucous membrane and internal sphincter. Contains internal rectal venous plexus and lymphatic.

The Ischiorectal fossa

- It is a wedge spaced space situated one on either side of the anal canal below the pelvic diaphragm. The base is directed downwards towards the skin. It is 5 to 6 cm deep, anterioposteriorly 5 cm, and 2.5 cm side to side , lying below the levator ani muscles and on either side of anal canal. Post anal space connects the two fossae posteriorly by a horse shoe path. The space is filled with loose areolar tissue and loosely arranged large loculi of fat. The infection of this space leads to abscess formation and are least painful because swelling can occur without tension.

Boundaries - (1) Base is formed by the skin.
(2) Apex - Is formed by meeting of Obturator fascia with the inferior fascia of the pelvic diaphragm (anal fascia). The line corresponds to the origin of levator ani from the lateral pelvic wall.
(3) Anteriorly - The fossa is limited by the posterior border of perineal membrane.
(4) Posteriorly - (a) lower border of the gluteus maximus and (b) Sacro tuberous ligament.
(5) Lateral wall is vertical and is formed by (a) Obturator internus with Obturator fascia, and medial surface of ischial tuberosity below the attachment of Obturator fascia.
(6) Medial wall - Slopes upwards and laterally and is formed by (a) external anal sphincter with fascia covering it in the lower part and (b) levator ani with anal fascia in the upper part.

Recesses - These are narrow extension of the fossa beyond its boundaries.
1. Anterior recess - Extends forwards above the urogenital diaphragm almost up to the posterior surface of the body of pubis.
2. Posterior recess - It is smaller than anterior recess. It extends deep to sacrotuberous ligament.
3. Horse shoe recess (deep post anal space/Post Sphincteric space) - Connects the two fossae behind the anal canal.

Contents of the Ischiorectal fossa -
1. Ischiorectal pad of fat.
2. Inferior rectal nerve and vessels.
3. Posterior scrotal / labial nerve and vessels.
4. Pudendal canal and its contents.
5. Perineal branch of fourth sacral nerve.
6. Perforating cutaneous branch of S2 , S3 nerve .

Pudendal Canal

- It is a fascial tunnel present in the lower part of the lateral wall of the Ischiorectal fossa , just above the sacrotuberous ligament . Pudendal canal is formed by splitting of the fascia lunata.The fascial wall of the canal is fused laterally to the Obturator fascia, medially to the perineal fascia and inferiorly with the sacrotuberous ligament. It contains pudendal nerve and the internal pudendal vessels.

Pudendal Nerve -It arises from sacral plexus in the pelvis from spinal nerves S2, S3, S4. In the posterior part of the pudendal canal the pudendal nerve gives off inferior rectal nerve and then it divides into to terminal branches - (a) perennial nerve and (b) dorsal nerve of penis.The inferior rectal nerve pierces the medial wall of pudendal canal, crosses the Ischiorectal fossa from lateral to medial side and supplies the external sphincter, the skin around the anus and the wall of the anal canal below the pectinate line.

Internal Pudendal Artery - It is smaller of the two terminal division of the anterior trunk of internal iliac artery. It gives off a branch in the posterior part of the pudendal canal - inferior rectal artery, then it divides in two terminal branches (a) perineal artery and (b) artery to penis.

Lunate Fascia - It arches over the Ischiorectal fat, begins laterally at the pudendal canal and after arching the Ischiorectal fat fuses medially with the fascial covering of external anal sphincter. The fascia divides the Ischiorectal space into (a) supra tegmental space above (b) tegmental space below

Applied Anatomy -
1. The two Ischiorectal fossae allow distension of rectum and anal canal during passage of faeces.
2. Both the Perianal and Ischiorectal spaces are common site of abscesses. Poor blood supply and coarse lobulated fat predispose it for infection. The Ischiorectal abscess may be the result of spread of infection from the nearby area - skin, lumen of bowel or perirectal tissue above the levator ani or through the blood or lymphatic. They can be excised fearlessly because of the poor vascularity of the fossa. Abscesses in this region are - (a) Perianal abscess (b) Ischiorectal abscess (c) Supra levator abscess
(d) Sub mucous abscess.
When making incision for drainage of abscesses care must be taken to avoid injury to inferior rectal neuromuscular bundle to prevent paralysis of external sphincter.

3. The Ischiorectal fat acts as a cushion like support of rectum and anal canal. Loss of this fat in debilitating diseases like diarrhea in children may results in prolapse of the rectum.
4. The occasional gap between the origin of levator ani and the Obturator fascia is known as the hiatus of Schwalbe. Rarely pelvic organs may herniate through this gap resulting in an Ischiorectal hernia.
5. Tension in gluteus maximus as during standing compresses fat of the fossa around anal canal contributing to fecal continence.
6. Pudendal Nerve - (a) Sphincteric Incontinence - Injury to the inferior rectal branch of the pudendal nerve causes Sphincteric incontinence. ( b) Pudendal Block - The pudendal nerve is infiltrated with local anaesthetic solution where it crosses the ischial spine. The ischial spine is palpated through the vagina / rectum and the needle is inserted through the perineum, medial to the ischial tuberosity to anaesthetize the perineum. Further sensory branches of the perineum are derived from the ilio inguinal nerve, the perineal branch of the posterior cutaneous nerve of the thigh and the genital branch of the genito femoral nerve. This means that when complete perineal anesthesia is required an injection of local anaesthetic must be given around the anus.

7. Fournier's Gangrene - An uncommon but potentially lethal complication of Ischiorectal abscess.
8. Through the inter Sphincteric longitudinal fibers the inter Sphincteric abscess tracks down and gives rise to Perianal abscess.


Pelvic Floor

It is important to the surgeon to understand the anatomy of the levator ani muscle because it constitute the sphincter mechanism of the anal canal and also because its accurate division is important step in the operation of rectal excision. The levator ani is a broad thin muscle attached to the inner surface of the side of pelvis and united medially with its fellow of the opposite side to form the greater part of the floor of the pelvic cavity.
It is formed by the pelvic diaphragm which consists of levator ani and coccygeus. It resembles a hammock or a gutter because it slopes from either side towards the median plane where it is pierced by urethra, anal canal and vagina in females.
The pelvic diaphragm separates the perineum from the pelvis.
Pelvic fascia covers both the surface of the pelvic diaphragm forming superior and inferior layers. The inferior layer of the pelvic fascia is called the anal fascia. Which forms the medial
wall of ischiorectal fossa .The superior layer is loosely arranged between the peritoneum and the pelvic floor. The loose nature of the fascia favors a rapid spread of pelvic cellulites & supra levator abscess develops. The muscles which comprises the pelvic floor are - Levator ani and Coccygeus with corresponding muscles of the opposite side forms the pelvic diaphragm. When examined more carefully it is seen to consist of three parts.
1. Illiococcygeus.
2. Pubococcygeus.
3. Puborectalis

Illiococcygeus - This is a very thin muscle which arises from the ischial spine and posterior part of the white line (tendinous arch) of obturator fascia. The fibers run backwards downwards and medially to be attached to anococcygeal raphe. It is a median fibrous band which runs between the anus and superior surface of coccyx to be inserted to the first piece of coccyx and last segment of sacrum.

Pubococcygeus - This arises from the back of pubis and anterior part of the white line of the obturator fascia, it is directed horizontally backwards on either side of the rectum to fuse with the fellow of the opposite side to form a broad band - anococcygeal raphe.

Nerve supply - The levator ani is supplied by (1) Branch from the fourth sacral nerve.
(2) Perineal division of the pudendal nerve.

Action - (a) The levator ani and coccygeus closes the posterior part of the pelvic outlet.
(b) During coughing, sneezing and lifting heavy weight the levator ani and coccygeus resist increased intra abdominal pressure and helps to maintain continence of the bowel and bladder.

Puborectalis - The name is some times applied to those fibers of the pubococcygeus which unit with the corresponding fibers of the opposite side to form a sling behind the rectum at the anorectal junction. It arises from the lower part of the back of symphysis pubis and superior fascia of the urogenital diaphragm, runs backward along the side of the rectum to join with its fellow immediately behind the ano-rectal junction and form a strong U-shaped loop which slings the rectum to the pubis.

Anorectal Ring - This term was coined by Milligan and Morgan in 1934 to denote the functionally important ring of muscle which surrounds the junction of the rectum and anal canal. It is composed of internal sphincter, deep part of the external sphincter which completely encircle the junction and puborectalis sling on the posterior & lateral aspect. As a result the ring is stronger posteriorly and laterally than it is anteriorly.
Recognition of the anorectal ring is of paramount importance in the treatment of abscesses and fistulas in the anal region. As its complete division inevitably results in rectal incontinence while its preservation despite the sacrifice of all the rest of sphincter musculature at lease ensures that there will be no gross lack of control, though minor degree of incontinence may results.