Rectum and anal canal

Sigmoid colon

• Runs from the pelvic brim to the rectosigmoid junction at about S3, on the inverted-V sigmoid mesocolon. At the apex of the V lies the intersigmoid recess, with the left ureter behind it at the common iliac bifurcation.
• A long mesocolon on a narrow root allows sigmoid volvulus (coffee-bean loop from the left pelvis). The sigmoid is the narrowest colon with the most solid stool, so a carcinoma here obstructs early.
• Supplied by sigmoid branches of the IMA. The IMA becomes the superior rectal artery where it crosses the left common iliac artery. The rectosigmoid (Sudeck point) is a watershed.
• Rectosigmoid junction: taeniae spread out into a complete longitudinal coat; mesentery and appendices epiploicae end.

Rectum and its coverings

• About 12–15 cm long, following the sacral curve. Three transverse rectal folds; the largest, most constant middle fold on the right wall roughly marks the anterior peritoneal reflection.
• Peritoneum covers the front and sides of the upper third, the front only of the middle third, and none of the lower third. Perforation of the lower rectum (after polypectomy) therefore gives extraperitoneal gas that tracks into the retroperitoneum, not a pneumoperitoneum.
• Mesorectum: fat, vessels and nodes around the rectum, enclosed by the mesorectal fascia, seen on MRI as a thin low-signal line. This is the plane of total mesorectal excision; a tumour within 1 mm of it threatens the circumferential resection margin.
• In front: Denonvilliers fascia (rectoprostatic septum) in men; rectovaginal septum in women. Behind: presacral fascia and the retrorectal (presacral) space, normally only about 1 cm; widening displaces the rectum forwards on a lateral barium film.

• Low rectal tumours below the peritoneal reflection can spread to lateral pelvic (internal iliac and obturator) nodes, which lie outside the mesorectal fascia.

Blood supply, veins and nerves

• Superior rectal vein drains to the IMV and portal system: upper anal canal and rectal cancers spread to the liver. Middle and inferior rectal veins drain to the IVC (lungs). Their meeting is a portosystemic anastomosis.
• Parasympathetic: pelvic splanchnic nerves (S2–S4) supply from the distal transverse colon down, so the sigmoid keeps its supply after vagotomy. Somatic: inferior rectal nerve (pudendal) to the external sphincter and perianal skin; it can be stretched in a long second stage or cut across the ischioanal fossa.

Anal canal

• About 4 cm long, beginning where the rectum passes through the puborectalis sling. The anorectal angle is about 90° at rest, sharper on squeezing, and opens out when puborectalis relaxes at defecation.

• Between the pectinate line and the intersphincteric groove lies the pecten: non-keratinised squamous epithelium without hair or glands.
• Internal sphincter: thickened circular smooth muscle, involuntary; hypoechoic ring on endoanal ultrasound; divided in lateral sphincterotomy.
• External sphincter: skeletal, with subcutaneous, superficial (attached to the perineal body in front and the anococcygeal ligament behind) and deep parts. With puborectalis it forms the anorectal ring, which must not be divided.
• Internal haemorrhoids lie at 3, 7 and 11 o'clock (lithotomy).

Spaces and fistulae

• Ischioanal fossa: fat with septa that lets the canal distend; pudendal canal in its lateral wall. Supralevator (pelvirectal) space: above levator ani, below peritoneum.
• Fistula MRI: tracts are bright on T2. An intersphincteric tract (commonest) runs down between the sphincters from the dentate line; transsphincteric tracts cross the external sphincter; suprasphincteric tracts pass over puborectalis.
• Obstetric tears: 3a under half the external sphincter thickness, 3b over half, 3c internal sphincter also torn, 4 anal epithelium torn.

Defecography and prolapse

• Full-thickness rectal prolapse: concentric circular folds, two layers of wall. Mucosal prolapse shows radial folds.
• A weak puborectalis lets the anorectal junction descend and the angle open (descending perineum). Solitary rectal ulcer lies on the anterior wall with internal intussusception or failed relaxation on straining. Enterocele: small bowel descending between vagina and rectum.

Development

• The urorectal septum divides the cloaca into rectum and urogenital sinus and forms the perineal body. Faulty division gives anorectal malformations: in a boy, a pouch ending above the pubococcygeal line on a prone cross-table lateral film is a high lesion, usually with a rectourethral fistula into the prostatic urethra (meconium in urine); in a girl, a vestibular fistula. A bulging membrane at the anal site is an unruptured anal membrane.

Remember Above the pectinate line: portal, visceral, internal iliac; below: systemic, somatic, inguinal.

Exam trap The lower third of the rectum has no peritoneum, so its perforation gives retroperitoneal, not intraperitoneal, gas.

Exam trap After both internal iliac arteries are tied the rectum survives on the superior rectal artery from the IMA.