Sigmoid Colon Rectum and Anal Canal

I. SIGMOID COLON (Figure 10-1)

A. General features. The sigmoid colon is the segment of the large intestine between the descending colon and the rectum. Its primary function is storage of feces. It begins at vertebral level SI (sacral promontory; pelvic inlet) and ends at S3 (rectosigmoid junction), where teniae coli (longitudinal bands of smooth muscle) are replaced by a complete circular layer of smooth muscle of the rectum. The sigmoid colon is intraperitoneal, being suspended by the sigmoid mesocolon. The left ureter and left common iliac artery lie at the apex of the sigmoid mesocolon. The arterial supply-is from the inferior mesenteric artery via the sigmoid arteries and rectosigmoid artery. Venous drainage is to the sigmoid veins inferior mesenteric vein ► hepatic portal system.

B. Clinical considerations

1. Hirschsprung disease (aganglionic megacolon) occurs when neural crest cells do not form the myenteric plexus in the sigmoid colon and rectum during embryologic development. This condition results in absence of peristalsis, fecal retention, and abdominal distension.

2. Diverticulosis is the presence of diverticula (abnormal pouchs or sacs). It is most commonly found in the sigmoid colon in patients older than 60 years of age. It is associated with a low-fiber, modern, Western diet. Perforation or inflammation of the diverticula results in diverticulitis. Clinical signs of diverticulitis include: pain in the left lumbar region, a palpable inflammatory mass in the left lumbar region, fever, leukocytosis, ileus, and peritonitis.

3. Flexible sigmoidoscopy permits examination of the sigmoid colon and rectum. During sigmoidoscopy, the large intestine may be punctured if the angle at the rectosigmoid junction is not negotiated properly. At the rectosigmoid junction, the sigmoid colon bends in an anterior direction and to the left. During sigmoidoscopy, the transverse rectal folds (Houston valves) also must be negotiated.

4. Colostomy. The sigmoid colon often is used in a colostomy because of the mobility provided by the sigmoid mesocolon (mesentery). An ostomy is an intestinal diversion that brings a portion of the gastrointestinal tract out through the rectus abdominis muscle. A colostomy may ablate the pelvic nerve plexus, resulting in loss of ejaculation or loss of erection in men, retention of urine in the bladder, and decreased peristalsis in the remaining colon.

Sigmoid Colon Rectum

Figure 10-1. (>4) Sagittal view of the male pelvis. The sigmoid colon (SC) extends from vertebral level S1 to S3, suspended by the sigmoid mesocolon (M), and ends at the rectosigmoid junction. The rectum (R) and ampulla of the rectum (AR) are shown along with the transverse rectal folds (TF). The rectum ends at the anorectal junction (dotted line), at the tip of the coccyx, where the puborectalis muscle (PR) maintains a 90° perineal flexure. The anal canal is divided into upper (U) and lower (L) regions by the pectinate line. RV= rectovesical pouch. B= bladder; PC= peritoneal cavity; RA = rectus abdominus; 7= testis. (B) Lateral barium radiograph. Dotted line = rectosigmoid junction. (C) Anteroposterior barium radiograph. 1 = sigmoid colon; 2 = rectum; 3 = ampulla of the rectum; 4 = transverse fold (Houston valve); 5= perineal flexure (90°); 6= sacrum; 7= transverse colon; 8 = terminal ileum; 9 = cecum; 10 = ileocecal valve; 11 = ascending colon. (A adapted with permission from Moore KL: Clinically Oriented Anatomy 3rd ed. Baltimore, Williams & Wilkins, 1992; B and C adapted with permission from Fleckenstein P, Tranum-Jensen J: Anatomy in Diagnostic Imaging. Philadelphia, WB Saunders, 1996, p 268.)

Figure 10-1. (>4) Sagittal view of the male pelvis. The sigmoid colon (SC) extends from vertebral level S1 to S3, suspended by the sigmoid mesocolon (M), and ends at the rectosigmoid junction. The rectum (R) and ampulla of the rectum (AR) are shown along with the transverse rectal folds (TF). The rectum ends at the anorectal junction (dotted line), at the tip of the coccyx, where the puborectalis muscle (PR) maintains a 90° perineal flexure. The anal canal is divided into upper (U) and lower (L) regions by the pectinate line. RV= rectovesical pouch. B= bladder; PC= peritoneal cavity; RA = rectus abdominus; 7= testis. (B) Lateral barium radiograph. Dotted line = rectosigmoid junction. (C) Anteroposterior barium radiograph. 1 = sigmoid colon; 2 = rectum; 3 = ampulla of the rectum; 4 = transverse fold (Houston valve); 5= perineal flexure (90°); 6= sacrum; 7= transverse colon; 8 = terminal ileum; 9 = cecum; 10 = ileocecal valve; 11 = ascending colon. (A adapted with permission from Moore KL: Clinically Oriented Anatomy 3rd ed. Baltimore, Williams & Wilkins, 1992; B and C adapted with permission from Fleckenstein P, Tranum-Jensen J: Anatomy in Diagnostic Imaging. Philadelphia, WB Saunders, 1996, p 268.)

II. RECTUM (see Figure 10-1)

A. General features. The rectum is a segment of the large intestine located between the sigmoid colon and the anal canal. It begins at vertebral level S3 and ends at the tip of the coccyx (anorectal junction), where the puborectalis muscle forms a U-shaped sling that causes a 90° perineal flexure. The ampulla of the rectum lies just above the pelvic diaphragm and generates the urge to defecate when feces move into the rectum. The rectum contains three transverse rectal folds (Houston valves) formed by the mucosa, submucosa, and inner circular layer of smooth muscle. These folds permanently extend into the lumen of the rectum. The arterial supply is chiefly from the inferior mesenteric artery via the superior rectal artery. Venous drainage is mainly to the superior rectal vein ~y inferior mesenteric vein hepatic portal system.

B. Clinical considerations

1. Familial adenomatous polyposis (FAP) is the archetype of adenomatous polyposis syndromes. In FAP, 500—2000 polyps typically (60% of cases) carpet the mucosal surface of the rectosigmoid colon; these polyps invariably become malignant. Malignant polyps are irregular in shape, sessile, and more than 2 cm in diameter. They exhibit sudden growth, and the base is broader than the height. FAP is an autosomal dominant disease that involves a mutation in the APC anti-oncogene. The progression from a small polyp to a large polyp is associated with a mutation in the ras proto-oncogene. The progression from a large polyp to metastatic carcinoma is associated with mutations in the DCC anti-oncogene and the p53 anti-oncogene.

a. Gardner syndrome is a variation of FAP that is characterized by adenomatous polyps and multiple osteomas.

b. Turcot syndrome is a variation of FAP in which patients have adenomatous polyps and gliomas.

2. Colonic adenocarcinoma invariably develops in patients with FAP. It accounts for 98% of all cancers in the large intestine. Mutations in the hereditary nonpolyposis colorectal cancer (HNPCC) gene, which codes for a DNA repair enzyme, have been implicated in some cases. Clinical findings include: fatigue, weakness, change in bowel habits, and weight loss. Right-sided tumors are associated with iron deficiency anemia. Left-sided tumors are associated with obstruction and bloody stools. It is a clinical maxim that iron deficiency anemia in an older man indicates adenocarcinoma of the colon until shown otherwise. Metastasis occurs most commonly to the liver, because the sigmoid veins and superior rectal veins drain into the hepatic portal system. A posterior metastasis may involve the sacral nerve plexus, causing sciatica.

3. Ulcerative colitis is a type of idiopathic inflammatory bowel disease. It always involves the rectum and extends proximally for varying distances. The inflammation is continuous, that is, there are no "skip areas" such as those seen in Crohn disease. Clinical signs include: bloody diarrhea with mucus and pus, malaise, fever, weight loss, and anemia. Ulcerative colitis may lead to toxic megacolon.

4. Rectal prolapse is the protrusion of the full thickness of the rectum through the anus. (It should be distinguished from mucosal prolapse, which is the protrusion of just the rectal mucosa through the anus). Clinical findings include: bowel protruding through the anus, bleeding, anal pain, mucous discharge, and anal incontinence caused by stretching of the internal and external anal sphincters or stretch injury to the pudendal nerve.

Sphincter Stretching
Feature Upper Anal Canal Lower Anal Canal

Arterial supply

Superior rectal artery (branch

Inferior rectal artery (branch

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Responses

  • miriam buccho
    How large can rectum stretch?
    6 years ago
  • tesmi
    Can stretching of the sigmoid mesentery heal?
    6 years ago
  • mustafa
    What vertebral level does rectum becomes anal canal?
    6 years ago
  • alex
    What vertebral level does the sigmoid colon begin?
    6 years ago
  • jackson
    Where is the rectosigmoid junction located?
    6 years ago
  • Tommaso Onio
    How does feces move through the sigmoid rectal junction?
    6 years ago
  • annett kortig
    What is the area between the sigmoid colon and the anal canal?
    5 years ago

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