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susceptible. An intact ileocecal valve, colon, or both decreases the amount of residual small bowel needed.

2.TPN (see Chapter 1, II E 2) is essential postoperatively. The small bowel will hypertrophy with time, and most patients can be weaned from TPN gradually. For refractory cases, long-term parenteral nutrition (home TPN) is available.

3.Oral nutrition. In order to ensure that oral nutrition is adequate, attention should be paid to several points.

a.The total calories ingested must increase to compensate for the portion that is not absorbed.

b.A low-residue or elemental diet is needed. An elemental diet contains only components that are directly absorbed by the intestinal mucosa without any enzymatic digestion (mediumand short-chain triglycerides, monosaccharides and disaccharides, monopeptides and dipeptides), plus vitamins and minerals.

c.Antiperistaltic agents and histamine2 (H2)-receptor antagonists or proton pump inhibitors should be given.

d.Fat and water soluble vitamin supplementation is needed.

e.Parenteral vitamin B12 is needed if the distal ileum has been resected.

f.Calcium and magnesium supplementation should be given.

g.Medium-chain triglycerides should replace dietary fats because they do not require micelles for absorption.

4.Surgical therapy is available, consisting of reversal of a short segment of distal small bowel to slow the intestinal transit time if adequate oral nutrition cannot be attained.

G

Radiation injury to the small bowel occurs in two phases.

1.Acute-phase injury is caused by mucosal injury. Symptoms, which include nausea, vomiting, and diarrhea, are transient. Rarely, bleeding or perforation occurs and requires surgery.

2.Chronic effects appear months to years later and are caused by an obliterative vasculitis. The symptoms and signs are similar to those associated with a recurrent malignancy and, indeed, this possibility must always be fully evaluated.

a.Minor symptoms—abdominal pain, malabsorption, and diarrhea—require symptomatic therapy only.

b.Major complications that require surgery include bowel obstruction (unrelieved by decompression with a nasogastric or long tube), perforation, abscess, fistula, and hemorrhage. Hemorrhage may be caused by mucosal erosion or by an enteroarterial fistula.

c.Surgery is technically difficult owing to fibrosis and scarring. With resection or bypass, unirradiated bowel must be used for any anastomosis. Even so, the anastomosis or surgical wound is likely to break down with subsequent fistula formation or other complications.

Chapter 13

Colon, Rectum, and Anus

Scott D. Goldstein

Paul A. Mancuso

I Introduction

AAnatomy

1.Colon. The colon, or large intestine, is approximately 3–5 ft in length and is divided into several parts: the cecum, ascending colon, transverse colon, descending colon, and sigmoid colon.

a.Arterial blood supply (Fig. 13-1)

1.Branches from the superior mesenteric artery supply the cecum, ascending colon, and proximal transverse colon.

2.Branches of the inferior mesenteric artery supply the distal transverse, descending, and sigmoid colon.

b.Venous drainage

1.The inferior mesenteric vein carries blood from the left side of the colon to the splenic vein.

2.The superior mesenteric vein drains the right side of the colon, joining the splenic vein to form the portal vein.

c.Lymphatic drainage

1.Lymph channels generally follow the arterial blood vessels.

2.Metastases from colon cancers generally spread through lymphatic paths in progressive fashion. The nodes closest to the cancer are involved first, and more distant nodes become involved as the disease advances.

d.Bowel wall

1.Layers consist of mucosa, submucosa, muscularis, and visceral peritoneum (serosa).

2.There are no villi in the colonic mucosa; the crypts of Lieberkühn are the distinguishing histologic feature of the colonic mucosa.

3.The outer longitudinal muscle of the colon is incomplete; it forms three distinct bands, the tenia coli.

4.Haustra are the outpouchings of the colonic wall between the tenia coli.

2.Rectum. The rectum extends from the sigmoid colon to the anus and is approximately 15 cm in length.

a.Arterial blood supply (Fig. 13-1)

1.The superior rectal artery, which is the terminal branch of the inferior mesenteric artery, supplies the upper and middle rectum.

2.The lower portion of the rectum is supplied with arterial branches from the internal iliac artery: the middle rectal arteries and inferior rectal arteries.

b.Venous drainage

1.The superior rectal veins drain the upper and middle rectum, communicating with the portal vein via the inferior mesenteric vein.

2.The middle rectal veins drain the lower rectum and anal canal, emptying into the vena cava via the internal iliac veins.

3.Note that tumors in the rectum may metastasize into venous channels that enter either the portal system (portal vein) or the systemic system (vena cava).

c.Lymphatic drainage

1.Lymph from the upper and middle rectum flows in channels that parallel the arterial supply and is filtered by the inferior mesenteric nodes.

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FIGURE 13-1 Arterial supply of right (ascending) colon via branches of superior mesenteric artery; left (descending) colon and rectum via branches of inferior mesenteric artery. Distal rectum supplied by branches from hypogastric artery.

2.Lymph from the distal rectum flows into channels adjacent to the middle and inferior rectal arteries. These channels drain to iliac nodes.

d.Bowel wall

1.In contrast to the colon, the rectum is lined by complete layers of inner circular and outer longitudinal muscle.

2.The proximal third of the rectum is covered with peritoneum, but the rectum descends beyond the peritoneal cavity, and the lower third has no peritoneal covering.

3.The valves of Houston, usually three in number, are mucosal folds that project into the lumen of the rectum.

3.Anus. The anus is the terminal portion of the intestinal tract. It is surrounded by two muscular tubes, which are involved in the mechanism of continence. The anus is also enveloped by the puborectalis, which is palpable by digital examination of the anus as the anorectal ring.

a.The anal canal is lined by anoderm, a specialized epithelium that is devoid of hair follicles, sebaceous glands, or sweat glands but has a rich nerve supply. The junction between the anoderm and perianal skin is the anal verge.

b.The colonic mucosa joins the anoderm at the dentate line, located approximately 1.5 cm above the anal verge.

c.A transitional zone of 6–12 mm in length resides above the dentate line. In this zone, squamous epithelium gradually changes to cuboidal epithelium and then to columnar epithelium.

d.The columns of Morgagni are longitudinal mucosal folds located just above the dentate line, where they meet to form the anal crypts.

e.Small anal glands are present beneath the anoderm; these glands are located between the internal and external sphincters and communicate with the anal crypts via anal ducts.

f.The anal canal is surrounded by two muscular sphincters, which provide continence.

1.The internal sphincter—a continuation of the inner circular muscle of the rectum—is a smooth muscle with involuntary control and autonomic innervation.

2.The external sphincter is a striated muscle under voluntary control with somatic innervation.

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BPhysiology

1.Absorption of water and electrolytes

a.The colon receives from 900–1,500 mL of ileal chyme each day, of which all but 100–200 mL are absorbed. Sodium is actively absorbed across the colonic mucosa, whereas potassium moves into the colonic mucosa by passive diffusion.

b.Water is absorbed passively, accompanying sodium molecules across the mucosa.

2.Bacterial fermentation of undigested carbohydrates in the colon produces short-chain fatty acids, which provide energy for sodium transport across the colonic mucosa.

3.Storage of feces

a.Nondigestible waste is stored in the colon until voluntary evacuation occurs.

b.Approximately one third of the dry weight of feces consists of bacteria. Each gram of feces contains 1011 to 1012 bacteria, with anaerobes being 100 to 10,000 times more prevalent than aerobes.

1.Bacteroides, an anaerobic bacterium, is the most common colonic organism.

2.Escherichia coli is the most common colonic aerobe.

4.Colonic gas can come from three sources: swallowed air, intraluminal production, and diffusion from the blood. Five gases constitute 98% of colonic gas: nitrogen, oxygen, carbon dioxide, hydrogen, and methane.

II Evaluation of the Colon, Rectum, and Anus

A History

The history provides very important information in the evaluation. A properly taken history most often establishes the diagnosis or at least suggests it. Inquiries should include the following:

1.Bleeding: passage of bright red blood (hematochezia) or dark, tarry stools (melena)

2.Pain, either abdominal or anal

3.Presence of an anal or perianal mass

4.Rectal discharge

5.Change in bowel habit

6.Incontinence

7.History of cancer, both personal and family

8. History of colorectal polyps or inflammatory bowel disease, both personal and family

B Physical examination

1.Pertinent aspects of the physical examination should be directed by the patient's complaints. Abdominal complaints require a standard, thorough abdominal examination. The examination for anorectal problems may require modification, depending on the patient's symptoms. For example, the patient with severe anal pain may not be able to tolerate examination with an anoscope or proctosigmoidoscope. If the cause of the pain is revealed by simple inspection (e.g., a thrombosed hemorrhoid), further methods of examination may not be necessary.

2.Anorectal examination is usually performed with the patient in the left lateral position and includes four basic steps:

a.Inspection. Skin abnormalities, masses, protrusions, and drainage sites should be noted.

b.Palpation. The perineum, anal canal, and lower rectum should be gently palpated with a gloved, well-lubricated index finger. Sphincter tone, areas of tenderness, and any masses should be noted.

c.Anoscopy with a small anoscope provides the best method to evaluate fissures, hemorrhoids, anal papillae, or other anal canal lesions.

d.Proctosigmoidoscopy. The rigid 20-cm proctosigmoidoscope remains a valuable instrument for studying the rectum. Often, the examination can be accomplished without preparation. A single enema may occasionally be required. The rectal mucosa should be inspected for any abnormalities such as ulceration, granularity, or tumors.

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C Radiographic studies

1.Barium enema remains the most cost-effective method of identifying colon pathology. Unfortunately, it cannot reliably detect small tumors and thus is not effective for screening patients for cancer.

2.Water-soluble contrast enema. If colonic perforation is suspected, barium enema is contraindicated; extravasation of barium and feces can cause a severe peritonitis with a high mortality rate. The water-soluble material, although safer, is quickly diluted, giving these studies lesser diagnostic quality than barium studies.

3.Computed tomography (CT) scan is an excellent method of evaluating the patient suspected of having diverticulitis (contrast enemas may worsen the inflammation in such situations). Pericolic inflammation may be revealed, as well as abscesses, which may be drained under CT scan guidance. The CT scan is also useful for detecting metastases in patients with known colorectal cancer.

4.Magnetic resonance imaging (MRI) appears to offer little advantage over CT scans for evaluation of colorectal disease, with the possible exception of distinguishing between recurrent cancer and fibrosis in postoperative situations.

5.Defecography is a dynamic radiologic study by which the distal colon and rectum are imaged as the patient eliminates barium. It is useful in studying disorders of defecation.

D

Flexible endoscopy permits a more extensive evaluation of the bowel than is possible with short, rigid instruments. Smaller lesions as well as mucosal irregularities can be evaluated more accurately with a flexible endoscope than with a barium enema.

1.Flexible sigmoidoscopy permits examination of the rectum and sigmoid colon (and occasionally the descending colon) with a flexible instrument that is 65 cm in length.

2.Colonoscopy enables evaluation of the entire colonic mucosal surface in more than 90% of patients. The flexible instrument used is 160 or 185 cm in length. Lesions can be biopsied and polyps removed with this instrument. Indications for its use include:

a.Evaluation of abnormalities noted on barium enema

b.Evaluation and surveillance of inflammatory bowel disease

c.Differential diagnosis between diverticular disease and cancer

d.Presence of a polyp (or history of previous polyp)

e.Gastrointestinal symptoms (e.g., bleeding, abdominal pain, iron deficiency anemia) not clarified by contrast studies

f.Follow-up of patients with prior colon cancer

g.Acute lower gastrointestinal bleeding

h.Reduction of sigmoid volvulus

i.Evaluation of the entire colon in a patient with known colorectal cancer to evaluate for synchronous lesions

E Fecal occult blood determination

Stool is placed on guaiac-impregnated paper. If hemoglobin is present, a blue color appears when a peroxide-containing developer is added.

1.A daily loss of approximately 20 mL of blood into the gastrointestinal tract is required to consistently produce a positive result on a fecal occult blood test.

2.Red meat, turnips, radishes, tomatoes, aspirin, nonsteroidal anti-inflammatory drugs, and iron may cause false-positive results.

3.Vitamin C (ascorbic acid) can cause false-negative results.

4.A properly performed, positive fecal occult blood test requires adequate investigation of the gastrointestinal tract to determine the cause.

F Anorectal physiologic studies

The exact role of anorectal physiologic studies in the clinical setting has yet to be determined. Often, a thorough history and physical examination are adequate to determine the diagnosis and the treatment. However, further studies are sometimes required to define a problem.

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1.Anorectal manometry provides information concerning anal sphincteric tone and the ability of the sphincter to contract. It can also document the presence of the normal rectosphincteric reflex, which is absent in Hirschsprung's disease.

2.Electromyography (pudendal nerve conduction velocity) may provide evidence of injury to the pudendal nerves that supply the anal sphincter.

G

Endorectal ultrasound may provide information in certain anorectal disorders, including:

1.The depth of invasion into the bowel wall by rectal cancer

2.The site of anal sphincter injury in the incontinent patient

3.The path of complicated anal fistulas

IIIBowel Preparation

The colon must be adequately cleansed before surgical resection or before studies such as barium enema and colonoscopy. Feces must be removed from the bowel lumen; and, for surgery, the bacterial population must be reduced to minimize the risk of infection. Many methods are available to accomplish these goals, but almost all use a combination of cathartics, enemas, and antibiotics. The type of preparation depends somewhat on specific requirements (e.g., only mechanical cleansing is