Chapter 12

Small Intestine

Karen A. Chojnacki

David J. Maron

I Introduction

AAnatomy

1.External structure. The small intestine is the length of bowel that extends from the pylorus to the cecum.

a.The duodenum, which is retroperitoneal, extends from the pylorus to the ligament of Treitz.

b.The jejunum (proximal 40%) and ileum (distal 60%), which are intraperitoneal, make up the remainder of the small intestine.

c.The total length of small bowel is approximately 3 m (the duodenum measures 30 cm; the jejunum is 110 cm; and the ileum is 160 cm).

2.Vasculature. The arterial supply to the small intestine is primarily from the jejunal and ileal branches of the superior mesenteric artery (except the duodenum, which is also supplied by the branches of the celiac axis).

a.Jejunal mesenteric arteries have only one or two arcades and a long vasa recta (the small arteries directly adjacent to the bowel wall).

b.Ileal arteries have multiple arcades that extend closer to the bowel and have short vasa recta.

3.Layers of the wall of the small intestine

a.The mucosa consists mostly of absorptive columnar epithelium and mucous-producing goblet cells. The absorption of nutrients takes place through the epithelial cells that cover the intestinal villi and have a total surface area of approximately 500 m2. Mucosal cells proliferate rapidly and have a life span of 5 days.

b.The submucosa is the strongest layer and provides strength to an intestinal anastomosis. It contains nerves, Meissner's plexus, blood vessels, lymphoid tissue (Peyer patches), and fibrous and elastic tissue.

c.The muscularis—the muscle layer—consists of an outer longitudinal layer and an inner circular layer with Auerbach's myenteric plexus of ganglion cells in between.

d.The serosa is the outermost layer and derives embryologically from the peritoneum.

4.Internal structure

a.Spiral folds of mucosa and submucosa, also known as plicae circulares or valvulae conniventes, are more prominent proximally.

b.The jejunum is larger in diameter, thicker walled, has more prominent plicae circulares, and has less mesenteric fat than the ileum.

c.The lymphoid tissue (Peyer patches) becomes more prominent distally in the ileum.

BPhysiology

The primary functions of the small intestine are digestion and absorption. All ingested food and fluid, plus secretions from the stomach, liver, and pancreas, reach the small intestine. The total volume may reach 9 L/day, and all except 1–2 L will be absorbed.

1. Motility

a.Two types of contractions occur after a meal.

1.To-and-fro motion mixes chyme with digestive juices and provides prolonged exposure to the absorptive mucosa.

2.Peristaltic contractions move food distally.

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b.In the fasting state, a strong contraction begins in the duodenum and occurs every 2 hours (migrating motor complex). This completes the emptying of residual food from previous meals.

c.Parasympathetic stimulation promotes contractions, whereas sympathetic stimulation inhibits them.

2.Absorption. Vitamins, fat, protein, carbohydrates, water, and electrolytes are all absorbed in the small intestine.

a.Water is absorbed throughout the small intestine, although most water is absorbed in the jejunum. Passive absorption is the mechanism.

b.Electrolytes

1.Potassium is absorbed by passive diffusion through intercellular pores in the jejunum.

2.Sodium is actively transported, and once a gradient is established, chloride follows passively.

3.Calcium is actively transported in the jejunum (enhanced by vitamin D and parathyroid hormone).

4.Iron is absorbed as the ferrous (reduced) ion Fe2+. Conversion of the ferric to the ferrous ion is enhanced by an acid milieu and the presence of reducing substances in the diet, such as vitamin C (ascorbic acid). Absorption is via active transport in the duodenum and jejunum, and 10%–26% (maximally) of dietary iron is absorbed.

c.Fat absorption occurs mainly in the jejunum. Fat is digested by pancreatic lipase and becomes emulsified in bile salt micelles. The micelles release fatty acids and monoglycerides to the epithelial cells. After absorption, the epithelial cells resynthesize triglycerides, which are assembled into chylomicrons and transported into the lymphatics. (All other absorbed nutrients are transported directly into the portal venous system.)

d.Carbohydrates are digested by salivary and pancreatic amylase. Enzymes of the mucosal cell surface further reduce sugars to the monosaccharides galactose and glucose, which are absorbed by active transport, and fructose, which is absorbed by diffusion.

e.Protein digestion begins in the stomach (by pepsin) and continues in the small bowel by pancreatic proteases. The process is completed at the brush border, yielding tripeptides, dipeptides, and amino acids. All are absorbed by active transport.

f.The fat-soluble vitamins A, D, E, and K are absorbed from micelles by the mucosa. Vitamin B12 is complexed with intrinsic factor and absorbed in the distal ileum. Vitamin C, thiamine, and folic acid are actively transported. The remaining water-soluble vitamins are absorbed by passive diffusion.

IISmall Bowel Diseases

A Small bowel obstruction (SBO)

1.Causes

a.Adhesions account for more than half of SBO cases. Adhesions from lower abdominal procedures carry a higher risk of causing obstruction.

b.Hernias of all types can cause obstruction. Femoral hernias are particularly prone to incarceration and bowel necrosis.

c.Obstruction can be caused by malignancy, most commonly adenocarcinoma or lymphoma.

d.Other less likely causes include gallstone ileus (which is caused by obstruction of the ileocecal valve by a gallstone), Crohn's disease (see II C 5 a), intussuception, and volvulus.

2.Symptoms include crampy abdominal pain, nausea and vomiting, and abdominal distention. Patients with complete bowel obstruction typically do not pass any flatus or bowel movements.

3.Diagnosis

Abdominal x-rays show dliated loops of small bowel on flat plate and air-fluid levels on upright films. Air in the colon may represent early complete or incomplete obstruction.

Small bowel follow-through and computed tomography (CT) scan can also be used to localize the point of obstruction and to determine the nature of the lesion.

4.Treatment includes resuscitation with intravenous (IV) fluids, nasogastric tube decompression, and placement of a urinary catheter to monitor urine output. Abdominal exploration is

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performed in patients with peritoneal signs, leukocytosis, fever, or failure of resolution of obstructive symptoms.

B Tumors of the small intestine

1.Benign neoplasms are usually asymptomatic. They are ten times more common than are malignant tumors (autopsy data). Surgery is indicated for bleeding, obstruction, or intussusception.

a.Adenomas are rare in the small intestine.

1.Duodenum is the most common site of small bowel adenomas.

2.There are three types of small bowel adenomas: tubular, villous, and Brunner's gland.

3.Villous adenomas have the highest malignacy potential.

4.Adenomas are usually discovered incidentally or as a source of gastrointestinal (GI) bleeding, obstruction, or intussusception.

5.Adenomas are treated by endoscopic or surgical resection.

b.Hamartomatous polyps are found in patients with Peutz-Jeghers syndrome (mucocutaneous pigmentation accompanied by widespread intestinal polyposis). There is little malignant potential in this syndrome.

c.Juvenile (retention) polyps are benign hamartomas and not true neoplasms. They are more common in the rectum and usually autoamputate.

d.Gastrointestinal stromal tumors (GIST) are mesenchymal neoplasms of the small bowel formerly called leiomyomas.

1.These tumors are most commonly benign and present with bleeding, obstruction, or intussusception.

2.Their origin is the cells of Cajal. They stain positive for CD 117 on immunohistochemical analysis.

3.The most common site is the ileum.

4.Ten to 30% of GIST tumors are malignant. Malignancy is based on tumor size greater than 5–10 cm, >10 mitotic figures per 50 high-power fields, necrosis, and/or the presence of metastases.

5.Treatment is wide surgical resection. Metastases should be debulked, if possible, for palliation. Imatinib mesylate, a tyrosine kinase inhibitor, has recently been shown to effective for patients with unresectable or metastatic GIST.

e.Other benign small bowel tumors include lipomas, hemangiomas, fibromas, and neurofibromas.

2.Malignant neoplasms

a.Overview

1.Incidence. The most common tumors are adenocarcinoma (40%), carcinoid (30%), lymphoma (20%), and sarcoma. Metastases from other intraabdominal malignancies are common, especially with peritoneal carcinomatosis. Metastases from extra-abdominal malignancies are rare except for the tendency of malignant melanoma to metastasize to the small bowel.

2.Symptoms. Malignant tumors constitute 75% of symptomatic small bowel tumors and usually present with bleeding, diarrhea, perforation, or obstruction (which may be caused by intussusception).

3.Diagnosis is frequently made late in the course of disease because symptoms are often subtle and insidious in onset.

4.Treatment involves segmental resection, including adequate margins proximally and distally and as much mesentery as possible without compromising the blood supply to the remaining small intestine.

b. Adenocarcinoma is most common in the duodenum and proximal jejunum.

1.Adenocarcinomas of the first and second portions of the duodenum are best managed by Whipple pancreaticoduodenectomy. Unresectable tumors at this location should be palliated by gastrojejeunostomy or stents.

2.Tumors of the distal duodenum and small bowel should be managed by wide local resection of the bowel and intervening mesentary.

3.Metastases are often found at the time of presentation.

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4.For patients with node-negative disease, 5-year survival may be as high as 80%. For those with nodepositive disease, 5-year survival is only 10%–15%.

c.Carcinoid tumors are derived from enterochromaffin cells. These cells are part of the amine precursor uptake and decarboxylation system (APUD cells) and secrete various vasoactive amines. Carcinoid tumors are most common in the appendix, followed by the small bowel (usually the ileum) and the rectum. Other sites are uncommon.

1.The prognosis is related to the presence of metastases at diagnosis, which is strongly associated with the size of the tumor. Both carcinoid tumors and metastases grow slowly, and prolonged survival is common.

a.Tumors less than 1 cm in diameter (75% of total) have a 2% incidence of metastases.

b.Tumors larger than 1 cm (20% of total) have a 50% rate of metastases.

c.Tumors larger than 2 cm (5% of total) have an 80%–90% rate of metastases.

2.Only small bowel carcinoids tend to be multicentric (30%).

3.Carcinoid syndrome—flushing, diarrhea, bronchoconstriction, and tricuspid and pulmonary valvular disease—is caused by serotonin and other vasoactive substances secreted by the tumor.

a.These substances are cleared by the liver, thus the syndrome can only occur in patients with liver metastases (which drain into the systemic veins) or primary extraintestinal carcinoids (e.g., bronchial carcinoids). Approximately 10% of patients with small bowel carcinoids develop the syndrome.

b.Diagnosis is confirmed by finding elevated urinary levels of 5-hydroxyindoleacetic acid (5-HIAA), which is the breakdown product of serotonin.

c.Treatment includes resection of the primary tumor and resection or “debulking” of metastases. Liver metastases can be resected (when solitary) but usually require palliative local therapies (e.g., intra-arterial chemotherapy, hepatic arterial chemoembolization, or local destruction) (Chapter 14, I E).

d.Prognosis. The overall 5-year survival rate for patients with small bowel carcinoids is 70%. If liver metastases are present at diagnosis, the 5-year survival rate is 20%.

d.Small bowel lymphomas may present as primary neoplasms or as part of a disseminated lymphoma. Small bowel lymphomas usually arise in the ileum.

1.Most primary lesions are non-Hodgkins B-cell lymphomas.

2.There is an increased incidence of small bowel lymphomas in patients who are immunosuppressed or

have inflammatory conditions such as Crohn's disease.

3.The most common symptoms are abdominal pain, fatigue, and weight loss. Diffuse adenopathy is uncommon.

4.Complications include bowel perforation, hemorrhage, obstruction, and intussusception.

5.Treatment is wide local resection. Liver biopsy and distant nodal biopsies are done for accurate staging.

6.Chemotherapy and local radiation are used for patients with disseminated disease.

7.Overall 5-year survival rates are 20%–40%.

e. Leiomyosarcoma is the most common of the small bowel sarcomas.

C

Crohn's disease (regional enteritis; granulomatous ileitis) is a chronic, transmural granulomatous inflammatory disease that can involve any area of the GI tract. Its cause is unknown.

1.Distribution. The small bowel alone is involved in 25% of patients, both the small and the large bowel in 50%, and the colon alone in almost 25%. The distal ileum (the most common site) is involved in 70% of all cases, which accounts for the older name, terminal ileitis.

2.Diagnosis

a.The peak age of onset is between the second and fourth decades.

b.Symptoms include abdominal pain, diarrhea (usually not bloody), lethargy, fever, weight loss, and anorectal disease. Anal fissures, fistulas, ulcers, or perirectal abscesses are seen in 50% of patients with colonic involvement and in 20% of patients with small bowel disease.

c.Signs include an abdominal mass, anemia, and malnutrition. Extraintestinal manifestations include inflammatory ocular (uveitis, iritis), joint (arthralgias, arthritis), skin

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(erythema nodosum, pyoderma gangrenosum), and biliary (primary sclerosing cholangitis) conditions.

d.Radiographic findings on contrast study characteristically include segmental areas of stricture separated by “skip areas” of uninvolved bowel (string sign), cobblestone appearance of the mucosa, and fistulas.

e.Gross appearance includes thickened, shortened mesentery, grayish-pink to purple discoloration of the bowel, and fat wrapping (circumferential growth of mesenteric fat around the bowel wall).

f.Pathologically, there is mucosal ulceration that progresses to transmural inflammation, and noncaseating granulomas are found in the bowel wall and regional lymph nodes.

3.Differential diagnosis includes ulcerative colitis, lymphoma, and infectious enteritides (tuberculosis; amebiasis; and

Yersinia, Campylobacter, and Salmonella infections).

4.Medical treatment includes a low-residue diet, aminosalicylates, prednisone, oral antibiotics (sulfasalazine, metronidazole), sedatives, antispasmodics, and, when necessary, total parenteral nutrition (TPN). Although antibiotics and steroids are helpful in acute active disease, their effectiveness in preventing relapses has not been established.

a.Infliximab, a chimeric IgG1 monoclonal antibody that binds to tumor necrosis factor-alpha, helps enterocutaneous fistulas close and results in improvement in 80% of chronic refractory patients, almost half of whom achieve short-term remission (8–12 weeks).

5.Surgical treatment is reserved for complications of the disease and ultimately becomes necessary in 70% of cases. Surgical therapy is conservative—resecting as little bowel as necessary. Margins of resection need only be to grossly uninvolved bowel. If resection is hazardous, bypass or exclusion of the involved segment may be necessary.

a.Intestinal obstruction, which is usually caused by stricture and inflammation, is the most common indication for surgery. Short strictures can be repaired by a stricturoplasty, thus avoiding resection.

b.Abscesses and fistulas are also common. Abscesses may be intraor retroperitoneal. Fistulas (see Chapter 2,

VII) form from bowel to skin, bladder, vagina, urethra, or other loops of bowel.

c.Perianal disease is best treated with oral metronidazole therapy.

1.Perirectal abscesses require drainage. Anal fistulas and fissures may need surgery if they are multiple or severe.

2.In general, surgery for perianal disease should be as limited as is practical, because wound healing in these patients is poor and recurrence is common.

d.Perforation, hemorrhage, intractable symptoms, cancer, and growth retardation (in children) are less common indications for surgery.

6.The prognosis is only fair. Approximately 50% of patients who require surgery will require it again within 5 years. The likelihood of a recurrence after each reoperation is again approximately 50%.

7.There is an increased risk of small bowel and colon adenocarcinoma associated with the severity and chronicity of inflammation.

D Diverticular disease

1.Duodenal diverticula are common (seen on 10%–20% of upper GI radiographs), but more than 90% are asymptomatic.

a.Approximately 70% of duodenal diverticula are in the periampullary region.

b.Periampullary diverticula can impair the emptying of bile through the ampulla, resulting in cholangitis, pancreatitis, and common bile duct stones.

2.Jejunoileal diverticula are rare.

a.They may cause obstruction (from intussusception), bleeding, or perforation.

b.They may also cause malabsorption owing to bacterial overgrowth within the diverticulum.

3.Meckel's diverticulum is the most common diverticulum of the GI tract (incidence 2%). It is located approximately 2 ft from the ileocecal valve. The male:female ratio is 2:1. Over a lifetime, approximately 95% of Meckel's diverticula will remain asymptomatic.

a.Meckel's diverticula are true diverticula.

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b.Most symptomatic Meckel's diverticula occur in children younger than age 2 and usually cause bleeding. Bleeding is due to heterotopic gastric mucosa in the diverticulum, which causes “peptic” ulceration in adjacent ileal mucosa. Heterotopic pancreatic mucosa may also be found within a Meckel's diverticulum.

c.Problems in adults include bowel obstruction (from intussusception), bleeding, and acute diverticulitis, which may be indistinguishable from appendicitis.

d.Management of asymptomatic Meckel's divericulum is controversial. Relative indications for resection include patient age <40, diverticulum >2 cm in length, fibrous bands between the diverticulum and the umbilicus, or mesentery.

E Small bowel fistulas

(see Chapter 2, VII)

F

Short bowel syndrome is a complication of extensive small bowel resection.

1.Symptoms. It is characterized by malabsorption with diarrhea and excessive loss of fat and protein in the stool. Inadequate absorption of water, electrolytes, minerals, and vitamins also invariably occurs. The length of bowel necessary to avoid the syndrome is variable, but patients with less than 100 cm (3.5 ft) of small bowel are most

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.