Chapter 39. The Intestines: I. Structure & Function; Malabsorption Syndrome; Intestinal Obstruction

The intestine begins at the pylorus and ends at the anorectal junction. It is divided into the small and large intestine, which are separated by the ileocecal valve. The small intestine is composed of the duodenum, jejunum, and ileum and is about 6 meters long in adults; the large intestine is composed of the cecum, ascending, transverse, descending, and sigmoid colon and the rectum, totaling about 1.5 meters in length in adults.

The intestinal wall has four layers:

1.  Mucosa, which is lined by glandular epithelium. The small intestine is characterized by the presence of villi and crypts (Figure 39-1). The villi increase the surface area for absorption. The colonic mucosa has no villi and is composed only of crypts. The crypts contain proliferating cells that continually divide to replace lost surface epithelial cells.

2. Submucosa, which contains blood and lymphatic vessels and the submucosal nerve plexus.

3. Muscularis externa, which is composed of 2 layers in the small intestine. In the large intestine, the longitudinal muscle is attenuated to form the taenia coli. The muscle is responsible for propulsive peristalsis. The myenteric plexus of nerves is situated between the 2 muscle layers and provides the neural impetus to peristalsis.

4. Serosa, which is the peritoneal lining in those parts of the intestine that lie in the peritoneal cavity.

The intestine digests and absorbs essential components from ingested food, eliminating the waste at defecation. Digestion is effected in the upper small intestine by enzymes contained in the secretions of intestinal juice, pancreatic juice, and bile. The small molecules resulting from digestion—monosaccharides, amino acids, and fatty acids—are absorbed in the small intestine. The colon absorbs water from the liquid ileal effluent to form solid feces.

Malabsorption

See Malabsorption Syndrome.

Intestinal Obstruction

Mechanical obstruction may result from (1) lesions outside the intestine that compress or constrict the intestine, eg, fibrous adhesions in the peritoneal cavity and hernial sacs; (2) intramural lesions such as fibrous strictures and neoplasms; (3) volvulus of the intestine (see Volvulus); (4) intussusception (see Intussusception; and (5) intraluminal foreign bodies.

Failure of peristalsis may be the result of paralysis of the intestinal smooth muscle (paralytic ileus) or abnormalities of the myenteric plexus, eg, congenital megacolon (Hirschsprung's disease), or smooth muscle diseases, eg, familial visceral myopathy. Paralysis occurs after abdominal surgery, in patients with peritoneal inflammation, and in severe acute intestinal inflammation (eg, toxic megacolon).

Intestinal obstruction has the following physiologic consequences:

1. Failure of propulsive movement of intestinal contents, leading to constipation and absence of flatus.

2. Accumulation of food and fluid secretions in the intestine proximal to the obstruction. This leads to dilation of bowel loops, which become filled with fluid, food, and gas; hypovolemia and electrolyte imbalance; abdominal distention; and vomiting. Rarely, with massive dilation, perforation occurs; this is most common in the cecum.

3. Increased peristalsis when the obstruction is mechanical, resulting in colicky abdominal pain and increased bowel sounds on abdominal auscultation. In cases where intestinal obstruction is due to muscle paralysis, pain does not occur and bowel sounds are absent.

Intestinal Perforation

Complete disruption of the bowel wall permits leakage of luminal contents into the peritoneal cavity. When perforation occurs in a part of the intestine whose wall is not covered by serosa, eg, the posterior wall of the descending colon and rectum, the luminal contents leak into the pericolic fat, causing pericolic abscess.

Many diseases may lead to free perforation into the peritoneal cavity. The effect of intestinal perforation depends on the site of the perforation. Perforation of a duodenal ulcer produces chemical peritonitis due to the acid content. Perforation of the intestine distal to the first part of the duodenum leads to bacterial peritonitis due to the intestinal bacterial flora. In all of these types of perforation, gas enters the peritoneal cavity and can be detected under the diaphragm on x-ray.

Intestinal Hemorrhage

Bleeding into the lumen of the intestines may be manifested in several ways: (1) Bright red blood per rectum occurs with rapid bleeding and when the source of bleeding is near the rectum. (2) Passage of dark stools containing altered blood admixed with stools occurs with bleeding originating in the proximal colon and small intestine. (3) Passage of tarry black stools occurs when bleeding is in the stomach and duodenum and the blood is altered by the action of acid. (4) Iron deficiency anemia may develop with chronic slow bleeding and is associated with occult blood in stools.

Protein Loss in the Intestine (Protein-Losing Enteropathy)

Increased loss of protein in the intestine may result in hypoproteinemia and edema.

Diarrhea

Diarrhea is the passage of fluid feces and occurs when the rate of movement of intestinal contents is increased so that complete digestion and absorption of fluid in the intestine fails to occur. The volume of feces is usually greatly increased in diarrhea, leading to increased frequency of evacuation and increased loss of water and electrolytes.

The mechanisms in the production of diarrhea are summarized in Table 39-1. Diarrhea may result from (1) increased fluid secretion into the intestine (secretory diarrhea); (2) the presence of increased amounts of osmotically active substances in the intestinal lumen; (3) inflammation of the intestine—mainly the small intestine; (4) increased peristalsis, eg, resulting from stimulation of smooth muscle by serotonin in carcinoid syndrome and in irritable bowel syndrome; (5) failure of colonic water absorption, eg, after surgical removal of the colon; and (6) malabsorption syndrome (Table 39-2).

Steatorrhea is a specific form of diarrhea defined by the presence of excessive fat (> 6 g/d) in the feces. Most patients with steatorrhea have diarrhea. Steatorrhea is a typical clinical manifestation of maldigestion or malabsorption of fat.

Dysentery

Dysentery is a specific type of diarrhea characterized by the passage of small amounts of bright red blood and mucus at frequent intervals. The volume of feces is usually small. Dysentery is the characteristic symptom of acute colonic inflammation. When the rectum is inflamed, dysentery is accompanied by a constant and painful desire to defecate (tenesmus).

Endoscopy

Upper endoscopy permits evaluation of the duodenum up to the ligament of Treitz. Sigmoidoscopy permits evaluation of the rectum and sigmoid colon. Colonoscopy permits evaluation of the entire colon and, in some cases, the terminal ileum. The rest of the small intestine is inaccessible to direct examination by endoscopy.

Radiography

Administration of barium and visualization of its passage through the small intestine by fluoroscopy permits evaluation of the mucosal pattern of the small intestine. Barium enema permits examination of the colonic mucosa. Computerized tomography of the abdomen is not very effective in detecting mass lesions of the intestinal wall but is the best available radiologic method.

Biopsy

Endoscopic biopsies may be obtained from any lesion that is detected at endoscopy. Biopsy of the jejunum, which is important for the diagnosis of malabsorption, may be performed by means of a biopsy capsule that is swallowed. The capsule has a spring mechanism that cuts off a piece of jejunal mucosa when activated. Laparoscopy permits visualization and biopsy of serosal and intraperitoneal lesions.

Normal Absorption in the Small Intestine

Absorption of Fat

(Figure 39-2)

Luminal Phase

Fifty to 100 g of fat is ingested daily by an adult, mainly as long-chain triglycerides. Pancreatic lipase hydrolyzes triglycerides to fatty acids and monoglycerides. The fatty acids and monoglycerides, along with fat-soluble vitamins, are complexed with bile acids to form a globular structure called a micelle.

Cellular Phase

The micelles dissociate at the surface of the intestinal mucosal cell, and the fatty acids, monoglycerides, and fat-soluble vitamins move into the cell while the bile acids remain in the lumen. The bile acids are finally reabsorbed in the terminal ileum and pass to the liver for reexcretion (the enterohepatic circulation of bile acids).

Fat absorption occurs mainly in the upper jejunum, and about 100 cm are required for normal fat absorption. In the intestinal cells, monoglycerides are further hydrolyzed by mucosal cell lipase. The fatty acids are then reconverted to triglycerides in the endoplasmic reticulum and complexed with protein, phospholipid, and some cholesterol to form chylomicrons. Chylomicrons exit the cell at the antiluminal border of the mucosal cell, enter intestinal lymphatics, and pass to the jugular vein via the thoracic duct.

Medium-chain triglycerides (containing 6 to 10 fatty acids), which normally form a small component of fat in the diet, are handled differently. They are better absorbed because they are more completely hydrolyzed and form micelles more readily than long-chain triglycerides. Limited absorption (30% of an oral dose) is also possible without formation of micelles, the medium chain triglycerides passing directly into the mucosal cell, where they are hydrolyzed by cellular lipase into fatty acids. Medium-chain fatty acids can pass directly into the portal circulation without reesterification and formation of chylomicrons. For these reasons, medium-chain triglycerides are useful in the treatment of some patients with malabsorption syndrome.

Absorption of Protein & Carbohydrate

Proteins and carbohydrates are hydrolyzed by enzymes in saliva (amylase), gastric juice (pepsin), pancreatic juice (amylase, trypsin, and chymotrypsin), and intestinal juice (disaccharidases, carboxypeptidases) into amino acids and monosaccharides. Because they are hydrophilic molecules, these substances are absorbed into the mucosal cells and pass into the portal venous radicals.

Definition of Malabsorption Syndrome

Malabsorption syndrome is a clinical syndrome characterized by increased fecal excretion of fat (steatorrhea) and the systemic effects of deficiency of vitamins, minerals, protein, and carbohydrates (Table 39-3). The complete clinical syndrome of malabsorption may not be present in all patients. Some patients have malabsorption of specific types of dietary constituents.

Steatorrhea is passage of soft, yellowish, greasy stools containing an increased amount of fat. The presence of steatorrhea is established when fat excretion exceeding 6 g/d is demonstrated in a 72-hour stool sample. The high fat content of the stool makes it float in water, and flushing it down the toilet may be difficult—a feature in a patient's history that is highly suggestive of steatorrhea.

Pathophysiology of Malabsorption

(Table 39-2)

Deficiency of Pancreatic Enzymes

Pancreatic juice contains numerous enzymes. Pancreatic lipase is essential for complete fat digestion, and its absence leads to steatorrhea. Pancreatic amylase and trypsin are important for carbohydrate and protein digestion, but their function can be taken over by other enzymes in intestinal juice.

Absence of pancreatic lipase is usually the result of primary pancreatic disease, commonly chronic pancreatitis. Rarely, lipase secretion is normal but its activity is decreased, either because of increased gastric acidity (eg, Zollinger-Ellison syndrome) or because rapid gastric emptying dilutes pancreatic enzymes.

Deficiency of Bile Acids

Bile acid deficiency occurs when bile does not enter the intestine, as in biliary obstruction, or as a result of bile salt deconjugation in the intestinal lumen resulting from overgrowth of bacteria. The small intestine is free of bacteria in about 50% of individuals; in the remainder, small numbers of bacteria (up to 1000 organisms per gram of luminal contents) are present. The small intestine becomes colonized with bacteria when there is stasis of luminal contents from any cause.

A third mechanism of bile acid deficiency is failure of reabsorption of bile acids in the terminal ileum, due either to absence or disease of that segment of intestine.

Deficiency of bile acids results in failure of micelle formation, interfering with absorption of fat and fat-soluble vitamins. Medium-chain triglycerides, proteins, and carbohydrates are absorbed normally.

Abnormalities in the Absorptive Mucosa

Quantitative and qualitative abnormalities in the absorptive mucosa are a common cause of malabsorption. Quantitative abnormalities occur (1) when there is a reduction in the length of small intestine due to extensive surgical resection; or (2) when the surface area of villi decreases, as occurs in villous atrophy (Figure 39-3) (celiac disease, tropical sprue, giardiasis, radiation, etc) or when the villous surface is abnormal, as in severe giardiasis.

Qualitative abnormalities of the villi such as amyloidosis and Whipple's disease also result in malabsorption.

Decreased Intestinal Transit Time

Digestion and absorption require a certain minimum amount of time. In conditions where there is increased intestinal motility such as carcinoid syndrome (serotonin stimulates smooth muscle) and after gastrectomy, there may be malabsorption.

Failure of Removal of Intestinal Triglyceride

Triglycerides formed in the mucosal cell must be complexed with a carrier protein to form chylomicrons before they can enter the intestinal lymphatics. This fails in abetalipoproteinemia, a rare autosomal recessive inherited disease characterized by decreased serum betalipoprotein levels and abnormal erythrocytes. The triglyceride accumulates in the intestinal mucosal cell (fatty change), which can be seen on intestinal biopsy.

Synthesized chylomicrons pass from the intestine into the lymphatics. Obstruction of lymphatics, as in intestinal lymphangiectasia and extensive intestinal lymphomas, may cause malabsorption. In macroglobulinemia, IgM is precipitated in lymphatics and may obstruct them.

Diseases Causing Malabsorption

Celiac Disease (Nontropical Sprue; Gluten-Induced Enteropathy)

Celiac disease is caused by the action of acidic peptides contained in the gliadin fraction of the wheat protein gluten on the intestinal mucosa. The exact mechanism of damage is not certain, but immunologic hypersensitivity seems most likely. Serum contains IgA antibodies to gliadin in most patients, and antibody titers fall when these patients are maintained on a gluten-free diet. The possibility that the disease is a direct toxic effect of gliadin on the mucosal cells has not been ruled out.

Susceptibility to gluten-induced intestinal damage is rare and has a familial tendency; 80–90% of patients have the histocompatibility antigen human leukocyte antigen (HLA)-B8 or HLA-DR3.

Celiac disease occurs commonly in children; onset in adults is not rare. Celiac disease is associated with dermatitis herpetiformis (see Chapter 61: Diseases of the Skin).

Pathology

(Figure 39-3)

The changes of celiac disease are restricted to the small intestinal mucosa. The basic abnormality is thought to be an increased rate of loss of epithelial cells. The crypt cell shows increased activity but cannot keep pace with the loss of cells, resulting in progressive decrease in height of villi (villous atrophy). The crypts are hyperplastic and become elongated. The epithelial cells show decreased cytoplasm and mucus, appearing cuboidal. There is associated lymphocytic infiltration of the epithelium, suggesting that cell-mediated immunologic mechanisms may have a part in the genesis of the lesion. Immunologic studies show the presence of anti-gliadin IgA antibodies in the mucosa.

Intestinal biopsy shows a decrease in the villus:crypt ratio. Normally (Figure 39-1), the villi are 3–4 times the height of the crypts (ratio of 3–4:1); in celiac disease, the ratio decreases progressively (partial, subtotal, and total villous atrophy). The shortened villi are also wider than normal (spade-like rather than finger-like) and infiltrated by chronic inflammatory cells. In severe celiac disease, the villi are completely atrophic (ratio of 0:1, or total villous atrophy; Figure 39-3).

Villous atrophy is a nonspecific abnormality resulting from increased loss of epithelial cells due to any cause. Total villous atrophy rarely occurs except in celiac disease.

Clinical Features & Diagnosis

Celiac disease presents with severe malabsorption syndrome. The diagnosis may be suggested by an abnormal mucosal pattern on small bowel follow-through radiologic studies and confirmed by the findings of total villous atrophy and epithelial abnormalities on jejunal biopsy.

Withdrawal of gluten from the diet produces dramatic improvement in both clinical symptoms and histologic changes in the small intestine. The ultimate diagnostic test is the demonstration of reversal of histologic changes after the patient has been on a gluten-free diet for 6 months.

Complications

Celiac disease is associated with an increased incidence of intestinal malignant lymphoma, most commonly of T cell origin. It is not known whether removal of gluten from the diet eliminates the risk of lymphoma.

Tropical Sprue

Tropical sprue is an acquired disease that is commonly seen in the Caribbean, Far East, and India. It is thought to result from chronic bacterial infection of the small intestine because treatment with broad-spectrum antibiotics is often successful.

Tropical sprue occurs in adults and is characterized by a severe malabsorption syndrome in which folic acid deficiency is often a dominant feature.

Jejunal biopsy shows villous atrophy that is usually partial. Total villous atrophy is rare. The correlation between the degree of villous atrophy and malabsorption is poor.

Whipple's Disease (Intestinal Lipodystrophy)

Whipple's disease is a rare disease characterized pathologically by distention of the lamina propria of the small intestine by macrophages with abundant pale foamy cytoplasm (Figure 39-4). The macrophages contain large numbers of bacilli (seen on electron microscopy) that produce a fine granular staining with periodic acid-Schiff (PAS) reagent. Infiltration of the mucosa by these cells results in a gross increase in size of the villi, which gives the mucosal surface a coarse appearance resembling the pile of a shaggy rug.

Treatment with antibiotics causes improvement of symptoms as well as disappearance of the bacillary bodies, suggesting that this is also a form of bacterial infection. The specific organism or organisms responsible have not been identified.

The disease occurs mainly in males over 30 years of age. Presentation is with severe malabsorption syndrome. The diagnosis may be made by demonstration of the typical macrophages on jejunal biopsy.

Forty percent of patients have the abnormal macrophages in tissues other than the small intestine—usually lymph nodes, commonly mesenteric; rarely, the spleen, liver, kidney, lungs, heart, and brain may be affected. Fever is present in 30% of patients, and polyarthritis and polyserositis (pleural effusion and ascites) are common. Extraintestinal Whipple's disease may sometimes occur in the absence of intestinal symptoms.

Intestinal Atresia

Atresia is a rare congenital disorder consisting of failure of development of the lumen in one part of the intestine. The jejunum and ileum are most commonly affected, and multiple bowel segments may be affected. Atresia presents with intestinal obstruction in the first week of life. Surgical correction is curative.

Imperforate Anus

Imperforate anus is a common congenital abnormality that results from failure of the endodermal hindgut to open at the anal dimple. Varying degrees of abnormality result, sometimes associated with fistulous communications with the bladder, urethra, or vagina.

Malrotations

The intestine rotates in a specific manner during development. Malrotation is characterized by failure of the cecum to reach its normal position in the right lower quadrant. The importance of malrotation is that the abnormal mobility of the cecum predisposes to twisting (volvulus) and intestinal obstruction.

Meckel's Diverticulum

Meckel's diverticulum is persistence of the intestinal end of the omphalomesenteric duct. It is present in 2% of the population (the most common congenital anomaly of the gastrointestinal tract), is found within 2 feet (1 m) of the ileocecal valve, and is usually 2 inches (5 cm) long.

The diverticulum is usually lined by small intestinal mucosa. About 40% of cases have heterotopic gastric mucosa or pancreatic tissue (Figure 39-5). Peptic ulceration may occur when there is gastric mucosa. Bleeding from an ulcerated Meckel diverticulum is an important cause of chronic intestinal blood loss, resulting in iron deficiency anemia. Other complications include infection (Meckel's diverticulitis), which presents a clinical picture very similar to that of acute appendicitis and may result in perforation and peritonitis. Volvulus may occur in relation to Meckel's diverticulum, especially when it is attached to the umbilicus by the obliterated omphalomesenteric duct. Rarely, the diverticulum inverts into the ileum and acts as the apex of an intussusception.

Congenital Megacolon (Hirschsprung's Disease)

Hirschsprung's disease is caused by failure of development of ganglion cells in the myenteric and submucosal plexuses of the colon. The aganglionic segment usually starts at the anorectal junction and extends proximally for a variable distance. In 90% of cases, aganglionosis is restricted to the rectum. Rarely, the aganglionic segment is longer, and in exceptional cases the entire colon lacks ganglion cells.

The absence of ganglion cells in the myenteric plexus results in failure of peristalsis in the affected segment. This segment remains narrow and spastic and represents a zone of functional intestinal obstruction. The colon proximal to the aganglionic segment dilates—often massively—leading to abdominal distention (Figure 39-6).

Most affected children present soon after birth with failure to pass meconium followed by distention and vomiting. In a few cases, the onset is delayed. Without treatment, death occurs from fluid and electrolyte imbalance or from perforation of a massively dilated cecum.

The diagnosis may be made by demonstrating absence of ganglion cells in the submucosa of an adequate rectal biopsy. Absence of ganglion cells in the submucosa correlates well with absence of ganglion cells in the myenteric plexus. Treatment consists of surgical removal of the aganglionic segment. Because aganglionosis may extend from the narrow segment to involve part of the dilated segment, confirmation of the presence of ganglion cells by frozen-section examination is necessary at surgery to make certain that the entire aganglionic segment has been removed.

Blood Supply to the Intestine

The intestine is supplied by 3 major arteries that arise from the aorta: (1) the celiac artery, which supplies the stomach, duodenum, liver, and pancreas; (2) the superior mesenteric artery, which supplies the rest of the small intestine and the ascending and transverse colon; and (3) the inferior mesenteric artery, which supplies the distal colon to the level of the rectum. The lowest part of the rectum receives its arterial supply from pelvic arteries.

The small intestine is entirely dependent on the mesenteric arteries for its blood supply; the colon receives a supplementary supply from retroperitoneal arteries and is much less susceptible to infarction, but more commonly it shows chronic ischemic damage of the mucosa (ischemic colitis).

The main mesenteric arteries supply a large segment of bowel, and their sudden occlusion causes infarction in a large area. Beyond the initial branches there is an extensive collateral circulation via the arterial arcades at the mesenteric border of the intestine, which is why obstruction of a secondary branch of a mesenteric artery usually does not cause infarction. From the arterial arcades, small arteries penetrate the intestinal wall; these are functional end-arteries, and their occlusion results in small areas of intestinal infarction.

Slow occlusion of the main mesenteric arteries, particularly the inferior mesenteric artery, does not produce infarction because adequate collaterals can develop. Generally, extensive arterial disease must be present before there is evidence of ischemia if the arterial disease causes slowly progressive narrowing.

Acute Small Intestinal Infarction

Etiology

Sudden Occlusion of the Superior Mesenteric Artery

(50% of cases.) This is due to atherosclerosis in over 90% of cases. Other causes of superior mesenteric artery occlusion include (1) vasculitis—most commonly polyarteritis nodosa, which commonly affects the branches of the main artery; (2) aortic dissection; (3) embolism from mural cardiac thrombi and valvular vegetations in patients with cardiac disease; and (4) fibromuscular dysplasia of mesenteric arteries.

Nonocclusive Infarction

(25% of cases.) Intestinal infarction may occur without physical obstruction of the mesenteric arteries in severe shock, where there is prolonged vasoconstriction in the intestinal arterial circulation.

Mesenteric Vein Occlusion

(25% of cases.) Intestinal infarction due to venous occlusion may be caused by (1) obstruction of the superior mesenteric vein as a result of thrombosis (in hypercoagulable states such as polycythemia vera and with oral contraceptive use) or (2) infiltration by malignant neoplasms.

Pathology

Intestinal infarction due to obstruction of the main superior mesenteric artery or vein at the root of the mesentery involves a large part of the small intestine (Figure 39-7). Affected bowel loops are hemorrhagic, appearing purple to black. The demarcation of viable and nonviable bowel on gross examination at surgery may be difficult.

Microscopically, there is transmural necrosis of the intestinal wall. This is rapidly followed by bacterial infection from the lumen and acute inflammation, leading to wet gangrene. The distinction between arterial and venous infarction cannot be made on examination of the bowel; it is necessary to dissect the vessels at the root of the mesentery or to have angiographic evidence of arterial occlusion.

Intestinal infarction associated with vasculitis and nonocclusive infarction characteristically causes multiple discontinuous areas of intestinal necrosis. Commonly the mucosa, which is the layer most sensitive to ischemia, is selectively involved by the necrosis. Transmural necrosis does not usually occur.

Clinical Features

Intestinal infarction occurs most often in elderly individuals with severe atherosclerosis. Embolism occurs in patients with cardiac diseases in which valve vegetations (infective endocarditis) or mural thrombi occur (myocardial infarction, mitral stenosis, atrial fibrillation).

In a few patients, infarction may be preceded by the syndrome of intestinal ischemia (called abdominal angina). This is characterized by abdominal pain 15–30 minutes after a meal—when intestinal oxygen demand is greatest. This may result in a fear of eating and subsequent weight loss. Identification of this syndrome may permit arterial reconstruction and prevent the occurrence of intestinal infarction.

Infarction is characterized by a sudden onset of severe abdominal pain accompanied by fever, vomiting, and abdominal distention. There is functional intestinal obstruction with absent bowel sounds. Bloody diarrhea may be present in some patients as a result of sloughing of the necrotic mucosa. The disease progresses very rapidly, with shock and peritonitis due to bacterial permeation of the necrotic bowel wall occurring within 1–2 days after onset of symptoms. Without treatment, there is a 100% mortality rate because the dead intestinal wall cannot regenerate and rapidly perforates. Even with emergency surgery, the mortality rate remains high (50–75%). In patients who survive, malabsorption may result if a large part of the small intestine has been resected.

Nonocclusive infarction occurs in shocked patients and is characterized by abdominal pain and bloody diarrhea. Treatment is by correction of the shock state, with restoration of intestinal perfusion. Because only the mucosa is necrotic in most of these cases, regeneration is possible, and the bowel returns to normal.

Ischemic Colitis

Ischemic colitis is caused by extensive atherosclerotic narrowing of the inferior mesenteric artery and the collateral arteries that supply the colon. The splenic flexure and rectosigmoid area are most commonly involved.

Ischemic colitis is characterized by patchy mucosal necrosis in the affected region because the mucosa is most susceptible to ischemia. Necrosis is accompanied by acute inflammation and ulceration. The condition is usually self-limited, with healing by fibrosis and epithelial regeneration. Rarely, fibrosis may be so extensive as to cause narrowing (stricture) of the colon.

Clinically, patients present with an acute onset of fever, left-sided abdominal pain, and bloody diarrhea. Colonoscopy and histologic examination of biopsies show nonspecific necrosis, inflammation, and ulceration. The differentiation from other inflammatory lesions of the colon (infections, idiopathic inflammatory bowel disease) is difficult.

Angiodysplasia

Angiodysplasia has been recently recognized as a common and important cause of intestinal bleeding. It occurs in elderly patients, probably as an acquired degenerative change. It commonly involves the right side of the colon but may occur anywhere in the gastrointestinal tract.

Angiodysplasia is characterized by the presence of numerous dilated, tortuous, thin-walled blood vessels in the mucosa and submucosa of the colon. Rupture of these vessels causes painless bleeding, which can be sudden and severe. The diagnosis is best made by angiography, which shows the vascular malformation bleeding into the lumen. Surgical resection of the involved segment of colon is usually necessary to control the bleeding.

Pathologic demonstration of angiodysplasia requires injection of silicone rubber into the mesenteric arteries to distend the vessels before the specimen is fixed. If this is not done, the dilated vessels collapse after removal from the body and cannot be identified microscopically.

Hemorrhoids

Hemorrhoids are varicose dilations of the hemorrhoidal veins. Two types occur: (1) internal hemorrhoids, which are above the anorectal junction, arising from the superior and middle hemorrhoidal veins; and (2) external hemorrhoids, which are in the anal canal, arise from the inferior hemorrhoidal vein, and are covered by squamous epithelium.

Hemorrhoids are very common over the age of 50 years. They are often associated with chronic constipation and pregnancy. Rare associations are portal hypertension and rectal carcinoma. Uncomplicated hemorrhoids are asymptomatic. Complications, which include bleeding, thrombosis, prolapse, and strangulation, may cause severe pain, itching, and rectal bleeding.

Abdominal Hernias

An abdominal hernia is the protrusion of a sac of peritoneum through a defect or weakness in the abdominal wall. Hernias occur (1) through the internal inguinal ring (indirect inguinal hernia; Figure 39-8), (2) through the posterior wall of the inguinal canal (direct inguinal hernia), (3) into the femoral canal (femoral hernia), (4) around the umbilicus (periumbilical hernia), (5) through areas weakened by surgical scars (incisional hernia), (6) in the posterior abdominal wall (lumbar hernia), and (7) through the diaphragm (diaphragmatic hernia).

Abdominal hernias are common, especially inguinal, umbilical, and incisional types. The peritoneal sac of a hernia may contain a variety of abdominal tissues, commonly omentum and intestine and more rarely bladder. When intestine constitutes part of the contents, intestinal obstruction (obstructed hernia) may occur as well as strangulation—occlusion of venous drainage and arterial supply to the segment because of constriction at the neck, causing infarction and gangrene (Figure 39-8).

Intussusception

Intussusception is telescoping of one segment of bowel into another (Figure 39-9). Once the intussusception begins, peristalsis pushes the intussusceptum farther distally. The common location for an intussusception is the terminal ileum.

Intussusception occurs in weaning infants and in adults with polypoid tumors, commonly lipomas or leiomyomas. During weaning, exposure to new antigens or infectious agents is believed to cause hypertrophy of lymphoid follicles in the terminal ileum. The hypertrophied follicles are pushed into the lumen as the apex of an intussusception. In adults it is the tumor that acts as the apex.

Intussusception causes intestinal obstruction. The blood supply to the intussusceptum (the segment of bowel that telescopes into the distal segment), which must pass along its outer wall, frequently becomes constricted at the neck, leading to hemorrhagic infarction and gangrene.

Clinical presentation is with abdominal pain, vomiting, and passage of blood per rectum. The intussuscepted bowel may be palpable as a sausage-shaped mass in the abdomen.

Volvulus

A volvulus is a twisting of the intestine about the axis of its mesentery or around an abnormal fibrous band. Abnormal fibrous bands usually result from previous local peritonitis followed by formation of fibrous adhesions between loops of bowel during repair. Complete twisting leads to intestinal obstruction and often strangulation of the vascular supply, causing infarction.

Volvulus commonly occurs in the sigmoid colon in elderly individuals. Cecal volvulus occurs in younger people who have developmental malrotation of the bowel.

Diverticulosis

A diverticulum is an outpouching of the mucosa of the intestine. Diverticula are classified as true or false. The walls of true diverticula have all the layers of the intestine; congenital diverticula such as Meckel's diverticula are true diverticula. False diverticula, which are really mucosal herniations through a weakened intestinal wall, often at a point of deficiency in the muscularis externa, are lined by mucosa and fibrous tissue and do not have muscle (Figure 39-10). Most acquired diverticula are false diverticula (see Meckel's Diverticulum). Other intestinal diverticula are discussed below.

Jejunal Diverticulosis

This disease is relatively uncommon and is characterized by the presence of multiple diverticula in the jejunum. Bacterial overgrowth in the static contents of the diverticula causes bile acid deconjugation and malabsorption.

Colonic Diverticulosis

(Figure 39-10)

Colonic diverticulosis is common in developed countries, being present in over 50% of patients over the age of 60 years. The sigmoid colon is most commonly involved. The diverticula are false diverticula which occur in a double vertical row along the antimesenteric taenia coli.

The rarity of diverticula in underdeveloped countries has led to the suggestion that they are the result of a diet deficient in fiber. Such a diet produces a stool that is small and hard, which requires a high luminal pressure for evacuation. With high-fiber diets, the stool is soft and bulky, and defecation occurs with relatively low intraluminal pressures. High intraluminal pressure causes the mucosa to herniate at the point of greatest weakness, which is the point of penetration of the muscle wall by the arteries along the antimesenteric taenia coli.

Most cases of sigmoid diverticulosis are asymptomatic, but the following complications may occur: Infection (diverticulitis) causes fever, leukocytosis, and left-lower-quadrant abdominal pain (left-sided appendicitis). Extension of inflammation to the pericolic fat may occur, resulting in pericolic abscesses or rupture into the peritoneal cavity. Intestinal obstruction may occur as a result of marked pericolic fibrosis resulting from healed diverticulitis. Fistulas occur most commonly between the sigmoid colon and the urinary bladder. Hemorrhage may occur as a result of erosion of blood vessels in the wall of the diverticulum, causing the passage of bright red blood per rectum or slower occult bleeding, causing iron deficiency anemia. Diverticulitis is second only to hemorrhoids as a cause of rectal bleeding in elderly patients.

Acute Appendicitis

Acute appendicitis is the most common surgical emergency. It occurs at all ages, with a peak incidence in young adulthood.

In most cases, inflammation of the appendix is preceded by obstruction of the appendiceal lumen (Figure 39-11) by a fecalith (hardened mass of feces), by kinking of the wall, or by submucosal lymphoid hyperplasia. Stagnation distal to the obstruction permits multiplication of colonic bacterial flora, including potential pathogens such as Escherichia coli, Streptococcus faecalis, and anaerobic bacteria. These bacteria then invade the mucosa and appendiceal wall, causing acute inflammation.

Pathology

Acute inflammation of the mucosa is followed by ulceration and inflammation of the muscular wall. Neutrophils are the dominant cells. Suppuration commonly occurs. Grossly, the inflamed appendix appears swollen and red, with a surface that has lost its normal shiny appearance and frequently is covered by a fibrinopurulent exudate. In a minority of cases, the external appearance is normal, and the diagnosis is confirmed only on histologic examination by finding neutrophil infiltration of the mucosa and wall.

Clinical Features

Patients present with acute onset of fever, abdominal pain, and vomiting. Pain is initially periumbilical (referred pain) but becomes localized to the right lower quadrant when the pain-sensitive parietal peritoneum is involved. Tenderness and guarding (reflex muscle spasm during palpation) are commonly present. The peripheral blood shows neutrophilic leukocytosis. The diagnosis must be made on the basis of the clinical features because laboratory and radiologic features are nonspecific. Surgical treatment (appendectomy) is curative.

Complications

Local extension of the inflammatory process may involve the periappendiceal tissues and result in an inflammatory mass (phlegmon) in the right lower quadrant or in appendiceal abscess. Perforation into the peritoneal cavity may result in acute peritonitis, and distant abscesses may form—commonly in the rectovesical pouch or subphrenic region. Infection of the portal venous radicals—very rare—causes portal vein thrombophlebitis with multiple liver abscesses (pylephlebitis suppurativa).