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The Small Intestine

The small intestine extends from the pylorus to the ileocecal valve and, depending on its muscle tone, is from 3.5 to 6.5 m long. It is divided into three regions:
  1. The duodenum extends to the ligament of Treitz.
  2. The jejunum is the proximal 40% of the remainder of the small intestine.
  3. The ileum is the distal 60%.
  1. The duodenum is almost entirely retroperitoneal and therefore fixed. Q
  2. The small intestinal wall has four layers: mucosa, submucosa, muscularis, and serosa.
Congenital Disorders
Atresia and Stenosis Cause Neonatal Intestinal Obstruction
  1. Atresia: Atresia is defined as a complete occlusion of the intestinal lumen.
  2. Stenosis: This is an incomplete stricture, which narrows but does not occlude, the lumen.
  3. Meckel Diverticulum Causes Bleeding, Obstruction, and Perforation
  1. Meckel diverticulum, caused by persistence of the vitelline duct and, on the antimesenteric ileal border, 60 to 100 cm from the ileocecal valve in adults. It is the most common and the most clinically significant congenital anomaly of the small intestine. Two thirds of patients are younger than 2 years.
  1. Meckel diverticulum is about 5 cm long
  2. Meckel diverticulum is a true diverticulum. It possesses all the coats of normal intestine; the mucosa is similar to that of the adjoining ileum. Most Meckel diverticula are asymptomatic and discovered only as incidental findings at laparotomy for other causes or at autopsy. Of the minority that becomes symptomatic, about half contain ectopic gastric, duodenal, pancreatic, biliary, or colonic tissue.
Clinical Features
The complications of Meckel diverticulum are several.
  1. Hemorrhage: The most common complication is bleeding. Meckel diverticula are responsible for half of all lower gastrointestinal hemorrhage in children.
  2. Intestinal obstruction: The diverticulum may be a lead point for intussusception and so cause intestinal obstruction.
  3. Diverticulitis: Inflammation of a Meckel diverticulum (i.e., diverticulitis) leads to symptoms indistinguishable from those of appendicitis.
  4. Perforation: Peptic ulceration, either in the diverticulum or in the ileum, may cause perforation, and lead to rapidly spreading peritonitis. Q
  5. Fistula: A fecal discharge from the umbilicus may be observed.
Two substances are preferentially absorbed by the distal small intestine: bile salts and vitamin B12. Q
Pathogenesis: Q
  1. Microvilli: Abnormal function of the microvilli may be primary, as in primary disaccharidase deficiencies; or secondary, when there is damage to the villi, as in celiac disease (sprue). Q
  1. Absorptive Area: The surface area may be diminished by (1) small bowel resection (short bowel syndrome), (2) gastrocolic fistula (bypassing the small intestine), or (3) mucosal damage due to a number of small intestinal diseases (celiac disease, tropical sprue, and Whipple disease).
  1. Clinical Features
    Malabsorption may be either Specific Or Generalized.
    1. Specific or isolated malabsorption refers to an identifiable molecular defect that causes malabsorption of a single nutrient. Examples of this group are the disaccharidase deficiencies (notably lactase deficiency) and deficiency of gastric intrinsic factor, which causes malabsorption of vitamin B12 and consequently pernicious anemia.
    2. Generalized malabsorption describes a condition in which absorption of several or all major nutrient classes is impaired.
Celiac Disease: Reflects an Immune Response to Gluten in Cereals
Celiac disease (celiac sprue, gluten-sensitive enteropathy) is characterized by (a) generalized malabsorption, (b) small intestinal mucosal lesions and (c) prompt clinical and histopathologic response to withdrawal of gluten-containing foods from the diet. Q
  1. Celiac disease is worldwide and affects all ethnic groups.
  2. There is a slight female predominance, 1.3:1. It may be seen any time after cereals are introduced into the diet.
  3. Most cases are diagnosed during childhood, although the disease may become clinically apparent for the first time as late as the seventh decade of life.
Genetic predisposition and gliadin exposure are crucial factors in the development of celiac disease.
Role Of Cereal Proteins: Q Experiments on successfully treated, asymptomatic patients with celiac disease have shown that ingestion or instillation of wheat, barley, or rye flour into the small intestine is followed by the clinical and histopathologic features of celiac sprue.
Genetic Factors: Q 90% of patients with celiac disease carry the histocompatibility antigen human leukocyte antigen (HLA)-B8 and a comparable frequency has been reported for HLA DR8 and DQ2.
  1. A microscopic finding in small bowel biopsies that can precede the more characteristic findings is an intraepithelial lymphocytic infiltrate involving crypts and surface epithelium in normal appearing villi. Q
  2. The hallmark of fully developed celiac disease is a flat mucosa, with (a) blunting or total disappearance of villi, (b) damaged mucosal surface epithelial cells with numerous intraepithelial lymphocytes (T cells), and (c) increased plasma cells in the lamina propria but not in deeper layers.
  3. The most severe histologic abnormalities in untreated celiac disease usually occur in the duodenum and proximal jejunum.
Clinical Features
  1. Fully developed celiac disease is characterized by generalized malabsorption.
  2. In adults, iron deficiency anemia resistant to oral therapy is often the clue to celiac disease.
  3. Collagenous sprue refers to a rare disorder characterized by the deposition of collagen in the lamina propria of the small bowel. Mimics celiac disease but does not respond to removal of gluten from the diet. The prognosis in collagenous sprue is grave: all reported patients have died of the disease.
Whipple Disease Is a Rare Infection of the Small Bowel
  1. Malabsorption is the most prominent feature of Whipple disease.
  2. The disease is systemic, and other clinical findings include fever, increased skin pigmentation, anemia, lymphadenopathy, arthritis, pericarditis, pleurisy, endocarditis, and central nervous system involvement.
  1. Whipple disease typically shows infiltration of the small bowel mucosa by macrophages packed with small, rod-shaped bacilli. Q
  2. The causative organism is one of the actinomycetes, Tropheryma whippelii.
  3. Circulating cells expressing CD11b, a cell-adhesion and complement-receptor molecule on macrophages, are reduced. CD11b is involved in activating macrophages to kill intracellular pathogens. Q
  1. The bowel wall is thickened and edematous, and mesenteric lymph nodes are usually enlarged.
  2. Villi are flat and thickened villi, and the lamina propria is extensively infiltrated with large foamy macrophages whose cytoplasm is filled with large glycoprotein granules that stain strongly with periodic acid Schiff (PAS).  Q
Tropical Sprue
Is A Disease Of Unknown Etiology That Causes Folate Deficiency
Laboratory findings include increased fecal fat, impaired D-xylose absorption, megaloblastic anemia, and decreased disaccharidase activity in the intestinal mucosa.
Gastrointestinal Stromal Tumors
  1. GISTs occur throughout the small intestine but most often in the jejunum. They grow as intramural masses covered by intact mucosa and are similar to those in other locations.
  2. Intestinal obstruction is uncommon, but volvulus may be a complication.
  3. Small intestinal GIST tumors are more likely to behave aggressively than their gastric counterparts.
  1. Epidemiology
  1. They account for half of all malignant small bowel tumors.  Q
  2. Most are located in the duodenum and jejunum. The majority occurs in middle-aged persons, and there is a moderate male predominance. Q
  3. Crohn disease of the small bowel is a risk factor for adenocarcinoma. Most common site – ilium.
  4. Familial adenomatous polyposis, HNPCC syndrome (Lynch syndrome), and celiac disease are additional risk factors.
Pathology And Clinical Features
  1. Adenocarcinoma of the small intestine may be polypoid or ulcerative or simply annular and stenosing. It can cause intestinal obstruction are intussusception. Q
  2. Occult bleeding is common.
  3. If adenocarcinoma of the duodenum involves the papilla of Vater, (ampullary carcinoma). It can cause obstructive jaundice or pancreatitis. Q
Primary Intestinal Lymphoma
  1. Primary lymphoma originates in nodules of lymphoid tissue normally present in the mucosa and superficial submucosa, termed mucosa-associated lymphoid tissue (MALT). Q
  2. Predisposing factor – celiac disease, immunodeficiency.
Carcinoid Tumor (Neuroendocrine Tumors)
  1. The term carcinoid tumor has been largely replaced by the term neuroendocrine tumors (NETs). These tumors are all considered malignant, but usually with low metastatic potential.
  2. The gut is the most common site for NETs (the bronchus is the next most common site).
  3. The appendix is the most common gastrointestinal site of origin, followed by the rectum.
  4. They are also seen in association with the multiple endocrine neoplasia (MEN) syndromes, MEN I.
  1. Macroscopically, small carcinoid tumors present as submucosal nodules covered by intact mucosa.
  2. The cut surface is firm and white to yellow.
  3. Microscopically, these neoplasms appear as nests, cords and rosettes of uniform small, round cells. Abundant eosinophilic cytoplasm contains cytoplasmic granules, which by electron microscopy are typically of the neurosecretory type.
Clinical Features
  1. Carcinoid syndrome features includes: Episodic flushing, bronchospasm, cyanosis, telangiectasia, and skin lesions, right-sided cardiac valvular disease. Diarrhea is thought to be caused by serotonin.
  2. After its release into the blood, serotonin is metabolized to 5-hydroxyindoleacetic acid (5-HIAA) by monoamine oxidase either in the tumor or in other tissues. Urine 5-HIAA is a diagnostic test for the carcinoid syndrome.
Metastatic Tumors
  1. The most common malignant tumors in the small intestine are metastatic.
  2. Lung and female genital organs and skin (melanomas) are the most frequent primary sites of small-intestinal metastases.Q

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