Porth's Essentials of Pathophysiology, 4e

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Disorders of Gastrointestinal Function

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the lower right quadrant, which is confined to a small area approximately the size of the fingertip. It usually is located at approximately the site of the inflamed appen- dix. The person with appendicitis often is able to place his or her finger directly over the tender area. Rebound tenderness, which is pain that occurs when pressure is applied to the area and then released, and spasm of the overlying abdominal muscles are common. Diagnosis is usually based on history and findings on physical examination. Ultrasonography or CT may be used to confirm the diagnosis. 41 Treatment consists of surgical removal of the appendix. Complications include peritonitis, localized periappendiceal abscess formation, and septicemia. Disorders of Intestinal Motility Intestinal motility, or the movement of contents through the gastrointestinal tract, is controlled by neu- rons located in the submucosal and myenteric plexuses of the gut (see Chapter 28). The axons from the cell bodies in the myenteric plexus innervate the circular and longitudinal smooth muscle layers of the gut. These neurons receive impulses from local receptors located in the mucosal and muscle layers of the gut and extrinsic input from the parasympathetic and sympathetic ner- vous systems. As a general rule, the parasympathetic nervous system tends to increase the motility of the bowel, whereas sympathetic stimulation tends to slow its activity. The large intestine has sphincters at both ends: the ileocecal sphincter, which separates it from the small intestine; and the anal sphincter, which prevents the movement of feces to the outside of the body. About 1500 mL of fluid and undigested food normally pass through the ileocecal valve into the large intestine each day. Most of the water and electrolytes in the fluid are absorbed in the colon, usually leaving less than 100 mL to be excreted in the feces. 2 The large intestine can absorb a maximum of 5 to 8 L of fluid and electrolytes each day. When the total amount entering the large intestine through the ileocecal valve or by way of intes- tinal secretion exceeds this amount, the excess appears in the feces as diarrhea. Diarrhea The usual definition of diarrhea is excessively frequent passage of stools. Diarrhea can be acute or chronic and can be caused by infectious organisms, food intoler- ance, drugs, or intestinal disease. In developing coun- tries, diarrhea is a common cause of mortality among children younger than 5 years of age, with an estimated 2 million deaths annually. 42 Even though diarrheal dis- eases are less prevalent in the United States than in other countries, they place a burden on the health care system. Acute Diarrhea. Acute diarrhea is predominantly caused by infectious agents and follows a self-limited course of less than 2 weeks. 43 Acute diarrhea is com- monly divided into noninflammatory (large-volume)

and inflammatory (small-volume) diarrhea, based on the characteristics of the diarrheal stool. Enteric organ- isms cause diarrhea in several ways. Some organisms are noninvasive and do not cause inflammation, but secrete toxins that stimulate fluid secretion. 44,45 Others invade and destroy intestinal epithelial cells, thereby altering fluid transport so that secretory activity continues while absorption activity is halted. Noninflammatory diarrhea is associated with large-volume watery and nonbloody stools, perium- bilical cramps, bloating, and nausea or vomiting. It is commonly caused by toxin-producing bacteria (e.g., enterotoxigenic E. coli, S. aureus, Vibrio cholerae ) or other agents (e.g., viruses, Giardia ) that disrupt the normal absorption or secretory process in the small bowel. Prominent vomiting suggests viral enteritis or S. aureus food poisoning. 32 Although typically mild, diarrhea that originates in the small intestine can be voluminous and result in dehydration with hypoka- lemia and metabolic acidosis. Because tissue invasion and inflammation do not occur, leukocytes are not present in the feces. Inflammatory diarrhea is usually characterized by the presence of fever and bloody diarrhea. It is caused by bacterial invasion of intestinal cells (e.g., Shigella, Salmonella , Yersinia , and Campylobacter ) or the toxins associated with the previously described C. difficile or E. coli O157:H7 infection. Because infections associated with these organisms predominantly affect the colon, the diarrhea is small in volume (<1 L/day) and is associ- ated with lower abdominal pain and the urgent desire to defecate. Infectious dysentery must be distinguished from acute ulcerative colitis, which may present with bloody diarrhea, fever, and abdominal pain. Diarrhea that persists for 14 days is usually not caused by bacte- rial pathogens (except for C. difficile ), and the person should be evaluated for chronic diarrhea. Chronic Diarrhea. Diarrhea is considered to be chronic when the symptoms persist for 3 to 4 weeks in children or adults and 4 weeks in infants. Chronic diarrhea is often associated with conditions such as irritable bowel and inflammatory bowel syndromes, malabsorption dis- orders, endocrine disorders (hyperthyroidism, diabetic autonomic neuropathy), or radiation colitis. There are four major causes of chronic diarrhea: presence of hyper- osmotic luminal contents, increased intestinal secretory processes, inflammatory conditions, and infectious processes 8 (Chart 29-1). A condition called factitious diarrhea is caused by indiscriminate use of laxatives or excessive intake of laxative-type foods. In osmotic diarrhea, water is pulled into the bowel by the hyperosmotic nature of its luminal contents. It occurs when osmotically active particles are not absorbed. In persons with lactase deficiency, the lactose in milk can- not be broken down and absorbed. Magnesium salts, which are contained in milk of magnesia and many antacids, are poorly absorbed and cause diarrhea when taken in sufficient quantities. Another cause of osmotic diarrhea is decreased transit time, which interferes with absorption.