Porth's Essentials of Pathophysiology, 4e

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Disorders of Gastrointestinal Function

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Acute Intestinal Obstruction Intestinal obstruction refers to impaired movement of intestinal contents in a cephalocaudal direction. The condition can be acute or chronic and may affect the small intestine or colon. In contrast to chronic obstruc- tions, which often involve the colon and may last for weeks or months, acute obstructions usually present as severe disorders of the small intestine that are poten- tially lethal if not recognized early. 50,51 Acute intesti- nal obstruction can be mechanical or nonmechanical, resulting from paralytic obstruction of the ileus. Mechanical obstruction can result from a number of conditions, intrinsic or extrinsic, that encroach on the patency of the bowel lumen (Fig. 29-10). Major inciting causes include an external hernia (i.e., inguinal, femo- ral, or umbilical) and postoperative adhesions. 52 Less common causes are strictures, tumors, foreign bodies, intussusception, and volvulus. 7 Intussusception involves the telescoping of bowel into the adjacent segment (Figs. 29-10A and 29-11). It is the most common cause of intestinal obstruction in children younger than 2 years of age. The most common form is intussusception of the terminal ileum into the right colon, but other areas of the bowel may be involved. In most cases, the cause of the disorder is unknown. 52 The condition can also occur in adults when an intraluminal mass or tumor acts as a traction force and pulls the segment along as it telescopes into the distal segment. Volvulus refers to a complete twisting of the bowel on an axis formed by its mesentery (see Fig. 29-10B). It can occur in any por- tion of the gastrointestinal tract, but most commonly involves the cecum, followed by the sigmoid colon. Mechanical bowel obstruction may be a simple obstruc- tion, in which there is no alteration in blood flow, or a strangulated obstruction, in which there is impairment of blood flow and necrosis of bowel tissue. Paralytic, or adynamic, obstruction of the ileus results from neurogenic or muscular impairment of peristalsis. Paralytic ileus is seen most commonly after abdominal surgery. It also accompanies inflammatory conditions of the abdomen, intestinal ischemia, pelvic fractures, and back injuries. It occurs early in the course of peritonitis and can result from chemical irritation caused by bile, bacterial toxins, electrolyte imbalances as in hypokale- mia, and vascular insufficiency. The major effects of intestinal obstruction are abdominal distention and loss of fluids and electro- lytes 51 (Fig. 29-12). Distention is further aggravated by the accumulation of gases and fluid proximal to the site of obstruction. Approximately 70% to 80% of these gases are derived from swallowed air, and because this air is composed mainly of nitrogen, it is poorly absorbed from the intestinal lumen. As the process continues, the distention moves proximally (i.e., toward the mouth), involving additional segments of bowel. Either form of intestinal obstruction eventually may lead to stran- gulation (i.e., interruption of blood flow), gangrenous changes in the bowel wall, and, ultimately, perforation of the bowel. The increased pressure in the intestine tends to compromise mucosal blood flow, leading to

Constipation can occur as a primary disorder of intestinal motility, as a side effect of drugs, as a problem associated with another disease condition, or as a symptom of obstructing lesions of the gastrointestinal tract. Some common causes of constipation are failure to respond to the urge to defecate, inadequate fiber in the diet, inad- equate fluid intake, weakness of the abdominal muscles, inactivity and bed rest, pregnancy, and hemorrhoids. The pathophysiology of constipation can be classified into three broad categories: normal-transit constipation, slow-transit constipation, and disorders of defecation. Normal-transit constipation (or functional constipation) is characterized by perceived difficulty in defecation and usually responds to increased fluid and fiber intake. Slow-transit constipation, which is characterized by infrequent bowel movements, is often caused by altera- tions in intestinal innervation. Hirschsprung disease is an extreme form of slow-transit constipation in which the ganglion cells in the distal bowel are absent because of a defect that occurred during embryonic development; the bowel narrows at the area that lacks ganglionic cells. Although most persons with this disorder present in infancy or early childhood, some with a relatively short segment of involved colon do not have symptoms until later in life. Defecatory disorders are most commonly due to dysfunction of the pelvic floor or anal sphincter. Diseases associated with chronic constipation include neurologic diseases such as spinal cord injury, Parkinson disease, and multiple sclerosis; endocrine disorders such as hypothyroidism; and obstructive lesions in the gastrointestinal tract. Drugs such as nar- cotics, anticholinergic agents, calcium channel blockers, diuretics, calcium (antacids and supplements), iron sup- plements, and aluminum antacids tend to cause consti- pation. Elderly people with long-standing constipation may develop dilation of the rectum, colon, or both. This condition allows large amounts of stool to accumulate with little or no sensation. Constipation, in the context of a change in bowel habits, may be a sign of colorectal cancer. Diagnosis of constipation usually is based on a his- tory of infrequent stools, straining with defecation, the passing of hard and lumpy stools, or the sense of incom- plete evacuation with defecation. Rectal examination is used to determine whether fecal impaction, anal stric- ture, or rectal masses are present. Constipation as a sign of another disease condition should be ruled out. Tests that measure colon transit time and defecatory function are reserved for refractory cases. The treatment of constipation usually is directed toward relieving the cause. A conscious effort should be made to respond to the defecation urge. A time should be set aside after a meal, when mass movements in the colon are most likely to occur, for a bowel movement. Adequate fluid intake and bulk in the diet should be encouraged. Moderate exercise is essential, and persons on bed rest benefit from passive and active exercises. Laxatives and enemas should be used judiciously. They should not be used on a regular basis to treat simple constipation because they interfere with the defecation reflex and actually may damage the rectal mucosa.