# The Merck Manual of Diagnosis & Therapy - Irritable Bowel Syndrome



## JeanG (Oct 20, 1999)

The URL for this article is: http://www.merck.com/pubs/mmanual/section3/chapter32/32a.htm Irritable Bowel Syndrome(Spastic Colon)A motility disorder involving the entire GI tract, causing recurring upper and lower GI symptoms, including variable degrees of abdominal pain, constipation and/or diarrhea, and abdominal bloating.Etiology The cause of irritable bowel syndrome (IBS) is unknown. No anatomic cause can be found. Emotional factors, diet, drugs, or hormones may precipitate or aggravate heightened GI motility. Some patients have anxiety disorders, particularly panic disorder; major depressive disorder; and somatization disorder. However, stress and emotional conflict do not always coincide with symptom onset and recurrence. Some patients with IBS appear to have a learned aberrant illness behavior; ie, they tend to express emotional conflict as a GI complaint, usually abdominal pain. The physician evaluating patients with IBS, particularly those with refractory symptoms, should investigate for unresolved psychologic issues, including the possibility of sexual or physical abuse (see also Approach to the Patient in Ch. 21).Pathophysiology In IBS, the circular and longitudinal muscles of the small bowel and sigmoid are particularly susceptible to motor abnormalities. The proximal small bowel appears to be hyperreactive to food or parasympathomimetic drugs. Small- bowel transit is variable in patients with IBS, and changes in bowel transit time often do not correlate with symptoms. Intraluminal pressure studies of the sigmoid show that functional constipation can occur when haustral segmentation becomes hyperreactive (ie, increased frequency and amplitude of contractions); in contrast, diarrhea is associated with diminished motor function.Excess mucus production, which often occurs in IBS, is not related to mucosal injury. Its cause is unclear, but it may be related to cholinergic hyperactivity.Hypersensitivity to normal amounts of intraluminal distention exists, as does a heightened perception of pain in the presence of normal quantity and quality of intestinal gas. The pain of IBS seems to be caused by abnormally strong contraction of the intestinal smooth muscle or by increased sensitivity of the intestine to distention. Hypersensitivity to the hormones gastrin and cholecystokinin may also be present. However, hormonal fluctuations do not correlate with clinical symptoms. The caloric density of food intake may increase the magnitude and frequency of myoelectrical activity and gastric motility. Fat ingestion may cause a delayed peak of motor activity, which can be exaggerated in IBS. The first few days of menstruation can lead to transiently elevated prostaglandin E2, resulting in increased pain and diarrhea. This is not caused by estrogen or progesterone but by the release of prostaglandins.Symptoms and Signs IBS tends to begin in the second and third decades of life, causing bouts of symptoms that recur at irregular periods. Onset in late adult life is rare. Symptoms usually occur in the awake patient and rarely rouse the sleeping patient. Symptoms can be triggered by stress or the ingestion of food.Features of IBS are pain relieved by defecation, an alternating pattern of bowel habits, abdominal distention, mucus in the stool, and sensation of incomplete evacuation after defecation. The more symptoms that are present, the likelier that the patient has IBS. In general, the character and location of pain, precipitating factors, and defecatory pattern are distinct for each patient. Variations or deviations from the usual symptoms may suggest intercurrent organic disease and should be thoroughly investigated. Patients with IBS may also have extraintestinal symptoms (eg, fibromyalgia, headaches, dyspareunia, temporomandibular joint syndrome).Two major clinical types of IBS have been described. In constipation-predominant IBS, constipation is common, but bowel habits vary. Most patients have pain over at least one area of the colon, associated with periodic constipation alternating with a more normal stool frequency. Stool often contains clear or white mucus. The pain is either colicky, coming in bouts, or a continuous dull ache; it may be relieved by a bowel movement. Eating commonly triggers symptoms. Bloating, flatulence, nausea, dyspepsia, and pyrosis can also occur.Diarrhea-predominant IBS is characterized by precipitous diarrhea that occurs immediately on rising or during or immediately after eating. Nocturnal diarrhea is unusual. Pain, bloating, and rectal urgency are common, and incontinence may occur. Painless diarrhea is not typical and should lead the physician to consider other diagnostic possibilities (eg, malabsorption, osmotic diarrhea).Diagnosis Diagnosis of IBS is based on characteristic bowel patterns, time and character of pain, and exclusion of other disease processes through physical examination and routine diagnostic tests. Standardized criteria have been developed for IBS. The Rome criteria for IBS includes abdominal pain relieved with defecation and a varying pattern of altered stool frequency or form, bloating, or mucus. The key to diagnosis is effective history taking, which requires attention to directed, but not controlled, elaboration of the presenting symptoms, history of present illness, past medical history, family history, familial interrelationships, and drug and dietary histories. Equally important are the patient's interpretation of personal problems and overall emotional state. The quality of patient-physician interaction is key to diagnostic and therapeutic efficacy.On physical examination, patients with IBS generally appear to be healthy. Palpation of the abdomen may reveal tenderness, particularly in the left lower quadrant, at times associated with a palpable, tender sigmoid. A routine digital rectal examination should be performed on all patients, and a pelvic examination on women.Stool examination for occult blood (preferably a 3-day series) should be performed. Routine testing for ova and parasites or a stool culture is rarely indicated without a supporting travel history or supporting symptoms (eg, fever, bloody diarrhea, acute onset of severe diarrhea).Proctosigmoidoscopy with a flexible fiberoptic instrument should be performed. Introduction of the sigmoidoscope and air insufflation frequently trigger bowel spasm and pain. The mucosal and vascular pattern in IBS usually appears normal. In patients with chronic diarrhea, particularly older women, mucosal biopsy can rule out possible microscopic colitis, which has two variants: collagenous colitis, seen on trichrome stain as increased submucosal collagen deposition, and lymphocytic colitis, characterized by increased numbers of mucosal lymphocytes. The mean age of presentation for these disorders is 60 to 65 yr, with a female predominance. Similar to IBS, presentation involves nonbloody, watery diarrhea. Diagnosis can be made via rectal mucosal biopsy.Laboratory examination should include a CBC; ESR; 6- and 12-channel biochemical profile (sequential multiple analyses 6 and 12), including serum amylase; urinalysis; and thyroid-stimulating hormone. An abdominal sonogram, barium enema x-ray, and upper GI esophagogastroduodenoscopy or colonoscopy may be selectively used, based on the history, physical examination, patient age, and follow-up evaluations. However, these studies should be undertaken only when less invasive and less expensive studies reveal objective abnormalities.Diagnosis of IBS should never preclude suspicion of intercurrent disease. Changes in symptoms may signal another disease process. For example, a change in the location, type, or intensity of pain; a change in bowel habits; constipation and diarrhea or vice versa; and new symptoms or complaints (eg, nocturnal diarrhea) may be clinically significant. Other symptoms that require investigation include fresh blood in the stool, weight loss, very severe abdominal pain or unusual abdominal distention, steatorrhea or noticeably foul-smelling stools, fever or chills, persistent vomiting, hematemesis, symptoms that wake the patient from sleep (eg, pain, the urge to defecate), or a steady progressive worsening of symptoms. Patients > 40 yr are more likely than younger patients to have an intercurrent organic illness.Differential Diagnosis Common illnesses that may be confused with IBS include lactose intolerance, diverticular disease, drug-induced diarrhea, biliary tract disease, laxative abuse, parasitic diseases, bacterial enteritis, eosinophilic gastritis or enteritis, microscopic (collagenous) colitis, and early inflammatory bowel disease. The bimodal age distribution of patients with inflammatory bowel disease makes it imperative to evaluate both younger and older patients for these conditions. In patients > 40 yr with a change in bowel habits, particularly those with no previous IBS symptoms, colonic polyps and tumors must be excluded by colonoscopy. In patients > 60 yr, ischemic colitis should be considered.Pelvic examination in women helps rule out ovarian tumors and cysts or endometriosis, which may mimic IBS. Hyperthyroidism, carcinoid syndrome, medullary cancer of the thyroid, vipoma, and the Zollinger-Ellison syndrome are possibilities in patients with diarrhea. Patients with constipation and no anatomic lesion should be evaluated for hypothyroidism or hyperparathyroidism. If the patient's history and laboratory studies suggest malabsorption, absorption tests should rule out tropical sprue, celiac disease, and Whipple's disease. Finally, elimination disorders (eg, pelvic floor dyssynergia) should be considered as a cause of constipation in patients who report excessive straining on defecation.Treatment Therapy is supportive and palliative. A physician's sympathetic understanding and guidance are of overriding importance. The physician must explain the nature of the underlying condition and convincingly demonstrate to the patient that no organic disease is present. This requires time for listening and explaining normal bowel physiology and the bowel's hypersensitivity to stress, food, or drugs. These explanations form the foundation for attempting to reestablish regular bowel routine and individualizing therapy. The prevalence, chronicity, and need for continuing care of IBS should be emphasized. Psychologic stress and anxiety or mood disorders should be sought, evaluated, and treated (see Chs. 187 and 189). Regular physical activity helps relieve stress and assists in bowel function, particularly in patients who present with constipation.In general, a normal diet should be followed. Patients with abdominal distention and increased flatulence may benefit from dietary reduction or elimination of beans, cabbage, and other foods containing fermentable carbohydrates. Reduced intake of apple and grape juice, bananas, nuts, and raisins may also lessen the incidence of flatulence. Patients with evidence of lactose intolerance should reduce their intake of milk and dairy products. Bowel function may also be disturbed by the ingestion of sorbitol, mannitol, fructose, or combinations of sorbitol and fructose. Sorbitol and mannitol are artificial sweeteners used in dietetic foods and as drug vehicles, whereas fructose is a common constituent of fruits, berries, and plants. Patients with postprandial abdominal pain may try a low-fat diet supplemented with increased protein.Increasing dietary fiber can help many patients with IBS, particularly those with constipation. A bland bulk-producing agent may be used (eg, raw bran, starting with 15 mL [1 tbs] with each meal, supplemented with increased fluid intake). Alternatively, psyllium hydrophilic mucilloid with two glasses of water tends to stabilize the water content of the bowel and provide bulk. These agents help retain water in the bowel and prevent constipation. They also can reduce colonic transit time and act as a shock absorber to prevent spasm of the bowel walls against each other. Fiber added in small amounts may also help reduce IBS-induced diarrhea by absorbing water and solidifying stool. However, excessive use of fiber can lead to bloating and diarrhea. Fiber doses must therefore be adjusted to individual patient needs.Anticholinergic (antispasmodic) drugs (eg, hyoscyamine 0.125 mg 30 to 60 min before meals) may be used in combination with fiber agents. The use of narcotics, sedative hypnotics, and other drugs that produce dependency is discouraged. In patients with diarrhea, diphenoxylate 2.5 to 5 mg (one to two tablets) or loperamide 2 to 4 mg (one to two capsules) may be given before meals. Chronic use of antidiarrheals is discouraged because tolerance to the antidiarrheal effect may occur. Antidepressants (eg, desipramine, imipramine, and amitriptyline 50 to 150 mg daily) help many patients with either type of IBS. In addition to constipation and diarrhea, abdominal pain and bloating are relieved by antidepressants. These drugs can also reduce pain by down-regulating the activity of spinal cord and cortical afferent pathways arriving from the intestine. Finally, certain aromatic oils (carminatives) can relax smooth muscle and relieve pain caused by cramps in some patients. Peppermint oil is the most commonly used agent in this class.


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