Irritable bowel syndrome (IBS) is characterized by chronic abdominal pain and altered bowel habits. It affects 10 to 15 percent of the U.S. population and represents up to 50 percent of all referrals to gastroenterologists.

The cause is unknown. To date, no physiologic or psychological cause has been identified. Investigation has centered on abnormal gastrointestinal action, hypersensitivity of the gastrointestinal nerves, microscopic inflammation, infection, carbohydrate or bile acid malabsorption, and emotional stress, but clinical studies thus far are inconclusive.

Symptoms

• Abdominal pain is the predominant symptom.
• Altered bowel habits are also often present. This may occur as diarrhea, constipation, or alternating diarrhea and constipation.
• Other symptoms include bloating, incomplete evacuation, nausea, difficulty swallowing, reflux, and heartburn.
• IBS may also be accompanied by painful menstrual cycles, sexual dysfunction, fibromyalgia, and urinary symptoms.

Risk Factors

• About half of cases occur in individuals younger than 35.
• Women are affected twice as often as men.

Irritable Bowel Syndrome: Diagnosis and Treatment

Diagnosis

• A careful history and physical examination are essential to uncover underlying causes and to avoid unnecessary and costly diagnostic testing.
  
  The examining physician should attempt to identify foods, nutrients or additives (e.g., lactose, sorbitol, saccharin, and sucralose), and medications (e.g., antacids, calcium channel blockers, and anticholinergics) that are related to symptoms.
  
  It is also important to look for worrisome symptoms that suggest gastrointestinal disease, which may require diagnostic testing. Examples include bloody stools, weight loss greater than 10 pounds, family history of colon cancer, recurring fever, anemia, and severe diarrhea.
• The Rome II criteria have been designed to create a standardized system for diagnosis, but the usefulness of these criteria has not been fully established. The criteria include:
  • At least 12 weeks of continuous recurrent abdominal pain that is relieved by defecation, and/or a change in the consistency, frequency, or form of stool
  • Abnormal stool passage (straining, urgency, or feeling of incomplete evacuation)
  • Passage of mucus
  • Bloating or abdominal distention
• In appropriate patients, laboratory studies may include blood testing, thyroid function tests, 24-hour stool collection, and stool testing for infection.
• Colonoscopy may be useful to rule out inflammatory bowel disease and colon cancer, especially in patients over 50. In younger patients with symptoms of irritable bowel syndrome, colonoscopy is not usually necessary.

Treatment

There is no specific cure. However, nutritional interventions and medications are effective in some patients to reduce the symptoms.

• Avoid possible food triggers, including lactose and artificial sweeteners (e.g., sorbitol, saccharin, and sucralose).
• Diarrhea can be treated with loperamide (Imodium), cholestyramine (Questran), or other antidiarrheal medications.
• Constipation can be treated with fiber supplementation, laxatives, or other medications.
• Abdominal pain may respond to antispasmodic agents (e.g., mebeverine, dicyclomine, or hyoscyamine) or tricyclic antidepressants (e.g., amitriptyline).
• Recent studies suggest that antibiotic therapy (e.g., rifaximin) can be useful in certain cases, especially in patients with bacterial overgrowth and diarrhea.
• Psychological interventions are often helpful. A recent review of randomized, controlled trials of psychological treatments found that eight out of 12 treatments showed positive responses, mainly reductions in pain and diarrhea. Treatment guidelines published by the American Gastroenterology Association suggest that cognitive-behavioral treatment, interpersonal psychotherapy, hypnosis, and stress management/relaxation are effective in reducing abdominal pain and diarrhea.

Irritable Bowel Syndrome: Nutritional Considerations

IBS is a complex illness that is frequently exacerbated by stress and, possibly, by poor diet. As with some other intestinal diseases, it may be more common in individuals consuming Western diets than in persons consuming the high-fiber, low-fat diets that are traditional in developing societies. Both dietary and psychological interventions have resulted in symptomatic improvements, and it is likely that patients will benefit most from a combination of medical, nutritional, and behavioral approaches. The following measures may be helpful:

• Increased insoluble fiber (e.g., bran cereal, whole grain breads): Several studies have revealed that increasing bran fiber intake decreases bloating, constipation, and diarrhea in patients with IBS. However, a placebo effect appears to account for some of those benefits, and some patients experience worsening symptoms (e.g., bloating) with bran fiber treatment.
  
  Other investigations have indicated that fiber types other than wheat bran (e.g., partially hydrolyzed guar gum) are more effective. Additional controlled clinical trials are needed in which different types of fiber are compared for their effectiveness. 
• Elimination diets for patients with adverse food reactions: Salicylates, amines, and glutamates in foods are suspected of causing symptoms of IBS. Among the foods that contain these compounds are milk, eggs, and wheat, the three foods that most frequently cause IBS flare-ups. Diets that eliminate these foods have been shown to help roughly half of patients.
  
  Further, an intervention in which beef, wheat, and dairy products were eliminated significantly reduced symptoms, which may be due to a dramatic decrease in intestinal gas production. Many cases of irritable bowel symptoms are probably due to lactose intolerance. For this reason, eliminating milk and dairy is an especially useful intervention. 
• Probiotic therapy: A number of studies have indicated differences in intestinal microbe populations between irritable bowel sufferers and control subjects, suggesting that antibiotic treatments may play a causative role. Repopulating the intestinal tract with "friendly" bacteria may be of benefit. Most studies have suggested a benefit from probiotic treatment with Lactobacillus plantarum, Bifidobacterium breve, Streptococcus faecium, and combinations of these with other organisms.
• Peppermint oil: Enteric-coated peppermint oil capsules have been evaluated in controlled clinical trials and found helpful in reducing the symptoms of IBS in more than half of patients overall, and in 75 percent of children.