Irritable bowel syndrome (IBS) is a
functional gastrointestinal disorder that can present with a wide array of symptoms that make treatment difficult. Current
therapies are directed at relieving symptoms of
abdominal pain or discomfort, bloating,
constipation, and
diarrhea. Pharmacologic agents used to treat IBS-associated
pain include myorelaxants,
peppermint oil, and peripherally acting
opiates.
Dicyclomine and
hyoscyamine, the two myorelaxants available in the United States, have not been proven effective in reducing
abdominal pain in patients with IBS. The efficacy of
peppermint oil is debated, but methodological problems with existing studies preclude definitive judgment.
Loperamide is ineffective for relief of
abdominal pain. For IBS patients with excessive abdominal bloating, a small number of studies suggest that bacterial eradication with gut-directed
antibiotics and bacterial reconstitution with nonpathogenic probiotics may reduce
flatulence. For
constipation-predominant (C-IBS) symptoms, current treatment options include fiber supplementation,
polyethylene glycol, and
tegaserod. Soluble fibers (
ispaghula,
calcium polycarbophil,
psyllium) are more effective than insoluble fibers (
wheat bran, corn fiber) in alleviating global symptoms and relieving
constipation, although fiber in general has marginal benefit in treatment of overall IBS symptoms.
Polyethylene glycol increases bowel frequency in chronic
constipation, but its overall efficacy against IBS is unclear.
Tegaserod, a 5-HT(4) agonist, demonstrates superiority over placebo in improving bowel frequency and stool consistency and alleviating
abdominal pain and bloating in women with C-IBS. Overall global symptoms are modestly improved with
tegaserod when compared with placebo. Additional agents under investigation for C-IBS include the
ClC(2) chloride channel opener
lubiprostone,
mu-opioid receptor antagonist
alvimopan, and 5-HT(4) agonist
renzapride. For
diarrhea-predominant (D-IBS) symptoms, available
therapies include
loperamide,
alosetron, and
clonidine.
Alosetron, a 5-HT(3) antagonist, is superior to placebo for reducing bowel frequency, improving stool consistency, and relieving
abdominal pain in women with D-IBS. However,
alosetron is available under a restricted license because of concerns for
ischemic colitis and severe
constipation necessitating
colectomy.
Clonidine may be helpful in alleviating global symptoms for D-IBS patients.