IBS Treatment with Antidiarrheals and Antispasmodics
For some patients with IBS, after eating, the transit time through the intestines is rapid and results in osmotic diarrhea. It is called osmotic diarrhea because as the food goes through the intestine, it is not absorbed well and draws water into the intestines to equalize water. These patients are referred to as diarrhea-predominant IBS. Antidiarrheal agents are used to slow the transit time. Loperamide, diphenoxylate, alosetron, and tricyclic antidepressants (TCAs) can function as antidiarrheals.
Loperamide improves the symptoms of diarrhea, urgency, and fecal soiling. It is preferred over diphenoxylate when the IBS is chronic because diphenoxylate can enter the brain. With long term use, a patient using diphenoxylate can become addicted. Loperamide does not penetrate the brain and so avoids this potential problem. It should be used cautiously in people with ulcerative colitis or pseudomembranous colitis, because it can cause toxic megacolon (a dangerous and massive swelling of the colon). Diphenoxylate inhibits GI motility and slows excess GI propulsion, allowing more fluid and electrolytes to be absorbed in the feces.
Alosetron and other 5HT3 receptor antagonists (or blockers) in general work by blocking specific neuron receptors, consequently slowing the motility of the small and large intestines. They also reduce water secretion. These actions together help prevent diarrhea. Alosetron is the strongest antagonist on the market and is the only one in this class approved by the FDA for treatment of women with severe diarrhea-predominant IBS. It is prescribed after unsuccessful trials with other treatments. Patients and physicians have to be enrolled in the manufacturer-sponsored and FDA-approved monitoring program before it is prescribed.
Tricyclic antidepressants (TCAs) include amytriptyline, nortriptyline, imipramine, desipramine, and trimipramine. They are popular in the treatment of chronic pain syndromes such as fibromyalgia. For IBS, the predominant effect of TCAs is pain control, not relief of psychological symptoms. At low doses, they decrease the pain sensation of a distended abdomen. This is done by increasing the pain threshold in the GI system. The TCAs also reduce pain by slowing gastrointestinal motility and reducing stool frequency.
Antispasmodics are among the most prescribed medications for people with IBS. These medications include hyoscamine and dicyclomine. They are both anticholinergics and work by blocking the actions of acetylcholine. Acetylcholine is a neurotransmitter that has many actions, including decreasing heart rate and blood pressure. In the gut, it causes not a decrease, but an increase in motility (spasm) and increased secretions (e.g., increased saliva, sweating, and urination)
Antispasmodics like hyoscamine and dicyclomine block the effects of acetylcholine. So they are important in decreasing spasms and motility. They are first line agents for the treatment of pain in people with IBS. Because these drugs counter the effects of acetylcholine (which increases secretions), some of the adverse effects to watch for when using antispasmodics include dry mouth, constipation, and urinary retention.
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