A 72-year-old man comes to the office for evaluation of constipation. His stools have been hard and pelletlike for as long as he can remember. The patient has a bowel movement every 3-4 days and frequently strains when using the bathroom. He has had associated abdominal discomfort but no hematochezia, melena, vomiting, or unexpected weight changes. The symptoms have not improved despite fiber supplementation. Vital signs are within normal limits. The abdomen is mildly distended with decreased bowel sounds. In addition to increasing water consumption, the patient is advised to try bisacodyl for constipation. What is the primary mechanism of action of this medication?

Question

A 72-year-old man comes to the office for evaluation of constipation.  His stools have been hard and pelletlike for as long as he can remember.  The patient has a bowel movement every 3-4 days and frequently strains when using the bathroom.  He has had associated abdominal discomfort but no hematochezia, melena, vomiting, or unexpected weight changes.  The symptoms have not improved despite fiber supplementation.  Vital signs are within normal limits.  The abdomen is mildly distended with decreased bowel sounds.  In addition to increasing water consumption, the patient is advised to try bisacodyl for constipation.  What is the primary mechanism of action of this medication?

Quizplus · Accepted Answer

11ec160b_3bcd_63ce_9249_cd0b81da8d6c_MD0001_00 Bisacodyl is a stimulant laxative that is commonly used to treat constipation and neurogenic bowel; it is also indicated for bowel preparation prior to endoscopic procedures (eg, colonoscopy). Bisacodyl is hydrolyzed in the colon to its active form, which directly stimulates the enteric nerves within the colonic myenteric plexus, thereby increasing peristalsis and enhancing colonic motility. It also has some minor secretory effects in the small intestine, where it increases production of cyclic AMP, indirectly increasing secretion of chloride and bicarbonate while inhibiting sodium resorption. Side effects include diarrhea, mild abdominal cramping, and electrolyte disturbances (eg, hypokalemia). Other stimulant laxatives include senna and castor oil. (Choice A) Docusate reduces the surface tension of stool, enabling water and fat to enter and soften it. (Choice B) Magnesium hydroxide, polyethylene glycol, and lactulose are osmotically active agents that are poorly absorbed by the intestines. These medications draw water into the intestinal lumen, thereby softening stool and making it easier to pass. (Choice C) Lubiprostone is an agonist of the ClC-2 chloride channel located on the apical membrane of the intestine, which increases chloride secretion into the intestinal lumen. Sodium and water follow chloride into the lumen, resulting in increased intestinal fluid content. (Choice D) Methylnaltrexone is a mu-opioid receptor antagonist designed to treat opioid-induced constipation. It does not cross the blood-brain barrier, which allows it to be used without inducing opiate-withdrawal symptoms. Educational objective: Bisacodyl is a commonly used stimulant laxative that stimulates the enteric neurons within the colonic myenteric plexus, thereby increasing peristaltic activity and enhancing colonic motility. __________ References: Management of constipation in older adults. (http://www.ncbi.nlm.nih.gov/pubmed/26371734) Treatment of chronic constipation: current pharmacologic approaches and future directions. (http://www.ncbi.nlm.nih.gov/pubmed/19138759)

A 72-Year-Old Man Comes to the Office for Evaluation of Constipation