Pharmacology
Summary
Constipation, characterized by straining, hard stools, and less than three bowel movements per week, is prevalent among the elderly and debilitated. A step-up therapeutic approach is recommended: beginning with psyllium, a bulk-forming laxative that swells by absorbing water, then moving to surfactant agents like docusate which promote water and lipid infiltration into the stool. Osmotic laxatives such as magnesium compounds, lactulose, and polyethylene glycol (PEG) draw water into the intestines, with the latter particularly useful for pre-procedure colonic cleansing. Magnesium-based laxatives should be used with caution in patients with chronic kidney disease. Additionally, lactulose treats hepatic encephalopathy by inhibiting ammonia absorption in cirrhosis patients, a process complemented by the antibiotic rifaximin. Stimulant laxatives, including senna and bisacodyl, directly stimulate the enteric nervous system and can lead to melanosis coli and other complications such as hypokalemia, salt, and protein loss. In cases of severe constipation, interventions like suppositories or enemas might be necessary. Targeted medications for specific constipation types include lubiprostone for chronic idiopathic constipation (CIC), opioid-induced constipation (OIC), and constipation-dominant IBS in adult women. Linaclotide functions by increasing chloride secretion and luminal fluid, accelerating intestinal transit, while opioid antagonist drugs cater to opioid-induced constipation.
Lesson Outline
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FAQs
Constipation, characterized by straining, hard stools, and fewer than three bowel movements per week, is especially prevalent in elderly and debilitated patients. Laxatives alleviate constipation by promoting bowel movements. A stepwise therapeutic approach to constipation begins with a bulk-forming agent, progressing to a stool softener, then an osmotic laxative, followed by a stimulant laxative, and in severe cases, an enema or suppository.
Surfactant agents, also known as stool softeners, enable the penetration of water and lipids into the stool, making it softer and easier to pass. Docusate is a popular stool softener and is sometimes used alongside senna. In hospital settings, docusate helps prevent constipation and reduces straining.
Osmotic laxatives are non-absorbable substances drawing water into the intestines, resulting in distention and peristalsis. Examples include magnesium compounds (like magnesium hydroxide and magnesium citrate), polyethylene glycol (PEG), and lactulose. Magnesium-based laxatives are effective for acute constipation relief and chronic constipation prevention. However, caution is advised for patients with chronic kidney disease to avoid hypermagnesemia. Magnesium-based osmotic laxatives can also interfere with the absorption of chelating drugs like tetracyclines. PEG is commonly used for colonic cleansing prior to procedures.
Lactulose, while an osmotic laxative, plays an important role in managing hepatic encephalopathy, a neurological complication in cirrhosis due to ammonia buildup. It prevents ammonia absorption by being metabolized by intestinal bacteria, leading to acidic metabolites. This results in an acidic pH shift, transforming ammonia (NH3) into ammonium (NH4+), which is then excreted. Rifaximin, a minimally absorbed antibiotic, can also be used to target ammonia-producing intestinal bacteria in hepatic encephalopathy treatment.
Stimulant laxatives, like senna and bisacodyl, incite the enteric nervous system and elevate colonic secretions to expedite bowel movements. Prolonged use of senna can lead to melanosis coli, a benign discoloration of the colon. Bisacodyl is available in both oral and rectal formulations. Extended use of stimulant laxatives can lead to complications such as hypokalemia, salt, and protein loss.
