Constipation, bowel clearance
The 5HT4 agonist prucalopride activates 5HT4 receptors to increase gastrointestinal motility.
Bulking agents act by retaining water in the gut lumen, softening the faeces and promoting peristalsis. Onset of action is generally 12 to 24 hours after oral administration. They are safe except in patients with strictures in whom obstruction may be precipitated. Adequate hydration should be maintained. Sodium containing agents should be avoided in patients with heart or renal failure.
Faecal softeners ease straining and are useful in patients with painful anal or rectal conditions such as anal fissure, haemorrhoids or proctitis. Paraffin lubricates the passage of faeces. It is hardly absorbed and is relatively safe, rarely being aspirated and causing lipoid pneumonia. However, its use should be restricted to the temporary relief of constipation. Repeated use is not recommended and caution is advised in patients with swallowing difficulties. It is contraindicated in children under three years old. Docusate sodium has a detergent action which reduces surface tension. It may enhance absorption of drugs, especially paraffin. Arachis oil can be given as an enema to soften impacted faeces but should not be used in people with hypersensitivity to peanut or soya.
Osmotic laxatives, whether taken orally or rectally, draw fluid into the bowel and so ease constipation. They should always be given with plenty of water, and are not recommended for chronic use. Magnesium salts have an additional effect of stimulating intestinal motility. They are hardly absorbed and are preferable to sodium salts, oral administration of which must be avoided in renal, heart or hepatic failure. Glycerol suppositories are gentle and useful in the elderly. Lactulose has a microbial and pH effect as well as an osmotic action, which increases stool output. Polyethylene glycol rapidly cleanses the bowel prior to radiography or colonoscopy, and can be used for chronic constipation and to relieve faecal impaction. Total evacuation can usually be achieved within four hours.
Stimulant laxatives can be administered orally or rectally to increase colonic motility. They may cause colicky pain and in the long term hypokalaemia or cathartic colon. They should only be used prior to surgery or instrumentation, or in severe constipation provided obstruction, impaction or colonic dilatation have been excluded. They should not be used in children or pregnancy. Onset of action is generally six to 12 hours after oral administration. In some preparations they are combined with softeners. Senna has the gentlest action of the group. Dose adjustment of bisacodyl can be difficult. Dantron is absorbed in the small intestine and is therefore less potent. Sodium picosulfate is very powerful, but the liquid preparation allows fine dose adjustment. Sodium hydrogen carbonate + sodium dihydrogen phosphate suppositories release carbon dioxide when they come into contact with moisture, activating intestinal movement and triggering bowel evacuation within 15—30 mins.
Methylnaltrexone and naloxegol are peripheral opioid-receptor antagonists. They are used to treat constipation caused by opioids in patients who have not responded adequately to other laxatives.