PAC3421 Study Guide - Final Guide: Myenteric Plexus, Laxative, Fecal Impaction
21 Jun 2018
School
Department
Course
Professor
Constipation
1. Describe the rationale for laxative drug use.
-Relieve symptoms
-Prevent symptoms (especially with opioid use)
-Restoration of normal defecation pattern
-Avoidance of straining
2. With examples, describe FIVE different types of laxatives and their respective mechanism of action. (5 marks)
-Bulk-forming laxative (e.g. ispaghula husk, psyllium and sterculia): absorb water in the colon to increase faecal bulk,
which stimulates peristaltic activity; LESS effective in chronically constipated AND non-ambulant AND opioid user ->
prefer to use stimulant/osmotic laxative instead
-Osmotic laxative (e.g. MgSO4, macrogol 3350 with electrolyte, sorbitol, PEG, saline): create osmotic load by drawing
water into intestinal lumen which lubricates faecal material to facilitate passage; suitable for long-term use; useful in
people with faecal impaction (+/- suppositories or enemas)
-Stimulant laxative (e.g. bisacodyl, senna, sodium picosulfate): directly stimulates nerve endings in colonic mucosa to
increase intestinal motility. May also cause accumulation of water and electrolytes in the colonic lumen; not suitable
for long-term use
-Stool softener (e.g. docusate, poloxamer): softens stool by assisting mixture of water into faeces. BUT lack of
evidence as sole agent in adults; so normally use in combination (e.g Coloxyl (i.e. docusate) + SENNA)
-Lubricant (e.g. liquid paraffin, glycerol): mixture of OSMOTIC, STIMULANT & STOOL-SOFTENING EFFECTS;
particularly useful in children with faecal impaction; must be taken at least 2 hours before lying down
-Opioid antagonist (e.g. methylnaltrexone): peripherally acting competitive mu-opioid receptor antagonist to block the
constipating effects of opioids in the GIT and has limited ability to cross BBB, therefore does not antagonise their
analgesic effects; ONLY used when other regular laxatives are inadequate
-Prokinetic agents (e.g. prucalopride): a 5HT4 receptor agonist that increases GI motility to provide propulsive force for
defecation; for chronic idiopathic condition
*note: Bulk-forming effect only apparent within 24 hours and require 2-3 days for full effect
3. Cisapride:
a. Inhibits the actions of dopamine in the GI tract
b. Selectively promotes the release of acetylcholine in the myenteric plexus
c. Promotes gastric emptying by stimulating 5HT4 receptors in the myenteric plexus
d. Both B and C
e. All of the above
4. Apart from pharmacotherapy approach, what other dietary and lifestyle changes you can advise for an adult patient
with constipation?
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