Expansion
Slow transit, outlet obstruction or normal transit with hard stool
Expansion
Mechanisms
- Normal transit: hard stool from low fibre and fluid intake; the commonest
- Slow transit: reduced colonic motility. Ageing, immobility, hypothyroidism, hypercalcaemia, hypokalaemia, Parkinson disease, diabetic autonomic neuropathy, opioids and anticholinergics
- Outlet obstruction: pelvic floor dyssynergia, rectocele, Hirschsprung disease, mechanical obstruction
Laxative classes by mechanism
- Bulk forming (ispaghula): increase stool mass and stimulate peristalsis; need adequate fluid
- Osmotic (macrogol, lactulose): retain water in the lumen. Lactulose also acidifies the colon, hence its use in encephalopathy
- Stimulant (senna, bisacodyl): increase motility and secretion
- Softeners (docusate) and lubricants
- Peripheral opioid antagonists (naloxegol, methylnaltrexone) for opioid-induced constipation, acting on gut receptors without reversing analgesia
Red flags requiring investigation: new constipation over 50, weight loss, rectal bleeding, anaemia, or a change in stool calibre, since colorectal carcinoma must be excluded.