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Constipation

Red flags

  • Absolute constipation + colicky pain + vomiting + distension - bowel obstruction, not simple constipation - do not give stimulant laxatives
  • New urinary retention/overflow incontinence with faecal impaction - cauda equina until excluded if with leg weakness/saddle anaesthesia
  • Faecal impaction with overflow diarrhoea mistaken for loose bowels - do not treat with antidiarrhoeals
  • Severe, unremitting abdominal pain - consider perforation/ischaemia in a very distended colon

Differential by mechanism

By cause - usually multifactorial in palliative patients
  • Opioid-induced constipation - near-universal, dose-independent tolerance does not develop (unlike nausea/sedation)
  • Other drugs - anticholinergics, 5-HT3 antagonists (ondansetron), iron, some chemotherapy (vincristine)
  • Disease-related - tumour mass effect, hypercalcaemia, spinal cord/cauda equina compression, autonomic neuropathy
  • Debility-related - immobility, poor oral intake, dehydration, low fibre intake, weakness to strain, lack of privacy/altered toileting
  • Metabolic - hypercalcaemia, hypokalaemia, hypothyroidism

Focused history

  • Baseline bowel pattern vs current - frequency, consistency (Bristol stool chart), straining, incomplete evacuation
  • Time since last bowel motion, flatus (passing flatus argues against complete obstruction)
  • Overflow/spurious diarrhoea in a constipated patient - easily missed
  • Full medication review, especially opioid dose changes
  • Diet, fluid intake, mobility, and privacy/dignity barriers to normal toileting

Focused examination

  • Abdominal palpation for faecal loading, distension, bowel sounds
  • PR exam - confirms impaction, assesses for a rectal mass, and distinguishes hard impacted stool (needs disimpaction) from an empty rectum with higher loading (needs oral therapy)
  • Neurological exam (perianal sensation, tone) if any suggestion of cord/cauda equina involvement

Investigation strategy

  • Usually clinical - PR exam is the key bedside investigation
  • Abdominal X-ray if diagnosis unclear or obstruction suspected (limited value in confirming simple constipation alone)
  • Calcium, TFT if metabolic cause suspected

Management

Prevention - the actual standard of care
  • Co-prescribe a laxative with every regular opioid, started at the same time, not reactively
  • Address modifiable contributors - hydration, mobility, privacy, toileting position
First-line - combine mechanisms
  • Stimulant (senna) + softener (docusate/coloxyl), or osmotic (macrogol/movicol) - typical combination for opioid-induced constipation
  • Titrate to a comfortable regular bowel habit, not a fixed dose
  • Avoid bulk-forming laxatives (psyllium/fibre supplements) in patients with poor fluid intake or low mobility - risk of worsening obstruction
Refractory opioid-induced constipation despite optimised standard laxatives
  • PAMORAs (peripherally acting mu-opioid receptor antagonists) - methylnaltrexone (subcutaneous) or naloxegol (oral)
    • Target the peripheral gut effect without reversing central analgesia
    • Contraindicated if bowel obstruction suspected/confirmed
Established impaction
  • PR measures (glycerin/bisacodyl suppository, phosphate enema) if impaction confirmed on PR exam
  • Manual evacuation as a last resort
If obstruction suspected
  • Stop stimulant laxatives and prokinetics - see bowel obstruction pathway (hyoscine, dexamethasone, octreotide)

Traps

  • Treating overflow (spurious) diarrhoea with antidiarrhoeals - worsens the underlying impaction
  • Waiting for constipation to develop before starting a laxative with a new opioid
  • Bulk-forming laxatives in an immobile, poorly hydrated patient
  • Assuming tolerance to opioid-induced constipation will develop over time - it generally does not, unlike nausea and sedation
  • Missing cauda equina as a cause of new constipation with urinary retention in a patient with spinal metastases

Talk track

1. Prevent, don't just treat

  • "Every opioid prescription gets a laxative alongside it - I don't wait for constipation to appear."

2. PR exam settles the plan

  • "That tells me disimpaction versus oral titration versus something more sinister."

3. Combine mechanisms

  • "Stimulant plus softener, or an osmotic - matched to what's actually driving it."

4. Know when to escalate to a PAMORA

  • "Refractory opioid-induced constipation on optimised laxatives gets methylnaltrexone or naloxegol - once I've excluded obstruction."

5. Never confuse obstruction with simple constipation

  • "Colic, vomiting, and distension change the whole plan - stimulants come off the table."

8 of 8 sections written · drafted 2026-09-13