Overflow Diarrhoea

Overflow diarrhoea is the leakage of liquid stool around an impacted faecal bolus in the rectum, leading to a paradoxical "diarrhoea" in a patient who is chronically constipated.

• Mechanism: rectal distension causes persistent relaxation of the internal anal sphincter via the rectoanal inhibitory reflex, allowing liquid contents from proximal bowel to bypass the impaction. Rectal sensation is blunted over time, reducing the urge to defaecate.
• High risk groups: older adults and care home residents, immobility, dementia, neurological disease (Parkinson disease, multiple sclerosis, spinal cord injury, stroke), diabetes with autonomic neuropathy, hypothyroidism, hypercalcaemia, opioid and anticholinergic use, antipsychotics, tricyclics, calcium channel blockers, iron, aluminium containing antacids, children with functional constipation (encopresis).
• Complications: faecal incontinence, urinary retention, stercoral ulceration with perforation, bowel obstruction, delirium, skin breakdown, urinary tract infection.

• Frequent small, loose, offensive stools or staining, often involuntary and unnoticed, accompanied by a history of constipation or infrequent hard stools.
• Lower abdominal discomfort or distension, anorexia, nausea, urinary symptoms (frequency, retention), new confusion in the elderly.
• Examination: palpable faecal loading in the left iliac fossa; digital rectal examination shows a rectum full of hard stool or, occasionally, empty if impaction is higher in the sigmoid colon; reduced anal tone.
• Red flags: rectal bleeding, weight loss, change in bowel habit over several weeks in those aged over 50, severe abdominal pain, vomiting, peritonism, fever.

• Clinical diagnosis from history and digital rectal examination.
• Plain abdominal X-ray when examination is unclear: stool loading in the colon and rectum, assess for obstruction.
• Bloods: urea and electrolytes, calcium, glucose, thyroid function, full blood count.
• Exclude obstruction, colorectal malignancy (colonoscopy or CT colonography if red flags) and stercoral colitis.
• Drug review to identify constipating agents.

• Disimpaction first, followed by maintenance (stool softeners and osmotic laxatives before stimulants).
• Adult regimen for disimpaction:
  • Macrogol (polyethylene glycol 3350 with electrolytes) 8 sachets daily dissolved in 1 litre of water, taken over 6 hours, for up to 3 days (caution in heart failure, renal impairment, and in those unable to swallow safely).
  • If the stool is hard: sodium docusate 200 mg orally three times daily, or use glycerol (glycerin) 4 g suppositories rectally once or twice daily to soften the hard stool; bisacodyl 10 mg suppository rectally once daily.
  • Arachis oil enema (not in nut allergy) 130 mL rectally at night to soften, followed by phosphate enema 118 mL next morning, or sodium citrate micro enema 5 mL, or manual evacuation under analgesia in the elderly or frail where other measures fail.
  • Avoid phosphate enemas in renal impairment, elderly, dehydration and heart failure (risk of hyperphosphataemia).
• Maintenance after disimpaction:
  • Macrogol 1 to 3 sachets daily (macrogol 3350 13.8 g per sachet) in 125 mL water, titrated to produce one soft stool daily.
  • Lactulose 15 mL orally twice daily as an alternative (onset of 48 hours).
  • Senna 7.5 to 15 mg orally at night, or bisacodyl 5 to 10 mg orally at night, as a stimulant when the stool is soft but propulsion is poor.
  • Prucalopride 2 mg orally once daily (1 mg in those over 65 or with severe renal or hepatic impairment) for chronic constipation resistant to laxatives (contraindicated in bowel obstruction, perforation, inflammatory bowel disease).
  • For opioid induced constipation: naloxegol 25 mg orally once daily (12.5 mg in renal impairment) on an empty stomach, or methylnaltrexone 12 mg subcutaneously on alternate days (8 mg if weight 38 to 61 kg).
• Do not give antidiarrhoeals (loperamide, codeine); they worsen impaction.
• General: fluid 1.5 to 2 litres daily, dietary fibre 25 to 30 g daily, toileting routine 20 to 30 minutes after meals with feet on a stool, mobility, remove constipating drugs.
• Admit if: stercoral perforation, obstruction, delirium with failed outpatient disimpaction, severe dehydration.
• Follow up at 1 to 2 weeks.