Megarectum

Megarectum is persistent dilatation of the rectum, defined as a rectal diameter over 6.5 cm at the pelvic brim on a lateral radiograph. It causes faecal retention, overflow incontinence and soiling.

Most cases are in children and young adults with chronic constipation, and in people with neurological or developmental disorders.

Causes are chronic functional constipation and rectal faecal impaction (the commonest), Hirschsprung disease, anorectal malformations, spinal cord disease, learning disability, long term opioid use and psychological factors.

Pathophysiology.

Prolonged faecal loading stretches the rectal wall, reduces sensation and contractility, and impairs the rectoanal inhibitory reflex, so the urge to defecate is lost.

Infrequent large stools, soiling and faecal incontinence from overflow, abdominal distension, a palpable left iliac fossa mass, a large faecal filled rectum on examination and urinary symptoms.

Red flags.

• Acute obstruction or perforation.
• Stercoral ulcer with bleeding.
• Neurological signs in the legs or perineum.

Digital rectal examination and abdominal examination. A lateral abdominal radiograph or contrast enema measures rectal diameter. Anorectal manometry shows a raised sensory threshold with intact or absent rectoanal inhibitory reflex; absent reflex needs rectal biopsy for Hirschsprung disease. Defecography and spinal imaging are used when indicated.

Differential diagnosis.

• Hirschsprung disease.
• Chronic idiopathic megacolon.
• Rectal prolapse or rectocele.
• Neurogenic bowel.
• Rectal tumour.

Clear the rectum, then maintain regular emptying.

• Glycerol suppository: 4 g rectally once daily for up to 3 days to start; maximum 4 g daily; monitor for rectal irritation; avoid in rectal bleeding.
• Sodium phosphate enema (disimpaction): 1 enema (118 mL) rectally once daily for up to 3 days; maximum 1 enema daily; monitor electrolytes; avoid in renal impairment, dehydration, children under 3 years, and bowel obstruction; check for inflammatory bowel disease.
• Macrogol 3350 with electrolytes: 1 to 3 sachets orally once daily for maintenance; maximum 8 sachets daily for impaction over up to 3 days; monitor electrolytes; avoid in obstruction; caution in heart failure and renal impairment.
• Bisacodyl: 10 mg as a suppository or 5 to 10 mg orally once daily at night; maximum 20 mg daily; monitor cramps; avoid in obstruction.

Add toilet training, biofeedback and transanal irrigation. Manual evacuation under anaesthesia may be needed. Surgery (rectal resection with coloanal anastomosis, or a Duhamel type procedure) is used for refractory megarectum with persisting symptoms.

Monitoring. Bowel diary and soiling episodes; rectal examination at follow up.

Referral and escalation. Colorectal surgery and paediatric surgery for refractory cases or suspected aganglionosis.

Special groups. Children need weight based dosing of laxatives and behavioural support. Neurological patients need a structured bowel programme.