Chronic Idiopathic Megacolon

Chronic idiopathic megacolon is persistent dilatation of the colon with constipation and no mechanical obstruction, enteric aganglionosis or other identified cause. Radiological criteria are a rectosigmoid diameter over 6.5 cm at the pelvic brim, a caecum over 12 cm, or an ascending colon over 8 cm.

Uncommon. It affects children and adults, with onset often in childhood and a long history of constipation and soiling.

Causes to exclude are Hirschsprung disease, Chagas disease, drugs (opioids, anticholinergics, antipsychotics), metabolic and endocrine disease (hypothyroidism, diabetes), neurological disease, scleroderma, amyloidosis and obstruction.

Pathophysiology.

Reduced enteric neuron or interstitial cell of Cajal numbers, and impaired motility, allow stool retention, which stretches and further weakens the colon. Chronic suppression of the defecation urge is a recognised contributor.

Long standing constipation, abdominal distension, faecal impaction, overflow incontinence, abdominal pain and bloating. Complications are volvulus, stercoral ulceration and perforation, and faecal obstruction.

Red flags.

• Severe pain with vomiting or no flatus (volvulus or obstruction).
• Peritonitis.
• Fever with distension.
• Rectal bleeding or weight loss.

Abdominal radiograph and CT show a dilated, faecal loaded colon. Contrast enema shows dilatation without a transition zone. Anorectal manometry shows a present rectoanal inhibitory reflex (it is absent in Hirschsprung disease). Rectal suction or full thickness biopsy with acetylcholinesterase staining excludes aganglionosis. Send thyroid function, calcium, glucose, Chagas serology where relevant, and review drugs. Colonic transit studies and defecography are optional.

Differential diagnosis.

• Hirschsprung disease.
• Chagas megacolon.
• Colonic pseudo obstruction.
• Functional constipation.
• Toxic megacolon.
• Drug induced dilatation.

Stop causative drugs, establish a toileting routine, and escalate laxatives.

• Macrogol 3350 with electrolytes: 1 to 3 sachets dissolved in water orally once daily for maintenance; continuous; maximum 8 sachets daily for faecal impaction over up to 3 days; monitor electrolytes and fluid balance; avoid in obstruction, perforation and ileus; use caution in heart failure and renal impairment with the impaction regimen.
• Bisacodyl: 5 to 10 mg orally once daily at night, or 10 mg as a suppository; maximum 20 mg daily; monitor for cramps and electrolyte loss; avoid in obstruction.
• Prucalopride: 2 mg orally once daily; reassess at 4 weeks; maximum 2 mg daily; monitor mood and diarrhoea; use 1 mg daily in people over 65 years and in severe renal impairment (creatinine clearance under 30 mL/min) or severe hepatic impairment; avoid in obstruction or perforation.

Disimpaction by enemas or manual removal is needed first. Biofeedback treats associated evacuation disorders. Endoscopic decompression and fixation address volvulus. Subtotal colectomy with ileorectal anastomosis is reserved for intractable disease with proven slow transit and an intact rectum.

Monitoring. Bowel diary, symptoms and electrolytes; imaging for acute change.

Referral and escalation. Gastroenterology, and colorectal surgery for volvulus, refractory disease or perforation.