Megacolon

Megacolon is a severe, abnormal dilatation of the colon, and is classified as acute or chronic. There are many etiologies of megacolon, including neuropathic and dysmotility conditions, severe infections, ischemia, and inflammatory bowel disease. Toxic megacolon is an acute form of megacolon with systemic toxicity, and carries the highest morbidity and mortality. Common symptoms include abdominal distention, pain, bloody diarrhea, or obstipation. Diagnosis depends on the underlying cause, and is usually established with a combination of the patient’s history, laboratory findings, and imaging studies. Patients with chronic megacolon may require laxatives, enemas, and bowel training. Management for acute megacolon includes supportive care, decompression, and potential surgery.

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Overview

Definition and classification

Megacolon is a severe dilatation of the colon secondary to impaired motility or an inflammatory process. The condition is classified based on the time course and duration:

  • Acute megacolon
  • Chronic megacolon

Epidemiology

  • Exact incidence is unknown.
  • All ages may be affected.
  • Incidence depends on the underlying disease:
    • Congenital aganglionic megacolon: 
      • 1 in 5,000 live births
      • Men > women
    • Toxic megacolon:
      • 4.3% in patients with Clostridium difficile colitis
      • 1%–10% in inflammatory bowel disease (IBD)

Etiology of acute megacolon

  • Toxic megacolon (non-obstructive colonic dilation with systemic toxicity)
    • IBD
      • Ulcerative colitis
      • Crohn’s disease
    • Ischemic colitis
    • Infectious colitis
      • C. difficile colitis (most common)
      • Salmonella
      • Shigella
      • Campylobacter
      • Escherichia coli O157
      • Cytomegalovirus (CMV) colitis (usually in immunodeficient patients)
      • Entamoeba histolytica
  • Acute colonic pseudo-obstruction (Ogilvie’s syndrome)
    • Severe systemic illness
    • Surgery (most commonly from cesarean section or hip surgery)
    • Trauma
    • Spinal anesthesia
    • Medications
      • Opiates
      • Anticholinergics
      • Calcium channel blockers

Etiology of chronic megacolon

  • Congenital aganglionic megacolon (Hirschsprung’s disease)
    • Most cases associated with mutations in the RET proto-oncogene
    • Sometimes associated with other genetic syndromes: 
      • Down’s syndrome
      • Multiple endocrine neoplasia (MEN) type 2A or 2B
      • Waardenburg syndrome
  • Acquired 
    • Chronic idiopathic constipation (most common)
    • Neuropathies 
      • Diabetic
      • Spinal cord injury
      • Parkinson’s disease
      • Chagas disease (through destruction of enteric neurons)
    • Myopathies: Duchenne muscular dystrophy
    • Rheumatologic disorders 
      • Scleroderma
      • Systemic lupus erythematosus
      • Dermatomyositis and polymyositis

Pathophysiology

Toxic megacolon

  • Mucosal inflammation:
    • Causes the release of inflammatory mediators → induction of nitric oxide synthase → nitric oxide generation by macrophages and smooth muscle cells → smooth muscle relaxation → colon dilation
    • Extends to the smooth muscle layer → paralysis of smooth muscle → colon dilation
  • Potential precipitating factors:
    • Hypokalemia
    • Medications:
      • Opiates
      • Anticholinergics
      • Antidepressants
      • Bowel preparation solutions or barium
    • Abrupt discontinuation of steroids or mesalamine in IBD

Acute colonic pseudo-obstruction

  • Exact mechanism is unknown.
  • Impairment of parasympathetic fibers S2–S4 has been implicated.
  • Progressive distention and ↑ tension on the colonic wall

Congenital aganglionic megacolon

  • Failure of neural crest cell migration during the embryonic intestinal development → distal colon is aganglionic and nonfunctioning → distension of the colon proximal to the aganglionic segment
  • Nerve plexuses affected: 
    • Meissner (submucosal plexus)
    • Auerbach (myenteric plexus)
  • The amount of colon affected varies.

Chronic acquired megacolon

  • Incompletely understood
  • Neurologic or muscular dysfunction → disruption in normal peristalsis → progressive colon dilation
  • Megacolon and megarectum from chronic constipation is associated with:
    • ↑ rectal compliance and elasticity
    • Blunted rectal sensation
    • ↑ threshold of anal sphincter relaxation

Clinical Presentation

Symptoms

  • Common symptoms:
    • Abdominal distention
    • Obstipation
  • Toxic megacolon:
    • Bloody diarrhea (most common)
    • Sudden cessation of diarrhea should also be concerning.
    • Abdominal pain
    • Malaise
  • Congenital aganglionic megacolon:
    • Majority will present in the neonatal period.
    • Bilious emesis
    • Failure to pass meconium or stool
    • Feeding intolerance and failure to thrive

Physical exam

  • Common findings:
    • Abdominal distension
    • Tympanic abdomen
  • Toxic megacolon:
    • Tachycardia
    • Fever 
    • Hypotension
    • Altered mental status
    • Abdominal distension
    • Lower abdominal tenderness
    • Possible evidence of peritonitis
      • Diffuse abdominal tenderness
      • Rebound tenderness
      • Rigidity
  • Congenital aganglionic megacolon:
    • Tight anal sphincter
    • Release of stool and gas with digital examination

Diagnosis

Laboratory evaluation

These studies help evaluate the severity of the disease, complications, and potential causes.

  • General findings:
    • Hypokalemia (gastrointestinal loss)
    • Metabolic alkalosis (dehydration)
    • Metabolic acidosis and ↑ lactic acid are concerning for colonic ischemia.
    • ↑ blood urea nitrogen (BUN):creatinine ratio (dehydration)
  • Findings in toxic megacolon:
    • Complete blood count (CBC):
      • Leukocytosis with left shift
      • Anemia (due to gastrointestinal blood loss)
    • ↑ erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP)
    • Stool studies:
      • Culture for bacteria and parasites
      • C. difficile toxin

Imaging

  • Abdominal X-ray 
    • Colon dilatation (> 6 cm)
    • Significant stool retention may be seen.
    • May show transition zone between dilated colon and narrow rectum in Hirschsprung’s disease
    • Air-fluid levels can be present.
    • Toxic megacolon findings:
      • Right and transverse colon dilation are the most prominent.
      • Loss of normal haustral pattern
      • Mucosal ulcerations
  • Computed tomography (CT) with oral and intravenous (IV) contrast
    • Colonic dilatation (> 6 cm)
    • Free air indicates perforation.
    • May show fecal impaction or stool burden in cases associated with chronic constipation
    • Toxic megacolon:
      • Loss of haustral pattern
      • Segmental colonic wall thinning
      • Nodular pseudopolyps (deep mucosal ulcerations)
      • Ischemia (if present)
  • Colonic transit studies
    • Assesses colonic motility in chronic megacolon
    • Options:
      • Radiopaque marker study
      • Wireless motility capsule

Other studies

  • Endoscopy 
    • For toxic megacolon:
      • Complete colonoscopy is avoided due to the high risk of perforation
      • Limited sigmoidoscopy may help establish an underlying etiology (C. difficile CMV, IBD)
    • For chronic megacolon:
      • Colonoscopy can rule out obstruction.
      • Biopsy can be performed if the etiology is unknown.
  • Studies for Hirschsprung’s disease:
    • Rectal biopsy
      • Gold standard
      • Absence of ganglion cells
    • Anorectal manometry
      • Lack of internal anal sphincter relaxation with rectal balloon dilation
      • Less accurate in neonates and people with chronic constipation
    • Contrast enema: transition zone between narrowed rectum and proximal dilated colon

Diagnostic criteria for toxic megacolon

  • Radiographic evidence of colon distension
  • And ≥ 3 of the following:
    • Fever
    • Tachycardia
    • Leukocytosis
    • Anemia
  • And ≥ 1 of the following:
    • Dehydration
    • Altered mental status
    • Electrolyte abnormalities
    • Hypotension

Management

Toxic megacolon

  • General management:
    • Patients should be monitored in an intensive care unit (ICU).
    • Serial abdominal exams
    • Laboratory studies and abdominal X-rays every 12 hours 
    • NPO (nothing by mouth)
    • IV hydration and electrolyte replacement
    • Nasogastric decompression, if vomiting or small bowel dilatation is present
    • Discontinue any antimotility, opiates, or anticholinergic medications.
    • IV broad-spectrum antibiotics
      • Reduce septic complications
      • Cover for potential peritonitis from perforation
  • Address the specific cause:
    • C. difficile colitis:
      • Oral vancomycin + metronidazole (oral or intravenous)
      • Vancomycin enemas (can cause perforation)
      • Fecal transplant
    • IBD:
      • Intravenous glucocorticoids
      • Infliximab or cyclosporine (2nd-line)
  • Surgical management:
    • Indications:
      • If no clinical improvement within 48–72 hours
      • Perforation
      • Peritonitis or worsening abdominal exam
      • Worsening colonic distention
      • Ischemia or necrosis
      • Massive colonic hemorrhage
      • Worsening systemic toxicity (fever, hemodynamics, or mental status)
      • Abdominal compartment syndrome or abdominal hypertension
    • Procedures:
      • Preferred in IBD: subtotal colectomy with end-ileostomy
      • Preferred in C. difficile:
        • Total abdominal colectomy
        • Diverting loop ileostomy with colonic lavage

Surgical findings for toxic megacolon related to C. difficile colitis

Image: “Toxic megacolon” by University of Pittsburgh Department of Pathology. License: CC BY 3.0

Acute colonic pseudo-obstruction

  • Initial management (for stable patients without peritonitis in 1st 48–72 hours and a cecal diameter < 12 cm):
    • Rectal tube for decompression
    • Stop offending medications (opiates, anticholinergics, calcium channel blockers).
    • Treat underlying illness.
    • Follow with serial physical exams and X-rays every 12–24 hours.
  • Pharmacologic interventions:
    • Indications:
      • No improvement in 72 hours
      • Cecal diameter > 12 cm
    • IV neostigmine (acetylcholinesterase inhibitor):
      • Bolus dosing or continuous infusion
      • 89% respond to a single dose.
      • Patients need to be in a monitored setting.
    • Methylnaltrexone: if the obstruction is precipitated by opiates
  • Colonoscopic decompression:
    • Indications:
      • No response to neostigmine
      • Contraindications to neostigmine
    • Has a 3% perforation rate
  • Surgery:
    • Indications: 
      • Failure of non-surgical management
      • Peritonitis (ischemia or perforation)
    • Procedures:
      • Primary anastomosis (stable patients)
      • Total abdominal colectomy with end ileostomy (ischemia or perforation)

Chronic megacolon

  • General management:
    • Supportive treatment as needed (hydration, electrolyte correction)
    • Stop offending medications (opiates, anticholinergics).
    • Nasogastric or rectal tube decompression, if requiring hospitalization
    • Empty the bowel.
      • Manual disimpaction may be required for fecal impaction.
      • Osmotic laxatives
      • Suppositories
      • Enemas 
    • Strict bowel retraining program
      • Scheduled defecation times
      • Increase physical activity.
      • Consume bulking agents (high fiber).
  • Surgical management:
    • For severe cases that are unresponsive to medical management
    • Hirschsprung’s disease:
      • Definitive therapy
      • Resection of aganglionic segment with primary anastomosis

Differential Diagnosis

  • Mechanical large bowel obstruction: interruption in the flow of intraluminal contents caused by intrinsic or extrinsic compression of the colonic lumen. Common etiologies include colorectal cancer and volvulus. Patients present with abdominal distention, obstipation, nausea, and vomiting. The diagnosis is established with imaging. Management includes bowel rest, decompression, and surgery.
  • Small bowel obstruction: an interruption of intraluminal contents through the small bowel due to a mechanical or functional problem. Patients present with abdominal pain, distention, nausea, and vomiting. Imaging will show involvement of the small bowel, although the colon may also be distended in functional obstruction. Most cases will resolve with supportive care.
  • Appendicitis: inflammation of the appendix. Early appendicitis can present with diffuse colicky pain, but pain and tenderness eventually localize to the right lower quadrant. Diagnosis is established by CT scan. Treatment includes antibiotics and surgery.
  • Diverticulitis: inflammation of the colonic diverticula. Patients present with crampy lower abdominal pain, and may have constipation. Diverticulitis is frequently associated with fever and leukocytosis. A CT scan shows the characteristic inflammatory findings. Treatment includes bowel rest, antibiotics, and occasionally surgery.

References

  1. Sheth S.G., Lamont T. (2020). Toxic megacolon. Retrieved 13 December 2020, from https://www.uptodate.com/contents/toxic-megacolon
  2. Wald A. (2020). Etiology and evaluation of chronic constipation in adults. Retrieved 13 December 2020, from https://www.uptodate.com/contents/etiology-and-evaluation-of-chronic-constipation-in-adults 
  3. Wesson D.E., Esperanza Lopez M. (2019). Congenital aganglionic megacolon (Hirschsprung disease). Retrieved 13 December 2020, from https://www.uptodate.com/contents/congenital-aganglionic-megacolon-hirschsprung-disease
  4. Cochran, W.J. (2019). Hirschsprung disease. [online] MSD Manual Professional Version. Retrieved December 19, 2020, from https://www.msdmanuals.com/professional/pediatrics/congenital-gastrointestinal-anomalies/hirschsprung-disease
  5. Manuel, D. and Piper, M.H. (2019). Chronic megacolon. In Anand, B.S. (Ed.), Medscape. Retrieved December 19, 2020, from https://emedicine.medscape.com/article/180955-overview
  6. Lin, B., and Wu, G.Y. (2018). Toxic megacolon. In Cagir, B. (Ed.), Medscape. Retrieved December 19, 2020, from https://emedicine.medscape.com/article/181054-overview

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