What is Megacolon?
Megacolon is the excessive dilation (distension) of the colon, also known as the large intestine.
Anatomy of the Colon
The colon is the portion of the intestine that begins at the ileocecal valve (where the small intestine ends) and terminates at the anus. It is composed of six anatomical segments:
- Cecum with the appendix
- Ascending colon
- Transverse colon
- Descending colon
- Sigmoid colon
- Rectum
In megacolon, the massive distension most commonly affects the sigmoid-rectal segment (the terminal portion of the colon), though in some cases the entire colon may be involved.
Classification
Megacolon is broadly classified as:
- Congenital: present from birth (for example, Hirschsprung’s disease)
- Acquired: developing secondary to a variety of underlying causes
Common Symptoms
Regardless of the underlying cause, the most frequent symptoms of megacolon include:
- Constipation (stipsi), often severe and prolonged
- Abdominal distension with a visibly enlarged abdomen
- Abdominal pain
- General malaise and fatigue
- Blood in the stool
In toxic megacolon, additional systemic symptoms such as fever, nausea, vomiting, and confusion may be present (see below).
Diagnosis
Diagnosis is based on clinical history and physical examination, supported by instrumental investigations:
- Abdominal X-ray: to visualize bowel distension and detect signs of perforation
- Contrast enema (barium or water-soluble): particularly useful in congenital megacolon
- CT scan of the abdomen: for detailed assessment of the extent and cause of dilation
- Colonoscopy: selectively used (contraindicated in toxic megacolon)
- Rectal biopsy is essential for confirming Hirschsprung’s disease
Treatment
Treatment of megacolon is causally directed; it targets the identification and elimination of the underlying cause. Initial management includes pharmacological and supportive therapy. However, in severe or refractory cases, surgical intervention (partial or total colectomy, which is the surgical removal of the dilated, non-functional segment of colon) may be required.
When treated promptly and appropriately, the prognosis of megacolon is generally excellent.
Causes and Symptoms
Congenital Megacolon
Congenital megacolon, more commonly known as Hirschsprung’s disease, is a condition affecting approximately 1 in 5,000 newborns. It is significantly more common in males, with a male-to-female ratio of approximately 4:1.
Pathophysiology
Hirschsprung’s disease occurs when ganglion cells are congenitally absent from the colon wall. This absence involves both the myenteric plexus of Auerbach and the submucosal plexus of Meissner. Typically, the rectosigmoid segment is affected, but the condition can also involve longer segments. These nerve ganglia play a crucial role in producing the coordinated muscular contractions of peristalsis, which are the wave-like movements that propel intestinal contents forward.
Without these ganglion cells, the affected bowel segment remains permanently contracted and unable to relax, creating a functional obstruction. Intestinal contents cannot pass through the aganglionic segment, causing progressive accumulation and massive dilation of the colon proximal to the obstruction.
The condition occurs because neural crest cells fail to migrate fully into the developing colon during embryogenesis. This process typically takes place between the 5th and 12th weeks of gestation. Mutations in the RET proto-oncogene are identified in a significant proportion of familial cases.
Clinical Presentation
Hirschsprung’s disease may present at different ages:
- Neonatal period: The most common early sign is delayed passage of meconium (the newborn’s first stool) beyond 48 hours after birth. In healthy full-term newborns, meconium is typically passed within the first 24–48 hours of life. Failure to do so should always prompt evaluation for Hirschsprung’s disease.
- Infancy and early childhood: In some cases, particularly those with short-segment involvement, the diagnosis may be delayed. The presentation can include:
- Chronic diarrhea (paradoxically, explosive diarrhea can occur when liquid stool bypasses the obstruction)
- Proctitis (inflammation of the rectum)
- Abdominal distension
- Failure to thrive and growth retardation
- Recurrent episodes of bowel obstruction
In severe or unrecognized cases, Hirschsprung-associated enterocolitis, a life-threatening inflammatory complication, may develop.
Acquired Megacolon: Toxic and Non-Toxic
Acquired megacolon is further subdivided into:
- Toxic megacolon
- Non-toxic megacolon
Toxic Megacolon
Toxic megacolon is a severe, life-threatening complication of certain inflammatory and infectious colonic diseases. It is characterized by extreme colonic dilation (typically defined as a transverse colon diameter exceeding 6 cm on abdominal X-ray) combined with systemic toxicity.
Primary causes include:
-
Chronic inflammatory bowel diseases (IBD):
- Crohn’s disease
- Ulcerative colitis, the most common underlying cause of toxic megacolon
-
Infectious colitis:
- Clostridium difficile, also known as pseudomembranous colitis. This condition is increasingly recognized as a major cause of toxic megacolon, especially in hospitalized patients and those who have recently undergone antibiotic treatment
- Other bacterial or parasitic infections (e.g., Salmonella, Campylobacter, Entamoeba histolytica)
Pathophysiology of toxic megacolon:
The inflammatory process disrupts the normal muscular tone of the colon wall, causing it to dilate massively. Within the distended colon, not only feces but also toxic metabolites accumulate and are reabsorbed into the bloodstream, rather than being eliminated. This, combined with alterations in the intestinal microbiome and disturbances in fluid-electrolyte balance and acid-base homeostasis, leads to the development of systemic toxicity.
Systemic symptoms of toxic megacolon include:
- Nausea and vomiting
- Altered mental status / confusion
- Fever (often high-grade)
- Tachycardia
- Dehydration
- Hypotension (in severe cases, signs of septic shock)
Toxic megacolon is a medical and potential surgical emergency with a mortality rate that can reach 20–30% if not promptly managed, particularly in cases complicated by colonic perforation.
Non-Toxic Megacolon
Non-toxic megacolon is caused by mechanical obstruction of the intestinal lumen or by conditions that impair normal intestinal motility, without the systemic inflammatory/toxic component. Unlike toxic megacolon, it does not carry the same degree of immediate systemic danger, but can still lead to serious complications if left untreated.
Common causes of non-toxic megacolon include:
-
Fecal impaction (fecaloma): A hard, compact mass of dried feces that accumulates in the rectosigmoid region, most commonly in elderly, debilitated, or chronically constipated patients. Dehydration, immobility, and chronic laxative use contribute to its development.
-
Colorectal neoplasm (colon cancer): A growing tumor can progressively narrow and eventually obstruct the intestinal lumen, causing upstream constipation and colonic distension.
-
Intestinal paralysis (paralytic ileus / Ogilvie’s syndrome): Loss of normal colonic motility without mechanical obstruction. This can be secondary to:
- Postoperative state: transient ileus is nearly universal in the first days after abdominal surgery
- Neurological diseases, including Parkinson’s disease, multiple sclerosis, and spinal cord injury
- Drug toxicity: medications that slow intestinal motility include opioids, anticholinergics, and calcium channel blockers
- Alcohol or drug intoxication
- Metabolic disturbances, such as hypothyroidism, hypokalemia, and hypomagnesemia, are important considerations
Diagnosis
Clinical Assessment
Diagnosis begins with a thorough clinical history (anamnesis) and physical examination. Key elements include:
- Duration and severity of constipation
- History of inflammatory bowel disease, recent antibiotic use, or prior abdominal surgery
- Presence of systemic symptoms (fever, confusion, tachycardia)
- Family history, particularly for Hirschsprung’s disease, is important
Physical examination typically reveals a distended, tympanitic abdomen, often with diffuse tenderness. Signs of peritoneal irritation, such as rigidity or rebound tenderness, may indicate imminent or established perforation. This condition is a surgical emergency.
Imaging Studies
-
Abdominal X-ray (plain film): The first-line imaging study. It demonstrates bowel distension and abnormal air-fluid levels. Free air under the diaphragm, known as pneumoperitoneum, signals intestinal perforation. This condition is a life-threatening complication and requires emergency surgery.
-
Contrast Enema (barium or water-soluble enema): Particularly valuable in congenital megacolon. In Hirschsprung’s disease, it typically shows a classic transition zone between the narrow aganglionic segment and the dilated proximal colon.
-
CT scan of the abdomen: Provides detailed information on the location, extent, and cause of colonic dilation, and can identify complications such as perforation, ischemia, or neoplasm. Should be performed alongside colonoscopy when mechanical obstruction is suspected.
-
Colonoscopy: Indicated when mechanical obstruction is suspected, and useful for obtaining biopsies in cases of suspected malignancy. Colonoscopy is absolutely contraindicated in toxic megacolon, due to the markedly elevated risk of iatrogenic perforation of the already fragile and inflamed colonic wall.
-
Digital rectal examination and rectal biopsy: Together with contrast enema, these are the three fundamental investigations for the diagnosis of congenital megacolon (Hirschsprung’s disease). Rectal biopsy confirming the absence of ganglion cells in the submucosal and myenteric plexuses is the diagnostic gold standard.
Treatment
Acute Management of Toxic Megacolon
The initial management of toxic megacolon is directed at preventing further colonic distension and avoiding the catastrophic complication of perforation.
Immediate supportive measures include:
- Nil by mouth (NPO): The patient is kept fasting to rest the bowel.
- Nasogastric tube insertion: This procedure helps decompress the upper gastrointestinal tract and prevents further gas accumulation.
- Intravenous fluid resuscitation, used to correct dehydration and hemodynamic instability.
- Restoration of electrolyte balance: correction of hypokalemia, hypomagnesemia, and other electrolyte disturbances.
- Acid-base correction: Metabolic acidosis or alkalosis may require specific intervention.
Pharmacological therapy:
- Corticosteroids are the cornerstone of treatment for IBD-related toxic megacolon. Intravenous corticosteroids, such as hydrocortisone or methylprednisolone, help reduce intestinal inflammation.
- Antibiotics: Broad-spectrum intravenous antibiotics are administered to prevent or treat bacterial translocation and sepsis. In C. difficile-associated toxic megacolon, specific treatment with oral vancomycin or fidaxomicin is required.
- Discontinuation of motility-inhibiting drugs: Any medications that slow intestinal peristalsis must be immediately stopped. These include:
- Antidiarrheal agents (e.g., loperamide)
- Anticholinergic drugs
- Calcium channel blockers
- Opioid analgesics
Biological therapies:
In IBD-related toxic megacolon refractory to corticosteroids, rescue medical therapy with:
- Cyclosporine (intravenous), particularly in ulcerative colitis, may be considered.
- Infliximab (anti-TNF biological agent).
may be attempted before proceeding to surgery, though these are typically administered in specialized centers under close monitoring.
Surgical Treatment
If medical management fails to produce adequate improvement within 48–72 hours, or if there is any sign of perforation, peritonitis, or clinical deterioration, emergency surgical intervention becomes the only viable option.
Surgical options include:
- Partial or total colectomy: The dilated, non-functional segment of colon is surgically removed. In emergency settings, a subtotal colectomy with end ileostomy (Hartmann’s procedure) is typically preferred, as it avoids the risks of primary anastomosis in an acutely ill, inflamed bowel.
- Intestinal anastomosis: When conditions permit, continuity of the bowel is restored after resection.
- Ileostomy: In rare or particularly severe cases, a loop of ileum is brought to the skin surface to divert the fecal stream, allowing the remaining bowel to heal. Ileostomy may be temporary (reversed in a subsequent procedure) or permanent.
Surgical Treatment of Congenital Megacolon
In Hirschsprung’s disease, definitive treatment is always surgical. The procedure involves:
- Resection of the aganglionic bowel segment: The segment lacking nerve ganglia is surgically removed.
- Pull-through anastomosis: The normally innervated, healthy proximal colon is brought down and connected to the anal canal. This procedure restores bowel continuity.
Several surgical techniques exist for the pull-through procedure (Swenson, Soave, Duhamel), and the operation can often be performed laparoscopically, even in neonates. When performed by an experienced pediatric surgeon, outcomes are excellent in the majority of cases.
Treatment of Non-Toxic Megacolon
Treatment is directed at the underlying cause:
- Fecal impaction: Manual disimpaction, enemas, laxatives, and hydration
- Colorectal neoplasm: Endoscopic or surgical resection of the tumor, with restoration of bowel continuity
- Paralytic ileus: Correction of underlying metabolic disturbances, discontinuation of offending drugs, early mobilization, and in selected cases neostigmine (a cholinesterase inhibitor that stimulates colonic motility) or colonoscopic decompression
Prognosis
When megacolon is diagnosed early and treated appropriately, the prognosis is generally excellent. Delays in diagnosis or treatment, particularly for conditions like toxic megacolon, significantly worsen outcomes. This is because such delays increase the risk of colonic perforation, peritonitis, and sepsis.
Sources and Bibliography
- Dionigi R. Chirurgia. Basi teoriche e chirurgia generale – Chirurgia specialistica, Vols. 1–2. Ed. Elsevier.
- Harrison’s Principles of Internal Medicine, Vol. 1 (17th Ed., McGraw-Hill, 2009).
- Jalan KN, Sircus W, Card WI, et al. “An experience with ulcerative colitis”. Gastroenterology, 1969. (Original criteria for toxic megacolon diagnosis)
- Autenrieth DM, Baumgart DC. “Toxic megacolon”. Inflammatory Bowel Diseases, 2012.
- Langer JC. “Hirschsprung disease”. Current Opinion in Pediatrics, 2013.
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) – Hirschsprung Disease
- Orphanet – Hirschsprung Disease
- American College of Gastroenterology – Toxic Megacolon
