Megacolon is a condition where the colon (large intestine) becomes abnormally enlarged. It can lead to a severe and life-threatening situation if not treated promptly. Megacolon can be diagnosed by observing the size of the colon on an abdominal x-ray scan. A colon diameter greater than 12 centimeters at the cecum should be classified as megacolon.
Pathophysiology
The exact mechanism by which megacolon develops is not known. However, the end result is the same: severely decreased intestinal motility causes a buildup of feces, air, and intestinal secretions in the colon, which presents as dilation of the colon.
In acute, non-toxic megacolon, there is damage to the autonomic nervous system. In chronic megacolon, there is inherent neurological and/or muscular dysfunction in the bowels. In toxic megacolon, there is reduced smooth muscle activity, likely as the result of inflammation.
Types of Megacolon
Megacolon can be classified as acute or chronic depending on whether the dilation is temporary or ongoing. All cases of acute megacolon are acquired, whereas chronic megacolon can be both acquired or congenital.
Acute megacolon can be further categorized depending on whether there is inflammation of the colon. If inflammation is present, this usually results in systemic toxicity, so the resulting condition is referred to as toxic megacolon. If no inflammation is present, the resulting condition is referred to as Ogilvie syndrome, or just acute megacolon.
Hirschsprung's Disease
Megacolon can also be caused by some congenital disorders, as is true in the case of Hirschsprung’s disease, where functional obstruction of the intestines is often observed.
It is a condition present at birth where the large intestine (colon) is missing nerve cells called ganglion cells, particularly in the lower part of the colon and rectum. The defect occurs in 1 in 5000 live births. This absence of nerve cells prevents the muscles in that part of the bowel from contracting and relaxing properly to allow stool to pass through, leading to severe constipation, intestinal blockage, and enlargement of the colon.
It is caused by a failure of nerve cells to migrate properly during fetal development. Genetics may play a role.
Symptoms include chronic constipation, abdominal swelling, vomiting, failure to pass stool (especially after birth), and poor feeding.
Diagnosis is usually diagnosed in infancy using tests like a barium enema, rectal biopsy, or manometry to confirm the absence of ganglion cells.
With timely treatment, most children recover well, but long-term issues like constipation or infections may persist in some cases.
Infection
One of the most common causes of megacolon is infection. This includes bacterial infections such as Clostridium difficile, Salmonella, Shigella, and Campylobacter, as well as parasitic infections such as Trypanosoma cruzi (commonly known as Chagas disease) and Entamoeba histolytica.
Disease
Megacolon can also be caused by a variety of neurological and systemic diseases. Common neurological causes are diabetic neuropathy and Parkinson's disease, while systemic causes include some muscular dystrophies, scleroderma, and systemic lupus erythematosus.
Medication
In rare cases, megacolon may be the adverse effect of a medication. Most notably, drugs such as risperidone, clozapine, and loperamide are associated with increased risk of megacolon.
Other
Other possible causes of megacolon include electrolyte imbalances (e.g. hypokalemia) and hypothyroidism.
Symptoms
Common symptoms of megacolon include constipation, bloating, and abdominal pain or tenderness. In more severe cases, hard fecal masses called fecalomas may also be present.
Depending on the cause, megacolon may have additional symptoms. In toxic megacolon, usually caused by infection, additional symptoms include fever, tachycardia, and shock.
A contrast enhanced CT scan is used to confirm these findings, additionally showing the colon is free of mechanical obstruction. If a CT scan is not possible, colonoscopy can be performed to verify the colon is free of mechanical obstruction. However, in toxic megacolon, colonoscopy should not be performed due to high risk of perforating the colon.
Toxic megacolon
Toxic megacolon is a rare but life-threatening condition that occurs when the colon rapidly dilates and becomes severely inflamed. It is often a complication of inflammatory bowel diseases (IBD), such as ulcerative colitis or Crohn's disease, but can also result from infections like Clostridium difficile colitis. In toxic megacolon, the colon is unable to move gas and stool, leading to a buildup that can cause the colon to rupture, resulting in a medical emergency. Less common causes include radiation colitis or certain medications.
Symptoms include severe abdominal pain and distension, fever and rapid heart rate, bloody diarrhea, dehydration, and shock (if the colon ruptures).
Toxic megacolons require prompt medical intervention, and management depends on the underlying cause of the condition. If the colon does not respond to medication or perforation occurs, a colectomy (removal of part or all of the colon) may be required.
Treatment
Treatment for megacolon starts by addressing the underlying cause (such as the offending medication or disease), if known. In acute megacolon, all food and drink should be withheld and a nasogastric tube placed. If non-toxic, neostigmine should be administered, and if necessary, the colon itself should be decompressed by means of a colonoscopy. If toxic, steroids and broad-spectrum antibiotics should be given.
In chronic megacolon, both dietary and pharmacological methods should be used to increase intestinal motility. Laxatives and enemas may also be used to prevent fecal impaction.
If the patient does not respond to these treatments within one to three days, it may be necessary to use surgery to remove all or part of the colon. Following colectomy, options include ileorectal anastomosis and ileostomy.
Is Megacolon Reversible?
In most cases, megacolon is reversible. This depends on whether it is possible to treat the underlying cause. In acute and toxic megacolon, the underlying cause can usually be treated or otherwise resolves itself naturally, so dilation of the colon is reversed. However, chronic megacolon involves a progressive or congenital pathophysiology that cannot be resolved.
With early detection and treatment, the outcome can be positive. However, delayed treatment can result in serious complications like sepsis, shock, or death
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