Megacolon: What Is It, Causes, Symptoms, Treatment, and More

What Is Megacolon?

Megacolon is an abnormal dilation of the colon that is not caused by mechanical obstruction. It is usually accompanied by symptoms such as abdominal discomfort, but may result in serious complications (colonic perforation, peritonitis, and/or sepsis) if left untreated.

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.

What Causes Megacolon?

Megacolon has a wide range of causes, including infection, disease, medication, and various congenital disorders. It may also occur following a major surgery; however, the condition is often idiopathic, which means the exact cause is not known. 

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.

Congenital disorders

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.

Other

Other possible causes of megacolon include electrolyte imbalances (e.g. hypokalemia) and hypothyroidism.

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.

There are several suggested mechanisms by which intestinal motility might be decreased:

• 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. This may be related to increased nitric oxide synthesis.

What Are the Symptoms of Megacolon?

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. In disease-related cases of megacolon, additional symptoms are those of the disease itself.

How Is Megacolon Diagnosed?

Megacolon can be diagnosed by observing the size of the colon on an abdominal x-ray scan. Most physicians agree that a colon diameter greater than 12 centimeters at the cecum should be classified as megacolon.

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.

How Is Megacolon Treated?

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.

Summary

Megacolon is an abnormal dilation of the colon that can be categorized as acute, toxic, or chronic. Acute megacolon, also known as Ogilvie syndrome, is associated with damage to the autonomic nervous system and often occurs in ill or postoperative patients with no clear cause. Toxic megacolon is characterized by concurrent systemic toxicity, usually as the result of colon inflammation following an infection. Chronic megacolon is caused by bowel dysfunction as a result of neurological or muscular disorders.

In all three types, symptoms include constipation, bloating, and abdominal pain. Diagnosis of megacolon relies on the use of x-ray and CT scans, and a variety of treatment options can be employed depending on the cause and nature. In rare cases, surgery may be needed to treat megacolon.