Toxic megacolon is defined as acute toxic colitis with dilation of the colon. Characteristics of this condition include nonobstructive segmental or total dilation of the colon to greater than 6 cm and signs of systemic toxicity including fever, tachycardia, hypo- or hyperthermia, leukocytosis, electrolyte abnormalities, or altered mental status.

The most common underlying etiologies for toxic megacolon are inflammatory bowel disease (IBD) and infectious colitis, especially pseudomembranous colitis secondary to Clostridioides difficile infection. Toxic megacolon can result from a number of infectious colitis organisms, including Salmonella, Shigella, Campylobacter, Yersinia, Entamoeba histolytica, cytomegalovirus, rotavirus, and fungal infections. Ischemic colitis, radiation colitis, or colitis secondary to chemotherapy are less likely to lead to toxic megacolon.

The prevalence of toxic megacolon from IBD has declined since the advent of medications that better treat the underlying inflammatory activity in IBD. Toxic megacolon will commonly present in the first 3 months to 3 years of IBD diagnosis. In C difficile infection, approximately 1% of hospitalized patients will develop toxic megacolon.

Patients with toxic megacolon present with symptoms of acute colitis including abdominal pain, sepsis / systemic inflammatory response syndrome (SIRS), and bloody diarrhea, although the diarrhea may decrease as colonic dilation occurs. Physical examination often reveals abdominal tenderness and distension and hemodynamic signs of a systemic stress response.