Approach Considerations
In general, toxic megacolon should be suspected in patients presenting with abdominal pain with distention and acute-onset chronic diarrhea. The diagnosis is made based on the clinical picture of systemic toxicity along with evidence of colon dilatation more than 6 cm. [4]
The most common criteria used to diagnose toxic megacolon are the ones proposed by Jalan et al in 1969, [43] as follows:
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Radiographic evidence of colonic dilatation: The classic finding is a more than 6-cm dilatation in the transverse colon
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Any three of the following: Fever (>101.5°F [>38°C]), tachycardia (>120 beats/min), neutrophilic leukocytosis (>10.5 x 103/µL), or anemia
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Any of the following: Dehydration, altered mental status, electrolyte abnormality, or hypotension
Complete blood cell (CBC) count
Patients with toxic megacolon may develop leukocytosis with a left shift. Patients can also present with leukemoid reaction. Additionally, bloody diarrhea results in anemia.
Although the presence of an increased white blood cell (WBC) count contributes to the diagnosis of toxic megacolon—and most investigators believe that the absence of a high WBC count makes defining a disease as toxic megacolon difficult—an abnormally low count, or even a WBC count that is within normal limits, does not rule out toxic megacolon. This is because in immunosuppressed or extremely toxic patients, the WBC count actually may be normal or low.
Chemistry panel
Electrolyte disturbances are very common in toxic megacolon secondary to inflammatory diarrhea, steroid use, and ongoing gastrointestinal losses. The inflamed colon is unable to reabsorb salt and water. Acid-base disorders have been described (eg, metabolic alkalosis) with the large volume loss. [48] Metabolic acidosis can occur with advanced disease and the development of bowel ischemia.
Nutrition and coagulation panels
A coagulation panel should be ordered in the event that surgery is required. A nutrition panel, in accordance with the physician's practice, is helpful in assessing the patient's nutritional status. Hypoalbuminemia is common at the later stages of the disease due to protein wasting, decreased hepatic production, and severe inflammation.
ESR and CRP
Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) levels are usually elevated. Although these findings are not specific to toxic megacolon, they may support the diagnosis.
Histology
Pathology in cases of toxic megacolon demonstrates acute inflammation involving all layers of the colon. Variable amounts of necrosis and degeneration are present. Infiltration by inflammatory cells (neutrophils, macrophages, and lymphocytes) is noted. The myenteric and submucosal plexuses are usually preserved.
Stool studies
Stool cultures, examination for ova and parasites (O&P), and as well as testing for C difficile toxin should be obtained.
Radiography
Plain abdominal radiographs are essential for the diagnosis and management of toxic megacolon. Repeated serial abdominal plain films are necessary to monitor the progress of the disease and the efficacy of treatment.
Radiographic findings include the following:
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Air-fluid levels in the colon with an absent or abnormal haustral pattern
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Deep mucosal ulcerations and possible pseudopolyp projections into the colon lumen
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The presence of air in the small bowel on x-ray: Can be predictive of toxic megacolon [49]
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Segmental parietal thinning: Pathognomic for toxic megacolon [3]
Comparison with old baseline films, if available, is helpful. Avoid barium enema studies in a patient who is severely toxic; the potential for perforation is considerable.
The following image depicts a dilated (>6 cm) transverse colon.
Toxic Megacolon. A 22-year-old man presented with abdominal pain, passage of blood and mucus per rectum, abdominal distention, fever, and disorientation. Findings from sigmoidoscopy confirmed ulcerative colitis. Abdominal radiographs obtained 2 days apart show mucosal edema and worsening of the distention in the transverse colon. The patient's clinical condition deteriorated over the next 36 hours despite steroid and antibiotic therapy, and the patient had to undergo a total colectomy and ileostomy.
Loss of colonic haustrations, possible "thumbprinting," is shown below
Toxic Megacolon. Plain abdominal radiograph from a patient with known ulcerative colitis who presented with an acute exacerbation of his symptoms. This image shows thumbprinting in the region of the splenic flexure of the colon.
Severe dilatation and a featureless transverse and sigmoid colon are demonstrated in the following image.
Toxic Megacolon. A 45-year-old woman presented with acute pain and colitis of uncertain etiology. The radiograph shows an increase in gaseous distention of the distal transverse colon, now measuring up to 80 mm in diameter (it was 60 mm 2 days previous). The transverse colon and sigmoid colon are both featureless, consistent with severe colitis. There is fecal loading in the right side of colon. No evidence of free gas or pneumatosis intestinalis is noted. The appearance is concerning for toxic megacolon. Courtesy of Craig Hacking, MBBS, Associate Professor, Royal Brisbane and Women’s Hospital (Radiopaedia.org, rID: 79194).
Ultrasonography
Ultrasonography of the intestines appears to be a potential diagnostic test for toxic megacolon. [50] In one study, investigators demonstrated similar findings in four cases, including the following:
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Complete loss of haustra coli of the colon
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Hypoechoic and thickened bowel walls with irregular internal margins in the sigmoid and descending colon
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Marked dilatation of the transverse colon (>6 cm), a finding that correlated well with the plain radiograph of the abdomen
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Hypoechoic and thin (< 2 mm) bowel walls without haustra coli in the dilated colon; in patients who underwent surgery, the postoperative pathohistologic findings of the bowel walls correlated with the ultrasonographic features observed before surgery
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Slight dilatation of ileal bowel loops (diameter >18 mm) with increased intraluminal gas and fluid
Computed Tomography Scanning
Computed tomography (CT) scanning should be obtained in patients in whom the diagnosis of toxic megacolon is being considered. This imaging modality may identify a local or contained perforation. [4] If the diagnosis remains unclear or the cause of toxicity is thought to be an abscess, CT scanning may be helpful.
Imbriaco and Balthazar found that CT scanning was more accurate than plain radiography in detecting severe colitis. [51] CT scans may show diffuse colonic wall thickening, thickened haustra with alternating bands of high and low density (‘‘accordion sign’’), a multilayered appearance caused by different densities of edematous submucosa, or hyperemic mucosa (‘‘target sign") and peri colic stranding. CT scanning was superior in showing the severity and length of colitis as well as the presence of dilatation. [51]
Toxic Megacolon. Computed tomography scan from a patient with pseudomembranous colitis demonstrating the classic accordion sign.
Endoscopy
Endoscopy can provide a further advantage for evaluating the etiology of toxic megacolon such as identifying cytomegalovirus (CMV) or pseudomembranous colitis, especially when clinical suspicion exists despite negative stool studies for C difficile.
Endoscopy may take the form of flexible sigmoidoscopy or colonoscopy. However, if clinical concern for toxic megacolon exists, the examination should not progress beyond the sigmoid colon, if at all. The scope should be advanced only as far as necessary for diagnosis. Air insufflation should be minimal. Sigmoidoscopy can miss the diagnosis of CMV infection because the lesions can be localized in the cecum or ascending colon. [52]
According to some experts, colonoscopy is generally justified only if the patient has no or minimal inflammation of the sigmoid colon or rectum. Perforation is an obvious potential complication of this approach. [12]
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Toxic Megacolon. A 22-year-old man presented with abdominal pain, passage of blood and mucus per rectum, abdominal distention, fever, and disorientation. Findings from sigmoidoscopy confirmed ulcerative colitis. Abdominal radiographs obtained 2 days apart show mucosal edema and worsening of the distention in the transverse colon. The patient's clinical condition deteriorated over the next 36 hours despite steroid and antibiotic therapy, and the patient had to undergo a total colectomy and ileostomy.
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Toxic Megacolon. A 72-year-old woman presented with vomiting and abdominal distention. The supine (right) and erect (left) plain abdominal radiographs show gross dilatation of the colon with multiple air-fluid levels. On further questioning, the patient revealed that she was taking diuretics for hypertension. Blood biochemical tests revealed markedly lowered potassium levels. After potassium replacement therapy, the patient's pseudo-obstruction completely resolved.
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Toxic Megacolon. Gross pathology specimen from a case of pseudomembranous colitis demonstrating characteristic yellowish plaques.
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Toxic Megacolon. Computed tomography scan from a patient with pseudomembranous colitis demonstrating the classic accordion sign.
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Toxic Megacolon. Plain abdominal radiograph from a patient with known ulcerative colitis who presented with an acute exacerbation of his symptoms. This image shows thumbprinting in the region of the splenic flexure of the colon.
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Toxic Megacolon. Increased postrectal space is a known feature of ulcerative colitis.
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Toxic Megacolon. A 45-year-old woman presented with acute pain and colitis of uncertain etiology. The radiograph shows an increase in gaseous distention of the distal transverse colon, now measuring up to 80 mm in diameter (it was 60 mm 2 days previous). The transverse colon and sigmoid colon are both featureless, consistent with severe colitis. There is fecal loading in the right side of colon. No evidence of free gas or pneumatosis intestinalis is noted. The appearance is concerning for toxic megacolon. Courtesy of Craig Hacking, MBBS, Associate Professor, Royal Brisbane and Women’s Hospital (Radiopaedia.org, rID: 79194).
