What are the criteria for diagnosing megacolon?

Diagnostic Criteria for Megacolon

Megacolon is definitively diagnosed by radiographic evidence of colonic distention with a transverse colon diameter >5.5 cm, along with clinical signs of systemic toxicity in the case of toxic megacolon. 1

Types of Megacolon and Their Diagnostic Criteria

Toxic Megacolon

• Defined by radiographic evidence of total or segmental colonic distention >6 cm, presence of systemic toxicity, and inflammatory (or infectious) etiology 1, 2
• Plain abdominal radiograph showing transverse colonic dilation >5.5 cm is the most established radiological definition 3
• CT findings that are pathognomonic include:
  • Air-filled colonic distension >6 cm
  • Abnormal haustral pattern
  • Segmental colonic wall thinning 4
• Clinical signs of systemic toxicity must be present to differentiate from other forms of megacolon 1
• Most commonly associated with inflammatory bowel disease but can also occur with infectious colitis like Clostridium difficile 1

Acute Non-Toxic Megacolon (Ogilvie's Syndrome)

• Cecal dilatation above 12 cm without underlying inflammatory condition 5
• Sigmoid colon dilation above 6.5 cm measured at the pelvic brim 5
• Occurs in hospitalized patients with serious underlying medical or surgical conditions 6
• Absence of the inflammatory/infectious component that characterizes toxic megacolon 6

Chronic Megacolon

• Persistent dilatation and lengthening of the colon in the absence of organic disease 7
• Radiological sigmoid diameter of approximately 10 cm 7
• Clinical presentation includes chronic constipation, distension, abdominal pain and/or gas distress 7
• May be congenital (Hirschsprung's disease) or represent end-stage refractory constipation 5

Diagnostic Approach

Initial Imaging

• Plain abdominal radiograph is the acceptable first study to detect toxic megacolon 3
• CT scan provides additional information and is particularly useful for detecting complications:
  • Perforation
  • Abscess formation
  • Ascending pylephlebitis (portal vein thrombosis) 3, 1
• In a study of 18 patients with toxic megacolon, CT scans revealed abdominal complications in four patients that were missed clinically and on plain abdominal films 3

Differential Diagnostic Features

• Toxic megacolon primarily affects the transverse colon, unlike colonic obstruction where cecal dilation is the primary concern 1
• CT can differentiate toxic megacolon from severe acute colitis without megacolon by identifying:
  • Segmental colonic wall thinning (seen only in toxic megacolon)
  • Air-filled colonic distension >6 cm
  • Nodular pseudopolyps 4
• Uncomplicated severe acute colitis more frequently shows diffuse colonic wall thickening and submucosal edema 4

Clinical Significance and Complications

• Perforation in the transverse colon of patients with toxic megacolon carries a high mortality rate (27-57%) 1, 2
• Progression of colonic dilatation is an indication for urgent surgical intervention 1, 2
• Persistent fever after 48-72 hours of steroid therapy should raise suspicion for local perforation or abscess formation 1, 2
• Indications for urgent surgery include:
  • Free perforation
  • Massive hemorrhage
  • Increasing transfusion requirements
  • Increasing signs of toxicity
  • Progression of colonic dilatation 1, 2

Diagnostic Algorithm

1. Obtain plain abdominal radiograph to assess for colonic dilatation >5.5 cm in transverse colon 3
2. If dilatation is present, assess for signs of systemic toxicity 1
3. Consider CT scan to:
   • Confirm diagnosis
   • Detect complications
   • Differentiate from other causes of colonic dilatation 3, 1
4. Evaluate for underlying cause (inflammatory bowel disease, infectious colitis) 1
5. Monitor colonic diameter progression closely as increasing dilation indicates need for urgent intervention 1, 2