Toxic Megacolon

Diagnostic criteria and morphology

To diagnose toxic megacolon using imaging modalities, it is necessary to rely on strict criteria. The main sign is colonic dilatation, the diameter of which may exceed 8.5 cm. Furthermore, a crucial condition is the complete absence of haustration. If preserved haustra are visualized on images, this diagnosis is invalid.

The most pronounced changes are usually localized in the transverse colon. This is explained by anatomical features: when the patient is in the supine position, this segment becomes the most dependent part of the abdominal cavity, and it is precisely where the bulk of intraluminal gas accumulates, causing overdistension.

Morphologically, the intestinal wall undergoes severe changes. The images reveal so-called "mucosal islands" (mucosal islands), which are edematous fragments of preserved mucosa indicating an extremely extensive pattern of ulceration. The bowel wall itself thinned and acquires a friable consistency often compared to "wet blotting paper". This condition of the wall critically increases the risk of spontaneous perforation.

Imaging Methods

In clinical practice, a comprehensive approach to bowel imaging is applied:

• Radiography: Serial plain abdominal radiographs are extremely useful for continuous monitoring of dilatation dynamics and timely detection of free gas.
• MRI: Magnetic resonance imaging (especially T2-weighted images in axial and coronal planes) effectively demonstrates bowel lumen expansion, its wall thickening (mural thickening), as well as concomitant perimural mesentery edema. In addition, MRI makes it easy to detect the accumulation of free fluid in the pelvic cavity.

When evaluating images, it is important to pay attention to the state of the small intestine. If its pronounced gas distension is determined, this is regarded as an extremely unfavorable prognostic sign indicating a low probability of success for medical (conservative) treatment.

Colonic perforation

Perforation is one of the most formidable complications. Most often it occurs against the background of an acute attack of ulcerative colitis (UC), typically within the first year from disease manifestation. The direct cause is deep ulceration of the wall, which can be either localized or develop as part of toxic megacolon.

There are two main types of perforation:

1. Free perforation: Easily recognized by the presence of free gas in the abdominal cavity on standard plain radiographs.
2. Sealed perforation (sealed perforation): It cannot be reliably detected using conventional radiography. In such cases, computed tomography (CT) is required, which has high sensitivity for finding extraluminal gas.

Interestingly, in Crohn's disease, free perforation is extremely rare. This is due to the chronic transmural nature of inflammation, which gradually leads to the adhesion of the affected area to adjacent structures. As a result, localized pericolic abscesses are formed, the detection of which also requires cross-sectional imaging modalities (CT or MRI).

Differential diagnosis of strictures

In addition to acute dilatation, inflammatory diseases can lead to the formation of strictures. The causes of colonic narrowing are diverse: from physiological spasm and surgical anastomoses to malignant neoplasms, ischemia, and diverticular disease.

Smooth muscle relaxants are used to differentiate functional spasm from true pathology, although physiological sphincters (e.g., Cannon's point) may persist.

On double-contrast barium enema, a classic benign stricture has a smooth lumen with smoothly tapering ends (tapering ends). A malignant stricture, by contrast, is characterized by an irregular lumen and overhanging edges (shouldered ends), although scirrhous carcinoma can mimic a benign process.

Features of strictures in inflammatory bowel diseases:

• Ulcerative colitis: Characterized by significant hypertrophy of the muscularis mucosae and submucosal fat thickening. Changes in smooth muscle lead to generalized shortening of the bowel. In patients with a long history of UC (in 10–20% of cases), localized strictures form in the left hemicolon.
• Crohn's disease: Strictures are typically asymmetric. Pouch-like protrusions (sacculation) are often observed, arising secondary to ulcers on the antimesenteric border.
• Diverticular disease: An important distinction from the malignant process is the preservation of mucosal folds and the presence of spicule-like necks of compressed diverticula.

Cross-sectional imaging (CT and MRI) is indispensable for assessing wall thickness, the pattern of contrast enhancement, and searching for extracolonic findings (e.g., signs of malignancy: irregular protruding areas or asymmetry).

Class and exam questions

What else to study on this topic

• Mechanisms of «mucosal islands» formation
• Causes of false-positive diagnoses of malignant colonic strictures
• The role of smooth muscle relaxants in the differential diagnosis of strictures
• Features of detecting sealed perforations using computed tomography
• Differential diagnosis of ischemic pouch-like protrusions and strictures in Crohn's disease

Other topics in the section "Diagnostic Radiology of the Abdominal Organs"

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