Differential diagnosis by localization
The topography of the narrowing provides the radiologist with crucial clues about the nature of the pathological process. Various diseases have preferred sites of involvement in the colon:
- Radiation strictures: their location directly depends on the zone of previous irradiation. However, in clinical practice, they are almost always found in the rectosigmoid region.
- Endometriosis: when the bowel is involved in the pathological process, the typical site for lesion formation is the anterior wall of the rectosigmoid region.
- Ischemic strictures: the most characteristic localization is the area of the splenic flexure (flexura splenica).
Pseudodiverticula and bowel wall changes
Strictures are frequently accompanied by the formation of pseudodiverticula — sac-like outpouchings of the intestinal wall.
- In Crohn's disease (Crohn's disease), such outpouchings are the result of fibrotic changes during the healing of eccentrically located ulcers.
- Similar deformations can also be detected in the presence of ischemic strictures.
- Pseudodiverticula extremely rarely accompany other forms of colitis. The most important differential sign: they never form in ulcerative colitis.
- Wide divericula with an unusual "square" shape are specific to scleroderma. They occur in areas of local weakness of the bowel wall.
An additional specific sign of Crohn's disease on gadolinium-enhanced MRI is the comb sign (comb sign). It is visualized as engorgement of the mesenteric fat around the straight vessels (vasa recta) proximal to the stricture.
Lipomatous diseases of the colon
Fatty tumors and infiltrations can also cause lumen deformation and mimic strictures or mass lesions.
- Infiltration of the ileocecal valve: leads to its diffuse enlargement. The surface of the valve may remain smooth or acquire a lobulated structure.
- Solitary lipomas: the colon is a favored site of localization (about 2/3 of all gastrointestinal lipomas). Most often, these are single lesions in the right colon. They grow in the submucosal layer, so their luminal surface is smooth (the mucosal line along the edge of the lesion is not visualized). When larger than 4 cm, they can cause pain, bleeding, or intussusception. The optimal diagnostic method is CT. Fatty tissue is easily recognized by its specific density: from –80 to –120 Hounsfield units.
- Pelvic lipomatosis (Pelvic lipomatosis): a rare pathology of unclear etiology. Excessive proliferation of adipose tissue in the lesser pelvis leads to compression of the rectum and urinary bladder. On a plain radiograph, the pelvis appears abnormally radiolucent, and the sacral contours are traced exceptionally clearly. The diagnosis is confirmed on CT by a diffuse increase in the volume of pelvic adipose tissue.
Pneumatosis Coli
In this condition, gas cysts form in the submucosal and subserosal layers. The exact etiology is unknown, but the pathogenesis is based on micro-ruptures of the mucous membrane through which gas or gas-forming bacteria penetrate deep into the wall.
The formed cyst sustains itself through balanced gas diffusion inward and outward. Interestingly, prolonged oxygen therapy can alter diffusion gradients, leading to the collapse of such cavities.
- Clinical presentation: the process is often asymptomatic, but may present with diarrhea or constipation. Superficial erosions sometimes cause rectal bleeding. Rupture of a cyst with the development of pneumoperitoneum is extremely rare.
- Imaging: images show tightly clustered gas bubbles 1–2 cm in diameter with well-defined contours. The left half of the colon is more frequently affected. Intramural location is clearly confirmed by imaging methods.
- Differential diagnosis: the picture should not be confused with necrotizing enterocolitis. In necrosis, multiple small foamy gas collections or crescentic linear shadows parallel to the wall are visible in the affected bowel segment, and in severe cases, gas in the portal vein.
Volvulus and imaging strategy in appendicitis
When assessing the causes of intestinal obstruction and strictures, it is important to keep acute surgical conditions in mind.
Volvulus (Volvulus) is possible only in those segments that have a mesentery. The most frequent localization is the sigmoid colon. The cecum, transverse colon, and splenic flexure are also at potential risk.
Appendicitis (Appendicitis) is the most common cause of acute abdominal surgical pathology (affecting about 9% of men and 7% of women, usually adolescents and young adults).
- Imaging is indicated only when the clinical diagnosis is doubtful.
- Optimal strategy: begin examination with ultrasound. CT is ordered only if ultrasound results are equivocal. This reduces radiation exposure in young patients while maintaining high predictive value.
- CT reveals an expanded, blind-ending appendix, inflammatory stranding of the surrounding adipose tissue (fat stranding), and the presence of an appendicolith.
- MRI is a very promising method for pregnant women and in pediatric practice, especially in diffusion-weighted imaging (DWI) mode.
Briefly: answers to topical questions
What are the main causes of ischemic strictures in the splenic flexure?
The main cause of ischemic strictures in the splenic flexure is necrosis of the submucosal and muscular layers of the intestinal wall. The splenic flexure (splenic flexure) is the most common location for ischemic colitis because this anatomical region represents a watershed zone of blood supply. The intestinal mucosa is most sensitive to ischemia, yet it retains the capacity for regeneration. Strictures form specifically when there is deeper injury — necrosis of the submucosal and muscular layers. Meanwhile, low-flow states (low-flow states) more frequently involve the right colon rather than the splenic flexure.
What is the normal thickness of the colonic wall on CT evaluation?
The normal thickness of the colonic wall on CT evaluation is less than 3 mm. In inflammatory bowel diseases, this parameter increases pathologically:
- Ulcerative colitis (Ulcerative colitis) — wall thickness is typically 5–8 mm (averaging 7.8 mm).
- Crohn's disease (Crohn disease) — a more pronounced thickening is observed, averaging up to 11 mm.
Measurement of intestinal wall thickness can also be performed using magnetic resonance imaging, ultrasound, or conventional radiography.
For the differential diagnosis of pseudodiverticula: "UC — NONE". In ulcerative colitis, sac-like outpouchings of the bowel wall never occur.
Class and exam questions
Are pseudodiverticula present in ulcerative colitis?
No, they never occur in ulcerative colitis, which is a crucial differential sign.
What does a solitary colonic lipoma look like on CT?
It has a smooth luminal surface, and its fatty content is clearly identified by a density of –80 to –120 Hounsfield units (averaging –100 HU).
What is the "comb sign"?
This is a sign of Crohn's disease on contrast-enhanced MRI, representing engorgement of the mesentery around the straight vessels proximal to the stricture zone.
What imaging diagnostic strategy is preferred when appendicitis is suspected?
Ultrasound is the primary method. CT is ordered only for equivocal ultrasound results to reduce radiation exposure in young patients.
What else to study on this topic
- Radiological signs of intussusception associated with large solitary lipomas.
- Specific CT signs of necrotizing enterocolitis and portal venous gas.
- Imaging features of various types of intestinal volvulus (sigmoid, cecal).
- Application of diffusion-weighted imaging (DWI) in MRI diagnosis in pregnant women.
- Pathogenesis of pneumatosis cystoides coli and the effect of oxygen therapy on gas cysts.
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