Unlike ulcerative colitis, Crohn's disease can affect any area of the GI tract, but the most common sites of involvement are the terminal ileum and colon.
The rectum is characteristically spared in Crohn's disease, as opposed to ulcerative colitis, in which the rectum is always involved.
Crohn's disease is characterized by transmural inflammation involving the entire thickness of the bowel wall, as opposed to inflammation of only the mucosal and submucosal layers seen in ulcerative colitis. Chronic transmural inflammation can lead to fissures, fistulas, or strictures.
50% of individuals with Crohn's disease demonstrate noncaseating granulomas, which are aggregates of giant cell macrophage derivatives. Findings of granulomas are highly specific for Crohn's disease.
Crohn's disease is characterized by the presence of multiple, separate, sharply delineated regions of disease, resulting in skip lesions. This is different from the continuous stepwise inflammation seen in ulcerative colitis.
Sparing of regions of mucosa in between the patchy distribution of affected areas results in a coarsely textured cobblestone appearance, which is characteristic of Crohn's disease.
Chronic transmural inflammation can cause development of fissures in the bowel wall, which can lead to fistulas and abscesses.
Chronic transmural inflammation can cause development of fistulas between the bowel and other organs including enteroenteric, enterovaginal, enterocutaneous, and enterovesicle fistulas.
Perianal disease is common in Crohn's disease. The major perianal complications include fissures, fistulas, abscesses, and stenosis and can cause anal pain, purulent discharge, bleeding, and incontinence.
Hyperplasia of the subserosal and mesenteric adipose tissue can lead to the appearance of creeping fat, which is highly characteristic of the serosal surface.
On histologic examination, Crohn's disease is characterized by transmural inflammation with multiple large lymphoid aggregates in the mucosa and submucosa.
String sign is a medical term for a radiographic finding in which a patient is given barium to swallow and X rays are subsequently taken of the patient's stomach and intestines. String sign signifies severe narrowing of a loop of bowel, causing a thin stripe of contrast. Differential diagnosis for string sign includes Crohn's disease, hypertrophic pyloric stenosis, and colon cancer.
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