Under a microscope, biopsies of the affected colon may show mucosal inflammation, characterized by focal infiltration of neutrophils, a type of inflammatory cell, into the epithelium. This typically occurs in the area overlying lymphoid aggregates. These neutrophils, along with mononuclear cells, may infiltrate the crypts, leading to inflammation (crypititis) or abscess (crypt abscess).Granulomas, aggregates of macrophage derivatives known as giant cells, are found in 50% of cases and are most specific for Crohns disease. The granulomas of Crohns disease do not show "caseation", a cheese-like appearance on microscopic examination characteristic of granulomas associated with infections, such as tuberculosis. Biopsies may also show chronic mucosal damage, as evidenced by blunting of the intestinal villi, atypical branching of the crypts, and a change in the tissue type (metaplasia). One example of such metaplasia, Paneth cell metaplasia, involves the development of Paneth cells (typically found in the small intestine and a key regulator of intestinal microbiota) in other parts of the gastrointestinal system. Diagnosis
The diagnosis of Crohns disease can sometimes be challenging, and many tests are often required to assist the physician in making the diagnosis. Even with a full battery of tests, it may not be possible to diagnose Crohns with complete certainty; a colonoscopy is approximately 70% effective in diagnosing the disease, with further tests being less effective. Disease in the small bowel is particularly difficult to diagnose, as a traditional colonoscopy allows access to only the colon and lower portions of the small intestines; introduction of the capsule endoscopy aids in endoscopic diagnosis.
Penetrating disease creates abnormal passageways (fistulae) between the bowel and other structures, such as the skin. Inflammatory disease (or nonstricturing, nonpenetrating disease) causes inflammation without causing strictures or fistulae. Endoscopy
A colonoscopy is the best test for making the diagnosis of Crohns disease, as it allows direct visualization of the colon and the terminal ileum, identifying the pattern of disease involvement. On occasion, the colonoscope can travel past the terminal ileum, but it varies from person to person. During the procedure, the gastroenterologist can also perform a biopsy, taking small samples of tissue for laboratory analysis, which may help confirm a diagnosis. As 30% of Crohns disease involves only the ileum, cannulation of the terminal ileum is required in making the diagnosis. Finding a patchy distribution of disease, with involvement of the colon or ileum, but not the rectum, is suggestive of Crohns disease, as are other endoscopic stigmata. The utility of capsule endoscopy for this, however, is still uncertain. A "cobblestone"-like appearance is seen in approximately 40% of cases of Crohns disease upon colonoscopy, representing areas of ulceration separated by narrow areas of healthy tissue. Radiologic tests
A small bowel follow-through may suggest the diagnosis of Crohns disease and is useful when the disease involves only the small intestine. Because colonoscopy and gastroscopy allow direct visualization of only the terminal ileum and beginning of the duodenum, they cannot be used to evaluate the remainder of the small intestine.