The presentation depends mainly on the site of the disease but also on whether there is a tendency to perforate or fistulate rather than to fibrose and stricture, probably determined by genetic factors. Terminal ileal and ileocolonic disease present with right iliac fossa pain, often with an associated mass. This can present acutely, mimicking acute appendicitis or - more commonly - chronically, when symptoms may closely mimic the irritable bowel syndrome. It can also present subtly, as short stature III a child or adolescent, as weight loss with minimal abdominal symptoms or even as nephrotic syndrome due to amyloidosis.
Colonic Crohn's disease is distinguishable from ulcerative colitis by the presence of skip lesions, rectal sparing, perianal skin tags or fistulas and/or granulomata on biopsy, but in up to one-third of cases the distinction is unclear and in this situation it is usually better not to be dogmatic about the diagnosis.
Initial diagnosis is usually made by barium studies and/or endoscopy. Radiolabelled white cell scanning may be helpful if radiological evidence of small-intestinal disease is equivocal. Serum C-reactive protein concentration is raised in about 95% of cases with active disease. Magnetic resonance scanning is very useful for evaluation of perianal fistulas and abscesses.
Extra-intestinal manifestations of the disease include erythema nodosum, episcleritis and reactive arthritis. With the exception of ankylosing s ondylitis, for which Crohn's disease - like Ulcerative Colitis - is a risk factor, the other extra-intestinal problems usually resolve with appropriate treatmenfofthe underlying bowel disease and do not usually require specific treatment. Sclerosing cholangitis is extremely rare, particularly if colonic disease is absent.