What is a reasonable differential for this patients bowel changes?
Features are those of a terminal ileitis, the most common cause being Crohn's disease. The differential is relatively broad and includes infection (e.g. Yersinia and Mycobacterium), backwash ileitis of ulcerative colitis, vasculitides and malignancies. (see related articles for a more detailed list).
What finding on bone window helps favour one of these entities? What is a little atypical?
Ankylosis of the right sacroiliac joint further suggests that this is Crohn's disease (with IBD associated arthropathy), although typically involvement of the SIJs is bilateral.
What is the typical distribution of bowel involvement in Crohn's disease?
Skip lesions are characteristic (thus the alternative name regional enteritis). Small bowel is involved in 70 - 80% of patients, with both small and large bowel involved in 50%. Large bowel only involvement is seen in 15 - 20%.
What imaging tests are available for the assessment of Crohn's disease?
CT and MR enteroclysis are similar in sensitivity for active inflammation (89% vs 83% respectively) and both are somewhat better than small bowel follow-through (67 - 72%). The lack of ionizing radiation from MRI would make it a better option, however the availability of MRI is limited in many countries.
Greater than 30 cm contiguous segment of terminal ileitis with luminal narrowing mural thickening and mesenteric venous dilatation (known as the comb sign). There is dilatation of the more proximal small bowel consistent with a degree of obstruction.
In addition there is right sacroiliitis with partial ankylosis.