Idiopathic inflammatory disease of the alimentary tract that may present anywhere in the GI tract; most commonly found in the terminal ileum (60%), but may be limited to the colon in 15–20%, proximal small bowel 10%:

• Transmural disease
• May involve multiple regions of the intestine in between normal sections (skip lesions)

Epidemiology

Incidence

• Annual incidence of 3–7 cases per 100,000
• In US, more common in whites than African Americans or Asians
• Predominant age: 15–25 years, 2nd smaller peak in ages 55–65 years
• Female > Male
• 2–4 × increased risk in Ashkenazi Jewish ethnicity

Risk Factors

Cigarette smoking (2 × higher risk in smokers)

Genetics
15% of patients have 1st-degree relatives with inflammatory bowel disease, and develop the disease with similar patterns and similar age of onset.

Pathophysiology

• Segmental disease with patchy distribution and variable severity
• Strictures commonly present and occasionally prevent passage of the endoscope
• Apthous ulcers found on mucosal surfaces
• Histologic features: Transmural inflammation, crypt abcesses, noncaseating granulomas

Etiology

• Combination of genetic factors, environmental factors, and immunologic abnormalities:
  • IBD locus on chromosome 16 - CARD15/NOD2, also on chromosomes 5q, 6p, and 19
• Idiopathic, immune-mediated Th-1 cells organize cell-mediated response, which involves tumor necrosis factor, interferon, and interleukin 12

Commonly Associated Conditions

• Arthritis similar to rheumatoid and spondylitis
• Variety of skin lesions, erythema nodosum, nonspecific rashes, pyoderma gangrenosum
• Uveal tract disease rare but related
• Gallstones increased with ileal disease; sclerosing cholangitis in ~ 10%
• Increased risk of both colorectal cancer and small bowel cancer: RR = 5–20 (1)