70: Inflammatory Bowel Disease

Key Points

• Disease summary:

  • The inflammatory bowel diseases (IBDs) are prototypic complex genetic disorders, involving multiple interacting genetic loci and environmental factors that trigger disease.

  • IBD is comprised of two inter-related disorders, Crohn disease (CD), and ulcerative colitis (UC), as well as indeterminate colitis (IC). IC describes the 5% to 10% of IBD involving only the colon and is neither classic CD nor UC.

  • Clinical presentations of UC and CD overlap; both present with diarrhea, bloody diarrhea, abdominal pain, most extraintestinal manifestations, weight loss, and fever.

  • Anatomic involvement: CD can involve any portion of the gastrointestinal (GI) tract, most commonly the terminal ileum (5%-75%) and colon (20%-50%). CD causes transmural inflammation in all four layers of bowel wall; extension through the bowel wall can cause fistulae, abscesses, or perianal complications. Strictures secondary to fibrosis are relatively common. UC is localized to varying lengths of continuous inflammation proceeding proximally from the rectum; a discontinuous cecal patch may be present. UC is limited to mucosal and submucosal inflammation, and does not cause perianal or intra-abdominal disease; strictures are much less common than in CD.

• Differential diagnosis:

  • Infectious colitis (bacterial, viral, protozoal), pseudomembranous colitis (Clostridium difficile toxin), and ischemic colitis

• Monogenic forms:

  • No single gene cause of IBD is known to exist except perhaps for a rare form of CD (see IL-10 pathway later).

• Family history:

  • An affected first-degree relative confers a relative risk of 30 to 40 for CD and 10 to 20 for UC. Overlapping risk occurs since first-degree relatives of patients with CD have relative risk of 3.9 for developing UC.

• Twin studies:

  • Monozygotic twins have a 20% to 50% concordance rate in CD, much lower in UC.

• Environmental factors:

  • Luminal microbes or microbial products are implicated as “environmental” triggers for IBD.

• Genome-wide associations:

  • Many associations exist. Disease-associated genetic variants (single-nucleotide polymorphisms [SNPs]) provide insight into disease pathogenesis; testing for SNPs is not yet clinically validated to diagnose or guide management of IBD.

• Pharmacogenomics:

  • Testing for common thiopurine S-methyltransferase (TPMT) variants to guide management has proven validity in some clinical circumstances.

Diagnostic Criteria and Clinical Characteristics

Diagnostic Criteria for IBD

Diagnostic evaluation should include

• Stool studies including fecal leukocytes, culture for appropriate bacterial and parasitic cultures, assays for C difficile toxin, appropriate tests for possible viral etiologies (cytomegalovirus) and Mycobacterium tuberculosis.

• Colonoscopy with biopsy and ileum cannulation. Granulomas are present on endoscopic biopsies in approximately 10% of CD, crypt abscesses are indicative of UC.

• Serologic markers may be helpful; anti-Saccharomyces cerevisiae antibodies (ASCA) and antineutrophil cytoplasmic antibodies with perinuclear staining (p-ANCA) have high sensitivity for IBD in combination compared to other causes of intestinal inflammation, but sensitivity varies for different ethnic groups (much lower, eg, in regions endemic for tuberculosis). ASCA shows high specificity for CD, and p-ANCA for UC, although patients with CD limited to the colon ...