Crohn’s disease

 Crohn’s disease is a chronic inflammatory GI disease characterized by transmural granulomatous inflammation. It  may affect any part  of the gut, but favors the terminal ileum and proximal colon. Unlike UC, there may be unaffected bowel between areas of active disease (skip lesions).

Cause:   Likely multifactorial etiology involving interplay of environmental factors, genetic susceptibility, and immune system dysregulation. Mutations of the NOD2/CARD15 gene increases risk autosomal recessive.

Prevalence: 319/100,000. Active smoking is associated with worse outcomes Smoking increases risk 3–4-fold.  

Symptoms and signs  
Diarrhea, abdominal pain, and weight loss are common. Fever, malaise, anorexia occur with active disease. Look for aphthous ulceration, abdominal tenderness, right iliac fossa mass, perianal abscesses/fistulae/skin tags, anal/rectal strictures.
Extraintestinal signs:   Clubbing, erythema nodosum, pyoderma gangrenosum, conjunctivitis, episcleritis, iritis, large joint arthritis, sacroiliitis, ankylosing spondylitis, fatty liver, primary sclerosing cholangitis, cholangiocarcinoma (rare), renal stones, osteomalacia, malnutrition, amyloidosis.

Complications  
Small intestinal obstruction; toxic dilatation (colonic diameter >6 cm); abscess formation (abdominal, pelvic, or ischiorectal); fistulae, e.g., colovesical (bladder), colovaginal, perianal, enterocutaneous; perforation; rectal hemorrhage; colonic carcinoma.

Tests  
Blood:   CBC, ESR, CRP, BMP, LFT, blood culture. Serum iron, B 12 , and red cell folate if anemia.

Markers of activity:  Hb ↓ ;  ↑ ESR;  ↑ CRP;  ↑ WBC; ↓ albumin, fecal calprotectin, fecal lactoferrin.
Stool microscopy/culture and Clostridium difficile toxin (cdt)   to exclude infectious diarrhea ( C. difficile, Salmonella, Shigella, Campylobacter, E. coli ).
Colonoscopy and biopsy   should be performed even when the mucosa is macroscopically normal (20% have microscopic granulomas).
Small bowel imaging:   To detect proximal and distal small bowel disease (strictures, small bowel dilatation, inflammatory mass, abscess or fistula). Studies include small bowel follow-through, CT  enterography,  MR enterography.  
Barium enema:   may show “cobblestoning’, “rose thorn” ulcers, and colonic strictures with rectal sparing.