Small Bowel
Obstruction, Crohn disease, Meckel, neuroendocrine and other small-bowel tumors, and mesenteric ischemia.
High-Yield Points
- Small bowel obstruction: adhesions = most common cause in the US (hernia worldwide and in patients without prior surgery); no prior surgery + no hernia → suspect malignancy. CT shows a transition point with distal decompression.
- Uncomplicated adhesive SBO → NGT + IV fluids + water-soluble (Gastrografin) challenge (reaching cecum predicts resolution). Signs of strangulation/closed-loop (fever, peritonitis, ↑lactate, reduced mural enhancement) → urgent operation.
- Postoperative ileus vs SBO: CT differentiates — ileus = uniform dilatation, no transition point; treat with electrolyte correction, fewer opioids, alvimopan.
- Crohn disease — transmural inflammation, skip lesions, cobblestoning, noncaseating granulomas, creeping fat; terminal ileum most common. Surgery is bowel-sparing: short fibrotic strictures → strictureplasty; resection margins only need 2 cm of normal bowel. Flare ladder: steroids → infliximab (also heals fistulas).
- Meckel diverticulum — rule of 2s, true diverticulum (vitelline duct), painless lower-GI bleed in a child from ectopic gastric mucosa → technetium-99m pertechnetate (Meckel) scan.
- Adult intussusception = pathologic (often malignant) lead point → segmental resection (do NOT reduce; unlike pediatric, which is reduced by enema).
- Carcinoid — appendix/ileum; carcinoid syndrome (flushing, diarrhea, right-sided/tricuspid valvular disease) requires liver metastases (bypasses hepatic clearance). 24-h urinary 5-HIAA = gold standard; chromogranin A screens; octreotide scan localizes. Appendiceal: <1 cm → appendectomy; ≥2 cm → right hemicolectomy.
- Acute mesenteric ischemia — pain out of proportion; CT angiography first. Embolus (AF, distal to SMA origin) → embolectomy; thrombosis (atherosclerotic origin) → bypass; venous → heparin; NOMI (low-output/pressors) → intra-arterial papaverine + restore perfusion. Peritonitis → laparotomy + resect.
- Short bowel syndrome (<180 cm) → early enteral feeding (drives adaptation) + PPI for hypergastrinemia; judge permanent TPN after 1–2 years. Terminal-ileum resection → B12 deficiency, bile-salt diarrhea, gallstones, calcium oxalate kidney stones.
- Small-bowel neoplasms: adenocarcinoma most common malignant (periampullary duodenum) → carcinoid → lymphoma → GIST least. GIST = CD117/c-kit positive; high-risk (>5 cm or >5 mitoses/50 hpf) → imatinib. Celiac disease → T-cell lymphoma. Peutz–Jeghers (AD, STK11) — mucocutaneous pigmentation + hamartomas + intussusception.
Key Tables / Differentials
| Mesenteric ischemia | Setting | CTA clue | Treatment |
|---|---|---|---|
| Embolic | Atrial fibrillation | Cutoff distal to SMA origin | Operative embolectomy |
| Thrombotic | Atherosclerosis | Calcified origin occlusion | Aorto-SMA bypass |
| Venous | Hypercoagulable | No SMV/portal filling | Heparin (anticoagulation) |
| NOMI | Low output, pressors | Diffuse vasospasm | Intra-arterial papaverine |
| Crohn | Ulcerative colitis | |
|---|---|---|
| Pattern | Skip lesions, mouth→anus | Continuous, rectum up |
| Depth | Transmural, granulomas | Mucosa/submucosa |
| Classic | Fistula, creeping fat, cobblestone | Lead-pipe colon, toxic megacolon |
| Surgery | Bowel-sparing (strictureplasty) | Curative proctocolectomy |
Board Pearls
- "Pain out of proportion to exam" → acute mesenteric ischemia → CTA.
- "Target/sausage sign in an adult" → intussusception with a tumor lead point → resect.
- "Necrolytic"… (that's glucagonoma — pancreas); here: flushing + diarrhea + tricuspid regurgitation → metastatic carcinoid.
- Glutamine = primary fuel of enterocytes; terminal ileum absorbs B12, bile acids, folate.
Classic Vignette Triggers
- "Prior abdominal surgery + crampy pain + bilious vomiting + transition point" → adhesive SBO.
- "Child, painless melena, normal abdomen" → Meckel → pertechnetate scan.
- "Celiac patient, new small-bowel mass, weight loss" → enteropathy-associated T-cell lymphoma.
- "Spindle cells, CD117/c-kit positive, gastric or jejunal mass" → GIST → imatinib if high-risk.
- "Lip/buccal pigmentation + recurrent intussusception" → Peutz–Jeghers (STK11).