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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
Key points
- Most common cause in adults: postoperative adhesions; always check for hernias
- Classic: colicky abdominal pain, vomiting, distension, obstipation
- CT abdomen/pelvis identifies transition point, cause, closed-loop obstruction, ischemia, or perforation
- Initial management: NPO, IV fluids, electrolyte correction, NG decompression, surgical consultation
- Peritonitis, fever, leukocytosis, lactic acidosis, continuous pain, or closed-loop obstruction suggests strangulation
Overview
SBO occurs when intestinal contents cannot pass through the small intestine. Causes include adhesions, hernias, malignancy, Crohn strictures, volvulus, intussusception, and gallstone ileus. Strangulation compromises mesenteric blood flow and can rapidly progress to necrosis and perforation.
Epidemiology
Adhesions from prior abdominal surgery cause most SBO in the United States. Hernias are important without prior surgery. Crohn disease causes inflammatory or fibrotic strictures. Gallstone ileus occurs in older adults after cholecystoenteric fistula.
Clinical Features
Symptoms
Colicky crampy abdominal pain
Bilious vomiting, earlier in proximal SBO
Abdominal distension
Obstipation or inability to pass flatus
Constant severe pain, fever, or syncope suggests strangulation
Signs
High-pitched bowel sounds early
Abdominal distension and tympany
Hernia on groin/abdominal wall exam
Peritonitis, fever, tachycardia, hypotension
Investigations
First-line
CBC, CMP, lactateLeukocytosis, alkalosis, AKI, lactate if ischemia
Abdominal/hernia examDo not miss incarcerated hernia
Pregnancy testBefore imaging in reproductive-age patients
Second-line
CT abdomen/pelvis with IV contrastBest test; transition point and ischemia signs
Abdominal X-rayDilated loops and air-fluid levels but less accurate
Specialist
Water-soluble contrast challengePredicts nonoperative resolution in adhesive SBO
SurgeryStrangulation, peritonitis, closed-loop, ischemia, perforation, or failed conservative therapy
1
Initial management
- NPO, IV isotonic fluids, electrolyte correction
- NG tube for significant vomiting/distension
- Analgesia, antiemetics, early surgical consultation
2
Nonoperative
- Many adhesive partial SBOs without ischemia can be observed
- Serial exams/labs/urine output
- Water-soluble contrast if stable
3
Operative
- Immediate surgery for peritonitis/strangulation/closed-loop/ischemia/perforation
- Resect nonviable bowel and correct cause
- Crohn obstruction may need steroids if inflammatory or surgery/dilation if fibrotic
Complications
- Strangulation: Ischemia, necrosis, lactic acidosis.
- Perforation: Peritonitis and sepsis.
- Aspiration: Vomiting and gastric distension.
- AKI/electrolyte derangement: Third spacing and vomiting.
USMLE Step 2 CK Exam Tips
- 1Prior abdominal surgery + SBO = adhesions
- 2No prior surgery + SBO = look for hernia or malignancy
- 3Do not give oral laxatives in mechanical SBO
- 4Continuous pain, fever, leukocytosis, lactate, or peritonitis = strangulation
- 5Gallstone ileus: pneumobilia + SBO + ectopic stone
- 6Crohn can cause SBO from terminal ileal stricture
- 7NG decompression is supportive; surgery depends on cause and ischemia risk
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