Small Bowel Obstruction
General Surgery
Illness script · General Surgery
Small Bowel Obstruction
Mechanical blockage of small intestinal lumen causing failure of distal passage of contents, most commonly from adhesions.
This illness script for Small Bowel Obstruction covers predisposing factors, classic presentation, mechanism, workup, management, and the clinical pivots that separate it from look-alikes—written for USMLE Step 1 and clerkship reasoning.
01
Predisposing factors
- Prior abdominal/pelvic surgery (adhesions) — #1 cause in developed world
- Hernias (incarcerated inguinal/femoral) — #1 cause in those without prior surgery
- Crohn disease (strictures, inflammatory mass)
- Malignancy (extrinsic compression or intraluminal tumor)
- Volvulus or intussusception (classic in pediatrics/adults with lead point)
- Age extremes, prior peritonitis, radiation history
02
Presentation
- Colicky, crampy periumbilical pain — episodic waves every 4–5 min
- Nausea and bilious vomiting — prominent and early
- Obstipation (no flatus or stool) — hallmark of complete obstruction
- Abdominal distension, high-pitched 'tinkling' or rushing bowel sounds early
- Fever, tachycardia, peritoneal signs = red flag for strangulation
- Prior surgical scar on exam is a key clue toward adhesive etiology
03
Pathophysiology
- Mechanical blockage → proximal bowel distension with gas and fluid
- Distension → increased intraluminal pressure → impaired venous outflow → edema
- Strangulation: arterial compromise → ischemia → perforation and peritonitis
- Closed-loop obstruction (two-point block) dramatically accelerates ischemia risk
04
Diagnostics
- Plain abdominal X-ray (upright + supine): dilated small bowel (>3 cm) with air-fluid levels, paucity of colonic gas — first-line
- CT abdomen/pelvis with IV contrast: gold standard — identifies transition point, closed loop, ischemia (pneumatosis)
- Transition point on CT distinguishes partial vs. complete and guides surgery
- Water-soluble contrast (Gastrografin) challenge: diagnostic and therapeutic in partial/adhesive SBO
- Labs: CBC, BMP, lactate — elevated lactate suggests ischemia/strangulation
05
Management
- NPO, IV fluids (aggressive resuscitation), NG tube decompression — initial management
- Foley catheter for urine output monitoring; serial abdominal exams
- Partial/adhesive SBO: nonoperative trial 24–48 h (Gastrografin challenge speeds resolution)
- Complete SBO, failed nonoperative trial, or any signs of strangulation → urgent operative exploration
- Strangulation/perforation/closed-loop = surgical emergency — do not delay; broad-spectrum antibiotics
06
Clinical pivots
How to separate this script from the look-alikes that show up on exams and on the wards.
Large Bowel Obstruction
Dilated colon (>6 cm) on imaging, competent ileocecal valve → no small bowel dilation; obstipation predominates over vomiting
Ileus (Paralytic)
Gas throughout entire bowel including colon/rectum uniformly; no transition point on CT; follows surgery/illness, not mechanical
Mesenteric Ischemia
Pain out of proportion to exam from the start; may have no distension; CT shows bowel wall thickening/pneumatosis without clear mechanical transition point
Incarcerated Hernia
Palpable, tender, irreducible groin or abdominal wall mass identifies the lead point — do not miss on exam
Keep reading
Full library- Sickle Cell Vaso-Occlusive CrisisAcute pain crisis in sickle cell disease caused by sickling of RBCs occluding microvasculature, causing ischemia and infarction.
- Status EpilepticusSeizure lasting ≥5 minutes OR ≥2 seizures without return to baseline consciousness between them.
Educational use only. This illness script is a study framework, not medical advice. Confirm decisions with current guidelines and your clinical supervisors.