Small bowel obstruction (SBO) is one of those “don’t miss it” GI topics that shows up everywhere on Step exams: surgery-style vignettes, electrolyte/acid–base twists, and imaging interpretation. The good news: SBO follows a predictable script—mechanical blockage → proximal dilation → vomiting + dehydration → ischemia risk—and if you know the patterns (adhesions vs hernia vs tumor, early vs late findings), you can reliably pick the right diagnosis and next step.
Quick Definition (Step-ready)
Small bowel obstruction = mechanical blockage of the small intestine that prevents passage of intestinal contents.
- Most common causes (adult):
- Adhesions (postoperative) — #1 overall
- Hernias (incarcerated/strangulated)
- Tumors (less common than large bowel obstruction, but still tested)
- Other HY causes:
- Crohn disease strictures (terminal ileum)
- Gallstone ileus (pneumobilia + SBO)
- Intussusception (more pediatric, but can occur in adults—often malignancy lead point)
- Volvulus (classically large bowel, but can involve small bowel)
Core distinction: SBO is mechanical. (Differentiate from ileus, which is functional paralysis.)
Pathophysiology (Why the symptoms happen)
Think of SBO in three linked processes:
1) Proximal dilation + hyperperistalsis
- Bowel proximal to obstruction dilates with:
- swallowed air
- GI secretions
- bacterial gas
- Early on, bowel tries to push past the blockage → crampy, colicky abdominal pain and high-pitched “tinkling” bowel sounds.
2) Fluid shifts → hypovolemia + electrolyte derangements
- Fluid gets “third-spaced” into the bowel lumen and bowel wall → hypovolemia
- Vomiting causes:
- Hypochloremic, hypokalemic metabolic alkalosis (especially with proximal SBO)
- Late/severe SBO can progress to metabolic acidosis if bowel ischemia develops (lactate).
3) Strangulation risk (the danger zone)
If the obstruction compromises blood flow → ischemia → necrosis → perforation → sepsis.
Strangulation is more likely with:
- Closed-loop obstruction (e.g., volvulus, hernia trapping a loop)
- Incarcerated hernia
- Severe/continuous pain and systemic toxicity
Clinical Presentation (Classic vignette cues)
Symptoms
- Abdominal pain
- Early: crampy, intermittent (peristaltic waves)
- Concerning: constant, severe (ischemia/strangulation)
- Vomiting
- Early and prominent in proximal SBO
- Later in distal SBO
- Distension
- More prominent in distal SBO
- Obstipation (no stool or flatus) suggests complete obstruction (late finding)
Exam
- High-pitched bowel sounds early; can become hypoactive late
- Dehydration signs: tachycardia, orthostasis, dry mucosa
- Look for scars (adhesions), palpate hernias (inguinal/femoral/umbilical)
- Peritoneal signs (guarding/rebound) + fever = worry for perforation/ischemia
Diagnosis (What to do, what you’ll see)
Step 1: Labs (supportive, risk stratification)
Not diagnostic, but helps assess severity:
- CMP: hypokalemia, hypochloremia, AKI from dehydration
- CBC: leukocytosis (can be stress or infection)
- Lactate: rising suggests ischemia
Step 2: Imaging (high yield)
CT abdomen/pelvis with IV contrast is the most useful test in most stable adults.
Key imaging findings
| Modality | SBO Findings | HY Notes |
|---|---|---|
| Abdominal X-ray | Dilated small bowel loops + air–fluid levels | Quick screen; less sensitive than CT |
| CT A/P | Transition point, proximal dilation, distal collapse | Best for cause + complications |
| Contrast study (water-soluble) | Can be diagnostic and sometimes therapeutic | Often used in adhesive SBO algorithms |
How to tell small vs large bowel on imaging
- Small bowel: central; valvulae conniventes traverse the entire lumen (“stack of coins”)
- Large bowel: peripheral; haustra do not span full lumen
“Red flag” CT findings for strangulation/ischemia
- Bowel wall thickening, pneumatosis intestinalis
- Portal venous gas
- Mesenteric edema, decreased enhancement
- Closed-loop configuration
SBO vs Ileus (very testable)
| Feature | Small Bowel Obstruction (mechanical) | Ileus (functional) |
|---|---|---|
| Pain | Colicky/crampy | Mild, diffuse discomfort |
| Bowel sounds | High-pitched early | Decreased/absent |
| X-ray | Air–fluid levels, dilated small bowel, transition | Diffuse dilation of small + large bowel, no transition point |
| Causes | Adhesions, hernia, tumor | Post-op, opioids, hypokalemia, severe illness |
Management (Step-wise + NBME logic)
Initial management (almost always first)
ABCs + resuscitate
- NPO
- IV fluids (isotonic)
- Electrolyte repletion (esp K+)
- NG tube decompression if significant vomiting/distension
- Analgesia + antiemetics
- Antibiotics if concern for strangulation/perforation (broad spectrum)
Nonoperative vs operative (the decision point)
Most uncomplicated adhesive SBO can be treated conservatively at first.
Trial of nonoperative management (common for adhesions) if:
- Stable vitals
- No peritonitis
- No ischemia signs
- No closed-loop on CT
Immediate surgery if any of the following:
- Peritonitis
- Strangulation/ischemia suspected (fever, leukocytosis, rising lactate, continuous pain)
- Closed-loop obstruction
- Incarcerated/strangulated hernia
- Failure of conservative management (clinical worsening or no improvement)
Common exam “next step” stems
- “Prior abdominal surgery + crampy pain + vomiting + high-pitched bowel sounds”
→ Adhesive SBO → NPO + IV fluids + NG tube → CT if needed/available - “Tender irreducible groin mass + SBO symptoms”
→ Incarcerated hernia → urgent surgery - “Pneumobilia + SBO + ectopic gallstone”
→ Gallstone ileus → surgery
High-Yield Etiologies & Their Associations
Adhesions (most common overall)
- History of abdominal surgery (C-section, appendectomy, hysterectomy, bowel surgery)
- Can occur years later
- Often managed initially without surgery unless complicated
Hernias (high risk for strangulation)
- Femoral hernia: classically in older women; higher strangulation risk
- Inguinal hernia: more common overall
- Always examine groin in SBO.
Crohn disease strictures
- Recurrent, subacute obstruction symptoms
- Terminal ileum involvement common
- May have weight loss, diarrhea, fistulas, aphthous ulcers
Gallstone ileus (classic triad-ish imaging clue)
- Gallstone passes through cholecystoenteric fistula → obstruction (often at ileocecal valve)
- Rigler triad:
- SBO
- Pneumobilia
- Ectopic gallstone
Intussusception
- Telescoping bowel → obstruction + ischemia risk
- Children: episodic pain + currant jelly stools, sausage-shaped mass
- Adults: often lead point (tumor)
Complications You Must Recognize
- Strangulation → ischemia/necrosis
- Perforation → peritonitis
- Sepsis
- Electrolyte disturbances and AKI from dehydration
Clinical clues suggesting ischemia/strangulation:
- Constant (non-colicky) severe pain
- Fever, tachycardia out of proportion
- Leukocytosis
- Metabolic acidosis or elevated lactate
- Peritoneal signs
First Aid Cross-References (where this lives in FA logic)
Exact page numbers vary by edition, but SBO concepts map to these First Aid buckets:
- GI Pathology → Intestinal obstruction
- Post-op complications (ileus vs mechanical obstruction)
- Acid–base disorders (vomiting → metabolic alkalosis)
- Abdominal masses/hernias (risk of incarceration/strangulation)
- Inflammatory bowel disease (Crohn strictures → SBO)
Use FA as your “trigger list,” then rely on CT findings + management algorithms to answer clinical questions.
Rapid Review (What Step wants you to pick)
- Most common cause of SBO in adults: adhesions
- Early bowel sounds: hyperactive/high-pitched
- Proximal SBO: more vomiting, less distension
- Distal SBO: more distension, later vomiting
- Vomiting acid loss: hypochloremic, hypokalemic metabolic alkalosis
- Most useful imaging in stable adult: CT with IV contrast
- Peritonitis/strangulation/closed-loop/hernia: surgery now
Mini Self-Check (1-minute practice)
-
Post-op patient with abdominal distension, minimal pain, absent bowel sounds, diffuse gas in small + large bowel
→ Ileus (supportive: NPO, electrolytes, mobilize, reduce opioids) -
Prior laparotomy patient with crampy abdominal pain, vomiting, high-pitched sounds, air–fluid levels
→ Adhesive SBO (NPO, IV fluids, NG; CT; surgery if complicated) -
SBO symptoms + fever + constant severe pain + rising lactate
→ Strangulation/ischemia (urgent surgery)