Plain film: dilated loops and air-fluid levels · tap to expand
CT: adhesive SBO, dilated above a transition point · tap to expand
CT: SBO from an incarcerated femoral hernia (82F) · tap to expand
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Small Bowel Obstruction
A pipe gets clogged, the gut backs up, and fluid floods the wrong way. Most of the time you wait it out with bowel rest. Miss the loop that is dying, and the patient dies with it.
A 58-year-old woman who had a hysterectomy years ago arrives with 2 days of crampy belly pain that comes in waves, repeated green (bilious) vomiting, and a swollen, drum-tight abdomen. She has not passed gas or stool since yesterday. You set your stethoscope down and hear high-pitched, tinkling rushes. She has no fever, and the belly is soft and non-tender.
What is going on, and what is your first move?
Gastroenteritis: give oral fluids and discharge
Adhesive small bowel obstruction: make her NPO and place a nasogastric tube
Large bowel obstruction: rush to colonoscopy
Paralytic ileus: restart her diet and observe
How It Breaks
The Clogged Pipe
Small bowel obstruction is a plumbing problem. Something blocks the pipe, the contents pile up behind it, and the backed-up pressure starts wrecking the wall. Follow the chain and the whole disease falls out of it.
→Something blocks the lumen (a band, a hernia, a tumor, a stone). Gas and fluid cannot get past.
→Bowel upstream of the block dilates and the gut cranks up peristalsis to push past it. That fight is the crampy, colicky pain that comes in waves.
→Backed-up contents have nowhere to go, so they come back out the top: bilious vomiting early (proximal), or feculent vomiting late (distal).
→The distended wall stops absorbing and starts secreting. Litres of fluid pour into the lumen and get vomited away: this is third-spacingⓘThird-spacing means fluid leaves the bloodstream and gets trapped where it does no good (here, the gut lumen). The patient looks dry, tachycardic, and oliguric even though their belly is full of fluid. and it drives hypovolemia, tachycardia, and a contraction (hypokalemic, hypochloremic) metabolic alkalosis.
→Now the dangerous fork: if wall pressure keeps climbing, it squeezes the veins shut first. Blood gets in but cannot leave, the loop swells, congests, and the arterial supply finally chokes off. That is strangulation, and the bowel begins to die.
Key Fact
Simple obstruction blocks flow but blood supply is intact. Strangulated obstruction has lost its blood supply and is becoming ischemic. The entire management question is really one question: is this loop still alive?
Tap each card. Front is the culprit. Back is why the board cares.
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Adhesions
Scar bands from old surgery
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Adhesions
#1 cause overall of SBO in adults.
Needs a prior insult: previous abdominal or pelvic surgery, or old peritonitis.
Scar tissue forms a band the bowel kinks over.
Most resolve with NPO + NG decompression.
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Hernia
A loop trapped in a wall defect
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Hernia
#1 cause when there is NO surgical history.
Bowel slips into a defect and gets stuck (incarcerated), then loses blood supply (strangulated).
Always examine the groin: inguinal and femoral.
Femoral hernia (below and lateral to the pubic tubercle) is classic in older women and strangulates often.
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Malignancy
A tumor narrowing the lumen
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Malignancy
Tumor narrows the lumen or seeds the peritoneum (carcinomatosis).
In adults, can act as a lead point for intussusception.
Think of it in an older patient with weight loss and no surgery or hernia.
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Crohn Stricture
Inflammation narrows the pipe
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Crohn Stricture
Transmural inflammation scars the wall into a narrow stricture.
Classic in a young patient with known Crohn disease.
Often the terminal ileum.
Acute flares can also cause a functional block from edema.
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Gallstone Ileus
A stone plugs the ileum
tap to flip
Gallstone Ileus
Big stone erodes a cholecystoenteric fistula, then impacts at the ileocecal valve.
Rigler triad: SBO + pneumobilia (air in the biliary tree) + an ectopic stone.
Elderly patient with longstanding gallstones.
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Intussusception
Bowel telescopes into itself
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Intussusception
One segment telescopes into the next; CT shows a target or sausage sign.
In children: usually idiopathic, reduced with an air-contrast enema.
In adults: assume a pathologic lead point (tumor) and resect.
Board Trap
The single most tested fact: adhesions are #1 overall, but in a patient who has never had surgery, the answer is a hernia. The stem hands you the discriminator on purpose. Surgical scar in the history = adhesions. Clean abdomen with a groin bulge = hernia. Always check the groin.
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At the Bedside
Four Cardinal Signs
Every classic SBO stem is built from the same four findings. Learn the quartet and the diagnosis announces itself before you ever order a film.
1Crampy, colicky pain in waves: the bowel straining against the block. Pain that turns constant and focal is the alarm for strangulation.
2Vomiting:bilious and early if the block is proximal, feculent and late if it is distal (contents sit and ferment).
3Distension: a swollen, tympanitic (drum-like) belly. The more distal the block, the bigger the belly.
4Obstipation: no stool AND no gas. This is the hallmark of a complete obstruction; a partial block may still pass some flatus.
The Sound Clue
Early SBO gives high-pitched, tinkling, hyperactive bowel soundsⓘThe gut is fighting hard against the block, so you hear loud, musical, tinkling rushes. As the bowel tires out and dies, sounds fade to silent. Ileus is silent from the start. as the gut fights the block. A silent abdomenAbsent bowel sounds late in obstruction can mean the bowel has tired out or infarcted. From the start, silence points to ileus instead. means either a lazy paralytic ileus, or a late, exhausted, possibly dying bowel.
The three look-alikes every stem tries to blur together:
Feature
Small Bowel Obstruction
Paralytic Ileus
Large Bowel Obstruction
Mechanism
Mechanical block in the small gut
Gut is paralyzed, nothing blocking
Mechanical block in the colon
Pain
Crampy, colicky, in waves
Mild, diffuse, not colicky
Crampy, lower, more gradual
Vomiting
Early, bilious (or feculent if distal)
Variable, often little
Late or absent
Bowel sounds
High-pitched, tinkling early
Absent or hypoactive
Variable
Common trigger
Adhesions, hernia
Recent surgery, opioids, low K+, sepsis
Cancer, volvulus, diverticular stricture
Film
Dilated central small bowel, air-fluid levels, little colon gas
Diffuse dilation of small bowel AND colon, gas to the rectum
Dilated peripheral colon with a distal cutoff
Board Trap
A post-op patient with a distended, quiet belly and no crampy pain is ileus, not SBO: do not rush to the OR. Fix the potassium, stop the opioids, and wait. The crampy, tinkling, mechanical picture is what earns a tube and a surgeon.
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Reading the Films
Confirm It and Stage It
The plain film makes the call fast and cheap. The CT tells you the cause, the level, and the one thing that changes everything: whether the bowel is being strangled.
→Upright / supine abdominal film first. The SBO triad: dilated central loops > 3 cm, stacked air-fluid levels (the stepladder), and little or no colonic gas because gas cannot pass the block.
→Small bowel shows valvulae conniventesFolds that cross the FULL width of the small bowel lumen. Colonic haustra only cross part way. This is how you tell small bowel from colon on a film. crossing the whole lumen; colon shows partial haustra. That is how you know it is small bowel.
→CT abdomen and pelvis with contrast is the workhorse: it finds the transition point (dilated above, collapsed below), names the cause, and flags ischemia (bowel wall thickening, poor enhancement, mesenteric edema, a closed loop).
→Free air under the diaphragm means perforation: stop reading and call surgery.
Board Trap
Reach for water-soluble (Gastrografin) contrast, not barium, when perforation is on the table. Barium that leaks into the peritoneum causes a brutal chemical peritonitis and can solidify proximal to a block. Water-soluble contrast is safe and, in partial SBO, doubles as treatment.
Walk the read. Pick before you peek.
Upright film: dilated central loops, stacked air-fluid levels, almost no gas in the colon. What is it?
SBO. The gas stops at the blockage, so the colon is empty while the small bowel above it balloons up with stacked air-fluid levels. Gas everywhere including the colon would be ileus. Empty colon plus stepladder levels is never normal.
Different patient, post-op day 3: the small bowel AND the colon are both diffusely dilated, with gas all the way down to the rectum. No transition point. What is it?
Ileus. Nothing is mechanically blocking, so gas spreads through the whole gut and reaches the rectum. A mechanical block would create a cutoff with an empty bowel beyond it. Treat the ileus: correct electrolytes, stop opioids, wait.
SBO confirmed on film, patient is stable. You need the cause, the level, and a look for ischemia. Best next test?
CT with contrast. It localizes the transition point, names the cause, and shows the strangulation findings a plain film cannot. Barium risks chemical peritonitis if there is perforation; colonoscopy looks at the colon, the wrong organ for a small bowel block.
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The Plan
Drip, Suck, and Decide
Most adhesive partial obstructions resolve with bowel rest. The job is to support the patient, watch like a hawk, and recognize the exact moment the case turns surgical.
→NPO. Nothing by mouth: stop adding to the traffic jam.
→NG tube to suction. Decompress the dilated proximal bowel: this relieves vomiting, drops the distension, and lowers wall pressure. Patients call it the worst and best part of the admission.
→IV fluids to replace the litres lost to third-spacing and vomiting, and correct electrolytes (the classic hypokalemic, hypochloremic metabolic alkalosis).
→Serial abdominal exams and labs. You are hunting one thing: the first sign the bowel is dying.
Operate Now When
Any of these cancels the waiting game and sends the patient to the OR: strangulation signs (fever, tachycardia, constant focal pain, peritonitis, rising lactate and leukocytosis), a closed-loop obstruction, a complete obstruction that will not resolve, or free air. A hernia that will not reduce also goes to surgery.
Run the management call. Commit, then reveal.
SBO confirmed. The patient spikes a fever, goes tachycardic, develops constant focal tenderness with guarding, and lactate is rising. Next step?
Operate. This is strangulation: the trapped loop has lost its blood supply and is becoming necrotic. Waiting or imaging-shopping lets it perforate. Antibiotics are an adjunct, not a substitute for removing dead bowel.
Different patient: stable partial SBO, passing a little flatus, benign abdomen, normal lactate, no fever. Initial management?
Bowel rest. A stable partial SBO without strangulation signs is managed nonoperatively first; the majority resolve. You only cut for strangulation, closed loop, complete non-resolving obstruction, or failure of the trial.
That partial-SBO patient still has high NG output, persistent distension, and no improvement after 60 hours, but the abdomen stays benign. Next step?
Contrast challenge. Past the 48 to 72 hour window with no progress, a water-soluble study is diagnostic and therapeutic: if the hyperosmolar contrast reaches the colon within about 24 hours, the obstruction is likely to resolve; if it does not, that predicts failure and the patient needs surgical exploration. Open-ended waiting just risks occult strangulation.
Punch Line
Small bowel obstruction is a waiting game with one trap door. Drip (fluids), suck (NG), and watch. The instant the bowel shows it is dying, or a partial block refuses to budge past 48 to 72 hours, the waiting is over.
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Prove It
Board Walkthrough
Original clinical vignettes, 5 dealt per round, answer choices shuffled, never-repeat within a round. Tap a wrong answer first to see why it almost works, then read the glowing clues.
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Medically reviewed by Kaitlyn Cocuzzo, MD and Fatima Ali, DO · Last updated July 1, 2026 at 10:03 PM ET
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