Bowel Obstruction
- Bowel obstruction is a traffic jam in the gut: something blocks the tube, so fluid and gas pile up behind the blockage and the bowel swells.
- The key questions are always: small bowel or large bowel? Where's the blockage (the "transition point")? And is the bowel still alive?
- Dilated loops with air–fluid levels are the classic sign; CT usually finds the transition point and the cause.
- The scary version is a closed loop or signs the bowel is dying — those buy a fast-track to the operating room.
Imagine a hose with a kink in it. Water keeps coming from the tap, but it can't get past the kink, so the hose upstream balloons out and the part downstream goes flat and empty. That's a bowel obstruction in one sentence. The gut is a long muscular tube that's supposed to push everything one direction, and when something stops the parade, the backup is exactly what you'd predict.
Our job on imaging is rarely "is there an obstruction?" alone. It's a checklist: which bowel, where's the blockage, what's causing it, and — the one that ruins everyone's afternoon — is the trapped bowel still healthy?
Small bowel vs large bowel: read the wallpaper
The first fork in the road is small bowel obstruction (SBO) versus large bowel obstruction (LBO), because they have totally different culprits and you tell them apart by the pattern on the wall of the bowel and where the loops sit.
Small bowel has thin folds that march all the way across the lumen like the rings on a worm — radiologists call these valvulae conniventes. Large bowel has thicker, incomplete pouches (haustra) that only reach partway across. Small bowel tends to live centrally; the colon frames the edges of the abdomen.
| Feature | Small bowel | Large bowel |
|---|---|---|
| Wall folds | Thin, cross the whole lumen | Thick, partial (haustra) |
| Usual location | Central abdomen | Around the periphery |
| Caliber that's "too big" | Loosely, above ~3 cm | Loosely, above ~6 cm (cecum higher) |
| Common causes | Adhesions, hernias | Tumor, volvulus, stricture |
Those size numbers are rules of thumb, not magic thresholds — real bowel is fuzzy, and a borderline loop in a sick patient matters more than a millimeter.
The transition point: find the kink
The single most useful thing to locate is the transition point — the spot where fat, swollen bowel suddenly becomes flat, decompressed bowel. That abrupt change is the blockage. Upstream the loops are dilated and fluid-filled; downstream they're collapsed and empty. Walk the bowel until the caliber drops off a cliff, and you've found the scene of the crime.
On a plain film you can often suspect obstruction from dilated loops and air–fluid levels (gas floating on fluid, like a half-finished soda left on the counter). But the plain film is a blunt instrument — start with reading the abdominal radiograph to get the gestalt, then lean on the abdominal CT, which usually nails the transition point and the cause.
The most common cause of small bowel obstruction is adhesions — scar-tissue bands from prior surgery that tether and pinch the bowel. The tell on CT is a transition point with nothing else there to explain it: no mass, no hernia, just an abrupt nip. The second classic cause is a hernia, so always check the groin and the abdominal wall — a loop of bowel squeezed through a small defect is a transition point hiding in plain sight.
Is the bowel still alive? The part that actually matters
Most obstructions are simple: blocked, swollen, uncomfortable, but the bowel is still getting blood. Those can often be managed without rushing to surgery. The emergency is when the blockage also strangles the blood supply — then the bowel starts to die, which marches toward bowel ischemia and perforation.
The most feared setup is a closed-loop obstruction: a segment pinched shut at both ends, like tying off both ends of a balloon and then squeezing. There's nowhere for the pressure to vent, the loop swells fast, and its blood supply gets throttled. On CT it often looks like a tense, fluid-filled C- or U-shaped loop with the vessels converging toward a single twist point.
Signs that bowel is in trouble — and that the patient may need the operating room soon — include a closed loop, a bowel wall that stops enhancing with contrast (it's not getting blood), wall thickening, surrounding fat stranding and fluid, and gas in the bowel wall (pneumatosis). These are the findings you call the surgeon about, not the ones you tuck into the bottom of the report.
Don't confuse obstruction (a mechanical block) with ileus (the bowel just stops squeezing — no block). Classic ileus dilates everything, small and large bowel alike, with no transition point — often after surgery or with electrolyte trouble. Obstruction has a kink and a flat segment beyond it; ileus is uniformly lazy bowel from end to end.
Also: free air under the diaphragm means a hole somewhere — that's pneumoperitoneum, a different alarm bell that can accompany a late, perforated obstruction.
Large bowel obstruction has its own villains
When the colon is the one backing up, think differently. The two headliners are an obstructing tumor (a cancer slowly walling off the lumen) and a volvulus — a loop of colon that twists on itself like a balloon animal, most famously in the sigmoid colon. Volvulus has a memorable plain-film look: a hugely dilated loop ballooning up out of the pelvis.
There's a catch unique to the colon. If the valve between small and large bowel holds firm, gas can't escape backward, so pressure builds in a closed system and the cecum (the floppy, thin-walled start of the colon) blows up like the weakest part of an over-inflated tire. A tense, ballooning cecum is the one that's about to rupture, so its size earns special attention.
Three questions, every time: small or large bowel, where's the transition point, and is the bowel still alive? Nail those and you've said everything that changes what happens to the patient.
If you remember nothing else: a bowel obstruction is a kinked hose, the transition point marks the kink, and the only true emergency is a hose that's also losing its blood supply. Find the block, then ask whether the bowel beyond it is suffering — that second question is the whole reason we look so carefully.