Appendicitis
- The appendix is a small dead-end tube off the cecum; appendicitis is what happens when its single exit gets plugged and it can't drain.
- On CT the classic combo is a dilated, fluid-filled appendix with a thick enhancing wall and angry, dirty-looking fat around it.
- An appendicolith (a little calcified plug) is a helpful clue, but it doesn't have to be there.
- CT is the workhorse; ultrasound is first-line in kids and pregnancy to dodge radiation; MRI is the backup, especially in pregnancy.
- The thing you're hunting for is perforation: free fluid, free air, or a walled-off abscess means the wall has given way.
Imagine a balloon with exactly one opening, and someone ties a knot in that opening. The balloon keeps making fluid on the inside, nothing can get out, and pressure climbs until the whole thing is tense, swollen, and miserable. That, in one sentence, is appendicitis. Everything else on this page is just how we see that tied-off balloon on a scan, and how worried we should be about it bursting.
What the appendix actually is
The appendix is a narrow, blind-ended tube that hangs off the cecum (the first part of the colon). "Blind-ended" is the whole problem: it has one way in and no way out. When that opening gets blocked, secretions back up, pressure rises, the wall swells, and bacteria throw a party. Block the drain on a sink and it overflows; block the appendix and it can't overflow, so it just inflates and gets inflamed.
What does the blocking? Often a little hardened plug of stool called a fecalith or appendicolith (think a tiny calcified pebble). Sometimes it's lymphoid tissue swelling up, sometimes a tumor in adults, and honestly sometimes we never find the culprit. This usually shows up as part of the acute abdomen — the classic right-lower-quadrant pain that started near the belly button and migrated.
How we image it
For most adults, the answer is a contrast-enhanced abdominal CT. It's fast, it's reliable, and it finds the appendix even when it's hiding behind loops of bowel. The trade-off is radiation, which matters most in young patients.
So the modality you pick depends a lot on who's in front of you:
| Patient | First choice | Why |
|---|---|---|
| Average adult | CT with contrast | Fast, accurate, sees complications. |
| Child | Ultrasound first | Avoids radiation; kids have less fat to obscure the appendix. |
| Pregnant patient | Ultrasound, then MRI | No ionizing radiation; MRI when ultrasound can't see it. |
Ultrasound uses a "graded compression" trick: the sonographer gently presses to squish away normal, gassy bowel. Normal bowel flattens and slides out of the way. An inflamed appendix won't compress — it's tense and tender, like poking a bruise, so it stays put under the probe.
The radiation point is not trivial in young people; it's the whole reason we reach for ultrasound and MRI first. If you want the bigger picture on that, see pregnancy and pediatric dose.
What you're looking for on the scan
Here's the mental checklist. A normal appendix is a thin, collapsible noodle. An inflamed one is the opposite of all those things:
- Dilated. It's swollen and fatter than it should be.
- Wall thickening and enhancement. The wall gets thick and lights up brightly with contrast — inflammation means more blood flow.
- Fat stranding. The fat around it stops looking clean and black and goes hazy and streaky, like someone smudged it with a thumb. This "dirty fat" is the body's inflammation bleeding into the neighborhood.
- Appendicolith. That calcified plug, bright white on CT. Nice to see, not required.
- Fluid. A little free fluid nearby is common.
The single most useful move is simply finding the appendix and following it to its tip. A normal appendix that's thin, collapsed, and surrounded by clean fat essentially rules appendicitis out. Half the battle is just tracing the whole tube.
When it has burst (or is about to)
The reason we care so much is perforation — the balloon popping. Once the wall gives way, the picture changes, and so does the surgery. Look for:
- A focal wall defect — a break in the enhancing wall.
- Free air outside the bowel (always abnormal; see pneumoperitoneum).
- An abscess — a walled-off pocket of pus, often with a rim that enhances.
- More extensive fat stranding and free fluid — the mess has spread.
Perforated and non-perforated appendicitis can be managed very differently — sometimes drainage and antibiotics first rather than straight to the operating room. So "is it perforated?" isn't a footnote; it changes the plan.
The mimics that fool you
Right-lower-quadrant pain has a long list of imposters, and the appendix isn't always the villain. A few classic traps:
Right-sided diverticulitis, inflamed bowel from inflammatory bowel disease, ovarian and tubal problems in women, and even a normal appendix sitting next to an unrelated inflamed structure can all masquerade as appendicitis. The cleanest tiebreaker: actually identify the appendix. If it's normal, go hunting elsewhere instead of blaming it by association.
The neighboring entity worth knowing is diverticulitis — same idea (a small outpouching gets blocked and inflamed), just a different pouch in a different part of the colon. And distended bowel loops in the area can raise the question of bowel obstruction, which has its own look entirely.
The one thing to remember
Appendicitis is a plugged, dead-end tube that can't drain, swells, and eventually bursts. On imaging: a fat, bright-walled appendix sitting in dirty, stranded fat. Find the appendix, follow it to the tip, and then ask the only question that changes the surgery — has it perforated yet?