Aortic Dissection
- A dissection is a tear in the inner wall of the aorta — blood burrows into the wall itself and splits it into two channels (a true and a false lumen).
- The thing you hunt for on a contrast CT is the intimal flap: a thin line floating inside the aorta, dividing it into two.
- Location is everything. If the tear involves the ascending aorta (Stanford type A), it's a surgical emergency. If it spares the ascending aorta (type B), it's often managed medically.
- The first-line test is a CT angiogram of the chest, abdomen, and pelvis with IV contrast. Missing this kills people fast.
- Don't confuse it with an aneurysm — that's a bulge in the wall; a dissection is a split in the wall. Different problem entirely.
Imagine your garden hose has three layers, and one day a tiny crack opens on the inside layer. Water under pressure doesn't just leak out — it sneaks between the layers and starts peeling them apart, carving a brand-new tunnel inside the wall of the hose. That, in one sentence, is an aortic dissection. The aorta hasn't burst (yet); it's been split into a hose-within-a-hose.
This is one of the great "don't-miss" diagnoses in radiology, partly because it's genuinely lethal and partly because it loves to disguise itself as a heart attack, a kidney stone, or just a bad day.
The wall has layers, and that's the whole story
The aorta has three layers: a thin inner lining (the intima), a thick muscular middle (the media), and an outer coat (the adventitia). A dissection begins when the intima tears and high-pressure blood dives into the media, splitting it lengthwise.
Now you have two channels:
- The true lumen — the original pipe, the one that's supposed to be there.
- The false lumen — the new tunnel the blood just carved inside the wall.
Separating them is a flap of peeled-off intima, flapping in the breeze of pulsatile blood flow. On imaging, that flap is your smoking gun.
This is a completely different beast from an aortic aneurysm, which is a balloon-like widening of the wall. An aneurysm is a pipe that's gotten too fat; a dissection is a pipe that's been split into two. They can coexist and they can both rupture, but conceptually, don't blur them together.
Type A vs Type B: the only classification you must know
There are a couple of naming systems for dissections, but the one that drives decisions is Stanford, and it's beautifully simple — it only cares about one thing: does the ascending aorta have a flap in it?
| Stanford type | What's involved | Why you care |
|---|---|---|
| Type A | Involves the ascending aorta (regardless of where the tear started) | Surgical emergency — risk of rupture into the pericardium, blocking the coronaries, or wrecking the aortic valve. |
| Type B | Spares the ascending aorta (starts beyond the left subclavian artery) | Usually managed medically with blood-pressure control, unless complications appear. |
The mental shortcut: A = Ascending = surgery now. B = Below = blood-pressure meds (usually). It's not perfectly precise, but it's the right instinct.
A type A dissection can tear backward toward the heart and bleed into the sac around it, causing cardiac tamponade — the heart gets squeezed and can't fill. This is why "ascending involvement" flips the whole management plan from "pills" to "operating room." If you see a flap in the ascending aorta, that's a phone-call-to-surgery finding.
How we image it
The workhorse is a CT angiogram (CTA) — a CT timed to catch a bolus of iodinated IV contrast lighting up the aorta. The contrast fills the lumens bright white, and the flap shows up as a dark line cutting across that brightness. (For the general logic of how a chest CT is built and read, see approach to chest CT.)
A crucial detail: you generally want a non-contrast scan first, then the contrast-enhanced one. The non-contrast scan is how you spot an intramural hematoma — blood that's seeped into the wall without an obvious flap, showing up as a crescent of higher density along the aortic wall. It's a close cousin of dissection and part of the same "acute aortic syndrome" family. Skip the non-contrast images and you can skate right past it.
How not to miss it (and what fools you)
The classic clinical story is sudden, severe, tearing or ripping chest or back pain that's often described as the worst the patient has ever felt. But clinical stories lie, which is exactly why imaging matters.
The two traps I see most: (1) Calling the dissection but forgetting to scroll up and confirm whether the ascending aorta is involved — that's the detail that changes everything. (2) Mistaking a pulsation or streak artifact in the ascending aorta for a real flap. A true flap is a consistent line you can track across multiple slices; a motion artifact tends to vanish or change shape between images. When unsure, look for the flap continuing through several slices and into the aortic root.
A dissection can also do collateral damage by sending the false lumen across the mouths of branch vessels, choking off blood to the kidneys, gut, or legs — so once you've found the flap, your job isn't done. You trace it from top to bottom and ask, at every branch: true lumen feeding this, or false?
This entity sits inside the broader category radiologists call acute aortic syndrome — dissection, intramural hematoma, and penetrating ulcer — three overlapping ways the aortic wall fails. They share imaging, urgency, and the same first question: is the ascending aorta involved?
If you remember nothing else: a dark line floating inside a bright aorta is a dissection until proven otherwise, and where that line lives decides whether your patient goes to the pharmacy or the operating room.