Carotid Stenosis Grading
- Carotid stenosis grading exists to answer one question: how narrow is the carotid, and does that narrowing put the brain at risk?
- Ultrasound is the everyday screening tool — it grades stenosis mostly by how fast the blood is moving through the narrowed spot (peak systolic velocity), not by directly measuring the lumen.
- CTA and MRA show the actual shape of the narrowing and the lumen you can measure with a ruler — they're the tiebreakers and the surgical roadmap.
- The classic threshold that changes management is roughly 70% narrowing in a symptomatic patient — that's the cutoff where fixing it tends to beat leaving it alone.
- Report the degree of stenosis, the method used to measure it, and whether the vessel is truly occluded (no flow) versus nearly occluded (a trickle) — those last two are managed very differently.
The carotid arteries are the two main on-ramps delivering blood up the neck and into the brain. When plaque builds up and narrows one of them, you've got a partially blocked highway, and the worry isn't just slow traffic — it's that a chunk of debris breaks loose, rides upstream, and causes a stroke. Our job in the reading room is to put a number on that narrowing, because that number is what decides whether someone gets a knife, a stent, or just a statin and a handshake.
Why we grade it at all
Here's the thing nobody tells you up front: we don't grade carotid stenosis because the number is interesting. We grade it because somewhere around the 70% mark in a symptomatic patient, the math flips — the risk of the surgery to clean out the artery becomes smaller than the risk of leaving it alone. Below that, the plumbing is ugly but the operation usually isn't worth it. So the whole elaborate grading apparatus exists to sort patients onto one side or the other of that line.
"Symptomatic" is doing heavy lifting here. It means the narrowing on this side has already thrown a clot that caused a TIA or stroke. A symptomatic 70% artery and an asymptomatic 70% artery are treated as different beasts, even though the picture looks identical.
Ultrasound: grading by speed, not by ruler
Carotid ultrasound is the front door — cheap, no radiation, no contrast. But it grades stenosis in a way that surprises people. It mostly doesn't measure the width of the opening. It measures how fast the blood is sprinting through it.
Think of a garden hose. Put your thumb over the end and the same water suddenly shoots out fast and far. A narrowed artery does the same thing: squeeze the channel and the blood has to speed up to push the same volume through. So the peak systolic velocity (PSV) — the top speed of blood at the tightest point — climbs as the stenosis worsens. We anchor the grade to velocity thresholds, often with a backup ratio comparing the speed at the narrowing to the speed in the normal internal-carotid further down.
The garden-hose logic breaks at the extremes. A near-total occlusion can have a tiny, slow trickle — low velocity — that a velocity-only reading might wave off as mild. And a very tight, nearly occluded artery is the one that's most dangerous. Always look at the actual color flow and lumen, not just the speedometer.
CTA and MRA: measuring the lumen with a ruler
When ultrasound is equivocal, or before anyone operates, we reach for cross-sectional imaging — CTA or MRA. These show the anatomy: the lumen, the plaque, the shape of the narrowing, and how the vessel looks above and below it.
Here's where measurement gets a footnote. The standard way to express stenosis is the NASCET method: you compare the diameter of the narrowest residual lumen to the diameter of the normal internal carotid downstream, past the bulb where the artery has settled to its true caliber. It is genuinely counterintuitive — you're not comparing the narrowing to the bulb right next to it, you're comparing it to a calmer stretch further up. Using the wrong denominator is one of the most common ways to over- or under-call a stenosis.
| Method | What you compare the narrow lumen to | Quirk |
|---|---|---|
| NASCET | Normal internal carotid downstream of the stenosis | The reference standard; smaller denominator than you'd guess |
| ECST | The estimated original vessel wall at the stenosis | Reads as a higher percentage for the same artery |
Because NASCET and ECST use different yardsticks, the same artery gets two different percentages. A "70% NASCET" and a "70% ECST" are not the same narrowing. Always state which method you used — a number without a method is a number nobody can act on.
Occluded versus near-occluded: the distinction that matters most
The single most important call you can make is whether the artery is completely occluded (no flow at all, a dead-end road) or near-occluded (a faint thread of flow still getting through). They look almost identical on a quick glance and they are managed in nearly opposite ways — you generally can't reopen a chronically, fully occluded carotid surgically, whereas a near-occlusion may still be a candidate. Get this wrong and you send the patient down the wrong path entirely.
When you suspect total occlusion on ultrasound, confirm with CTA or MRA before you commit it to the report. A whisper of contrast filling the distal vessel changes "occluded" to "near-occluded" — and changes the whole plan.
What the report should actually say
A carotid stenosis report isn't a vibe; it's a structured answer. State the side, the degree of stenosis as a percentage, the method behind that percentage, and explicitly whether the vessel is patent, near-occluded, or occluded. If you used ultrasound, anchor the grade to the velocity criteria; if CTA or MRA, give the NASCET measurement. Note the plaque if it's relevant (a soft, ulcerated plaque is more sinister than a smooth calcified one). For the fine points of the Doppler technique itself, the Doppler ultrasound page goes deeper.
Do that, and you've handed the surgeon and the neurologist exactly what they need: not a pretty picture, but a clean number with a clear meaning and the confidence to act on it.