I got this really great question from one of my students, and it got me thinking about how important it is to have really clear definitions of pathologic conditions. These three conditions – aneurysm, thrombosis, and stenosis – are totally different things. And yet they can sometimes co-occur, or one can cause another – so it can become confusing!

I thought I’d share the question and my answer here, because I’m sure there are other students who are having trouble understanding these disorders.

Here’s the question:

I was reviewing the Blood Vessel Pathology lecture notes from this past week and was having a bit of trouble differentiating between aneurysm, thrombosis and stenosis. I’ve written what I believe to be the differences, but would you mind giving me some feedback on if this is correct?

“An aneurysm is when a clot occurs, widening the blood vessel to unhealthy proportions due to high blood pressure and or atherosclerosis, and it may rupture with no warning signs, leading to internal bleeding. The difference between aneurysm and thrombosis is that aneurysm causes damage to the lining wall of the blood vessel. Thrombosis is clotting of a blood vessel without damage to the walls. Stenosis is narrowing of the artery to cause clotting, and it comes with the warning sign of severe chest pain.”

Great question!! You’re on the right track – but there are some things in your statement that aren’t quite right – so I’ll give you my definitions and then comment on what you wrote.

Aneurysm

An aneurysm is an abnormal widening (or dilation, or outpouching) of a blood vessel. It’s focal in nature, which means that it’s just in one place; you can point to where it is (it’s not like the entire vessel is just a little bit wider). Here’s an image of a normal vessel and a vessel with an aneurysm:

Aneurysms can be caused by lots of things (like trauma and atherosclerosis), or they can be congenital. Sometimes aneurysms just sit there and never cause any problems. But sometimes they get larger and larger, and the vessel wall weakens to the point where it eventually ruptures.

Thrombosis

A thrombosis (or thrombus) is an abnormal blood clot. It’s not just a normal little blood clot formed to repair a hole in a vessel – it’s a blood clot that’s been made when it isn’t needed. The most common place for a thrombus is in the deep veins of the legs – but you can form a thrombus anywhere in the body.

It’s not good to have a thrombus for a few reasons:

• If it’s big enough, the thrombus can block blood flow through the vessel, and the tissues fed by that vessel can be damaged or even die as a result.
• Thrombi can weaken and damage the vessel wall, leading to other problems (like aneurysms, or even rupture of the vessel if it gets weak enough).

Here’s a related term: embolus. An embolus is a blood clot that’s floating in the blood (maybe it broke off from a thrombus in the leg, or maybe it formed on its own somewhere). The point is that it is mobile, and it’s going to move with the blood until it gets to a vessel that’s too small for it to pass through, and it will lodge there. If the embolus is tiny, you may not notice anything clinically. But if the embolus is big enough to block off an important vessel (say, one of the vessels in the brain), that means that the tissue fed by that blood vessel won’t get blood, and it will die.

Stenosis

Stenosis just means “narrowing.” It can be used to describe abnormal narrowing of lots of different structures in the body (like heart valves and the spine). When a blood vessel is stenotic, that means its lumen is smaller than normal.

There are many possible causes of stenosis in vessels. Here are some common ones: atherosclerosis (formation of plaques that take up space and narrow the lumen), thrombosis (formation of an abnormal clot that takes up space within the vessel lumen), and vasculitis (inflammation of the vessel).

Like the other abnormalities we talked about above, stenosis can be asymptomatic if it is mild. But if a vessel is very stenotic (for example, if the vessel lumen is only 20% of its normal diameter), that can impair blood flow enough to cause serious problems to the tissue downstream. This is particularly a problem if the vessel feeds the heart or the brain; in these places, restriction of blood flow can cause severe symptoms (or even death).

Why these things are confusing

These three conditions are distinct and separate entities – but they can occur together, and they can also occur sequentially – and this can be confusing. For example, if you have a thrombus in a vessel, that can weaken the vessel wall enough to cause an aneurysm. Or you can have a thrombus that simply sits there and takes up space in the vessel lumen, causing stenosis of the vessel.

So the best way to approach this is to make sure you understand what each of these disorders is – and then once you have that down, you can go on to learn about what causes them and what they can lead to.

Back to the statement part of the question – my comments are in blue.

An aneurysm is when a clot occurs, widening the blood vessel to unhealthy proportions due to high blood pressure and or atherosclerosis, and it may rupture with no warning signs, leading to internal bleeding. You’re correct in saying that an aneurysm is a widening of a blood vessel that may be caused by high blood pressure or atherosclerosis, and that it may rupture. And it’s true that aneurysms can be caused by abnormal blood clots (thrombosis) – but just to clarify – not all aneurysms are caused by clots. The main point is that an aneurysm is an abnormal widening of a blood vessel – and there are many potential causes. The difference between aneurysm and thrombosis is that aneurysm causes damage to the lining wall of the blood vessel. Thrombosis is clotting of a blood vessel without damage to the walls. No; the difference between aneurysm and thrombosis is that an aneurysm is an abnormal dilation/widening of a blood vessel, whereas a thrombosis is a blood clot that forms within a blood vessel. Both aneurysms and thromboses can damage the vessel wall. Stenosis is narrowing of the artery Yes! to cause clotting Not exactly. Stenosis is just the narrowing of a vessel lumen; it doesn’t necessarily cause the formation of a blood clot. However, thrombosis (abnormal clotting) can lead to stenosis (narrowing of the vessel lumen)! This is where you have to be really strict about your definitions, otherwise it gets confusing! and it comes with the warning sign of severe chest pain Sometimes! If the stenotic vessel is one that supplies the heart, and if the stenosis is moderately severe (meaning that the lumen is narrowed enough to decrease the amount of blood that can flow through the vessel), then the patient will experience chest pain (because there’s less blood flow to the heart than usual). This is a warning sign – it tells you that the tissue isn’t getting quite enough blood flow, and you better go see a cardiologist and get those vessels looked at. However, if the stenosis is really severe (like if the lumen is only 10% of its normal diameter), then almost no blood is getting through, and that may be enough to actually cause tissue death (myocardial infarction, or heart attack). In this case, the chest pain the patient experiences isn’t just a warning sign – it’s a sign that the tissue is actually dying right now.