Systems Pathology | Pathology Student

What’s the relationship between aneurysm, thrombosis, and stenosis?

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.

What exactly does “storiform” mean?

Do you know what a “storiform” pattern is? Yeah, neither did I when I was a medical student. However, that term did get thrown around in pathology lectures a lot, without any description or definition. There are lots of terms like this – so I’m gonna just go ahead and create a new category called:

Words Pathologists Use In Lecture As If You Know What They Mean.

I think it’s important to pause and define these terms, because otherwise this is what happens: given the sheer volume of stuff you’re supposed to learn, and the minimal amount of time you have to accomplish this task, you’re not going to look up every word you have a nagging doubt about. You’re going to infer the meaning from whatever was said in lecture, and wind up with a fuzzy and probably incorrect definition. And then someone will ask you about it on rounds, and it will be frustrating.

SO. We’ll start with “storiform” today – and I’ll keep adding posts about terms in this category as I run across them. Please email me if you find a word like this! Then we’ll wind up with a nice glossary of these formerly-unexplained terms, and you’ll look like the star you are when one of these terms comes up on rounds.

First, a little Latin

Storiform comes from the Latin storea (woven mat) and formis (form, or pattern) – so technically, storiform means “having the pattern of a woven mat.” When we use “storiform” in pathology, though, it has a more specific meaning. It refers to a tumor pattern consisting of spindle cells in a pinwheel-shaped arrangement (radiating out from a central core).

Ready to see the labeled image? Okay, scroll down….

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Keep going…

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Here’s the same image, faded out a bit so you can see the pinwheel-shaped areas outlined in black:

Okay, they’re not perfectly symmetrical pinwheels, but they do look pinwheel-ish, with tumor cells radiating out from a central core region.

One last thing, now that we’ve got the pinwheels down…if you go back and look at the unmarked image again, you might be able to imagine that the cells are arranged like a woven mat, like the Latin term suggests.

I hope you feel comfortable with “storiform” now. That’s one less undefined term!

How do gallstones form?

There are two types of gallstones: cholesterol stones and pigment stones. If you didn’t know anything about gallstones, you’d guess (rightly so) that cholesterol stones are made up of cholesterol. And you’d also probably guess that you get cholesterol stones when there’s too much cholesterol around. But how, exactly? And pigment stones – what are those made of? Pigment?

Turns out there are very good explanations for all of these questions. Let’s take a look.

Cholesterol gallstones

Cholesterol gallstones contain – not surprisingly – cholesterol. And they arise when there’s more cholesterol around than the gallbladder can handle. But what does this actually mean?

A tiny bit of basic science here. Under normal conditions, cholesterol is soluble in bile because it binds to bile salts (which are water-soluble) and lecithins (which are water-insoluble). These guys both act like detergents, and cholesterol is dispersed within the bile, and everything’s cool.

But what happens if there’s too much cholesterol around? If the concentration of cholesterol exceeds the solubilizing capacity of bile, then cholesterol will nucleate into solid cholesterol crystals, which can over time get big enough to form stones.

Pigment gallstones

These stones are made of unconjugated bilirubin (mixed with calcium salts). They’re called pigment gallstones because they’re dark brown to black in color (compared to cholesterol stones, which are usually pale yellowish-greenish in color).

The two main conditions in which you see pigment stones are chronic hemolytic anemia and infection of the biliary tract. Why would these conditions lead to an accumulation of unconjugated bilirubin in the bile? In order to answer that, let’s quickly review bilirubin metabolism in the bile itself.

Normally, the liver conjugates bilirubin and dumps it into the bile. So the bile contains just conjugated bilirubin, then, right? Wrong! About 1% of the bilirubin in bile undergoes deconjugation while it’s still in the biliary tree (betcha didn’t know that!). Bile is then dumped into the gut, where bacterial-glucuronidases convert most of the remaining conjugated bilirubin into its unconjugated form.

Back to the causes of pigmented stones. If you have an infection in the biliary tree, and the infectious agent makes glucuronidase, then you’ll end up deconjugating more bilirubin than normal…and over time, that unconjugated bilirubin can accumulate and form stones.

The other main cause of pigmented stones is chronic hemolysis. If you’re busting open lots of red cells, all that heme gets transformed into bilirubin, which the liver conjugates and dumps into the bile. So the bile contains a lot more bilirubin than usual. Most of that bilirubin remains conjugated – but around 1% is turned into unconjugated bilirubin right there in the biliary tree. If you’re making a lot more bilirubin than normal, that 1% is significant – and over time, that excess of unconjugated bilirubin can be enough to lead to pigment stones.

The Pathology Course Cometh. Part 1: Books.

Next week marks the start of classes for a lot of medical, dental, nursing, physician assistant, medical technology, veterinary, and other allied health students. (more…)

Latin, Greek, and a quick review of lymphangioma

I was putting together some thoughts about lymphangioma the other day, and I got to wondering where the name came from. I love it when there’s a Latin or Greek word root that makes sense…and I found more than I hoped for when I looked into lymphangioma. (more…)

Hereditary hemorrhagic telangiectasia, simplified.

Hereditary hemorrhagic telangiectasia is one of a handful of diseases that I could never really get into my head. I don’t know why, because the name is so descriptive – it tells you exactly what the distinctive features of the disease are.

Maybe it’s because my professors used the older name for this disease – Osler-Weber-Rendu disease – which is about as non-descriptive a name as you can get. Good thing we’re getting away from eponymous disease names. They’re charming, but they make life really hard for students.

Back to hereditary hemorrhagic telangiectasia. Let’s take a closer look at this name – because once you know what the words mean, you’ll understand the disease without having to use brute memorization.

We’ll look at the words out of order, so that the explanation makes sense.

Hereditary: This is an autosomal dominant disorder. For boards (I can’t imagine an exam question on this, but who knows?), you might want to know that the mutated genes in this disorder encode parts of the TGF-β signaling pathway.

Telangiectasia: This is a great name, because everything you need to know is right there in the name. Too bad we don’t have time to cover some simple Greek and Latin word roots in medical school, because once you know a few, you can easily figure out what words mean. Telangiectasia comes from three Greek words:

τέλος (telos): “end” (For example: telomeres are the little things on the ends of chromosomes.)
ἀγγεῖον (angeion): “vessel” (angio- is almost always used to indicate a blood vessel. For example: an angiogram is an imaging study that looks at blood vessels.)
ἐκτείνειν (ekteinein) “to extend” (any time you see ectasia or ectasis, it means an abnormal dilation of a tubular structure. For example: bronchiectasis means dilation of bronchi.)

So telangiectasias are abnormal dilations of the very ends (tiny capillaries or venules) of blood vessels. They often occur in the skin and mucous membranes, and sometimes they look like tiny little spiders, but sometimes they just look like red blotches (check out the image above). Telangiectasias can also occur in the respiratory, gastrointestinal, and urinary tracts. Telangiectasias are not neoplastic (there aren’t any new, neoplastic vessels); they’re just malformations of existing vessels.

Hemorrhagic: Sometimes telangiectasias can rupture, causing nosebleeds, GI bleeding or hematuria.

That’s it! Now you know 🙂

What’s the difference between an aortic dissection and a false aneurysm?

Q. I’m a little confused about the difference between an aortic dissection and a false aneurysm.

In diagrams of aortic dissection, it looks like all three layers of the vessel have been damaged and blood is leaking out of the vessel BUT still contained by connective tissue etc. Isn’t that what a false aneurysm is? So what’s the difference between the two?

A. I can see what you mean – diagrams of aortic dissections can be misleading!

Diagrams of aortic dissection (like this one above, from Wikipedia) often focus on the three types of dissections. The point of these diagrams is just to show the place in the aorta where the dissection begins (near the heart vs. more distally). They can be kind of misleading, though, if they don’t clearly depict how the blood tunnels through the wall of the aorta. Like you described, it can look more like the aorta has actually ruptured all the way through, and the blood is collecting outside the aorta.

So let’s quickly review the definitions of false aneurysm and dissection.

In a false aneurysm, all three walls of the vessel have been broken through, and blood collects just outside the vessel. It doesn’t tunnel down or up into the wall of the vessel at all. After some time, the blood organizes and becomes firm – kind of like a blood clot – and it prevents further blood from escaping the damaged vessel.

In a dissection, only the inner portion of the vessel wall is damaged. Blood enters into that damaged area, and tunnels up or down within the wall of the vessel. Unlike a false aneurysm, in which blood bursts through all three layers of the vessel, in a dissection, the outer layers of the vessel are still intact, and blood forms a channel within the vessel wall itself.

So you had the right idea! It was just the diagram that threw you off. In the diagram above, the arrangement of the two little white arrows incorrectly implies that blood is busting all the way through the aorta at a single point – but the rest of the aortic wall looks intact. To really show what’s happening in an aortic dissection, the wall of the aorta should be more clearly depicted, and the second arrow should point up or down within the wall itself, showing the path of blood as forms a tunnel within the vessel wall.