- There's two main types of stroke.
Ischemic stroke which is where brain tissue dies
because it's not getting enough blood
and therefore not getting enough oxygen to stay alive.
This is often caused by some type
of obstruction of a blood vessel.
And hemorrhagic strokes which is where
a blood vessel in the brain bursts.
And neither of them are particularly good
because they both cause brain tissue to die off.
So in this video, we'll look at ischemic strokes
which actually make up about 75% of all strokes.
And let me orient you to what this over here is.
This is an underside view of the brain
just so you can get a really good look
at the blood vessels that originate underneath
the brain and supply it with blood.
So, there's three major causes of ischemic strokes
and we'll sort of go through each one in turns.
So, the first one we'll talk about is an embolism
or an embolic cause of stroke.
And let me just bring in my guy here
to help us out with visualizing.
So we're looking at the underside.
An embolism is some type of traveling mass
in your blood stream
and it's most often a little blood clot.
So, in an embolic stroke,
this little blood clot will travel up
to one of your cerebral arteries and get stuck
and cut off your blood circulation to that area.
So let's say it get stuck somewhere about here.
It's traveled up through the internal carotid artery
and is lodged here in the middle cerebral artery.
Now, parts of the brain served by that MCA,
that middle cerebral artery,
are all gonna start to die off.
So, where would a clot come from?
Well, they often come from the heart.
And I know that sounds like a line
from a Boyz II Men song.
But you can literally get blot clots forming
within your heart.
So, either on the valves or within the walls
of usually your left atrium or your left ventricle
that then break off away from the walls of the heart
to travel up to your brain and then
they get stuck just like you see here.
Where else do emboli come from?
Well, you might have some cholesterol built up
in one of your carotid arteries
or maybe in your basilar artery
as part of a process called atherosclerosis.
And from these collections of cholesterol that build up,
you can have little pieces of cholesterol
that sort of flick off.
And just as we saw before,
they sort of travel up into your brain
and they get stuck.
So, this little piece of cholesterol
has gotten stuck in one of our
anterior cerebral arteries here.
And so, it will choke off blood supply
to the distribution of this anterior cerebral artery.
So we'll have death of brain tissue on this side.
And remember the anterior cerebral artery
supplies blood to some of the more midline
parts of the brain.
So that's the first major type of ischemic stroke,
ones caused by emboli.
Now, another major type of ischemic stroke
are ones called thrombotic strokes.
So, how did those happen?
So, it's similar to embolic strokes in that
a blood vessel that supplies
a part of the brain gets completely blocked.
But the difference is that emboli arise
away from the brain.
Whereas in a thrombosis,
a problem actually arises in that blood vessel
and it gets blocked off at that very spot.
So, let me show you some examples to clarify this.
So, we talked about how you can get some
carotid atherosclerosis happening here.
The build up of a fatty cholesterol deposit
inside your arterial wall.
Actually, let's show it happening in the basilar artery
because it's a bit easier to see on the underside picture.
So, you get this build up of cholesterol
and this actually the most common reason
for a thrombotic stroke.
And eventually, that cholesterol deposit
is gonna sort of crack open.
It's gonna break open which actually causes
a blood clot to form on top of it.
And that blood clot can get really, really big,
and so, as you can see in this picture
it actually got so big that blood can't
even get through this basilar artery anymore.
And so obviously, no blood is being allowed
to get through into the cerebral circulation
from at least the basilar artery
which will pretty quickly cause a stroke.
So, that's an example of a thrombotic stroke.
A disruption of blood caused by a problem
with that blood vessel locally.
And actually, this atherosclerosis commonly happens
in this middle cerebral artery here.
And of course, possibly on the other side's
middle cerebral artery.
Inside these internal carotid arteries
ride as they sort of turn into
the middle cerebral arteries
and in this basilar artery here.
So these are three key locations
that atherosclerosis happens.
So that's atherosclerosis
and that generally happens
in larger arteries in the brain.
So, the ones I sort of pointed out to you already.
But there's another sort of subtype of thrombotic strokes
and this one happens in primarily
smaller vessels of the brain
and these are called lacunar strokes.
So let's actually look at these vessel here.
I'm gonna just draw in a few tiny little vessels, okay.
And now, we'll blow one up.
So, that's our small vessel.
If you have hypertension
which is longstanding high blood pressure,
then the walls of these small blood vessels
can't really handle the pressure and the force with
which red blood cells travel through them with.
Because this really high blood pressure
puts a lot of strain and stress
on these blood vessel walls.
So, what these blood vessels do in response
is they sort of thicken up.
They get a lot thicker.
And so immediately, you can kinda see that
not very much blood is gonna get through them
as they start to get thicker and thicker, right?
And if the high blood pressure isn't dealt with,
they're just gonna get thicker.
And so, you can imagine that now
not very much blood is gonna be able
to get out of these blood vessels
because not really that much blood
can get through these blood vessels.
And so, what do you think might happen
to the piece of brain that this blood vessel serves?
Well, it's not really gonna be very happy.
It's not gonna get much oxygen
and so, it's gonna die off.
So that's a lacunar stroke,
a small vessel stroke.
You can also get tiny bits of atherosclerosis
happening in the small vessels
which would block them off to some extent.
But really, hypertension and this wall-thickening
is the major cause of these lacunar strokes.
So, the last type of ischemic stroke
I want to talk to you about
is something called the watershed stroke.
Now what does that mean?
Well, let's just go over some basics first.
The blood flow within your brain
goes from inside to out
and from outside to in.
So, you have some redundancy there
to make sure that all of this brain area is covered.
It's all getting enough blood.
But let's say that something happened
to reduce total blood flow to your brain.
So for example, if you got two large atheromas
or atherosclerotic deposits in your
internal carotid arteries.
Okay?
So not that much blood.
Actually, really little blood is getting
through up to your brain, all right?
Well, the total amount of blood that's getting
to your brain is gonna go down, right?
You'll be in a state of what we call hypoperfusion.
Hypo meaning lowered and perfusion
just refers to how much blood
your brain is being filled with.
So as a result of all this,
you're still gonna have a little bit
of cerebal blood flow.
But you're not gonna have that much, right?
So, we'll make these arrows a bit smaller.
You get some blood flow on this side
from outside to in but you're
not gonna have as much
because overall you're getting less blood
up to your brain, right?
Because of these two blockages.
And so, what do you notice here?
I'm sure you noticed that there's this huge area
in the middle that is not getting
that blood supply from either circulation now.
>From either the inside-to-out
or the outside-to-in circulation.
So this area will start to die off.
And actually in watershed infarcs
there's a classic pattern almost exactly
like what I'm drawing here.
And so, as a result of that
this area is colloquially called the watershed area.
Areas where say this part, this half,
is normally perfused by this blood supply
and this half is normally perfused
by this blood supply.
But when you sort of scale both
blood supplies back a bit,
this watershed area is prone to being damaged.
And in this case, this person has developed
a watershed stroke.
So, these are three main causes of ischemic stroke:
Embolism, thromboses, and watershed strokes
caused by hypoperfusion.