The last space-occupying bleed or hemorrhage is known as the subarachnoid hemorrhage.
It’s important for us to understand the structural and clinical considerations of this form
of bleed. This will be a bleed specifically into the subarachnoid space which is the area
between the subarachnoid membrane and the pia mater. This is a normal space that contains
the cerebrospinal fluid. The most common cause of a subarachnoid hemorrhage is going to be
a cerebral aneurysm. However, head trauma or head injury can also result in a subarachnoid
hemorrhage. In this illustration, we are showing the circle of Willis. We see a little dilatation
right in through here within the circle of Willis. If we zoom in, we have this particular view
and we see an artery at the circle of Willis. Here’s a branching point. An aneurysm within
the circle of Willis can develop at branching points. If this aneurysm ruptures, that will
result then in a subarachnoid hemorrhage. The vascular source of a subarachnoid hemorrhage
with respect to an aneurysm rupture is within the circle of Willis. So, if there is a rupture
of the aneurysm, where within the circle of Willis would you most commonly find a subarachnoid
hemorrhage? The most common point at which a berry aneurysm will develop is at the branching
point between the anterior cerebral and the anterior communicating arteries. That would be
in the area that we see here. This is your anterior cerebral artery. It originates from the internal
carotid shown here and then courses medially. We see its continuation at this point.
Then we see the opposite one over here, again coursing medially. Between the short distance
between the right and the left anterior cerebral arteries, we have an anterior communicating
artery. So right at that branching point and through here is going to be the most common
location of a berry aneurysm that if it ruptures would be the source of the subarachnoid
hemorrhage. Berry aneurysms can occur elsewhere where you have branching points, so along the
course of the middle cerebral artery that we see here. You’ll have arteries that will branch off
of it. At some of those branching points, you can have a berry aneurysm. Those tend to be
more proximal. The third one to point out would be the branching point that exists between
the internal carotid artery and the posterior communicating arteries. That would be shown
in this location. Here’s your internal carotid. Here’s your posterior communicating artery.
So at this branching point, you could also have a berry aneurysm. So those are the three
most common sites at which this can develop and again, the most common one is at the
branching point between the anterior cerebral and your anterior communicating arteries.