Subarachnoid hemorrhage

There are two main types of stroke: a hemorrhagic stroke, which occurs when an artery ruptures and bleeds within the brain, and an ischemic stroke, which occurs when an artery gets blocked.

Hemorrhagic strokes can be further split into two types, an intracerebral hemorrhage which is when bleeding occurs within the cerebrum, and a subarachnoid hemorrhage which is when bleeding occurs between the pia mater and arachnoid mater of the meninges - the inner and middle layers that wrap around the brain.

We’ll focus on subarachnoid hemorrhage, which can quickly lead to death if they’re left untreated.

Subarachnoid hemorrhages can lead to a pool of blood under the arachnoid mater that increases the intracranial pressure and prevents more blood from flowing into the brain.

Ok - let’s start with three protective layers of the brain called meninges.

The inner layer of the meninges is the pia mater, the middle layer is the arachnoid mater, and the outer layer is the dura mater.

Between the arachnoid mater and the pia mater is the subarachnoid space, which houses cerebrospinal fluid, or CSF.

CSF is a clear, watery liquid which is pumped around the spinal cord and brain, cushioning them from impact and bathing them in nutrients.

This space is also where the arteries that supplies the brain travel, and it is the location of the blood brain barrier where CSF and the vascular system can exchange nutrients.

The brain has a few regions - the most obvious is the cerebrum, which is divided into two cerebral hemispheres, each of which has a cortex - an outer region - divided into four lobes including the frontal lobe, parietal lobe, temporal lobe, and the occipital lobe.

There are also a number of additional structures - including the cerebellum, which is down below, as well as the brainstem which connects to the spinal cord.

The right cerebrum controls muscles on the left side of your body and vice versa.

The frontal lobe controls movement, and executive function, which is our ability to make decisions.

The parietal lobe processes sensory information, which lets us locate exactly where we are physically and guides movements in a three-dimensional space.

The temporal lobe plays a role in hearing, smell, and memory, as well as visual recognition of faces and languages.

Finally, there’s the occipital lobe which is primarily responsible for vision.

Within the cortex are deeper structures like the internal capsule, which is like a highway that allows information to flow through neurons that are going to and from the cerebral cortex.

There’s also the basal ganglia, which helps control smooth movement and cognitive function, along with the cerebellum.

The cerebellum also helps with muscle coordination and balance.

And finally, there’s the brainstem, which plays a vital role in functions like heart rate, blood pressure, breathing, intestinal motility, and consciousness.

The brain receives blood from the left and right internal carotid arteries, as well as the left and right vertebral arteries, which come together to form the basilar artery.

The internal carotid arteries turn into the left and right middle cerebral arteries which serve the lateral portions of the frontal, parietal, and temporal lobes of the brain.

Each of the internal carotid arteries also give off branches called the anterior cerebral arteries which serve the medial portion of the frontal and parietal lobes and connect with one another with a short little connecting blood vessel called the anterior communicating artery.

Meanwhile, the vertebral arteries and basilar artery give off branches to supply the cerebellum and the brainstem.

In addition, the basilar artery divides to become the right and left posterior cerebral artery which mainly serve the occipital lobe and some of the temporal lobe as well as the thalamus.

Finally, the internal carotid arteries each give off a branch called the posterior communicating artery which attaches to the posterior arteries on each side.

So together, the main arteries and the communicating arteries complete what’s called the Circle of Willis - a ring where blood can circulate from one side to the other in case of a blockage.

Three things can cause a subarachnoid hemorrhage.

The first and most common cause of subarachnoid hemorrhages is aneurysms, which is when a blood vessel has weak walls and starts to bulge out to about one and a half times larger than its normal diameter.

The most common aneurysms in the brain are saccular cerebral aneurysms which have a characteristic rounded shape on one side of the artery and are also called berry aneurysms.

Most saccular cerebral aneurysms arise in the anterior half of the circle of Willis whereas only a few arise in the posterior half.

Some genetic disorders like Marfan syndrome cause a defect in the connective tissues of arteries and they can also predispose individuals to having saccular aneurysms.

Aneurysms can burst open when there’s an increase in intracranial pressure, like what you might feel if you’re moving a large sofa into the living room.