Subarachnoid Space

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The subarachnoid plays a crucial role in the brain and the cerebrospinal fluid of the brain. So, what is the subarachnoid space, and why is it so important in the brain?

The area between the pia mater and the arachnoid membrane is known as the subarachnoid space. It is home to branches of the brain's arteries and veins as well as delicate connective tissue trabeculae, intercommunicating channels, and cerebrospinal fluid (CSF).

The cavity of the subarachnoid space in a normal body is fairly tiny. The subarachnoid space is an important area in the meninges. However, as with many body parts, it is not immune to trauma or spontaneous irritation. Here's what to know about the subarachnoid space in the brain.

What Membrane Layers Is The Subarachnoid Space Between?

3 membranes surround the brain and spinal cord, known as meninges. These are the dura mater, arachnoid mater, and pia mater. The function of these membranes is to generally provide structural support for the brain and to keep the cerebrospinal fluid contained.

The outermost membrane is the dura mater. It is a thick, tough layer adhering to the skull on one side and the second layer, the arachnoid mater, on the other side. This membrane ensures that the brain and spinal cord are provided with an extra protective layer. It also attaches the brain to the skull and spinal cord to the vertebral column, so neither gets shaken.

Not only that, but it provides a venous drainage system where blood can exit the brain.The second layer, arachnoid mater, is named as such due to how its appearance and consistency mimic that of a spider's cobweb. Therefore, it's no surprise that it is not as substantial as the dura mater. This membrane is a delicate, avascular membrane superior to the pia but inferior to the dura mater.

It envelopes the brain and spine, and its function is to contribute to the cerebrospinal's fluid circulation throughout the body's central nervous system. There are strands of tissue, referred to as arachnoid trabeculae, that stretch between the arachnoid mater and the pia mater, the third membrane.

The function of these strands is to help suspend the brain in place. This is where the subarachnoid space comes in. It is situated between the arachnoid and the pia mater. The cerebrospinal fluid, significant blood vessels, and cisterns make up the subarachnoid space.

Per the anatomy of the spinal cord and brain surface, the cisterns are larger pockets of CSF caused by the split of the arachnoid from the pia mater. This is because, in certain spaces, the subarachnoid space widens to form cavities, thus forming cisterns.

As previously mentioned, the distance between the pia mater and the arachnoid membrane is known as the subarachnoid space and consists of the arachnoid trabecular, a network of the brain's arteries and veins, intercommunicating channels, and cerebrospinal fluid.

However, there is the spinal subarachnoid space connected to the intracranial subarachnoid space in the spine between the arachnoid and pia mater. It terminates at the point of the sacrum vertebra and interacts with the subarachnoid space through the foramen magnum.[2]

What Are Subarachnoid Cisterns?

As mentioned above, cisterns are also what make up the subarachnoid space. However, what are they truly? First, it is worth noting that the spinal and cranial arachnoid mater is quite similar in functions and structure. There are, however, certain features that the cranial arachnoid mater has that are specifically unique to it. 

This is where the subarachnoid cisterns come in. They are a prominent feature of the cranial arachnoid mater. They can be referred to as spaces situated in the subarachnoid space where cerebrospinal fluid pools, and there are many nerves and vessels that exit toward the skull foramina.

The pooling of the cerebrospinal fluid in these cisterns is because the pia mater of the cranial is being tightly adhered nook and cranny, otherwise referred to as fissure and contour, to the surface of the brain. In contrast, the cranial arachnoid mater, like the cranial dura mater, only loosely envelops the immediate surface of the brain.

The difference in how tightly the three different membranes envelop the brain allows for large gaps to exist between the pia and arachnoid mater in places where the pia goes down into a sulcus. However, as with many things, there is an exception regarding cistern formation within the cranial subarachnoid spaces.

This exception is in the lumbar cistern, which is situated at the termination of the canal of the spine, extending from the vertebral level and down to the sacral foraminal level. Simply put, the lumbar cistern is located in the dural sac of the spinal cord. This is typically where cerebrospinal fluid is drawn during lumbar procedures.[1]

What Passes Through The Subarachnoid Space?

Several important things pass through the subarachnoid space. These are the spinal cord roots, the cranial nerves, and the veins and arteries from the brain and the spinal cord.[1]

What Is The Function Of The Subarachnoid Space?

As the subarachnoid space is a space formed by the arachnoid mater and contains the thin fibrous filaments called trabeculae that hold the arachnoid and pia mater together, it is not surprising that its function will be aligned with that of the two impermeable membranes.

One of its functions is to provide a pathway for the circulation of the cerebrospinal fluid it is filled with. It is also responsible for absorption around the space and the spinal cord.[1]

Are There One Or Two Subarachnoid Spaces In The Brain?

There is just one subarachnoid space. The place between the pia and the arachnoid mater that houses the CSF. It wraps completely around the brain, penetrates the ventricles, and descends the spine to the second sacral vertebra. It even reaches the retina along the optic nerves. How can one subarachnoid space have two names if there is only one?

This is because the spinal cord is completely encircled by the subarachnoid space. Therefore, despite the subarachnoid space being continuous, the space located in front of the spinal cord is referred to as the ventral subarachnoid space, and the space situated behind the spinal cord is the dorsal subarachnoid space.[3]

What Is A Subarachnoid Hemorrhage?

Now that we know that the brain is encased by three meninges, it is important to know what brain problems can occur in it. To understand the mechanisms of severe acute bleeding, it is key to understand the anatomy of the arachnoid mater.

As mentioned above, there are thin filaments called trabeculae that transverse the subarachnoid space and subsequently blend with the pia mater. The arachnoid layer is not only delicate, but it is thin and transparent.

This is imperative as it serves to cushion the central nervous system. As it doesn't follow the fissures and contours of the brain, that space between the pia and the arachnoid mater contains cerebral vasculature.

Should one of these cerebral vasculatures get damaged, it can lead to the subarachnoid filling with blood. This is what is referred to as a subarachnoid hemorrhage. When blood fills up in the subarachnoid space, it irritates the adjacent membrane layers. When this happens, there is a classical triad of symptoms of meningism.

These symptoms are photophobia, neck stiffness, and severe headache. However, meningism is not particularly specific to blood within the subarachnoid space. This means that it can actually be caused by just about anything that irritates and inflames the meninges. It is also typically associated with bacterial meningitis.

Types Of Subarachnoid Hemorrhages

Subarachnoid hemorrhage can either be spontaneous or traumatic. When it happens to be due to non-traumatic or spontaneous causes, it is more commonly due to a rupture of an intracranial berry aneurysm. However, the majority of subarachnoid hemorrhages are because of trauma.

They are the most common as they make up roughly 85% of the non-traumatic subarachnoid hemorrhage. An aneurysm is known as a bulging of the blood vessel wall. This bulging typically weakens the blood vessel wall, and it may rupture. When it does, the blood will leak into the subarachnoid space.

In the brain, there is what is called a Circle of Willis, and it has main branches. A berry aneurysm is considered a circular aneurysm. It frequently happens between the junction of the major intracranial arteries at the branching places from the Circle of Willis.

An aneurysm rupturing is possibly a catastrophic event. However, it can be treated endovascularly with the help of interventional radiologists through the use of coils or stents. It can also be treated by neurosurgeons who stop the bleeding by clipping the aneurysm's neck.

Another type of spontaneous subarachnoid hemorrhaging should be kept in mind. It is called Perimesencephalic hemorrhage. The name suggests that this is blood in the subarachnoid systems, particularly around the midbrain. In this kind of subarachnoid hemorrhage, there is no cerebral aneurysm identified.

Furthermore, the cause of the bleeding is considered venous in origin. In cases of this kind of subarachnoid hemorrhage, blood is typically seen anterior to the midbrain as well as the Pons. Because it is thought to be venous as the origin of the blood, the prognosis is tons better than a subarachnoid hemorrhage that occurs because of a ruptured arterial cerebral aneurysm.

Additionally, other causes of blood in the subarachnoid space are less frequent. These are the Arteriovenous Malformations, inflammatory causes such as cerebral vasculitis, and non-traumatic bleed that is secondary to anticoagulation therapy.[3]

How Do Doctors Diagnose A Subarachnoid Hemorrhage?

There are many ways that doctors can diagnose whether or not a patient has a subarachnoid hemorrhage. The first thing they will do is want to obtain a non-contract CT scan of the head. What they will be looking for is the classic subarachnoid pattern. There will be blood that will show up on the CT scan in several places. These would be the sulci, the different types of cisterns, and the ventricles.

Doctors usually give a modified fisher score if they detect the subarachnoid hemorrhage pattern. This score is what determines what the risk is of the patient developing what is called a vasospasm, which is the second most common mortality in patients with subarachnoid hemorrhage.

Once the CT scans come back and shows that there is, in fact, a subarachnoid hemorrhage, the next test that will be done will be a CT angiogram of the head and the neck. This is because it will give a clear look at the vessels so doctors can detect if there is a part that has an aneurysm that may have ruptured. However, if the aneurysm is quite tiny, it can be missed by this CT.

A digital subtraction angiogram is the next best test that triumphs over the other two. This is the standard gold test as it gives direct visualization of the vessels in a 3D image. However, this is quite invasive, and there are risks of arteries being punctured.

The last thing that will be done is when the subarachnoid hemorrhage is not showing on the CT, but the doctor highly suspects that the patient has one. It is called a lumbar puncture. This is because the subarachnoid space extends to the spinal cord, as mentioned in the cistern's explanation.

For this reason, it is possible to tap into the subarachnoid space via the lumbar to take a sample of the cerebrospinal fluid. If that fluid has blood, then the patient has subarachnoid hemorrhaging.[3]

What Is The Treatment For A Subarachnoid Hemorrhage?

The most important thing to do is to get the airway, breathing, and circulation going when a patient is diagnosed with subarachnoid hemorrhage. This is because bleeding in the brain increases intracranial pressure, which pushes down on the brain stem, and decreases respiratory activity. Other treatments include external ventriculation drainage, coiling or clipping, etc.[2]


The subarachnoid is situated in between the inner and middle brain membranes. It has several central roots, brain, spinal arteries, veins, etc., that go through it. It is imperative in the pathway creation of cerebrospinal fluid. Unfortunately, any slight irritation can lead to severe consequences like a subarachnoid hemorrhage.

Reference this article:

Practical Psychology. (2022, September). Subarachnoid Space. Retrieved from

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