Enveloping the brain and the spinal cord are three membranous layers known as the meninges. From superficial to deep, these layers are the dura mater, arachnoid mater, and pia mater. The meninges function to protect the brain and the spinal cord from mechanical trauma, support the cranial vasculature and form a continuous cavity through which the cerebrospinal fluid (CSF) circulates.
These membranes collectively define three clinically important potential spaces: the epidural, subdural, and subarachnoid spaces. This article will focus on the subarachnoid space.
|Definition||A space between arachnoid mater and pia mater, surrounding brain and spinal cord|
|Contents||CSF, arachnoid trabeculae, cerebral arteries and veins, cranial and spinal nerves (intracranial and intravertebral portions)|
|Cisterns||Posterior cerebellomedullary cistern, interpeduncular cistern|
|Main functions||House CSF to cushion brain and spinal cord, provide nutrients, remove waste
Support and stabilize brain and spinal cord
- Anatomical structure
- Subarachnoid cisterns
- Subarachnoid hemorrhage
The subarachnoid space lies between the membranous layers of the arachnoid mater and the pia mater and surrounds the brain and the spinal cord. The subarachnoid space of the brain is known as the intracranial subarachnoid space or the cerebral subarachnoid space, while the subarachnoid space of the spinal cord is known as the spinal subarachnoid space. The intracranial subarachnoid space is continuous with the fourth ventricle of the brain via the median aperture and paired lateral apertures. The spinal subarachnoid space begins at the foramen magnum (where it communicates with the spinal subarachnoid space) and extends to terminate at the level of the S2 vertebra. The distal portion of the spinal cord (the cauda equina) is located at the base of the spinal subarachnoid space within a subarachnoid cistern known as the lumbar cistern.
Find out more about the meningeal layers by watching the video below.
The subarachnoid space is characterized by a lattice-like, spider web appearance due to the presence of a filamentous network of trabeculae and varies greatly in depth depending on location, resulting in formation of subarachnoid cisterns. The two main elements contained in the subarachnoid space amongst the trabeculae are the cerebrospinal fluid and neurovasculature.
The subarachnoid space is filled with CSF which is produced by the choroid plexus within the ventricles of the brain. CSF flows through the ventricles of the brain and is received into the subarachnoid space from the fourth ventricle via the posterior cerebellomedullary cistern.
Extending through the depth of the subarachnoid space are delicate connective tissue trabeculae. Formed by extensions of the deep layers of the arachnoid mater, the collagen-reinforced trabeculae cross the subarachnoid space to reach the pia mater and essentially function to stitch the pia mater and arachnoid mater together.
Trabeculae vary in their shape and size and range from single or branched strands to tree shaped structures. Ultimately, they work together to form a trabecular network which takes on the appearance of honeycomb. The lattice arrangement of the trabeculae allows for the unobstructed passage and flow of CSF within the subarachnoid space and functions to loosely connect the arachnoid and pia mater as well as stabilize and suspend the brain and the spinal cord in one place. As a result, the trabeculae of the subarachnoid space contribute to the intracranial and intraspinal biomechanics.
Also located within the subarachnoid space are the large cerebral arteries and veins, as well as the intracranial and intravertebral components of the cranial and spinal nerves, which traverse the trabeculae. Neurovascular structures of the subarachnoid space have been found to lie in close proximity to the arachnoid trabeculae for protective purposes. The arachnoid trabeculae provide a supporting framework to blood vessels of the subarachnoid space and therefore offer a degree of protection to these vital structures.
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Subarachnoid cisterns are formed due to the differently arranged pia and arachnoid mater layers. Pia mater conforms to the contours of the brain and invests each sulcus and gyrus. As a result pia mater maintains contact with the brain at all times. Dissimilarly, arachnoid mater is much looser and does not generally follow the sulci and gyri of the brain. Therefore, the subarachnoid space is not of uniform depth across the entire brain. It varies from place to place within the central nervous system. In areas where the pia mater and arachnoid mater are not in close contact the space expands and the trabecular network is not as dense or plentiful as in other areas. Here we can find expanded areas filled with CSF known as the subarachnoid cisterns from the Latin word for ‘box’. All subarachnoid cisterns can freely communicate with each other and the rest of the subarachnoid space. There are several cisterns in the subarachnoid space:
- Pericallosal cistern
- Cistern of lamina terminalis
- Cistern of lateral cerebral fossa (sylvian fossa)
- Chiasmatic cistern (suprasellar cistern)
- Interpeduncular cistern
- Quadrigeminal cistern (superior cistern)
- Pontine cistern
- Pontocerebellar cistern
- Posterior cerebellomedullary cistern
- Lumbar cistern (spinal subarachnoid space)
While there are many cisterns within the subarachnoid space, the largest of the two are the posterior cerebellomedullary cistern and the interpeduncular cistern.
Posterior cerebellomedullary cistern
The posterior cerebellomedullary cistern, also known as the cisterna magna, is the largest cistern within the subarachnoid space. It lies between the cerebellum and the posterior aspect of the medulla oblongata, which is where it gets its name. This large cistern is continuous above with the fourth ventricle and below with the subarachnoid space of the spinal cord. The posterior cerebellomedullary cistern contains a number of neurovascular structures which include the vertebral arteries, the posterior inferior cerebellar arteries, the glossopharyngeal, vagus, accessory and hypoglossal cranial nerves, and the choroid plexus. This cistern acts as a conduit in the passage of CSF from the fourth ventricle to the subarachnoid space.
The interpeduncular cistern is located at the base of the brain at the junction where the arachnoid mater stretches between the two temporal lobes, occupying the interpeduncular fossa. Located within this cistern is the optic chiasm as well as some important neurovascular structures which include: the basilar artery, the posterior cerebral and thalamoperforating arteries, the anterior pontomesencephalic vein and the proximal portions of the oculomotor nerves.
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The primary function of the subarachnoid space is to house CSF which cushions the brain and the spinal cord whilst also providing nutrients and removing waste. The structural components of the subarachnoid space, such as the arachnoid trabeculae, also function to provide support and stabilization to the brain and spinal cord. The subarachnoid space also provides an expanse for important neurovascular structures to pass through as they supply the brain and spinal cord.
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Injury, rupture and/or damage to the cerebral vessels within the subarachnoid space may result in a subarachnoid hemorrhage. As its name suggests, a subarachnoid hemorrhage refers to bleeding within the subarachnoid space. Most subarachnoid hemorrhages are caused by injury to the cerebral vessels, however, a small number of cases may be caused by vascular malformations such as arterial blockages or aneurysms.
A buildup of blood within the subarachnoid space may lead to an increase in intracranial pressure and subsequent herniation. Furthermore, an increase in subarachnoid pressure may impede the vascular structures within this area leading to obstructed blood flow to the brain.
Characteristic signs of a subarachnoid hemorrhage include sudden onset ‘thunder-clap’ headaches, nausea, seizures and diplopia (double vision).
A diagnosis of subarachnoid hemorrhage is achieved through the use of CT imaging or through analysis of CSF by lumbar puncture.
Treatment of subarachnoid hemorrhage differs depending on the cause and severity, but may involve repair of the damaged vessel and/or pain medication. Left untreated, a subarachnoid hemorrhage may lead to brain damage or death.
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