Subarachnoid Space

• The subarachnoid space is the space between the arachnoid mater and pia mater. It’s filled up with CSF which enters it from the ventricular system of the brain.
• It is traversed by thread like trabeculae going from the arachnoid to pia, supplying it a spider’s web look, for this reason the name arachnoid (arachnoid = like spider’s web) and creates a type of fluidfilled up sponge. The arteries and veins of the brain are located in this space.
• The subarachnoid space around the brain interacts with all the ventricular system only via the foramina in the roof of the 4th ventricle and is constant with the subarachnoid space around the spinal cord in the foramen magnum.
• The cerebrospinal fluid in the subarachnoid space functions as a mobile buffer to disperse and equalize pressures inside the skull.

Subarachnoid Cisterns

The enlargements of subarachnoid space, in particular situations, around the brain are referred to as subarachnoid cisterns. They include large amount of CSF creating ‘pools of CSF’. The cisterns are seen where the brain, closely covered with pia mater, is located at some distance from the arachnoid lining the dura mater. The cisterns serve as a water bed to the brain.

The different types of subarachnoid cisterns are as follows:

• Cerebellomedullary cistern (cisterna magna): It’s the largest cistern and created by the arachnoid, bridging the period between the medulla oblongata and the infe-rior surface of the cerebellum. Consequently, it is located in the triangle created by the cerebellum, the medulla oblongata and the occipital bone. It’s directly continuous inferiorly with the posterior part of the spinal subarachnoid space.

The cerebellomedullary cistern is easily reachable to a needle inserted anterosuperiorly via the posterioratlanto-occipital membrane, between the posterior arch of atlas and the posterior margin of the foramen magnum. For that reason, it’s used for cisternal puncture if lumbar puncture isn’t possible to remove CSF by clinicians for curative and diagnostic functions.

This cistern is triangular in sagittal section. It’s the only cistern which directly interacts with the ventricular system of the brain via openings (foramen of Magendie and foramina of Luschka) in the roof of the 4th ventricle.

• Pontine cistern (cisterna pontis): This is an extensive subarachnoid space on the ventral surface of the pons and includes the basilar artery and its branches. It’s continuous below with the subarachnoid space of the spinal cord, behind with the cerebellomedullary cistern and rostrally with the interpeduncular cistern.
• Interpeduncular cistern (basal cistern): It’s created by the arachnoid mater bridging across the 2 temporal lobes on the inferior aspect of the brain and includes the circle of Willis (circulus arteriosus).

Interpeduncular cistern is constant laterally with the subarachnoid spaces enclosing the middle and posterior cerebral arteries and anteriorly with the subarachnoid spaces around the anterior cerebral arteries.

• Cistern of lateral sulcus/fossa (sylvian cistern): It is located in front of every temporal post and is composed by the arachnoid mater bridging the lateral sulcus. It includes the middle cerebral artery.
• Cistern of great cerebral vein (cisterna superior or cisterna ambiens): It takes up the period between the splenium of corpus callosum and the superior surface of cerebellum. This cistern includes the great cerebral vein of Galen and pineal gland and is widely used as a neurosurgical landmark.

The different cisterns are large pools of CSF and communicate freely with every other. The pulsations of arteries inside CSF help to drive the CSF from cisterns onto the superolateral surfaces of the cerebral hemispheres and after that into arachnoid villi and granulations.