Subarachnoid

Subarachnoid is an anatomical term that describes structures relating to the subarachnoid space, the potential space between the arachnoid mater and the pia mater that surrounds the brain and spinal cord. This space contains cerebrospinal fluid and the major vessels supplying the central nervous system, and it serves as a conduit for CSF circulation surrounding neural tissue.

In the cranium, the subarachnoid space envelops the brain and communicates with the ventricular system via the openings of the fourth ventricle, specifically the foramina of Luschka and Magendie. It forms cisternal enlargements such as the cisterna magna and prepontine cistern, where CSF can be sampled or observed more easily. The subarachnoid space also extends around the cranial nerves and is continuous with the spinal subarachnoid compartment.

The spinal subarachnoid space extends within the vertebral canal from the foramen magnum to the second sacral level. Within the spinal canal, the space contains the cauda equina and is typically widest in the lumbar region, where lumbar puncture can access CSF.

Functionally, the subarachnoid space provides buoyancy and protection for the brain and spinal cord, while enabling the circulation and exchange of nutrients and waste via cerebrospinal fluid. CSF is produced mainly by the choroid plexus, flows into the subarachnoid space, and is reabsorbed into venous blood through arachnoid granulations into the dural venous sinuses.

Clinically, involvement of the subarachnoid space is notable in subarachnoid hemorrhage, usually from rupture of intracranial aneurysms or trauma, presenting with sudden severe headache and meningeal signs. Diagnosis relies on imaging and CSF analysis, and treatment depends on the underlying cause. Other conditions affecting this space include infections and inflammatory arachnoiditis.