The subarachnoid space (SAS) is a fluid-filled cavity surrounding the brain and spinal cord, protecting the central nervous system (CNS). This space contains cerebrospinal fluid, blood vessels, and connective tissue structures. It extends the entire length of the CNS and facilitates several biological processes essential for neurological health and function.
Anatomical Placement of the Space
The subarachnoid space is defined by the three membranes, known as the meninges, that encase the brain and spinal cord. These layers are named, from outermost to innermost: the dura mater, the arachnoid mater, and the pia mater. The dura mater is a tough, dense connective tissue layer lying directly beneath the skull and vertebral column.
The arachnoid mater is the middle layer, situated beneath the dura mater. The subarachnoid space lies beneath the arachnoid mater and is bounded on its inner side by the pia mater. The pia mater closely adheres to the surface of the brain and spinal cord, following all of its contours and grooves.
The SAS is the interval between the arachnoid and pia mater, and its name literally refers to the “spider-like” appearance of its contents. A network of fine, web-like connective tissue strands, called arachnoid trabeculae, spans the gap, providing structural support throughout the space. Because the pia mater follows the brain’s surface tightly while the arachnoid mater bridges over deeper grooves, the depth of the space is not uniform. Enlarged pockets called subarachnoid cisterns are formed where the membranes are widely separated, containing larger volumes of fluid.
The Essential Function of Cerebrospinal Fluid
The subarachnoid space is continuously filled with Cerebrospinal Fluid (CSF), a clear, colorless liquid produced primarily by the choroid plexus within the brain’s ventricles. The body generates 400 to 600 milliliters of CSF daily, though only about 150 milliliters circulate at any given time. This continuous production and circulation ensures the fluid is constantly refreshed.
A primary role of CSF is providing buoyancy to the brain, reducing its effective weight significantly. This buoyancy prevents the brain from being crushed against the base of the skull. The CSF also acts as a shock absorber, cushioning the brain and spinal cord against sudden movements and external impacts.
After circulating through the ventricular system, the CSF exits into the subarachnoid space to bathe the entire CNS surface. It is then reabsorbed into the bloodstream through specialized structures called arachnoid granulations, which project into the dural venous sinuses. Beyond physical protection, the fluid maintains chemical stability, supplying nutrients to nervous tissue and actively removing metabolic waste products. This waste removal, including the clearance of proteins and cellular debris, is partially facilitated by the glymphatic system, a network that utilizes the CSF for brain cleansing.
Clinical Relevance: Subarachnoid Hemorrhage
The subarachnoid space contains the major cerebral arteries and veins that supply the brain, making it a site of clinical concern if bleeding occurs. A subarachnoid hemorrhage (SAH) happens when blood enters this space, usually due to the rupture of a blood vessel. This condition requires immediate medical intervention.
The most frequent cause of spontaneous SAH is the rupture of a cerebral aneurysm, a weakened, bulging area in a blood vessel wall. Trauma, such as a severe head injury, is another common cause, particularly in younger individuals. When blood spills into the SAS, it rapidly increases pressure inside the skull, which can lead to brain cell damage and permanent disability.
The primary symptom of SAH is a sudden-onset headache, often described as the “worst headache of their life” or a “thunderclap headache.” Other symptoms include nausea, vomiting, neck stiffness, and a decreased level of consciousness. Blood in the space can also lead to delayed complications, such as cerebral vasospasm, where blood vessels narrow, restricting blood flow. Prompt diagnosis, often involving a CT scan and sometimes a lumbar puncture, is necessary for managing SAH.
Diagnostic Use in Medical Procedures
The accessibility of the subarachnoid space makes it a target for certain diagnostic and therapeutic medical procedures. The most common procedure accessing this space is the lumbar puncture. This technique involves inserting a needle into the lower back to withdraw a sample of CSF.
The procedure is performed in the lumbar region, typically between the L3 and L5 vertebrae, to safely enter the lumbar cistern, an enlarged part of the SAS. Since the spinal cord ends higher up, the needle does not risk damaging nervous tissue. The collected CSF sample is analyzed to diagnose conditions such as meningitis, certain cancers, or neurological disorders like multiple sclerosis.
A lumbar puncture also allows clinicians to measure the opening pressure of the CSF, which gauges intracranial pressure within the skull. Elevated pressure can indicate blockages in CSF circulation or the presence of a mass lesion. Furthermore, the subarachnoid space is sometimes used to inject medications, such as chemotherapy drugs or anesthetics, allowing them to bypass the blood-brain barrier and act directly on the central nervous system.