The human brain is protected within the skull by membranes and a specialized fluid. This system has multiple layers, and among them is the subarachnoid space, a fluid-filled area surrounding the brain and spinal cord. This space is a component of the central nervous system’s protective mechanism. Understanding its structure is the first step in appreciating its role in neurological health.
Anatomy of the Subarachnoid Space
The brain and spinal cord are enveloped by three protective membranes known as the meninges. From the outermost layer inward, they are the dura mater, the arachnoid mater, and the pia mater. The dura mater is a tough, fibrous layer that attaches to the inner surface of the skull. Beneath it lies the arachnoid mater, a thinner membrane that does not adhere to the contours of the brain. The innermost layer, the pia mater, is a membrane that closely follows the surface of the brain and spinal cord.
The subarachnoid space is the interval located between the arachnoid mater and the pia mater. This area is maintained by a network of web-like connective tissue strands called trabeculae, which bridge the gap between the two membranes. This web-like structure gives the arachnoid layer its name, derived from the Greek word for spider. The space is filled with cerebrospinal fluid and also houses the major blood vessels that supply and drain the brain.
To visualize this arrangement, imagine the brain as a fruit. The tough outer skin represents the dura mater, a thin film on the fruit’s surface is the pia mater, and a layer between them is the arachnoid mater. The subarachnoid space is the fluid-filled gap between the arachnoid and pia mater layers.
The Role of Cerebrospinal Fluid
The subarachnoid space is filled with cerebrospinal fluid (CSF), a clear liquid that performs several functions. Its primary role is providing buoyancy, which reduces the brain’s effective weight from about 1,500 grams to only 25 grams. This allows the brain to float, preventing its lower portions from being compressed by its own weight against the skull.
The CSF also acts as a shock absorber, dampening the effects of sudden movements and minor trauma. When the head sustains an impact, the fluid distributes the force. This reduces the risk of the brain colliding with the interior of the skull.
The CSF contributes to the chemical stability and waste clearance of the brain. It circulates through the ventricles and the subarachnoid space, delivering nutrients and removing metabolic byproducts. This circulation functions like a lymphatic system for the brain, flushing away waste that could otherwise accumulate and disrupt neural function.
Subarachnoid Hemorrhage
A subarachnoid hemorrhage (SAH) is a medical emergency involving bleeding into the subarachnoid space. The most common cause of a spontaneous SAH is a ruptured cerebral aneurysm, which is a weak, bulging spot on a brain blood vessel. When an aneurysm bursts, blood is released under arterial pressure into the space.
Less frequent causes of SAH include traumatic brain injuries that damage blood vessels. Arteriovenous malformations (AVMs), which are tangled connections between arteries and veins, can also rupture and cause bleeding. The presence of blood in the subarachnoid space increases pressure on the brain and can damage brain cells.
The most prominent symptom of an SAH is a sudden, severe headache, often called a “thunderclap headache” and described as the “worst headache of my life.” Other symptoms can accompany the headache, and their severity depends on the extent of the bleeding. These include:
- Nausea and vomiting
- A stiff neck
- Sensitivity to light
- Blurred or double vision
- Loss of consciousness
Diagnosing and Treating a Hemorrhage
When an SAH is suspected, immediate medical evaluation is required. The primary diagnostic tool is a non-contrast computed tomography (CT) scan of the head, which is highly effective at detecting blood in the subarachnoid space. A CT scan can confirm a diagnosis quickly, allowing for prompt treatment.
If a CT scan is negative but symptoms strongly suggest an SAH, a lumbar puncture (spinal tap) may be performed. This procedure involves collecting a sample of cerebrospinal fluid from the lower back. The fluid is then analyzed for red blood cells, which confirms bleeding into the space.
The goal of treatment for an SAH from a ruptured aneurysm is to stop the bleeding and prevent it from happening again. One method is surgical clipping, an open neurosurgical procedure where a small metal clip is placed across the neck of the aneurysm. This clip blocks blood flow into it.
A less invasive alternative is endovascular coiling. A catheter is guided through blood vessels from the groin to the aneurysm in the brain. Tiny platinum coils are then deployed into the aneurysm, causing the blood to clot and sealing it off. The choice between clipping and coiling depends on the aneurysm’s characteristics and the patient’s health.