The subarachnoid space is a fluid-filled cavity located between the membranes that cover the central nervous system. This continuous space exists around both the brain and the spinal cord. It is part of the nervous system’s protective layers, contributing to its physical and physiological safeguarding.
Anatomical Location and Structure
The brain and spinal cord are enveloped by three protective membranes called the meninges: the dura mater, arachnoid mater, and pia mater. The subarachnoid space is the interval between the arachnoid mater and the pia mater. This space surrounds the brain and extends down the entire length of the spinal cord, terminating at the level of the second sacral vertebra.
Connecting the arachnoid and pia mater across this space is a delicate network of fibrous, web-like filaments called arachnoid trabeculae. These trabeculae give the layer its spider-like name and create a supportive, lattice-like framework that helps to suspend the brain and spinal cord. This structural arrangement allows for the circulation of fluid throughout the space.
While the subarachnoid space is a narrow gap, it widens in certain areas to form larger pools called subarachnoid cisterns. These cisterns are created where the arachnoid and pia mater are more widely separated. One of the largest is the cerebellomedullary cistern, located between the cerebellum and the medulla oblongata. Another is the lumbar cistern, found in the lower back, which contains the cauda equina nerve roots.
Primary Functions of the Subarachnoid Space
The primary role of the subarachnoid space is to contain cerebrospinal fluid (CSF), a clear liquid that bathes the central nervous system. This fluid is produced within the brain’s ventricles and circulates throughout the subarachnoid space. An adult has about 150 ml of CSF circulating at any time, and it is renewed four to five times daily through constant production and reabsorption.
One of the main functions of the CSF is to act as a shock absorber. It cushions the brain and spinal cord, protecting them from trauma from sudden movements or impacts. This fluid layer provides a buffer between the nervous tissue and the bony structures of the skull and vertebral column.
The CSF also provides buoyancy, which reduces the effective weight of the brain. While the brain weighs around 1,500 grams, its buoyancy in the CSF reduces its net weight to approximately 25 to 50 grams. This prevents the brain’s lower portions from being compressed under their own mass, which protects nerve tissue and blood vessels from damage.
Beyond physical protection, the CSF facilitates the removal of metabolic waste from the brain and spinal cord. It functions similarly to a lymphatic system for the central nervous system. The fluid also transports nutrients, hormones, and other substances necessary for nervous tissue health.
Associated Medical Conditions
Subarachnoid Hemorrhage
A subarachnoid hemorrhage (SAH) is a type of stroke that occurs when there is bleeding into the subarachnoid space. The most frequent cause of a spontaneous SAH is the rupture of a cerebral aneurysm, a weak, bulging spot on the wall of a blood vessel in the brain. Head trauma is another common cause.
The defining symptom of a subarachnoid hemorrhage is a sudden, severe headache, often described as the “worst headache of their life” and called a thunderclap headache. Other symptoms can include nausea, vomiting, a stiff neck, sensitivity to light, confusion, and loss of consciousness. Because blood can mix with and irritate the CSF, it can lead to complications like hydrocephalus, where blocked CSF drainage increases pressure inside the skull.
Meningitis
Meningitis is the inflammation of the meninges, which affects the membranes and the cerebrospinal fluid within the subarachnoid space. It is most often caused by a bacterial or viral infection but can also result from fungal or non-infectious causes. When pathogens enter the space, the immune system’s response causes inflammation, leading to symptoms like fever, headache, and a stiff neck.
Inflammation from meningitis can spread to blood vessels in the brain, potentially causing blood clots or damaging brain tissue. The infection causes the CSF to become cloudy with immune cells, and the pressure within the subarachnoid space often becomes elevated. Distinguishing between bacterial and viral meningitis is an important diagnostic step, as the bacterial form is severe and requires immediate antibiotic treatment.
Diagnostic and Therapeutic Procedures
Lumbar Puncture
Medical professionals can access the subarachnoid space to diagnose and treat conditions. The most common procedure is the lumbar puncture, or spinal tap. This test involves inserting a needle into the lower back’s subarachnoid space, between the L3/L4 or L4/L5 vertebrae, to withdraw a sample of cerebrospinal fluid. This location is chosen because the spinal cord ends at a higher level, minimizing the risk of injury.
The collected CSF sample can be analyzed for signs of disease. The presence of blood in the fluid can confirm a subarachnoid hemorrhage. Analysis of white blood cells, proteins, and glucose levels can help diagnose meningitis and distinguish between bacterial and viral causes. The CSF pressure can also be measured, which is often elevated in cases of meningitis or hydrocephalus.
Spinal Anesthesia
The subarachnoid space is also used for administering spinal anesthesia. In this procedure, local anesthetic is injected into the cerebrospinal fluid in the lumbar region. This injection blocks nerve impulses, causing a temporary loss of sensation and muscle paralysis in the lower half of the body. Spinal anesthesia is commonly used for surgeries involving the lower limbs, pelvis, or abdomen, and can be an alternative to general anesthesia.