The brain is protected by several layers, including the subdural space. This space is typically a narrow, potential gap within the skull, not usually open but able to expand under certain conditions.
The Brain’s Protective Layers
The brain and spinal cord are enveloped by three protective membranes called meninges: the dura mater, arachnoid mater, and pia mater. Each has distinct characteristics.
The outermost layer, the dura mater, is a thick, tough membrane directly beneath the skull. It consists of two connective tissue layers, one adhering to the skull and the other to the arachnoid mater. This layer contains dural venous sinuses, which drain blood and cerebrospinal fluid from the brain.
Beneath the dura mater lies the arachnoid mater, a delicate, web-like membrane. This middle layer lacks blood vessels and nerves, with fine projections connecting it to the innermost layer. It aids in cerebrospinal fluid circulation and absorption, which cushions the brain.
The innermost layer, the pia mater, adheres tightly to the brain and spinal cord surface, following every contour. Small blood vessels pass through it to supply brain tissue. These three meningeal layers provide a comprehensive protective system for the brain.
Locating the Subdural Space
The subdural space is located between the dura mater and the arachnoid mater. In a healthy individual, this area is not an open cavity but a “potential space.” This means the dura and arachnoid layers are normally pressed together, with little actual gap.
A “potential space” can be compared to an uninflated balloon, where the sides are collapsed. An actual space only forms if fluid or other substances accumulate, pushing these layers apart. While it may contain a thin film of fluid, it becomes noticeable only during trauma or certain medical conditions.
The Clinical Significance of the Subdural Space
The subdural space is a common site for medical conditions, particularly bleeding. A subdural hematoma, the most frequent issue, occurs when blood collects within this potential space. This bleeding often results from tearing bridging veins that cross the subdural space to drain blood from the brain into the dural venous sinuses.
Subdural hematomas typically develop after a head injury, where sudden forces can stretch and rupture these veins. The accumulated blood expands the collapsed subdural space, creating pressure on the underlying brain tissue. This pressure can interfere with brain function and lead to serious complications. In older individuals, brain shrinkage can increase tension on these veins, making them more susceptible to tearing from minor trauma.