The brain and spinal cord, which form the central nervous system, are delicate structures requiring robust physical protection. This protection is provided by the bony skull and a series of three specialized membranes known collectively as the meninges. These coverings act as a supportive framework and contain the fluid that cushions the brain against sudden movement. Understanding the arrangement of these layers is fundamental to locating the subdural space and appreciating its significance.
The Protective Layers of the Brain
Dura Mater
The meninges consist of three distinct layers situated between the skull and the brain tissue. The outermost layer is the dura mater, meaning “tough mother” in Latin, reflecting its composition as a thick, dense fibrous membrane. The dura mater adheres closely to the inside of the skull, forming a strong barrier. It also houses a network of large veins known as the dural venous sinuses.
Arachnoid Mater
The arachnoid mater is the middle layer of the meninges. This membrane is much thinner and more delicate than the dura and is described as web-like due to its fine strands of connective tissue. It functions primarily as a barrier, preventing the passage of substances between the cerebrospinal fluid and the bloodstream.
Pia Mater
The innermost layer is the pia mater, meaning “tender mother.” This thin, translucent membrane closely adheres to the surface of the brain, following every contour and groove. It contains numerous small blood vessels that extend into the brain tissue, providing necessary blood supply.
Precise Location and Nature of the Subdural Space
The subdural space is the anatomical area situated directly between the dura mater and the arachnoid mater. Specifically, it lies deep to the inner layer of the dura mater and superficial to the arachnoid mater.
In a healthy individual, this is considered a “potential space,” not an open, fluid-filled cavity. The dura and arachnoid membranes are normally pressed together with only a very thin film of serous fluid between them. This thin fluid allows for minimal gliding movement, accommodating slight shifts of the brain within the skull.
The subdural space is distinct from the subarachnoid space, which is a true anatomical space located beneath the arachnoid mater and continuously filled with cerebrospinal fluid. The subdural space only becomes an actual, noticeable space when abnormal material, such as blood, collects there.
Why This Space Matters Clinically
The primary clinical significance of the subdural space relates to the formation of a subdural hematoma (SDH). This collection of blood occurs when trauma causes the separation of the dura and arachnoid layers. This separation turns the potential space into an actual space filled with blood, which then puts pressure on the underlying brain tissue.
The most common cause is the tearing of bridging veins that traverse this area. These fragile veins run from the brain’s surface, cross the subdural space, and empty into the dural venous sinuses. Sudden rotational forces or rapid acceleration/deceleration of the head can stretch and rupture these veins.
Because bridging veins carry low-pressure venous blood, the bleeding can sometimes be slow, allowing a hematoma to develop over days or weeks. This collection of blood is a serious medical event that leads to increased pressure within the skull and subsequent neurological damage.