Subdural hematoma

Subdural hemorrhage can be broken down. Sub means below, “dural” which refers to the outermost protective layer of the brain, which is called dura mater, and “hemorrhage” refers to bleeding. So, a subdural hemorrhage is when there’s bleeding below the dura mater.

OK - let’s start with some basic brain anatomy. The brain is protected by 3 thin layers of tissue called the meninges which covers the brain and spinal cord.

The inner layer of the meninges is the pia mater, the middle layer is the arachnoid mater, and the outer layer is the dura mater. The pia and arachnoid maters, are also called leptomeninges.

Between the leptomeninges, there’s the subarachnoid space, which houses cerebrospinal fluid, or CSF.

CSF is a clear, watery liquid which is pumped around the spinal cord and brain, cushioning them from impact and bathing them in nutrients. The outer membrane, the dura mater consists of two layers.

The internal layer of the dura mater lies above the arachnoid mater - the two are separated by the subdural space. The external layer of the dura mater adheres to the inner surface of the skull.

These two layers of the dura mater travel together, but at certain spots, the internal layer of the dura mater separates from the external one to form the meningeal folds.

The meningeal folds help divide the sections of the brain like the falx cerebri which separates the two hemispheres of the cerebrum, and the tentorium, which covers the cerebellum and separates it from the cerebrum. The subdural space plays a major role in venous blood drainage in the brain.

The surface of the brain is supplied by numerous arteries in the subarachnoid space that provides oxygen rich blood to the brain.

After the brain tissue has taken up the oxygen and nutrients, the blood drains into superficial cerebral veins, or bridging veins, that also sit in the subarachnoid space.

These veins travel through the arachnoid mater, pass through the subdural space and penetrate the inner layer of the dura mater to drain into the dural venous sinuses located between the two layers of the dura mater.

Eventually the blood in the venous sinuses drain into the internal jugular vein and returns to the heart.

So, the main cause of a subdural hemorrhage is a rupture of the bridging veins located in the subdural space.

When a blood vessel is damaged and there’s active bleeding, it’s called a hemorrhage, and the collection of blood that results is called a hematoma.

Some people, like the elderly often have brain atrophy where their brain shrinks in size, and that means that the bridging veins are stretched across a wider space where they are largely unsupported - a bit like a long rickety bridge across a mountain pass.

In infants and in individuals that abuse alcohol the veins are thin walled, and therefore more likely to break.

Subdural hematomas are common in head traumas like falling in a wet bathtub and whacking your head or in shaken baby syndrome where a baby is violently shaken, making their head whip back and forth damaging the bridging veins.

Subdural hematomas can also occur after an acceleration-deceleration injury like speeding on the road and then suddenly slamming the brakes.

When that happens, the seatbelt will stop the body and head from moving forwards but the momentum will carry the brain forward where it impacts the front of the skull.

Right after that, the head moves backwards and hits the headrest causing the brain to impact the back of the skull. That rapid back and forth movement once again causes damage to the bridging veins.