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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.
Regardless of the mechanism, once a bridging vein is torn, the blood pools in the subdural
space creating a hematoma.
Since the damaged bridging veins are under low pressure, the bleeding can be slow causing
delayed onset of symptoms which might develop over the course of days to weeks as the hematoma
gradually expands.
So an acute subdural hematomas causes symptoms within 2 days, a subacute subdural hematoma
causes symptoms between 3 and 14 days, and a chronic subdural hematomas causes symptoms
after 15 days.
The hematoma can compress the brain and cause increased intracranial pressure.
A large subdural hematoma on one side of the skull can cause a midline shift which is a
displacement of the whole brain towards the opposite side of the skull.
The increased intracranial pressure can also cause the brain to herniate.
There can be a supratentorial herniation where the cerebrum is pushed against the skull or
the tentorium, and an infratentorial herniation where the cerebellum is pushed against the
brainstem.
Both of these can be lethal.
Supratentorial herniations can compress the arteries that nourish the brain leading to
an ischemic stroke and infratentorial herniations can compress the vital area in the brainstem
that control consciousness, respiration, and heart rate.
The most common symptoms of a subdural hemorrhage are loss of consciousness right after the
injury or in the ensuing days to weeks as the hematoma increases in size.
Other symptoms include headaches and vomiting.
Sometimes there can be focal neurological symptoms like muscle weakness or sensory problems
based on the location of the hematoma.
Diagnosis of a subdural hemorrhage is typically done with an imaging study.
On a CT scan, an acute subdural hematoma shows up as a hyperdense mass meaning that it looks
"more white" than the surrounding healthy brain tissue, whereas a chronic subdural hematoma
shows up as a hypodense masses which is "less white" than the surrounding brain tissue.
An "acute on chronic" bleeding is a combination of hyperdense and hypodense, which is seen
in individuals who have a rebleed in the bridging veins after a chronic hematoma has already
formed.
The blood in a subdural hemorrhage is between the inner and outer layers of the dura mater.
So on a CT scan, subdural hematomas follow the contour of the brain and form a crescent-shape
and cross suture lines.
In contrast, epidural hemorrhages cause blood to build up between the outer layer of the
dura mater and the skull.
Since the outer layer of the dura mater adheres tightly to the skull at sutures, epidural
hematomas don't cross suture lines and they push on the brain forming a biconvex shape.
Small subdural hematomas are drained by placing a small tube called catheter, through a drilled
hole in the skull.
Large subdural hematomas require a craniotomy, which is when part of the skull bone is removed
in order to remove accumulated blood below.
Alright, as a quick recap.
A subdural hemorrhage is a bleeding below the dura mater, caused by the rupture of the
bridging veins found within the subdural space.
Subdural hemorrhages are common in trauma, and in some groups, like the elderly, babies,
and individuals that abuse alcohol.
On a CT scan, an acute subdural hemorrhage forms a hyperdense blood collection, whereas
a chronic subdural hemorrhage forms a hypodense blood collection.
Subdural hematomas cross suture lines and follow the contour of the brain forming a
crescent-shape, unlike epidural hematomas which don't cross suture lines and push
on the brain forming a biconvex shape.
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