This hematoma is a slow bleed and occurs due to a tear of bridging veins. Bridging veins travel through the subarachnoid space and subdural space. They can be injured leading to bleeding in the subdural space.
The venous system is lower pressure than the arterial system and the bleeding is thus slow. Patients usually present within 48 hours of injury but may not present for days to weeks after injury.
The subdural space exists between the dura mater and arachnoid layer. This is where blood accumulates in a subdural hematoma (below the dura).
Subdural hematoma appears crescent-shaped on CT versus epidural hematoma, which is lens-shaped.
Unlike epidural hematoma, subdural bleeds can cross suture lines. This is a common finding on imaging.
As blood accumulates and pressure increases, subdural hematoma can lead to midline shift of the brain.
Due to brain atrophy and shrinking that occurs with aging, more tension is placed on the bridging veins. The brain can move more thus stretching the bridging veins and making them more susceptible to tear and injury.
Acceleration-deceleration injuries can lead to subdural hematoma. As the brain continues to move in the CSF after acceleration or deceleration, tearing of the bridging veins may occur.
Babies have thin-walled bridging veins and are highly susceptible to subdural hematoma, particularly when they are shaken. Subdural hematoma in a baby is abuse until proven otherwise.
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