Subdural haemorrhage
Bridging veins shear between the mobile cortex and the fixed dural sinuses, bleeding beneath the dura to form a crescentic collection that crosses sutures but not the falx.
Definition
Subdural haemorrhage is blood between the dura and arachnoid, usually from torn bridging veins. It is staged by age as acute, subacute or chronic. Those labels are a radiological convention tracking the collection's changing density on CT, not a NICE definition. They matter because the chronic form is a different disease with a different operation.
First principles
A venous source produces two different diseases
Bridging veins rupture when a sudden change in head velocity moves the brain against the dura. A large tear bleeds fast enough to compress the brain within hours. A small one may seal, then organise into a membrane-bound collection that leaks and rebleeds for weeks. Acute and chronic subdural are not one disease at two speeds: they present differently, are operated on differently and have different outcomes.
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