Extradural haemorrhage
Trauma tears the middle meningeal artery against the thin temporal bone, and arterial pressure strips the dura off the skull, so the collection grows in hours rather than days.
Definition
Extradural haemorrhage is a collection of blood between the inner table of the skull and the outer, periosteal layer of the dura mater, usually arterial and usually traumatic. It is recognised radiologically as a biconvex extra-axial collection that does not cross suture lines.
First principles
Arterial pressure is why an extradural is the fast one
Most extradural bleeds are arterial, usually a branch of the middle meningeal artery torn by a temporal or pterional fracture. Arterial pressure peels the dura off the inner table of the skull, so mass effect develops over minutes to hours. That speed is why NICE guideline NG232 on head injury puts a 1-hour clock on the scan once consciousness is impaired. It is also why prompt evacuation can still give an excellent functional outcome. Up to 10% are venous instead, from a torn dural sinus or diploic vein, and those fill more slowly.
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