A pocket of blood that forms between the skull and the tough outer layer of the brain’s protective cover, called the dura mater, is known as an epidural hematoma. Epidural means outside the dura, and hematoma means mass of blood. Epidural hematomas are not as common as subdural hematomas, and are most often the result of bleeding from higher-pressure arteries. The more common subdural hematomas result from bleeding of lower pressure veins.
The bleeding forms a pocket of blood that increases the pressure inside the skull. As the pocket of blood, or hematoma, grows, the pressure keeps getting higher and pushes on the brain. This pressure can damage the brain, and in some cases even push part of the brain through the hole in the bottom of the skull that the spinal column passes through. This is called herniation which is likely to be fatal.