An epidural hematoma is a pocket of blood that forms immediately outside the dura mater. The dura mater is the fibrous outermost sheath or membrane that encloses the brain and spinal cord. Epidural means outside the dura, and hematoma means mass of blood.
Epidural hematomas usually form when a violent blow breaks a blood vessel in the space outside the dura mater, whether in the skull or in the spinal column. In the skull, the vessel most often responsible for epidural hematoma is the middle meningeal artery.
Blood from the broken vessel forms a pressurized pocket of blood, like a large, internal blood blister. The growing hematoma pushes against the rigid bone of the skull or spinal column and thus exerts pressure on the dura mater, which in turn pushes on the brain or spinal cord. This pressure may stretch and tear blood vessels or even force the brain to herniate (i.e., partially squeeze out) through the foramen magnum, the hole in the bottom of the skull through which the spinal cord enters, or through the tentorium cerebelli, the part of the dura mater that covers the cerebellum and supports the occipital lobes from below. Herniation of the brain is likely to be fatal.
Epidural hematomas are less common than subdural hematomas, which are the most common mechanism of fatal brain damage in head trauma. They are also distinguished from intracranial hematomas, volumes of blood that collect inside the brain rather than at its surface.
Larry Gilman, The Gale Group Inc., Gale, Detroit,