Epidural hematomas occur with trauma or skull fractures that tear meningeal arteries (especially middle meningeal artery branches). Blood from the tear dissects the outer layer of the dura from the skull, forming a space-occupying mass in what was normally only a potential space.
The hematoma may compress adjacent brain tissue, producing localized signs, and may also cause herniation of distant brain regions across the free edge of the tentorium cerebelli (a transtentorial herniation) or across the falx cerebri (a subfalcial herniation). Such herniation may produce changes in consciousness, breathing, and blood pressure, and altered motor, pupillary, and other neurological signs. It may be fatal. Severe head trauma in an adult may tear bridging veins that lead from the brain through the subarachnoid space and into the dural sinuses, especially the superior sagittal sinus. The subsequent venous bleeding dissects the arachnoid membrane from the inner layer of the dura, and the blood accumulates as a subdural hematoma. The subdural space is normally only a potential space. Some of the proteins and other solutes in the hematoma attract edema, adding fluid accumulation to the hematoma and further exacerbating the space-occupying nature of the bleed. A subdural hematoma also may be associated with bleeding directly into the brain, an intracerebral hematoma.