An EDH is a collection of blood which forms between the outer layer of dura (actually the periosteum) and the inner table of the skull vault. As it is essentially a subperiosteal haematoma, it is limited by the sutures, as opposed to SDHs which will cross these with impunity (click here for example). Conversely, an EDH can cross venous sinuses as long as there is no suture there also.
Again, due to their location, it is not difficult to understand why EDHs tend to occur in young patients: anyone who has assisted in craniotomies on older patients will know how much more stuck to the bone an old person’s dura is, and how much more frequent dural tears are.
The source of bleeding is typically a torn meningeal artery, usually middle meningeal, from an associated fracture. As the dura is stripped from the bone, pain is caused. In about 5 - 10% of patients the EDH is posterior fossa. Occasionally an EDH can form due to venous blood, typically a torn sinus with associated fracture.
Prognosis is in general quite good, as long as the clot is evacuated in time.
Reference: Osborn A. Diagnostic imaging: Brain Amirsys 2004
Credit: Dr Frank Gaillard