Subdural hematoma (SDH) is a much more ominous injury than is EDH, almost always associated with significant underlying brain damage. The typical trauma is a fall or MVA, where the head—and thus the brain—is in motion and strikes a stationary surface. In this scenario the brain continues in motion when the head itself is stopped, meaning the brain smashes against the inside of the skull. The inside of the skull is an unforgiving place for a brain in motion.
SDH is usually venous bleeding, caused by the tearing of small to moderate sized veins coursing across the surface of the brain itself. These vessels ooze more than pulsate, and often the bleeding stops before long—though not always. Even if the bleeding stops, the underlying damage to the brain produces swelling, which takes up crucial space inside the closed skull. The modest sized SDH is also taking up crucial space and for this reason needs to be removed.
Unfortunately, the brain damage is already done at the time of the injury and cannot be reversed by the surgeon. Thus, although surgery is necessary and life-saving, the patient is often severely compromised and rarely recovers to normal. At best, a prolonged period of rehab will be necessary.
Note that this is different from chronic SDH, which is a chronic, slowly expanding blood clot which sometimes forms over the surface of the elderly brain. Its treatment is removal and recovery is generally good, though it also has a tendency to recur.