A subdural hematoma (SDH) is a type of bleeding in which a collection of blood usually associated with a traumatic brain injury gathers between the inner layer of the dura mater and the arachnoid mater of the meninges surrounding the brain. It usually results from tears in bridging veins that cross the subdural space.Subdural hematomas may cause an increase in the pressure inside the skull, which in turn can cause compression of and damage to delicate brain tissue. Acute subdural hematomas are often life-threatening. Chronic subdural hematomas have a better prognosis if properly managed.In contrast, epidural hematomas are usually caused by tears in arteries, resulting in a build-up of blood between the dura mater and the skull. The third type of brain hemorrhage, known as a subarachnoid hemorrhage, causes bleeding into the subarachnoid space between the arachnoid mater and the pia mater.