A blood clot outside the brain is known as a subdural hematoma. Severe head injuries are the most common cause. A subdural hematoma can cause life-threatening bleeding and increased pressure on the brain. Some come to a halt and then vanish, while others require surgical drainage. Blood collects between the tissue layers that surround the brain in a subdural hematoma. The dura is the outermost layer of the body. Bleeding happens in between the dura and the next layer, the arachnoid, in a subdural hematoma. The bleeding occurs beneath the skull and outside the brain, rather than inside the brain. Blood pooling, on the other hand, puts extra pressure on the brain. The pressure on the brain causes symptoms. For example, a subdural hematoma can cause unconsciousness and death if the pressure inside the skull climbs to dangerously high levels.