A subdural hematoma is a collection of blood outside the brain. Subdural hematomas are usually caused by severe head injuries. The bleeding and increased pressure on the brain from a subdural hematoma can be life-threatening. Some subdural hematomas stop and resolve spontaneously; others require surgical drainage.
What Is a Subdural Hematoma?
In a subdural hematoma, blood collects between the layers of tissue that surround the brain. The outermost layer is called the dura. In a subdural hematoma, bleeding occurs between the dura and the next layer, the arachnoid.
The bleeding in a subdural hematoma is under the skull and outside the brain, not in the brain itself. As blood accumulates, however, pressure on the brain increases. The pressure on the brain causes a subdural hematoma's symptoms. If pressure inside the skull rises to very high level, a subdural hematoma can lead to unconsciousness and death.
Causes of Subdural Hematoma
Subdural hematoma is usually caused by a head injury, such as from a fall, motor vehicle collision, or an assault. The sudden blow to the head tears blood vessels that run along the surface of the brain. This is referred to as an acute subdural hematoma.
People with a bleeding disorder and people who take blood thinners are more likely to develop a subdural hematoma. A relatively minor head injury can cause subdural hematoma in people with a bleeding tendency.
In a chronic subdural hematoma, small veins on the outer surface of the brain may tear, causing bleeding in the subdural space. Symptoms may not be apparent for several days or weeks. Elderly people are at higher risk for chronic subdural hematoma because brain shrinkage causes these tiny veins to be more stretched and more vulnerable to tearing.
Subdural hematoma can also rarely result from a complication of a spinal tap.