A subdural hematoma is a buildup of blood on the surface of the brain. The blood builds up in a space between the protective layers that surround your brain.
Your brain sits within a bony skull. Inside your skull and over the brain there are three layers called the meninges. These layers cover and protect the brain. The outermost covering is just inside the skull. It's a tough, fibrous layer called the dura mater, or usually just dura. On the inside of the dura is a layer called the arachnoid. Closest to the brain is a membrane called the pia. Cerebrospinal fluid (CSF) fills the space inside the meninges. CSF also helps cushion your brain.
The dura lines the skull and surrounds the brain. In a subdural hematoma, the blood seeps between the dura and the arachnoid layers. It collects inside the brain’s tough outer lining. This bleeding often comes from a blood vessel that breaks within the space around the brain. This most often happens because of a head injury. The injury can be mild. The blood may press against the brain and damage the tissue. A subdural hematoma can be life-threatening.
A subdural hematoma is not uncommon. It happens in some people who get a severe head injury. Younger men are most likely to get these types of injuries. Older adults are at higher risk for a subdural hematoma, even with mild head injury. This is because the veins surrounding the brain are more likely to tear. About a third of subdural hematomas happen in older adults. In babies, subdural hematomas are often from shaken baby syndrome.