A subdural hematoma combined with a brain hernia (in white) courtesy James Heilman, MD, via Wikipedia
A subdural hematoma is a common complication of an injury to the skull where blood accumulates between the outer layer of the cerebellum (the "arachnoid layer") and the inner layer of the skull (the "dura"). The bleeding is almost always from the veins that bridge this space within the skull. They can be life threatening if serious as they increase intracranial pressure, but all subdural hematomas require medical attention. If a person suffers a head injury, they should be routinely given an MRI or CT scan to rule out the condition. They are more common to occur in the very young and the very old.
Unlike an epidural hematoma, which usually progresses quickly and has external symptoms, subdural hematomas can often be "ticking time bombs". As the veins are not always under pressure, they can progress slowly and not show other symptoms until days or even weeks after a head injury. In addition, a subdural hematoma will show no sign on the outside of the skull.
Once a subdural hematoma is detected in a scan, if it is not serious the patient can be monitored and it will often correct itself. However, if the hematoma is serious, it usually requires some type of surgery. This can range from minimally invasive to opening up the skull to stem the bleeding. However, in rare cases, the hematoma may be inoperable and all that can be offered is palliative care.