Accumulation of blood in the subdural space between the dura mater and the arachnoidal layer of the meninges. This condition primarily occurs over the surface of a CEREBRAL HEMISPHERE, but may develop in the spinal canal (hematoma, subdural, spinal). Subdural hematoma can be classified as the acute or the chronic form, with immediate or delayed symptom onset, respectively. Symptoms may include loss of consciousness, severe headache, and deteriorating mental status.