A hematoma is a collection of blood resulting from internal bleeding; a subdural hematoma (SDH) is a type of traumatic brain injury. Subdural hematomas occur when blood collects between the covering of the brain (the dura) and the brain itself (specifically, the arachnoid layer of the meninges). SDH, which is caused by tears in veins surrounding the brain, differs from typical hematomas, which are usually caused by tears in arteries. Subdural hematomas may result from traumas to the head or from an aneurysm (the spontaneous rupturing of a blood vessel in the brain).

One of the dangers of hematomas is an increase in intracranial pressure (ICP). This can force the delicate tissue of the brain against the abrasive inside of the skull, causing considerable pain and damage. The pressure may result in the slurring of speech, difficulty walking or dizziness; if not treated it can culminate in a coma or death. Severe head trauma can result in an Acute Subdural Hematoma (ASDH), which is a medical emergency and is often fatal. Subdural hematomas occur more frequently in alcoholics and individuals over the age of 50. SDH can be diagnosed by MRI or CAT scan.