The meningeal layers covering the brain consist of an outermost layer (the dura mater), a middle layer (the arachnoid membrane), and an inner layer (the pia mater). Between these layers are potential spaces where fluid can collect. Hemorrhage above the dura is called an epidural hematoma. Hemorrhage below the dura is called a subdural hematoma (SDH), and hemorrhage below the arachnoid membrane is called subarachnoid hemorrhage. Bleeding within brain tissue itself is called intraparenchymal hemorrhage, and bleeding into the ventricles is intraventricular hemorrhage. The prognosis and management of these findings are different, so misnomers such as “head bleed” should be avoided when describing an intracranial hemorrhage. Subdural hematomas can grossly be divided into acute, subacute, and chronic. The duration of an acute hematoma is less than 3 days; that of subacute hematoma is from 3–20 days. A chronic hematoma persists for more than 21 days.

Acute subdural hematoma

Historically, the occurrence of acute SDH has been posited to be due to tearing of the bridging veins. This condition can often result from the direct extension of bleeding from a lacerated or contused brain. The most common reported causes of acute SDH are motor vehicle collisions, falls, and assault. This type of injury is seen 3–5 times more often in males than in females; median age is generally in the 40s. As compared with patients with epidural hematoma, patients with acute subdural hematoma are more likely to have associated parenchymal damage and a higher morbidity and mortality. An important category of subdural hematoma patients are those on anti-coagulation therapy. Patients taking anti-coagulants are at risk for intracranial hemorrhage following low-impact injuries such as ground level fall or low-velocity motor vehicle collision. Furthermore, in this patient population, SDH often develops spontaneously and without evidence of antecedent trauma.