Subdural hematomas are usually from the venous vasculature, and are often specifically as a result of tearing the cortical veins that bridge the subdural space en route to the dural sinuses. Because the inner dural layer and the arachnoid are not firmly attached the bleeding can spread over a large area and can form a distinctive crescent shape.

These extra-axial bleeds are often the result of blunt trauma that causes the brain to shift rapidly within the skull. Typically, it is the older population where these are most common, because cerebral atrophy can lead to more room for the brain to move, and more stress placed upon the bridging veins. Another unique characteristics of subdural hematomas is that they will not cross the dural reflections formed by the falx cerebri and tentorim (they will cross sutural margins which helps differentiate subdual from epidural bleeds). Also, a subdual bleed can be chronic with episodes of rebleeding which leads to a layering of blood as new bleeding occurs and old blood is being absorbed called a hematocrit effect.