A subdural hematoma is a collection of blood in the space between the outer and middle layers of the covering of the brain (the meninges). It is most often caused by torn, bleeding veins on the inside of the brain covering after a blow to the head.

A subdural hematoma most often affects people who are prone to falling. Only a slight bump on the head (or even a fall to the ground without hitting the head) may be enough to tear veins in the brain, often without fracturing the skull. There may be no external evidence of the bruising on the brain's surface.

Small subdural hematomas may not be very serious, and the blood that seeps out can be slowly absorbed over several weeks. Larger hematomas, like a big bruise, can gradually enlarge over several weeks even after the bleeding has stopped. This enlargement can...