I've been hearing about bleeding in the brain after a TBI. Can you explain how this happens and why it's so dangerous? What exactly is a subdural hematoma?
Let's start with the basics. A hematoma is a tumor-like collection of blood, usually clotted, located outside a blood vessel. The subdural space is located between the dura mater (the outermost, toughest, and most fibrous of the three membranes covering the brain and spinal cord) and the arachnoid membrane (the middle of these three membranes). The epidural space is located on or outside the dura mater.
Now, to answer your question, a subdural hematoma (illustrated below) is basically bleeding into the space between the brain cover (dura mater) and the brain itself. More specifically, there are blood vessels running through the brain and in the spaces between the outside of the brain and the inside of the skull. During a brain injury, any of these vessels can tear and bleed. Ruptured vessels running in the subdural space — typically veins — cause subdural hematomas. Ruptured vessels running through the epidural space — typically arteries — cause epidural hematomas. Both types of bleeding take up space in the skull and in so doing squeeze down on the brain. Because arteries are under pressure, epidural hematomas leak quickly and compress the brain rapidly while subdural hematomas leak much more slowly.
Because the brain is enclosed within the rigid skull, it gets compressed by this leaking blood. With enough bleeding, the brain gets compressed so much that oxygen-rich blood is prevented from flowing into the brain tissue. This lack of oxygen causes additional brain swelling. This brain swelling, added to the blood leaking from the torn vessel, forces the brain down through the small hole at the base of the skull called the foramen magnum. The parts of the brain that come in direct contact with the bone around this opening get so compressed that they stop working. Because these brain areas control breathing and heart rate, death can result.
Getting to the hospital quickly is the best way to diagnose and treat this bleeding, which usually requires surgery to remove the blood and relieve the pressure on the brain.
Dr. Bazarian is an emergency physician with a strong research interest in traumatic brain injury. He is associate professor of Emergency Medicine, Neurology, and Neurosurgery at the Center for Neural Development and Disease, University of Rochester Medical Center.