TBI Inform: Introduction to Brain Injury Tom Novack, PhD, UAB Traumatic Brain Injury Model System/Center for Neuro Skills, TBI Resource Guide (page 2 of 3) Page 2 of 3

Pathology

What happens to the brain as a result of a traumatic brain injury? The brain is about 3 to 4 pounds of soft tissue that is extremely delicate. Nature views the brain as a very important organ, providing it with several protective layers. The brain is composed of about 15 to 20 billion neurons plus other support cells. To understand what happens in head injury, you have to know about the skull as well as the brain. The brain is essentially floating, supported in fluid within the skull. The brain tissue is soft and therefore can be compressed, pulled, and stretched. When there is sudden acceleration and deceleration, such as in a car crash or fall, the brain can move inside the skull.

Localized Injury

Localized injury means that a particular area of the brain is injured. A good example of this is contusion (bruising) to the brain in a particular area. When there is acceleration and deceleration the brain can strike the inside of the skull. It can bounce back and forth, hitting the back of the skull and the front of the skull, causing bruising to the brain. Because of the way that the temporal and the frontal lobes fit in the skull, these are the areas of the brain most likely to be contused. Another type of localized injury is a hematoma. This is when a blood clot forms. Again the movement of the brain within the skull is the culprit, causing the blood vessels that cover the surface of the brain to be pulled, stretched, or torn. This will cause bleeding. A relatively small amount of bleeding is often referred to as a subarachnoid hemorrhage. If larger blood vessels are torn you might have a subdural hematoma or an epidural hematoma on the surface of the brain. These are named differently because of where they occur in relation to protective layers of the brain. An intracerebral hematoma or hemorrhage is when there is bleeding within the brain tissue itself. One of the major problems caused by hematomas is that they press the soft tissue of the brain, pushing it out of the way. Eventually, if the pressure is sufficient and treatment (usually surgery) is not done, the person may die.

Diffuse Axonal Injury

The brain is composed of billions of neurons, very small specialized cells. Axons extend from each neuron cell body in the brain. The axons allow communication from one neuron to another neuron. A neuron creates an electrical signal by way of a chemical reaction. That signal is sent down the axon of the neuron to excite more neurons. Everything our brains do for us is based on groups of neurons working together. When the brain moves, such as occurs in an acceleration/deceleration event, axons can be pulled, stretched, and torn. If there is sufficient injury to the axon, the cell will not survive. That happens all over the brain, not just in one area, which is why it is called diffuse axonal injury. This kind of injury does not show up on a CT or MRI scan because the scan is not a microscope that can see these tiny cells. Diffuse axonal injury is often diagnosed on the basis of the person’s symptoms. Any time there is a loss of consciousness associated with a trauma event, there will be DAI. The more severe the DAI, usually the longer the duration of loss of consciousness.

Secondary Brain Injury

Localized and diffuse axonal injuries occur at the time of injury and there is nothing that physicians can do to reverse those injuries. Instead, the goal of the treatment team in the hospital is to prevent any further, or secondary, injury to the brain. One example of secondary injury is hypoxia, or not getting sufficient oxygen to the brain. This can occur when the person is not breathing or their blood pressure is too low. The end result is further brain injury.

Another problem leading to secondary brain injury can be increased intracranial pressure, which can come from significant swelling of the brain, often referred to as edema. Physicians try to bring the pressure down because if the edema is great enough, it prevents blood flow into the tissue. To measure intracranial pressure the physician may insert a ventriculostomy. A small hole is bored through the skull and a tube is inserted into a fluid cavity (we normally have several in our brain) that allows for measurement of pressure. The ventriculostomy also has a valve that will open if pressure becomes too high, allowing fluid to drain from the brain and thus lowering the pressure. To better control intracranial pressure and deal with other medical issues the injured person may be sedated and given paralytic agents, drugs that temporarily paralyze the skeletal muscles. While receiving these drugs the person will not be able to move his/her arms or legs. Because chest muscles are also affected the person is unable to breathe independently and so must be on a respirator.

Incidence of Traumatic Brain Injury

Based on information from the Centers for Disease control, the incidence of traumatic brain injury is about 85 cases per 100,000 population, which means about 1.4 million people in the United States experience TBI each year. Fortunately, 85-90% of these are mild in degree. Each year about 235,000 people are hospitalized with a diagnosis of TBI and 50,000 do not survive. To put this in perspective, it is helpful to understand how TBI compares to the occurrence of other disorders. With spinal cord injury, the incidence rate is about 4 per 100,000 and for cerebral palsy it is about 10 per 100,000. While the incidence of stoke approaches that of TBI, it usually occurs in an older population.

TBI is an injury that often affects a younger population. The occurrence of TBI peaks with age groups below 5 years, between 15-24 years, and over 70 years. Many of the people experiencing TBI are in the midst of school or just beginning adult life.

About half of those with very severe TBI do not survive. Of those who die, 50% do so within the first couple of hours after the injury. Trauma is still the leading cause of death from people ages 1 to 44. An estimated 70,000 people who survive TBI experience impairment due to the TBI, causing lasting problems in daily living.

From University of Alabama at Birmingham. Used with permission. www.uab.edu/tbi.

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