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Understanding TBI, Part 1: What Happens to the Brain During Injury and in the Early Stages of Recovery from TBI?

Thomas Novack, PhD and Tamara Bushnik, PhD , Model Systems Knowledge Translation Center

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Understanding TBI, Part 1: What Happens to the Brain During Injury and in the Early Stages of Recovery from TBI?
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What is a brain injury?

Traumatic brain injury (TBI) refers to damage to the brain caused by an external physical force such as a car accident, a gunshot wound to the head, or a fall. A TBI is not caused by something internal such as a stroke or tumor, and does not include damage to the brain due to prolonged lack of oxygen (anoxic brain injuries). It is possible to have a TBI and never lose consciousness. For example, someone with a penetrating gunshot wound to the head may not lose consciousness.

Commonly accepted criteria established by the TBI Model Systems (TBIMS)* to identify the presence and severity of TBI include:

  • Damage to brain tissue caused by an external force and at least one of the following:
    • A documented loss of consciousness
    • The person cannot recall the actual traumatic event (amnesia)
    • The person has a skull fracture, post-traumatic seizure, or an abnormal brain scan due to the trauma

Causes of TBI

A 2004 study of emergency department records by the Centers for Disease Control indicated that the leading cause of brain injury is falls (28%) followed by car crashes (20%) and being struck by an object (19%). However, if you focus only on moderate to severe TBI (those injuries that require admission to a neurointensive care unit), car crashes are the most frequent cause of TBI, followed by gunshot wound, falls, and assault.

Types of injuries

The brain is about 3–4 pounds of extremely delicate soft tissue floating in fluid within the skull. Under the skull there are three layers of membrane that cover and protect the brain. The brain tissue is soft and therefore can be compressed (squeezed), pulled, and stretched. When there is sudden speeding up and slowing down, such as in a car crash or fall, the brain can move around violently inside the skull, resulting in injury.

Closed versus open head injury
Closed means the skull and brain contents have not been penetrated (broken into or through), whereas open means the skull and other protective layers are penetrated and exposed to air. A classic example of an open head injury is a gunshot wound to the head. A classic closed head injury is one that occurs as the result of a motor vehicle crash.

In a closed head injury, damage occurs because of a blow to the person’s head or having the head stop suddenly after moving at high speed. This causes the brain to move forward and back or from side to side, such that it collides with the bony skull around it. This jarring movement bruises brain tissue, damages axons (part of the nerve cell), and tears blood vessels. After a closed head injury, damage can occur in specific brain areas (localized injury) or throughout the brain (diffuse axonal injury).

Damage following open head injury tends to be localized and therefore damage tends to be limited to a specific area of the brain. However, such injuries can be as severe as closed head injuries, depending on the destructive path of the bullet or other invasive object within the brain.

Primary versus secondary injuries
Primary 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. Below are some primary injuries.

Secondary injuries occur after the initial injury, usually within a few days. Secondary injury may be caused by oxygen not reaching the brain, which can be the result of continued low blood pressure or increased intracranial pressure (pres¬sure inside the skull) from brain tissue swelling.

  • Skull fracture occurs when there is a breaking or denting of the skull. Pieces of bone pressing on the brain can cause injury, often referred to as a depressed skull fracture.
  • Localized injury means that a particular area of the brain is injured. Injuries can involve bruising (contusions) or bleeding (hemorrhages) on the surface of or within any layer of the brain.
  • Diffuse axonal Injury (DAI) involves damage throughout the brain and loss of conscious¬ness. DAI is a “stretching” injury to the neurons (the cell bodies of the brain) and axons (fibers that allow for communication from one neuron to another neuron). Everything our brains do for us depends on neurons communicating. When the brain is injured, axons can be pulled, stretched, and torn. If there is too much injury to the axon, the neuron will not survive. In a DAI, this happens to neurons all over the brain. This type of damage is often difficult to detect with brain scans.

Secondary injuries occur after the initial injury, usually within a few days. Secondary injury may be caused by oxygen not reaching the brain, which can be the result of continued low blood pressure or increased intracranial pressure (pressure inside of the skull) from brain tissue swelling.

Measuring the severity of TBI

“Severity of injury” refers to the degree or extent of brain tissue damage. The degree of damage is estimated by measuring the duration of loss of consciousness, the depth of coma and level of amnesia (memory loss), and through brain scans.

The Glasgow Coma Scale (GCS) is used to measure the depth of coma. The GCS rates three aspects of functioning: eye opening, movement and verbal response. Individuals in deep coma score very low on all these aspects of functioning, while those less severely injured or recovering from coma score higher. A GCS score of 3 indicates the deepest level of coma, describing a person who is totally unresponsive. A score of 9 or more indicates that the person is no longer in coma, but is not fully alert. The highest score (15) refers to a person who is fully conscious.

Understanding TBI was developed by Thomas Novack, PhD and Tamara Bushnik, PhD in collaboration with the Model System Knowledge Translation Center. Portions of this document were adapted from materials developed by the University of Alabama TBIMS, JFK Johnson Rehabilitation Institute, Baylor Institute for Rehabilitation, New York TBIMS, Moss TBIMS, and from Picking up the pieces after TBI: A guide for Family Members, by by Angelle M. Sander, PhD, Baylor College of Medicine (2002). Copyright © 2010 by University of Washington/MSKTC. 

Please check the MSKTC site for any recent updates on this article.

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