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What happens to the brain at the point of injury?
Traumatic brain injury (TBI) refers to damage or destruction of brain tissue due to a blow to the head, resulting from an assault, a car crash, a gunshot wound, a fall, or the like.
In closed head injury, damage occurs because the person receives a blow to the head that whips the head forward and back or from side to side (as in a car crash), causing the brain to collide at high velocity with the bony skull in which it is housed. This jarring bruises brain tissue and tears blood vessels, particularly where the inside surface of the skull is rough and uneven; damage occurs at (and sometimes opposite) the point of impact. Thus, specific areas of the brain - most often the frontal and temporal lobes - are damaged. This focal damage often can be detected through MRI and CAT scans.
In closed head injury, the rapid movement of the brain can also stretch and injure neuronal axons - the long threadlike arms of nerve cells in the brain that link cells to one another, that link various parts of the brain to each other and that link the brain to the rest of the body. This widespread axonal injury interrupts functional communication within and between various brain regions and sometimes between the brain and other body parts. However, this type of diffuse damage typically cannot be detected through currently available imaging technology (but with new developments, this may change). Its existence is very clear, however, in the widespread effects it has on the individual's functioning.
In sum, after a closed head injury, damage can occur both in specific brain areas (due to bruising and bleeding) and also be found throughout the brain (due to stretched or destroyed axons). The results of a closed head injury tend to affect broad areas of the individual's functioning, primarily due to the diffuse axonal injury. The extent of damage is correlated with the force of the blow to the head; for example, a head forced into a car windshield at high speed will tend to sustain more tissue damage than when the car is traveling at a slower speed.
Open head injury, the second type of TBI, occurs when the skull is penetrated, for example by a bullet. Damage following open head injuries tends to be focal, not diffuse, and the implications for subsequent impairment tend, also, to be focal and limited. 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.
What happens immediately after TBI?
Immediately following TBI, two types of effects are seen. First, brain tissue reacts to trauma and to tissue damage with a series of biochemical and other physiological responses. Substances that once were safely housed within the cells now flood the brain. These processes further damage and destroy brain cells, in what is called secondary cell death.
The second type of effect is seen in the individual's functioning. For those with more severe injuries, loss of consciousness (LOC) occurs at the time of trauma, lasting from a few minutes or hours to several weeks or even months. Lengthy LOC is referred to as coma. In such serious injuries, the first few days after trauma may also produce negative changes in respiration (breathing) and motor functions.
As an individual regains consciousness (those with the severest injuries may never do so), a variety of neurologically based symptoms may occur: irritability, aggression and other problems. Post-traumatic amnesia (PTA) is also typically experienced when an injured person regains consciousness. PTA refers to the period when the individual feels a sense of confusion and disorientation - Where am I? What happened? - and an inability to remember recent events.
As time passes, these responses typically subside, and the brain and other body systems again approach physiological stability. But, unlike tissues such as bone or muscle, the neurons in the brain do not mend themselves. New nerves do not grow in ways that lead to full recovery. Certain areas of the brain remain damaged, and the functions that were controlled by those areas may emerge as challenges in the individual's life.
Before discussing in greater detail what happens to the person after injury, which depends to great extent on the severity of injury, "severity" needs to be defined (in the next question).
What is meant by "severity of injury?"
Typically, "severity of injury" refers to the degree of brain tissue damage. Although the degree of such damage cannot be directly measured, it is estimated typically by measuring the duration of loss of consciousness (LOC) and the depth of coma (and sometimes by the length of PTA).
The scale most commonly used to measure the depth of coma is the Glasgow Coma Scale (GCS). The GCS is used to rate three aspects of functioning: eye opening, motor response, 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.
Severity of injury is typically categorized into three levels: mild (or minor), moderate and severe. A commonly used rule of thumb is that mild injury refers to LOC of less than 20 minutes and an initial GCS of 13-15. Typically, an initial GCS of 9-12 defines a moderate injury and 3-8 a severe injury.
Although initial "severity" measures may generally predict long-term impairment, initial severity scores do not correlate well with negative consequences in a person's life. The effects of TBI on individuals and the meaning of those effects depend upon a wide variety of factors, only one of which is initial "severity of injury."
How long does recovery take?
Recovery after injury is usually quite different for those with moderate-to-severe injuries versus those with mild injuries. And, as must be constantly kept in mind, recovery varies greatly from person to person. Thus, recovery will not be the same for any two people with TBI.
In mild TBI, one person may recover quickly and completely, while another may experience significant challenges even several years after injury. (Recovery after mild TBI is discussed more fully in a later question, What Problems Emerge after a Mild TBI?)
From Mount Sinai Medical Center. www.mssm.edu.