Overview

Definition
Traumatic brain injury (TBI) occurs whenever a physical trauma causes damage to the brain. The brain is injured as a direct result of the external forces that it receives.

Incidence and Prevalence
The most common causes of TBI in the United States are transportation-related crashes, falls, and assaults. Overall, approximately 1.4 million people sustain a TBI in the U.S. each year. The vast majority of these injuries (75 percent) are classified as mild (sometimes referred to as concussions) and usually are treated in an emergency room. More severe brain injuries result in 50,000 deaths and 235,000 hospitalizations each year. TBI often causes long-term disability. An estimated 5.2 million people in the U.S. (about 2 percent of the population) will need help with daily living as a result of a TBI.

Types of TBI
Traumatic brain injuries are often classified by the type of trauma that causes them. The two primary types are blunt or penetrating injuries. Penetrating injuries occur whenever an object actually pierces the skull and enters the brain. An example would be a gunshot wound to the brain. Blunt injuries cause brain damage without penetrating the skull (although they might cause cracks or fractures in the skull).  Blunt injuries often involve a direct impact to the head (for instance, being struck in the head with an object). However, a direct impact is not necessary for a brain injury to occur. Even without impact, the brain can be damaged by acceleration-deceleration forces. These forces are present whenever a person who has been moving quickly (such as in a car) suddenly stops. An example would be a motor vehicle collision in which the head did not hit the steering wheel or windshield. Acceleration-deceleration forces often cause more damage to the brain than direct impact. Most causes of TBI involve both direct impact and acceleration-deceleration forces (for instance, a fall from a height).

Levels of severity
Traumatic brain injuries are classified as mild, moderate, or severe based on the initial change in the level of consciousness. A mild TBI results in a brief period of confusion or loss of consciousness, whereas moderate or severe injuries result in deeper and more prolonged periods of unconsciousness. In clinical practice, the distinction between mild, moderate, and severe brain injuries is made through the use of the Glasgow Coma Scale (GCS). 

Table 1. Glasgow Coma Scale (GCS)

This scale scores patients based on their performance in three areas: moving their arms or legs, opening their eyes, and talking. The examination of the patient is performed as soon as possible after the injury (usually in the emergency room). The lowest score possible is a 3 (1 in each area) and the highest possible score is 15 (the higher the score, the lesser the injury and the better the patient). The initial GCS score is 3-8 in severe TBI, 9-12 in moderate TBI, and 13-15 in mild TBI. 

Table 2. Severity of TBI Based on GCS Score

Although the initial GCS score has some relationship to prognosis, many other factors can affect outcome after TBI. In particular, people who have severe brain injuries (based on their initial GCS score) can still have very good outcomes. Likewise, persons who have had a moderate or even mild TBI may have long-term disabilities as a result of their injuries. Prognosis is discussed in more detail below.

Basic Anatomy
In order to understand the effects of brain injury, it is useful to have a basic understanding of the anatomy of the brain and the parts of the head that cover it. There are several layers that separate the brain from the outside world. 

Directly under the scalp is the skull. Below the skull, there is a thick, fibrous layer known as the dura that covers the brain. The space between the dura and the brain is filled by cerebrospinal fluid (CSF). Finally, directly covering the brain is a thin layer known as the arachnoid layer. In addition to the cerebrospinal fluid that surrounds the brain, there are hollow spaces in the brain that are also filled with CSF. These spaces are called ventricles.

The brain itself is made of a thin outer layer and the much larger area inside.

The outer layer is known as the cerebral cortex and is often referred to as the gray matter. The inside or interior of the brain is known as the white matter. The white matter is mainly made up of long fibers (known as axons) that carry chemical and electrical messages from one part of the brain to another. The gray matter or cortex is made up of cells known as neurons. Different areas of the cortex control different functions.

For instance, some areas of the cortex control our ability to move our bodies. Others allow us to receive information from our senses of sight, hearing, touch, etc. Still other parts of the cortex control our thinking and emotions.