Primary Injuries
Damage to the brain after traumatic brain injury (TBI) is the result of several different processes. These can be classified as to whether they are a direct result of the trauma (for instance, bruising to the brain) or occur later because of other processes (such as damage caused by low blood- oxygen levels). Direct brain damage can be classified as either focal or diffuse. Focal injuries affect particular parts of the brain (such as the front of the brain) and are usually the result of direct bruising or contusions. The effect of a contusion depends on the part of the brain that is injured. For instance, a contusion to the part of the brain that controls language might make it difficult for a person to speak. Another type of focal injury is bleeding (hemorrhage) and is named based on where it occurs in the brain. Sometimes the bleeding occurs outside of the brain, that is, between the brain and the skull. These hemorrhages have different names, depending on the exact location of the bleeding. The most common are epidural, subdural, and subarachnoid hemorrhages. Hemorrhages can also occur in the brain itself and are known as intracerebral hemorrhages

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Although focal injuries such as contusions or hemorrhages are important, most of the brain damage after TBI is widespread (diffuse). In particular, the white matter of the brain is almost always injured during a TBI. This is a result of the acceleration-deceleration forces that occur in most types of trauma (for instance, in a motor vehicle collision). Because these forces cause shearing of the fibers (axons) that make up the white matter, this type of damage is sometimes referred to as shear injury. Another common name for this type of damage is diffuse axonal injury (DAI). DAI is responsible for most of the problems caused by traumatic brain injury. The more severe the brain injury, the more extensive is the DAI. Unlike contusions and hemorrhages, which are easily seen on CT scans (“cat scans”) of the brain, DAI is often hard to detect on CT scans. However, studies have shown that most cases of prolonged unconsciousness are the result of extensive DAI, whether it is detected on CT scan or not.

The focal and diffuse injuries discussed so far are a result of the physical damage to the brain. However, much of the damage to the brain is a result of chemical changes in the brain that occur after trauma. Many toxic substances are released in the brain that further increase the amount of damage. Currently, there is no method available in routine clinical practice to measure these substances. And there are no definitive treatments yet to reduce these chemical and inflammatory changes.

Secondary Injuries
Primary injury to the brain typically occurs or begins at the moment of impact. However, other problems might develop later that can increase the amount of damage to the brain. Most of these secondary injuries are diffuse. Examples include damage to the brain that can be caused by not receiving enough blood or oxygen. This might happen if someone stopped breathing or had low blood pressure at any time after the injury. Other processes that can potentially worsen the brain injury include fevers, high blood sugars, cerebrospinal fluid infections, etc. It is important to be aware of both the primary and any secondary injuries in order to get a clear picture of the extent of brain damage a person has suffered.