Mechanisms of TBI


Lt Col Reynolds
When we talk about mechanisms of traumatic brain injury, we’re referring to the cause of injury and the resulting physiological or structural damage. There are four primary mechanisms of TBI: direct impact, sudden or rapid acceleration and deceleration, penetrating injury, and blast injury. Dr. Freeman, can you help us understand a little bit more about the mechanisms of brain injury?

Dr. Freeman
Sure thing, Dr. Reynolds. Direct impact means the head has struck something, like the ground or a windshield, or an object has struck the head, like a baseball or a fist. Sudden or rapid acceleration and deceleration happens when nothing has come in direct contact with the head, but the brain inside the skull still experiences violent motion. This is what happens in the case of a whiplash injury, when the head moves rapidly forward and backward. Whiplash that causes a rapid turning motion of the head results in a rotational injury.

A penetrating injury is caused by high-velocity projectiles, such as bullets or shrapnel, or objects of lower velocity, such as knives or bone fragments from a skull fracture, being driven into the brain. 

A blast injury is caused by the impact from a pressure wave generated by an explosion or direct trauma to the head resulting from a blast.

There are three primary types of damage that can result from these external forces, including:

  • Diffuse axonal injury
  • Focal contusions, and
  • Hematomas, or bleeding, in or around the brain

Diffuse axonal injury, or DAI, refers to the widespread damage to the brain’s white matter. White matter is composed of bundles of axons, which, like the wires in a computer, connect different areas of the brain to one another. DAI is the result of shearing forces, which stretch, twist, or tear these axon bundles. DAI is often microscopic, meaning it cannot be detected using standard neuroimaging techniques. When DAI is severe, however, it can be seen on an MRI. DAI can occur throughout the brain, and is typically associated with mild TBI, but it can also occur with more severe TBIs.

Focal contusions are bruises or swelling in small, specific areas of the brain. These are commonly referred to as coup injuries, which is when the bruising is directly under the site of impact, and contrecoup injuries, when the bruising happens on the side of the brain opposite the site of impact. Coup-contrecoup injuries are also possible, where the brain bounces back and forth, causing bruises on both sides of the brain.

Hematomas, or bleeding, in or around the brain is another type of damage that can happen in TBI. An epidural hematoma, or EDH, involves bleeding into the area between the skull and the dura mater, which is the tough outer layer of the brain’s protective covering.  A subdural hematoma, or SDH, involves bleeding between the dura mater and the next protective layer, the arachnoid mater. Subarachnoid hemorrhage means there is bleeding beneath the arachnoid mater. Bleeding into the brain itself is called an intracerebral hematoma, or ICH. Any bleeding surrounding or within the brain is a serious medical concern.