Mechanisms of Injury
What is the physiological impact of a traumatic brain injury?
A traumatic brain injury (TBI) can occur when there is a force on the head that results in penetration of the skull (aka open-head injury), or when there is a force to the head that leaves the skull intact but results in injury to the brain tissue (aka closed-head injury).
External mechanical forces that can cause TBI
- Blow to head (e.g., fall onto pavement, head impacting windshield in a car accident, assault, baseball striking head)
- Acceleration-deceleration forces, wherein no direct impact is required (e.g., a restrained passenger coming to a sudden stop when a car strikes a tree or telephone pole)
- Projectile missile (e.g., bullet)
- Concussive forces (e.g., blast waves from an explosion)
Neurological events that may occur as a result of trauma to the head
- Direct injury or bruising (contusion) to surface brain tissue.
- Trauma to the blood vessels of the brain causing bleeding (hematoma).
- A hematoma (bleeding) can damage the brain tissue it contacts, and can increase pressure in the brain.
- As a consequence of contusions and bleeding, intracranial pressure may rise, which can shift and put pressure on brain tissue due to the confined space of the skull.
- Adding to the building pressure is the possibility of edema, swelling of brain tissue due to trauma.
- Typically associated with acceleration-deceleration injuries, there may be shearing or twisting of axonal fibers (white matter tracts) in the brain.
- This occurs most often in the frontal and temporal lobes.
- Midline structures (e.g., corpus callosum, anterior commissure) may also be affected, which can disrupt communication between left and right cerebral hemispheres.
- Extensive shearing can cause Diffuse Axonal Injury (DAI), a serious condition that may be associated with coma and poor outcome.
- Changes in neurotransmitter (i.e., chemical communicative agents in the brain) release
- Increase in glucose metabolism in the context of decreased cerebral blood flow leads to an energy deficit in the brain
- Hypoxia (i.e., decreased oxygen to the brain)
- Changes in the flow of cerebral spinal fluid (CSF; e.g., hydrocephalus)
Brain Regions Commonly Affected in TBI
- Coup injury -- damage occurring at the point of head trauma impact (e.g., where the head hit the ground in a fall) as the brain slides forward within the skull and impacts the inside of the skull.
- This can occur even if there is no direct external impact to the head (e.g., sudden deceleration in a motor vehicle accident).
- Countercoup injury -- damage occurring when the head/brain rebounds from the initial force and either impacts an object (e.g., headrest in a motor vehicle accident) or simply rapidly moves in the opposite direction.
- The frontal lobes rest on bony hills of the skull just above the eyes. When the brain moves within the skull, these “hills” can cause a grating action against the brain tissue, leading to injury.