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.