Types and Mechanisms of Moderate to Severe TBI
There are two types of TBI. Open head, or "penetrating," TBIs are caused by objects such as bullets and knives that penetrate the skull and injure the underlying brain. Closed head, or "non-penetrating," injuries are more common and occur when the brain moves rapidly back and forth within the bony confines of the skull. Closed-head injuries are the focus of this page.
Closed-head injuries fall into two categories: focal and diffuse. Each exerts its own type of pathological process on the brain.
Diffuse Closed-Head Injuries
The most common (and most severe) type of injury to the brain is the diffuse injury. Oftentimes, a moderate to severe diffuse injury can result in
diffuse axonal injury (DAI), which typically occurs when severe, angular deceleration forces operate on the brain (e.g., when a child hits a solid object such as a concrete floor after a fall from a balcony). This type of injury causes the shifting and rotation of the brain inside the bony confines of the skull, resulting in microscopic tears in the nerve fibers (axons) of the brain
(see illustration). DAI usually results in coma and is associated with long-term problems in neuropsychological and behavioral functioning. Neuroimaging (e.g., CT or MRI) done later on reveals tears and very small hemorrhages in the brain tissue. DAI can sometimes occur in milder forms of TBI such as concussion, where the duration of lost consciousness is less than six hours.
Focal Closed-Head Injuries
Focal brain injuries can occur to any part of the brain under the site of impact. The most common place these injuries occur is on the under surfaces of the frontal and temporal lobes. The focal injury can occur on one or both sides of the brain and vary in size and depth. Occasionally, the site of injury to the brain is opposite the site of impact. This is termed a
coup-contrecoup injury and occurs most often when the back of the head is struck ("coup") such as after a fall. In these cases, the most common area of damage is in the anterior frontal and temporal lobes ("contrecoup")
(see illustration). Injuries to these portions of the brain often give rise to predictable problems in
executive functions,
attention and
memory.
Any TBI can create both primary and secondary effects on the brain. Primary effects are those that occur at the time of the injury and are usually irreversible. Examples of primary damage include the tearing of the dura, laceration of brain tissue or intracranial hemorrhage. Secondary damage can be caused by complications related to the primary damage or from independent metabolic or systemic events. Examples of secondary effects include brain swelling (edema), hypotension, anemia, increased intracranial pressure, hypoxic-ischemic injury, hematoma and seizures. Secondary damage can add "insult to injury," by adding to the effects of the primary injury. For more detailed information about focal brain injuries, DAI and the general neuropathology of TBI, see Graham (1999).