Head injuries (or otherwise called brain injuries) have been traditionally classified into three categories, mild, moderate and severe. Mild head injuries are typically brain injuries that do not involve loss of consciousness for more than 20 minutes. Moderate involve significant loss of consciousness, but not do not involve extended coma. Severe brain injuries are those that involve a coma for a substantial length of time. For our treatment of severe brain injury, see http://waiting.com
The problem with these classifications schemes that define the severity of the brain injury in terms that relate to a period of loss of consciousness, is that they severely understate the risk factors associated with mild brain injury. Mild brain injury, which is also synonymous with concussion, can leave a person completely and totally disabled. Even though a brain injury may not involve a significant disruption of the part of the brain that triggers consciousness, it can involve severe damage to either specific parts of the cerebral cortex, or disruptive damage to the white matter of the brain.
Focal versus Diffuse Brain Damage. Brain injury is usually broken out into two geographic classes, focal and diffuse. A focal brain injury involves damage to a specific area of the brain, and in mild brain injuries, this can be a very small area. Diffuse damage means the damage is spread out throughout the brain, but the pathology in any one area is large enough for specific pathology in that one area to be identifiable.
Mild Focal Injury. Again the term mild here is something of a misnomer. The type of focal injury which would be classified as mild, would typically be a focal injury that does not involve a significant portion of one of the major lobes of the brain, but can still be identified as existing, because it has compromised a specific function of that particular part of the brain. Most of the significant mild focal injuries, involve injuries to the frontal lobes, particularly the underside of the frontal lobes. The reason these relatively small areas of damage can become disabling, is that the underside of the brain hold particularly important functions in terms of adult like behaviors and productivity.
Diffuse Injury. Diffuse injury to the white matter is referred to as diffuse axonal injury. An axon the long skinny wire like part of a neuron, that transmits the electrical impulse from the cell nucleus of the neuron, to the next part of the brain or nervous system, that must receive that signal, for the appropriate function to occur. Neuron’s are microscopic, and axons, even smaller. Typically an axon can only be seen by an electronic microscope. While there are massive numbers of these microscopic axons transmitting signals throughout the white matter of the brain, injury to even thousands of these axons in the same area, may not be concentrated enough pathology for it to show up on even a high resolution MRI. For more information on Diffuse Axonal Injury, click here.
Most of the controversy in brain injury cases involves battles about whether or not a mild brain injury even occurred and if so, whether it was severe enough to leave any last deficits. The reason such controversy exists is that most mild brain injuries do not involve clear cut loss of consciousness. For most of the 20th century, a identifiable loss of consciousness was required in order for there to be the diagnostic possibility of a brain injury. While this issue began to change as considerable research on axonal injury evolved between 1971 and 1990, the significant definition change occurred in 1992, with the publishing by the American Congress of Rehabilitation Medicines definition of Mild Traumatic Brain Injury. See http://subtlebraininjury.com/noloc.html
“It is not necessary to have a loss of consciousness to suffer permanent brain injury.”
Source: Definition of Mild Traumatic Brain Injury Developed by the Mild Traumatic Brain Injury Committee of the Head Injury Interdisciplinary Special Interest Group of the American Congress of Rehabilitation Medicine. J Head Trauma Rehabil 1993:8(3):86-87
In that definition, loss of consciousness was only one of four acute symptoms, that could form the basis of a diagnosis of concussion or MTBI. Those four events are:
Loss of Consciousness (of less than 20 minutes);
A change in mental state;
Amnesia, for events both before OR after the event; or
Focal neurological deficits.
With this 1992 definition, the medical community began to look at brain injury differently, and in subsequent years, the American Academy of Neurology and the CDC, adopted similar definitions. Now, no recognized organization still maintains the Loss of Consciousness is a prerequisite for a diagnosis of brain injury, but there are still holdouts. One of the challenges of being a brain injury attorney, is finding ways to get defense neurologists to admit that what it says in the old textbooks about loss of consciousness, is no longer good medicine or good science.
This discussion here has used the classical term of mild traumatic brain injury. However, this has been used strictly in the context of the definitional scheme that is laid out throughout our three tiered classification of brain injury. I have been since creating the web page http://subtlebraininjury.com in 1999, using the word subtle brain injury© to describe MTBI.