An excellent study in the Journal of Neurotrauma (Browne KD et al. 2011) has shed light on another aspect of symptom onset – unconsciousness. Many insurance companies and some unread doctors, will insist that no brain injury can have occurred in an accident whereby the victim did not lose consciousness. They would likewise suggest that the primary method of brain injury in trauma, diffuse axonal injury, can likewise not occur unless the victim has a loss of consciousness. However, Dr. Browne and his colleagues using pigs, very rapidly rotated their heads in different directions without striking an object. This motion alone was found to cause diffuse axonal injury at levels consistent with mild traumatic brain injury. Interestingly, they found that rotation in the axial plane (being hit in the rear or front of a vehicle moving forward to back) produced unconsciousness in the pigs, while forces in which the head rotated along the croronal plane did not produce a sustained loss of consciousness. A week after the trauma the brains of the pigs were looked at and it was shown that both types of rotation produced similar and significant amount of DAI in both pigs, while the axial plane pigs brain stem showed signs of injury.
Therefore, it can now be said that loss of consciousness during an accident is a feature of the direction of movement of the head more than it is about the seriousness of the eventual brain injury found. Damage to the neurons of the brain can occur and do occur in the context of a mild traumatic brain injury with or without the unconsciousness that arises from damage or pressure to the brain stem. Loss of consciousness is not a equirement for brain injury nor for the presence or absence of DAI damage in the brain.
This study sheds light on why certain car accidents tend to knock out the occupants, while others do not. It is well known that angular rotational forces (e.g. a car hit from the side and not the rear) can injure the brain more readily.