As stated earlier, the most common cause of closed head injury is automobile accident which involves acceleration - deceleration injury to the brain. As one can imagine, in automobile accidents, focal, multifocal, and diffuse injury to brain tissue may occur. The type and extent of damage will be directly related to the speed and direction of head movements (linear or rotational acceleration) and the time and distance of deceleration. The typical head on automobile accident results in the individual's head moving forward and stopping (due to hitting the steering wheel, dashboard, or windshield, or being restrained by the seatbelt and airbag), and then, after stopping, moving backwards to land against the headrest. In this scenario, the brain, in fact, moves forward within the cranium and may rapidly impact against the irregular frontal and temporal bones which can directly damage associated tissue through contusion (bruising) or hemorrhage. After the skull and brain have terminated their forward movement, the brain can bounce backward in the opposite directly and impact (in this case) the occipital bone. This movement of the brain inside the skull in the opposite direction of the first impact is referred to as the contra coup effect. This opposite area of impact is referred to as the contra coup site, which can in fact, result in as much or more damage to brain tissue as the original site of impact. In automobile accidents where the occupant may be thrown in several directions within the vehicle, there may be multiple impact and contra coup sites. The secondary effects of such closed head injuries may include hemorrhaging (bleeding) which can occur intracerebrally from stress to the vascular system, or hematomas (collections of blood typically noted between the skull and brain associated with meningial tearing: subdural and epidural hematomas or subarachnoid hemorrhage), and edema (swelling) of the brain.
Hemorrhage may cause brain tissue necrosis based upon lack of oxygen saturation (ischemia), increased intracranial pressure (ICP), and the direct toxic effect of blood on brain tissue when it is not confined to arteries, vessels, and capillaries (it is treated as a foreign body in the brain by the immune system). Edema also creates ICP which can also directly effect brain tissue (which we will discuss later this semester: Figure: Necrosis of Brain Tissue in the Inferior Frontal Lobes and Anterior Right Temporal Lobe Secondary to Moderate Closed Head Injury with Hemorrhage