Primary and secondary injury


Primary and secondary Brain injury in Chino Hills

MRI scan showing damage due to brain herniation after TBI

A large percentage of the people killed by brain trauma do not die right away but rather days to weeks after the event; rather than improving after being hospitalized, some 40% of TBI patients deteriorate. Primary brain injury(the damage that occurs at the moment of trauma when tissues and blood vessels are stretched, compressed, and torn) is not adequate to explain this deterioration; rather, it is caused by secondary injury, a complex set of cellular processes and biochemical cascades that occur in the minutes to days following the trauma. These secondary processes can dramatically worsen the damage caused by primary injury and account for the greatest number of TBI deaths occurring in hospitals.

Secondary injury events include damage to the blood–brain barrier, release of factors that cause inflammation, free radical overload, excessive release of the neurotransmitter glutamate (excitotoxicity), influx of calcium and sodium ions into neurons, and dysfunction of mitochondria. Injured axons in the brain’s white matter may separate from their cell bodies as a result of secondary injury,  potentially killing those neurons. Other factors in secondary injury are changes in the blood flow to the brain; ischemia (insufficient blood flow); cerebral hypoxia (insufficient oxygen in the brain); cerebral edema (swelling of the brain); and raised intracranial pressure (the pressure within the skull). Intracranial pressure may rise due to swelling or a mass effect from a lesion, such as a hemorrhage.  As a result, cerebral perfusion pressure (the pressure of blood flow in the brain) is reduced; ischemia results. When the pressure within the skull rises too high, it can cause brain death or herniation, in which parts of the brain are squeezed by structures in the skull.  A particularly weak part of the skull that is vulnerable to damage causing extradural haematoma is the pterion, deep in which lies the middle meningeal artery, which is easily damaged in fractures of the pterion. Since the pterion is so weak, this type of injury can easily occur and can be secondary due to trauma to other parts of the skull where the impact forces spreads to the pterion.


Source: Wikipedia