Primary and secondary brain injury

Primary and secondary brain injury are ways to classify the injury processes that occur in brain injury. In traumatic brain injury (TBI), primary injury occurs during the initial insult, and results from displacement of the physical structures of the brain. On the other hand, secondary injury occurs gradually and may involve an array of cellular processes. Secondary injury, which is not caused by mechanical damage, can result from the primary injury or be independent of it. The fact that people sometimes deteriorate after brain injury was originally taken to mean that secondary injury was occurring. It is not well understood how much of a contribution primary and secondary injuries respectively have to the clinical manifestations of TBI.

Primary and secondary injuries occur in insults other than TBI as well, such as spinal cord injury and stroke.

In TBI, primary injuries result immediately from the initial trauma. Primary injury occurs at the moment of trauma and includes contusion, damage to blood vessels, and axonal shearing, in which the axons of neurons are stretched and torn. The blood brain barrier and meninges may be damaged in the primary injury, and neurons may die. Cells are killed in a nonspecific manner in primary injury. Tissues have a deformation threshold: if they are deformed past this threshold they are injured. Different regions in the brain may be more sensitive to mechanical loading due to differences in their properties that result from differences in their makeup; for example, myelinated tissues may have different properties than other tissues. Thus some tissues may experience more force and be more injured in the primary injury. The primary injury leads to the secondary injury.

Secondary injury is an indirect result of the injury. It results from processes initiated by the trauma. It occurs in the hours and days following the primary injury and plays a large role in the brain damage and death that results from TBI. Unlike in most forms of trauma a large percentage of the people killed by brain trauma do not die right away but rather days to weeks after the event. In addition, rather than improving after being hospitalized as most patients with other types of injuries do, about 40% of people with TBI deteriorate. This is often a result of secondary injury, which can damage neurons that were unharmed in the primary injury. It occurs after a variety of brain injury including subarachnoid hemorrhage, stroke, and traumatic brain injury and involves metabolic cascades.

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