Traumatic brain injury (tbi) is classically defined as an insult to the brain caused by an external physical force with a resulting alteration in brain function. The insult can be the result of blunt trauma, penetrating, acceleration-deceleration, or blast wave injuries. The result of such an impact varies highly from patient to patient and has been correlated with the severity of injury.1 TBI is considered a leading cause of death and long-term disability worldwide and affects all age groups and genders; however, children and the elderly are often considered at a higher risk of experiencing a TBI.2 Despite the tremendous effort invested in TBI research, the ability to minimize chronic neurologic deficits in TBI patients has remained extremely limited. This is likely a result of the vast complexity of the pathophysiology of a TBI. In fact, it is necessary to continue to explore and understand the underlying mechanisms of a TBI and how these mechanisms relate to functional outcomes.
This chapter summarizes the current understanding of the pathophysiology of brain injury. The pathophysiology can be divided into primary and secondary injury. Primary injury is the immediate result of the traumatic event resulting from mechanical forces. Secondary injury is a result of the primary injury and results in a deleterious cascade of complex pathways causing secondary injury.
The chapter is divided into two sections: the pathophysiology of primary brain injury and the resultant secondary brain injury. The first section illustrates the various pathologies associated with the primary brain injury, that is, those that result from the initial physical or nonphysical impact to the brain. The second section illustrates the complexity of secondary brain injuries, as well as the pathophysiological change that occurs in the hours, days, and months after the initial neurologic insult.
PATHOPHYSIOLOGY OF PRIMARY BRAIN INJURY
A multitude of studies have been performed in an attempt to classify the various types of traumatic brain injury. The results of these studies have demonstrated that primary brain injuries fall into one of the following categories based on physical mechanism of injury: focal injuries, diffuse injuries, or a heterogeneous mixture of the two.3–7
Focal brain injuries are defined as injuries that occur in a localized area of the brain and are the result of direct mechanical forces applied to the skull. These forces can result in skull fracture, contusions, hematomas (subdural, epidural, and intraparenchymal), and hemorrhages both in the subarachnoid and intracranial spaces.
One of the most immediate effects of blunt force trauma to the skull is the potential for a resulting skull fracture. Skull fractures are classified by the location and force of the impact, as well as the size and shape of the object that made the impact to the skull. The types of fractures that can be ...