Basic Neuroscience of Neurotrauma

Percussion Concussion

The FP injury model produces a brief 22-ms deformation of the exposed cortex. A craniotomy is performed, and a plastic modified Luer-Lok injury tube is attached to the exposed dura and sealed. A Plexiglas cylinder filled with sterile saline is attached to the injury tube via a metal fitting. A pendulum is released on the other end and at impact produces a pressure wave that results in a bolus injection of saline onto the exposed dura. By varying the distance of the pendulum swing, several degrees of injury severity—mild (1.1–1.5 atm), moderate (2.0–2.3 atm), and severe (2.4–2.6 atm)—can be studied in a reproducible fashion. This is an important model characteristic because of the heterogeneous nature of human TBI and the possibility that treatment strategies may vary with injury severity. The central, lateral, and parasagittal FP models are characterized by brief behavioral responsiveness (e.g., coma), metabolic alterations, changes in local cerebral blood flow (LCBF), blood–brain barrier (BBB) permeability, and behavioral deficits. The central FP model tends to involve variable and small contusions in the vicinity of the fluid pulse as well as scattered axonal damage mostly limited to the brainstem. In contrast, lateral and parasagittal FP is characterized by a lateral cortical contusion that is remote from the impact site. Evidence of axonal damage is seen throughout the white matter tracts in the ipsilateral cerebral hemisphere, and tissue tears are seen at gray matter/white matter interfaces. Hippocampal damage is pronounced in the lateral and parasagittal FP injury models, with little brainstem damage. The FP injury model thus produces a range of disorders including contusion, widespread axonal injury, and selective neuronal necrosis.

Another commonly used rodent TBI model is controlled cortical impact injury. In this model, a bone flap is removed and the impact device is vertically driven into the cerebral cortex to produce tissue displacement. This model produces a well-demarcated cortical contusion with variable degrees of hippocampal involvement depending on velocity and deformation depth. An advantage of the controlled cortical impact model is that it can be used in mice and allows the testing of genetically altered mice to help determine the cause-and-effect relationships between gene expression and cell injury.

Acceleration Concussion

Inertial acceleration models can produce pure acute subdural hematomas and diffuse axonal injury. These models accelerate the head of the animal in one plane followed by a rapid deceleration, resulting in movement of the brain within the cranial cavity. The inertial acceleration models are designed to mimic motor vehicle accidents. Tissue-tear hemorrhages occur in the central white matter, and gliding contusions occur in the parasagittal gray matter/white matter junctions. These models are characterized by a variable period of coma and axonal damage in the upper brainstem and cerebellum. For the impact acceleration model, the animal is placed on a foam bed with a metal disk attached to the exposed skull. A weight is then dropped from a specified height to produce brain injury. This model also produces prolonged coma and widespread axonal damage. However, these models are characterized by variable and somewhat uncontrolled skull fractures.

Human head injury is never as pure as an experimental model, and the total human injury condition may not be addressed adequately with a single animal model. Therefore the use of complicated models in assessing pathomechanisms after TBI can provide valuable data. Many of these studies have added secondary insults such as hypotension (shock), hypoxia, or hyperthermia after the primary injury to more adequately mimic human head trauma. However, this added component to the model can result in complex data interpretation. Thus, once a particular feature of human brain injury has been produced in an experimental model, the pathogenesis of the injury can be critically investigated.

Military Models

The incidence of brain injury is currently estimated to be as high as 40%–70% of combat casualties. Advancements in protective body armor have limited the number of high-velocity impact injuries to the brain, but the use of improvised explosive devices has caused blast injury to become more prevalent. Military-relevant models have only recently been developed, and much work is needed to fully explore pathomechanisms induced by these models ( ; ). Two models of significance are the penetrating ballistic brain injury (PBBI) model and a blast injury model using a metal tube with a small amount of plastic explosive at one end. The PBBI model mimics a bullet wound by expanding and contracting a balloon that has been inserted into the brain. Injury severity and placement of the probe can be varied so that different types of bullet injuries can be studied. PBBI produces robust histopathological damage and increases in intracranial pressure (ICP) and hemorrhage. In addition, sensorimotor deficits are observed in this model, along with seizure activity. Blast injury models that can mimic whole-body blast or local blast injury have been developed ( ). This type of trauma produces cognitive deficits as well as ultrastructural and oxidative damage to the hippocampus. Axonal damage is also present, as indicated by increases in phosphorylated neurofilament proteins. These military-relevant injury models have the potential to provide valuable information for managing and treating combat casualties.

In Vitro Models

A shortcoming of animal models is that they preclude a critical assessment of individual cell responses to trauma. In animal experiments, for example, the cellular response to injury may be a consequence of both primary and secondary events initiated by a complex cascade of cellular interactions. To critically investigate the consequences of injury on a specific cell type in the absence of confounding cellular and systemic factors, several in vitro cell culture models have been developed. Models range from scratching the culture with a pipette tip to inducing cellular deformation by stretching cultured cells. Most recently, these models have incorporated multiple cell types including neurons, astrocytes/glia, and endothelial cells in order to study cellular interactions after injury ( ).

Using these approaches, investigators have found that trauma induces a wide range of primary cellular alterations. Astrocytic responses include hyperplasia, hypertrophy, and increased glial fibrillary acidic protein content. Increases in intracellular calcium occur, which are blocked by specific receptor antagonists. Traumatized astrocytes also produce interleukins (ILs) and neurotrophic factors. Neonatal cortical neurons that are stretched undergo delayed depolarization that depends on the activation of specific receptor populations. Combined mechanical trauma and metabolic impairment in vivo also induces N -methyl- d -aspartate (NMDA) receptor–dependent neuronal cell death and caspase-3–dependent apoptosis. In vitro experimental approaches provide novel data concerning intracellular signaling cascades, mechanisms underlying cellular responses to trauma, and the role of specific cell types in the pathophysiology of brain trauma ( ).

Temporal Patterns of Neuronal Death

The neuropathological sequelae of experimental and human TBI have been well described ( ). In experimental TBI, temporal patterns of neuronal damage have also been characterized. As early as 6 hours after cortical contusion injury, the contused tissue appears edematous and pyknotic neurons are apparent at the injury site. By 8 days, a cortical cavity surrounded by a border containing necrotic tissue, a glial scar, or both has developed. The temporal profile of neuronal damage after parasagittal FP brain injury has also been assessed with light and electron microscopy. As early as 1 hour after impact, dark shrunken neurons indicative of irreversible damage are seen in cortical layers overlying the gliding contusion, which displays BBB breakdown to protein tracers. Ultrastructural studies demonstrate that early BBB dysfunction results from mechanical damage of small venules in vulnerable regions, including the external capsule. In some brain regions, focal sites of acute neuronal damage are associated with extravasated protein, whereas neuronal damage in other regions appears to occur without overt BBB breakdown. Astrocytic swelling is observed early after injury, with increased glial fibrillary acidic protein immunoreactivity apparent at later times in some areas, demonstrating histopathological damage ( Fig. 60.1 ). In terms of neuroprotection, the acuteness of this damage limits the potential for therapeutic interventions directed against the early neuronal and glial response to TBI.

More subacute patterns of neuronal injury have been documented in various TBI models. At 3 days after moderate parasagittal FP brain injury, scattered necrotic neurons are present throughout the frontoparietal cerebral cortex remote from the impact site ( Fig. 60.2 ). In addition, selective neuronal damage is seen in the CA3 and CA4 hippocampal subsectors, the dentate hilus, and lateral thalamus ipsilateral to the trauma. These patterns of selective neuronal damage are associated with a well-demarcated contusion overlying the lateral external capsule. Ultrastructural changes consistent with apoptosis have been described after TBI; therefore delayed patterns of neuronal cell death may involve necrotic and programmed cell death processes. Additional nonapoptotic cell death mechanisms have also been identified, including necroptosis and pyroptosis, both being demonstrated in models of TBI. Some studies have shown that, should apoptosis fail to execute, these alternative nonapoptotic forms of cell death may be activated ( ). In addition to these cell death mechanisms in some TBI studies, autophagy has been associated with various forms of cell death in other conditions ( ). Numerous interconnections between autophagy and the other cell death mechanisms have been described in models of TBI. Pyroptosis, a caspase 1–dependent form of cell death, is an inflammatory type of cell death associated with cell swelling and rapid membrane breakdown. In this cell death mechanism, caspase-1 is activated by a multiprotein complex termed the inflammasome that also responds to molecular and biochemical toxins and viral RNA. Recent evidence in models of TBI has shown that inflammasome-induced pyroptotic cell death occurs in models of TBI and can lead to neuronal inflammatory cell death ( ). Finally, all other cell death mechanisms—commonly termed caspase-independent cell death ( ), resulting from mitochondrial membrane permeabilization—have also been described in models of TBI and have additional distinct morphological and biochemical characteristics. It is important to understanding the underlying cellular molecular mechanisms of cell death after TBI because they present potentially new targets for therapeutic intervention that may reduce the degree of both cell death and axonal damage, thus improving functional outcomes. Continued investigations into underlying factors that lead to these distinct cell death mechanisms and new therapeutic interventions that target cell vulnerability in models of TBI could lead to new improvements and acute neuroprotection as well as reducing TBI-induced progressive neural degenerative disorders.

Selective Neuronal Vulnerability

Damage to the hippocampus is commonly reported in autopsy studies of head-injured patients. Clinical and experimental studies describe cognitive abnormalities thought to be associated with hippocampal dysfunction. In an acceleration model of brain injury in nonhuman primates, CA1 hippocampal histopathological damage was reported in the majority of animals. CA1 damage was not produced by secondary global ischemia, elevated ICP, or seizure activity alone. In this regard, CA1 hippocampal damage is not routinely reported in other TBI models, including cortical contusion and moderate FP injury. However, midline FP injury followed by a delayed sublethal global ischemic insult leads to CA1 neuronal damage. The studies indicate the importance of injury severity on outcome and the vulnerability of the posttraumatic brain to secondary insults. Finally, the dentate gyrus, bilateral dentate hilus, and CA3 hippocampus have also been reported to be selectively damaged in FP models.

Thalamic damage after brain injury is described in clinical and experimental studies. In human brain injury, the loss of inhibitory thalamic reticular neurons is proposed to underlie some forms of attention deficits. In radiographic studies of patients with TBI using magnetic resonance imaging, relationships between injury severity, lesion volume, ventricle-to-brain ratio, and thalamic volume have been reported. Patients with moderate to severe injuries have smaller thalamic volumes and greater ventricle-to-brain ratios than patients with mild to moderate injuries. Decreased thalamic volumes suggest that subcortical brain structures may be susceptible to transneuronal degeneration after cortical damage. Focal damage to thalamic nuclei seen after long-term ( ) FP injury may result from progressive circuit degeneration after axonal damage, neuronal cell death, or lack of neurotrophic delivery.

Progressive Damage

Only recently has the progressive nature of the histopathological consequences of TBI been appreciated (Bramlett and Dietrich, 2015). At 2 months after moderate parasagittal FP injury, significant atrophy of the cerebral cortex, hippocampus, and thalamus is apparent in histological sections ( Fig. 60.3 , A ). Progressive tissue loss in the cortex and hippocampus at various times up to 1 year after lateral FP injury has been reported. Similar findings have been found in a model of controlled cortical impact injury. At 3 weeks and 1 year after injury, analysis demonstrated a significant hemispheric volume loss and expansion of the ipsilateral lateral ventricle. Thus atrophy of gray matter structures is associated with significant enlargement of the lateral ventricle.

Ventricular expansion not associated with hydrocephalus or increased ICP is felt to be a sensitive indicator of structural damage and an indirect measure of white matter atrophy. Indeed, ventricular size has been correlated with memory disturbances; patients with the highest ventricular volumes demonstrated significantly lower memory scores. Recent findings provide direct evidence of progressive white matter damage after FP brain injury ( ). At 1 year after TBI, severe atrophy of specific tracts including the external capsule and cerebral peduncle was documented (see Fig. 60.3 , B ). This white matter vulnerability may result from direct trauma-induced axonal damage as well as recently described oligodendrocyte vulnerability after TBI. In experiments where long-term survival has been evaluated in both animal models and human tissue, patterns of chronic microglial activation have been identified; this underlines the progressive nature of the inflammatory response after TBI ( ). The importance of a progressive injury cascade after TBI in terms of other neurological conditions merits consideration. For example, if mild head trauma leads to a progressive reduction in neuronal reserve, would the person who sustained such trauma be more susceptible than others to neurodegenerative processes associated with aging? Recent findings indicate that TBI may also be a risk factor for the development of age-associated neurodegenerative disorders including Alzheimer disease, Parkinson disease, amyotrophic lateral sclerosis, and multiple sclerosis ( ). The observation that pathological processes which potentially affect long-term outcome may be active days or even months after injury provides new targets to improve outcome after central nervous system (CNS) injury. This new way of assessing and treating acute injuries is also important as we think about how TBI may enhance the vulnerability of the aging brain and underlying mechanisms responsible for syndromes including chronic traumatic encephalopathy (CTE) ( ). Transgenic models of neurodegenerative diseases are now being produced and investigated to clarify novel injury cascades that may underlie the increased vulnerability of the postinjured brain to patterns of progressive injury ( ).